ORGANIC MATERIALS WITH ACENOID AND IPTYCENE STRUCTURES
BY
ZHIHUA CHEN
B.S., Chemistry
Tsinghua University, 1998
Submitted to the Department of Chemistry
in Partial Fulfillment of the Requirements for the Degree of
DOCTOR OF PHILOSOPHY
at the
MASSACHUSETTS INSTITUTE OF TECHNOLOGY
September 2007
C 2007 Massachusetts Institute of Technology. All Rights Reserved.
IN,
/
Signature of Author:
Department of Chemistry
August 30, 2007
Certified by:
SJU
Timothy M. Swager
Thesis Supervisor
Accepted by:
S~SACHUSETTS INSTITUTE
OF TECHNOLOGY
SEPI7 2007
LIBRARIES
Robert W. Field
Chairman, Departmental Committee on Graduate Studies
This doctoral thesis has been examined by a Committee of the Department of Chemistry as
follows:
Professor Rick L. Danheiser:
- V
Chairman
6
Professor Timothy M. Swager:
Daresis Advisor
Professor Sarah E. O'Connor:
Department of Chemistry
ForBingling and my parents
ORGANIC MATERIALS WITH ACENOID AND IPTYCENE STRUCTURES
BY
ZHIHUA CHEN
Submitted to the Department of Chemistry
on August 31, 2007 in Partial Fulfillment of the
Requirements for the Degree of
Doctor of Philosophy in Chemistry
ABSTRACT
Chapter 1. The synthesis of a group of alkoxy-substituted para-acenequinones and their
photophysical properties in solution and liquid crystal are reported. Polarized absorption and
fluorescence measurements demonstrate that these acenequinones have excellent alignment
with nematic LC hosts, indicating their potential as dichroic dyes for guest-host liquid crystal
displays. In addition, the sensitivity of emission to the substitution allows the tuning of
emission by functionalization of the acenequinone chromophore.
Chapter 2. The synthesis of a series of fluorine- and alkyl/alkoxy-functionalized tetracenes
using N-methyl-1,2,4,5-tetrafluoroisoindole as a synthetic building block is reported. The
incorporation of fluorine functionalities was found to induce significant face-to-face
molecular n-stacking in their crystal structures. Electrochemical behaviors and UV-vis
absorbance spectroscopy results of these materials are also discussed. It was demonstrated
that the substitution of alkyl/alkoxy groups on the main chain not only provided better
solubility in common organic solvents, but also subtly tuned the crystal structures and
electrochemical behaviors.
Chapter 3. A self-polymerizable AB-type monomer for Diels-Alder (D-A) polymerization
was prepared, and its polymerization was carried out in the melt phase and at high pressure in
solution. The former method generated only low molecular weight polymer, but the latter one
offered an efficient polymerization with increased molecular weight, due to the effect of highpressure on reactions with a negative activation volume. A pyridinium p-toluenesulfonate
catalyzed dehydration reaction of the D-A polymer led to a novel aromatic ladder polymer,
poly(iptycene), which is soluble in common organic solvents and stable up to 350 oC. The
NMR and UV-vis spectra of these polymers match the spectra of their corresponding model
compounds, and their synthesis is also reported.
Chapter 4. Two diamino functionalized iptycene monomers were successfully synthesized
via two synthetic routes: a direct nitration of triptycene followed by a reduction with
hydrazine and D-A reaction between a benzo-fused 1,4-endoxide and 2,6-diaminoanthracene
followed by a strong acid catalyzed dehydration. Their applications in the synthesis of novel
triptycene-containing polyimides and polyureas were investigated and the resulting polymers
were characterized by NMR, FT-IR, and UV-vis absorption spectroscopy.
Chapter 5. Iptycene type quinoxaline and thienopyrazine monomers were successfully
synthesized via a condensation between 10-dihydro-9,10-ethanoanthracene-11,12-dione and
the corresponding diamines. Copolymers based on fluorene and these new iptycene
monomers were prepared via Suzuki coupling reaction, and they exhibited good solubility in
appropriate organic solvents. These copolymers are fluorescent both in solution and the solid
state, emitting blue, greenish-blue, and red color, due to the electronic properties of the
iptycene comonomers. The difference in their absorption and emission spectra was attributed
to the donor-acceptor charge transfer interactions and/or polymer backbone conformation
change induced by steric effects. Moreover, the spectroscopic data clearly demonstrated the
insulating effect of iptycene units, which prevented the aggregation of the polymer chains and
formation of excimers in the solid state.
Thesis Supervsior: Timothy M. Swager
Title: Department Head, Professor of Chemistry
Table of Contents
D edication .................................................................................
........................................
3
Abstract ....................................................................................
.........................................
4
Table of Contents ...........................................................................
List of Figures ..............................................................................
..................................
6
.....................................
9
List of Tables ........................................................................................................................
List of Schem es ....................................................................................
12
.......................... 13
Chapter 1: Acenequinones as Dyes for Guest-Host Liquid Crystal Displays ................
Introduction ......................................................................................
..............................
Results and Discussion ............................................................................. ......................
Conclusion ........................................................................................
..............................
Experim ental Section .......................................................... ............................................
References and Notes .............................................................................. .......................
14
15
24
33
33
44
Chapter 2: Syntheses of Soluble, vt-Stacking Tetracene Derivatives for OFETs ............
Introduction .......................................................................................
.............................
Results and D iscussion ............................................................................. ......................
Conclusion .............................................................
...................................................
Experim ental Section .......................................................... ............................................
References and N otes ..............................................................................
.......................
48
49
58
67
68
79
Chapter 3: Synthesis and Characterization of a Novel Poly(iptycene) Ladder Polymer ....84
Introduction ........................................................
............................................................ 85
R esults and D iscussion ......................................................................................................
87
Conclusion .........................................................................................................
............. 99
Experim ental Section .....................................................................................................
99
References and N otes ..................................................................................................
109
Chapter 4: Synthesis and Characterization of Triptycene-Containing Polyimides and
Polyureas .........................................................................................................
............. 112
Introduction ............................................................
................................................. . 113
Results and Discussion ..............................................................
..................................... 125
Conclusion ............................................................
................................................. . 139
Experim ental Section ...................................................................................................
140
R eferences and N otes ............................................................................
....................... 147
Chapter 5: Synthesis and Properties of Fluorene-Iptycene Based Polyheteroarylenes .... 153
Introduction ........................................................................................................................ 154
R esults and D iscussion .......................................................................... ....................... 161
C onclusion .......................................................................................
............................. 171
Experim ental Section ............................................................................. ....................... 172
References and Notes ........................................
177
C urriculum V itae ................................................................................................................
184
Acknowledgements ........................................
185
Appendix 1: NMR Spectra for Chapter 1 .....................................
186
Appendix 2: NMR Spectra for Chapter 2 .....................................
...........
201
Appendix 3: NMR Spectra for Chapter 3 .....................................
...........
221
Appendix 4: NMR Spectra for Chapter 4 .....................................
...........
235
Appendix 5: NMR Spectra for Chapter 5 .......................................................................
247
List of Figures
Chapter 1
Figure 1.1. Schematic illustration of phase transitions of a thermotropic LC consisting of rod
shape molecules .........................................................
................................................
16
Figure 1.2. Schematic illustration of nematic and semectic LC phases of rod-shaped
molecules, and columnar LC phase of disk-shaped molecules. ...................................
. 17
Figure 1.3. Schematic illustration of the operation of a simple TN-LCD cell (adapted from
reference 5) . .. ........................................................................................................
18
Figure 1.4. Schematic illustration of the operation principle of a GH-LCD cell ..............
21
Figure 1.5. Commercial products based on GH technologies: (A) variable transmittance visor
for airborne helmet; (B) ski goggle with switchable tint." ....................................... 22
Figure 1.6. Uv-vis absorption spectra (A) and fluorescence spectra (B) of acenequinones in
toluene (5: red; 10: green; 11: blue; 14: black) ...............................................................
28
Figure 1.7. Polarized absorption (A) (solid line: A,,; dotted line: Al) and fluorescence spectra
(B) (solid line: III; dotted line: I_) of 5 in MLC-6884. The vertical dotted-dashed lines
indicate the wavelengths of dichroic ratios reported. .....................................
....... 30
Figure 1.8. Fluorescent images of a test cell containing 5 in E7 (A: no applied voltage; B: 9
V applied voltage). The sample was excited with a 365 nm light from a hand-held UV-lamp.
....................................................................
32
Figure 1.9. Schematic diagram of the LC test cell .......................................................... 35
Figure 1.10. Schematic diagram of the measurement system used to determine the degree of
polarized emission and the calculation of dichroic ratio and order parameter based on
polarized fluorescence ...........................................................................................
35
Chapter 2
Figure 2.1. Typical field effect transistor configurations: (A) Top contact device; (B) Bottom
contact device . .....................................................
.......... ..................................... . . . 50
Figure 2.2. Schematic representation ofp- and n-channel field effect transistor. 4a ........ 51
Figure 2.3. Schematic representation of a field effect transistor. .................................... 52
Figure 2.4. Herringbone-like packing motif of pentacene. 7 ............................................... 56
Figure 2.5. Packing diagrams of 5a-d show that the molecules stack in a slipped cofacial
motif. (Left: view along the short molecular axis; Right: view along the long molecular
axis.) . .......
...........................................................................................................
60
Figure 2.6. Packing diagram of 5a (view along the a-axis). .......................................
62
Figure 2.7. Packing diagram of 5b (view along the a-axis). ................................................ 62
Figure 2.8. Packing diagram of 5ec (view along the b-axis). ....................................
.63
Figure 2.9. Packing diagram of 5d (view along the a-axis). ......................................
63
Figure 2.10. Packing diagram of 5d, showing the 1410 angle between molecules in
neighboring crystal packing columns. ......................................................
64
Figure 2.11. CVs of compounds 5a-d and tetracene performed erformed in 0.1 M solution of
TBAPF6 in CH 2C12, with a Pt electrode, a scan rate of 100 mV/s, and ferrocene as the
internal standard. ...................................................................................................................
66
Figure 2.12. UV-vis absorption spectra of tetracene and 5a-d measured in CH 2C12 . . ... ..... 67
Chapter 3
Figure 3.1. (A) Molecular structure of triptycene; (B) Schematic representation of internal
free volume in triptycene; (C) Space filling mode of triptycene molecule. ...................... 85
Figure 3.2. (A) Space filling model of a poly(iptycene) ladder polymer;
(B) Proposed
structure of interlocked poly(iptycene) and linear polymer. .......................................
86
Figure 3.3. DSC trace of monomer 5 showing melt endotherm and thermolysis exotherm.
.................................................................................................................................................
90
Figure 3.4. 1H NMR (Solvent: CDCl 3) spectra of model compound 10 (top) and polymer P1
(M, = 16,400 Da, PDI = 3.6) (bottom). ..........................................................................
93
Figure 3.5. UV-vis absorption spectra of model compound 10 and polymer P1 (Mn = 16,400
Da, PDI = 3.6) measured in methylene chloride. .................................................................. 94
Figure 3.6. 'H NMR (Solvent: CDC13) spectra of model compound 11 (top) and polymer P2
(Mn = 16,300 Da, PDI = 2.5) (bottom). ..................................................................
Figure 3.7.
13
.
.. 96
C NMR (Solvent: CDC13) spectra of model compound 11 (top) and polymer P2
(Mn = 16,300, Da, PDI = 2.5) (bottom). ......................................................................
.97
Figure 3.8. UV-vis absorption spectra of model compound 11 and polymer P2 (Mn=16,300
Da, PDI = 2.5) measured in methylene chloride. ............................................................ 97
Figure 3.9. UV-vis absorption spectra of model compound 13 and low molecular weight
polymer P2 (Mn = 6,000 Da, PDI = 2.3), with inset showing the proposed structure of
polym er with one end group. ................................................................................................. 98
Chapter 4
Figure 4.1. (A) General structure of linear imide functional group; (B) An example of
aromatic imide functional group; (C) Molecular structure of polyimide in Kapton and Vespel;
(D) Molecular structure of polyimide in Meldin. ....................................
113
Figure 4.2. Schematic illustration of the formation of polyamic acid at the first stage. .... 114
Figure 4.3. Schematic illustration of the formation of polyimide from polyamic acid. ..... 114
Figure 4.4. Examples of 25-micron polyimide films as insulation materials. 9 ................. 117
Figure 4.5. Schematic illustration of the preparation of polyureas and polyurethanes. ..... 120
Figure 4.6. Sprayable polyurea coating for roof protection.23 ............................................ 122
Figure 4.7. FTIR spectrum of polyimide PI-2. .....................................
129
Figure 4.8. FTIR spectrum of polimide PI-6. .....................................
129
Figure 4.9. 1H NMR spectrum of PI-2 in DMSO-d6 and peak assignment. ................... 130
Figure 4.10. 1H NMR spectrum of PI-6 in DMSO-d6 and peak assignment. ................ 131
Figure 4.11. 19F NMR spectra of PI-2 (A) and PI-6 (B) in DMSO-d6 . ............. .......... .
.
131
Figure 4.12. FTIR spectra of polyamic acid PA-2 (A) and polyimide PI-2 from thermal
im idization (B). .......................................................................
. . ........................................... 133
Figure 4.13. Comparison of FTIR spectrum of PI-2 prepared from chemical imidization and
therm al im idization (soluble part). ....................................................................................
133
Figure 4.14. TGA thermograms of polyimide PI-2 and PI-6 obtained at a scan rate of 10
oC/min under nitrogen . .....................................................
............................................ 135
Figure 4.15. UV-vis absorption spectra of PI-2 and PI-6 in a chloroform solution (A) and as
thin film (B) .....................................................
... ................................................ . . 136
Figure 4.16. 'H NMR spectrum of PU-1 in DMSO-d6 and peak assignment. ................ 138
Figure 4.17. FTIR spectrum of polyurea PU-1.
...............................................................
138
Chapter 5
Figure 5.1. Calculated (frontier) energy levels of oligothiophenes with n = 1-4 and of
polythiophene, where Eg = band gap. 8 .................................................................................
155
Figure 5.2. Molecular orbital interaction in donor (D) and acceptor (A) moieties leading to a
donor-acceptor monomer with a reduced band gap (adapted from ref. 10b). .................. 157
Figure 5.3. Examples of acceptors for donor-acceptor type conjugated polymers ............ 158
Figure 5.4. Images of the synthesized fluorene-iptycene based conjugated polymers. ...... 164
Figure 5.5. 1H NMR spectra of the (A) PFTP; (B) PFQ; (C) PFP in CDCl 3, in which labels
of * and # correspond to CHC13 and H20, respectively. .....................................
164
Figure 5.6. Normalized UV-vis absorption spectra of PFP, PFQ and PFTP in chloroform.
....................................................................
16 6
Figure 5.7. Normalized fluorescence spectra of PFP, PFQ and PFTP in chloroform. ..... 167
Figure 5.8. Normalized UV-vis absorption spectra of PFP, PFQ and PFTP films spin-casted
from their chloroform solution .......................................
170
Figure 5.9. Normalized fluorescence spectra of PFP, PFQ and PFTP films spin-casted from
their chloroform solution ..................................................
170
Figure 5.10. Fluorescent images of thin films of PFTP, PFQ, and PFP under UV irradiation
(365 nm ). ........................................................................................................
. . . ....
171
List of Tables
Chapter 1
Table 1.1. Absorption and Emission Maxima and Quantum Yields of Acenequinones in
T oluene ............................................................................................
................................ 28
Table 1.2. Spectral Characterizations, Dichroic Ratios, and Order Parameters
Acenequinones in M LC-6884 ............................................................................................
of
31
Chapter 2
Table 2.1. Comparison between the Intermolecular Distances and Pitch/Roll Displacements
of 5a-d ...................................................................................................................................
64
Table 2.2. Optical Band Gap and CV Data of Tetracene and 5a-d ...................................
65
Chapter 3
Table 3.1. Summary of Hyperbaric Polymerization Data ........................................
94
Chapter 5
Table 5.1. Summary of Physical Properties of Copolymers PFP, PFQ to PFTP .......... 169
List of Schemes
Chapter 1
Scheme 1.1. Synthesis of pentacenequinone 5 .........................................
...... 24
Scheme 1.2. Synthesis of hexacenequinone 10 and 11 .....................................
............
25
Scheme 1.3. Synthesis of tetrafluoroanthraquinone 7 ......................................
....... 26
Scheme 1.4. Synthesis of heptacenequinone 14 ................................................................ 26
Chapter 2
Scheme 2.1. Syntheses of 5a-d ........................................................... 58
Scheme 2.2. Syntheses of 3a-d ........................................................
59
Chapter 3
Scheme 3.1. Synthesis of m onom era .....................................................................................
Scheme 3.2. Synthesis of model compounds a .....................................
...
88
............ 89
Scheme 3.3. Syntheses of polymers .....................................................
92
Chapter 4
Scheme 4.1. Synthesis of Daminotriptycene 2 .....................................
125
Scheme 4.2. Synthesis of Diaminotriptycene 6 .....................................
126
Scheme 4.3. Synthesis of Polyimides .....................................
127
Scheme 4.4. Synthesis of Polyureas .....................................
137
Chapter 5
Scheme 5.1. Synthesis of triptycene-type quinoxaline 2 ....................................
162
Scheme 5.2. Synthesis of triptycene-type thieno[3,4-b]pyrazine monomer 5 .................... 162
Scheme 5.3. Synthesis of fluorene-triptycene based copolymers .................
163
CHAPTER 1
Acenequinones as Dyes for Guest-Host
Liquid Crystal Displays
Adapted from:
Chen, Z.; Swager, T. M. Org. Lett. 2007, 9, 997.
Introduction
Liquid Crystals
The three common states of matter are solid, liquid, and gas. Crystalline solids are
highly ordered and the constituting atoms or molecules stay in a fixed position and orientation
with a small amount of variation from vibrations. Such arrangements result from the large
attractive forces holding the molecules in place and therefore a solid is difficult to deform. On
the contrary, molecules in the liquid and gas phase have no fixed position and orientation and
they are free to move in a random fashion. In addition, a liquid can be easily deformed and gas
molecules can spread out to fill any container that holds them. The differences between these
three states can be attributed to the temperature of the substance, which is a measure of the
randomness of the molecules. Increasing temperature will cause the transition from a solid to a
liquid and then to a gas, and decreasing the temperature will reverse the phase transitions.'
Liquid crystals (LCs) are a class of materials that can exhibit intermediate
thermodynamic phases between the crystalline solid and simple liquid." 4 In contrast to the
three common states of matter, molecules in LC phase possess orientational (and weak
positional) order. Therefore, LCs reveal several physical properties of crystals, but flow like
fluids.'" When a substance shows LC behavior in a temperature region between the solid and
liquid states, this type of material is called thermotropic LCs (Figure 1.1). In blends of different
components, their phase transitions may depend on both the temperature and the concentration
of one component of the mixture.
If the LC phase of a mixture is dependent on the
concentration in a non-LC solvent, it is called lyotropic LC. While thermotropic LCs are
presently mostly used for technical applications, lyotropic LCs are important for biological
systems.' Not all substances, however, can have a liquid crystal phase. It has been recognized
that molecules that tend to have a thermotropic LC phase have a rigid central region and
flexible ends.1, 2 This leads to two major subclasses of LCs based on the molecular structures:
rod-shaped and disk-shaped molecules (Figure 1.2). In addition, polymers and polymer
!In,
tt
Crystal
Liquid
Crystal
Liquid
Temperature
Figure 1.1. Schematic illustration of phase transitions of a thermotropic LC consisting of rod
shape molecules.
solutions can also exhibit LC phases."' 2 Regardless of their molecular structures, LCs can also
be classified according to their symmetries and degree of long-range order. In the simplest case,
the molecules possess only orientational but no positional long-range order. This type of LCs is
called nematic, and this name was given as a result of the thread-like textures observed under
polarizing microscope. 1-3 The direction of preferred alignment can be described by a unit
vector, the so-called LC director. Smectics are another common type of LCs, which have a
soapy texture and retains some positional order with the molecules arranged in layers. In
addition, smectic phases are characterized by additional degrees of positional order, which will
not be discussed here. Discotic molecules often form columnar types of LC arrangements (see
examples in Figure 1.2).
Nematics
Semectics
Columnar
Figure 1.2. Schematic illustration of nematic and semectic LC phases of rod-shaped molecules,
and columnar LC phase of disk-shaped molecules.
The most remarkable features of LCs for applications are their anisotropic optical
properties, which mean that the properties of LC materials depend on the direction from which
the measurements are made. Since light is an electromagnetic wave, the behavior of light as it
passes through a liquid crystal will also strongly depend on the direction of its propagation and
polarization with respect to the director of the LC. The anisotropies of LC materials arise from
the positional or orientational order in the LC phase. This kind of orientational order gives
birefringence that can be manipulated by magnetic, electric or optical fields, leading to huge
magneto-optical, electro-optical and opto-optical effects.'
For example, applying a weak
electric field to a liquid crystal molecule with a permanent electric dipole will cause the dipole
to align with the field. If the molecule did not originally have a dipole, then it is induced when
the field is applied. Either of these situations has the ultimate effect of aligning the director of
the liquid crystal with the electric field being applied. In these cases, the order of the liquid
crystal has not increased, and we have only achieved alignment of the director.
Liquid Crystal Displays
The most successful application of liquid crystal materials is liquid crystal displays
(LCDs), which are well known in pocket calculators, digital cameras, and flat screens of laptop
computers. This application takes advantage of electro-optical effects of LCs. As compared
b
a
3
I
Polarizer
ectric
Id
Alignment lay
1 .1 qI
II
I
rriz(
D
Figure 1.3. Schematic illustration of the operation of a simple twisted nematic liquid crystal
display (TN-LCD) cell (adapted from ref. 5) (ITO: indium tin oxide).
to the traditional cathode-ray tube (CRT) displays, LCDs offer several advantages that make
them ideal for several applications. Firstly, LCDs are flat, and they use less power than that
required by CRTs. They are easier to read and more pleasant to work with for long periods of
time than most ordinary video monitors. Although there were several tradeoffs, such as limited
view angle, brightness, or contrast, as well as high manufacturing cost, the continuing research
and development has made these limitations less significant. Computer monitors using these
techniques have replaced CRT monitors to a large extent, and LCD TVs that use LCD
technology for its visual output have achieved satisfactory picture quality and have captured
part of the TV market from traditional CRTs. 6
LCDs based on the twisted nematic (TN) mode are the most common flat panel
displays. 5' 6 A schematic representation of a TN LCD cell is shown in Figure 1.3. Basically,
each pixel of an LCD consists of a layer of molecules aligned between two transparent
electrodes and two polarizing filters, the axes of polarity of which are perpendicular to each
other. The surfaces of the electrodes that are in contact with the liquid crystal material are
treated with rubbed polyimide films (please see Chapter 4 for more details) so as to align the
liquid crystal molecules in a particular direction.5 Before applying an electric field, the
orientation of the liquid crystal molecules is determined by the alignment at the surfaces. In a
TN device, the surface alignment directions at the two electrodes are perpendicular to each
other, and therefore, the molecules arrange themselves in a helical structure. Because the liquid
crystal material is birefringent, light passing through one polarizing filter is rotated by the liquid
crystal helix as it passes through the liquid crystal layer, allowing it to pass through the second
polarized filter (Figure 1.3 (a)). When a voltage is applied across the electrodes, a torque acts to
align the liquid crystal molecules parallel to the electric field, distorting the helical structure. If
the applied voltage is large enough, the liquid crystal molecules are completely untwisted and
the polarization of the incident light is not rotated at all. This light will then be polarized
perpendicular to the second filter, and thus be completely blocked and the pixel will appear
black (Figure 1.3 (b)). By controlling the voltage applied across the liquid crystal layer in each
pixel, light can be allowed to pass in varying amounts, correspondingly illuminating the pixel.
When the electric field is turned off, LC molecules will relax back into its twisted structure and
light will again be able to pass through. In some displays, the polarizers are parallel to each
other, thus reversing the on and off states. If red, green, and blue colored filters are used on
groups of three pixels, a color display can be created.1
Guest-Host Effect and Guest-Host Liquid Crystal Displays
Guest-host liquid crystal displays (GH-LCDs) that use a LC/dye mixture as the active
material have received much attention since their invention in the 1960s because of their wider
viewing angle, daylight readability, and high stability in harsh environments. 7-10 Guest-host
displays operate by the absorption of light by dichroic dye molecules oriented in a liquid
crystal. A schematic representation of the operational principle of GH-LCD cell is shown in
Figure 1.4. Similar to the TN-LCD device, a typical GH-LCD cell consists of a mixture of a
dichroic dye (guest) and LC medium (host), which is sandwiched into a thin layer between two
transparent plates. However, the rubbing directions of alignment layers (unidirectional rubbed
polyimide film) coated on the plates are parallel with each other. This feature will lead to a
homogenous alignment of the LC molecules in the cell if no voltage is applied (Figure 1.4).
The dichroic dyes used in GH-LCDs are designed such that they tend to align preferentially
along the director of the LC layer. When a voltage is applied, reorientation of the LC director
causes a corresponding reorientation of the dissolved dye molecules (Figure 1.4). The dichroic
dyes adsorb light more along one molecular axis than the others. Usually the optical transition
moments of the dye also align with the LC directors, and, the color intensity of the cell can be
controlled through the direction of LCs. An important difference with the widely used
TN-LCD is the fact that a TN-LCD owes its optical switching to the presence of two external
polarizers, while the GH-LCD involve the LC layer in the light absorption process.
Voltage ON
Voltage OFF
Rubbed
poiyimnide
O01001>
400W
Am=*460owr
I Ilý,,f 4Orr
"iiA
_~I Om
S
uI
Incident light
ITO
LC molecules
Dye molecules
Incident light
Figure 1.4. Schematic illustration of the operation principle of a GH-LCD cell.
Therefore, the optimal performances of GH-LCDs depend on the dichroic properties of dyes,
their solubility in LC host, and stability under various environments. Anthraquinone and azo
derivatives are the two major classes of dyes receiving most intensive study.10
Although the GH electro-optic effect was first filed for patent in 1965, this technology
got off to a slow start owing to the poor contrast of original devices and lack of suitable LCs and
dichroic dyes. The development of room-temperature LC materials and photochemically stable
anthraquinone dyes with high dichroic ratios offered an opportunity of revival of interest in the
1970s. Dramatic improvement of GH technology came from the efforts of White and Taylor,
who introduced a device without polarizers and demonstrated that excellent optical
performance could be achieved if dichroic dyes with higher order parameter were combined
with an LC mixture that had a helical structure of small molecular pitch. 8b In the 1980s,
high-performance dichroic dyes, which combined the good photochemical stability of the early
anthraquinone dyes with the high order parameters of the early azo dyes, were reported in the
8th
International Liquid Crystal Conference. 9b Many of these new anthraquinone-based dyes
had high solubility in common LC hosts. Today ready-to-use black guest-host mixtures are
commercially available. 10
The main attractive features of GH devices are a very wide viewing angle, high
brightness, no need for polarizers, and ability to produce colored displays. The most common
configuration for GH-LCDs at present is a display with negative contrast (light characters on a
(A)
(B)
Figure 1.5. Commercial products based on GH technologies: (A) variable transmittance visor
for airborne helmet; (B) ski goggle with switchable tint."1
dark background). This may be considered to be identical to all emissive display technologies
in common use today. Generally, the GH-LCD is best suited for display applications where a
lower information density is acceptable (direct drive applications) and a wide viewing angle,
high brightness, or multiple color contrast is desired. TN-LCDs are best suited for higher
information density applications such as portable terminals and other such alpha numeric or
limited graphic applications. GH-LCDs have reached the point where they can begin to create
their own market because they allow for the designer flexibilities previously not readily
obtainable.10 Color can be used in more ways than increasing the "perceived value" or
"fashionability" of consumer products.
The use of color designation, especially in
pseudoanalog meter and bar graph types of applications, greatly enhances the functionality of
products. Recently, a company called AlphaMicron in Kent, OH has developed interesting
ski-goggles, visors for pilots, and digital mirrors for automobiles based on GH-LCD
technologies (Figure 1.5)."
Recently, fluorescent dye-based GH-LCDs, which combine the excellent hues and high
brightness levels of emissive displays with the desirable features of LCDs, have been proposed
as a less energy consuming display for portable electronics. 12 ,13 For fluorescent dyes in LCs,
the fluorescence intensity can be controlled in a similar way as absorption. Therefore, the
development of fluorescent GH-LCDs requires synthesis of fluorescent dyes with a high
dichroic ratio, a high quantum yield, and a strong emission in the visible region. Additionally,
the rod-like shape of nematic LC molecules favors alignment of elongated, rod-like dye
molecules along the direction of long molecular axis of nematic LC molecules. In this study,
we report emissive linear para acenequinone dyes with large dichroic ratios in LC mixtures.
We describe the syntheses of a group of alkoxy-substituted acenequinone derivatives, their
solution absorption and emission spectra, and polarized absorption and emission spectra of
their LC solutions. These compounds demonstrated strong orientation properties and have
excellent potential as fluorescent dyes for GH-LCDs.
Results and Discussion
Synthetic methodologies for preparing linear para acenequinones have been previously
reported, 14- 16 and the Diels-Alder (D-A) reaction between isobenzofurans (or their analogues)
and linear 1,4-quinones is one of the more efficient approaches.15 In this work, we employed
3,6-di-2-pyridyl-1,2,4,5-tetrazine (DPT) and anthracene 1,4-endoxide 3 to generate an
isonaphthofuran, which is subsequently trapped by a quinone via a D-A mechanism. 17 The D-A
adducts are then easily converted into corresponding acenequinones.
Scheme 1.1. Synthesis of pentacenequinone 5
HO
Br
n-C 6H1 3 1
K2CO3,DMF
C6H 3 0
Br
fura n, PhLi,
rTH
F, OC,
HO:
Br
85C
C6H130
Br
(89%)
1
o-/70)
•O170)
C6 H130
N••
(94%)
C6H13 0
o
C6H130
C6H 30
O
1,4-naphthoquinone
DPT, toluene, 100 OC
4 (2 isomers)
PPTS,
(CH3 CO)20, 80 OC
C6H1
(77%)
C6H1
5
0
Scheme 1.2. Synthesis of hexacenequinone 10 and 11
O
DPT, toluene, 100 OC
3 +
X
X
C6H130
X
C6H130
O
6 (X=H)
7 (X=F)
X
8 (X=H)
9 (X=F)
PPTS,
(CH 3 00)20,800 C
CH 130
p-TsOH,
C6H130
0
toluene, 100 C
X
X
0
10 (X=H), 48% (2 steps)
11 (X=F), 20% (2 steps)
X
This methodology avoids direct use of isobenzofuran/isonaphthofuran compounds, which are
difficult to synthesize and are generally unstable. It has also allowed us to prepare various
substituted acenequinones.
The synthetic route to a para pentacenequinone is outlined in Scheme 1.1. The starting
6,7-dibromo-2,3-dihydroxynaphthalene
(1),
which
was
prepared
from
2,3-dihydroxynaphthalene in two steps via a literature procedure, 18 was converted to 2 by a
Williamson ether synthesis. Intermediate 2 was treated with phenyllithium (PhLi) in presence
of excess of furan, at low temperature, to produce 3. The generation of 4 was successfully
accomplished by treating 3 with DPT, in the presence of 1,4-naphthoquinone.
Pentacenequinone 5 was then obtained from dehydration of 4, which could be realized by
employing acetic acid, 1m p-toluenesulfonic acid (p-TsOH)/toluene 15
or pyridinium
p-toluenesulfonate (PPTS)/acetic anhydride. 19 We found the PPTS approach affords clean
product in a satisfactory yield by a simple filtration. Compound 5 is an orange/red solid, with
high melting point and excellent thermal and photochemical stability.
Scheme 1.3. Synthesis of tetrafluoroanthraquinone 7
c
OH
NaCI, AICl3
180 OC-220C
FOC
(81%)
F
OH
F
0
OH
F
O
OH
12
F
1) NaBH 4 , CH3OH, rt
F
2) HCI, rt
F
0
F
(75%)
0
0
Scheme 1.4. Synthesis of heptacenequinone 14
DPT,
4+
1/2
toluene, 100 OC
C6H13
(71%)
CH
6
i3
p-TsOH, toluene, 10(0 OC
SH13
(21%)
iH13
Similar methodology allowed us to successfully synthesize two para hexacenequinones
(10 and 11) from the corresponding 1,4-anthraquinones (6 and 7) as the dienophiles (Scheme
1.2). Dienophile 6,7,8,9-tetrafluoro-1,4-anthraquinone (7) was synthesized previously from the
oxidation of corresponding 1,4-hydroquinone. 20 Here we report a more convenient route, as
shown in Scheme 1.3.
A Friedel-Crafts bis-cycloacylation of 1,4-dihydroxybenzene and
3,4,5,6-tetrafluorophthalic anhydride was conducted in a melt of aluminum chloride/sodium
chloride (AlC13/NaC1) to give tetrafluoroquinizarin 12.21
Reduction of 12 with sodium
borohydride (NaBH 4) and a subsequent dehydration with concentrated aqueous HCl afforded 7
as a dark-brown solid.22
Using 1,4-benzoquinone as the limiting reagent, we prepared a tetraalkoxy-substituted
heptacenequinone (14) (Scheme 1.4). Interestingly, the only detected products are exo/exo and
exo/endo isomers of 13.
Dehydrating both isomers yields heptacenequinone 14 as a
yellow/orange solid.
A solution of the acenequinones possess a characteristic strong absorption band at
350-400 nm and a second absorption band at 420-500 nm (Figure 1.6 (A)), both of which can
be assigned as '(7r, 7t*) absorptions. 23 When compared to their non-substituted hydrocarbon
analogues, these acenequinone compounds have broadened absorption peaks that are
red-shifted by about 20-30 nm. These differences are the result of intramolecular charge
transfer (ICT) character between electron-withdrawing quinoid moiety and electron-donating
alkoxy sidechains.23
350
400
450
500
Wavelength (nm)
450 500 550 600 650 700
Wavelength (nm)
Figure 1.6. Uv-vis absorption spectra (A) and fluorescence spectra (B) of acenequinones in
toluene (5: red; 10: green; 11: blue; 14: black).
Table 1.1. Absorption and Emission Maxima and Quantum Yields of Acenequinones in
Toluene
compd
5
.max (nm)
366
Lem (nm)
(Irela
515
0.30
493
0.21
519
0.24
484
0.18
475
10
369
465
11
378
482
14
376
470
a Fluorescence quantum yield relative to coumarin 153 ((Dre = 0.38).
All the acenequinone derivatives are fluorescent in solution, with the relative quantum
yields ranging from 0.18 to 0.30 (Table 1.1). Figure 1.6 (B) displays the fluorescence spectra of
the acenequinones in toluene. The fluorescence spectra reveal that the emission maximum
(Xem) of 10 has a 30 nm blue-shift as compared to that of 5. This blue shift results from the
reduced ICT character of 10, because the naphtho-fused quinoid group has weaker
electron-withdrawing properties than the benzo-fused quinoid group in 5.13a Such an effect was
also demonstrated by the 35 nm red-shift of the Xem of 11 and the 10 nm blue-shift of the Xem of
14, with respect to the Xem of 10. Clearly, the fluorine groups significantly enhance the ICT
character in
1 1 .13d
These results show that photophysical properties of para acenequinones can
be tuned by attaching suitable functional groups.
The orientation properties of acenequinone dyes in LCs host were investigated via the
measurement of polarized absorption and fluorescent spectra in aligned test cells. Samples
were prepared by loading dye/LC fluids (0.1-0.5wt% dye in MLC-6884) into LC test cells.
The dye/LC mixture forms a homogenously aligned structure, and the polarized UV-vis
absorption and emission spectra were determined.
A
0
C
0
o*
.0
0
.0
C
r,-
350
400
450
500
550
500
Wavelength (nm)
550
600
650
700
Wavelength (nm)
Figure 1.7. Polarized absorption (A) (solid line: All; dotted line: A±) and fluorescence spectra
(B) (solid line: III; dotted line: IL) of 5 in MLC-6884. The vertical dotted-dashed lines indicate
the wavelengths of dichroic ratios reported.
The polarized absorption spectra of acenequinone 5 in LC host are displayed in Figure
1.7 (A). The anisotropy of molecular absorption transition moments is in agreement with our
preliminary calculations that reveal the transitions of the two major absorption bands are
aligned with the long molecular axis. 24 Therefore, dichroic ratio (DA) and order parameter (SA)
of a dye in LC can be calculated by using the following equations:
DA = A, / A±
SA = (A,, - A±) / (A, + 2A±)
where A,, and A± represent the absorbance for parallel and perpendicular irradiation with respect
to LC director.' 0 As summarized in Table 1.2, those compounds demonstrated dichroic ratios
in the range of 8-10, indicating their excellent alignment in the LC host. It has been reported
that dyes with dichroic ratio greater than 8 are appropriate for practical use.13b
Figure 1.7 (B) shows the representative polarized fluorescence spectra. The dichroic
ratio (DF) and order parameter (SF) based on polarized fluorescence can be calculated by using
the following equations:
DF= II / II
SF = (Il - IL) / (1// + 2IL)
where II and Ii represent the emission intensity for parallel and perpendicular irradiation with
respect to LC director.2 5 Order parameter values from the polarized fluorescence are similar
Table
1.2.
Spectral
Characterizations,
Dichroic Ratios,
and Order Parameters
Acenequinones in MLC-6884
compd
(nm)
max
DA
SA
em
(nm)
DF
SF
5
368
8.1
0.70
522
9.2
0.73
478
8.8
0.72
373
8.8
0.72
504
10.4
0.76
470
9.6
0.74
383
8.9
0.72
532
10.1
0.75
485
10.0
0.75
380
8.7-12.8a
0.72-0.80a
_b
_b
_b
10
11
14
477
7.7-8.8 a
0.69-0.72a
aThe exact DA and SA Of 14 could not be obtained due to the strong interference from test cell. b
The DF and SF of 14 could not be determined due to its low relative quantum yield and poor
solubility in LCs.
Figure 1.8. Fluorescent images of a test cell containing 5 in E7 (A: no applied voltage; B: 9 V
applied voltage). The sample was excited with a 365 nm light from a hand-held UV-lamp.
to those from the polarized absorption spectra (Table 1.2). Compounds 10 and 11 have higher
aspect ratios and as expected have higher orientation properties than 5. This feature agrees with
the classic model that elongation of dichroic dye molecules should enhance their orientation in
nematic LCs. The fluorescence maxima of acenequinones in LCs were found to have a small
blue-shift (7-13 nm) as compared to those in toluene solution. This feature is likely due to the
different dielectric properties of each medium.
The reorientation of dyes under electric field was demonstrated in a test cell (Figure
1.8), containing a solution of 0.2wt% 5 in E7 (a commercial LC mixture with positive dielectric
anisotropy). Because the test cell is coated with parallel rubbed polyimide films, the LC/dye
mixture forms a homogeneously aligned texture, with the long molecular axis of LCs and dyes
parallel with rubbing direction. Therefore, acenequinone dyes in this orientation give strong
emission when they are excited by a UV light (Figure 1.8 (A)). Upon application of an electric
field, the LC director and acenequinone backbones align normal to the surface of the test cell.
Consequently, the transition dipole of dye compound is parallel to the direction of incident
light, resulting in decreased projection of the transition dipole along the electric vector of
incident UV light and minimal absorption and emission (Figure 1.8 (B)). 13e,26 The polarized
fluorescence is rapidly recovered upon removal of the voltage.
Conclusion
In summary, we have reported a convenient synthesis of substituted para acenequinones
by employing DPT and an anthracene 1,4-endoxide. The polarized absorption and fluorescence
results demonstrate that acenquinones align with nematic LC hosts, indicating their potential as
dichroic dyes for GH-LCDs. In addition, the sensitivity of the emission to the substitution
allows the tuning of fluorescence by functionalization of the acenequinone chromophore.
Experimental Section
Materials: Anhydrous tetrahydrofuran (THF) and toluene were purchased from Mallinckrodt
Baker Inc.
Furan was distilled from K2C0
3
prior to use.
Starting materials,
2,3-dibromo-6,7-dihydroxynaphthalene (1)18 and 1,4-anthraquinone (6)22, were prepared
following literature procedures. All other reagent-grade starting materials were purchased from
Aldrich, Lancaster, or Alfa Aesar, and used without further purification. Liquid crystal test
cells with 10 tm gap and parallel rubbed polyimide coatings were purchased from E.H.C. Co.
Ltd, Tokyo, Japan. MLC-6884 is a liquid crystal mixture (negative dielectric anisotropy) for
active matrix addressed ECB liquid crystal displays (LCDs) and it was purchased from EMD
Chemicals Inc. E7 is a liquid crystal mixture with positive dielectric anisotropy and it was
donated by Merck Chemicals Ltd. (UK). The composition of E7 is shown in Chart 1.1.
Chart 1.1. Molecular composition of E7
N
-_
-
N
51%
N-
..
8%
-
25%
N
-
16%
General Methods and Instrumentation: Column chromatography was performed using silica
gel (40-63 rim) from SiliCycle. NMR (1H, 13C, 19F) spectra were recorded on Varian
Mercury-300 MHz, Varian Inova-500 MHz or Bruker Advance-400 MHz spectrometers. The
1H and 13C chemical shifts are given in unit of 8 (ppm) relative the tetramethylsilane (TMS)
where 8(TMS) = 0 and are referenced to the residual solvent. The 19F NMR chemical shifts are
reported in ppm relative to hexafluorobenzene (8= -164.9 ppm). High-resolution mass spectra
(HRMS) were obtained on a Bruker Daltonics APED II 3T FT-ICR-MS using electron impact
ionization (EI) or electrospray ionization (ESI). Melting points were measured on a Mel-Temp
II apparatus (Laboratory Devices Inc.) and were not corrected. UV-vis spectra were obtained
from Hewllett-Packard 8452A diode array UV-visible spectrophotometer or Cary 50
UV/Visible spectrometer. Fluorescence spectra were measured with a SPEX Fluorolog-r3
fluorometer (model FL112, 450W xenon lamp). Fourier Transform infrared (FT-IR) spectra
were measured on a Perkin-Elmer model 2000 FT-IR spectrophotometer using the Spectrum v.
2.00 software package.
Polarized UV-vis absorption and fluorescence measurement: A solution ofacenequinone in
MLC-6884 (0.1-0.5wt%) was loaded into a LC test cell via capillary action. The test cell
consists of two glass plates coated with transparent electrodes (Indium tin oxide) (1 cm x 1 cm)
and parallel rubbed polyimide thin films (Figure 1.9). The polarized UV-vis absorption spectra
Side View
Uni-directionallv
rubbedpolvimide
10 "in
ITO laver
LC/dve fill
direction
Top View
- 0
Rub direction
Figure 1.9. Schematic diagram of the LC test cell.
Excite Vertical or Horizontal
S
Dichroic Ratio (N) :
I
Iv
Gx IvH
-
I1
_V
Isotropic
sample
amatar SI_ a
SF-
-I
I+21
(I I)-I(I 1 )+ 2
N
A - 1
NF +2
Figure 1.10. Schematic diagram of the measurement system used to determine the degree of
polarized emission and the calculation of dichroic ratio and order parameter based on polarized
fluorescence.
were obtained by irradiating the cell with polarized lights parallel and perpendicular to the
rubbing direction of the test cell. Polarized fluorescence measurements were carried out
according to the procedure previously reported by Breen et al.25 A schematic description of the
set-up for measuring polarized fluorescence is shown in Figure 1.10.
2,3-Dibromo-6,7-bis(hexyloxy)naphthalene (2): In an oven-dried 100 mL round-bottom flask
equipped with a stir bar were combined 1 (2.1 g, 6.6 mmol), 1-iodohexane (3.0 mL, 20 mmol),
potassium carbonate (9.8 g, 71 mmol), 18-crown-6 (0.1 g) and nitrogen-bubbled DMF (50 mL).
The mixture was stirred under Ar at 85 oC for 5 d. After cooling to room temperature, the
reaction mixture was poured into 200 mL water and the product was extracted with CH 2C12.
The organic layer was washed by dilute NH 4Cl aqueous solution and saturated NaCl aqueous
solution, and then dried over MgSO 4. Solvent was removed in vacuo and the residue was
purified by column chromatography on silica gel with CH 2C12 :hexane (1:5 up to 1:3, v/v),
yielding a white solid as the product (3.2 g, 87%), which is pure enough for next step. A small
portion of the product was recrystallized to obtain analytical pure sample for melting point
measurement. mp 66-67 OC (methanol). 'H NMR (300 MHz, CDC13) 7.92 (s, 2H), 6.96 (s, 2H),
4.08 (t, J=6.6 Hz, 4H),1.90 (m,4H), 1.52 (m,4H), 1.38 (m,8H), 0.93 (m,6H).
13C
NMR (100
MHz, CDC13) 150.6, 130.6, 129.4, 119.4, 106.3, 69.1, 31.8, 29.1, 25.9,22.8, 14.2. FT-IR (KBr)
v/cm': 2956, 2925, 2852, 1506, 1466, 1405, 1381, 1347, 1250, 1164, 1043, 994, 945, 881.
HRMS (EI) calcd for C22 H30Br 20 2 (M÷) 484.0608, found 484.0608.
1,4-Epoxy-6,7-bis(hexyloxy)-1,4-dihydroanthracene
(3): Under Ar, a stirred mixture of 2
(2.5 g, 5.1 mmol), furan (25 mL) and anhydrous THF (80 mL) was cooled by an ice-water bath.
A solution of phenyllithium (PhLi) in cyclohexane/ether (6 mL, 1.5-1.7 M) was then added
dropwise over a course of 4 h. After the addition of PhLi, the mixture was stirred at 0 oC for an
additional 2 h, and then allowed to warm to room temperature slowly and stirred overnight.
Methanol (4 mL) was added slowly to quench the reaction. The reaction mixture was poured
into water and the product was extracted by CH 2C12 . The organic layer was washed by
saturated NaCl aqueous solution and dried over MgSO 4. Solvent was removed in vacuo.
Purification by column chromatography on silica gel with CH 2Cl 2 :hexane (2:1 up to 3:1, v/v)
afforded the title compound as a white solid (1.8 g, 89%), which is pure enough for next step. A
small portion of the product was recrystallized to obtain analytical pure sample for melting
point measurement. mp 102-103 oC (methanol). 'H NMR (400 MHz, CDC13) 7.46 (s, 2H),
7.07 (s, 2H), 6.97 (s, 2H), 5.78 (s, 2H), 4.09 (t, J=6.4 Hz, 4H), 1.89 (m, 4H), 1.53 (m, 4H), 1.38
(m, 8H), 0.93 (t, J= 6.8 Hz, 6H).
13
C NMR (100 MHz, CDC13) 149.4, 142.9, 142.0, 127.0,
117.8, 109.4, 82.1, 69.1, 31.8, 29.3, 25.9, 22.8, 14.2. FT-IR (KBr) v/cm-': 2954, 2929, 2856,
1617, 1507, 1453, 1384, 1251, 1153, 1068, 983, 866, 841. HRMS (EI) calcd for C26H340 3 (M+)
394.2503, found 394.2512.
exo/endo-6,13-Epoxy-5a,6,13,13a-tetrahydro-9,10-bis(hexyloxy)pentacene-5,14-dione (4):
A mixture of 3 (0.44 g, 1.1 mmol), 3,6-di-2-pyridyl-1,2,4,5-tetrazine (DPT) (0.27 g, 1.1 mmol),
1,4-naphthoquinone (0.18 g, 1.1 mmol) and toluene (50 mL) was stirred at 100 oC under Ar for
4 d. After cooling to room temperature, the reaction mixture was concentrated on a rotary
evaporator. The residue was dissolved in CH 2C12 and then washed in sequence with HCI
aqueous solution (5%), water and saturated NaCl aqueous solution. Solvent was removed in
vacuo. Purification by column chromatography on silica gel with ethyl acetate:hexane (1:6 up
to 1:4, v/v) afforded both endo (0.12 g) and exo product (0.43 g) as a slightly yellow solid
(94%). Exo: 'H NMR (400 MHz, CDC13) 8.15 (dd, J= 6.0, 3.2 Hz, 2H), 7.77 (dd, J= 6.0, 3.2
Hz, 2H), 7.64 (s, 2H), 7.12 (s, 2H), 5.86 (s, 2H), 4.10 (t, J= 6.4Hz, 4H), 3.12 (s, 2H), 1.90 (m,
4H), 1.54 (m,4H), 1.39 (m,8H), 0.94 (t, J=6.8 Hz, 6H).
13C
NMR (100 MHz, CDC13) 195.6,
149.8, 140.2, 135.6, 134.7, 128.4, 127.4, 116.9, 108.6, 85.4, 69.0, 53.1, 31.8, 29.2, 25.9, 22.8,
14.2. FT-IR (KBr)v/cm': 2956, 2920, 2856, 1669, 1589, 1502, 1460, 1390, 1296, 1271, 1248,
1158, 972, 866, 850, 821. HRMS (EI) calcd for C 34 H 3 80 5 (M +) 526.2714, found 526.2708;
Endo: 'HNMR (400 MHz,CDC13)7.63 (dd, J=6.0, 3.2 Hz,2H), 7.29 (dd, J= 6.0, 3.2 Hz,2H),
7.27 (s, 2H), 6.84 (s, 2H), 5.93 (dd,J=3.2, 2.0 Hz, 2H), 3.97 (t, J=6.8 Hz, 4H), 3.86 (dd, J=
3.2, 2.0 Hz, 2H), 1.82 (m,4H), 1.47 (m,4H), 1.34 (m,8H), 0.90 (t, J=6.8 Hz, 6H).
13C
NMR
(100 MHz, CDC13) 194.8, 149.4, 137.6, 134.1, 133.9, 128.2, 126.4, 118.5, 108.2, 82.9, 68.8,
50.9, 31.7, 29.1, 25.8, 22.7, 14.2. FT-IR (KBr) v/cm-': 2952, 2930, 2856, 1678, 1617, 1594,
1502, 1454, 1390, 1300, 1271, 1248, 1191, 1158, 1014, 985, 940, 895, 854. HRMS (EI)calcd
for C34H38 0 5 (M+) 526.2714, found 526.2737.
9,10-Bis(hexyloxy)pentacene-5,14-dione
(5): A mixture of 4 (0.31 g, 0.59 mmol), pyridinium
p-toluenesulfonate (PPTS) (1.48 g, 5.9 mmol) and acetic anhydride (10 mL) was stirred at 80
oC
for 16 h. After the reaction mixture was cooled to room temperature, the precipitate was
collected by filtration, washed thoroughly with methanol and dried in vacuum, affording the
title compound as a red/orange solid (0.23 g, 77%). A small portion of the product was
recrystallized to obtain analytical pure sample for melting point measurement. mp 219-220 oC
(CH 2Cl 2/methanol). 'H NMR (400 MHz, CDCl 3) 8.71 (s, 2H), 8.30 (dd, J=5.6, 3.2 Hz, 2H),
8.20 (s, 2H), 7.74 (dd, J=6.0, 3.2 Hz, 2H), 7.05 (s, 2H), 4.10 (t, J=6.4 Hz, 4H), 1.92 (m,4H),
1.56 (m,4H), 1.42 (m,8H), 0.96 (t, J= 6.8 Hz, 6H). 13C NMR (100 MHz, CDC13) 182.9, 151.6,
134.9, 133.9, 131.2, 130.8, 130.5, 127.8, 127.5, 127.2, 105.9, 68.9, 31.8, 29.1, 26.0, 22.8, 14.3.
FT-IR (KBr)v/cm-': 2953, 2929, 2854, 1665, 1570, 1489, 1432, 1324, 1300, 1223, 1152.
HRMS (EI)calcd for C34 H36 0 4 (M÷) 508.2608, found 508.2624.
exo/edo-7,14-Epoxy-6a,7,13,13a-tetrahydro-10,113,13a-tetrahyd-bis(hexyloxy)hexacene-6,15-dione
(8):
This was prepared from 3 (0.72 g, 1.82 mmol), DPT (0.44 g, 1.86 mmol), and
1,4-anthraquinone (0.39 g, 1.87 mmol) in toluene (70 mL) in the same manner as described for
4, yielding 8 (0.80 g, 76%) as a yellow solid. Exo: 'H NMR (300 MHz, CDC13) 8.73 (s, 2H),
8.09 (dd, J= 6.3, 3.3 Hz, 2H), 7.71 (dd, J= 6.3, 3.3 Hz, 2H), 7.69 (s, 2H), 7.15 (s, 2H), 5.94 (s,
2H), 4.12 (t, J= 6.6 Hz, 4H), 3.26 (s, 2H), 1.91 (m,4H), 1.54 (m,4H), 1.39 (m,8H), 0.94 (m,
6H). ' 3 C NMR (125 MHz, CDC13) 195.9, 149.9, 140.4, 135.5, 131.4, 130.3, 129.8, 129.7,
128.5, 116.9, 108.7, 85.9, 69.1, 53.6, 31.8, 29.3, 25.9, 22.8, 14.3. FT-IR (KBr) v/cm- : 2954,
2925, 2856, 1677, 1614, 1507, 1456, 1390, 1295, 1276, 1251, 1191, 1153, 1093, 1021, 872,
851, 803. HRMS (ESI) calcd for C38H410 5 (M+H +) 577.2949, found 577.2939; Endo: 'H
NMR (400 MHz, CDC13) 8.12 (s, 2H), 7.57 (dd, J= 6.0, 3.2 Hz, 2H), 7.31 (dd, J= 6.0, 3.2 Hz,
2H), 7.27 (s, 2H), 6.66 (s, 2H), 5.99 (dd, J= 3.2, 2.0 Hz, 2H), 3.92 (dd, J= 3.2, 2.0 Hz, 2H), 3.74
(m,4H), 1.63 (m,4H), 1.30 (m,4H), 1.28 (m,8H), 0.84 (t, J= 6.8 Hz, 6H). 13C NMR (125
MHz, CDCl3) 194.5, 149.1, 137.8, 134.5, 130.0, 129.5, 129.0, 128.5, 127.9, 118.3, 107.9, 82.9,
68.4, 51.0, 31.5, 28.9, 25.6, 22.5, 14.0. FT-IR (KBr) v/cm-': 2954, 2929, 2853, 1677, 1617,
1852, 1507, 1456, 1390, 1302, 1264, 1198, 1156, 1090, 1037, 942, 870, 854. HRMS (ESI)
calcd for C 38H 410 5 [M+H]÷ 577.2949, found 577.2960.
10,11-Bis(hexyloxy)hexacene-6,15-dione
(10): A mixture of 8 (0.31 g, 0.54 mmol),
p-toluenesulfonic acid monohydrate (p-TsOH
H20) (60 mg) in toluene (20 mL) was stirred at
100 oC under Ar for 13 h. After cooling to room temperature, most of solvent was removed in
vacuo. The residue was dissolved in CH 2C12 and the product was washed with H2 0 and
saturated NaCl aqueous solution, dried over MgSO 4 , and concentrated on a rotary evaporator.
Purification by column chromatography on silica gel with CH 2 Cl 2 :hexane (2:1, slowly up to
5:1, v/v) afforded the title compound as a yellow/orange solid (0.19 g, 63%). A small portion of
the product was recrystallized to obtain analytical pure sample for melting point measurement.
mp 235-236 oC (CH 2Cl 2/methanol). 1H NMR (400 MHz, CDCl 3) 8.84 (s, 2H), 8.81 (s, 2H),
8.26 (s, 2H), 8.04 (dd, J=6.0, 3.2 Hz, 2H), 7.64 (dd, J= 6.4, 3.2 Hz, 2H), 7.07 (s, 2H), 4.09 (t,
J= 6.8 Hz, 4H), 1.91 (m, 4H), 1.55 (m, 4H), 1.41 (m, 8H), 0.96 (t, J=6.8 Hz, 6H).
13C
NMR
(125 MHz, CDCl 3) 183.0, 151.7, 135.3, 131.3, 131.0, 130.8, 130.2, 129.7, 129.4, 128.7, 127.1,
105.9, 69.0, 31.8, 29.1, 26.0, 22.9, 14.3. FT-IR (KBr) v/cm-1 : 2950, 2925, 2852, 1673, 1570,
1490, 1457, 1429, 1393, 1308, 1228, 1201, 1155, 988, 930. HRMS (ESI) calcd for C38H3 80 4Na
[M+Na] ÷ 581.2662, found 581.2686.
1,2,3,4-Tetrafluoro-10,11-bis(hexyloxy)hexacene-6,15-dione
(11): This was prepared from 3
(0.37 g, 0.94 mmol), DPT (0.22 g, 0.93 mmol), and 6,7,8,9-tetrafluoro-1,4-anthraquinone (7)
(0.26 g, 0.93 mmol) in toluene (40 mL) in the same manner as described for 8. The crude
product 9 was directly used for the dehydration step as described for 5, employing PPTS (1.0 g,
4.0 mmol) and acetic anhydride (15 mL), to afford the title compound as a yellow solid (0.12 g,
20% over two steps). A small portion of the product was recrystallized to obtain analytical pure
sample for melting point measurement. mp 332-334 OC (CH 2 Cl2/methanol).
1H
NMR (300
MHz, CDC13) 8.78 (s, 2H), 8.68 (s, 2H), 8.10 (s, 2H), 6.91 (s, 2H), 4.07 (t, J=6.6 Hz, 4H), 1.94
(m, 4H), 1.57 (m, 4H), 1.44 (m, 8H), 0.98 (t, J=6.9 Hz, 6H).
19F
NMR (282 MHz, CDC13)
-144.5 (d), -151.1 (d). FT-IR (KBr) v/cm-': 2952, 2933, 2856, 1680, 1667, 1573, 1514, 1491,
1468, 1429, 1361, 1309, 1264, 1222, 1092, 1009, 992. HRMS (EI) calcd for C38H34 F4 0 4 (M+)
630.2388, found 630.2378.
1,2,3,4-Tetrafluoro-5,8-dihydroxyanthraquinone (12): A mixture of AlC13 (5.0 g) and NaCi
(1.1 g) was pulverized using a mortar and pestle under a nitrogen atmosphere. The mixture was
then transferred into an oven-dried 50 mL Schlenk flask under Ar, and heated to 180 oC. An
intimate
mixture
of
3,4,5,6-tetrafluorophthalic
anhydride
(1.1
g,
4.8
mmol),
1,4-dihydroxybenzene (0.6 g, 5.4 mmol) and A1Cl 3 (2.6 g) was added to the above melt. The
heating temperature was raised to 220 oC and held there for 1.5 h. The hot mixture was poured
over crushed ice and 40 mL concentrated HCI aqueous solution was then added. The resulting
red/purple suspension solution was filtered, affording a brown solid as the product (1.2 g, 81%),
which was used for next step, without further purification. mp 230-231 oC.
1H
NMR (400
MHz, (CD 3)2CO) 12.37(s, 2H), 7.45 (s, 2H). '9 F NMR (282 MHz, (CD 3)2CO) -133.0 (m),
-141.8 (m). FT-IR (KBr) v/cm-: 3448, 1632, 1605, 1510, 1449, 1412, 1379, 1301, 1264, 1210,
1118, 968, 782, 750. HRMS calcd for C14H4 F4 0 4 (M+) 312.0040, found 312.0037.
6,7,8,9-Tetrafluoro-1,4-anthraquinone (7): Under Ar, sodium borohydride (NaBH 4) (0.65 g)
was added in portions to a stirred solution 12 (1.0 g, 3.2 mmol) in methanol (30 mL), which was
cooled by an ice-water bath. After the addition of NaBH 4, the reaction mixture was refluxed for
24 h. The reaction mixture was then cooled to room temperature and poured into water (60
mL). The resulting solution was acidified with concentrated HCI solution. The precipitate was
collected by filtration and dried in a vacuum, affording the title compound as a dark/brown
solid (0.67g, 75%), which was used for next step, without further purification. mp > 180 OC
(decomposed).
1H
NMR (300 MHz, CDC13) 8.87 (m,2H), 7.16 (s, 2H). 19F NMR (282 MHz,
CDC13) -145.3 (d), -151.6 (d). FT-IR (KBr)v/cm-l: 3064, 3022, 1661, 1674, 1605, 1512, 1472,
1375, 1300, 1123, 1066, 1056, 963, 855. HRMS (EI) calcd for C14H4F40 2 (Me) 280.0142,
found 280.0147.
(exo/endo)/(endo/endo)-[6,17],[8,15]-Diepoxy-6,6a,7a,8,15,15a,16a,17-octahydro-2,3,11,1
2-tetrahexyloxyheptacene-7,16-dione
(13): This was prepared from 5 (0.47 g, 1.19 mmol),
DPT (0.28 g, 1.18 mmol), and 1,4-benzoquinone (61 mg,0.59 mmol)intoluene (50 mL) inthe
same manner as described for 4, yielding both endo/endo product (0.22 g) and exo/endo product
(0.14 g) as a white solid (totally 71%). The endo/endo product was not isolated. Exo/Exo: 1H
NMR (300 MHz, CDC13) 7.44 (s, 4H), 7.08 (s, 4H), 5.52 (m,4H), 4.10 (t, J= 6.6 Hz, 8H), 2.43
(m 4H), 1.91 (m,8H), 1.54 (m,8H), 1.39 (m, 16H), 0.93(m, 12H).
13
C NMR (100Hz, CDC13)
206.7, 150.1, 138.0, 128.5, 118.4, 108.5, 82.7, 69.0, 52.8, 31.7, 29.2, 25.9, 22.8, 14.2. FT-IR
(KBr)v/cm': 2952, 2927, 2859, 1682, 1617, 1502, 1457, 1390, 1293, 1252, 1158, 1043, 988,
+) 867.4806, found 867.4796;
937, 899, 854. HRMS (ESI) calcd for C 54H 680 8Na (M+Na
Exo/Endo: 1H NMR (300 MHz,CDC13) 7.56 (s,
2H), 7.46 (s,
2H), 7.06 (s,
2H), 7.05 (s,
2H),
5.78 (m,2H), 5.67 (s, 2H), 4.06 (m,8H), 3.93 (m,2H), 2.20 (s, 2H), 1.86 (m,8H), 1.50 (m,8H),
1.36 (m,16H), 0.91 (m,12H).
13C
NMR (100 MHz, CDC13) 207.4, 149.9, 149.7, 139.4, 138.9,
128.4, 128.3, 119.3, 116.7, 108.5, 85.0, 81.4, 69.0, 68.9, 56.9, 53.0, 31.7, 29.2, 25.9, 22.8, 14.2.
FT-IR (KBr) v/cm-': 2952, 2930, 2856, 1704, 1617, 1508, 1454, 1390, 1300, 1252, 1155, 1001,
924, 873, 850. HRMS (ESI) calcd for C54H680sNa [M+Na] + 867.4806, found 867.4777.
2,3,11,12-Tetrahexyloxyheptacene-7,16-dione (14): This was prepared in the same manner as
described for 10, employing 13 (0.18 g, 0.21 mmol), p-TsOH-H20 (0.15 g, 0.79 mmol), and
toluene (12 mL). A yellow/orange solid was obtained as the product (36 mg, 21%). A small
portion of the product was recrystallized to obtain analytical pure sample for melting point
measurement. mp 232-234 oC (CH 2C12/methanol). 1H NMR (400 MHz, CDC13) 8.87 (s, 4H),
8.27 (s, 4H), 7.10 (s, 4H), 4.13 (t, J= 6.8 Hz, 8H), 1.93 (m, 8H), 1.57 (m, 8H), 1.41 (m, 16H),
0.96 (t, J= 6.8 Hz, 12H).
13
C NMR (125 MHz, CDC13) 183.2, 151.7, 131.3, 131.2, 130.6,
129.3, 127.1, 106.0, 69.1, 31.8, 29.2, 26.0, 22.9, 14.3. FT-IR (KBr) v/cm-': 2953, 2928, 2858,
1670, 1570, 1490, 1463, 1426, 1317, 1210, 1158. HRMS (EI) calcd for C54H640 6 (M+)
808.4696, found 808.4706.
References and Notes
(1) Collings, P. J. Liquid Crystals: Nature's Delicate Phase of Matter; Princeton University
Press: Princeton, NJ, 1990.
(2) Collings, P. J.; Hird, M. Introduction to Liquid Crystals: Chemistry and Physics; Taylor &
Francis: London, 1997.
(3) http://en.wikipedia.org/wiki/Liquid_crystal (accessed August 30, 2007).
(4) Brown, G. H.; Shaw, W. G. Chem. Rev. 1957, 57, 1049.
(5) Hoogboom, J.; Rasing, T.; Rowan, A. E.; Nolte, R. J. M. J. Mater. Chem. 2006, 16, 1305.
(6) Pauluth, D.; Tarumi, K. J. Mater. Chem. 2004, 14, 1219.
(7) Scheffer, T. J. Phil. Trans. R. Soc. Lond. A 1983, 309, 189.
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G. N. J. Appl. Phys. 1974, 45, 4718.
(9) (a) Raj, D. Mater. Chem. Phys. 1996, 43, 204. (b) Hathaway, K. J.; Henderson, E. C.; Koch,
G. C. Proc. SPIE 1983, 3, 87.
(10) Bahadur, B. In Liquid Crystals: Applications and Uses, Bahadur, B.; Eds.; World Scientfic:
Singapore, 1992; Vol. 3, pp 65-208.
(11) http://www.alphamicron.com/ (accessed August 30, 2007).
(12) Baur, G.; Stieb, A.; Meier, G. Mol. Cryst. Liq. Cryst. 1973, 22, 261.
(13) (a) Zhang, X.; Yamaguchi, R.; Moriyama, K.; Kadowaki, M.; Kobayashi, T.; Ishi-I, T.;
Thiemann, T.; Mataka, S. J Mater. Chem. 2006, 16, 736. (b) Zhang, X.; Gorohmaru, H.;
Kadowaki, M.; Kobayashi, T.; Ishi-I, T.; Thiemann, T.; Mataka, S. J. Mater. Chem. 2004, 14,
1901. (c) Bojinov, V. B.; Grabchev, I. K. Org. Lett. 2003, 5, 2185. (d) Matsui, M.; Suzuki, M.;
Mizuno, K.; Funabiki, K.; Okada, S.; Kobayashi, T.; Kadowaki, M. Liq. Cryst. 2004, 11, 1463.
(e) Iwanaga, H.; Naito, K. Liq. Cryst. 2000, 27, 115.
(14) (a) Goodings, E. P.; Mitchard, D. A.; Owen, G. J. Chem. Soc., Perkin Trans. 1 1972, 1310.
(b) Meng, H.; Bendikov, M.; Mitchell, G.; Helgeson, R.; Wudl, F.; Bao, Z.; Siegrist, C.; Kloc,
C.-H. Adv. Mater. 2003, 15, 1090. (c) Payne, M. M.; Delcamp, J. H.; Parkin, S. R; Anthony, J. E.
Org. Lett. 2004, 6, 1609. (d) Payne, M. M.; Parkin, S. R.; Anthony, J. E. J. Am. Chem. Soc. 2005,
127, 8028.
(15) (a) Smith, J. G.; Dibble, P. W.; Sandbom, R. E. J. Org. Chem. 1986, 51, 3762. (b) Smith, J.
G.; Fogg, D. E.; Munday, I. J.; Sandborn, R. E.; Dibble, P. W. J. Org. Chem. 1988, 53, 2942. (c)
Miller, G. P.; Briggs, J. TetrahedronLett. 2004, 45, 477.
(16) (a) Martin, N.; Behnisch, R.; Hanack, M. J. Org. Chem. 1989, 54, 2563. (b) Almlif, J. E.;
Feyereisen, M. W.; Jozefiak, T. H.; Miller, L. L. J. Am. Chem. Soc. 1990, 112, 1206. (c) Thomas,
A. D.; Miller, L. L. J. Org. Chem. 1986, 51, 4160. (d) Christopfel, W. C.; Miller, L. L. J. Org.
Chem. 1986, 51, 4169.
(17) (a) Warrener, R. N. J. Am. Chem. Soc. 1971, 93, 2346. (b) Chan, S.-H.; Yick, C.-Y.; Wong,
H. N. C. Tetrahedron2002, 58, 9413. (c) Yick, C.-Y.; Chan, S.-H.; Wong, H. N. C. Tetrahedron
Lett. 2000, 41, 5957.
(18) Cammidege, A. N.; Chambrier, I.; Cook, M. J.; Garland, A. D.; Heeney, M. H.; Welford, K.
J. Porphyr.Phthalocya.1997, 1, 77.
(19) Chen, Z.; Amara, J. P.; Thomas, S. W.; Swager, T. M. Macromolecules 2006, 39, 3202.
(20) Swartz, C. R.; Parkin, S. R.; Bullock, J. E.; Anthony, J. E.; Mayer, A. C.; Malliaras, G. G.
Org. Lett. 2005, 7, 3163.
(21) (a) Krapcho, A. P.; Getahun, Z.; Avery, K. J. Jr. Synth. Commun. 1990, 20, 2139. (b)
Sakamoto, Y.; Suzuki, T.; Kobayashi, M.; Gao, Y.; Fukai, Y.; Inoue, Y.; Sato, F.; Tokito, S. J.
Am. Chem. Soc. 2004, 126, 8138.
(22) Hua, D. H.; Tamura, M.; Huang, X.; Stephany, H. A.; Helfrich, B. A.; Perchellet, E. M.;
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2907.
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(24) The computation was carried out by Gaussian, employing ZINDO on the DFT B3LYP 631G(d) optimized geometry. Gaussian 03, Revision C.02. M. J. Frisch, G. W. Trucks, H. B.
Schlegel, G. E. Scuseria, M. A. Robb, J. R. Cheeseman, J. A. Montgomery, Jr., T. Vreven, K. N.
Kudin, J. C. Burant, J. M. Millam, S. S. Iyengar, J. Tomasi, V. Barone, B. Mennucci, M. Cossi,
G. Scalmani, N. Rega, G. A. Petersson, H. Nakatsuji, M. Hada, M. Ehara, K. Toyota, R. Fukuda,
J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, M. Klene, X. Li, J. E. Knox,
H. P. Hratchian, J. B. Cross, C. Adamo, J. Jaramillo, R. Gomperts, R. E. Stratmann, O. Yazyev,
A. J. Austin, R. Cammi, C. Pomelli, J. W. Ochterski, P. Y. Ayala, K. Morokuma, G. A. Voth, P.
Salvador, J. J. Dannenberg, V. G. Zakrzewski, S. Dapprich, A. D. Daniels, M. C. Strain, O.
Farkas, D. K. Malick, A. D. Rabuck, K. Raghavachari, J. B. Foresman, J. V. Ortiz, Q. Cui, A. G.
Baboul, S. Clifford, J. Cioslowski, B. B. Stefanov, G. Liu, A. Liashenko, P. Piskorz, I.
Komaromi, R. L. Martin, D. J. Fox, T. Keith, M. A. Al-Laham, C. Y. Peng, A. Nanayakkara, M.
Challacombe, P. M. W. Gill, B. Johnson, W. Chen, M. W. Wong, C. Gonzalez, and J. A. Pople,
Gaussian, Inc., Wallingford CT, 2004.
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(26) Zhu, Z.; Swager, T. M. J. Am. Chem. Soc. 2002, 124, 9670.
CHAPTER 2
Syntheses of Soluble, a-Stacking Tetracene
Derivatives for OFETs
Adapted from:
Chen, Z.; Muller, P.; Swager, T. M. Org.Lett. 2006, 8, 273.
Introduction
Organic Field Effect Transistors
Field-effect transistors (FETs) based on inorganic materials are the basic building
units for modem digital integrated circuits and rely on an electric field to control the
conductivity of "channels" in a semiconductor material.' An FET can be constructed from a
number of semiconductors and silicon is by far the most common semiconductor used. Some
other materials have also been used to achieve products with unique properties. For example,
gallium arsenide (GaAs) has electron mobility and drift velocities that are far higher than the
standard doped silicon. Amplifiers designed with GaAs FET devices have a much higher
frequency response and lower noise factor than those based on standard silicon-based FETs.2
Organic semiconductors, including small molecules and polymers, represent a new channel
material for FETs, and studies in the past two decades have achieved great advancements. 3 In
addition to the FETs, organic semiconductors have attracted much attention in applications,
such as organic light emitting diodes (OLEDs), solar cells, electrochemical cells, and organic
memories.4 This interest is based in part on their potential for lightweight, low cost, and
flexible devices.
Basically, organic field effect transistor (OFET) devices are based on the conventional
configuration of their inorganic counterpart.1, 4 A typical device is composed of three main
components: (1) source, drain, and gate electrodes; (2) a dielectric layer; and (3) an active
semiconductor layer.
Within the basic FET design, there are two types of device
configurations: top contact and bottom contact (Figure 2.1). The former involves building the
source and drain electrodes onto a preformed semiconductor layer, whereas the latter is
constructed by depositing the semiconductor layer over the source and drain electrodes. Each
of these devices has particular advantages and disadvantages in the fabrication process. 3 -4
The FET operates by the effects of an electric field on the flow of electrons through a single
type of semiconductor material.1, 4 Current moves within the FET in a channel, from the
source contact to the drain contact, and it is controlled by the electric field generated by a gate
terminal (gate-source voltage: VGS). The channel materials can be either n-type or p-type
semiconductors. Accordingly, the device is named as either an n-channel or p-channel device
(Figure 2.2), respectively. In n-channel devices, electrons flow so the drain potential must be
higher (more positive) than that of the source (drain-source voltage: VDS > 0). In p-channel
devices, the flow of holes (carrier with positive charge) requires that VDS < 0.1,4 The current
I.
I
1
I
I
8
(A)
I
I
I
I
-
(B)
-
I
I
Figure 2.1. Typical field effect transistor configurations: (A) Top contact device; (B) Bottom
contact device.
__r~Vos
:**gas
vos> 0
VGas< 0 V
Vas
+"-r·*r····r··
c~~~rl.
+·.···r
..........
~r·
I
I
VoS,>OVV:
VneOV
nk4-\
In-rr
r
-I
uh
Al
1'
I
I
ON-Channel (N-Type) Operation
P-Channel (P-Type) Operation
SOrganic Semiconductor
Figure 2.2. Schematic representation ofp- and n-channel field effect transistor. 4 a
between source and drain electrode is measured when no voltage is applied to the gate
electrode (off current) and when a voltage is applied to the gate electrode (on current). The
basic relationships describing the source drain current of a FET device can be express as the
following equations:
(IDS Un(IDS )sat =
VGs -V
?
2L
-
VSD
I(VGs - VT )2
where (IDS)lin and (IDS)sat are the source-drain currents in the linear and saturation regimes,
respectively. The value ~ is the field-effect carrier mobility of the semiconductor, W is the
channel width, L is the channel length, Ci is the capacitance per unit area of the insulator
layer, Vr is the threshold voltage, VDs is the source-drain voltage, and VSG is the source-gate
Length
.Smicnductor
Insulator
,Substrate
Figure 2.3. Schematic representation of a field effect transistor.
voltage. 4 a The mobility, 4 , describes how easily charge carriers can move within the active
layer under the influence of an electric field. Therefore, it is directly related to the switching
speed of the device. This parameter can be extracted from current-voltage measurements, and
is ideally as large as possible. Typical values range from 0.1-1 cm 2 /Vs for amorphous-silicon
devices, and the best organic materials have been reported to achieve mobilities of 1-10
cm2Ns. 4 The on/off ratio, defined as the ratio of the on current and off current, is indicative
of the switching performance of OFETs.
Devices with on/off ratio higher than 106 are
generally suitable for most applications.
Although the current OFETs cannot compete with most conventional inorganic
technologies, they should find applications in low-performance radio-frequency technology,
disposable sensors, and integrated optoelectronic devices, such as pixel drives and switching
elements in displays. Polymer Vision has successfully implemented pentacene based activematrix backplanes in its rollable display prototypes. These organic devices have acceptable
levels of performance and stability in an introductory application and are the first devices
nearing commercialization. Many groups have also demonstrated potential utility of OFETs
for OLED backplanes on flexible substrates of fairly large size.3 The advantages of organic
systems include solution phase fabrication and good compatibility with flexible plastic
substrates, which meet the current requirements for developing low-cost, large area,
lightweight, and flexible devices. These requirements can not be achieved by conventional Si
technology, because it involves high-temperature and high-vacuum deposition processes and
sophisticated photolithographic patterning methods. Since the report of the first OFET in
1986, there have been great advances in materials purification, device fabrication methods,
and development of higher-performance organic semiconducting materials, which have
improved the existing technologies and expanded the scope of potentially realizable
applications.4' 5 Recent improvements in device performance have rapidly expanded OFETs
from niche markets, making them targets for a wider range of applications.6 The numerous
applications presented thus far have already shown the potential of OFETs, which will
continue to grow.
Although challenges exist for large-scale manufacturing, their rapid
development shows great promise for their future in organic electronics.
Much of this progress of OFETs has been a result of the understanding of structureproperty relationships of the active channel materials. 4a,4c Molecular tuning based on this
understanding has yielded materials with better solubility for spin-coating, desirable
supramolecular order for higher mobility, and improved stability towards oxygen and
light.4,7,8 In addition to the challenges presented by fabrication, particular attention must be
paid to the design of materials that will meet the performance demands of a OFET in its
parent applications. In well-ordered inorganics, such as single-crystal Si, the delocalization of
electrons over equivalent sites leads to a band-type mode of transport, with charge carriers
moving through a continuum of energy levels in the solid. In an organic transistor, the active
layer is comprised of a thin film of highly conjugated small molecules or polymers, which
have a different nature of charge transport than their inorganic counterpart.
The well
recognized mechanism is hopping between discrete, localized states of individual molecules. 9
The presence of impurities or inconsistencies in a structure may result in "traps" that alter the
relative energy levels, and inhibit the flow of charge carriers. Hence, to achieve acceptable
performance, organic semiconductors must satisfy general criteria related to both the carrier
injection and current-carrying characteristics.
Basically, the highest occupied molecular
orbital (HOMO) or lowest unoccupied molecular orbital (LUMO) energies of the individual
molecules must match the Fermi levels of electrodes for efficient charge carrier injection and
transport.8 In addition, the materials must exhibit unique molecular packing with sufficient
overlap of frontier orbitals in solid state and the molecules should be preferentially oriented
on the FET substrate to facilitate most efficient charge transport along the direction of
intermolecular
7tn-n
stacking. Since impurity and traps in organic semiconductors have been
found to significantly reduce device performance, the materials should be easily processed to
achieve an extremely high purity. In the past decades, novel organic semiconductors with
excellent device performances have been developed. 4,7 Pentacene, a-sexithiophene, and poly3-hexylthiophene are good examples of p-type organic semiconductors, and perfluorinated
copper-phthalocyanine, N,N'-dialkyl substituted perylenetetracaboxydiimide derivatives, and
poly(benzobisimidazobenzophenanthroline), are typical n-type organic materials. 4' 7
Acenes
Acenes or oligoacenes are a class of organic polycyclic aromatic hydrocarbons made
up of linear fused benzene rings (Chart 2.1). The linear acenes from naphthalene to heptacene
have been synthesized. 10 However, higher members of this series have proved too reactive to
isolate under ordinary conditions and they have been mainly the subject of many theoretical
studies and controversial discussions." The larger acene representatives have potential
Chart 2.1. Molecular structures of acenes
n
n = 0, naphthalene
n = 1,anthracene
n = 2, tetracene
n = 3, pentacene
n = 4,hexacene
n = 5, heptacene
interest in electronic applications and are actively researched in chemistry. 7 In addition, the
oligoacene units are a basic building unit of graphite and carbon nanotubes, and investigation
of their electronic properties is helpful for understanding the structure and properties of the
latter materials.7a Oligoacenes, typically pentacene and tetracene, have been widely studied
as the channel materials for OFETs. 7 Petancene-based devices were reported to show field
effect mobilities of greater than 2 cm 2V-'s -1, which are comparable to the performance of
amorphous-silicon thin film devices. 12 Recent reports of mobilities up to 35 cm 2V-'s' for
high-purity pentacene single crystals, however, give hope that further investigation will
continue to pay off for organic semiconductors.'
3
It is generally believed that the charge
carrier mobility in organic materials at room temperature is dominated by
7-n
interactions
between molecules via a charge hopping transport mechanism. 9 Therefore, the molecular
stacking in solid state has a strong influence on the field effect mobility. It is well recognized
that the intermolecular distance and the extent of 7c-orbital overlap between neighboring
molecules are two important parameters for investigating the
D~III)BPB
Figure 2.4. Herringbone-like packing motif of pentacene. 7
electrical properties of organic semiconductors. 14 However, the herringbone-like stacking
superstructure of oligoacenes (Figure 2.4) is not the optimum packing for high mobility
devices, because the edge-to-face packing reduces the In-overlap between adjacent
molecules. 15 Recently, several novel organic semiconducting compounds with cofacial
7n-
stacking arrangement have been synthesized and their devices showed promising chargecarrier mobilities. 16 The excellent intrinsic electrical properties of oligoacenes have also
inspired chemists to design and synthesize oligoacene derivatives with improved n-stacking in
the solid-state.1 7 -20 Additionally, the often poor solubility of oligoacenes in organic solvents
at room temperature has been a hurdle in fully realizing the advantages of organic electronics,
such as low-cost patterning and low-temperature processing. Therefore, it is necessary to
develop new, soluble, 7n-stacking organic semiconductors for OFETs.2 1
In this work, we report a family of partially fluorinated and alkyl/alkoxy substituted
tetracene molecules that are soluble in common organic solvents and have significant ntstacking motifs in crystal lattices. Fluorine has been applied extensively to tune the electronic
and structural properties of organic semiconducting materials, due to its strong
electronegativity and significantly small size.17a,22 Recently, Watson and coworkers have
reported that partial fluorination overcame the herringbone crystal packing of tetracyclic
aromatic compounds, leading to a cofacial packing with a n-distance of 3.37 A. 22a Partially
fluorinated silylethynyl pentacenes were also found to reduce the intermolecular distances. 17a
Incorporation of long alkyl or alkoxy chains is widely used to improve the solubility of small
molecules and polymers, and to facilitate formation of semiconductor thin films by costeffective solution deposition techniques.23 In our studies, we found that the regiochemistry of
the side chain attachment on the tetracene backbone not only improved the solubilities, but
also tuned the electrochemical properties and packing motifs. Furthermore, the incorporation
of both donor and acceptor substituents to the main chain was added to induce a strong dipole
moment. Strong dipole-dipole interactions between neighboring molecules were previously
demonstrated to afford n-stacking with short intermolecular distance and self-assembly
properties. 19
Results and Discussion
The key steps of our synthetic strategy for preparing tetracene derivatives, as outlined
in Scheme 2.1, were adapted from a methodology previously reported by Gribble. 24 Nmethyl-4,5,6,7-tetrafluoroisoindole (2), which is a moderately stable compound and should be
prepared freshly, could be easily prepared from a reaction between 1 and 3,6-di(2-pyridyl)1,2,4,5-tetrazine, via a Diels-Alder (D-A) reaction and subsequent two thermally allowed
retro D-A reactions.25 In the presence of 2, dibromonaphthalenes 3a-d were treated with
phenyllithium (PhLi) at 0 oC, leading to the formation of corresponding naphthalyne, which
readily reacted with 2 via the D-A reaction. Amines 4a-d were obtained as colorless or
slightly yellow solids in 40-65% yields. The deamination was carried out by the treatment of
Scheme 2.1. Syntheses of 5a-d
F H3C
3,6-(Di-2-pyridyl)-
F
F
F H3C
R
R'
R
R'
PhLi, THF, 0 OC
F
F
CH 2C 2 ,rt
F
NCH 3
F
R
R'
N
F
F
F
3
R'
4a-d
3a-d
F
NaOH (50%, aq)
FN`
benzyltriethylammonium F
chloride, CHC13, rt
R
R'
y
F
R
R'
a: R =H,
b: R = OC6 H 13 ,
c: R= H,
d: R =C8H17,
R'= OC 6H13
R'= H
R'= C8 H17
R'= H
yield (3 steps)
18%
35%
21%
15%
5a-d
an aqueous solution of NaOH (50%, w/w) to the stirred chloroform solution of 4a-d and
phase-transfer catalyst benzyltriethylammonium chloride at room temperature. This reaction
was proposed to proceed by way of the in situ formed dichlorocarbene, which combined with
4a-d to form ammonium ylide intermediates. 26 The aromatic acenes were then generated
from the rapid cheletropic loss of methyl isocyanide dichloride. Tetracene derivatives 5a-d
were obtained as red or orange crystals from recrystallization in hexane, with yields of 1535% over 3 steps. The solubility of 5a-d in organic solvents, such as methylene chloride,
chloroform and toluene, was found to be much better than their parent tetracene, owing to the
long linear side chains.
Scheme 2.2. Syntheses of 3a-d
O0
Br,
zO
0
NI_
,1
2) K2CO3 , C6 H
1 3 Br
DMF, 85 OC
64%
Br
uH
ether, rt
toluene, reflux
+
n
I 'a2 2v 4,
OC 6H 13
Br
Br
Br
74 o/
OC6H13
117/
6
O
O
3a
OC6Hi3
OC 6H13
1) Na2S 20 4, H20, ether, rt
NBS (2.leq)
Br
2) K2 C03, C6H13Br, DMF, 85 OC
88%
CH 2C22, rt, 78%
Br
7
0O
Br
BrK
Br toluene, n-BuLi
00C, 64%
C8 H17
3b
8
C8H17
Br
OC 6H13
OC 6H1 3
C0
8 H17
Br
CsH17
TiC4, Zinc, THF, 0 OC-reflux
Br
88%
C8 H17
10a
9a
C8H17
17
3c
C8H17
furan
Br
toluene, n-BuLi
-300C, 78%
Br
C8H17
TiCl4 , Zinc, THF, 0 OC-reflux
95%
Br
C8H17
C8H17
10b
3d
Scheme 2.2 shows the synthetic routes to dibromonaphthalenes (3a-d), which are used
in Scheme 2.1.
Hexyloxy-substituted naphthalenes were prepared from corresponding
naphthoquinone (6 and 7) via a reduction to naphthalenehydroquinone and then alkylation
under basic conditions. 27 The bromination of 1,4-bis(hexyloxy)naphthalene (8) was carried
out by a treatment of N-bromosuccinimide (NBS) in methylene chloride.28 Both 3a and 3b
were obtained as colorless crystals. Dioctyl-substituted dibromonaphthalenes (3c-d) were
(Sa
j=:Vr
j=:VF
(5b'
(5c)
(5d)
Figure 2.5. Packing diagrams of 5a-d show that the molecules stack in a slipped cofacial
motif. (Left: view along the short molecular axis; Right: view along the long molecular axis.).
prepared in two steps with good yields.
In the presence of an excess of the furans,
tetrabromobenzenes (9a-b) were treated with n-butyllithium (n-BuLi) at low temperature,
leading to the 1,4-endoxides (10a-b) via a D-A reaction between corresponding benzyne and
furans.29 Low-valent Ti, prepared in situ from a reaction between zinc and TiC14, was applied
for the deoxygenation of 10a-b, 29'30 to give 3c-d as white solids.
The molecular and crystal structures of 5a-d were determined by X-ray structure
analysis on single crystals, which were obtained by solvent diffusion at low temperature. The
packing diagrams of 5a-d are displayed in Figure 2.5. All four compounds show primarily
face-to-face slipped n-stacking in crystals (Figure 2.6-2.9), and the selected structural data are
summarized in Table 2.1. The hexyloxy-substituted molecules, 5a and 5b, adopt anti-parallel
cofacial arrangement. The n-stacked column has regular alternating intermolecular distances
(Table 2.1), forming a face-to-face n-stacks of n-dimers. 14 In the crystal structure of 5a, we
found that the two hexyloxy chains are in a conformation which extends them out of the
aromatic plane, holding the n-dimer like a shield. The remarkably short intermolecular
distances (3.22 A and 3.24 A) indicate the existence of strong intermolecular interactions,
which are expected to enhance the charge carrier mobility.
The relatively large
intermolecular distances in 5b may be attributed to the repulsion from the side chains, which
have a kink in their otherwise extended conformation.
However, the relative small roll
displacement 4 (0.99 A and 0.89 A) is suggestive of large n-orbital overlap.' 5 In contrast to the
crystal structures of 5a and 5b, octyl-substituted 5c crystallizes in a parallel cofacial
molecular arrangement, with the intermolecular distance of 3.35 A and roll displacements of
1.15 A. The molecules in each column are slipped lengthwise by about one benzene ring.
Compound 5d has a face-to-face anti-parallel nt-stacking motif, similar to that of 5a and 5b.
However, the stacks of 5d are packed with alternating columns that are at an angle of 1410
between the short axes of the molecules in neighboring columns (Figure 2.10). Interestingly,
5d exhibits the shortest interplanar distance (3.18 and 3.20 A) among these four compounds,
Figure 2.6. Packing diagram of 5a (view along the a-axis).
Figure 2.7. Packing diagram of 5b (view along the a-axis).
Figure 2.8. Packing diagram of 5c (view along the b-axis).
Figure 2.9. Packing diagram of 5d (view along the a-axis).
but it has the biggest pitch and roll displacement, which is expected to reduce the extent of norbital overlap. 14,15 These displacements are probably due to the out of plane conformation of
the octyl chains. The crystal features of 5a-d clearly demonstrate the effect of substitution
regiochemistry on molecular packings. 3 1 A close inspection of the nt-dimers in 5a, 5b and 5d
reveals that the electron-poor ring (fluorinated ring) faces to the electron-rich ring (hexyloxyor octyl-substituted ring), indicating the effect of arene-perfluoroarene interaction on the
Table 2.1. Comparison between the Intermolecular Distances and Pitch/Roll Displacements
of 5a-d
compd
d (A)a
dp (A)b
dr (A)c
5a
3.22
1.37
1.16
3.24
1.68
1.36
3.44
3.41
0.99
3.53
1.21
0.89
5c
3.35
3.46
1.15
5d
3.18
1.22
1.45
3.20
4.68
1.75
5b
a
Intermolecular distance. b Pitch displacement. c Roll displacement (see ref. 4).
Figure 2.10. Packing diagram of 5d, showing the 1410 angle between molecules in
neighboring crystal packing columns.
molecular stacking.32
A similar packing motif was previously observed in the crystal
structure of 2,3,4,5,6-pentafluorodiphenyldiacetylene by Grubbs. 33 Extensive studies on the
interactions of phenyl and perfluorophenyl groups, especially the 1:1 mixture of benzene and
hexafluorobenzene, have supported that this co-facial stacking arrangement results from the
electrostatic attractions between adjacent molecules. 32 34
Our results suggest that the
noncovalent electrostatic interactions between donor and acceptor substituted arenes can be
9 35
employed to design novel 7n-stacking organic semiconductors.' '
The electrochemical behaviors of tetracene and compounds 5a-d (Figure 2.11)
together with the spectroscopic data (Figure 2.12) are summarized in Table 2.2. All five
compounds show one quasi-reversible reduction wave with onset reduction potentials
between -2.05 and -1.84 eV (vs Fc/Fc÷ couple). The cyclic voltammogram (CV) of 5a
exhibits two separate one-electron reversible oxidation waves at E1/2 = 0.39 V and 0.97 V.
Table 2.2. Optical Band Gap and CV Data of Tetracene and 5a-d
compd
Ego p (eV)a
Eoxonset V)b
Eredonset (V)b
tetracene
2.57
0.41
-2.05
2.46
5a
2.25
0.31
-1.92
2.23
5b
2.34
0.44
-1.84
2.28
5c
2.47
0.56
-1.89
2.45
5d
2.40
0.48
-1.91
2.39
EgEChem
(eV)c
Calculated from the onset of UV-vis absorbance spectra measured in CH 2C12 solution.
Performed in 0.1 M solution of TBAPF 6 in CH 2C12, Pt electrode, scan rate of 100 mV/s,
ferrocene as internal standard. cCalculated from the onset of CVs.
a
Similarly, 5b has two sequential oxidation waves over the range of scan rate investigated (10100 mV/s). However, the return reduction processes were scan rate dependent, displaying the
expected two waves (0.47 and 0.66 V) at slow scan rates (10 mV/s) and a unexpected third
wave with large potential shifts (0.19, 0.45 and 0.65 V) at 100 mV/s. The electrochemical
irreversibility of 5b probably indicates slow electron transfer kinetics and/or strong
intermolecular interactions. Both the CVs of 5c and 5d show one reversible oxidation wave
and a second quasi-reversible oxidation wave, with the first waves at E1/2 = 0.65 V and 0.58 V
for 5c and 5d, respectively. We found the introduction of both fluorine and hexyloxy groups
offered slight change on oxidation potentials, as compared to the E1/2 potential of tetracene.
However, 5c and 5d exhibited relatively higher oxidation potentials, due to the modest
Our CV studies reveal that the position of
electron-donating ability of the alkyl group.
alkoxy/alkyl substituents could significantly tune the electrochemical properties of these
molecules. The bandgaps, determined from the onset of oxidation and reduction (scan rate =
I
1.6x10
'
I
'
I
'
1
'
I
'
I
'
I
'
I
'
F
OC 6H1 3
F
OC 6H1 3
I
"
F
8.0x1 0 6
0.0
-8.0x10 "
F
OC6H13
F
OC6H1 3
F
C8 H17
F
C8 H17
U
-1.6x10
F
C8 H17
F
C8H,7
"
-2.4xl 0 "
-2.5 -2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0 1.5
su
ls
na
tu
s.
.
.
a
s
s
.
Potential (V)
Figure 2.11. CVs of compounds 5a-d and tetracene performed in 0.1 M solution of TBAPF6
in CH 2C12 , with a Pt electrode, a scan rate of 100 mV/s, and ferrocene as the internal standard.
5a
S5b
(0
(.
c
(U
.0
L.
5d
0,
U)
Tetracene
SI
350
400
I
I
I
450
500
550
600
Wavelength (nm)
Figure 2.12. UV-vis absorption spectra of tetracene and 5a-d measured in CH 2C12.
100 mV/s), correspond with the values calculated from the onset of UV-vis absorption spectra
(Figure 2.12) measured in methylene chloride solution (Table 2.2). The small bandgaps of
these compounds are expected to make them promising candidates for use in organic
electronics.
Conclusion
In summary, we have shown the syntheses of a series of partially fluorinated tetracene
derivatives with N-methyl-1,2,4,5-tetrafluoroisoindole
as a synthetic building block.
Compounds 5a, 5b and 5d were found to crystallize with antiparallel, cofacial-stacked
column superstructures, while 5ec exhibited a slipped parallel 7t-stacking arrangement in
crystals. We find that the substitution of alkyl/alkoxy groups on the main chain not only
provided better solubility in common organic solvents, but also subtly tuned the crystal
structures and electrochemical behaviors.
It will be very interesting to grow high quality
single crystals and study the intrinsic charge carrier mobilities, as well as to study the device
performance of OFETs based on these tetracene derivatives.
Experimental Section
Materials:
Anhydrous
tetrahydrofuran
(THF),
toluene,
acetonitrile
(CH 3CN)
and
dichloromethane (CH 2 C12) were purchased from Mallinckrodt Baker Inc. Furan was distilled
over K2C0 3 and NBS was recrystallized from water prior to use. Known compounds 1,2,3,4tetrafluoro-5,8-dihydro-5,8-N-methylaminonaphthalene
(1),38
6,7-dibromo-1,4-
naphthoquinone (6), 39 1,2,4,5-tetrabromo-3,6-dioctylbenzene (9b)40 and 2,5-dioctylfuran41
were prepared following literature procedures. All other reagent grade starting materials were
purchased from Aldrich, Lancaster, or Alfa Aesar, and used without further purification.
Silica gel (40-63 tim) was obtained from SiliCycle. Neutral alumina (58 angstroms) was
purchased from Alfa Aesar and basic alumina (chromatographic grade, pH: 9.0-10.5, 80-325
mesh) was purchased from EMD Chemicals Co.
General methods and instrumentation: NMR ('H,
Varian Mercury-300 MHz, Varian Inova-500
spectrometers.
The 1H and
13C, 19F)
spectra were recorded on
MHz or Bruker Advance-400 MHz
13
C chemical shifts are given in unit of 6 (ppm) relative the
tetramethylsilane (TMS) where 8 (TMS) = 0, and referenced to the residual solvent. The
19F
NMR chemical shifts are reported relative to trichlorofluoromethane or hexafluorobenzene as
the reference.
Melting points were measured on a Mel-Temp II apparatus (Laboratory
Devices INC) and were not corrected. High-resolution mass spectra (HRMS) were obtained
on a Bruker Daltonics APED II 3T FT-ICR-MS using electron impact ionization (EI). The X-
ray crystal structure was determined with a Siemens Platform three-circle diffractometer
coupled to a Bruker-AXS Smart Apex CCD detector with graphite-monochromated Mo Kao
radiation (A = 0.71073
A),
performing (p- and o-scans. All structures were solved by direct
methods using SHELXS 36 and refined against F 2 on all data by full-matrix least squares with
SHELXL-97. 37 All non-hydrogen atoms were refined anisotropically. All hydrogen atoms
were included into the model at geometrically calculated positions and refined using a riding
model. The isotropic displacement parameters of all hydrogen atoms were fixed to 1.2 times
the U value of the atoms they are linked to (1.5 times for methyl groups).
The
electrochemistry studies were performed using Autolab PGSTAT 10 potentiostat (Eco
Chemie) in a three-electrode cell configuration under nitrogen atmosphere.
voltammetry (CV)
experiments
were carried
out in 0.1
Cyclic
M tetrabutylammonium
hexafluorophosphate (TBAPF 6) supporting electrolyte in CH 2C12. A platinum button (1.6 mm
in diameter) and platinum gauze were used as the working and counter electrodes,
respectively. The potential was measured against a Ag/AgNO 3 (10 mM) in 0.1M TBAPF 6 in
CH 3CN. The reported potentials are versus the ferrocene/ferrocenium (Fc/Fc+) couple with
observed half-wave potentials between 250 mV and 300 mV, obtained by adding a crystal of
ferrocene to the solution. UV-vis spectra were obtained from Hewlett-Packard 8452A diode
array UV-visible spectrophotometer.
General procedure for syntheses of 4a-d:24
A solution of 3,6-(di-2-pyridyl)-1,2,4,5-tetrazine (1 equiv.) and 1,2,3,4-tetrafluoro-5,8dihydro-5,8-N-methylaminonaphthalene 1 (1 equiv.) in 50 mL CH 2C12 was stirred overnight
under Ar at room temperature. The mixture was filtered through a short column of neutral
A120 3. The filtrate was concentrated in vacuo, and the slightly yellow residue was dried
under vacuum. Dibromonaphthalene 3a-d (0.8 equiv.) and anhydrous THF (50 mL) were then
added. Under Ar atmosphere, a solution of phenyllithium (1.6 M in cyclohexane/ether; 1
equiv.) was then added dropwise to the above mixture at 0 oC over 3 h. After addition, the
mixture was stirred at 0 oC for an additional 1 h, and then allowed to warm to room
temperature slowly and stirred overnight. The mixture was poured into water and extracted
by CHC13 . The organic layer was washed by brine and dried over Na2 SO4. Solvent was
removed in vacuo. Purification by column chromatography (basic A120 3) afforded colorless
or slightly yellow sticky solids.
13-Methyl-1,2,3,4-tetrafluoro-7,10-bis(hexyloxy)-5,12-dihydronaphthacene-5,12-imine
(4a): This compound was prepared by following the general procedure described above,
employing 1 (1.7 g, 7.4 mmol), 3a (2.5 g, 5.2 mmol), phenyllithium (1.6 M in
cyclohexane/ether, 4.0 mL, 6.4 mmol), THF (130 mL).
The common workup and
chromatography afforded 1.1 g of 4a as a white/tan sticky solid (40%). 1H NMR (300 MHz,
CDC13) 8.17 (s, 2H), 6.72 (s, 2H), 5.37 (s, 2H), 4.05 (m, 4H), 2.29 (s, 3H), 1.91 (m, 4H), 1.56
(m, 4H), 1.41 (m, 8H), 0.95 (m, 6H). 19F NMR (282 MHz, CDC13) -140.9 (br), -142.2 (br), 153.8 (br), -156.2 (br).
13
C NMR (125 MHz, CDCl3) 149.2, 125.1, 105.8, 69.8, 68.8, 36.5,
31.9, 29.6, 26.2, 22.8, 14.3. HRMS calcd for C31H35F4NO2 (Me) 529.2598, found 529.2590.
13-Methyl-1,2,3,4-tetrafluoro-6,11-bis(hexyloxy)-5,12-dihydronaphthacene-5,12-imine
(4b): This compound was prepared by following the general procedure described above,
employing 1 (0.77 g, 3.4 mmol), 3b (1.2 g, 2.5 mmol), phenyllithium (1.6 M in
cyclohexane/ether, 2.6 mL, 4.2 mmol), THF (50 mL). The common workup and
chromatography afforded 0.7 g of 4b as a colorless sticky solid (55%). 'H NMR (300 MHz,
CDC13) 8.12 (dd, J= 6.3, 3.3 Hz, 2H), 7.50 (dd, J= 6.3, 3.3 Hz, 2H), 5.63 (s, 2H), 4.25 (m,
2H), 4.07 (m, 2H), 2.28 (s, 2H), 1.95 (m, 4H), 1.61 (m, 4H), 1.43 (m, 8H), 0.97 (t, J=6.9 Hz,
6H). 19F NMR (282 MHz, CDC13) -144.2 (br), -145.4 (br), -156.9 (br), -159.2 (br). 13C NMR
(125 MHz, CDC13) 128.8, 126.6, 122.7, 74.8, 67.8, 36.5, 31.8, 30.6, 26.0, 22.8, 14.2. HRMS
calcd for C31H35F4NO2 (M+) 529.2598, found 529.2597.
13-Methyl-1,2,3,4-tetrafluoro-7,10-dioctyl-5,12-dihydronaphthacene-5,12-imine
(4c):
This compound was prepared by following the general procedure described above, employing
1 (0.73 g, 3.2 mmol), 3e (1.2 g, 2.4 mmol), phenyllithium (1.6 M in cyclohexane/ether, 2.0
mL, 3.2 mmol), THF (60 mL). The common workup and chromatography afforded 0.6 g of
4c as a white/tan sticky solid (45%). 'H NMR (300 MHz, CDC13) 8.00 (s, 2H), 7.24 (s, 2H),
5.41 (s, 2H), 3.00 (m, 4H), 2.33 (s, 3H), 1.72 (m, 4H), 1.31 (m, 20H), 0.92 (t, J= 6.9 Hz, 6H).
19F
NMR (282 MHz, CDC13) -141.0 (br), -142.2 (br), -153.4 (br), -155.9 (br). ' 3 C NMR (125
MHz, CDC13) 137.5, 131.1, 126.6, 70.0, 36.5, 33.5, 32.1, 31.1, 30.0, 29.7, 29.5, 22.9, 14.3.
HRMS calcd for C35H43F4N (M+) 553.3326, found 553.3334.
13-Methyl-1,2,3,4-tetrafluoro-6,11-dioctyl-5,12-dihydronaphthacene-5,12-imine
(4d):
This compound was prepared by following the general procedure described above, employing
1 (0.81 g, 3.53 mmol), 3d (1.44 g, 2.8 mmol), phenyllithium (1.6 M in cyclohexane/ether, 2.1
mL, 3.4 mmol), THF (50 mL). The common workup and chromatography afforded 0.66 g of
4d as a white/tan sticky solid (42%). 'H NMR (300 MHz, CDCl 3) 7.96 (dd, J= 6.6, 3.6 Hz,
2H), 7.48 (dd, J= 6.3, 3.3 Hz, 2H), 5.48 (s, 2H), 3.07 (t, J= 7.8 Hz, 4H), 2.31 (s, 3H), 1.63
(m, 4H), 1.51 (m, 4H), 1.31 (m, 16), 0.91 (m, 6H).
142.1 (br), -153.6 (br), -156.0 (br).
13C
19F
NMR (282 MHz, CDCl 3) -140.7 (br), -
NMR (125 MHz, CDC13) 131.4, 125.9, 124.8, 69.1,
36.5, 32.1, 31.3, 30.4, 29.9, 29.7, 29.5, 22.9, 14.3.
HRMS calcd for C35H4 3F4N (M)
553.3326, found 553.3320.
General procedure for the preparation of 5a-d:2 4'26
Under Ar, benzyltriethylammonium chloride (40 mg) was added in one portion to a stirred
solution of amine (4a-d) (1.5 mmol) in chloroform (40 mL). An aqueous NaOH solution
(50%, 2 mL) was then added dropwise.
The resulting mixture was stirred at room
temperature overnight. The reaction mixture was poured into an aqueous HCI solution (3 M,
80 mL) and extracted by CHC13 . The organic layer was washed by water and brine in
sequence, and dried over K2C0 3. The solvent was removed in vacuo. Purification by flash
column chromatography (silica gel) and recrystallization from hexane afforded red or orange
crystals.
1,2,3,4-Tetrafluoro-7,10-bis(hexyloxy)tetracene
(5a): This compound was prepared by
following the general procedure described above, employing 4a (1.09 g, 2.06 mmol),
benzyltriethylammonium chloride (50 mg), NaOH (50% aqueous, 1.2 mL), CHC13 (50 mL).
The common workup, chromatography and recrystallization afforded 0.45 g of 5a as red
needle-like crystals (44%). mp 141-142 oC (hexane). 1H NMR (300 MHz, CDCl 3) 8.61 (s,
2H), 8.39 (s, 2H), 6.44 (s, 2H), 4.11 (t, J=6.6 Hz, 4H), 2.02 (m, 4H), 1.67 (m, 4H), 1.50 (m,
8H), 1.02 (m, 6H).
19F
NMR (282 MHz, CDCl 3) -149.8 (m, 2F), -158.8 (m, 2F).
13
C NMR
(100 MHz, CDC13) 148.3, 128.9, 126.5, 121.1, 120.0, 101.7, 68.4, 32.0, 29.6, 26.3, 23.0, 14.4.
HRMS calcd for C30H32F40 2 (M÷) 500.2333, found 500.2325.
1,2,3,4-Tetrafluoro-6,11-bis(hexyloxy)tetracene
(5b): This compound was prepared by
following the general procedure described above, employing 4b (0.69 g, 1.3 mmol),
benzyltriethylammonium chloride (50 mg), NaOH (50% aqueous, 0.9 mL), CHCl 3 (40 mL).
The common workup, chromatography and recrystallization afforded 0.42 g of 5b as red
needle-like crystals (64%). mp 111-112 TC (hexane).
1H
NMR (300 MHz, CDCl 3) 9.09 (s,
2H), 8.29 (dd, J= 6.8, 3.0 Hz, 2H), 7.48 (dd, J= 6.9, 3.0 Hz, 2H), 4.26 (t, J= 6.3 Hz, 4H),
2.11 (m, 4H), 1.75 (m, 4H), 1.49 (m, 8H), 1.00 (m, 6H). 19F NMR (282 MHz, CDC13) -149.0
(m, 2F), -157.8 (m, 2F).
13
C NMR (125 MHz, CDCl 3) 147.9, 125.9, 125.3, 124.2, 123.0,
115.4, 76.8, 32.0, 30.9, 26.2, 23.0, 14.3. HRMS calcd for C30H32F40 2 (M + ) 500.2333, found
500.2329.
1,2,3,4-Tetrafluoro-7,10-dioctyltetracene (5c): This compound was prepared by following
the
general
procedure
described
above,
employing
4c
(0.6
g,
1.08
mmol),
benzyltriethylammonium chloride (40 mg), NaOH (50% aqueous, 0.8 mL), CHC13 (30 mL).
The common workup, chromatography and recrystallization afforded 0.26 g of 5c as
red/orange crystals (45%). mp 86-87 oC (hexane). 'H NMR (300 MHz, CDC13) 8.63 (s, 2H),
8.60 (s, 2H), 7.19 (s, 2H), 3.10 (t, J= 7.8 Hz, 4H), 1.85 (m, 4H), 1.54 (m, 4H), 1.34 (m, 16),
0.93 (t, J= 6.6 Hz, 6H). 19F NMR (282 MHz, CDCl 3) -149.5 (m, 2F), -158.4 (m, 2F).
13C
NMR (100 MHz, CDC13) 137.0, 132.1, 129.0, 125.2, 123.6, 120.1, 33.4, 32.2, 30.4, 30.2, 29.8,
29.6, 23.0, 14.4. HRMS calcd for C34H40F4 (M+) 524.3061, found 524.3055.
1,2,3,4-Tetrafluoro-6,11-dioctyltetracene (5d): This compound was prepared by following
the
general
procedure
described
above,
employing
4d
(0.65
g,
1.18
mmol),
benzyltriethylammonium chloride (40 mg), NaOH (50% aqueous, 0.9 mL), CHC13 (40 mL).
The common workup, chromatography and recrystallization afforded 0.22 g of 5d as
red/orange crystals (35%). mp 114-115 oC (hexane). 'H NMR (300 MHz, CDC13) 8.95 (s,
2H), 8.27 (dd, J = 6.9, 3.0 Hz, 2H) 7.50 (dd, J = 6.9, 3.0 Hz, 2H) 3.66 (t, J = 8.4 Hz, 4H),
1.86(m, 4H), 1.65 (m, 4H), 1.46 (m, 4H) 1.33 (m, 12H), 0.92(t, J= 6.6 Hz, 6H). 19F NMR
(282 MHz, CDCl 3) -150.0 (m, 2F), -158.4 (m, 2F).
13C
NMR (75 MHz, CDC13) 134.7, 129.8,
128.1, 125.8, 125.6, 117.2, 32.1, 31.6, 30.5, 29.7, 29.6, 28.7. HRMS calcd for C34H40F4 (M÷)
524.3061, found 524.3068.
6,7-Dibromo-1,4-bis(hexyloxy)naphthalene (3a): A mixture of 6 (4.3 g, 13.6 mmol) and
Na2 S20
4
(35 g) in 300 mL water and 500 mL ethyl ether was vigorously shaken periodically
for about one hour. Organic layer was separated, washed by brine, and dried over Na2 SO 4.
The solvent was removed in vacuo, leading to slight yellow solid. After drying in vacuum, the
product was used directly in the next step. Under Ar, 250 mL DMF was added to a mixture of
the solid product, K2 C0
3
(18 g, 130 mmol), and 18-crown-6 (0.1 g). After stirring for 10
minutes at room temperature, 1-iodohexane (8 mL, 54.2 mmol) was added. The resulting
mixture was then heated to 85 TC and stirred at that temperature for 4 days. After cooling, the
mixture was poured into water and extracted by CH 2C12. The organic layer was washed by
dilute aqueous NH 4Cl solution, water and brine in sequence, and then dried over MgSO 4 and
concentrated in vacuo.
Purification by column chromatography (silica gel, neat hexane
ramping to CH 2 C12:hexane = 1:9) and recrystallization from hexane yielded colorless crystals
(4.7 g, 71%). mp 84-85 TC (hexane). 'H NMR (400 MHz, CDCl 3) 8.48 (s, 2H), 6.68 (s, 2H),
4.05 (t, J= 6.8 Hz, 4H), 1.90 (m, 4H), 1.54 (m, 4H), 1.40 (m, 8H), 0.95 (m, 6H). 13C NMR
(100 MHz, CDC13) 147.8, 127.2, 126.4, 122.2, 105.7, 68.8, 31.8, 29.4, 26.1, 22.8, 14.3.
HRMS calcd for C22H30Br 20 2 (M+) 484.0607, found 484.0613.
1,4-Bis(hexyloxy)naphthalene (8): This compound was prepared by following the procedure
as described for 3a, employing 1,4-naphthoquinone (14.8 g, 93.5 mmol), Na2S20 4 (76 g),
K2C0 3 (119 g, 862 mmol), 18-crown-6 (0.13 g), 1-iodohexane (38 ml, 260 mol), and DMF
(350 mL). The usual workup and recrystallization (from methanol) gave 21.3 g of 8 as
colorless crystals (73%). mp 52-53 TC (methanol). 'H NMR (400 MHz, CDC13) 8.26 (dd, J =
6.4, 3.2 Hz, 2H), 7.51 (dd, J= 6.4, 3.2 Hz, 2H), 6.70 (s, 2H), 4.09 (t, J=6.4 Hz, 4H), 1.92 (m,
4H), 1.58 (m, 4H), 1.41 (m, 8H), 0.96 (m, 6H).
13C
NMR (100 MHz, CDCl 3) 149.0, 126.8,
125.8, 122.1, 104.5, 68.7, 31.9, 29.6, 26.2, 22.9, 14.3. HRMS calcd for C22H320 2 (M)
328.2397, found 328.2393.
2,3-Dibromo-1,4-bis(hexyloxy)naphthalene (3b): A mixture of 8 (5.19 g, 15.8 mmol) and
NBS (6.2 g, 34.8 mmol) in 100 mL CH 2C12 was stirred at room temperature for 24 h. The
reaction mixture was washed with a dilute Na 2S20 4 aqueous solution and brine. The organic
layer was dried over Na2SO4 and concentrated in vacuo.
Purification by column
chromatography (Silica gel, hexane:CH 2Cl 2 = 4:1) afforded a colorless solid (6.65 g, 87%),
which is pure enough for next step. A small portion of the product was recrystallized to
obtain analytical pure sample for melting point measurement. mp 36-38 OC (methanol). 'H
NMR (300 MHz, CDC13) 8.08 (dd, J= 6.6, 3.3 Hz, 2H), 7.57 (dd, J=6.6, 3.3 Hz, 2H), 4.06 (t,
J = 6.6 Hz, 4H), 1.97 (m, 4H), 1.60 (m, 4H), 1.41 (m, 8H), 0.95 (m, 6H).
13
C NMR (125
MHz, CDC13) 150.4, 128.7, 127.4, 123.0, 116.6, 74.7, 31.9, 30.4, 25.9, 22.9, 14.3. HRMS
calcd for C22H30Br 20 2 (M ) 484.0607, found 484.0599.
1,4-Epoxy-1,4-dihydro-6,7-dibromo-1,4-dioctylnaphthalene (10a): To a stirred solution of
2,5-dioctylfuran (10.7 g, 36.6 mmol) and 1,2,4,5-tetrabromobenzene (9a) (9.0 g, 22.9 mmol)
in 500 mL anhydrous toluene under Ar at 0 oC was added a solution of n-butyllithium (n-BuLi)
(1.6 M in hexane, 18.6 mL, 29.8 mmol) dropwise over 6 h via a syringe pump. After the
addition of n-BuLi, the reaction mixture was stirred at 0 oC for additional 2 h, and then
allowed to warm to room temperature slowly and stirred overnight.
quenched by the slow addition of methanol (5 mL).
through a pad of celite.
The reaction was
The resulting mixture was filtered
The solvent was removed in vacuo. Purification by column
chromatography (silica gel, neat hexane then hexane:CHCl 3 = 5:1) yielded colorless oil as
clean product (7.75 g, 64%). 'H NMR (300 MHz, CDCl 3) 7.31 (s, 2H), 6.74 (s, 2H), 2.19 (m,
4H), 1.57 (m, 4H), 1.29 (m, 20H), 0.90 (m, 6H). ' 3 C NMR (100 MHz, CDCl 3) 154.2, 145.7,
124.4, 120.5, 91.9, 32.1, 30.3, 29.7, 29.5, 29.4, 24.9, 22.9, 14.4. HRMS calcd for C26H38Br 2 0
(M÷) 524.1284, found 524.1291.
1,4-Epoxy-1,4-dihydro-6,7-dibromo-5,8-dioctylnaphthalene
(10b) This compound was
prepared by following the procedure as described for 10a, employing the following materials:
1,2,4,5-tetrabromo-3,6-dioctylbenzene (9b) (6.38 g, 10.3 mmol), furan (11 mL), n-BuLi (1.6
M in hexane, 7.4 mL, 11.8 mmol), and toluene (150 mL). This reaction was carried out at -30
oC. The common workup and chromatography gave 4.2 g of 10b as a colorless solid (78%),
which is pure enough for next step. A small portion of the product was recrystallized to obtain
analytical pure sample for melting point measurement.
mp 60-61 "C (methanol). 'H NMR
(400 MHz, CDC13) 7.03 (s, 2H), 5.76 (s, 2H), 2.81 (m, 4H), 1.50 (m, 4H), 1.30 (m, 20H) 0.90
(m, 6H).
13 C
NMR (100 MHz, CDC13) 147.8, 142.9, 135.1, 124.7, 82.0, 35.0, 32.1, 30.0, 29.8,
29.6, 29.4, 22.9, 14.3. HRMS calcd for C26H38Br 2 0 (M+ ) 524.1284, found 524.1279.
2,3-Dibromo-5,8-dioctylnaphthalene (3c): Under Ar, TiCl4 (11 mL, 100mmol) was added
carefully and slowly to a suspension of zinc dust (10.2 g, 156 mmol) in anhydrous THF (250
mL) at 0 oC. The resulting mixture was heated to reflux for 10 minutes. After cooling back to
0 oC by an ice-water bath, a solution of 10a (7.75 g, 14.7 mmol) in anhydrous THF (100 mL)
was added dropwise over a course of 2 h. The reaction mixture was refluxed for 15 h. After
cooling, the reaction mixture was poured into 400 g of crushed ice and extracted by CH 2C1 2.
The organic layer was washed with water and brine, dried over MgSO 4 , and concentrated in
vacuo.
Purification by column chromatography (silica gel, hexane) yielded a white solid
(6.61g, 88%), which is pure enough for next step.
A small portion of the product was
recrystallized to obtain analytical pure sample for melting point measurement. mp 50-51 OC
(THF/methanol). 1H NMR (400 MHz) 8.33 (s, 2H), 7.28 (s, 2H), 2.97 (pt, J= 7.6 Hz, 4H),
1.72 (m, 4H), 1.42 (m, 4H) 1.33 (m, 16H), 0.91 (m, 6H). 13C NMR (100 MHz, CDC13) 136.5,
132.5, 129.6, 127.1, 121.7, 33.0, 32.1, 30.9, 29.9, 29.7, 29.5, 22.9, 14.3. HRMS calcd for
C26H38Br 2 (M÷) 508.1335, found 508.1352.
2,3-Dibromo-1,4-dioctylnaphthalene (3d): This compound was prepared by following the
procedure as described for 3c, employing the following materials: 10b (5.32 g, 10.1 mmol),
TiC14 (7.6 mL, 69.3 mmol), zinc (7.15 g, 0.11 mol), THF (200 mL). The usual workup,
chromatography and recrystallization (methanol/THF) gave 4.9 g of 3d as a white solid (95%).
mp 44-45 oC (THF/methanol).
1H
NMR (400 MHz, CDC13) 8.05 (dd, J = 6.4, 3.2 Hz, 2H),
7.55 (dd, J= 6.4, 3.2 Hz, 2H), 3.34 (pt, J= 8.0 Hz, 4H), 1.66 (m, 4H), 1.54 (m, 4H), 1.37 (m,
16H), 0.91 (m, 6H).
13
C NMR (100 MHz, CDC13) 139.2, 131.9, 126.8, 125.7, 125.5, 35.1,
32.1, 30.3, 29.6, 29.5, 22.9, 14.4. HRMS calcd for C26H38Br 2 (M+) 508.1335, found 508.1337.
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CHAPTER 3
Synthesis and Characterization of a Novel
Poly(iptycene) Ladder Polymer
Adapted from:
Chen, Z.; Amara, J. P.; Thomas, S. W.; Swager, T. M. Macromolecules2006, 39, 3202.
Introduction
The term "iptycene" was coined by Hart as the base terminology to describe a family
of molecules containing a number of arene units joined together to form the bridges of [2.2.2]
bicyclic ring systems.'
The prefixes (e.g. tri- and penti-) indicate the number of
interconnected arene planes. Since the first synthesis of triptycene (Figure 3.1) by Bartlett in
Intprnmal
r···u·
Frem
r · rr
Vn
u
Triptycene
V)
A
IAD
(C)
Figure 3.1. (A) Molecular structure of triptycene; (B) Schematic representation of internal
free volume in triptycene; (C) Space filling mode of triptycene molecule
1942, 2 a variety of iptycene molecules have been synthesized and investigated. 1' 3 6 The rigid
framework of iptycenes has been useful for the study of intramolecular charge transfer,7
atropisomerism 8 and molecular gears. 9 Recently, iptycenes and iptycene-containing polymers
were found to be able to direct molecular organizations in liquid crystal and polymer matrices,
using the minimization of the free volume between the arene rings of iptycene frameworks as
an organizational driving force. 6c,10
The insertion of fused iptycene units into a polymer
backbone was also found to improve solubility, 11' 12 enhance mechanical properties' 3 and
lower the dielectric constants 14 of the polymer.
More interestingly, the incorporation of
iptycene units into conjugated polymers improves their solid state luminescence quantum
yields, stability, and thereby their exciton transport properties. 11,15 The latter is the result of
the three dimensional shape-persistence of iptycene units, which helps to isolate the polymer
backbones from each other to avoid self-quenching often observed in condensed phases.
Although
iptycenes and iptycene-containing polymers have been extensively
investigated, the reports of polymers composed solely of iptycene scaffolds, which are
referred to as poly(iptycene)s, are rare. 11,16-18 Poly(iptycene), which contains both the rigid
iptycene unit and a double-stranded ladder polymer structure, is expected to be a two-
\'ý ýý
ý)Iý)v?ý
I
II
If
I1/
I
Ill
1
1
II
I
I(I~
/
(A)
/I1/
IJI
)
(B)
Figure 3.2. (A) Space filling model of a poly(iptycene) ladder polymer; (B) Proposed
structure of interlocked poly(iptycene) and linear polymer (adapted from ref. 18).
dimensional disc with a shape-persistent structure and high degrees of internal free
volume. 11'19 The free volume of these structures has been demonstrated to display unique
organization in stretched polymer films.' 8
It has been proposed that interweaving a
poly(iptycene) ladder polymer with a linear polymer can produce an inherently puncture and
penetration resistant material (Figure 3.2).
Iptycenes are typically prepared by Diels-Alder (D-A) reactions between anthracene or
higher acenes
(as the diene) and various dienophiles,
such as quinones,
arynes
(dehydrobenzenes) and benzofused endoxides. The quinone route generally requires multistep syntheses,2, 3a' 5c and the aryne reactions require harsh conditions and result in low
yields. 1,3a,3b
In contrast, the reactions between anthracene derivatives and benzofused
endoxides represent an efficient method for preparing triptycene precursors that can be readily
dehydrated.' Our lab has previously demonstrated that a closely related ladder polymer could
be prepared from a triptycene-containing AB-type monomer, which bears both anthracene and
endoxide groups. 18 Since AB-type monomers contain both the diene and dienophile, they
possess the perfect stoichiometry for a D-A polymerization. In the present paper, we report
the synthesis of a new AB-type monomer, 1,4-epoxy- 5,12-dihexyloxy-1,4-dihydrotetracene
(5) (Scheme 3.1), and its polymerization in a neat melt and under high pressure in solution. A
novel poly(iptycene) ladder polymer was successfully prepared from the dehydration of the
above D-A polyaddition polymer.
Results and Discussion
Monomer Synthesis
Scheme 3.1 illustrates the synthetic route to 5, an AB-type monomer containing both the
anthracene and endoxide functionality, as diene and dienophile, respectively.
The 1,4-
anthraquinone (1), which was prepared from quinizarin according to a literature procedure, 20
was converted into 1,4-dihexyloxyanthracene (3) by reduction with sodium dithionate and
subsequent alkylation. The hexyloxy side chain was introduced to improve the polymer
solubility because aromatic ladder polymers generally have poor solubility in organic
solvents. 19 The brominated intermediate (4) was prepared by stirring a mixture of 3 and Nbromosuccinimide (NBS) in N,N-dimethylformamide (DMF) at room temperature.18 In the
presence of excess furan, treatment of 4 with phenyllithium (PhLi) in anhydrous
tetrahydrofuran (THF) afforded 5 as slightly yellow needle-like crystals. The yield over five
steps is 15%.
Scheme 3.1. Synthesis of monomer a
OH
O
OH
0
a
b
\\\
c
82%
85%
4
63%
OH
0
1
2
OC6H13
UL;6 13
e
45%
76%
OC6 H13
3
N
OC6H13
OC 6H13
4
5
Reagents and conditions: (a) (1) NaBH 4 , MeOH, rt-reflux; (2) HC1, rt. (b)
Na2 S2 0 4, p-dioxane, H20, rt; (c) n-C 6H13Br, K2 C0 3, KI,18-crown-6, DMF,
85 oC; (d) NBS, DMF, rt; (e) furan, THF, PhLi, 0 oC.
a
Syntheses of Model Compounds
For spectroscopic references, we synthesized model compounds 10 and 11 (Scheme
3.2), which resemble the repeating unit of the initial D-A polyaddition polymer and the
expected poly(iptycene) after dehydration, respectively. The synthetic strategy was adapted
from a procedure previously reported by Hart.'
Compound 8 was prepared by the D-A
reaction of anthracene and an aryne generated in-situ from 7. Compound 9 was prepared from
8 by a procedure similar to that used for the synthesis of 5. The final step to model compound
10 involved a D-A reaction between anthracene and endoxide (9), which gave a single adduct
Scheme 3.2. Synthesis of model compoundsa
OH
Br
Br
Br
I
OC6H13
Br
a
88%
Br
Br
Br
Br
OH
OC6 H13
5
7
6
b
29%
/
C6 H 13
c
-Br
62%
13
Br
C6H130
B
7
8
e
d
81%
OC6H13
74%
CrH,,O
11
5+
9
-
81%
C6H1 3
C6H130
12
-
2%OC6H13
C6H130
13
a Reagents and conditions: (a) K 2C0 3, n-C 6HI 3Br, 18-crown-6, DMF, 70 oC; (b) anthracene,
toluene, n-BuLi, 0 oC; (c) furan, toluene, n-BuLi, -30 oC; (d) anthracene, xylene, 140 OC; (e)
pyridinium p-toluenesulfonate, acetic anhydride, 130 oC;(f) xylene, 140 oC; (g) pyridinium p-
toluenesulfonate, acetic anhydride, 120 oC.
10 in 81% yield. In the 'H NMR spectrum of 10 (Figure 3.4), three singlets at 2.22, 4.36 and
5.01 ppm are assigned to the three aliphatic methine protons, respectively. These features are
consistent with the NMR results of previously reported similar molecules,
4g,4h,4j
which were
proposed to form a rigid framework with dihedral angles close to 900 between adjacent sets of
nonequivalent bridgehead protons, resulting in almost no coupling.
Single crystal x-ray
structural analysis confirmed that molecules bearing such NMR spectral features are the exoadduct of the D-A reaction between anthracene and endoxide. 21 The dehydration of 10
produced model compound 11, as a white solid. Model compound 13, which resembles the
terminal tetracene group of expected poly(iptycene) chain, was prepared in two steps. A D-A
reaction between 5 and anthracene afforded the single adduct 12, which demonstrated the
similar 'H NMR spectral features as 10 and could be attributed as the exo-adduct. Finally, the
dehydration of 12 afforded model compound 13 as a yellow solid.
67 "C
/
Second heating
First Heating
_1
35 KJ/mol
11
KJ/mol
'
'I
)
-1
50
100
150
200
250
300
Temperature (OC)
Figure 3.3. DSC trace of monomer 5 showing melt endotherm and thermolysis exotherm.
Thermal Analysis by Differential Scanning Calorimetry (DSC)
DSC was used to investigate the onset and breadth of various thermally induced
transitions and reactions. Figure 3.3 displays the DSC trace of compound 5, showing the D-A
polymerization. On heating 5, a sharp endotherm peak was seen at 67 OC, which corresponds
to the melting point of 5. Further heating generated a large broad exotherm peak with onset
temperature at 162 OC and maximum at 215 oC, which could be attributed to an 111 kJ/mol
exothermicity from the D-A reaction of 5. The large breadth of the exotherm peak indicates
the slow rate of the D-A reaction. After heating to 290 oC and cooling to 30 oC, no subsequent
crystallization was observed. Furthermore, heating of the sample back to 300 oC did not
reveal any endothermic melting or exothermic transitions, indicating that the monomer was
entirely consumed in the first heating cycle. Analysis of the DSC sample by gel permeation
chromatography (GPC) confirmed that polymerization had occurred, yielding a mixture of
oligomers with the number average molecular weight (Mn) = 2,800 Da and polydispersity
index (PDI) = 2.8.
Preparation of Polymer P1
Following the DSC result, we carried out a polymerization of 5 in the neat melt by
heating the solid at 170 oC (Scheme 3.3). Unfortunately, only low molecular weight polymers
with Mn = 5,400-6,000 Da were obtained.
Since iptycene molecules generally have high
melting points, the low molecular weight can be attributed to the high melting point of the
formed oligomers. Additionally, attempts to polymerize 5 in o-dichlorobenzene at 180 OC
under atmospheric pressure only afforded a mixture of oligomers.
Scheme 3.3. Syntheses of polymers
OC 6 H13
neat, 170 oC
O
OR
or
0 |
OC6H13
THF, 129-146 Kpsi
145 OC
80-85%
R
RO
R = n-C6H13
P1
5
pyridinium
p-toluenesulfonate
acetic anhydride,
140 OC
82-88%
R = n-C6 H13
It is well known that the application of pressure accelerates some reactions whose
transition states present a net contraction in volume. 22
AVW = -RT(alnk / aP)T
The D-A reaction is strongly pressure accelerated because it has large negative activation
volumes AV+ (-25 - -45 cm3 molf).
22
d,2 2e Hence, the polymerization of 5 under high pressure
in the solution phase was investigated,' 8 and the results are summarized in Table 3.1. Four
monomer solutions of different concentrations in THF were heated at 145 oC under the
pressures of 129-146 Kpsi for 5 hours (Scheme 3.3). As expected, increased pressure yielded
polymers (P1, Scheme 3.3) with a higher degree of polymerization, and the higher monomer
concentrations led to polymers with higher molecular weight, which is typical for
condensation polymerizations. However, the molecular weight could not be further improved
by increasing the monomer concentration, because the final reaction mixture from 1.5 M
monomer solution was found to be viscous gel-like material. P1 was obtained as a white
q _-CH3r
w.C: -
HC
r
i
p/q
a-h
Im
kn
O
"j
16/h
Ha
SHb
c.f
d
H2C-CH 2
g H2 C-CH2
i
1
CH3
k
Figure 3.4. 1H NMR (Solvent: CDC13) spectra of model compound 10 (top) and polymer P1
(Mn = 16,400 Da, PDI = 3.6) (bottom).
solidand is soluble in methylene chloride, chloroform and THF. Both the 1H and 13C NMR
spectra of P1 were identical to those of the polymer obtained from the melt. Figure 3.4
displays the 1H NMR spectra of P1 and the peak assignments, together with the spectrum of
model compound 10. The spectral features of P1 are in accord with those of 10. The broad
and unstructured signals of P1 reflect the ribbon structure produced by D-A polymerization
with a complex sequence of stereoisomers. 23
Specifically, the asymmetrical broad peak
observed at 6.7--7.3 ppm suggests that the D-A polymerization proceeds with the formation of
stereoisomers. As indicated earlier, model compound 10 was obtained as single adduct and
the endoxide reacts exclusively on the exo surface.
The stereochemical disorder in the
polymer is therefore likely due to reaction at the nonequivalent faces of the anthracene in 5.
This is also consistent with the two broad peaks at 1.8-2.5 ppm, which can be assigned to the
methine protons (Hd of P1, Figure 3.4) in different isomeric configurations. Only one singlet
was observed for the corresponding methine proton of 10. A complicated
13 C
spectrum of P1,
which demonstrates more aromatic carbon atoms than those of one repeating unit, provides
further support for isomeric structures in P1. However, the stereochemical possibilities in P1
are numerous and can not be conclusively determined. Although we only observe exo D-A
3.0
2.52.01.5
1.0
P1
0.5
0.0
10
.........
_____._
•~~~~~~~
200
0
42
260
280
300
320
340
.
360
380
400
Wavelength
(nm)
Figure 3.5. UV-vis absorption spectra of model compound 10 and polymer P1 (Mn = 16,400
Da, PDI = 3.6) measured in methylene chloride.
Table 3.1. Summary of Hyperbaric Polymerization Data
[M] a
(M)
Temperature
(oC)
Time
(h)
Pressure
(psi)
P1
M b (Da)
PDI
P2
Mnb (Da)
PDI
1
0.50
145
5
128,900
6,100
2.2
n/a
n/a
2
0.88
145
5
139,600
9,400
2.7
10,900
2.4
3
1.01
145
5
145,800
11,100
3.3
12,600
2.6
4
1.50
145
Monomer concentration.
polystyrene standards.
5
145,800
16,400
3.6
16,300
2.5
Molecular weights determined by GPC in THF against
adduct in the synthesis of 10 and 12, the high pressure conditions and other macromolecular
structural constraints could lead to some endo D-A product. 23 There are further different
orientations of the dienophile (endoxide group) with respect to each face of each diene
(anthracene group) for a total of four potential stereoisomers for endo D-A linkage as well as
four possible stereoisomers for exo D-A linkage. Upon conversion to P2, both the endo and
exo linkage give the same stereoisomers and hence as shown in Figure 3.6, the NMR
spectrum of P2 is less complex. Interestingly, the UV-vis absorption spectrum of P1 is
almost identical to that of the model compound 10 (Figure 3.5).
Preparation of Polymer P2
We have found that the dehydration of a D-A polymer similar to P1 was unsuccessful
by either catalytic p-toluenesulfonic acid in toluene or direct thermolysis, and incomplete
dehydration and decomposition of the polymer were observed. With careful optimization, the
dehydration of P1 was realized by employing pyridinium p-toluenesulfonate and acetic
anhydride (Scheme 3.3). Polymer P2 was obtained as light yellow solid with no significant
change of molecular weight compared to its precursor polymer P1 (Table 3.1). It is readily
soluble in organic solvents including methylene chloride, chloroform and THF, due to both
the side chain and three-dimensional iptycene units. Thermogravimetric analysis (TGA) of P2
revealed thermal stability up to 350 oC under helium atmosphere.
Successful dehydration of P1 to P2 was confirmed by comparison of the 1H and
13
C
NMR spectra of P2 and model compound 11 (Figure 3.6 and Figure 3.7). Compared to P1,
the 1H spectrum of P2 is well resolved, partly due to the elimination of stereochemistry by
dehydration. The peak at 7.90 ppm in the 'H spectrum of P2 is attributed to the proton on
naphthalene unit, and the other two aromatic peaks at 7.36 and 6.95 ppm are assigned to the
two aromatic protons on the benzene ring. The signal of bridgehead proton is observed at
5.88 ppm, which is typical for iptycene molecules. The signal at 5.49 ppm in the spectrum of
11 does not have a counterpart in the polymer spectrum because it is from the other
bridgehead proton (Hg) of 11 (Figure 3.6). Accordingly, the signal at 54.1 ppm in the 13 C
spectrum of 11, which is attributed to the above mentioned bridgehead carbon atom (Cm), also
has no counterpart in the '3 C spectrum of P2 (Figure 3.7). Other peaks observed at 3.93, 2.02,
1.70, 1.52 and 1.06 ppm in the 1H spectrum of P2 are assigned to the protons of hexyloxy side
fg
. I.&
k/i
iii
AL
-
·
-U1-
--
I-..
i
°
•,p.,i
Figure 3.6. 'H NMR (Solvent: CDC13) spectra of model compound 11 (top) and polymer P2
(Mn = 16,300 Da, PDI = 2.5) (bottom).
I
0
/h
"i
-I
k
Figure 3.7.
13C
m,
e
0
ba
NMR (Solvent: CDCl 3) spectra of model compound 11 (top) and polymer P2
(Mn = 16,300, Da, PDI = 2.5) (bottom).
0.0
Wavelength (nm)
Figure 3.8. UV-vis absorption spectra of model compound 11 and polymer P2 (Mn=16,300
Da, PDI = 2.5) measured in methylene chloride.
0.30
0.25
OR
Rn
S0.20-
RO
R=n-CH
13
SP2
0 0.150-
400
420
440
460
480
500
5 520540
560
580
600
Wavelength (nm)
Figure 3.9. UV-vis absorption spectra of model compound 13 and low molecular weight
polymer P2 (M, = 6,000 Da, PDI = 2.3), with inset showing the proposed structure of polymer
with one end group.
chain. More informative results came from the
13C
spectrum of P2, which has an excellent
match with that of model compound 11 (Figure 3.7). The P2 spectrum shows signals of eight
aromatic carbons and seven saturated carbons, consistent with the structure of repeating unit
of the expected poly(iptycene). Based on the 13C peak assignments of model compound 11,
which are made by using the combined techniques of distortionless enhancement by
polarization transfer (DEPT) and heteronuclear multiple-bond correlation (HMBC), we can
make the complete 13C peak assignments of P2 as illustrated in Figure 3.7.
Figure 3.8 shows the UV-vis absorption spectra of model compound 11 and P2 in
methylene chloride. The spectrum of P2 shows a considerable bathochromic shift of the
absorption Xmax as compared with compound 11, which may result from the extended
hyperconjugation between the arene rings of iptycene units. 24 Interestingly, the UV-vis
absorption spectrum of a sample of the low molecular weight P2 (Mn = 6,000 Da, PDI =2.3),
shown in Figure 3.9 together with the spectrum of model compound 13, clearly demonstrates
the features of a tetracene unit, which is believed to be the end group of the polymer chain.
This feature may be useful for studying the alignment of these polymers in liquid crystals and
polymeric matrices.
Conclusion
A bifunctional AB-type monomer (5) capable of undergoing D-A polymerization was
synthesized
and its thermally induced
polymerization
was
investigated by DSC.
Polymerization of 5 in the melt phase yielded only low molecular weight D-A polymer.
However, running the polymerization under increased pressure in solution yielded polymers
(P1) with a higher degree of polymerization.
Additionally, a novel poly(iptycene) ladder
polymer (P2) was successfully produced from the dehydration of the precursor polymer (P1),
employing pyridinium p-toluenesulfonate and acetic anhydride. The structures of both P1 and
P2 were assigned by correlating their NMR spectra with model compounds.
Experimental Section
Materials. Anhydrous THF and toluene were purchased from Mallinckrodt Baker Inc. NBS
was recrystallized from water. Anthracene was recrystallized from hexane and furan was
distilled from K2C0 3 prior to use. Compound 1,4-anthraquinone (1) was prepared according
to literature procedures. 20 All other chemicals of reagent grade were obtained from Aldrich
Chemical Co. or Alfa Aesar, and used without further purification.
General methods and instrumentation. All synthetic manipulations were performed under
an argon atmosphere using standard Schlenk techniques unless otherwise noted.
High-
pressure reactions were carried out on a custom-built isostatic compaction system from
Harwood Engineering Company, Inc. (Walpole, MA). 18
Column chromatography was
performed using silica gel (40-63 ptm) from SiliCycle. NMR spectra were obtained on Varian
Mercury-300 MHz, Bruker Advance-400 MHz or Varian Inova-500 MHz instruments. The
1H and
13 C
chemical shifts are given in units of 8 (ppm) relative to tetramethylsilane (TMS)
where 8 (TMS) = 0, and referenced to residual solvent. Experiments of
13
C distortionless
enhancement by polarization transfer (DEPT) and 'H- 13 C heteronuclear multiple-bond
correlation (HMBC) on compound 11 were carried out on a Varian Inova-500 MHz
instrument.
High-resolution mass spectra (HRMS) were obtained on a Bruker Daltonics
APED II 3T FT-ICR-MS using electron impact ionization (EI). Melting points were measured
on a Mel-Temp II apparatus (Laboratory Devices INC) and were not corrected. Polymer
molecular weights were determined on a HP series 1100 GPC system in THF (1mg/mL
sample concentration) at room temperature vs polystyrene standards. Transition temperatures
were determined by DSC using a TA Instruments Q1000 DSC at scan rates of 10 oC/min.
TGA was carried out with TA Instruments Q50 under helium at a scan rate of 20
vis spectra were obtained
from Hewlett-Packard
8452A diode
oC/min.
UV-
array UV-visible
spectrophotometer.
Synthesis of monomer
1,4-Dihydroxyanthracene (2). A solution of Na 2 S2 0 4 (30 g) in p-dioxane (170 mL) and
nitrogen-bubbled water (170 mL) was added to 1,4-anthraquinone (8.6 g, 41.3 mmol) under
100
Ar. After the mixture was stirred at 25 oC for 3.5 h, an extra Na2S20 4 (16 g) was added in one
portion. The mixture was stirred for an additional 4 h, and then poured into 400 mL water.
The precipitate was collected by filtration, washed with water and dried in vacuum, leading to
a greenish yellow solid as crude product (7.4 g, 85%), which was used without further
purification. mp 170-172 OC. 'H NMR (300 MHz, DMSO-d 6) 9.57 (s, 2H), 8.66 (s, 2H), 8.08
(dd, J= 6.3, 3.3 Hz, 2H), 7.46 (dd, J= 6.6, 3.3 Hz, 2H), 6.62 (s, 2H).
13 C
NMR (75 MHz,
DMSO-d6) 145.4, 130.5, 128.5, 125.4, 125.2, 120.7, 105.6. HRMS calcd for C14H, 00 2 (M÷)
210.0675, found 210.0665.
1,4-Dihexyloxyanthracene (3). In an oven-dried 250 mL round-bottom flask equipped with a
stir bar were combined 2 (4.0 g, 19.0 mmol), 1-bromohexane (9.4 mL, 67.0 mmol), potassium
carbonate (21 g, 152.2 mmol), 18-crown-6 (0.1g) and nitrogen-bubbled DMF (150 mL). The
mixture was stirred under Ar at 60 oC for 5 d. After cooling to room temperature, the mixture
was poured into 300 mL water and the product was extracted with CH2C1 2. The organic layer
was washed by dilute NH 4 Cl aqueous solution and saturated NaCl aqueous solution, and then
dried over MgSO 4. Evaporation of the solvent yielded a brown solid which was recrystallized
from methanol to yield yellow needle-like crystals (4.5 g, 63%). mp 69-70 oC (methanol). 'H
NMR (300 MHz, CDC13) 8.80 (s, 2H), 8.06 (dd, J= 6.3, 3.3 Hz, 2H), 7.48 (dd, J= 6.3, 3.3
Hz, 2H), 6.60 (s, 2H), 4.16 (t, J= 6.5 Hz, 4H), 2.00 (m, 4H), 1.63 (m, 4H), 1.44 (m, 8H), 0.96
(t, J = 6.8 Hz, 6H).
13
C NMR (100 MHz, CDC13) 149.2, 131.8, 129.0, 126.2, 125.8, 121.2,
102.3, 68.8, 32.1, 29.8, 26.5, 23.1, 14.6. HRMS calcd for C26 H34 0 2 (M+) 378.2553, found
378.2556.
101
2,3-Dibromo-1,4-dihexyloxyanthracene (4). A mixture of 3 (4.4 g, 11.6 mmol), NBS (4.5 g,
25.3 mmol) and nitrogen-bubbled DMF (90 mL) was stirred under Ar at room temperature for
5 h. The mixture was poured into 450 mL dilute NH4Cl aqueous solution and the product was
extracted by CH 2C12. The extracts were combined and washed by 0.1 N aqueous solution of
NaOH, water and saturated NaCi aqueous solution in sequence, and then dried over MgSO 4
and concentrated in vacuo.
Purification by column chromatography on silica gel with
hexane:CH 2Cl 2 (4:1, v/v) afforded 2.8 g (45%) product as light yellow solid. A small portion
of the product was recrystallized to obtain analytical pure sample for melting point
measurement. mp 56-57 OC (hexane). 'H NMR (300 MHz, CDC13) 8.65 (s, 2H), 8.05 (dd, J=
6.3, 3.3 Hz, 2H), 7.54 (dd, J= 6.6, 3.3 Hz, 2H), 4.17 (t, J= 6.6 Hz, 4H), 2.05 (m, 4H), 1.68
(m, 4H), 1.46 (m, 8H), 0.97 (t, J= 6.8 Hz, 6H).
13
C NMR (100 MHz, CDCl 3) 150.7, 132.5,
129.0, 127.1, 126.9, 122.4, 115.3, 74.8, 32.2, 30.7, 26.2, 23.1, 14.6.
HRMS calcd for
C26H32Br 20 2 (M+) 534.0764, found 534.0785.
1,4-Epoxy-5,12-dihexyloxy-1,4-dihydrotetracene
(5). Under Ar, a stirred mixture of 4 (2.8
g, 5.2 mmol), furan (30 mL) and anhydrous THF (80 mL) was cooled by an ice-water bath. A
solution of PhLi (4.5 mL, 1.5-1.7 M) was then added dropwise over a course of 5 h. After the
addition of PhLi, the mixture was stirred at 0 oC for an additional 2 h, and then allowed to
warm to room temperature slowly and stirred overnight. Methanol (2 mL) was added slowly
to quench the reaction. The reaction mixture was poured into water and the product was
extracted by CH 2C12 . The organic layer was washed by saturated NaCl aqueous solution and
dried over MgSO 4. Solvent was removed in vacuo. Purification by column chromatography
on silica gel with CH 2Cl 2 :hexane (2:1, v/v) and recrystallization from hexane afforded the
product as slightly yellow needle-like crystals (1.76 g, 76%). mp 62-63 OC (hexane). 1H
102
NMR (300 MHz, CDC13) 8.58 (s, 2H), 8.00 (dd, J= 6.3, 3.3 Hz, 2H), 7.47 (dd, J= 6.5, 3.3
Hz, 2H), 6.95 (s, 2H), 6.13 (s, 2H), 4.22 (m, 4H), 1.96 (m, 4H), 1.62 (m, 4H), 1.44 (m, 8H),
0.96 (t, J = 6.8 Hz, 6H).
13
C NMR (75 MHz, CDC13) 143.0, 140.0, 132.0, 128.6, 127.6,
125.9, 125.7, 121.6, 80.5, 74.2, 31.9, 30.5, 26.1, 22.9, 14.3. HRMS calcd for C 30H36 0 3 (M+ )
444.2659, found 444.2658.
Syntheses of model compounds
1,2,4,5-Tetrabromo-3,6-dihexyloxybenzene
(7). In an oven-dried 500 mL round-bottom
flask equipped with a stir bar were combined 1,2,4,5-tetrabromohydroquinone (6) (10.9 g,
25.6 mmol), 1-bromohexane (15 mL, 107 mmol), potassium carbonate (38 g, 275 mmol), 18crown-6 (0.1 g) and nitrogen-bubbled DMF (290 mL). The mixture was stirred under Ar at 70
TC for 5 d. The workup procedure is similar to that of the preparation of 3. Recrystallization
from CH 2CH 2/methanol
yielded colorless
crystals
(13.3
g, 88%). mp 50-51 OC
(CH 2CH 2/methanol). 1H NMR (300 MHz, CDC13) 3.98 (t, J = 6.6 Hz, 4H), 1.88 (m, 4H),
1.51 (m, 4H), 1.38 (m, 8H), 0.93 (t, J = 6.8 Hz, 6H). ' 3 C NMR (75 MHz, CDC13) 152.1,
121.6, 73.8, 31.9, 30.1, 25.7, 22.8, 14.3. HRMS calcd for C1 8H26Br 40 2 (M÷) 589.8661, found
589.8677.
2,3-Dibromo-1,4-dihexyloxytriptycene (8). To a stirred solution of 7 (3.0 g, 5.0 mmol) and
anthracene (2.1 g, 12 mmol) in anhydrous toluene (180 mL) under Ar at 0 oC was added
dropwise a solution of n-butyllithium (n-BuLi) (3.5 mL, 1.6 M, 5.6 mmol) over a course of
3.5 h. After the addition of n-BuLi, the reaction mixture was allowed to warm to room
temperature slowly and stirred overnight. The mixture was quenched by water, and extracted
with diethyl ether. The organic layer was washed with water and saturated NaCl aqueous
103
solution, and dried over MgSO 4 .
recrystallized in acetone.
The solvent was evaporated and the residue was
The colorless crystals (unreacted anthracene) were removed by
filtration. The filtrate was concentrated and the residue was chromatographed on silica gel
with hexane, affording product as colorless crystals (0.9 g, 29%), which are pure enough for
next step. A small portion of the product was recrystallized to obtain analytical pure sample
for melting point measurement. mp 73-75 TC (methanol).
1H
NMR (500 MHz, CDC13) 7.40
(dd, J= 5.3, 3.3 Hz, 4H), 7.04 (dd, J= 5.5, 3.0 Hz, 4H), 5.75 (s, 2H), 3.94 (t, J= 6.8 Hz, 4H),
1.98 (m, 4H), 1.64 (m, 4H), 1.46 (m, 8H), 0.99 (t, J = 7.0 Hz, 6H).
13
C NMR (125 MHz,
CDC13) 149.0, 144.6, 139.8, 125.8, 124.1, 118.0, 75.3, 49.0, 32.0, 30.4, 26.1, 22.9, 14.4.
HRMS calcd for C32H36Br 20 2 (M+) 610.1077, found 610.1071.
6,11(1',2')-Benzeno-1,4-epoxy-5,12-dihexyloxy-1,4,6,11-tetrahydrotetracene
(9).
To a
stirred solution of 8 (0.69 g, 1.1 mmol) and furan (8 mL) in anhydrous toluene (80 mL) under
Ar at -30 oC was added dropwise a solution of n-BuLi (0.8 mL, 1.6M, 1.3 mmol) over a
course of 3 h. After the addition of n-BuLi, the mixture was stirred at -30 oC for an additional
2 h, and then allowed to warm to room temperature slowly and stirred overnight. The reaction
mixture was quenched by water and diluted with 150 mL diethyl ether. The product was
washed with water and saturated NaCl aqueous solution, and dried over MgSO 4 . The solvent
was evaporated, and the residue was chromatographed on silica gel with CH 2Cl 2 :hexane (1:1,
v/v, slowly up to 2:1), affording product as a colorless, highly viscous liquid (0.36 g, 62%).
1H
NMR (300 MHz, CDC13) 7.36 (dd, J= 5.1, 3.3 Hz, 4H), 6.98 (m, 6H), 5.82 (pt, 2H), 5.71
(s, 2H), 3.94 (m, 4H), 1.84 (m, 4H), 1.57 (m, 4H), 1.43 (m, 8H), 0.97 (m, 6H).
13C
NMR (100
MHz, CDC13) 146.0, 145.8, 144.6, 142.9, 137.8, 136.5, 125.2, 123.8, 81.0, 74.9, 48.4, 31.9,
30.4, 26.1, 22.9, 14.3. HRMS calcd for C36H400 3 (M+ ) 520.2972, found 520.2964.
104
exo-5,16(1 ',2'):8,13(1",2")-Dibenzeno-6,15-epoxy-7,14-dihexyloxy-5,5a,6,8,13,15,15a,16octahydrohexacene (10). A solution of 9 (0.36 g, 0.7 mmol) and anthracene (0.20 g, 1.1
mmol) in xylene (15 mL) was refluxed for 3 d. The reaction mixture was concentrated and
the residue was chromatographed on silica gel with CH 2 Cl 2 :hexane (1:1, v/v), affording the
product as a colorless solid (0.40 g, 81%), which is pure enough for next step. A small
portion of product was recrystallized to obtain analytical sample for melting point
measurement. mp 108-110 oC (methanol). 1H NMR (500 MHz, CDC13) 7.33 (dd, J= 5.2, 3.2
Hz, 2H), 7.31 (dd, J= 5.2, 3.2 Hz, 2H), 7.28 (dd, J
=
5.2, 3.2 Hz, 2H), 7.18 (dd, J = 5.2, 3.2
Hz, 2H), 7.15 (dd, J= 5.2, 3.2 Hz, 2H), 6.98 (m, 4H), 6.93 (dd, J= 5.2, 3.2 Hz, 2H), 5.68 (s,
2H), 5.01 (s, 2H), 4.36 (s, 2H), 3.95 (m, 4H), 2.22 (s, 2H), 1.87 (m, 4H), 1.60 (m, 4H), 1.46
(m, 8H), 1.02 (t, J= 7.0 Hz, 6H).
13
C NMR (100 MHz, CDC13) 145.7, 145.6, 144.2, 142.7,
141.6, 138.2, 135.9, 126.2, 125.9, 125.2, 125.17, 124.0, 123.8, 123.76, 123.68, 80.0, 74.7,
49.2, 48.4, 47.6, 31.9, 30.4, 26.1, 23.0, 14.4. HRMS calcd for C50 H50 0 3 (M+ ) 698.3754,
found 698.3740.
5,16(1 ',2'):8,13(1",2")-Dibenzeno-7,14-dihexyloxy-5,8,13,16-tetrahydrohexacene
(11). A
mixture of 10 (76 mg, 0.11 mmol), pyridinium p-toluenesulfonate (270 mg, 1.1 mmol) and
acetic anhydride (4 mL) was stirred under Ar at 120 oC for 15 h. The mixture was then
allowed to cool to room temperature.
The white precipitate was collected by filtration,
washed thoroughly with methanol and dried in vacuum (55 mg, 74%). A small portion of the
product was recrystallized to obtain analytical pure sample for melting point measurement.
mp 256-258 oC (methanol). 1H NMR (500 MHz, CDC13) 7.90 (s, 2H), 7.39 (m, 8H), 6.99 (m,
8H), 5.83 (s, 2H), 5.49 (s, 2H), 3.97 (t, J=6.8 Hz, 4H), 2.03 (m, 4H), 1.69 (m, 4H), 1.50 (m,
105
8H), 1.03 (t, J= 7.0 Hz, 6H). 13C NMR (100 MHz, CDC13) 145.8, 144.9, 144.8, 142.4, 133.3,
125.7, 125.6, 123.9, 123.8, 116.5, 75.9, 54.1, 48.2, 32.1, 30.8, 26.2, 23.0, 14.4. HRMS calcd
for C50 H480 2 (M÷) 680.3649, found 680.3667.
exo-5,16(1',2')-Benzeno-6,15-epoxy-7,14-dihexyloxy-5,5a,6,15,15a,16hexahydropentacene (12). A mixture of 5 (88 mg, 0.20 mmol) and anthracene (86 mg, 0.48
mmol) in xylene (5 mL) was refluxed at 140 oC for 3 h. The solvent was then removed in
vacuo, and the residue was chromatographed on silica gel with CH 2C12:hexane (1:2, v/v,
slowly up to 1:1), affording product as a colorless solid (100 mg, 81%), which is pure enough
for next step. A small portion of the product was recrystallized to obtain analytical pure
sample for melting point measurement. mp 77-79 OC (methanol).
1H
NMR (400 MHz,
CDC13) 8.66 (s, 2H), 8.03 (dd, J= 6.4, 3.2 Hz, 2H), 7.49 (dd, J= 6.8, 3.2 Hz, 2H), 7.42 (dd, J
= 5.2, 3.2 Hz, 2H), 7.33 (dd, J= 5.2, 3.2 Hz, 2H), 7.26 (dd, J== 5.6, 3.2 Hz, 2H), 7.09 (dd, J=
5.6, 3.2 Hz, 2H), 5.40 (s, 2H), 4.58 (s, 2H), 4.26 (m, 4H), 2.62 (s, 2H), 2.03 (m, 4H), 1.71 (m,
4H), 1.52 (m, 8H), 1.06 (t, J = 7.0 Hz, 6H).
13
C NMR (100 MHz, CDCl 3) 143.9, 141.5,
141.1, 131.5, 129.4, 128.5, 127.7, 126.3, 126.0, 125.6, 124.1, 123.8, 121.6, 80.1, 73.9, 49.6,
47.7, 31.9, 30.5, 26.1, 22.9, 14.4. HRMS calcd for C44 H46 0 3 (M+) 622.3442, found 622.3422.
5,16(1',2')-Benzeno-7,14-dihexyloxy-5,16-dihydrohexacene
(13). A mixture of 12 (0.10 g,
0.16 mmol), pyridinium p-toluenesulfonate (0.40 g, 1.6 mmol) and acetic anhydride (5 mL)
was stirred at 120 oC for 13 h. After cooling the mixture to room temperature, the yellow
precipitate was collected by filtration and washed thoroughly with methanol (30 mg). The
filtrate was concentrated and chromatographed on silica gel with CH 2 Cl 2 :hexane (1:4, v/v),
leading to the second fraction of product as a yellow solid (30 mg) (total yield = 62%). A
106
small portion of the product was recrystallized to obtain analytical pure sample for melting
point measurement. mp 150-152 OC (hexane). 1H NMR (400 MHz, CDC13) 8.82 (s, 2H),
8.15 (s, 2H), 8.00 (dd, J= 6.4, 3.2 Hz, 2H), 7.49 (dd, J= 5.2, 3.2 Hz, 4H), 7.39 (dd, J= 6.8,
3.2 Hz, 2H), 7.09 (dd, J= 5.6, 3.2 Hz, 4H), 5.57 (s, 2H), 4.19 (t, J= 6.8 Hz, 4H), 2.13 (m,
4H), 1.75 (m, 4H), 1.51 (m, 8H), 1.03 (t, J = 7.0 Hz, 6H).
13
C NMR (125 MHz, CDC13)
147.3, 144.1, 140.5, 131.1, 128.8, 126.0, 125.5, 124.9, 124.1, 123.1, 121.4, 116.0, 76.1, 53.9,
32.1, 30.9, 26.2, 23.0, 14.4. HRMS calcd for C4 4 H 4 4 0 2 (M+ ) 604.3336, found 604.3360.
Synthesis ofpolymers
P1. Procedurefor melt polymerization. Monomer (5) (160 mg, 0.36 mmol) was heated at 160
oC
under Ar for 3 d. After cooling, the mixture was dissolved in THF (-1 mL) and the
product was precipitated into methanol. The precipitate was collected by centrifuge, washed
by methanol and dried in vacuum, leading to a white solid as product (150 mg, M, = 5,400
Da, PDI = 1.9).
Generalprocedurefor solution state polymerization under high pressure. Under a nitrogen
atmosphere in a glove box, a solution of 5 (130 mg) in THF (0.33 mL, 0.88 M) was
transferred into a short Teflon® tube (inner diameter = 5 mm), which was sealed by insertion
of two glass rods at its ends. The sealed tube was placed into the hypobaric reactor, which
was equipped with a thermocouple and a pressure measuring system.
The reaction was
carried out under a pressure of 139,600 psi at 145 OC for 5 h. After cooling, the reaction
mixture was added dropwise to 10 mL methanol, leading to the precipitation of a white solid
material. The white precipitate was collected by centrifuge, washed by methanol and dried in
107
vacuum (110 mg, 85%, Mn = 9,400 Da, PDI = 2.7). 'H NMR (500 MHz, CDCl 3) 6.70-7.32
(m,br, 4H), 4.90 (br, 2H), 4.59 (br, 2H), 3.84 (br, 4H), 2.22 (br, 2H), 2.00 (br, 2H), 1.78 (br,
4H), 1.41 (br, 12H), 0.98 (br, 6H).
13
C NMR (125 MHz, CDCl 3) 144.4, 142.9, 142.2, 136.1,
133.4, 125.8, 123.6, 80.0, 74.6, 48.7, 41.3, 31.9, 30.4, 26.1, 22.9, 14.3.
P2. Generalprocedure of the dehydration of P1. A mixture of P1 (100 mg, Mn = 16,400 Da,
PDI = 3.63), pyridinium p-toluenesulfonate (580 mg) and acetic anhydride (9 mL)was stirred
under Ar at 130 oC for 23 h.
After cooling to room temperature, a light yellow solid
precipitated. The precipitate was collected by filtration, washed thoroughly by methanol and
dried in vacuum, affording a light yellow solid as crude product (84 mg, 87%). Further
purification involved the precipitation of a THF solution in methanol, and the solid was
collected by centrifuge and dried in vacuum (Mn = 16,300 Da, PDI = 2.5). 'H NMR (500
MHz, CDC13) 7.93 (br, 2H), 7.36 (br, 2H), 6.95 (br, 2H), 5.88 (br, 2H), 3.93 (br, 4H), 2.02 (br,
4H), 1.70 (br, 4H), 1.52 (br, 8H), 1.06 (br, 6H). '3 C NMR (100 MHz, CDC13) 145.8, 144.3,
141.7, 132.7, 126.0, 125.8, 123.9, 116.6, 75.9, 48.1, 32.1, 30.8, 26.3, 23.0, 14.4.
108
References and Notes
(1) Hart, H.; Bashir-Hashemi, A.; Luo, J.; Meador, M. A. Tetrahedron 1986, 42, 1641.
(2) Bartlett, P. D.; Ryan, M. J.; Cohen, S. G. J Am. Chem. Soc. 1942, 64, 2649.
(3) For reviews, see: (a) Skvarchenko, V. R.; Shalaev, V. K.; Klabunovskii, E. I. Russ. Chem.
Rev. (Engl. Transl.) 1974, 43, 951. (b) Hart, H. PureAppl. Chem. 1993, 65, 27. (c) Hopf. H.
Classics in Hydrocarbon Chemistry: Syntheses, Concepts, Perspectives; Wiley-VCH: New
York, 2000; pp 385-389.
(4) (a) Shahlai, K.; Hart, H. J. Org. Chem. 1991, 56, 6905. (b) Shahlai, K.; Hart, H.; BashirHashemi, A. J Org. Chem. 1991, 56, 6912. (c) Shahlai, K.; Hart, H. J. Am. Chem. Soc. 1990,
112, 3687. (d) Shahlai, K.; Hart, H. J. Org. Chem. 1989, 54, 2615. (e) Chen, Y.-S.; Hart, H.
J. Org. Chem. 1989, 54, 2612. (f) Shahlai, K.; Hart, H. J. Am. Chem. Soc. 1988, 110, 7136.
(g) Luo, J.; Hart, H. J. Org. Chem. 1987, 52, 3631. (h) Luo, J.; Hart, H. J. Org. Chem. 1987,
52, 4833. (i) Bashir-Hashemi. A.; Hart, H.; Ward, D. L. J. Am. Chem. Soc. 1986, 108, 6675.
(j) Hart, H.; Raju, N.; Meador, M. A.; Ward, D. L. J. Org. Chem. 1983, 48, 4357. (k) Hart,
H.; Shamouilian, S.; Takehira, Y. J. Org. Chem. 1981, 46, 4427.
(5) (a) Sugihashi, M.; Kawagita, R.; Otsubo, T.; Sakata, Y.; Misumi, S. Bull. Chem. Soc.
Jpn. 1972, 45, 2836. (b) Butler, D. N.; Gupta, I. Can. J. Chem. 1978, 56, 80. (c) Patney, H. K.
Synthesis 1991, 694. (c) Ito, S.; Murashima, T.; Ono, N. 1 Chem. Soc., Perkin Trans. 1 1997,
3161. (d) Godinez, C. E.; Zepeda, G.; Mortko,C. J.; Dang, H.; Garcia-Garibay, M. A. J. Org.
Chem. 2004, 69, 1652.
(6) (a) Kingsborough, R. P.; Swager, T. M. Angew. Chem., Int. Ed. 2000, 39, 2897. (b)
Williams, V. E.; Swager, T. M. Macromolecules 2000, 33, 4069. (c) Long, T. M.; Swager, T.
M. Adv. Mater.2001, 13, 601.
109
(7) (a) Nakazawa, T.; Murata, I. J. Am. Chem. Soc. 1977, 99, 1996.
(b) Iwamura, H.;
Makino, K. J. Chem. Soc., Chem. Commun. 1978, 720. (c) Murata, I. PureAppl. Chem. 1983,
55, 323. (d) Wasielewski, M. R.; Niemczyk, M. P.; Johnson, D. G.; Svec, W. A.; Minsek, D.
W. Tetrahedron 1989, 45, 4785. (e) Wiehe, A.; Senge, M. O.; Hurreck, H. Leibigs Ann./Recl.
1997, 1951. (f) Wiehe, A.; Senge, M. O.; Schafer, A.; Speck, M.; Tannert, S.; Kurreck, H.;
Roder, B. Tetrahedron 2001, 57, 10089. (g) Springer, J.; Kodis, G.; de la Garza, L.; Moore,
A. L.; Moore, T. A.; Gust, D. J. Phys. Chem. A 2003, 107, 3567.
(8) Oki, M. In Topics in Stereochemistry; Allinger, N. L., Eliel, E. L., Wilen, S. H., Eds;
Wiley: New York, 1983; Vol. 14, p 1.
(9) (a) Kelly, T. R. Acc. Chem. Res. 2001, 34, 514. (b) Guenzi, A.; Johnson, C. A.; Cozzi,
F.; Mislow, K. J. Am. Chem. Soc. 1983, 105, 1438. (c) Kawada, Y.; Iwamura, H. J. Am.
Chem. Soc. 1983, 105, 1449.
(d) Kawada, Y.; Iwamura, H.; Okamoto, Y.; Yuki, H.
Tetrahedron Lett. 1983, 24, 791. (e) Koga, N.; Iwamura, H. J. Am. Chem. Soc. 1985, 107,
1426.
(10) (a) Long, T. M.; Swager, T. M. J. Am. Chem. Soc. 2002, 124, 3826. (b) Zhu, Z.; Swager,
T. M. J. Am. Chem. Soc. 2002, 124, 9670. (c) Ohira, A.; Swager, T. M. Macromolecules 2007,
40, 19.
(11) Swager, T. M.; Long, T. M.; Williams, V.; Yang, J.-S. Polym. Mater. Sci. Eng. 2001, 84,
304.
(12) Zhao, D.; Swager, T. M. Org. Lett. 2005, 7, 4357.
(13) (a) Tsui, N. T.; Paraskos, A. J.; Torun, L.; Swager, T. M.; Thomas, E. L. Macromolecules
2006, 39, 3350. (b) Tsui, N. T.; Torun, L.; Pate, B. D.; Paraskos, A. J.; Swager, T. M.;
Thomas, E. L. Adv. Funct. Mater.2007, 17, 1595.
110
(14) (a) Long, T. M.; Swager, T. M. J. Am. Chem. Soc. 2003, 125, 14113. (b) Amara, J. P.;
Swager, T. M. Macromolecules 2004, 37, 3068.
(15) (a) Yang, J.-S.; Swager, T. M. J. Am. Chem. Soc. 1998, 120, 5321.
(b) Yang, J.-S.;
Swager, T. M. J. Am. Chem. Soc. 1998, 120, 11864. (c) Tovar, J. D.; Rose, A.; Swager, T. M.
J. Am. Chem. Soc. 2002, 124, 7762.
(16) Fang, T. Ph.D. Dissertation, University of California, Los Angeles, CA, 1986.
(17) Perepichak, D. F.; Bendikov, M.; Meng, H.; Wudl, F. J. Am. Chem. Soc. 2003, 125,
10190.
(18) Thomas, S. W.; Long, T. M.; Pate, B. D.; Kline, S. R.; Thomas, E. L.; Swager, T. M. J.
Am. Chem. Soc. 2005, 127, 17976.
(19) SchUilter, A. D. Adv. Mater. 1991, 6, 282.
(20) Hua, D. H.; Tamura, M.; Huang, X.; Stephany, H. A.; Helfrich, B. A.; Perchellet, E. M.;
Sperfslage, B. J.; Perchellet, J.-P.; Jiang, S.; Kyle, D. E.; Chiang, P. K. J. Org. Chem. 2002,
67, 2907.
(21) (a) Kohnke, F. H.; Slawin, A. M. Z.; Stoddart, J. F.; Williams, D. J. Angew. Chem., Int.
Ed. 1987, 26, 892. (b) Ashton, P. R.; Brown, G. R.; Isaacs, N. S.; Giuffrida, D.; Kohnke, F.
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(22) (a) MacCabe, J. R.; Eckert, C. A. Acc. Chem. Res. 1974, 7, 251. (b) Jenner, G. Angew.
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111
CHAPTER 4
Synthesis and Characterization of TriptyceneContaining Polyimides and Polyureas
112
Introduction
What is Polyimide?
The term "polyimides" refers to a class of heterochain polymers that contain imide
group (Figure 4.1) in the main chain. 1-3 According to the molecular structures, polyimides
can be categorized into two general types: linear polyimides, which are made by combining
imides into long chains (Figure 4.1 (A)) and, aromatic heterocyclic polyimides, which
incorporate benzimide in the backbone (Figure 4.1 (B)). Commercial polyimides, which
O O
Raa ýNNI Rc
Rb
(A)
(B)
(C)
(D)
Figure 4.1. (A) General structure of linear imide functional group;
aromatic imide functional group;
(B) An example of
(C) Molecular structure of polyimide for Kapton and
Vespel; (D) Molecular structure of polyimide for Meldin.
113
include Kapton, Kaptrex, Meldin, Vespel and Plavis (Figure 4.1 (C) and (D)), have been in
mass production since 1955.
Specifically, aromatic polyimides and other aromatic
heterocyclic polymers were intensively investigated during the 1960s in order to obtain hightemperature resistant materials.2
General Preparation of Polyimides
The synthesis of polyimides can be dated back to 1908, when Marson Bogert reported
the polycondensation of ethers or anhydrides of 4-aminophthalic acid.4 Since then, many
synthetic routes for preparing polyimides have been reported.2 However, the most common
way is the two-stage process of the polyacylation of dianhydrides and diamines. 2' 3 At the first
stage, a liquid phase reaction between selected dianhydrides and diamines generates a solution
of polyamic acids, the prepolymers of polyimides (Figure 4.2). The second stage of the
+
I-R
H2N-R-NH2
Polyamic acid
Figure 4.2. Schematic illustration of the formation of polyamic acid at the first stage.
Li
HI
--HN
U
IN
1
Polyimide
Polyamic acid
Figure 4.3. Schematic illustration of the formation of polyimide from polyamic acid.
114
n
process involves a chemical cyclization, which is also called imidization (Figure 4.3). This
step can be carried out in solution phase with the aid of chemical dehydrating reagents and
catalysts. Acetic anhydride/pyridine system is most widely used for chemical imidization,
because it does not degrade polyamic acid, even at high temperature, and the reaction can also
proceed at rather high rates at substantially low temperatures, beginning with room
temperature. In addition, the polyamic acid solution prepared at the first stage is often used to
cast various products, such as films, fibers, coatings, and molding powders. 3 In this case, the
second stage of the synthesis is carried out in cast products by thermal treatment of polyamic
acids, which removes solvents and induces the transformation of polyamic acid to polyimide.3
In addition to traditional wet chemical polymer manufacturing process, different gas phase
methods have also been applied. For example, an alternative polyimide deposition method
based on vapor deposition polymerization (VDP) allows formation of solventless films
through vacuum co-evaporation of a dianhydride and a diamine. 5 The resultant films are
subsequently thermally cured to form a polyimide film and the film properties can be
controlled by changing the evaporation rates. It has been reported that the dielectric constant
of this polyimide film is lower than that of the one obtained by traditional solution deposition
of polyamic acid. Interestingly, a recent report demonstrated that the VDP method could
prepare a polyimide thin film via a solution-free process as dielectric layer for organic field
effect transistors.6
Applications of Polyimides
Polyimides are one of the most important classes of polymers used in the
microelectronics and photoelectronics industries. Because of their high thermal stability,
115
chemical resistance, and good mechanical and electric properties, polyimides are often
applied in photoresists, passivation and dielectric films, soft print circuit boards, and
alignment films within displays.'
Polyimides, especially aromatic polyimides, have been well recognized as high
thermally stable engineering plastics, and have been widely used as a reliable material. 1' 7 In
the 1960s, polyimides, as well as other heterocyclic and aromatic polymers, were intensively
investigated for applications in the aerospace industry.1- 2 The aromatic and heterocyclic rings
of polyimides offered rigid structures with high glass transition (Tg) and strong linkages,
which allowed good resistance in harsh environment. Recently, the dispersion of carbon
nanotubes into polyimides at a relatively low loading level was reported to provide
environmentally durable materials with electrical conductivity and excellent thermo-optical
properties. These composite materials also exhibited excellent flexibility and stability to
harsh mechanical manipulation. 8 The high thermal stability of polyimides also extended their
applications in the automotive industry, which requires organic materials with good
thermomechanical properties up to 200-220
OC.I
116
Figure 4.4. Examples of 25-micron polyimide films as insulation materials. 9
In electrical component industries, polyimides are often used for flexible cables and as
an insulating film on magnetic wires.1-2 For example, in a laptop computer, the cable that
connects the main logic board to the display (which must flex every time the laptop is opened
or closed) is often a polyimide base with copper conductors. The semiconductor industry uses
polyimide as a high-temperature adhesive.
Currently there is an increasing demand for
insulation of wires working above 180 oC.
Polyimide is one of the three heterocyclic
polymers widely in use. 2
Polyimides were introduced for the production of electronic components mainly for
passivation at the end of the 1970s. 2 However, they are more interesting as materials for
electronic and electro-optical applications because of their excellent processability and
dielectric and thermomechanical properties.10 -11 Currently polyimides have been accepted as
dielectrics for the integrated circuits and multichip modulus fabrications, owing to their ease
of processing, high thermal stability, low stress/coefficient of thermal expansion, excellent
electrical
properties,
and
adhesion
to
117
substrates.
Kapton
pyromellitic
dianhydride/oxydianiline (PMDA/ODA), which has a dielectric constant of 3.2 when
measured at 1 MHz, is one of the most widely employed polyimides. 1-2 However, there is
current demand for dielectric materials with dielectric constants approaching 2.0 or below in
order to reduce the signal delays, drive voltages, and power consumption of today's high
powered integrated circuits.13a Among the various approaches for developing polyimides
with lower dielectric constants, the introduction of fluorinated groups into polyimide
structures has been most effective. This methodology reduces dielectric constant mainly by
reducing the electronic polarizability. 12 However, the lowest dielectric constant obtained with
fluorinated polyimide is about 2.5. Recently, a new concept has been developed with an aim
to reduce the dielectric constant below 2.13 This concept is based on the generation of
nanoscopic porosity in the polyimide films, because the dielectric constant of air is 1. A
variety of approaches have been reported for developing porous polyimides, such as the use of
foaming agents, the generation of foaming agents from partial degradation, the inclusion of
hollow microspheres, and block copolymers with "nanofoam". 12c,14 It has been proposed that
the void generation allows the formation of dielectric films with dielectric constant ranging
between 1 and 2, depending on the ratio of air to polymer.2' 13a
The conventional multilayer processing of polyimide films by dry or wet-etch often
increases the number of process steps and decreases manufacturing throughput.
Photosensitive polyimides could be used to save several steps in the process of
microelectronics, because the dielectric has to be patterned in order to obtain the bias needed
for the connection.1d,2 However, it is still a challenging problem, because the photosensitivity,
contrast, control of the thickness, and dielectric constant have yet to be definitely solved.
118
Probably the most widely known application of polyimides in the electro-optic area is
their use as alignment coating for liquid crystal displays (LCDs), because they offer excellent
thermal and mechanical properties and provide for stable LC alignment. 15 In the current LCD
industry, a polyimide alignment layer, which is unidirectionally rubbed with a cloth, is coated
on the top and bottom electrodes of a LCD cell. Rubbing process changes the topography of
the film surface and it ensures the chain orientation of the polyimide responsible for the
anisotropic orientation of LC molecules, which we have described in Chapter 1. Basically,
the LC alignment is a result of interactions between LC molecules and polyimides in the
alignment layer surface. 1 ' 16 The operation of LCDs is based on the uniform alignment of LC
molecules and its change upon electric field. Hence, the nature of LC alignment and the
electro-optical properties of LCDs are strongly dependent on the chemical and physical
features of the polyimide film.
In addition to the applications discussed above, polyimides are also interesting
materials for nonlinear optical devices, 17 electroluminescent devices, 18 and optical fiber
waveguides.
10,19
Polyurea and Polyurethane
Polyureas and polyurethanes are extremely versatile classes of polymers. 20- 21 A variety
of raw materials, synthetic methodologies, and application technologies have been developed
to prepare useful materials for many applications. Initially, their development by Otto Bayer
and his colleagues came from the impetus for competing with DuPont's polyamides and
polyesters in the 1940s. 2 1 Basically, polyurea can be prepared from a polycondensation
119
reaction between an isocyanate and an amine, and polyurethane is the product of a reaction
between an isocyanate and a polyol (Figure 4.5).
The first prepared polyureas from
hexamethylene diamine and hexamethylene diisocyanate (HDI) were not suitable for the
preparation of fibers or thermoplastics. However, the new raw materials developed by I. G.
Farben (the predecessor company of Bayer AG) during 1940s afforded various polyurea and
polyurethane products such as adhesives, rigid foams, elastomers and coatings.21
A large
number of isocyanates have been developed and are commercially available (Chart 4.1).
Furthermore, polyurea and polyurethane coatings/elastomers can also be derived from the
reaction product of an isocyanate component and a synthetic resin blend component.
The
isocyanate can be aromatic or aliphatic in nature, and it can be monomer or polymer (quasiprepolymer or prepolymer). The resin blend can be made up of amine/hydroxyl-terminated
polymer resins
0O
H2N,_./-NH2
N
*
In
H2N
•_../NH2
,
H
Urea linkage
OCN
Pnlv lran
,.NCO
D
Diisocyanate
N
Diol
Diol
I
H
n
Urethane linkage
Polyurethane
Figure 4.5. Schematic illustration of the preparation of polyureas and polyurethanes.
120
Chart 4.1. Examples of commercially important isocyanates 21
NCO
OC,2
OCN
CH2
NCO
4.4'-Diisocyanato dicyclohexylmethane
(hydrogenated MDI. H12MDI, rMDI)
4.4', 2,4' and 2.2'-Methylene diphenyl diisocyanate
(MDI)
OCN
NCO
NCO
OCN
CH -1
NCO
Hexamethylene diisocyanate (HDI)
H3C
CH2 Sn= 1-4
CH 3
OCN - CH2 3
Polymeric MDI (PMDI)
NCO
H3 C
Isophorone diisocyanate (IPDI)
CH 3
O
NCO
OCN
(CH 2)6NCO
N
OCN(CH 2) 6- N
=O
4
(CH 2 )6NCO
2.4 and 2.6-Toluene diisocyanate
(TDI)
Isocyanurate trimer of HDI
(HDI trimer)
NCO
O
H
C- N-
OCN(CH2) 6-
(CH,) 6 NCO
N
C- N - (CH 2)6NCO
NCO
o
H
Biuret of HDI
Naphthalene diisocyanate
(NDI)
or chain extenders. 20 Since the 1980s, polyurea and polyurethane have been among the most
advanced resins for many applications, especially for modem coatings. In addition, polyureas
stand out in industrial coatings for their versatility, strength, moisture insensitivity, heat and
fire resistance, and longevity. For example, they are widely used in severe environments
owing to their good chemical resistance to hydrocarbons and hydrogen sulfide gas, such as
industrial sewage and wastewater coatings for rehabilitation of concrete. 22 The short reaction
121
time and good adhesion properties of polyureas also enable them to be used as caulk and
sealant products. Polyurethanes are generally used as casting materials and foams, and their
Figure 4.6. Sprayable polyurea coating for roof protection.23
attractive features include good longevity and relatively low cost. Polyurethane foams are
often used as soft fiber stuffing that fills sofas and mattresses and hard fibers are employed to
insulate buildings and repair cracks in houses. However, it is important to point out that the
preparation of polyurethane requires the use of a catalyst to complete the reaction in a timely
manner.
122
Polyurea/Polyurethane hybrid formulations can be defined as the result of a chemical
reaction between an isocyanate and a mixture of polyol and amine reactants.
These
formulations generally provide an intermediate polyurea that displays many of the same
properties of a polyurea. However, hybrid formulations can also display some of the negative
problems associated with polyurethane chemistry. In coatings formulation, hybrids generally
contain a polyether/polyester polyol and a primary amine resulting in a chemical backbone
comprised of amine and hydroxyl functionality.
Iptycene-Containing Polymers
As we described in Chapter 3, iptycene is a class of bridged aromatic molecules with
attractive rigid frameworks, unique intramolecular cavities (internal free volume), and
exceptional thermal stability. Recently, polymers incorporated with iptycene units in their
backbones have been found to exhibit interesting properties, such as improved solubility in
common organic solvents,24 directing polymer chain organizations in liquid crystal,25
enhanced mechanical properties, 26 and low dielectric constants. 27 These structure-property
relationships have inspired us to further investigate the effect of iptycene functionalization on
the conventional polymer materials.
Aromatic polyimides have outstanding thermal resistance, good mechanical properties
and excellent dielectric properties necessary for use in microelectronics as the interlayer
dielectrics in integrated circuit fabrication. 12-14,28 However, many aromatic polyimides often
suffer from their intractability and poor solubility, leading to poor processability.19c
We
attempt to address this issue by incorporating three-dimentional bulky iptycene groups into
123
the polyimide backbone.
In addition to providing solution processable polymers, we
envisioned that the high degree of internal free volume induced by iptycene units may lower
the dielectric constant of the resulting polymer materials.
Recently, elastomeric-like polyurea and polyurethane attracted attention for their
interesting mechanical behavior, which is believed to be related to the microstructure of the
polymers. 29 Their mechanical properties at very high strain rates is particularly interesting
because it is critical to enhance the structural survivability of polyurea and polyurethane
protective coatings during high rate loading events. Previous studies by the Swager and
Thomas groups have demonstrated that the sterically interconnected polymer-chain network
created by the minimization of the internal free volume of iptycene-contaning polyesters
significantly enhanced the material's mechanical properties via a molecular threading and
molecular interlocking mechanism. 26 Therefore, it is of particular interest to synthesize
iptycene-containing polyurea and polyurethane and study the effect of iptycene corporation on
the polymer's mechanical properties.
In this chapter, we report the synthesis and characterization of two 2,6diaminotriptycenes and their application on the synthesis of novel polyimides and polyureas.
The typical characterizations of these itpycene-containing polyimides and polyureas are also
discussed.
124
Results and Discussion
Monomer Synthesis
As shown in Scheme 4.1, the synthesis of monomer 2,6-diaminotriptycene (2) was
carried out by a nitration of triptycene followed by a reduction of the nitro groups. Nitration
of triptycene with nitric acid in acetic anhydride led to a mixture of reaction products, which
were identified as 2,6-dinitrotriptycene (la) as the major production and 2,7-dinitrotriptycene
(lb) as the minor one. 30 The separation of these two products was feasible because lb has
better solubility than la in benzene and other solvents. The predominant P-nitration was
previously attributed to resonance effects, steric factors, and/or strained-ring effects and has
been extensively discussed by Klanderman and Perkins. 30 d,31
The reduction of
dinitrotriptycene to diaminotriptycene was successfully achieved by using hydrazine and
Scheme 4.1. Synthesis of Daminotriptycene 2
(CH3CO) 20
HNO3 (70%)
/
0 oC-rt
/
Ia, 39%
hydrazine
raney-Ni
THF, 60 OC
93-95%
H2 N
H2N2
NH2
2
125
NO2
0 2N
NU
lb, 10%
2
Raney nickel, and the product was obtained as a white solid. 30a,30c, 32 Scheme 4.2 shows the
synthesis of a second diaminotriptycene (6) via a Diels-Alder (D-A) reaction between
anthracene derivative and benzofused 1,4-endoxide, which has been proved to be an efficient
way for preparing various iptycene compounds. 33 Compound 2,6-diaminoanthracene was
prepared in one-step from the reduction of 2,6-diamno-anthraquinone using zinc and sodium
hydroxide, according to a previously reported route. 33 Reaction of 3 with dienophile 4 in 1,2dichlorobenzene at refluxing temperatures gave a single D-A adduct 5, which was dehydrated
in the presence of perchloric acid, leading to diaminotriptycene 6 as a white solid.34 Both the
monomers were characterized by 1H and 13C NMR spectroscopy as well as high resolution
mass spectroscopy.
Scheme 4.2. Synthesis of Diaminotriptycene 6
H2N
4
NH2
.NH2 Zn, NaOH, H20
rt-reflux
H2•
40%
H2N
o-dichlorobenzene
140 0
3
C
80%
HCI0 4 , EtOH
NH2
reflux
68%
/
NH2
2I
6
Polyimide Synthesis
The preparation of polyimides (PI-2 and PI-6) by a conventional two-step
polymerization method, which involves a ring-opening polyaddition and a subsequent ring126
closing
imidization,
is
outlined
in
Scheme
4.3.
In
this
work,
only
4,4'-
(hexafluoroisopropylidene)diphathalic anhydride (6FDA), which has been widely used for
preparing polyimides as dielectric materials, was employed as the dianhydride monomer,
because the trifluoromethyl groups have been proved to lower the dielectric constants 12 and
polyimides prepared from 6FDA have been demonstrated to show excellent thermal and
mechanical properties, solubility as well as forming colorless thin films. 2 Reaction between
Scheme 4.3. Synthesis of Polyimides
'lr
IH
0
~
H2N-
F3CCF
3
C
+
~NH
2
/
o
NMP, rt
C
acetic anhydride, pyridine, rt-1 10 OC
H(
'COOH
poly(amic acid)
PA-2 or PA-6
6:
Polyimide
PI-2: Mn = 20.6K Da, PDI = 1.8
PI-6: Mn = 28.9K Da, PDI = 1.8
127
diaminotriptycene and 6FDA in N-methyl-2-pyrrolidone (NMP) at room temperature led to a
viscous solution of polyamic acid (PA-2 and PA-6). The chemical imidization of polyamic
acids with a dehydrating reagent such as a mixture of acetic anhydride and pyridine (or
triethylamine) was very effective in obtaining the desired polyimides. 35 Both polyimides
were obtained as a white/gray solid. Polyimides often exhibit yellowish to brownish in solid
state because of the formation charge-transfer complexes between electron-donating amino
units and electron-accepting pyromellitimide or phthalimide units. 2 For PI-2 and PI-6, the
bulky three-dimensional triptycene units may significantly prevent the charge-transfer
complex formation. 24
Polyimide characterizations
The resulting polyimides PI-2 and PI-6 exhibited number-average molecular weight
(Mn) of 20.6 K Da (polymer disperse index (PDI) = 1.8) and 28.9 K Da (PDI = 1.8),
respectively, as measured by gel permeation chromatography (GPC) with respect to
polystyrene standards. The structures of the obtained polyimides were verified by Fourier
transform infrared spectroscopy (FTIR) and 'H NMR spectroscopy. As shown in Figure 4.7
and Figure 4.8, both spectra clearly show the typical IR absorption features of a polyimide.
The asymmetric and symmetric stretching modes of imide unit's carbonyl group of PI-2 are
observed at 1785 and 1726 cm -' (for PI-6: 1786 and 1727 cm'), respectively.
Other
characteristic absorption bands of polymide are observed at 1373/1371 cm-1 (stretching
vibration of imide C-N bond) and 720/722 cm "1 (bending vibration of carbonyl group on
imide).36 The band at 1479 cm'1 of both PI-2 and PI-6's spectrum can be attributed to the
128
100
90
80
70
60
50
40
4000
3500
3000
2500
2000
1500
1000
500
1500
1000
500
Wavenumber (cm1)
Figure 4.7. FTIR spectrum of polyimide PI-2.
140
120
S100
(Us
(I
r
I-
20
4000
3500
3000
2500
2000
Wavenumber (cm"')
Figure 4.8. FTIR spectrum of polimide PI-6.
129
C
b
a
I
8.5
'
I
8.0
'
I
7.5
'
I
7.0
'
I
6.5
"
I
6.0
'
ppm
Figure 4.9. 'H NMR spectrum of PI-2 in DMSO-d 6 and peak assignment.
skeletal stretching vibration mode of aromatic rings.36c,37 Multibands in the range of 11001400 cm-' can be contributed by the absorption of C-F vibrations.
No observation of
characteristic absorption bands of polyamic acid at around 3360 cm- (N-H stretching
vibration) and 1650 cm- (amide carbonyl stretching vibration) indicates that polyamic acids
are completely imidized.36
The success of imidization was also evidenced by the NMR spectroscopy studies.
Figure 4.9 and Figure 4.10 illustrate the 'H NMR spectra of PI-2 and PI-6 in dimethyl-d 6
sulfoxide (DMSO-d6 ), which clearly demonstrate all the peaks of the aromatic protons of
polymers' expected structures. No aromatic carboxylic acid proton peaks and aromatic amide
proton peaks were detected, indicating the complete conversion of polyamic acid to polyimide.
Complete peak assignments of the 'H NMR spectra, which are made by using gradient-
130
ha
I
*
8.5
c
b
I
II
I
I
I
8.0
7.5
7.0
6.5
6.0
ppm
Figure 4.10. 'H NMR spectrum of PI-6 in DMSO-d 6 and peak assignment.
-62.7
(A): PI-2
._
0
_-`-I-~I--i
-40
-20
-2
I
'lldi
".. I-N
. II
..
(B): PI-6
I
~
0
Figure 4.11.
-20
9F
-80
-60
."•i
-100
llljII
-120
[
I~ '. _
-140
-160 ppm
-62.7
-*
I
.40
~
~
i
-60
~
I I
-80
I . . I ..
I
-100 -120 -140 -160
NMR spectra of PI-2 (A) and PI-6 (B) in DMSO-d 6.
131
ppm
correlation spectroscopy (gCOSY) measurements, are shown in the insets of Figure 4.9 and
4.10. In 19F NMR measurements, a sharp single peak was observed at -63.2 ppm (Figure
4.11), providing further evidence for the successful synthesis of triptycene-containing
polyimides. 36
Thermal Imidization
A thermal induced imidization of PA-2 was also investigated by FTIR spectroscopy.
Figure 4.12 (Part A) shows the IR spectrum of PA-2, which demonstrates the characteristic
absorption bands of a polyamic acid. The broad band centered at 3400 cm-1 is attributed to
the overlap of amide N-H stretching and carboxylic acid O-H stretching vibration modes.
The absorption bands of amide C=0 stretching and carboxylic acid C-O stretching were
observed at 1654 cm- ' and 1718 cm-1,respectively. The band at 1540 cm 1 is assigned to the
absorption of amide N-H deformation mode.
12b,35e,36
The thermal induced imidization was
carried out via a stepped thermal curing of a cast film of polyamic acid on silica wafer.3 8 The
FTIR spectrum of the resulting polyimide PI-2 is shown in Figure 4.12 (part B), which clearly
demonstrated the characteristic bands of polyimide.
The small peak at 1858 cm- was
36 39
attributed to the symmetrical stretching of carbonyl group of unreacted anhydride group. a,
In order to further characterize the polymer after thermal curing, the polyimide film was
treated with tetrahydrofuran (THF) and the soluble part was purified by precipitation in
methanol.
Interestingly, this sample exhibited an IR spectrum identical to the one of
polyimide prepared from chemical imidization (Figure 4.13). GPC analysis showed that the
soluble part of thermally cured film contained polyimide with Mn = 8.2 K Da.
132
.00
o'0
U
I-
I-r
4000
3500
2500
3000
1500
2000
1000
500
Wavenumber (cm")
Figure 4.12. FTIR spectra of polyamic acid PA-2 (A) and polyimide PI-2 from thermal
imidization (B).
0
U
E
(u
r
I--
2000
1800
1600
1400
1200
Wavenumber
1000
800
600
(cm1)
Figure 4.13. Comparison of FTIR spectrum of PI-2 prepared from chemical imidization and
thermal imidization (soluble part).
133
Properties of Polyimides
The triptycene-containing polyimides exhibited good solubility in various solvents,
such as THF, NMP, dimethylacetamide, N,N-dimethylformamide (DMF), and DMSO at room
temperature, partly owing to the incorporation of bulky triptycene units into the polymer main
chain.24 Thermal stability of the polyimides were investigated by thermogravimetric analysis
(TGA), and the results are displayed in Figure 4.14. TGA tests were run from 30 to 800 oC, at
a heating rate of 10 oC/min under nitrogen atmosphere. Both PI-2 and PI-6 showed a weight
loss of 5-10% below 400 oC, which may be attributed to the removal of moisture and terminal
amine-group. 40 The thermal weight loss starting at around 500 oC corresponded to the
decomposition of the polymer main chain. This good thermal stability may be due to the
presence of rigid triptycene units in the polymer backbone.34 The amount of carbonized
residue (char yield) of both PI-2 and PI-6 was about 60% at 800 OC. The high char yields of
these polyimides can be ascribed to their high aromatic contents.
Differential scanning
calorimetry (DSC) studies on PI-2 and PI-6 did not detect any glass transition and melting
point up to 350 oC, which indicates these polyimides have excellent thermal properties
suitable for high temperature applications.
134
i
100
T
I
1
I
300
400
I
I'
I
I
I
I
!
500
600
700
800
'
90
80
.- 70
60
SPI-6
50
40
--- U-
0
100
200
I
'
900
Temperature (OC)
Figure 4.14. TGA thermograms of polyimide PI-2 and PI-6 obtained at a scan rate of 10
oC/min under nitrogen.
The optical properties of the polyimides were studied using UV-vis spectroscopy. Figure 4.15
illustrates the normalized UV-vis absorption spectra of PI-2 and PI-6 in a chloroform solution
and as a cast film on a quartz plate. Both polyimides exhibit absorption bands in the region of
200-350 nm. The films have an onset absorption wavelength at around 330 nm, which is
lower than that of Kapton (450 nm).41 This may be interpreted by the reduction of
intermolecular CTC, owing to the bulky triptycene units in the polymer backbone.
135
4)
'NW
0C
0
V
'a
0
0
0
<"o
z
10
Wavelength (nm)
Figure 4.15. UV-vis absorption spectra of PI-2 and PI-6 in a chloroform solution (A) and as
thin film (B).
Synthesis ofpolyureas
Similar to aromatic polyamides, aromatic polyureas have been recognized as highly
stable materials with high melting temperatures and poor solubility in organic solvents, owing
to their high crystallinity caused by strong hydrogen bonding between polymer chains. 2 1,42 In
this work, we chose methylene diphenyl 4,4'-diisocyanate (MDI) as the diisocyanate for the
synthesis of triptycene containing aromatic polyurea.
According to the typical synthetic
methodology, a mixture of 2,6-diaminotriptycene (2) and MDI in anhydrous DMF was stirred
at room temperature for 24 hours, leading to a highly viscous liquid (Scheme 4.4). Quenching
the reaction mixture in methanol afforded a white solid material (PU-1) that could not be
136
Scheme 4.4. Synthesis of Polyureas
OCNI
H
NCO
H
Hn
DMF, rt
OCN
H H2N
N
/
/
2
-
NH 2
NCO
DMF rt
DMFrt
2
PU-2: Mn
DMF, rt
OCN
/-
NCO
0
HN
=
10.0 K Da, PDI = 1.6
H
H
PU-3: Mn = 8.3 K Da, PDI = 1.8
easily dissolved in common organic solvents, including DMF. However, the material can be
dissolved in DMF and DMSO at elevated temperature. No precipitation or curding were
observed for the DMSO solution upon cooling and standing at room temperature for weeks.
This enables us to further characterize the resulting polyurea PU-1. Figure 4.17 illustrates the
1H NMR spectrum of PU-1, which has excellent match with the expected polymer structure.
A complete peak assignment was made with the help of gCOSY NMR measurement, as
shown in the inset of Figure 4.16. FTIR spectroscopy study clearly demonstrated the
137
e
9 h
DMF
I
'
9.0
I
'
8.5
I
'
8.0
7 I
I
7.5
7.0
"
I
I
6.5
6.0
'
I
5.5 ppm
Figure 4.16. 'H NMR spectrum of PU-1 in DMSO-d 6 and peak assignment.
100
95
90
60
4000
3500
3000
2500
2000
Wavenumber (cm
Figure 4.17. FTIR spectrum of polyurea PU-1.
138
1500
1)
1000
500
characteristic absorption bands of a polyurea (Figure 4.17). The band centered at 3393 cm-1
and the strong band at 1660 cm - can be unambiguously attributed to the N-H stretching
mode and the C=O stretching mode of urea units, respectively.
The MDI's methylene
(-CH2-) stretching absorption was observed at around 2927 cm-1 and the band at 1411 cm l
was attributed to its deformation mode. Multiple bands in the range of 1470-1600 cm -1 were
contributed by the aromatic ring absorptions and amide II absorptions.43
GPC analysis
indicated PU-1 has a high molecular weight with a large PDI (Mn = 36.3 K Da, PDI = 5.0). In
order to improve the solubility and processibilty, soft segments were induced into the main
chain by using two aliphatic diisocyanates, 4,4'-diisocyanatodicyclohexylmethane (H12MDI)
and hexamethylene diisocyanate (HDI), to synthesize triptycene containing polyureas
(Scheme 4.4). Although PU-2 and PU-3 have better solubility than PU-1, only low molecular
weight PU-2 and PU-3 were obtained using the similar reaction conditions. This could be
attributed to the lower reactivity of aliphatic diisocyanates relative to that of the aromatic
diisocyanates. Both PU-2 and PU-3 are characterized by FTIR and 1H NMR spectroscopy.
Increasing the reaction temperature to 50 oC did not improve the molecular weight. It will be
necessary to optimize the reaction conditions for polyurea synthesis from aliphatic
diisocyanates in the future.
In addition, an application of both aromatic and aliphatic
isocyanates in one polyurea system may be of particular interest.
Conclusion
Diamine monomer 2 and 6 were successfully synthesized via a direct nitration of
triptycene followed by a reduction with hydrazine and a D-A reaction between 1,4-endoxide
and diaminoanthracene followed by a strong acid catalyzed dehydration. Their applications in
139
the synthesis of novel triptycene-containing polyimides and polyureas are investigated and the
resulting polymers are characterized. In order to clearly understand the effect of triptycene
units on these polymers, it is necessary to further study their properties that are directly
related to their specific applications, such as dielectric constants and mechanical properties,
and to compare them with corresponding non-iptycene analogues.
Experimental Section
Materials. Anhydrous solvents NMP, pyridine, DMF, and o-dichlorobenzene were purchased
from Aldrich Chemical Co. Inc. Known compound 2,6-diaminoanthracene (1) was prepared
according to a literature procedure.33 All other chemicals of reagent grade were obtained
from Aldrich Chemical Co. or Alfa Aesar, and used without further purification.
General methods and instrumentation. All synthetic manipulations were performed under
an argon atmosphere using standard Schlenk techniques unless otherwise noted. Column
chromatography was performed using basic alumina (chromatographic grade, pH: 9.0-10.5,
80-325 mesh) from EMD Chemicals Inc. and silica gel (40-63 gtm) from SiliCycle. NMR
spectra were obtained on Varian Mercury-300 MHz, Bruker Advance-400 MHz or Varian
Inova-500 MHz instruments. The 1H and
13C
chemical shifts are given in units of 8 (ppm)
relative to tetramethylsilane (TMS) where 8(TMS) = 0, and referenced to residual solvent. 19F
NMR chemical shifts are reported relative to trichlorofluoromethane (CFC13) (8(CFC13) = 0)
as the reference. Gradient-correlation spectroscopy NMR spectra were obtained on Varian
Inova-500 MHz instrument.
High-resolution mass spectra (HRMS) were obtained on a
Bruker Daltonics APED II 3T FT-ICR-MS using electron impact ionization (EI) or
140
electrospray ionization (ESI). Melting points were measured on a Mel-Temp II apparatus
(Laboratory Devices Inc.) and were not corrected.
Polymer molecular weights were
determined on a HP series 1100 GPC system in DMF or THF (1mg/mL sample concentration)
vs polystyrene standards. Transition temperatures were determined by DSC using a TA
Instruments Q1000 DSC at scan rates of 10 oC/min.
TGA was carried out with TA
Instruments Q50 under nitrogen at a scan rate of 10 oC/min. UV-vis spectra were obtained
from Hewlett-Packard 8452A diode array UV-visible spectrophotometer. Fourier Transform
infrared
(FTIR) spectra were measured
on a Perkin-Elmer model 2000 FTIR
spectrophotometer using the Spectrum v. 2.00 software package.
Syntheses of monomers
2,6-Dinitrotriptycene (la). A solution of triptycene (13.9 g, 54.6 mmol) in acetic anhydride
(280 mL) was cooled by a water bath. Nitric acid (68-70%, 21 mL) was then added dropwise.
After addition, the mixture was stirred at room temperature for 2 h and then poured into water.
The resulting mixture was stirred vigorously overnight and the precipitate was collected by
filtration, washed with water, and dried in vacuum. This slightly yellow solid was added to
250 mL benzene. The solid residue was filtered off and recrystallized from ethyl acetate
twice, affording the product as a white solid (6.4 g, 39%). The benzene solution and ethyl
acetate solution were combined and concentrated on a rotary evaporator. The residue was
purified by column chromatography on basic alumina with benzene and afforded a white solid.
Further purification involved a recrystallization from ethyl acetate, leading to a clean white
material (1.8 g, 9.7%), which was found to be the minor product - 2,7-dinitrotriptycene.
Major product (la, 2, 6-dinitrotriptycene): mp > 350 OC (ethyl acetate). 1H NMR (300 MHz,
141
DMSO-d 6 ) 8.37 (d, J=2.4 Hz, 2H), 8.00 (dd, J= 8.1, 2.1 Hz, 2H), 7.75 (d, J= 8.4 Hz, 2H),
7.54 (dd, J= 5.4, 3.3 Hz, 2H), 7.10 (dd, J= 5.4, 3.3 Hz, 2H), 6.18 (s, 2H).
13C
NMR (100
MHz, DMSO-d 6) 151.6, 145.9, 145.2, 142.9, 126.1, 125,0, 124.6, 121.8, 119.1, 51.7. HRMS
calcd for C20H12N20 4 (M÷) 344.0792, found 344.0781.
dinitrotriptycene): mp 253-255 OC.
Minor product (Ib, 2,7-
H (400 MHz, DMSO-d6) 8.34 (d, J = 2.4 Hz, 2H), 7.99
(dd, J=8.0, 2.0, 2H), 7.76 (d, J= 8.0 Hz, 2H), 7.55 (m, 2H), 7.10 (m, 2H), 6.20 (s, 1H) 6.15
(s, 1H).
13C
NMR (100 MHz, DMSO-d6) 146.3, 145.2, 143.3, 142.5, 126.1, 125.1, 124.7,
124.6, 121.7, 118.9, 52.1, 51.5.
HRMS calcd for C20H12N2 0 4 (M+) 344.0792, found
344.0798.
2,6-Diaminotriptycene (2). To a solution of 2,6-dinitrotriptycene (0.22g, 0.64 mmol) in THF
(40 mL) was added hydrazine monohydrate (1 mL) and a spatula tip of Raney Ni. The
mixture was stirred at 60 OC under Ar for 14 h. After cooling to room temperature, the
mixture was filtered through a pad of celite and solvent was removed in vacuo. The residue
was treated with CH 2C12/ethyl acetate (1:1), and the resulting white solid was collected by
centrifuge and then dried in a vacuum (0.11 g). The soluble part was concentrated and then
purified by column chromatography on silica gel with CH 2C12 :ethyl acetate (2:1, up to 1:1,
v/v), affording a white solid as the second part of product (0.06 g. Totally, 0.17 g, 93%). A
small portion of the product was recrystallized to obtain analytical pure sample for melting
point measurement. mp 219-221 OC (CH 2C12). 1H NMR (400 MHz, DMSO-d6) 7.30 (dd, J=
5.2, 3.2 Hz, 2H), 6.97 (d, J= 8.0 Hz, 2H), 6.92 (dd, J= 5.6, 3.2 Hz, 2H), 6.62 (d, J= 2.0 Hz,
2H), 6.09 (dd, J= 8.4, 5.6 Hz, 2H), 5.12 (s, 2H), 4.84 (s, 4H).
142
3C
NMR (100 MHz, DMSO-
d6) 146.9, 146.5, 145.8, 133.0, 124.4, 123.7, 122.9, 110.3, 108.7, 52.2. HRMS calcd for
C20H 16N2 [M+H] + 285.1386, found 285.1392.
exo-2-Amino-5,12(1 ',2')-(4'-amino)benzeno-6,11-epoxy-5,5a,6,11,1 la,12hexylhydrotetracene (5). A solution of 2,6-diaminoanthracene (3) (0.37 g, 1.8 mmol) and
1,4-dihydro-1,4-epoxynaphthalene (4) (0.28 g, 1.8 mmol) in o-dichlorobenzene (25 mL) was
refluxed for 4 d. After cooling to room temperature, the reaction mixture was concentrated by
rotary evaporation and the residue was chromatographed on silica gel with CHCl 3 :ethyl
acetate (1:1, v/v), affording the product as a slightly yellow solid (0.50 g, 80%), which is pure
enough for next step. A small portion of the product was recrystallized to obtain analytical
pure sample for melting point measurement. mp > 200 oC (CHCl3/hexane) (decomposed). 1H
NMR (500 MHz, CDC13) 7.16 (dd, J= 5.5, 3.5 Hz, 2H), 7.06 (dd, J= 5.5, 3.0 Hz, 2H), 7.04
(d, J= 8.5 Hz, 1H) 6.98 (d, J= 7.5 Hz, 1H), 6.68 (d, J= 2.5 Hz, 1H), 6.60 (d, J= 2.5 Hz, 1H),
6.45 (dd, J= 8.0, 2.0 Hz, 1H), 6.31 (dd, J= 8.0, 2.0 Hz, 1H), 4.95 (s, 1H), 4.94 (s, 1H), 4.19
(m, 2H), 3.45 (s, 4H), 2.20 (m, 2H).
13C
NMR (125 MHz, CDCl 3) 147.2, 147.1, 146.2, 144.6,
144.4, 143.4, 134.6, 131.8, 126.3, 124.2, 124.1, 118.8, 112.1, 112.0, 111.5, 111.3, 81.4, 81.3,
49.5, 48.6, 46.9, 46.8. HRMS calcd for C24H20N2 0 [M+1] ÷ 353.1648, found 353.1657.
2-Amino-5,12(1',2')-(4'-amino)benzeno-5,12-dihydrotetracene
(6). A mixture of 5 (0.36 g,
1.0 mmol), ethanol (20 mL), and perchloric acid (70%, 4 mL) was refluxed under Ar for 7 h.
The mixture was then allowed to cool to room temperature and poured into ice-water (100
mL). The resulting mixture was neutralized with aqueous NaOH solution and extracted with
CH C12.
Organic layers were combined, washed with brine, and dried over anhydrous Na2 SO .4
2
143
Solvent was removed in vacuo and the residue was treated with cold CH 2C12, and the slight
yellow/white solid was collected by filtration and dried in vacuum (0.12 g). Filtrate was
concentrated and the residue was purified by column chromatography on silica gel with
CH 2C12:ethyl acetate (7:1, up to 6:1, v:v), affording the second portion of product (0.11g,
totally 0.23 g, 68%). A small portion of the product was recrystallized to obtain analytical
pure sample for melting point measurement. mp 233-235 oC (CH 2C12). 1H NMR (500 MHz,
CDC13) 7.71 (s, 2H), 7.69 (dd, J= 6.0, 3.5 Hz, 2H), 7.36 (dd, J= 6.0, 2.5 Hz, 2H), 7.16 (d, J=
8.0 Hz, 2H), 6.79 (d, J = 2.5 Hz, 2H), 6.30 (dd, J = 8.0, 2.5 Hz, 2H), 5.29 (s, 2H), 3.52 (s,
4H). '3 C NMR (125 MHz, CDCl 3) 146.6, 144.3, 142.9, 135.1, 131.9, 127.6, 125.6, 124.3,
121.2, 111.6, 111.3, 53.1. HRMS calcd for C24H18N 2 [M+1] + 335.1543, found 335.1547.
Polyimide synthesis
PI-2: To a stirred solution of 5 (0.28 g, 1.0 mmol) in 5 ml of NMP was added 4,4'(hexafluoroisopropylidene)diphthalic anhydride (6FDA) (0.44 g, 1.0 mmol) gradually. The
reaction mixture was stirred at the ambient temperature for 24 h to form a viscous liquid. A
mixture of pyridine (0.5 mL) and acetic anhydride (1 mL) was added to the above reaction.
The resulting mixture was stirred at room temperature for an additional 1 h and then was
heated to 110 oC for 4.5 h. After cooling to room temperature, the reaction mixture was
poured into methanol (200 mL). The precipitate was collected by filtration and washed
thoroughly with methanol. Further purification involved the precipitation of a THF solution
in methanol, Soxhlet extraction with methanol for 2 d, and a vacuum drying at 100 OC, leading
to a white/gray solid as the product (0.53 g, 78%, Mn = 20.6 K Da, PDI = 1.8). 1H NMR (400
MHz, DMSO-d 6) 8.14 (d, J= 7.2 Hz, 2H), 7.94 (s, br, 2H), 7.72 (s, 2H), 7.63 (d, J= 7.6 Hz,
144
2H), 7.55 (s, 2H), 7.53 (s, 4H), 7.11 (d, J= 7.6 Hz, 2H), 7.06 (s, 2H), 5.84 (s, 2H). ' 9F NMR
(282 MHz, DMSO-d6 ) -62.7 (s, 6F). FT-IR (KBr) v/cm-': 1785, 1726, 1623, 1478, 1436,
1372, 1298, 1257, 1210, 1142, 1105, 985, 853.
Polyimide film of PI-2 was prepared by casting a solution of polyamic acid in NMP on a
silicon wafer at room temperature and then by heating at 80 oC (under Ar), 150 TC (under Ar),
250 oC (under air), and 300 oC (under air) each for 1 h.
PI-6: PI-6 was obtained in a similar manner as that for the synthesis of PI-2, employing 6
(0.25 g, 0.75 mmol), 6FDA (0.32 g, 0.72 mmol), NMP (5 mL), pyridine (0.4 mL) , and acetic
anhydride (0.8 mL). (0.46 g, 83%, Mn = 28.9 K Da, PDI = 1.8). 'H NMR (400 MHz, DMSOd6) 8.14 (s, br, 2H), 7.97 (s, 2H), 7.94 (br, 2H), 7.80 (s, 2H) 7.72 (s, 2H), 7.69 (s, 2H), 7.62 (s,
2H), 7.43 (s, 2H), 7.16 (d, J= 7.2 Hz, 2H), 5.96 (s, 2H).
19F
NMR (282 MHz, DMSO-d 6) -
62.7. FT-IR (KBr) v/cm-': 1786, 1727, 1622, 1478, 1434, 1371, 1297, 1257, 1210, 1193,
1143, 1104, 985, 964, 881, 837, 744, 721.
Polyurea synthesis
PU-1: To a stirred solution of 2 (0.45 g, 1.6 mmol) in anhydrous DMF (4 mL), which was
cooled by an ice-water bath, was added dropwise a solution of MDI (0.40 g, 1.6 mmol) in
anhydrous DMF (6 mL) under Ar. The resulting mixture was stirred at room temperature for
24 h, and then quenched by a slow addition to methanol. The white precipitate was collected
by filtration, washed with methanol, and dried under a vacuum (0.85 g, 100%) (Mn = 36.3 K
Da, PDI = 5.0). 'H NMR (400 MHz, DMSO-d 6) 8.53 (s, 2H), 8.51 (s, 2H), 7.64 (s, 2H), 7.40
(s, 2H), 7.32 (br, 4H), 7.29 (br, 2H), 7.08 (d, J= 7.6 Hz, 4H), 6.97 (s, 2H), 6.90 (d, J= 7.6 Hz,
145
2H), 5.75 (s, 2H), 5.49 (s,2H). FT-IR (KBr) v/cm-1 : 3392, 2927, 1660, 1597, 1508, 1475,
1411, 1308, 1233, 1098, 1018, 814, 745, 662.
PU-2: This polymer was prepared by following the procedure as described for PU-1,
employing 2 (0.28 g, 0.98 mmol), H12MDI (90%, mixture of isomers, 0.29 g, 0.98 mmol), and
DMF (6 mL). A white solid was obtained as the product (0.50g, 89%). (Mn = 10.0 K Da, PDI
= 1.6). 'H NMR (400 MHz, DMSO-d 6) 8.21 (s, 1H), 8.17 (s, 1H), 7.57 (s, 1H), 7.53 (s, 1H),
7.37 (s, 2H), 7.22 (m,2H), 6.96 (s, 2H), 6.82 (m,2H), 6.19 (d, J= 5.6 Hz, 1H), 5.93 (d, J=
6.4 Hz, 1H), 5.39 (s, 2H) 1.82 (br, 2H), 1.65 (br, 2H), 1.46 (br, 6H) 1.23 (br, 2H) 1.07 (br,
6H), 0.89 (br, 2H). FT-IR (KBr) v/cm-': 3339, 2925, 2851, 1661, 1596, 1547, 1478, 1324,
1226, 1151, 1097, 746.
PU-3: This polymer was prepared by following the procedure as described for PU-1,
employing 2 (0.37 g, 1.30 mmol), HDI (0.22 g, 1.30 mmol), and DMF (9 mL).A white solid
was obtained as the product (0.50g, 85%) (Mn = 8.3 K Da, PDI = 1.4). 'H NMR (400 MHz,
DMSO-d 6) 8.27 (s, 2H), 7.57 (s, 2H), 7.37 (s, 2H), 7.21 (d, 2H), 6.95 (s, 2H), 6.83 (d, J= 7.6
Hz,2H), 6.05 (2, 2H), 5.40 (s, 2H) 3.02 (m,br, 4H), 1.38 (s, br, 4H) 1.25 (s, br, 2H). FT-IR
(KBr) v/cm': 3315, 2931, 1856, 1654, 1594, 1552, 1477, 1415, 1235, 1094, 818, 749.
146
References and notes
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152
CHAPTER 5
Synthesis and Properties of Fluorene-Iptycene
Based Polyheteroarylenes
153
Introduction
Band Gap Engineering of Conjugated Polymers
Organic n-conjugated polymers constitute a class of promising semiconducting
materials having demonstrated utility in device applications, including light-emitting diodes, 1
photovoltaic cells,2 field effect transistors, 3 and nonlinear optics.4 In addition, fluorescent
conjugated polymers have demonstrated high sensitivity in chemical and biological sensor
schemes because of the ability of conjugated polymers to create large signal amplification
relative to small molecule chemosensors. 5 This property has been proposed to result from the
delocalization and rapid diffusion of excitons through polymer chains in solution and in thin
films. 5
The advantages of conjugated polymers can be attributed to their interesting properties
that combine the physical properties of polymers with those of semiconductors to obtain
unique and novel materials. Furthermore, it is very attractive for synthetic chemists that their
electronic and/or optical properties can be extensively tuned by structure modification. 6 An
additional advantage of using polymers as sensor materials emerges from the modular nature
of polymers; i.e., the structure and sequence of the repeating units within polymers can be
widely varied and modified, which allows the polymers to be tailored for diverse targets and
potentially achieve high selectivity.
In the field of materials science, band gap engineering is very important because the
band gap is one of the most important factors controlling a material's physical properties.
Similarly, band gap engineering of organic t-conjugated polymers also plays a crucial role in
154
Sý/+n
3.00.0 -
LUMO -.
:onduction band
-3.0
Energy
[eV]
-6.0-
falence band
HOMO --
-9.0-12.0
-
n=
1
2
-
3
-I
4
00
Figure 5.1. Calculated (frontier) energy levels of oligothiophenes with n = 1-4 and of
polythiophene, where Eg = band gap. 8
optimizing the performance of optoelectronic devices.
The band gap of conventional
inorganic semiconductors is defined as the energy difference between the edges of the
conduction band and valence band. The band structure of a conjugated polymer is similar to
that of its inorganic analogues, which have their electrons organized in bands rather than in
discrete levels, and it originates from the interaction of nt-orbitals of the repeating units
throughout the polymer backbone (Figure 5.1). 6 -8
Interactions between the 7n-electrons of
neighboring polymer chains lead to a three-dimensional band structure. According to the
band theory for inorganic semiconductors, the highest occupied band and the lowest
unoccupied band of a conjugated polymer, which originate respectively from mixing of the
highest occupied molecular orbitals (HOMOs) and lowest unoccupied molecular orbitals
(LUMOs) of each repeating unit, are called the valence band and conduction band of the
155
polymer, and the energy difference between these two bands is referred to as the band gap
(Eg).
The band gap of a conjugated polymer is determined by many different parameters,
such as bond length alternation, conjugation length, aromaticity, resonance energy, and interring torsion angle. 6-7 However, subtle influence on the band gap can be achieved by several
synthetic approaches. 8-10 The introduction of side groups is the easiest way to manipulate the
band gap of a given polymer via mesomeric and induced electron effects. 9 For example,
electron-donating groups raise the valence band level and electron-withdrawing groups lower
the conduction band level. The most widely used electron withdrawing groups are cyano and
nitro groups. Flexible side groups, such as alkoxy and alkyl substituents, are also often
employed to improve the solubility of a polymer. Furthermore, the effects of these groups on
the polymer's organization in the solid phase have been demonstrated to significantly
influence its electronic and optical properties, which is exemplified by regioregular poly(3hexyl thiophene).3i
Another effective way to manipulate the band structure of a conjugated polymer is the
incorporation of donor (electron rich)-acceptor (electron poor) units into the polymer
backbone, which has been an important approach to tune conjugated polymers' electronic and
optoelectronic properties.',10- 11 Polymers with alternating donor and acceptor units are also
called push-pull polymers. The low band gaps of push-pull polymers are attributed to the
interaction of the donor and acceptor moieties, which enhances the double bond character
between repeating units and stabilizes the low band gap quinonoid forms within the polymer
156
backbones.8 Recent molecular orbital calculations have shown that the hybridization of the
energy levels of the donor and acceptor moieties result in donor-acceptor systems with
unusually low HOMO-LUMO separation. 12 If the HOMO levels of the donor and the LUMO
levels of the acceptor moiety are close in energy, the resulting band structure will show a low
energy gap as illustrated in Figure 5.2. On the other hand, it is also accepted that the band gap
of such copolymers can decrease significantly if there is a big difference between the
electronegativities of the donor and acceptor moieties.
LUMO -'-"'
.o o
S"
Energy
LUMO
-
HOMO
HOMO
D
D-A
A
Figure 5.2. Molecular orbital interaction in donor (D) and acceptor (A) moieties leading to a
donor-acceptor monomer with a reduced band gap (adapted from ref. 10b).
Therefore, designing extremely low Eg polymers requires strong donors and acceptors.
Commonly employed donors are pyrrole,l1bl
i
c
thiophene, 13 fluorene, 14 phenylenella
and
vinylene,' 5 and these electron rich units also allow numerous chemical modifications, leading
to various interesting conjugated polymers. The acceptor moieties that have been reported for
157
low band
gap
polymers
include
pyridine, 16 bithiazole, 17
naphthoselenadiazole, 18
indenofluorene, 19 quinoxaline,2 0 2,1,3-benzothiadiazole, 21 and perylene 22, with some of their
molecular structures shown in Figure 5.3. Further reduction of band gaps by enhancing the
strength of donor and acceptor moieties via strong orbital interactions has also been reported.
For example, the electron withdrawing power of
N
N
thieno[3,4-b]pyrazine
N
N
quinoxaline
NS.N
2,1,3-benzothiadiazole
Figure 5.3. Examples of acceptors for donor-acceptor type conjugated polymers.
quinoxaline or 2,1,3-benzothiadiazole could be further increased by fusion of another
pyrazine or thiadiazole ring onto the vacant sites of the phenyl ring to yield
pyrazinoquinoxaline, thiadiazoloquinoxaline and benzobis(thiadiazole). 23-24
In short, the
concept of donor-acceptor copolymerization offers several possibilities to design conjugated
polymers whose electronic properties and band gaps can be tuned by varying the electron
donor or acceptor strength of the monomers.
Iptycene-Containing Conjugated Polymers
Iptycenes, especially triptycene and pentiptycene derivatives, have been extensively
employed in our laboratory to design and synthesize novel functional materials.25 Conjugated
polymers containing these rigid, three-dimensional itpycene scaffolds have been found to
display improved photostability and quantum yields in the solid state, as well as enhanced
158
solubility in common organic solvents. These enhancements were proposed to result from
spatial isolation of the polymer backbones that significantly reduces interchain interaction in
the solid state, which is believed to play a dominant role in determining the thin film optical
properties of conjugated polymers.26 Upon introducing pentiptycene units into the conjugated
polymer, the three-dimensional bulky structures sterically isolate the polymer backbones and
thereby reduce interchain electron/orbital coupling and self-quenching, which often
accompanies these interactions. These features afforded novel, highly-fluorescent conjugated
polymers, such as poly(p-phenyleneethynylene)s (PPEs), which have been demonstrated to be
ultrasensitive sensory materials. Sensor applications have been developed to take advantage
of the ultrasensitive fluorescent quenching response of PPEs to vapors of electron-accepting
analytes, such as 2,4,6- trinitrotoluene (TNT) and 2,4-dinitrotoluene (DNT).25 -j The detection
mechanism is fluorescence quenching through nonbonding electrostatic associations between
the electron-rich polymer and the electron-poor nitroaromatic quenchers.
In addition, the
rigid elaborated scaffolds also introduce additional free volume in the solid-state material,
which facilitates rapid analyte diffusion into the polymer film to achieve faster and largeramplitude responses. This large free volume inherent to the iptycene scaffolds has also been
25
exploited to achieve enhanced chromophore and liquid crystal alignments. a-c
On the basis of the advantages of iptycene-containing PPEs, we have been interested
in extending this design concept into other conjugated systems, which have also shown
excellent properties for numerous applications. 6 Polyfluorenes (PFs) have been widely
studied for polymer light-emitting diodes (PLEDs) because of their processibility, high
quantum yield, and good charge transport properties. 14 However, the PLED application of
159
PFs often suffers from their poor electron-transporting property, which results in a large
electron-injection barrier and unbalance of charge carrier transport. Recent work found that
copolymerization of fluorene with various electron-accepting and/or electron-transporting
moieties could solve this problem. 23 Hence, fluorene-acceptor based conjugated copolymers
have attracted considerable attention.2 7 In addition to the improvement of charge injection
and transport, the light-emitting color and efficiency of PLEDs based on the fluorene-acceptor
copolymers were found to be easily tuned by the acceptors. No other polymer class offers the
full range of color with high efficiency, low operating voltage, and high lifetime when applied
in a device configuration. Interestingly, photovoltaic devices with good efficiencies and field
effect transistors with high electron mobility based on these kinds of copolymers were also
recently demonstrated. 27cd
Inspired by the above considerations, we designed and synthesized three fluoreneiptycene based conjugated polymers, and investigated their properties. In this work, we
present the synthesis of two triptycene-type quinoxaline and thienopyrazine acceptor units and
the synthesis of the corresponding iptycene-type donor-accepter copolymers using
palladium(0)-catalyzed Suzuki coupling reaction. These two copolymers were also compared
with a regular fluorene-triptycene copolymer. The three copolymers have iptycene groups of
different electronic properties installed in each repeating unit, and this unique feature
represents a key difference between them and the previously reported copolymers. This
difference is envisioned to explore both the iptycene "insulation" effect and the tuning of the
band structure of conjugated polymers.
160
Results and Discussion
Monomer Synthesis
The triptycene-type monomers 2 and 5 were prepared from the condensation of a
diketone with corresponding diamines (Scheme 5.1 and 5.2).28 The diketone compound 1 was
prepared by employing commercially available vinylene carbonate and anthracene, a route
that has been previously employed to prepare soluble acene precursors because the diketone
functionality can be easily removed by heating or irradiation. 29 The synthesis of 1 starts from
a Diels-Alder reaction of anthracene and vinylene carbonate, followed by a base assisted
hydrolysis in a methanol/tetrahydrofuran(THF)/water mixture, leading to the diol product in
high yield.29d,30 Finally, an activated Swern oxidation using trifluoroacetic anhydride and
dimethyl sulfoxide (DMSO) gave the target product 1 as a yellow solid.29a,2 9 d,30a,3 1 Monomer
2 was obtained as a white solid by condensing 1 with 3,6-dibromo-l,2-diaminobenzene,
which was prepared from the reduction of dibromobenzothiazole.2 8 a,32 A similar condensation
between diketone 1 and 3,4-diaminothiophene hydrochloride (3),33 which was easily prepared
from a nitration of 2,5-dibromothiophene followed by a reduction with tin metal under acidic
conditions, 33a afforded iptycene type thieno[3,4-b]pyrazine 4 as a white solid. Finally, a
typical bromination reaction of 4 with N-bromosuccinimide (NBS) in N,N-dimethylformide
(DMF) gave the target thienopyrazine monomer 5 as a bright yellow solid.34 All of these new
compounds are characterized by 1H and
13C
NMR spectroscopy, as well as high resolution
mass spectroscopy.
161
Scheme 5.1. Synthesis of triptycene-type quinoxaline 2
+
O
NaOH, MeOH
reflux
THF, H20, rt
92%
98%
Br
~
NH2
NH2
DMSO, (CF3CO) 20, Et3N
Br
CH2CI2, -60 OC-rt
-N
EtOH, CH3 COOH
'N
125 0 C
85%
98%98%
Br
Br
2
Scheme 5.2. Synthesis of triptycene-type thieno[3,4-b]pyrazine monomer 5
-Br
B
Br
S
HNO 3, H2SO4
0 OC-rt
56%
NO2
0 2N
Br i
S
Br
NH
3CI
Nu3CI
CIH
3i +
ClH3N
Sn, HCI
0 OC-rt
40%
S
NBS, DMF, rt
-N
83%
N
EtOH, CH3COOH,
CH 3COONa, 125 OC
86%
Br
S
Br
5
Polymer Synthesis
The synthetic strategy employed for the preparation of copolymers is based on the
Suzuki reaction, using 9,9-dioctylfluorene-2,7-bis(trimethyleneboronate) as the organoboron
reagent.3 5 The polymerization was carried out in a mixture of toluene and aqueous potassium
162
carbonate (2 M) containing a catalytic amount of tetrakis(triphenylphosphine)palladium(0)
(Pd(PPh3)4) and aliquat® 336 (methyltrioctylammonium chloride) under vigorous stirring at
105 oC for 72 h (Scheme 5.3).17,20,27a,36 For comparison, a copolymer (PFP) containing
regular triptycene was also prepared from
1,4-diiodotriptycene (6), which was prepared
according to a reported procedure. 37 All these copolymers were obtained in high yields and
are readily dissolved in common organic solvents, such as chloroform, THF, toluene, and
chlorobenzene. After standard purifications and drying, PTP, PTQ and PFTP were obtained
as white, greenish yellow and dark red solids, respectively (Figure 5.4), indicating their
different photophysical properties. Their molecular weights as determined by Gel permeation
Scheme 5.3. Synthesis of fluorene-triptycene based copolymers
-N
811
0
Br
Pd(PPh 3)4 , K2CO3,
toluene, H20,
711
aliquat 336, 105 OC
-
/
-N
94%
C8H17
C48 H17
N
Br
2
PFQ: Mn= 20.6 K Da, PDI = 2.3
\
n
C8H17 CH 17
O
-N
Br +
B
B
o
'N-
Pd(PPh 3) 4, K2 C03 ,
toluene, H20,
'\0
aliquat 336, 105 OC
84%
Br
PFTP: Mn = 11.6 K Da, PDI = 1.9
C
9
O
CAH17 C8H1 7
19
\/
Pd(PPh 3)4, K2C03 ,
toluene, H20,
aliquat 336, 105 oC
/
C8H1 7
85%
PFP: Mn= 26.9 K Da, PDI = 2.2
163
Figure 5.4. Images of the synthesized fluorene-iptycene based conjugated polymers.
i
I
8
7
6
5
4
3
2
1
ppm
Figure 5.5. 1H NMR spectra of the (A) PFTP; (B) PFQ; (C) PFP in CDC13, in which labels
of * and # correspond to CHC13 and H2 0, respectively.
164
chromatography (GPC) against polystyrene standards are within the range of 12 K-27 K Da,
corresponding to degrees of polymerization of about 17-42 units and polydispersity index
(PDI) of around 2.
The copolymers show high thermal stability, with decomposition
temperature up to 400 oC as estimated by thermogravimetric analysis (TGA) (Table 5.1). The
molecular structures of these copolymers were verified by 'H NMR spectroscopy (Figure 5.5),
and the spectroscopic data are in good agreement with their structures.
Polymer Characterization
The normalized optical absorption and fluorescence spectra of PFP, PFQ, and PFTP
in dilute chloroform solution are shown in Figure 5.6 and Figure 5.7, respectively. PFP has
an absorption maximum at 335 nm corresponding to the 7t-n'* transition of the polymer
backbone that is blue-shifted by 50 nm and 17 nm relative to the absorption of PF
homopolymer3 8 and fluorene-phenylene copolymer, 39 respectively.
The impact of steric
effects was previously invoked to explain the difference in electronic properties of conjugated
polymers that differ only in the shape of the repeating unit, because the steric repulsion
between adjacent repeating units tends to increase their torsional angle, leading to weaker
conjugation and bigger band gap. 10 ,27a,39a,39d In this case, the observed blue shift may arise
from the repulsive interaction between the bridgehead hydrogen of triptycene and the C-1 or
C-3 hydrogen of fluorene. Both PFQ and PFTP have multiple absorption bands in solution
(Figure 5.6), and the highest energy absorption bands at 328 and 345 nm, respectively, may be
assigned to the fluorene segments of both polymers. 41 Additional long wavelength bands are
observed at 395 and 530 nm for PFQ and PFTP, respectively. The substitution of phenylene
by quinoxaline or thienopyrazine units in the polymer backbone is observed to cause a
165
significant red-shift in the absorption band edge of these polymers relative to PFP. This
difference is attributed to the intramolecular charge transfer (ICT) interactions that lead to an
increase in the effective conjugation length and a reduction of band gap, because quinoxaline
and thienopyrazine are good acceptors. 10' 27 Previous studies by Kitamura and coworkers
estimated the LUMO of quinoxaline and thienopyrazine at -0.90 and -1.41 eV, respectively,
which indicates that thienopyrazine is a stronger acceptor that is expected to have stronger a
ICT interaction than quinoxaline. 11b This is consistent with our observation that PFTP has a
larger red-shift than PFQ.
However, it is also possbile that the (five-membered ring)
thiophene of thienopyrazine could result in a smaller torsional angle with fluorene than the
(six-membered ring) phenylene of quinoxaline, which enhances the efficiency of ICT
0.0
250
300
350
400
450
500
Wavelength (nm)
550
600
650
Figure 5.6. Normalized UV-vis absorption spectra of PFP, PFQ and PFTP in chloroform.
166
interaction and leads to a smaller band gap.27a,39,41 The optical band gaps of PFP, PFQ and
PFTP, determined from the onset of absorption, are 3.43, 2.84, and 2.02 eV, respectively
(Table 5.1).
These three copolymers exhibited different emission colors when in solution, as
shown in Figure 5.7. The emission bands of PFP, PFQ and PFTP have maxima at 381, 472,
and 622 nm, respectively, roughly corresponding to blue, greenish-blue and red light. Similar
to their absorption spectra, this difference in the emission properties between the three
copolymers can be understood in terms of the strength of donor-acceptor interaction and/or
the increased planarity of thiophene-containing polymers, which leads to longer conjugation
length and a smaller band gap. 27,39a The varied photophysical properties of PFP, PFQ and
PFTP also demonstrate that the photophysical properties of fluorene-based copolymers can
0.8
0.6
0.4
0.2
0.0
350
400
450
500
550
600
650
700
750
800
Wavelength (nm)
Figure 5.7. Normalized fluorescence spectra of PFP, PFQ and PFTP in chloroform.
167
be easily tuned using suitable co-monomers, indicating the flexibility of structure
manipulation of conjugated polymers. The emission bands of all polymers are relatively
narrow, without significant vibronic peaks/shoulders, which may be smeared out by large
inhomogeneous optical broadening of polymers.4 2
Additionally, the spectra demonstrate
significant red-shift of the emission with respect to absorption (Stokes shift), valued at 46, 77,
and 92 nm for PFP, PFQ and PFTP, respectively, which indicates the presence of a large
conformation difference between the ground and the excited states of the copolymers. This
may be explained by the structural twisting of polymer backbone from ground state steric
effects, while the polymer backbone at excited state has more quinonoid character, which
induces planarization."
Furthermore, the progressively larger Stokes shift from PFP, PFQ
to PFTP suggests that the Stokes shift is increased by the donor-acceptor charge transfer
interactions. Table 5.1 summarizes the quantum yields and fluorescence lifetimes of PFP,
PFQ to PFTP, together with other photophysical properties. The large Stokes shifts observed
are also consistent with the fluorescence lifetime results (Table 5.1), which indicate that the
copolymers have long lived excited states that allow for excitons to be transported to lower
energy segments in the polymer chain.44 The presence of a large Stokes shift in conjugated
polymers has been considered as an advantage for designing polymer laser materials because
the red-shifted emission will limit self-absorption. 45
168
Table 5.1. Summary of Physical Properties of Copolymers PFP, PFQ to PFTP
Chloroform Solution
m
Polymers
PFP
Thin film
d
Zmax
Zmax
*
Eg,op
(nm)
(nm)
(ns)
(eV)c
335
381
0.48; 100%
3.43
Ore
0.81a
max
430
xe
(nm)
(nm)
(ns)
337
391
0.34; 92%
0.76; 8%
PFQ
328
472
2.46; 100%
2.84
0.70a
440
395
PFTP
345
530
a Fluorescence
328
478
396
622
2.17; 78%
2.02
0 .2 5 b
4.01; 22%
405
345
523
1.70; 86%
3.04; 14%
623
1.10; 75%
3.19; 25%
quantum yield relative to quinine sulfate in 0.1 N H2S0 4 aqueous solution (Dre
= 0.54). b Fluorescence quantum yield relative to cresyl violet perchlorate in methanol (Dre =
0.54).43 c Band gap estimated from the onset of absorption spectrum in chloroform. d
Decomposition temperature estimated by TGA analysis.
The absorption and emission spectra of PFP, PFQ and PFTP in the solid state (Figure
5.8 and 5.9) are almost identical to their spectra in solution, indicating the success of polymer
chain insulation by iptycene units. 25d,25i-j,3 7 Further evidence came from the absence of any
shoulder peaks and/or tails at the longer wavelength range, which are often observed for
various conjugated polymers and are proposed to result from strong polymer inter-chain
interactions, leading to formation of aggregates and excimers. 27 Figure 5.10 shows the
fluorescent images of thin films of PFP, PFQ and PFTP irradiated by a 365 nm UV light.
The moderately strong and pure emissions indicate that these copolymers retain their pristine
single-chain photophysical properties in a thin film.
169
0.0
300
350
400
450
500
550
600
650
Wavelength (nm)
Figure 5.8. Normalized UV-vis absorption spectra of PFP, PFQ and PFTP films spin-casted
from their chloroform solution.
350
400
450
500
550
600
650
700
750
800
Wavelength (nm)
Figure 5.9. Normalized fluorescence spectra of PFP, PFQ and PFTP films spin-casted from
their chloroform solution.
170
Figure 5.10. Fluorescent images of thin films of PFTP, PFQ, and PFP under UV irradiation
(365 nm).
Conclusion
Iptycene type quinoxaline and thienopyrazine monomers (2 and 5) were successfully
synthesized via a condensation between 10-dihydro-9,10-ethanoanthracene-11,12-dione (1)
and corresponding diamines. Three fluorene-based copolymers (PFTP, PFQ, and PFP)
containing different iptycene units were prepared via Suzuki coupling reaction, and they
exhibited good solubility in appropriate organic solvents. These copolymers are fluorescent
in both solution and the solid state, emitting blue, greenish-blue, and red color, respectively.
The difference in absorption/emission spectra was attributed to the donor-acceptor charge
transfer interactions and conformation change in the polymer backbone induced by steric
effects. Moreover, we clearly demonstrated the insulation effect of iptycene units, which
prevented both the aggregation of polymer chain and formation of excimers in the solid state.
The future work includes electrochemical studies and further tuning the electronic and optical
properties of this class of copolymers.
171
Experimental Section
Materials. Vinylene carbonate, anthracene, glacial acetic acid, trifluoroacetic anhydride,
anhydrous
DMSO,
metal
tin,
NBS,
9,9-dioctylfluorene-2,7-bis(trimethyleneborate),
tetrakis(triphenylphosphine)palladium(0), potassium carbonate, and methyltrioctylammonium
chloride (aliquat" 336) were purchased from Alfa Aesar, TCI America, Strem Chemical Inc.,
or Aldrich Chemical Co., and used without further purification. Anhydrous solvents used in
the reactions were purchased from Mallinckrodt Baker Inc. Known compounds 9,10-dihydro9,10-ethanoanthracene- 11,12-dione
3,6-dibromo- 1,2-diaminobenzene, 2 8a,32
(1),29-31
3,4-
diaminothiophene dihydrochloride (3),33a and 1,4-diiodotriptycene (6) 37 were prepared
according to literature procedures.
General methods and instrumentation. All synthetic manipulations were performed under
an argon atmosphere using standard Schlenk techniques unless otherwise noted. Column
chromatography was performed using basic alumina (pH: 9.0-10.5; 80-325 mesh) from EMD
Chemicals Inc. or silica gel (40-63 ýtm) from SiliCycle. NMR spectra were obtained on
Bruker Advance-400 MHz or Varian Inova-500 MHz instruments. The 'H and
13
C chemical
shifts are given in units of 8 (ppm) relative to tetramethylsilane (TMS) where 8 (TMS) = 0
and are referenced to residual solvent. High-resolution mass spectra (HRMS) were obtained
on a Bruker Daltonics APED II 3T FT-ICR-MS using electron impact ionization (EI) or
electrospray ionization (ESI). Melting points were measured on a Mel-Temp II apparatus
(Laboratory Devices INC) and were not corrected.
Polymer molecular weights were
determined on a HP series 1100 GPC system in THF (1mg/mL sample concentration) at room
temperature vs polystyrene standards. TGA was carried out with TA Instruments Q50 under
172
nitrogen at a scan rate of 10 oC/min. UV-vis spectra were obtained from Hewlett-Packard
8452A diode array UV-visible spectrophotometer. Fluorescence spectra were measured with
a SPEX Fluorolog-r2 fluorometer (model FL112, 450W xenon lamp). Fluorescence quantum
yields in chloroform solutions were determined relative to quinine sulfate solution in 0.1 N
H2 S0
4
or cresyl violet perchlorate in methanol. Time-resolved fluorescence measurements
were acquired by exciting the samples with 170 femtosecond pulses at 330, 390, or 530 nm
from a Coherent RegA Ti:Sapphire amplifier. The resulting fluorescence was spectrally and
temporally resolved with a Hamamatsu C4770 Streak Camera system.
Synthesis of monomers
5,12-diaza-6,11-benzeno-1,4-dibromo-6,11-dihydrotetracene
(2). A mixture of 9,10-
dihydro-9,10-ethanoanthracene- 1,12-dione (1) (0.91 g, 3.9 mmol) and 2,3-diamino-1,4dibromobenzene (1.03 g, 3.9 mmol) in glacial acetic acid (30 mL) and ethanol (30 mL) was
heated for 20 h at 125 oC. Solvent was removed in vacuo, and the residue was recrystallized
from acentonitrile, affording a white/slightly yellow solid as the product (1.54 g, 85%).
Further purification involved column chromatography on silica gel with CH 2Cl 2 :hexane (2:5,
up to 1:1, v/v), affording a white solid as clean product. mp 292-294 oC (acetonitrile). 'H
NMR (400 MHz, CDC13) 7.75 (s, 2H), 7.58 (dd, J= 5.6, 3.2 Hz, 2H), 7.15 (dd, J= 5.6, 3.2 Hz,
2H), 5.85 (s, 2H).
13 C
NMR (100 MHz, CDC13) 159.0, 141.7, 138.4, 132.9, 127.1, 125.5,
123.2, 54.8. HRMS calcd for C20H12N20 4 [M+H] ÷ 462.9440, found 462.9426.
1,4-diaza-5,10-benzeno-5,10-dihydroanthra[2,3-c]thiophene
(4). A mixture of 9,10-
dihydro-9,10-ethanoanthracene- 11,12-dione (1) (0.50 g, 2.1 mmol), 3,4-diaminothiophene
173
hydrochloride (0.40 g, 2.1 mmol), and CH 3 COONa (0.35 g, 4.3 mmol) in glacial acetic acid
(20 mL) and ethanol (20 mL) was stirred at 125 oC for 18 h. Solvent was removed in vacuo,
and the residue was taken with CH 2C12 . The mixture was washed with dilute aqueous
NaHCO 3 and saturated aqueous NaC1, dried over Na2 SO 4, and concentrated on a rotary
evaporator. Purification by column chromatography on silica gel with CH 2Cl 2 :hexane (2:1,
v/v) afforded a white solid as the product (0.57 g, 86%), which is pure enough for next step. A
small portion of the product was recrystallized to obtain analytical pure sample for melting
point measurement. mp > 280 OC (acetonitrile) (decomposed). 'H NMR (400 MHz, CDCl3)
7.68 (s, 2H), 7.53 (dd, J= 5.2, 3.2 Hz, 2H), 7.17 (dd, J= 5.6, 3.2 Hz, 2H), 5.47 (s, 2H).
3C
1
NMR (100 MHz, CDCl 3) 156.2, 141.3, 140.2, 127.2, 125.2, 116.7, 55.4. HRMS calcd for
C20H12N2 S [M+1] ÷ 313.0794, found 313.0790.
1,4-diaza-5,10-benzeno-5,10-dihydroanthra[2,3-c]-(2',5'-dibromo)thiophene
(5).
A
solution of NBS (3.6 g, 20 mmol) in DMF (100 mL) was added drop wise to a solution of 4
(3.0 g, 9.6 mmol) in DMF (250 mL) that was cooled by an ice-water bath. After addition, the
ice-water bath was removed and the mixture was allowed to warm to room temperature and
stirred for 24 h. The mixture was poured into water (400 mL) and the product was extracted
with CHCl 3. Organic layers were combined, washed with saturated aqueous NaC1, dried over
anhydrous Na2 SO 4 , and concentrated by rotary evaporation. The residue was recrystallized
from CH 2C12 , leading to yellow crystals (3.08 g). Mother liquor from recrystallization was
concentrated and purified by column chromatography on silica gel with CH 2Cl 2 :hexane (1:1,
v/v), affording a second crop of product (0.65 g, totally 3.73 g, 83%). mp > 200 oC (CH 2C1 2)
(decomposed). 'H NMR (400 MHz, CDC13) 7.53 (dd, J= 5.2, 3.2 Hz, 2H), 7.19 (dd, J= 5.6,
174
3.2 Hz, 2H), 5.60 (s, 2H).
13C
NMR (125 MHz, CDCl 3) 157.6, 140.7, 138.3, 127.4, 125.3,
103.9, 54.6. HRMS calcd for C20H1 oBr 2N2S [M+1]÷ 468.9004, found 468.8994.
Polymer synthesis
PFQ: Under Ar, an aqueous solution of potassium carbonate (2 M, 7 mL) was added to a
mixture
of 9,9-dioctylfluorene-2,7-bis(trimethyleneboronate)
(190
mg, 0.34 mmol),
compound 2 (158 mg, 0.34 mmol), Pd(PPh 3)4 (7.0 mg, 0.006 mmol), Aliquat® 336 (60 mg),
and toluene (15 mL) in a 25 mL Schlenk flask, which was equipped with a condenser. The
reaction mixture was stirred for 72 h at 105 oC under Ar, and then it was cooled to room
temperature and taken with chloroform. The resulting mixture was washed with water and
saturated aqueous NaC1, dried over anhydrous Na2 SO 4, and concentrated on a rotary
evaporator. The residue was dissolved in a small amount of chloroform and the solution was
precipitated in methanol and acetone. A slightly green solid was collected by centrifugation
and dried in a vacuum. (0.23 g, 94% yield). 1H NMR (400 MHz, CDCl 3), 6(ppm): 7.99 (d, br,
2H), 7.93 (s, br, 2H), 7.83 (s, br, 2H), 7.79 (d, br, 2H), 7.56 (s, br, 4H), 7.18 (s, br, 4H), 5.65
(s, 2H), 2.18 (br, 4H), 1.00-1.50 (m, br, 24H), 0.83 (s, 6H).
PFTP: This was prepared according to the procedure described for the synthesis of PFQ
using 5 (88.5 mg, 0.19 mmol), 9,9-dioctylfluorene-2,7-bis(trimethyleneboronate) (105 mg,
0.19 mmol), Pd(PPh 3)4 (5.0 mg, 0.0043 mmol), Aliquat® 336 (40 mg), aqueous potassium
carbonate (2 M, 5 mL), and toluene (10 mL) (110 mg, 84%).
1H
NMR (400 MHz, CDCl 3), 6
(ppm): 8.34 (d, br, 2H), 8.03 (s, br, 2H), 7.90 (d, br, 2H), 7.60 (m, br, 4H), 7.22 (s, br, 4H),
5.58 (s, 2H), 2.17 (br, 4H), 1.00-1.30 (s, br, 20H), 0.94 (s, br, 4H), 0.78 (m, br, 6H).
175
PFP: This was prepared according to the procedure described for the synthesis of PFQ using
6 (46.4 mg, 0.092 mmol), 9,9-dioctylfluorene-2,7-bis(trimethyleneboronate) (51.0 mg, 0.092
mmol), Pd(PPh 3)4 (3.0 mg, 0.0026 mmol), Aliquat® 336 (20 mg), aqueous potassium
carbonate (2 M, 2 mL), and toluene (4 mL) (50 mg, 85%). 'H NMR (400 MHz, CDC13), 6
(ppm): 8.06 (d, br, 2H), 7.59 (m, br, 4H), 7.39 (s, br, 4H), 7.27 (overlap with CHC13 peak, 2H),
7.10 (s, br, 4H), 5.90 (s, 2H), 2.29 (s, br, 4H), 1.10-1.40 (m, br, 20H), 1.00 (S, br, 4H), 0.80 (t,
6H).
176
References and notes
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183
Curriculum Vitae
ZHIHUA CHEN
EDUCATION
2002-2007 Massachusetts Institute of Technology, Cambridge, MA
Candidate for Ph.D., Organic Chemistry
Advisor: Professor Timothy M. Swager
Thesis Title: "Organic Materials with Acenoid and Iptycene Structures"
1999-2002 National University of Singapore, Singapore
M.S., Physical Chemistry
1994-1998 Tsinghua University, Beijing, China
B.S., Chemistry
RESEARCH EXPERIENCE
2002-2007 Massachusetts Institute of Technology, Cambridge, MA
Graduate Research Assistant
* Studied fluorene-iptycene based donor-acceptor type conjugated polymers.
* Synthesized and characterized iptycene-containing polyimides and polyureas.
* Examined the effect of perfluoroalkyl sidechains on the photophysical properties of conjugated
polymers.
* Synthesized acenequinones as fluorescent dyes for guest-host liquid crystal displays.
* Designed and synthesized a novel poly(iptycene) ladder polymer with rigid and shape-persist
structure.
* Developed a family of solution processable, n-stacking fluorinated tetracene derivatives for organic
field effect transistors.
1999-2002 National University of Singapore, Singapore
Graduate Research Assistant
* Conducted surface analysis of small molecules on metal and metal oxide single crystal surfaces.
1997-1999 Tsinghua University Beijing, China
Undergraduate Research Assistant
• Developed and evaluated a series of La-Ce-Cu three-way-catalysts for controlling emission of
exhaust gases from automobiles.
PUBLICATIONS
* "Synthesis and characterization of fluorescent acenequinones as dyes for guest-host liquid crystal
displays" Chen, Z.; Swager, T. M. Org.Lett. 2007, 9, 997-1000.
* "Synthesis of a novel poly(iptycene) ladder polymer" Chen, Z.; Arama, J. P.; Thomas, S. W.;
Swager, T. M. Macromolecules 2006, 39, 3202-3209.
* "Syntheses of soluble, n-stacking tetracene derivatives" Chen, Z.; Miuller, P.; Swager, T. M. Org.
Lett. 2006, 8, 273-276.
* "Efficient growth of ordered thin oxide films on Ni(1 11) by NO2 oxidation" Yeo, B.-S.; Chen, Z.;
Sim, W.-S. Surf Sci. 2004, 557, 201-207.
* "Surface functionalization of Ni(l11) with acrylate monolayers" Yeo, B.-S.; Chen, Z.; Sim, W.-S.
Langmuir 2003, 19, 2787-2794.
184
Acknowledgements
I owe my most sincere gratitude to my research advisor Professor Timothy M. Swager, who
has undoubtedly become one of the most influential persons in my life. I have been highly impressed
with his enthusiasm for chemistry, dedication to his career, and his inexhaustible ideas. Under his
guidance, the past few years have not only been the most rewarding but an enjoyable period of time.
Tim's influences upon me had a tremendous impact on my philosophy of scientific research, and will
continue to play an important role throughout my future career.
I would also like to thank my other committee members, Professor Rick L. Danheiser and
Professor Sarah E. O'Connor, for their valuable suggestions and encouragement during my studies.
Without a doubt, the Swager Group has been my unending resource of great chemistry,
interesting conversation, and fun times. I want to take this opportunity to thank Brad and Jocelyn for
all kinds of help they provided during my first year in Swager group, especially the laboratory
management skills. Many thanks must go to Changsik, who spent more time with me than with his
wife in the past five years, I guess. I especially appreciate him helping me on organizing and
managing the "isolated" lab in Bldg. 13 and basement of Bldg. 18. Jean, who could provide any help I
needed, must be highly acknowledged. His willingness to assist me in daily endeavors never dimmed,
no matter how demanding the task at hand. I wish he will enjoy the time in Japan and marry a
Japanese girl! I would also like to thank Dahui and Anne for their helps on experiments and paper
writings. Johan and Paul are appreciated for teaching me so much on liquid crystals. I am grateful to
Trisha for various helps, especially on photophysics and instrumental measurements. I am also
thankful to my master, Karen, for her encouragement and assistance, while I was not a good apprentice.
Kathy is high acknowledged for much assistance, especially proofreading my thesis. I must "Gan
Xie" Gu Hongwei, Zhang Wei, Yang Yong, and Wang Fei for many helps and advices. I will
remember the fun time we had at Weihua Nightclub.
I wish to express my appreciation to all the former and present Swager group members for
their help and companionship over the years: Shiwei, Michael, Bruce, Nate, Craig, Sandra, Elena,
Kazu (2), Akihiro, Tekashi, Koji (2), Paul (B), Koushik, Julian, Jeewoo, John, Sam, Gigi, Hyuna,
Jessica, Youngmi, Evgueni, Eric, Brett, Scott, Andrew, and Becky. There are many more people who
have helped me in one way or another in the past years, and I apologize for not being able to mention
all of them.
Additionally, I am immensely grateful to the DCIF staff: Li Li, Dave, Mark, Hongjun, Bob,
and Jeff, without whom I would not have completed the work reported here and elsewhere. Great
thanks must go to Dr. Peter Muiller and Dr. Steven Kooi for their assistance on single crystal X-ray
studies and fluorescence lifetime studies. Prof. Jean-Luc Bredas, Prof. Theodore Goodson, and Prof.
Brian J. Bacskai are acknowledged for their efforts on collaborative projects. Dr. Sen Liu at Bell Labs
is especially appreciated for his help on device studies. I am thankful to Huang Hao, Song Xiaogen,
and Wu Qin for their help on many aspects of my researches.
I'd like to take this opportunity to thank my parents, brother, sister-in-law, parents-in-law, and
Bingyan for their love, support, and encouragement. Finally, I owe the most to my wife, Bingling, for
this degree. She is the one who for the past five years helped me through every obstacle and shared
every bit of my sadness and joy, frustration and triumph.
185
Appendix 1
NMR Spectra for Chapter 1
186
1H
NMR Spectrum of 2
1
8
7
)
ppm
13
C NMR Spectrum of 2
L·Y·- m
JE; S?_•'; •.2;~ •-..\2 _Lu w0I
9 iI mew
m.Y~~l*IYY··
u · minmruu* d"YY m ,- ·· ·- m-ruLu~rC*r*-~
· --- --··
.........
200
180
160
140
120
100
ppm
187
I80
80
I
60'
60
40
' 20I
20
0
0
'H NMR Spectrum of 3
1!
5
5
6
6
7
7
8
8
i
I:
Lu U ~
4
4
I
I
3
I
2
1
'
I
0
ppm
13
C NMR Spectrum of 3
Y-YLLI YL-· ~ LIII_·II~·I-~L.~·I-I
I1I*·UYJIIL-YILC~~II1_1
I~-LY ·illr ilil·i*i~YY*Y-I~I·d
·
·
·
·
200
180
160
140
120
100
ppm
188
80
"U-
60
40
20
""FIRM
0
1H
NMR Spectrum of 4 (exo)
I
10
·
,
I
9
Ii
I
8
.
·
7
6
·
5
i;
rll If.
)I
·
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4
3
80
60I
~
2
1I
40
20
00I
ppm
13C
NMR Spectrum of 4 (exo)
· · · · · · _______~~~
200
180
160
140
120
100
ppm
189
60
'4I
I
I
0
H NMR Spectrum of 4 (endo)
I;
- -.1
8
7
P
6
5
J~I
i LII
-j,~
~
4
I ~~
I 11
__J
3
2
1
0
ppm
13C NMR
Spectrum of 4 (endo)
r-Ir*·l·~L~-~ur~uYuu~L'·*rYY~·-~u_·.1~C(~
r0
[-NI..
,$w..
..... I.
...
.
a*
No
1
200
180
160
140
120
100
ppm
190
-,..~rl..,...· Y
Y
80
*
1
60
*
1
40
*
i 0 -Iw
*
1
20
,.*L[..
I
0
IH NMR Spectrum of 5
1
·
10
·
9
JII I
ij
8
7
6
5
ii
I
I
4
3
2
60
40
'
I
ppm
13
C NMR Spectrum of 5
2(
ppm
191
ii W
IVL
'
\
I
1
I
'
0
]H NMR Spectrum of 8 (exo)
cUU~ '"
.I
U
10
9
8
7
6
I
ii
L-
4
5
3
2
1
0
ppm
13
C NMR Spectrum of 8 (exo)
1
· YYII
200
Ij
--1-··-1--·-·1_1-·
·1·11-1
·l·-I·-LL-··I··-I·
-UIY·IY·*·- -Y1···l_· · IIL-LWY·LIYY·
180
160
140
120
100
ppm
192
80
_ L ,,,L ..........
al.....
......
J. I•L.
-... ...
I
60
40
20
0
'H NMR Spectrum of 8 (endo)
10
13C NMR
9
________
,
L
_
7
8
5
6
I~tI
A J
ppm
4
3
2
- j
1
0
Spectrum of 8 (endo)
a"MMO
Y
I
200
,
.
l
180
'
I
160
"
I
IrMlop-POIN
I Y
0M
140
'
I
120
'
u u 14'YWJl
won~CkYI~rm
kir
w
lrll
---I
100
ppm
193
80
60
40
20
0
'HNMR Spectrum of 10
Ii~
UJL
10
Lil
9
8
7
I1I,
.
6
1.1-I-I-
5
ppm
4
L.
L~UJ
3
2
•II
k
U
L
1
J--
I
I
0
'3C NMR Spectrum of 10
~y~y·u·*.lr
..uru,~·r
.yu·r I·r.-.
Uln.·u~l-l~urrl ...I-....·*·.~.l~u.L~ry
··--.L·r-···
lly.·YI mI gI a"
meamemmo··~-·1·
I
200
180
160
140
120
100
ppm
194
8O0
60
40
20
I
0
1H
NMR Spectrum of 11
.. .
·
. . .
·
.-
w
·
_
· I
·
_r
· ·
·_
·
__
·
I
SL
1
I
4
·
hAl
*
I1,
~
'.%
'-V
I
I
I
3
2
1
.~~
ppm
'9F NMR Spectrum of 11
•L, Ilhti , h· I11 i1l1lkl]·
Ijiu1flJ
",1
IL
"1"
" P
I
-20
I
-40
~I'l•l
~
I
-,IlIII.,I
..
.
!JYL2Ai
. A" I&AL
L"III MLIl"IL
I...w
J
I
-60
-
I
-80
, ... . . ....
· II1
'
"I I"" ' Im
rmw"Inlrqrnlll•1•I~l•'P
I rvgmanm
I
"VIi'IrlrlWP/"
li" l;rIt'll'
l"lll
I
-100
ppm
195
-120
-140
-160
-180
1H
NMR Spectrum of 12
I
15
'
I
14
ii
I
'
13
I
12
,
'
11
I
'
10
1\
'
I
9
8
I
7
'
6
;
5
I
'
4'
3
4
3
-Ll-ý
I
2
'
1
I
0
ppm
' 9F NMR Spectrum of 12
0
-20
-40
-60
-80
ppm
196
-100
-120
-140
-160
1H
NMR Spectrum of 7
I
10
-
I-
I
9
8
I
7
6
4
5
'
3
I
---
I
~-I
2
1
ppm
19F
NMR Spectrum of 7
I
I
-20
-40
Aw"_L
-60
-80
ppm
197
-100
-120
-140
.
-160
'H NMR Spectrum of 13 (exo/endo)
k
I
'
I
'
j.
I
9
10
I
8
'
LI I, .
.
L
I
'
7
iI
i
I,
I
I
6
5
·I
3
II
w-uJr
-~ Jr
I
'
'
2
'
I
1
0
W..
WMAMW
ppm
13C
NMR Spectrum of 13 (exo/endo)
1_
220
1
200
1/
*1.1'
180
W--. -- . .--*..- -.I,
40
i
160
140
120
100
ppm
198
80
60
40
40
20
20
'H NMR Spectrum of 13 (exo/exo)
I
I
10
'
J
__L
-L
I
I
I
9
8
ij
6 45
7
ppm
3
2
__
1
13C NMR Spectrum of 13 (exo/exo)
220
200
180
160
140
120
100
ppm
199
80
60
-I.0 0 wo one"^
"wowWILLII*
w
I
40
20
I
0
1H
NMR Spectrum of 14
k
10
9
8
7
6
5
4
-I
3
11
2
I
~1
1
0
ppm
13C
NMR Spectrum of 14
L7.iInsmmumwwauY
m
200
mIn
I
1*1*1*1'1*
180
160
140
120
100
ppm
200
80
60
i I I I IY-.
40
20
Appendix 2
NMR Spectra for Chapter 2
201
'H NMR spectra of 4a
Fi
I
'
10
I
I
8
6
I
I
4
2
ppm
' 9F NMR spectra of 4a
-40
-80
-120
202
-160
ppm
13C NMR
spectra of 4a
IM
-u
--.
-I~~---·
I--· ·.Y·_~··L
-~
I1
200
1H NMR
100
150
mmRN"aw
~---Z -I··~..-· _·1~~_L_
ppm
50
spectra of 5a
I
---
10
6
-
4
203
-
J L-- a I
·
2
I
ppm
19F
NMR spectra of 5a
.Lr~~nup~r.~~lI·eull·l~~~I'W·llu~~,.WwU:~~lu ltlMM.M
vý`VFý
T-1- ""11. I " I - - - r " - 11""1
·
'1""
p '
-- 1
-80
-80
-40
13
Ji ll~ F'" "
·
1
-120
,"
1',
I
1
1
r
-160
I
ppm
C NMR spectra of 5a
Cll.rur.r~w~~u
.*~~Ylls·-~YIILLY.~I~·JL~·IL·LLL~U~~Jy~
I·
J
200
150
LS~~GUIII··ICkIYY·1(IY·
·
100
204
(IYII~LLJ· YrirYrblYL~LLLL·+uL1U~Y~I*LYJ·L·I·l·
~
ppm
1H
NMR spectra of 4b
I
I
10
I
8
I
6
I
I
4
2
ppm
'9F NMR spectra of 4b
0
-50
-100
-150
205
-200 ppm
13
C NMR spectra of 4b
200
1H
Y
ppm
50
100
150
NMR spectra of 5b
I
I
6
4
206
•m
2
IL
ppm
19F NMR
spectra of 5b
~ul~ulLullrlL~w~.nr~.lulu~lLuY~I*~lu~
.. ..
..
·Luu·-iu-~u·ul.u
......
.._..
-120
-80
-40
am
•LYYM
I
t II
"
"...
-160
m
ppm
'3 C NMR spectra of 5b
200
150
100
207
50
ppm
1H
NMR spectra of 4c
10
8
6
4
ppm
2
19
F NMR spectra of 4c
ilrr~r~mmm~
TIT"
'1*1
-40
-80
-120
208
-160
ppm
13
C NMR spectra of 4c
200
150
100
ppm
50
H NMR spectra of 5c
S10
-----
'
4
4
I
10
209
ie:l
~
Li
ppm
19F
NMR spectra of 5c
·L_
--·._-.··~·
·I--I·_I·-·--··
· · I~~LY-·· ···.·-·
-40
-80
· _I _··I· · L~__Ill_·.l·I.IL-
13
L·_-L·--L
.·-·I~1-_·I--··-·1I
a k"9mw9=
-160
.120
ppm
C NMR spectra of 5c
Ll- a.AýkLiLi 6
11ý110101-11
-11' 1 100"
I
200
--
lI
lI
150
100
210
JAM
I
50
i
S
Mk
J.
ppm
1H
NMR spectra of 4d
it9
10
19
/iI
6
8
4
ppm
2
F NMR spectra of 4d
-40
-80
-120
211
-160
ppm
13
C NMR spectra of 4d
200
1H
150
100
ppm
50
NMR spectra of 5d
1iii
I
jib
I
I
I
II
I
4I
4
212
ilrI
-
G~
ppm
19F NMR
spectra of 5d
M-0
ski
-
0 W1
n
-80
-40
13C NMR
is-4-
I
-80
me
-120
-I
i.0
"~'
*WM"
~I
-160
ppm
spectra of 5d
U·IYLIII
~YL·I-·I~·~yy~l
I
-M;, - ---WYJIII
YLL-·I~Y_~
·YI·-·IULYLWY·
II~~LYI-LYII*~I
··LY.W1
LI YY-V_·I
L··L~L_~·-C·Y1-_IM
I
Ow'I mm..."
200
150
100
213
ppm
'H NMR spectra of 3a
1~
I
*
10
13C
I
*
8
IKýI
fl
1
II r~
ILU
I
L
I
-
WJJ\.
ppm
6
NMR spectra of 3a
200
150
100
214
50
ppm
1H
NMR spectra of 8
··
Juu·
·· ·
tI
·
;~-
ppm
6
8
13C
Ii
nf i
NMR spectra of 8
200
150
100
215
50
ppm
1H NMR spectra of 3b
I
10
13C
2
4
6
8
ppm
NMR spectra of 3b
200
150
100
216
50
ppm
1H
NMR spectra of 10a
i ~
_
luJ
W...
ppm
10
13C
-
NMR spectra of 10a
Ilill ........ Iidi ......
iI
200
I
......................... . ... . .....
150
........... .....
I
100
217
.... .. . .
......................................
l_li·--U_·
WONI
ON" it
ppm
1H NMR
spectra of 3c
ii:
~''" '~ '
I
'
ppm
13
C NMR spectra of 3c
~~~~,~
~__~~~_._~_..~....
ST
200
~_....L-·.,
~·opt-·-·~
· ·--
I
I
150
100
218
-
~_~,~~.~~,._....
I.~.,........~. Yu ---u...
J. ·· · ·-·-~·YL
ppm
1H NMR
spectra of 10b
1141!
I~
ppm
13
C NMR spectra of 10b
200
150
100
219
50
ppm
1H
NMR spectra of 3d
i
IYY
10
13
a
·
I
·
I
I
I
It
I
ppm
8
C NMR spectra of 3d
200
150
100
220
50
ppm
Appendix 3
NMR Spectra for Chapter 3
221
'H NMR spectrum of 2
·
I Ih
Iln_·
·
·
·
·1
. .
,
,
·
·
0 ppm
13C NMR spectrum of 2
lu.yuullruu~ulurru*ru~.lulr·n~lr..
.IrYrlUI.I
···rl···IL
·I1·_I~_I~-III·_-LY
·~·
L·l
U·ll·IAl .u I h~lY~-~ i .. .. .. .... .I
- -- · · r---·----·rn~r-~Y~r
·
.......
.......
rr· -~·l·-,~nml-l· I~m'rrl··rr*····lr~ln·llr
·~·rl·r~r
yl-r~l~r·~·r·~~-l*~·PrF~~7·~·C~·Pr*rCII~
·rpr~~P~p7~~F
·
180
180
I
160
I40
140
'
I
120
'
I
I
8
80
100
222
I
60
40
20
ppm
1H
NMR spectrum of 3
-
--I
·
j
I-p
-·
I
·
6
8
0
ppm
13
C NMR spectrum of 3
I
180
7."•-7_._"_".•;•._•.
1 I
160
4
-·' '.·· .
- :.'M-01.2010
,6Ow. . .
I
2I
140
120
.
··--.. .1· t IIi t| I.l.....A..
.. . .....
.
.......
. I
.. .
I
"
I
80
100
223
60
40
J0
"I"'""
WA
MM"l
SI
P'
20
ppm
1H
NMR spectrum of 4
-..
II
A
.
I
6
8
13C
I
I
4
2
'I
0
pI
p
ppm
NMR spectrum of 4
.I--·--I-e
-·-Ill·l-Y-l·
180
160
-IIYY---YY.*lrY·-
140
120
-·IIYY····Y~Y
100
· Y L~·
80
224
-II*·L-LI~·I·LI1-~-·L3
-Y~LYI·I *A~ 0r amr
60
40
20
9....
ppm
1H NMR
spectrum of 5
K
~JLi
8
8I
13C NMR
0
ppm
spectrum of 5
''''"'''"'"'~~'"'"'
-
180
"" '' "''"" "'
~'''''"'""'
r-'-.
I
160
jlj ~ U.-.
""~
111 I"111r-nI-M--m
140
,
'
r,.
r
ucuYI Y
YYlllýT4-TF
" ,
.
'
60
120
100
80
225
60
r
ii 1 I
L-'Ru-Fr~~~u·
.-,r F0 9 -1.
40
40
20
20
~.,r.
pI
ppm
1H
NMR spectrum of 7
,
d
--
I0
0
10
ppm
13C NMR spectrum of 7
I
I
180
160
'
I
140
120
100
80
226
60
40
20
ppm
1H
NMR spectrum of 8
?1L
_
·
I
1111
·
·
·
·
·
0
13
-1
ppm
C NMR spectrum of 8
I
180
'
I
16(0
I
140
' 120
120
100
i
I
80
100
227
'
I
60
*
I
40
'
I
20
ppm
1H
NMR spectrum of 9
LIdl
-
·
-·
·
0
6
8
ppm
13
C NMR spectrum of 9
i
.. I
1..1.
.11[_'
11
-~I~-·---·----- ~·---~-~--~~~~~~-II -~~
--~~~~----·
·-- ---~~~~-----~~~
-~ --~·--~~~~~
~-----~-~ ~-~~~----~----~~ ~~~----------~------~180
160
140
120
100
80
228
60
40
20
ppm
1H
NMR spectrum of 10
I
I
iLI
*
8
13C
6
0
ppm
NMR spectrum of 10
I----~-II··YI·-LIC
11 I 01
UIYYL-1Y··~·-Yn~n~n~n~n~n~n~n~n~n~n~n~n~
.
.
I I
180
160
140
120
I
.
·-
1
100
1
80
229
.
. · Ili_
. . ,,-, .
60
60
. owl'
.. ,
."
40I
40
.
"
, 0 .O.
20
N.......
I20P
ppm
1H NMR spectrum of 11
I
13
I
*
I
*
•
2
4
6
8
I
!
•
0
ppm
20
ppm
C NMR spectrum of 11
180
160
140
120
80
100
230
60
40
IH NMR
spectrum of 12
I
13C
'
jI L
' ,jý
I
·
I
4I
4
I
'I
'
2
0
ppm
NMR spectrum of 12
I-YYU~Y-~~~~"L`~Y-~~'-1--1·1
I
rI
180
160
140
120
I
'
100
231
FM909MAIM=~r
I
I
80
60
MMOMM*
=9MMEMMEM
I
r
'
40
ppm
1H
NMR spectrum of 13
III
-13C
·
·
·
·
I
I
4
2
ppm
NMR spectrum of 13
~-~
-~---~-----~-~----~
--- -------,I~----------II,---,-------- ,-----180
160
140
120
100
80
232
60
40
20
ppm
'3 C DEPT spectra of 11
cH3 carbons
-1-~ 7~-~1- -1-
-1 `
---
CH2Carbons
CH Carbons
all protonated Carbons
145
140
135
130
233
125
120
ppm
IH-13C HMBC spectra of 11
F2
(pp
5.4
5.6
5.8
6.0
6.2
6.4
6.6
6.8
7.0
7.2
7.4
-
7.6
7.8
8. 05.
.
145
.
140
..
.
135
234
.
..
130
Fl (ppm)
.
125
120
Appendix 4
NMR Spectra for Chapter 4
235
1H
NMR spectrum of la
-. 1 I
Y_
Y
'
1
I
i
y
I
ppm
8
13
C NMR spectrum of la
180
160
140
120
100
236
80
60
40
20
ppm
1H NMR
spectrum of lb
_
Irl
Pl
·
I·
·
·
1
I~
··I
2
8
13C
I
ppm
NMR spectrum of lb
180
160
140
120
100
237
80
60
40
20
PPm
'H NMR spectrum of 2
* H,O
*CH2CI
2
10
13C
8
2
6
4
2
ppm
NMR spectrum of 2
A_
L.
I
I
I
I
180
160
140
120
*: CH 2C 2
I
'
I
'
'
100
238
80
I
60
I
I
I
40
20
ppm
'H
NMR spectrum of 5
* ethyl acetate
IIL*
i 111
...
......
.
I
13C
·
7
·
ppm
NMR spectrum of 5
180
160
140
120
100
239
80
60
40
20
ppm
H NMR spectrum of 6
* : ethyl acetate
,,
8I
8
13C
I
I
6
4
A
'
''
I
ppm
2
NMR spectrum of 6
* : ethyl acetate
180
160
140
120
100
240
80
60
40
20
ppm
IH NMR spectrum of PI-2
hlýI
I
I
KIL
I
8
fP1H~ NM6pcru
I
6
I
ppm
4
IH NMR spectrum of PI-6
i.ll
SI
I
I
I
ppm
241
1H
gCOSY NMR spectrum of PI-2
F2
(ppM)
7 . 17 .2-
7 .3-
7 . 4-
7.5-
7.6-
7.7-
7.8-
7.9-
o
FT ---
-T
@
8.0-
8.1-
8.28.2
8.1
8.0
0, 7.8
7.9
7.7
7.6
Fl (ppm)
242
7.5
7.4
7.3
7.2
7.1
1H
gCOSY NMR spectrum of PI-6
I
'
~
F2
7.2
7.3
C.,
7.4
-I'
7.5 t
7.6A
7.
7
K-,i
-
7.87.9-
4
8.0-
8.1-
12'j
O.L7
8.38.1
8.0
7.9
7.8
7.7
7.6
Fl (ppm)
243
7.5
7.4
7.3
7.2
IH
NMR spectrum of PU-1
AI
LIIJLJJWY
I'
I
8
6
I
10
1H
IB
IA
4
I
*: DMF
I
I
ppnn
2
NMR spectrum of PU-2
DMF
IF
m
8
6
4
244
2
4-,
4
ppm
1H
NMR spectrum of PU-3
*: DMF
I ,,
I
10
-)JAJU-JL'LJL
I
I
8
,
___)\JJ
I
6
4
245
.1
AI
iý..j Ia
A
I
2
ppm
1H
gCOSY NMR spectrum of PU-1
lýi"p ý-F2
(ppm
6. 8-
7.0
cS
7.2
Av
7.4
'1
i
7.6
7.8
8.0
8.2
8.4
8.6-
~I
8.88.8
8.6
8.4
8.2
8.0
7.8
7.6
Fl (ppm)
246
7.4
7.2
7.0
6.8
Appendix 5
NMR Spectra for Chapter 5
247
1H NMR
spectrum of 2
LI
-I-I
8
10
13C
4
6
2
ppm
40
20 ppm
NMR spectrum of 2
200
180
160
140
120
100
248
80
60
1H NMR
spectrum of 4
II
I
m
·
g
·
·
·
ppm
10
13
C NMR spectrum of 4
200
180
160
140
120
100
249
80
60
40
20
ppm
1H
NMR spectrum of 5
I
Cr
I
8
13C
L----
I
'
k.,i - i
-IY
I
ppm
6
NMR spectrum of 5
·-·
200
--·----- '·- --- ~-~-~~"--
180
160
-- ·~
!!n!.
140
r --- · ----- ·--·
'--
120
100
250
-·--------·~--·
80
·-------
60
--------
1
40
' '-
20
ppm
1H
NMR spectrum of PFQ
'1
I
hI
A
I
8
It
1K.
I*
6
I
4
A/~
U
2
I
v
ppm
'H NMR spectrum of PFTP
I
·
·
8
·
·
_
6
251
·
_
4
2
ppm
1H
NMR spectrum of PFP
I
J A4
v
1nA
r
~c~J
h/K
u/
ppm
252