Synthesis and photoresponse of helically folded poly(phenyleneethynylene)s bearing azobenzene moieties in the main chains

Article abstract of DOI:10.1021/ma4002947

Novel optically active poly(phenyleneethynylene)s bearing azobenzene moieties in the main chains [poly(1-2m), poly(1-2p)] were synthesized by the Sonogashira-Hagihara coupling polymerization of 3′,5′-diiodo- 4′-hydroxy-N-α-tert-butoxycarbonyl-d-phenylglycine hexylamide (1) with 3,3′-diethynylazobenzene (2m) and 4,4′-diethynylazobenzene (2p). The corresponding polymers [poly(1-2m), poly(1-2p)], with number-average molecular weights of 10700 and 9400, were obtained in 70% and 86% yields, respectively. CD and UV-vis spectroscopic analyses revealed that poly(1-2m) and poly(1-2p) formed predominantly one-handed helically folded structures in CHCl₃/THF mixtures. Poly(1-2m) underwent a reversible conformational change between folded and unfolded structures upon UV and visible irradiation, as a result of trans-cis isomerization of the azobenzene moieties. On the other hand, poly(1-2p) showed very little conformational transformation or azobenzene isomerization. The formation of helical structures was supported by conformational analysis based on the molecular mechanics (MM), semiempirical molecular orbital (MO), and density functional theory (DFT) methods.

Full text of DOI:10.1021/ma4002947

Article
pubs.acs.org/Macromolecules
Synthesis and Photoresponse of Helically Folded
Poly(phenyleneethynylene)s Bearing Azobenzene Moieties in the
Main Chains
†
‡
,§
Hiromitsu Sogawa, Masashi Shiotsuki, and Fumio Sanda*
Department of Polymer Chemistry, Graduate School of Engineering, Kyoto University, Katsura Campus, Nishikyo-ku, Kyoto
15-8510, Japan
6
*
S Supporting Information
A B S T R A C T : N o v e l o p t i c a l l y a c t i v e p o l y -
phenyleneethynylene)s bearing azobenzene moieties in the
(
main chains [poly(1−2m), poly(1−2p)] were synthesized by
the Sonogashira−Hagihara coupling polymerization of 3′,5′-
diiodo-4′-hydroxy-N-α-tert-butoxycarbonyl-D-phenylglycine
hexylamide (1) with 3,3′-diethynylazobenzene (2m) and 4,4′-
diethynylazobenzene (2p). The corresponding polymers
[
poly(1−2m), poly(1−2p)], with number-average molecular
weights of 10700 and 9400, were obtained in 70% and 86%
yields, respectively. CD and UV−vis spectroscopic analyses
revealed that poly(1−2m) and poly(1−2p) formed predom-
inantly one-handed helically folded structures in CHCl /THF mixtures. Poly(1−2m) underwent a reversible conformational
3
change between folded and unfolded structures upon UV and visible irradiation, as a result of trans−cis isomerization of the
azobenzene moieties. On the other hand, poly(1−2p) showed very little conformational transformation or azobenzene
isomerization. The formation of helical structures was supported by conformational analysis based on the molecular mechanics
(
MM), semiempirical molecular orbital (MO), and density functional theory (DFT) methods.
3
3,49−51
INTRODUCTION
thynylene-p-phenyleneethynylene)s,
and examined
■
their secondary structures. These polymers form folded helical
structures in nonpolar solvents such as CHCl , as a result of the
amphiphilicity afforded by the hydrophobic exterior (alkyl
chains and phenyleneethynylene main chain) and hydrophilic
interior (hydroxy groups). It should be noted that the
amphiphilic balance is opposite from that of typical poly-
phenyleneethynylene) derivatives reported so far. We further
synthesized poly(m-phenyleneethynylene)s containing side-
chain azobenzene moieties that are helically folded in nonpolar
solvents. The higher order structures of the polymers
collapsed to some extent by UV irradiation and were
reconstructed by visible irradiation, caused by photoisomeriza-
tion of the azobenzene moieties. The degree of change of
higher order structure was not large, probably because the cis-
trans isomerization of the azobenzene units did not directly
twist the main chain.
Herein we report the synthesis of D-hydroxyphenylglycine-
derived novel optically active poly(pheyleneethynylene)s
containing azobenzene moieties in the main chains with
m,m′- and p,p′-linkages by the Sonogashira−Hagihara coupling
polymerization of the corresponding monomers (Scheme 1).
Photoinduced large conformational changes are expected
Photoresponsive materials are attracting interest because of
1
−4
3
their applicability to optical memories,
molecular ma-
5
,6
7−11
12−14
chines,
tors,
recognition materials,
catalysts,
etc. Azobenzene is the most widely used photo-
responsive compound due to its highly efficient reversible cis/
actua-
1
5−18
19
trans photoisomerization. The more stable trans-azobenzene
undergoes isomerization upon UV irradiation to cis-azoben-
zene, and the isomerization is reversed by visible irradiation or
heat. This occurs reversibly with high quantum yields with
change in molecular shape from extended coplanar (trans) to
twisted (cis) forms and resulting dipole moment change from 0
to 3 D, respectively.
52
(
33
Azobenzene units are often incorporated into main chains of
oligomers/polymers to control the higher order structures.
20−24
Besides this, control over higher order structures of conjugated
polymers by external stimuli is also gaining considerable
attention, because of the possibility for development of stimuli-
responsive photoelectrically functional materials. There are
various reports about the synthesis of conjugated polymers with
photocontrollable higher order structures containing azoben-
25
2
6−33
zene moieties, either on the side chains
or the main
3
4−37
chains.
Among various conjugated polymers, ortho- and
meta-phenyleneethynylene derivatives are representative poly-
mers that form folded helical structures by amphiphilic
Received: February 8, 2013
Revised: May 12, 2013
3
8−48
balance.
Recently, we synthesized a series of D-
hydroxyphenylglycine-/L-tyrosine-derived poly(m-phenylenee-
©
XXXX American Chemical Society
A
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Scheme 1. Sonogashira−Hagihara Coupling Polymerization of D-Hydroxyphenylglycine-Derived Diiodophenylene Monomer 1
with 3,3′- and 4,4′-Diethynylazobenzene Monomers 2m and 2p
because the azobenzene units are completely contained in the
main chains instead of the side chains as in our previous
polymers. We discuss the chiroptical and photoresponsive
properties of the polymers based on CD and UV−vis
spectroscopic analysis, as well as MM, semiempirical MO and
DFT calculations.
azobenzene. After that, it was dissolved in THF (5 mL), and 1.0 M
solution of tetrabutylammonium fluoride in THF (25 mL) was added
to the solution. The resulting mixture was concentrated in vacuo, and
3
3
the residual mass was dispersed in CHCl and water. The organic layer
3
was washed with 1.0 M HCl and saturated NaCl(aq), dried over
anhydrous MgSO , and then filtered. The filtrate was concentrated,
4
and the residual mass was purified by silica gel column
chromatography eluted with hexane/CHCl = 19/1 (v/v) to obtain
3
5
5
1
EXPERIMENTAL SECTION
2m as an orange solid in 28%. Mp 156−158 (lit. 157−159 °C). H
■
NMR (400 MHz, CDCl ): δ 3.13 (s, 2H, −CCH), 7.47 (t, J = 7.2
Measurements. Proton (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. Mass spectra were measured on a
JEOL JMS-SX102A mass spectrometer. Number- and weight-average
molecular weights (M and M ) of polymers were determined by size
3
Hz, 2H, Ar), 7.59 (d, J = 7.6 Hz, 2H, Ar), 7.89 (d, J = 7.6 Hz, 2H, Ar),
8
8
.03 (s, 2H, Ar). 13C NMR (100 MHz, CDCl ): δ 78.06 (−CCH),
3
2.83 (−CCH), 123.1, 123.5, 126.3, 129.1, 134.5, 152.1 (Ar).
+
HRMS (m/z): [M + H] calcd for C H N , 231.0922; found,
1
6
11
2
2
31.0919.
Polymerization. All the polymerizations were carried out in a glass
n
w
tube equipped with a three-way stopcock under nitrogen. A typical
experimental procedure for polymerization 1 with 2m is given. A
solution of 1 (218 mg, 0.400 mmol), 2m (92.0 mg, 0.400 mmol),
Pd(PPh ) Cl (14.4 mg, 0.020 mmol), PPh (10.4 mg, 0.040 mmol),
exclusion chromatography (SEC, Shodex columns KF805 × 3) eluted
with tetrahydrofuran (THF) calibrated by polystyrene standards at 40
°
C. CD and UV−vis absorption spectra were recorded on a JASCO J-
3
2
2
3
8
20 spectropolarimeter. Dynamic light scattering (DLS) measure-
CuI (11.4 mg, 0.060 mmol), and Et N (0.8 mL) in DMF (1.2 mL) was
3
ments were performed using a Malvern Instruments Zetasizer Nano
stirred at 80 °C for 24 h. The resulting mixture was poured into
MeOH/acetone [9/1 (v/v), 300 mL] to precipitate the polymer. It
was separated by filtration using a membrane filter (ADVANTEC
H100A047A) and dried under reduced pressure.
ZS at 25 °C. The measured autocorrelation function was analyzed
using a cumulant method. The hydrodynamic radii (R ) of the
h
polymers were calculated from the Stokes−Einstein equations.
Materials. Unless stated otherwise, reagents and solvents were
purchased and used without purification. Trimethylsilylacetylene was
provided as a gift from Shin-Etsu Chemical Co., Ltd. 3,3′-
Spectroscopic Data for the Polymers. Poly(1−2m). IR (KBr):
3
421, 3331, 3064, 2952, 2926, 2857, 2215, 1671, 1476, 1366, 1160,
1
5
3
54
798, 687, 519. H NMR (400 MHz, CDCl ): δ 0.82−1.79 [br, 20H,
Dibromoazobenzene and 4,4′-diethynylazobenzene (2p) were
3
−
−
5
CONHCH (CH ) CH , −NHCOOC(CH ) ], 3.08−3.53 [br, 2H,
synthesized according to the literature reports. Et N and N,N-
2
2
4
3
3 3
3
CONHCH (CH ) CH ], 5.03−5.38 (br, 1H, −CHCONH−),
dimethylformamide (DMF) used for polymerization were distilled
prior to use.
2
2
4
3
.52−6.88 [br, 3H, −CONHCH (CH ) CH , −NHCOOC(CH ) ,
2
2
4
3
3 3
−
OH], 7.10−8.38 (br, 10H, Ar). Poly(1−2p): IR (KBr): 3422, 3339,
Photoirradiation. Photoirradiation was carried out with a 400 W
high-pressure mercury lamp 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 < λ < 400 nm or with an L-42 filter (Toshiba) filter
for irradiation at 420 nm < λ. The sample solution was held in a quartz
cell (10 mm width) placed 20 cm from the lamp.
3
8
2
2
5
−
060, 2956, 2928, 2857, 2204, 1676, 1491, 1366, 1260, 1228, 1161,
−1 1
49, 804, 490 cm . H NMR (400 MHz, CDCl ): δ 0.82−1.79 [br,
3
0H, −CONHCH (CH ) CH , −NHCOOC(CH ) ], 3.08−3.53 [br,
2
2
4
3
3 3
H, −CONHCH (CH ) CH ], 5.03−5.38 (br, 1H, −CHCONH−),
2
2
4
3
.52−6.88 [br, 3H, −CONHCH (CH ) CH , −NHCOOC(CH ) ,
2
2
4
3
3 3
OH], 7.10−8.38 (br, 10H, Ar).
Computation. The MM calculations were carried out using the
Synthesis of 3,3′-Diethynylazobenzene (2m). 3,3′-Dibromoa-
5
6
zobenzene (3.40 g, 10.0 mmol), PdCl (PPh ) (0.168 g, 0.24 mmol),
Merck molecular force field (MMFF94) with Wave function, Inc.,
Spartan ′10 version 1.1.0, running on a Macintosh computer. The
58
2
3 2
PPh (0.252 g, 0.96 mmol), and CuI (0.274 g, 1.44 mmol) were fed
3
5
7
into a two-neck flask, which was flushed with dry nitrogen. THF (20
semiempirical MO calculations (PM6, ZINDO/S) and DFT
calculations were performed with the GAUSSIAN 09 program,
59
mL) and Et N (15 mL) were added to the solution, and then
3
trimethylsilylacetylene (3.30 mL, 23.9 mmol) was added dropwise to
the solution. The mixture was stirred at 50 °C for 72 h. The resulting
mixture was concentrated in vacuo and the residual mass was washed
EM64L-G09 Rev C.01 running on the supercomputer system,
Academic Center for Computing and Media Studies, Kyoto University.
Theoretical CD and UV−vis spectra were simulated by the ZINDO/S
6
0−64
with Et O to extract the product. The organic phase was washed with
method
in the GAUSSIAN 09 program. The low-energy
2
1
.0 M HCl, and saturated NaCl aq., dried over anhydrous MgSO , and
transition states of 20 were predicted under the condition of a CI
4
then filtered. The filtrate was concentrated, and the residual mass was
purified by silica gel column chromatography eluted with hexane/
EtOAc = 9/1 (v/v) to obtain crude 3,3′-bis(trimethylsilylethynyl)-
number of 20 × 20, including each oscillator strength (f) and rotatory
strength (R ) in velocity form. The simulated CD and UV−vis
vel
spectra were produced by using the R − and f−wavelength data with
vel
B
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a wavelength-based Gaussian function of 14 nm tentatively used for a
half of 1/e-bandwidth, respectively.
hand, poly(1−2p) exhibited intense CD signals both in CHCl
3
and THF, and in CHCl /THF mixtures with various
3
compositions (Figure 2 and Figure S4, Supporting Informa-
RESULTS AND DISCUSSION
■
Polymerization. The Sonogashira−Hagihara coupling
polymerizations of 1 with diethynylazobenzenes 2m and 2p
were performed to obtain the corresponding polymers with
azobenzene chromophore in the main chains. Poly(1−2m) and
poly(1−2p) with M ’s of 10,700 and 9,400 were obtained in 70
n
and 86% yields, respectively (Table 1). Polymers with almost
Table 1. Sonogashira−Hagihara Coupling Polymerization of
a
1
with 2m and 2p
polymer
b
c
c
monomer
yield (%)
Mn
M /Mn
w
1
+ 2m
+ 2p
poly(1−2m)
poly(1−2p)
70
86
10700
9400
1.7
1.8
1
a
Conditions: [1] = [2m] = [2p] = 0.20 M, [PdCl (PPh ) ] = 0.010
0
0
0
2
3 2
M, [PPh ] = 0.020 M, [CuI] = 0.030 M, DMF/Et N = 3/2 (v/v), 80
3
3
b
c
°
C, 24 h. Insoluble part in MeOH/acetone = 9/1 (v/v). Estimated
by SEC measured in THF, polystyrene calibration.
Figure 2. CD and UV−vis spectra of poly(1−2p) measured in CHCl
and THF (c = 0.03 mM) at 20 °C.
3
the same M ’s were obtained irrespective of the way of
n
substitution. The DP’s of poly(1−2m) and poly(1−2p) are
tion). The CD and UV−vis signals of the polymers did not
change over the concentration range of 0.03−0.3 mM or after
calculated to be 18 and 16 from the M ’s and formula weights
n
of the monomer units. The helix turns are estimated to be 2−3
65
filtration using a membrane filter with a pore size of 0.45 μm.
4
9
(
6−1 helix). The solubilities of the two polymers were
Dynamic light scattering (DLS) measurements of the sample
solutions showed no signal assignable to polymer aggregates.
These results indicate that the CD signals do not originate from
chiral aggregation but from unimolecularly folded helical
different. Poly(1−2m) and poly(1−2p) were soluble in CHCl ,
3
THF and DMF. Poly(1−2m) was soluble in CH Cl and
2
2
toluene as well, while poly(1−2p) was not. The solubility of
poly(1−2m) was comparatively higher than that of poly(1−
66
structures with predominantly one-handed screw sense. The
2
p), presumably because the main chain of poly(1−2m) is less
λmax of poly(1−2p) at 390 nm is 76 nm longer than that of
poly(1−2m) likely due to the p,p′-linked azobenzene units.
The CD intensities of poly(1−2m) varied according to the
stiff than that of poly(1−2p). This will be discussed later in this
paper.
Chiroptical Properties of the Polymers. The CD and
UV−vis spectroscopic analysis was performed to obtain
information about higher order structures of the polymers. As
shown in Figure 1, poly(1−2m) showed intense split-type CD
signals in the absorption region of the main chain chromophore
composition of CHCl /THF mixtures. Figure 3 shows the plot
3
in CHCl /THF = 7/3 mixture, while the polymer did not show
3
such intense peaks in either CHCl or THF alone. On the other
3
Figure 3. Plot of g-values of poly(1−2m) at the [θ] wavelengths
max
measured in CHCl /THF mixtures with various compositions (c =
3
0
.03 mM) at 20 °C.
of the Kuhn dissymmetry factor g (=Δε/ε, in which Δε = [θ]/
6
7
3
,298) of poly(1−2m) observed in CHCl /THF with various
3
compositions at the [θ]
wavelengths. The g-values give
max
quantitative information associated with the degree of
68
preferential screw sense. As shown in Figure 3, the g-value
reached a maximum at a ratio of CHCl /THF = 7/3. Poly(1−
3
Figure 1. CD and UV−vis spectra of poly(1−2m) measured in
CHCl , THF, and CHCl /THF = 7/3 (c = 0.03 mM) at 20 °C.
2p) exhibited the similar trend, although it showed intense CD
signals both in CHCl3 and THF (Figure S4, Supporting
3
3
C
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Information). As listed in Table 2, the g-values of poly(1−2p)
were almost five times larger than those of poly(1−2m) at the
behavior. A solution of poly(1−2m) in CHCl /THF = 7/3
3
[the composition at which poly(1−2m) exhibited the largest
CD intensities] was irradiated with UV-light through a suitable
transmission filter using a high pressure mercury lamp. The
intensities of the CD signals gradually decreased upon UV
irradiation, and approached a minimum after 16 min (Figure
4). The intensity of the UV−vis signal around 315 nm also
Table 2. Absolute g-Values of Poly(1−2m) and Poly(1−2p)
|
g| × 104
polymer
first Cotton
second Cotton
2.49
a
poly(1−2m)
poly(1−2p)
5.07
25.1
b
11.7
b
a
Measured in CHCl /THF = 7/3 (c = 0.03 mM). Measured in
3
CHCl /THF = 6/4 (c = 0.03 mM).
3
wavelengths of both the first and second Cotton effects. It is
considered that the one-handedness is more predominant in
69
the poly(1−2p) helix than in the poly(1−2m) helix. The CD
intensities of oligomeric phenyleneethynylene foldamers
70
significantly depend on molecular weights. As mentioned
above, the DP of poly(1−2p) is smaller than that of poly(1−
2
m), while the g-values of poly(1−2p) are larger than those of
71
poly(1−2m). It is likely that the helix-forming ability of
poly(1−2p) is higher than that of poly(1−2m).
Intramolecular hydrogen bonds and π-stacking are the key
factors for stabilizing helically folded structures of some
poly(phenyleneethnylene)s, in a fashion similar to that of
other artificial helical polymers, including polyisocyanide,
3
3,49,72−74
polyisocyanate, polyacetylene, etc.
Solution-state IR
Figure 4. CD and UV−vis spectra of poly(1−2m) upon UV-light
spectra of poly(1−2m), poly(1−2p), and monomer 1 were
irradiation (300 < λ < 400 nm) for 16 min measured in CHCl /THF =
3
7
/3 (c = 0.03 mM) at 20 °C.
measured in CHCl /THF = 7/3 and CHCl under diluted
3
3
conditions (20 mM) to determine the presence/absence of
intramolecular hydrogen bonding (Table 3). All the com-
decreased, apparently due to the decrease of absorption based
on the π−π* transition band of the trans-azobenzene units.
Although no increase was observed corresponding to the n−π*
of cis-azobenzene, it appears that the trans-azobenzene moieties
isomerized into the cis-form, weakening the chirality of the
polymer molecule. In helical polymers, the decrease of CD
intensity represents the transformation of a helix into a random
structure and/or loss of predominance of one-handedness while
keeping the total helix content constant. In the present case, it
is likely that the decrease of CD intensities is caused by the
collapse of helically folded structures. It has been reported that
the trans-to-cis photoisomerization of azobenzene units in the
main chains of oligo(phenyleneethynylene) derivatives disor-
ders the helically folded structures due to the loss of π−stacking
interactions between the folded main chains induced by loss of
Table 3. Solution-state IR Spectroscopic Data (CO
a
Absorption Peaks) of the Monomer and Polymers
wavenumber (cm−1)
solvent
compound
carbamate
amide
CHCl /THF = 7/3
1
1709
1703
1701
1700
1678
1671
1677
1652
3
poly(1−2m)
1
CHCl3
poly(1−2p)
a
c = 20 mM.
pounds exhibited two strong absorption peaks assignable to
CO stretching vibrations of the amide and carbamate groups.
Poly(1−2m) exhibited the carbamate and amide CO peaks
3
4,36,37
planarity.
This is predictable from the fact that trans-
azobenzene is linear and planar, while cis-azobenzene is bent. In
the present study, the CD intensities of poly(1−2m) decreased
by as much as 71% after UV irradiation for 16 min. This is a
much larger effect than that of the analogous polymer (10%)
with azobenzene moieties on the side chains under the same
−1
−1
at 1703 and 1671 cm , which were lower by 6 and 7 cm
compared with those of 1, respectively. On the other hand, the
carbamate CO peak of poly(1−2p) was observed at only 1
−1
cm lower wavenumber than that of 1, while the amide CO
−1
33
peak was observed at 25 cm lower than that of 1. The sample
solutions were sufficiently dilute to avoid intermolecular
conditions. This remarkable difference is caused by
introducing the azobenzene moieties into the main chain.
Subsequent irradiation with visible light resulted in almost
full recovery of the initial CD and UV−vis spectra (Figure S6,
Supporting Information), indicating that poly(1−2m) refolded
into a helix with isomerization of azobenzene units from cis to
trans. Further reversibility was confirmed upon repeated UV-
and visible irradiation. As shown in Figure 5, the CD and UV−
vis intensities were reversible upon alternating UV and visible
irradiation for 16-min intervals without remarkable decom-
position. This indicates that the folded helical structure was
repeatedly deformed and reformed even after 8 cycles of
alternating UV and visible irradiation. On the contrary, UV
75
interaction as mentioned above. These results indicate that
poly(1−2m) forms intramolecular hydrogen bonds between
the carbamate and amide groups in CHCl /THF = 7/3
3
mixture. The hydrogen bonds in poly(1−2p) involve amide−
amide interactions in CHCl . It is assumed that the way of
3
hydrogen bonding affects the helix forming ability of the
polymers. More detailed information about the hydrogen
bonding will be discussed in the Conformational Analysis part
below.
The CD and UV−vis spectra of the polymers were measured
upon photoirradiation to examine the photoisomerization
D
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localized at azobenzene is decreased in conjunction with the
enhanced stiffness of the main chain by extension of
76−78
conjugation involving azobenzene.
Azobenzene-containing
oligo(phenyleneethynylene)s undergo photoisomerization
more efficiently when they take random coil structures
compared with helically folded structures because the below
and above conjugated plane of unimolecular polymer chain in
37
helix state were strongly interacted in dilute concentration.
Consequently, poly(1−2p) hardly undergoes photoisomeriza-
tion, while poly(1−2m) easily does. It is possible to estimate
the photoisomerization ratio of trans-azobenzene moieties from
79
the decrease in the ratio of π−π* absorption. In the present
study, however, this measurement was complicated by the
overlap of absorption peaks of the azobenzene units and the
conjugated main chain. We tried to determine the isomer-
1
ization ratio by H NMR spectroscopy and SEC measurement,
but unfortunately we failed to obtain clear information.
Conformational Analysis. There are several reports
regarding the conformational analysis of poly-
Figure 5. Plot of [θ]max and εmax after repeated photoirradiation cycles:
UV (300 < λ < 400 nm) 16 min; then visible (420 nm < λ) 16 min
33,49
(phenyleneethynylene)s using the MM
and molecular
80−82
dynamics simulations.
In the present study, the geometries
repeated 8 times; measured in CHCl /THF = 7/3 (c = 0.03 mM) at
3
were optimized first with the MM method using the
20 °C.
56
MMFF94, second with the semiempirical MO method
using the PM6 Hamiltonian, and finally with the DFT method
at the M06-2X/6-31G* level for selected conformers. The
M06-2X functional was employed because it is superior
compared to the commonly used B3LYP functional in
estimating noncovalent interactions including π-stacking and
irradiation did not induce CD or UV−vis spectral changes in
poly(1−2p) (Figure 6). After UV irradiation for 16 min, the
8
3,84
hydrogen bonding,
both of which are essential for
stabilizing the helically folded structures of the present
poly(phenyleneethynylene) derivatives. Charts 1−3 summarize
the possible stereoregular right- and left-handed helical
conformers of poly(1−2m) and poly(1−2p) (12-mer), both
of whose chain ends are terminated with hydrogen atoms. The
hexyl groups are replaced with methyl groups to save CPU
time. Trans zigzag conformers (Chart 4) were also calculated
for comparison. The torsional angles of the main chains of
right- and left-handed helices were set to +6° and −6° per
phenylene unit, respectively, and the azobenzene units were set
to a planar structure in the initial geometries.
As shown in Chart 1, poly(1−2m) forms helices with short
diameters when the two ethynylene groups at the 3,3′-positions
of the azobenzene moiety adopt the “cis” geometry. (Note that
the azobenzene moieties are always in the “trans” config-
urations in these optimized geometries.) One turn consists of
two monomer units (denoted 2−1 in Chart 1). In the chart,
symbols “R” and “L” represent right- and left-handed helices,
respectively. Four stereoregular conformers are possible as
right-handed helices considering the directions of the
azobenzene and amide/carbamate moieties. Symbols “O” and
“I” represent the directions of the azobenzene moieties along
the conjugated plane of the main chain, corresponding to
bending outside and inside viewed from the top positions.
Symbols “A” and “B” represent the two possible directions of
the amide/carbamate moieties. In a similar way, four stereo-
regular conformers are possible as left-handed helices. After
geometry optimization by the MMFF94 method using the
constraints described above, the amide and carbamate N−H
groups of a monomer unit formed stereoregular intramolecular
hydrogen bonds with the CO groups of the amide and
carbamate groups two units earlier (i + 2 → i N−H···OC
hydrogen bonding), respectively. Next, the geometries were
fully optimized without any constraints by the PM6 method. In
Figure 6. CD and UV−vis spectra of poly(1−2p) upon UV irradiation
(
300 < λ < 400 nm) for 16 min measured in CHCl (c = 0.03 mM) at
3
20 °C.
intensities of the CD and UV−vis signals decreased by only
7% and 1%, respectively. Monomer 2p was photoisomerized
upon UV and visible irradiation; the UV−vis absorption at the
decreased 51% upon UV irradiation for 8 min compared
1
λ
max
with its initial state (Figure S8, Supporting Information).
The large difference in photoresponse between poly(1−2p)
and 2p is explained by an extended azobenzene chromophore
of poly(1−2p) due to the para-connected phenyleneethynylene
34
unit, which has a lower efficiency of isomerization. This linear
repeating unit has a much stronger tendency to π-stack (ideally
with itself) that introduces a higher barrier for its isomerization.
When azobenzene is incorporated into the main chain of a
conjugated polymer at the p,p′-positions, the π-electrons of
azobenzene are delocalized through the conjugated main chain.
As a result, the efficiency of the UV-induced π−π* transition
E
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a
Chart 1. Possible Regulated 2-1 Helical Conformers of 12-mers of Poly(1−2m)
a
Ten monomer units are omitted at the wavy lines. The torsional angles of the main chains of right- and left-handed helices were set to +6° and −6°
per phenylene unit, respectively, and the azobenzene units were set to a planar structure in the initial geometries.
a
Chart 2. Possible Regulated 6-1 Helical Conformers of 12-mers of Poly(1−2m)
a
Six monomer units are omitted at the wavy lines. The torsional angles of the main chains of right- and left-handed helices were set to +1° and −1°
per phenylene unit, respectively, and the azobenzene units were set to a planar structure in the initial geometries.
F
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a
Chart 3. Possible Regulated 6-1 Helical Conformers of 12-mers of Poly(1−2p)
a
Six monomer units are omitted at the wavy lines. The torsional angles of the main chains of right- and left-handed helices were set to +1° and −1°
per phenylene unit, respectively, and the azobenzene units were set to a planar structure in the initial geometries.
Chart 4. Trans Zigzag Conformers of 12-mers of Poly(1−
m) and Poly(1−2p)
four possible stereoregular conformers according to the
directions of the azobenzene and amide/carbamate moieties
in right-handed helices, and four left-handed helices as well.
The symbols “R/L”, “O/I” and “A/B” are employed in the
same manner as with the 2−1 helices mentioned above.
Stereoregular hydrogen-bonding strands were formed after
geometry optimization with constraints described above by the
MMFF94 method; the amide and carbamate N−H groups of a
monomer unit formed stereoregular intramolecular hydrogen
bonds with the CO groups of the amide and carbamate
groups six units earlier (i + 6 → i N−H···OC hydrogen
bonding), respectively. The hydrogen bonding strands were
intact after full geometry optimization without any constraints
by the PM6 method in a fashion similar to the 2−1 helices
mentioned above. Judging from the relative energy values
calculated by the PM6 method (Table 4), 2−1 helical
conformer m-2−1-R-I-A is most likely among the 16 possible
stereoregular conformers for poly(1−2m). The geometries of
stable conformers were further optimized by the DFT method
at the M06-2X/6-31G* level to confirm the similar trend with
the PM6 method. Although PM6 is a semiempirical method,
and therefore it is exempt from long CPU time compared with
DFT, PM6 is reliable enough for preliminarily estimating the
stable conformers of the present polymers involving π-stacking
2
every case, the regulated i + 2 → i N−H•••OC hydrogen
bonding strands were intact after the semiempirical calculations.
On the other hand, poly(1−2m) forms helices with long
diameters when the two ethynylene groups at the 3,3′-positions
of the azobenzene adopt the “trans” geometry, as shown in
Chart 2.
The torsional angles of the main chains of right- and left-
handed helices were set to +1° and −1° per phenylene unit,
respectively, and the azobenzene units were set to a planar
structure in the initial geometries. One turn consists of six
monomer units (denoted 6−1 helices in Chart 2). There are
85
and hydrogen bonding. The right-handed helical structure of
m-2−1-R-I-A coincides with the first-positive and second-
negative exciton coupling observed in the CD spectra of
poly(1−2m) shown in Figure 1. The helical pitch after full
optimization with DFT was 3.8 Å, which is consistent with the
turns of the helix being near van der Waals contact and
86
comparable to π-stacking between aromatic rings. The trans
zigzag conformer was unstable compared with m-2−1-R-I-A by
as much as 132.6 kJ/(mol·unit) at the M06-2X/6-31G* level,
G
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Table 4. Relative Energies for the Possible Conformers of 12-mers of Poly(1−2m) and Poly(1−2p)
relative energy [kJ/
mol·unit)]
relative energy [kJ/
relative energy [kJ/
(
(mol·unit)]
(mol·unit)]
conformer of poly(1−
m)
M06-2X/6-
conformer of poly(1−
M06-2X/6-
conformer of poly(1−
M06-2X/6-
a
b
a
b
a
b
2
PM6
31G*
2m)
PM6
31G*
2p)
PM6
31G*
m-trans zigzag
m-2−1-R-O-A
m-2−1-R-O-B
m-2−1-R-I-A
m-2−1-R-I-B
m-2−1-L-O-A
m-2−1-L-O-B
m-2−1-L-I-A
m-2−1-L-I-B
40.2
4.6
132.6
c
p-trans zigzag
p-R-O-A
p-R-O-B
p-R-I-A
59.1
32.7
33.9
33.2
33.7
47.5
50.6
50.1
28.7
153.9
m-6−1-R-O-A
m-6−1-R-O-B
m-6−1-R-I-A
m-6−1-R-I-B
m-6−1-L-O-A
m-6−1-L-O-B
m-6−1-L-I-A
m-6−1-L-I-B
14.8
11.7
13.2
10.7
15.4
9.6
c
80.7
2.2
6.4
0.0
c
44.0
c
c
81.8
c
0.0
6.2
47.4
c
p-R-I-B
11.8
4.1
c
p-L-O-A
p-L-O-B
p-L-I-A
101.0
c
13.1
c
44.7
41.8
c
10.1
5.7
13.2
15.5
c
13.9
p-L-I-B
72.3
a
b
Converted using the value of m-2−1-R-I-A [66.6 kJ/(mol·unit)] as zero. Converted using the value of m-2−1-R-I-A [−4405952.4 kJ/(mol·unit)]
c
as zero. Not calculated.
8
7
definitely due to the absence of π-stacking and hydrogen
bonding.
shown in Figure 8. We can say that the geometry
optimization by PM6 followed by the DFT calculation
In contrast to poly(1−2m), poly(1−2p) cannot form 2−1
helices because of the geometrical restriction. Poly(1−2p)
possibly forms the 6−1 helices presented in Chart 3. The
symbols “R/L”, “O/I”, and “A/B” are used as described above
for poly(1−2m) helices. Among the possible eight helical
structures, p-L-I-B seems to be most stable based on the
relative energies calculated by the PM6 and M06-2X/6-31G*
method. However, the left-handed helical structure does not
coincide with the exciton coupling of the CD spectra shown in
Figure 2.
The CD and UV−vis spectra of the polymers were
theoretically simulated by the ZINDO/S method to obtain
more structural information. Among the conformers listed in
Table 4, p-R-I-A successfully gave theoretical CD and UV−vis
spectra (Figure 7) that agree well with the observed ones
Figure 8. Conformation of p-R-I-A after geometry optimization by
M06-2X/6-31G*.
(
Figure 2). The theoretical CD spectrum shows the positive
first and negative second CD signals assignable to exciton
coupling based on the main chain chromophore twisted in
right-handed helices. Thus, it is likely that poly(1−2p) adopts a
stereoregular 6−1 helically folded conformation like p-R-I-A, as
reasonably predicts the conformations of the present polymers,
and the ZINDO/S method is effective for simulating the CD
and UV−vis spectra. We also tried time-dependent DFT
calculations but could not obtain the results within the
88
acceptable CPU time.
CONCLUSIONS
■
In the present study, we have synthesized novel optically active
poly(phenyleneethynylene)s containing azobenzene moieties in
the main chains [poly(1−2m) and poly(1−2p)] by the
Sonogashira−Hagihara coupling polymerization of D-hydrox-
yphenylglycine-derived monomer 1 with 3,3′- and 4,4′-
diethynylazobenzenes (2m, 2p). Poly(1−2m) was folded into
a predominantly one-handed helix in CHCl /THF = 7/3, but
3
the helix structure is less predominant in CHCl and THF. On
3
the other hand, poly(1−2p) was folded into a helix both in
CHCl and THF, and their mixtures. The λ of poly(1−2p)
3
max
occurs at a longer wavelength than that of poly(1−2m).
Solution state IR measurements revealed the existence of
intramolecular hydrogen bonds between the amide and/or
carbamate groups at the side chains, and higher hydrogen-
bonding strength in poly(1−2p) than that in poly(1−2m), in
agreement with the higher helicity of poly(1−2p). The folded
structure of poly(1−2m) was disrupted upon UV irradiation,
which induced the photoisomerization of the azobenzene units
Figure 7. CD and UV−vis spectra of p-R-I-A. One possible conformer
of poly(1−2p) simulated by the ZINDO/S (n
= 20) method using
states
the geometries optimized by M06-2X/6-31G*. The red lines indicate
Rvel and f values. ε, Δε, R , and f of the polymer molecule are divided
vel
by 12 as the values per monomer unit.
H
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from the planar trans-form to the bent cis-form. The deformed
poly(1−2m) refolded into a helix upon visible irradiation as a
result of isomerization of the azobenzene units from the cis-
form to the trans-form. This reversible photoinduced conforma-
tional change of poly(1−2m) was further confirmed by cycles
of alternating UV and visible irradiation. Thus, incorporation of
m,m′-linked azobenzene units in the main chain is effective in
achieving reversible photoresponsive conformational changes of
poly(phenyleneethynylene)s. Interestingly, poly(1−2p)
showed very little cis−trans photoisomerization of the
azobenzene units and polymer conformation. This is explain-
able from the stiff main chain, which is more strongly stabilized
by hydrogen bonding and extended conjugation through p,p′-
linked azobenzene units compared with the m,m′-counterpart
grateful to Prof. Kazunari Akiyoshi and Prof. Shinichi Sawada at
Kyoto University for DLS measurement, Prof. Yoshihito Inai at
Nagoya Institute of Technology, Prof. Mikihito Takenaka and
Prof. Hideo Ohkita at Kyoto University for their guidance in
CD and UV−vis simulation, and Prof. Kenneth B. Wagener and
Dr. Kathryn R. Williams at the University of Florida for their
helpful suggestions and comments.
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dx.doi.org/10.1021/ma4002947 | Macromolecules XXXX, XXX, XXX−XXX