Synthesis of novel optically active poly(phenyleneethynylene- aryleneethynylene)s bearing hydroxy groups. Examination of the chiroptical properties and conjugation length

Article abstract of DOI:10.1021/ma401730e

Novel optically active poly(phenyleneethynylene-aryleneethynylene)s bearing hydroxy groups with various arylene units [poly(1-2), poly(1-3a), poly(1-3b), poly(1-4)] were synthesized by the Sonogashira-Hagihara coupling polymerization of (S)-3,5-diiodo-4-h

Full text of DOI:10.1021/ma401730e

Article
pubs.acs.org/Macromolecules
Synthesis of Novel Optically Active Poly(phenyleneethynylene−
aryleneethynylene)s Bearing Hydroxy Groups. Examination of the
Chiroptical Properties and Conjugation Length
Hiromitsu Sogawa,†,∥ Yu Miyagi,† Masashi Shiotsuki,‡ and Fumio Sanda*,§
†Department of Polymer Chemistry, Graduate School of Engineering, Kyoto University, Katsura Campus, Nishikyo-ku, Kyoto
615-8510, Japan
‡Molecular Engineering Institute, Kinki University, Kayanomori, Iizuka, Fukuoka 820-8555, Japan
§Department of Chemistry and Materials Engineering, Faculty of Chemistry, Materials and Bioengineering, Kansai University, 3-3-35
Yamate-cho, Suita, Osaka 564-8680, Japan
S
* Supporting Information
ABSTRACT: Novel optically active poly(phenyleneethynylene-
aryleneethynylene)s bearing hydroxy groups with various arylene
units [poly(1−2), poly(1−3a), poly(1−3b), poly(1−4)] were
synthesized by the Sonogashira−Hagihara coupling polymerization
of (S)-3,5-diiodo-4-hydroxy-C6H4CONHCH(CH3)COOC12H25
(1) with HCC−Ar−CCH [2 (Ar = 1,4-phenylene), 3a (Ar
= 2,7-naphthylene), 3b (Ar = 1,4-naphthylene) and 4 (Ar = 1,6-
pyrenylene), and the optical properties were compared. Polymers
with number-average molecular weights (Mn) of 5,300−11,300
were obtained in 88−94% yields. CD and UV−vis spectroscopic
analysis revealed that all the polymers formed predominantly one-
handed helical structures in THF. The order of absorption maxima (λmax) of the polymers was poly(1−3a) < poly(1−2) <
poly(1−3b) < poly(1−4). Poly(1−2), poly(1−3a), poly(1−3b), and poly(1−4) emitted blue, purplish blue, green and yellow
fluorescence, respectively.
from that of typical helically folded poly(m-phenyleneethyny-
lene) derivatives reported so far.
INTRODUCTION
■
Conjugated polymers with regulated higher order structures
show useful properties including molecular recognition, chiral
catalysis and chemical sensing together with electronic and
optical properties.1 Poly(phenyleneethynylene) is a typical
conjugated polymer that features photoelectric properties,
photoluminescence, and electroluminescence. The conjugation
length of the polymer backbone largely affects the absorption
and luminescent properties.2 Poly(phenyleneethynylene)s have
a nature to adopt a folded helical conformation. m-Linked
oligo(phenyleneethynylene)s bearing tetraethylene glycol moi-
eties at the side chains are folded into helices in polar solvents
such as acetonitrile/water based on the amphiphilicity
originating from the hydrophilic side chains and hydrophobic
main chains.3 Many efforts have been made to synthesize such
helical phenyleneethynylene polymers, and to investigate the
chiroptical properties.4−20 We have recently revealed that D-
hydroxyphenylglycine- and L-tyrosine-derived poly(m-phenyl-
eneethynylene-p-phenyleneethynylene)s form helically folded
structures due to π-stacking between phenylene moieties,
intramolecular hydrogen bonding between the amide groups at
the side chains, and amphiphilicity caused by hydrophobic
exterior (alkyl groups and phenyleneethynylene main chain)
and hydrophilic interior (hydroxy groups).21−25 It is note-
worthy that the amphiphilic balance of our polymers is opposite
Naphthalene26,27 and pyrene28,29 are flat-shaped aromatic
molecules that gather much interest because of their photo-
electric functions. Extension of conjugation length is expected
by introducing these condensed aromatic rings into the main
chains of π-conjugated polymers, when the aromatics are linked
at proper positions.30−35 For instance, 1,4-naphthylene and 1,6-
pyrenylene linkages are effective for this purpose,31,32,34,35 while
2,7-naphthylene linkage is not because of its kinked structure
and inefficient resonance.33 There are several reports on the
synthesis of π-conjugated polymers containing pyrene moieties
at the main chains,33−36 and also optically active helical
polymers containing pyrene moieties at the side chains.37−41
Nevertheless, there is no report concerning chiral conjugated
polymers containing pyrene moieties at the main chains as far
as we know.
There are many reports regarding the change of conjugation
length of helical polymers in response to external stimuli and/
or additives.42−45 On the other hand, there are only a few
examples that control the conjugation length of helical
Received: August 19, 2013
Revised: October 19, 2013
Published: November 4, 2013
© 2013 American Chemical Society
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Scheme 1. Sonogashira−Hagihara Coupling Polymerization of Diiodophenylene Monomer 1 with Diethynylarylene Monomers
2, 3a, 3b, and 4
step without purification. 1H NMR (400 MHz, CDCl3): δ 0.88 (t, J =
7.2 Hz, 3H, −CH2CH3), 1.23−1.41 [m, 21H, −OCH2(CH2)9−,
−CH3], 1.44 [s, 9H, −C(CH3)3], 1.63−1.65 (m, 2H, −CH2CH3),
polymers by adjusting the conjugation of monomer repeating
units.46−48 Herein we report the synthesis of novel optically
active poly(phenyleneethynylene−phenyleneethynylene) poly-
4.06−4.18 [m, 2H, −OCH2(CH2)9−], 4.26−4.29 (m, 1H, −CH−),
(1−2), poly(phenyleneethynylene−napthyleneethynylene)s
5.31 (br, 1H, −NH−). 13C NMR (100 MHz, CDCl3): δ 13.76
poly(1−3a) and poly(1−3b), and poly(pheyleneethynylene−
pyrenyleneethynylene) poly(1−4) containing hydroxy groups
(−CH2CH2CH3), 18.21 (−CH3), 22.39 (−CH2CH2CH3), 25.54
(−CH2CH2CH3), 27.99 [−C(CH3)3], 28.94, 29.06, 29.22, 29.28,
by the Sonogashira−Hagihara coupling polymerization of the
corresponding monomers (Scheme 1). This paper discusses the
effects of phenylene, naphthylene, and pyrenylene units on the
chiroptical and fluorescence properties, conjugation length of
the polymers, and secondary structures based on DFT
calculations as well as CD, UV−vis, and fluorescence
spectroscopic analysis.
29.34, 29.36, 29.40 [−(CH2)7−], 31.63 (−OCH2CH2−), 53.12
(−CH−), 62.31 (−OCH2CH2−), 79.18 [−C(CH3)3], 163.18
[−COOC(CH3)3], 173.11 (−COOCH2−).
Dodecyl (S)-2-(4-Hydroxy-3,5-diiodobenzamido)propanoate
(1). Trifluoroacetic acid (TFA, 10 mL, 135 mmol) was added to a
solution of N-tert-butoxycarbonyl-L-alanine dodecyl ester (13.2 g, 35.5
mmol) in THF (15 mL) at 0 °C, and the resulting mixture was stirred
at room temperature overnight. The resulting solution was
concentrated in vacuo to obtain L-alanine dodecyl ester trifluoroacetate
salt as a viscous liquid. Et3N (7.00 mL, 50.2 mmol) was added to a
solution of L-alanine dodecyl ester trifluoroacetate salt (3.71 g, 10.0
mmol) in THF, and then the resulting mixture was stirred at room
temperature for 2 h. 4-Hydroxy-3,5-diiodobenzoic acid (3.89 g, 10.0
mmol) and TRIAZIMOCH (3.24 g, 10.0 mmol) were added to the
solution subsequently, and the resulting mixture was stirred at room
temperature overnight. It was washed with 1.0 M HCl, saturated
NaHCO3 aq., and saturated NaCl aq., dried over anhydrous MgSO4,
and then filtered. The filtrate was concentrated, and the residual mass
was purified by silica gel column chromatography eluted with CHCl3/
hexane =4/1 (v/v), followed by recrystallization to obtain dodecyl (S)-
2-(4-hydroxy-3,5-diiodobenzamido)propanoate as a white solid in
26%. Mp 104−105 °C. [α]D +14° (c = 0.10 g·dL−1, THF). IR (KBr):
3445, 3058, 2918, 1740, 1625, 1537, 1447, 1389, 1349, 1315, 1173,
1127, 765, 703 cm−1. 1H NMR (400 MHz, CDCl3): δ 0.88 (t, J = 7.2
Hz, 3H, −CH2CH3), 1.23−1.52 [m, 21H, −OCH2(CH2)9−, −CH3],
1.63−1.65 (m, 2H, −CH2 CH3 ), 4.14−4.20 [m, 2H,
−OCH2(CH2)9−], 4.71−4.75 (m, 1H, −CH−), 6.05 (br, 1H,
−NH−), 6.58 (s, 1H, −OH), 8.12 (s, 2H, Ar). 13C NMR (100
MHz, CDCl3): δ 14.04 (−CH2CH2CH3), 18.34 (−CH3), 22.58
(−CH2CH2CH3), 25.72 (−CH2CH2CH3), 28.43, 29.10, 29.24, 29.28,
29.40, 29.46, 29.52 [−(CH2)7−], 31.81 (−OCH2CH2−), 48.61
(−CH−), 65.87 (−OCH2CH2−), 82.11, 129.48, 138.39, 156.39
(Ar), 163.44, (−NHCO−), 173.39 (−COOCH2−). Anal. Calcd for
C22H33I2NO4: C, 41.99; H, 5.29; N, 2.23. Found: C, 41.71; H, 5.28; N,
2.19.
EXPERIMENTAL SECTION
■
Measurements. Proton (400 MHz) and Carbon-13 (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-MS700 mass spectrometer. Specific rotations ([α]D) were
measured on a JASCO DIP-1000 digital polarimeter. Number- and
weight-average molecular weights (Mn and Mw) of polymers were
determined by size 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-820 spectropolarimeter, and fluorescence
spectra were recorded on a FP-750 spectrometer.
Materials. Unless stated otherwise, reagents and solvents were
purchased and used without purification. 4-[4,6-Dimethoxy-1,3,5-
triazin-2-yl]-4-methylmorpholinium chloride (TRIAZIMOCH) and
trimethylsilylacetylene were provided as gifts from Tokuyama Co., Ltd.
and Shin-Etsu Chemical Co., Ltd., respectively. 2,7-Diethynylnaph-
thalene (3a)49 and 1,4-diethynylnaphthalene (3b)50 were synthesized
according to the literature reports. Et3N and N,N-dimethylformamide
(DMF) used for polymerization were distilled prior to use.
Monomer Synthesis. N-tert-Butoxycarbonyl-L-alanine Do-
decyl Ester. 1-Ethyl-3-(3-dimethylaminopropyl)carbodiimide hydro-
chloride (EDC•HCl, 11.5 g, 60.0 mmol), 4-dimethylaminopyridine
(DMAP) (0.732 g, 6.00 mmol), and 1-dodecanol (11.2 g, 60.0 mmol)
were added to a solution of N-tert-butoxycarbonyl-L-alanine (9.45 g,
50.0 mmol) in CH2Cl2 (80 mL) at 0 °C, and the resulting mixture was
stirred at room temperature overnight. It was washed with 0.5 M HCl,
saturated NaHCO3 aq., and saturated NaCl aq., dried over anhydrous
MgSO4, and then filtered to afford N-tert-butoxycarbonyl-L-alanine
dodecyl ester in quantitative yield. The product was used in the next
1,6-Diethynylpyrene (4). 1,6-Dibromopyrene (2.16 g, 6.00
mmol), Pd(PPh3)2Cl2 (0.213 g, 0.60 mmol), PPh3 (0.314 g, 2.40
mmol), and CuI (0.342 g, 3.60 mmol) were fed into a two-neck flask,
ant it was flushed with dry nitrogen. THF (40 mL) and Et3N (15 mL)
were added to the solution, and then trimethylsilylacetylene (4.20 mL,
30.0 mmol) was added dropwise to the solution. The mixture was
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stirred at 50 °C for 72 h. The resulting mixture was concentrated in
vacuo and the residual mass was washed with Et2O to extract the
product. The organic phase was washed with 1.0 M HCl, and saturated
NaCl aq., dried over anhydrous MgSO4, and 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) and
subsequently by preparative HPLC (eluent CHCl3) to obtain 1,6-
bis(trimethylsilylethynyl)pyrene. After that, it was dissolved in CHCl3
(60 mL), and then a suspension of K2CO3 (2.76 g, 20.0 mmol) in
MeOH (30 mL) was added to the solution. The resulting mixture was
concentrated in vacuo, and the residual mass was dispersed in CHCl3
and water. The organic layer was washed with 1.0 M HCl, and
saturated NaCl aq., dried over anhydrous MgSO4, and then filtered.
The filtrate was concentrated, and the residual mass was purified by
recrystallization from CHCl3 and THF to obtain 3 as a brownish solid
in 77%. No mp was observed up to 172 °C (decomposition). IR
(KBr): 3294, 2096, 1601, 1571, 1433, 1292, 1181, 840, 643, 597 cm−1.
1H NMR (400 MHz, CDCl3): δ 3.64 (s, 2H, −C≡CH), 8.13−8.20 (m,
6H, Ar), 8.61−8.63 (m, 2H, Ar).51 HRMS. (m/z): [M]+ calcd for
C20H10, 250.0783; found, 250.0783.
Polymerization. All the polymerizations were carried out in a glass
tube equipped with a three-way stopcock under nitrogen. A typical
experimental procedure for polymerization of 1 with 2 is given. A
solution of 1 (188 mg, 0.300 mmol), 2 (37.8 mg, 0.300 mmol),
Pd(PPh3)2Cl2 (10.5 mg, 0.015 mmol), CuI (11.8 mg, 0.045 mmol),
PPh3 (5.70 mg, 0.030 mmol), and Et3N (1.2 mL) in DMF (1.8 mL)
was 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.
a wavelength-based Gaussian function of 40 nm tentatively used for a
half of 1/e-bandwidth.
RESULTS AND DISCUSSION
■
Polymerization. The Sonogashira−Hagihara coupling
polymerization of diiodophenylene monomer 1 with dieth-
ynylarylene monomers 2, 3a, 3b, and 4 was performed in DMF
at 80 °C for 24 h to obtain the corresponding polymers
[poly(1−2), poly(1−3a), poly(1−3b) and poly(1−4)] with
moderate molecular weights (Mn) in the range 5300−11300 in
88−94% yields (Table 1). The Mn of poly(1−3a) was relatively
Table 1. Sonogashira−Hagihara Coupling Polymerization of
a
1 with 2, 3a, 3b and 4
polymer
b
c
c
monomer
yield (%)
Mn
Mw/Mn
1 + 2
poly(1−2)
poly(1−3a)
poly(1−3b)
poly(1−4)
90
94
88
90
11300
5300
1.3
2.1
1.4
1.3
1 + 3a
1 + 3b
1 + 4
7500
11300
a
Conditions: [1]0 = [2]0 = [3a]0 = [3b]0 = [4]0 = 0.10 M,
[Pd(PPh3)2Cl2] = 0.0050 M, [CuI] = 0.0025 M, [PPh3] = 0.0010 M,
b
DMF/Et3N = 3/2 (v/v), 80 °C, 24 h. Insoluble part in MeOH/
c
acetone =9/1 (v/v). Estimated by SEC measured in THF,
polystyrene calibration.
low, probably due to the steric hindrance caused by the 2,7-
naphthylene linkage. Poly(1−2), poly(1−3a) and poly(1−3b)
were soluble in common organic solvents such as CH2Cl2,
CHCl3, THF, DMSO and DMF. On the other hand, poly(1−
4) was insoluble in CH2Cl2, and partly insoluble in CHCl3,
DMSO and DMF, likely due to the poor solubility of the rigid
pyrene skeleton.
Spectroscopic Data for the Polymers. Poly[(S)-1−2]. IR (KBr):
3338, 3064, 2925, 2852, 2208, 1735, 1654, 1509, 115, 838, 753, 518
cm−1 1H NMR (400 MHz, CDCl3): δ 0.81−0.90 (br, 3H,
.
−CH2CH3), 1.21−1.34 [br, 21H, −OCH2(CH2)9−, −CH3], 1.58−
1.73 (br, 2H, −CH2CH3), 4.10−4.31 [br, 2H, −OCH2(CH2)9−],
4.70−4.95 (br, 1H, −CH−), 6.50−8.07 (br, 8H, −NH−, −OH−, Ar).
Poly(1−3a). IR (KBr): 3297, 3058, 2922, 2852, 2211, 1736, 1654,
1
1509, 1450, 1340, 1205, 1170, 1093, 840, 800, 752, 470 cm−1. H
Chiroptical Properties of the Polymers. Figure 1 shows
the CD and UV−vis spectra of the polymers measured in THF
NMR (400 MHz, CDCl3): δ 0.80−0.94 (br, 3H, −CH2CH3), 1.15−
1.34 [br, 21H, −OCH2(CH2)9−, −CH3], 1.50−1.69 (br, 2H,
−CH2CH3), 4.21−4.30 [br, 2H, −OCH2(CH2)9−], 4.84−4.90 (br,
1H, −CH−), 6.46−8.32 (br, 10H, −NH−, −OH−, Ar).
Poly(1−3b). IR (KBr): 3328, 3058, 2923, 2852, 2200, 1736, 1654,
1509, 1451, 1189, 1161, 1092, 761 cm−1. 1H NMR (400 MHz,
CDCl3): δ 0.80−0.90 (br, 3H, −CH2CH3), 1.15−1.34 [br, 21H,
−OCH2(CH2)9−, −CH3], 1.53−1.72 (br, 2H, −CH2CH3), 4.08−4.31
[br, 2H, −OCH2(CH2)9−], 4.78−4.96 (br, 1H, −CH−), 6.26−8.60
(br, 10H, −NH−, −OH−, Ar).
Poly(1−4). IR (KBr): 3423, 3040, 2921, 2851, 2195, 1735, 1654,
1509, 1457, 1341, 1091, 841, 817 cm−1. 1H NMR (400 MHz, CDCl3):
δ
0.72−0.88 (br, 3H, −CH2CH3), 1.15−1.34 [br, 21H,
−OCH2(CH2)9−, −CH3], 1.53−1.72 (br, 2H, −CH2CH3), 4.26−
4.41 [br, 2H, −OCH2(CH2)9−], 4.83−5.02 (br, 1H, −CH−), 6.19−
8.87 (br, 12H, −NH−, −OH−, Ar).
Computation. The MM calculations were carried out using the
Merck molecular force field52 (MMFF94) with Spartan ’10 version
1.1.0, running on a Macintosh computer. The ZINDO/S and density
functional theory (DFT) calculations were performed with the
Gaussian 09 program53 running on the supercomputer system of the
Academic Center for Computing and Media Studies of Kyoto
University. The DFT54 method with Becke’s three-parameter hybrid
functional55 and the LYP correlation functional (B3LYP)56 were
utilized in conjunction with the 6-31+G* basis set to fully optimize
geometries. Theoretical CD and UV−vis spectra were simulated by the
ZINDO/S method57−61 or by the TD-DFT method. The low-energy
transition states of 20 were predicted under the condition of a CI
number of 20 × 20, including each oscillator strength (f) and rotatory
strength (Rvel) in velocity form. The simulated CD and UV−vis
spectra were produced by using the Rvel− and f−wavelength data with
Figure 1. CD and UV−vis spectra of poly(1−2), poly(1−3a),
poly(1−3b), and poly(1−4) measured in THF (c = 30 μM) at 20 °C.
at 20 °C. Poly(1−3a), poly(1−2), poly(1−3b), and poly(1−4)
exhibited CD signals in the absorption regions of the main
chain chromophores around 250−350, 280−400, 300−450 and
350−500 nm, respectively. These CD signals undoubtedly
come from the chiral ordered structures of the polymers, since
chiral monomer 1 shows the λmax at 286 nm and no CD signal
(Figure S1, Supporting Information). The CD and UV−vis
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signals were intact after filtering the polymer solutions using a
membrane filter with a pore size of 0.45 μm. It is supposed that
the CD signals do not originate from chiral aggregation but
unimolecular chirality, i.e., folded helical conformations with
predominantly one-handed screw sense.62 The absorption
maxima (λmax’s) were remarkably dependent on the arylene
units. The λmax of 2,7-naphthylene linked poly(1−3a) was the
shortest (320 nm) among the four polymers, presumably due
to the most kinked linkage. The λmax’s of 1,4-naphthylene
linked poly(1−3b) and 1,6-pyrenylene-linked poly(1−4) were
25 and 50 nm longer than that of poly(1−2), respectively,
indicating the longer conjugation length as predicted from the
condensed aromatic rings with the linkages parallel to the main
chains.
the polymers are folded into helical structures in THF, and the
response to temperature and solvent depends on the structure
of arylene unit and linking positions.
It is suggested that poly(1−2) prefers the left-handed helical
structure to the right-handed one from the conformational
analysis and simulation of CD spectra.64 In the present study,
the information on the predominance of helical sense of
poly(1−3b) and poly(1−4) was obtained in a similar fashion.
The 24-mers of poly(1−3b) and poly(1−4) were constructed
with both chain ends terminated with hydrogen atoms. The
torsional angles of the main chains were set to −1° and +1° per
phenylene unit corresponding to the left- and right-handed
helices as the initial geometries. The geometries were optimized
by the MM method (MMFF94). The left-handed helical 24-
mers of poly(1−3b) and poly(1−4) were estimated to be 10.47
and 0.21 kJ/mol•unit more stable than the right-handed
counterparts, respectively [Figure S4: the optimized conformers
of poly(1−3b) and poly(1−4)]. The unimodal and bisignate
CD curves were satisfactorily simulated for poly(1−3b) and
poly(1−4), respectively, by the ZINDO/S method using these
conformers (Figure S5). Although the wavelengths were
simulated at longer wavelengths than the observed ones, the
theoretical CD well supports the predominance of left-handed
helical conformers of both poly(1−3b) and poly(1−4) in spite
of the difference of conjugation units.65
Figure 2 shows the Δεmax values measured in THF at various
temperatures, elucidating the thermal stability of helical
Some poly(phenyleneethynylene)s stabilize the helical
conformation by intramolecular hydrogen bonds between the
pendent groups.20,21,23,66,67 Solution-state IR spectra of the
present polymers were measured together with that of
monomer 1 in CHCl3 under diluted conditions (c = 3
mM)68 to determine the presence/absence of intramolecular
hydrogen bonding (Table 2). In every case, a negligibly small
Figure 2. Plot of |Δεmax| value of poly(1−2), poly(1−3a), poly(1−
3b), and poly(1−4) measured in THF (c = 30 μM) at various
temperatures.
conformation of the polymers.63 The Δεmax of poly(1−2)
and poly(1−4) decreased 24% and 3% by raising temperature
from 0 to 60 °C, while those of poly(1−3a) and poly(1−3b)
decreased 72% and 39%, respectively. The temperature-
responsive change was reversible for poly(1−2) and poly(1−
4), but irreversible for poly(1−3a) and poly(1−3b). The
stabilities of conformations of the polymers were largely
different. The UV−vis signals of poly(1−3a) and poly(1−3b)
changed upon heating (Figure S2) simultaneously with the CD
change. It is likely that the folded helical structures were
disrupted, and turned into different structures such as trans-
zigzag form as increasing the temperature. Once this thermo-
induced transformation occurred, refolding into helices seems
to be difficult.
The conformation of dynamic helical polymers is largely
affected by solvent polarity in most cases, especially when the
polymers contain polar functional groups such as amide and
hydroxy groups that interact strongly with polar solvents.1f,h In
the present study, poly(1−4) exhibited almost the same CD
signals in THF/MeOH mixtures irrespective of the composi-
tion, although the λmax was slightly blue-shifted as increasing the
MeOH content (Figure S3). Poly(1−3a) exhibited the same
trend regarding the solvent effect. On the other hand, the CD
signal intensity of poly(1−2) increased as increasing the
MeOH content, while that of poly(1−3b) decreased. The
shape of the UV−vis signals of poly(1−3b) observed in THF/
MeOH = 8/2 mixture almost coincided with that observed at
60 °C in THF. Poly(1−3b) seems to turn into another
structure presumably trans-zigzag form not only by heating but
also by raising MeOH content. These results indicate that all
Table 2. Solution-State IR Spectroscopic Data (CO
a
Absorption Peaks) of Monomer 1 and the Polymers
wavenumber (cm−1) CO
compound
ester
amide
1
1730
1729
1731
1731
1730
1659
1659, 1642
1652
poly(1−2)
poly(1−3a)
poly(1−3b)
poly(1−4)
1652
1644
a
Measured in CHCl3 (c = 3 mM).
difference was observed between the wavenumbers of ester
CO absorption peaks of the polymer and 1. On the other
hand, the amide CO absorption peaks of poly(1−2),
poly(1−3a), poly(1−3b) and poly(1−4) were observed at
7−17 cm−1 lower wavenumbers than that of 1, indicating the
presence of intramolecular hydrogen bonds at the amide groups
between the side chains in these polymers, and the ester CO
groups do not participate in the hydrogen bonds. Two amide
CO absorption peaks with a ratio of 1:1 were observed in the
IR spectrum of poly(1−2); one was the same wavenumber as
that of 1 (1659 cm−1) and the other was 17 cm−1 lower. In this
case, the amide groups seem to be partly free from hydrogen
bonds. This may explain the smallest CD intensity of poly(1−
2) among the polymers as shown in Figure 1. The helical
structure of poly(1−2) appears to be incomplete due to lack of
stabilization by intramolecular hydrogen bonds between the
amide groups.
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The fluorescence spectra of the present polymers were
measured in THF excited at the main chain based absorption
maxima (Figure 3). Poly(1−3a), poly(1−2), poly(1−3b), and
Chart 1. Structures of Model Compounds
Figure 3. Fluorescence spectra of poly(1−2), poly(1−3a), poly(1−
3b), and poly(1−4) measured in THF (c = 0.6−3.0 μM) at room
temperature excited at λmax (342, 320, 367, and 392 nm).
poly(1−4) emitted purplish blue, blue, green and yellow
fluorescence in quantum yields ranging from 18% to 34%
(Table 3 and Figure 4).69 The emission maximum (λemi) was
a
Table 3. Optical Data of the Polymers
b
polymer
λmax (nm)
λemi (nm)
Φemi (%)
poly(1−2)
poly(1−3a)
poly(1−3b)
poly(1−4)
342
320
367
392
446
408
504
535
31
19
34
18
a
b
Measured in THF. Measured using anthracene in EtOH (Φemi
27%) or quinine sulfate in 0.5 M H2SO4 (Φemi = 54%) as a standard.
=
Figure 5. Excited-state parameters and UV−vis spectra simulated for
M2, M3a, M3b, and M4. The f values (bars) are predicted at the
B3LYP/6-31+G* level.
Figure 4. Photograph of THF solutions (c = 3.0 μM) of poly(1−3a),
poly(1−2), poly(1−3b), and poly(1−4) (from left to right) under
irradiation of light (365 nm).
red-shifted in consonance with the red-shift of the λmax. Thus,
the polymers emitted variously colored fluorescence depending
on the arylene units.
Figure 6. Relationship between the λmax of M2, M3a, M3b, and M4
simulated at the B3LYP/6-31+G* level and observed λmax of the
corresponding polymers [poly(1−2), poly(1−3a), poly(1−3b), and
poly(1−4)]. The data points are colored in accordance with Figure 5.
Simulation of UV−Vis Spectra. The UV−vis spectra of
the model compounds (M2, M3a, M3b, and M4 in Chart 1)
for the polymers were simulated using the TD-DFT method at
the B3LYP/6-31+G* level to obtain further information about
the conjugation length.70 Figure 5 depicts the oscillator
strength (f) values and UV−vis spectra simulated from the f
values and their positions, where a half Gaussian (1/e)-
bandwidth (Δ/2) was assumed to be 40 nm.71 The λmax values
were simulated to be 338, 399, 445, and 469 nm for M3a, M2,
M3b, and M4, respectively, in harmony with the red-shift of
observed λmax’s of the polymers as plotted in Figure 6. The
most intense f originates from the electron transition from the
HOMO to LUMO. The good correlation indicates the
properness of TD-DFT calculation on simulating the UV−vis
spectra of phenyleneethynylene polymers. The trend of λmax
(M3a < M2 < M3b < M4) completely agrees with that of
HOMO levels of M2, M3a, M3b, and M4 as listed in Table 4.
It appears that the HOMO levels reflect the degree of
delocalization of π-electron, which is understood from the
shapes of HOMO as illustrated in Figure 7. On the other hand,
no clear relation was confirmed between the LUMO levels and
72
λmax
.
The experimentally observed λmax values were positioned at
shorter wavelengths than those simulated by the TD-DFT
method, in a manner similar to several conjugated com-
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(phenyleneethynylene-naphthyleneethynylene)s, and poly-
(phenyleneethynylene-pyrenyleneethynylene) bearing hydroxy
groups [poly(1−2), poly(1−3a), poly(1−3b), and poly(1−4)]
by the Sonogashira−Hagihara coupling polymerization of
optically active diiodophenylene monomer 1 with the
corresponding diethynylarylene monomers 2, 3a, 3b, and 4.
CD and UV−vis spectroscopic analysis revealed that all the
polymers formed predominantly one-handed folded helical
structures in THF. The order of λmax values of the polymers was
poly(1−3a) < poly(1−2) < poly(1−3b) < poly(1−4). It
appeared that the 1,4-naphthylene and 1,6-pyrenylene units in
the polymer main chains extended the conjugation length
compared with phenylene unit, while the 2,7-naphthylene unit
rather shortened the conjugation length. Poly(1−2) and
poly(1−4) formed a thermally stable helical structure in
THF, while poly(1−3a) and poly(1−3b) lost the regulated
helical structures to some extent upon heating. Poly(1−3a) and
poly(1−4) kept the CD spectroscopic patterns in THF/MeOH
irrespective of the MeOH content. On the other hand, poly(1−
2) increased the CD signal intensity upon increasing the
MeOH content, while poly(1−3b) remarkably diminished the
CD signals. Namely, the formed polymers showed different
temperature- and solvent-responses depending on the arylene
Table 4. Energy Levels of HOMO and LUMO and Band
Gaps (Δ) of Model Compounds M2, M3a, M3b and M4
a
Δ
polymer
HOMO (eV)
LUMO (eV)
(eV)
(nm)
M2
M3a
M3b
M4
−5.66
−5.81
−5.49
−5.07
−2.25
−2.10
−2.44
−2.21
3.39
3.71
3.05
2.86
366
334
406
433
a
Calculated at the B3LYP/6-31+G* level.
pounds.73,74 Another possible reason is the degrees of twisting
of the main chains. The main chains of the polymers are twisted
due to the folded helical structures. On the contrary, the main
chains of model compounds M2, M3a, M3b, and M4 almost
exist on a plane to maximize the conjugation. Anyway, we could
obtain clear evidence for the effect of arylene structures on the
conjugation length of the present polymers based on the TD-
DFT calculations.
CONCLUSIONS
In the present study, we have synthesized optically active novel
poly(phenyleneethynylene- phenyleneethynylene), poly-
■
Figure 7. Shapes of HOMO (bottom) and LUMO (top) of M2, M3a, M3b, and M4 calculated at the B3LYP/6-31+G* level.
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ASSOCIATED CONTENT
* Supporting Information
■
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S
CD and UV−vis spectra of monomer 1 measured in THF at 20
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S2), measured in THF/MeOH mixtures with various
compositions at 20 °C (Figure S3), possible conformers of
poly(1−3b) and poly(1−4) (Figure S4); simulated CD and
UV−vis spectra of poly(1−3b) and poly(1−4) (Figure S5),
photograph of the polymer solutions in THF (Figure S6),
structures of achiral and chiral model compounds for UV−vis
simulation (Chart S1), simulated UV−vis spectra of achiral and
chiral model compounds (Figure S7), transmittance of the
polymer solutions measured at 633 nm in THF (Table S1), and
Cartesian coordinates of geometries of M2, M3a, M3b, and M4
optimized at the B3LYP/6-31+G* level. This material is
available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail: (F.S.) sanda@kansai-u.ac.jp.
■
(16) Inoue, M.; Teraguchi, M.; Aoki, T.; Hadano, S.; Namikoshi, T.;
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Present Address
∥(H.S.) Department of Organic and Polymeric Materials,
Tokyo Institute of Technology, O-okayama, Meguro-ku, Tokyo
152−8552, Japan
(19) Chen, Y.; Malkovskiy, A.; Wang, X.; Lebron-Colon, M.;
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Notes
The authors declare no competing financial interest.
(20) Banno, M.; Yamaguchi, T.; Nagai, K.; Kaiser, C.; Hecht, S.;
Yashima, E. J. Am. Chem. Soc. 2012, 134, 8718−8728.
(21) Liu, R.; Shiotsuki, M.; Masuda, T.; Sanda, F. Macromolecules
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ACKNOWLEDGMENTS
■
This research was supported by JSPS Research Fellowships of
Young Scientists and a Grant-in-Aid for Science Research (B)
(22350049) from the Japan Society for the Promotion of
Science, and by a Grant-in-Aid for Scientific Research on
Innovative Areas “New Polymeric Materials Based on Element-
Blocks (No. 2401)” from the Ministry of Education, Culture,
Sports, Science, and Technology, Japan.
(22) Liu, R.; Sogawa, H.; Shiotsuki, M.; Masuda, T.; Sanda, F.
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1235−1255.
8904
dx.doi.org/10.1021/ma401730e | Macromolecules 2013, 46, 8896−8904