Synthesis and fluorescent properties of model compounds for conjugated polymer containing maleimide units at the main chain

Article abstract of DOI:10.1002/pola.24792

2,3-Diaryl substituted maleimides as model compounds of conjugated maleimide polymers [poly(RMI-alt-Ar) and poly(RMI-co-Ar)] were synthesized from 2,3-dibromo-N-substituted maleimide (DBrRMI) [R= cyclohexyl (DBrCHMI) and n-hexyl (DBrHMI)] and aryl boronic acid using palladium catalysts. To clarify structures of conjugated polymer containing maleimide units at the main chain, ¹³C NMR spectra of 2-aryl or 2,3-diaryl substituted maleimides were compared with those of N-substituted maleimide polymers. Copolymers obtained with DBrRMI via Suzuki-Miyaura cross-coupling polymerizations or Yamamoto coupling polymerizations were dehalogenated structures at the terminal end. This dehalogenation may contribute to the low polymerizability of DBrRMIs. On the other hand, the π-conjugated compounds showed high solubility in common organic solvents. The N-substituents of maleimide cannot significantly affect the photoluminescence spectra of 2,3-diaryl substituted maleimides derivatives. The fluorescence spectra of poly(RMI-alt-Ar) and poly(RMI-co-Ar) varied with N-substituents of the maleimide ring. When exposed to ultraviolet light of wavelength 352 nm, a series of 1,4-phenylene- and/or 2,5-thienylene-based copolymers containing N-substituted maleimide derivatives fluoresced in a yellow to blue color. It was found that photoluminescence emissions and electronic state of π-conjugated maleimide derivatives were controlled by aryl- and N-substituents, and maleimide sequences of copolymers.

Full text of DOI:10.1002/pola.24792

Synthesis and Fluorescent Properties of Model Compounds for
Conjugated Polymer Containing Maleimide Units at the Main Chain
KENJIRO ONIMURA, MIEKO MATSUSHIMA, MUNETOSHI NAKAMURA, TATSUYA TOMINAGA,
KAZUHIRO YAMABUKI, TSUTOMU OISHI
Department of Applied Chemistry, Graduate School of Science and Engineering, Yamaguchi University,
2-16-1 Tokiwadai, Ube, Yamaguchi 755-8611, Japan
Received 10 November 2010; accepted 23 May 2011
DOI: 10.1002/pola.24792
Published online 8 June 2011 in Wiley Online Library (wileyonlinelibrary.com).
ABSTRACT: 2,3-Diaryl substituted maleimides as model com-
pounds of conjugated maleimide polymers [poly(RMI-alt-Ar)
and poly(RMI-co-Ar)] were synthesized from 2,3-dibromo-N-
substituted maleimide (DBrRMI) [R¼ cyclohexyl (DBrCHMI) and
n-hexyl (DBrHMI)] and aryl boronic acid using palladium cata-
lysts. To clarify structures of conjugated polymer containing
maleimide units at the main chain, 13C NMR spectra of 2-aryl
or 2,3-diaryl substituted maleimides were compared with those
of N-substituted maleimide polymers. Copolymers obtained
with DBrRMI via Suzuki-Miyaura cross-coupling polymeriza-
tions or Yamamoto coupling polymerizations were dehalogen-
ated structures at the terminal end. This dehalogenation may
contribute to the low polymerizability of DBrRMIs. On the other
hand, the p-conjugated compounds showed high solubility in
common organic solvents. The N-substituents of maleimide
cannot significantly affect the photoluminescence spectra of
2,3-diaryl substituted maleimides derivatives. The fluorescence
spectra of poly(RMI-alt-Ar) and poly(RMI-co-Ar) varied with N-
substituents of the maleimide ring. When exposed to ultravio-
let light of wavelength 352 nm, a series of 1,4-phenylene- and/
or 2,5-thienylene-based copolymers containing N-substituted
maleimide derivatives fluoresced in a yellow to blue color. It
was found that photoluminescence emissions and electronic
state of p-conjugated maleimide derivatives were controlled by
aryl- and N-substituents, and maleimide sequences of copoly-
C
V
mers. 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym
Chem 49: 3550–3558, 2011
KEYWORDS: conjugated polymer; copolymerization; fluorescen-
ces; N-substituted maleimide
INTRODUCTION p-Conjugated compounds exhibit many
interesting and useful properties, such as strong absorption,
fluorescence, and conductivity. The properties of fluores-
cence are influenced by structures, substituents, molecular
weights, conformations of the compounds, and sequence of
monomer unit at conjugated polymers. The fluorescent com-
pounds showing various colors are most useful. Polymers
with a p-conjugated structure such as polyacetylene, poly-
arylene, polyarylenevinylene, polythiophene and their
derivatives, has become of interest in polymer chemistry
and material science in recent years. Synthetic methods
have been reported to prepare p-conjugated polymers using
transition metal catalyzed polymerization, polycondensa-
tion, and Wittig reaction. As one of the useful polymeriza-
tion method, dehalogenation carbon–carbon coupling poly-
merization by zerovalent palladium or nickel complexes has
been regarded.
thalimide and peryleneimide, which are known to be n-type
organic semiconducting materials for organic transistors.1–4
For instance fluorescence zinc ion sensors based on malei-
mide derivatives have been reported by Cola and coworkers5
Chen and coworkers reported on the efficient and bright red
organic light-emitting diode (OLED) based on nondoping red
emitter a N-methyl-2,3-diarylmaleimide.6,7
The authors reported synthesis of conjugated copolymers
containing N-substituted maleimide using transition metal
catalysis.8 But number-average molecular weights (Mn) of
copolymers obtained by coupling-polymerizations of N-sub-
stituted 2,3-dibromomaleimide (DBrRMI) with diboronic
acid derivatives or dihalide aryl compounds were 680–
4000 by GPC analyses. It is necessary to investigate the
structures of copolymer to clarify the low polymerizabil-
ities of DBrRMI. Dehalogenation carbon–carbon coupling
polymerization such as Suzuki-Miyaura or Yamamoto cou-
pling reactions proceeded by insertions of a transition
metal for halogen–carbon bond, trans-metalations, and
reductive eliminations, repeatedly. But a side reaction may
occur with a dehalogenation and hydrogenation at growth-
terminal.
On the other hand, maleimide moieties can be regarded as
cis olefin, repeating unit s of cis configurational polyacety-
lene, which have been known as materials of organic elec-
tronic devices. Maleimide is an electron-deficient heterocyclic
ring and similar to the larger members of phthalimide, naph-
Correspondence to: K. Onimura (E-mail: onimura@yamaguchi-u.ac.jp)
C
V
Journal of Polymer Science Part A: Polymer Chemistry, Vol. 49, 3550–3558 (2011) 2011 Wiley Periodicals, Inc.
3550
WILEYONLINELIBRARY.COM/JOURNAL/JPOLA

ARTICLE
CHART 1
In this article, we prepared 2,3-diaryl-N-substituted maleimide
(2,3-DArRMI) from DBrRMI such as model compounds of con-
jugated copolymers containing N-substituted maleimide
(Chart 1). The relationship between the properties of fluores-
cence and structures of the conjugated copolymers was inves-
tigated by a detailed comparison of model compounds.
ylamine, potassium carbonate and 2,20-bipyridyl (a,a0-) were
purchased from Kishida Reagents Chemicals. Hexamethyldisi-
lazane was purchased from AZmax. 1,4-Diiodobenzene was
purchased from Merck. Bromine, triethylamine, anhydrous
magnesium sulfate were purchased from Nakalai Tesque.
Bis(1,5-cyclooctadiene) nickel (0), phenylboronic acid, 4-
biphenylboronic acid, zinc chloride, sodium sulfate were
purchased from Kanto Chemical. n-Hexane, ethyl acetate,
tetrahydrofuran, N,N-dimethylformamide, CH2Cl2 and CHCl3
were dried according to the standard procedure and distilled
under nitrogen.
EXPERIMENTAL
Measurements
1H (270 MHz) and 13C (68 MHz) nuclear magnetic resonance
(NMR) spectra were recorded on a JEOL-EX270 apparatus
(JEOL) spectrometer using tetramethylsilane (TMS) (1H
Analytical thin-layer chromatography (TLC) was performed
on Merck silica gel plate 60F254. Column chromatography
was performed with silica gel 60 (0.063–0.200 mm, Merck).
NMR, d 0.00) or CDCl3 (
13C NMR, d 77.0) as internal refer-
ence peaks at room temperature. Splitting patterns are desig-
nated as s (singlet), d (doublet), t (triplet), q (quartet), m
(multiplet), and br (broad). The photoluminescence (PL)
spectra were recorded on a FP-6300 Jasco 4100 (Jasco)
spectrophotometer. Melting points (mp) were determined on
a Yanagimoto micro melting point apparatus. Elemental ana-
lysis was carried out using Micro Corer JM10 (J-Science Lab).
The absolute PL quantum yields were obtained using an
Absolute PL Quantum Yield Measurement System (C9920;
Hamamatsu Photonics K.K.).
RMI was prepared from maleic anhydride and the corre-
sponding primary amine by usual methods.9,10 Compounds
DBrRMI were prepared following literature procedures.11
Synthesis of 2-Aryl and 2,3-diaryl-N-substitued
Maleimide
2-Phenyl-N-cyclohexylmaleimide (2-PhCHMI)
Typical Procedure. A 50 mL three-necked, round-bottomed
flask was equipped with magnetic stirring bar and fitted
with a glass stopper and a nitrogen inlet. The flask was
charged with toluene (15 mL), ethanol (10 mL), and
saturated aqueous sodium hydrogen carbonate (6.5 mL).
2-Bromo-N-cyclohexylmaleimide (1.01 g, 2.97 mmol) and
benzene-1,4-diboronic acid (0.45 g, 3.68 mmol) were added
in single portions, followed by tetrakis-(triphenylphosphine)-
palladium(0) (0.17 g, 0.15 mmol). The reaction mixture was
vigorously stirred at 90 ꢀC for 5 hrs under nitrogen atmos-
phere. After cooling to room temperature, the mixture was
Materials
All chemicals were used without any further purification.
Tetrakis-(triphenylphosphine)palladium(0), 2-thiophenebor-
onic acid, and 1,5-cyclooctadiene were purchased from TCI.
N-Benzyl-2,3-dibromomaleimide and benzene-1,4-diboronic
acid were purchased from Aldrich. Maleic anhydride, dode-
cylamine, 2,5-thiophendiboronic acid, 2,5-dibromothiophene
were purchased from Wako Pure Chemical Industries. n-Hex-
MODEL COMPOUNDS FOR CONJUGATED MALEIMIDE POLYMERS, ONIMURA ET AL.
3551

JOURNAL OF POLYMER SCIENCE PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
added to distilled water (30 mL) and then extracted with
ethyl acetate (50 mL ꢁ 2). The combined organic extract
phase was washed with saturated aqueous NaCl (50 mL ꢁ
3). After drying over anhydrous magnesium sulfate, the fil-
trate concentrated in vacuo. The residue was purified by
chromatography on silica gel, using n-hexane/ethyl acetate
(20/1 (v/v), Rf ¼ 0.17) to afford PhCHMI (0.65 g, 2.55
mmol) as a pale yellow solid.
TMS): 0.88 (3H, t, J ¼ 6.6 Hz, ACH3), 1.25–1.32 (18H, m, do-
decyl), 1.66 (2H, t, J ¼ 7.0 Hz, dodecyl), 3.62 (2H, t, J ¼ 7.3
Hz, ACH2A), 7.14 (2H, dd, J ¼ 4.7 Hz, -TP), 7.58 (2H, dd, J ¼
5.0 Hz, -TP), 7.83 (2H, dd, J ¼ 3.6 Hz, -TP).
2-(Biphenyl-4-yl)-N-cyclohexylmaleimide (2-BPhCHMI)
Yield 83%. yellow solid. m.p. 147–148 ꢀC. Rf ¼ 0.14 [silica
gel TLC; n-hexane/ethyl acetate ¼ 20/1 (v/v)]. Anal. Calc for
C
22H21NO2; C: 79.73, H: 6.39, N: 4.23, Found; C: 80.05, H:
Yield 65%. pale yellow solid. m.p. 142–143 ꢀC. 1H NMR
(CDCl3) d (ppm from TMS): 1.21–1.48 (3H, m, cyclohexyl),
1.65–1.70 (3H, m, cyclohexyl), 1.73–1.88 (2H, m, cyclohexyl),
2.05–2.18 (2H, m, cyclohexyl), 3.92–4.04 (1H, m, ACHA),
6.67 (1H, s, HC¼¼CPh), 7.42–7.48 (3H, m, APh), 7.88–7.92
(2H, m, APh).
1
6.67, N: 4.89. H NMR (CDCl3) d (ppm from TMS): 1.22–1.43
(3H, m, cyclohexyl), 1.71–1.75 (3H, m, cyclohexyl), 1.84–1.89
(2H, m, cyclohexyl), 2.05–2.20 (2H, m, cyclohexyl), 3.93–4.06
(1H, m, ACHA), 6.70 (1H, s, HC¼¼CBPh), 7.37–7.53 (4H, m,
-Ph), 7.61-7.71 (5H, m, -Ph).
2-(Biphenyl-4-yl)-N-dodecylmaleimide (2-BPhDMI)
ꢀ
2-Phenyl-N-dodecyllmaleimide (2-PhDMI)
Yield 78%. Pale yellow solid. m.p. 90–92 C. Rf ¼ 0.09 [silica
Yield 85%. brown solid. m.p. 44–45 ꢀC. Rf ¼ 0.24 [silica gel
gel TLC; n-hexane/ethyl acetate ¼ 20/1 (v/v)]. Anal. Calc for
TLC; n-hexane/ethyl acetate ¼ 20/1 (v/v)]. Anal. Calc for
C28H35NO2; C: 80.54, H: 8.45, N: 3.35, Found; C: 80.13, H: 8.16,
N: 3.70. 1H NMR (CDCl3) d (ppm from TMS): 0.88 (3H, t, J ¼ 6.6
Hz, ACH3), 1.07–1.30 (18H, m, dodecyl), 1.55–1.64 (2H, m,
dodecyl), 3.58 (2H, t, J ¼ 7.3 Hz, ACH2A), 6.75 (1H, s,
HC¼¼CBPh), 7.35–7.60 (4H, m, APh), 7.62–7.72 (5H, m, APh).
13C NMR (CDCl3) d (ppm from TMS): 14.10, 22.64, 26.76, 28.55,
29.30, 29.55, 31.86, 38.00, 123.25, 127.02, 127.42, 128.05,
128.88, 129.00, 139.75, 143.03, 143.59, 170.49, 170.74.
C
22H31NO2; C: 77.38, H: 9.15, N: 4.10, Found; C: 77.81, H:
1
8.86, N: 4.29. H NMR (CDCl3) d (ppm from TMS): 0.88 (3H,
t, J ¼ 6.6 Hz, ACH3), 1.23–1.30 (18H, m, dodecyl), 1.62–1.65
(2H, m, dodecyl), 3.57 (2H, t, J ¼ 7.3 Hz, ACH2A), 6.72 (1H,
s, HC¼¼CPh), 7.43–7.49 (3H, m, -Ph), 7.91–7.94 (2H, m, APh).
13C NMR (CDCl3) d (ppm from TMS): 14.09, 22.64, 26.75,
28.55, 29.12, 29.48, 29.57, 31.86, 38.01, 123.79, 128.53,
128.76, 128.85, 130.97, 143.52, 170.46, 170.69.
2,3-Bis(biphenyl-4-yl)-N-cyclohexylmaleimide
(2,3-DBPhCHMI)
2,3-Diphenyl-N-cyclohexylmaleimide (2,3-DPhCHMI)
Yield 78%. pale yellow solid. m.p. 152–154 ꢀC. Rf ¼ 0.15
[silica gel TLC; n-hexane/ethyl acetate ¼ 20/1 (v/v)]. Anal.
Calc for C22H21NO2; C: 79.73, H: 6.39, N: 4.23, Found; C:
Yield 88%. yellow solid. m.p. 183–184 ꢀC. Rf ¼ 0.13 [silica gel
TLC; n-hexane/ethyl acetate ¼ 20/1 (v/v)]. Anal. Calc for
C
34H29NO2; C: 84.44, H: 6.04, N: 2.90, Found; C: 83.69, H: 6.41,
1
N: 3.08. 1H NMR (CDCl3) d (ppm from TMS): 1.32–1.41 (3H, m,
cyclohexyl), 1.77–1.81 (3H, m, cyclohexyl), 1.86–1.91 (2H, m,
cyclohexyl), 2.18–2.21 (2H, m, cyclohexyl), 4.08–4.09 (1H, m,
ACHA), 7.34–7.50 (8H, m, APh), 7.61–7.64 (10H, m, APh).
77.51, H: 6.78, N: 4.67. H NMR (CDCl3) d (ppm from TMS):
1.22–1.44 (3H, m, cyclohexyl), 1.67–1.78 (3H, m, cyclohexyl),
1.84–1.89 (2H, m, cyclohexyl), 2.09–2.23 (2H, m, cyclohexyl),
4.00–4.11 (1H, m, ACHA), 7.34–7.37 (6H, m, APh), 7.44–
7.47 (4H, m, APh).
2,3-Bis(biphenyl-4-yl)-N-dodecylmaleimide (2,3-DBPhDMI)
Yield 96%. yellow solid. m.p. 111–113 ꢀC. Rf ¼ 0.13 [silica
gel TLC; n-hexane/ethyl acetate ¼ 20/1 (v/v)]. Anal. Calc for
2,3-Diphenyl-N-dodecylmaleimide (2,3-DPhDMI)
ꢀ
Yield 85%. yellow solid. m.p. 38–39 C. Rf ¼ 0.11 [silica gel
TLC; n-hexane/ethyl acetate ¼ 20/1 (v/v)]. Anal. Calc for
C
40H43NO2; C: 84.32, H: 7.61, N: 2.46, Found; C: 84.57, H:
1
C
28H35NO2; C: 80.54, H: 8.45, N: 3.35, Found; C: 80.02, H:
7.49, N: 2.67. H NMR (CDCl3) d (ppm from TMS): 0.88 (3H,
t, J ¼ 6.6 Hz, ACH3), 1.23–1.36 (18H, m, dodecyl), 1.70–1.72
(2H, m, dodecyl), 3.67 (2H, t, J ¼ 7.2 Hz, ACH2A), 7.34–7.49
(8H, m, APh), 7.59–7.66 (10H, m, APh). 13C NMR (CDCl3) d
(ppm from TMS): 14.12, 22.67, 26.85, 28.63, 29.33, 29.60,
31.89, 38.48, 127.02, 127.17, 127.66, 127.82, 128.84, 130.30,
135.35, 140.05, 142.41, 170.89.
1
8.76, N: 3.59. H NMR (CDCl3) d (ppm from TMS): 0.88 (3H,
t, J ¼ 6.6 Hz, ACH3), 1.25–1.32 (18H, m, dodecyl), 1.58–1.67
(2H, m, dodecyl), 3.64 (2H, t, J ¼ 7.3 Hz, ACH2A), 7.32–7.39
(5H, m, APh), 7.43–7.49 (5H, m, APh). 13C NMR (CDCl3) d
(ppm from TMS): 14.05, 22.61, 26.79, 28.54, 29.27, 29.53,
31.83, 38.38, 127.08, 128.54, 129.64, 132.01, 135.95, 170.70.
2,3-Bis(thiophen-2-yl)-N-cyclohexylmaleimide
(2,3-DTPCHMI)
Copolymer containing RMI [poly(RMI-alt-Ar) and poly(RMI-
co-Ar)] were prepared following literature procedures.8
Yield 92%. Brown oil. Rf ¼ 0.14 [silica gel TLC; n-hexane/ethyl
acetate ¼ 20/1 (v/v)]. 1H NMR (CDCl3) d (ppm from TMS):
1.21–1.42 (3H, m, cyclohexyl), 1.67–1.76 (3H, m, cyclohexyl),
1.84–1.88 (2H, m, cyclohexyl), 2.07–2.21 (2H, m, cyclohexyl),
3.95–4.10 (1H, m, ACHA), 7.13 (2H, dd, J ¼ 5.0 Hz, -TP), 7.58
(2H, dd, J ¼ 5.1 Hz, -TP), 7.81 (2H, dd, J ¼ 3.8 Hz, -TP).
2,3-Bis(thiophen-2-yl)-N-dodecylmaleimide (2,3-DTPDMI)
Yield 92%. Brown oil. Rf ¼ 0.14 [silica gel TLC; n-hexane/
ethyl acetate ¼ 20/1 (v/v)]. 1H NMR (CDCl3) d (ppm from
RESULTS AND DISCUSSION
Synthesis and Characterization of Model Compounds
2-Aryl and 2,3-diaryl-N-substitued maleimide compounds (2-
ArRMI and 2,3-DArRMI) were prepared by Suzuki-Miyaura
cross-coupling reaction using Pd(PPh3)4 and K2CO3 as
catalyst (Scheme 1). The DBrRMIs were synthesized in
two steps from N-substitued maleimides.9 The first step
involved bromination with bromine in acetic acid, and then
3552
WILEYONLINELIBRARY.COM/JOURNAL/JPOLA

ARTICLE
SCHEME 1 Synthetic route of 2-
Aryl, and 2,3-diaryl substituted RMI.
dehydrobromination in the presence of triethylamine to pro-
vide 2-bromo-N-substitued maleimide (BrRMI; 30–96% yield)
which was used in the Suzuki-Miyaura coupling reaction
described later. These BrRMIs were then treated with bromine
in the presence of sodium acetate to give 2,3-DBrRMIs with
moderate yields (85–97% yield). 2-ArRMI and 2,3-DArRMI
were synthesized from mono- and dibromomaleimides with
aryl boronic acid derivatives via a Suzuki-Miyaura coupling
reaction (Chart 1). The synthesis results of 2-ArRMI and 2,3-
DArRMI are summarized in Table 1. 2-Phenyl and 2,3-di-
phenyl substituted maleimides (2-PhRMI and 2,3-DPhMI), and
the 2-biphenyl and 2,3-bis-p-biphenyl substituted maleimides
(2-BPhRMI and 2,3-DBPhMI) were obtained with excellent
yields as a pale yellow or yellow solid. Coupling reactions of
DBrRMI with 2-thiopheneboronic acid were carried out to
give 2,3-bis(thiophen-2-yl)-N-substituted maleimide (2,3-
DTPRMI) as brown oil (yield 92%; Run 7 and 8). When the
mono-bromomaleimide derivatives were used instead, a low
yield and byproducts were observed (run 5 and 6). The 2-
TPRMIs could not be purified by column chromatography on
silica gel. On the other hand, copolymer (poly(RMI-co-Ar))
conjugated by 2-aryl and 2,3-diaryl-N-substitued maleimides
were continuously prepared following literature procedures.8
polymer obtained by coupling-polymerizations were very
low. To clarify the reason for its low polymerizabilty and
structures of conjugated polymer containing maleimide units
at the main chain, 13C NMR spectra of 2-aryl or 2,3-diaryl
substituted maleimides were compared with those of N-sub-
stituted maleimide polymers. Figure 1 showed 13C NMR
spectra of maleimide derivatives and copolymers. The signals
at 124 ppm of 2-PhDMI (G) and 2-BPhDMI (I) are assigned
to an unsubstituted double bond carbon (ArAC¼¼CAH; Ar ¼
phenyl or biphenyl) at the maleimide ring. 13C NMR spectra
of poly(DMI-alt-Ph) (E) slightly showed a peak similar to an
unsubstituted double bond carbon at about 124 ppm (peak
e1). The result means that an activated bromine atom of mal-
eimide derivatives was substituted for a hydrogen atom dur-
ing coupling polymerizations using DBrRMIs. This debromi-
nation may contribute to the low polymerizability of
DBrRMIs and diboronic acid/dihalide compounds. BrDMI (B)
exhibited peaks because of C¼¼O around 165.32 and 168.60
ppm. DBrDMI (C) exhibited peaks because of C¼¼O around
163.91 ppm. The peaks at 132 ppm of poly(DMI-alt-Ph) (E)
were assigned to a bromine atom substituted double bond
carbon (AC¼¼CABr) at the terminal polymer. The peaks at
165 and 170 ppm were assigned to C¼¼O having bromine at
the terminal polymer and C¼¼O bonded phenyl groups at
2,3-positions of maleimide, respectively. Judged from com-
parison with 13C NMR spectra of poly(DMI-alt-Ph) (E),
BrDMI (B), and DBrDMI (C), the terminal end of poly(DMI-
alt-Ph) existed both unpolymerizable hydrogen atom and
polymerizable bromine atom.
NMR Characterization of ArRMI and BArRMI, and
Poly(RMI-co-Ar)
The authors reported synthesis of conjugated copolymers
containing N-substituted maleimide using transition metal
catalysis.8 But number-average molecular weights (Mn) of co-
MODEL COMPOUNDS FOR CONJUGATED MALEIMIDE POLYMERS, ONIMURA ET AL.
3553

JOURNAL OF POLYMER SCIENCE PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
TABLE 1 Synthesis and Spectroscopic Data of Aryl Substituted
The two peaks at around 171 ppm of 2-PhDMI (G) and 2-
BPhDMI (I) were assigned to carbonyl groups (C¼¼O) having
one aryl group at 2-positions of maleimide, which appeared
in different chemical shifts because of having each hydrogen
atom and aryl group at 2,3-positions of maleimide. According
to expanded 13C NMR spectra of poly(DMA-co-Ph), the multi-
ple peaks (f3) were observed at 170–171 ppm. The results
suggest poly(DMA-co-Ph) includes units of different substitu-
ents at 2,3-positions of maleimide.
RMI and Copolymers
RMI or
Yield
(%)
Amax
(nm)a
kmax
(nm)a
UF
(%)b
Run
Copolymer
1
2-PhCHMI
85
78
85
92
–
290.5, 339.5
289.0, 339.0
295.5, 362.5
293.5, 363.0
–
460
459
498
498
–
5.0
2
2-PhDMI
10
77
80
3
2,3-DPhCHMI
2,3-DPhDMI
2-TPCHMI
4
On the other hand, a new peak (f4) of poly(DMA-co-Ph) (F)
was observed at around 177 ppm. According to 13C NMR
spectra of termaleimide derivatives,11 no peaks of carbonyl
carbons of maleimide ring were observed at lower magnetic
field over 171 ppm. The f4 peak was assigned to carbonyl
carbon of structures such as neighboring maleimide ring and
phenyl groups (Chart 2). Judging from the results of
NMR spectra of 2-ArRMI and 2,3-DArRMI, and conjugated
copolymer containing maleimide units, Suzuki-Miyaura cross-
coupling polymerizations and Yamamoto coupling poly-
merizations gave alternating copolymers and random
copolymers, respectively.
5
–
–
6
2-TPDMI
–
–
–
7
2,3-DTPCHMI
2,3-DTPDMI
2-BPhCHMI
2-BPhDMI
92
92
83
78
88
96
17
295.0, 433.0
295.0, 439.5
306.5, 378.5
295.5, 368.5
294.5, 391.5
306.5, 403.5
298.5
548
548
466
466
532
534
543
31
27
97
8
9
10
11
12
13c
100
94
2,3-DBPhCHMI
2,3-DBPhDMI
97
poly(CHMI-
18
alt-Ph)
14c
15c
16c
a
poly(CHMI-
6.7
55
293.0
313.5
294.5
517
546
547
1.2
23
0.5
alt-TP)
Optical Properties of Conjugated Polymer Containing N-
Substituted Maleimides and 2-ArRMI and 2,3-DArRMI
The UV-vis and PL spectra excited at the wavelength 300 nm
of the poly(CHMI-alt-PhMI) were measured in THF solution
(concentration: 5.0 ꢁ 10ꢂ3–5.0 ꢁ 10ꢂ7 mol/L) (Fig. 2). The
UV-vis spectrum could not be measured under a high con-
centration (5.0 ꢁ 10ꢂ3 mol/L) because of strong absorption
[Fig. 2(A)]. The emission maximum (kmax) of the polymer is
536 nm [Fig. 2(B)]. The emission of light is in the yellow
region. The PL characteristics of the polymer in THF were
also investigated at different solution concentrations (5.0 ꢁ
10ꢂ3–5.0 ꢁ 10ꢂ7 mol/L). PL spectra measured under 5.0 ꢁ
10ꢂ4–5.0 ꢁ 10ꢂ5 mol/L showed similar fluorescence peaks,
but those of a dilute solution (5.0 ꢁ 10ꢂ6–5.0 ꢁ 10ꢂ7 mol/
L) exhibited very weak peaks. At 5.0 ꢁ 10ꢂ4 mol/L concen-
tration, the PL intensity is found to be seven times higher
than at 5.0 ꢁ 10ꢂ7 mol/L concentration. The appropriate
values of concentration for measurements of UV-vis and PL
spectra were 5.0 ꢁ 10ꢂ4–5.0 ꢁ 10ꢂ5 mol/L. The results indi-
cate that the PL intensities increase with decreasing solution
concentration i.e. fluorescence quenches at higher polymer
solution concentration. This can be explained by concentra-
tion quenching effects due to nonemissive excimer com-
plexes most easily formed in the more planar conjugated
structure. Accordingly, the UV-vis and PL spectra of copoly-
mer and model compounds were measured under 5.0 ꢁ
10ꢂ5 mol/L.
poly(CHMI-
co-Ph)
poly(CHMI-
57
co-TP)
Concentration: 5.0 ꢁ 10ꢂ4 mol/L in THF.
Concentration: 1.0 ꢁ 10ꢂ5 mol/L in CH2Cl2.
Ref. 8.
b
c
The signals at around 144 ppm of 2-PhDMI (G) and 2-
BPhDMI (I) shifted to a high magnetic field (135 ppm) for
2,3-DPhDMI (H) and 2,3-DBPhDMI (J). This peaks are
assigned to two aryl groups substituted double bond carbons
(ArAC¼¼CAAr; Ar ¼ phenyl or biphenyl). On the other hand,
poly(DMI-alt-Ph) (E) exhibited similar peaks at around 134
ppm (e3). And 13C NMR spectra of poly(DMI-alt-Ph) (E), 2,3-
DPhDMI (H), and 2,3-DBPhDMI (J) showed similar peaks at
171 ppm. The peaks were assigned to carbonyl groups
(C¼¼O) having aryl groups at 2,3-positions of a maleimide.
Judging from the results of 13C NMR spectra, poly(DMI-alt-
Ph) obtained by the Suzuki-Miyaura cross-coupling reaction
consists of alternate arrangement of maleimides ring and
1,4-phenelene groups.
The peak at around 135 ppm of 2,3-DPhDMI (H) and 2,3-
DBPhDMI (J) dipicted similar peak (f1) of poly(DMA-co-Ph)
(F) obtained via Yamamoto coupling polymerizations.
And 13C NMR spectra of poly(DMA-co-Ph) exhibited multi-
ple peaks at 139–141 ppm. The multiple peaks (f2) at
139–141 ppm of poly(DMA-co-Ph) were similar to the
peaks of 2-BPhDMI (I) and 2,3-DBPhDMI (J). Judging from
13C NMR spectra of biphenyl,12 2-BPhDMI (I), and 2,3-
DBPhDMI (J), the peaks f1 and f2 were assigned to olefin
carbon bonded by phenyl groups at 2,3-positions of malei-
mide and carbons of neighboring phenylene groups (-Ph-
Ph-), respectively.
Relationships between the photoluminescences and the
structures of conjugated compounds containing maleimide
derivatives were investigated by the assignment of the
molecular structure.
The UV-vis and fluorescence spectra of 2-ArRMI and 2,3-
DArRMI in THF solution were summarized in Table 1 and
Figure 3. All 2-ArRMI and 2,3-DArRMI displayed absorption
maxima spectra at around 290–300 nm in the ultraviolet,
3554
WILEYONLINELIBRARY.COM/JOURNAL/JPOLA

ARTICLE
FIGURE 1 13C NMR spectra of DMI (A), BrDMI (B), DBrDMI (C), poly(DMI) (D), poly(DMI-alt-Ph) (E), poly(DMI-co-Ph) (F), 2-PhDMI
(G), 2,3-DPhDMI (H), 2,3-DPhDMI (I), and 2,3-DBPhDMI (J) (R ¼ AC12H25, Ph ¼ Phenyl, BPh ¼ p-Biphenyl) in CDCl3.
which were assigned to p-p* transition of the olefin at the
maleimide ring [Fig. 3(A)]. Further, absorption bands at
340–440 nm are due to the p-p* transition of the aryl units
in 2,3-position. Therefore, the UV absorptions of N-substi-
tuted maleimides containing phenyl groups or biphenyl
groups showed slight red-shifts with increasing the number
MODEL COMPOUNDS FOR CONJUGATED MALEIMIDE POLYMERS, ONIMURA ET AL.
3555

JOURNAL OF POLYMER SCIENCE PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
CHART 2
FIGURE 2 Comparison of the UV-vis spectra (A) and PL spectra
(B) of poly(CHMI-alt-RMI) measured in THF solution under vari-
ous concentrations (5.0 ꢁ 10ꢂ3–5.0 ꢁ 10ꢂ7 mol/L). [Color figure
can be viewed in the online issue, which is available at
wileyonlinelibrary.com.]
of phenyl units. The UV absorptions of 2,3-DTPRMI were
observed more red-shifts than that of phenyl substituted
maleimide derivatives (2,3-DPhRMI) due to the different
band gap energy between the phenyl and thiophene rings.
No differences of UV absorptions were observed between N-
cyclohexyl and n-dodecyl substituents of 2-ArRMI and 2,3-
DArRMI. Figure 3(B) illustrates PL spectra of 2-ArRMI and
2,3-DArRMI in THF. All 2-ArRMI and 2,3-DArRMI emitted
strong fluorescence under ultraviolet irradiation. Figure 3(C)
shows fluorescence of 2-ArRMI and 2,3-DArRMI in THF (5.0
ꢁ 10ꢂ4 mol/L) under near-ultraviolet irradiation (352 nm,
15 W). When exposed to ultraviolet light of wavelength
about 352 nm, 2-PhRMI and 2-BPhRMI fluoresced a blue
color. The photoluminescence spectra of 2,3-DPhRMI with
the maximum intensity at 498 nm were also red-shifted by
about 40 nm relative to that (460 nm) of 2-PhRMI, emitted a
light blue fluorescence. Similarly photoluminescence spectra
(534 nm) of 2,3-DBPhRMI were also red-shifted by about 68
nm relative to that (466 nm) of 2-BPhRMI. The red-shift
bands increase with increasing phenyl group unit. The
FIGURE 3 UV-vis spectra (A), fluorescence spectra (B), and fluorescence images (C) of model compounds in THF.
3556
WILEYONLINELIBRARY.COM/JOURNAL/JPOLA

ARTICLE
to the n-p* electronic transition of the main chain. Alternat-
ing copolymers bearing bulky substituents such as a phenyl,
a benzyl, and a cyclohexyl group gave the photoluminescence
at 542 nm. The maximum emission spectra (kmax) of alter-
nating copolymers bearing linear substituents such as a
n-hexyl and a n-dodecyl were blue-shifted about 35 nm rela-
tive to that of poly(RMI-alt-Ph)s (R ¼ Ph, Bn, CH). The val-
ues of kmax for five poly(RMI-alt-TP)s containing thiophene
moieties were blue-shifted about 30–40 nm relative to that
of poly(RMI-alt-Ph)s containing phenylene units.
On the other hand, all five poly(RMI-co-Ph)s obtained by
Yamamoto coupling polymerizations show similar absorption
spectra at 310–335 nm and 385–405 nm, which are attribut-
able to the p-p* and n-p* electronic transitions of the olefin
of a maleimide ring and aryl group units at 2,3-positions
[Fig. 4(C)]. The intensities of absorption of poly(RMI-co-Ph)s
at 350–400 nm were stronger than those of corresponding
alternating copolymers (poly(RMI-alt-Ph)). From the integra-
tion values of proton peaks by 1H NMR measurement, we
could estimate that the ratios of RMI to 1,4-phenylene units
were 35:65.8 The increasing intensities of absorption are at-
tributable to different increase of phenylene unit, that is, the
presence of continuous 1,4-phenylene segments.
Figure 5 depicts the relative PL intensities of (poly(RMI-alt-
Ph)), (poly(RMI-alt-TP)), and (poly(RMI-co-Ph)) in THF
solution. The intensities of photoluminescence of copolymers
having N-phenyl group at maleimide ring were very small. This
resulted from a fluorescence decay by a phenyl group. Judging
from the results of absorption and emission spectra, the photo-
physical properties of copolymers are influenced by N-sub-
stituents, kinds of comonomer unit, and monomer sequences.
Figure 6 shows the photoluminescence spectra of poly(HMI-
alt-Ph) measured in the used solvents such as acetonitrile,
methanol, dichloromethane, tetrahydrofuran, ethyl acetate,
chloroform, diethyl ether, toluene. The maximum emission
spectra showed slight blue-shift of 9 nm relative to those
measured in solution, and the blue-shifts became progres-
sively greater with polar solvents. The photoluminescence
emissions of the copolymers were influenced by solvents.
FIGURE 4 Normalized absorption and emission spectra of pol-
y(RMI-alt-Ph) (A), poly(RMI-alt-TP) (B), and poly(RMI-co-Ph) (C)
in THF. [Color figure can be viewed in the online issue, which
is available at wileyonlinelibrary.com.]
The absolute PL quantum yields (UF) of 2-ArRMI, 2,3-
DArRMI, poly(CHMI-alt-Ar), and poly(CHMI-co-Ar) in CH2Cl2
are summarized in Table 1. The UFs for mono phenyl substi-
tuted maleimide and dithienyl maleimides were very low
(Run 1, 2, 7 and 8 in Table 1). Maleimides derivatives having
two pheny groups and two biphenyl groups showed high
UFs (Run 3, 4, and 9–12 in Table 1). The UF values of four
copolymers were lower than those of the corresponding
model compounds. The reason may be associated with the
differences in the chain stereoregularity and/or packing
arrangements of the polymers in solution. The conjugated
polymer backbones such as polyacetylene are generally
quenching sites for light emission. Most of excited energy by
UV irradiation may be dispersed by the main chain and car-
bonyl groups of maleimide polymers via nonradiative decay,
thus resulting in low photoluminescence efficiency in the
polymers. The UFs of poly(CHMI-alt-Ph), and poly(CHMI-co-
results were probably due to the extension of p-conjugation.
Furthermore, photoluminescence spectra of 2,3-DTRMI bear-
ing two 2-thienyl groups were observed more red-shifts than
that of phenyl or biphenyl substituted maleimide derivatives,
showed a yellow color.
Figure 4 illustrate absorption spectra of alternating poly-
(RMI-alt-Ph) (A) and poly(RMI-alt-TP) (B) obtained by
Suzuki-Miyaura cross-coupling polymerizations and random
copolymers (poly(RMI-co-Ph) (C) obtained by Yamamoto
coupling polymerizations in THF. All five poly(RMI-alt-Ph)s
show similar absorption spectra with peaks locating at 280–
315 nm, which are attributable to the p-p* electronic transi-
tion of the olefin of a maleimide ring and aryl group units at
2,3-positions. The absorption bands at 350–400 nm are due
MODEL COMPOUNDS FOR CONJUGATED MALEIMIDE POLYMERS, ONIMURA ET AL.
3557

JOURNAL OF POLYMER SCIENCE PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
FIGURE 5 Relative PL intensity of poly(RMI-alt-Ph) (A), poly(RMI-alt-TP) (B), and poly(RMI-co-Ph) (C) in THF. [Color figure can be
viewed in the online issue, which is available at wileyonlinelibrary.com.]
Ph) were larger than that of poly(CHMI-alt-TP), and poly-
(CHMI-co-TP). These results are consistent with the model
compounds having thienyl groups. The presence of thiophene
moiety in the model compounds and copolymer backbone is
responsible for low fluorescence efficiency.
exposed to ultraviolet light of wavelength 352 nm, a series of
1,4-phenylene- and/or 2,5-thienylene-based copolymers con-
taining N-substituted maleimide derivatives fluoresced in a
yellow to blue color in a THF solution. It was found that photo-
luminescences and electronic state of p-conjugated maleimide
derivatives were controlled by aryl- and N-substituents, sol-
vents, and maleimide sequences of copolymers.
The fluorescence spectra of 2-ArRMI and 2,3-DArRMI in THF
solution were influenced by kinds of substituents at 2,3-posi-
tion of a maleimide ring, rather than at the N-substituent.
The photoluminescences are attributable to the extension of
p-conjugation for substituted aromatic groups. On the other
hand, the fluorescence spectra of poly(RMI-alt-Ar) and poly-
(RMI-co-Ar) varied with the N-substituent of a maleimide
ring. The p-conjugation was affected by the torsional angle
of planar conjugated bond at the main chain of copolymers
because of sterical repulsion of bulky substituents.
The authors thank Masami Watanabe and Hideaki Nagashima
at Idemitsu Kosan. for measurements of the absolute PL quan-
tum yields (UF) and useful comments on this work.
REFERENCES AND NOTES
1 Katz, H. E.; Lovinger, A. J.; Johnson, J.; Llock, C.; Siegrist, T.;
Li, W.; Lin, Y.-Y.; Dodabalapur, A. Nature 2000, 404, 478–481.
2 Malenfant, P. R. L.; Dimitrakopoulos, C. D.; Gelorme, J. D.;
Kosbar, L. L.; Graham, T. O.; Curioni, A.; Andreoni, W. Appl
Phys Lett 2002, 80, 2517–2519.
CONCLUSIONS
2,3-Diaryl substituted maleimide derivatives with a fixed cis-
stilbenoid structure were synthesized in three steps using N-
substituted maleimides as a starting material, and their p-con-
jugated polymers were prepared by Pd or Ni catalysts. When
3 Unni, K. N. N.; Pandey, A. K.; Nunzi, J.-M. Chem Phys Lett
2005, 407, 95–99.
4 Chen, H. Z.; Shi, M.-M.; Aernouts, T.; Wang, M.; Borghs, G.;
Heremans, P. Sol. Energy Mater Sol Cells 2005, 87, 521–527.
5 Kaletas, B. K.; Williams, R. M.; Konig, B.; De Cola, L. Chem
¨
Commun 2002, 776–777.
6 Yeh, H.-C.; Wu, W.-C.; Chen, C.-T. Chem Commun 2003,
404–405.
7 Wu, W.-C.; Yeh, H.-C.; Chan, L.-H.; Chen, C.-T. Adv Mater
2002, 14, 1072–1075.
8 Onimura, K.; Matsushima, M.; Yamabuki, K.; Oishi, T. Polym
J 2010, 42, 290–297.
9 Reddy, P. Y.; Kondo, S.; Toru, T.; Ueno, Y. J Org Chem 1997,
62, 2652–2654.
10 Reddy, P. Y.; Kondo, S.; Fujita, S.; Toru, T. Synthesis 1998,
999–1002.
11 Choi, D.-S.; Huang, S.; Huang, M.; Barnard, T. S.; Adams, R.
D.; Seminario, J. M.; Tour, J. M. J Org Chem 1998, 63, 2646–2655.
FIGURE 6 Normalized emission spectra of poly(HMI-alt-Ph) in
different solvents. [Color figure can be viewed in the online
issue, which is available at wileyonlinelibrary.com.]
12 Gobert, F.; Combrisson, S.; Platzer, N.; Ricard, M. Org Magn
Reson 1976, 8, 293–298.
3558
WILEYONLINELIBRARY.COM/JOURNAL/JPOLA