Synthesis and self-assembly of rod-coil diblock molecules of oligophenylenevinylene-poly(ethylene oxide)

Article abstract of DOI:10.1246/cl.2000.586

A series of amphiphilic rod-coil diblock molecules containing oligophenylenevinylene have been synthesized and demonstrated various aggregate behaviors in selected solvents. They can nano-separate to form either domain structures or stripes in as-cast sta

Full text of DOI:10.1246/cl.2000.586

586
Chemistry Letters 2000
Synthesis and Self-Assembly of Rod-Coil Diblock Molecules of
Oligophenylenevinylene-Poly(ethylene oxide)
Huiming Xiong, Lidong Qin, Jingzhi Sun, Xi Zhang,* and Jiacong Shen
Key Lab of Supramolecular Structure & Spectroscopy, Department of Chemistry, Jilin University, Changchun 130023, P. R. China
(Received January 11, 2000; CL-000035)
A series of amphiphilic rod-coil diblock molecules containing
oligophenylenevinylene have been synthesized and demonstrated
various aggregate behaviors in selected solvents. They can nano-
separate to form either domain structures or stripes in as-cast state.
obtained by mixing compound 1 (6.4 mmol) with p-hydroxyben-
zaldehyde (7.2 mmol) and anhydrous potassium carbonate (12.8
mmol) in 50 mL dry acetone. A little amount of 18-crown-6 was
added as phase-transition catalyst. The mixture was heated under
reflux for 36 h under nitrogen and then filtered after cooled to
room temperature. The solvent was removed, 15 mL of water
added to the residue and the resulting solution was extracted with
methylene choloride. The organic extract was washed with water,
dried over anhydrous magnesium sulfate, and filtered. The sol-
vent was removed under vacuum, and the crude product was puri-
fied by column chromatography (silica gel G, anhydrous chloro-
form-methanol (3:1) as eluent). The synthesis of 4-styrylbenzyl-
triphenylphosphonium bromide 3 was according to reference.5
The target compound 4 (abbreviated as OPV-3/PEO350) was
obtained by using Wittig reaction: compound 2 (1.3 mmol) and 3
(1.3 mmol) were mixed in 40 mL dry tetrahydrofuran. Potassium
tert-butoxide (2 mmol) was dissolved in dry tetrahydrofuran and
added dropwise to the suspension on an ice-bath for about 2 h.
The mixture was stirred on ice-bath for 36 h, then filtered. The
solvent was removed under vacuum at room temperature. Yellow
compound was obtained and purified by column chromatography
(silica gel G, anhydrous chloroform-cyclohexane (5:1) as eluent),
recrystallized at least three times from ethanol and dried under
vacuum to give the desired product. (Yield, 30-40%). OPV-
3/PEO750 was purified by recrystallized from anhydrous diethyl
Rod-coil diblock copolymers consisting of a rigid rod and a
flexible coil have been studied both experimentally and theoret-
ically in recent years.1 The unique self-assembly behaviors of
rod-coil diblock copolymers exhibit as they can form various
supramolecular structures. Incorporation of functional group
into rod-coil system is anticipated to create a new class of mate-
rial. Recently, poly(phenylene vinylene) (PPV) and its deriva-
tives have attracted considerable attention due to their applica-
tions and fundamental research interest.2 It is shown that
chains aggregation and intermolecular interaction have dramat-
ic effects on the photophysical properties of the polymers.3 So,
design and control of supramolecular structure and morphology
become critically important. Conjugated oligomers as models
for the related polymer have aroused extensive interest.4 Here,
we report synthesis of a series of amphiphilic rod-coil diblock
molecules, the rod is oligophenylene vinylene(OPV-3), the coil
is poly(ethylene oxide)(PEO). It will then discuss the spectro-
scopic studies of aggregates of this kind of rod-coil amphiphilic
diblock molecule in selected solvents and its supramolecular
structures based on nanophase separation in as-cast state.
1
ether. The purity of compounds was confirmed by H NMR,
GPC and IR spectroscopy.6
Little work has been reported on aggregation of
poly(phenylenevinylene) oligomers in solution because of their
solubility problem. However, the solubility of this kind of rod-
coil system is greatly enhanced due to the connection of the PEO
coil, which enables us to investigate their self-assembly behav-
iors in selected solvents. The compounds can dissolve in chloro-
form well, and in water or cyclohexane up to a concentration of
an order of 10-5 M. The UV-Vis absorption spectra of OPV-
The synthetic route is shown in Scheme 1. All the solvents
were dried, freshly distilled and bubbled with nitrogen before
used. The methyloxy poly(ethyleneoxy) ethyl bromide 1 was
originally prepared via bromination of the corresponding methy-
loxypoly(ethyleneoxy)ethyl alcohol (20 mmol) with PBr3 (30
mmol) in 30 mL dry dioxane. The methyloxypoly(ethyleneoxy)-
ethyl alcohol (Mw 350, 750) were commercially obtained from
Fluka and dried under vacuum at 60 °C overnight before used.
The 4-[methyloxypoly(ethyleneoxy)]-benzaldehyde 2 was
Copyright © 2000 The Chemical Society of Japan

Chemistry Letters 2000
587
3/PEO350 show large difference depending on the solvents, as
shown in Figure1. From the UV-Vis absorption spectrum in
chloroform (Figure 1a), we can find two absorption bands
peaked around 316 nm and 361 nm, respectively. In water the
absorption maximum is at about 300 nm (Figure 1b). In cyclo-
hexane, the absorption band peaked at about 361 nm (Figure 1c).
For OPV-3/PEO750, it shows minor substituent effect on
absorbance and almost similar spectroscopic behaviors.
The microphase separation behaviors of the copolymers
were studied by using transmission electron micrograph (TEM).
For TEM studies, thin films were prepared by casting the cyclo-
hexane solution (1 × 10-5 M) on carbon-coated copper grid and
evaporating in a solvent atmosphere, then exposing it to vacu-
um for about one day and not staining. As shown in Figure 3,
the micrograph of OPV-3/PEO750 clearly shows aggregate
domains that are approximate 3-4 nm in diameter. It is also
found that the morphology is dependent on coil fraction. For
OPV-3/PEO350, it can form stripes on the scale of nanometer
and the length can extend to several tens of nanometers. These
patterned materials based on nanophase separation are antici-
pated leading to novel photophysical and electronic properties
and further research is underway.
To identify these absorption bands which originate from dif-
ferent aggregate states, we examine temperature dependent UV-
Vis absorption and host-guest complexes of OPV-3/PEO350 with
β-cyclodextrin (β-CD) in aqueous solution. It is believed that β-
CD can only form 1:1 inclusion complex with oligo(phenylene
vinylene).7 OPV-3/PEO350 and β-CD (mole ratio 1:200) were
dissolved in water and stirred for two weeks, however, the absorp-
tion spectrum was not found any change. The peak positions of
spectra remained almost the same even to relatively low concen-
tration or upon heating, which suggests that this species may be
pretty stable. To adjust the polarity of the solvent, ethanol was
added. By increasing the solution temperature from 23 oC to 84
oC, we found that the absorption band peaking around 300 nm
decreased, meanwhile a absorption band at about 361 nm
appeared and its intensity increased gradually, as shown in Figure
2. This suggests that absorption around 361 nm should originate
from monomers. An isosbectic point was also found from the
temperature dependent absorption spectra. It indicated a direct
conversion of these two species. Furthermore, when we added the
β-CD to the mixed solution (water : ethanol 3:1), stirred overnight
and precipitated β-CD from the solution by chloroform, washed
the precipitates several times, dried it under vacuum, then dis-
solved the β-CD into water again, we found that besides the
absorption at 300 nm, there still existed an absorption band
peaked around 361 nm. It was greatly different from that in pure
water without β-CD since the aggregates are stable and can’t exist
as monomers in it. This can also support the conclusion that the
absorption at about 361 nm originates from monomers. In this
case it formed 1:1 inclusion complex with β-CD. In water, the
absorption peaking around 300nm which is much blue-shifted
compared to that of monomers is indicative of formation of H-
aggregates. It is very similar to those obtained for LB films.8 In
cyclohexane, the rod-coil compound absorbing at about 361 nm
exists majorly as monomers. In chloroform, the absorption is
blue-shifted to 316 nm compared to that of monomers, suggesting
formation of a type of H-aggregate.
The authors give the thanks to National Natural Science
Foundation of China for the financial support.
References and Notes
1
a) A. Haperin, Macromolecules, 23, 2724 (1990). b) D. R. M.
Williams and G. H. Fredrickson, Macromolecules, 25, 3561 (1992).
c) L. H. Radzilowski and S. I. Stupp, Macromoleules, 27, 7747
(1994). d) S. A. Jenekhe and X. L. Chen, Science, 279, 1903 (1998).
e) M. Lee, B. K. Cho, H. Kim, J. Y. Yoon, and W. C. Zin, J. Am.
Chem. Soc. 120. 9168 (1998). f) J. T. Chen, E. L. Thomas, C. K.
Ober, and G. –p. Mao, Science, 273, 343 (1996). g) W. J. Li, H. B.
Wang, L. P. Yu, T. L. Morkved, and H. M. Jaeger, Macromolecules,
32, 3034 (1999).
2
3
a) J. H. Burroughes, D. D. C. Bradley, A. R. Brown, R. N. Marks, K.
Mackay, R. H. Friend, P. L. Burn, and A. B.Holmes, Nature, 347,
539 (1990). b) D. Braun and A. J. Heeger, Appl. Phys. Lett., 58, 1982
(1991).
J. Cornil, D. A. dos Santos, X. Crispin, R. Silbey, and J. L. Brédas, J.
Am. Chem. Soc., 120, 1289 (1998).
4
5
K. Müllen and G. Wegner, Adv. Mater., 10, 433 (1998).
R. Schenk, H. Gregorius, K. Meerholz, J. Heinze, and K. Müllen, J.
Am. Chem. Soc., 113, 2634 (1991).
6
Selected data OPV-3/PEO350: Mw/Mn = 1.10; (1H NMR 400 MHz,
CDCl3): ɬ3.379 (s,3H), 3.5-3.8 (24H), 3.871 (t,2H,J=4.4 Hz), 4.146
(t, 2H, J = 4.4 Hz), 6.916 (d, 2H, J = 8.4 Hz), 6.952-7.114 (4H),
7.257 (t, 1H, J = 7.6 Hz), 7.364 (t, 2H, J = 7.6 Hz), 7.448 (d, 2H, J =
8.8 Hz), 7.490 (s, 4H), 7.523 (d, 2H, J = 7.6 Hz); IR(KBr cm-1): CH2
2871, 2924, C-O-C 1113, 1063, C-O-Ar 1251, trans –CH=CH- 968,
3022; OPV-3/PEO750: Mw/Mn = 1.09; ɬ3.379 (s, 3H) 3.5-3.8
(60H), 3.872 (t, 2H, J = 4.8 Hz), 4.158 (t, 2H, J = 4.8 Hz), 6.917 (d,
2H, J = 8.4 Hz), 6.953-7.115 (4H), 7.259 (t, 1H, J = 7.2 Hz), 7.364
(t, 2H, J = 7.6 Hz), 7.449 (d, 2H, J = 8.8 Hz) 7.491 (s, 4H), 7.524 (d,
2H, J = 7.2 Hz); IR(KBr cm-1): CH2 2870, 2924, C-O-C 1113, 1062,
C-O-Ar 1253, trans –CH=CH- 968, 3024.
7
8
X. D. Song, C. Geiger, M. Farahat, J. Perlstein, and D. G. Whitten, J.
Am. Chem. Soc., 119, 12481 (1997).
a) A. Watakabe, H. Okada, and T. Kunitake, Langmuir, 10, 2722
(1994). b) Results unpublished.