Synthesis and optical properties of 1,1-binaphthyl-thiophene alternating copolymers with main chain chirality

Article abstract of DOI:10.1039/c2jm35594a

A series of axially chiral 1,1-binaphthyl-thiophene copolymers were synthesized by Migita-Kosugi-Stille polycondensation and electrochemical polymerization. The polymers exhibit circular dichroism not only in the ultraviolet region but also in the visible region, both in solution and film states. Optical and electrochemical properties of the polymers are in good agreement with theoretical results obtained by density functional theory calculations. Although effective conjugation of the polymers was cleaved at the orthogonally oriented two naphthalene rings in the 1,1-binaphthyl unit, the electrochemically synthesized polymer film shows good electrochemical redox behavior with repeatable changes in optical absorption and circular dichroism. The changes in circular dichroism may reflect a change in the dihedral angle of the binaphthyl unit caused by reorientation of conjugated main chains between oxidized and neutral states. The Royal Society of Chemistry 2012.

Full text of DOI:10.1039/c2jm35594a

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PAPER
Synthesis and optical properties of 1,1-binaphthyl–thiophene alternating
copolymers with main chain chirality†
*
Kohsuke Kawabata and Hiromasa Goto
Received 17th August 2012, Accepted 14th September 2012
DOI: 10.1039/c2jm35594a
A series of axially chiral 1,1-binaphthyl–thiophene copolymers were synthesized by Migita–Kosugi–
Stille polycondensation and electrochemical polymerization. The polymers exhibit circular dichroism
not only in the ultraviolet region but also in the visible region, both in solution and film states. Optical
and electrochemical properties of the polymers are in good agreement with theoretical results obtained
by density functional theory calculations. Although effective conjugation of the polymers was cleaved
at the orthogonally oriented two naphthalene rings in the 1,1-binaphthyl unit, the electrochemically
synthesized polymer film shows good electrochemical redox behavior with repeatable changes in optical
absorption and circular dichroism. The changes in circular dichroism may reflect a change in the
dihedral angle of the binaphthyl unit caused by reorientation of conjugated main chains between
oxidized and neutral states.
1,10-binaphthyl derivatives well-known as chiral molecules with
Introduction
axial chirality into conjugated polymer backbones.21–26
In this study, we report synthesis of a series of binaphthyl–
thiophene copolymers via Migita–Kosugi–Stille poly-
condensation and electrochemical polymerization. Modifying
copolymerized thiophene units gave fine-tuned optical and elec-
trochemical properties of resulting polymers, which are in good
agreement with theoretical results obtained by density functional
theory (DFT) calculations. The electrochemically synthesized
polymers exhibited good electrochemical redox behavior result-
ing in electrochromism accompanied by repeatable changes in
CD. Thus tuning chiroptical activity via redox driven mechanical
change in the dihedral angle of binaphthyl units in the main chain
can be achieved.
Conjugated polymers are of great interest as nonlinear optical,
fluorescent, and chromic materials.1–3 Optical properties of
conjugated polymers can be modulated by modifying the
chemical structure of the polymers.4 Among various kinds of
conjugated polymers, polythiophene derivatives are the most
promising because of their thermal and environmental stability
and structural versatility,5 examples of which include, poly-
(3-alkylthiophene)s,6 poly(3,4-ethylenedioxythiophene)s7 and
polyisothianaphthenes.8,9 The optical properties of conjugated
polymers are also tunable by electrochemical redox processes
which lead to repeatable changes in the electronic state and
chemical structure of the polymers.
Chiral polymers have also attracted much attention because of
their chiroptical activity,10 asymmetric separation11 and recog-
nition properties,12 and various types of chiral structure.
Particularly, chiral conjugated polymers can exhibit circular
dichroism (CD) in the visible region, which can be applied to
chiroptical materials. To date, a great number of chiral conju-
gated polymers, such as chiral side chain type,13–16 helical main
chain type,17 and chiral aggregation type polymers,18–20 have been
synthesized. One of the most effective ways to induce such chiral
structures in polymers is to introduce chiral moieties into the
polymer structures. To date, several conjugated polymers with
main chain axial chirality were synthesized by introducing
Experimental
Characterization
The chemical structures of the monomers were confirmed by 1H
and 13C NMR spectroscopy with AVANCE-600 (Bruker), JNM-
ECS 400 (Jeol), and JNM-EX 270 (Jeol) spectrometers. The
chemical shifts in deuterated chloroform were reported against a
tetramethylsilane internal standard. Ultraviolet-visible (UV-vis)
absorption spectra were taken with a UV-3100PC (Shimadzu).
The concentration of the polymers was 2 mg Lꢀ1 in THF.
Polymer films were cast from chloroform solutions. CD
measurements were performed with a JASCO J-720. Photo-
luminescence (PL) spectra were collected employing an F-4500
spectrometer (Hitachi). IR absorption spectra were obtained
with an FT/IR 550 (JASCO) by using the KBr method. Matrix-
assisted laser desorption ionization time-of-flight mass spec-
trometry (MALDI-TOF-MS) analysis was conducted using a
Division of Materials Science, Faculty of Pure and Applied Sciences,
University of Tsukuba, Tsukuba, Ibaraki, 305-8573, Japan. E-mail:
gotoh@ims.tsukuba.ac.jp; Fax: +81-298-53-4490
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c2jm35594a
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TOF/TOF 5800 (AB SCIEX) with dithranol as a matrix. Cyclic
voltammetry measurements were carried out with a mAUTO-
LAB TYPE III (ECO Chemie). Electrolyte solutions contained
0.1 M of TBAP in acetonitrile. Molecular weights of the poly-
mers were determined by GPC against polystyrene standards by
using THF as the solvent with 5 mm MIXED-D column (Polymer
Laboratories), PU-980 HPLC pump (Jasco), and MD-915
multiwavelength detector (Jasco).
the mixture under reduced pressure gave isothianaphthene (4) as
a white solid (1.00 g, 7.5 mmol, yield ¼ 75%). Freshly sublimated
compound 4 (1.00 g, 7.5 mmol) and TMEDA (2.17 g, 18.6 mmol)
were dissolved in THF (22 mL) under an argon atmosphere. To
the mixture was added 2.5 M n-butyllithium in hexane (7.0 mL,
ꢁ
17.5 mmol) at ꢀ78 C through a dropping funnel over 10 min,
and then stirred vigorously at room temperature for 1 h. The
mixture was cooled to ꢀ78 ꢁC again, and a solution of trime-
thylstannyl chloride (3.80 g, 19.0 mmol) in THF (10 mL) was
added dropwise to the mixture. The mixture was slowly warmed
up to room temperature and stirred for 12 h. The mixture was
poured into a saturated ammonium hydrogen carbonate aqueous
solution (70 mL) and separated. The aqueous layer was extracted
with ether twice and the combined organic layer was dried over
magnesium sulfate. After evaporation of the solvent, recrystal-
lization from petroleum ether (30–70 ꢁC) gave a pale yellow solid
Monomer synthesis
2,5-Di(trimethylstannyl)thiophene (1). To a solution of thio-
phene (2.10 g, 25.0 mmol) and N,N,N0,N0-tetramethylethylene-
diamine (TMEDA) (11.62 g, 100 mmol) in THF (50 mL) was
added 1.6 M n-butyllithium in hexane (32.8 mL, 52.5 mmol)
through a dropping funnel over 10 min at ꢀ78 ꢁC under an argon
atmosphere. After addition of the butyllithium, the reaction
mixture was refluxed at 50 ꢁC for 1 h to afford a white slurry, and
1
(1.73 g, 3.75 mmol, yield ¼ 50%). H NMR (400 MHz, d from
ꢁ
TMS (ppm), CDCl3): d 0.49 (s, 18H, –CH3), 7.07 (dd, 2H, 5,6-H,
J ¼ 3.0, 6.6 Hz), 7.67 (dd, 2H, 4,7-H, J ¼ 2.8, 6.8 Hz). 13C NMR
(100 MHz, d from TMS (ppm), CDCl3): d 8.0, 122.6, 123.9, 137.0,
146.7.
cooled to ꢀ78 C again. To the slurry was added a solution of
trimethylstannyl chloride (10.46 g, 52.5 mmol) in THF (30 mL)
dropwise over 10 min. Then, the mixture was gradually warmed
up to room temperature and stirred for 12 h. After the reaction,
water (50 mL) was added to the mixture and the mixture was
extracted with ether. The organic layer was dried over magne-
sium sulfate and filtered. After evaporation of the solvent,
recrystallization from methanol gave a white solid (2.43 g, 10.6
2,5-Di(tributylstannyl)-3,4-ethylenedioxythiophene (6). To a
solution of 3,4-ethylenedioxythiophene (3.48 g, 23.5 mmol) and
TMEDA (10.90 g, 93.8 mmol) in THF (50 mL) was added 1.6 M
n-butyllithium in hexane (31.0 mL,ꢁ 49.6 mmol) through a
dropping funnel over 20 min at ꢀ78 C under an argon atmo-
sphere. The mixture was stirred for 1 h at room temperature to
afford a white slurry. To the slurry was added tributylstannyl
chloride (16.5 g, 50.7 mmol) in THF (10 mL) dropwise at ꢀ78 ꢁC,
and the mixture was slowly warmed up to room temperature.
After stirring for 12 h, water (100 mL) was added and the solu-
tion was extracted with ether twice. The organic layer was dried
over magnesium sulfate. After evaporation of the solvent, the
residual liquid was purified by silica gel column chromatography
(eluent: hexane–triethylamine ¼ 19/1) to afford a colorless oil
(0.94 g, 1.4 mmol, yield ¼ 6%), however a large part of the target
compound was decomposed through the column. 1H NMR (270
MHz, d from TMS (ppm), CDCl3): d 0.87 (t, 18H, –CH3, J ¼ 7.26
Hz), 1.05–1.10 (m, 12H, –CH2–CH3), 1.26–1.39 (m, 12H, –CH2–
C2H5), 1.50–1.62 (m, 12H, –CH2–C3H7), 4.11 (s, 4H, O–C2H4–
O). 13C NMR (68 MHz, d from TMS (ppm), CDCl3): d 10.5,
13.7, 27.2, 29.0, 64.5, 115.6, 148.0.
1
mmol, yield ¼ 43%). H NMR (400 MHz, d from TMS (ppm),
CDCl3): d 0.37 (s, 2H, –CH3), 7.37 (s, 2H, 3,5H(thiophene)). 13
C
NMR (100 MHz, d from TMS (ppm), CDCl3): d 8.2, 135.8, 143.0.
1,3-Dihydro-benzo[c]thiophene (2). To a refluxed solution of
sodium sulfide nonahydride (26.14 g, 109 mmol) in ethanol
(250 mL) and water (50 mL) was added 1,2-bischloro-
methylbenzene (12.31 g, 70.3 mmol) through a Soxhlet extractor
and the mixture was refluxed for 40 min. After evaporation of the
solvent, the residual tan-colored oil was extracted with
dichloromethane and water, and the organic layer was dried over
magnesium sulfate. Evaporation of the solvent in vacuo afforded
a yellow solid (5.91 g, 43.4 mmol, yield ¼ 62%). This compound
was used in the next step without further purification. 1H NMR
(400 MHz, d from TMS (ppm), CDCl3): d 4.27 (s, 4H, –CH2–),
7.18–7.27 (m, 4H, 3,4,5,6-H). 13C NMR (100 MHz, d from TMS
(ppm), CDCl3): d 38.0, 124.7, 126.7, 140.3.
1,3-Dihydro-benzo[c]thiophene 2-oxide (3). To a solution of
compound 2 (1.88 g, 13.8 mmol) in ethanol (47 mL) was added a
solution of sodium periodate (2.96 g, 13.8 mmol) in water
(39 mL) dropwise at 0 ꢁC with vigorous stirring. After stirring for
3 h, a white inorganic salt was filtered off. Evaporation of the
solvent followed by recrystallization from ethyl acetate–hexane
gave a colorless plate (1.53 g, 10.1 mmol, yield ¼ 73%). 1H NMR
(400 MHz, d from TMS (ppm), CDCl3): d 4.16 (d, 2H, C–H (the
opposite side of oxygen atom), J ¼ 16.2 Hz), 4.30 (d, 2H, C–H
(the same side of oxygen atom, J ¼ 16.3 Hz), 7.31–7.34 (m, 2H,
4,5-H), 7.38 (dd, 2H, 3,6-H, J ¼ 3.4, 5.4 Hz). 13C NMR (100
MHz, d from TMS (ppm), CDCl3): d 59.4, 126.6, 128.4, 135.1.
5,5-Ditrimethylstannyl-2,20-bithiophene (7). To a solution of
2,20-bithiophene (4.0 g, 24.1 mmol) in THF (110 mL) was added
1.6 M n-butyllithium in hexane (35 mL, 56.0 mmol) dropwise at
ꢁ
ꢀ78 C, and then the mixture was stirred at room temperature
for 1 h. The mixture was cooled to ꢀ78 ꢁC again, and trime-
thylstannyl chloride (11.6 g, 58.2 mmol) in THF (10 mL) was
added dropwise. The mixture was gradually warmed up to room
temperature and stirred for 12 h. The mixture was extracted with
ether twice, and the organic layer was dried over magnesium
sulfate. Purification by silica gel column chromatography
(eluent: hexane) followed by recrystallization from ethanol gave
1
a white solid (5.37 g, 10.9 mmol, yield ¼ 46%). H NMR (400
1,3-Di(trimethylstannyl)isothianaphthene (5). Compound
3
MHz; CDCl3; TMS) d 0.38 (s, 18H, –CH3), 7.08 (d, 2H, J ¼ 3.3
Hz), 7.27 (d, 2H, J ¼ 3.36 Hz). 13C NMR (100 MHz; CDCl3;
TMS) d 8.24, 124.86, 135.85, 137.07, 143.04.
(1.51 g, 9.9 mmol) and aluminum oxide powder (2.40 g, 23.6
mmol) were mixed and finely crushed in a mortar. Sublimation of
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4,40-n-Dioctyl-2,20-bithiophene (8). To a solution of 3-octyl-
thiophene (7.85 g, 40 mmol) and TMEDA (5.12 g, 44 mmol) in
THF (120 mL) was added n-butyllithium (1.6 M in hexane)
(25 mL, 40 mmol) dꢁropwise over 5 min at 0 ꢁC. Then, the mixture
was refluxed at 50 C for 1 h. After the mixture was cooled to
toluene solution was refluxed or 24 h. After the reaction, the
crude product was purified by silica gel (containing 10 v/v% of
potassium carbonate) column chromatography (eluent: hexane–
chloroform ¼ 6/4) to afford a colorless oil (0.096 g, 0.12 mmol,
yield ¼ 61%). 1H NMR (400 MHz; CDCl3; TMS) d 0.87–1.29 (m,
42H, –C9H18–CH3), 1.39–1.43 (m, 4H, –O–CH2–CH2–), 3.91–
3.99 (m, 4H, –O–CH2–), 7.08 (dd, 2H, 4-H(thiophene), J ¼ 3.6,
4.8 Hz), 7.18 (d, 2H, 3,30-H(binaphthyl), J ¼ 8.8 Hz), 7.26 (dd,
2H, 5,50-H(thiophene), J ¼ 1.2, 5.2 Hz), 7.34 (dd, 2H, 3,30-
H(thiophene), J ¼ 1.0, 3.4 Hz), 7.42 (d, 2H, 4,40-H(binaphthyl),
J ¼ 9.2 Hz), 7.48 (dd, 2H, 7,70-H(binaphthyl), J ¼ 1.6, 8.8 Hz),
7.94 (d, 2H, 7,70-H(binaphthyl), J ¼ 8.8 Hz), 8.06 (d, 2H, 5,50-
H(binaphthyl), J ¼ 2.0 Hz). 13C NMR (100 MHz; CDCl3; TMS)
d 14.15, 22.72, 25.70, 29.20, 29.39, 29.53, 29.56, 29.66, 31.94,
69.70, 116.29, 120.45, 122.80, 124.30, 124.37, 124.72, 126.08,
128.00, 129.27, 129.29, 129.47, 133.50, 144.82, 154.78.
ꢁ
ꢀ78 C, copper(II) chloride (5.92 g, 44 mmol) was added in one
portion, and stirred at ꢀ78 ꢁC for 40 min. Then, the reaction
mixture was gradually warmed up to room temperature, and
stirred for 6 h. The reaction was quenched with 2 M hydrochloric
acid (80 mL). The reaction mixture was extracted with ether and
dried over magnesium sulfate. Purification by silica gel column
chromatography followed by recrystallization from hexane gave
1
a pale yellow solid (3.60 g, 9.2 mmol, yield ¼ 46%). H NMR
(400 MHz, CDCl3): d 0.88 (t, 6H, J ¼ 6.9 Hz), 1.27–1.31 (m,
20H), 1.61 (quintet, 4H, J ¼ 7.4 Hz), 2.56 (t, 4H, J ¼ 7.6 Hz), 6.76
(d, 2H, J ¼ 1.2 Hz), 6.98 (d, 2H, J ¼ 1.4 Hz). 13C NMR (100
MHz, CDCl3): d 14.11, 22.68, 29.27, 29.33, 29.44, 30.41, 30.54,
31.90, 118.71, 124.82, 137.39, 144.00.
A general procedure for Migita–Kosugi–Stille polymerization
reaction
4,40-Dioctyl-5,50-bistrimethylstannyl-2,20-bithiophene (9). To a
solution of 8 (2.74 g, 7.0 mmol) in dry THF (120 mL) was added
n-butyllithium (1.6 M in hexane) (9.7 mL, 15.5 mmol) dropwise
Ditrialkylstannylated monomers (0.1 mmol), compound 10
(0.078 g, 0.1 mmol) and tetrakis(triphenylphosphine)palla-
dium(0) (Pd(PPh3)4) (0.006 g, 0.005 mmol) were dissolved in
toluene (1 mL), and the mixture was refluxed at 90 ꢁC. After 72 h,
the mixture was washed with methanol (200 mL) followed by
centrifugation and decantation of the supernatant solution. This
procedure was repeated three times. The residue was dried
in vacuo to afford target polymers: P(Bn-T) (38 mg, yield ¼ 54%),
P(Bn-ITN) (20 mg, yield ¼ 26%), P(Bn-EDOT) (37 mg, yield ¼
49%), P(Bn-BT) (20 mg, yield ¼ 27%), and P(Bn-B8T) (66 mg,
yield ¼ 66%).
ꢁ
ꢁ
over 5 min at ꢀ78 C. After stirring at ꢀ78 C for 15 min, the
mixture was warmed up to room temperature and stirred for 1 h.
ꢁ
After the reaction mixture was cooled to ꢀ78 C again, trime-
thyltin chloride (3.29 g, 16.5 mmol) in THF (16 mL) was added to
the mixture over 5 min. The mixture was gradually warmed up to
room temperature and stirred for 8 h. After the reaction, water
(100 mL) was added to the mixture, and the organic layer was
extracted with ether and dried over magnesium sulfate. Evapo-
ration followed by drying in vacuo gave a yellow oil (4.43 g, 6.2
mmol, yield ¼ 88%). 1H NMR (400 MHz, CDCl3): d 0.37 (s,
18H), 0.88 (t, 6H, J ¼ 6.9 Hz), 1.27–1.31 (m, 20H), 1.58 (quintet,
4H, J ¼ 7.5 Hz), 2.54 (t, 4H, J ¼ 7.8 Hz), 7.11 (s, 2H). 13C NMR
(100 MHz, CDCl3): d 7.88, 14.12, 22.69, 29.26, 29.54, 29.62,
31.89, 32.01, 32.87, 125.98, 130.93, 142.71, 151.60.
Electrochemical polymerization
EP(Bn-BT) was synthesized by electrochemical polymerization
of compound 11 using repeated potential cycling with a three-
electrode system which consisted of an ITO coated glass working
electrode, an Ag/Ag+ reference electrode, and a platinum wire
counter electrode. The electrolyte solution consisted of TBAP
(0.1 M) and the monomer (1.0 mM) in acetonitrile solution. The
scan rate was 100 mV sꢀ1. After polymerization, a yellow thin
film was obtained on the ITO electrode.
(R)-6,60-Dibromo-2,20-didodecyloxy-1,10-binaphthyl (10).
A
solution of (R)-6,60-dibromo-1,10-bi-2,20-naphthol (2.00 g, 4.47
mmol), 1-bromododecane (2.24 g, 9.00 mmol), potassium
carbonate (2.48 g, 18.0 mmol), and potassium iodide (3.00 g,
18.0 mmol) in 2-butanone (50 mL) was refluxed for 24 h. After
the reaction, the mixture was extracted with ether and dried over
magnesium sulfate. The crude product was purified by silica gel
column chromatography (eluent: chloroform–hexane ¼ 1/1) to
afford a colorless oil (1.168 g, 1.50 mmol, yield ¼ 33%). 1H NMR
(400 MHz, CDCl3): d 0.87–1.43 (m, 46H, –C10H20–CH3), 3.86–
3.98 (m, 4H, –O–CH2–), 6.97 (d, 2H, 8,80-H(binaphthyl), J ¼ 8.8
Hz), 7.25 (dd, 2H, 7,70-H(binaphthyl), J ¼ 2.4, 9.2 Hz), 7.40 (d,
2H, 3,30-H(binaphthyl), J ¼ 9.2 Hz), 7.82 (d, 2H, 4,40-
H(binaphthyl), J ¼ 9.2 Hz), 7.99 (d, 2H, 5,50-H(binaphthyl), J ¼
1.6 Hz). 13C NMR (100 MHz; CDCl3; TMS) d 14.15, 22.72,
25.66, 29.14, 29.30, 29.40, 29.51, 29.54, 29.67, 29.69, 31.96, 69.56,
116.42, 117.22, 120.05, 127.13, 128.36, 129.76, 130.20, 132.57,
154.77.
Calculations
The highest occupied molecular orbital (HOMO), the lowest
unoccupied molecular orbital (LUMO) and their energy levels of
model compounds for the polymers in the ground state were
calculated by the density functional theory (DFT) method
employing Becke’s three-parameter set with Lee–Yang–Parr
correlation functional (B3LYP) and the 6-31G* basis set, which
is implemented in Spartan ’04 package.
Results and discussion
Synthesis of monomers
(R)-2,20-Didodecyloxy-6,60-di(2-thienyl)-1,10-binaphthyl (11). A
solution of 10 (0.156 g, 0.20 mmol), 2-tributylstannylthiophene
(0.157 g, 0.42 mmol), and Pd(PPh3)4 (0.01 g, 0.008 mmol) in
A series of bistrialkylstannylated monomers (1, 5, 6, 7, 9) and
(R)-6,60-dibromo-2,20-didodecyloxy-1,10-binaphthyl (10) were
synthesized
as
monomers
for
Migita–Kosugi–Stille
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polycondensation reactions (Scheme 1). The bistrialkylstanny-
lated monomers were prepared via double-lithiation of corre-
sponding thiophene derivatives at ꢀ78 ꢁC followed by quenching
with trialkylstannyl chloride. 10 was synthesized via Williamson
etherification from (R)-6,60-dibromo-1,10-bi-2,20-naphthol and 1-
bromododecane. (R)-2,20-Didodecyloxy-6,60-di(2-thienyl)-1,10-
binaphthyl (11) was synthesized via Stille cross coupling reaction
between 10 and 2-tributylstannylthiophene as a monomer for
electrochemical polymerization.
Scheme 2 Synthesis of polymers.
Table 1 Polymerization results
Mn (ꢂ103)a,b Mw (ꢂ103)a,c Mw/Mn
Yieldd (%)
a
Polymer
P(Bn-T)
P(Bn-ITN)
P(Bn-EDOT) 4.4
P(Bn-BT)
P(Bn-B8T)
4.2
3.9
5.8
4.6
5.6
7.7
4.6
1.4
1.2
1.3
1.6
1.4
54
26
49
27
66
4.8
3.9
a
b
Values against polystyrene standards. Number average molecular
c
d
weight. Weight average molecular weight. Calculated by (Ws/(Wm
Nm)), Ws: weight of the polymer sample (g), Wm: weight of monomer
repeating units of the polymers (g molꢀ1), and Nm: amount of the used
monomer (mol).
ꢂ
Polymerization
Migita–Kosugi–Stille polycondensation reactions between 6,60-
dibromo-2,20-dioctyloxy-1,10-binaphthyl and a series of bis-
trialkylstannylated monomers in the presence of a palladium
catalyst were carried out (Scheme 2). The resulting polymers
were soluble in THF and chloroform. Table 1 summarizes the
results of the polymerization reactions. The GPC measurements
(against a polystyrene standard) using THF as an eluent for the
polymers show number-average molecular weights of around
3900–4800 g molꢀ1 and weight-average molecular weights of
around 4600–7700 g molꢀ1. The chemical structures of the
1
polymers were confirmed by H NMR spectroscopy (Fig. S4–
S8†) and MALDI-TOF-MS (Fig. S9–S13†).
Infrared (IR) absorption spectroscopy
Fig. 1 shows IR spectra of the polymers and the binaphthyl
monomer 10. The polymers show relatively strong absorptions at
2924 and 2852 cmꢀ1 due to C–H stretching of a long alkyl chain
attached to the binaphthyl units. P(Bn-B8T) shows intense
absorption at this band because of an additional long alkyl chain
in the dioctylbithiophene units. The polymers also show an
absorption band at 1092 cmꢀ1, which is due to C–O–C stretching
Scheme
1
Synthesis
of
monomers
(TMEDA;
N,N,N0,N0-
tetramethylethylenediamine).
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at long wavelengths and show red-shift in maximum emission
wavelengths due to development of the conjugation. P(Bn-T)
shows absorption bands at 297 nm and 364 nm and emission
bands at 419 nm and 444 nm, while P(Bn-BT) shows an
absorption band at 404 nm and emission bands at 462 nm and
492 nm. These results are in good agreement with previous
reports on binaphthyl-mono and bithiophene copolymers in
spite of different molecular weights,36,37 indicating that effective
conjugation lengths of the polymers are independent of mole-
cular weight more than several thousands because of limited
conjugation between the orthogonally oriented binaphthyl units.
P(Bn-BT) has a longer effective conjugation in the main chain
than P(Bn-T) because of an increase in the number of thiophene
rings in the monomer unit. On the other hand, P(Bn-B8T) with
flexible alkyl chains as substituents shows an absorption band at
a short wavelength (350 nm) as compared to P(Bn-BT) and P(Bn-
T) although P(Bn-B8T) has two thiophene rings in the monomer
repeat unit. This can be due to the fact that steric hindrance of
alkyl side chains of the thiophene units increases the dihedral
angle between thiophene and naphthalene rings.
Optimized geometry, HOMO, and LUMO of model
compounds in the gas state, which consisted of the thiophene
spacers sandwiched with two 2,20-dimethoxy-1,10-binaphthalene,
were calculated by a DFT method (Fig. 4). The dihedral angle in
optimized geometry of a model compound for P(Bn-B8T) was
calculated to be 46ꢁ, whereas 26ꢁ and 23ꢁ were calculated for the
model compounds of P(Bn-T) and P(Bn-BT), respectively. This
result can explain why P(Bn-B8T) has a shorter effective conju-
gation length as compared to P(Bn-BT) and P(Bn-T), whereas
the dihedral angle between EDOT and naphthalene rings in
P(Bn-EDOT) was calculated to be 15ꢁ. P(Bn-EDOT) shows an
absorption band at 377 nm, which is more intense than that of
P(Bn-T). Generally, isothianaphthene units in conjugated poly-
mers reduce bandgap energy due to conjugation over the fused
benzene ring and stabilization of the quinonoid structure of the
thiophene moiety.38 The DFT calculation results for P(Bn-ITN)
gave the smallest energy gap (3.03 eV) among the polymers,
despite a wide dihedral angle of 40ꢁ between isothianaphthene
and naphthalene rings. Indeed, as expected, the absorption onset
of P(Bn-ITN) in THF is located at longer wavelengths (482 nm)
than for other polymers. However, intensity of the absorption
band at long wavelengths is somewhat low. It should be noted
that in the DFT calculation results for the model compounds,
HOMOs and LUMOs were localized only at coplanar naph-
thalene–thiophene–naphthalene moieties, which indicated
conjugation breaking at the orthogonally oriented naphthalenes.
This limited conjugation is supported by the fact that P(Bn-ITN)
and 1,3-bis-(6-dodecyloxy-naphthalen-2-yl)-isothianaphthene
(compound 13 in ESI†), and P(Bn-B8T) and 5,50-bis-(6-dodecy-
loxy-naphthalen-2-yl)-4,40-dioctyl-[2,20]bithiophenyl (compound
14 in ESI†) show absorption onsets at the same wavelengths,
respectively (Fig. S1 and S2†). Hence, the naphthalene–thio-
phene–naphthalene moieties can be regarded as discrete
chromophores.
Fig. 1 Infrared absorption spectra of the polymers and monomer 10.
in the binaphthyl units. Particularly, P(Bn-EDOT) shows a
relatively strong absorption band at that wavenumber because of
the ethylenedioxy group of the EDOT units. P(Bn-T), P(Bn-BT),
and P(Bn-B8T) show absorption bands at 795–800 cmꢀ1 due to
C–H bending vibration (out of plane) at b-positions of the
thiophene unit.27,28 P(Bn-EDOT) shows a strong absorption
band at 1092 cmꢀ1. An absorption band of P(Bn-ITN) at 1659
cmꢀ1 is assignable to skeletal vibration of a fused benzene ring of
the isothianaphthene unit. These characteristic absorption bands
indicate that the polymers have desired chemical structures.
Optical properties
Fig. 2 shows CD, UV-vis absorption, and PL spectra of the
monomer 10 and the polymers in THF solution and film states.
Absorption maximum and onset and emission maximum wave-
lengths are summarized in Table 2. The binaphthyl monomer
shows a clear couplet with a negative Cotton effect at 245 nm and
a positive one at 233 nm (negative couplet), which corresponds to
a strong optical absorption band at around 240 nm. This couplet
is due to the coupling between 1Bb transitions of two naphthalene
rings of the binaphthyl monomer. The relationship between CD
spectra and conformation of binaphthyl was well discussed both
theoretically and experimentally in the literature.29–35 The sign of
the couplet is determined not only by the (R) or (S) configuration
but also the dihedral angle between the two naphthalene rings.
As for the monomer 10 bearing the (R)-configuration, the
observed negative couplet at around 240 nm indicates that the
dihedral angle between two naphthalene rings is smaller than
100ꢁ. The polymers in THF solution show new absorption bands
In the CD spectra, the polymers show a negative couplet
around 240 nm, indicating that the polymers maintain the axial
conformation at the binaphthyl moiety after polymerization,
whereas the polymers show weak Cotton effects at long wave-
lengths, which are attributable to exciton coupling between
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Fig. 2 CD (upper), UV-vis absorption (solid), and photoluminescence (dashed) spectra of the polymers in THF solution (black) and in film (red) states.
naphthalene–thiophene–naphthalene chromophores. In the film
state, absorption spectra of the polymers were red-shifted. This
red shift was due to an increase of coplanarity in the naphtha-
lene–thiophene–naphthalene unit and development of p-stack-
ing between the chromophores, which are derived from
aggregation in the solid state. The red shift is also observable in
the CD and the PL spectra of the polymers. The negative sign
and splitting width of the couplet at 240 nm of the polymers in
the film state show the same values as the polymers in the solu-
tion, indicating that the dihedral angle between two naphtha-
lenes is not changed after the film formation. The red-shift value
in the PL spectra is greater than that in the UV-vis absorption
spectra after the film formation due to interchromophoric
interaction in the film state.39 Among the series of the polymers,
P(Bn-B8T) shows relatively small changes in absorption, CD,
and PL spectra after film formation. This indicates that the main
chains of P(Bn-B8T) are entangled and have weak inter-
chromophoric interactions in both solution and film states
because of steric hindrance of long alkyl chains at octylthiophene
units. While, in the film, other polymers show increase of the CD
intensity at longer wavelength couplets as compared to the
couplet at 240 nm after film formation, especially for P(Bn-
EDOT). This is due to the strong interchromophoric interaction
of P(Bn-EDOT) which has high coplanarity in the conjugated
chromophore. In P(Bn-T) and P(Bn-BT), negative Cotton effects
were observed at long wavelengths in solution, while the first
positive and new second negative Cotton effects were observed at
long wavelengths in the film.
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Table 2 Maximum absorption wavelengths (lmax), absorption onset
wavelengths (lonset), and maximum emission wavelengths (lem) of the
polymers in THF solution and in the film state
Table
3
Oxidation potentials (Eonset,ox
)
(vs. Fc/Fc+), maximum
absorption wavelength (lmax) in the THF solutions and onset wavelength
(lonset) of optical absorption, HOMO and LUMO values and the energy
gaps (DE) determined from the results of cyclic voltammetry and optical
absorption spectra of the polymers
lmax
(nm) (nm)
lonset lmax,film
(nm)
lonset,film
(nm)
lem lem,film
(nm) (nm)
Polymer
a
b
c
Polymer
Eonset,ox (V) EHOMO (eV) DEopt (eV) ELUMOd (eV)
P(Bn-T)
239
424
482
426
234
429
392
438
419
444
377
470
(sh)b
499
P(Bn-T)
P(Bn-ITN)
0.65 V
0.98 V
P(Bn-EDOT) 0.44 V
ꢀ5.45
ꢀ5.78
ꢀ5.24
ꢀ5.37
ꢀ5.29
2.89
3.16
2.83
2.63
2.83
ꢀ2.56
ꢀ2.62
ꢀ2.41
ꢀ2.74
ꢀ2.46
297
364
240
341
(sh)a
409
237
285
299
367
240
336
(sh)a
P(Bn-ITN)
442
P(Bn-BT)
P(Bn-B8T)
0.57 V
0.49 V
a
Onset oxidation potentials of the polymers calibrated with ferrocene.
The values were calculated from the onset oxidation potentials of the
b
P(Bn-EDOT)
230
300
427
448
469
496
polymers with respect to Fc/Fc+ under the assumption that the Fc/Fc+
redox potential is 4.8 eV below the vacuum level. Calculated from the
c
(sh)b
(sh)b
d
onset wavelength of optical absorption of the polymers. Calculated
377
234
404
240
359
384
243
408
238
353
from optical bandgap energy and onset oxidation potential of the
polymers.
P(Bn-BT)
463
428
472
438
462
492
465
525
550
498
P(Bn-B8T)
a
Absorption shoulder. b Emission shoulder.
Fc/Fc+ redox potential is 4.8 eV below the vacuum level,40 the
HOMO energy levels of the polymers can be calculated by using
the following equation: EHOMO ¼ ꢀ(Eonset,ox + 4.8) eV. The
LUMO energy levels of the polymers can be calculated using
optical energy gaps (DEopt) of the polymers by the following
equation: ELUMO ¼ EHOMO + DEopt. The HOMO and LUMO
energy levels estimated by the electrochemical method, and
optical energy gaps are summarized in Table 3. A comparison of
the experimental results with results obtained by DFT calcula-
tions in an energy diagram is summarized in Fig. 4. Experi-
mentally estimated HOMO and LUMO values show good
agreement with those of DFT calculation results except for P(Bn-
ITN). Experimentally, P(Bn-ITN) has somewhat a deep HOMO
value and a large energy gap as compared to the DFT calculation
results. This can be due to differences between the gas phase
conditions employed in the DFT calculations and the solid state
conditions employed in the measurements.
Voltammetric behavior of the polymers
In order to examine electrochemical properties of the polymers,
cyclic voltammetry of the polymers in the film state was carried
out by using an Ag/Ag+ reference electrode in 0.1 M tetrabutyl
ammonium perchlorate (TBAP) acetonitrile solution. Fig. 3
shows cyclic voltammograms of the polymers calibrated with the
ferrocene/ferrocenium (Fc/Fc+) redox potential. Among the
polymers, P(Bn-EDOT) has the lowest oxidation potential of
0.44 V (vs. Fc/Fc+) because of electro-donating ethylenedioxy
substituents in the EDOT units. Under the assumption that the
Electropolymerized P(Bn-BT) (EP(Bn-BT))
P(Bn-BT) was synthesized by electrochemical polymerization
(Scheme 2). Fig. 5 shows cyclic voltammograms for electro-
chemical polymerization of compound 11. An oxidation current
onset of the first cycle corresponds to the oxidation potential of
the monomer of 0.69 V (vs. Fc/Fc+). After the second cycle
oxidation current onset was shifted to 0.57 V (vs. Fc/Fc+), and
the current response intensity increased with an increase in the
number of cycles, indicating the progress of polymerization
reaction. Generally, as polymerization reaction proceeds, the
oxidation current onset decreases due to extension of the effec-
tive conjugation length with growth of the main chain. However,
in the case of EP(Bn-BT), where EP stands for electro-
polymerization, no decrease in oxidation current onset was
observed after the second cycle, indicating saturation of the
effective conjugation. This result also supports the conclusion
that the effective conjugation of the polymer was cleaved at the
orthogonally oriented two naphthalene rings. The oxidation
onset potential at 0.57 V (vs. Fc/Fc+) is observed for EP(Bn-BT).
This observation is in good agreement with the results for the
chemically polymerized P(Bn-BT).
Fig. 3 Cyclic voltammograms of the polymers cast on a platinum
disc electrode (0.1 M TBAP in acetonitrile solution with a scan rate of
100 mV sꢀ1).
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Fig. 4 HOMO and LUMO and their energy levels of model compounds calculated by the DFT method and those of the polymers in the film state
estimated from observed oxidation potentials and optical band gaps.
which is characteristic of dC–H at b-positions of 2,5-substituted
thiophenes appeared after the polymerization.27,28 These results
confirmed that electrochemical polymerization of compound 11
afforded EP(Bn-BT) via a coupling reaction between a-positions
of thiophene units of the monomers.
MALDI-TOF-MS measurements using dithranol as a matrix
was carried out to evaluate molecular weight of EP(Bn-BT).
Fig. 7 shows the MALDI-TOF-MS spectrum of EP(Bn-BT). The
spectrum shows a certain pattern with a periodicity of 785 of m/z,
which corresponds to the molecular weight of the monomer unit
(785.20). In the measurements, the peaks up to 7065 of m/z,
which correspond to 9 mers, were detected. The peak intensity of
the spectra decreased with an increase in the m/z value. This
result indicates that the MALDI-TOF-MS measurements for the
Fig. 5 Cyclic voltammograms for electrochemical polymerization of 11
in 0.1 M TBAP acetonitrile solution using an Ag/Ag+ reference electrode
electrosynthesized polymer can evaluate molecular weights of the
low molecular weight fraction of the polymer.
with a scan rate of 100 mV sꢀ1
.
EP(Bn-BT) was obtained as an electroactive film deposited on
an indium-tin-oxide (ITO) glass electrode, which exhibits
repeatable color change by the electrochemical redox process
(electrochromism) accompanied by a repeatable change in CD.
Fig. 8 shows CD and UV-vis absorption spectra of the
EP(Bn-BT) film at various applied potentials during the elec-
trochemical redox process. The spectra at short wavelengths
(<300 nm) could not be obtained because of the optical
Fig. 6 shows the IR spectra of compound 11 and EP(Bn-BT) in
the reduced state. EP(Bn-BT) shows the same absorption bands
as those of the chemically polymerized P(Bn-BT), and absorp-
tion bands similar to those of the monomer. However, an
absorption band at 692 nm observed for the monomer (dC–H at
a-positions of thiophenes) disappeared after electrochemical
polymerization. Additionally, an absorption band at 796 nm
Fig. 6 IR absorption spectra of 11 and EP(Bn-BT).
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Fig. 7 MALDI-TOF-MS spectrum of EP(Bn-BT).
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absorption of the ITO glass electrode in the ultra-violet range. In
the reduced state, the polymer shows an absorption band at 410
nm due to p–p* transition of naphthalene–bithiophene–naph-
thalene chromophores, which is similar to that of chemically
polymerized P(Bn-BT) in the film state. However, in the CD
spectra, EP(Bn-BT) shows a negative couplet at around 410 nm,
which is the opposite sign to that of P(Bn-BT) in the film state
although the polymer possesses the same chemical structure and
stereo-configuration of the binaphthyl unit. The differences in
the CD spectra suggest that the chemically and electrochemically
polymerized polymers have different aggregation structures in
the film, which are derived from the differences in the film
formation processes. The P(Bn-BT) film was prepared by a cast
method, while the EP(Bn-BT) film was directly deposited by the
electrochemical oxidation process on the substrate via radical
coupling reaction. During an oxidation process, EP(Bn-BT)
showed a new absorption band at around 600–700 nm, which is
accompanied by a decrease in the intensity of the absorption
band at short wavelengths. This new band is due to formation of
a radical cation in the naphthalene–bithiophene–naphthalene
unit. Simultaneously, in the CD spectra, a new negative couplet
appeared at around 600–700 nm, and the intensity of the original
negative couplet at 410 nm decreased. The new negative couplet
can be due to interchromophoric exciton coupling between the
radical cationic naphthalene–bithiophene–naphthalene units.
During the reduction process, a change in the UV-vis and the CD
spectra was observed in the reverse direction against the elec-
trochemical oxidation process. The dihedral angle of the
binaphthyl unit can be tuned by a possible mechanical change in
the form of the main chain upon the redox process. Fig. 9 shows
optimized geometries of the model compound for P(Bn-BT) in
both neutral and radical cationic states obtained by the DFT
calculation. These geometries show differences in the length and
coplanarity of the naphthalene–bithiophene–naphthalene unit in
the neutral and radical cationic states. The naphthalene–bithio-
phene–naphthalene unit in the radical cationic state has a shorter
length and a more planar conformation than that in the neutral
state because of structural contribution from quinonoid char-
acter in the radical cationic state rather than benzenoid character
in the neutral state.41 In the film, this reorientation by the redox
process can drive mechanical change in the main chain, resulting
Fig. 8 CD and UV-vis spectra of EP(Bn-BT) at various applied
potentials during oxidation (top) and reduction (bottom) processes.
Fig. 9 Optimized geometries of model compound for P(Bn-BT) in the neutral state (a) and radical cationic state (b).
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Fig. 10 Possible mechanism of structural change of EP(Bn-BT) in the film by electrochemical redox processes.
in a change of the dihedral angle of the binaphthyl unit as shown
References and notes
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EP(Bn-BT) through electrochemical oxidative coupling reaction,
the planar conformation of the chromophores in the radical
cationic state can lead to a better packing structure of the
chromophores than that in the P(Bn-BT) film prepared by the
cast method.
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of Tsukuba and Glass Work Shop of University of Tsukuba.
This research was supported by the Japan Society for the
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