Synthesis, optical and electrochemical properties of arylenevinylene-based π-conjugated polymers with imidazolium units in the main chain

Article abstract of DOI:10.1002/pola.24618

Arylenevinylene-based π-conjugated polymers containing imidazolium cationic units in the main chain and their model compounds were synthesized and characterized in terms of optical and electrochemical properties. 9,9-Bisoctylfluorene, 2,5-bisdodecyloxyben

Full text of DOI:10.1002/pola.24618

Synthesis, Optical and Electrochemical Properties of Arylenevinylene-
Based p-Conjugated Polymers with Imidazolium Units in the Main Chain
MASAYA TOBA, TAKUYA NAKASHIMA, TSUYOSHI KAWAI
Graduate School of Materials Science, Nara Institute of Science and Technology, Ikoma, Nara 630-0192, Japan
Received 22 November 2010; accepted 29 January 2011
DOI: 10.1002/pola.24618
Published online 1 March 2011 in Wiley Online Library (wileyonlinelibrary.com).
ABSTRACT: Arylenevinylene-based p-conjugated polymers con-
taining imidazolium cationic units in the main chain and their
model compounds were synthesized and characterized in terms
of optical and electrochemical properties. 9,9-Bisoctylfluorene,
2,5-bisdodecyloxybenzene, and 3-dodecylthiophene were intro-
duced as arylene units with different donor characteristics to
evaluate the effect on the highest occupied molecular orbital-
lowest unoccupied molecular orbital (HOMO-LUMO) gap
energy. The UV–vis and fluorescence spectra of cationic poly-
mers and model compounds with iodide counter anion exhib-
ited a significant blue shift with respect to the parent neutral
molecules. X-ray single crystal analysis for model compounds
revealed that the effective p-conjugation length of cationic
model compounds decreased compared to the neutral model
compounds by means of twisted conformation directed by CH-p
interactions between N-methyl groups of imidazolium and
neighboring aryl units. The cyclic voltammetry measurement
suggested the negative shift of LUMO levels by the conversion
of imidazole to imidazolium, indicating the electron-accepting
C
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characteristics of cationic imidazolium unit. 2011 Wiley Period-
icals, Inc. J Polym Sci Part A: Polym Chem 49: 1895–1906, 2011
KEYWORDS: conjugated polymers; copolymerization; electro-
chemistry; fluorescence
INTRODUCTION In the past two decades, p-conjugated
oligomers and polymers have been widely studied for func-
tional and practical applications such as light source,1–6 elec-
tronic devices,7–11 and sensors.12–16 Considerable effort has
been made for the design and synthesis of functional p-conju-
gated oligomers and polymers whose electronic properties
can be modulated with external stimuli. As their electrical and
optical properties are largely determined dominantly by the
expansion of p-conjugation systems on the main chains, mod-
ulation of the coplanarity and bond alternation in the p-conju-
gated main chain by means of photoirradiation and chemicals
has been studied.17–29 In this regard, p-conjugated polymers
with heteroaromatic units containing imine nitrogen, for
example, thiazole,30,31 pyridine,32–35 and benzimidazole,36–38
in their main chain have been attracting considerable interest,
because their properties can be modulated by metal binding,
protonation, or quaternization at the N-position. For example,
Curtis et al.39,40 demonstrated that optical and electrical prop-
erties of p-conjugated polymers with thiazolium units
obtained by N-alkylation of the thiazole ring in the main chain
are considerably different from those of the corresponding
neutral thiazole-based polymers. Swager and coworkers41,42
investigated conducting polymers containing pyridinium units
in the main chain. The quaternization of the pyridine to pyri-
dinium provided polymers with longer effective p-conjugation
lengths by stretching their conformation due to the electro-
static repulsion, and the resulting polymer showed electron-
accepting (n-type) property with high conductivity and carrier
mobility.42 From this point of view, quaternization at the N-
position in the heteroaromatic units would afford a new route
toward charged p-conjugated backbones that have specific op-
tical and electrical properties.
Imidazole is an aromatic heterocycle containing imine nitro-
gen, which acts as an important building unit for biomolecules
and has attractive chemical properties such as formation of
coordination compounds and effective hydrogen bonding.43,44
Hence, it is considered to be a building unit for p-conjugated
molecular and polymer systems in recent studies.45,46 Imidaz-
ole is easily converted into imidazolium cation, which has a
cationic charge by protonation or quaternization of the N-
position similar to thiazole and pyridine. The imidazolium
ring, a cationic N-heteroaromatic ring containing quaternized
nitrogen atom, has, thus, been interested as a building block
for self-assembled, supramolecular systems and specific ion/
photomodulation units.47–52 It is also recognized as the most
popular structure for the cationic moiety of ionic liquids.53 In
the field of conjugated polymers, Leclerc and coworkers54
demonstrated that polythiophene derivatives with imidazo-
lium units in the side chain could be used to detect iodide
anions and nucleic acids by fluorescence signal amplifica-
tion.55,56 Recently, we have investigated the phenyleneethyny-
lene and thienyleneethynylene-based p-conjugated copolymers
with imidazolium units in the main conjugated chain.57 In
that report, the optical properties of p-conjugated polymers
were modulated by introducing imidazolium units in the main
Additional Supporting Information may be found in the online version of this article. Correspondence to: T. Kawai (E-mail: tkawai@ms.naist.jp)
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Journal of Polymer Science Part A: Polymer Chemistry, Vol. 49, 1895–1906 (2011) 2011 Wiley Periodicals, Inc.
SYNTHESIS, OPTICAL AND ELECTROCHEMICAL PROPERTIES, TOBA, NAKASHIMA, AND KAWAI
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JOURNAL OF POLYMER SCIENCE PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
chain compared to the corresponding imidazole-based conju-
gated polymers depending on partnering aryl units. That is,
for a phenyleneethynylene-based p-conjugated polymer, char-
acteristic blue shift was observed both for absorption and flu-
orescence bands on the quaternization of imidazole-1,5-yl
units to imidazolium, which was attributed to the distortion
on the conjugated main chain on the introduction of cationic
charge.45 On the other hand, thienyleneethynylene-based p-
conjugated polymers showed a characteristic red shift by con-
verting the imidazole units to cationic imidazolium due to the
formation of donor–acceptor (D–A) structures. Quaternization
at the N-position in imidazole-based polymer, namely intro-
ducing the cationic charges in the main chain of p-conjugated
moieties is expected to modulate not only the optical proper-
ties but also the electrical properties. However, p-conjugated
polymers, particularly those with N,N0-dialkylated imidazolium
units in the main chain, have met limited success. Further-
more, the effects on the frontier energies by introducing cati-
onic imidazolium units in the main chain for arylenevynylene-
based conjugated polymers, that are known to display a num-
ber of utility properties, have never been studied to the best
of our knowledge.
grams of thin films of the polymers were done by using l-
AUTOLAB TYPE3 potentiostat/galvanostat equipped with
electrochemical analysis system software in an argon-filled
glove box. A platinum wire was used as a counter electrode,
Ag/Agþ electrode was used as a reference electrode and an
indium tin oxide (ITO)-coated glass was used as a working
electrode. Thin film of the polymers on ITO-coated glass
electrode was prepared by casting the chloroform solution of
polymers on the ITO-coated glass electrode. The measure-
ments were conducted in a solution of 0.1 M tetramethylam-
monium hexafluorophosphate in acetonitrile at a scan rate of
50 mV/s at room temperature in an argon-filled glove box.
Materials
Benzaldehyde (Wako, >98.0%), 2,20-thenil (Aldrich, >98.0%),
3-dodecylthiophene (TCI, >98.0%), 9,9-dioctylfluorene (TCI,
>98.0%), hydroquinone (TCI, >98.0%), dodecyl bromide
(Wako, >97.0%), 2-thiophenemethanol (TCI, >98.0%), 2-bro-
mofluorene (TCI, >98.0%), diethyl(3-methoxybenzyl)phosph-
onate (TCI, >98.0%), iodomethane (Wako, >95%), and poly-
stylene standard (Chemco Scientific) were used as received
without further purification.
Syntheses
In this article, we report the preparation, optical, and elec-
trochemical properties of arylenevinylene-based p-conju-
gated polymers with imidazolium units in their main chain.
Model compounds that have monomeric unit structures of
corresponding polymers were also synthesized to discuss the
effect of cationic charge on their electronic structures and
conformations by means of spectroscopy and single crystal
X-ray analyses. Shifts of the highest occupied molecular or-
bital (HOMO) and the lowest unoccupied molecular orbital
(LUMO) energy levels of p-conjugated polymers on introduc-
ing cationic charges are also investigated by electrochemical
measurements.
2-Phenyl-4,5-di(thiophen-2-yl)-1H-imidazole (1)
A mixture of 2,2-thenil (4.0 g, 18 mmol), benzaldehyde (1.9
g, 18 mmol), ammonium acetate (9.5 g, 0.13 mol), and glacial
acetic acid (80 mL) was refluxed overnight under N2 atmos-
phere. The reaction mixture was poured into 300 mL of dis-
tilled water. The resulting white precipitate was filtered,
washed with ethanol for several times, and dried in vacuo to
give 1 (5.3 g, 96%) of a white solid. This material was used
without further purification.
1-Methyl-2-phenyl-4,5-di(thiophen-2-yl)-1H-imidazole (2)
A dried four-neck round-bottomed flask was charged with 1
(4.8 g, 16 mmol), anhydrous tetrahydrofurane (THF) (150
mL), and NaH (1.1 g, 46 mmol). The resulting mixture was
refluxed under N2 atmosphere for 1 h. Iodomethane (2.7 g,
19 mmol) was added to the resulting mixture and further
refluxed for 5 h. The reaction was then quenched by adding
water (50 mL), extracted by dichloromethane (250 mL), and
washed with saturated Na2SO3 solution (180 mL). The or-
ganic layer was separated and dried over MgSO4. After re-
moval of the solvent, the crude product was purified with
the alumina column chromatography using dichloromethane
as an eluent to yield 2 (3.5 g, 67%) as a white solid.
EXPERIMENTAL
General
1H NMR (300 MHz) and 13C NMR (75 MHz) spectra were
recorded on a JEOL JNM-AL300 instrument. High resolution
mass spectroscopy (HRMS) was measured by JEOL JMS-700
MStation. Elemental analyses were performed for C, H, and N
by Perkin Elmer, 2400 II CHNS/O. UV–vis absorption and flu-
orescence spectra were obtained on a JASCO V-670 spectro-
photometer and a JASCO FP-6500 fluorescence spectropho-
tometer, respectively. Absolute fluorescence quantum yields
of polymers and model compounds were measured using a
Hamamatsu C9920-02. Thin films for solid state measure-
ments were cast from chloroform solutions onto glass slides,
and the UV–vis and fluorescence spectra were recorded after
thermal drying at 70 ꢀC for 1 h. X-ray crystal structures
were given by using a Rigaku R-AXIS RAPID/S. Gel permea-
tion chromatography (GPC) was carried out (COSMO SIL
5GPC-60 Waters column, particle size: 5 lm, pore size: 60 Å)
using chloroform as an eluent with a flow rate of 3.5 mL/
min at room temperature using polystylene standard as a
calibrant (Detector: JASCO UV-2086). The cyclic voltammo-
1H NMR (300 MHz, CD2Cl2): d (ppm) 3.51 (s, 3H), 6.91 (t,
1H), 6.98–6.99 (d, 1H), 7.13–7.15 (d, 1H), 7.24–7.25 (m, 2H),
7.49–7.52 (m, 3H), 7.62 (d, 1H), 7.68–7.72 (m, 2H). 13C NMR
(75 MHz, CD2Cl2): d (ppm) 32.8, 121.3, 123.1, 123.8, 127.1,
127.8, 128.6, 129.0, 129.1, 130.2, 130.4, 130.9, 135.9, 137.6,
148.5.
1,3-Dimethyl-2-phenyl-4,5-di(thiophen-2-yl)-
1H-imidazol-3-ium iodide (2þ)
To a dried four-neck round-bottomed flask, 2 (0.15 g, 0.45
mmol), acetonitrile (75 mL), and iodomethane (0.66 g, 4.5
mmol) were added. The resulting mixture was refluxed
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ARTICLE
overnight under N2 atmosphere. The reaction was quenched
by adding water (50 mL) and dichloromethane (250 mL).
The organic layer was separated and dried over MgSO4. After
removal of the solvent, the product was recrystallized from
hexane to yield 2þ (0.18 g, 90%) as a white solid.
ature. The reaction was quenched by adding water (50 mL)
and dichloromethane (250 mL). The organic layer was sepa-
rated and dried over MgSO4. After removal of the solvent,
the crude product was purified with the alumina column
chromatography using dichloromethane as an eluent to yield
MT (0.37 g, 85%) as a bright yellow solid.
1H NMR (300 MHz, CDCl3): d (ppm) 3.65 (s, 6H), 7.11–7.14
(m, 2H), 7.50–7.52 (m, 2H), 7.65–7.70 (m, 5H), 8.11–8.13 (d,
2H). 13C NMR (75 MHz, CDCl3): d (ppm) 34.9, 121.7, 123.9,
127.8, 128.0, 129.8, 129.9, 131.4, 123.7, 133.7.
1H NMR (300 MHz, CD2Cl2): d (ppm) 3.57 (s, 3H), 6.88 (d,
2H), 7.00–7.05 (m, 5H), 7.14–7.33 (m, 7H), 7.51–7.53 (d,
3H), 7.71–7.73 (d, 3H). 13C NMR (75 MHz, CDCl3): d (ppm)
33.4, 121.0, 121.3, 122.1, 122.9, 124.0, 124.5, 125.3, 126.2,
127.1, 127.4, 128.1, 128.2, 129.0, 129.3, 129.5, 130.8, 132.2,
135.9, 137.4, 140.9, 142.4, 143.0, 146.2. EI-HRMS (m/z),
Calcd. for C30H22N2S4 [Mþ]: 538.0666. Found 538.0668.
4,5-Bis(5-bromothiophen-2-yl)-1-methyl-2-phenyl-
1H-imidazole (3)
To a three-neck round-bottomed flask, 2 (1.0 g, 3.1 mmol),
chloroform (380 mL), and N-bromosuccinimide (1.2 g, 6.5
mmol) were added. The resulting mixture was stirred overnight
at room temperature, and then quenched by adding water (100
mL). The organic layer was separated and dried over MgSO4.
After removal of the solvent, the crude product was purified
with the alumina column chromatography using dichlorome-
thane as an eluent to yield 3 (910 mg, 61%) as a yellow solid.
1H NMR (300 MHz, CD2Cl2): d (ppm) 3.54 (s, 3H), 6.88 (m,
2H), 6.98 (d, 1H), 7.19 (d, 1H), 7.49 (q, 3H), 7.68–7.72 (d,
2H). 13C NMR (75 MHz, CD2Cl2): d (ppm) 32.8, 121.3, 123.1,
123.8, 127.1, 127.8, 128.6, 129.0, 129.1, 130.2, 130.4, 130.9,
135.9, 137.6, 148.5.
1,3-Dimethyl-2-phenyl-4,5-bis(5-((E)-2-(thiophen-2-yl)
vinyl)thiophen-2-yl)-1H-imidazol-3-ium (MTþ)
To a dried four-neck round-bottomed flask, MT (20 mg, 0.037
mmol), acetonitrile (75 mL), and iodomethane (0.20 g, 1.4
mmol) were added. The resulting mixture was refluxed
overnight under N2 atmosphere. The reaction was quenched by
adding water (50 mL) and dichloromethane (100 mL). The
organic layer was separated and dried over MgSO4. After
removal of the solvent, the product was washed with hexane
and methanol several times to yield MTþ (24 mg, 95%) as a yel-
low solid.
1H NMR (300 MHz, CDCl3): d (ppm) 3.68 (s, 6H), 6.98–7.06
(m, 10H), 7.20 (d, 2H), 7.50 (d, 2H), 7.70 (d, 3H), 8.12 (d,
2H). 13C NMR (75 MHz, CDCl3): d (ppm) 30.1, 121.2, 121.8,
122.5, 122.9, 123.5, 124.4, 125.7, 126.8, 127.1, 127.4, 128.2,
128.6, 129.2, 129.3, 129.5, 130.8, 132.7, 135.9, 137.4, 141.1,
142.6, 143.0, 145.7.
5,50-(1-Mthyl-2-phenyl-1H-imidazole-4,5-diyl)
dithiophene-2-carbaldehyde (4)
3 (0.30 g, 0.72 mmol) was added dropwise to a well-stirred
solution of anhydrous THF (20 mL), tetramethylethylenedi-
amine (0.2 mL, 1.5 mmol), and n-BuLi (1.1 mL, 1.6 M in
hexane, 1.7 mol) under N2 atmosphere at ꢁ78 ꢀC. The
ꢀ
mixture was stirred at ꢁ78 C for 1 h. Anhydrous dimethyl-
4,5-Bis(5-(3-methoxystyryl)thiophen-2-yl)-1-
methyl-2-phenyl-1H-imidazole (MB)
formamide (0.56 mL, 7.2 mmol) was added, and the mixture
ꢀ
was warmed to 20 C over 10 h. The reaction was quenched
To a dried four-neck round-bottomed flask, 4 (0.20 g, 0.53
mmol), diethyl(3-methoxybenzyl)phosphonate (0.27 g, 1.1
mmol), and anhydrous THF (30 mL) were added. KOtBu
(0.14 g, 1.6 mmol) was added slowly, and the resulting mix-
ture was stirred overnight at room temperature. The reac-
tion was quenched by adding water (50 mL) and dichloro-
methane (250 mL). The organic layer was separated and
dried over MgSO4. After removal of the solvent, the crude
product was purified with the alumina column chromatogra-
phy using dichloromethane as an eluent to yield MB (0.18 g,
59%) as a yellow solid.
by adding ethyl acetate (50 mL), water (50 mL), dichlorome-
thane (250 mL), and saturated Na2SO3 solution (180 mL).
The organic layer was separated and dried over MgSO4, and
the solvent was removed to provide a crude product as an
orange solid. The crude product was purified with the alu-
mina column chromatography using dichloromethane as an
eluent to yield 4 (0.20 g, 75%) as a bright yellow solid.
1H NMR (300 MHz, CDCl3): d (ppm) 3.59 (s, 3H), 7.33 (s,
1H), 7.38 (s, 1H), 7.52 (m, 3H), 7.62 (m, 1H), 7.70 (d, 2H),
7.90 (s, 1H), 9.83 (s, 1H), 10.01 (s, 1H). 13C NMR (75 MHz,
CDCl3): d (ppm) 33.3, 122.7, 124.8, 128.8, 129.1, 129.4,
129.7, 132.2, 136.5, 137.0, 138.7, 141.7, 146.7, 146.8, 150.1,
182.7, 182.8. EI-HRMS (m/z), Calcd. for C20H14N2O2S2 [Mþ]:
378.0497. Found 378.0493. Anal. Calcd. for C20H14N2O2S2: C,
63.47; H, 3.73; N, 7.40. Found: C, 63.72; H, 3.71; N, 7.14.
1H NMR (300 MHz, CD2Cl2): d (ppm) 3.57 (s, 3H), 3.80 (s, 3H),
3.83 (s, 3H), 5.31–5.91 (m, 5H), 7.01–7.33 (m, 11H), 7.50–7.53
(m, 3H), 7.70–7.72 (m, 2H). 13C NMR (75 MHz, CD2Cl2): d
(ppm) 33.41, 55.6, 111.5, 111.9, 113.6, 114.1, 119.2, 119.4,
122.1, 122.4, 122.7, 123.9, 127.3, 127.6, 127.6, 129.0, 129.1,
129.3, 129.5, 129.7, 130.0, 130.2, 130.8, 132.1, 135.9, 137.6,
138.5, 139.0, 141.3, 146.6, 149.0, 160.4. EI-HRMS (m/z), Calcd.
for C36H30N2O2S2 [Mþ]: 586.1749. Found 586.1741.
1-Methyl-2-phenyl-4,5-bis(5-((E)-2-(thiophen-2-yl)vinyl)
thiophen-2-yl)-1H-imidazole (MT)
To a dried four-neck round-bottomed flask, 4 (0.30 g, 0.80
mmol), diethyl thiophen-2-ylmethylphosphonate58 (0.37 g,
1.6 mmol), and anhydrous THF (45 mL) were added. Potas-
sium t-butoxide (KOtBu) (210 mg, 2.3 mmol) was added
slowly to the mixture and stirred overnight at room temper-
4,5-Bis(5-(3-methoxystyryl)thiophen-2-yl)-1,3-dimethyl-2-
phenyl-1H-imidazol-3-ium (MBþ)
To a dried four-neck round-bottomed flask, MB (24 mg,
0.041 mmol), acetonitrile (75 mL), and iodomethane (0.20 g,
SYNTHESIS, OPTICAL AND ELECTROCHEMICAL PROPERTIES, TOBA, NAKASHIMA, AND KAWAI
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JOURNAL OF POLYMER SCIENCE PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
1.4 mmol) were added. The resulting mixture was refluxed
overnight under N2 atmosphere. The reaction was quenched
by adding water (100 mL) and dichloromethane (150 mL).
The organic layer was separated and dried over MgSO4. After
removal of the solvent, the product was washed with hexane
and methanol several times to yield MBþ (23 mg, 92%) as a
yellow solid.
ne)diphosphonate (0.15 g, 0.29 mmol), and anhydrous THF
(5.5 mL) were added. KOtBu (51 mg, 0.57 mmol) was added
slowly, and the resulting mixture was stirred for 3 days
under N2 atmosphere at room temperature. The reaction
mixture was poured into a large amount of methanol and
subjected to centrifugal separation to precipitate the polymer
PT (82 mg, 45%) as a red powder.
1H NMR (300 MHz, CD2Cl2): d (ppm) 3.78 (s, 3H), 3.81 (s,
3H), 3.84 (s, 6H), 5.31–5.90 (m, 5H), 7.01–7.31 (m, 11H),
7.50–7.57 (m, 3H), 8.07 (d, 2H). 13C NMR (75 MHz, CD2Cl2):
d (ppm) 31.59, 55.2, 111.0, 111.3, 113.1, 114.2, 118.8, 119.4,
122.5, 122.7, 123.1, 127.3, 127.6, 127.6, 129.0, 129.1, 129.3,
129.5, 129.7, 130.0, 130.2, 130.4, 132.1, 136.6, 137.6, 138.0,
139.7, 140.6, 146.0, 154.0, 160.2.
1H NMR (300 MHz, CDCl3): d (ppm) 0.62 (br), 0.81 (br),
1.06 (br), 1.61 (br), 1.99 (br), 3.60 (s), 6.97–7.23 (m), 7.39–
7.73 (m). Number-avarage molecular weight (Mn) ¼ 1.2 ꢂ
103, polydispersity index (PDI) ¼ 3.3.
PTþ
To a dried four-neck round-bottomed flask, PT (30 mg,
0.047 mmol/monomer unit), acetonitrile (25 mL), chloro-
form (50 mL), and iodomethane (500 mg, 3.5 mmol) were
added. The resulting mixture was refluxed for 1 day under
N2 atmosphere. After removal of the solvent, the product
was washed with hexane and methanol several times to yield
PTþ (37 mg, 90%) as a red solid.
4,5-Bis(5-((E)-2-(9,9-dimethyl-9H-fluoren-2-yl)vinyl)
thiophen-2-yl)-1-methyl-2-phenyl-1H-imidazole (MF)
To a dried four-neck round-bottomed flask, 4 (0.40 g, 1.07
mmol), diethyl (9,9-dimethyl-9H-fluoren-2-yl)methylphospho-
nate (0.74 g, 2.13 mmol), and anhydrous THF (60 mL) were
added. KOtBu (0.28 g, 3.1 mmol) was added slowly, and the
resulting mixture was stirred overnight at room temperature.
The reaction was quenched by adding water (70 mL) and
dichloromethane (100 mL). The organic layer was separated
and dried over MgSO4. After removal of the solvent, the
crude product was purified with the alumina column chro-
matography using dichloromethane as an eluent to yield MF
(0.57 g, 70%) as a yellow solid.
1H NMR (300 MHz, CDCl3): d (ppm) 0.83 (br), 1.20 (br), 1.77
(br), 3.69 (br), 6.67–6.82 (br), 7.52 (br), 7.68 (br), 8.19 (br).
PB
To a dried four-neck round-bottomed flask, 4 (51 mg, 0.13
mmol), tetraethyl (2,5-bis(dodecyloxy)-1,4-phenylene)bis(me-
thylene)diphosphonate (0.10 g, 0.13 mmol), and anhydrous
THF (2.5 mL) were added. KOtBu (24 mg, 0.27 mmol) was
added slowly, and the resulting mixture was stirred for 3
days under N2 atmosphere at room temperature. The reac-
tion mixture was poured into a large amount of methanol
and subjected to centrifugal separation to precipitate the
polymer PB (82 mg, 45%) as an orange powder.
1H NMR (300 MHz, CD2Cl2): d (ppm) 1.54–1.56 (m, 12H), 3.60
(s, 3H), 6.95–7.75 (m, 27H). 13C NMR (75 MHz, CD2Cl2): d
(ppm) 27.3, 47.1, 54.5, 120.2, 120.4, 120.5, 120.6, 120.8, 121.2,
121.8, 122.4, 123.0, 126.0, 126.2, 127.3, 127.4, 127.6, 127.8,
128.1, 128.9, 129.0, 129.3, 129.4, 130.2, 132.2, 136.2, 136.8,
139.1, 139.2, 139.7, 141.6, 146.9, 154.4, 154.6, 154.7. EI-HRMS
(m/z), Calcd. for C52H42N2S2 [Mþ]: 758.2789. Found 758.2789.
1H NMR (300 MHz, CDCl3): d (ppm) 0.85 (br), 1.24 (br),
1.50 (br), 1.86 (br), 3.59 (s), 4.02 (br), 6.91–7.19 (m), 7.22–
7.34 (m), 7.34–7.50 (br), 7.71–7.73 (br). Mn ¼ 1.7 ꢂ 103,
PDI ¼ 2.5.
4,5-Bis(5-((E)-2-(9,9-dimethyl-9H-fluoren-2-yl)vinyl)
thiophen-2-yl)-1,3-dimethyl-2-phenyl-1H-imidazol-3-ium
iodide (MFþ)
PBþ
To a dried four-neck round-bottomed flask, MF (37 mg,
0.040 mmol), acetonitrile (75 mL), and iodomethane (0.20 g,
1.4 mmol) were added. The resulting mixture was refluxed
overnight under N2 atmosphere. The reaction was quenched
by adding water (50 mL) and dichloromethane (250 mL).
The organic layer was separated and dried over MgSO4. After
removal of the solvent, the product was washed with hexane
and methanol several times to yield MFþ (33 mg, 92%) as a
yellow solid.
To a dried four-neck round-bottomed flask, PB (30 mg,
0.047 mmol/monomer unit), acetonitrile (25 mL), chloro-
form (50 mL), and iodomethane (500 mg, 3.5 mmol) were
added. The resulting mixture was refluxed for 1 day under
N2 atmosphere. After removal of the solvent, the product
was washed with hexane and methanol several times to yield
PBþ (37 mg, 90%) as a red solid.
1H NMR (CDCl3): d (ppm) 0.86 (br), 1.24 (br), 1.48 (br),
1.81 (br), 3.72 (s), 4.01 (br), 6.94–7.07 (m), 7.53 (br), 7.73
(br), 8.07 (br).
1H NMR (300 MHz, CD2Cl2): d (ppm) 1.47–1.51 (m, 12H),
3.72 (s, 6H), 6.95–7.75 (m, 25H), 8.15 (d, 2H). 13C NMR (75
MHz, CD2Cl2): d (ppm) 27.3, 47.1, 54.5, 120.2, 120.4, 120.5,
120.6, 120.8, 121.2, 121.8, 122.4, 123.0, 126.0, 126.2, 127.3,
127.4, 127.6, 127.8, 128.1, 128.7, 129.1, 129.4, 130.2, 131.2,
136.6, 137.2, 139.0, 139.5, 142.3, 146.5, 154.0, 154.2, 154.5.
PF
To a dried four-neck round-bottomed flask, 4 (0.11 g, 0.28
mmol), tetraethyl (9,9-dioctyl-9H-fluorene-2,7-diyl)bis(methy-
lene)diphosphonate59 (0.19 g, 0.28 mmol), and anhydrous
THF (5.2 mL) were added. KOtBu (49 mg, 0.55 mmol) was
added slowly, and the resulting mixture was stirred for 3
days under N2 atmosphere at room temperature. The reac-
tion mixture was poured into a large amount of methanol
PT
To a dried four-neck round-bottomed flask, 4 (0.11 g, 0.29
mmol), tetraethyl (3-dodecylthiophene-2,5-diyl)bis(methyle-
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SCHEME 1 Syntheses of monomers and polymers.
and subjected to centrifugal separation to precipitate the
polymer PF (0.12 g, 58%) as a yellow powder.
1H NMR (300 MHz, CDCl3): d (ppm) 0.60 (br), 0.78 (br),
1.03 (br), 1.71 (br), 1.98 (br), 3.74 (br), 4.00 (br), 7.08–7.18
(m), 7.29–7.31 (m), 8.08 (br).
1H NMR (300 MHz, CD2Cl2): d (ppm) 0.62 (br), 0.81 (br),
1.06 (br), 1.61 (br), 1.99 (br), 3.60 (s), 4.00 (br), 6.97–7.23
(m), 7.39–7.73 (m). Mn ¼ 2.9 ꢂ 103, PDI ¼ 2.9.
RESULTS AND DISCUSSION
PFþ
Polymerization
To a dried four-neck round-bottomed flask, PF (50 mg,
0.064 mmol/monomer unit), acetonitrile (25 mL), chloro-
form (50 mL), and iodomethane (0.50 g, 3.5 mmol) were
added. The resulting mixture was refluxed for 1 day under
N2 atmosphere. After removal of the solvent, the product
was washed with hexane and methanol several times to yield
PFþ (53 mg, 90%) as a red solid.
Our synthetic strategy for polymerization via the Wittig-
Horner route involved syntheses of the appropriate poly-
meric system from the corresponding dialdehydes and
diphosphonate esters, as illustrated in Scheme 1. The Mn of
PT, PB, and PF were evaluated by GPC measurement with
polystylene standard to be 1.2 ꢂ 103 with 3.3 of PDI, 1.7 ꢂ
103 with 2.5 of PDI and 2.9 ꢂ 103 with 2.9 of PDI,
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SCHEME 2 Syntheses of model compounds.
respectively. The neutral polymers, PT, PB, and PF were
readily converted to the cationic polymers PTþ, PBþ, and
PFþ, respectively, by the N-methylation with methyliodide.
The conversion ratios of quaternization were more than
90% for all polymers, which were estimated by comparing
results are summarized in Table 1. The neutral polymer PT
showed absorption bands at 454 nm [Fig. 1(a)]. The kabs of
PB and PF were found at 446 and 423 nm, respectively [Fig.
1(b,c)]. These absorption bands are responsible for the p–p*
transition in their p-conjugated main chain. In the normal-
ized fluorescence spectra, the kem of PT, PB, and PF were
found at 590, 579, and 545 nm, respectively. Both the
absorption and fluorescence maximum wavelengths shifted
to longer wavelength in the order of PF, PB, and PT, indicat-
ing intrachain donor–acceptor (D–A) structures. That is, as
the donor strengths of aryl units in the p-conjugated main
chain increases in the order of 9,9-bisoctylfluorene, 2,5-bis-
dodecyloxybenzene, and 3-dodecylthiophene, the optical
1
the integral of a H NMR signal of N,N0-dimethyl protons,
which appeared as a single peak at about 3.7 ppm, with that
of the terminal methyl protons in the long alkyl chains teth-
1
ered to aryl units at 0.9 ppm. The H NMR signal of N,N0-di-
methyl protons was shifted to a lower magnetic field by 0.1
ppm compared with N-methyl signals of the neutral poly-
mers PT, PB, and PF because of the introduction of cationic
charges on the main chain.57
Syntheses of Model Compounds
We also applied the Wittig-Horner reaction to give model
compounds, MT, MB, and MF as shown in Scheme 2. Cationic
model compounds MTþ, MBþ, and MFþ were obtained by N-
methylation of MT, MB, and MF, respectively. The molecular
TABLE 1 Optical Properties of the Conjugated Polymers in
Chloroform
Fluorescence
1
structures of model compounds were confirmed by H NMR,
UV–Vis
Emission
Quantum
Yield (Uf, %)a
13C NMR, HRMS, and elemental analysis. Because of the
decreased electronic density of the N-heteroaromatic ring by
N-quaternization, the chemical shifts of the N,N0-dimethyl
groups on the MFþ, MTþ, and MBþ shifted to lower magnetic
field to be 3.7 ppm from 3.5 to 3.6 ppm for neutral com-
pounds MF, MT, and MB.
Polymers
(kabs, nm)
(kem, nm)
PT
PB
PF
PTþ
PBþ
PFþ
a
454
446
423
436
428
402
590
579
545
569
544
527
15
20
21
10
12
12
Optical Properties of Polymers
The UV–vis absorption and fluorescence spectra of neutral
polymers (PT, PB, and PF) and cationic polymers (PTþ, PBþ,
and PFþ) were measured in chloroform (1.0 ꢂ 10ꢁ6 M), and
Absolute fluorescence quantum yield evaluated using an integrating
sphere.
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ARTICLE
parison with the neutral polymers. These results can be
attributed to the restriction of the effective p-conjugation
length at the imidazolium units in the cationic polymers. The
absorption and fluorescence maximum wavelengths are in
the order of PFþ, PBþ, and PTþ, similarly to that for neutral
polymers. Fluorescence quantum yields of PTþ, PBþ, and
PFþ were estimated to be 10, 12, and 12%, respectively,
which were substantially lower than the corresponding neu-
tral polymers. The low fluorescence quantum yields of cati-
onic polymers might be attributed, in some extent to the
external heavy-atom effect of iodide counter anion.57
UV–vis absorption and fluorescence spectra of thin films for
polymers are shown in Figure 2, and results are summarized
FIGURE 1 Absorption and fluorescence spectra of (a) PT (solid
line) and PTþ (dashed line), (b) PB (solid line) and PBþ (dashed
line), and (c) PF (solid line) and PFþ (dashed line). These poly-
mers are dissolved in chloroform at the concentration of 1.0 ꢂ
10ꢁ6 M.
bandgap decreases in that order. Absolute fluorescence quan-
tum yields of PT, PB, and PF, which are also summarized in
Table 1, were measured to be 15, 20, and 21%, respectively.
The low Uf of PT suggested that these donor–acceptor-type
polymers, that is, polymers having intramolecular charge
transfer nature, could suppress fluorescence probability.60
On N-methylation, the absorption bands of cationic polymers
shifted to shorter wavelengths with respect to the corre-
sponding neutral polymers to be 436, 428, and 402 nm for
PTþ, PBþ, and PFþ, respectively (Fig. 1). Furthermore, the
PTþ, PBþ, and PFþ gave fluorescent peaks at 569, 544, and
527 nm, respectively, which also showed blue shifts in com-
FIGURE 2 Absorption and fluorescence spectra of (a) PT (solid line)
and PTþ (dashed line), (b) PB (solid line) and PBþ (dashed line), and
(c) PF (solid line) and PFþ (dashed line) in solid state (films).
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tion profile for 2þ gave no apparent peak above 250 nm
where the chloroform is transparent, the onset of absorption
spectrum is compared with that of 2 (Fig. 4). The peak onset
was 356 nm for 2, whereas that for 2þ was 338 nm, indicat-
ing the restriction of the effective p-conjugation length and
consequent increase in p–p* transition energy by the intro-
duction of cationic charge on N-methylation of the imidazole
ring. The restriction of p-conjugation system would be partly
explained considering crystal structures of 2 and 2þ, which
were obtained by recrystallization from hexane/chloroform
solutions (Fig. 5). As shown in Figure 5(a), for the neutral
model compound 2, the dihedral angle between a side
thienyl ring (T2) and the central imidazole ring (Im) is 21ꢀ,
TABLE 2 Optical Properties of Conjugated Polymers
in Solid State
Fluorescence
UV–Vis
(kabs, nm)a
Emission
(kem, nm)a
Quantum
Yield (Uf, %)a,b
Polymers
PT
PB
PF
PTþ
PBþ
PFþ
a
450
430
407
438
422
393
657
600
550
633
586
542
11
18
10
8
10
10
Cast films on glass slides.
b
Absolute fluorescence quantum yield evaluated using an integrating
sphere.
in Table 2. The kabs of PT, PB, and PF were 450, 430, and
407 nm, respectively. Though the kabs of neutral polymers in
the solid state were close to or slightly shorter than those in
chloroform, the onsets of absorption spectra were shifted to
longer wavelength in comparison with solution state, and
the half bandwidths were broadened. Additionally, the kem of
PT, PB, and PF in the solid state were at 657, 600, and 550
nm, respectively, which are at apparently longer wavelength
compared with each solution state (see Table 1). These
results suggest the partial aggregation of polymers in the
thin films. The lower absolute fluorescence quantum yields
for polymer films than those of solutions also support the
possibility of aggregation in the solid state. The both absorp-
tion and fluorescence maxima are in the order of PF, PB, and
PT as well as the trend observed for solution state. As
observed for chloroform solutions, the cationic polymers also
showed blue shifts in both absorption and fluorescence spec-
tra in comparison with the corresponding neutral polymers,
indicating the restriction of effective p-conjugation length in
the solid state.
Optical Properties of Model Compounds
The optical properties of neutral model compounds (MT,
MB, MF, and 2) and cationic model compounds (MTþ, MBþ,
MFþ, and 2þ) were also studied by the UV–vis absorption
and fluorescence spectroscopy in chloroform (Fig. 3 and Ta-
ble 3). The values of kabs for MT, MB, and MF were 375,
350, and 374 nm, respectively, and those for cationic model
compounds, MTþ, MBþ, and MFþ were 354, 341, and 367
nm, respectively. Furthermore, kem of MT, MB, and MF were
542, 530, and 545 nm, respectively, and those of cationic
model compounds, MTþ, MBþ, and MFþ were 510, 514, and
524 nm, respectively. The blue shifts for both absorption and
fluorescence spectra for cationic model compounds in com-
parison with the corresponding neutral compounds were
attributed to the shortening of effective p-conjugation and
observed for the corresponding polymers.
To discuss the origin of shortening of effective p-conjugation
length on the introduction of cationic charge, more simplified
units 2 and 2þwere investigated by absorption and fluores-
cent spectroscopy and X-ray crystallography. Because absorp-
FIGURE 3 Absorption and fluorescence spectra of (a) MT (solid
line) and MTþ (dashed line), (b) MB (solid line) and MBþ
(dashed line), and (c) MF (solid line) and MFþ (dashed line) in
chloroform (concentration ¼ 1.0 ꢂ 10ꢁ6 M).
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TABLE 3 Optical Properties of Model Compounds
in Chloroform
Quantum
Yield
(Uf, %)a,b
UV–Vis
(kabs, nm)a
Emission
(kem, nm)a
Compounds
MT
MB
MF
MTþ
MBþ
MFþ
2
375
350
374
354
341
367
356c
338c
542
530
545
510
514
524
429
431
17
28
28
14
19
15
d
–
2þ
–
d
FIGURE 4 Absorption and fluorescence spectra of 2 (solid line)
and 2þ (dashed line) in chloroform (concentration ¼ 1.0 ꢂ 10ꢁ6
a
In chroloform.
M).
b
Absolute fluorescence quantum yield evaluated using an integrating
sphere.
c
Onset wavelength.
p-conjugation system was disconnected at all connections
between each aryl unit. The distances between N-methyl
protons (H9, H10, and H11) and C(16) of T3 ring and
between the other N-methyl protons at the opposite side
(H6, H7, and H8) and C(12) of T4 ring were both about 2.7
Å, which clearly suggests that the intramolecular CH-p inter-
actions partly stabilizes the twisted molecular conformation.
These intramolecular interactions should be also effective in
solution state and stabilize the twisted conformation in com-
bination with steric hindrance between the central imidazo-
lium ring and neighboring thiophenes. Thus, N,N0-dimethyla-
tion of imidazole unit for conjugated polymers and the
corresponding monomers induces the restriction of p-conju-
gation length in the main chain due to the twisting between
aryl rings, resulting in blue shifts of absorption and
d
Not measured.
whereas the dihedral angle between T1 and Im is 71ꢀ. The
p-conjugation is, therefore, restricted between T2 and Im
and mostly disconnected between T1 and Im. Additionally,
the crystal structure of 2 indicates that the distance between
N-methyl protons(H1, H2, and H3) and C(11) of T1 is about
2.7 Å, which is shorter than the sum of the van der Waals
radii of methyl proton (1.2 Å) and the carbon of thienyl ring
(1.7 Å). Thus, the intramolecular CH-p interaction would be
partly responsible for the twisted conformation between T1
and Im. For the cationic 2þ, as shown in Figure 5(b), the di-
hedral angles between T3 and Imþ and between T4 and Imþ
are 80ꢀ and 66ꢀ, respectively. These values indicate that the
FIGURE 5 ORTEP drawings of (a) 2 and (b) 2þ in crystals. Thermal ellipsoids were shown at the 50% probability levels. Hydrogen
atoms are omitted for clarity except for methyl protons.
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JOURNAL OF POLYMER SCIENCE PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
fluorescence spectra. Despite significant blue shift for
absorption band from 2 to 2þ, emission spectra for 2 and
2þ are almost comparable (Fig. 4). Although the reason for
that is not clear at this moment, it might be attributed to the
large difference of molecular conformation or interaction
with counter anion (Iꢁ) for 2þ between its ground state and
excited state in comparison with 2.50 The large Stokes shift
for 2þ compared with
2 in Figure 4 supports this
hypothesis.
Electrochemistry
The electrochemical properties of the neutral polymers (PT,
PB, and PF) and cationic polymers (PTþ, PBþ, and PFþ)
were then investigated through cyclic voltammetry (CV) to
evaluate the change of the HOMO and LUMO energy levels
from the reference energy level of half-wave potential of fer-
rocene/ferrocenium redox couple (4.8 eV below the vacuum
level)61 and energy gaps by introducing cationic charges in
the main chain. Figure 6 shows the cyclic voltammograms of
neutral and cationic polymers, and the results are listed in
Table 4. From the reduction process, as shown in Figure 6,
the onset reduction potentials of PT, PB, and PF were found
at ꢁ1.83, ꢁ1.90, and ꢁ1.84 V versus Ag/Agþ, respectively,
and those of PTþ, PBþ, and PFþ were found at ꢁ1.01,
ꢁ0.99, and ꢁ1.12 V versus Ag/Agþ, respectively. This result
indicates that the electron accepting capability of cationic
imidazolium unit is stronger than that of neutral imidazole
unit. In the similar manner, we compared the reduction
potentials of model compounds 2 with 2þ (see Supporting
Information, Fig. S28) and found that the onset reduction
potentials of 2þ were evaluated to be ꢁ2.11 V versus Ag/
Agþ, which was higher by 0.10 V compared with that of 2
(the onset reduction potential of 2 was evaluated to be
ꢁ2.21 V versus Ag/Agþ). Meanwhile, the onset oxidation
potentials of PT, PB, and PF were evaluated to be 0.33, 0.44,
and 0.40 V versus Ag/Agþ, and the HOMO-LUMO energy
gaps (Eg) of neutral polymers PT, PB, and PF were then cal-
culated to be 2.16, 2.35, and 2.24 eV, respectively, from the
difference in redox potentials. These values are in good
agreement with those estimated from onsets of absorption
peaks (Fig. 1). Because the oxidation peaks of iodide counter
anion appeared above 0.3 V versus Ag/Agþ for all cationic
polymers, we found the difficulty in the direct determination
of HOMO energy of cationic polymers from the oxidation
onset of p-conjugated backbone. Alternatively, we compared
the oxidation peak positions of cationic polymers with those
of the corresponding neutral polymers. The oxidation peak
of p-conjugated system of cationic polymer was observed as
the third oxidation peak, which follows the first and second
oxidation peaks of iodide at about 0.5 and 1.0 V, respectively.
The oxidation peaks appeared at 1.44, 1.39, and 1.45 V for
PTþ, PBþ, and PFþ, respectively, which are apparently higher
than those observed for the corresponding neutral polymers
by about 0.2 V (Fig. 6). The introduction of cationic charge
made the p-conjugated system resistant to oxidation. We
also estimate the HOMO energy levels of cationic polymers
from the optical band gaps (taken as the absorption onsets
of polymers in Fig. 1). As shown in Figure 1, the absorption
FIGURE 6 Cyclic voltammograms of (a) PT (solid line) and PTþ
(dashed line), (b) PB (solid line) and PBþ (dashed line), and (c)
PF (solid line) and PFþ (dashed line). Supporting electrolyte:
0.1
M
of
tetramethylammonium
hexafluorophosphate
(TMAPF6) in acetonitrile. Scan rate: 50 mV/s.
onsets of PTþ, PBþ, and PFþ were 535, 515, and 486 nm,
respectively, and Eg of cationic polymers PTþ, PBþ, and PFþ
were consequently calculated to be 2.32, 2.41, and 2.57 eV,
respectively. These values are substantially larger than those
for the corresponding neutral polymers, reflecting the
restriction of p-conjugation. The HOMO energy levels of cati-
onic polymers were then calculated to be ꢁ6.03, ꢁ6.14, and
ꢁ6.17 eV for PTþ, PBþ, and PFþ, respectively (see Support-
ing Information Fig. S29), from the values of Eg (optical) and
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TABLE 4 Electrochemical Properties and Bandgap Energies of the Conjugated Polymers
Eox (V vs.
Ag/Agþ)a
Ered (V vs.
Ag/Agþ)a
HOMO
(eV)b
LUMO
(eV)b
Eg
Compounds
(eV)
PT
PB
PF
PTþ
PBþ
PFþ
2
0.33
0.44
0.40
ꢁ1.83
ꢁ1.90
ꢁ1.84
ꢁ1.01
ꢁ0.99
ꢁ1.12
ꢁ2.14
ꢁ2.00
ꢁ5.05
ꢁ5.16
ꢁ5.12
ꢁ2.89
ꢁ2.82
ꢁ2.88
ꢁ3.71
ꢁ3.73
ꢁ3.60
ꢁ2.51
ꢁ2.61
2.16
2.34
2.24
c
c
–
–
2.32d
2.41d
2.57d
2.77
c
c
–
–
c
c
–
–
0.56
ꢁ5.28
2þ
–
–
2.96d
c
c
a
Onset of oxidation and reduction curve.
b
HOMO ¼ [ꢁ(Eonset,
ꢁ 0.08) ꢁ 4.8] eV and LUMO ¼ [ꢁ(Eonset,
ꢁ 0.08) ꢁ 4.8] eV, where 0.08 V is the
ox
red
value for the half-wave potential of ferrocene/ferrocenium versus Ag/Agþ and 4.8 eV is the energy level of
ferrocene below the vacuum.
c
Not determined due to the oxidation of Iꢁ.
d
Estimated from optical energy gap.
measured Ered. The increase of ionization potential for cati-
onic polymers is consistent with the shift of oxidation poten-
tial to positive direction on the introduction of cationic
charge. Although the direct impact of the introduction of cat-
ionic charge on the conjugated system was difficult to be
studied due to the conformational change, the LUMO state
certainly shifted to negative direction by more than 0.7 eV,
indicating the enhancement of electron affinity. The design of
monomer containing imidazole ring, which prohibits the con-
formational twisting on N,N0-dialkylation of imidazole unit,
would be desirable for p-conjugated polymers with a sub-
stantially narrow bandgap and a low LUMO energy, and
those studies are currently under way by these laboratories.
This work was partly supported by a Grant-Aid for Science
Research (B) from the Ministry of Education, Culture, Sports,
Science, and Technology (MEXT) of Japan.
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