Controlled homeotropic and homogeneous orientations for nanoscale phase-separated domain of light-emitting amphiphilic block copolymer bearing a 2,5-diarylthiazole moiety

Article abstract of DOI:10.1246/cl.2008.272

An amphiphilic block copolymer composed of poly(ethylene oxide) and poly(methacrylate ester) bearing a donor-acceptor-type 2,5-diarylthiazole in the side chain is prepared by atom-transfer radical polymerization. The precursor 2,5-diarylthiazole monomer i

Full text of DOI:10.1246/cl.2008.272

272
Chemistry Letters Vol.37, No.3 (2008)
Controlled Homeotropic and Homogeneous Orientations for Nanoscale
Phase-separated Domain of Light-emitting Amphiphilic Block Copolymer
Bearing a 2,5-Diarylthiazole Moiety
Atsunori Mori,ꢀ1 Junichi Shikuma,1 Motoi Kinoshita,2 Tomiki Ikeda,2 Masahiro Misaki,3 Yasukiyo Ueda,1
Motonori Komura,2 Sadayuki Asaoka,2 and Tomokazu Iyodaꢀ2
1Department of Chemical Science and Engineering, Kobe University, 1-1 Rokkodai, Nada, Kobe 657-8501
2Chemical Resources Laboratory, Tokyo Institute of Technology, 4259 Nagatsuta, Yokohama 226-8503
3National Institute of Advanced Industrial Science and Technology (AIST), 1-1-1 Higashi, Tsukuba 305-8565
(Received December 3, 2007; CL-071341; E-mail: amori@kobe-u.ac.jp; iyoda.t.aa@m.titech.ac.jp)
An amphiphilic block copolymer composed of poly(ethyl-
dium catalyst in the presence of AgF. The block copolymer 3
was then synthesized by atom-transfer radical polymerization
(ATRP)4 using an ꢀ-bromo ester of poly(ethylene oxide)
composed of 114 monomer units as the macroinitiator 2 and
ene oxide) and poly(methacrylate ester) bearing a donor–accep-
tor-type 2,5-diarylthiazole in the side chain is prepared by atom-
transfer radical polymerization. The precursor 2,5-diarylthiazole
monomer is prepared by transition-metal-catalyzed CH arylation
reactions. A thin film of the block copolymer formed by spin
coating exhibited nanoscale phase separation with homeotropi-
cally oriented domains, while a film formed by the friction trans-
fer method showed domains with homogeneous orientation con-
firmed by AFM. Spectroscopic studies of the film, utilizing the
luminescent characteristics of 2,5-diarylthiazole, showed polar-
ized light emission as a result of the liquid crystalline ordering.
a
complex of 1,1,4,7,10,10-hexamethyltriethylenetetramine
(HMTETA) and CuCl. The polymerization of 1 took place at
80 ꢁC for 24 h in a living manner with a monomer/macroinitiator
ratio of (9:1–48:1). Under the reaction conditions, the corre-
sponding block copolymer was obtained with Mn ¼ 9800{
28500 and Mw=Mn ¼ 1:08{1:10 (Scheme 1).5
UV–vis absorption and luminescence measurements were
carried out on the corresponding homopolymer, which was
formed from 1 by radical polymerization employing AIBN as
an initiator. The polymer showed ꢁmax values of 338 nm (ab-
sorption) and 416 nm (luminescence; ꢀ ¼ 0:19). The results
suggest that the spectroscopic characteristics of the low molecu-
lar weight 2,5-diarylthiazoles are maintained in the side-chained
copolymer.1 Further characterization of the amphiphilic block
copolymer 3 were carried out on 3d, which had a monomer/
initiator ratio of 48:1. DSC analysis of polymer 3d revealed
endothermic peaks at 36, 146, and 255 ꢁC upon heating, assigned
to phase transition of the PEO domain, transition of the
methacrylate domain to a liquid crystalline phase, and transition
to an isotropic phase, respectively.5 Polarizing microscope
observations also suggested the formation of a smectic phase
at 248 ꢁC. Other polymers having different monomer/initiator
ratios (9:1–48:1) also showed similar behavior.5
We have recently shown that controlled CH arylation of
thiazole leads to a facile synthesis of 2,5-diarylated thiazoles.
It was also found that 2,5-diarylthiazole derivatives bearing do-
nor–acceptor-type substituents show strong photoluminescent
and liquid crystalline characteristics.1–3 These findings prompted
us to investigate introduction of the photoluminescent 2,5-
diarylthiazole moiety into the side chains of polymers to enable
formation of a functional organic thin film.
On the other hand, we have also been studying the character-
istics of amphiphilic block copolymers composed of hydrophilic
poly(ethylene oxide) and hydrophobic poly(methacrylate) seg-
ments. We have found that liquid crystal copolymers bearing a
pendant azobenzene in the hydrophobic part show remarkable
nanoscale phase-separation.4 Recently, our interest has centered
on the synthesis of amphiphilic block copolymers bearing a
pendant 2,5-diarylthiazole, the liquid crystalline characteristics
of which should induce orientation of a thin film of the polymers
and lead to remarkable light emission. We herein report prelimi-
nary results on the preparation of light-emitting amphiphilic
block-copolymers with controlled molecular weight and molec-
ular weight distribution, and present characterization data for a
thin film of the copolymers, which indicates ordered nanoscale
phase separation.
We then envisaged the formation of a thin film of the block
copolymer 3d. The film was fabricated on a silicon wafer by
spin-coating a chloroform solution of 3d. The spectroscopic
characteristics of the film were similar to those in the corre-
sponding homopolymer solution of the diarylthiazole. Nanoscale
phase separation of oriented domains was observed upon
annealing of the thin film at 180 ꢁC for 24 h. UV–vis absorption
O
CuCl / HMTETA
MeO
Br
1
+
O
Preparation of a monomer bearing a 2,5-diarylthiazole
moiety 1 (Chart 1) in a methacrylate side chain was carried
out by tandem CH arylation, a method which we published
recently,1 using a palladium/copper catalyst system and a palla-
114
Anisole
80 °C, 24 h
2
O
Me
CO
MeO
O
114
n
N
3a
(n=9), 3b (n=20),
(n=28), 3d (n=48)
F3C
O(CH2)6O
3c
S
N
O
CF3
O(H2C)6O
:
1
S
Chart 1.
Scheme 1.
Copyright Ó 2008 The Chemical Society of Japan

Chemistry Letters Vol.37, No.3 (2008)
273
a)
b)
b)
a)
Figure 2. a) AFM image (phase mode) of the thin film of 3d
prepared by the friction transfer method. The arrow shows the
drawing direction. b) Luminescence spectra of the thin film
with perpendicular (upper) and parallel (lower) polarized light
(excitation wavelength: 330 nm).
Figure 1. a) UV–vis (left) and luminescence (right) spectra of 3d
in a thin film before (black) and after (red) annealing at 180 ꢁC
for 24 h. b) AFM image (phase mode) of the thin film of 3d.
and luminescence spectra of the film, were lower in intensity
after annealing, as shown in Figure 1a. The absorption and
luminescence spectra, with ꢁmax values at 319 and 487 nm,
respectively, reduced in intensity, suggesting that homeotropic
orientation of the liquid crystalline mesogen had occurred.
AFM analysis also indicated nanoscale phase separation, as
shown in Figure 1b, with the sample exhibiting a regular dotted
structure, with the dots 18 nm in diameter (PEO domain as
the dark region) separated by 26 nm on average. The structure
suggests hexagonal-packed cylinders of homeotropically aligned
liquid crystals, which would be induced by the orientation of the
liquid crystalline mesogen moiety.5,6 This is the first time to
the best of our knowledge that the orientation of a hydrophobic
domain bearing a photoluminescent chromophore has been
identified.
diarylthiazole moiety in the side chain was synthesized by
palladium-catalyzed CH arylation reactions and atom-transfer
living radical polymerization protocols. A thin film of the
polymer exhibited homogeneous and homeotropic orientations
were successfully obtained for thin films of the polymer with
nanoscale phase separation. The homogeneous orientation
observed in the thin film, which was generated by the friction
transfer method, was found to induce remarkable polarized light
emission.9
This work was partially supported by a Grant-in-Aid for
Scientific Research in Priority Area, Advanced Molecular
Transformation of Carbon Resources, by Ministry of Education,
Culture, Sports, Science and Technology, Japan.
We have recently reported that we were able to form a thin
film with homogeneous orientation of the amphiphilic block
copolymer bearing an azobenzene moiety when the thin film
was fabricated by spin coating on the rubbing polyimide plate.
The homogeneous orientation was observed along the rubbing
direction.7 Encouraged by which, we attempted a similar prepa-
ration for 3d in the present work. However, such alignment
was not observed in the 2,5-diarylthiazole pendant block
copolymers.
References and Notes
1
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T. Ikeda, Org. Lett. 2004, 6, 2011. c) K. Kobayashi, M. S. Mohamed
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See also: a) S. Pivsa-Art, T. Satoh, Y. Kawamura, M. Miura, M.
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¨
Homogeneous orientation of the photoluminescent 2,5-
diarylthiazole domain was found to be possible using a friction
transfer technique, in which a thin film is formed from a pellet
of the block copolymer.8 Figure 2a shows an AFM image of a
film formed by the friction transfer method, which shows nano-
scale phase separation with homogeneous orientation of the
diarylthiazole domain suggesting the PEO domain as the dark re-
gion and the thiazole pendant hydrophobic domain as the light
region, respectively. In sharp contrast to films of main-chain-
type liquid crystalline polymers formed by friction transfer,
which have orientation parallel to the drawing direction,8d
films of the side-chain-type polymer 3d, which have not been
previously examined, showed orientation perpendicular to the
drawing direction.
Also worthy of note is the observation that polarized
luminescence is induced as a result of the homogeneous orienta-
tion. As shown in the luminescence spectra acquired under
polarized excitation at 330 nm (Figure 2b), the ratio of parallel/
perpendicular intensities was found to be 1/13. The result
supports perpendicular alignment of thiazole mesogen toward
the friction drawing, whereas alignment of the azobenzene
moiety was parallel to the rubbing direction.
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Measurements of images of AFM of the cross section and TEM support
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8
9
Supporting Information is available electronically on the CSJ-Journal
Web site, http://www.csj.jp/journals/chem-lett/index.html.
In summary, amphiphilic block copolymer bearing a 2,5-
Published on the web (Advance View) February 2, 2008; doi:10.1246/cl.2008.272