Synthesis, chiroptical properties, and photoresponsiveness of optically active poly(m -phenyleneethynylene)s containing azobenzene moieties

Article abstract of DOI:10.1021/ma200281e

The Sonogashira-Hagihara coupling polymerization of 3′,5′- diiodo-4′-hydroxy-N-α-tert-butoxycarbonyl-d-phenylglycine ethyl-, hexyl-, and laurylamides 1a-c with p-nonsubstituted, cyano, hexyl, and methoxy 3,5-diethynylazobenzenes 2a-d was carried out to ob

Full text of DOI:10.1021/ma200281e

ARTICLE
pubs.acs.org/Macromolecules
Synthesis, Chiroptical Properties, and Photoresponsiveness of
Optically Active Poly(m-phenyleneethynylene)s Containing
Azobenzene Moieties
Hiromitsu Sogawa, Masashi Shiotsuki, Hideki Matsuoka, and Fumio Sanda*
Department of Polymer Chemistry, Graduate School of Engineering, Kyoto University, Katsura Campus, Nishikyo-ku, Kyoto 615-8510,
Japan
S Supporting Information
b
ABSTRACT: The SonogashiraꢀHagihara coupling polymeriza-
tion of 30,50-diiodo-40-hydroxy-N-R-tert-butoxycarbonyl-D-phe-
nylglycine ethyl-, hexyl-, and laurylamides 1aꢀc with p-
nonsubstituted, cyano, hexyl, and methoxy 3,5-diethynylazoben-
zenes 2aꢀd was carried out to obtain optically active novel
poly(m-phenyleneethynylene) with Mw values in the range from
6900 to 15 400 in 62ꢀ84% yields. CD and UVꢀvis spectroscopic
data indicated that the polymers adopted thermally stable helical
conformations in CH2Cl2 and N,N-dimethylformamide. Poly-
(1bꢀ2a) further formed a chirally aggregated structure. The
azobenzene moieties of the polymers underwent reversible photoisomerization upon UV- and visible-light irradiation, accompany-
ing the changes of the higher-order structures.
’ INTRODUCTION
between trans and cis forms upon UV- and visible-light
irradiation.24 This conformational change triggers not only
geometric change but also chemical properties such as dipole
moment. Among various kinds of external stimuli, photoirradia-
tion is practically useful for constructing sensing materials
because of easy control over the irradiation wavelength, time,
and intensity.25ꢀ30 Azobenzene-containing π-conjugated helical
polymers are applicable to intelligent materials that possess
photoresponsiveness in addition to the electronic and optical
properties, as mentioned above.31ꢀ34
Herein we wish to report the synthesis of novel hydroxyphe-
nylglycine-derived poly(m-phenyleneethynylene)s containing
azobenzene moieties (Scheme 1) and investigation of the effects
of the alkyl chain lengths or substituent of azobenzene moieties
on the higher-order structure, together with the solvent effect.
We also report the reversible photoresponsive conformational
changes of the polymers.
Biomacromolecules such as proteins and DNA commonly have
one-handed helical structures based on the homochirality of the
components. Their functions and biological activities are generated
by their well-defined higher-order structures. Artificial helical
polymers have been extensively synthesized by imitating naturally
derived helices.1 The development of artificial helical polymers may
encourage better understanding of the mechanisms for complicated
and elegant functions of biopolymers. π-Conjugated helical poly-
mers such as polyisocyanides,2 polyacetylenes,3 polythiophenes,4
poly(phenylenevinylene)s,5 and poly(phenyleneethynylene)s6
have attracted much attention because of their potential of practical
applications including molecular recognition materials,7ꢀ12 chiral
catalysts,13ꢀ15 and chemical sensors16ꢀ18 using their unique elec-
tronic and optical properties based on the secondary structures.
Among them, poly(m-phenyleneethynylene)s substituted with
polar groups tend to folded into helical structures in polar solvents
based on the amphiphilic property between the hydrophilic side
chains and hydrophobic main chain.1b,19ꢀ21 We have recently
found new examples of hydroxyphenylglycine- and tyrosine-based
poly(m-phenyleneethynylene-p-phenyleneethynylene)s that adopt
helical conformations consisting of hydrophobic exterior (alkyl
groups and phenyleneethynylene main chain) and hydrophilic
interior (hydroxy groups) in nonpolar solvents.22,23 The helix
formation of these polymers is based on the amphiphilic balance
opposite from that of poly(m-phenyleneethynylene) derivatives
reported so far.
’ EXPERIMENTAL SECTION
Measurements. 1H (400 MHz) and 13C (100 MHz) NMR spectra
were recorded on a JEOL EX-400 or a JEOL AL-400 spectrometer. IR
spectra were measured on a JASCO FT/IR-4100 spectrophotometer.
Melting points (mp) were measured on a Yanaco micro melting point
apparatus. Elemental analysis was done at the Microanalytical Center of
Kyoto University. Mass spectra were measured on an Applied Biosystems
Received: February 8, 2011
Meanwhile, azobenzene is one of the best-known photore-
sponsive molecules undergoing reversible photoisomerization
Revised:
Published: April 08, 2011
March 21, 2011
r
2011 American Chemical Society
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Scheme 1. SonogashiraꢀHagihara Coupling Polymerization of Monomers 1aꢀc with 2aꢀd
analytical and spectroscopic data are described in the Supporting
Information. N,N-Dimethylformamide (DMF) and Et3N used for
polymerization were distilled prior to use.
Scheme 2. Synthesis of Monomers 2aꢀd
Polymerization. All polymerizations were carried out in a glass
tube equipped with a three-way stopcock under nitrogen. A typical
experimental procedure for polymerization 1a with 2a is given below.
A solution of 1a (164 mg, 0.30 mmol), 2a (69.1 mg, 0.30 mmol),
Pd(PPh3)2Cl2 (4.2 mg, 6.0 μmol), CuI (6.9 mg, 36 μmol), PPh3 (6.3
mg, 24 μmol), and Et3N (0.6 mL) in DMF (0.9 mL) was stirred at 80 °C
for 24 h. The resulting mixture was poured in MeOH/acetone [4/1
(v/v), 150 mL] to precipitate a polymer. It was separated by filtration
using a membrane filter (ADVANTEC H020A047A) and dried under
reduced pressure. The spectroscopic data of the polymer together with
those of the other polymers are described in the Supporting Information.
Voyager Elite MALDI mass spectrometer or a JEOL JMS-SX102A mass
spectrometer. Number- and weight-average molecular weights (Mn and
Mw) of polymers were determined by GPC (Shodex columns K803, K804,
K805) eluted with CHCl3 or (Shodex columns KF805 ꢁ 3) eluted with
THF calibrated by polystyrene standards at 40 °C. CD and UVꢀvis
absorption spectra were recorded on a JASCO J-820 spectropolarimeter.
Dynamic light scattering (DLS) measurements were carried out with an
Otsuka Electronic SLS-7000 goniometer with a GC-1000 correlator. The
time-correlation functions were analyzed by double exponential methods.
The measurements were performed at four different scattering angles, and
the diffusion coefficient D was calculated from the slope of the straight line
in the decay rate Γ versus q2 plot, with q as the scattering vector. The
hydrodynamic radii of scatterers were evaluated by the StokesꢀEinstein
equation.
’ RESULTS AND DISCUSSION
Monomer Synthesis and SonogashiraꢀHagihara Cou-
pling Polymerization. 3,5-Diethynylazobenzene monomers
2aꢀd were synthesized by the route illustrated in Scheme 2. First,
3,5-diiodoazobenzene derivatives 3aꢀc were synthesized by the
condensation of 3,5-diiodoaniline with the corresponding nitroso-
benzene derivatives in CH3COOH. Compound 3d was synthesized
by the diazo coupling reaction of 3,5-diiodoaniline with phenol,
followed by the formation of methyl etherification of 4-(3,5-
diiodophenylazo)phenol because the reaction using 4-nitrosoani-
sole was unsatisfactory. Subsequently, the SonogashiraꢀHagihara
coupling reaction of 3aꢀd with trimethylsilylacetylene and desilyla-
tion from the ethynyl group were carried out to obtain monomers
2aꢀd. All monomers were characterized by 1H, 13C NMR, and IR
spectroscopies besides elemental analysis or high-resolution mass
spectrometry.
The SonogashiraꢀHagihara coupling polymerization of 1a with
2a, 1b with 2aꢀd, and 1c with 2a was carried out in DMF at 80 °C
for 24 h to obtain the corresponding polymers [poly(1aꢀ2a)ꢀ
poly(1cꢀ2a)] with Mw values in the range of 6900ꢀ15 400 in
62ꢀ84% yields, as listed in Table 1.38 Except for poly(1bꢀ2b), the
polymers were soluble in common organic solvents such as
CH2Cl2, CHCl3, THF, DMSO, and DMF. Poly(1bꢀ2b) was
insoluble in CH2Cl2 and CHCl3 and partially soluble in THF,
DMSO, and DMF.
Photoirradiation. Photoirradiation was carried out with a 400 W
high-pressure mercury lamp equipped with a power source (HB-400,
Fuji Glass Work) at room temperature. The appropriate wavelengths
were selected either with a Pyrex glass and a UV-D33S filter (Toshiba)
for irradiation at 300 nm < λ < 400 nm or with an L-42 filter (Toshiba)
for irradiation at 420 nm < λ. Sample solutions were fed in a quartz cell,
and it was placed 20 cm apart from the lamp. The photoisomerization
was monitored by UVꢀvis absorption spectroscopy.
Materials. 30,50-Diiodo-40-hydroxy-N-R-tert-butoxycarbonyl-D-phe-
nylglycine ethylamide (1a), 30,50-diiodo-40-hydroxy-N-R-tert-butoxycar-
bonyl-D-phenylglycine hexylamide (1b), 30,50-diiodo-40-hydroxy-N-R-
tert-butoxycarbonyl-D-phenylglycine laurylamide (1c), 3,5-diiodoaniline,
4-nitrosobenzonitrile, and 4-nitrosohexylbenzene were prepared ac-
cording to the literature.23,35ꢀ37 Monomers 2aꢀd were newly synthe-
sized according to Scheme 2. The details of the monomer synthesis and
Chiroptical Properties of the Polymers. The CD and
UVꢀvis spectra of the polymers were measured in CH2Cl2 at
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Table 1. SonogashiraꢀHagihara Coupling Polymerization of
1aꢀc with 2aꢀda
polymer
monomer
c
c
yield (%)b
Mw
Mw/Mn
1aþ2a
1bþ2a
1bþ2b
1bþ2c
1bþ2d
1cþ2a
poly(1aꢀ2a)
poly(1bꢀ2a)
poly(1bꢀ2b)
poly(1bꢀ2c)
poly(1bꢀ2d)
poly(1cꢀ2a)
83
62
65
68
84
72
11 900
15 400
8800d
6900
1.9
1.8
1.5d
1.8
1.9
1.6
14 000
10 900
a Conditions: [1aꢀc] = [2aꢀd] = 0.20 M, [PdCl2(PPh3)2] = 0.0040 M,
[CuI] = 0.024 M, [PPh3] = 0.016 M, Et3N/DMF = 2/3 (v/v), 80 °C, 24
h. b MeOH/acetone = 4/1 (v/v) insoluble part. c Determined by GPC
eluted with CHCl3, polystyrene calibration. d Eluted with THF.
Figure 2. CD and UVꢀvis spectra of poly(1bꢀ2a) measured in
CH2Cl2 (c = 0.030 mM) at 20 °C before and after filtration using a
membrane with 0.45 μm pore size.
Figure 3. DLS results for solutions of poly(1bꢀ2a) 0.10 wt % in
CH2Cl2 measured at a scattering angle of 90° at 25 °C. Left: Time
correlation functions for the scattered filed. Solid lines are double-
exponential fits. Right: Decay rate Γ versus q2 plot for poly(1bꢀ2a) for
the fast mode (b) and slow mode (O). The excellent linearity in this plot
guarantees that both of these two modes correspond to the translational
diffusion. The diffusion coefficients of the particles were evaluated from
the slopes of the straight lines for the fast mode.
Figure 1. CD and UVꢀvis spectra of poly(1aꢀ2a), poly(1bꢀ2a), and
poly(1cꢀ2a) measured in CH2Cl2 (c = 0.030 mM) at 20 °C.
20 °C to obtain information on the secondary structures. Poly-
(1aꢀ2a), poly(1bꢀ2a), and poly(1cꢀ2a) exhibited CD signals
at 250ꢀ450 nm, as shown in Figure 1. They showed UVꢀvis
absorption at the same region (λmax around 315 nm) as the CD
signals. The λmax values of the polymers appeared red-shifted by
76ꢀ79 nm compared to those of 1aꢀc. It is considered that
these CD and UVꢀvis absorption peaks come from the con-
jugated m-phenyleneethynylene backbone. The absorption de-
rived from the azobenzene moieties seems to overlap with that of
the main chains because the λmax of 2a was 320 nm as well.
Hence, it is considered that poly(1aꢀ2a), poly(1bꢀ2a), and
poly(1cꢀ2a) adopt helical conformations with predominantly
one-handed screw sense in CH2Cl2 in a manner similar to the
analogous poly(m-phenyleneethynylene-p-phenyleneethynylene)s.23
The CD intensity of poly(1cꢀ2a) was higher than that of
poly(1aꢀ2a) probably because of the longer alkyl chains, which
are effective to stabilize the helical structure by enhancing the
hydrophobicity of the helix exterior and/or hydrophobic inter-
action between the side chains.
poly(1cꢀ2a), as shown Figure 2. This result suggests that poly-
(1bꢀ2a) formed the chiral aggregates exhibiting the different CD
signals from those of nonaggregated states. Namely, it is likely that
the intense chirality of the aggregates is induced by chiral arrange-
ment of the chiral helical polymers. Because the UVꢀvis intensities
of the sample before and after filtration were almost the same, it is
likely that the population of the aggregates is very small. The CD
and UVꢀvis spectra of poly(1aꢀ2a) and poly(1cꢀ2a) samples
did not change at all before and after filtration.
We measured the DLS of a CH2Cl2 solution of poly(1bꢀ2a) to
elucidate the formation of aggregates. From the correlation func-
tions depicted in Figure 3, a hydrodynamic radius (Rh) could be
evaluated from both fast and slow modes. From the fast mode, the
presence of aggregated particles with an Rh around 385 nm was
confirmed. The slow mode indicates the presence of very large
aggregates whose size is ∼2000 nm, which is larger by the order of
magnitude. According to the scattering principle, the scattering
intensity is proportional to the volume of the scatterer, and hence
the number population of the large aggregates should be 10ꢀ3 or
less, which leads us to the conclusion that the fast mode is the main
It should be noted that the CD spectrum of poly(1bꢀ2a) was
different from those of poly(1aꢀ2a) and poly(1cꢀ2a). We filtered
the sample solution using a membrane with a pore size of 0.45 μm
and then measured the CD and UVꢀvis spectra of the sample
again. Interestingly, the CD signal of poly(1bꢀ2a) became nearly
identical to those of the other two polymers, poly(1aꢀ2a) and
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Table 2. Solution-State IR Spectroscopic Data (Amide and
Carbamate CdO Absorption Peaks) of the Monomers and
Polymersa
wavenumber (cmꢀ1
)
compound
1a
amide CdO
carbamate CdO
1683
1654
1682
1653
1682
1654
1707
1701
1707
1701
1706
1701
poly(1aꢀ2a)
1b
poly(1bꢀ2a)
1c
poly(1cꢀ2a)
a Measured in CH2Cl2 (c = 10ꢀ20 mM).
Figure 4. CD and UVꢀvis spectra of poly(1bꢀ2a) measured in THF
and DMF (c = 0.030 mM) at 20 °C.
As mentioned above, poly(1bꢀ2a) formed chiral aggregates in
CH2Cl2, whereas poly(1aꢀ2a) and poly(1cꢀ2a) did not. It is
presumed that the chain length of alkyl groups of poly(1bꢀ2a)
match the requirement for inducing aggregation. The CD intensity
slightly increased as the sample concentration was increased in
CH2Cl2 as shown in Figure 5, left. An interesting phenomenon was
observed in DMF, as shown in Figure 5, right. At a concentration of
0.010 mM, a nonaggregated CD pattern was observed. At
0.025 mM, an aggregated CD pattern appeared, and the intensity
increased as the concentration was increased to 0.035 mM. It was
confirmed that the critical aggregation concentration of poly-
(1bꢀ2a) exists between 0.010 and 0.025 mM in DMF.
The helical structures of poly(m-phenyleneethynylene)s are
commonly susceptible to temperature.40,41 We measured the CD
spectra of poly(1aꢀ2a), poly(1bꢀ2a), and poly(1cꢀ2a) in
CH2Cl2 at 0ꢀ40 °C to find negligibly small changes (Figure
S5 of the Supporting Information). The Kuhn’s dissymmetry
factors (g = Δε/ε, in which Δε = [θ]/3298) of poly(1aꢀ2a) and
poly(1cꢀ2a) decreased only 6 and 5% by raising temperature
from 20 to 40 °C, and that of poly(1bꢀ2a) did not decreased at
this temperature range.42 The g values gives quantitative infor-
mation associated with the degree of preferential screw sense.43
The temperature dependence of g values of the present polymers
is very small compared with that of the previously reported
poly(m-phenyleneethynylene) derivatives. For example, the g
value of a poly(phenyleneethynylene) tethering tetraethylene
glycol decreases by 17% from 20 to 40 °C.40
Therefore, the higher-order structures of the polymers are
thermally stable compared with the poly(m-phenyleneethynylene)s
reported so far. This is explainable by the presence of intramole
cular hydrogen bonding between the amide and carbamate groups
at the side chains, which was confirmed by the solution state IR
spectra of the polymers measured in CH2Cl2 at diluted concentra-
tions (Table 2). The carbonyl absorption peaks of amide and
carbamate groups of the polymers were observed at 28ꢀ29 and
5ꢀ6 cmꢀ1 lower than those of the monomers 1aꢀc, respectively.
Poly(1bꢀ2c) and poly(1bꢀ2d) also exhibited CD signals, as
shown Figure 6, indicating that they also formed chiral higher-order
structures. Poly(1bꢀ2b) exhibited no CD signal in THF and DMF
(Figure S6 of the Supporting Information). The electron-with-
drawing character of the CN groups seems to affect the amphiphilic
balance, preventing the polymer from adopting a helical conforma-
tion with predominantly one-handed screw sense.
Figure 5. CD and UVꢀvis spectra of poly(1bꢀ2a) measured in
CH2Cl2 (left) and DMF (right) at various concentrations at 20 °C.
component, about which we should discuss. The filtration of the
sample solution resulted in the disappearance of the DLS modes
assignable to aggregated particles (fast mode) in addition to the
large aggregates (slow mode). It is concluded that the aggregated
particles have chirality, showing the intense CD signals.39
Figure 4 shows the CD and UVꢀvis spectra of poly(1bꢀ2a)
measured in THF and DMF. The previously reported hydroxy-
phenylglycine-based
poly(m-phenyleneethynylene-p-phenyle-
neethynylene)s exhibited Cotton effect on the CD spectrum,
indicating the formation of predominantly one-handed helical
conformation in THF, whereas no CD signal was observed in
DMF.32 Poly(1bꢀ2a), on the contrary, exhibited intense CD
signals in either of the solvents. Poly(1bꢀ2a) seems to have a
high ability to form a chiral higher-order structure compared with
the reported hydroxyphenylglycine-based poly(m-phenyleneethy-
nylene-p-phenyleneethynylene)s. The all meta-linked structure of
poly(1bꢀ2a) may be preferable for the polymer to adopt a
folded structure. In addition, the azobenzene moieties of poly-
(1bꢀ2a) may participate in intramolecular π-stacking between
the phenylene groups of the main chain, resulting in the high
ability of helix formation. Poly(1aꢀ2a) and poly(1cꢀ2a) also
exhibited CD signals in CH2Cl2, THF, and DMF (Figure S4 of
the Supporting Information).
Only poly(1bꢀ2a) formed a chirally aggregated structure in
CH2Cl2 among the polymers containing of monomer 1b unit.
This is probably because the affinity of poly(1bꢀ2a) to CH2Cl2
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is smaller than that of poly(1bꢀ2c) and poly(1bꢀ2d) having
hexyl and methoxy groups, respectively. Poly(1bꢀ2c) and poly-
(1bꢀ2d) showed λmax values longer than that of poly(1bꢀ2a),
moieties is unfavorable for aggregation compared with the more
linear main chain before irradiation. No absorption attributable to
nꢀπ* transition of cis-azobenzene unit was observed around
440 nm probably because of the low photoisomerization ratio,
which was estimated to be 7% based on change of the UVꢀvis
absorption.45 The low degree of photoisomerization is attributable
to the overlap of πꢀπ* transition band of the phenyleneethynylene
main chain with that of the trans-azobenzene units.19 It is con-
sidered that the UV-light was absorbed not only by the azobenzene
units but also by the conjugated main chain, preventing the effective
photoisomerization. This assumption seems to be reasonable from
the photoisomerization data of poly(1bꢀ2c) and poly(1bꢀ2d).
Namely, it was calculated that the trans fcisisomerizationratiosof
azobenzene units of poly(1bꢀ2c) and poly(1bꢀ2d) were 17 and
31%, respectively, which were higher than that of poly(1bꢀ2a). As
mentioned above, the πꢀπ* transition bands of azobenzene
moieties of poly(1bꢀ2c) and poly(1bꢀ2d) are positioned at
longer wavelength regions than that of poly(1bꢀ2a). It is con-
sidered that the azobenzene moieties of the former two polymers
can absorb UV-light with higher efficiencies because the overlaps of
the absorption bands with those of phenyleneethynylene back-
bones are smaller. Interestingly, the CD signals of poly(1bꢀ2c)
and poly(1bꢀ2d) around 450 nm increased with UV-light irradia-
tion. This result suggests that the polymers induced a certain
regulated structure according to the photoisomerization of the
azobenzene moieties. Visible-light irradiation to the UV-light
irradiated samples resulted in complete recovery of the initial CD
and UVꢀvis spectroscopic patterns. Reversible conformational
changes were confirmed in all polymers.
as shown in Figure 6. Poly(1bꢀ2d) showed the longest λmax
,
corresponding to the λmax order of the monomers (2a: 320 nm,
2c: 335 nm, 2d: 354 nm). This is quite reasonable because the
πꢀπ* transition band of azobenzene is shifted to a longer
wavelength region by introducing electron-donating groups.44
Figure 7 depicts the changes of CD and UVꢀvis spectra of
poly(1bꢀ2a), poly(1bꢀ2c), and poly(1bꢀ2d) with UV-light
irradiation. The polymer solutions in CH2Cl2 were irradiated with
a 400 W high-pressure mercury lamp through a suitable filter to
exclude the light of the wavelength below 300 nm and above
400 nm. The UVꢀvis absorption of poly(1bꢀ2a) at 320 nm
decreased as UV-light irradiation, accompanying a decrease of CD
intensity at 340 nm, and took photostationary state after 8 min.
These spectroscopic changes indicate that the trans-azobenzene
moieties isomerized into cis form, resulting in partial collapse of
a chirally aggregated structure. It seems that the bended main
chain of photoirradiated poly(1bꢀ2a) due to the cis-azobenzene
Chart 1. Structure of 18-mers of Poly(1aꢀ2a)a
Figure 6. CD and UVꢀvis spectra of poly(1bꢀ2a), poly(1bꢀ2c), and
poly(1bꢀ2d) measured in CH2Cl2 (c = 0.030 mM) at 20 °C. Poly-
(1bꢀ2b) was insoluble in CH2Cl2.
aArows indicate a torsional angle per phenylene unit.
Figure 7. CD and UVꢀvis spectra of poly(1bꢀ2a) (left), poly(1bꢀ2c) (center), and poly(1bꢀ2d) (right) measured in CH2Cl2 (c = 0.030 mM) with
irradiation at 300 < λ < 400 nm at 20 °C.
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Chart 2. Possible Regulated Conformers of Poly(1aꢀ2a)-18-mera
aFifteen monomer units are omitted at the wavy lines. The values are the energies in kJ/mol unit of the conformers after geometry optimization by the
3
MMFF94 method.
Conformational Analysis. The conformations of poly
(m-phenyleneethynylene)s have been analyzed by several methods
because they have been found to fold into helices under certain
conditions. Amine-46,47and ester-functionalized48 poly(m-phenyl-
eneethynylene)s energetically prefer helical conformations to
coiled and extended ones in water, wherein surrounding water
molecules play an important role to fold the polymer chains
according to the molecular dynamics simulations. When the
molecule adopts a helical conformation, the interaction between
the polar side chains and solvents becomes maximum and
π-stacking interaction between phenylene units as well. The
unfavorable contact between the hydrocarbon backbone and
polar solvents becomes minimum simultaneously.1b
Judging from the nature of poly(m-phenyleneethynylene)s, it is
likely that the polymers in the present study also adopt folded
helical structures with stacked phenylene moieties. We examined
the conformation of the polymers based on the molecular me-
chanics method (MMFF94).49 We first constructed helical 18-
mersofpoly(1aꢀ2a), whose both chain ends were terminated with
hydrogen atoms, as illustrated in Chart 1. The torsional angles of
main chains of left- and right-handed helices were set to ꢀ6 and
þ6° per phenylene unit at the initial geometries, respectively.
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with the CdO groups of the amide and carbamate groups three
units earlier (i þ 3 f i NꢀH OdC hydrogen bonding),
3 3 3
respectively. Accordingly, the conformer is surrounded by regulated
three hydrogen-bonding strands formed at the amide groups, and
the other three hydrogen-bonding strands formed at the carbamate
groups. The interatomic distances of the hydrogen and oxygen
(NꢀH OdC) participating in hydrogen bonding between the
3 3 3
(i þ 3)th and ith units at the amide and carbamate moieties were
1.76 and 1.79 Å on average, respectively. The shorter interatomic
distance between the amide groups may explain the larger shift of
carbonyl absorption of the solution state IR from the monomer to
polymer than that of carbamate as listed in Table 2. The hydroxy
groups inside the helix cannot hydrogen bond each other because
the distance is too long (3 to 4 Å). The helical pitch after geometry
optimization was 3.8 Å, which was the same as the value before
optimization. This value is consistent with the turns of the helix
being near van der Waals contact and comparable to π-stacking
between aromatic rings.47
’ CONCLUSIONS
In this Article, we have demonstrated the synthesis of novel
photoresponsive optically active poly(m-phenyleneethynylene)s
by the SonogashiraꢀHagihara coupling polymerization of D-phe-
nylglycine-derived m-diiodobenzenes with 3,5-diethynylazoben-
zene derivatives. CD and UVꢀvis spectroscopic studies revealed
that the polymers except for poly(1bꢀ2b) formed thermally stable
chiral higher-order structures in CH2Cl2, THF, and DMF. Unlike
the reported poly(m-phenyleneethynylene)s, the present polymers
could form chiral higher-order structures in nonpolar CH2Cl2 and
in polar DMF as well. The presence of intramolecular hydrogen
bonds between the amide and carbamate groups at the side chains
seems to be effective to stabilize the higher-order structures. The
trans-azobenzene moieties of the polymers isomerized into cis
formsupon UV-light irradiation, accompanying the conformational
changes of the polymers. Poly(1bꢀ2a) formed a chirally aggre-
gated structure in CH2Cl2, which partially collapsed according to
trans f cis photoisomerization. Poly(1bꢀ2c) and poly(1bꢀ2d)
showed other CD signals at regions of nꢀπ* transition band of cis-
azobenzene moieties after UV-light irradiation, indicating the
formation of chiral structures different from those before photo-
irradiation. The degree of photoisomerization was improved by
introducing a para-substituent in the azobenzene. The azobenzene
moieties of the polymers reversibly isomerized from cis to trans
upon visible-light irradiation, recovering the initial higher-order
structures.
Figure 8. Top and side views of a possible conformation (right-handed
bꢀa in Chart 2) of poly(1aꢀ2a)-18-mer. The phenyleneethynylene main
chain and OꢀH groups are illustrated by the space filling models, and the
hydrogen atoms other than NꢀH and OꢀH moieties are omitted for
enhancement of visibility. The green dotted lines represent hydrogen
’ ASSOCIATED CONTENT
bonds (NꢀH OdC) between the amide groups and carbamate
S
Supporting Information. Experimental procedure for
3 3 3
b
groups. Geometries were optimized by the MMFF94 method.
monomer synthesis, analytical, and spectroscopic data of the mono-
mers and polymers, 1H NMR spectrum of 2a (Figure S1), 1H NMR
spectrum of poly(1bꢀ2a) (Figure S2), DLS results for poly(1aꢀ2a)
and poly(1cꢀ2a) (Figure S3), CD and UVꢀvis spectra of poly-
(1aꢀ2a) and poly(1cꢀ2a) (Figure S4), plots of g of poly(1aꢀ2a),
poly(1bꢀ2a), and poly(1cꢀ2a) at 0ꢀ40 °C (Figure S5), and CD
and UVꢀvis spectra of poly(1bꢀ2b) (Figure S6). This material is
available free of charge via the Internet at http://pubs.acs.org.
Chart 2 illustrates the eight possible regulated conformers. Symbols
a and b in the pattern column represent the two ways of directions
(þ180° and ꢀ180°) of the carbamate/amide and azobenzene
moieties along the conjugated plane of the main chain.
After geometry optimization, the right-handed helical 18-mers
became 3.2ꢀ6.3 kJ/mol unit more stable than the left-handed
3
counterparts of patterns aꢀa, aꢀb, bꢀa, andbꢀb. Amongtheeight
conformers, the right-handed bꢀa in Chart 2 was the most stable.
As shown in Figure 8, the amide and carbamate NꢀH groups of a
monomer unit formed regulated intramolecular hydrogen bonds
’ AUTHOR INFORMATION
Corresponding Author
*E-mail: sanda@adv.polym.kyoto-u.ac.jp.
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dx.doi.org/10.1021/ma200281e |Macromolecules 2011, 44, 3338–3345

Macromolecules
ARTICLE
’ ACKNOWLEDGMENT
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This research was supported by a Grant-in-Aid for Science
Research (B) (22350049) and Kyoto University Global COE
Program “International Center for Integrated Research and Ad-
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Education, Culture, Sports, Science, and Technology, Japan, and
The Kurata Memorial Hitachi Science and Technology Foundation.
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