Synthesis of optically active conjugated polymers bearing m-terphenylene moieties by acetylenic coupling polymerization: Chiral aggregation and optical properties of the product polymers

Article abstract of DOI:10.1021/ma402123w

The acetylenic coupling polymerization of D-hydroxyphenylglycine-derived m-terphenylene diynes 1-5 using Pd/Cu catalyst gave the corresponding polymers [poly(1)-poly(5)] with M_n = 12 000-60 000 in 53-89% yields. The polymers were soluble in THF

Full text of DOI:10.1021/ma402123w

Article
pubs.acs.org/Macromolecules
Synthesis of Optically Active Conjugated Polymers Bearing
m‑Terphenylene Moieties by Acetylenic Coupling Polymerization:
Chiral Aggregation and Optical Properties of the Product Polymers
Yu Miyagi,† Hiromitsu Sogawa,‡ Masashi Shiotsuki,§ and Fumio Sanda⊥,*
†Department of Polymer Chemistry, Graduate School of Engineering, Kyoto University Katsura Campus, Nishikyo-ku, Kyoto
615-8510, Japan
‡Department of Organic and Polymeric Materials, Tokyo Institute of Technology, O-okayama, Meguro, Tokyo 152-8552, 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: The acetylenic coupling polymerization of D-
hydroxyphenylglycine-derived m-terphenylene diynes 1−5
using Pd/Cu catalyst gave the corresponding polymers
[poly(1)−poly(5)] with Mn = 12 000−60 000 in 53−89%
yields. The polymers were soluble in THF and DMF. CD and
UV−vis spectroscopic analysis revealed that p,p′-phenyl-
eneethynylene-linked poly(1), poly(3), and poly(5) formed
chiral higher-order structures in THF/H2O mixtures, while
m,m′-phenyleneethynylene-linked poly(2) and poly(4) did
not. The sign of CD signal of poly(1) was reasonably
predicted by time-dependent density functional calculations of
the model system. The polymers emitted fluorescence with quantum yields ranging from 0.2−14.8%.
powerful tool for synthesizing conjugated diyne polymers that
show useful photoelectrical properties.
INTRODUCTION
■
Conjugated polymers such as poly(acetylene)s,1 poly-
(phenylene)s,2 poly(phenylenevinylene)s,3 and poly-
(phenyleneethynylene)s4 attract considerable attention because
of their useful properties: electrical conductivity, paramagnetic
susceptibility, optical nonlinearity, photoconductivity, and
fluorescence emission. Conjugated polymers substituted with
optically active groups tend to form chiral higher order
structures, i.e., predominantly one-handed helices or chiral
aggregates, due to the small mobility of the conjugated main
chains. In some cases, the higher order structures are further
stabilized by intra- and/or intermolecular interactions, such as
hydrogen bonding and π-stacking.5
The p-terphenylene-linkage is often used as a key unit for
constructing molecules with rigid conformation, including α-
helix mimic-,10 pillar-,11 triangle-,12 and cage-shaped13
structures, due to the rod-like rigid nature. p-Terphenylene-
containing molecules have also been examined as photoelectri-
cally functional materials.14 In a similar fashion, the m-
terphenylene-linkage is used as a building block for assembling
bent-shaped molecules.15 Various p- and m-linked terpheny-
lene-based polymers have been synthesized, primarily for
preparation of photoelectrically functional polymers with
controlled secondary and assembled structures.16
The acetylenic coupling reaction is a useful method for
synthesizing diacetylene compounds.6 Since the first report on
cupper (Cu)-mediated oxidative acetylenic coupling developed
by Glaser in the 19th century, various modifications have been
reported, including the Eglington coupling using excess
In this study, we report the synthesis of novel hydrox-
yphenylglycine-derived conjugated polymers bearing terpheny-
lene moieties by acetylenic coupling polymerization and an
investigation of influence of the phenyleneethynylene moieties
on the higher-order structures. We analyzed the aggregation of
the polymers by dynamic light scattering (DLS) and proposed a
mechanism using density functional theory (DFT) calculations
of the model compounds.
7
Cu(OAc)2 and Hay coupling using a catalytic amount of
CuCl−N,N,N′,N′-tetramethylethylenediamine.8 A milestone
work on acetylenic coupling was the development of palladium
(Pd)-catalyzed coupling reactions reported by Rossi et al.9 In
recent years, variations of Pd-catalyzed coupling reactions of
terminal alkynes have been utilized for synthesizing diaryl and
dialkyl diynes, and applied to the synthesis of natural products
and polymers. Pd-catalyzed coupling polymerization is a
Received: October 15, 2013
Revised: February 12, 2014
Published: February 21, 2014
© 2014 American Chemical Society
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Macromolecules
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3′,5′-Di(4″-ethynylphenyl)-4′-hydroxy-N-α-tert-butoxycarbonyl-
D-phenylglycine Hexylamide (1). 4-[(Trimethylsilyl)ethynyl]-
phenylboronic acid pinacol ester (5.41 g, 18.0 mmol), Cs2CO3 (7.82
g, 24.0 mmol) and Pd2(dba)3 (166 mg, 0.182 mmol) were added to a
solution of 9 (3.61 g, 6.01 mmol) in acetone/H2O (60 mL/18 mL)
under nitrogen, and the resulting mixture was heated with refluxing for
18 h at 65 °C. The resulting solution was extracted with ethyl acetate,
and the organic layer was washed with 0.5 M HCl, dried over
anhydrous MgSO4, and then filtered. The filtrate was concentrated,
and the residual mass was purified by silica gel column
chromatography with CHCl3/hexane/ethyl acetate =38/1/1 (v/v/v)
and preparative HPLC to obtain 1 as a white powder in 39% yield.
Mp: 111 °C. [α]D: −82° (c = 0.10 g/dL, THF). IR (in CHCl3): 3546,
3294, 2930, 2858, 2107, 1657, 1165 cm−1. 1H NMR (400 MHz,
CDCl3): δ 0.83 (t, J = 6.3 Hz, 3H, −CH2CH3), 1.20−1.41 [m, 17H,
−C(CH3)3, (CH2)4], 3.13 (s, 2H, −CCH), 3.20−3.25 (m, 2H,
−NCH2), 5.16 (br, 1H, C*H), 5.38 (br, 1H, CONH), 5.87 (br, 1H,
NHCH2), 5.98 (br, 1H, OH), 7.27−7.57 (10H, Ar). 13C NMR (100
MHz, CDCl3): δ 13.9 (−CH2CH3), 22.4, 26.4, 29.4, 31.4, 39.8
[(CH2)4], 28.3 [−C(CH3)3], 57.8 (C*H), 78.0 (−CCH), 80.1
[−C(CH3)3], 83.3 (−CCH), 121.7, 128.6, 128.8, 129.2, 131.3,
132.5, 137.4, 149.2 (Ar), 155.3 (CONH), 170.1 (OCONH). HRMS.
(m/z): [M + H]+ calcd for C35H39N2O4, 551.2904; found, 551.2901.
3′,5′-Di(3″-ethynylphenyl)-4′-hydroxy-N-α-tert-butoxycarbonyl-
D-phenylglycine Hexylamide (2). This compound was synthesized
from 9 and 3-[(trimethylsilyl)ethynyl]phenylboronic acid pinacol ester
in a manner similar to 1. Yield: 28% (white powder). Mp: 77−78 °C.
[α]D: −87° (c = 0.10 g/dL, THF). IR (KBr): 3298, 2928, 2858, 2107,
1656, 1163 cm−1. 1H NMR (400 MHz, CDCl3): δ 0.79 (t, J = 6.3 Hz,
3H, −CH2CH3), 1.15−1.37 [m, 17H, −C(CH3)3, (CH2)4], 3.06 (s,
2H, −CCH), 3.22−3.17 (m, 2H, −NCH2), 5.39 (br, 1H, C*H),
5.62 (br, 1H, CONH), 6.11 (br, 1H, NHCH2), 6.77 (br, 1H, OH),
7.29−7.57 (10H, Ar). 13C NMR (100 MHz, CDCl3): δ 13.9
(−CH2CH3), 22.4, 26.3, 29.3, 31.3, 39.7 [(CH2)4], 28.2 [−C(CH3)3],
57.4 (C*H), 77.7 (−CCH), 80.0 [−C(CH3)3], 83.2 (−CCH),
122.6, 128.2, 128.6, 129.6, 131.1, 131.2, 132.8, 137.2, 149.2 (Ar), 155.4
(CONH), 170.3 (OCONH). HRMS. (m/z): [M + H]+ calcd for
C35H39N2O4, 551.2904; found, 551.2899.
3′,5′-Di(4″-ethynylphenyl)-4′-hydroxy-N-α-tert-butoxycarbonyl-
D-phenylglycine N-Hexylmethylamide (3). This compound was
synthesized from 10 and 4-[(trimethylsilyl)ethynyl]phenylboronic
acid pinacol ester in a manner similar to 1. Yield: 39% (white solid).
Mp: 168−169 °C. [α]D: −107° (c = 0.10 g/dL, THF). IR (KBr):
3410, 3292, 2929, 2860, 2107, 1640, 1166 cm−1. 1H NMR (400 MHz,
CDCl3): δ 0.83 (t, J = 6.3 Hz, 3H, −CH2CH3), 1.21−1.42 [m, 17H,
−C(CH3)3, (CH2)4], 2.92−2.93 (ds, 3H, −NCH3), 3.13 (s, 2H, −C
CH), 3.23−3.56 (m, 2H, −NCH2), 5.39 (br, 1H, C*H), 5.51−5.59
(br, 1H, CONH), 6.06 (br, 1H, OH), 7.29−7.57 (10H, Ar). 13C NMR
(100 MHz, CDCl3): δ 13.9 (−CH2CH3), 22.4, 26.4, 31.5, 33.6, 35.1
[(CH2)4], 28.4 [−C(CH3)3], 48.4 (NCH3), 54.3 (C*H), 78.0 (−C
CH), 79.7 [−C(CH3)3], 83.3 (−CCH), 121.7, 128.6, 129.3, 130.8,
131.4, 132.6, 137.4, 149.1 (Ar), 155.0 (CONH), 169.7 (OCONH).
HRMS. (m/z): [M + H]+ calcd for C36H41N2O4, 565.3061; found,
565.3056.
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. Mass spectra were measured on a Thermo Scientific
Exactive 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 GPC (TSK gel α-3000) using a solution of LiBr (10 mM) in N,N-
dimethylformamide (DMF) as the eluent with polystyrene standards
at 40 °C. CD and UV−vis absorption spectra were recorded on a
JASCO J-820 spectropolarimeter. DLS measurements were performed
using a Malvern Instruments Zetasizer Nano ZS at 25 °C. The
measured autocorrelation function was analyzed using a cumulant
method. The Z-average values of the polymers were calculated from
the Stokes−Einstein equations.
Materials. 3′,5′-Diiodo-4′-hydroxy-N-α-tert-butoxycarbonyl-D-phe-
nylglycine hexylamide (9) was prepared according to the literature.17
Reagents, including di-tert-butyl dicarbonate [(Boc)2O, Tokuyama], 4-
(4,6-dimethoxy-1,3,5-triazine-2-yl)-4-methylmorpholinium chloride
(TRIAZIMOCH, Tokuyama), anhydrous THF (Wako, water max.
0.001%), Pd(OAc)2 (Aldrich, assay 99.9%), CuI (Wako, 99.5%), and
1,4-diazabicyclooctane (DABCO) (Wako, 95.0%) were used as
received.
Monomer Synthesis. 3′,5′-Diiodo-4′-hydroxy-N-α-tert-butoxy-
carbonyl-D-phenylglycine N-Hexylmethylamide (10). (Boc)2O (82.2
g, 377 mmol) and Na2CO3 (38.1 g, 359 mmol) were added to a
solution of D-4-hydroxyphenylglycine (50.0 g, 299 mmol) in 1,4-
dioxane/H2O (500 mL/500 mL) at 0 °C, and the resulting mixture
was stirred at room temperature overnight. 1,4-Dioxane was removed
by evaporation, and the residual solution was carefully acidified with
citric acid to pH = 3 and extracted with ethyl acetate. The organic layer
was washed with water and saturated NaCl(aq), dried over anhydrous
MgSO4, and then filtered. The filtrate was concentrated to obtain 4-
hydroxy-N-α-tert-butoxycarbonyl-D-phenylglycine (6) as a viscous
liquid. After that, TRIAZIMOCH (12.9 g, 46.8 mmol) and N-
hexylmethylamine (6.10 mL, 40.0 mmol) were added to a solution of
6 in ethyl acetate (200 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. The filtrate was concentrated
to obtain 4-hydroxy-N-α-tert-butoxycarbonyl-D-phenylglycine N-hex-
ylmethylamide (8) as a yellow solid. After that, NaIO4 (9.66 g, 45.1
mmol) and NaCl (10.6 g, 181 mmol) were added to a solution of 8 in
acetic acid/H2O (135 mL/15 mL) at room temperature, and the
resulting mixture was stirred at room temperature for 15 min. KI (22.5
g, 135 mmol) was added to the solution at 0 °C, and then the resulting
mixture was stirred at room temperature overnight. It was washed with
water, 1 M Na2S2O3(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 with CHCl3/hexane = 4/1 (v/v) as the eluent to
1
obtain 10 as a yellow powder in 44% yield. H NMR (400 MHz,
3′,5′-Di(3″-ethynylphenyl)-4′-hydroxy-N-α-tert-butoxycarbonyl-
D-phenylglycine N-Hexylmethylamide (4). This compound was
synthesized from 10 and 3-[(trimethylsilyl)ethynyl]phenylboronic
acid pinacol ester in a manner similar to 2. Yield: 16% (white solid).
Mp: 73−74 °C. [α]D: −113.8° (c = 0.10 g/dL, THF). IR (KBr): 3408,
3292, 2957, 2929, 2859, 2372, 2109, 1707, 1638 cm−1. 1H NMR (400
MHz, CDCl3): δ 0.82 (t, J = 6.3 Hz, 3H, −CH2CH3), 1.20−1.47 [m,
17H, −C(CH3)3, (CH2)4], 2.91−2.92 (ds, 3H, −NCH3), 3.10 (s, 2H,
−CCH), 3.20−3.57 (m, 2H, −NCH2), 5.50 (br, 1H, C*H), 5.59
(br, 1H, CONH), 6.04 (br, 1H, OH), 7.27−7.65 (10H, Ar). 13C NMR
(100 MHz, CDCl3): δ 14.0 (−CH2CH3), 22.5, 26.4, 31.5, 33.7, 35.1
[(CH2)4], 28.4 [−C(CH3)3], 48.4 (NCH3), 54.6 (C*H), 77.7 (−C
CH), 79.6 [−C(CH3)3], 83.1 (−CCH), 122.7, 128.3, 128.7, 129.2,
129.6, 130.5, 131.3, 132.8, 137.0, 149.1 (Ar), 154.8 (CONH), 169.6
(OCONH). HRMS. (m/z): [M + H]+ calcd for C36H41N2O4,
565.3061; found, 565.3057.
CDCl3): δ 0.87 [t, J = 1.6 Hz, 3H, −N(CH2)5CH3], 1.24−1.42 [m,
17H, −C(CH3)3, (CH2)4], 2.86 and 2.94 (s, 3H, −NCH3), 3.25−3.48
(m, 2H, −NCH2−), 5.35−5.40 (m, 1H, NHCO), 5.86 (s, 1H, C*H),
6.05 (m, 1H, −OH), 7.69 (s, 2H, Ar).
3′,5′-Diiodo-4′-methoxy-N-α-tert-butoxycarbonyl-D-phenylgly-
cine Hexylamide (11). Compound 9 (1.20 g, 2.0 mmol) and K2CO3
(0.41 g, 3.0 mmol) were dissolved in acetone (7.5 mL). MeI (0.19 mL,
3.0 mmol) was added to the solution at 0 °C, and the resulting mixture
was refluxed for 3 h. The mixture was poured into iced water, and it
was extracted with CHCl3. The organic layer was washed with
saturated aqueous NaCl, dried over anhydrous MgSO4, filtered and
then concentrated to obtain 11 as a white solid. Yield: 97%. 1H NMR
(400 MHz, CDCl3): δ 0.87 (t, J = 6.4 Hz, 3H, −CH2CH3), 1.24−1.42
[m, 17H, −C(CH3)3, (CH2)4], 2.85, 2.94 (ds, 3H, −NCH3), 3.23−
3.53 (dq, J = 6.8 Hz, 2H, −NCH2−), 5.36−5.40 (m, 1H, NHCO),
5.87 (s, 1H, C*H), 6.05 (m, 1H, −OH), 7.69 (s, 2H, Ar).
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Scheme 1. Synthesis of Monomers 1−5
Scheme 2. Acetylenic Coupling Polymerization of Monomers 1−5
3′,5′-Di(4″-ethynylphenyl)-4′-methoxy-N-α-tert-butoxycarbonyl-
D-phenylglycine Hexylamide (5). This compound was synthesized
from 11 and 4-[(trimethylsilyl)ethynyl]phenylboronic acid pinacol
ester in a manner similar to 1. Yield: 41% (white solid). Mp: 112−113
°C. [α]D: −68.1° (c = 0.10 g/dL, THF). IR (KBr): 3296, 2957, 2931,
2371, 2107, 1655, 1508 cm−1. 1H NMR (400 MHz, CDCl3): δ 0.81 (t,
J = 5.2 Hz, 3H, −CH2CH3), 1.16−1.39 [m, 17H, −C(CH3)3, (CH2)4],
3.08 (s, 3H, −OCH3), 3.12 (s, 2H, −CCH), 3.17−3.22 (m, 2H,
−NCH2), 5.42 (br, 1H, C*H), 6.11 (br, 1H, CONH), 6.53 (br, 1H,
NHCH2), 7.30−7.45 (10H, Ar). 13C NMR (100 MHz, CDCl3): δ 14.0
(−CH2CH3), 22.5, 26.4, 29.4, 31.4 [(CH2)4], 28.3 [−C(CH3)3], 57.0
(C*H), 60.4 (−OCH3), 77.7 (−CCH), 80.2 [−C(CH3)3], 83.5
(−CCH), 121.0, 128.9, 129.1, 131.8, 134.7, 135.1, 138.4 (Ar), 154.5
(CONH), 170.0 (OCONH). HRMS. (m/z): [M + H]+ calcd for
C36H41N2O4, 565.3061; found, 565.3056.
Polymerization. All polymerizations were carried out in a glass
tube equipped with a three-way stopcock under oxygen. In a typical
experiment, a solution of Pd(OAc)2 (0.9 mg, 2 μmol) in THF (1 mL)
and a solution of CuI (0.76 mg, 2 μmol) in THF (1 mL) were added
to a mixture of a monomer (0.2 mmol) and 1,4-diazabicyclooctane
(0.3 mmol) under oxygen, and the resulting solution was kept at 0 °C
for 6−48 h. Then, the reaction mixture was poured into a large volume
of MeOH to precipitate the polymer. It was separated by filtration
using a membrane filter (ADVANTEC H100A047A) and dried under
reduced pressure.
Spectroscopic Data of the Polymers. Poly(1). IR (KBr): 3530,
3297, 2927, 2856, 2372, 2209, 1656, 1162 cm−1. 1H NMR (400 MHz,
CDCl3): δ 0.80 (br, 3H, −CH2CH3), 1.13−1.35 [m, 17H, −C(CH3)3,
(CH2)4], 7.40−8.06 (br, 10H, NHCO, NHCH2, OH, Ar).
Poly(2). IR (KBr): 3530, 3413, 2929, 2858, 2369, 2217, 1671, 1163
1
cm−1. H NMR (400 MHz, CDCl3): δ 0.84 (br, 3H, −CH2CH3),
1.23−1.40 [m, 17H, −C(CH3)3, (CH2)4], 7.35−7.78 (br, 10H,
NHCO, NHCH2, OH, Ar).
Poly(3). IR (KBr): 3530, 2409, 2928, 2869, 2369, 2202, 1648, 1166
1
cm−1. H NMR (400 MHz, CD3OD): δ 0.85 (br, 3H, −CH2CH3),
1.23−1.40 [m, 17H, −C(CH3)3, (CH2)4], 2.94−2.95 (ds, 3H,
−NCH3), 7.35−7.62 (br, 10H, NHCO, OH, Ar).
1
Poly(4). IR (KBr): 3422, 2927, 2370, 2212, 1655, 1165 cm−1. H
NMR (400 MHz, CDCl3): δ 0.84 (br, 3H, −CH2CH3), 1.23−1.39 [m,
17H, −C(CH3)3, (CH2)4], 2.88−2.95 (ds, 3H, −NCH3), 7.34−7.78
(br, 10H, NHCO, OH, Ar).
Poly(5). IR (in CHCl3): 3301, 2928, 2857, 2371, 2208, 1656, 1163
1
cm−1. H NMR (400 MHz, CDCl3): δ 0.84 (br, 3H, −CH2CH3),
1.23−1.39 [m, 17H, −C(CH3)3, (CH2)4], 3.11 (br, 3H, −OCH3),
7.62−7.91 (br, 10H, NHCO, NHCH2, Ar).
Computation. All calculations were performed with the
GAUSSIAN 09 program,18 EM64L-G09 Rev C.01, running on the
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a
Table 1. Acetylenic Coupling Polymerization of Poly(1)−Poly(5)
b
c
c
run
monomer
solvent
[M]0 (M)
temp (°C)
time (h)
yield (%)
Mn
Mw/Mn
1
1
1
1
1
1
1
1
2
3
4
5
CH2Cl2
DMF
DMF
DMF
DMF
THF
THF
THF
THF
THF
THF
0.05
0.05
0.05
0.05
0.10
0.10
0.10
0.10
0.10
0.10
0.10
30
30
0
24
3
36
57
8
d
d
d
2
d
3
2
17 000
34 000
d
2.3
4.0
d
4
0
24
24
24
48
48
6
8
5
0
e
6
0
70
59
75
53
89
86
12 000
22 000
60 000
18 000
43 000
20 000
3.2
2.6
5.4
3.6
4.2
4.6
7
0
8
0
9
0
10
11
0
24
0
4
a
b
c
Conditions: Pd(OAc)2:CuI:1,4-diazabicyclooctane =1:1:150, [M]0/[Pd(OAc)2] = 50. MeOH-insoluble part. Estimated by SEC (10 mM LiBr
d
e
solution in DMF, polystyrene standards). Insoluble in common organic solvents. Not determined.
supercomputer system, Academic Center for Computing and Media
Studies, Kyoto University. The energies were calculated by the
integrated molecular orbital and molecular mechanics method
(ONIOM),19 in which density functional theory (DFT)20 with the
M06-2X functional21 in conjunction with the 6-31G* basis set and the
molecular mechanics (MM) method with universal force filed (UFF)22
were employed for higher and lower layers, respectively. Transition
electric dipole moment and excitation energies were calculated by the
time-dependent (TD) DFT method23 with M06-2X/6-31G*. The
spectra were simulated by calculating 20 excited states, using Gaussian
band shapes with a half bandwidth of 20 nm.
polymeric mass precipitated at the late stage of the polymer-
ization. In this case, the polymer yield decreased to 59%, while
the Mn increased to 22,000. Monomers 2−5 were polymerized
in THF at 0 °C as well, wherein the polymerization times were
adjusted to obtain DMF- and THF-soluble polymers in good
1
yields (runs 8−11). The IR and H NMR spectra of the
polymers were very consistent with the structures linked by
diacetylene moieties. The IR spectral patterns of the solvent-
soluble and insoluble polymers were almost identical. It seems
that the solvent-insoluble parts are high molecular weight
polymers, and there is no significant structural difference
between the solvent-soluble and insoluble polymers.
RESULTS AND DISCUSSION
■
Chiroptical Properties of Poly(1)−Poly(5). CD and
UV−vis spectra of the polymer samples (runs 7−11) were
measured to obtain information on the chiroptical properties.
Poly(1) exhibited intense CD signals at the absorption region
of the main chain chromophore around 340−380 nm in THF−
H2O mixtures, while monomer 1 did not (Figure 1). This result
Monomer Synthesis and Polymerization. Scheme 1
illustrates the synthetic procedures for monomers 1−4. First,
compounds 9 and 10 were synthesized by protection of the
amino group of 4-hydroxy-D-phenylglycine with a tert-
butoxycarbonyl group, amidation of the carboxy group and
3,5-diiodination of the phenyl group. Then, monomers 1−4
were synthesized by the Suzuki−Miyaura coupling of 9 and 10
with 3- and 4-[(trimethylsilyl)ethynyl]phenylboronic acid
pinacol esters. Monomer 5 was synthesized via 11, which was
obtained by methyl etheration of the phenol group of 9. The
1
monomer structures were confirmed by IR, H and 13C NMR
spectroscopy, high-resolution mass spectrometry, and elemen-
tal analysis.
The acetylenic coupling polymerization of 1−5 was carried
out using Pd and Cu catalysts in the presence of DABCO in
CH2Cl2, DMF and THF at 0 and 30 °C under oxygen for 4−48
h (Scheme 2). The corresponding polymers [poly(1)−
poly(5)] with Mn’s ranging from 12 000 to 60 000 were
obtained in 8−89% yields as listed in Table 1. First, the
polymerization of 1 was carried out in CH2Cl2 at 30 °C for 24
h, [M]0 = 0.05 M (run 1). The polymer was obtained in 36%
yield, but it was insoluble in common organic solvents
including DMF and THF. When DMF was used as a solvent,
the polymer yield increased to 57%, but the obtained polymer
was still solvent-insoluble (run 2). A DMF- and THF-soluble
polymer was obtained by decreasing the temperature and time
to 0 °C and 2 h (Run 3), although the yield was low (8%).
Extension of polymerization time to 24 h did not improve the
polymer yield (run 4). Increase of initial monomer concen-
tration to 0.10 M resulted in formation of a solvent-insoluble
polymer (run 5). On the other hand, the polymerization of 1 in
THF with [M]0 = 0.10 M at 0 °C for 24 h gave a DMF- and
THF-soluble polymer with Mn = 12,000 in 70% yield (run 6).
When the polymerization time was extended to 48 h (run 7), a
Figure 1. CD and UV−vis spectra of poly(1) and monomer 1
measured in THF−H2O mixture (H2O content 50%, c = 0.04 mM) at
room temperature.
indicates that poly(1) forms a chiral higher-order structure in
the solvent. Poly(1), poly(3), and poly(5) exhibited strong
intensity CD signals, while the signals for poly(2) and poly(4)
were less intense (Figures S1 and S2, Supporting Information).
The Kuhn dissymmetry factor24 (g = Δε/ε) of poly(1),
poly(3), and poly(5) at the ε maxima increased with increasing
H2O content in THF−H2O mixtures to reach a maximum at
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H2O concentrations of 50%, 80% and 50%, respectively (Figure
2). The g-values decreased with further increase in H2O
Figure 3. CD and UV−vis spectra of poly(1) measured in THF−H2O
mixture (H2O content 50%, c = 0.04 mM) at various temperatures.
Figure 2. g-values of poly(1)−poly(5) at the ε maxima measured in
THF−H2O mixtures with various compositions (c = 0.04 mM) at
room temperature.
(H2O content 50%) (Figure 4). The CD intensity around 360
nm increased with increasing concentration of poly(1). The
content. On the other hand, the g-values of poly(2) and
poly(4) remained small irrespective of H2O content. These
results suggest that p-linked poly(1), poly(3), and poly(5) form
chiral higher-order structures possibly aggregation in THF−
H2O mixture, and the chiral intensities depend on the solvent
composition, while m-linked poly(2) and poly(4) do not form
such chiral structures. It is considered that the m- and p-
linkages of the phenyleneethynylene moieties remarkably
influence the formation of higher-order structures, in a manner
similar to m- and p-phenylene-linked polyfluorene.25 In THF,
poly(1) and poly(3) exhibited negligibly small CD signals. On
the other hand, poly(5) exhibited intense CD signals, which
were still observed after filtration with a membrane filter
(Figure S3). It seems that poly(5) adopts a unimolecularly
chiral higher order structure such as helix in THF. The CD
intensity (g-value) of poly(5) became high by the addition of
H2O mixture as shown in Figure 2, presumably due to the
formation of aggregates as mentioned above. It is likely that the
aggregation only partly contributes to the CD signal, and
therefore the UV signal does not show the typical feature of an
aggregation band. Poly(5) exhibited a positive CD signal at 371
nm, 6 nm longer wavelength than that of poly(1), and a
negative CD signal at 332 nm in THF/H2O = 1/1 (Figure S1),
which was not observed for poly(1). The way of aggregation
poly(5) seems to be somewhat different from that of poly(1) in
the solvent, definitely due to the difference between −OMe and
−OH but the concrete reason is unclear. Poly(3) having
−(CO)NCH3− group showed a weak CD signal compared
with poly(1) having −(CO)NH− group. It indicates that the
amide hydrogen bond plays a truly major role in forming a
higher order structure, as opposed to the carbamate.
Figure 4. CD and UV−vis spectra of poly(1) measured in THF−H2O
mixture (H2O content 50%) with various concentrations.
CD and UV−vis signals of poly(1) disappeared after filtration
using a membrane filter with a pore size of 0.45 μm (Figure 5).
Next, the CD and UV−vis spectra of poly(1) were measured
in THF/H2O solvent (H2O content 50%) at various temper-
atures (Figure 3). The intensity of the CD signal around 360
nm gradually decreased as temperature increased from 0 to 60
°C, while the UV−vis signal maintained almost the same
pattern. Once the temperature was raised to 60 °C, the CD
signals did not return to the original values when the samples
were cooled to 20 °C. This result indicates that the higher-
order structure of poly(1) irreversibly changes when the
temperature is raised.
Figure 5. CD and UV−vis spectra of poly(1) measured in THF−H2O
mixture (H2O content 50%, c = 0.04 mM) before and after filtration
using a membrane filter with a pore size of 0.45 μm.
The CD and UV−vis spectra of poly(1) were further
measured at various concentrations in THF−H2O mixture
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These results indicate that the polymers do not adopt a
unimolecularly chiral higher-order conformation, such as a
predominantly one-handed helix, but that they form chiral
aggregates.
Table 3. On the other hand, m-linked poly(2) and poly(4)
emitted almost no fluorescence. Similar trends were observed in
a
Table 3. Optical Data of Poly(1)−Poly(5)
Previous researchers have reported that m-phenyleneethyny-
lene oligomers carrying nonpolar chiral side chains maintain the
same conformation and aggregation state in nonpolar
solvents.26 Aggregation plays an important role in the twist
sense bias that coincides with the occurrence of optical activity,
and hydrophobic interactions are used for controlling the
ordering of the oligomers. In the present study, DLS
measurements were performed to obtain information on
aggregation of the polymers. No particle was determined in a
poly(1) solution in THF, indicating the absence of aggregates.
On the contrary, the presence of particles with Z-average =113
nm was confirmed in a poly(1) solution in THF−H2O mixture
(H2O content 50%). In a similar fashion, particles with Z-
averages ranging from 70 to 421 nm were observed in the cases
of poly(2)−poly(4), as summarized in Table 2. The Z-values of
b
c
polymer
λabs (nm)
λemi (nm)
Φemi (%)
poly(1)
poly(2)
poly(3)
poly(4)
poly(5)
337.5
335.5
336.0
334.0
332.0
383, 404
−
383, 404
14.8
0.4
12.5
0.2
−
373, 490
8.1
a
b
c
Measured in THF. Excited at the λabs in the left column. Measured
using anthracene as a standard (Φemi = 0.27) in EtOH. The values
were corrected using the refractive indices of THF and EtOH.
the CD and UV−vis spectra of poly(1)−poly(5) as mentioned
above. The fluorescence quantum yields of aggregated
conjugated polymers tend to be small compared with those
of less-aggregated states28 and monomeric model compounds,
due to radiationless deactivation and/or intersystem crossing.29
In the present study, poly(5) chirally aggregated in THF and
emitted fluorescence, but the intensity was lower than that of
poly(1) and poly(3), neither of which forms chiral aggregates
in THF. This is also supported by the fluorescence spectra of
monomer 1 measured in THF and THF/H2O = 1/1 (Figure
S5). In THF, monomer 1 showed a single fluorescence peak at
386 nm. In THF/H2O = 1/1, the peak intensity decreased and
another peak assignable to aggregate-derived fluorescence
appeared at 510 nm.
Conformational Analysis. In order to obtain information
on aggregation of the polymers, DFT calculations of aggregates
of monomers 1 and 2 were carried out as model systems at the
M06-2X/6-31G* level. M06-2X functional is superior com-
pared with commonly used B3LYP functional in estimating
noncovalent interactions including π-stacking and hydrogen
bonding. Two-layer ONIOM calculations were employed to
save CPU time; tert-Bu and hexyl groups were placed in the low
layer assigned to MM calculations. Chart 1 shows how
aggregation of the monomers was modeled. Dummy atoms
were placed at the center of the tetrasubstituted benzene rings
a
Table 2. DLS Measurement of Poly(1)−poly(5)
H2O content of solvent (THF−H2O
Z-average
(nm)
polymer
mixture, %)
PDI
poly(1)
0
50
20
80
20
b
113
235
70
b
0.101
0.069
0.040
0.141
c
poly(2)
poly(3)
poly(4)
poly(5)
421
20
c
a
b
c
c = 0.04 mM. No particle was determined. Could not be
determined.
m-linked polymers [poly(2), poly(4)] were larger than those of
the p-linked counterparts [poly(1), poly(3)], presumably
because aggregate formation is favorable for the largely bent
structures of the m-linked polymers. m-Linked poly(2) and
poly(4) possibly adopt multiple conformations in contrast to
the p-linked counterparts [poly(1) and poly(3)].27 The former
two polymers showed no CD signal in THF/H2O upon raising
the H2O content, while the latter two polymers showed CD
signals assignable to chiral aggregation as summarized in Figure
2. It is apparent that the former two polymers also formed
aggregates judging from the DLS data (Table 2), but the
aggregation is not chirally regulated.
Chart 1. Aggregation of 1 (p-,p′-Diethynyl) or 2 (m-,m′-
a
Diethynyl) Molecules
The p-linked polymers poly(1), poly(3), and poly(5)
emitted blue colored fluorescence in THF (Figure 6), with
fluorescence quantum yields of 8.1% to 14.8%, as listed in
a
X: dummy atom placed at the center of the tetrasubstituted benzene
ring. Green and blue dotted lines represent π-stacking and hydrogen
bonding, respectively. The intermolecular distance between the two
molecules is illustrated longer than the starting geometries of the DFT
calculations for clarity.
Figure 6. Fluorescence spectra of poly(1)−poly(5) measured in THF
(c = 0.004 mM) at room temperature, excited at the λmax
.
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Figure 7. Initial and optimized geometries of tetrameric aggregate of 1, top and side views. Dummy atoms are indicated in violet. In the initial
geometry, adjacent molecules are separated by 3.73 Å and rotated by +18.0° at the dummy atoms. The high and low layers for the ONIOM are
indicated with tube and wireframe models, respectively.
to construct the initial geometry accurately. The carbonyl
groups of amides and carbamates were located vertical to the
tetrasubstituted benzene rings in an antiparallel manner to form
regulated hydrogen-bonding strands.
bonding between amide/carbamate moieties, and π-stacking
between the benzene rings.
DFT calculations were also carried out from starting
geometries rotated by −18°. As shown in Figure 8, the
optimized geometries were less stable compared to the
counterparts started from +18° for both 1 and 2. This result
is explained by the distance between the molecules. As
summarized in Table 4, the average interatomic distance of
N−H···OC of amides and carbamate moieties ranged from
1.82 to 2.09 Å, indicating the presence of hydrogen bonding.
There was no clear difference of N−H···OC distances of the
optimized geometries between the conformers initially with X−
X torsions of +18.0° and −18.0° in the series of 1 and 2. On the
other hand, the X···X distances between the conformers
initialized at +18.0° torsion were around 3.8 Å, shorter than
those of −18.0° counterparts (X···X distances >4.0 Å in most
cases). The X···X distances (3.8 Å) of the former aggregates
were comparable to the values observed for π-stacking between
aromatic rings.31 Consequently, the conformers initialized at
+18.0° torsion are more efficiently stabilized by π-stacking
between the benzene rings compared to the conformers
initialized at −18.0°. It is likely that this kind of aggregation
also occurs between the polymer molecules. The relative
energies of 2−6-mers of 2 are slightly smaller than those of the
corresponding 2−6-mers of 1 in Figure 8. This may explain the
larger aggregation tendency, as well as the smaller fluorescence
quantum yield of poly(2) compared to poly(1).
The left two pictures in Figure 7 are the starting geometries
of the tetrameric aggregate of 1, top and side views, in which
adjacent molecules are separated by 3.73 Å (X···X distance) and
rotated by +18.0° (torsional angle at X−X) at the dummy atom
placed at the center of the tetrasubstituted benzene ring.30
Thus, the aggregate forms a columnar shaped helically stacked
structure. After geometry optimization, the average distance
between the dummy atoms became 3.94 Å (the right two
pictures in Figure 7, top and side views). It is considered that π-
stacking contributes to association. The average rotation
became +19.4°, almost the same as the starting geometry
(+18.0°). The >N−H···OC< distance at the amide groups
between adjacent molecules was 1.86 Å on average after
geometry optimization, indicating the presence of intermolec-
ular hydrogen bonding between the amide groups. In a similar
fashion, the >N−H···OC< distance at the carbamate groups
between adjacent molecules became 1.97 Å on average,
indicating the presence of hydrogen bonding between the
carbamate groups as well. The specific data of the dimeric−
hexameric aggregates are listed in Table 4. Figure 8 depicts the
relationship between the energy per molecule and number (n)
of molecules of 1. The energy decreased with increasing n, and
is likely to be saturated around n = 6. The molecules were
stabilized by association based on intermolecular hydrogen
The TD-DFT method is a cost-efficient and accurate method
to simulate electronic spectra of optically active molecules.32
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Figure 9 shows the CD and UV−vis spectra of a stacked 4-mer
of 1 simulated by the TD-DFT (M06-2X/6-31G*). The
Table 4. Data of Geometries of Dimeric (n = 2)−Hexameric
(n = 6) Aggregates of 1 and 2 Optimized by M06-2X/6-31G*
average interatomic
distance (Å)
N−H···OC
a
torsional angle of X···X
a
compound
n
amide carbamate X···X
(deg)
1 (+18.0)
2
3
4
5
6
2
3
4
5
6
2
3
4
5
6
2
3
4
5
6
2.02
1.87
1.86
1.83
1.85
2.03
2.02
1.89
1.91
1.82
1.99
1.86
1.82
1.86
1.90
1.98
1.85
1.86
b
2.00
1.91
1.97
1.90
1.96
2.04
1.89
1.87
1.88
1.99
2.09
2.03
1.87
1.97
1.95
1.89
1.94
2.00
b
3.81
3.83
3.94
3.87
3.86
3.85
4.27
4.02
4.70
4.14
3.66
3.88
3.75
3.90
4.03
3.81
4.21
4.07
b
+13.7
+22.6
+19.4
+29.3
+25.6
+14.6
−38.0
−34.0
−28.8
−29.5
+5.6
1 (−18.0)
2 (+18.0)
2 (−18.0)
+29.7
+36.2
+27.5
+21.0
−0.6
−29.6
−16.8
b
b
b
b
b
a
X is a dummy atom positioned at the center of the benzene ring.
Not determined.
b
Figure 9. TD-DFT (M06-2X/6-31G*) simulated CD and UV−vis
spectra of tetrameric aggregates of 1 (Rvel and f are divided by 4 as the
values per monomer unit, nstates = 20, half-width wavelength of
Gaussian distribution =20 nm). (a) Aggregate with a right-handed
twist. (b) Aggregate with a left-handed twist.
Figure 8. Relationship between the energy per molecule and number
conformers initiated from +18° (right-handed) and −18° (left-
handed) are predicted to show CD signals with positive and
negative signs at 220−350 nm, respectively. As depicted in
Figure 1, poly(1) exhibits a positive CD signal around 350 nm.
As shown in Figure 10, the π-electrons are delocalized over the
phenyleneethynylene moieties of the four molecules of 1, which
□
●
(n) of molecules in aggregates of 1 ( ) and 2 ( ). Energy standard: n
= 1. Top: Optimized from geometries with +18° rotation. Bottom:
Optimized from geometries with −18° rotation.
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the CD simulation of the model system for chirally aggregated
polymer molecules. The structural differences between the p-
and m-linkages of the phenyleneethynylene moieties remark-
ably influenced the formation of higher-order structures. While
m-linked poly(2) and poly(4) emitted negligibly small
fluorescence, p-linked poly(1), poly(3), and poly(5) emitted
fluorescence with quantum yields ranging from 8.1−14.8%. The
DLS and fluorescence spectra in conjunction with DFT
calculations indicate a much greater aggregation tendency for
the more largely bent structures of m-linked poly(2) and
poly(4), compared with the structures of p-linked poly(1),
poly(3) and poly(5).
ASSOCIATED CONTENT
* Supporting Information
■
S
Additional CD and UV−vis spectra of poly(1)−poly(5)
(Figures S1−S3) and monomers 1 and 2 (Figure S4), UV−
vis and fluorescence spectra of 1−5 (Figures S5−S7), optical
data of 1−5 (Table S1), and Cartesian coordinates for 1, 2 and
the dimeric−hexameric aggregates, initial and M06-2X/6-
31G*-optimized conformers. This material is available free of
charge via the Internet at http://pubs.acs.org.
Figure 10. Shapes of MO 586 (the third highest occupied MO) of a
right-handed twist tetrameric aggregate, corresponding to Figure 7,
right. Hydrogen atoms are omitted for clarity.
AUTHOR INFORMATION
■
Corresponding Author
*(F.S.) E-mail: sanda@kansai-u.ac.jp.
supports the presence of π-stacking. This MO 586 is the third
highest occupied MO, which strongly affects the excited state
corresponding to the absorption peak at 286 nm (Rvel = 319, f =
0.0912) shown in Figure 9 (a). The simulated positive CD
signal around 290 nm is assignable to the absorption due to the
phenyleneethynylene moieties stacked with a right-handed
twist. It is therefore likely that poly(1) aggregates also have a
right-handed twist in a fashion similar to the model system. The
simulated positive CD peak assignable to the phenyl-
eneethynylene moieties is positioned at ca. 80 nm shorter
wavelength compared with poly(1). This is explained by the
short conjugation of the tetrameric aggregate compared with
the polymer. In fact, monomer 1 exhibits λmax at a wavelength
80 nm lower than that of poly(1) as shown in Figure 1, which
agrees well with the value of the difference simulated by the
TD-DFT.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This research was supported by 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. The authors are
grateful to Prof. Kazunari Akiyoshi and Prof. Shinichi Sawada
for DLS measurement 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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