White light-emitting devices based on star-shape polymers with a bisindolylmaleimide core

Article abstract of DOI:10.1021/ma101007x

A new type of star-shape polymers employing bisindolylmaleimide dye (2a-c) as the core and poly(2,7-fluorene) (PF) and/or poly(2,7-carbazole) (PC) as the arms were synthesized. These materials exhibited dual emissions consisting of an intensive blue lumin

Full text of DOI:10.1021/ma101007x

Macromolecules 2010, 43, 5925–5931 5925
DOI: 10.1021/ma101007x
White Light-Emitting Devices Based on Star-Shape Polymers
with a Bisindolylmaleimide Core
Zhenghuan Lin,†,‡ Yan-Duo Lin,† Cheng-Ya Wu, Po-Ting Chow,§ Chia-Hsing Sun, and
Tahsin J. Chow*,†
†Institute of Chemistry, Academia Sinica, Taipei 115, Taiwan, ‡Department of Chemistry and Engineering,
Huanghuai University, Zhumadian 463000, China, §Department of Chemistry, National Taiwan University,
Taipei 106, Taiwan, and Department of Chemistry, Soochow University, Taipei 111, Taiwan
Received October 20, 2009; Revised Manuscript Received June 12, 2010
ABSTRACT: A new type of star-shape polymers employing bisindolylmaleimide dye (2a-c) as the core and
poly(2,7-fluorene) (PF) and/or poly(2,7-carbazole) (PC) as the arms were synthesized. These materials
exhibited dual emissions consisting of an intensive blue luminescence from PF or PC and an orange emission
from maleimide a result of partial energy transfer between the two. Highly efficient white light emitting
devices were fabricated using a single emitting film made by spin-coating method. The electroluminescence
(EL) properties of the devices were investigated from several directions, such as the loading amount of male-
imide core, the concentration dependent spectral changes, the difference in the composition of arms, and the
substituent effect in the indole segment, etc. A typical device based on the star-shape polymer MF001
containing 0.01 mol % of core exhibited a maximal luminous efficiency of 7.2 cd/A and external quantum effi-
ciency of 3.2%. The device based on MMF001 with a methyl substituent on the indole group can be improved
to reaching a maximal brightness of 11450 cd/m2, and that based on MFC1001 with arms comprising equal
amount of PF and PC can be boosted to a maximal power efficiency of 4.8 lm/w. All devices can be fabricated
readily and turned on at a considerable low voltage (<5 V).
Introduction
intermolecular interactions. Wang and co-workers8 have repor-
ted ina recent article that a star-shapepolymer emitted white light
White light organic light-emitting diodes (WOLEDs) have
attracted considerable attention in the past decade due to their
potential applications in flat panel display and lighting applica-
tions.1 With small molecules as emitters, WOLEDs have shown
remarkable quantum efficiencies,2 yet in practice, their applica-
tions have been severely hindered by the high cost of fabricating
complicated multilayer devices. Devices employing a single emit-
ting layer can be made by doping blue, green, and red dyes into a
common host; in such a design it is crucial to control energytrans-
fers among different dopants.3 Recently, our group has employed
hole transporting material (HTM) as a host containing 1% of a
orange dopant to develop a simple WOLED device, ITO/dye
(1%):NPB/TPBI/LiF/Al.4 The white light intensity exceeded
104 cd/m2; nevertheless, there still left plenty of room for further
improvements.
Recently, white polymeric LEDs (WPLEDs) have demon-
strated their great potential in future applications for their easy
processing at low cost.5 There are two major ways to get a white
light emission from WPLEDs. The first is to blend red and blue
polymer dyes together to form a single film.6 However, these
devices usually face the difficulties of low efficiency and/or color
instability due to intrinsic phase separation. Another way is to
prepare single polymer systems incorporating a red component
into a blue polymer chain.7 The optimal performance of these
systems was comparable to those made with small molecules, and
their high color stability has rendered them extremely promising
in practical aspect.
with a notable luminous efficiency of 7.06 cd/A. In this report, we
describe a new series of star-shape white electroluminescent poly-
mers containing an orange core made of maleimide and three blue
arms made of either polyfluorene or polycarbazole. Thin films
made of these materials emit white light, which can be fabricated
into WPLED with remarkable quantum efficiency.
Experimental Section
Materials. Solvents were freshly distilled from appropriate
drying agents prior to use. Commercial reagents were used without
further purification unless otherwise stated. Precoated thin-layer
chromatographic plates (TLC) and silica gel (230-400 mesh)
were purchased from Merck. Compound 1a-c,9 2a-b,4 3, and 410
were synthesized according to reported procedures.
Characterization. Fast atom bombardment (FAB) mass spec-
tra were recorded on a Jeol JMS 700 double-focusing spectro-
meter. Microanalyses were completed on a Perkin-Elmer 2400
elemental analyzer. NMR spectra were measured in CDCl3 on a
Bruker AVA300 FT-NMR spectrometer; 1H and 13C chemical
shifts were quoted relative to the internal standard tetramethyl-
silane. UV-vis spectra were obtained on a HP-8453 spectro-
photometer. The PL and EL properties were probed on a Hitachi
F-4500 fluorescence spectrophotometer. The fluorescence quan-
tum yields were determined in CH3CN solutions at 293 K against
Nile red as a reference (Φp =0.78).11 Cyclic voltammetry mea-
surements were carried out on a BAS 100B electrochemical
analyzer. A conventional three-electrode configuration consisting
of a glassy carbon working electrode, a Pt-wire counter electrode
and a Ag/AgNO3 reference electrode was used. The supporting
electrolyte was [Bu4N]PF6 (0.1 M). Ferrocene was added as an
internal standard in each set of measurements, and all potentials were
quoted with reference to the ferrocene-ferrocenium (Fc/Fcþ)
Star-shape polymers have exhibited unusual properties for
their highly branched and globular features capable of reducing
*Corresponding author. E-mail tjchow@chem.sinica.edu.tw.
r 2010 American Chemical Society
Published on Web 06/28/2010
pubs.acs.org/Macromolecules

5926 Macromolecules, Vol. 43, No. 14, 2010
Lin et al.
couple at a scan rate of 100 mV s-1. The oxidation potential
104.73, 109.24, 120.31, 120.71, 121.27, 121.78, 122.06, 125.76,
127.46, 130.61, 131.82, 131.93, 135.78, 135.96, 136.79, 137.86,
171.11. HRMS (FAB): [M]þ calcd, 859.0045; found, 859.0044.
2,3-Bis(N-(400-bromobenzyl)-20-phenyl-30-indolyl)-N-(40-bromo-
benzyl)maleimide (2f). Physical data of 2f (yield 49%). 1H NMR
(CDCl3, 300 MHz): δ 4.58 (2H, s, Ar-CH2), 4.91-5.03 (4H, m,
2ꢀAr-CH2), 6.46 (3H, br, Ar-H), 6.74 (4H, d, J 8.10, Ar-H),
7.01-7.07 (8H, m, Ar-H), 7.10 (2H, br, Ar-H), 7.17 (6H, br,
Ar-H), 7.29-7.33 (4H, m, Ar-H), 7.35-7.39 (3H, m, Ar-H).
13C NMR (CDCl3, 100 MHz): δ 40.96, 47.24, 105.11, 110.41,
120.89, 121.14, 121.40, 122.78, 126.71, 127.66, 127.91, 128.44,
129.67, 130.15, 131.33, 131.54, 131.80, 132.74, 136.01, 136.38,
137.03, 141.66, 170.24. HRMS (FAB): [M]þ calcd, 983.0358;
found, 983.0361.
General Procedure for Suzuki Polymerization. To a mixture of
monomer 2, 3, and/or 4 under a nitrogen atmosphere was added
Aliquat 336 (0.10 g, 0.25 mmol), aqueous K2CO3 (2 M, 2.5 mL),
and toluene (6 mL). To it was added tetrakis(triphenylphosphine)-
palladium (0.046 g, 0.04 mmol) while under nitrogen, and then
the mixture was refluxed for 36 h. The polymerization was com-
pleted by refluxing with benzeneboronic acid (0.11 g, 1.0 mmol)
for 6 h and with bromobenzene (0.10 mL, 1.0 mmol) for 8 h. The
mixture was cooled to room temperature and poured into methanol.
The precipitates were collected by filtration, dried, and then dis-
solved in dichloromethane. The solution was washed with water
and dried over anhydrous MgSO4. After most of the solvent being
removed, the residue was poured into methanol with stirring to
give polymer fiber. The polymer was further purified by extracting
with acetone for 24 h using a Soxhlet apparatus. The final product
was obtained after drying in vacuum with a yield of 40-55%.
MF001. Polymer 2d (0.08 mg, 1 ꢀ 10-4 mmol) and 3 (0.6 g,
1 mmol) were used in the polymerization. 1H NMR (300 MHz,
CDCl3): δ 0.82 (10H, br), 1.14 (20H, br), 2.13 (4H, br), 7.68 (4H,
d, J 8.1), 7.83 (2H, d, J 10.5). GPC: Mw =14500; PDI=1.61.
MF015. Polymer 2d (1.25 mg, 1.5ꢀ10-3 mmol) and 3 (0.6 g,
1 mmol) were used in the polymerization. 1H NMR (300 MHz,
CDCl3): δ 0.82 (10H, br), 1.14 (20H, br), 2.12 (4H, br), 7.68 (4H,
d, J 7.8), 7.83 (2H, d, J 9.9). GPC: Mw =21300; PDI=1.85.
MF200. Polymer 2d (16.69 mg, 2 ꢀ 10-2 mmol) and 3 (0.6 g,
1 mmol) were used in the polymerization. 1H NMR (300 MHz,
CDCl3):δ0.82(10H,br),1.14(20H,br),2.11(4H,br),7.68(4H,br),
7.83 (2H, br). GPC: Mw =21400; PDI=1.88.
3
(Eox) was used to determine the HOMO energy level using the
equation EHOMO=-(Eox þ 4.8) eV, which was calculated to the
value of ferrocene (-4.8 eV).12 Thermal analyses were perfor-
med with a Perkin-Elmer TGA6 thermal analyzer. Differential
scanning calorimetry was done on a Perkin-Elmer DCS-7 instru-
ment. The weight-average molecular weights (Mw) and the poly-
dispersity index (PDI) of soluble polymers were determined by
gel-permeation chromatography (GPC) with a Waters 1515 instru-
ment. The polystyrene and THF were used as standards and as
eluent, respectively.
Devices Fabrication. Indium-tin oxide (ITO) with a sheet resis-
tance of <50 Ω was used as the substrate that was prepatterned by
photolithography,13 giving an effective device size of 4 mm2. It was
washed by detergent, deionized water and alcohol in sequence,
followed by oxygen plasma cleaning. A 40 nm thick layer of poly-
(ethylenedioxythiophene):poly(styrenesulfonic acid) (PEDOT:PSS)
was spin-coated onto the ITO glass substrate, followed by baking
at 120 °C for 20 min. The polymer layer (70 nm) was then spin-
coated on top of PEDOT:PSS layer, from a fresh toluene solution
(10 mg/mL) through a Teflon filter (0.2 μm), and dried for 2 h at
90 °C under vacuum. An electron-transporting layer, 2,20,200-(1,3,5-
benzeneriyl)tris(1-phenyl-1H-benzimidazole) (TPBI), was grown
through thermal evaporation using ULVAC cryogenics at a cham-
ber pressure of 10-6 Torr. Finally, a thin layer of LiF (1 nm) was
deposited onto the TPBI layer, followed by a layer of aluminum
(120 nm).14 The current-voltage and light-intensity measurements
were done on a Keithley 2400 source meter and a Newport 1835C
optical meter equipped with a Newport 818-ST silicon photodiode.
All the fabrication and characterization of devices was carried out
at ambient atmosphere.
General Procedure of Core and A3 Monomer Synthesis. Com-
pound 1 (1.00 mmol) was mixed with NaH (0.086 g, 3.6 mmol) in
dried THF (7 mL) under a nitrogen atmosphere, and the solu-
tion turned gradually from red to dark blue. After the mixture was
stirred with a magnetic bar for 15 min, benzylic bromide (3.6 mmol)
was added to the mixture, while the solution began to turn red.
The mixture was stirred for another 3 h at room temperature.
The reaction was quenched by the addition of water (200 mL)
and filtered. The filtrate was dissolved in dichloromethane and
dried over MgSO4. The crude product was purified by silica gel
column chromatography with dichloromethane/hexane (1:3) as
eluant to afford compound 2 as red solids.
MMF001. Polymer 2e (0.09 mg, 1ꢀ10-4 mmol) and 3 (0.6 g,
1 mmol) were used in the polymerization. 1H NMR (300 MHz,
CDCl3): δ 0.82 (10H, t), 1.14 (20H, br), 2.12 (4H, br), 7.68 (4H,
d, J 8.5), 7.83 (2H, br). GPC: Mw =19900; PDI=1.79.
PMF001. Polymer 2f (0.10 mg, 1 ꢀ 10-4 mmol) and 3 (0.6 g,
1 mmol) were used in the polymerization. 1H NMR (300 MHz,
CDCl3): δ 0.82 (10H, t), 1.14 (20H, br), 2.12 (4H, br), 7.68 (4H,
d, J 8.1), 7.83 (2H, d, J 10.8). GPC: Mw =14500; PDI=1.65.
MFC1001. Polymer 2d (0.08 mg, 1ꢀ10-4 mmol), 3 (0.3 g, 0.5
mmol) and 4 (0.3 g, 0.6 mmol) were used in the polymerization.
1H NMR (300 MHz, CDCl3): δ 0.81-0.91 (33H, m), 1.04-1.13
(59H, m), 1.25-1.64 (27H, m), 2.12-2.24 (6H, m), 4.35 (4H, br),
7.37 -7.85 (29H, m), 8.17 (4H, br). GPC: Mw=11600; PDI=1.59.
MFC2001. Polymer 2d (0.08 mg, 1ꢀ10-4 mmol), 3 (0.2 g, 0.3
mmol) and 4 (0.4 g, 0.8 mmol) were used in the polymerization.
1H NMR (300 MHz, CDCl3): δ 0.81-0.95 (17H, m), 1.02-1.40
(36H, m), 1.52-1.65 (25H, m), 2.15-2.27 (7H, m), 4.36 (4H, br),
7.49-7.88 (18H, m), 8.23 (5H, br). GPC: Mw=12400; PDI=1.41.
MF001, MF015, MF200, MMF001, and PMF001. All com-
pounds showed similar NMR spectra. A typical spectrum of
MF001: 13C NMR (CDCl3, 75 MHz): δ 14.06, 22.59, 23.91,
29.22, 30.03, 31.79, 40.38, 55.33, 119.96, 121.48, 126.16, 127.19,
140.02, 140.49, 151.80. Polymers MFC1001 and MFC2001
2,3-Bis(N-benzyl-20-phenyl-30-indolyl)-N-benzylmaleimide (2c).
Physical data of 2c (yield 69%). 1H NMR (CDCl3, 400 MHz): δ
4.72 (2H, s, Ar-CH2), 5.00-5.14 (4H, m, 2 ꢀ Ar-CH2), 6.58
(3H, br, Ar-H), 6.96 (5H, br, Ar-H), 7.02-7.15 (9H, m, Ar-H),
7.20 (4H, br, Ar-H), 7.26-7.28 (2H, m, Ar-H), 7.30 (2H, br,
Ar-H), 7.31-7.33 (4H, m, Ar-H), 7.37 (4H, br, Ar-H). 13C
NMR (CDCl3, 100 MHz): δ 41.54, 47.77, 105.05, 110.52, 120.25,
120.87, 122.50, 125.94, 126.77, 127.17, 127.26, 127.44, 127.80,
128.22, 128.38, 128.61, 129.78, 131.54, 132.70, 137.12, 137.21,
137.43, 141.73, 170.49. HRMS (FAB): [M]þ calcd, 749.3042;
found, 749.3036.
2,3-Bis(N-(400-bromobenzyl)-30-indolyl)-N-(40-bromobenzyl)male-
imide (2d). Physical data of 2d (yield 70%). 1H NMR (CDCl3,
300 MHz): δ 4.77 (2H, s, Ar-CH2), 5.24 (4H, s, 2 ꢀAr-CH2),
6.72 (2H, t, J 7.3 Ar-H), 6.96 (6H, t, J 7.8, Ar-H), 7.04 (2H, t,
J7.6, Ar-H), 7.14 (2H, d, J8.2 Ar-H), 7.32 -7.45 (8H, m, Ar-H),
7.67 (2H, s, Ar-H). 13C NMR (CDCl3, 75 MHz): δ 41.20, 49.90,
106.51, 109.86, 120.31, 121.67, 121.79, 122.19, 122.60, 126.28,
127.06, 128.42, 130.35, 131.71, 131.75, 131.95, 135.48, 135.87,
136.16, 171.74. HRMS (FAB): [M]þ calcd, 830.9732; found,
830.9739.
2,3-Bis(N-(400-bromobenzyl)-20-methyl-30-indolyl)-N-(40-bromo-
benzyl)maleimide (2e). Physical data of 2e (yield 65%). 1H NMR
(CDCl3, 300 MHz): δ 1.98 (6H, br, Ar-CH3), 4.82 (2H, s, Ar-
CH2), 5.12-5.18 (4H, m, 2 ꢀ Ar-CH2), 6.55 (4H, br, Ar-H),
6.93 (2H, br, Ar-H), 7.10-7.21 (9H, m, Ar-H), 7.38-7.49 (5H,
m, Ar-H). 13C NMR (CDCl3, 75 MHz): δ 12.32, 41.38, 46.11,
showed similar NMR spectra. The spectrum of MFC1001: 13
C
NMR (CDCl3, 75 MHz): δ 11.00, 14.03, 22.58, 23.11, 23.98,
24.50, 28.87, 29.23, 29.70, 30.09, 31.22, 31.77, 39.60, 40.46, 47.40,
55.34, 107.50, 107.96, 118.79, 119.19, 119.98, 120.55, 121.50, 121.97,

Article
Macromolecules, Vol. 43, No. 14, 2010 5927
Scheme 1
126.18, 126.46, 127.19, 128.39, 128.50, 128.76, 132.19, 139.57,
140.04, 140.18, 140.53, 141.10, 142.20, 151.82.
Results and Discussion
Synthesis of Monomers and Polymers. The chemical struc-
tures and synthetic scheme of the designed polymers are
shown in Scheme 1. The bisindolylmaleimide derivatives were
chosen as the core because of their intensive red to orange
colors.9,15 Their long wavelength emissions are derived from
charge transfer (CT) transitions, where the indole moieties
act as electron donors and the maleimide carbonyl groups as
electron acceptors. The presence of two cross-intercepting
dipoles reduced the net polarity of molecules. Their round
shape geometry along with flexible conformation reduced
their tendency of crystallization, therefore resulted in high
glass transition temperatures. Poly(2,7-fluorene) (PF) and
poly(2,7-carbazole) (PC) are widely used as blue emitting
dyes with large band gaps. They were selected as the arms
of the star-shape polymers for their blue emission and out-
standing electroluminescence (EL) efficiency.16 Three kinds
of polymers, i.e., MF001, MC001, and MFC1001 were syn-
thesized, where the capital letters denote maleimide (M),
fluorene (F), and carbazole (C), respectively. The last three
digits in MF001 and MC001 denote the content of the orange
core in mol %. For MFC1001, the first number of the last
four digits denotes a molar ratio of carbazole to fluorene.
The maleimides 2a-c were synthesized readily through a
substitution reactions with benzyl bromide from compounds
1a-c with three kinds of substituents at C(2) of the indoles.
In order to extend the chain lengths on the nitrogen substi-
tuents, a substituent need to be introduced at the para-
position of the benzyl groups. The maleimide-based core
units 2d-f were synthesized in a similar manner with 4-bromo-
benzyl bromide in yields of 50-70%. Star-shape polymer
were built up starting from the A3 type units 2d-f, which
coupled with AB type monomers 3 or/and 4 by a one-pot
Suzuki polycondensation reaction. By changing the feed
ratios of M, F, and C units, a series of star-shape polymers
were synthesized as listed in Scheme 1. All polymers are solu-
ble in common organic solvents, such as toluene, chloroform
Figure 1. Absorption (left) and emission (right) spectra of compounds
2a-c in dioxane. The excitation wavelength was 457 nm.
and THF, except MC001 and MC060. The chain length of
arms might not be the same, as there was no control in the
process of the polycondensation. In addition to the star-
shaped polymers, there were unavoidably some detached
arms coexisting in the media. The materials were used as
obtained without further purification. The weight-average
molecular weights (Mw) of these soluble polymers were
estimated by gel permeation chromatography (GPC) using
polystyrenes as internal standards, while the results revealed
the range of 11600-21400 with polydispersity indexes (PDI)
of 1.41-1.88.
Physical Properties of the Core. The absorption spectra of
maleimides 2a-c in dioxane are shown in Figure 1. A high
energy absorption band <325 nm is known to derive from a
π-π* transition localized mainly on the indole groups.15b
The low energy absorption bands at >325 nm can be assig-
ned to the transitions involving both indole and maleimide
moieties, because neither of them alone displays absorp-
tion at wavelength longer than 325 nm. The spectrum of 2a
displays two broad absorption bands at 381 and 459 nm.

5928 Macromolecules, Vol. 43, No. 14, 2010
Lin et al.
The spectra of 2b and 2c are similar to each other, with a broad
band at 463 and 450 nm, respectively, along with a shoulder
at 406 and 400 nm. The difference of 2a from 2b and 2c may
be attributed to a more flexible conformation of the former
through rotations along the C (maleimide)-C (indole) bond.
Upon irradiation at 457 nm in dioxane, compounds 2a-c
yield intensive orange fluorescence at λmax 566-576 nm with
quantum yields 0.32-0.61 (Table 1). It was well-recognized
that these emissions come from charge transfer transitions,
so that they exhibit apparent solvatochromic effect in diffe-
rent environments.9
Oxidation potentials were measured by cyclic voltamme-
try (CV) in THF with Ag/AgCl as a reference. The half-wave
potentials of an oxidative sweep were measured to be 0.52,
0.62, and 0.51 eV for 2a-c, respectively. The highest occu-
pied molecular orbital (HOMO) levels were obtained by a com-
parison with the ionization potential of ferrocene at 4.8 eV
(Table 1). The HOMO-LUMO (lowest unoccupied mole-
cular orbital) band gaps were estimated at the interception
wavelength of absorption and emission spectra, and were
found to be 2.33 (2a), 2.35 (2b) and 2.37 (2c) eV. The absorp-
tion spectra of 2a-c overlapped with the emission spectra
of fluorene and carbazole (ESI S1), thus assured a feasible
energy transfer. From the energy level diagram it was also
found that the positions of HOMO and LUMO energy levels
of 2a-c lay within the corresponding band region of either
fluorene or carbazole. Therefore, energy transfer from the
outer arms of the star-shape polymer to the central core is
energetically favorable. By controlling the loading amounts
of 2d-f with respect to the fluorene or carbazole units, poly-
mers suitable for producing white emission can be prepared
plausibly.
emissions at ca. 420 nm deriving from PF, except MF200
which showed a minor shoulder at 570 nm (Figure 2). The
emission spectra in the solid state displayed a distinctly
different appearance. In films the relative intensity of the
yellow emissions at 540-555 nm became much more pro-
nounced comparing to the blue bands at ca. 430 nm. The blue
component diminished nearly to nil when the ratio of M
reached a level of 2%, i.e., in the case of MF200 (Figure 2). It
is evident that intermolecular energy transfer plays a major
role in the solid state as a result of closer stacking of mole-
cules. A substantial red shift of the yellow bands was also
observed along with the increase of the M ratio. It can be
explained by a gradual increase of the polarity of the media,
because the polarity of maleimide is substantially higher than
that of fluorene. The influence of substituent was also explo-
red by examining the emission spectra of polymers MMF001
and PMF001, in which a methyl or a phenyl group was added
onto the indole moieties of maleimide (Scheme 1). In their PL
spectra, they all exhibit dual emissions at both 445 and 537 nm,
indicating a negligible effect of substituent groups on the
indoles (ESI S3).
The energy transfer characteristics of the polymers were
further examined at different concentrations. As mentioned
above, these materials were composed of both the star-shape
polymers and also some detached polyfluorene arms. In
highly diluted solutions, ET was confined as an intramole-
cular process, while each unit of MF200 emits a red light in
the solution and each detached polyfluorene arm emits a blue
light. The combined emission was a broad band spectrum
with double maxima. The relative intensities between the red
and the blue bands maintained a constant independent to
concentration (Figure 3a). When the concentration increa-
sed, e.g., over 0.01 mg/mL (ca. 10-6 M) in toluene, intermole-
cular processes started to appear. The intensity of red band
increased at the expenses of the blue band. The trend can be
clearly depicted by the normalized spectra given in Figure 3a.
The lowest red/blue ratio at high dilution was determined by
the relative amount of detached polyfluorene arms. For the
purpose of comparison, a reference mixture was prepared by
blending 2a into pure polyfluorene with the same ratio as
that preparing MF200. At high concentration this mixture
displayed a similar behavior as that of MF200. However, at
low concentration the red emission diminished to nil as a
result of lacking of intramolecular ET, which was feasible
only in star-shape polymers (Figure 3b).
Physical Properties of the Polymers. By changing the
molar ratio of the core 2a, three kinds of star-shape polymers
MF001, MF015, and MF200 were synthesized. Their absorp-
tion spectra exhibited a strong absorption band at ca. 384 nm
in toluene as well as in the solid state, which derives mainly
from the fluorene segments (ESI S2).16a The absorption band
deriving from the maleimide core cannot be distinguished
due to its low content. The luminescence spectra of polymers
MF001 and MF015 in toluene exhibited dominant blue
Table 1. Physical Data of Compounds 2a-ca.
absorption
(nm)
emission
(nm)
HOMO
(eV)
LOMO
(eV)
compound
Φp
The polarity of carbazole is higher than fluorene; there-
fore, it was anticipated that replacing the PF side arms by PC
may induce a red shift on the emission of the M core.4,17 A
slight red shift is helpful for adjusting the emission spectrum
2a
2b
2c
298, 381, 459
284, 463
295, 450
576
575
566
0.42
0.32
0.61
5.32
5.42
5.31
2.99
3.07
2.94
a. Spectra taken in dioxane, excitation wavelength 457 nm.
Figure 2. Emission spectra of polymers MF001, MF015, and MF200 in solid film (left) and toluene (right, at 0.01 mg/mL) upon excitation at 380 nm.
The photographs in the inset correspond to the spectra with different core concentrations.

Article
Macromolecules, Vol. 43, No. 14, 2010 5929
Figure 3. (a) Concentration dependent emission spectra of MF200 in toluene excited at 380 nm. The intensity of red component increases along with
the increase of concentration, as a result of intermolecular energy transfer. The ratio of red/blue stays invariant at high dilution while energy transfer
processes are limited within each molecular strand. (b) Emission spectra of a material prepared byblending2a into polyfluorene with the samecore/arm
ratio as MF200. It displays a similar concentration dependent behavior at high concentration. At low concentration the red band diminished as a
consequence of lacking of significant intramolecular energy transfer. All the spectra in parts a and b were normalized at the blue emission band for easy
comparison.
toward a standard white light. Polymers with PC side arms,
e. g. MC001 and MC060, were synthesized in this regard.
Comparing to the spectrum of MF001, their long wavelength
emissions appeared at ca. 583 nm, which did exhibit a consi-
derable red shift with respect to the PF polymers (ESI S4).
However, the solubility of PC polymers was considerably
lower than those of the PF polymers. For example, the solu-
bility of MC001 and MC060 was so low that it was difficult to
make workable film through solution processes. To overcome
the solubility problem, copolymer MFC1001 and MFC2001
were prepared by mixing fluorene with carbozole together.
According to the relative integrations in the 1H NMR spectra
of MFC1001 and MFC2001, the actual ratios of C to F in
polymer arms were estimated to be 0.65 and 1.25, respecti-
vely (ESI S5). These polymers did express better solubility in
most organic solvents. In their emission spectra (Figure 4),
the red bands deriving from maleimide appeared at 546 and
552 nm, which did exhibited a ca. 20 nm red shift comparing
Figure 4. Emission spectra of polymers MF001, MFC1001, andMFC2001
in solid film.
to that of MF001.
The thermal properties of these star-shape polymers were
investigated by thermogravimetric analysis (TGA) and dif-
ferential scanning calorimetry (DSC) under a nitrogen atmos-
phere. All polymers showed high thermal stability. Accord-
ing to the TGA plot of MF001, only a slight weight loss
(<5%) can be detected before 423 °C (ESI S6), and only at a
higher temperature the polymer chain started to collapse. A
glass transition temperature (Tg) of MF001 was detected at
112 °C as shown in a DSC sweep (Figure S6 inset). The rela-
tively high values of Tg and Td (thermal decomposition tem-
perature) help to suppress morphological changes during the
operation of devices.
White Electroluminescence from Single Films. To investi-
gate the EL characteristics of these star-shape polymers,
devices were fabricated with the configuration of ITO/PED-
OT:PSS (40 nm)/polymer (70 nm)/TPBI (30 nm)/LiF (1 nm)/Al
(120 nm). In this device, PEDOT:PSS was employed as a
hole-injecting material and TPBI as an electronic-transpor-
ter as well as hole-blocker. All polymers exhibited dual emis-
Figure 5. Electroluminescence spectra ofpolymersMF001, MF015 and
sions, whereas the relative intensities between the two depends
on the relative amount of maleimide core. The EL spectra deri-
ved from the devices made with MF001, MF015, and MF200
are compared in Figure 5. The spectrum of MF001 displayed
a broad band with fwhm (full width at half-maximum) ca.
155 nm ranging from 415 to 570 nm. Its CIE coordinate at
(0.23, 0.33) was a little away from standard white light (0.33,
0.33) due to the blue shift of maleimide emission in a nonpolar
MF200 in WPLEDs.
environment. Increasing the content of maleimide in the poly-
mers from 0.01% mol (MF001) to 0.15% (MF015) and 2%
mol (MF200), the red component was shifted successfully to
539 and 561 nm. The CIE coordinates at (0.30, 0.40) and
(0.36, 0.40) moved closer to the standard white light. However,
the increase of loading amount of the M core resulted to a

5930 Macromolecules, Vol. 43, No. 14, 2010
Table 2. Performance of Devices Made with Star-Like Polymers
Lin et al.
maximum brightness
(cd/m2)
maximum luminous
efficiency (cd/A)
maximum power
efficiency (lm/W)
maximum quantum
efficiency (%)
polymer
turn on (v)
CIE (x, y)
MF001
MF015
MF200
MMF001
PMF001
MFC1001
MFC2001
5.0
5.3
5.7
4.5
4.5
4.0
4.0
7728
4962
3960
11450
1190
8994
6483
7.20
4.16
3.95
6.69
2.93
6.09
4.61
3.48
2.38
2.07
4.09
1.68
4.78
3.22
3.21
1.89
1.52
2.62
1.10
2.42
1.85
(0.23, 0.33)
(0.30, 0.40)
(0.36, 0.40)
(0.28, 0.37)
(0.28, 0.39)
(0.26, 0.36)
(0.29, 0.36)
Figure 6. Plots of current density I (0) and brightness L (b) versus
voltage V of device made with MMF001.
Figure 7. Plots of luminance efficiency, power efficiency, and external
quantum efficiency versus current density of device made with MMF001.
decrease of EL efficiency along with an increase of turn-on
voltages (Table 2). The device made with MF001 performed
the best, i.e., with luminance, power and quantum efficien-
cies of 7.20 cd/A, 3.48 lm/W, and 3.21%, respectively. The
maximal brightness reached to 7728 cd/m2. When the con-
tent of M was increased to 2% mol, maximum brightness and
efficiencies reduced to almost half those of MF001. Such
differences may be ascribed to a concentration quenching of
maleimides, along with possible charge-trapping effect.8,18
The substituent effect of maleimide core was investigated
in the next stage by changing the substituent on the indole
groups.9 Polymers MMF001 and PMF001 were synthesized
with methyl and phenyl substituents attached onto the C(20)
position of the indoles in M. With increased bulkiness of the
group, the relative intensity of the maleimide emission became
higher (ESI S7). It seems that the energy transfer from poly-
fluorene to maleimide increased in the order of PMF001 >
MMF001>MF001. A slight red shift was also observed on
the long wavelength band, although the amount of shift may
not sufficient enough for composing a pure white light. The
device made with MMF001 displayed an emission with a CIE
coordinate of (0.28, 0.37), which is getting closer to a white
light than that of (0.23, 0.33) made with MF001. Their EL pro-
perties are shown in Table 2. The device made from MMF001
displayed a maximum luminance efficiency 6.69 cd/A, power
efficiency 4.09 lm/W and quantum efficiency 2.62% with
turn-on voltage 4.5 V. Moreover, their broad band EL spec-
tra were quite stable in a voltage range of 6-15 V (ESI S8).
According to the I-V-L plots in Figure 6, the brightness can
reach to 11450 cd/m2 at 21 V with a low turn-on voltage 4.5 V.
Plots of device performance versus current density are given
in Figure 7. The luminance, power and quantum efficiency
decreased only mildly with an increasing of current density.
Carbazole is known to be a blue emitter with good hole
transporting ability, while its molecular structure is more
polar than fluorene. Replacing the fluorene units in polymer
chains with carbazole units was able to increase the polarity
Figure 8. Electroluminescence spectra of polymers MF001, MFC1001
and MFC2001 in WPLEDs.
of the polymer films. In their emission spectra of copolymers
MFC1001 and MFC2001, the maleimide band indeed showed a
red shift comparing to that of MF001. The devices made with
MFC1001 and MFC2001 by spin coating method displayed
broad band emissions with CIE coordinates at (0.26, 0.36)
and (0.29, 0.36) (Figure 8). Their performances are compa-
tible with those made with MF001 (Table 2), e.g. with similar
brightness, lower turn-on voltage, yet slightly lower quan-
tum efficiency. The maximum brightness, luminance, power
and quantum efficiencies of MFC1001 were measured to be
8994 cd/m2, 6.90 cd/A, 4.78 lm/W, and 2.42%, respectively,
along with a turn-on voltage at 4.0 V. Their EL spectra was
stable with respect to the applied bias in the range of 6-10 V
(ESI S9). The plots of inter-relationships among voltage,

Article
Macromolecules, Vol. 43, No. 14, 2010 5931
current density, brightness, and efficiency of the device made
with MFC1001 can be found in the Supporting Information
(ESI S10, S11). According to such a performance, it is appa-
rent that inserting carbazole units into the arms of star-shape
polymers is a promising way to improve their EL properties
toward the design of white light devices. Further improve-
ments can be expected along this direction.
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In summary, we have developed an efficient way of preparing
star-shape polymers using bisindolylmaleimide as the core for
producing white light LEDs. These materials exhibit blue emis-
sion from polyfluorene and yellow to red emission from the
maleimide. The spectra of devices can be adjusted by changing
the molar ratio of maleimide to fluorene. The range of a ratio in
0.01-2% has been examined, while the electroluminescence dis-
played dual bands covering the wavelength region of 400-700 nm.
In highly diluted solutions, the materials displayed a bluish color
due to the low content of maleimide, yet in condensed phases
such as solid films the materials emit white light as a result of
significant intermolecular energy transfer. To further adjust the
wavelength toward standard white light, carbazole can be added
into the polymer chains to red shift the long wavelength compo-
nentn. The devices made with them performed remarkable well,
and in CIE coordinate the color approached reasonably close to
the standard white light. The materials made by blending carba-
zole and fluorene together has the advantages of high solubility
for easy processing, ready accessibility and fabrication, good ther-
mal stability and reliable color under wide operating voltages, etc.
These star-shape polymers have revealed promising results for
producing white light from a single film of polymers.
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Acknowledgment. This work was supported by Academia
Sinicaandthe NationalScienceCouncil ofthe RepublicofChina.
Chem. Soc. 2004, 126, 7041. (b) Dierschke, F.; Grimsdale, A. C.;
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Supporting Information Available: Figures showing absorp-
tion spectra of polymer MF001, MF015, and MF200 both in
solutions and in solid films, emission spectra of PF and PC in
CH2Cl2, emission spectra of polymers MF001,MMF001, PMF001,
MC001, and MC060 in solid films, 1H NMR spectra of MFC1001
and MFC2001, TGA and DSC plots of MF001, HOMO and
LUMO energy levels of 2a-c, PF and PC, electroluminescence
of polymers MF001,MMF001,PMF001,and MFC1001, and plots
of device parameters made with MFC1001. This material is avail-
able free of charge via the Internet at http://pubs.acs.org.
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