Fullerene C60 Containing Liquid Crystalline Codendrimers :
Synthesis, Characterization and Application
Natalia Yevlampieva *, Nikolai Beljaev *, and Robert Deschenaux +
*
V.A. Fock Institute of Physics, St. Petersburg State University,
198504 St. Petersburg, Russian Federation
+
Institut de Chimie, Université de Neuchâtel, 2009 Neuchâtel, Switzerland
ABSTRACT
Liquid crystalline fullerene C60 containing dendritic compounds of different design
have been investigated. Mesomorphic and molecular properties of poly(benzyl
ether)/poly(aryl ester) codendrimers bearing mesogenic groups have been analyzed
based on the results of theirs study by electrooptical Kerr effect and total polarity
determination in dilute solutions and on the results of modelling by quantum chemical
semiempirical method PM3. The strategy of the design connected with the inducing of
asymmetry to the dendritic core using fulleropyrrolidine as the branching centre and
different chemical structure dendrons as fullerene addends was concluded the most
promising for stimulation of self-organization of codendrimer molecules in the
condensed phase. Specific fullerene microsegregation detected in the mesophase of
poly(benzyl ether)/poly(aryl ester) codendrimers have been explained by the
molecular anisometry and stiffness, responsible for the orientation of these
compounds as a whole in solution under the influence of external electric fields and
in condensed phases.
INTRODUCTION
Macromolecules of dendritic structure have received general acceptance as the
nanosized building blocks in modern supramolecular chemistry and material science
concerning to organic electronics. Normally, dendrons and dendrimers have well
defined chemical structure, 3D-shape and terminal periphery, suitable for further
modification, for instance, by mesogenic end-groups. The mesogenic groups’
interactions are able to stimulate self-organization and formation of well ordered
condensed phase of macromolecules that is important for producing of thin specially
organized photosensitive films and conducting materials [1]. Due to such properties
mesomorphic dendrimers have a lot of preferable advantages in comparison with the
ordinary linear structure polymers. Not far ago a fruitful idea to combine the
polyester/polyether dendrons bearing cyanobiphenyl mesogenic end-groups with
fullerene C60 as the branching center into the hybrid macromolecules had been
realized, and synthetic methodology based on the modular approach for synthesis of
dendritic compounds had been developed [2, 3]. Novel molecular design permitted
significantly to reinforce the variability of physico-chemical properties of dendritic
compounds and gave a possibility to create multifunctional materials with tunable
properties [2-4]. An appearance of mesomorphic fullerene containing dendrimers
initiated the research activity in the fields of plastic solar cells [5], organic light
emitting diodes [6], photoactive dyads and polyades [7, 8]. The above-enumerated
applications are directly connected with electron accepting properties of C60 [7].
Liquid crystalline fullerene containing dendritic compounds belong to
relatively new class of multicomponent macromolecules which structure-properties
and self-organization ability continue to be under consideration [9, 10]. From strategic
2-117
point of view the sphere-like shape of dendrimer is an obstacle for free ordering of
terminal mesogenic end-groups bonded to dendritic core. Closed to spherical
distribution of mesogenic end-groups around the core of dendrimer leads to the partial
loss of theirs mesogenic ability. Traditional incorporation of long aliphatic spacers
between the core and periphery is not very good decision in this case because of the
initial unity of molecule practically disappears due to a separate behavior of dendritic
fragment and mesogenic end-groups in such molecule. The acceptable solution of this
problem may be achieved by another strategy of synthesis based on a special shift of
the shape of dendritic core to more asymmetric one.
The present contribution is devoted to investigation of structure-properties
relations for fullerene C60 containing codendrimers composed of two different type
dendrons (Fig. 1a) or dendrons of different generations (Fig. 1b). Poly(aryl esters) and
poly(benzyl ethers) have been selected for the synthesis of codendrimers due to well
known ability of these compounds to form Langmure, Langmure-Blodgett, multy- or
monolayer films [1]. Mesomorphic behavior of poly(aryl esters)/poly(benzyl ethers)
codendrimers bearing mesogenic end-groups is also discussed.
Determination of permanent dipole values and the study of electrooptical Kerr
effect [11] in dilute solutions have been used as the basic experimental methods for
investigation of molecular properties of dendritic samples. Quantum-chemical
simulation have been applied for the analysis of molecular polarity, for estimation of
optical polarizability of building blocks of hybrid macromolecules, and for Kerr effect
data interpretation. Quantum chemical calculations have been performed by
semiempirical method PM3 in the framework of HyperChem program [12].
EXPERIMENTAL
Dendrons D1, D2 (Fig. 1a) have been synthesized as described earlier [13, 14]. Two
approaches have been utilized for the synthesis of fullerene containing dendritic
compounds 1- 4. First approach is based on 1, 3-dipolar cycloaddition reaction of
addends with fullerene C60 [15] and leads to fulleropyrrolidine derivatives (Fig. 1b,
compounds 1-3). More details of this approach application for the synthesis of
fulleropyrrolidines can be found in [3, 4]. The second approach, based on the addition
of dendrons to C60 by applying Bingel reaction [16], produces methanofullerene
derivatives (Fig. 1b, compound 4).
Molecular properties of codendrimers 1-3, compound 4 and dendrons D1, D2
have been investigated in dilute benzene solutions at 21 oC.
The permanent dipole moment values μ of compounds have been determined
by Guggenheim-Smith method [17].This method is derived from the experimental
determination of the dielectric permittivity increment (ε-εo)/c, where (ε-εo) is the
difference between the dielectric permittivity of the solution and solvent, and from the
determination of the squared refractive index increment (n2-no2)/c, where n and no are
the refractive indices of the solution and solvent, respectively, and c is the solute
concentration. Dielectric permittivity measurements were performed by a resonance
technique at a frequency of 700 kHz using a standard capacity meter E12-1 and
cylindrical titanium capacitor having its own capacity of 92.86 pF. Refractive indices
n were determined using Pulfrich refractometer (IRF-23, Russia) with the line 578 nm
corresponding to the wavelength of Hg-lamp. The permanent dipole moments were
calculated according to equation (1).
2-118
μ2 = 27kT M [(ε-εo) / c - (n2-no2) / c] / [4πNA(εo2+2)2]
(1)
Here M is molecular mass, k is Boltsmann constant, T is absolute temperature, and NA
is Avogadro’s number.
Linear concentration dependencies of (ε-εo) and (n2-no2) were observed for solutions
of all compounds under investigation. The increments (ε-εo)/c and (n2-no2)/c were
determined from the slopes of the mentioned dependencies. Values of increments are
reported in Table 1.
Electrooptical properties of the compounds have been studied by equilibrium
Kerr-effect method in radio frequency rectangular pulsed electric field [11, 18]. The
specific electrooptical Kerr constant K and molar electrooptcal Kerr constant KM,
connected with each other by equation (2), were determined for dendritic compounds.
KM=
6n0 MK
,
(n02  2) 2 ( 0  2) 2
(2)
n  n0
) c 0 ;
(Δn - Δno) is the difference between optical
E 2c
birefringence of solution with the solute concentration c and optical birefringence of
solvent, respectively; E is the electric field strength; the subscript c → 0 is
symbolizing K value determination at the condition of infinite dilution. The others
parameters of eq. (2) have been explained above.
where
K= (
The optical birefringence in solutions of 1-4, D1 and D2 under the treatment of the
rectangular pulsed electric field have been measured with the impulse duration of 1
ms in the voltage range 0-1000 V. The compensatory technique with the photoelectric
registration of optical birefringence value Δn was applied. The thin mica plate
compensator having its own optical phase difference 0.01x2π was used. Glass cell
with the titanium semi-cylindrical electrodes of 2 cm in length and with the gap
between electrodes of 0.05 cm was employed. He-Ne laser (1.5 mW power) operating
at 632.8 nm was used as the light source.
The variation of optical birefringence value Δn as a function of E2 for different
concentrations of 3 are shown in Fig. 2. No deviation from Kerr low (according to
which, optical birefringence Δn is proportional to E2 in molecular dispersed liquids)
was observed in solutions of 1-4, D1 and D2.
The dependences of (Δn-Δno/E2c) as a function of solute concentration are shown in
Fig. 3. The (Δn-Δno/E2c)c→o values were obtained at c = 0 and used for the calculation
of KM according to equation (2).
Compounds D1, 3 and 4 had liquid crystalline properties. Types of
mesophases and phase transition temperatures have been determined using polarized
microscope with the temperature gradient control 0.2 grad per minute. Characteristics
of mesomorphic properties of the investigated samples are presented in Table 2. Glass
transition temperature had not been detected for compounds 3, 4. Mesomorphic
properties of dendritic compounds were not similar to same one of low molecular
liquid crystal analogues to mesogenic groups of 3, 4, as follows from Table 2.
RESULTS AND DISCUSSION
2-119
Realization of the declared in the introduction part strategy of synthesis of C 60
containing dendrimers have been started with fulleropyrrolidine derivatives that were
composed of different generation number poly(benzyl ether) dendrons (Fig. 1b,
compounds 1, 2). Determination of the permanent dipole moments and electrooptical
properties of codendrimers 1, 2 have shown that molecular properties continue to stay
very similar (see, please, μ and KM of 1, 2 in Table 1) when generation numbers and
total number of polar groups significantly vary in these compounds. Thus, we have
received experimental evidence that spherical shape of dendritic core will determine
properties of codendrimers 1, 2 before its modification by mesogenic end-groups. By
other words, this experience have shown that different generations of the same
structure dendrons are not able to induce a significant change of the shape of
dendritic molecule which sub-units (dendrons) are bonded to fullerene surface.
The next step in realization of the declared strategy was connected with the
synthesis of fulleropyrrolidine derivative 3 composed of different chemical structure
dendrons (Fig. 1b). Poly(benzyl ether) dendron D2 of the third generation and the
second generation of poly(aryl ester) dendron D1 of practically equal to each other
hydrodynamic dimensions (see, please, d values in Table 1) have been selected for the
synthesis of compound 3. Form asymmetry in the architecture of compound 3 was
induced not only by the difference in chemical structure of dendrons bonded to C60,
but also by the difference in molecular mass of D1 and D2 ( Table 1). Architecture of
compound 3 was realized so, that it has a heavy “head”(D2) and strongly polar “tail”
(D1), and that is similar to typical low molecular liquid crystal molecules, but at the
level of dendritic structure molecules.
The latter strategy brought an interesting result. The mesomorphic behavior
and supramolecular ordering in mesophase of compounds 3 was not the same if
compared with compound 4 in which structure two poly(aryl ester) dendrons D1 both
bearing cyanobiphenyl mesogenic end-groups were bonded to fullerene surface (Fig.
1b) using traditional synthetic strategy. Methanofullerene derivative 4 and
fulleropyrrolidine derivative 3 have appeared significantly different thermotropic
liquid crystalline properties as one can see in Table 2. Dendrimer 4 has smectic A and
very short nematic phase, when codendrimer 3 possesses rectangular columnar phase
with the formation of fullerene layers between the dendrons detected by X-rays
diffraction [4]. There was not detected fullerene segregation in mesophases of
compound 4.
Experimental data, received for D1, D2 and 3, 4, permit us to analyze
molecular properties of codendrimer 3 in detail and to explain the difference of
mesomorphic properties of 3 and 4. First of all, it may be pointed out that D1, D2 and
3, 4 have large in value permanent dipole moments, including compound D2 which
has not mesogenic end-groups. Each mesogenic group of D1, 3 and 4 has large in
value permanent dipole moment of 7.2 D according to quantum chemical calculation.
Experimentally it was detected that compounds 3 and 4 are characterized practically
equal to each other total polarity (see, please, μ in Table 1) in spite of number of
strongly polar mesogenic groups in their content differ twice. Due to dendrons D1 and
D2 have closed in value hydrodynamic diameters d, the compounds 3 and 4 also have
similar size (d in Table 1), but the mass and polarity distribution are non equivalent in
3 and 4, that have been said above. These facts reflect an importance of structural
difference between 3 and 4, but it is not enough sufficient to explain the difference in
mesomorphic behavior of these compounds (Table 2).
2-120
Additional information on specificity of intermolecular organization of
compound 3 and 4 has been received from theirs electrooptical properties.
Elecrooptical Kerr constant KM of the substance directly depends on polarity, optical
anisotropy and on the structural geometry of its molecules [11, 18]. It is well known
that molar Kerr constant KM is an additive value in the case when separated fragments
of molecule are able to be independent in their orientations under the treatment of
external electric field [11, 18]. Multicomponent compounds 3 and 4 can be easily
divided on some separate sub-units due to theirs individual structure. Because of this,
it is possible to estimate the freedom degree of sub-units in 3 and 4 by means of
comparison of experimental molar Kerr constants with the corresponding values
calculated according to the additive scheme. It is self-evident that electrooptical
properties of separated sub-units in objects under consideration need to be known.
Quantum chemical modeling has been used for this purpose.
Before the calculation of KM an important remark need to taken into
consideration in relation to difference in the chemical structure of dendrons D1 and
D2. The total polarity and the dependent on polarity electrooptical properties of
dendron D1 are fully determined by mesogenic end-groups in contrast to dendron D2.
Modeling has shown that the central part of D1 is highly symmetric nonpolar
fragment (Fig. 4, Table 3). It means that electrooptical Kerr effect in solution of
compound 4 similar to D1 in a great measure will be connected with inputs of
mesogenic groups.
KM, cal value of single mesogenic group (its chemical structure may be seen in Fig. 1a)
as well as KM, cal values of the model compound corresponding to the central part of
D1 and of the fullerene derivatives analogues to the fullerene containing central
fragment of compounds 3 and 4 (named FP and MF, correspondingly, Fig. 5) have
been calculated with parameters accumulated in Table 3.
Estimation of molar Kerr constant KM, cal according to additive scheme for
multicomponent compounds 3 and 4 has been done as a sum (eq. (3)).
KM, cal = Σ KMiWi ,
(3)
i
where KMi is the molar Kerr constant of i-fragment and Wi is its weight fraction
value.
The inputs of eight mesogenic groups, MF–sub-unit, and two polyaryl ester fragments
(Fig. 4) without mesogenes have been taken for calculation of KM, cal for compound 4
through eq. (3). Correspondingly, the inputs of the fourth mesogenic groups, D2 (its
experimental value KM was used), polyaryl ester fragment and FP–sub-unit have been
taken for calculation of KM, cal for compound 3.
The result of calculation presented in the last column of Table I shows very good
coincidence between the calculated and experimental values of molar Kerr constant
for compound 4; and at the same time an absence of coincidence can be declared for
compound 3. This reveals that the rotational freedom of dendrons in compound 3 is
significantly restricted. Furthermore, the fact that the experimental dipole moment
value of 3 is practically the sum of the polarities of its both dendrons (see, please, μ
column in Table 1) is another evidence of the structural stiffness of 3. Indeed, this
situation is reached because D1 and D2 are stiffly linked in 3, and rotate
synchronically in the external pulsed (in the case of Kerr effect study) and in the
2-121
external sinusoidal (in the case dielectric measurements) electric fields which were
used in the framework of this study. Due to stiffness the molecules’ packing in the
mesophase of compound 3 will have a macroscale character and will differ from the
same process in the mesophase of compounds 4, having relatively free and mobile
sub-units. Molecular stiffness well explain the specific segregation of sub-units
detected by X-rays diffraction in the mesophase of compound 3, where
microsegregation of fullerene have been detected [4].
CONCLUSION
Mesomorphic fullerene C60 containing dendritic compounds of different architecture
design have been compared based on their solution and mesomorphic properties
Novel self-organization type have been detected and explained for poly(benzyl
ether)/poly(aryl ester) codendrimer with fulleropyrrolidine as the branching center. It
was shown that the core asymmetry and rigid linkage of dendrons in such
codendrimers are responsible for molecular and mesomorphic properties of these
compounds. Fullerene containing mesomorphic poly(benzyl ether)/poly(aryl ester)
codendrimers can be considered as successful example of design of liquid crystalline
substances reproducing anisometry-principle, peculiar to low molecular liquid
crystals, at the level of dendritic structure macromolecules.
REFERENCIES
1. Lehn J- M: Supramolecular Chemistry. Weinheim- New York, VCH, 1995.
2. Chuard T, Deschenaux R : “Design, mesomorphic properties, and supramolecular
organization of [60] fullerene-containing termotropic liquid crystals”. J. Mater. Chem.
2002 12 (7) 1944-1951.
3. Lenoble J, Maringa N, Campidelli S, Donnio B, Guillon D, Deschenaux R : “
Liquid crystalline fullerodendrimers which display columnar phases”. Org. Lett. 2006
8 (9) 1851-1854.
4. Lenoble J, Campidelli S, Maringa N, Donnio B, Guillon D, Yevlampieva N,
Deschenaux R : “Liquid-crystalline Janus-type fullerodendrimers displaying tunable
smectic-columnar mesomorphism”. J. Am. Chem. Soc. 2007 129 (32) 9941-9952.
5. Winder Ch, Muhlbacker D, Neugebauer H, Sariciftci N, Brabec Ch., Janssen R,
Hummelen, H: “Polymer solar cells and infrared light emitting diods: dual function
low bandgap polymer”. Mol. Cryst. Liq. Cryst. 2002 385 93-100.
6. Nakamura E, Isobe H: “Functionalized fullerenes in water. The 10 years of their
chemistry, biology and nanoscince ”. Acc. Chem. Res. 2003 36 (11) 807-815.
7. Guldi D, Zerbetto F, Georgakilas V, Prato M: “Ordering Fullerene Materials at
Nanometer Dimension“. Chem. Soc. Rev. 2005 38 (1) 38-43.
8. Figuera-Duarte T, Gegout A, Nirengarten J.-F : “Molecular and supramolecular
C60-oligophenylenevinylene conjugates” .Chem. Commun. 2007 (2) 109-119.
9. Peroukidis S, Vanakaras A, Photinos D:“Molecular modeling of liquid crystalline
self-organization of fullerodendrimers” . J. Phys. Chem. B. 2008 112 (40) 1276112767.
10. Yang M, Wang W, Lieberwirth I, Wenger G: “Multiple H-bonds directed selfassembly of amphiphilic and plate-like codendromer with Junus faces at water-air
interface” J. Am.Chem.Soc. 2009 131 (17) 6283-6292.
11. Tsvetkov V.N: Rigid-chain polymers. New York, Plenum, 1989.
12. Stewart J. J. P: “A Semiempirical Molecular Orbital Program”. J. ComputerAided Mol. Design. 1990 4 (1) 1-105.
2-122
13. Dardel B, Guillon D, Heinrich B, Deschenaux R: “Fullerene containing liquid
crystalline dendrimers“. J. Mater. Chem. 2001 11 (11) 2814-2818.
14. Deschenaux, R., Donnio, B., Guillon D :“Liquid crystalline fullerodendrimers“.
New J. Chem. 2007 31 (7) 1064-1070.
15. Prato M., Maggini M:“Fulleropyrrolidines: a family of full-fledged fullerene
derivatives“. Acc. Chem. Res. 1998 31 (9) 519-526.
16. Bingel C:“Cyclopropanierung von fullerene”. Chem Ber. 1993 126 (8) 19571959.
17. Oehme F: Dielektrische messmethooden zur quantitativen analyse und fur
chemische strukturbestimmungen. Weinheim, Verlag Chemie, 1962.
18. O’Konsky Ch. T: Molecular Electro-optics. New-York, Marcel Dekker, 1978.
19. Yevlampieva N, Dardel B, Lavrenko P, Deschenaux R: “Electrooptical properties
of liquid-crystalline fullerene containing dendrimers in solutions”. Chem. Phys. Lett.
2003 382 (1-2) 32-40.
20. Yevlampieva N, Beljaev N, Lavrenko P, Deschenaux R: “Mesomorphic poly(aryl
ester)/poly(benzyl ether) dendrimers/co-dendrimers with C60 as the core”. Mol. Cryst.
Liq. Cryst. 2009 506 34-46.
21. Rjumtsev E, Yevlampieva N, Kovshik A: “The influence of aliphatic spacers
position on molecular polarity and dielectric properties of liquid crystalline
substances”. Russian J. Phys. Chem. 1995 69 (5) 934-939.
CAPTIONS TO FIGURES
Figure 1. Chemical structure of poly(aryl ester) dendron of the second generation D1,
poly(benzyl ether) dendron of the third generation D2 (a), codendrimers 1-3 and
compound 4 (b).
Figure 2. Variation of the optical birefringence (Δn) versus E2 for codendrimer 3 in
benzene at different solute concentrations: (1) pure solvent , (2) 0.353, (3) 0.635 and
(4) 1.057·10-2 g cm-3.
Figure 3. Concentration dependence of (Δn – Δn0)/E2c for 3, 4, D1 and D2 in
benzene.
Figure 4. Model of the central part of poly(aryl ester) dendron D1.
Figure 5. Model compounds FP and MF analogues to the fullerene-containing central
part of compounds 3 and 4, correspondingly.
Table 1. Molecular weight (M), hydrodynamic diameter (d), permanent dipole
moment (μ), dielectric permittivity increment (ε-εo)/c, squared refractive index
increment ((n2-no2)/c), and molar Kerr constant (KM) of samples 1-4, D1 and D2.
Compound
M*
d**
Μ
(ε-εo)/c
(n2-no2)/c
Å
Debye
cm3·g-1
cm3·g-1
KM, exp·108
KM, cal·108
cm5· (300V)-2 cm5 · (300V)-2
· mol-1
· mol-1
D1
2733
33
14.2±0.8
9.4±0.2
0.10±0.01
3.6
3.7
D2
4228
34
6.4
1.3
0.09
0.46
-
2-123
1
4097
-
8.1
2.1
0.10
0.99
-
2
7347
-
8.5
1.4
0.10
0.99
-
3
7679
45
19.2
6.2
0.13
11.4
1.8
4
5878
44
18.6
7.6
0.15
12.5
11.5
*
Molecular mass M corresponds to structural formula; ** data from [19, 20].
Table 2. Mesomorphic properties of compounds under investigation.
Compound
C6H13-Ph-COO-Ph-Ph-CN *
Phase transition temperatures, oC
Tg→N 65o I 218o
D1
Tg →SA 34o I 182o
4
SA →N 183o I 184o
3
Columnar→ I 152o
N– nematic; SA – smectic A; Tg – glass state; I – isotropic state.
* low molecular nematic liquid crystal analogues to mesogenic groups of D1 [21].
Table 3. Dipole moment (μ), mean polarizability value (bmean), anisotropy of optical
polarizability (Δb) calculated by quantum chemical semiempirical method PM3 for
fully optimized model compounds presented in Figs. 4 and 5 and for compound
analogues to mesogenic group, and theirs molar Kerr constants (KM).
Compound
μ, D
bmean х1024,
cm3
Δb х1024, cm3
КМх1010 *
MF
2.96
78.4
11.54
5.52
FP
1.18
86.9
21.49
19.12
Model of the
central part of
D1
0.01
38.0
47.35
93
Model of
mesogenic
group
7.20
39.4
50.99
220
* Kм = 2 π NA (θ1+ θ2) [11] ,
where θ1 = (45 kT)-1[(b1-b2)2 + (b2-b3)2 + (b3- b1)2] and
2-124
θ2 = (45 k2 T2)-1 [(μ12-μ22)( b1-b2) + (μ22-μ32)( b2-b3) + (μ32-μ12)(b3- b1)];
here bi are the main values of optical polarizability tensor, μi are the projections of
dipole moment in the coordinate system, which abscissa-axis coincides with the
principal direction of optical polarizability; i = 1, 2, 3;
bmean=(b1+b2+b3)/3;
Δb= [(b1-b2)2 + (b2-b3)2 + (b3- b1)2] ½.
Figure 1a.
2-125
1
2
4
3
Figure 1b.
2-126
2,5
4
2,0
3
nx10
8
2
1,5
1,0
1
0,5
0,0
0,1
0,2
2
0,3
-4
2
0,4
0,5
-2
E x10 ((300V) cm )
4
8
3
6
D1
10
5 -1
((n-n0)/E c) x10 , cm g (300 V)
-2
Figure 2.
2
4
2
D2
0,0
0,5
1,0
2
1,5
c x10 , g cm
-3
Figure 3.
Figure 4.
2-127
2,0
CH3
N
FP
CO2
C 2H 5
C 2H 5
CO2
MF
Figure 5.
2-128
CO2
C 2H 5