GRAFTED POLY(PHENYLENE-ETHYNYLENE): OPTICAL AND OPTOELECTRONIC
APPLICATIONS
BY
CRAIG A. BREEN
B.A., CHEMISTRY
MIDDLEBURY COLLEGE, MIDDLEBURY, VERMONT; 2000
SUBMITTED TO THE DEPARTMENT OF CHEMISTRY IN PARTIAL FULFILLMENT OF THE
REQUIREMENT FOR THE DEGREE OF
DOCTOR OF PHILOSOPHY IN CHEMISTRY
MASSACHUSETTS tNSTfJTE'
OF TECHNOLOGY
AT THE
MASSACHUSETTS
INSTITUTE OF TECHNOLOGY
JUN
2 1 2005
LIBRARIES
JUNE2005
© 2005 MASSACHUSETTS INSTITUTE OF TECHNOLOGY. ALL RIGHTS RESERVED.
SIGNATURE OF AUTHOR:
B1MENT
OF UCEMISTRY
MAY24, 2005
CERTIFIED BY:
TIMOT CY. SWAGER
PROFESSOR OF CHEMISTRY
THESIS SUPERVISOR
ACCEPTED BY:
ROBERT W. FIELD
CHAIRMAN, DEPARTMENTAL COMMITTEE ON
GRADUATE STUDENTS
ARCHIVeS
This doctoral thesis has been examined by a Committee of the Department of
Chemistry as follows:
Professor Moungi G. Bawendi
,)
Chairman
Professor Timothy M. Swager
Thesis-Advisor
Professor Edwin L. Thomas
Professor Vladimir Bulovic
/
2
/
To my family and the lovely Sandra Rifai - soon to be Sandra Breen
3
Grafted Poly(Phenylene-Ethynylene): Optical and Optoelectronic Applications
By
Craig A. Breen
Submitted to the Department of Chemistry on May 24, 2005 in partial fulfillment
of the requirements for the Degree of Doctor of Philosophy in Chemistry
ABSTRACT
Poly(phenylene-ethynylene) (PPE), a fully conjugated polymer system,
exhibits high solution state quantum yields, narrow emission profiles, and wide
band gaps allowing for blue emission, making them ideal candidates for display
applications. Unfortunately, PPEs have received little attention in solid-state
optical and optoelectronic applications due to aggregation phenomena, which
significantly reduces solid-state emission efficiencies and limits the miscibility of
PPEs with other materials systems.
Furthermore, the acetylene linkage in PPEs
limits the redox properties of these polymers making charge injection, especially
holes, difficult without material degradation. However, this dissertation details
the development of a grafted PPE system that demonstrates enhanced optical
and optoelectronic behavior by circumventing the negative aspects of traditional
PPEs outlined above. The basic luminescent properties of conjugated polymer
systems are
understanding,
outlined in Chapter 2.
Building upon this fundamental
we move to the design and synthesis of a new grafted PPE
system described in Chapter 3. The synthetic modification of the PPE is used for
domain-specific incorporation into a cylindrical morphology block copolymer host
matrix, which is reported in Chapter 4.
This work also details the design and
fabrication of new PPE based organic light emitting devices (OLEDs). Chapter 5
discusses the development of a new hybrid OLED system whereby energy
transfer from a hole-transport host to grafted PPEs results in efficient, blue PPE
electroluminescence (EL) that matches the solid-state PL. Moreover, the grafting
process is completely modular, allowing for further modification. Chapter 6
details the introduction of a charge transport moiety, which is directly grafted to a
PPE backbone structure, enabling an entirely polymeric, single layer device,
capable of achieving efficient, narrow blue EL. The culmination of these results
in Chapter 7 solidifies that PPEs, in combination with a modular grafting
technique, can now be accessed as viable light-emitting materials for OLED
applications.
Thesis Supervisor: Timothy M. Swager
Title: Professor of Chemistry
4
Acknowledgments
I like to consider myself a fairly simple individual.
consider myself
poetic or eloquent.
I do not, however,
The following is by no means
comprehensive, nor would I ever assume that the words I will use in the following
segment even begin to repay those for the richness and enjoyment that they
have brought into my life. With that said, I will do my best to thank the many
people that have shaped my graduate experience at MIT.
I would like to begin by thanking my advisor, Tim, for all of his energy and
passion for developing "big picture" ideas. His creativity and desire to design and
synthesize multifunctional materials systems while always keeping real-world
applications of the materials in mind, truly has shaped my development as a
scientist. In addition to his constant encouragement, my thanks also go out to
Prof. Ned Thomas for giving me the opportunity to work closely with his research
group.
Dr. Deng and Dr. Breiner, post-docs in Ned's group, made my block
copolymer roll casting experience much more fun than just watching cumene
evaporate.
I'm also thoroughly indebted to Prof. Vladimir Bulovic. I consider
myself extremely fortunate to have had the opportunity to work with him and his
research group. Vladimir's intellect and technical abilities are only overshadowed
by his warm and friendly
nature.
My transition
into device fabrication
and
analysis would not have been possible, nor nearly as enjoyable, without the help
of Yaakov Tischler, a graduate student in Vladimir's group.
Many of the fond memories I have from these past five years come from
intramural sports. With all due humility and respect, let me be so bold as to say
5
that the athleticism and raw competitiveness exhibited by the crazy group of guys
that constituted The Little Lebowski Urban Achievers (otherwise known as LLUA)
intramural association, has never been, nor will likely ever be, matched by
another intramural organization
at MIT.
To Rene, Scottie, Philip, Monkey, JP,
Chudi, Aaron and Andy Aguirre, Cody, ZB (a.k.a. Brent Fisher), Drew, Chris,
Jose, Big John, Andy, CJ, Akin, Neil, Mike and Peewee - It was a great run.
I
hope you all had half as much fun as I did. And remember boys, it is only IM's.
Lastly, but most certainly not least, I must thank my family.
Everything I
have accomplished is entirely due to the love and support of my family.
I have
learned an enormous amount from my younger brother Eric. His love of life and
unwavering convictions are truly inspiring.
confidence
My parents' and grandparents'
in me, in addition to their willingness
to "always
be there," is
unparalleled. My dad has taught me the value of education and why practicality,
while not everything, is extremely important. As for my mom, well, lets just say
I'm a momma's boy.
My mom is very special and has set the bar high for my
future wife. Speaking of which, that brings me to the sweet and lovely Sandra.
While the amount of time she has been in my life up to this point is relatively
short, she has most certainly had the greatest impact on my graduate experience
and on my life as a whole.
Probably the best way to sum up my love for Sandra
is the standard cliche; when you meet the right one, you just know. I definitely
knew. Deciding to ask her to marry me was the easiest decision I have ever
made.
My life-long mission will be to hopefully make her as happy as she has
made me. Sure sounds like a nice life to me.
6
Table of Contents
Title Page
.........
.........
Signature Page
.........
Dedication
Abstract
.. .......
Acknowledgements .........
Table of Contents .........
.........
List of Figures
List of Schemes
.........
List of Tables
.........
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... . . ..
... .. . .. .
.. ..
... .. . .. .
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......... ........... 5
......... ........... 7
......... ........... 11
......... ........... 21
......... ........... 22
Chapter 1: Development of a Quantum Efficient Solid-State
Poly(Phenylene-Ethynylene) System: The Quest for the
Elusive Blue OLED
................................................
1.1 Introduction - Display Technologies .....................
1.1.A Liquid Crystal Displays
.....................
1.1.B Organic Light Emitting Devices
1.2 OLEDs - Towards Full Color Displays
............
1.3 The Elusive Blue OLED
1.3.A Overview
............
..............................
.......................................
1.3.B Inorganic Systems
..............................
1.3.C Small Molecule OLED Systems
............
1.3.D Organic/Inorganic Hybrid Systems............
1.3.E Polymer Based LEDs or PLEDs
1.4 Poly(Phenylene-Ethynylene)
1.4.A Electroluminescence
1.4.B Conclusions
(PPE)
............
.....................
from PPEs
............
.......................................
1.5 Thesis Outline
................................................
1.6 References
................................................
7
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24
25
27
31
32
32
34
34
36
37
39
40
48
49
51
Chapter 2: Conjugated Polymer Systems and the Basic Theory of
Organic Luminescent Phenomena ..........
...................
... 55
2.1 Introduction - Conjugated Polymers ..................................................
56
2.2 Luminescence in Organic Materials ..................................................
61
2.2.A Radiative Transitions
..................................................
62
2.2.B Non-Radiative Transitions ..................................................
64
2.2.C Franck-Condon Shift .........
2.3 Electronic Energy Transfer
.........
.........
............................
...........................
65
2.3.A Forster Energy Transfer
..................................................
2.3.B Dexter Energy Transfer
............................
2.4 Energy Transfer in Conjugated Polymers
.. 66
.........
68
......................
69
.........................................
70
2.5 Energy Levels and Transitions of Coupled Systems (or Aggregates)
..... 71
2.6 Conclusions
...................................................................
74
2.7 References
...................................................................
76
Chapter 3: Polystyrene Grafted Poly(Phenylene-Ethynylene) via Atom
Transfer Radical Polymerization (A TRP)
................................
80
3.1 Abstract ..................................................................
3.2 Introduction - Grafted Conjugated Polymers
................................
3.3 Atom Transfer Radical Polymerization and its Mechanism .........
3.4 Results and Discussion
3.4.A Polymer Synthesis
...................
3.5 Conclusions
...................................................................
3.6 Experimental Section ............................
3.7 References
..........................
............................
.........
............................
...............................................
8
81
..............83
..................
............................
3.4.B Polymer Characterization
79
.. 86
86
......................
88
93
94
. 100
Chapter 4: Polarized Photoluminescence from Poly(p-phenyleneethynylene) via a Block Copolymer Nanotemplate ...................
101
4.1 Abstract ...............................................................
..... . . . .. . . 102
4.2 Introduction - Polarized Photoluminescent Materials
..... . . . .. . . 103
4.2.A Overview
.............................................
4.2.A Block Copolymers
..... . . . .. . . 103
....................................
4.2.C Alignment of Conjugated Polymers
..... . . . .. . . 103
.........
..... . . . .. . . 106
4.3 A New Guest/Host System
....................................
..... . . . .. . . 109
4.4 Results and Discussion
....................................
111
4.4.A Overview
.............................................
111
..... . . . .. . .
4.4.B Roll Cast Processing ....................................
113
4.4.C Roll Cast Film - Structural Characterization
115
4.4.D Roll Cast Film - Polarization Spectroscopy
..... . . . .. . . 118
4.4.E Roll Cast Film - Anisotropy ...........................
123
4.4.F Mechanically Tunable Polarized Emission
127
4.5 Conclusions
......................................................
129
4.6 Experimental Section .............................................
130
4.7 References
131
......................................................
Chapter 5: Highly Efficient Blue Electroluminescence from
Poly(Phenylene-Ethynylene) via Energy Transfer from
a Hole Transport Matrix
5.1 Abstract ...............................................................
5.2 Introduction
135
..................................
...
.. .. . .. ... ..
......................................................
5.3 Results and Discussion
5.3.A Overview
....................................
137
... ... . . .. . . 138
.............................................
5.3.B Photoluminescence Energy Transfer Study
136
138
.. ... . . .. . . . .
141
5.3.C Electroluminescence of TPD/PPE Systems
143
5.3.D Operational Analysis of TPD/PPE Devices
148
9
5.4 Conclusions
151
.....................................................
5.5 Experimental Section .....................................................
152
5.6 References
156
...........................................................................
Chapter 6: Blue Electroluminescence
from Oxadiazole Grafted
Poly(Phenylene-Ethynylene) ................................................
6.1 Abstract
159
160
.....................................................
6.2 Introduction
161
.....................................................
6.3 Results and Discussion
6.3.A Overview
...................
162
......................................
162
..................................
............................
6.3.B Polymer Characterization
165
................................................
6.3.C Photophysical Characterization
166
.......................................
6.3.D Electroluminescence of Oxadiazole Grafted PPEs
............
170
6.3.E Operational Analysis of Oxadiazole Grafted PPE Devices.........
172
6.4 Conclusions
.........
.............................. 175
........................... .........
........... 176
176..........................
6.5 Experimental Section .........
6.6 References
..................
...................
.........
.......
182
Chapter 7: Conclusions and Future Directions .......................................
185
7.1 Overview .........
186................................
186
7.2 Conclusions
.........
7.3 Future Work
...........................................................................
188
7.4 References
.........
192...................
192
Curriculum Vitae
...........................................................................
..............................................................
............................
10
186
195
List of Figures
Chapter
1
Figure 1.1 Schematic representation of a liquid crystal display. Adopted from
http://www.plasma.com/classroom/what is tft Icd.htm.
26
.......................
Figure 1.2 Top: Sony display which shows off the remarkably thin monitor that
can be developed
with OLED technology.
(Adopted from Universal
Display
Corporation website.) Bottom: Inset of a 40-inch Seiko Epson prototype OLED
Schematic of ink-jet printed OLED display.
display.
website:
http://www.epson.co.ip/e/newsroom/news
Figure 1.3
Clockwise from left:
Adopted from Epson
.... 29
2004 05 18.htm
1) Personal MP3 music player launched by
Delta Electronics that incorporates a green PLED display for the control and
indication of all system operations. 2) Philips has developed the new 639 mobile
telephone using 'Magic Mirror' technology.
The clam-shell design provides a
mirror for checking personal grooming when it is closed.
Incoming calls are
indicated by the display showing through the mirror - a highly effective use of a
secondary display.
3) A men's shaver sold by Philips uses a PLED display
showing information including shaving time remaining before recharge.
MicroEmissive
Displays has developed
the NHJ 3-in-1 Camera.
camera, movie camera and MP3 player all in one.
4)
It is a still
It has 3.2M pixels, USB
connection and a PLED display which has one quarter the power consumption of
an
LCD.
Adopted
from
Cambridge
http://www.cdtltd.co.uk/technoloqy/42.asp
Technologies
website:
........................................
30
Display
Figure 1.4 A typical CIE diagram. CIE stands for the Commission Internationale
de 'Eclairage, which is an international committee for the establishment of color
standards. The CIE model and the CIE Chromaticity Diagram define the different
variations of color. The dark triangle overlayed on the CIE diagram represents
the various colors that can be achieved using a standard CRT computer monitor.
The CIE coordinates for red, green, and blue phosphors used in a standard CRT
11
monitor are (x = 0.61; y = 0.34), (x = 0.25; y = 0.62), and (x = 0.15; y = 0.063)
respectively. .......................................................
Figure 1.5
33
Chemical structures of common wide-band gap small molecule
materials for blue color emission. The structure for TPD is show in a, while that
of a-NPD is shown in b.
35
......................................................
Figure 1.6 Chemical structure of a blue phosphorescent emitter, Flrpic.
Figure 1.7
.... 36
EL from QD-LED devices with (a) and without (b) hole-blocking
layers. Plots c and d display the ability of these systems to produce red and
green EL, while plot e displays new blue (CdS)ZnS quantum dot PL and initial
QD-LED studies with this material.
.................................................
37
Figure 1.8 Chemical structures of PPV type systems used in PLEDs. The
standard PPV structure is shown in a, while PPV systems with interrupted
conjugation are shown in b, c, and d.
Figure 1.9
.................................................
Examples of various PPP structures.
A fluorine unit has been
incorporated into b. .......................................................
Figure 1.10
(dashed
Figure 1.11
39
Chemical structure, absorbance (solid line), PL (dotted line), EL
line), and operational
OPPE/ITO)
38
analysis
of PPE based
LEDs (Cathode/O41
......................................................
Top: Structures of EHO-OPPE, a poly(2,5-dialkoxy-p-phenylene-
ethynylene) derivative, poly(N,N'-diphenylbenzidine diphenylether) (poly-TPD),
and a tetrameric spiroquinoxaline (spiro-qux). Bottom: Schematic view of the
device structures investigated: 1) single-layer EHO-OPPE, 2) two-layer polyTPD/EHO-OPPE,
3) single-layer
blend,
and 4) blend
layer with additional
electron-transport/hole-blocking layer (ETHBL), and of their corresponding
energy-level diagrams; the band-edge energy levels were determined by cyclic
voltammetry (CV). ......................................................
42
Figure 1.12 Examples of poly(phenylene ethynylene)s. A 2,5-pyridinediyl unit is
incorporated in a while b is a poly(3,4-dialkyl-1,6-phenylene-ethynylene).
.... 44
Figure 1.13 Electroluminescence from AI/PPE/ITO devices. Plot a shows the
results for the PPE system in Figure 1.1lla while plot b shows the results for the
PPE in Figure 1.1lb.
...................................................................
12
44
Figure 1.14
Top:
Chemical structure of the meta-phenylene ethynylene
polymer, 5a and 5b. Bottom:
ITO/PEDOT/5b/Ca/AI
Solid-state PL of polymer 5b and EL spectra for a
LED at 12 and 17 V.
45
.........................................
Figure 1.15 The synthesis of the naphthalene unit and naphthalene containing
PPEs is shown in a.
Plot b shows the PL for PPEs containing
increasing
amounts of naphthalene unit (5a-5d where 5d contains the largest naphthalene
content). Plot c reveals spectrally integrated EL emission intensity for a PPEbased OLED with (solid symbols) and without (open symbols) a PEDOT hole
injection layer while d shows the normalized EL spectra of hexyl-dodecyl-PPE
where the emissive layer is spin cast from toluene and from CHCI3/toluene (1:1).
Plot e shows the normalized EL spectra for devices containing PPEs 5b and 5c.
...................................................................................................
.... 47
Chapter 2
Figure 2.1 Three repeat units of important conjugated polymer structures..... 56
Figure 2.2 Peierls distortion of an isolated chain of equidistant atoms. Crystal
lattice, electron density of states and electron dispersion relation without
distortion (a) and with distortion (b).
.................................................
57
Figure 2.3 Bond alternation in leads to Peierls gap. Overlap of the pz orbitals of
the double bonds in polyacetylene results in the formation of a delocalized electron system along the polymer backbone.
Bonding and antibonding
T
orbitals form the equivalent of conduction and valence bands, responsible for the
metallic and semiconducting character of these materials.
Figure 2.4
......................
58
a) Irradiation of a fluorescent polymer excites an electron from the
HOMO to the LUMO. In a typical conjugated polymer, two new energy states are
generated upon relaxation within the original HOMO-LUMO energy gap and are
each filled with one electron of opposite sign (singlet excited state). The excited
polymer may then relax to the ground state with emission of light at a longer
wavelength than that absorbed (photoluminescence).
b) In a polymer LED,
electrons are injected into the LUMO (to form radical anions) and holes into the
13
HOMO (to form radical cations) of the electroluminescent
polymer. The resulting
charges migrate from polymer chain to polymer chain under the influence of the
applied electric field.
When a radical anion and radical cation combine,
generating an exciton-polaron, singlet and triplet excited states are formed, of
which singlets can radiatively combine, emitting light
Figure 2.5 Jablonski diagram.
Figure 2.6
...............................
60
61
..........................................
Schematic diagram representing exciton spin and symmetry.
The
total possible spin states of an exciton can be spatially anti-symmetric (S = 1)
also known as triplet states or they can be spatially symmetric (S = 0) also known
as singlet states. There are three possible triplet configurations and only one
singlet configuration.
...................................................................
63
Figure 2.7 Energy diagram of the electronic ground state and first excited state
for a diatomic molecule, schematically illustrating the Frank-Condon or Stokes
shift
66
..............................................................................................
Figure 2.8 Schematic of the Frster and Dexter processes resulting in singletsinglet energy transfer.
69
..........................................
Figure 2.9 Exciton splitting in dimers of various geometries. Orientations of
monomer transition dipoles are represented by short arrows. Dipole-forbidden
transitions are denoted by dotted lines.
.. 74
........................................
Chapter 3
Figure 3.1 Plot a shows UV/vis analysis (300-700 nm) of ungrafted PT (5) and
grafted PT (6) in solution and cast films.
The fluorescence
of ungrafted and
grafted PT in solution (excited at 420 nm) and cast films (excited at 445 and 420
nm respectively) are shown in plots b and c respectively.
......................
83
Figure 3.2 GPC elugrams of the PPE Backbone (black), PS grafted PPE (blue),
and cleaved PS grafts (green).
The plot to the left shows the elugram for the
grafted PPE measured using the various detection wavelengths available with
the GPC system, i.e. Refractive Index (RI), Ultra-violet (250 nm), and Visable
(450 nm).
89
..........................................
14
Figure 3.3
1H
NMR characterization of polymers 3, 4, and 5. Signals normalized
on the aromatic methoxy proton signal. ........................................
91
Figure 3.4 Right: Photophysical analysis of the PPE backbone and grafted PPE
systems. Absorption profiles for the grafted (a) and ungrafted (b) PPEs are
plotted in both solution and solid-state. The photoluminescence for the grafted
(c) and ungrafted (d) PPEs are also shown.
Left: A comparison is made between
the quantum yields of the grafted and ungrafted systems in both solution and
solid state.
92
........................................
Chapter 4
Figure 4.1 Phase diagram for a simple A-B di-block copolymer system.
... 105
Figure 4.2 Polarized absorption (A) and photoluminescence (B) of an oriented
film (draw ratio,
= 80) of a 2 weight % EHO-OPPE-UHMW-PE
blend.
The
spectra were recorded with polarizers oriented parallel (solid line) and
perpendicular (dashed line) to the orientation direction of the polymer blend films.
The inset in (B) shows at a magnification of 70 the spectrum recorded
perpendicular to the orientation direction.
......................................
107
Figure 4.3 Schematic structures of PL display devices. (A) Device in which the
light emitted by the polarized PL layer is switched. (B) Bicolor device in which
the light emitted by the polarized PL layer is switched. (C) Device where the UV
excitation light is switched. Arrows at the left indicate the polarization directions
of the respective elements.
Devices A and B constituted a commercially
available, standard TN cell fitted with a linear sheet polarizer. Device C was
based on a laboratory UV-active TN cell and a UV-active sheet polarizer
(Polaroid HNP-B) producing linearly polarized light. All devices used a Bioblock
UV lamp (VL-4LC).
Photographs (right) of a PL display device like that shown in
A compared to a similar commercial, direct view LCD. Also shows a photograph
of the bicolor display device where the MEH-PPV-based polarized layer was
omitted in the upper left part of the display.
15
.......................................
108
Figure 4.4 Polarized absorption and emission spectra of oriented films. Spectra
were obtained by irradiating the sample with vertically (solid line) and horizontally
(dashed line) polarized light. Emission spectra were obtained by exciting the
films at 365 nm. a, Binary UHMW PE/DMC blend; b, Binary UHMW PE/EHOOPPE blend; c, ternary
UHMW PE/EHO-OPPE/DMC
blend.
Photophysical
processes and optical emission. d-f, Schematic representation for uniaxially
oriented films of the binary reference blends of UHMW PE/DMC (d), UHMW
PE/EHO-OPPE
(e) and the ternary
UHMW PE/EHO-OPPE/DMC
Arrows indicate polarizations of incident and emitted light.
Figure 4.5
blend (f).
....................
109
Schematic showing the alignment of PS grafted PPE by
sequestration inside the PS domains of a SIS triblock copolymer. The grafted
PPE was co-assembled into PS cylinders within the PI matrix.
Figure 4.6 Schematic diagram of the roll cast apparatus.
...........
.....................
112
114
Figure 4.7 Cross-sectional transmission electron microscopy (TEM) images and
small angle x-ray scattering (SAXS) data. (a) SIS triblock copolymer blended
with 0.1% (w/w) PS grafted PPE after simple casting.
overall microdomain
Small grains and a lack of
ordering are apparent in the TEM image and an isotropic
scattering ring is evident in the SAXS pattern. (b) Same system after roll casting.
The TEM view is along the axis of the cylinders showing the PS cylinders (white
circular dots) form a long-range ordered hexagonal lattice in the P matrix.
The
SAXS pattern with the incident x-ray beam orthogonal to the cylinders shows the
'-/,
3, and
V4 reflections
of the hexagonal lattice of cylinders.
The
FWHM of the first reflection is only 10 degrees, indicating the high axial
alignment achieved by the roll cast process.
Figure 4.8
......................................
117
Schematic diagram of the measurement system used to determine
the degree of polarized emission generated from the roll cast PS grafted PPE
doped block copolymer films.
Figure 4.9
........................................
Polarized absorption (a) and photoluminescence
119
(b) of a roll cast
oriented film of 0.1 weight % PS-PPE doped in SIS. The spectra were obtained
with polarizers oriented parallel (solid line) and perpendicular (dashed line) to the
PS cylinder orientation of the block copolymer host. The emission spectrum in
16
(b), measured perpendicular to the polarizers (dashed line), is corrected for
instrumental polarization dependence.
........................................
120
Figure 4.10 Artistic representation of the PS grafted PPE alignment within the
PS domain of the block copolymer host matrix. Intramolecular energy transfer, or
energy migration, from the less aligned PPE segments to highly aligned
segments allows this particular system to absorb a significant amount of
unpolarized light and transfer this energy to on particular polarization.
... 122
Figure 4.11 Polarized photoluminescence of a simple cast (a) and roll cast film
m. The excitation wavelength was X =
(b). The thickness of both films was - 40
400 nm and the sample was irradiated perpendicular to the surface of the
film ..................................................................
123
Figure 4.12 Angular dependence of anisotropy r (0) of a simple cast film and a
roll cast film at
= 472 nm.
The thickness of both films was - 80
m. The
nonzero anisotropy in the isotropic film arises due to the inherent polarizability of
the PPE guest molecule. Due to the relatively short lifetime of the PPE system,
the polarized excitation results in emitted light that retains a small, non-zero
polarized emission component.
............................
............
124
Figure 4.13 Thickness dependence of anisotropy at X = 472 nm of a roll cast
film.
The excitation wavelength was X = 400 nm and the thickness of the film
varies between -40
is -10%.
m to -470
m. The variance in the anisotropy measured
....................................................................................
126
Figure 4.14 Change of anisotropy with applied transverse strain. The film (-100
m) was initially deformed to 240% and then released. The data shown is for
the second loading/unloading cycle.
................................................
128
Chapter 5
Figure 5.1
Diagram of a standard heterojunction OLED device architecture.
Electrons are injected from the cathode into an electron transport layer while
holes are injected from the anode into a hole transport layer. The charges travel
to the center of the device where they reach one another forming excited states,
17
or excitons, on the molecules that comprise the recombination region within the
device. This region can simply the interface of the hole transport and electron
transport layers or, it can be another material that constitutes the desired
luminescent layer.
region,
these
Once the excitons are formed within the recombination
excited
states
recombine, producing light.
i.e. the electrons
and holes can
........................................................
139
can relax,
Figure 5.2 Solid-state absorption and photoluminescence spectra of the various
TPD/PPE blends. Dashed lines indicate absorption spectra. Photoluminescence
spectra were obtained by irradiating the samples with X = 360 nm and X = 420
nm excitation wavelengths. Plots (a) and (b) correspond to blends of ungrafted
PPE (O-PPE and P-PPE respectively), while (c) and (d) correspond to blends of
grafted PPE (GO-PPE and GP-PPE respectively).
Figure
5.3
In the system
containing
142
.............................
ungrafted
PPEs,
aggregated
PPE
molecules (red) phase separate from the TPD host molecules (yellow) and thus
do not undergo energy transfer, displaying a significant spectral component due
to TPD in the PL energy transfer study. The grafted PPEs (blue), however, are
completely miscible with the TPD host and thus exhibit efficient energy transfer
resulting in spectrally pure PPE photoluminescence.
..............................
144
Figure 5.4 Schematic diagram of an organic vacuum deposition system. .... 145
Electroluminescence spectra (solid line) overlayed with the
Figure 5.5
corresponding photoluminescence spectra (dashed
TPD/PPE-LEDs.
line) of the various
Inset, plot of the EL and PL of a TPD control device.
Spectra
for ungrafted PPE devices are shown in plots (a) and (b) (O-PPE and P-PPE
respectively)
while (c) and (d) correspond to devices containing grafted PPE
The TPD/PPE layer is spin-cast onto
(GO-PPE and GP-PPE respectively).
clean, indium tin oxide (ITO) coated glass substrates with a thickness of 50 nm
for ungrafted PPE blends and 40 nm for grafted PPE blends. This is followed by
thermal evaporation of a 20-nm-thick film of 3-(4-biphenyl)-4-phenyl-5-tbutylphenyl-1,2,4-triazole (TAZ), a 30-nm-thick film of tris-(8-hydroxyquinoline)
aluminum) (Alq 3 ), and a 1-mm-diameter,
100-nm-thick
Mg:Ag (10:1 by mass)
cathode, with a 30-nm Ag cap. The spin casting and device manipulation during
18
fabrication is conducted in a dry nitrogen environment, with moisture and oxygen
content of less than 5 p.p.m. All EL measurements are done in air ...........
147
Figure 5.6 External quantum efficiency versus current density for the TPD/PPE
devices and the TPD control device shown in Figure 2. Inset shows the currentvoltage behavior for the various devices. The peak efficiency of the GP-PPE
device is r7 = 1.6% at 0.1 mA cm -2 and 7.5 V which corresponds to a luminance
efficiency of 3.3 cd A-1. The peak efficiency of the GO-PPE device is - = 1.1% at
0.1 mA cm 2 and 7.6V which corresponds to a luminance efficiency of 1.7 cd A-1 .
The ungrafted PPE devices had peak efficiencies of r = 0.006% and
0.0007%
at 100 mA cm -2 and 16.5V which corresponded
=
to a luminance
efficiencies of 0.013 cd A-1 and 0.001 cd A-1 for the O-PPE and P-PPE systems
respectively. The TPD control device had a peak efficiency of r = 0.8% at 8 mA
cm -2 and 7.5 V giving a luminance efficiency of 0.72 cd A-' .
.....................
149
Chapter 6
Figure 6.1 Normalized solution-state (a) and solid-state (b) photoluminescence
spectra of the various ungrafted PPEs. Inset, plots of the absorption spectra for
each case. Solution-state measurements were conducted in chloroform solution.
Solid-state measurements were obtained using polymer thin-films spin-cast onto
glass substrates from chloroform solution (2mg/mL). In all cases, samples were
excited at their respective absorption maxima (see Table 6.1).
...........
167
Figure 6.2 Normalized solution-state (a) and solid-state (b) photoluminescence
spectra of the various oxadiazole grafted PPEs. Inset, plots of the absorption
spectra for each case.
Solution-state measurements were conducted in
chloroform solution. Samples were irradiated with X = 385 nm light in order to
prevent excitation, and thus emission, from the oxadiazole grafts. Solid-state
measurements were obtained using polymer thin-films spin-cast onto glass
substrates from chloroform solution (2mg/mL). The films were all irradiated near
the oxadiazole absorption maxima (
= 318) with X = 320 nm light in order to
observe energy transfer from the oxadiazole grafts to the PPEs.
19
...........
169
Figure
6.3
Electroluminescence
spectra
(solid
line)
overlayed
with
the
corresponding solid-state photoluminescence spectra (dashed line) and solutionstate photoluminescence spectra (dotted line) of the various oxadiazole grafted
PPEs. Spectra for the all dialkoxy substituted PPEs G1-G3 are shown in plots
(a)-(c) while plots (e) and (f) correspond to pentiptycene containing PPEs G4 and
G5 respectively. Plot (d) shows the EL for devices containing ungrafted PPEs 4
(dotted line) and 5 (dot-dash line) for comparison.
.............................
171
Figure 6.4 External quantum efficiency versus current density for the oxadiazole
grafted PPEs. Inset shows the current-voltage behavior for the various devices.
The entirely dialkoxy substituted PPEs G1-G3 had peak efficiencies of r =
0.05%, 0.07%, and 0.11% respectively with corresponding luminance efficiencies
of 0.17 cd A-' , 0.21 cd A 1, and 0.22 cd A-1.
pentiptycene
containing
The peak efficiency
of the
= 0.23% at 12.2 mA cm 2 and 9.9V
PPE G4 was
corresponding to a luminance efficiency of 0.40 cd A 1' while the peak efficiency
of PPE G5 was i = 0.29% at 13.6 mA cm '2 and 11.4V corresponding
to a
luminance efficiency of 0.34 cd A-1.
173
...............................................
20
List of Schemes
Chapter 3
Scheme 3.1
Synthetic scheme of polythiophene (PT) graft copolymer
synthesis..........................................................................................
82
Scheme 3.2 The ATRP mechanism utilizing copper ion as a metal catalyst. .. 85
Scheme 3.3
Synthesis scheme for synthesizing PPE monomer with terminal
hydroxyl substituents.
........................................
..................
Scheme 3.4 Synthetic scheme for the ATRP grafting process.
.............
87
87
Chapter 5
Scheme 5.1 Chemical structures for the materials used in the TPD/PPE hybrid
OLED devices.
..........................................
................
140
Chapter 6
Scheme
devices.
6.1
Oxadiazole
grafted
PPE backbone
structures
used in PLED
..........................................................
163
Scheme 6.2 Synthetic scheme for the vinyl oxadiazole synthesis. ............
163
Scheme 6.3 Schematic of the oxadiazole grafting process. .....................
163
21
List of Tables
Chapter 3
Table 3.1
Polymer Characterization ..................
......................
90
Polymer Characterization. ..................
......................
113
Polymer Characterization. ..................
......................
140
Polymer Characterization. ..................
......................
164
Chapter 4
Table 4.1
Chapter 5
Table 5.1
Chapter 6
Table 6.1
22
Chapter
1
Development of a Quantum Efficient Solid-State
Poly(Phenylene-Ethynylene) System:
The Quest for the Elusive Blue OLED
1.1 Introduction - Display Technologies
Electronic, optical, and optoelectronic applications of organic materials
have emerged as important aspects of modern technologies that continue to
shape the information and communication of our society.
The ability to
synthetically tune the materials properties of organics, combined with the
relatively low cost of processing and excellent mechanical properties these
systems present, has ushered in a new era of semiconducting
device platforms in various commercial applications.
organic based
Presently, the most
promising short-term realization of marketable technology revolves around the
implementation of organic optical and optoelectronic systems in flat-panel display
technologies.
For decades display technology meant the implementation of cathode ray
tubes (CRTs). In the 1980s, the arrival of electronic games and cell phones
resulted in the spread of primitive light emitting diode (LED) and liquid crystal
displays (LCDs) that provided the advantage of more cost-effective, durable, and
lighter display platforms. With high-definition television (HDTV) now a reality,
there has been a burst of activity with new types of thinner, higher-resolution
screens based on LCDs, plasma, and even micro-electro-mechanical
(MEMS) being developed.
Organic based materials
systems
and systems, such as
molecular and polymeric organic light emitting devices (OLEDs and PLEDs), as
well as carbon-nanotube field effect displays, are now emerging as important
new directions for display technologies.
These new platforms present the
possibility for lower-cost and better performance than the established CRT, LCD,
24
and plasma displays. Many display manufacturing companies are utilizing these
systems in the hopes that they may become viable contenders for television and
information technology (IT) markets, as well as perhaps catalyze the
development of revolutionary new flexible displays that will address the needs of
the burgeoning mobility market.
1.1.A Liquid Crystal Displays
Typical LCDs make use of twisted nematic (TN) liquid crystals.
In a TN
display, nematic liquid crystal molecules are placed between two plates of glass
with microgrooves
at right angles to each other.
The molecules experience a
twist as shown in Figure 1.1. By placing cross polarizers around the LC cell, light
produced from a backlight traveling through the display will follow the twisted
orientation of the molecules and be transmitted. If an electric field is applied
across the TN display, the liquid crystal molecules tend to line up parallel to the
applied field throughout the material. The molecules are no longer able to twist
the light traveling through the display. Consequently, the cross polarizers
effectively block the light traveling through the display and the display appears
black. In this way, varying the voltage applied to the liquid crystals will determine
the intensity of light transmitted through the device. Finally, color filters placed
above the LC cell allow the system to generate a full color display.
25
4~~~~~"
I~~~
I~~~~~~~ic
l
,
'
t.I
1
XP
I
$
M, :
Figure 1.1 Schematic representation of a liquid crystal display. Adopted from
http://www.plasma.com/classroomlwhat is tft Icd.htm.
LCDs are currently the most dominant flat panel display platform.
can be made much thinner and more power efficient than CRTs.
They
They are also
much lighter and more durable than plasma-based flat-screen technology.
Furthermore, the maturation of LCD manufacturing continues to reduce the cost
of these systems and has been a major driving force for the implementation of
LCD technology in the portable electronics industry.
However, LCDs still suffer
from certain drawbacks. Since the light modulation in these systems involves the
movement of liquid crystal molecules under an applied field, they can suffer from
slow response times that result in "ghost" images when rendering video rate
images.
In addition,
the molecular
movement
can
be dependent
upon
temperature, which can pose problems in terms of stability in hand-held, portable
26
electronics.
Due to the fact that the display requires polarizers, LCDs have
restricted viewing angles; however, much progress has been made to reduce or
even eliminate this problem. Finally, the display itself is not emissive, requiring
the filtering of light through polarizers and color filters. This is an inherently
inefficient process whereby typically >50% of the light emitted from the backlight
is not used.
In an effort to alleviate the power consumption and energy efficiency
issues related to LCDs, research in the area of photoluminescent (PL) polarizers
has begun. By generating a material that emits polarized light (i.e. replacing or
augmenting the LCD backlight) the need for one of the cross polarizers would be
eliminated and result in an increase in the overall device efficiency. Furthermore,
color tuning of the polarized emitter would also eliminate the need for color
filtering, further enhancing the device efficiency.
Various organic materials
systems have been explored for use as photoluminescent polarizers. Chapter 4
discusses some of the latest research in this area.
1.1.B Organic Light Emitting Devices
Organic light emitting devices have the potential to provide better
performance at lower costs than LCDs.
OLED displays are visually more
attractive than LCDs. They are able to display full-color, video-rate imagery with
much faster response times (<< 1 microsecond) even at low temperatures,
exhibit wider viewing angles (based on their Lambertian emission), and emit
brighter, more color-saturated emission. Today, the OLED color gamut covers
27
70-75% of the National Television System Committee (NTSC) spectrum while
typical LCD displays cover only 40-60%.1
Improvements in new materials and
device architectures have also led to operating lifetimes of tens of thousands of
hours. In addition, OLEDs are an inherently emissive technology - they emit
light as a function of their electrical operation - eliminating the need for
backlights, filters, and polarizers, and thus have the potential to be more energy
efficient than comparable LCDs. OLEDs are also intrinsically lower in cost to
manufacture than LCDs, but today that manufacturing capability is less mature.
As capacity increases and manufacturers reach yield targets, this advantage
should emerge to benefit the marketplace. Finally, the much thinner and lighter
weight form factor is another highly desirable advantage of OLED technology,
especially for mobile electronic products.
Commercial OLED products can already be seen in today's marketplace.
In the late 1990s, Tohoku Pioneer Corporation introduced the first OLED display
product into an automotive audio component.
Companies such as Sony
Corporation and Seiko Epson Corporation have unveiled full color display
prototypes, which are shown in Figure 1.2. Seiko Epson used ink-jet printing to
deposite the organic material used in the display (Figure 1.2 Bottom).
Other
manufacturers, such as RiTdisplay Corporation, Philips Mobile Display Systems,
Samsung OLED, and Teco Optronics Corporation have begun to launch OLED
products for use in cell phones, digital still cameras, games and other consumer
products. Examples of products incorporating polymer based PLED displays are
shown in Figure 1.3.
28
Figure 1.2 Top: Sony display which shows off the remarkably thin monitor that
can be developed
with OLED technology.
(Adopted from Universal
Display
Corporation website.) Bottom: Inset of a 40-inch Seiko Epson prototype OLED
display.
Schematic of ink-jet printed OLED display.
Adopted from Epson
website: http://www.epson.co.jp/e/newsroom/news_2004_05_18.htm
29
I
re
i
*0
^%
00'~~~~~'
Figure 1.3
Clockwise from left:
1) Personal MP3 music player launched by
Delta Electronics that incorporates a green PLED display for the control and
indication of all system operations.
2) Philips has developed the new 639 mobile
telephone using 'Magic Mirror' technology. The clam-shell design provides a
mirror for checking personal grooming when it is closed. Incoming calls are
indicated by the display showing through the mirror - a highly effective use of a
secondary display.
3) A men's shaver sold by Philips uses a PLED display
showing information including shaving time remaining before recharge.
MicroEmissive
Displays has developed the NHJ 3-in-1 Camera.
camera, movie camera and MP3 player all in one.
4)
It is a still
It has 3.2M pixels, USB
connection and a PLED display which has one quarter the power consumption of
an
LCD.
Adopted
from
Cambridge
http://www.cdtltd.co.uk/technoloy/42
.asp.
30
Display
Technologies
website:
1.2 OLEDs - Towards Full Color Displays
Inorganic electroluminescent materials have been known for many years,
and LEDs based on these materials have been commercially available since the
early 1960s.
At about the same time, in 1965, electroluminescence
molecular organic materials was discovered where 50
(EL) from
m to 1mm crystals of
anthracene were used to generate EL with an efficiency of 8%.2 Despite the
impressive efficiency, the operating voltages were impractical, requiring 50-1000
V. It was not until 1987, the first fabrication of vacuum deposited OLEDs, that
OLED devices exhibiting brightnesses and operating voltages suitable for
commercial flat panel display applications were achieved.3
Soon thereafter, in 1990, electroluminescence from conjugated polymers
was demonstrated by Burroughes et a. 4
another added advantage.
The polymeric systems presented
Inorganic semiconductors and organic small
molecule materials require relatively expensive high vacuum sublimation or vapor
deposition techniques, which are not well suited to fabrication of large-area
devices. Polymers, on the other hand, can be readily deposited from solution as
thin films
over
large areas
by spin-coating
or
printing techniques.5 ' 6
Consequently, considerable research has been conducted over the past decade
in the area of polymer based organic light emitting devices (PLEDs).7 - 9 Chapters
6 and 7 present some of the latest results in new polymer/small molecule hybrid
OLEDs (Chapter 6) and PLEDs (Chapter 7).
31
1.3 The Elusive Blue OLED
1.3.A Overview
In order for OLEDs to become viable candidates for use in flat panel
displays, they must demonstrate red, green, and blue (RGB) emission, with good
color saturation
brightness,
(defined by CIE coordinates
high efficiency
- shown in Figure 1.4),10 high
(i.e., low operating voltage and current), and long
operational stability or lifetime. Minimal performance requirements for indoor and
portable display applications are brightnesses of - 100 cd/m2 at an operating
voltage between 5 and 15 V and a continuous operational lifetime of at least
10,000h. l
The eventual success of OLED technology may also be dictated by the
overall applicability of the platform to capture other potential markets. One area
involves the use of OLEDs as a large-area, conventional light source.
Development of high power blue OLEDs opens up a novel route for making white
light LEDs. 2 White light can be generated by mixing light from separate red,
green, and blue LEDs, but a more compact route involves exciting phosphors
with high-intensity blue LEDs. There are many applications for white light LEDs,
such as backlights for full color liquid crystal displays and replacements for
conventional light bulbs and lamps.
32
__
I
U.9
I
520
:
.I
I
I
'
I
II
--
--
a
0.8
B
I
0.7
Ill
!
0.6
=.
!
0.5
mB
.
I
>,
0.4
II
0.3
II
0.2
!
0.1
JB
II
0.0
JB
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
X
Figure 1.4 A typical CIE diagram. CIE stands for the Commission Internationale
de I'Eclairage, which is an international committee for the establishment of color
standards. The CIE model and the CIE Chromaticity Diagram define the different
variations of color. The dark triangle overlayed on the CIE diagram represents
the various colors that can be achieved using a standard CRT computer monitor.
The CIE coordinates for red, green, and blue phosphors used in a standard CRT
monitor are (x = 0.61; y = 0.34), (x = 0.25; y = 0.62), and (x = 0.15; y = 0.063),
respectively.
33
Unfortunately,
blue EL has been the slowest in development,
both in
traditional inorganic materials as well as organic small molecule and polymeric
systems.
In order for OLEDs to become a mature, marketable technology,
efficient, wide band gap, blue emitter materials and devices must be developed.
The following sections will detail the current prospects for blue LEDs in a wide
range of material systems.
1.3.B Inorganic Systems
Only until recent developments using group III nitrides, specifically gallium
nitride (GaN) and its alloys with indium and aluminum, has the production of
inorganic UV and blue-light-emitting diodes and lasers been possible. Epitaxial
growth of GaN onto lattice-mismatched substrates with minimal defect density
was made possible by metal organic vapor phase epitaxy (MOVPE) and has
paved the way for commercial development.1 2 However, the fabrication process
is quite expensive and these devices are fragile and still suffer from stability and
lifetime issues.
1.3.C Small Molecule OLED Systems
Wide band gap EL has been achieved in small molecule OLEDs using
materials such as N,N'-diphenyl-N,N'-bis(3-methylphenyl)-[1,1'-biphenyl]-4,4'diamine (TPD) shown in Figure 1.5a.
One of the major drawbacks
of this
particular system has been a low glass transition temperature (Tg) causing thin
films of this material to crystallize over time. Attempts to raise the Tg have led to
34
the development of N,N'-Di-naphthalen-1-yl-[N,N'-diphenyl-biphenyl]-4,4'-diamine
(a-NPD)
(Figure 1.5b).1 3
These devices still suffer from low efficiencies and
reduced operational stability.
a
Figure 1.5
b
Chemical structures of common wide-band gap small molecule
materials for blue color emission. The structure for TPD is show in a, while that
of a-NPD is shown in b.
Attempts to improve upon the operational efficiency of these OLED
devices has sparked the development of blue phosphorescent emitters, such as
Flrpic shown in Figure 1.6, that take advantage of efficient emission from the
more abundant triplet states (in addition to singlet states) that are produced
during device operation (see Section 2.2.A).1 4 1 5
Unfortunately, present
materials, such as Flrpic, do not exhibit good color saturation, resulting in a more
"sky blue" emission. Another major challenge for these systems is finding a
suitable wide band gap host. The triplet state for typical host materials is often
lower in energy than the triplet state of the phosphorescent guest making energy
transfer to the guest endothermic and thus fundamentally limiting the operating
efficiency of the devices.
35
L
Figure 1.6 Chemical structure of a blue phosphorescent emitter, Flrpic.
1.3.D Organic/Inorganic Hybrid Systems
In an effort to improve upon the color saturation and stability problems in
OLED systems, the incorporation of inorganic nanocrystals into organic/inorganic
hybrid devices has recently been explored.
The synthesis of semiconductor
quantum dots (QDs) with narrow size distributions and high luminescence
efficiencies has enabled the development of new QD light emitting devices (QDLEDs), which utilize an emissive species with size tunable emission that spans
the visible region of the spectrum.
Efficient red and green-light-emitting
QD-LEDs with (CdSe)ZnS core-shell
nanocrystals with a full-width-at-half-maximum
(FWHM) of approximately 30 nm
have been realized (see Figure 1.7).16,17 However, blue emitting CdSe has
proved elusive, requiring quantum dots smaller than 2 nm, which are difficult to
synthesize with narrow size-distribution and good quantum efficiency. Recently,
the development of (CdS)ZnS nanocrystals with luminance efficiencies of 2030% along with narrow blue emission from 460 to 480 nm and a FWHM < 28 nm
36
(Figure 1.7) has been achieved. 1 8
These new systems, incorporated
into the
appropriate device architecture, may provide new, stable, blue QD-LEDs.
I
I'
i
400 450
500 55
6
50
400
450 500 550
800
650
c
sQ
c-
.N
E
z
400
500
600
Wavelength [nm]
400
500
600
Wavelength [nm]
3xn)D
''
MX
.S.,
Figure 1.7
-4(w
W
Ir,
EL from QD-LEDs with (a) and without (b) hole-blocking
layers.
Plots c and d display the ability of these systems to produce red and green EL,
while plot e displays new blue (CdS)ZnS quantum dot PL and initial QD-LED
1 6 '1 8
studies with this material.
1.3.E Polymer Based LEDs or PLEDs
Most of the success in the development of PLEDs revolves around the
use of poly(phenylene-vinylene) (PPV) materials which typically emit in the green
region of the spectrum (Figure 1.8a). Consequently, attempts to generate blue
EL from PLEDs have been made using PPV oligimers and PPV-type materials
37
with interrupted conjugation (see Figure 1.8).19 These systems suffer from the
drawback that emission usually occurs from the more conjugated segments and
is therefore broadened and red-shifted.
a
MeO
OMe
o-I
CH)
O~~~e~
N-N
H3
O
H
d
Figure 1.8 Chemical structures of PPV type systems used in PLEDs. The
standard
PPV structure
is shown in a, while PPV systems with interrupted
conjugation are shown in b, c, and d.1 9
The high HOMO-LUMO gap required for blue emission has brought the
focus
of research
systems,
shown
in blue
in Figure
emitting
1.9.
PLEDs to poly(para-phenylene)
PPP based
PLEDs have
achieved
(PPP)
high
efficiencies.2 0 However, strong vibrational coupling in these systems causes the
38
emission profiles to be broadened, reducing the degree of color saturation.
Furthermore, due to the planar conformational arrangement often adopted by
these systems in the solid state (especially enhanced after annealing), the device
operation is unstable with yellow-emission observed over time due to interchain
and t-stacking
interactions. 1 9
OCloH21
f~~t~~
a
Figure 1.9
b
R CH 1 3
R = CH 3 0CH 2CH
R = C8 H1 7
Examples of various PPP structures.
2
C H 2CH
2
A fluorine unit has been
incorporated into b.1 9
1.4 Poly(Phenylene-Ethynylene)
(PPE)
Another fully conjugated polymer system, poly(phenylene-ethynylene)
(PPE), exhibits high solution state quantum yields as well as narrower emission
profiles than both PPVs and PPPs. The band-gap of these systems can also be
tailored to achieve a more blue emission relative to PPVs.1 9 To date, however,
PPEs have received little attention as potential OLED materials due to reduced
emission efficiencies in the solid state, which has been attributed to aggregation
phenomena facilitated by the long persistence length2 1 and strong n- stacking of
the PPE polymer backbone.
Furthermore, the acetylene linkage in PPEs limits
the redox properties2 2 of these polymers contributing to a large potential barrier
39
to charge injection, especially for holes.
In this section we will discuss the
current status of PPE based LEDs, presenting the seminal work in the literature.
To conclude, we will entertain possible improvements that will provide motivation
for the research in this thesis. The goal will be to develop new wide-band gap
PPE based LEDs that will utilize the efficient, narrow, blue emission inherent in
solution-state PPE systems, ultimately providing a new platform for the
generation of blue EL.
1.4.A Electroluminescence from PPEs
Initial studies on PPE based LEDs were conducted
workers.23
26
by Shinar and co-
They used dialkoxy substituted PPE systems that exhibited yellow
emission with maxima around 600 nm and disappointing operating efficiencies.
Long-range order of the materials in the solid-state was assumed to lead to the
formation of excimer complexes which provide non-emissive decay channels for
the excited states.
By varying the side chain substitution
along the PPE
backbone, Weder et al. were able to disrupt the coplanar arrangement of the
conjugated polymer backbones resulting in relatively higher solid-state quantum
yields.2 7
Incorporation
of these materials into LED devices resulted in more
promising efficiencies and operating voltages (dependent on cathode metal),
which are shown in Figure 1.10.28 Unfortunately, these systems still displayed
broad emission profiles characteristic of the strong aggregation phenomena of
PPEs (Figure 1.10).
40
R
I
,I
R = O
'
i
11
,, '
,
I,
/
O-OPPE:80
-
0
-~~~~~
*
2
....
- v
"
,
EHO-OPPE:
r%
-
rI = U
-
r
'
-
_ /
300
500
400
600
700
o00
Wavelength[nmn
Table1
EL characrcritic, r' substitutedpoly(p-phenylene :thynylem ). EtIO-OPPEand O-OPPE. singl-iaye:LEDs
Emniringiayer
EHO-OPPE
EHO-OPPE
EHO-OPPE
O-OPPE
O-OPPE
Cathodematerial
A:
Ca
C:
A!
Ca
EL hreaholdvoltagc (V)
External quantumefficiencyfr%
Max. brightaess cd/n:
10.8
0.035
80
14.5
0.023
8
19
0.015
33
!1.0
0.032
142
0.020
35
Figure 1.10
Chemical structure, absorbance
SO
(solid line), PL (dotted line), EL
(dashed line), and operational analysis of PPE-based LEDs (Cathode/OOPPE/ITO).28
Further improvements on the system above were made in an attempt to
balance charge injection and isolate the region of exciton recombination away
from the organic/electrode interface.1 1
This was accomplished by introducing
other charge transport species, such as a hole-transporting layer of poly-TPD
and an electron-transporting and hole-blocking layer of spiroquinoxaline (shown
in Figure 1.11).29 While these heterojunction devices displayed improved
operation (operating voltage, efficiency, etc.), the authors did not present any EL
spectra, suggesting that these changes were unable to improve upon the overall
color saturation achievable in these particular PPE systems.
41
I
0
0
--
//- O
-/ (
/
/.
./
OPPE
.
-.
-.
N--'
.,~~~
~~
::
:ri
~
- -,.-'~I..,*- [
.
\poly-TP
poly-TPD
i.:\,:.
.1
-
....
.
1:'"'1 '"
spiro-Qux
Device 2
Device 1:
Device 3.
Device 4:
Ir-
>-2.4
t>:
M
-3.6
4.2
4.8
_
-3.6
3
4.2
4.2
Al
4.8
4.8
A
FI
2
ITO
ITO
-5.1
'
4.2
Al
4.8
ITO
I 8
-5.8
-5.8
-2 8
t
L_
5.8L
-6.5
Figure 1.11
Top: Structures of EHO-OPPE, a poly(2,5-dialkoxy-p-phenylene
ethynylene) derivative, poly(N,N'-diphenylbenzidine diphenylether) (poly-TPD),
and a tetrameric spiroquinoxaline (spiro-qux). Bottom: Schematic view of the
device structures investigated: 1) single-layer EHO-OPPE, 2) two-layer polyTPD/EHO-OPPE,
3) single-layer
blend, and 4) blend
layer with additional
electron-transport/hole-blocking layer (ETHBL), and of their corresponding
energy-level diagrams; the band-edge energy levels were determined by cyclic
voltammetry (CV).2 9
42
Attempts to achieve further blue shifted emission in PPE systems have
Incorporation of a 2,5-pyridinediyl
also been conducted.
the PPE backbone
emission
(410
results in blue-green
nm)
was
obtained
unit (Figure 1.12a) into
emission (XMAX= 480 nm). 30
from
Blue
poly(3,4-dialkyl-1,6-phenylene-
ethynylene)s, the structure of which is shown in Figure 1.12b.31 Steric twisting of
the polymer chain, as well as electronic interactions, reduce the effective
conjugation length and thus increase the HOMO-LUMO gap in these systems,
allowing for blue emission.
Unfortunately, these same steric interactions
increase the vibronic coupling of the system causing undesirable spectral
broadening (see Figure 1.13b).
A similar synthetic approach was taken by Chu et a/.32 Instead of a 1,6- or
ortho-phenylene
linkage in the PPE backbone as shown in Figure 1.12b, Chu et
al. introduced a 3,4- or meta-phenylene linkage into a standard para-phenyleneethynylene system (Figure 1.14). The meta-linkage breaks conjugation within
these systems and introduces a bent angle along the polymer backbone. The
authors noted that this modified the chain stiffness, leading to a simultaneous
improvement in both solubility and luminescence efficiency. This system was
explored in an electroluminescent device and exhibited an external quantum
efficiency of 0.013%.
Although this synthetic modification lead to improved
luminance efficiencies in the solid-state, it was unable to completely eliminate
aggregation phenomena, as seen from the broad PL and EL spectra shown in
Figure 1.14. Moreover, similar to the ortho- system, the meta-linkage probably
43
also introduced increased vibronic coupling that lead to further broadening of the
emission profile.
H 2 5 C12
oC 12 H25
C1 2 H 2
5
C12H25
b
a
Figure 1.12 Examples of poly(phenylene ethynylene)s. A 2,5-pyridinediyl unit is
incorporated
in a while b is a poly(3,4-dialkyl-1,6-phenylene
ethynylene).
. ·-_
1.2
1
0.
J
..9, CJ'g
,
e0.8
e-
_
0.6
'0.5
90.4
0.4
0.6
'E
-j.0o
E 0.2
0
01
n
400
500
600
700
wavelength(nm)
3Sf
400
450
wavehng
sOG
55W
<(nm)
b
a
Figure 1.13 Electroluminescence from Al/PPE/ITO devices. Plot a shows the
results for the PPE system in Figure 1.12a while plot b shows the results for the
PPE in Figure 1.12b. 30 '3 1
44
I
..
5
( a: Rl= H, R = H; b: RL=OC4Hg, R2=CH3 )
1,0
0.8
Ci
p
.4
4_
0.6
10
Q
N
0.4
0
z
z
02
0.0
40W
450
500
550
600
850
700
Wavelengti (nm)
Figure 1.14
Top:
Chemical structure of the meta-phenylene ethynylene
polymer, 5a and 5b. Bottom: Solid-state PL of polymer 5b and EL spectra for a
ITO/PEDOT/5b/Ca/AI
LED at 12 and 17 V.32
45
Another synthetic approach
has been used by Bunz and coworkers. 2 2
Planarity in the main chain alone can effect a significant decrease in the HOMOLUMO gap (rotation of phenylene
ethynylenes
< 1 kcal/mol).3 3
Therefore,
intrachain effects may be involved in the red-shift observed when going from
solution to the solid-state in PPE systems. However, planarity of the polymer
backbone in the solid state also allows the phenyl rings of neighboring polymers
to pack closely (according to X-ray powder diffraction)34 and thus enables
interchain interactions that can also significantly red-shift the emission of these
systems.
Consequently, in order to achieve blue solid-state emission, they
attempted to prohibit both interchain interactions and planarization, thus forcing
emission from individual polymer chains, as is the case in solution.
To this end,
they synthesized PPE copolymers possessing sterically obtrusive aryl units,
namely,
3,7-di-tert-butylnaphthalene,
in
the
hopes
that
increased naphthalene content in the polymer backbone would result in
diminished planarization and aggregate formation (Figure 1.15a).
The observed external quantum efficiencies for these devices were 0.10.15%, much higher than those seen previously. However, the overall luminance
efficiencies for these devices were quite poor, 0.1 cd A'1 for polymer 5b and 0.02
cd A1 for polymer 5c (Figure 1.15). In addition, only one of the polymer systems
(5c) was able to generate predominantly blue emission. Moreover, all of these
devices exhibited the characteristically broad spectral features that plagued the
previously reported systems.
46
a
Schenwe . Syntihesisof Naphtlulene Monomer
K10.
Pd(Ph) 2C.i:
Cul. pipend
X',
XI,?
I i
--r
l
ITOIo
Ir
fOIPPIIu
0
°
10
1o0
i
0
Cj
a
_.a
I--
-----
O
3
10
16
,
_
0e
I
30
-
34
34
Voltage,V
Schenle 2. CoolInlerization
via Alkxne
Mletathlleii
,-.
-
C
JV1
C,1 li ,
fl
d
from toluana
rTE fromnC"C I olven
(
I
0
1
a
a)C0 4 IrVS
ebQ,
.DW,.4
*
"~~~~
to~~~~brU-b*-y
0
o
hll
I
-PE
1.0
o00
00
see
7oo
WavelengthnmJ
I
e
1..
ITOIPET
-
6c
LIF
1
TOCEDTb
I b LlFI Al
0.1
"l-i
b
S
260
I
i
O.0
CL
-Sd
u
g 0.4
.3
200
c
150-
0.2
100
0.3
3 so0
400
4s0
00o
io650
0
50
Wavelngth. nm
0
400
450
500
550
Wavelenlh (nm)
eO
Figure 1.15 The synthesis of the naphthalene unit and naphthalene containing
PPEs is shown in a.
Plot b shows the PL for PPEs containing
increasing
amounts of naphthalene unit (5a-5d where 5d contains the largest naphthalene
content). Plot c reveals spectrally integrated EL emission intensity for a PPEbased OLED with (solid symbols) and without (open symbols) a PEDOT hole
injection layer while d shows the normalized EL spectra of hexyl-dodecyl-PPE
where the emissive layer is spin cast from toluene and from CHCI3/toluene (1:1).
Plot e shows the normalized EL spectra for devices containing PPEs 5b and
5c. 2 2
47
It is reasonable to assume that the bulky tert-butyl naphthalene units
helped to prevent aggregation and interchain interactions in these systems.
However, the "break" in linearity introduced in the polymer backbone by the
presence of the naphthalene units most likely increased the vibrational manifold
as well as induced electronic perturbations in the conjugation.
As a result, a
broader emission profile was observed, similar to the 3,4-dialkyl systems
discussed above.3 1
1.4.B Conclusions
Insight gained from the previous work on PPE-based LEDs discussed
above leads to seemingly conflicting system requirements. In order to benefit
from the efficient, narrow, wide-band gap emission characteristic of PPE
systems, intrachain planarization as well as interchain interactions must be
minimized in the solid-state. Such a scenario will allow for a PPE thin-film that
will preserve its solution state emission and quantum efficiency due to isolation of
individual PPE chains and the elimination of non-radiative pathways.
However, this same scenario requires disordered films of conjugated
polymer, which will inevitably limit their conduction properties due to charge
trapping.
This
behavior has been
explored extensively when
using
polythiophene (PT) systems for organic field effect transistor systems and helps
to explain why regio-regular, well-ordered, tightly-packed PT films exhibit
superior conduction properties.3 5 This, in conjunction with the fact that oxidation
and reduction of the acetylene linkage in the PPE backbone typically leads to
48
material degradation, helps to explain why the previous PPE-based LEDs
displayed poor operating efficiencies and spectral purity.
Consequently, the development of a successful PPE-LED system
presents an interesting challenge - we are left with a scenario where optimization
of the thin-film emission properties of a particular PPE material will result in
further reduction of the charge transport properties.
The following chapters
present our particular attempts to provide solutions to this very problem. We
have chosen to focus on implementing modular materials properties into a new
PPE system whereby we can exploit the desirable emission characteristics of
PPEs while introducing increased materials compatibility. The realization of such
a system allows for the incorporation of additional materials that can provide the
necessary charge conduction properties while still benefiting from enhanced
solid-state optical properties of the PPE. The result is efficient, color saturated,
blue electroluminescence from PPE-based LEDs that has not been achieved to
date.
1.5 Thesis Outline
An introduction to conjugated polymers followed by the basic theory
behind the luminescence of conjugated polymers, such as PPEs, will be
presented in Chapter 2. This will provide a fundamental background for the
topics that will be discussed in the remaining chapters. Chapter 3 will detail the
synthesis of a new grafted PPE system as well as discuss the enhanced solidstate photophysical properties exhibited by the new system.
49
The next three chapters will detail the various optical and optoelectronic
applications of the grafted PPE. Chapter 4 introduces a new guest/host system
for the generation of polarized photoluminescence whereby the grafted PPE is
the optically active guest material.
The increased materials compatibility
introduced to the PPE system via the grafting process utilized in this chapter is
further showcased in Chapter 5. Introducing a small molecule hole-transport
host material enables the development of a new PPE based LED system that is
able to achieve efficient, color-saturated blue electroluminescence. In Chapter 6,
a commonly used charge transport moiety is directly grafted to the PPE, enabling
an entirely polymeric, single-layer LED which exhibits spectrally pure blue EL,
superior to any of the previously reported PPE-LED systems.
Finally, the dissertation will conclude with a summary of the results
presented, as well as an outline of additional areas in the field that warrant future
research efforts.
50
1.6 References
[1] Universal Display Corporation Website:
http://www. universaldisplay.com/marketplace.htm
[2] Helfrich, W.; Schneider, W. G. Phys. Rev. Lett. 1965, 14, 229.
[3] Tang, C. W.; VanSlyke, S. A. Appl. Phys. Lett. 1987, 51, 913.
[4] Burroughes,
J. H.; Bradley,
D. D. C.; Brown, A. R.; Marks,
R. N.; Mackay,
R.
N.; Friend, R. H.; Burns, P. L.; Holmes, A. B. Nature 1990, 347, 539.
[5] Friend, R. H.; Gymer, R. W.; Holmes, A. B.; Burroughes, J. H.; Marks, R. N.;
Taliani,
C.; Bradley,
D. D. C.; Dos Santos,
D. A.; Bredas,
J. L.; Logdlund,
M.;
Salaneck, W. R. Nature 1999, 397, 121.
[6] Friend, R. H. Pure Appl. Chem. 2001, 73, 425.
[7] Greenham, N. C.; Friend, R. H. Solid State Phys. 1995, 49, 1.
[8] Rothberg, L. J.; Lovinger, A. J. J. Mater. Res. 1996, 11, 3174.
[9] Sheats, J. R.; Antoniadis, H.; Hueschen, M.; Leonard, W.; Miller, J.; Moon, R.;
Rottman, D.; Stocking, A. Science 1996, 273, 884.
[10] Colorimetry, 2nd. ed.; CIE: Vienna, 1986; Vol. Publication No. 15.2.
[11] Bulovic, V.; Burrows, P. E.; Forrest, S. R. Semiconductors
and Semimetals
2000, Ch. 5, Vol. 64, pg. 256.
[12] Martin, R. Chem. Ind. 1999, 5, 173.
[13] Kato, T.; Mori, T.; Mizutani, T. Thin Solid Films 2001, 393, 109.
[14] Tokito, S. J. Photopolym. Sci. Tech. 2004, 17, 307.
51
[15] Ren, X.; Li, J.; Holmes, R. J.; Djurovich, P. I.; Forrest, S. R.; Thompson, M.
E. Chem. Mater. 2004, 16, 4743.
[16] Coe, S.; Woo, W-. K.; Bawendi, M.; Bulovic, V. Nature 2002, 420, 800.
[17] Coe-Sullivan, S.; Woo, W-. K.; Steckel, J. S.; Bawendi, M.; Bulovic, V. Org.
Elec 2003, 4, 123.
[18] Steckel, J. S.; Zimmer, J. P.; Coe-Sullivan, S.; Scott, N. E.; Bulovic, V.;
Bawendi, M. G. Angew. Chem. Int. Ed. 2004, 43, 2154.
[19] Kraft, A.; Grimsdale, A. C.; Holmes, A. B. Angew. Chem. Int. Ed. 1998, 37,
402.
[20] Yang, Y.; Pei, Q.; Heeger, A. J. J. Appl. Phys. 1996, 79, 934.
[21] Cotts, P. M.; Swager, T. M.; Zhou, Q. Macromolecules 1996, 29, 7323.
[22] Pschirer, N. G.; Miteva, T.; Evans, U.; Roberts, R. S.; Marshall, A. R.; Neher,
D.; Myrick, M. L.; Bunz, U. H. F. Chem. Mater. 2001, 13, 2691.
[23] Swanson, L. S.; Lu, F.; Shinar, J.; Ding, Y. W.; Barton, T. J. Proc. SPIE
1993, 1910, 101.
[24] Shinar, J.; Swanson, L. S. Proc. SPIE 1993, 1910, 147.
[25] Swanson, L. S.; Shinar, J.; Ding, Y. W.; Barton, T. J. Synth. Met. 1993,
55,
1.
[26] Chen, W.; Maghsoodi,
I-.; Barton, T. J.; Cerkvenik, T.; Shinar, J. Polym.
Prepr. (Am. Chem. Soc., Div. Polym. Chem.) 1995, 36, 495.
[27] Weder, C.; Wrighton, M. S. Macromolecules 1996, 29, 5157.
[28] Montali, A.; Smith, P.; Weder, C. Synth. Met. 1998, 97, 123.
52
[29] Schmitz, C.; Posch, P.; Thelakkat, M.; Schmidt, H.-W.; Montali, A.; Feldman,
K.; Smith, P.; Weder, C. Adv. Funct. Mater. 2001, 11, 41.
[30] Tada, K.; Onoda, M.; Hirohata, M.; Kawai, T.; Yoshino, K. Jpn. J. Appl. Phys.
1996, 35, L251.
[31] Yoshino, K.; Tada, K.; Onoda, M. Jpn. J. Appl. Phys. 1994, 33, L1785.
[32] Chu, Q.; Pang, Y.; Ding, L.; Karasz, F. E. Macromolecules 2002, 35, 7569.
[33] Moore, J.S. Modern Acetylene Chemistry
1995, Stang, P., Diederich, F.,
Eds.; VCH: Weinheim, Germany.
[34] Bunz, U. H. F.; Enkelmann, V.; Kloppenburg, L.; Jones, D.; Shimizu, K. D.;
Claridge, J. B.; sur Loye, H. -C.; Lieser, G. Chem. Mater. 1999, 11, 1416.
[35] McCullough,
R. D.; Williams,
S. P.; Tristram-Nagle,
Ewbank, P. C.; Miller, L. Synth. Met. 1995, 69, 279.
53
S.; Jayaraman,
M.;
54
Chapter 2
Conjugated Polymer Systems and the Basic Theory of Organic
Luminescent Phenomena
2.1 Introduction - Conjugated Polymers
Inorganic semiconductor devices, such as transistors and light emitting
devices (LEDs), have been critical in the development of the electronic and
photonic technologies that have had a tremendous impact on the information age
of the 2 0th century.
Over the past decade, these industries have expanded their
materials base to organic materials, in particular 7t-conjugated small molecules
and polymers. These materials combine the optical and electronic properties of
semiconductors with the advantage for low-cost, synthetic tunability, ease of
processing, and mechanical flexibility of plastics.1
Polyacetylene
'
§ 2-
:-
H
/~~3
Polyphenylene
Polypyrrole
·
\Poy(phenylene-vinylene)
H
yPolythlophene
--
Poly(phenylene-ethynylene)
Figure 2.1 Three repeat units of important conjugated polymer structures.
Conjugated polymers, are extended structures consisting of covalentlybonded molecular units separated by alternating saturated single bonds and
unsaturated double or triple bonds. Chemical structures of several important
conjugated polymers are shown in Figure 2.1. The unsaturated bonds are
stronger than the single bonds due the presence of additional
-bonds, and
consequently, are shorter in length. The bond alternation of short and long
56
bonds in the polymer backbone causes a lattice distortion. This results in a gap
in the electron density of states through the electron-lattice coupling, also known
as Peierls distortion.2 A schematic representation of Peierls distortion is shown
in Figure 2.2. All states below the gap are occupied and form the valence band,
while the states above the gap are empty and form the conduction band.
a) I a.
.
.
.
.
L.
EF
c
N ())
b)
0
I
I
-
kF
r/a
k
a
air
EF
EF
rr/2a
nFa
N (k)
,da
Ha k
Figure 2.2 Peierls distortion of an isolated chain of equidistant atoms. Crystal
lattice, electron density of states and electron dispersion relation without
distortion (a) and with distortion (b). 3
This gap in the electronic density of states can be illustrated using a
simple example of polyacetylene (Figure 2.3).
From a molecular orbital
standpoint, linking two CHo radicals to a (CH) 2 pair (connected by a double bond)
results in a linear combination of their respective molecular orbitals forming
57
bonding :c and antibonding * orbitals. Continuing this process and generating
the macromolecular polymer structure causes the 7t and * orbitals to split into
bands.
Thus, the top of the 7c-band or valence band is called the highest
occupied molecular orbital (HOMO) and the bottom of the n*-band or conduction
band is called the lowest unoccupied molecular orbital (LUMO).
A similar
interpretation can be extended to the other conjugated polymer systems shown
in Figure 2.1. The semiconducting properties of the polymers are characteristic
of the band gap energy associated with the HOMO-LUMO gap, which can be
determined by optical absorption.
metallic state
-
......'
(electrons
delocaluzed)
Peierts transition
insulating state
(conjugatred
double bonds)
Figure 2.3 Bond alternation in leads to Peierls gap. Overlap of the pz orbitals of
the double bonds in polyacetylene results in the formation of a delocalized electron system along the polymer backbone.
Bonding and antibonding n
orbitals form the equivalent of conduction and valence bands, responsible for the
metallic and semiconducting character of these materials.3
Conjugated polymers show semiconductor properties that differ from
those of conventional inorganic semiconductors.
When the polymer chain is
excited optically, an exciton is formed whereby the electron and hole that are
created can be very strongly correlated. The extended t-electron bonding of the
58
polymer chain (as well as its neighboring chains in the solid state) undergoes
reorganization in the vicinity of the excitation allowing the exciton to diffuse
through the system. This reorganization slightly lowers the overall energy of the
system as shown in Figure 2.4a. The system can then relax to the ground state
by emitting light, also known as photoluminescence (PL).
Under direct charge injection (Figure 2.4b), electrons are injected into the
LUMO, forming radical anions (also known as a negative polaron), while holes
are injected into the HOMO forming radical cations (also known as a positive
polaron). The resulting charges migrate from one polymer chain to the next
under the influence of the applied electric field. When a radical anion and a
radical cation combine on a single conjugated segment, neutral, excited singlet
and triplet states are formed. The singlet states can then radiatively recombine
resulting in electroluminescence
(EL).
The next sections discuss the basic theory of the luminescence
phenomena observed from conjugated organic systems. These areas provide
the fundamental background for the research discussed in the remaining
chapters.
59
a)
LUMO
_
""VV
~
hv
hv,
-
HOMO
singlet
excited state
b)
) Cathode
+e
-
M M
radical
anion
singlet
excited state
radical
cation
Ai ode
oe-
+
Figure 2.4 a) Irradiation of a fluorescent polymer excites an electron from the
HOMO to the LUMO. In a typical conjugated polymer, two new energy states are
generated upon relaxation within the original HOMO-LUMO energy gap and are
each filled with one electron of opposite sign (singlet excited state). The excited
polymer may then relax to the ground state with emission of light at a longer
wavelength than that absorbed (photoluminescence).
b) In a polymer LED,
electrons are injected into the LUMO (to form radical anions) and holes into the
HOMO (to form radical cations) of the electroluminescent polymer. The resulting
charges migrate from polymer chain to polymer chain under the influence of the
applied electric field.
When a radical anion and radical cation combine,
generating an exciton-polaron, singlet and triplet excited states are formed, of
which singlets can radiatively combine, emitting light.4
60
2.2 Luminescence in Organic Materials
Luminescence is the phenomena whereby the emission of light occurs
from a substance due to the radiative recombination of electronically excited
states. The processes of excitation and emission of light within an organic
material are often illustrated by a Jablonski diagram.5
A typical Jablonski
diagram is shown in Figure 2.5. The singlet ground, first, and second electronic
Each of these electronic
states are represented by So, S, and S 2, respectively.
energy levels contain a number of vibrational energy levels, denoted by 0,1,2,etc,
which in turn contain various rotational and translational energy levels as well
(not shown). An electron can be promoted to an excited state by absorbing
photons, phonons, dipole-dipole energy transfer, electron transfer, or by direct
electrical injection. After excitation, the system will tend to dissipate the energy,
allowing the electron to return to the ground state, via radiative or non-radiative
processes. The relative contribution of the various dissipative pathways to the
overall relaxation of the system is determined by the relative magnitudes of their
rate constants.
S2
A
Internal
Conversion
k
-C)
Si
Intersystem
Crossing
._
.
.
_
_
_
A,
..
Ill
-
hv/s
bs
Flu(orescence
So
o VAbs
fi
;_
0,O
I
_
I
hv FI
F
.
hvPh
Phosphorescence
r
.
I
i
t-
F
Figure 2.5 Jablonski diagram.
61
2.2.A Radiative Transitions
Depending on the nature of the excited state, the resultant radiative
components are divided into two categories,
phosphorescence.
namely, fluorescence
and
Fluorescence involves singlet excited states where the
electron in the excited orbital has spin opposite to that of its pair in the groundstate orbital (Figure 2.6). The relaxation of this excited state is spin-allowed,
occurring rapidly, resulting in the emission of a photon.
fluorescence are typically 109 s,
Emission rates for
and thus typical fluorescence
lifetimes are on
the order of 1 ns.
The radiative transition of triplet excited states, in which the electron in the
excited orbital has the same spin as the ground-state electron, is known as
phosphorescence (Figure 2.6).
Recombination of this excited state is spin-
forbidden and consequently the rates are slow (10 3
-
10 0
s'),
causing
phosphorescent lifetimes to be on the order of milliseconds to seconds with even
longer lifetimes possible.
Phosphorescent emission is generally shifted to longer wavelengths and
thus is a lower energy transition relative to fluorescence. Qualitatively, the shift
to lower energy can be explained by considering the spatial distribution of the
electron pair, or exciton, participating in the electronic transitions. According to
the exclusion principle, no two particles can occupy the same quantum
mechanical state. Hence two electrons with opposite spins will, on average, be
closer together than those with parallel spins.
62
Consequently, the average
coulombic interaction of the singlet state is larger and thus is higher in energy
than that of the triplet.
Phosphorescence has become a key component in the design of efficient
OLED materials and devices. The spin symmetry associated with excitons that
are formed in a spin-independent process (e.g. direct charge injection) results in
the formation of triplet and singlet configurations in the ratio of 3:1, as shown in
Figure 2.6. For standard OLED systems, the luminescence is predominantly
fluorescence due to singlet recombination, and thus, approximately 75% of the
electron-hole pairs are lost to triplet excitons, which do not decay radiatively with
high efficiency. Therefore, in order to improve upon the operational efficiency of
OLED devices, systems that display efficient phosphorescence have been
introduced, making use of the more abundant triplet excitons that are formed in
the device.
i
,
9=1?t)i
E
LUMO
HOMO
i
I
)+ I
= I t )-I
Tk
= 1, triplet
T)
S=
O,
singlet
Figure 2.6 Schematic diagram representing exciton spin and symmetry. The
total possible spin states of an exciton can be spatially anti-symmetric (S = 1)
also known as triplet states or they can be spatially symmetric (S = 0) also known
as singlet states. There are three possible triplet configurations and only one
singlet configuration.
63
2.2.B Non-Radiative Transitions
Non-radiative transitions, represented by wavy arrows in Figure 2.5, can
also participate in the excited-state dynamics of conjugated systems. Typically,
organics are excited to some higher vibrational level of either S or S2. An
electron in an excited vibrational level can loose its energy non-radiatively by
vibrational relaxation. In the solid-state, vibrational energy is lost to neighboring
molecules and phonon modes of the solid. This process occurs in 10-12 s or less,
rapidly relaxing the system to the lowest vibrational level of S1.
Electron transfer can also occur from the vibrational manifold of one
electronic state to another if they are sufficiently close in energy. This electron
transfer does not change the overall energy of the system and so no photon is
emitted. When the spin state of the system remains the same, this process is
calledinternalconversion.
If this electron transfer process involves the spin conversion of a singlet
excited state to a triplet state (e.g. T1 in Figure 2.5), it is referred to as
intersystem
crossing.
degree of spin-orbit
incorporating
The degree of intersystem crossing is dictated by the
coupling within the system.
Therefore,
heavy atoms such as metals typically
materials
display enhanced
phosphorescence quantum yields.
Excited organic systems, especially in the solid-state, can also undergo
non-radiative energy transfer to a neighboring molecule via dipole-dipole
interactions or electron exchange. These processes are known as Forster and
Dexter processes respectively, and will be discussed in Section 2.3.
64
2.2.C Franck-Condon Shift
Electronic transitions between states are depicted with vertical lines
(Figure 2.7) in order to illustrate the instantaneous nature of the absorption or
emission of a photon. These rapid transitions (-10-15 s) occur too quickly for a
significant nuclear displacement to occur. This is known as the Frank-Condon
principle.
Due to the fact that the excited state lifetimes for the radiative transitions
are much longer than non-radiative phonon emission, internal conversion is
generally complete prior to emission.
Thus, due to vibrational dissipation of
some of the excitation energy, luminescence typically results from a thermally
equilibrated excited state, namely, the lowest-energy vibrational state of Si. This
principle is known as Kasha's rule. Therefore, the energy of the emitted photon
as a result of radiative recombination is less than the energy of the absorbed
photon.
This energy difference is called the "Frank-Condon shift" and is
generally referred to as the "Stokes shift." Materials which have a more rigid
chemical structure and thus lower vibronic coupling of the excited state, such as
poly(phenylene-ethynylene
(the subject of this thesis), will exhibit relatively
smaller Stokes shifts and narrower linewidths associated with its luminescence.
65
Abso
Figure 2.7 Energy diagram of the electronic ground state and first excited state
for a diatomic molecule, schematically illustrating the Frank-Condon or Stokes
shift.
2.3 Electronic Energy Transfer
As mentioned in Section 2.2.B, organic systems in an excited state can
undergo non-radiative energy transfer to a neighboring species via Frster or
Dexter processes. In general, energy transfer can be represented by scheme
2.1. An excited donor molecule, D*, transfers its energy to an acceptor, A, which
in turn is promoted to an excited state A*:
D* + A
(2.1)
D + A*.
66
In doped films, exciton energy can be transferred from the host to the guest,
quenching emission from the host while increasing that of the guest. When this
process occurs among one molecular species, it is termed energy migration.
Both of these scenarios will be critical to findings in Chapters 4, 5, and 6.
The interaction between donor and acceptor (D* + A) is described by the
perturbation Hamiltonian, H.
Therefore, time-dependent perturbation theory
assigns the probability of evolution from (D* + A) to (D + A*) in terms of the
"Golden Rule:" 6
p oc p(j IH1lyf)2 ,
where i
(2.2)
is the wave function of the initial state (D* + A) and
f
is the wave
function of the final state (D + A*), and p is the density of initial and final states
capable of interacting.
For organic systems, which typically display broad
spectral features, p can be estimated by determining the spectral overlap of
donor luminescence and acceptor absorption. The probability, P, of equation
2.2, can be related to an experimental quantity, the rate constant of energy
transfer,
kET.
Furthermore, the electron energy transfer can take place by either
electrostatic interactions or direct exchange.
Consequently, the perturbation
Hamiltonian can be broken up into the electrostatic (Coulombic or Frster)
interaction and electron exchange (or Dexter) interaction.
67
2.3.A Frster Energy Transfer
The electrostatic dipole-dipole interaction was formulated by F6rster7 who
found that the rate of energy transfer depends on the distance R between the
donor and acceptor molecules given by:8
kET(R) = (1I)(R/R)
6,
(2.3)
where Ro is the F6rster Radius, and - is the average donor exciton lifetime for
radiative recombination, corresponding to rate kD
=
1/c.
When R = Ro then
kET
=
kD, and the probability that an exciton will recombine at the donor is equal to its
transfer probability. This critical distance R is given by the integral over all
frequencies v:
Ro6 = 1.25 x 1017 (PE/n4 ) JFD(v)aA(v)(dv/v
Here
E
4
).
(2.4)
is the quantum efficiency of donor emission, n is the refractive index of
the host, FD is the normalized emission spectrum of the donor, and aA is the
molar absorption extinction coefficient of the acceptor. The spin states of both D
and A are conserved in Frster energy transfer. Typical values for this critical
distance can be as large as 100 A.
68
2.3.B Dexter Energy Transfer
Dexter formulated electron exchange energy transfer and derived the
following relation 8
kET(R)
2
-W
eI
(2.5)
L fFD(v)A(v)dv
where R is the distance between donor and acceptor and L is a constant. Under
the Dexter mechanism, only the total spin of the donor acceptor system is
conserved, and thus a triplet state of the donor can be transferred to the
acceptor. Due to the fact that this process involves direct electron exchange, it is
only significant at short distances, typically on the order of 10 A. A schematic
representation of Forster and Dexter processes are shown in Figure 2.8.
Forster, Coulombic
(Long Range- 30-100 A)
,,,.,,,,,,,i+
I -~-.
-I
,
,
A
D*
(Short Range - 6-20 A)
D
-
Dexter, Electron Exchange
+
A*
+
D*
A
Figure 2.8 Schematic of the Forster and Dexter processes resulting in singletsinglet energy transfer.8
69
2.4 Energy Transfer in Conjugated Polymers
Energy transfer is a key process in the operation of a number of optical
and optoelectronic systems based on conjugated polymers. Current research in
these areas, including that conducted in our group, has prompted interest in
developing a detailed understanding of the mechanisms for energy migration in
conjugated polymers. One of the main issues involves the relative efficiencies of
interchain versus intrachain energy transfer processes, or in other words, Forster
versus Dexter energy transfer. There is still a significant amount of debate as to
the dominant mechanism in various systems discussed in the literature.1
9-
12
The scenarios that will be the focus of this thesis are two-fold. The first
involves the intrachain energy migration of isolated conjugated polymer chains,
whereby electronic excitations are transported through a hopping mechanism
along a single polymer chain with a rigid rodlike conformation and is most often
attributed to Dexter type processes. Depending on the chemical structure of the
conjugated polymer and on its "history" (sample preparation parameters, nature
of solvent, sample casting (Chapter 4) etc.), various conformations can form such
as defect cylinder or defect coil where
-n
interactions can arise from the
collapse of single conjugated chains.13 In other words, in the absense of chainchain interactions, de-excitation of single molecules mainly involves migration of
the excitons along the conjugated chains and radiative decay of the excited
species. Therefore, ensuring the isolation of conjugated polymer chains should
translate into enhanced radiative recombination within these materials and is the
70
motivation for the development and utilization of grafted poly(phenyleneethynylene) discussed in the following chapters.
The second scenario involves the use of guest-host systems, which have
proven attractive for the investigation of energy transfer processes. Literature
examples predominantly involve the use of conjugated polymers as the host
' 5 In this scenario,
materials with small molecule energy acceptor guests.12
141
energy transfer has been described to occur in a two step process.'5 The first is
thermally activated migration of the exciton within the donor polymer to a point
sufficiently close to the acceptor that resonance energy transfer from the host to
the guest can occur. The latter step is most often described in the framework of
the Frster model.7 Therefore, it is assumed that the energy transfer process
occurs after vibrational relaxation of the donor excited state. This rationale can
be applied similarly to the systems that will be discussed in Chapters 6 and 7,
however in these cases, a small molecule will be the donor species, while a
conjugated polymer will be used as the energy acceptor.1 6
2.5 Energy Levels and Transitions of Coupled Systems (or Aggregates)17 19
In addition to energy transfer, other important factors governing the
luminescence of organic materials arise when molecules are brought in close
proximity of one another, such as the case in the solid-state. The interaction of
two coupled molecules is often referred to as dimer formation.
There are two main scenarios involved in dimer formation.
The first
involves the interaction of an excited state of one molecule with the ground state
71
of another molecule of the same species, which is called an excimer (short for
"excited dimer"). When two different molecular species are involved, it is termed
an exciplex. An excimer or exciplex is an emissive excited state complex which
is delocalized over two molecular units.2 0 The ground state of the excimer is
dissociative, meaning the ground state of the dimer spontaneously dissociates
into two ground-state molecules. Furthermore, the excimer cannot be directly
excited optically. Photoexcitation in excimer-forming systems therefore, leads to
production of an intramolecular singlet exciton that later delocalizes over two
molecules, forming the excimer. Excimer formation typically results in strong
geometric distortion of the molecular configuration, which, when combined with
the dissociative nature of the ground state, leads to featureless, strongly Stokesshifted emission in comparison to the emission observed in dilute solution.20
While excimer formation only exists in the excited state, the second
scenario involves dimer formation that lead to ground-state interactions as well.
These are often called aggregate states. Upon aggregation, both the groundand excited-state wavefunctions are delocalized over several molecular units, or
in the case of conjugated polymers, over several polymer chains. Aggregates
are therefore directly accessible spectroscopically.
The wavefunction
delocalization leads to a red-shift in both emission and absorption compared to
that in solution. Unlike excimers, which tend to form spontaneously, aggregates
usually result from a forced proximity of two or more molecular species. While
the energy of the ground state is lowered upon aggregate formation, the first
excited state splits into several distinct levels.
72
The degree of radiative
recombination observed from these new excited states is determined by the
alignment of the individual dipoles of the molecules which form the aggregate
(see Figure 2.9). For the simple case of a two-molecule dimmer or aggregate,
the electronic transitional dipoles for the two molecules can either add
constructively to give an allowed transition (indicated by a solid arrow) or
destructively to yield a forbidden transition (indicated by a dashed arrow).
In the parallel
arrangement of the dimer, as shown in Figure 2.9,
absorption to a high-energy exciton state followed by rapid relaxation to a lower
energy exciton state results in a dipole-forbidden transition and thus
luminescence is very weak, translating to reduced quantum yields.
Dimer
emission spectra are also typically broader and less well defined as compared to
the monomer spectrum due to the presence of new vibrational modes associated
with the dimer. This scenario is very pronounced in solid-state phenomena of
poly(phenylene-ethynylene)
chapters of this dissertation.
and is the motivation for the work in the latter
Elimination of aggregation phenomena in these
systems should lead to preservation of solution-state emission profiles as well as
an increase in emission efficiency in the solid state.
In the case of head-to-tail and oblique dimer arrangements shown in
Figure 2.9, absorption leads to allowed radiative transitions and thus systems
displaying this arrangement in the solid state exhibit enhanced quantum yields.
This approach has been explored in our group to increase the solid-state
quantum efficiency of PPEs.
By introducing chiral groups onto the polymer
backbone substituents, an oblique packing of the polymer chains is induced,
73
resulting in enhanced quantum yields.2 1 Molecular systems that adopt a head-totail arrangement in solid state have been termed "J-aggregates."
These
materials typically display enhanced emission efficiencies in the solid state
relative to that of the monomer species in solution.
Parallel
1~~~~~~IW
t
t . --0
Head-to-Tail
V
c11AA
/___7
7
-
Oblique
_11/'
- -E't
f F_-
I
-dnu
Monomer
Dimer
Monomer
Dimer
Monomer
Dimer
Figure 2.9 Exciton splitting in dimers of various geometries. Orientations of
monomer transition dipoles are represented by short arrows. Dipole-forbidden
transitions are denoted by dotted lines.17
2.6 Conclusions
This chapter provided an introduction to conjugated polymers and the
basic theory behind their luminescent properties. In order to improve upon the
luminescent properties of existing conjugated polymer systems, especially PPEs,
one must understand the fundamental processes that dictate the solution-state
and solid-state emission profiles and quantum efficiencies associated with these
systems. Utilization of this understanding allows for the design and realization of
74
a new conjugated polymer system capable of achieving enhanced solid-state
luminescence and will be the subject of the next chapter.
75
2.7 References
[1] Bredas, J-. L.; Beljonne, D.; Coropceanu, V.; Cornil, J. Chem. Rev. 2004, 104,
4971.
[2] Peierls, R. E. Quantum Theory of Solids, Oxford University Press, London,
1955.
[3] Roth, S.; Carroll, D. One-Dimensional
Metals, WHILEY-VCH, Verlag GmbH &
Co. KGaA, Weinheim; 2004.
[4] Kraft, A.; Grimsdale, A. C.; Holmes, A. B. Angew. Chem. Int. Ed. 1998, 37,
402.
[5] Lakowicz J. R. Principles of Fluorescence Spectroscopy, New York: Kluwer
Academic; 1999.
[6] Turro, N. J. Modern Molecular Photochemistry, Sausalito CA: University
Science Books; 1991.
[7] Forster, Th. Ann. Phys. 1948, 2, 55.
[8] Bulovic, V.; Burrows, P. E.; Forrest, S. R. Semiconductors
and Semimetals
2000, Ch. 5, Vol. 64, pg. 256.
[9] Zhou, Q.; Swager, T. M. J. Am. Chem. Soc. 1995, 117, 12593.
[10] McQuade, D. T.; Pullen, A. E.; Swager, T. M. Chem. Rev. 2000, 100, 2537.
[11] McQuade, D. T.; Hegedus, A. H.; Swager, T. M. J. Am. Chem. Soc. 2000,
122,12389.
[12] Zheng, J.; Swager, T. M. Chem. Comm. 2004, 24, 2798.
[13] Yu, J.; Hu, D.; Barbara, P. F. Science 2000, 289,1327.
[14] Kuroda, K.; Swager, T. M. Chem. Comm. 2003, 1, 26.
76
[15] List, E. W. J.; Creely, C.; Leising, G.; Schulte, N.; Schliter,
A. D.; Scherf, U.;
Mullen, K.; Graupner, W. Chem. Phys. Lett. 2000, 325, 132.
[16] Breen, C. A.; Tischler, J. R.; Bulovic, V.; Swager, T. M. Adv. Mater2005,
in
press.
[17] Pope, M.; Swenberg, C. E. Electronic Processes in Organic Crystals and
Polymers, New York, Oxford University Press, 1999.
[18] Moliton, A.; Hiorns, R. C. Polym. Int. 2004, 53, 1397.
[19] Farchioni, R.; Grosso, G. Organic Electronic Materials: Conjugated Polymers
and Low Molecular Weight Organic Solids 2001, Springer Series in Materials
Science; Verlag Berlin Heidelberg.
[20] Birks, J. B. Photophysics of Aromatic Molecules 1970, Wiley Interscience;
London.
[21] Zahn, S.; Swager, T. M. Angew. Chem. 2002, 114, 4399.
77
78
Chapter 3
Polystyrene Grafted Poly(Phenylene-Ethynylene) via Atom
Transfer Radical Polymerization (A TRP)
Partially adapted from:
C. A. Breen, T. Deng, T. Breiner, E. L. Thomas, T. M. Swager, J. Am. Chem. Soc. 2003,
125, 9942.
3.1 Abstract
Polystyrene grafted poly(phenylene-ethynylene) has been synthesized by
atom transfer radical polymerization
(ATRP) in bulk styrene monomer using an
isobutyryl bromide initiator and a CuBr/bipyridine catalyst system. The resulting
grafted
polymer
system
was
characterized
using
gel
permeation
chromatography, proton nuclear magnetic resonance spectroscopy (1H NMR),
and steady-state photophysical spectroscopies. The presence of the polystyrene
grafts, eliminates intrinsic aggregation phenomena of the PPE backbone,
causing these systems to maintain both their solution-state photoluminescence
and quantum yield in the solid-state.
80
3.2 Introduction - Grafted Conjugated Polymers
In an effort to expand upon the scope of applications amenable to
poly(phenylene-ethynylene) (PPE) systems, this chapter discusses the synthetic
modification of a PPE conjugated backbone structure.
The idea is to generate a
PPE system that can be functionalized post-polymerization, rendering the system
with
new materials
properties, such
as increased solubility,
materials
compatibility due to miscibility with other polymeric systems, as well as enhanced
photophysical properties in the solid-state due to the elimination of aggregation
phenomena. This has been an area of particular interest to our research group,
as well as other researches exploring the use of conjugated polymer systems.1
Our group has explored the use of PPE systems that incorporate a pentiptycene
scaffold into the polymer backbone, which limits the degree of aromatic face-toface interactions. The result is a semi-porous polymer network in the solid-state
that exhibits only a 2-5 nm red-shift in emission spectra compared to that in the
solution state.2
The quantum yield of these systems in the solid state is also
improved, however, typically still exhibit a >50% reduction from that observed in
solution.
Other groups exploring the synthetic modification of another conjugated
polymer system, polythiophene (PT), have explored the funtionalization of PT
with other polymeric systems.3 '4 More specifically, Constanzo et. al. synthesized
a poly(methyl acrylate) (pMA) grafted PT system using ATRP (Scheme 3.1).5
Using this approach, they were able to achieve a densely functionalized polymer
system having controlled molecular weight, low-polydispersity, graft polymers.
81
They also noted that the presence of the pMA grafts separated the PT
backbones and inhibited any interchain r-stacking interactions, which improved
their photoluminescent properties (Figure 3.1).
We have chosen to explore a similar synthetic scheme in the hopes of
generating
a new polystyrene
grafted PPE system
using ATRP.
Such an
approach is very modular, allowing for synthetic flexibility in terms of the different
monomers that can be accessed via ATRP.
More importantly, this overall
synthetic route should provide a means for preserving the excellent solution-state
photophysical properties of PPE systems in the solid-state by eliminating intrinsic
aggregation phenomena of the conjugated PPE backbone, ultimately allowing for
new and interesting applications.
Scheme 3.15
Synthetic Schenle of Graft Copolymer Svnthesis
oOH
OH
OTMS
Br
Br
(3)
(2)
(1)
OTMS
(4)
d
(5)
(6I
*(_.C
8litiot, . Ia N S, 1111',() NI,' . 1. Ni(l 1111( ) i. LI
;-t-'lill)Zt
it'lg II 0lnid
.\ IR
tlldiliml,
(D
"1 ';)) ,
II
. lilt
I tF. ".h,()11
82
·
'. /(C1 :3.Nillq)p)(.I (I)1111L
. Nl. . I FAI
.
L.
D 1
.-.
1200 .
Ion
1000 800-
a
600
1.2
.
400
.__
i--- 5 - SolutionI
· ,
tion
0.8 E
z0
200
-j
400
0.6
500
550
600
650
700
7540
Wavelength (nm)
0.4 !
C
0.2
c(D
450
6104
4
0
Si -0.2
6
410 3
200
300
400
500
600
700
800
0
-
-
..
-
/\
i
- --
--
-
·
900
Wavelength (nm)
>.
o0:
\
450 500 550 600 650 700 750 800
Wavelength (nm)
Figure 3.1 Plot a shows UV/vis analysis (300-700 nm) of ungrafted PT (5) and
grafted PT (6) in solution and cast films.
The fluorescence
of ungrafted and
grafted PT in solution (excited at 420 nm) and cast films (excited at 445 and 420
nm respectively) are shown in plots b and c respectively. 5
3.3 Atom Transfer Radical Polymerization and its Mechanism
Atom
transfer
radical
polymerization
(ATRP)
is a controlled
radical
polymerization that allows for the synthesis of well-defined polymers with lowpolydispersity. This technique preserves end-group and side-chain functionality
which has lead to the design of new polymeric architectures such as block-,
breached-, and graft polymers.6
Traditional approaches used to obtain such
polymers utilize ionic polymerizations, which limits the nature of the monomers
83
due to the reactivity of the ionic intermediates and side-reactions that result
between functional groups during polymerization.
In addition, these systems
require rigorous exclusion of oxygen, water, and other typical solvent impurities
in order for a significant degree of polymerization to occur.
In contrast, radical polymerizations are compatible with many types of
functionalized
monomers
and solvent
systems.
Furthermore,
radical
polymerizations are tolerant to water and other impurities, which has made them
suitable for industrial large-scale production.
The reactivity of the radical
intermediate, however, is difficult to control due to irreversible termination
reactions, which results in reduced control of the polymer molecular weight.
The ATRP method has been developed to avoid these unfavorable
termination reactions by controlling the equilibrium between the radical and
dormant (non-radical) species in the presence of a metal catalyst. The ATRP
mechanism
utilizing copper ion as a metal catalyst is shown in Scheme 3.2.
First, an initiator (R-X) produces a radical species (Ro) when the copper/ligand
complex (Cu(l)/L) abstracts a halogen atom (X) from R-X to produce a X-Cu(ll)/L
species.
The generated radical initiator (R-) then reacts with the monomer
species and the propagation of the polymer chain begins. The chain terminated
radical species (R-Pm) is deactivated by the conversion of X-Cu(ll)/L to Cu(l)/L
and halogenation of the polymer (R-Pm-X). The propagation of the polymer chain
is continued through the repetition of this reversible activation/deactivation
process of the radical species. Due to the fact that the equilibrium that exists
between the radical and dormant forms (Equilibrium (2) in Scheme 3.2) favors
84
the dormant form, the radical concentration during polymerization is low,
significantly reducing termination processes that involve radical-radical coupling.
Multi-dentate amines have been widely used as ligands for the Cu ion in order to
stabilize the Cu(ll) species and provide solubility in organic solvents. The redox
potential of the Cu ion can be altered by ligand complexation and thus allows for
tunability of the catalytic activity of the system depending on the monomers and
conditions used.
Scheme 3.2
Initiation:
R-X
+
Cu(I)/L
R'
W-
-
+
X-Cu(II)/L
(1)
+ Monomers
Propagation:
R-Pm-X +
R-P
Cu(I)/L
+
X-Cu(II)/L
(2)
+Monomers
Termination:
R-Pm
+
R-Pm
m
R-Pm+n-R
(3)
The rate of initiation in the ATRP mechanism is another important factor
for obtaining polymers with well-defined molecular weights. The initiation rate
should be fast relative to the propagation rate, allowing all of the polymer chains
to begin growing at the same time. However, if the initiation rate is too large, the
85
large number of radical species produced in the initial stages of the reaction will
result in radical-radical termination. In practice, these considerations result in the
use of initiators whose chemical structure resemble those of the monomer unit in
order to avoid a mismatch in radical reactivity with the monomer.
The ATRP
equilibrium, combined with the initiation conditions, is vital to the success of a
controlled polymerization and should be taken into account in the design of new
monomer/catalyst systems.
3.4 Results and Discussion
3.4.A Polymer Synthesis
We functionalized
a dialkoxy substituted PPE with polystyrene (PS) by
grafting from a PPE macroinitiator via (ATRP).5 7'
The synthesis of the
diacetylene monomer with terminal hydroxyl side chains, 1, is shown in Scheme
3.3. The di-iodide hydroquinone is alkylated with 11-bromo-undecanol
to give
monomer la. 8 Alkynylation of la followed by standard deprotection affords the
desired monomer 1. Monomer I can then be polymerized with the diiodide 2
under Pd(O)catalyzed cross-coupling conditions to give the PPE backbone (3) as
shown in Scheme 3.4. The PPE was then functionalized with 2-bromoisobutyryl
bromide to give the ATRP macroinitiator 4. Subsequent ATRP polymerization of
styrene gave the polystyrene grafted PPE (PS-PPE) 5.
process is summarized in Scheme 3.4.
86
The entire grafting
Scheme 3.3
IOH
.K
OH
Br/
OH
10
=
2-Butanone
OH
TMS
OH
10,,
. TMS --
TMS
DIPA
Toluene
K 2 CO 3 , KI
OH
Pd(PPh 3)4
Cul
O
60°C, 24hrs.
Reflux
la
lb
.
_
.
.
H
KOH
MeOH/THF
Reflux
1
Scheme 3.4
ocI-'16
Pd(PPh3 ) 4
Cul
DIPA
Toluene
+oc
I
60 °C, 24h
3
2
1
0
Br
TEA
THF
Br
I
CuBr
I
Bipy
4
5
87
3.4.B Polymer Characterization
Table 3.1 contains characterization data for the synthesized polymers.
The polymers were characterized by gel permeation chromatography (GPC)
(Figure 3.2), nuclear magnetic
resonance
(NMR) spectroscopy
(Figure 3.3),
UVNis spectroscopy, and fluorescence spectroscopy (Figure 3.4).
GPC analysis reveals the expected molecular weight increase upon
grafting, as well as a lowering of the polydispersity of the grafted PPE system
relative to the ungrafted PPE backbone due to the addition of low polydispersity
polystyrene grafts. Further confirmation of the covalent linkage between the graft
polymers and the PPE backbone is provided when observing the elution volume
via multiple detection wavelengths. The GPC system is equipped with multiple
detectors, allowing the absorption of the sample to be monitored by refractive
index (RI), ultra-violet (at 250 nm), and visible (at 450 nm) wavelengths.
Since
the PPE backbone absorbs visible light (450 nm) while the polystyrene grafts
only absorb in the UV (250 nm), as long as all three detectors reveal absorption
profiles at the same elution volume, we can conclude that the PPE backbone and
PS grafts are covalently
linked.
Free PS grafts, or homo-polystyrene,
in the
sample would result in only UV absorption at a particular elution volume. Such a
scenario was not observed in these systems.
Finally, the atom transfer
polymerization of styrene was assessed by measuring the polydispersity of the
PS grafts (Table 3.1). The ester linkages of the ATRP initiator were cleaved post
polymerization by treating the grafted PPE with KOH in a refluxing THF/MeOH
mixture, allowing for GPC analysis of the free PS grafts. 5
88
GPC Elugram for PPE, PS-g-PPE, and
Cleaved PS Grafts
H
0
OC 16
CiRO
1oO
HO
1-
ci
F
C
0
So
.
0
b-
N
O
O
PDM,,
- 12().()()
PDI --1.8)
E
ii
10
15
20
25
30
*1
Elution Time (min.)
\1
(6.()040)
PD1[) I 17
GPC Elugram at Different Detection
Wavelengths for Grafted Polymer
RI
l
14
16
18
20
22
Elution Time (min.)
Figure 3.2 GPC elugrams of the PPE Backbone (black), PS grafted PPE (blue),
and cleaved PS grafts (green). The bottom plot shows the elugram for the
grafted PPE measured using the various detection wavelengths available with
the GPC system, i.e. Refractive Index (RI), Ultra-violet (250 nm), and Visable
(450 nm).
89
Table 3.1 Polymer Characterization
Photophysical Characterization
(Solid State)
GPC Analysis
Polymer
Mn
PPE Backbone (3)
PS Grafted PPE (5)
Cleaved PS Grafts
PDI
XMaxabs
XMaxem
(nm)
(nm)
(
56,000
2.70
480
495
0.10
120,000
1.80
445
475
0.60
6,000
1.17
--
--
--
The success of each step involved in the synthesis of the grafted polymers
could also be assessed through
1H
NMR analysis.
As shown in Figure 3.3, the
change in the chemical shift of the methylene protons adjacent to the terminal
hydroxyl group on monomer 1 was monitored. In the PPE backbone structure,
the resonance for these protons appears at about 3.6 ppm.
The covalent
attachment of the ATRP initiator (4) is verified due to the shift in this resonance to
4.15 ppm. This resonance is again shifted upon successful initiation of the PS
polymerization
(5) to 3.75 ppm.
Finally, the fact that there are no detectable
peaks corresponding to starting material present in any of the spectra shown in
Figure 3.3, confirms that each step in the reaction scheme is complete.
90
1H
NMR
PPE Backbone
Z
o
3
0.
ATRP Macroinitiator
ki
PS Grafted PPE
4.3
4.2
4.1
4.0
3.9
3.8
3.7
3.6
ppm
Figure 3.3
1H
NMR characterization of polymers 3, 4, and 5. Signals normalized
on the aromatic methoxy proton signal.
Further confirmation for the successful grafting process is provided upon
observation of the grafted PPEs photophysical behavior in the solid state. The
absorption and photoluminescence profiles for the PPE backbone and
polystyrene grafted PPEs are shown in Figure 3.4. In solution, the ungrafted and
grafted PPEs have essentially identical absorption and emission profiles. The
quantum efficiencies for the two systems in solution are also very similar with
quantum efficiencies of ' = 0.70 and 1 = 0.60 for the ungrafted and grafted
PPEs respectively (see Figure 3.4). The similarity in the luminescence properties
is expected considering that the two systems contain the same emitting
chromophore.
91
Solution State
0
:6
0
N
0O
C
E
0
0
_U)
U
C
Quantum Efficiencies ():
PPE
Backbone
C
.0
.2
E
W
PS Grafted
PPE
350
400
450
500
5'S
600
650
Wavelength (nm)
Solution
State
0.70
0.60
Solid State
-·-----I--------·----------1 ·---·-----·--------------··----·
Solid
State
0.10
0.60
0
o0
0
C
C
O
0
E
350
400
450
500
550
600
650
Wavelength (nm)
Figure 3.4 Right: Photophysical analysis of the PPE backbone and grafted PPE
systems. Absorption profiles for the grafted (a) and ungrafted (b) PPEs are
plotted in both solution and solid-state. The photoluminescence for the grafted
(c) and ungrafted (d) PPEs are also shown.
Left: A comparison is made between
the quantum yields of the grafted and ungrafted systems in both solution and
solid state.
92
In the solid-state, however, the ungrafted PPE absorption profile is redshifted and displays the appearance of a new band, which is often attributed to
aggregation phenomena.1
The PL spectrum for the ungrafted PPE is also red-
shifted and the emission efficiency for this system in the solid-state is
dramatically reduced to 'F = 0.10 relative to
= 0.70 in solution.
In contrast, the
grafted PPE absorption and emission profiles in solid-state match those in
solution.
Furthermore, the quantum yield of the grafted PPE, D = 0.60, is also
preserved in the solid-state.
These results confirm that the presence of the polystyrene grafts
eliminates the intrinsic aggregation of the PPE system allowing the grafted PPE
system to simultaneously maintain its solution state emission profile and
quantum yield in the solid-state. This is similar to the results seen for the PT
system discussed above.5 Now that we have developed a successful procedure
for the synthesis of polystyrene grafted PPEs, the remainder of this thesis will
present novel
applications that
benefit from
the
enhanced
solid-state
photophysical properties that this system provides.
3.5 Conclusions
The detailed synthesis of a new polystyrene grafted PPE system has been
presented. Facile synthetic modification of a PPE backbone containing terminal
hydroxy alkyl substituents with a known ATRP initiator allows for the generation
of a PPE macroinitiator. Subsequent ATRP polymerization of styrene using a
CuBr/bipyridine catalyst system affords controlled, low-polydispersity polystyrene
93
grafts covalently linked to a PPE conjugated polymer core. The presence of the
graft polymers increased the solubility and processability of the PPE backbone
by incorporating the miscibility and added materials compatibility into the system.
Most importantly, the grafts enable the PPE system to preserve its solution
conformation in the solid state, preventing a-orbital interactions. The elimination
of aggregation phenomena minimizes the nonradiative decay pathways for the
excited PPE backbone, allowing grafted PPE systems to maintain their solution
state emission and quantum yield in the solid state. The combination of added
materials compatibility and enhanced photophysical properties of the grafted
PPE system in the solid state will prove useful in the following chapters as we
extend the scope of applications amenable to PPE materials systems.
3.6 Experimental Section
General. H NMR spectra for monomers and polymers were recorded on a
Varian MERCURY (300 MHz) and a Varian VXR-500
(500 MHz) instrument
respectively, using deuterochloroform as reference or internal deuterium lock.
The chemical shift data for each signal are given in units of
tetramethylsilane
residual.
13
(TMS) where 6 (TMS) = 0, and referenced
C NMR spectra were recorded on Varian
(ppm) relative to
to the solvent
MERCURY
(75 MHz)
instrument using internal deuterium lock and proton decoupling. High resolution
mass spectra were obtained on a Finnigan MAT 8200 systems using sector
double focus and an electron impact source with an ionizing voltage of 70 V, and
with a Bruker DALTONICS APEX 11,3 Tesla, FT-ICR-MS with ESI source or
94
El/CI source. Infrared spectra were collected with a Nicolet Impact 400 FT-IR
spectrometer in KBr and were recorded in reciprocal centimeters (cm1 ,
wavenumbers). UV-visible absorption spectra were measured with a Cary 50
UVNisible spectrometer.
Fluorescence spectra were measured with a SPEX
Fluorolog- 2 fluorometer (model FL112, 450W xenon lamp).
Solution state
quantum yields were determined relative to Coumarin 314 in ethanol (
= 0.63)
and solid state quantum yields were determined relative to -10-2 M 9,10
diphenylanthracene in PMMA (
= 0.83).
The molecular weights of polymers
were determined by Gel Permeation Chromatography (GPC) running with THF
as the eluent versus polystyrene standards (PolySciences) using Hewlett
Packard series 1100 HPLC instrument equipped with a Plgel 5 mm Mixed-C (300
x 7.5 mm) column. Reagents were purified and dried by standard technique.9 All
air and water-sensitive synthetic manipulations were performed under an argon
atmosphere using standard Schlenk techniques. Anhydrous diisopropylamine
(DIPA) and toluene were purchased from Aldrich and used without further
purification. All other chemicals were of reagent grade and used as received.
1,4-Bis((11-undecanol)oxy)-2,5-diiodobenzene
(la):
In a 1000mL
round
bottom flask containing 400mL of 2-butanone were combined 2,5-diiodo-1,4dihydroxybenzene (10.33g, 0.0285 mol), 11-bromo-undecanol (21.51g, 0.0856
mol), K 2 CO3 (31.56g, 0.2283 mol), and KI (1.421g, 0.0086 mol).
The reaction
flask was equipped with a condensing tube and the mixture was heated to reflux
for 72 hrs.
The reaction mixture was filtered and the salts washed with
methylene chloride. The filtrate was washed several times with 0.1M NaOH,
95
followed by H20, and dried over MgSO4.
The solvent was removed under
reduced pressure and the residue was recrystallized from hexanes to give la as
a white powder in a 70% yield.
H NMR (300MHz)
H
7.17 (2H, s), 3.94 (4H, t, J
6.7), 3.65 (4H, t, J 6.6), 1.82 (4H, tt, J 6.6 and 6.0), 1.62-1.33 (34H, multiplet); 6c
122.6, 86.2, 70.3, 63.1, 32.9, 29.7, 29.59, 29.56, 29.5, 29.3, 29.2, 26.1, 25.8; m/z
(ESI) [Found: (M+Na) + 725.1515. C2 8 H48 12 0 4 requires M, 725.1534].
1 ,4-Bis((trimethylsilyl)ethynyl)-2,5-bis((1
-undecanol)oxy)benzene
(b):
Schlenk flask was loaded with la (4.0 g, 5.69 mmol), trimethylsilylacetylene
A
(3.22
cm 3 , 22.77 mmol), Cul (54 mg, 0.29 mmol) and Pd(PPh3 ) 4 (329 mg, 0.29 mmol)
in a mixture of dry DIPA and toluene (80 cm 3 , 1:2). The flask was degassed and
back-filled with argon three times before heating at 80 °C for 24 h. The reaction
mixture was dissolved in several volumes of methylene chloride, washed several
times with water and dried over MgSO4.
The solvent was removed under
reduced pressure and the residue was purified by column chromatography (20:1
to 3:1 hexane:EtOAc) to give lb as a pale yellow solid in a 76% yield.
H NMR
(300MHz) & 6.89 (2H, s), 3.95 (4H, t, J 6.4), 3.65 (4H, t, J 6.6), 1.801 (4H, tt, J
6.7 and 6.5), 1.63-1.32 (34H, multiplet); & 153.7, 117.0, 113.7, 100.9, 100.0,
69.3, 63.1, 32.9, 29.7, 29.6, 29.49, 29.47, 29.4, 26.1, 25.8, 0.1; m/z (ESI) [Found:
(M+Na) + 665.4376. C38 H660 4 Si2 requires M, 665.4392].
1,4-Diethynyl-2,5-bis((l 1 -undecanol)oxy)benzene (1): A 100mL Schlenk flask
was loaded with lb (2.5g, 3.9 mmol) and KOH (0.5g, 8.6 mmol).
96
The reaction
flask was degassed and backfilled with argon three times. A 40 mL mixture of
degassed MeOH and THF (1:3) was syringed into the reaction flask and the
reaction was allowed to proceed for 24h. The solvent was removed under
reduced pressure and the residue was dissolved in methylene chloride. The
solution was then washed with water (3x), brine (lx), and dried over MgSO4. The
solvent was removed under reduced pressure and the remaining pale yellow
solid (80% yield) was used without further purification.
H NMR (300MHz) H
6.96 (2H, s), 3.98 (4H, t, J 6.6), 3.65 (4H, t, J 6.6), 3.35 (2H, s), 1.81 (4H, tt, J 6.6
and 7.4), 1.61-1.32 (34H, multiplet);
6c
153.7, 117.5, 113.1, 82.4, 79.7, 69.6,
63.1, 32.9, 29.7, 29.59, 29.55, 29.5, 29.4, 29.2, 26.0, 25.8; m/z (ESI) [Found:
(M+Na) + 521.3591. C32 H50 0 4 requires M, 521.3601].
Polymerization of monomers 1 and 2 to give 3: A 25mL Schlenk tube was
loaded with monomer
diiodobenzene
1 (478 mg, 0.59 mmol), 2,5-bis(hexadecyloxy)-1,4-
(2) (300 mg, 0.60 mmol), 8 Cul (6 mg, cat.) and Pd(PPh 3) 4 (34 mg,
0.03 mmol) in a mixture of DIPA and toluene (12 cm3, 1:2). The flask was
degassed and back-filled with argon three times. The mixture was heated to
65°C for 48 h. The resulting mixture was cooled to room temperature and
washed with saturated NH4CI and brine.
The reaction solution was then
precipitated into acetone. This procedure was repeated twice. The precipitate
was collected and dried to give the desired polymer 5 as a red solid;
max
3407, 2922, 2851, 2361, 2340, 1513, 1467, 1429, 1387, 1275, 1213, and
1041;
6H
7.0 (4H, br s), 4.0 (8H, br t), 3.6 (4H, t, J 6.7), 1.9 (5H, br multiplet), 1.6-
97
1.5 (17H, br signal) 1.4-1.2 (72H, br signal), 0.9 (6H, t, J 6.9); GPC Mn 5.60 x
104 , Mw 1.51 x 105 , M/Mn 2.70.
Preparation of ATRP macroinitiator (4): The initial PPE, 3 (100mg, 0.19mmol
of -OH functionality), was added to a 50mL schlenk flask and the flask was
evacuated and backfilled with argon three times. Next, anhydrous triethylamine
(5mL) and THF (10mL) were syringed into the reaction mixture. The solution
was cooled to 0°C in an ice bath followed by the dropwise addition of 2bromoisobutyryl bromide (0.24mL, 1.9 mmol).
The reaction was allowed to
return to room temperature and stir for 3 hours. The solvents were concentrated
under reduced pressure and the residue was precipitated into acetone and
filtered.
The filtrate was washed with water several times to remove the
triethylamine salts. The precipitate was collected and dried to give the ATRP
macroinitiator
4 as a dark red solid; Vmax 2923, 2852, 2360, 2341, 1735, 1513,
1466, 1429, 1388, 1275, 1213, 1162, 1108, and 1024; 6H 7.0 (4H, br s), 4.2 (4H,
t, J 6.6), 4.0 (8H, br t), 1.9 (12H, br s), 1.8 (8H, br signal), 1.7 (8H, br tt), 1.5 (8H,
br signal), 1.4-1.3 (68H, br signal), 0.9 (6H, t, J 6.9).
ATRP polymerization of polystyrene from 4 to give PS grafted PPE (5): A
25mL Schlenk tube was loaded with the ATRP macroinitiator, 4 (30mg, 0.04
mmol), CuBr (6.3mg, 0.04 mmol), and 2,2-dipyridyl (Bipy) (17mg, 0.11 mmol) in
a glove box. Approximately 2mL of styrene (distilled from CaH2) was then
syringed into the reaction tube. The mixture was frozen, evacuated, and thawed
98
three times. The reaction flask was then heated in an oil bath to 90°C and
allowed to stir for 1 h. At the end of the reaction time, the reaction flask was
cooled in liquid N2 in order to quench the polymerization and then the mixture
was filtered and precipitated out of MeOH. The precipitate was then filtered,
washed several times with water, collected, and dried to give the PS grafted PPE
5 as a bright yellow solid; vmax3082, 3059, 3025, 2923, 2851, 2361, 2339, 1733,
1718, 1653, 1493, 1452, 757, 697, and 668;
H
7.2-6.9 (Aromatic br signal), 6.9-
6.3 (aromatic br signal), 4.0 (br t), 3.8 (br t), 2.0-1.7 (br signal), 1.6 (br s), 1.5-1.3
(br signal), 1.27 (br s), 1.0-0.8 (br t); GPC Mn 1.20 x 105 , M, 2.16 x 105 , MV/Mn
1.80.
99
3.7 References
[1] Bunz, U. H. F. Chem. Rev. 2000, 100, 1605.
[2] (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.
[3] (a) Alkan, S.; Toppare, L.; Hepuzer, Y.; Yagci, Y. J. Polym. Sci. A: Polym.
Chem. 1999, 37, 4218.
(b) Waugaman,
M.; Pratt, L.; Khan, I. Polym. Prepr.
1997, 38, 257. (c) Malenfant, P.; Frechet, J. Macromolecules 2000, 33, 3634.
[4] (a) Olinga, T.; Francios, B.; Makromol. Chem., Rapid Commun. 1991, 12,
575.
(b) Hallensleben,
M.; Hollwedel, F.; Stanke, D. Macromol. Chem. Phys.
1995, 196, 3535. (c) Ranieri, N.; Ruggeri, G.; Ciardelli, F. Polym. Int. 1999, 48,
1091.
[5] Costanzo, P. J.; Stokes, K. K. Macromolecules 2002, 35, 6804.
[6] (a) Matyjaszewski, K.; Xia, J. Chem. Rev. 2001, 101, 2921. (b) Kamigaito, M.;
Ando, T.; Sawamoto, M. Chem. Rev. 2001, 101, 3689.
[7] Borner,
H. G.; Beers,
K.; Matyjaszewski,
K.; Sheiko,
S. S.; Moller, M.
Macromolecules 2001, 34, 4375.
[8] Swager, T. M.; Gil, C. J.; Wrighton, M. S. J. Phys. Chem. 1995, 99, 4886.
[9] Perrin, D. D.; Amarego,
W. L. F.; Perrin, D. R. Purification of Laboratory
Chemicals; Pergamon: Oxford, 1988, 3 rd ed.
100
Chapter 4
Polarized Photoluminescence from Poly(p-phenylene-ethynylene) via
a Block Copolymer Nanotemplate
E
EE
Partially adapted from:
C. A. Breen, T. Deng, T. Breiner, E. L. Thomas, T. M. Swager, J. Am. Chem. Soc. 2003, 125,
9942.
T. Deng, C. A. Breen, T. Breiner, T. M. Swager, E. L. Thomas, Polymer, 2005.
4.1 Abstract
A new approach
based
on a conjugated
polymer/block
copolymer
guest/host system for the generation of polarized photoluminescence is
discussed.
The synthetically modified poly(p-phenylene-ethynylene) (PPE)
conjugated polymer, described in Chapter 3, is used for domain specific
incorporation into a cylindrical morphology block copolymer host matrix.
Subsequent ordering of the host nanostructure via roll cast processing templates
uniaxial alignment of the guest PPE. The ordered films are optically anisotropic
displaying both polarized absorption with a dichroic ratio of 3.0 at 440 nm and
polarized emission with a polarization ratio of 6.4 at 472 nm.
Deforming the
thermoplastic elastomer matrix in a direction perpendicular to the orientation
direction of the cylinders causes rotation of the PS cylinders and the PPE emitter
molecules.
This affords mechanically tunable polarized emission due to re-
orientation of the PPE containing PS cylinders as well as film thinning from
Poisson effects.
102
4.2 Introduction - Polarized Photoluminescent Materials
4.2.A Overview
Polarized luminescent materials1 have attracted attention for potential
uses in flat panel displays2 and other optoelectronic devices.3
polymers
(CPs),
such
as
poly(p-phenylene-vinylene)
(PPV) 4
Conjugated
and
poly(p-
phenylene-ethynylene) (PPE)2 '5 derivatives have been of particular interest for
use as polarized emitters due to the intrinsic anisotropy of their (quasi) 1dimensional electronic structure and ability to be processed in uniaxially oriented
blends by various techniques such as rubbing,6 friction transfer,7 tensile
drawing,4 and Langmuir-Blodgett film deposition.8
This work presents a new
approach to achieving polarized emission from CPs, specifically PPEs, by using
a block copolymer host matrix as a nanotemplate for the spatial orientation of
CPs.
This technique provides a means of generating large area films for
polarized photoluminescence as well as an opportunity to realize mechanically
tunable polarized emission.
4.2.B Block Copolymers
Block copolymers are comprised of two or more chemically distinct blocks
that are covalently linked into polymer chains. When the different blocks have a
strongly repulsive interaction, they tend to phase separate into microdomains to
minimize the overall free energy.9-12 Typically, the interaction between different
blocks varies inversely with temperature. Thus, cooling a block copolymer from
its high temperature homogeneous melt to below its order to disorder transition
103
(ODT) results in microphase separation. Alternatively, the repulsive interaction
between the blocks can be screened by the presence of a solvent. In this case,
solvent evaporation increases the effective interaction, and only induces
microphase separation when the system crosses the ODT concentration.
Microphase-separated
block
copolymers
can
form
numerous
morphologies depending on their composition. The simplest linear AB diblock
copolymers, for example, form sphere, cylinder, double gyroid, and lamellar
morphologies (Figure 4.1). Some complex block copolymers, e.g. miktoarm star
terpolymers, show many other morphologies.1
In all of these morphologies,
block copolymer microdomains self assemble into one-, two-, and threedimensional periodically ordered structures. In 1971, Keller et al. demonstrated
near single crystal texture development in an ABA triblock copolymer using the
flow field generated in a capillary rheometer.1 2 This discovery stirred interest in
the global ordering of block copolymer microdomains and as a result, various
13
methods have been developed over the past several decades.
16
A roll casting process for ordering block copolymer microdomains from a
solution was developed using the flow field generated between two counter
rotating rollers.1 4
It offers a convenient alternative route to large area ordered
block copolymer microdomain films without involving strong electric field and/or
high temperature processing. Roll casting is also a process that is amenable to
large-scale production and to high molecular weight polymers where the ODT is
well above the thermal degradation temperature.
104
Ability to Microphase-
BCC
parate
3D Structures
lam
Increasing Weight Fraction
Block A
Figure 4.1 Phase diagram for a simple A-B di-block copolymer system.
Highly organized
block copolymers
offer many
opportunities for
applications requiring such order for systematic control and precise property
manipulation.17 -20
For example, Ha et al. systematically studied a block
copolymer-layered silicate nanocomposite system in which the block copolymer
microdomains were ordered by roll-casting. 17 2' 1
Li et al. used block copolymer
templates to fabricate ordered GaAs nanostructures.1 8 Cheng et al. have used a
block copolymer combined with graphoepitaxy to produce a 2D mask for creation
of ultrahigh density 2D cobalt dot arrays as magnetic storage media.1 9 The use
of block polymers as a directing template for nanoparticles is analogous to the
situation to be discussed in this chapter. Here, instead of a nanoparticle, the
block copolymer is used to guide emitter molecules into specific domains and to
create alignment of these guest molecules.2 2 The role of block copolymers in a
host of nanotechnology related applications has been recently reviewed.2 0
105
4.2.C Alignment of Conjugated Polymers
Polymers offer a versatile matrix to host optically active molecules. For
example,
polarized emission from a dopant has previously been reported in
stretched polymer films.23'2 4
Gin et al. reported a self-assembly approach in
templating poly(phenylenevinylene) (PPV) into hexagonally ordered microdomain
structures.2 5 However, in their approach only a very low concentration of PPV
could be loaded into the template. Therefore, no polarized light emission data
could be measured since their system was not well ordered over a large region.
Weder et al. developed a photoluminescent polarizer using tensile drawing
of ultrahigh molecular weight polyethylene (UHMW PE) blended with PPEs
(Figure 4.2) and PPVs.2 a
By incorporating these films into commercial
liquid
crystal displays (LCDs), they were able to generate single color and bicolor LCD
displays as shown in Figure 4.3. These systems were limited by the low optical
density of the PL polarizer (2% w/w PPE) as well as the reduced emission
efficiency due to aggregation of the highly aligned PPE polymer chains.
In an attempt to improve upon the device performance, the group
introduced a non-anisotropic small molecule dopant. This resulted in an increase
in the amount of unpolarized light absorbed by the PL polarizer, while still
transferring energy to the aligned PPE emitter in order to generate polarized
light.2 b
Figure 4.4 shows the materials system and the observed polarized
emission.
106
'
--
---I-
0
Z
To
0c
1
C
U*
C)
I
, V
I
LI P
c
1l
E
I
_
_
400 500 600 700
-
0
S
C
0
b.
0.C
Ca
4
300
400
500
600
0
400
1
500
600
700
Wavelength (nm)
Figure 4.2 Polarized absorption (A) and photoluminescence (B) of an oriented
film (draw ratio,
= 80) of a 2 weight % EHO-OPPE-UHMW-PE
blend.
The
spectra were recorded with polarizers oriented parallel (solid line) and
perpendicular (dashed line) to the orientation direction of the polymer blend films.
The inset in (B) shows at a magnification of 70 the spectrum recorded
perpendicular to the orientation direction.2a
107
A
VieCer
Stase
I
oailzer
TN C ce I
EHO-OPPE P_ aeft
I
UV
A
4 t
arnp
4 4 A I
.F *
A
-r
B
V'len P
Stredtpck.Ifizo
-------.
TN C ce I
FHO OF'Pt P_ ayoer
MEH PPVP_L.bYcr
VV11MO
,:
.....................
[_ l ___
-
+
4,
.
VI* Vf'
EHC-OPPE PL
.
f
A
, ..,
aef
IN LC -.-l
_ ,
.
tV ;!('l.t
l-H
- t .. . ld. t.
UV~a",
'C,
.
UV lamCT-
+
'
A
C
Figure 4.3 Schematic structures of PL display devices. (A) Device in which the
light emitted by the polarized PL layer is switched.
the light emitted by the polarized PL layer is switched.
(B) Bicolor device in which
(C) Device where the UV
excitation light is switched. Arrows at the left indicate the polarization directions
of the respective elements.
Devices A and B constituted a commercially
available, standard TN cell fitted with a linear sheet polarizer. Device C was
based on a laboratory
UV-active TN cell and a UV-active
sheet polarizer
(Polaroid HNP-B) producing linearly polarized light. All devices used a Bioblock
UV lamp (VL-4LC).
Photographs (right) of a PL display device like that shown in
A compared to a similar commercial, direct view LCD. Also shows a photograph
of the bicolor display device where the MEH-PPV-based polarized layer was
omitted in the upper left part of the display.2 a
108
U
r--
n
)
0(,.
-
t.U
:6)
450
Jo0o
DMC
-! P F:'DMC
r EHO-OPE
jll
0
w8C
4tCnvi
-- --
-IA'A
"r)r
vq
eq
y
IdyaSe
011
f r
'4
r---~----DVC
AU.
4(J.
Ii3
2 CJ
d
fl
e
h
-
PPE : \
.
-"
.-
PE'
DMC
?PPE
Figure 4.4 Polarized absorption and emission spectra of oriented films. Spectra
were obtained by irradiating the sample with vertically (solid line) and horizontally
(dashed line) polarized light. Emission spectra were obtained by exciting the
films at 365 nm. a, Binary UHMW PE/DMC blend; b, Binary UHMW PE/EHO-
OPPE blend; c, ternary UHMW PE/EHO-OPPE/DMC blend.
Photophysical
processes and optical emission. d-f, Schematic representation for uniaxially
oriented films of the binary reference blends of UHMW PE/DMC (d), UHMW
PE/EHO-OPPE (e) and the ternary UHMW PE/EHO-OPPE/DMC blend (f).
Arrows indicate polarizations of incident and emitted light.2 b
4.3 A New GuestlHost System
While the system above shows excellent alignment of the guest PPE
emitter, the tensile drawing technique does not provide a viable route to
generating large area films. In addition, aggregation phenomena associated with
the PPE guest causes the overall emission efficiency of the system to be low. It
109
is the aim of this chapter to discuss the design and demonstration of a new
guest/host system for the generation of polarized photoluminescence.
The
ultimate goal is to simultaneously achieve high emission efficiencies while
providing a generalized method for generating large-area, highly aligned films
that can be used as photoluminescent polarizers.
The system developed here involves the self-assembly and global
orientation of a cylindrical morphology block copolymer host matrix by roll cast
processing that results in an ordered nanostructure which provides a template for
the alignment and spatial ordering of a guest PPE. Sequestering the grafted
PPE system discussed in Chapter 3 into the cylinder domain of a block
copolymer and globally orienting the host matrix results in the alignment of the
PPEs parallel to the long axis of the cylinders. The uniaxial orientation of the
PPEs within the block copolymer host gives rise to polarized absorbance and
photoluminescence.
In addition, we prepared a series of roll-cast grafted PPE
doped block copolymer thin films with various thicknesses and characterized
them using transmission electron microscopy (TEM), small angle x-ray scattering
(SAXS), and fluorescence spectroscopy. We compare the emission properties of
this highly anisotropic system with those of a corresponding simple cast isotropic
grafted PPE doped block copolymer system. We also present an initial study of
the strain dependence of the polarized emission for samples deformed
perpendicular to the orientation direction of the anisotropic film.
110
4.4 Results and Discussion
4.4.A Overview
Roll cast processing requires a rubbery matrix phase, and thus, we used a
polystyrene-polyisoprene-polystyrene
(SIS) triblock copolymer system with
styrene cylinders and an isoprene matrix.
Consequently, our approach was to
use the polystyrene grafted PPE system discussed in Chapter 3 as the doped,
optically active material.
Due to the presence of the polystyrene grafts, the
grafted PPE has selective miscibility to the cylinder phase of the block copolymer
host. Hence, by sequestering the guest preferentially into the cylinder domain
and globally orienting the host matrix, the resulting nanostructure should
template the orientation and spatial ordering of the guest PPE system.
The polymers used for this study are summarized in Table 4.1. The endto-end distance of a PS homopolymer with a molecular weight of 5,600 g/mol is
about 5 nm and the average contour length of a PPE emitter molecule is about
30 nm. The persistence length of similar PPEs was measured to be 15 nm.2 6
Therefore, the PS grafted PPE component can be pictured as a reasonably stiff,
30 nm long PS coated 5 nm diameter particle with an aspect ratio of roughly 5.
An illustration of the composite materials system described above (drawn
approximately to scale) is shown in Figure 4.5.
111
U-k
-~~~~~
I
k
-§
I
I ,/
'Is·JL//'?-"
PS
Figure 4.5
PI
PS
Schematic showing the alignment of PS grafted PPE by
sequestration inside the PS domains of a SIS triblock copolymer. The grafted
PPE was co-assembled into PS cylinders within the PI matrix.
112
Table 4.1 Polymer Characterization
Photophysical Characterization
Polymer
GPC Analysis
Mn
PDI
(Solid State)
kMaxAbs
(nm)
XMaxEm
(D
(nm)
PPE Backbone (3)
55,800
2.59
480
495
0.10
PS Grafted PPE (5)
80,000
1.62
445
475
0.60
5,600
1.17
--
--
--
101,000
1.05
Cleaved PS Grafts
PS/PI/PS
4.4.B Roll Cast Processing
The roll cast films consisted of PS grafted PPE doped in poly(styreneisoprene-styrene) (SIS) triblock copolymer.
The
block copolymer was
commercial grade Vector 4211 purchased from Dexco (Dexco Polymers, 12012
Wickshester, Houston, TX 77079). The triblock has a composition of 29 wt % PS
(26 vol %) as determined by NMR and a total molecular weight of 101 x 103 as
determined by GPC. Thus, the block molecular weights are (15-72-15) x 103 .
The isoprene block contained 92% 1,4 addition.2 7 The roll cast solutions were
prepared by making 40% (w/w) solutions of polymer in cumene. Initially, the roll
cast solution also contained THF to ensure a homogenous solution but the THF
was allowed to evaporate until only cumene remained. The PS grafted PPE was
blended into the roll cast solutions at 0.1 weight % of polymer. The viscous
polymer/cumene solution was then used directly for roll cast processing.
113
Motor
D,- D11.
E-: .I.e OLJLCI
%3WHIM55
XVIIVI
4_
uap
[
I
I I
k
//////////////A
Teflon
Roller
Motor
Teflon Roller
J
.k
i
Stainless Steel
Roller
I
Micrometer
Enclosure with
small vents
Figure 4.6 Schematic diagram of the roll cast apparatus.
The roll cast apparatus2 8 used to orient the composite films is shown in
Figure 4.6.
The apparatus consists of two
parallel rollers powered
independently. One roller is stainless steel, the other is Teflon, and the two are
separated by a micrometer-controlled gap.2 7
Typical processing conditions
involve covering the rollers uniformly with roll cast solution, setting the roller gap
anywhere from 0.05-0.4 mm, and matching the roller speeds to 8rpm.
The
apparatus is enclosed during the roll cast process in order to slow solvent
evaporation. The solution is initially coated onto both rollers.
Once enough
solvent evaporates, the polymer film transfers completely to the stainless steel
roller. At this point, a lengthwise cut is made to the film along the roller. The film
is allowed to dry for 24 hours before it is removed from the roller with isopropanol
and stored in between two Teflon plates. After roll cast processing, the resultant
114
films were approximately
1 mm thick and 6 x 16 cm 2 in area. The polystyrene
cylinders were oriented in the plane of the film and hence the films displayed the
expected mechanical anisotropy9 due to the oriented glassy styrene cylinders in
an elastic isoprene matrix. Simple cast, unoriented doped block copolymer films
were prepared by quiescently casting on glass slides using the same solution as
for the roll casting. During simple casting, glass slides were also covered with a
box to slow solvent evaporation.
4.4.C Roll Cast Film - Structural Characterization
Structural characterization of the guest/host films was conducted using
transmission electron microscopy (TEM) and small angle X-ray scattering
(SAXS).
Figure 4.7 shows the TEM images and the corresponding SAXS
patterns of both simple cast and roll cast films. To prepare the TEM sample, a
small piece of polymer was embedded in epoxy and then microtomed using a
Reichert-Jung
Ultracut FC 4E at temperatures
of -90
C (knife) and -110 °C
(sample) with a nominal section thickness of 70 nm.
The sections were
subsequently placed above a 4% aqueous solution of osmium tetraoxide for 2
hours to selectively stain the PI block. A thin carbon film was also evaporated
onto the sections to enhance electrical conductivity before performing TEM.
TEM was carried out on a JEOL 200 CX using a tungsten filament source at 200
keV accelerating voltage.
Energy Synchrotron
SAXS studies were performed at the Cornell High
Source (CHESS) using monochromatic
115
X-rays and a 2D
charge coupled detector (CCD). The selected X-ray wavelength was 1.51 A
while the sample to detector distance was 250 cm.
A TEM image of the simple cast film is presented in Figure 4.7a.
Figure
4.7b shows a TEM image of the roll cast film with the viewing direction along the
cylinder axis. After microphase separation, the PS cylinders in the simple cast
films arrange into small grains as evidenced in Figure 4.7a, whereas in roll cast
films, the PS cylinders formed a hexagonal lattice in PI matrix with excellent long
range order (Figure 4.7b). The SAXS patterns also demonstrate the isotropic,
polygranular nature of the simple cast films via the presence of uniform scattering
rings (Figure 4.7a insert). The long-range order of the PS cylinders in the roll
cast films is evidenced by the near single crystal texture of the SAXS patterns
(Figure 4.7b insert) as well as the clear presence of a hexagonally
packed
cylindrical structure noted in the TEM micrograph. The azimuthal full-width at
half-maximum (FWHM) of the (100) reflection of the roll cast film (the first peak in
Figure 4.7b insert) is approximately 10°.
In roll cast films, the flow field
generated induces the alignment of the PS cylinders along the flow direction.
Optical characterization, discussed in the next section, also demonstrates that
the PS grafted PPE molecules align along the flow direction.
116
b
0
125 nm
100 nm
Figure 4.7 Cross-sectional transmission electron microscopy (TEM) images and
small angle x-ray scattering (SAXS) data. (a) SIS triblock copolymer blended
with 0.1% (w/w) PS grafted PPE after simple casting. Small grains and a lack of
overall microdomain
ordering are apparent in the TEM image and an isotropic
scattering ring is evident in the SAXS pattern. (b) Same system after roll casting.
The TEM view is along the axis of the cylinders showing the PS cylinders (white
circular dots) form a long-range ordered hexagonal lattice in the PI matrix.
The
SAXS pattern with the incident x-ray beam orthogonal to the cylinders shows the
1,
3, and
-4
reflections of the hexagonal lattice of cylinders.
The
FWHM of the first reflection is only 10 degrees, indicating the high axial
alignment achieved by the roll cast process.
117
4.4.D Roll Cast Film - Polarization Spectroscopy
Photophysical characterization of the globally oriented roll cast films was
conducted. Polarized absorption was measured using a Cary 50 UVNisable
spectrometer. Polarized photoluminescence measurements were obtained using
a SPEX Fluorolog-
2 fluorometer (model FL112, 450W xenon lamp) equipped
with a 1935B polarization kit.
measurement
A schematic representation of the PL
setup is shown in Figure 4.8.
Before each measurement,
the
spectrometer was calibrated using a standard aqueous silica bead solution
(Dupont, Ludox).
Two different emission spectra, represented by Sv, and Svh
were obtained from a sample (vertically oriented cylinders relative to the
instrument polarizers) by rotating the polarization of the detection signal. The
subscripts correspond to the polarization of the emission and detection signals;
for example, Svh is the emission spectrum obtained from the sample with
vertically polarized excitation and horizontally polarized detection. In order to
correct for the instrumental polarization dependence associated with the
monochromator, a sample of the roll cast solution was used to determine the
instrumental correction factor, also known as the "G" factor.
determined
This value is
by measuring the ratio of Shy to Shh collected from the solution
sample. Thus, the polarized emission spectrum displayed in Figure 4.9 was
generated by multiplying the G factor by Svh,yielding the corrected I spectrum,
and then plotting the resultant spectrum against the 11spectrum, namely, Sv,.
The polarization ratios reported were then determined by taking a ratio of the
118
intensities at the peak emission wavelength for the II spectrum (Ivv) verses the
corrected I spectrum (vh), as shown in the following expression:
Polarization
(4.1)
Ratio = Ivv/G(Ivh), where G = Ihv/lhh
Excite Vertical or Horizontal
Light
Source
x
*,/
4
Samole
- ----i
IV
Polarization Ratio =
(G)(IvH)
lonochromator
A
HH
IHV
G = I
flr
Detector
W
IHH
or HV
Figure 4.8 Schematic diagram of the measurement system used to determine
the degree of polarized emission generated from the roll cast PS grafted PPE
doped block copolymer films.
119
1~
Z
cri
0
1C
.0
M
q,
e
0
0
42.
00d
C
0
U
S
LL
400
420
440
460
480
500
520
)
Wavelength(nm)
Wavelength(nm)
Figure 4.9
Polarized absorption (a) and photoluminescence
(b) of a roll cast
oriented film of 0.1 weight % PS-PPE doped in SIS. The spectra were obtained
with polarizers oriented parallel (solid line) and perpendicular (dashed line) to the
PS cylinder orientation of the block copolymer host. The emission spectrum in
(b), measured
perpendicular
to the polarizers
(dashed line), is corrected for
instrumental polarization dependence.
Polarized absorption and photoluminescence
spectra are shown in Figure
4.9. The observed dichroic ratio for the PPE doped roll cast films was 3.0 at X =
440 nm while the polarization ratio for the PPE doped roll cast films was 6.4 at X
= 472 nm. These values indicate that the transition dipoles of the guest PS
grafted PPE are aligned parallel to the cylinder axis of the block copolymer host
and thus emit polarized light with polarization parallel to the plane of the film.
Furthermore, the difference in the absorbance dichroic ratio as compared to the
emission polarization ratio suggests that this particular materials system is able
to absorb a significant fraction of unpolarized light and funnel the excitation into
120
polarized emission. This phenomena can be explained by intramolecular energy
transfer, or energy migration, from less aligned segments of conjugated polymer
to the highly aligned, and therefore more conjugated, segments of conjugated
polymer within the host cylinder domains. This rationale is supported by the fact
that as the excitation wavelength
is increased to X = 440 nm, i.e. towards the
band edge, the observed polarization ratio increases to 7.3 at X = 472 nm.2 9 The
lower-energy excitation directly excites the more aligned, longer-conjugation
length segments.
A schematic representation of the intramolecular energy
transfer observed in this system is shown in Figure 4.10.
The control sample, composed of an unoriented, simple cast grafted PPE
doped block copolymer film was also probed spectroscopically relative to the roll
cast film. Figure 4.1 la and Figure 4.1 lb are the polarized emission spectra of a
simple cast and roll cast film, respectively. Both films had a thickness of - 40
m. Figure 4.11a reveals, as expected, that the simple cast film does not exhibit
polarized emission. The roll cast film in Figure 4.1 1b, similar to that in Figure 4.9,
displays polarized emission with a polarization ratio of about 6.2, again
demonstrating that the roll cast processed block copolymer host matrix templates
the alignment of the guest PPE.
121
I
Lower Conjugation
Length Segments
;her Conjugation
.ngth Segments
Excitation at 400 nm:
Polarization Ratio
i
0. 4
xcitation at 440 nm:
Polarization Ratio -- 7.3
Wavelength (nm)
Figure 4.10 Artistic representation of the PS grafted PPE alignment within the
PS domain of the block copolymer host matrix. Intramolecular energy transfer, or
energy migration, from the less aligned PPE segments to highly aligned
segments allows this particular system to absorb a significant amount of
unpolarized light and transfer this energy to on particular polarization.
122
.1.d
en
a)
a
cn
-
4"
C
I0>
us
an
9
0
M
450
500
550
600
450
500
550
600
Wavelength (nm)
Figure 4.11 Polarized photoluminescence of a simple cast (a) and roll cast film
(b). The thickness of both films was - 40 mm. The excitation wavelength was k
= 400 nm and the sample was irradiated perpendicular to the surface of the film.
4.4.E Roll Cast Film - Anisotropy
The angular dependence of the optical anisotropy of grafted PPE doped
roll cast and simple cast films was also explored. Figure 4.12 shows the angular
dependence of anisotropy in a simple cast film and a roll cast film (both
measured at the peak position of the emitted radiation, i.e., at X = 472 nm). The
films were mounted on a rotation stage and rotated to specific angles before
each measurement. The angle between the excitation polarizer and the cylinder
axis is designated as q. The anisotropy "r" of the emitting light is defined as: 30
r () = (I,(0) - 1,(0)) / (11(0)+ 21,(0))
(4.2)
123
0.6
0.5
0.4
,
0
0.3
0.2
0 0
.1
-0.1
-0.2
-0.3
0
Angle ()
50
100
150
200
250
300
350
Between Excitation Polarizer and the Cylinder Axis
Figure 4.12 Angular dependence of anisotropy r () of a simple cast film and a
roll cast film at X = 472 nm. The thickness of both films was - 80 mm. The
nonzero anisotropy in the isotropic film arises due to the inherent polarizability of
the PPE guest molecule. Due to the relatively short lifetime of the PPE system,
the polarized excitation results in emitted light that retains a small, non-zero
polarized emission component.3 0
124
Since the cylinders have no overall orientational anisotropy in the simple
cast film, neither do the conjugated emitter molecules.
Consequently the
anisotropy does not change with angle as shown in Figure 4.12. In the roll cast
films, the anisotropy is maximized when q = 0 (the excitation polarizer is parallel
to the cylinder axis), and minimized for q = 90.
For an ideally ordered
guest/host film without scattering and other depolarization effects, the anisotropy
will be proportional to cos2 (q).3 1 The solid line in Figure 4.12 is a fit to r = (rmax rnmin)cos2 (q) + rmin, in which rmax is the maximum experimental
anisotropy and rmin
is the minimum.
The experimentally observed anisotropy values are smaller than the
theoretical values based on perfect alignment. One way the anisotropy can
decrease is due to depolarization of the emitted light by scattering within the
sample. In order to understand the origins of depolarization, we studied the
thickness dependence of the polarized emission in roll cast films by roll casting
films with thickness in the range of 0.04 mm to 0.5 mm. The FWHM of the (100)
reflection of all these films is in the range of 100 to 120, so we can assume any
decrease in the anisotropy is not due to alignment differences of the PS
cylinders.
Figure 4.13 shows the anisotropy (at X = 472 nm) of these films with
different thicknesses.
The decrease in anisotropy with the increase in film
thickness suggests that depolarization is mainly due to scattering inside the
polymer films. In a single scattering event, the anisotropy could decrease to 70%
of its original value.3 1 As the thickness of the film increases, the scattering
125
increases and the anisotropy decreases. If scattering is the only mechanism for
depolarization, then we would expect the anisotropy to decrease linearly with an
increase in sample thickness,3 1 which is in reasonable agreement with the data in
Figure 4.13. The picture that emerges is that the PS cylinders are better aligned
than the much smaller aspect ratio PS grafted PPE "particles" inside them (see
Figure 4.5).
0.6
0.5
.
0.4
0
0.3
0.2
0
100
200
300
400
500
Thickness (m)
Figure 4.13 Thickness dependence of anisotropy at
film. The excitation wavelength was
= 472 nm of a roll cast
= 400 nm and the thickness of the film
varies between -40 mm to -470 mm. The variance in the anisotropy measured
is -10%.
126
4.4.F Mechanically Tunable Polarized Emission
As noted before, this PS grafted PPE-SIS block copolymer system not
only has emission anisotropy, but also has mechanical anisotropy.2 7' 2 8 The film
is strong and stiff along the PS cylinder direction, while it is soft and easily
deformed normal to cylinder direction.
This mechanical anisotropy can be
rationalized as follows: when stretched parallel to the cylinder axis, the serial
arrangement of the PS cylinders in this direction prohibits deformation due to the
glassy nature of the PS. However, when stretched perpendicular to the cylinder
orientation, the parallel arrangement of the PS cylinders does not inhibit
deformation of the rubbery PI matrix. As force is applied normal to the cylinder
axis, the PS cylinders initially move apart and then eventually kink bands are
initiated and the cylinders rotate towards the stretch direction. At short times
after the release of force, the cylinders do not fully recover to their original
alignment and there is residual strain in the system. During the second (and
subsequent) loading/unloading cycle, however, the cylinders approximately
recover to their initial state at the beginning of the second cycle, and the film
shows nearly reversible deformation behavior.
Figure 4.14 shows the measured optical anisotropy as a function of
applied strain for the second loading and unloading cycle. When force is applied
normal to the original roll cast direction, the film thickness decreases and above
some strain, the PS cylinders buckle and rotate towards the stretching direction.
Consequently, the normalized optical anisotropy decreases, dropping by about
50% of the initial normalized anisotropy at a strain of about 200%.
127
After
releasing the force, the rubbery PI matrix pulls the PS cylinders back into their
original aligned states, recovering the initial anisotropy.
This experiment
demonstrates the potential of this system as a mechanically tunable
photoluminescent polarizer.
.
I
·
·
II
I
1.0 0.9 N
*<'
*A*
0.8-
zO
t
0.7-
0.
2
0.6-
0.54
0.4
-
I
0
_~~~~~~~~~~~~~~~~~~~~
50
I
100
150
I
200
200
250
Strain (%)
Figure 4.14 Change of anisotropy with applied transverse strain. The film (-100
mm) was initially deformed to 240% and then released. The data shown is for
the second loading/unloading cycle.
128
4.5 Conclusions
This materials system, combined with roll cast processing, provides
advantages over other techniques for achieving polarized photoluminescence.
Many of the systems and processes reported to date are not readily amenable to
large-scale production.1 In addition, the emission efficiencies of these systems
are often limited by aggregation phenomena found in highly aligned parallel
chains of conjugated polymer.1 In contrast, we have demonstrated the ability to
process large-area, uniform films that produce polarized PL. The grafted PPE,
used as the optically active material, intrinsically eliminates aggregation, and thus
the aligned films have enhanced emission efficiency in the solid state. Moreover,
due to the intramolecular energy transfer observed in these systems, the PSPPE-SIS block copolymer system is able to absorb unpolarized light and
efficiently transfer the excitation energy into light with one particular polarization.
This is very desirable for applications in LCD backlight technologies where
introducing photoluminescent polarizers can improve the overall power efficiency
of LCD devices.
In summary, we have demonstrated that polarized photoluminescence can
be obtained from a roll cast CP-block copolymer guest/host system. Synthetic
modification of an optically active PPE guest was conducted for domain specific
doping of a block copolymer host. Globally orienting the nanostructure of the host
provided a template for the alignment of the guest PPE. The host/guest system
gives rise to both polarized absorption and PL parallel to the cylinder axis of the
host, indicative of uniaxially oriented PPEs. The versatility of block copolymers
129
also enables the modulation of light emission in different ways, for example, via
photonic band gap manipulation with high molecular weight block
copolymers,3233or as we demonstrated here, via mechanical deformation.
4.6 Experimental Section
General. The synthesis of the monomers and polymers used here is detailed in
Chapter 3. UV-visible absorption spectra were measured with a Cary 50
UVNisible spectrometer. Fluorescence spectra were measured with a SPEX
Fluorolog-t2 fluorometer
(model FL112, 450W xenon lamp).
Polarized
absorption and emission spectra were acquired from these instruments equipped
with a Cary Fibre Optic Coupler accessory and a 1935B polarization kit,
respectively.
The poly(styrene-isoprene-styrene) (SIS) triblock copolymer was
commercial grade Vector 4211 purchased from Dexco (Dexco Polymers, 12012
Wickshester, Houston, TX 77079). The triblock has a composition of 29 wt % PS
(26 vol %) as determined by NMR and a total molecular weight of 101 x 103 as
determined by GPC. Thus, the block molecular weights are (15-72-15) x
10 3 .
The isoprene block contained 92% 1,4 addition.2 7 The roll cast block copolymer
film nanostructure was characterized by transmission electron microscopy (TEM)
using a JEOL 200CX TEM operated at 200 keV. All chemicals were of reagent
grade and used as received.
130
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H. H., Scott D. B., Gronski
W., Okamoto
S., Hashimoto
T.
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E., Boyce M. C., Thomas
T., Kwon Y., Chan E. P., Breiner T. E., Cohen R.
E. L. Macromolecules,
in press.
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132
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C. G.; Swager, T. M. J. Am. Chem. Soc. 2001, 123,
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133
134
Chapter 5
Highly Efficient Blue Electroluminescence from Poly(PhenyleneEthynylene) via Energy Transfer from a Hole Transport Matrix
0
0C120
C1 2
0
_
:oO
er,
00,0
C 12
C120
0
71
R
-
C
a
z
z
Wavelength(nm)
Partially adapted from:
C. A. Breen, J. R. Tischler, V. Bulovic, T. M. Swager, Adv. Mater. 2005 in press.
5.1 Abstract
Blue poly(phenylene-ethynylene)
(PPE) electroluminescence
in a new hybrid organic light emitting device (OLED).
is observed
Energy transfer from a
hole-transport host to polystyrene grafted PPEs is utilized to improve the poor
LED characteristics of traditional PPE-based systems, thus, demonstrating the
potential for PPEs as light-emitting materials for OLED applications.
136
5.2 Introduction
Recent research in polymer based organic light emitting devices (OLEDs)
has focused on the use of conjugated polymers (CPs), such as poly(phenylenevinylene)
(PPV)1
and
poly(para-phenylene)
(PPP), 2
due
to
their
high
photoluminescence (PL) efficiency in thin films. The conjugated polymer system
that is the focus of this thesis, poly(phenylene-ethynylene) (PPE), exhibits high
solution state quantum yields and typically possesses wider band gaps than its
PPV counterpart allowing for a more blue emission.lb
PPEs also exhibit
narrower emission spectra than both PPVs and PPPs, which is desirable for
saturated color rendering in display applications. To date, however, PPEs have
received little attention as potential OLED materials, 3 as discussed in Chapter 1,
due to aggregation phenomena that lead to reduced emission efficiencies in the
solid state.
Moreover, the acetylene linkage in PPEs limits the charge
conduction properties3 c of these polymers contributing to a large potential barrier
to charge injection.
The present study, however, demonstrates that by
circumventing the negative aspects of traditional PPEs outlined above, one can
realize efficient, blue PPE electroluminescence (EL).
Utilizing the new polystyrene grafted PPE system discussed in Chapter 3,
we are able to take advantage of this system's ability to prevent polymer
aggregation allowing for a dramatic increase in the solid-state quantum
efficiency.5
Furthermore, by eliminating the intrinsic aggregation of the PPE
backbone, grafted PPEs are miscible with other materials systems as seen in
Chapter 4,5 allowing for increased compatibility as well as an opportunity to
137
exploit energy transfer in the solid state. The concept explored in this chapter is
as follows: can we develop a hybrid system consisting of a grafted PPE and a
small molecule host matrix that can be used as the hole-injection/light
layer in a standard heterojunction
OLED architecture
emitting
(see Figure 5.1)?
By
developing this concept, we are able to further demonstrate the utility of grafted
PPEs relative to their ungrafted counter-parts by studying the energy transfer
from
a commonly
methylphenyl)-[1,
used hole transport
material,
N, N'-diphenyl-N,
N'-bis(3-
'-biphenyl]-4, 4'-diamine (TPD),7 8 in both PL and EL.
5.3 Results and Discussion
5.3.A Overview
Scheme 5.1 illustrates two dialkoxy PPEs, along with their respective
grafted counterparts we have synthesized for this study; one with a co-monomer
with para-substitution
(P-PPE and GP-PPE) and another with ortho-substitution
(O-PPE and GO-PPE), which further
blue shifts the emission. 6
Table 5.1
contains characterization data for the synthesized polymers. Comparing the
solution and solid-state photophysical data for these polymers demonstrates that,
as seen in Chapter 3, the grafting process isolates the PPE backbone causing
the grafted PPE to maintain its solution state emission and quantum yield in the
solid-state.
138
.'J
Cathode
--"-,
ETL
Electron Transport Layer
LL
Organic Layers
Luminescent Layer
HTL
ITO
.
.
.
Hole Transport Layer
..
Substrate
rhode:
Light Emission
Anc
Figure 5.1
Diagram of a standard heterojunction OLED device architecture.
Electrons are injected from the cathode into an electron transport layer while
holes are injected from the anode into a hole transport layer. The charges travel
to the center of the device where they reach one another forming excited states,
or excitons, on the molecules that comprise the recombination region within the
device. This region can simply be the interface of the hole transport and electron
transport layers or, it can be another material that constitutes the desired
luminescent layer.
Once the excitons are formed within the recombination
region, these excited states can relax, i.e. the electrons and holes can
recombine, producing light.
139
Scheme 5.1
OH
O C,20
0C1 2
GO-PPE
0C1 2
o
o{,0"
GP-PPE
rn P-PPE
+9H
N-N
TPD
N0--2N
O-e*
N
TAZ
A~113 Alq3
Table 5.1 Polymer Characterization
Polymer
Mn
O-PPE
GO-PPE
PDI
Xem
kabs
Xem
(nm)
(nmII)
(nm)
(nm)
Xabs
85,000
3.50
430
455
0.68
463
472
0.20
128,000
2.18
430
455
0.57
430
456
0.50
52,000
2.70
450
475
0.70
483
498
0.10
124,000
1.90
450
475
0.62
450
476
0.60
P-PPE
GP-PPE
Photophysical
Characterization
(Solution-State)
(Solid-State)
GPC Analysis
GPC - Gel Permeation Chromatography;
- Polydispersity Index;
abs -
M - Number average molecular weight; PDI
Maximum absorption wavelength;
emission wavelength;
em -
Maximum
- Photoluminescence quantum yield. * Measurements done in
chloroform as the solvent.
140
5.3.B Photoluminescence Energy Transfer Study
The luminescence of TPD has a significant spectral overlap with the
absorption of the PPEs used in this study. Thus, efficient Forster energy transfer
from the TPD to the PPEs should be observed provided that the two systems are
miscible. In order to investigate energy transfer to the various PPEs, a blend of
TPD and PPE (50:50 w/w) was made in chloroform
solution
(10mg/mL total
concentration) and deposited onto a glass substrate via spin-casting.
Spin-casting involves mounting the glass substrate on a rotating chuck. A
small amount of the TPD/PPE solution is dropped at the center of the substrate.
Rotating the substrate results in a centripetal force that spreads the solution
radially. Adhesion of the materials to the substrate, along with the surface
tension of the solvent, result in the formation of a thin uniform film.
This
technique is widely used due to the ease and time efficient nature of the
deposition process.
The steady-state absorption and photoluminescence measurements of the
spin-cast films are plotted in Figure 5.2. The absorption spectra of the ungrafted
PPEs (Figure 5.2a and 5.2b) reveal characteristic aggregation bands at
wavelength
= 465 nm and
= 484 nm for O-PPE and P-PPE respectively,
while the grafted PPEs (Figure 5.2c and 5.2d) maintain their solution state
absorption bands. The PL spectra were obtained by irradiating each film with X =
360 nm and
= 420 nm light, exciting the TPD and PPE respectively. The films
containing the ungrafted PPEs reveal a significant spectral contribution due to
TPD upon excitation at X = 360 nm. In addition, the emission intensity of the
141
A
_C
4,
.
4
a0
0.
._
.2
Wavelength (nm)
Figure 5.2 Solid-state absorption and photoluminescence spectra of the various
TPD/PPE blends. Dashed lines indicate absorption spectra. Photoluminescence
spectra were obtained by irradiating the samples with k = 360 nm and k = 420
nm excitation wavelengths.
Plots (a) and (b) correspond to blends of ungrafted
PPE (O-PPE and P-PPE respectively), while (c) and (d) correspond to blends of
grafted PPE (GO-PPE and GP-PPE respectively).
142
PPE is not significantly changed when selectively exciting the PPE (when
accounting for background TPD emission). This suggests that there is very little
energy transfer from TPD to PPE, likely due to spatial segregation of TPD and
PPE in the films (Figure 5.3). Ungrafted PPEs are, therefore, incompatible with
the TPD host, typical of traditional PPE systems.
In the grafted PPE case (Figure 5.2c and 5.2d) minimal TPD emission is
observed when excited at k = 360 nm and the significant increase in intensity of
PPE emission relative to that under X = 420 nm excitation illustrates that this
system is thoroughly mixed and undergoes efficient energy transfer. The grafted
PPEs are, therefore, compatible with TPD suggesting that the
mixed
TPD/grafted-PPE film may display superior spectral purity and operating
efficiency in an appropriate device architecture. A schematic diagram of the
solid-state interactions between the ungrafted and grafted PPEs with the TPD
host observed in the PL energy transfer study is shown in Figure 5.3.
5.3.C Electroluminescence of TPD/PPE Systems
The same materials systems were explored in electroluminescent devices
(device structure is shown in Figure 5.5). The hole transporting TPD/PPE layer
is deposited by spin casting the same blend used for the PL energy transfer
study described above onto indium tin oxide (ITO) coated glass substrates. This
is followed by thermal evaporation of a hole blocking layer composed of 3-(4biphenyl)-4-phenyl-5-t-butylphenyl-1,2,4-triazole (TAZ), an electron transport
layer of tris-(8-hydroxyquinoline)aluminum) (Alq3), (see Scheme 5.1 for chemical
143
Ungrafted PPEs
hVExcitation
r
L--11
Grafted PPEs
a~IC·
T eCI
~~~~~2,~
hYExcitation
~
-1VV
Sp
1
sIap.
I"VPPE
Vf:r\
[
[c3I
L~iir
L-
-
g,
I
I
I
I
L-
<I-
Figure 5.3 In the system containing ungrafted PPEs, aggregated PPE molecules
(red) phase separate from the TPD host molecules (yellow) and thus do not
undergo energy transfer, displaying a significant spectral component due to TPD
in the PL energy transfer
study.
The grafted
PPEs (blue), however,
are
completely miscible with the TPD host and thus exhibit efficient energy transfer
resulting in spectrally pure PPE photoluminescence.
144
structures) and metal electrodes. The introduction of a hole-blocking layer, such
as TAZ, was necessary in order to confine the recombination region of the device
to the TPD/PPE layer and ensure that only PPE emission is produced under
device operation. In the absence of a hole-blocking layer, an additional emission
component from the Alq3 electron-transport layer can be observed.
For small molecule organic materials, such as TAZ and Alq 3 used in these
devices, sublimation, or thermal evaporation, is another method for depositing
thin films. This technique is performed in a high vacuum environment and thus,
unlike the spin-casting method discussed above, has the added benefit of
eliminating the presence of impurities in the film. Vacuum deposition can be
performed in a system similar to the one shown in Figure 5.4.
Figure 5.4 Schematic diagram of an organic vacuum deposition system.9
145
A vacuum environment is achieved by evacuation of the system using a
turbo pump in conjunction with a roughing pump. Typical base pressures are 105
to
10
-7
boats.
Torr. Organic materials are loaded into molybdenum or tantalum source
Resistive heating of the materials is achieved by increasing current
through the boats.
Once a sufficient temperature
is reached, the material
sublimes/evaporates and is deposited onto the substrate suspended above. The
thickness of the resulting film is monitored using a quartz crystal microbalance
that is capable of measuring thicknesses to within -10A.
Control of the film
thickness is achieved by simply moving the substrate shutter in and out of the
path
of the
evaporated
material.
Co-deposition
can
be
performed
by
simultaneously subliming two materials. The thickness uniformity of the films can
be ensured by rotating the substrate stage during deposition.
Thermal
evaporation also provides a means for depositing various metals in order to
generate the necessary cathode structure for the optoelectronic devices. Placing
a shadow mask over the substrate during metal deposition allows for the
delineation of specific electrical contacts onto the system.
The TPD/PPE devices shown in Figure 5.5 were prepared using a similar
vacuum
deposition
system
as
the
one
depicted
in
Figure
5.4.
The
electroluminescence spectra of the resulting TPD/PPE devices are also shown in
Figure 5.5. For comparison, the inset in Figure 5.5 shows the emission spectrum
of a TPD control device.
The control was fabricated with the same device
architecture and using a TPD concentration
identical to that used to make the
TPD/PPE blends (5mg/mL). The devices containing ungrafted PPEs exhibit the
146
.
In
-1
a
I
0
u,
c
Wavelength (nm)
Figure 5.5
corresponding
Electroluminescence spectra (solid line) overlayed with the
photoluminescence spectra
(dashed line) of the various
TPD/PPE-LEDs. Inset, plot of the EL and PL of a TPD control device. Spectra
for ungrafted PPE devices are shown in plots (a) and (b) (O-PPE and P-PPE
respectively) while (c) and (d) correspond to devices containing grafted PPE
(GO-PPE and GP-PPE respectively).
The TPD/PPE layer is spin-cast onto
clean, indium tin oxide (ITO) coated glass substrates with a thickness of 50 nm
for ungrafted PPE blends and 40 nm for grafted PPE blends. This is followed by
thermal evaporation of a 20-nm-thick film of 3-(4-biphenyl)-4-phenyl-5-tbutylphenyl-1,2,4-triazole (TAZ), a 30-nm-thick film of tris-(8-hydroxyquinoline)
aluminum)
(Alq 3), and a 1-mm-diameter,
100-nm-thick
Mg:Ag (10:1 by mass)
cathode, with a 30-nm Ag cap. The spin casting and device manipulation during
fabrication is conducted in a dry nitrogen environment, with moisture and oxygen
content of less than 5 p.p.m. All EL measurements are done in air.
147
expected TPD emission component along with PPE emission. In addition, the 0PPE device (Figure 5.5a) also exhibits a strong spectral component centered at X
= 528 nm due to aggregate emission. The grafted PPE devices (Figure 5.5c and
5.5d) exhibit predominantly PPE emission with a minimal TPD component. The
EL closely matches the PL spectra resulting in spectrally pure, narrow blue PPE
electroluminescence that has not been obtainable with traditional PPE systems
to date.3 The emission for these devices corresponds to CIE coordinates (x =
0.17; y = 0.20) and (x = 0.17; y = 0.35) for the GO-PPE
and GP-PPE
respectively. This matches the performance of many present blue display subpixels and is close to the NTSC standard blue pixel coordinates of (x = 0.15; y =
0.07). Note that the CIE coordinates of the PL spectra are even bluer (GO-PPE:
x = 0.16; y = 0.16 GP-PPE: x = 0.14; y = 0.28) as the EL devices have not been
optimized for the weak microcavity effects that in these structures emphasize
green emission.1 0
5.3.D Operational Analysis of TPD/PPE Devices
The current-voltage-luminance characteristics of the EL devices are
plotted in Figure 5.6. The inset shows the current-voltage characteristics for the
four TPD/PPE devices, as well as the TPD control device.
The ungrafted PPE
devices have large leakage currents and high turn-on voltages (-10
V).
Furthermore, the external quantum efficiencies for these devices were poor, with
peak efficiencies of r = 0.006% and /-= 0.0007% at 100 mA cm -2 for the O-PPE
and P-PPE device respectively. The higher operational efficiency of the O-PPE
148
-
a0
1I
>t4
GP-PPE/
0.5
GO-F
'PE
0-,,
0o Conrol
C,
0C,
.4%fl,
-111
*
v
0.1
U
E
.6a
4
0.05
I
'I
O-PPE
0.005
E
0.01-
O-PPE
/
,.T.......%",.Lv,:.c:;......
.
E
Cy
O-.PE............../...
IE-41
...
U
IF-R
0. 5
1
0.0005
_
i
_.
40%fo
a
·
.
5
10
P-PPE
Voltage (V)
a
0.001
.
I
;O-PPE
0.001 -
a
..........
GPPE
......
a
CI
:3
m
O
<
0.01
0.1
I
1
·
10
100
Current Density (mA cm' )
Figure 5.6 External quantum efficiency versus current density for the TPD/PPE
devices and the TPD control device shown in Figure 2. Inset shows the currentvoltage behavior for the various devices. The peak efficiency of the GP-PPE
device is r = 1.6% at 0.1 mA cm '2 and 7.5 V which corresponds to a luminance
efficiency of 3.3 cd AN1. The peak efficiency of the GO-PPE device is 1 = 1.1% at
0.1 mA cm '2 and 7.6 V which corresponds to a luminance efficiency of 1.7 cd A-1 .
The ungrafted PPE devices had peak efficiencies of ir = 0.006% and i =
0.0007%
at 100 mA cm -2 and 16.5 V which corresponded
to a luminance
efficiencies of 0.013 cd A 1 and 0.001 cd A-1 for the O-PPE and P-PPE systems
respectively. The TPD control device had a peak efficiency of wr= 0.8% at 8 mA
cm '2 and 7.5 V giving a luminance efficiency of 0.72 cd A 1 .
149
vs. the P-PPE may be due to the higher PL quantum efficiency (see Table 5.1) of
the O-PPE. Another factor may be the additional electroluminescence of the 0PPE aggregate shown in Figure 5.3.
It should also be noted the results
presented for ungrafted PPE devices are the best performance out of a low yield
sample. Most devices with ungrafted PPEs on the same substrate would short,
have exceedingly high turn-on voltages (>30V), or simply not turn on. The low
yield is most likely due to phase separation of the TPD/PPE blend in conjunction
with the poor conduction properties of the PPE backbone. An effort to improve
upon the fidelity of these devices by reducing the amount of PPE in the TPD
blend resulted in strictly TPD luminescence.
The grafted PPE devices have a markedly better behavior. EL devices
with the two grafted PPEs have matching current-voltage characteristics and
behave similarly to the TPD control, differing only by a slightly higher operating
voltage. This can be attributed to a thicker hole-transport layer and charge
trapping due to poorer conduction properties introduced by the presence of the
PPE. Furthermore, in contrast to the ungrafted PPE devices, the grafted PPE
devices were entirely reproducible, resulting in high device yields and consistent
LED performance.
The external quantum efficiencies for the grafted PPE devices exceed that
of the control at low device luminances. The peak efficiencies were r = 1.6%
and r = 1.1% at 0.1 mA cm -2 corresponding to luminance efficiencies of 3.3 cd A
1 and 1.7 cd A'1 for the GP-PPE and GO-PPE devices respectively. These
devices maintained an efficiency equal to that of the control at its peak of
150
=
0.8% at 8 mA cm2 . At increasingly higher current densities the efficiencies of the
TPD/PPE devices dropped below that of the control.
This behavior can be
rationalized in the following manner: At low current densities excitons are formed
predominantly on the TPD host molecules and energy transfer allows for highly
efficient PPE emission.
As the current density is increased, charge is
increasingly trapped at the PPEs.
Subsequent energy transfer from TPD to
charged PPE sites results in a non-radiative Auger process decreasing the
external EL efficiency. Thus, the trend continues, since the increase in injected
charged species increases the exciton-polaron interaction."
This explanation is supported further when comparing the two grafted PPE
devices. At low current densities the GP-PPE device exhibits a higher external
quantum efficiency than that of the GO-PPE, likely due to less efficient exciton
energy transfer from the TPD to the GO-PPE, a result of reduced spectral
overlap. However, as the current density is increased the efficiencies of the two
devices become equivalent.
In other words, at higher current densities the
energy transfer from the TPD host plays a less significant role in the device
efficiency - due to an increase in direct exciton formation on the PPEs - and thus
the two grafted PPE devices exhibit essentially the same behavior.
5.4 Conclusions
The device performance of the TPD/PPE systems is in agreement with the
PL energy transfer study and demonstrates the utility of grafted PPE systems
over traditional PPEs. While the ungrafted PPE systems displayed poor device
151
operation, the grafted PPE devices exhibited efficient Frster energy transfer
resulting in spectrally
pure, blue EL with high external quantum
efficiencies.
Despite the drop in device efficiency with increasing current, the grafted PPE
devices are still considerably more efficient at higher luminances than their
ungrafted counterparts as well as other reported PPE systems.3 c
In summary, we have demonstrated efficient energy transfer from a hole
transport host to grafted PPEs yielding
electroluminescence.
highly quantum efficient blue
By grafting the PPE backbones and rendering them both
quantum efficient in the solid-state as well as miscible with a hole transport host,
we are able to remove the detrimental characteristics of traditional PPE-LED
systems and realize several orders of magnitude improvement in PPE
electroluminescence. Furthermore, the grafting process is completely modular.
This allows for synthetic flexibility in terms of tuning the PPE backbone structure
in order to access other emission wavelengths and materials properties. In light
of these results, we believe that PPEs can now be accessed as viable lightemitting materials for OLED applications.
5.5 Experimental Section
General:
The general synthesis of the ortho- 6 and para- 5 monomers as well as
identical procedures used for the preparation of the polymers5 described here
has been previously reported. The characterization of the polymers used for this
study can be found below.
H NMR spectra for polymers were recorded on a
152
Varian MERCURY (300 MHz) using deuterochloroform as reference or internal
deuterium lock. The chemical shift data for each signal are given in units of 6
(ppm) relative to tetramethylsilane
(TMS) where 6 (TMS) = 0, and referenced to
the solvent residual. Infrared spectra were collected with a Perkin Elmer 1000
FT-IR spectrometer in KBr and were recorded in reciprocal centimeters (cm 1' ,
wavenumbers).
UV-visible absorption spectra were measured with a Cary 50
UVNVisiblespectrometer. Photoluminescence
spectra were measured with a
SPEX Fluorolog-t2 fluorometer (model FL112, 450W xenon lamp). Solution state
quantum yields were determined relative to Coumarin 314 in ethanol (
= 0.63)
and solid state quantum yields were determined relative to -10-2 M 9,10
diphenylanthracene in PMMA (
= 0.83). The molecular weights of polymers
were determined by Gel Permeation Chromatography (GPC) running with THF
as the eluent versus polystyrene standards (PolySciences) using Hewlett
Packard series 1100 HPLC instrument equipped with a Plgel 5 mm Mixed-C (300
x 7.5 mm) column.
Polymer Characterization:
O-PPE:
1027 and 803;
max 3448,
6
H
2923, 2852, 1508, 1466, 1437, 1376, 1262, 1220,
7.0 (4H, br s), 4.0 (8H, br t), 3.6 (4H, t, J 6.7), 1.9 (8H, br
multiplet), 1.6-1.5 (16H, br signal) 1.4-1.2 (52H, br signal), 0.9 (6H, t, J 6.9); GPC
Mn 8.50 x 104 , Mw 2.80 x 105 , Mw/Mn 3.50.
O-PPE ATRP macroinitiator:
max2923, 2852, 1736, 1509, 1467, 1435,
1389, 1275, 1220, 1163, 1108, 1030 and 805;
153
6H
7.0 (4H, br s), 4.2 (4H, t, J 6.6),
4.0 (8H, br t), 1.9 (12H, br s), 1.8 (8H, br t), 1.7 (8H, br signal), 1.5 (8H, br
signal), 1.4-1.3 (52H, br signal), 0.9 (6H, t, J 6.9).
GO-PPE: vmax3082, 3060, 3025, 2922, 2851, 1942, 1869, 1780, 1726,
1601, 1493, 1452, 1262, 1027, 803, 756, 697, and 537;
6
H
7.2-6.9 (Aromaticbr
signal), 6.9-6.3 (aromatic br signal), 4.0 (br t), 3.8 (br t), 2.0-1.7 (br signal), 1.6 (br
s), 1.5-1.3 (br signal), 1.27 (br d), 1.0-0.8 (br t); GPC Mn 1.28 x
105,
Mw 2.79 x
105, Mw/Mn 2.18.
P-PPE: max 3423, 2923, 2852, 1513, 1468, 1428, 1389, 1340, 1262,
1215, 1026, 858, 802 and 721;
6
H
7.0 (4H, br s), 4.0 (8H, br t), 3.6 (4H, t, J 6.7),
1.9 (8H, br multiplet), 1.6-1.5 (16H, br signal) 1.4-1.2 (52H, br signal), 0.9 (6H, t,
J 6.9); GPC Mn 5.20 x 104, Mw 1.40 x 105, Mw/Mn 2.70.
2924, 2853, 1736, 1513, 1466, 1428,
P-PPE ATRP macroinitiator: Vmax
1389, 1275, 1213, 1163, 1108, 1030, 861, 803 and 722; 6H 7.0 (4H, br s), 4.2
(4H, t, J 6.6), 4.0 (8H, br t), 1.9 (12H, br s), 1.8 (8H, br t), 1.7 (8H, br signal), 1.5
(8H, br signal), 1.4-1.3 (52H, br signal), 0.9 (6H, t, J 6.9).
GP-PPE: vmax3081, 3059, 3026, 2924, 2852, 1943, 1870, 1801, 1730,
1601, 1493, 1453, 1262, 1027, 803, 758, 698, and 539;
6
H
7.2-6.9 (Aromatic br
signal), 6.9-6.3 (aromatic br signal), 4.0 (br t), 3.8 (br t), 2.0-1.7 (br signal), 1.6 (br
s), 1.5-1.3 (br signal), 1.27 (br signal), 1.0-0.8 (br t); GPC Mn 1.24 x 105, Mw 2.36
X 105 , MV/Mn 1.90.
154
Fabricationof electroluminescentdevices:
The devices were built on ITO coated glass substrates with a sheet
resistance of 20 Q cm 2 . The ITO glass was cleaned by ultrasonic agitation in
detergent solution, rinsed with DI water, acetone, boiled in trichloroethane, rinsed
with acetone, boiled in isopropyl alcohol, and N2 blown dry. Immediately prior to
material deposition, substrates were pretreated with UV/Ozone for 5 minutes.
The devices were fabricated by spin-casting (v: 3K; Acc: 10K; Duration: 60 sec.)
the TPD/PPE layer (50:50 w/w; 10 mg/mL total concentration), followed by
successive vacuum deposition of TAZ (20 nm), Alq 3 (30 nm), Ag:Mg (100 nm;
10:1 by weight), and Ag (30 nm). Prior to deposition, TAZ and Alq 3 were purified
by sublimation. A shadow mask with 1 mm2 openings was used to define the
metal electrodes. The EL spectra were recorded on an Acton Research Spectra
Pro
300i
spectrometer.
The
luminance-voltage
and
current-voltage
characteristics were measured on an Agilent-4156C Precision Semiconductor
Parameter Analyzer using a Newport 818-UV photodetector. All measurements
conducted at room temperature under ambient atmosphere.
155
5.6 References
[1] (a) Becker, H.; Spreitzer, H.; Kreuder, W.; Kluge, E.; Schenk, H.; Parker, I.;
Cao, Y. Adv. Mater. 2000, 12, 42. (b) Kraft, A.; Grimsdale, A. C.; Holmes, A. B.
Angew. Chem. Int. Ed. 1998, 37, 402. (c) Staring, E. G. J.; Demandt, R. C. J. E.;
Braun, D.; Rikken, G. L. J.; Kessener, Y. A. R. R.; Venhuizen, A. H. J. ; van
Knippenberg, M. M. F.; Bouwmans, M. Synth. Met. 1995, 71, 2179. (d) Zhang,
C.; Braun, D.; Heeger, A. J. J. Appl. Phys. 1993, 73, 5177. (e) Greenham, N. C.;
Moratti, S. C.; Bradley, D. D. C.; Friend, R. H.; Holmes, A. B. Nature 1993, 365,
628. (f) Burroughes,
J. H.; Bradley,
D. D. C.; Brown, A. R.; Marks,
R. N.; Mackay,
K.; Friend, R. H.; Burns, P. L.; Holmes, A. B. Nature 1990, 347, 539.
[2] (a) Yang, Y.; Pei, Q.; Heeger, A. J. J. Appl. Phys. 1996, 79, 934. (b) Stampfl,
J.; Tasch, St.; Leising, G.; Schert, U. Synth. Met. 1995, 71, 2125. (c) Jing, W-X.;
Kraft, A.; Moratti, S. C.; Gruner, J.; Cacialli, F.; Hamer, P. J.; Holmes, A. B.;
Friend, R. H.; Synth. Met. 1994, 67, 161. (d) Grem, G.; Leising, G. Synth. Met.
1993, 57, 4105.
[3] (a) Arias, E.; Maillou, T.; Moggio, I.; Guillon, D.; Le Moigne, J.; Geffroy, B.
Synth. Met. 2002, 127, 229. (b) Schmitz, C.; Posch, P.; Thelakkat, M.; Schmidt,
H-W.; Montali, A.; Feldman, K.; Smith, P.; Weder, C. Adv. Funct. Mater. 2001,
11, 41. (c) Pschirer, N. G.; Miteva, T.; Evans, U.; Roberts, R. S.; Marshall, A. R.;
Neher, D.; Myrick,
M. L.; Bunz, U. H. F. Chem. Mater. 2001,
13, 2691. (d) Zhan,
X.; Liu, Y.; Yu, G.; Wu, X.; Zhu, D.; Sun, R.; Wang, D.; Epstein, A. J. J. Mater.
Chem. 2001, 11, 1606. (e) Lee, S. H.; Nakamura, T.; Tsutsui, T. Org. Lett. 2001,
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3, 2005.
(f) Montali, A.; Smith, P.; Weder, C. Synth. Met. 1998, 97, 123. (g)
Hirohata, M.; Tada, K.; Kawai, T.; Onoda, M.; Yoshino, K. Synth. Met. 1997,
85,1273.
(h) Tasch, S.; List, E. J. W.; Hochfilzer, C.; Leising, G.; Schlichting, P.;
Rohr, U.; Geerts, Y.; Scherf, U.; Mullen, K.; Phys. Rev. B. 1997, 56, 4479.
(i)
Jeglinski, S. A.; Amir, O.; Wei, X.; Vardeny, Z. V.; Shinar, J.; Cerkvenik, T.; Chen,
W.; Barton, T. J. Appl. Phys. Lett. 1995, 67, 3960.
(j) Yoshino, K.; Tada, K.;
Onoda, M. Jpn. J. Appl. Phys. 1994, 33, 1785.
[4] Cotts, P. M.; Swager, T. M.; Zhou, Q. Macromolecules 1996, 29, 7323.
[5] Breen, C. A.; Deng, T.; Breiner, T.; Thomas, E. L.; Swager, T. M. J. Am.
Chem. Soc. 2003, 125, 9942.
[6] Zhu, Z. G.; Swager, T. M. Org. Lett. 2001, 3, 3471.
[7] Mattoussi, H.; Murata, H.; Merritt, C. D.; Lizumi, Y.; Kido, J.; Kafafi, Z. H. J.
Appl. Phys. 1999, 86, 2642.
[8] Coe, S.; Woo, W-K.; Bawendi, M.; Bulovic, V. Nature 2002, 420, 800.
[9] Bulovic, V. The Role of Excitons and Interfaces in Molecular Organic Devices
1998, Doctoral Thesis.
[10] Bulovic, V.; Khalfin, V. B.; Gu, G.; Burrows, P. E.; Garbuzov, D. Z.; Forrest,
S. R. Phys. Rev. B 1998, 58, 3730.
[11] (a) Tasch, S.; Kranzelbinder, G.; Leising, G.; Scherf, U. Phys. Rev. B 1997,
55, 5097. (b) Deussen, M.; Scheidler, M.; Bassler, H. Synth. Met. 1995, 73, 123.
157
158
Chapter 6
Blue Electroluminescence from Oxadiazole Grafted Poly(PhenyleneEthynylene)
Partially adapted from:
C. A. Breen, S. Rifai, V. Bulovic, T. M. Swager, Nano Lett. 2005 submitted.
6.1 Abstract
Blue poly(phenylene-ethynylene)
(PPE) electroluminescence
is achieved
in a single layer organic light emitting device (OLED). The polymeric system
consists of an oxadiazole grafted PPE, which combines the necessary charge
transport properties while maintaining the desirable efficient, narrow light emitting
properties of the PPE. Incorporation of a pentiptycene scaffold within the PPE
structure prevents ground-state and excited-state interactions between the
pendent oxadiazole units and the conjugated backbone.
160
6.2 Introduction
The promise of polymer based organic light emitting devices (PLEDs)
hinges on the cost-effective nature of their fabrication.
Multiple layers of
thermally evaporated materials can be eliminated when using a single polymer
thin film prepared by simple spin-coating, casting, or printing techniques.1
Present methods allow for the synthetic fine-tuning of polymer emission
wavelengths and materials properties. Hence, general materials solutions for
improved device fabrication and performance could have broad applicability.2
The previous chapter demonstrated the ability to generate efficient,
narrow, blue electroluminescence (EL) from a multilayer device utilizing energy
transfer from a small molecule host to polystyrene grafted poly(phenyleneethynylene) (PPE).3 The modularity of the grafting process can be exploited by
simply replacing the polystyrene with a material that incorporates the necessary
charge transport properties into the system. This would allow for fabrication of a
single-layer device exhibiting efficient PPE electroluminescence.
Oxadiazole
derivatives have been extensively investigated as electron conducting and hole
blocking materials in both small molecule and polymeric systems.4
Polymers
containing oxadiazole units in either the main5 or side chain6 have been widely
used in LEDs. This chapter discusses the development of an oxadiazole grafted
PPE system, which combines a charge transporting segment with a light emitting
conjugated backbone in a single polymeric system.
161
6.3 Results and Discussion
6.3.A Overview
Scheme 6.1 depicts the set of five PPEs we explored in this study. The
first three PPEs presented are entirely dialkoxy substituted.
Both of the co-
monomers used to construct the polymer are para substituted in 1. In order to
blue shift the emission, ortho substituted monomers 37 are used in PPEs 2 and 3.
A similar blue shift may be obtained by eliminating donating alkyloxy substituents
that lower the band-gap of the polymer by raising the HOMO. This is achieved
by replacing
one of the alkoxy-substituted
monomers
with a pentiptycene
containing monomer. Therefore, the solution-state photophysics of polymers 4
and 5 should be analogous to polymers 2 and 3 respectively.
However, thin films
4 and 5 should show enhanced fluorescence quantum yields, since the
integrated pentiptycene functionality limits aggregation.8
The vinyl oxadiazole
moiety used to generate the oxadiazole grafted PPEs was synthesized according
to a literature procedures and is outlined in Scheme 6.2. The oxadiazole grafting
process is schematically illustrated in Scheme 6.3.
The synthesis of the
oxadiazole grafted PPEs is analogous to the methodology covered in Chapter 3
where the polystyrene grafted PPE system was synthesized via an atom transfer
radical polymerization (ATRP).3' 10 For clarity, we show the PPE backbone with a
single hydroxy terminated alkoxy chain undergoing the ATRP reaction. The
PPEs in Scheme 1 undergo grafting at both hydroxyl groups. We will refer to the
grafted counterparts with a G before their respective ungrafted numbers (e.g. G1,
G2,... G5).
162
Scheme 6.1
OOH
10
14B
r'-
O
lIT7n
OC1 6
I\
I10"
0; C1 6
0C1 6
n
r
4
19H
OH
dl
H1%
H
r4RrO1
3
n
5
•=
Cl
Scheme 6.2
AICI3
+ C,-;,~3-·CHCH
Acetyl Chloride,
CH2 CI 2, Reflux
NaBH 4
~~n~-~_Q60-C-CH
Benzene, Ethanol
P4 0 1 0
OH
N-N
dHCI-1
C
+C~~10'~-~
OH
N-N
+0410Q-eHCCI-1
/
N-N
Benzene, Reflux
Overall 35% Yield
Scheme 6.3
0
Br
H
t
t
TEA
A
r
/
a
-TE
THF
CuBr
/
N-N
fo-N'I~ I
163
Bipy
N
Table 6.1 Polymer Characterization
Photophysical
Polymer
GPC Analysis
Characterization
(bolution-State)
(Solid-State)
,
PDI
-
'
kabs Xem
(nm) (nm)
(nm) (nm)
'abs
Xem
1
23,000
2.9
450 475 0.70
480 496 0.10
2
28,000
3.5
430 455 0.65
460 504 0.09
3
14,000
2.3
391
424 0.68
411
4
29,000
3.2
422
454
441 462 0.35
5
55,000
2.5
391
424 0.92
385 429
0.43
G1
41,000
1.7
432
486 0.65
432
0.18
G2
47,000
1.6
417
457 0.61
484 421 0.30
G3
31,000
1.5
391
424 0.60
385
461
0.54
G4
45,000
1.7
420 454 0.70
421
460
0.67
G5
98,000
1.8
390 424 0.91
386 430 0.85
GPC - Gel Permeation Chromatography;
- Polydispersity Index;
Xabs -
0.73
533
506
0.62
Mn- Number average molecular weight; PDI
Maximum absorption wavelength;
kem - Maximum
emission wavelength; <>- Photoluminescence quantum yield. *Measurements done in
chloroform as the solvent.
164
6.3.B Polymer Characterization
Table 6.1 summarizes the characterization data for the polymers. Gel
permeation chromatographic (GPC) analysis of the PPE systems reveals an
increase in the molecular weight of the polymers after grafting with oxadiazole.
In addition, similar to the findings in Chapter 3, the high polydispersity of the
ungrafted PPEs - typical for cross-coupling polymerizations - is lowered after the
grafting process, due to the addition of low polydispersity oxadiazole grafts by
controlled free-radical polymerization methods.1 0
Further evidence for the successful grafting process is obtained when
monitoring the absorption profile of the polymer samples as they are eluted from
the GPC columns.
The grafted PPE absorption displays the characteristic
absorption band for both the oxadiazole monomer (abmax = 318 nm) and the
conjugated PPE backbone at the same elution volume. This confirms that the
oxadiazole grafts are indeed covalently linked to the PPE backbone. It should
also be noted that despite very similar oxadiazole graft molecular weights (Mn =
2,900 - 3,300; PDI = 1.1) for all of the PPE systems, there was not a consistent
increase in the overall molecular weight, nor was the total molecular weight
increase very significant considering the size of the oxadiazole grafts. This may
be attributed to the more spherical shape of the grafted polymers that effectively
yield a lower molecular weight by GPC as well as the limited solubility of the PPE
macroinitiators1 0 in the oxadiazole monomer/solvent system used for the graft
polymerization (see experimental section for details). Limited solubility will result
in preferential grafting of the low molecular weight PPE chains, and thus the
165
observed total molecular weight for the grafted PPE systems is lower than
expected.
This latter effect also explains the variation in molecular weight
increase from the ungrafted backbone to the oxadiazole grafted PPE from one
polymer system to the next (see Table 6.1).
6.3.C Photophysical Characterization
The solution-state absorption and fluorescence spectra for the ungrafted
PPEs were recorded in chloroform
and are shown in Figure 6.1a and 6.1c
respectively. It was observed that both the absorption and emission (Table 6.1)
of 2 were blue shifted compared to the corresponding maxima for 1.
As
expected from the polymer substitution, the spectra of 4 overlap with those of 2.
By design, even wider band gaps are achieved in 3, where the dialkoxy units are
entirely ortho substituted. Again, the spectra of the pentiptycene containing 5
align with those of 3. Thin films of the ungrafted conjugated polymers were spincast from chloroform solution (2 mg PPE/mL).
photoluminescence
(PL)
results
are
graphed
The solid-state absorption and
in
Figure
6.1b
and
6.1d
respectively. The absorption spectra of thin films 1-3 are red-shifted and display
spectroscopic features arising from interchain 7t-orbital interactions generally
referred to as aggregation bands (Figure 6.1b). Aggregation phenomena cause
the solid-state PL of these polymers to be broadened and shifted significantly
towards longer wavelengths compared to the emission profiles observed in
solution.
Similarly, the quantum efficiency of these aggregated systems is
dramatically reduced in thin film. In contrast, the spectra of thin films 4 and 5 are
166
Solution State
I-
- --
-
- -
-
-
* -
Solid State
-
.
.
.
Ci
.
.
.
b ,'',
.
.
.
.I
.
I .
.
.I.I
·
I
C
0
o
I
/.~'
a.
00
1
.0s
2
3
Cl
0
N
II
4
M
'I,.·
I
I
iI
r~~~~~~~
'
5
E
i
z
rl
v
)
.
0
'-,......._..'.
.t4Y_,...
..
_
I
.
d
Ji
N
"I
9d
?
.
I
e
.1
I
i
I
.1
* i
II
____
d
,
A .'
z3
' '
SI
I
E
I
I
1
MIV
!
I
,
la
-__
I I
)'I
!I
I
II
m
III, !
j
II
-
I
I,
iiIi
I
~
I
.
.~~~~~~~_I
.
300 350
:
_
650 350 400 45i
00
Wavelength
(nm)
Figure 6.1 Normalized solution-state (a) and solid-state (b) absorption spectra of
the various ungrafted PPEs. The corresponding normalized photoluminescence
spectra
are
measurements
shown
were
in
plots
(c)
conducted
and
in
(d)
respectively.
chloroform
solution.
Solution-state
Solid-state
measurements were obtained using polymer thin-films spin-cast onto glass
substrates from chloroform solution (2mg/mL).
In all cases, samples were
excited at their respective absorption maxima (see Table 6.1).
167
almost identical to those in solution confirming that the pentiptycene monomer
limits interchain a-orbital interactions.8 However, these systems still experience a
drop in quantum efficiency going from solution to thin-film, albeit not as significant
as the entirely dialkoxy substituted PPEs 1-3 (Table 6.1).
Chapters
3, 4, and 5 have shown that the grafting process can also
eliminate the intrinsic aggregation of the PPE backbone, allowing PPEs without
pentiptycene scaffolds to maintain their solution state characteristics in the solidstate.3 1 0 The absorption and PL spectra for the grafted polymers are depicted in
Figure 6.2.
The solution state PL spectra were obtained by irradiating the
samples with k = 385 nm light to avoid excitation of the oxadiazole grafts.
However, in the solid-state measurements the oxadiazole units were excited with
= 320 nm light, in order to observe energy transfer to the PPE backbone.
The
solution PL spectra of PPEs G1-G3 are broader than their ungrafted
counterparts.
Furthermore, in the solid-state they are also red shifted and do not
retain their solution state quantum yield upon grafting as seen in the previous
three chapters.3' 9 We attribute this unexpected observation to ground-state and
excited-state interactions between the oxadiazole and the PPE backbone.
However, we note that the pentiptycene containing grafted PPEs (G4 and G5)
maintain their narrow, blue emission profiles and high quantum yields in the
solid-state. This suggests that in the same way the iptycene scaffold helps to
prevent PPE aggregation phenomena, it may also mitigate the interactions
between the oxadiazole grafts and the PPE backbone that lead to spectral
broadening.
168
Solution State
Solid State
CI
C
.
E
Z
az
Z
Wavelength (nm)
Figure 6.2 Normalized solution-state (a) and solid-state (b) absorption spectra of
the
various
oxadiazole grafted
PPEs.
The
corresponding normalized
photoluminescence spectra are shown in plots (c) and (d) respectively. Solutionstate measurements were conducted in chloroform solution.
Samples were
irradiated with X = 385 nm light in order to prevent excitation, and thus emission,
from the oxadiazole grafts.
Solid-state measurements were obtained using
polymer thin-films spin-cast onto glass substrates from chloroform solution
(2mg/mL). The films were all irradiated near the oxadiazole absorption maxima
(X = 318) with
= 320 nm light in order to observe energy transfer from the
oxadiazole grafts to the PPEs.
169
6.3.D Electroluminescence of Oxadiazole Grafted PPEs
The oxadiazole grafted PPE systems were explored in electroluminescent
devices; the device structure is shown in Figure 6.3.
ethylenedioxythiophene
First, a layer of poly-3,4-
(PEDOT) doped with polystyrene sulphonic acid was
spin cast onto indium tin oxide (ITO) coated glass substrates.
Next, a layer of
the grafted PPE was deposited by spin casting from CHCI 3 solution (10 mg/mL
(w/w)). Finally, metal electrodes were thermally evaporated onto the polymer
layers using a shadow mask.
The electroluminescence
devices are shown in Figure 6.3.
(EL) spectra of the oxadiazole
grafted PPE
In the case of polymers G1 and G2 (Figure
6.3a and 6.3b), the EL spectra (solid line) closely match the solid state PL
spectra (dashed line). The EL for polymer G3 deviates from the solid-state PL
due to what appears to be aggregate emission centered at
= 490 nm.
In all
three cases, the EL spectra are significantly red-shifted from the solution state PL
(dotted line). This shift to longer wavelengths, combined with the broadened
emission profile of these polymers, prevents these systems from realizing the
narrow blue emission that we were hoping to achieve. Nevertheless, the grafted
pentiptycene containing PPEs G4 and G5 (Figure 6.3d and 6.3e) maintain their
narrow emission profiles in EL. The EL closely matches the solid-state PL
spectra and is only slightly shifted from the solution-state PL. The emission for
these devices corresponds to CIE coordinates (x = 0.14; y = 0.21) and (x = 0.16;
y = 0. 13) for G4 and G5 respectively.
170
.c
,n
E
'a
z._0
Wavelength
Figure 6.3
(nm)
Electroluminescence spectra (solid line) overlayed with the
corresponding solid-state photoluminescence spectra (dashed line) and solutionstate photoluminescence spectra (dotted line) of the various oxadiazole grafted
PPEs. Spectra for the all dialkoxy substituted PPEs G1-G3 are shown in plots
(a)-(c) while plots (e) and (f) correspond to pentiptycene containing PPEs G4 and
G5 respectively. Plot (d) shows the EL for devices containing ungrafted PPEs 4
(dotted line) and 5 (dot-dash line) for comparison.
171
This spectrally pure, narrow blue PPE electroluminescence
is similar to
the results discussed in the previous chapter.3 In this case, however, the system
is a single polymer layer - eliminating the need for both a small molecule host as
well as thermally evaporated hole-blocking and electron-transporting layers.
Here again the presence of the pentiptycene scaffold limits the interaction of the
oxadiazole units with the conjugated PPE backbone allowing these systems to
simultaneously maintain the excellent luminescent properties of the PPEs while
harnessing the charge conduction properties of the pendent oxadiazole grafts.
To further support our observations, control devices containing ungrafted PPEs 4
and 5 were fabricated (see Figure 6.3d). The broad EL spectra demonstrate that
without the necessary charge transport properties, the pentiptycene scaffold
alone is not sufficient to generate narrow PPE electroluminescence.
6.3.E Operational Analysis of Oxadiazole Grafted PPE Devices
The current-voltage-luminance characteristics of the EL devices are
plotted in Figure 6.4. The inset shows the current-voltage characteristics for the
five oxadiazole grafted PPE devices. All of the grafted PPE systems displayed
superior operational efficiencies when compared to other traditional PPE singlelayer devices in the literature.1 0
had peak efficiencies
of
The entirely dialkoxy substituted PPEs, G1-G3,
= 0.05%, 0.07%, and 0.11% respectively
with
corresponding luminance efficiencies of 0.17 cd A-1, 0.21 cd A-1 , and 0.22 cd A-1 .
The pentiptycene containing PPEs G4 and G5 displayed even better behavior.
Devices incorporating these polymers had consistently lower leakage current,
172
, r
0.25
W
>-f.
dom
0.1-
a
_
/
/
-----....ml
-
_ a__-
-o
m
/
100
E
0.01-
.......
...........
C
-
1n
· ·- ··-
0.001
.U
·
· a;·-
G2......
1
.-----
G3
G4
G5
0.0001
0.0001
0.5
0.1
1
5
Voltage (V)
10
I
I
I
1
10
100
Current Density (mA cm
2)
Figure 6.4 External quantum efficiency versus current density for the oxadiazole
grafted PPEs. Inset shows the current-voltage behavior for the various devices.
The entirely dialkoxy substituted PPEs G1-G3 had peak efficiencies of
i
=
0.05%, 0.07%, and 0.11% respectively with corresponding luminance efficiencies
of 0.17 cd A-1 , 0.21 cd A-1 , and 0.22 cd A-1 .
pentiptycene
containing
PPE G4 was
The peak efficiency
of the
= 0.23% at 12.2 mA cm -2 and 9.9V
corresponding to a luminance efficiency of 0.40 cd A -1 while the peak efficiency
of PPE G5 was r = 0.29% at 13.6 mA cm
luminance efficiency of 0.34 cd A-1.
173
2
and 11.4V corresponding
to a
lower turn-on voltages and displayed higher external quantum efficiencies than
G1-G3.
Polymer G4 had a peak efficiency of r = 0.23% at 12.2 mA cm -2 and 9.9
V which corresponds to a luminance efficiency of 0.40 cd A-' . Polymer G5 had a
peak efficiency of r1= 0.29% at 13.6 mA cm -2 and 11.4 V which corresponds to a
luminance efficiency of 0.34 cd A-' . Polymers G4 and G5 also maintained higher
operational efficiencies over a larger range of current densities than the entirely
dialkoxy substituted PPEs. These observations emphasize the importance of the
ground-state and excited-state interactions between the oxadiazole grafts and
the conjugated
PPE backbone.
Only when the grafted PPE backbone
is
engineered with the pentiptycene scaffold is the system able to realize both the
desired narrow, blue emission as well as the high external quantum efficiency
from a single polymer layer. Such a finding is critical for future development of
PPE based LEDs.
Although the
oxadiazole-grafted PPE systems exhibited excellent
properties for a single-layer architecture, they suffered from relatively high
leakage currents, and turn-on voltages. In addition, the devices achieved their
maximum operational efficiency at high current densities (>10 mA cm-2 ). This
behavior is indicative of the fact that this PPE system does not incorporate a
balance in both hole and electron transport properties.
Since the pendant
oxadiazole units are primarily an electron transport material, a larger current
density is necessary in order to introduce enough holes into the system to
balance the
recombination.
build-up
of electrons
and thus,
realize efficient exciton
Furthermore, the build-up of electrons may also explain the
174
tendency for these systems to maintain their operational efficiencies over a large
range of current densities.
In other words, the characteristic decrease in
efficiency due to an increase in the exciton-polaron interaction3 '11 does not
become significant until a sufficient number of holes reach the recombination
region within the device.
These findings suggest that the overall device
performance may be improved provided that another material possessing the
necessary hole-transport properties is introduced into the system.
6.4 Conclusions
To conclude, we have further demonstrated the modularity of the PPE
grafting process developed in Chapter 33,8by simply replacing the graft monomer
with a vinyl functionalized oxadiazole moiety. Ground-state and excited-state
interactions between the pendent oxadiazole grafts and the conjugated PPE
backbone limit the overall device performance in the case of the entirely dialkoxy
substituted PPEs. By appropriately engineering the PPE backbone structure with
a pentiptycene scaffold, these interactions are removed allowing the oxadiazole
grafted systems to simultaneously incorporate the necessary charge transport
properties while maintaining the desired narrow, wide-band gap emission
characteristic of PPEs. Consequently, these systems are able to realize efficient,
narrow, blue PPE electroluminescence from a single polymer layer that has not
been achieved to date.
175
6.5 Experimental Section
General:
The general synthesis of the ortho-[7 ] and para-[10] monomers as well as
identical procedures used for the preparation of the ungrafted polymers7' 8' 10 and
ATRP macroinitiators1[ 0] described here has been previously reported. The vinyl
oxadiazole was synthesized following a literature method.[9]
A general
preparation for oxadiazole grafted PPEs as well as the characterization of the
polymers used for this study can be found below.
1H
NMR spectra were
recorded on a Varian MERCURY (300 MHz) using deuterochloroform as a
reference or an internal deuterium lock. The chemical shift data for each signal
are given in units of b (ppm) relative to tetramethylsilane
(TMS) where
(TMS) =
0, and are referenced to the solvent residual. Infrared spectra were collected
with a Perkin Elmer 1000 FT-IR spectrometer in KBr and were recorded in
reciprocal centimeters (cm-1, wavenumbers). UV-visible absorption spectra were
measured with a Cary 50 UVNisible spectrometer. Photoluminescence spectra
were measured with a SPEX Fluorolog-t2 fluorometer (model FL112, 450W
xenon lamp).
Solution state quantum yields were determined relative to
Coumarin 314 in ethanol (
0.55).
= 0.63) and quinine sulfate in 1 N sulfuric acid (
Solid state quantum yields were determined relative to
diphenylanthracene in PMMA (
=
10-2 M 9,10
= 0.83). The molecular weights of polymers
were determined by Gel Permeation Chromatography (GPC) running with THF
as the eluent versus polystyrene standards (PolySciences) using Hewlett
176
Packard series 1100 HPLC instrument equipped with a PIgel 5 mm Mixed-C (300
x 7.5 mm) column.
Polymer Synthesis:
General ATRP polymerization of vinyl oxadiazole to give oxadiazole
grafted PPEs (G1-G5): A 25mL Schlenk tube was loaded with the respective
PPE ATRP macroinitiator (see characterization data below) 9 (40mg, 0.03 mmol),
CuBr (9.5mg, 0.07 mmol), and 2,2-dipyridyl
(Bipy) (26mg, 0.17 mmol).
Approximately 750 mg of vinyl oxadiazole12 was dissolved in 2-3 mL of
anhydrous toluene and then syringed into the reaction tube. The mixture was
frozen, evacuated, and thawed three times. The reaction flask was then heated
in an oil bath to 1400°Cand allowed to stir for 72 h. At the end of the reaction
time, the reaction flask was cooled in liquid N2 in order to quench the
polymerization and then the mixture was filtered and precipitated out of
methanol. The precipitate was then filtered, washed several times with water,
collected, and dried.
The isolated polymer was re-dissolved in THF and
precipitated out of acetone to remove any oxadiazole homopolymer produced in
the reaction.
The precipitate was isolated by centrifugation due to partial
solubility in acetone.
The remaining pellet was re-dissolved in THF and
precipitated one last time out of methanol. The precipitate was filtered, collected
and dried to give oxadiazole grafted PPEs.
177
Polymer Characterization:
1: Reported previously;
9
1 ATRP macroinitiator:
GI:
GPC M, 2.30 x
104,
Mw 6.70 x 104, Mw/Mn 2.90.
Reported previously. 9
max 3026, 2924, 2853, 1725, 1613, 1488, 1269, 1067, 1005, 822,
713, and 558;
H
8.2-7.8 (Aromatic br signal), 7.8-7.2 (Aromatic br signal), 7.2-6.6
(Aromatic br signal), 6.5 (br signal), 4.0 (br signal), 2.6-1.7 (br signal), 1.4 (br s),
1.2 (br s), 0.85 (br signal); GPC Mn 4.10 x 104 , Mw 7.00 x 10 4 , Mw/Mn 1.70.
2:
max3406,
1041, 809, and 721;
2922, 2851, 1636, 1508, 1467, 1435, 1377, 1262, 1220,
H
7.2 (2H br s), 7.0 (2H br s), 4.2 (4H, br t), 4.0 (4H, br t),
3.6 (4H, br t, J 6.6), 1.8 (8H, br t), 1.5 (32H, br signal), 1.3 (52H, br signal), 0.88
(6H, br t, J 6.6); GPC Mn 2.80 x 10 4 , Mw 9.80 x 104, MW/Mn 3.5.
2 ATRP macroinitiator Vmax2924, 2852, 1736, 1508, 1466, 1434, 1371,
1275, 1220, 1162, 1109, 1029, 859, 816, and 721; 6 H 7.0 (br s), 4.2 (br signal),
4.0 (br t), 1.9 (s), 1.8 (br signal) 1.6-1.1 (br signal), 0.9 (br t); GPC Mn 5.20 x 104 ,
Mw 1.40 x
10 5 , Mw/Mn 2.70.
G2:
max 3027,
2923, 2851, 1725, 1612, 1487, 1267, 1067, 1005, 822,
712, and 557; 6 H 8.2-7.8 (Aromatic br signal), 7.8-7.2 (Aromatic br signal), 7.2-6.6
(Aromatic br signal), 6.5 (br signal), 4.1 (br multiplet), 2.2-1.9 (br signal), 1.8 (br
signal), 1.4 (br s), 1.2 (br s), 0.86 (br signal); GPC Mn 4.70 x 104, Mw 7.50 x 104,
M,/Mn 1.60.
3: Vmax3340, 2919, 2850, 1506, 1468, 1438, 1378, 1225, 1192, 1088, 807,
and 720;
H
7.2 (4H, br s), 4.2 (8H, br t), 3.6 (4H. br t, J 6.6), 1.8 (8H, br q), 1.5
178
(32H, br signal), 1.3 (52H, br signal), 0.88 (6H, br t, J 6.6); GPC M, 1.40 x
Mw 3.22 x
10 4,
10 4 ,
Mw/Mn 2.30.
Vmax2919, 2850, 1736, 1577, 1503, 1467, 1438,
3 ATRP macroinitiator.
1378, 1275, 1225, 1191, 1164, 1108, 1087, 1027, 808, and 721;
H
7.2 (4H, br
s), 4.16 (4H, br t, J 6.6), 4.0 (8H, br t), 1.9 (12H, br s), 1.8 (8H, br q), 1.7 (8H, br
q), 1.5 (12H, br signal), 1.4-1.2 (64H, br signal), 0.9 (6H, t, J 6.6).
G3: Vmax3026, 2923, 2851, 1724, 1613, 1487, 1268, 1067, 1005, 821,
712, and 557;
H
8.2-7.8 (Aromatic br signal), 7.8-7.2 (Aromatic br signal), 7.2-6.6
(Aromatic br signal), 6.5 (br signal), 4.1 (br signal), 2.0 (br signal), 1.8 (br signal),
1.4 (br s), 1.2 (br s), 0.85 (br s); GPC Mn 3.10 x 104, Mw 4.70 x 104, MW/Mn 1.50.
4:
max3422, 3068, 3021, 2924, 2852, 1500, 1458, 1381, 1279, 1198,
1023, 754, 673, and 568;
H
7.5 (6H, br signal), 7.05 (4H, br signal), 6.1 (4H, br
s), 4.5 (4H, br t), 3.6 (4H, br t, J 6.6), 2.2 (4H, br signal), 1.7 (4H, br signal), 1.47
(8H, br signal), 1.2 (28H, br signal); GPC Mn 2.90 x 10 4 , Mw 9.30 x 10 4, M/Mn
3.20.
4 ATRP macroinitiator.
Vmax3069,
3021, 2925, 2853, 1734, 1502, 1458,
1414, 1383, 1277, 1213, 1163, 1107, 1023, 860, 803, 754, 673, and 569;
H
7.5
(6H, br signal), 7.05 (4H, br signal), 6.12 (4H, br s), 4.5 (4H, br t), 4.1 (4H, br t, J
6.6), 2.2 (4H, br signal), 1.9 (12H, br s), 1.8(6H, br q), 1.6 (8H, br multiplet), 1.4
(8H, br signal), 1.3-1.0 (18H, br signal).
G4: Vmax3024, 2959, 2857, 1723, 1612, 1487, 1267, 1096, 1005, 821,
712, and 557;
H
8.2-7.8 (Aromatic br signal), 7.8-7.2 (Aromatic br signal), 7.2-6.6
(Aromatic br signal), 6.5 (br signal), 6.1 (br signal), 4.4 (br signal), 4.0 (br signal),
179
2.4-1.5 (br signal), 1.3 (br s), 1.1 (br s); GPC M, 4.50 x 104, Mw 7.70 x 104, M/M,
1.70.
5: Vmax3407, 3068, 3021, 2925, 2852, 1492, 1458, 1429, 1380, 1319,
1228, 1198, 1179, 1056, 754, 673, and 567;
H
7.5 (6H, br signal), 7.05 (4H, br
signal), 6.07 (4H, br s), 4.6 (4H, br signal), 3.6 (4H, br t, J 6.6), 2.17 (4H, br
signal), 1.6 (4H, br signal), 1.5 (8H, br multiplet), 1.4-1.0 (28H, br signal); GPC
Mn5.50 x
104,
Mw1.40 x 105, MMn 2.50.
5 ATRP macroinitiator
1Vmax
3068, 3020, 2926, 2852, 2679, 1732, 1494,
1458, 1430, 1370, 1275, 1228, 1163, 1108, 1023, 814, 754, 673, and 568;
6H
7.5
(6H, br signal), 7.1 (4H, br signal), 6.05 (4H, br s), 4.6 (4H, br t), 4.15 (4H, br t, J
6.6), 3.1 (4H, br signal), 2.1 (4H, br signal), 1.9 (12H, br s), 1.6 (6H, br q), 1.5
(6H, br multiplet), 1.3 (8H, br signal), 1.1 (16H, br signal).
G5:
max3024, 2925, 2853, 1723, 1612, 1487, 1265, 1096, 1005, 821,
712, and 567; 8H8.2-7.8 (Aromatic br signal), 7.8-7.2 (Aromatic br signal), 7.2-6.6
(Aromatic br signal), 6.5 (br signal), 6.0 (br signal), 4.6 (br signal), 4.0 (br signal),
3.5 (br signal), 2.2 (br signal), 1.6 (br signal), 1.4 (br s), 1.2 (br signal).
Fabrication of electroluminescent devices:
The devices were built on ITO coated glass substrates with a sheet
resistance of 20 Q cm-2 . The ITO glass was cleaned by ultrasonic agitation in
detergent solution, rinsed with DI water, acetone, boiled in trichloroethane, rinsed
with acetone, boiled in isopropyl alcohol, and N2 blown dry. Immediately prior to
material deposition, substrates were pretreated with UV/Ozone for 5 minutes.
180
The devices were fabricated by spin-casting
(v: 2.5K; Acc: 10K; Duration: 60
sec.) a layer of PEDOT:PSS, followed by thermal annealing at 175 C for 5
minutes.
This is followed by spin-cast deposition (v: 3K; Acc: 10K; Duration: 60
sec.) of the oxadiazole grafted PPE layer. Finally, metal cathodes consisting of
Ag:Mg (100 nm; 10:1 by weight), and Ag (30 nm) were thermally evaporated on
top of the oxadiazole grafted PPE layer. A shadow mask with 1 mm2 openings
was used to define the metal electrodes. The EL spectra were recorded on an
Acton Research Spectra Pro 300i spectrometer.
The luminance-voltage and
current-voltage characteristics were measured on an Agilent-4156C Precision
Semiconductor Parameter Analyzer using a Newport 818-UV photodetector. All
measurements conducted at room temperature under ambient atmosphere.
181
6.6 References
[1] (a) Friend, R. H.; Gymer,
N.; Taliani,
C.; Bradley,
R. W.; Holmes, A. B.; Burroughes,
D. D. C.; Dos Santos,
Salaneck, W. R. Nature 1999, 397, 121. (b)
J. H.; Marks,
D. A.; Bredas, J. L.; Logdlund,
R.
M.;
R. H. Friend, Pure Appl. Chem.
2001, 73, 425.
[2] Kraft, A.; Grimsdale, A. C.; Holmes, A. B. Angew. Chem. Int. Ed. 1998, 37,
402.
[3] Breen, C. A.; Tischler, J. R.; Bulovic, V.; Swager, T. M. Adv. Mater. 2005, .
[4] Strukelj, M.; Papadimitrakopoulos,
F.; Miller, T. M.; Rothberg, L. J. Science
1995, 267, 1969.
[5] (a) Li, X-C.; Holmes, A. B.;
Kraft, A.; Moratti, S. C.; Spencer, G. C. W.;
Cacialli, F.; Gruner, J.; Friend, R. H. J. Chem. Soc., Chem. Commun. 1995, 21,
2211.
(b) Pei, Q.; Yang, Y. Adv. Mater. 1995, 7, 559. (c) Gruner, J.; Friend, R.
H.; Huber, J.; Scherf, U. Chem. Phys. Lett. 1996, 251, 204.
Kaminorz, Y.; Brehmer, L. Synth. Met. 1997, 84, 449.
(d) Schulz, B.;
(e) Peng, Z.; Bao, Z.;
Galvin, M. E. Adv. Mater. 1998, 10, 680.
[6] Bao, Z.; Peng, Z.; Galvin, M. E.; Chandross, E. A. Chem. Mater. 1998, 10,
1201.
[7] Zhu, Z. G.; Swager, T. M. Org. Lett. 2001, 3, 3471.
[8] (a) Yang, J.-S.; Swager, T. M. J. Am. Chem. Soc. 1998, 120, 11864.
(b)
Yang, J.-S.; Swager, T. M. J. Am. Chem. Soc. 1998, 120, 5321. (c) Williams, V.
E.; Swager, T. M. Macromolecules 2000, 33, 4069.
182
[9] Kurfurst, A.; Lhotak,
P.; Nadenik,
P.; Raclova-Pavlicova,
F.; Kuthan, J.
Collect. Czech. Chem. Commun. 1991, 56, 1495.
[10] Breen, C. A.; Deng, T.; Breiner, T.; Thomas,
E. L.; Swager, T. M. J. Am.
Chem. Soc. 2003, 125, 9942.
[11] (a) Hirohata, M.; Tada, K.; Kawai, T.; Onoda, M.; Yoshino, K. Synth. Met.
1997, 85, 1273. (b) Montali, A.; Smith, P.; Weder, C. Synth. Met. 1998, 97, 123.
(c) Pschirer, N. G.; Miteva, T.; Evans, U.; Roberts, R. S.; Marshall, A. R.; Neher,
D.; Myrick,
M. L.; Bunz, U. H. F. Chem. Mater. 2001, 13, 2691.
Posch, P.; Thelakkat,
Weder,
C. Adv.
(d) Schmitz,
C.;
M.; Schmidt, H-W.; Montali, A.; Feldman, K.; Smith, P.;
Funct.
Mater.
2001,
11, 41.
(e)
Chu,
Q.;
Pang,
Y.
Macromolecules 2002, 35, 7569.
[12] (a) Tasch, S.; Kranzelbinder, G.; Leising, G.; Scherf, U. Phys. Rev. B 1997,
55, 5097. (b) Deussen, M.; Scheidler, M.; Bassler, H. Synth. Met. 1995, 73,123.
183
184
Chapter 7
Conclusions and Future Directions
7.1 Overview
While the commercialization of organic electronic and photonic devices is
beginning to permeate the flat panel display and portable electronics industries, a
considerable amount of research and development
still needs to be
accomplished. These organic systems suffer from many drawbacks that must be
improved if these materials are to become a mature technology.
The work presented in this thesis has attempted to address one of these
issues, namely, the development of a highly efficient, wide-band gap, color
saturated emitter that, when combined with novel materials solutions, can
demonstrate efficient, blue, electroluminescence.
We chose to focus on
poly(phenylene ethynylene) (PPE) systems due to their high solution state
quantum yields, narrow emission profiles, and potential for wide-band gap, blue
emission.
7.2 Conclusions
As outlined in Chapter 1, much of the research conducted to date on PPE
systems has been disappointing, and therefore, PPEs have been disregarded as
viable OLED materials.
The poor performance in these systems is widely
considered to be due to the strong
-.n interactions that result from aggregation
phenomena of the PPE backbone.
Aggregation leads to solid-state PPE
emission profiles that are broadened, red-shifted and exhibit a dramatically
reduced quantum yield compared to that in solution.
In order to realize an
effective PPE based LED, the development of a new approach was needed that
186
would eliminate the undesirable solid-state emission along with the poor charge
transport properties associated with the PPE conjugated backbone structure.
Chapter 3 outlined the modular synthetic modification of a PPE backbone
via ATRP.
Generation of a PPE macroinitiator and subsequent grafting of
polystyrene from the PPE backbone yielded a new grafted PPE system that
demonstrated enhanced solid-state properties. The presence of the PS grafts
prevented aggregation phenomena by isolating the individual PPE backbone
structure. This preserved the solution-state emission and quantum yield in the
solid-state.
Chapter 4 discussed the utilization of this grafted PPE system in a new
guest/host system for the generation of polarized photoluminescence.
This
research demonstrated that, not only did the PS grafts limit aggregation of the
PPE backbone, but they provided increased materials compatibility, rendering
the PPE miscible with other polymeric systems. Moreover, by demonstrating the
ability to develop an ordered block copolymer film with an efficient, optically
active PPE guest, new possibilities for other photonic device applications can be
envisioned.
This paradigm may be extended to high molecular weight block
copolymer systems, which exhibit photonic band-gaps in the visible wavelength
region, creating potential for emission and excited-state lifetime modulation that
have implications for sensor and solid-state lasing applications.
Ultimately, in Chapter 5, the added materials compatibility was extended
to a hybrid, small molecule/polymer LED structure capable of realizing efficient,
blue PPE electroluminescence, far superior to any of the previously reported
187
systems. By preventing aggregation phenomena, the grafted PPE was rendered
miscible with a small molecule hole transport host material, TPD. The TPD was
effectively
intercalated
into the grafted PPE film, introducing
the necessary
charge transport into the system while allowing for efficient Frster
energy
transfer from the TPD to the PPE, ultimately producing spectrally pure, blue PPE
EL.
The modularity of the grafting process was further demonstrated in
Chapter 6.
By simply replacing the styrene graft monomer with a vinyl
oxadiazole monomer, a charge transport moiety was directly grafted onto the
PPE backbone. An appropriately engineered PPE backbone resulted in a single
polymeric system, capable of generating efficient, blue EL in a simple, singlelayer device architecture.
The culmination of this work demonstrates the significant improvements
made in the development
of PPE based LEDs.
In fact, not only have we
generated PPE electroluminescence far superior to that of previously reported
systems, we have provided a new, viable polymer materials system that exhibits
blue emission comparable
to, if not better than, the current PPV and PPP
materials used for blue emission.
7.3 Future Work
While we have provided some interesting solutions to developing better
blue
electroluminescent
devices,
still
other
issues
remain
that
limit
the
applicability of organic electronic and photonic systems and warrant future
188
research efforts. There continues to be a lack of understanding associated with
charge injection at organic/electrode interfaces, which limits device performance.
The poor environmental stability to water, oxygen, and other impurities
characteristic of these systems, which contributes to material degradation under
continued device operation, is perceived to be one of the most significant barriers
to widespread commercialization of this technology.
Finally, operational
efficiencies of organic LEDs are still too low to solidify them as a dominant
contender in many industrial applications.
We can attempt to address a couple of the issues presented above with
the grafted PPE platform we have developed.
Immediate research can be
conducted to measure the lifetime of the LED systems discussed in Chapters 5
and 6 in order to assess their stability. The presence of the grafts may further
isolate the PPE backbone from exposure to water and oxygen and result in more
durable, longer lifetime blue devices.
Additional efforts to increase the stability of these systems may be done
by altering the PPE backbone structure. One technique used to prevent photobleaching and potentially limit degradation processes in optoelectronic devices of
various conjugated systems, involves the addition of fluorinated substituents that
are covalently linked to the polymer backbone.1
Unfortunately, the presence of
these groups can also significantly reduce the solubility of the material, limiting
the light emitting properties and eventual use and exploration of these materials
in electroluminescent devices. Therefore, application of the grafting process to
similarly functionalized PPE systems could provide a solution. The increased
189
solubility and added materials compatibility that grafted systems exhibit may
increase
the
impact
and
exposure
PPE
systems
could
have
on
the
understanding and improvement of stability in optoelectronic devices.
One of the efforts to increase the operating efficiency of polymer based
LEDs has been to develop phosphorescent polymer emitters. Predominately,
phosphorescent polymer systems that have been explored in electroluminescent
applications are unconjugated.2
While these systems introduce the increased
mechanical stability provided by the polymer film, they are still plagued by broad
emission profiles and are unable to achieve efficiencies comparable to small
molecule phosphorescent systems.
In addition, the development of phosphorescent conjugated polymers has
been of particular interest due to controversial, yet compelling, experimental3 '8
and theoretical9 1 4 evidence that in conjugated polymers, the limit of 25% internal
conversion of singlet states in EL devices (due to the four exciton states where
there are three triplet states and one singlet state - see Section 2.3.A) may be
exceeded. This introduces the possibility for producing highly efficient polymer
LEDs and raises fundamental questions about the mechanisms determining
exciton formation within these devices. For example, Wilson et al. reported a
singlet generation fraction close to 57% in a platinum-containing conjugated
polymer, while a much smaller value (22%) was inferred for the corresponding
monomer. 4 b
Unfortunately, phosphorescent conjugated polymers are typically plagued
by many of the same problems associated with traditional PPE systems that
190
have been discussed in this dissertation. Solid-state aggregation phenomena
dramatically reduces the phosphorescence emission efficiency, most likely due to
competition with much faster non-radiative decay processes, and thus have
limited the utilization of these materials in optoelectronic devices.
The
understanding and utilization of phosphorescent conjugated polymers may be
enhanced with the development of a phosphorescent grafted PPE system.
Grafted systems could provide isolation of individual polymer chains allowing for
solution-type phosphorescent emission and quantum yields in the solid state.
Furthermore, perhaps a new phosphorescent
PPE system could provide a new
route to generating color-saturated blue phosphorescent emission.
In closing, the area of organic electronics and photonics is still relatively
new. There is much research to be conducted, new discoveries to be made, and
many improvements to be realized. The multitude of synthetic techniques and
materials solutions available for the development and tuning of properties in
organic conjugated systems, holds great promise for continued success in these
areas.
191
7.4 References
[1] Kim, Y.; Swager, T. M. Chem. Comm. 2005, 3, 372.
[2] Furuta,
P. T.; Deng, L.; Garon, S.; Thompson,
M. E.; Frechet,
J. M. J. J. Am.
Chem. Soc. 2004, 126, 15388.
[3] Cao, Y.; Parker, . D.; Yu, G.; Zhang, C.; Heeger, A. J. Nature 1999, 397, 414.
[4] (a) Ho, P. K. H.; Kim, J.; Burroughes,
J. H.; Becher,
H.; Li, S. F. Y.; Brown, T.
M.; Cacialli, F.; Friend, R. H. Nature 2000, 404, 481. (b) Wilson, J. S.; Dhoot, A.
S.; Seeley, A. J. A. B.; Khan, M. S.; KOhler, A.; Friend, R. H. Nature 2001, 413,
828.
[5] (a) Wohlgenannt, M.; Tandon, K.; Mazumdar, S.; Ramasesha, S.; Vardeny, Z.
V. Nature 2001, 409, 494.
(b) Wohlgenannt,
M.; Jiang, X. M.; Vardeny, Z. V.;
Janssen, R. A. J. Phys. Rev. Lett. 2002, 88, 197401.
[6] Dhoot, A. S.; Giner, D. S.; Beljonne, D.; Shuai, Z.; Greenham, N. C. Chem.
Phys. Lett. 2002, 360, 195.
[7] Virgili, T.; Cerullo,
G.; Gadermaier,
G.; LUer, L.; Lanzani,
C.; Bradley,
D. D. C.
Phys. Rev. Lett. 2003, 90, 247402.
[8] Meulenkamp, E. A.; van Aar, R.; Bastiaansen, J. J. A. M.; van den Biggelaar,
M.; van Dijken, A.; Kiggen, N. M. M.;
A. J. M.; Borner, H.; Brunner, K.; Bichel,
Kilitziraki,
M.; de Kok, M. M.; Langeveld,
B. M. W.; Ligter,
M. P. H.; Vulto, S. E.
E.; van de Weijer, P.; de Winter, S. H. P. M. SPIE-Int. Soc. Opt. Eng. In press.
[9] Shuai, Z.; Beljonne, D.; Silbey, R. J.; Bredas, J. L. Phys. Rev. Lett. 2000, 84,
131.
192
[10] Kobrak, M. N.; Bittner, E. R. Phys. Rev. B 2000, 62, 11473.
[11] Karabunarliev, S.; Bittner, E. R. Phys. Rev. Lett. 2003, 90, 057402.
[12] Tandon, K.; Ramasesha, S.; Mazumdar, S. Phys. Rev. B. 2003, 67, 045109.
[13] Hong, T.; Meng, H. Phys. Rev. B 2003, 63, 075206.
[14] Bredas, J. L.; Beljonne, D.; Coropceanu,
104, 4971.
193
V.; Cornil, J. Chem. Rev. 2004,
194
CURRICULUM
CRAIG
VITAE
BREEN
SUMMARY
* Extensive experience in polymer chemistry and materials processing.
* Designed and fabricated novel organic light emitting devices (OLEDs) for display
applications.
* Developed a new composite polymer system for the generation of polarized
photoluminescence.
EDUCATION
* PhD, Physical Chemistry, Massachusetts Institute of Technology, June 2005
Cumulative GPA: 4.75/5
Thesis: "Grafted Poly(P-Phenylene-Ethynylene): Optical and Optoelectronic
Applications"
Advisor: Dr. T. M. Swager
* BA, High Honors in Chemistry, (summa cum laude) Middlebury College (VT), 2000
Cumulative GPA: 3.93/4.00
Thesis: "Intramolecular Radical Cyclization Reactions on Aromatic Systems"
Advisor: Dr. J. H. Byers
EMPLOYMENT HISTORY
September 2000-2005
Graduate Research Assistant, Massachusetts Institute of
Technology
Advisor: Dr. T. M. Swager
* Synthesized polystyrene grafted poly(p-phenylene-ethynylene) (PPE) via atom
transfer radical polymerization (ATRP).
* Developed a new guest/host system for the generation of polarized
photoluminescence using grafted PPE doped cylindrical phase tri-block
copolymers.
* Extended the methodology outlined above for the uniaxial alignment of carbon
nanotubes.
* Designed and constructed novel OLEDs using grafted PPEs as the emitting
material.
2004
Patent Research Consultant, Oblon, Spivak, McClelland,
Maier & Neustadt, PC (VA) Supervisor: Dr. J. P. Lavelleye
JD
* Conducted an extensive literature search for a previous synthesis of a
particular chemical compound (details omitted due to confidentiality
agreement).
* Assessed the present methodology involved in the synthesis of the compound
under investigation.
195
Graduate Teaching Assistant, Massachusetts Institute of
Technology
2000-2002
* Graduate Physical Organic Lecture Course, Spring 2002.
* Undergraduate General Chemistry, Fall 2002.
* Undergraduate Thermodynamics and Kinetics, Fall 2000-Spring 2001.
Summer
1999
Undergraduate Research Assistant, Massachusetts
Institute of Technology
Advisor: Dr. D. G. Cory
Designed radio-frequency
pulse sequences demonstrating
novel NMR
quantum information processing capabilities.
Summer 1998
Undergraduate Research Assistant, Middlebury College
(VT)
Advisor: Dr. J. H. Byers
* Developed new aromatic cyclizations using atom transfer radical reactions.
LABORATORYS KILLS
· Institute for Soldier Nanotechnologies (ISN):
-- Spectroscopy: Ultraviolet-visible (UV-vis) spectroscopy; Photoluminescence
(PL) spectroscopy; Photoluminescence Excitation (PLE) spectroscopy; Polarized
Luminescence spectroscopy; Nuclear Magnetic Resonance (NMR) spectroscopy;
Infrared (IR) spectroscopy.
-- Confocal Microscopy: Transmission and Polarization microscopy; Scanning
Laser confocal microscopy - UV Ar, Visible Ar, and HeNe lasers.
-- Analytical: Gas Chromatography-Mass Spectrometry (GC-MS); Gel
Permeation Chromatography (GPC); Differential Scanning Calorimetry (DSC);
ThermogravimetricAnalysis (TGA); Ellipsometry.
--
Synthesis: Monomer and Polymer synthesis
-- Material Processing: Spin Casting; Roll Casting.
* Laboratory for Organic Optics and Electronics (LOOE):
-- Ultra-High Vacuum System: Fabrication of OLEDs involving spin casting of
materials as well as material and metal electrode thermal evaporation.
-- Device Testing: CurrentNoltage (IN) Characteristics; Electroluminescence
analysis; Quantum Efficiency analysis.
AWARDS ANDSCHOLARSHIPS
* College Scholar (Highest Academic Rank at Middlebury), 1997-2000.
* American Institute of Chemists - Best Senior Chemist Award, 2000.
* Phi Beta Kappa Member, 1999.
* The Materials Processing Center and the Center for Materials Science and
Engineering Summer Research Scholarship, Massachusetts Institute of
Technology, 1999.
196
*
*
*
*
*
ACS Undergraduate Award in Analytical Chemistry, 1999.
ACS Polymer Education Committee Organic Chemistry Award, 1998.
Outstanding Freshman Award for Middlebury College, 1997.
CRC Press Freshman Chemistry Achievement Award, 1997.
National Youth Science Camp - Vermont Delegate, 1996.
RELEVANT ACTIVITIES
Varsity Basketball: Captain (2000), Middlebury College, 1996-2000.
Varsity Track and Field, Middlebury College, 1996-2000.
Intramural Athletics: 3-Time Basketball and 2-Time Football Champions (A-League)
Massachusetts Institute of Technology, 2000-Present.
Member of the American Chemical Society (ACS), 1998-Present.
Member of the Materials Research Society (MRS), 2002-Present.
P U B L I C A T I O N S AND P R E S E N T A T I O N S
Breen, C. A.; Rifai, S.; Bulovic, V.; Swager, T. M. (2005) Blue Electroluminescence from
Oxadiazole Grafted Poly(Phenylene-Ethynylene). Nano Lett. (Submitted). (Ph.D. work).
Rifai, S.; Breen, C. A.; Solis, D.; Swager, T. M. (2005) Facile In Situ Silver Nanoparticle
Formation in Insulating Polymer Matrices. Chem. Mater. (Submitted).
Breen, C. A.; Tischler, Y.; Bulovic, V.; Swager, T. M. (2005) Highly Efficient Blue
via Energy Transfer from a Hole
Electroluminescence from Poly(Phenylene-Ethynylene)
Transport Matrix. Advanced Materials (Accepted). (Ph.D. work).
Deng, T.; Breen, C. A.; Breiner,
T.; Swager,
T. M.; Thomas,
E. L. (2005)
Block
Copolymer Nanotemplates for Tunable Polarized Emission. Polymer (Publication
Pending) (Ph.D. work).
Breen, C. A.; Deng, T.; Breiner, T.; Thomas,
Photoluminescence
E. L.; Swager, T. M. (2003) Polarized
from Poly(p-phenylene-ethynylene)
via a Block Copolymer
Nanotemplate. J. Am. Chem. Soc. 125(33); 9942-9943 (Ph.D. work).
Price, M. D.; Fortunato, E. M.; Pravia, M. A.; Breen, C.; Kumaresean, S.; Rosenberg, G.;
Cory, D. G. (2001) Information Transfer on an NMR Quantum Information Processor.
Concepts in Magnetic Resonance 13(3), 151-158: (Summer 1999).
Breen, C. A.; Tischler, Y.; Bulovic, V.; Swager, T. M. "Color Saturated Quantum Efficient
Blue Organic Light Emitting Devices." MRS Fall Meeting,
Poly(P-Phenylene-Ethynylene)
Boston, MA 2003 (December 1-5, 2003). (Poster, Ph.D. work).
Breen, C. A.; Deng, T.; Breiner, T.; Thomas, E. L.; Swager, T. M. "A New Guest/Host
System for the Generation of Polarized Photoluminescence." ACS National Meeting,
New York, NY 2003 (September 7-11, 2003) (Oral Presentation Ph.D. Work).
Breen, C. A.; Breiner, T.; Deng, T.; Thomas, E. L.; Swager, T. M. "Polarized Emission of
Polystyrene Grafted Poly(P-Phenylene Ethynylene) in a Globally Oriented Block
Copolymer Matrix." MRS Fall Meeting, Boston, MA 2002 (December 2-6, 2002) (Poster,
Ph.D. work).
Breen, C. A.; Byers, J. H.; Kosterlitz, J. A. "Intramolecular
Reactions Involving Radical
Aromatic Substitution via Atom Transfer." ACS National Meeting, Boston, MA, 1998
(August 23-27, 1998) (Poster, B.A. work).
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