Achieving saturated red photoluminescence and electroluminescence with readily synthesized maleimide-arylamine copolymers

Article abstract of DOI:10.1016/j.tet.2006.07.096

Maleimide-based red fluorescent copolymers were easily synthesized from palladium catalyzed polycondensation of N-alkyl-3,4-bis(4-bromophenyl)maleimide with commercially available or readily prepared secondary aryldiamines. The copolymers were characteriz

Full text of DOI:10.1016/j.tet.2006.07.096

Tetrahedron 62 (2006) 9541–9547
Achieving saturated red photoluminescence
and electroluminescence with readily synthesized
maleimide-arylamine copolymers
a,
Li-Hsin Chan,a,b Yu-Der Leeb, and Chin-Ti Chen
*
*
aDepartment of Chemical Engineering, National Tsing Hua University, Hsinchu 30013, Taiwan, ROC
bInstitute of Chemistry, Academia Sinica, Taipei 11529, Taiwan, ROC
Received 29 April 2006; revised 26 July 2006; accepted 28 July 2006
Available online 22 August 2006
Abstract—Maleimide-based red fluorescent copolymers were easily synthesized from palladium catalyzed polycondensation of N-alkyl-3,4-
bis(4-bromophenyl)maleimide with commercially available or readily prepared secondary aryldiamines. The copolymers were characterized
by gel permeation chromatography, differential scanning calorimetry, cyclic voltammetry, UV–vis absorption, and fluorescence spectroscopy.
They showed brilliant red fluorescence in solution (toluene) with emission maximum in the range of 617–638 nm, although severely red-
shifted in thin films. With judicious selection of aryldiamine monomers, the red-shifting of the thin film fluorescence can be largely dimin-
ished. The structure and property (molecular weight, glass transition temperature, and fluorescence) relationships were analyzed and
deciphered as well. A light-emitting device has been fabricated with maleimide-arylamine copolymer in demonstrating the potentials for
saturated red polymer light-emitting diodes (PLEDs).
Ó 2006 Elsevier Ltd. All rights reserved.
1. Introduction
tend to be crystallinic and fluorescence-quenching in solid
state. Specifically, the precursor in preparing NPAMLMe
is a dibromo-substituted species, N-methyl-3,4-bis(4-bromo-
phenyl)maleimide (Br2ML), which is easily synthesized
from 4-bromophenylacetonitrile (Scheme 1).1,2 This is an
appropriate monomer in synthesizing high molecular weight
polyarylenes through Yamamoto-type Ni-catalyzed or
Suzuki or Heck-type Pd-catalyzed polycondensation. Br2ML
also can be a suitable monomer in Sonogashira-type Pd/Cu-
catalyzed co-polycondensation of arylalkynes. About half a
dozen of reports taking a good utilization of Br2ML or different
We have recently reported N-methyl-3,4-bis(4-(N-(1-naph-
thyl)phenylamino)phenyl)maleimide (NPAMLMe) as the
novel host light-emitter in the fabrication of rare non-dopant
red organic light-emitting diodes (OLEDs).1 The triaryl-
amine-substituted maleimide showed unusual amorphous
property (glass transition temperature, Tg, at 126 ꢀC) and
strong red fluorescence in solid state. This is pretty extra-
ordinary for red fluorophores, which usually possess extended
p-conjugation or dipolar donor–acceptor substituents and
R
N
R
N
KOtBu
RI
DMF
i) I2, THF
NaOCH3/HOCH3
ii) 3% HCl(aq) /THF
1-naphthylphenylamine
Pd(OAc)2
P(tBu)3
xylenes
H
N
Br
NaOtBu
O
O
O
O
O
O
40~50%
80~86%
71%
NC
Br
Br
Br
N
Br
N
R = methyl Br2ML
R = iso-butyl Br2MLib
R = methyl
NPAMLMe
Scheme 1. Synthesis of Br2ML, Br2Mlib, and NPAMLMe.
Keywords: Maleimide; Arylamine; Copolymer; Red fluorescence; Electroluminescence.
* Corresponding authors. Fax: +886 2 27831237; e-mail: cchen@chem.sinica.edu.tw
0040–4020/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2006.07.096

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L.-H. Chan et al. / Tetrahedron 62 (2006) 9541–9547
alkyl derivatives in the preparation of 3,4-diphenylmaleimide-
based polyaryl macromolecules have illustrated this point.3
Only yellow to orange fluorescent polymers derived from
maleimides have been known thus far.
of 100 mV/s at room temperature. A platinum wire was
used as counter electrode, and Ag/AgNO3 was employed
as reference electrode. Ferrocene was used for potential
calibration (all reported potentials are referenced against
ferrocene/ferrocenium, FOC) and for reversibility criteria.
Both solution and solid-state fluorescence quantum yields of
the model compounds and maleimide-based copolymer
were determined by integrating sphere method described by
de Mello et al.6 The thin films of the copolymers were spin-
casted from the toluene solution on the glass substrate and
subjected to vacuumed drying at room temperature for at
least 16 h before the experimental measurement.
In addition to non-doped red OLEDs, we have been inter-
ested in developing the readily prepared red fluorescent
polymers for applications in polymer light-emitting diodes
(PLEDs). Red light-emitting polymer is one of the three in-
dispensable components (red, green, and blue) in the fabrica-
tion of full color display based on OLEDs. However, when
compared with either green or blue light-emitting polymers,
red light-emitting polymer is relatively scarce. Moreover,
most of the known red light-emitting polymers are not easily
accessible and require lengthy synthesis procedure in the
preparation of suitable monomers.4 For generating long
wavelength red emission, it is necessary to attach arylamino
unit to the maleimide moiety shown by a series of fluorescent
3,4-diaryl-substituted maleimides.2 Pd-catalyzed aromatic
amination reactions, which are commonly employed in the
high-yield synthesis of triarylamines, will be promising for
synthesizing red fluorescent polymer based on N-alkyl-
3,4-bis(4-bromophenyl)maleimide bearing secondary aryl-
amines.5 Accordingly, herein we report the easy syntheses
and physical characterizations, including gel permeation
chromatography, differential scanning calorimetry, cyclic
voltammetry, UV–vis absorption, and fluorescence spectros-
copy, of a series of red fluorescent maleimides-arylamine co-
polymers. A preliminary test of the red electroluminescence
of the copolymers is reported here as well.
2.2. Syntheses of the monomers
Monomer Br2ML was prepared following the already pub-
lished procedure,2,7 and the synthesis of Br2MLib was also
conducted according to the known literature (Scheme 1),2,7
but iso-butyl iodide was used as the alkylating reagent.
OPhAtol and IPPhAtol were synthesized (with isolated yields
of 67 and 51%, respectively) by the amination of 4-tolyl
iodide with 4,40-oxydianiline or 4,40-(1,4-phenylenediiso-
propylide)bisaniline, respectively, by using (DPPF)PdCl2/
DPPF as the catalyst.8a OPhAnhx was prepared by a two-
step reaction. First, 4,40-oxydianiline was converted to a
diamide derivative in near quantitative yield with tert-butyl-
acetyl chloride, and then the amide groups were conveniently
reduced to secondary amine (with isolated yield of 63%)
using NaBH4/I2 in THF followed by the acidic treatment
with 3 N hydrochloric acid solution.8b
2.2.1. N-Isobutyl-3,4-bis(4-bromophenyl)maleimide,
Br2MLib. Yellow-greenish powder with a yield of 80%. 1H
NMR (DMSO, 400 MHz, d/ppm): 7.49 (d, 4H, J¼8.7 Hz),
7.34 (d, 4H, J¼8.7 Hz), 3.44 (d, 2H, J¼7.3 Hz), 2.11–2.04
(m, 1H), 0.93 (d, 6H, J¼6.7 Hz). 13C NMR (DMSO,
100 MHz, d/ppm): 170.4, 135.0, 132.0, 131.3, 127.2,
124.7, 45.8, 27.9, 20.1. Anal. Calcd for C20H17Br2NO2: C,
51.86; H, 3.70; N, 3.02. Found: C, 51.66; H, 3.57; N, 3.02.
2. Experimental
2.1. Materials and measurement
All chemicals were purchased from Aldrich and Acros
chemical companies and were used without any further
purification. All the solvents such as toluene, DMSO, and
THF were distilled after drying with appropriate drying
2.2.2. N,N0-Di-neo-hexyl-4,40-oxydianiline, OPhAnhx
.
˚
agents. The dried solvents were stored over 4 A molecular
sieves before usage. 1H and 13C NMR spectra were recorded
on a Bruker AV-400 MHz or AMX-400 MHz Fourier trans-
form spectrometer at room temperature. The number- and
weight-average molecular weight of polymers were deter-
mined by gel permeation chromatography (GPC) on a Waters
GPC-1515 with a 2414 refractive index detector, using THF
as the eluent and polystyrene as the standard. UV–vis
absorption and fluorescence spectra were recorded by
Hewlett–Packard 8453 and Hitachi F-450 spectrophoto-
meters, respectively. Glass transition temperatures (Tgs) of
the copolymers were determined by differential scanning
calorimetry (DSC) using a Perkin–Elmer DSC-6 analyzer
system. Thermal analyses were performed with a scanning
(both heating and cooling) rate of 10ꢀ/min in an atmosphere
of nitrogen. The temperatures at the intercept of the curves on
the thermogram (endothermic or exothermic) and the leading
baseline were taken as the estimates for the onset Tg. The
oxidation potentials of the copolymers were determined by
cyclic voltammetry (CV) using an Electrochemical Analyzer
BAS 100B. The electrochemical measurement of the copoly-
mers was performed in a 0.1 M tetrabutylammonium per-
chlorate (Bu4NClO4) solution in toluene with scanning rate
White powder with a yield of 63%. 1H NMR (DMSO,
400 MHz, d/ppm): 6.69 (d, 4H, J¼8.8 Hz), 6.49 (d, 4H,
J¼8.8 Hz), 5.16 (s, 2H), 2.93 (t, 4H, J¼8.1 Hz), 1.47–1.43
(m, 4H), 0.92 (s, 18H). 13C NMR (DMSO, 100 MHz,
d/ppm): 148.7, 144.0, 119.1, 113.6, 42.3, 40.3, 29.6, 29.4.
Anal. Calcd for C24H36N2O: C, 78.21; H, 9.85; N, 7.60.
Found: C, 78.29; H, 9.60; N, 7.57.
2.2.3. N,N0-Di(4-tolyl)-4,40-oxydianiline, OPhAtol. Off-
white powder with a yield of 67%. 1H NMR (DMSO,
400 MHz, d/ppm): 7.84 (s, 2H), 7.01–6.99 (m, 8H), 6.92–
6.85 (m, 8H), 2.20 (s, 6H). 13C NMR (DMSO, 100 MHz,
d/ppm): 150.4, 141.5, 139.4, 129.6, 128.0, 119.2, 118.2,
116.5, 20.3. Anal. Calcd for C26H24N2O: C, 82.07; H,
6.36; N, 7.36. Found: C, 82.10; H, 6.31; N, 7.39.
2.2.4. N,N0-Di(4-tolyl)-4,40-(1,4-phenylenediisopropyl-
ide)bisaniline, IPPhAtol. White powder with a yield of
51%. H NMR (DMSO, 400 MHz, d/ppm): 7.85 (br, 2H),
1
7.11 (s, 4H), 7.06–6.99 (m, 8H), 6.94–6.90 (m, 8H), 2.20
(s, 6H). 13C NMR (DMSO, 100 MHz, d/ppm): 147.6,
141.5, 141.1, 141.0, 129.5, 128.2, 127.1, 125.9, 117.0,

L.-H. Chan et al. / Tetrahedron 62 (2006) 9541–9547
9543
115.9, 41.3, 30.5, 20.3. Anal. Calcd for C38H40N2: C, 86.98;
H, 7.68; N, 5.34. Found: C, 86.87; H, 7.62; N, 5.22.
4H), 7.09–6.81 (m, 24H), 3.38 (br, 2H), 2.28 (s, 6H), 2.05
(br, 2H), 1.61 (s, 12H), 0.90 (s, 6H). Anal. Calcd for
(C58H55N3O2)n: C, 84.32; H, 6.66; N, 5.09. Found: C,
83.96; H, 6.59; N, 5.08.
2.3. General procedure of the copolymer synthesis
Following a pertinent procedure5 in the high-yield synthesis
of triarylamines, a mixture of Br2ML (0.42 g, 1.0 mmol),
appropriate aryldiamine (1.0 equiv amount), NaOtBu
(0.21 g, 2.2 mmol), P(tBu)3 (0.049 g, 0.2 mmol), and
Pd(OAc)2 (0.01 g, 0.04 mmol) in dry THF (8 mL) was
stirred at refluxing temperature under a nitrogen atmosphere
for about 2 days. After cooling to room temperature, the so-
lution mixture was then poured into a solution of methanol
and deionized water (10:1). The precipitates were collected
by filtration. The solid was redissolved in either chloroform
or dichloromethane and then reprecipitated by the addition
of either methanol or hexane. The reprecipitation procedure
was repeated at least three or four times, followed by Soxhlet
extraction using methanol first then hexane for 24 h. Yields
of all polymerization reactions were in a range of 63 and
84% depending on the aryldiamine used. All products
have been structurally characterized by NMR spectroscopy
and elemental analysis.
3. Results and discussion
3.1. Synthesis and characterization
The syntheses of red polymers shown in Scheme 2 proceeded
via a Pd-catalyzed polycondensation of N-methyl-3,4-bis-
(4-bromophenyl)maleimide with secondary aryldiamines,
such as N,N0-diphenyl-1,4-phenylenediamine (PhAPh), and
N,N0-diphenylbenzidine (BPhAPh) are commercially avail-
able and can be adopted in the polycondensation as well.
Other secondary aryldiamines, such as N,N0-di(4-tolyl)-
4,40-oxydianiline (OPhAtol), N,N0-di-neo-hexyl-4,40-oxydi-
aniline (OPhAnhx), and N,N0-di(4-tolyl)-4,40-(1,4-phenyl-
enediisopropylide)bisaniline (IPPhAtol), (Scheme 2) are
readily prepared.
The resulting copolymers, except PMLPhAPh, are readily
soluble in common organic solvents, such as toluene, chlo-
roform, dichloromethane, and THF. Their molecular weights
were determined by gel permeation chromatography (GPC)
using THF as the eluent and polystyrene standard for calibra-
tion. The results are listed in Table 1. These copolymers in
general show average molecular weight (Mw) from 5400 to
15,200 with polydispersity indices (Mw/Mn) of 1.4–3.9.
It is somewhat to our surprise that PMLPhAPh and
PMLBPhAPh have the highest molecular weights among
these copolymers prepared in this study. At this stage, we
can only surmise that difference in the reactivity of aryldi-
amine may be accounted for the varied molecular weight
of these copolymers, in addition to the solubility concern
of these copolymers.
2.3.1. PMLPhAPh. The copolymer was obtained as a deep
red semifibrous material with a yield of 62% after drying un-
der vacuum. 1H NMR (CDCl3, 400 MHz, d/ppm): 7.41–6.96
(m, 22H), 3.08 (s, 3H). Anal. Calcd for (C35H25N3O2)n: C,
80.89; H, 4.82; N, 8.09. Found: C, 80.53; H, 4.84; N, 7.99.
2.3.2. PMLBPhAPh. The copolymer was obtained as a deep
red fibrous powder with a yield of 84% after drying under
1
vacuum. H NMR (CDCl3, 400 MHz, d/ppm): 7.45–6.99
(m, 26H), 3.10 (s, 3H). Anal. Calcd for (C41H29N3O2)n: C,
82.66; H, 4.87; N, 7.06. Found: C, 82.28; H, 4.82; N, 7.01.
2.3.3. PMLOPhAtol. The copolymer was obtained as a red
powder with a yield of 65% after drying under vacuum. 1H
NMR (CDCl3, 400 MHz, d/ppm): 7.38 (d, 4H, J¼8.2 Hz),
7.07–6.86 (m, 20H), 3.08 (s, 3H), 2.29 (s, 6H). Anal. Calcd
for (C43H33N3O3)n: C, 80.72; H, 5.16; N, 6.57. Found: C,
80.48; H, 5.15; N, 6.44.
3.2. Thermal properties and optical absorption
characterization
The thermal properties of the copolymers were determined
by differential scanning calorimetry (DSC) as shown in
Table 1. The Tgs of these copolymers in general follow the
variation of Mw values, i.e., the higher the Mw values, the
higher the Tg values, although the structural flexibility is
the factor affecting Tg values as well.
2.3.4. PMLOPhAnhx. The copolymer was obtained as
a deep red powder with a yield of 80% after drying under
vacuum. 1H NMR (CDCl3, 400 MHz, d/ppm): 7.42 (d, 4H,
J¼8.4 Hz), 7.14 (d, 4H, J¼8.4 Hz), 7.02 (d, 4H,
J¼8.5 Hz), 6.64 (d, 4H, J¼8.5 Hz), 3.66 (br, 4H), 3.06
(s, 3H), 1.57 (br, 4H), 0.92 (s, 18H). Anal. Calcd for
(C41H45N3O3)n: C, 78.42; H, 7.17; N, 6.69. Found: C,
78.22; H, 7.21; N, 6.56.
The absorption spectra were measured from the copolymers
dissolved in toluene. All copolymers show very similar ab-
sorption spectra with peaks locating at 283–342 nm, which
are attributable to the p–p* electronic transitions of the
copolymer backbones. The additional absorption peaks at
485–490 nm are due to the n–p* transition of the diaryl-
maleimide units in the copolymer main chain.3f,3g From
the on-set absorption wavelengths of the absorption spectra,
the optical band gaps (Eopt) are estimated to be 2.05–
2.12 eV. The physical characterization and absorption spec-
tra are summarized in Table 1, Table 2, and Figure 1.
2.3.5. PMLIPPhAtol. The copolymer was obtained as a red
powder with a yield of 69% after drying under vacuum. 1H
NMR (CDCl3, 400 MHz, d/ppm): 7.37 (d, 4H, J¼8.3 Hz),
7.11–6.96 (m, 20H), 6.86 (d, 4H, J¼7.8 Hz), 3.06 (s,
3H), 2.28 (s, 6H), 1.62 (s, 12H). Anal. Calcd for
(C55H49N3O2)n: C, 84.25; H, 6.25; N, 5.36. Found: C,
83.99; H, 6.22; N, 5.37.
3.3. Fluorescent characterization
2.3.6. PMLibIPPhAtol. The copolymer was obtained as
a bright red powder with a yield of 63% after drying under
vacuum. 1H NMR (CDCl3, 400 MHz, d/ppm): 7.38 (br,
The most intriguing property of these copolymers is the long
wavelength fluorescence. All copolymers showed strong red

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L.-H. Chan et al. / Tetrahedron 62 (2006) 9541–9547
CH3
N
O
O
HN
NH
a
n
N
N
Br2ML
PhAph
PMLPhAph
CH3
N
O
O
HN
NH
a
n
N
N
Br2ML
BPhAph
PMLBPhAph
CH3
N
O
O
O
N
N
H
a
H
Br2ML
N
N
OPhAtol
O
n
PMLOPhAtol
CH3
N
O
O
O
N
N
H
a
H
Br2ML
N
OPhAnhx
N
O
n
R
N
H
N
PMLOPhAnhx
O
O
N
H
IPPhAtol
N
a
Br2ML or Br2MLib
N
n
R = methyl
R = isobutyl
PMLIPPhAtol
PMLibIPPhAtol
Scheme 2. Syntheses of red polymers derived from Br2ML (or Br2MLib) and secondary aryldiamines. (a) Reagents and conditions: THF, NaOtBu, P(tBu)3,
Pd(OAc)2, reflux, 48 h.
Table 1. Physical data of the copolymers
ab
max
Yield (%)
l
, nma (3, Mꢁ1 cmꢁ1
)
Tg (ꢀC)
Mw
Mn
PDIb
PMLPhAPh
62
84
65
80
69
63
311, 485 (10,200)
342, 485 (11,500)
303, 485 (8500)
283, 485 (8900)
303, 490 (7700)
300, 485 (7100)
201
222
189
168
200
163
15,200
14,500
5400
8200
12,400
9000
3900
6800
3300
5700
6400
5600
3.9
2.1
1.6
1.4
1.9
1.6
PMLBPhAPh
PMLOPhAtol
PMLOPhAnhx
PMLIPPhAtol
PMLibIPPhAtol
a
In toluene.
Poydispersity index (Mw/Mn).
b

L.-H. Chan et al. / Tetrahedron 62 (2006) 9541–9547
Table 2. Optical and Electrochemical Properties of the copolymers
9545
(nm)b
fd (%)
De (nm)
EOX versus EFOC
HOMOg (eV)
LUMOh (eV)
f
Eopt (eV)
em
max
l
Solutiona
Filmc
PMLPhAPh
638 (95)
631 (75)
626 (73)
617 (73)
624 (72)
620 (73)
694 (115)
677 (111)
667 (122)
672 (122)
661 (111)
633 (81)
4
13
16
24
31
30
56
46
41
55
37
13
0.46
0.65
0.68
0.53
0.67
0.67
2.05
2.06
2.11
2.12
2.10
2.10
ꢁ5.26
ꢁ5.45
ꢁ5.48
ꢁ5.33
ꢁ5.47
ꢁ5.47
ꢁ3.21
ꢁ3.39
ꢁ3.37
ꢁ3.21
ꢁ3.37
ꢁ3.37
PMLBPhAPh
PMLOPhAtol
PMLOPhAnhx
PMLIPPhAtol
PMLibIPPhAtol
a
In toluene.
Numbers in parenthesis denote the full-width-at-half-maximum of the emission spectra.
Spin-casted solid film from toluene solution and vacuum-dried.
b
c
d
e
f
Fluorescence quantum yield in toluene using f¼98% of rubrene in benzene as the reference.10
em
max
em
max
D ¼ l ðfilmÞ ꢁ l ðsolutionÞ.
Band gaps were calculated from the on-set absorption edge of the UV–Vis spectrum.
HOMO levels were calculated from E1/2 of the oxidation voltammograms by comparing with the ferrocene value of 4.8 eV below the vacuum level.9
LUMO levels were estimated from the HOMO energy levels and the optical band gap of the copolymers.
g
h
weight as indicated by its high PDI value. The relatively
rigid and short p-conjugation of the aryldiamine segment
causes the copolymer chain more folding, which is believed
to be the reason for the low solubility of PMLPhAPh. The
puckered copolymer chain also increases the chance of
intra-copolymer interaction even in diluted solution condi-
tion. Such interaction severely quenches the fluorescence
and the lowest solution fluorescence quantum yield (only
4%) observed for PMLPhAPh is as a consequence of the
copolymer chain structure.
PMLPhAph
1.2
1.0
0.8
0.6
0.4
0.2
0.0
PMLBPhAph
PMLOPhAtol
PMLOPhAnhx
PMLIPPhAtol
PMLibIPPhAtol
As shown for PMLPhAPh, fluorescence spectroscopic
properties are closely related to the chemical structure of
the copolymers. Copolymers with more extended p-conju-
gation structure will show longer fluorescence wavelength.
Furthermore, aryl substituent of the diamine moiety has
a stronger effect than the alkyl substituent in pushing the
300
400 500
Wavelength (nm)
600
700
Figure 1. UV–Vis absorption spectra of copolymers in toluene solution.
fluorescence toward longer wavelengths, such as 617 and
em
max
fluorescence in toluene solution with l
in the range of
em
max
626 nm of l
for PMLOPhAnhx and PMLOPhAtol in
617–638 nm (Fig. 2). Polymer PMLPhAPh is the exception
regarding the intensity of fluorescence, because it is the only
weak fluorescent in solution. Although fluorescence is
highly visible, the solution quantum yields of these polymers
were low to moderate (4–31%). PMLPhAPh possesses the
widest full-width-at-half-maximum (95 nm) and the longest
wavelength (638 nm) of the emission band. The unique
spectroscopic property of PMLPhAPh may be attributed to
its high molecular weight (the highest among all copoly-
mers) accompanying with wide distribution of the molecular
solution, respectively.
In thin films, when compared with solution fluorescence
spectra (Fig. 3), the red-shifting of the thin film fluorescence
is considerably large, in the range of 13–56 nm (Table 2).
With the exception of PMLibIPPhAtol, almost all copolymers
exhibit fluorescence beyond 660 nm (Table 2). Emission at
wavelength longer than 650 nm is not practical for the display
PMLPhAph
1.4
PMLBPhAph
PMLPhAph
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
1.2
PMLPhAtol
PMLBPhAph
PMLPhAtol
PMLPhAnhx
1.0
PMLPhAnhx
0.8
PMLIPPhAtol
0.6
0.4
PMLibIPPhAtol
PMLIPPhAtol
PMLibIPPhAtol
0.2
0.0
-0.2
500 550 600 650 700 750 800 850
Wavelength (nm)
500
550
600
650
Wavelength (nm)
700
750
800
850
Figure 3. Photoluminescence spectra of the polymers in thin films spin-
coated from toluene solution.
Figure 2. Photoluminescence spectra of the copolymers in toluene solution.

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L.-H. Chan et al. / Tetrahedron 62 (2006) 9541–9547
PMLIPPhAtol
application, because of the poor spectral response of the hu-
man eye at such long wavelength. Therefore, it is necessary
to prevent fluorescence in thin film from red-shifting for
practical concern.
PMLIibPPhAtol
1.0
0.8
0.6
0.4
0.2
0.0
EL
The red-shifting fluorescence in thin film is frequently
attributed to molecular aggregation, particularly involving
p–p interaction of aromatic moiety. Therefore, preventing
p–p interaction between copolymer chains should inhibit
the red-shifting of the fluorescence in thin film. Followings
are a few observations from the study that help in designing
copolymers with reduced aggregation in thin film. First,
PMLPhAPh has the worst red-shifting in its class of the
copolymer. The structural rigidity is the cause, but electronic
donor substituent, in this case, the para-substituted amino
group, may be another reason. Second, we notice that
PMLOPhAnhx has a larger red-shifting (D¼55 nm) fluores-
cence spectrum than its tolyl analog PMLOPhAtol
(D¼41 nm) (Table 2). This result suggests that the rigid
aryl substituent is better than flexible alkyl substituent in
preventing the copolymer aggregation in thin film. On the
other hand, by breaking up the p-conjugation as well as by
replacing of electronic donating oxygen-bridge with weaker
donor of isopropyl group, the extent of the red-shifting
fluorescence can be further reduced to D¼37 nm for
PMLIPPhAtol.
500
600
700
Wavelength (nm)
800
900
Figure 4. Fluorescence spectra of PMLIPPhAtol and PMLibIPPhAtol
both in toluene solution (dotted lines) and solid film (solid lines). EL spec-
trum of the device ITO/PMLibIPPhAtol/LiF/Al is also illustrated (diamond
symbols).
PMLBPhAph
PMLPhAph
PMLOPhAnhx
PMLIPPhAtol
PMLOPhAtol
Based on the above observations and inductions, PMLibIP-
PhAtol was designed and synthesized to reduce the red-
shifting of fluorescence in thin film. PMLibIPPhAtol can
be considered as the improved version of PMLIPPhAtol.
Having a longer and bulkier alkyl substituent on the nitrogen
of maleimide, PMLibIPPhAtol really exhibits the desired
improvement on fluorescence (Fig. 4), i.e., shorter fluores-
cence wavelength both in solution (620 nm, the second
shortest of all) and thin film (633 nm, the shortest of all)
(Table 2), smaller red-shifting in thin film (only 13 nm,
the smallest among all), and narrower full-width-at-half-
maximum (81 nm, the narrowest among all copolymers).
PMLibIPPhAtol
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6
Oxidation Potential (V Vs. Fc/Fc+)
Figure 5. Cyclic voltammograms (oxidation) of copolymers in toluene,
containing 0.1 M Bu4NClO4 versus Ag/Ag+ reference electrode, scan
rate: 100 mV/s.
these red fluorescent copolymers have a chance to be used
as the single layer light-emitting device (see below).
3.4. Electrochemical characterization and
HOMO–LUMO energy levels
Cyclic voltammetry (CV) was employed to investigate the
electrochemical behaviors of the synthesized copolymers
and to estimate their HOMO energy levels. Figure 5 depicts
cyclic voltammograms of the polymers in the oxidation
process. The copolymers showed reversible peak under oxi-
dation process, which indicates their electrochemical stabil-
ity in the hole-carrying process. Similar observations have
been found for the non-dopant red emitter NPANLMe as
before.1 The HOMO energy level of the polymers was cal-
culated using the ferrocene (FOC) value of ꢁ4.8 eV as the
standard.9 The electrochemical characteristics of the poly-
mers are summarized in Table 2. The HOMO energy levels
of the copolymers were estimated from ꢁ5.26 to ꢁ5.48 eV.
The slight difference may be rationalized by the electron-
donating ability of arenes to the nitrogen. By calculating
from HOMO levels and optical band gaps (from on-set
absorption energy), the LUMO energy levels of the copoly-
mers were found to be ꢁ3.21 to ꢁ3.39 eV. With the
appropriately aligned HOMO and LUMO energy levels to
the work function of the anode and cathode, respectively,
3.5. Application to PLED
PLED was fabricated for preliminary test of the red electro-
luminescence (EL). The device was constructed by spin-
casting (1500 rpm) of a toluene solution of a mixture
of PMLIibPPhAtol (10 mg/mL) followed by vacuum
(w10ꢁ5 Torr) deposition of LiF and Al, sequentially, on
PEDOT–PSS/ITO (indium tin oxide) glass substrate, where
PEDOT–PSS is the conducting polyethylene dioxythio-
phene/polystyrene sulfonate. The device had the simple con-
figuration of ITO/PEDT–PSS/PMLIibPPhAtol/LiF/Al. It
el
max
showed EL with l
at 656 nm (Fig. 4) corresponding to
CIEx,y of (0.66, 0.33), which is comparable with or better
than CIEx,y of (0.64, 0.33), the standard red color of National
Television System Committee (NTSC). For the single layer
device of PMLIibPPhAtol, the maximum electroluminance
was around 9 cd/m2, and the maximum luminous efficiency
was 0.01 cd/A (Fig. 6). As compared with PL emission, the
EL spectrum is red-shifted slightly (23 nm) and the band
width becomes broadened.

L.-H. Chan et al. / Tetrahedron 62 (2006) 9541–9547
9547
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8
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0
50
100
150
Current Density (mA/cm2)
200
Figure 6. L-I-Luminous efficiency characteristics of PMLibIPPhAtol
device.
4. Conclusion
We have shown that a new type of red fluorescent copolymers
based on arylamine-derived 3,4-diphenylmaleimide can be
easily synthesized. We have successfully achieved the
copolymers showing saturated red fluorescence in thin films.
Applications of the copolymer for saturated red PLEDs have
been demonstrated primarily. This study is valuable for
the further optimization of the readily prepared maleimide-
arylamine copolymers, in enhancing solubility, and fluores-
cence quantum yield in both solution and thin film.
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Acknowledgements
The authors would like to thank the National Science Coun-
cil of the Republic of China, Taiwan for financially support-
ing this research. Support of the National Tsing Hua
University and Academia Sinica is also appreciated.
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