Synthesis and light-emitting properties of carbazole-based copolymers bearing cyano-substituted arylenevinylene chromophore

Article abstract of DOI:10.1002/pola.23764

Two arylenevinylene compounds bearing the cyano group at α-position (6) and β-position (9) from the dialkoxylphenylene unit were synthesized, in which the molecular termini were functionalized with 3-bromocarbazole. The Suzuki coupling copolymerization of these compounds with 1,4-bis[(3′- bromocarbazole-9′-yl)methylene]-2,5-didecyloxybenzene and 9,9-dihexylfluorene-2,7-bis(boronic acid) was carried out to obtain copolymers (cp67 and cp97) containing the cyano-substituted arylenevinylene fluorophore of 7 mol %. Model compounds (6′ and 9′) corresponding to the arylenevinylene fluorophore were also prepared. The UV spectra of copolymers resembled that of homopolymer hp with no arylenevinylene segment in both CHCl₃ solution and thin film. The emission maxima of copolymers In CHCl₃ (394 nm) agreed with that of homopolymer indicating that the emission bands originated from the carbazole-fluorene-carbazole segment. The emission maximum wavelength of copolymer cp67 in thin film (477 nm) indicated fluorescence from the cyano-substituted arylenevinylene fluorophore because of the occurrence of fluorescence resonance electron transfer. In contrast, copolymer cp97 showed fluorescence at 528 nm to suggest the formation of a new emissive species such as a charge-transfer complex (exciplex).

Full text of DOI:10.1002/pola.23764

Synthesis and Light-Emitting Properties of Carbazole-Based Copolymers
Bearing Cyano-Substituted Arylenevinylene Chromophore
KOJI TAKAGI, TSUYOSHI NAKAGAWA, HIDENOBU TAKAO
Department of Materials Science and Engineering, Nagoya Institute of Technology, Gokiso, Showa, Nagoya 466-8555, Japan
Received 27 July 2009; accepted 28 September 2009
DOI: 10.1002/pola.23764
Published online in Wiley InterScience (www.interscience.wiley.com).
ABSTRACT: Two arylenevinylene compounds bearing the cyano
group at a-position (6) and b-position (9) from the dialkoxyl-
phenylene unit were synthesized, in which the molecular ter-
mini were functionalized with 3-bromocarbazole. The Suzuki
coupling copolymerization of these compounds with 1,4-
bis[(30-bromocarbazole-90-yl)methylene]-2,5-didecyloxybenzene
and 9,9-dihexylfluorene-2,7-bis(boronic acid) was carried out to
obtain copolymers (cp67 and cp97) containing the cyano-sub-
stituted arylenevinylene fluorophore of 7 mol %. Model com-
pounds (60 and 90) corresponding to the arylenevinylene
fluorophore were also prepared. The UV spectra of copolymers
resembled that of homopolymer hp with no arylenevinylene
segment in both CHCl3 solution and thin film. The emission
maxima of copolymers in CHCl3 (394 nm) agreed with that of
homopolymer indicating that the emission bands originated
from the carbazole-fluorene-carbazole segment. The emission
maximum wavelength of copolymer cp67 in thin film (477 nm)
indicated fluorescence from the cyano-substituted aryleneviny-
lene fluorophore because of the occurrence of fluorescence
resonance electron transfer. In contrast, copolymer cp97
showed fluorescence at 528 nm to suggest the formation of a
new emissive species such as a charge-transfer complex (exci-
C
V
plex). 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym
Chem 48: 91–98, 2010
KEYWORDS: carbazole; copolymerization; cyano-substituted ary-
lenevinylene; fluorescence; fluorescence resonance electron
transfer; light-emitting diodes
INTRODUCTION Since the discovery of light-emitting proper-
ties in the electroluminescent device using poly(p-phenylene-
vinylene),1 many conjugated polymers including polyfluor-
ene, polyphenylenevinylene, and polythiophene have been
developed for the purpose of commercialization of a poly-
mer-based full color display. White-light-emitting diodes
have recently drawn much attention as next generation flat
panel light sources. Poly(p-arylenevinylene)s are regarded as
the most important materials in part because the carbon–
carbon bond formation can occur by several methods (Wes-
sling route, Gilch route, and Wittig-Horner reaction) without
using expensive metal complexes. The introduction of appro-
priate donor and acceptor pairs,2 the control of stereochem-
istry (E or Z) in the vinylene moiety,3 and the intensive
reduction of conjugation length with insulating spacers4
affect the band gap energy, and thereby the emission color
of poly(p-arylenevinylene)s can be tuned. The attachment of
an electron-withdrawing cyano group can not only decrease
the lowest unoccupied molecular orbital (LUMO) level of
materials to impart the electron affinity but also induce the
bathochromic shift of photoluminescence and electrolumi-
nescence spectra.5 Generally, the extension of chain length is
a straightforward protocol to obtain longer wavelength emis-
sions. However, the maximum electron delocalization is
dominated by the chemical structure in the repeating unit,
and the fully conjugated system sometimes brings about
problems in the light emission of solid devices. The time-de-
pendent changes of an emission color were observed for
conjugated polymers with rigid backbone owing to inevitable
structural defects and/or chain aggregations.6 From these
viewpoints, poly(oligomer)s that join the merits of conju-
gated oligomer blocks having the minimized structural defect
with the long-term morphological stability of polymers are
attractive candidates for light-emitting materials.4,7 It is
known that cyano-substituted arylenevinylene oligomers
have potential enough to cover the wide-ranging visible
spectrum.8
Recently, we have reported the preparation of luminescent
poly(oligomer)s containing
a carbazole-fluorene-carbazole
segment separated by the p-xylylene insulating unit and their
optical properties.9 It is to be emphasized that copoly-
mers carrying a small amount of 2,5-bis(phenylethenyl)-4-
decyloxyanisole (PEDA) segment emitted photoluminescence
not only from the carbazole-fluorene-carbazole segment but
also from the PEDA one through the fluorescence resonance
energy transfer (FRET) especially in thin film. A copolymer
carrying the PEDA segment of only 5 mol % exhibited the
brightest emission among examined copolymers with the
PEDA segment ranging from 5 to 50 mol %. This result
Additional Supporting Information may be found in the online version of this article. Correspondence to: K. Takagi (E-mail: takagi.koji@nitech.ac.jp)
C
V
Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 48, 91–98 (2010) 2009 Wiley Periodicals, Inc.
PROPERTIES OF CARBAZOLE-BASED COPOLYMERS, TAKAGI, NAKAGAWA, AND TAKAO
91

JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
indicates the presence of energy-harvesting phenomenon,
namely, the excited energy gained by the carbazole-fluorene-
carbazole segment is efficiently concentrated into the PEDA
one. In this contribution, we will investigate the utility of
cyano-substituted arylenevinylene segments as the energy-
accepting and light-emitting fluorophore with the aim of pur-
suing the possibility of our FRET system.
1H NMR (d, CDCl3) 6.90 (2H), 3.96 (2H, t, J ¼ 6.5 Hz), 3.83
(3H, s), 3.69 (2H), 3.68 (2H), 1.78 (2H, m), 1.51–1.20 (14H),
0.87 (3H, t, J ¼ 6.6 Hz).
4-[(30-Bromocarbazole-90-yl)methylene]benzyl Chloride (4)
To a solution of 2 (2.0 g, 8.1 mmol) and p-xylylene chloride
(3.2 g, 18 mmol) in THF (20 mL) was added NaH (60% oil
suspension) (0.39 g, 9.8 mmol), and the mixture was heated
ꢀ
at 50 C for 3 h. The reaction system was filtered to remove
EXPERIMENTAL
inorganic salts and the filtrate was concentrated to dryness.
After an EtOAc soluble part was recovered, the crude prod-
uct was washed with Et2O and dried in vacuo to give color-
Materials
All reactions were performed under dry nitrogen atmosphere.
Dry tetrahydrofuran (THF) was purchased from Kanto Chemi-
cal. Tetrakis(triphenylphosphine)palladium(0) [Pd(PPh3)4] for
the Suzuki coupling polymerization was purchased from TCI.
Dimethylsulfoxide (DMSO), N,N-dimethylformamide (DMF),
and ethanol (EtOH) were dried and distilled according to the
general methods. The synthesis of 1,4-bis[(30-bromocar-
bazole-90-yl)methylene]-2,5-didecyloxybenzene, 9,9-dihexyl-
fluorene-2,7-bis(boronic acid), 2,5-bis(bromomethyl)-4-decyl-
oxyanisole (1), and 3-bromocarbazole (2) was reported
earlier.9 The preparation of homopolymer hp with no arylene-
vinylene segment was also described previously.9
ꢀ
less solid in 1.6 g (50% yield). Mp. 133–136 C.
1H NMR (d, CDCl3) 8.20 (1H, d, J ¼ 1.9 Hz), 8.04 (1H, d, J ¼
7.8 Hz), 7.52–6.99 (9H), 5.44 (2H, s), 4.49 (2H, s).
4-[(30-Bromocarbazole-90-yl)methylene]benzaldehyde (5)
A solution of 4 (1.6 g, 4.2 mmol) and sodium periodate
(1.3 g, 6.3 mmol) in DMF (40 mL) was heated to reflux for
6 h. The mixture was poured into water and extracted with
CH2Cl2. The organic phase was washed with saturated aq.
Na2S2O3, water, and brine in this order and then dried over
MgSO4. After removing solvents, the crude product was puri-
fied by SiO2 column chromatography (hexane/CH2Cl2 ¼ 1:2
in vol %, Rf ¼ 0.32) to give colorless solid in 0.50 g (34%
Instrumentation
ꢀ
1H nuclear magnetic resonance (NMR) spectra were
recorded on a Bruker Avance 200 FT-NMR spectrometer
using tetramethylsilane (d 0.00) as an internal reference
peak. Infrared (IR) spectra were recorded on a JASCO FTIR
460Plus spectrophotometer in the ATR method. Melting
points (Mp) were determined on a YANAGIMOTO micro melt-
ing point apparatus MP-J3. Elemental analyses (EA) were
performed on a Perkin-Elmer PE2400II by the CHN mode.
Gel permeation chromatographic (GPC) analyses were per-
formed on a Tosoh DP-8020 using tandem TSK Multipore
HXL-M columns (THF as an eluent, flow rate ¼ 1.0 mL/min,
40 ꢀC) equipped with a refractive index detector (RI-8010).
Number-averaged molecular weight (Mn) and molecular
weight distribution (Mw/Mn) were determined on the basis
of a calibration curve made from standard polystyrene sam-
ples. Ultraviolet (UV) and photoluminescence (PL) spectra
were collected on a Shimadzu UV-1650PC spectrophotometer
and a Shimadzu RF-5300PC spectrofluorometer, respectively,
using a 1 cm quartz cell or a 1 cm ꢁ 5 cm quartz plate. Flu-
orescence quantum yields (QYs) of copolymers in solution
were determined relative to quinine sulfate in 0.5 M H2SO4
having a QY of 0.55. Relative QYs in thin film were calculated
by assuming a QY of homopolymer hp in thin film to be 1.0.
yield). Mp. 144–147 C.
1H NMR (d, CDCl3) 9.92 (1H, s), 8.23 (1H, d, J ¼ 1.9 Hz),
8.07 (1H, d, J ¼ 7.4 Hz), 7.76 (2H, d, J ¼ 8.3 Hz), 7.52–7.12
(7H), 5.52 (2H, s).
(E,E0)-2,5-Bis[a-cyano-40-(300-bromocarbazole-900-yl)
methylene]styryl-4-decyloxyanisole (6)
To a solution of 3 (0.05 g, 0.14 mmol) and 5 (0.11 g, 0.29
mmol) in THF/EtOH (4 mL/2 mL) was added t-BuOK (0.04 g,
0.35 mmol), and the mixture was heated to reflux for 4 h. The
reaction system was poured into water and an aqueous phase
was extracted with EtOAc. The combined organic phase was
dried over MgSO4. After removing solvents, the crude product
was washed with acetone several times and dried in vacuo to
give yellow solid in 0.04 g (24% yield). Mp. 238–241 ꢀC.
1H NMR (d, CDCl3) 8.23 (2H, d, J ¼ 1.8 Hz), 8.07 (2H, d, J ¼
7.8 Hz), 7.79–7.75 (4H), 7.65–7.12 (16H), 7.05 (1H), 7.03
(1H), 5.54 (4H, s), 4.01 (2H, t, J ¼ 6.4 Hz), 3.87 (3H, s), 1.78
(2H, m), 1.47–1.14 (14H), 0.84 (3H, t, J ¼ 6.4 Hz). IR (cmꢂ1
,
ATR) 3049, 2923, 2852, 2213, 1596, 1489, 1473, 1456,
1422, 1349, 1331, 1300, 1274, 1227, 1206, 1124, 1034, 912,
857, 802, 789, 764, 738, 718, 656. Found: C, 70.47; H,
5.36%. Calcd for C61H54Br2N4O2: C, 70.79; H, 5.26%.
Monomer Synthesis
2,5-Bis(cyanomethyl)-4-decyloxyanisole (3)
A solution of 1 (3.0 g, 6.7 mmol) and sodium cyanide
(E,E0)-2,5-Bis(a-cyano-40-methyl)styryl-4-decyloxyanisole (60)
To a solution of 3 (0.10 g, 0.29 mmol) in THF (5 mL) were
added sodium amide (0.03 g, 0.73 mmol) and p-tolualdehyde
(0.09 g, 0.73 mmol), and the mixture was heated to reflux
for 2 h. After neutralization with 1 M HCl, an aqueous phase
was extracted with Et2O. The combined organic phase was
dried over MgSO4 and solvents were removed. The crude
product was purified by SiO2 column chromatography (hex-
ꢀ
(0.83 g, 17 mmol) in DMSO (30 mL) was heated at 50 C for
1 h and then at 85 ꢀC for 0.5 h. The mixture was poured
into water and extracted with EtOAc. The organic phase was
dried over MgSO4. After removing solvents, the crude prod-
uct was washed with MeOH several times and dried inꢀvacuo
to give colorless solid in 1.4 g (59% yield). Mp. 92–95 C.
ane/EtOAc
¼
4:1 in volume ratio, Rf
¼
0.58) and
92
INTERSCIENCE.WILEY.COM/JOURNAL/JPOLA

ARTICLE
recrystallization (MeOH/CH2Cl2) to give a yellow solid in
0.03 g (19% yield). Mp. 113–115.
added t-BuOK (0.15 g, 1.4 mmol), and the mixture was
heated to reflux for 4 h. The precipitated solid was collected
by the filtration that was washed with EtOH and dried in
vacuo to give orange solid in 0.21 g (70% yield). Mp. 139–
1H NMR (d, CDCl3) 7.80 (4H, d, J ¼ 8.1 Hz), 7.67 (1H, s), 7.56
(1H, s), 7.28 (4H, d, J ¼ 7.7 Hz), 7.09 (1H, s), 7.06 (1H, s), 4.05
(2H, t, J ¼ 6.4 Hz), 3.92 (3H, s), 2.42 (6H, s), 1.84 (2H, m),
1.52–1.16 (14H), 0.87 (3H, t, J ¼ 6.3 Hz). IR (cmꢂ1, ATR)
3031, 2922, 2852, 2212, 1605, 1513, 1499, 1461, 1410, 1392,
1355, 1278, 1215, 1040, 917, 879, 811, 744. Found: C, 81.02;
H, 7.82%. Calcd for C37H42N2O2: C, 81.28; H, 7.74%.
ꢀ
141 C.
1H NMR (d, CDCl3) 8.00 (1H, s), 7.94 (1H, s), 7.90 (1H, s),
7.86 (1H, s), 7.59 (4H, d, J ¼ 7.2 Hz), 7.25 (4H, d, J ¼ 7.2
Hz), 4.11 (2H, t, J ¼ 6.4 Hz), 3.95 (3H, s), 2.40 (6H, s), 1.84
(2H, m), 1.56–1.17 (14H), 0.87 (3H, t, J ¼ 6.1 Hz). IR (cmꢂ1
,
ATR) 3074, 3030, 2920, 2851, 2208, 1612, 1591, 1513,
1496, 1465, 1421, 1393, 1366, 1318, 1296, 1253, 1214,
1129, 1066, 1037, 988, 930, 905, 856, 811, 709, 691, 673.
Found: C, 80.97; H, 7.86%. Calcd for C37H42N2O2: C, 81.28;
H, 7.74%.
2,5-Diformyl-4-decyloxyanisole (7)
A solution of 1 (2.0 g, 4.4 mmol) and sodium periodate
(2.9 g, 13 mmol) in DMF (90 mL) was heated to reflux for
6 h. The mixture was poured into water and extracted with
CH2Cl2. The organic phase was washed with saturated aq.
Na2S2O3, water, and brine in this order and then dried over
MgSO4. After removing solvents, the crude product was puri-
fied by SiO2 column chromatography (hexane/CH2Cl2 ¼ 1:2
in vol %, Rf ¼ 0.55) to give yellow solid in 0.59 g (42%
Polymerization
Typical Example (cp67)
To
a
ternary mixture of 1,4-bis[(30-bromocarbazole-90-
yl)methylene]-2,5-didecyloxybenzene (79 mg, 87 lmol),
monomer 6 (4.8 mg, 4.6 lmol), and 9,9-dihexylfluorene-2,7-
bis(boronic acid) (39 mg, 92 lmol) in THF (5 mL) were
added Pd(PPh3)4 (1.1 mg, 0.92 lmol) and 2 M aq. Na2CO3
(0.92 mL). The mixture was heated to reflux for 48 h. After
pouring the mixture into water, an aqueous phase was
extracted with CH2Cl2. The combined organic phase was
washed with 1 M HCl and dried over MgSO4. The concen-
trated solution was poured into MeOH and the obtained
product was dried in vacuo to give a yellow powder in 0.09
g (89% yield).
ꢀ
yield). Mp. 88–90 C.
1H NMR (d, CDCl3) 10.53 (1H, s), 10.49 (1H, s), 7.45 (2H, s),
4.09 (2H, t, J ¼ 6.4 Hz), 3.97 (3H, s), 1.84 (2H, m), 1.51–1.22
(14H), 0.88 (3H, t, J ¼ 6.1 Hz).
4-[(30-Bromocarbazole-90-yl)methylene]benzyl Cyanide (8)
A solution of 4 (0.60 g, 6.7 mmol) and sodium cyanide (0.09
ꢀ
g, 1.9 mmol) in DMSO (20 mL) was heated at 50 C for 2 h
and then at 85 ꢀC for 2 h. The mixture was poured into
water and extracted with EtOAc. The organic phase was
dried over MgSO4. After removing solvents, the crude prod-
uct was purified by SiO2 column chromatography (the ratio
of hexane/CH2Cl2 was gradually varied from 2:3 (Rf ¼ 0.15)
to 0:1 in vol %) to give colorless solid in 0.40 g (68% yield).
1H NMR (d, CDCl3) 8.44 (2H, bs), 8.24 (2H, d, J ¼ 7.5 Hz),
7.90–7.20 (16H ꢁ 0.93 þ 26H ꢁ 0.07), 7.07 (2H ꢁ 0.07,
bs), 6.30 (2H ꢁ 0.93, bs), 5.62 (4H ꢁ 0.07, bs), 5.50 (4H ꢁ
0.93, bs), 4.01 (2H ꢁ 0.07, bs), 3.88 (3H ꢁ 0.07, bs), 3.59
(4H ꢁ 0.95, t, J ¼ 6.0 Hz), 2.13 (4H, bs), 1.90–1.74 (2H ꢁ
0.07, bs), 1.62–1.00 (32H ꢁ 0.93 þ 14H ꢁ 0.07), 0.96–0.68
(16H ꢁ 0.93 þ 13H ꢁ 0.07). Another copolymer (cp97) was
likewise synthesized using monomer 9 instead of monomer
6 and the polymer structure was confirmed by the 1H NMR
spectrum.
ꢀ
Mp. 92–95 C.
1H NMR (d, CDCl3) 8.22 (1H, d, J ¼ 2.0 Hz), 8.06 (1H, d, J ¼
7.8 Hz), 7.53–7.01 (9H), 5.46 (2H, s), 3.66 (2H, s).
(E,E0)-2,5-Bis[b-cyano-40-(300-bromocarbazole-900-yl)
methylene]styryl-4-decyloxyanisole (9)
To a solution of 7 (0.06 g, 0.19 mmol) and 8 (0.15 g, 0.40
mmol) in THF/EtOH (6 mL/3 mL) was added t-BuOK (0.05
g, 0.48 mmol), and the mixture was heated to reflux for 4 h.
The precipitated solid was collected by the filtration that
was washed with EtOH and dried in vacuo to give orange
solid in 0.18 g (91% yield). Mp. 292–295.
1H NMR (d, CDCl3) 8.43 (2H, bs), 8.24 (2H, d, J ¼ 7.6 Hz),
7.88–7.19 (16H ꢁ 0.93 þ 26H ꢁ 0.07), 6.85 (2H ꢁ 0.07,
bs), 6.30 (2H ꢁ 0.93, bs), 5.61 (4H ꢁ 0.07, bs), 5.50 (4H ꢁ
0.93, bs), 4.07 (2H ꢁ 0.07, bs), 3.90 (3H ꢁ 0.07, bs), 3.59
(4H ꢁ 0.93, t, J ¼ 6.0 Hz), 2.13 (4H, bs), 1.90–1.74 (2H ꢁ
0.07, bs), 1.67–1.00 (32H ꢁ 0.93 þ 14H ꢁ 0.07), 0.96–0.67
(16H ꢁ 0.93 þ 13H ꢁ 0.07).
1H NMR (d, CDCl3) 8.25 (2H, d, J ¼ 1.8 Hz), 8.09 (2H, d, J ¼
7.7 Hz), 7.95 (1H, s), 7.90 (1H, s), 7.85 (1H, s), 7.81 (1H, s),
7.64–7.13 (16H), 5.54 (4H, s), 4.06 (2H, t, J ¼ 6.5 Hz), 3.89
(3H, s), 1.78 (2H, m), 1.47–1.15 (14H), 0.86 (3H, t, J ¼ 6.4
Hz). IR (cmꢂ1, ATR) 3053, 2925, 2852, 2206, 1626, 1592,
1513, 1490, 1473, 1455, 1419, 1365, 1342, 1331, 1314,
1296, 1272, 1252, 1216, 1122, 1032, 913, 855, 840, 802,
787, 763, 738, 718, 692, 658, 625. Found: C, 70.61; H,
5.19%. Calcd for C61H54Br2N4O2: C, 70.79; H, 5.26%.
RESULTS AND DISCUSSION
Synthesis
The preparation route of monomers bearing a cyano-substi-
tuted arylenevinylene core and 3-bromocarbazole terminal
groups is shown in Scheme 1. The bromomethyl group in 1
was converted into the cyanomethyl group (3) and the
formyl group (7) by the substitution reaction and the oxida-
tion reaction, respectively. On the other hand, 2 was treated
with excess p-xylylene chloride to give 4 having the benzyl
chloride group that was further transformed into 5 and 8
(E,E0)-2,5-Bis(b-cyano-40-methyl)styryl-4-decyloxyanisole (90)
To a solution of 7 (0.17 g, 0.54 mmol) and 4-methylbenzyl
cyanide (0.15 g, 1.1 mmol) in THF/EtOH (12 mL/6 mL) was
PROPERTIES OF CARBAZOLE-BASED COPOLYMERS, TAKAGI, NAKAGAWA, AND TAKAO
93

JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
SCHEME 1 Synthetic pathway of monomers (6 and 9) carrying cyano-substituted arylenevinylene fluorophore (reagents are
omitted for clarity).
having the benzaldehyde group and the benzyl cyanide
group, respectively. Finally, the target monomers (6 and 9)
were obtained by the base-catalyzed Knoevenagel-type con-
densation using 3, 5, 7, and 8 in combination. The reactions
proceeded in a stereoselective manner giving rise to E,E0
products concerning the C¼¼C double bond.8(c) The cyano
group was introduced at a-position (6) and b-position (9)
from the dialkoxylphenylene unit. The model fluorophores
(60 and 90) replacing 3-bromocarbazole group in monomers
(6 and 9) with the hydrogen atom were also prepared simi-
larly from 3 with p-tolualdehyde and 7 with 4-methylbenzyl
cyanide, respectively, (Fig. 1).
at a-position was obtained. The copolymer had a good solu-
bility in solvents, such as CH2Cl2, THF, and toluene, thanks to
the nonconjugated methylene spacer on the carbazole nitro-
gen. The Mn and Mw/Mn determined by GPC analysis in THF
were 9700 and 2.4, respectively. The content of cyano-substi-
tuted arylenevinylene segment in copolymer cp67 (7 mol %)
was calculated by comparing the signal integral ratio of
methylene protons adjacent to the arylenevinylene segment
[Fig. 2(e)] with those adjacent to the didecyloxyphenylene
1
unit [Fig. 2(f)] in the H NMR spectrum. Likewise, copolymer
cp97 containing the arylenevinylene segment having the
cyano group at b-position was likewise synthesized using
1,4-bis[(30-bromocarbazole-90-yl)methylene]-2,5-didecyloxy-
benzene, monomer 9, and 9,9-dihexylfluorene-2,7-bis(boronic
acid) in the molar ratio of 95:5:100. The Mn and Mw/Mn
were 7300 and 1.6, respectively. The content of cyano-
The Suzuki coupling copolymerization using 1,4-bis[(30-bro-
mocarbazole-90-yl)methylene]-2,5-didecyloxybenzene, mono-
mer 6, and 9,9-dihexylfluorene-2,7-bis(boronic acid) was
conducted while keeping the molar balance of bromide and
boronic acid to unity. The feed ratio between 1,4-bis[(30-bro-
mocarbazole-90-yl)methylene]-2,5-didecyloxybenzene
and
monomer 6 was set to 95:5 in mol % to obtain a copolymer
having the energy-accepting fluorophore as the minor com-
ponent and to investigate the energy-harvesting phenom-
enon. After precipitating into MeOH, copolymer cp67 con-
taining the arylenevinylene segment having the cyano group
FIGURE 1 Chemical structure of model fluorophores 60 and 90.
FIGURE 2 1H NMR spectrum of copolymer cp67 in CDCl3.
94
INTERSCIENCE.WILEY.COM/JOURNAL/JPOLA

ARTICLE
FIGURE 3 Chemical structure of copolymers cp67 and cp97 and that of homopolymer hp.
substituted arylenevinylene segment in copolymer cp97 was
similarly calculated to be 7 mol %. For comparison, homo-
polymer hp with no arylenevinylene fluorophore was syn-
thesized from 1,4-bis[(30-bromocarbazole-90-yl)methylene]-
2,5-didecyloxybenzene and 9,9-dihexylfluorene-2,7-bis(bor-
onic acid) (Mn ¼ 7000, Mw/Mn ¼ 1.9). The chemical struc-
tures of polymers are illustrated in Figure 3.
ture were also observed at 411 nm in the PL spectra of
copolymers. The QYs of copolymers were 0.11 (cp67) and
0.13 (cp97) being comparable to that of homopolymer hp
(0.12). On the other hand, the QY of a similar copolymer
incorporating PEDA segment of 5 mol % was higher (0.29)
than that of homopolymer hp because of the occurrence of
intramolecular FRET and energy-harvesting phenomenon.9
To get information about whether FRET successfully occurs
or not in copolymers, the relationship between the emission
spectrum of homopolymer hp and the absorption spectra of
model fluorophores (60 and 90) in CHCl3 is compared in Fig-
ure 5. These spectra would correspond to the fluorescence
from the carbazole-fluorene-carbazole segment and the
Optical Properties
The absorption maxima of copolymers (cp67 and cp97) in
CHCl3 were equally detected at 345 nm (Fig. 4) and that of
homopolymer hp was observed at 344 nm (Fig. 4, inset). All
these absorption bands could be ascribed to the p–p* transi-
tion of the carbazole-fluorene-carbazole segment isolated by
the nonconjugated p-xylylene spacer. The shape of UV spec-
tra of copolymers and homopolymer resembled each other
because copolymers mostly consisted of carbazole-fluorene-
carbazole segment. The absorption maximum wavelengths
were shorter by a few nanometers than a fluorene trimer
interconnected at 2,7-positions (348 nm).10 The maximum
effective conjugation may be limited to the phenylene-fluo-
rene-phenylene sequence in the present case because fluo-
rene is connected to 3-position of carbazole ring. In the UV
spectrum of copolymer cp97, an additional small peak origi-
nated from the cyano-substituted arylenevinylene segment
was also observed at ꢃ430 nm, as implicated by the absorp-
tion maximum of model fluorophore 90 at 433 nm (vide
infra). The PL spectra of both copolymers in CHCl3 had peak
maxima at 394 nm when they were irradiated with a light at
345 nm, which would be originated from the carbazole-fluo-
rene-carbazole fluorophore. Similar to our previous report,9
the emission maxima of copolymers (cp67 and cp97) blue-
shifted from that of homopolymer hp (411 nm) by introduc-
ing a small amount of arylenevinylene segment into the
main chain. The peaks assignable to the vibronic fine struc-
FIGURE 4 UV (open mark) and PL (closed mark) spectra of
copolymers cp67 (circle) and cp97 (square) in CHCl3 (10ꢂ5 M).
Excitation wavelengths were 345 nm for both copolymers.
Inset shows UV (open triangle) and PL (closed triangle)
spectra of homopolymer hp in CHCl3 (10ꢂ5 M) (excitation wave-
length ¼ 344 nm).
PROPERTIES OF CARBAZOLE-BASED COPOLYMERS, TAKAGI, NAKAGAWA, AND TAKAO
95

JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
nated from the p–p* transition of the carbazole-fluorene-car-
bazole segment (346 nm) showed ignorable red-shift from
those in CHCl3 (Supporting Information Fig. S11). Owing to
the presence of nonconjugated p-xylylene spacer, a p–p
stacking aggregation between chromophores may be forbid-
den. The emission maximum of copolymer cp67 was
observed at 477 nm that agreed well with that of model flu-
orophore 60 in CHCl3 (Fig. 6). The emission from the carba-
zole-fluorene-carbazole segment having peak maximum at
394 nm was considerably quenched. Although model fluoro-
phore 60 has rather low QY in CHCl3 because of the nonpla-
nar conformation (vide supra), the QY can be dramatically
improved in the frozen state by the aggregation-induced
enhanced emission.11 These results would suggest that the
excited energy gained by the carbazole-fluorene-carbazole
segment is transferred to the cyano-substituted aryleneviny-
lene fluorophore through FRET to give a longer wavelength
emission. In the film state, not only the intrachain but also
the interchain FRET can take place because polymer chains
are close to each other. The emission maximum of copolymer
cp67 (477 nm) is close to that of model fluorophore 60 in
CHCl3 (473 nm) rather than in solid state12 (456 nm) indi-
cating that the cyano-substituted arylenevinylene fluoro-
phores are well dispersed in the polymer matrix. The rela-
tive QY of copolymer cp67 in thin film was roughly
calculated using homopolymer hp as a standard (QY ¼ 1.0).
Although the absolute QY cannot be determined, the light-
emitting potential of homopolymer hp in thin film is prob-
ably low as judged from the emission properties in solution
(QY ¼ 0.12). If the rate of FRET is slower than that of the
nonradiative decay of the carbazole-fluorene-carbazole seg-
ment in the excited state, the calculated QY of copolymer
cp67 does not exceed 1. In this case, the relative QY of co-
polymer cp67 was determined to be 1.6 to indicate the fast
FRET process. For the efficient energy-harvesting system, the
molar balance of energy-donating and energy-accepting
FIGURE 5 PL spectrum of homopolymer hp (closed triangle,
excitation wavelength ¼ 344 nm) along with UV spectrum of
model fluorophores 60 (open circle) and 90 (open square) in
CHCl3 (10ꢂ5 M).
absorption of the cyano-substituted arylenevinylene seg-
ments embedded in copolymers. The absorption spectrum of
model fluorophore 60 had two peak maxima at 310 and 366
nm, whereas that of model fluorophore 90 red-shifted to
have maxima at 356 and 432 nm indicating the wider conju-
gation system. Because the absorption spectrum of model
fluorophore 60 does not efficiently overlap with the emission
spectrum of homopolymer hp, the intramolecular FRET from
the carbazole-fluorene-carbazole segment to the cyano-sub-
stituted arylenevinylene one is supposed to rarely occur in
copolymer cp67. Furthermore, 1-cyano-trans-1,2-bis(40-meth-
ylbiphenyl)ethylene (CN-MBE) molecule is known to be
twisted in solution by the steric interaction of the cyano
group introduced at the vinylene moiety.11 Model fluoro-
phore 60 is thus likely to adopt nonplanar conformation and
have restricted effective conjugation length resulting in
rather low QY (0.006). Accordingly, if the intramolecular
FRET occurs to transfer the excited energy from the carba-
zole-fluorene-carbazole segment to the cyano-substituted
arylenevinylene one, photoluminescence from the energy-
accepting fluorophore cannot be observed. On the contrary,
the absorption spectrum of model fluorophore 90 nicely
overlaps with the emission spectrum of homopolymer hp to
enable the intramolecular FRET from the carbazole-fluorene-
carbazole segment to the cyano-substituted arylenevinylene
one. The QY of model fluorophore 90 is large (0.25) enough
to detect fluorescence emission at 504 nm (Supporting Infor-
mation Fig. S10). From the emission spectrum of copolymer
cp97 shown in Figure 4, however, no intramolecular FRET
seems to occur. It is now speculated that the arrangement of
dipole moments of two chromophores is not adequate and/
or the distance between energy-donating and energy-accept-
ing segments is far away for the efficient Fo¨rster-type FRET
process.
FIGURE 6 PL spectrum of homopolymer hp (closed triangle)
and those of copolymers cp67 (closed circle) and cp97 (closed
square) in thin film. Excitation wavelengths were 346 nm for
all polymers. Inset shows PL spectra of model fluorophores 60
(open circle, excitation wavelength ¼ 365 nm) and 90 (open
square, excitation wavelength ¼ 433 nm) in CHCl3 (10ꢂ5 M).
Finally, the UV and PL spectra of copolymers (cp67 and
cp97) were measured in thin film obtained by spin-coating a
toluene solution of copolymers (ca. 1 mg/mL) in 2000 rpm
for 60 s. In both copolymers, the absorption maxima origi-
96
INTERSCIENCE.WILEY.COM/JOURNAL/JPOLA

ARTICLE
chromophores is one of the most important factors.13 In co-
polymer cp67, one arylenevinylene fluorophore (energy
acceptor) is surrounded by many carbazole-fluorene-carba-
zole chromophores (energy donor) to efficiently collect an
excited energy. We have also synthesized another copolymer
cp611 containing the arylenevinylene segment having the
cyano group at a-position of 11 mol % (not described here),
which had decreased relative QY of 0.65 to further support
the presence of energy-harvesting phenomenon. In sharp
contrast, the emission spectrum of copolymer cp97 showed
bimodal peak consisted of 410 and 528 nm. The emission
band at 410 nm may be ascribed to fluorescence from the
carbazole-fluorene-carbazole segment. The emission maxi-
mum observed at 528 nm obviously red-shifted from that of
energy-accepting model fluorophore 90 in both CHCl3 solu-
tion (Fig. 6, inset) and thin film (Supporting Information Fig.
S12) (504 nm). The relative QY of copolymer cp97 in thin
film (0.63) calculated using homopolymer hp as a standard
(QY ¼ 1.0) was higher than that in CHCl3 (0.13) but unex-
pectedly lower compared with that of copolymer cp67 (1.6),
although model fluorophore 90 was a good light emitter
(QY ¼ 0.25 in CHCl3). These facts may suggest the genera-
tion of a new emissive species such as a charge-transfer
complex (exciplex) resulting from the photo-induced electron
transfer from the electron-rich carbazole-fluorene-carbazole
segment to the electron-poor arylenevinylene one having the
cyano group.14 This hypothesis was supported by the theo-
retical calculation of reference compounds at HF/31-G* level
of theory (Supporting Information). The reference compound
corresponding to the model fluorophore 90 (M90) had rather
planar structure compared with that of model fluorophore 60
(M60), namely, the dihedral angle made of central phenylene
ring and cyano-substituted vinylene group was 37ꢀ in M90,
whereas that was 48ꢀ in M60 (Supporting Information Fig.
S13). These data also agreed with the experimental results
in the absorption spectra of model fluorophores 60 and 90
(Fig. 5). The HOMO/LUMO energy levels were determined to
be ꢂ7.70 eV/1.55 eV and ꢂ7.56 eV/1.08 eV for M60 and
M90, respectively. The similar theoretical calculation was car-
ried out for the carbazole-fluorene-carbazole segment (MD
in Supporting Information Fig. S13), and HOMO/LUMO
energy levels were determined to be ꢂ6.66 eV/2.37 eV.
Accordingly, the energy level of HOMO of MD was closer to
that of LUMO of M90 than that of M60 to favor the more effi-
cient orbital interaction (charge transfer).
rene-carbazole segment, which indicated the insufficient
FRET and/or the low light-emitting potential of the energy-
accepting fluorophore. In thin film, fluorescence from copoly-
mer cp67 was observed at longer wavelength region through
the intramolecular and intermolecular FRET, and the relative
QY was improved because of the energy-harvesting phenom-
enon. The PL spectrum of copolymer cp97 might suggest a
new emissive species such as a charge-transfer complex
(exciplex) by the photo-induced electron transfer from elec-
tron-rich carbazole-fluorene-carbazole segment to electron-
poor arylenevinylene one having the cyano group.
REFERENCES AND NOTES
1 Burrousghes, J. H.; Bradey, D. D. C.; Brown, A. R.; Marks, R. N.;
Mackay, K.; Friend, R. H.; Burns, P. L.; Holmes, A. Nature 1990, 347,
539–541.
2
(a) Cornil, J.; Beljonne, D.; Dos Santos, D. A.; Bre´das, J. L. Synth Met
1996, 76, 101–104; (b) Thompson, B. C.; Kim, Y.-G.; Reynolds, J. R.
Macromolecules 2005, 38, 5359–5362; (c) Yang, L. Y.; Wang, C.; Li,
L. Q.; Janietz, S.; Wedel, A.; Hua, Y. L.; Yin, S. G. J Polym Sci Part A:
Polym Chem 2007, 45, 4291–4299; (d) Karastatiris, P.; Mikroyannidis,
J. A.; Spiliopoulos, I. K. J Polym Sci Part A: Polym Chem 2008, 46,
2367–2378; (e) Xun, S.; Zhou, Q.; Li, H.; Ma, D.; Wang, L.; Jing, X.;
Wang, F. J Polym Sci Part A: Polym Chem 2008, 46, 1566–1576.
3
(a) Son, S.; Dodabalapur, A.; Lovinger, A. J.; Galvin, M. E. Science
1995, 269, 376–378; (b) Holzer, W.; Penzkofer, A.; Gong, S. H.; Bleyer,
A.; Bradley, D. D. C. Adv Mater 1996, 8, 974–978; (c) Cassano, T.;
Tommasi, R.; Babudri, F.; Cardone, A.; Farinola, G. M.; Naso, F. Synth
Met 2003, 139, 827–830; (d) Katayama, H.; Nagao, M.; Nishimura, T.;
Matsui, Y.; Fukuse, Y.; Wakioka, M.; Ozawa, F. Macromolecules 2006,
39, 2039–2048; (e) Wang, F.; He, F.; Xie, Z.; Li, M.; Hanif, M.; Gu, X.;
Yang, B.; Zhang, H.; Lu, P.; Ma, Y. J Polym Sci Part A: Polym Chem
2008, 46, 5242–5250.
4
(a) Yang, Z.; Sokolik, I.; Karasz, F. E. Macromolecules 1993, 26,
1188–1190; (b) Brun, P. L.; Kraft, A.; Baigent, D. R.; Bradley, D. D. C.;
Brown, A. R.; Friend, R. H.; Gymer, R. W.; Holmes, A. B.; Jackson, R. W.
J Am Chem Soc 1993, 115, 10117–10124; (c) Hay, M.; Klavetter, F. L. J
Am Chem Soc 1995, 117, 7112–7118; (d) Padmanaban, G.; Nagesh,
K.; Ramakrishnan, S. J Polym Sci Part A: Polym Chem 2003, 41,
3929–3940.
5
(a) Greenham, N. C.; Moratti, S. C.; Bradley, D. D. C.; Friend, R. H.;
Holmes, A. B. Nature 1993, 365, 628–630; (b) Chen, S. A.; Chang, E. C.
Macromolecules 1998, 31, 4899–4907; (b) Yu, Y.; Lee, H.; VanLaeken,
A.; Hsieh, B. R. Macromolecules 1998, 31, 5553–5555; (c) Kraft, A.;
Grimsdale, A. C.; Holmes, A. B. Angew Chem Int Ed 1998, 37,
402–428; (d) Pinto, M. R.; Hu, B.; Karasz, F. E.; Akcelrud, L. Polymer
2000, 41, 2603–2611; (e) De Souza, M. M.; Rumbles, G.; Gould, I. R.;
Amer, H.; Samuel, I. D. W.; Moratti, S. C.; Holmes, A. B. Synth Met
2000, 111/112, 539–543; (f) Li, H.; Hu, Y.; Zhang, Y.; Ma, D.; Wang, L.;
CONCLUSIONS
Two cyano-substituted arylenevinylene fluorophores (6 and
9) sandwiched with 3-bromocarbazole were synthesized, in
which the cyano group was introduced at a-position (6)
and b-position (9) from the dialkoxylphenylene unit. The
Suzuki coupling copolymerization of these monomers with
1,4-bis[(3-bromocarbazole-9-yl)methylene]-2,5-didecyloxyben-
zene and 9,9-dihexylfluorene-2,7-bis(boronic acid) gave rise
to copolymers (cp67 and cp97) bearing arylenevinylene fluo-
rophores of 7 mol %. Copolymers in CHCl3 had the absorp-
tion and emission maxima originated from the carbazole-fluo-
Jing, X.; Wang, F.
3947–3953.
J Polym Sci Part A: Polym Chem 2004, 42,
6
(a) Jenekhe, S. A.; Osaheni, J. A. Science 1994, 265, 765–768; (b)
Weinfurtner, K.-H.; Fujikawa, H.; Tokito, S.; Taga, Y. Appl Phys Lett
2000, 76, 2502–2504; (c) List, E. J. W.; Gu¨ntner, R.; De Freitas, P. S.;
Scherf, U. Adv Mater 2002, 14, 374–378; (d) Gaal, M.; List, E. J. W.;
Scherf, U. Macromolecules 2003, 36, 4236–4237.
PROPERTIES OF CARBAZOLE-BASED COPOLYMERS, TAKAGI, NAKAGAWA, AND TAKAO
97

JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
7
(a) Chen, R.-M.; Chien, K.-M.; Wong, K.-T.; Jin, B.-Y.; Luh, T.-Y. J Am
10 Klaemer, G.; Miller, R. D. Macromolecules 1998, 31, 2007–2009.
Chem Soc 1997, 119, 11321–11322; (b) Miao, Y.-J.; Bazan, G. C. Mac-
romolecules 1997, 30, 7414–7418; (c) Kim, D. S.; Shim, S. C. J Polym
Sci Part A: Polym Chem 1999, 37, 2263–2273; (d) Jung, S. H.; Kim, H.
K.; Kim, S. H.; Kim, Y. H.; Jeoung, S. C.; Kim, D. Macromolecules 2000,
33, 9277–9288; (e) Kwak, G.; Masuda, T. Macromol Rapid Commun
2001, 22, 846–849.
11 An, B.-K.; Kwon, S.-K.; Jung, S.-D.; Park, S. Y. J Am Chem Soc 2002,
124, 14410–14415.
12 The spectra of model fluorophores in solid state were obtained af-
ter spin-coating a CHCl3 solution (20 mg/mL) of a binary blend with
PMMA (Mn ¼ 700,000) matrix on a quartz plate.
13 (a) Vamvounis, G.; Schulz, G. L.; Holdcroft, S. Macromolecules
2004, 37, 8897–8902;13 (b) Vamvounis, G.; Holdcroft, S. Adv Mater
2004, 16, 716–719.
8
(a) Detert, H.; Sugiono, E. Synth Met 2000, 115, 89–92; (b) Li,
C.-L.; Shieh, S.-J.; Lin, S.-C.; Liu, R.-S. Org Lett 2003, 5, 1131–1134; (c)
Belleteˆte, M.; Morin, J.-F.; Leclerc, M.; Durocher, G. J Phys Chem A
2005, 109, 6953–6959; (d) Ryu, H.; Subramanian, L. R.; Hanack, M.
Tetrahedron 2006, 62, 6236–6247.
14 (a) Shim, H. K.; Kang, I. N.; Jang, M. S.; Zyung, T.; Jung, S. D. Mac-
romolecules 1997, 30, 7749–7752; (b) Zhang, X.; Kale, D. M.; Jenekhe,
S. A. Macromolecules 2002, 35, 382–393; (c) Hwang, S.-W.; Chen, Y.
Macromolecules 2002, 35, 5438–5443.
9 Takagi, K.; Nakagawa, T.; Takao, H. J Polym Sci Part A: Polym Chem
2008, 46, 8141–8148.
98
INTERSCIENCE.WILEY.COM/JOURNAL/JPOLA