Conjugated polymers containing trifluoren-2-ylamine, trifluoren-2-ylbenzene and trifluoren-2-yltriazine for electroluminescence

Article abstract of DOI:10.1016/j.polymer.2012.11.050

A series of light-emitting conjugated polymers (LEPs) based on building blocks of electron-donating trifluoren-2-ylamine (TFA), electrically neutral 1,3,5-trifluoren-2-ylbenzene (TFB), and electron-withdrawing 2,4,6-trifluoren-2-yltriazine (TFT) were successfully synthesized via palladium-catalyzed Suzuki cross-coupling polycondensation. Their structure-property relationships were thoroughly studied. For P1 containing electron-withdrawing backbone and electron-donating pendants, their photophysical properties in solution are strongly dependent on the solvent polarity due to the intense intramolecular charge-transfer (ICT) interaction. In addition, their emission colors and energy levels could be effectively tuned by changing the structure of the main chain as well as the substituent at the pendent fluorene of TFT, TFB, and TFA. To evaluate their electroluminescence properties, double-layer devices with a configuration of ITO/PEDOT:PSS (40 nm)/emitting layer (EML) (70-80 nm)/TPBI (30 nm)/CsF (1.5 nm)/Al were fabricated, where the developed polymers were used as an EML and TPBI (2,2′,2″-(1,3,5-benzenetriyl)-tris-(1-phenyl-1H-benzimidazole)) was used as an electron-transport and hole-block layer. Deep-blue light emission was achieved for the device based on P1d, a polymer based on TFT and the second generation (G2) of carbazole dendrimer pendant, that shows a maximum current efficiency (CE) of 1.26 cd A^-1, corresponding to external quantum efficiency (EQE) of 1.27%, with Commission Internationale de L'Eclairage (CIE) coordinates of (0.16, 0.14). In comparison, due to the intense ICT interaction between the main chain and the side chain, light-green emission was achieved for the device based on P1c, a polymer based on TFT and bis(9,9-dioctyl-9H-fluoren- 2-yl)amine pendant, giving the highest maximum CE of 4.10 cd A^-1, corresponding to EQE of 2.45%, with CIE coordinates of (0.24,0.51), although either of the main chain and the side chain emits blue light.

Full text of DOI:10.1016/j.polymer.2012.11.050

Polymer 54 (2013) 162e173
Contents lists available at SciVerse ScienceDirect
Polymer
journal homepage: www.elsevier.com/locate/polymer
Conjugated polymers containing trifluoren-2-ylamine, trifluoren-2-ylbenzene
and trifluoren-2-yltriazine for electroluminescence
*
Hua Ye, Dongcheng Chen, Ming Liu, Xiao Zhou, Shi-Jian Su , Yong Cao
State Key Laboratory of Luminescent Materials and Devices (South China University of Technology) and Institute of Polymer Optoelectronic Materials and Devices, South China
University of Technology, Guangzhou 510640, China
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of light-emitting conjugated polymers (LEPs) based on building blocks of electron-donating
trifluoren-2-ylamine (TFA), electrically neutral 1,3,5-trifluoren-2-ylbenzene (TFB), and electron-
withdrawing 2,4,6-trifluoren-2-yltriazine (TFT) were successfully synthesized via palladium-catalyzed
Suzuki cross-coupling polycondensation. Their structureeproperty relationships were thoroughly
studied. For P1 containing electron-withdrawing backbone and electron-donating pendants, their pho-
tophysical properties in solution are strongly dependent on the solvent polarity due to the intense
intramolecular charge-transfer (ICT) interaction. In addition, their emission colors and energy levels
could be effectively tuned by changing the structure of the main chain as well as the substituent at the
pendent fluorene of TFT, TFB, and TFA. To evaluate their electroluminescence properties, double-layer
Received 28 August 2012
Received in revised form
15 November 2012
Accepted 19 November 2012
Available online 27 November 2012
Keywords:
Light-emitting conjugated polymer
Intramolecular charge-transfer
Electroluminescence
devices with
a configuration of ITO/PEDOT:PSS (40 nm)/emitting layer (EML) (70e80 nm)/TPBI
(30 nm)/CsF (1.5 nm)/Al were fabricated, where the developed polymers were used as an EML and
TPBI (2,20,200-(1,3,5-benzenetriyl)-tris-(1-phenyl-1H-benzimidazole)) was used as an electron-transport
and hole-block layer. Deep-blue light emission was achieved for the device based on P1d, a polymer
based on TFT and the second generation (G2) of carbazole dendrimer pendant, that shows a maximum
current efficiency (CE) of 1.26 cd Aꢀ1, corresponding to external quantum efficiency (EQE) of 1.27%, with
Commission Internationale de L’Eclairage (CIE) coordinates of (0.16, 0.14). In comparison, due to the
intense ICT interaction between the main chain and the side chain, light-green emission was achieved for
the device based on P1c, a polymer based on TFT and bis(9,9-dioctyl-9H-fluoren-2-yl)amine pendant,
giving the highest maximum CE of 4.10 cd Aꢀ1, corresponding to EQE of 2.45%, with CIE coordinates of
(0.24,0.51), although either of the main chain and the side chain emits blue light.
Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction
different energy band gaps are mixed together, energy transfer
occurs from the chromophore with the larger energy band gap to
Over the last two decades, design and synthesis of light-emitting
conjugated polymers (LEPs) as the active materials in polymer
light-emitting diodes (PLEDs) have been the subject of intense
academic and industrial research due to their great potential
applications in full-color flat-panel displays, back-lighting sources
for liquid-crystal displays, and next-generation solid-state lighting
sources [1e6]. Enormous efforts have been focused on their lumi-
nescence properties through molecular engineering [7]. It is well-
known that the emission color obviously depends upon the
energy band gap between the highest occupied molecular orbital
(HOMO) and the lowest unoccupied molecular orbital (LUMO)
energy levels of the chromophore. If two chromophores with
that with the smaller energy band gap, and emission from the latter
dominates [8]. In view of this effect, one method for color-tuning is
to blend a dye chromophore into a host. Doping the host with highly
fluorescent dyes has been proven to be an efficient strategy to
achieve high color purity of emission color and to increase the
efficiency of the devices [9,10]. Zhen et al. achieved pure blue
emission with extremely high maximum external quantum effi-
ciency (EQE) of 9.40% and Commission Internationale de l’Eclairage
(CIE) coordinates of (0.147, 0.139) by doping the emitter into a small
molecular host material of 4,40-bis(carbazol-9-yl)biphenyl (CBP)
[10]. However, the blend system like this tends to show phase
separation during the device operation and over time, and thus
leads to instability in the device performance [11]. An alternative
solution is to covalently attach the dye chromophore to the polymer
which could effectively prevent the aggregation of the dye chro-
mophore caused by the migration of dye inside the polymer matrix
* Corresponding author. Tel.: þ86 20 87114346; fax: þ86 20 87110606.
E-mail address: mssjsu@scut.edu.cn (S.-J. Su).
0032-3861/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.polymer.2012.11.050

H. Ye et al. / Polymer 54 (2013) 162e173
163
[12,13]. In addition, this approach facilitates the efficient energy
transfer from the polymer matrix to the dye chromophore and easily
confines the excitons to the chromophore [14], which acts as an
effective energy trap for excitons formed at the higher-energy
segments of the polymer matrix [15]. Significant efforts have been
made to tune the emission colors of the fluorene-based polymers
(PFs), in which fluorene, its oligomers, or polymers could be used as
the high energy chromophore segments because of their large
energy band gaps to generate other colors at longer wavelengths by
efficient energy transfer to the low-energy chromophore units. For
example, Ego et al. reported the emission color of PFs can be facilely
tuned across the whole visible spectrum through efficient energy
transfer to the chromophore units by incorporation of perylene dyes
in the polymer main chain, at the chain termini, or as side chains
[12]. Wang and co-workers designed and synthesized three kinds of
red electroluminescent (EL) polymers based on PF as the main chain
and the low-energy chromophore of 2,1,3-benzothiadiazole deriv-
atives with different emission wavelengths as red chromophore
units on the side chain [16]. So far, most of the color-tunable PFs have
been developed through the approach of energy transfer by incor-
porating low-energy band gap co-monomers into the main chain or
attaching dyes as pendent side groups [12e14,16]. Pu et al. devel-
oped four fluorescent dyes of bis(difluorenyl) amino-substituted
carbazole, pyrene, perylene, and benzothiadiazole derivatives, and
their emitting colors could be tuned from sky blue to deep red [17].
Very recently, we successfully designed and synthesized a series of
blue light-emitting conjugated polymers based on electron-
donating trifluoren-2-ylamine (TFA) as a building block and their
emission colors could be effectively tuned in the region of deep-blue
with CIE coordinates of (0.16, 0.12) and light-blue with CIE coordi-
nates of (0.18, 0.30) by introducing various substituents onto the TFA
as the pendants [18], indicating that color-tuning of the conjugated
polymers can be also achieved by controlling the effective conju-
gation length and introducing intramolecular charge-transfer (ICT)
interaction.
In this article, a series of LEPs based on building blocks of
electron-donating TFA, electrically neutral 1,3,5-trifluoren-2-
ylbenzene (TFB), and electron-withdrawing analogue of 2,4,6-
trifluoren-2-yltriazine (TFT) were designed and synthesized as
active materials of PLEDs. Among the three fluorene arms of these
building blocks, two fluorene arms combine with another fluorene
unit to be the main chain of the polymer, and the other one fluorene
arm acts as the side chain. The structures of the main chains of all
the polymers are nearly identical except for the distinct junctions of
the side chains to the backbones, changing from electron-donating
amine, to electrically neutral benzene, and electron-withdrawing
triazine. Moreover, substituted groups of electron-donors, like
carbazole, 3,6-di-tert-butyl carbazole, the second generation (G2)
of carbazole dendrimer, and bis(9,9-dioctyl-9H-fluoren-2-yl)
amine, were introduced onto the pendent fluorene as the side
chain. Their structureeproperty relationships were thoroughly
studied. As expected, their energy levels could be effectively tuned
by changing the junctions of the side chains to the backbones and
the pendent electron-donors. More importantly, this paper offers
an alternative approach to tune the emission color of LEPs by
introducing different building blocks from electron-acceptor to
electron-donor as the main chain and thus the ICT interaction
between the side chain and the main chain.
Chemical Co., and they were used as received unless otherwise
specified. All manipulations involving air-sensitive reagents were
performed in the atmosphere of dry argon. The solvents were
purified by routine procedure and distilled under dry argon before
being used. 2-Dibromo-9,9-dioctylfluorene (1) [19], 2-amimo-9,9-
dioctylfluorene (2) [18], 7-bromo-9,9-dioctyl-9H-fluorene-2-
carbonitrile (4) [20], 3,6-di-tert-butyl-9H-carbazole (7) [21], 3,6-
di-tert-butyl-9-(3-(3,6-di-tert-butyl-9H-carbazol-9-yl)-9H-carba-
zol-6-yl)-9H-carbazole (8) [21], 13a [18], 13b [18], and 2,7-
bis(4,4,5,5-tramethyl-1,3,2-dioxaborolan-2-yl)-9,9-dioctylfluorene
(14) [22] were prepared according to the reference procedures.
2.1.1. Synthesis of bis(9,9-dioctyl-9H-fluoren-2-yl)amine (3)
A mixture of tris(dibenzylideneacetone)dipalladium (Pd2(dba)3)
(98 mg, 0.11 mmol), diphenylphosphino-ferrocene (DPPF) (228 mg,
0.41 mmol), 1 (1.89 g, 4.03 mmol), 2 (1.63 g, 4.02 mmol), and t-BuOK
(1.68 g, 14.97 mmol) was stirred in toluene (40 ml) for 24 h at 110 ꢁC
under an argon atmosphere. After cooling to room temperature, the
reaction mixture was extracted with ethyl acetate. The organic
extracts were combined and washed with brine and dried with
anhydrous MgSO4. The precipitate was separated by filtration. The
solvent was removed under reduced pressure, and the residue went
through a silica-gel column to give a brown viscous liquid 2.00 g.
Yield: 62.6%. 1H NMR (300 MHz, DMSO),
d (ppm): 8.52 (s, 1H), 7.66 (t,
4H), 7.35 (d, 2H), 7.30e7.20 (m, 6H), 6.91 (d, 2H), 2.00e1.70 (m, 8H),
1.30e1.00 (m, 40H), 0.80e0.75 (t, 12H), 0.70e0.40 (m, 8H). 13C NMR
(CDCl3, 75MHz), d (ppm): 137.87,129.05,128.24,126.38,125.32, 55.00,
40.64, 35.47, 31.84, 30.21, 29.73, 29.36, 29.33, 23.90, 22.63, 21.46,14.09.
2.1.2. Synthesis of 2,4,6-tris(7-bromo-9,9-dioctyl-9H-fluoren-2-yl)-
[1,3,5]triazine (5)
Under a nitrogen atmosphere, a solution of 4 (8.95g, 18.1 mmol)
in 50 ml anhydrous chloroform (CHCl3) was added dropwise into
the trifluoromethanesulfonic acid at 0 ꢁC, then the mixture was
warmed to room temperature and stirred for 2 days. The reaction
was quenched with a solution of 6 ml ammonia monohydrate in
100 ml water, and the mixture was extracted with chloroform. The
extract was washed with brine and dried over anhydrous magne-
sium sulfate. The precipitate was separated by filtration. The
solvent was removed under reduced pressure, and the residue went
through a silica-gel column to give a yellow solid (8.95 g) in 97.2%
yield. 1H NMR (300 MHz, CDCl3),
d (ppm): 8.83 (d, 3H), 8.73 (s, 3H),
7.90 (d, 3H), 7.69 (d, 3H), 7.60e7.50 (m, 6H), 2.19e2.00 (m, 12H),
1.20e1.00 (m, 60H), 0.80e0.60 (m, 10H). 13C NMR (CDCl3, 75 MHz),
d
(ppm): 171.81, 154.12, 150.79, 144.47, 139.34, 135.84, 135.67,
130.25, 128.46, 126.42, 123.32, 122.26, 121.85, 119.88, 67.96, 55.64,
40.24, 31.77, 29.97, 29.70, 29.20, 25.61, 23.80, 22.58, 14.01. Calcd
C90H120N3Br3 1483.6, EI-MS (m/z): 1482.7 (Mþ).
2.1.3. Synthesis of 9-{7-[4,6-bis(7-bromo-9,9-dioctyl-9H-fluoren-2-
yl)-[1,3,5]triazin-2-yl]-9,9- dioctyl-9H-fluoren-2-yl}-9H-carbazole (9a)
Under an argon atmosphere, a mixture of carbazole (84 mg,
0.5 mmol), 5 (0.74 g, 0.5 mmol), CuI (0.49 g, 2.57 mmol), K2CO3
(0.49 g, 3.55 mmol), and 18-crown-6 (90 mg, 0.34 mmol) in 3.2 ml
o-dichlorobenzene was heated to 180 ꢁC and stirred for 24 h. After
cooling to room temperature, the mixture was poured into water
and extracted with dichloromethane. The extract was washed with
brine and dried over anhydrous magnesium sulfate. The precipitate
was separated by filtration. The solvent was removed under
reduced pressure, and the residue went through a silica-gel column
to give yellow-green solid (0.326 g) in 41.6% yield. 1H NMR
2. Experimental
2.1. Materials
(300 MHz, CDCl3), d (ppm): 8.92e8.88 (d, 1H), 8.87e8.83 (d, 2H),
8.80 (s, 1H), 8.75 (s, 2H), 8.21e8.18 (d, 2H), 8.08e8.00 (t, 2H), 7.95e
7.90 (d, 2H), 7.78e7.68 (m, 3H), 7.65e7.59 (m, 2H), 7.58e7.55
(m, 2H), 7.54e7.50 (m, 1H), 7.49e7.42 (m, 4H), 7.38e7.30 (m, 2H),
All reagents were obtained from Sigma Aldrich Chemical Co.,
Alfa Aesar Chemical Co., Aladdin Chemical Co., and HuiCheng

164
H. Ye et al. / Polymer 54 (2013) 162e173
2.23e2.00 (m, 12H), 1.30e1.00 (m, 60H), 0.90e0.60 (m, 30H). 13C
NMR (CDCl3, 75 MHz), (ppm): 171.83, 154.28, 154.14, 153.85,
151.49, 150.83, 150.62, 144.79, 144.59, 144.52, 140.98, 139.97, 139.53,
139.35, 137.39, 136.15, 135.84, 135.66, 135.54, 132.35, 130.27, 128.49,
126.43, 126.01, 123.49, 123.33, 122.30, 122.21, 121.91, 121.77, 120.46,
120.04, 109.81, 93.99, 55.67, 55.59, 40.30, 31.81, 30.07, 30.00, 29.74,
29.35, 29.25, 24.10, 23.83, 22.62, 14.07.
ethanol. The mixture was stirred for 18 h at room temperature and
was then poured into water and extracted with dichloromethane.
The extract was washed with brine and dried over anhydrous
magnesium sulfate. The precipitate was separated by filtration. The
solvent was removed under reduced pressure, and the residue went
through a silica-gel column to give glassy solid (3.57 g) in 60.1%
d
yield. 1H NMR (300 MHz, CDCl3),
d (ppm): 7.87 (s, 3H), 7.72e7.81
(m, 6H), 7.67 (m, 3H), 7.59e7.62 (m, 3H), 7.43e7.49 (m, 6H), 1.91e
2.08 (m, 12H), 0.98e1.39 (m, 60H), 0.77e0.92 (m, 18H), 0.62e0.67
2.1.4. Synthesis of 9-{7-[4,6-bis(7-bromo-9,9-dioctyl-9H-fluoren-2-
yl)-[1,3,5]triazin-2-yl]-9,9- dioctyl-9H-fluoren-2-yl}-3,6-di-tert-
butyl-9H-carbazole (9b)
(m, 12H). 13C NMR (CDCl3, 75 MHz),
d (ppm): 153.22, 151.26,
142.84, 140.52, 139.73, 130.08, 126.48, 126.27, 125.25, 121.76, 121.20,
121.15, 120.13, 55.62, 40.32, 31.75, 29.97, 29.18, 23.77, 22.58, 14.03.
Calcd C93H123Br3 1480.7, EI-MS (m/z): 1480.0 (Mþ).
9b was synthesized by following the similar procedure of 9a.
Yield: 42.5%. 1H NMR (300 MHz, CDCl3),
d (ppm): 8.91e8.86 (m,
2H), 8.84 (s, 1H), 8.79 (s, 1H), 8.75 (s, 2H), 8.19 (d, 2H), 8.04-7.98 (t,
2H), 7.93-7.90 (d, 2H), 7.76 (s,1H), 7.71 (s,1H), 7.68 (s,1H), 7.63e7.54
(m, 5H), 7.52 (d, 1H), 7.49 (d, 1H), 7.45e7.42 (m, 2H), 2.18e2.00 (m,
12H), 1.49 (s, 18H), 1.30e1.00 (m, 60H), 0.90e0.60 (m, 30H). 13C
2.1.9. Synthesis of 9-{7-[3,5-bis(7-bromo-9,9-dioctyl-9H-fluoren-2-
yl)phenyl]-9,9-dioctyl-9H- fluoren-2-yl}-9H-carbazole (12a)
12a was synthesized by following the similar procedure of 9a.
NMR (CDCl3, 75 MHz),
d
(ppm): 171.88, 171.81, 154.14, 153.73,
Yield: 34.7%. 1H NMR (300 MHz, CDCl3),
d (ppm): 8.19 (d, 2H), 7.98e
151.44, 150.81, 144.95, 144.49, 142.98, 139.97, 139.36, 139.26, 139.01,
137.91, 135.69, 135.37, 130.26, 128.50, 126.43, 125.51, 123.65, 123.49,
123.33, 122.28, 121.89, 121.68, 121.51, 119.94, 116.39, 109.25, 55.67,
55.63, 40.29, 34.79, 32.06, 31.88, 31.81, 30.06, 30.00, 29.73, 29.37,
29.31, 29.25, 26.94, 24.06, 23.82, 22.62, 14.07.
7.87 (m, 5H), 7.83e7.65 (m, 10H), 7.64e7.53 (m, 3H), 7.52e7.40 (m,
7H), 7.50e7.28 (m, 2H), 2.10e1.09 (m, 12H), 1.03e1.00 (m, 60H),
0.90e0.60 (m, 30H). 13C NMR (CDCl3, 75 MHz),
d (ppm): 140.75,
140.57,140.46,140.38, 140.02,139.82,139.75,136.56,136.00,132.20,
130.10, 126.52, 126.29, 125.93, 125.31, 123.43, 121.93, 121.87, 121.74,
121.53, 121.22, 120.95, 120.39, 120.28, 120.20, 119.90, 109.81, 92.68,
55.65, 55.58, 40.31, 31.77, 30.05, 29.98, 29.71, 29.30, 29.20, 24.06,
23.79, 22.61, 14.05.
2.1.5. Synthesis of 7-(4,6-bis(7-bromo-9,9-dioctyl-9H-fluoren-2-
yl)-1,3,5-triazin-2-yl)-N,N-bis(9,9-dioctyl-9H-fluoren-2-yl)-9,9-
dioctyl-9H-fluoren-2-amine (9c)
9c was synthesized by following the similar procedure of 3.
2.1.10. Synthesis of 9-{7-[3,5-bis(7-bromo-9,9-dioctyl-9H-fluoren-
2-yl)phenyl]-9,9-dioctyl-9H- fluoren-2-yl}-3,6-di-tert-butyl-9H-
carbazole (12b)
Yield: 44.4%. 1H NMR (300 MHz, CDCl3),
d (ppm): 8.85e8.80 (m,
3H), 8.74e8.70 (m, 3H), 7.96e7.80 (m, 3H), 7.71e7.50 (m, 9H), 7.42e
7.28 (m, 10H), 7.25e7.23 (m, 1H), 7.10e7.02 (m, 3H), 2.20e1.80 (m,
20H), 1.30e1.00 (m, 100H), 0.90e0.50 (m, 50H).
12b was synthesized by following the similar procedure of 9a.
Yield: 39.4%. 1H NMR (300 MHz, CDCl3),
d (ppm): 8.18 (s, 2H), 7.96e
7.87 (m, 5H), 7.83e7.74 (m, 5H), 7.73 (s, 1H), 7.68 (s, 2H), 7.63 (s, 1H),
7.60 (s, 1H), 7.58e7.54 (m, 2H), 7.52e7.45 (m, 6H), 7.42e7.39 (m,
2H), 2.10e1.09 (m, 12H), 1.51 (s, 18H), 1.03e1.00 (m, 60H), 0.90e
2.1.6. Synthesis of 9-{7-[4,6-bis(7-bromo-9,9-dioctyl-9H-fluoren-2-
yl)-[1,3,5]triazin-2-yl]-9,9- dioctyl-9H-fluoren-2-yl}-3,6-di-tert-
butyl-9-(3-(3,6-di-tert-butyl-9H-carbazol-9-yl)-9H-carbazol-6-yl)-
9H-carbazole (9d)
0.60 (m, 30H). 13C NMR (CDCl3, 75 MHz),
d (ppm): 153.24, 152.73,
151.92, 151.28, 142.94, 142.87, 142.83, 140.58, 140.27, 140.17, 139.75,
139.48, 139.35, 137.04, 130.10, 126.54, 126.27, 125.45, 125.33, 123.60,
126.40, 121.80, 121.49, 121.22, 120.88, 120.18, 116.36, 109.25, 55.65,
55.61, 40.37, 34.78, 32.07, 31.87, 31.80, 30.07, 30.01, 29.74, 29.35,
29.31, 29.24, 26.94, 24.03, 23.80, 22.64, 14.11.
9d was synthesized by following the similar procedure of 9a.
Yield: 34.3%. 1H NMR (300 MHz, CDCl3),
d (ppm): 8.95e8.92 (d, 1H),
8.88 (s, 1H), 8.86e8.84 (m, 2H), 8.76 (s, 2H), 8.29 (s, 2H), 8.18 (s, 5H),
8.09e8.06 (d, 1H), 7.94e7.92 (d, 2H), 7.81 (s, 2H), 7.73e7.68 (m, 4H),
7.66e7.62 (m, 2H), 7.56 (s, 3H), 7.53 (s, 1H), 7.50 (s, 1H), 7.48 (s, 3H),
7.39 (s, 3H), 7.36 (s, 1H), 2.30e2.00 (m,12H),1.48 (s, 36H),1.30e1.00
(m, 60H), 0.90e0.60 (m, 30H).
2.1.11. Synthesis of 7-(3,5-bis(7-bromo-9,9-dioctyl-9H-fluoren-2-
yl)phenyl)-N,N-bis(9,9-dioctyl-9H-fluoren-2-yl)-9,9-dioctyl-9H-
fluoren-2-amine (12c)
2.1.7. Synthesis of 1-(7-bromo-9,9-dioctyl-9H-fluoren-2-yl)
ethanone (10)
12c was synthesized by following the similar procedure of 9a.
Yield: 31.1%. 1H NMR (300 MHz, CDCl3),
d (ppm): 7.92e7.85 (m, 3H),
Under
a
nitrogen atmosphere,
a
solution of
1
(10.22 g,
7.82e7.72 (m, 6H), 7.71e7.64 (m, 5H), 7.63e7.53 (m, 6H), 7.52e7.42
(m, 3H), 7.35e7.27 (m, 8H), 7.25e7.20 (m, 1H), 7.08e7.01 (d, 3H),
2.10e1.08 (m, 20H), 1.04e1.00 (m, 100H), 0.90e0.50 (m, 50H). 13C
22.0 mmol) and anhydrous aluminum chloride (AlCl3) in 80 ml
dichloromethane (CHCl2) was cooled to 0 ꢁC. Then CH3COCl
(1.62 ml, 22.9 mmol) was added via syringe, the mixture was stirred
for 12 h at 0 ꢁC. The reaction was quenched with 2% NaOH solution.
The reaction mixture was extracted with ethyl acetate. The organic
extracts were combined and washed with brine and dried with
anhydrous MgSO4. The precipitate was separated by filtration. The
solvent was removed under reduced pressure, and the residue went
through a silica-gel column to give a yellow solid 9.36 g. Yield:
NMR (CDCl3, 75 MHz),
d (ppm): 151.04, 150.42, 142.77, 141.00,
140.82, 140.64, 140.39, 139.73, 135.96, 132.17, 130.06, 126.77, 126.48,
126.26, 125.20, 122.74, 121.72, 121.50, 121.16, 120.30, 120.16, 119.01,
92.61, 55.62, 55.55, 55.33, 55.10, 40.42, 40.33, 38.98, 34.14, 31.85,
31.75, 30.13, 29.96, 29.70, 29.44, 29.30, 29.18, 23.99, 23.76, 22.64,
22.59, 20.15, 19.17, 14.39, 14.09, 14.03, 11.37.
83.9%. 1H NMR (300 MHz, CDCl3),
d
(ppm): 7.95 (m, 1H), 7.94 (m,
2.1.12. General procedure for polymer synthesis
1H), 7.72 (m, 1H), 7.61 (m, 1H), 7.47e7.5 (m, 2H), 2.66 (s, 3H), 1.93e
2.02 (m, 4H), 1.02e1.11 (m, 20H), 0.76 (m, 6H), 0.53e0.59 (m, 4H).
Carefully purified 14 (0.5 mmol), 0.5 mmol 9a or 9b or 9c or 9d
or 12a or 12b or 12c, Pd(OAc)2 (3 mg), tricyclohexyl phosphine
(P(Cy)3) (6 mg), 1 ml Et4NOH and 1 ml deionized water were dis-
solved in 8e15 ml toluene. The mixture was heated to 90e100 ꢁC
with vigorous stirring for 48 h under an argon atmosphere. At the
end of polymerization, polymers were sequentially end-capped
with phenylboronic acid and bromobenzene in order to avoid
2.1.8. Synthesis of 1,3,5-tris(7-bromo-9,9-dioctyl-9H-fluoren-2-yl)
benzene (11)
Under an argon atmosphere, silicon tetrachloride (SiCl4) was
added dropwise into the solution of 10 (6 g, 11.8 mmol) in 80 ml

H. Ye et al. / Polymer 54 (2013) 162e173
165
a carrier trap and luminescence quenching center formation by end
groups in OLEDs [23]. The mixture was then poured into methanol.
The precipitated material was recovered by filtration and was
extracted with methanol and acetone utilizing a Soxhlet extractor
to remove oligomers and catalyst residues. The resulted polymers
were dried under vacuum. Yields: 50e80%.
N2 flow at a heating rate of 10 ꢁC minꢀ1. UVevis absorption spectra
were recorded on an HP 8453 spectrophotometer. Photo-
luminescent (PL) spectra were measured using a Jobin-Yvon spec-
trofluorometer. PL quantum efficiencies were determined on
a fluoyo SENS spectrofluorometer. Cyclic voltammetry (CV) was
performed on a CHI600D electrochemical workstation with a plat-
inum working electrode and a platinum wire counter electrode at
a scan rate of 50 mV sꢀ1 against a Ag/Agþ reference electrode with
an argon saturated anhydrous solution of 0.1 mol Lꢀ1 tetrabuty-
lammonium hexafluorophosphate (Bu4NPF6) in acetonitrile. The
polymer films for electrochemical measurements and PL quantum
efficiencies measurements were coated from their toluene
solutions.
2.1.12.1. P1a. 1H NMR (300 MHz, CDCl3),
d (ppm): 8.98e8.88 (m,
3H), 8.87e8.80 (m, 3H), 8.21 (d, 2H), 8.10e7.86 (m, 8H), 7.80e7.60
(m, 10H), 7.52e7.46 (m, 3H), 7.44e7.40 (m, 1H), 7.38e7.31 (m,
2H), 2.40e2.00 (m, 16H), 1.30e1.00 (m, 80H), 0.97e0.70 (m, 40H).
Anal. Calcd. for [C131H168N4]n: C, 87.47; H, 9.41; N, 3.11. Found: C,
85.90; H, 10.16; N, 2.93.
2.1.12.2. P1b. 1H NMR (300 MHz, CDCl3),
d
(ppm): 8.97e8.82 (m,
2.3. Device fabrication and characterization
6H), 8.19 (s, 1H), 8.10e7.83 (m, 9H), 7.79e7.68 (m, 7H), 7.66e7.60
(m, 2H), 7.53e7.42 (m, 5H), 2.40e2.00 (m, 16H), 1.50 (s, 18H),
1.30e1.00 (m 80H), 0.99e0.70 (m, 40H). Anal. Calcd. for
[C139H184N4]n: C, 87.36; H, 9.71; N, 2.93. Found: C, 84.97; H, 7.85; N,
2.63.
The fabrication process of the devices is as the following. The
indium tin oxide (ITO)-coated glass substrates were cleaned by
ultrasonic treatment in deionized water, acetone, detergent, and
isopropyl alcohol sequentially. After treatment with oxygen plasma,
a 40-nm-thin layer of poly-(3,4-ethylenedioxythiopene) : poly(-
styrenesulfonate) (PEDOT:PSS) (Bayer Baytron P 4083) was spin-
coated as a buffer layer. After being dried in a vacuum oven at
80 ꢁC for 12 h, the xylene solutions of the polymers prepared in
a nitrogen-filled drybox were spin-coated on the top of the
PEDOT:PSS film. The thickness of the spin-coated film was
controlled by regulating the spinning speed and the solution
concentration, measured with an Alfa Step 500 surface profiler
(Tencor). The low mass materials were vacuum deposited in
a vacuum of 1 ꢂ 10ꢀ4 Pa. A thin layer of CsF (1.5 nm) and subse-
quently 120-nm-thin layer of Al were vacuum deposited in
a vacuum of 1 ꢂ10ꢀ4 Pa as the cathode. Device performances were
measured inside a drybox. Current versus driving voltage charac-
teristics were recorded with a Keithley 236 source meter. Electro-
luminescent (EL) spectra were obtained by Oriel Instaspec IV CCD
spectrograph. Luminance was measured by a PR 705 photometer
(Photo Research). External quantum efficiencies were determined
by a Si photodiode with calibration in an integrating sphere (IS080,
Labsphere).
2.1.12.3. P1c. 1H NMR (300 MHz, CDCl3),
d
(ppm): 1H NMR
(300 MHz, CDCl3), (ppm): 8.92e8.71 (m, 6H), 8.02e7.45 (br, 20H),
d
7.44e7.28 (br, 9H), 7.23e7.14 (m, 3H), 2.30e1.80 (m, 24H), 1.40e
0.92 (m, 120H), 0.90e0.40 (m, 60H). Anal. Calcd. for
[C177H242N4]n: C, 87.64; H, 10.06; N, 2.31. Found: C, 85.54; H, 11.02;
N, 2.11.
2.1.12.4. P1d. 1H NMR (300 MHz, CDCl3),
d (ppm): 9.01e8.81 (br,
6H), 8.29 (s, 2H), 8.18 (s, 5H), 8.12e7.88 (br, 6H), 7.84e7.60 (m, 15H),
7.51e7.47 (d, 4H), 7.40e7.37 (d, 4H), 2.40e2.00 (m, 16H), 1.55 (s,
36H), 1.30e1.00 (m, 80H), 0.95e0.60 (m, 40H). Anal. Calcd. for
[C171H214N6]n: C, 87.26; H, 9.16; N, 3.57. Found: C, 83.97; H, 10.33; N,
3.29.
2.1.12.5. P2a. 1H NMR (300 MHz, CDCl3),
d (ppm): 8.19 (d, 2H),
8.00e7.65 (m, 24H), 7.62e7.56 (m, 2H), 7.51e740 (m, 4H), 7.39e7.30
(m, 3H), 2.30e2.00 (m, 16H), 1.30e1.00 (m, 80H), 0.95e0.60 (m,
40H). Anal. Calcd. for [C134H171N]n: C, 89.62; H, 9.60; N, 0.78. Found:
C, 87.30; H, 10.13; N, 0.75.
3. Results and discussion
2.1.12.6. P2b. 1H NMR (300 MHz, CDCl3),
d (ppm): 8.18 (s, 2H), 7.97
(s, 3H), 7.96e7.66 (m, 21H), 7.60e7.55 (m, 2H), 7.51 (s, 1H), 7.49e
7.43 (m, 3H), 7.41 (s, 1H), 2.30e2.00 (m, 16H), 1.49 (s, 18H), 1.30e
1.00 (m, 80H), 0.97e0.70 (m, 40H). Anal. Calcd. for [C142H187N]n:
C, 89.39; H, 9.88; N, 0.73. Found: C, 87.37; H, 10.47; N, 0.71.
3.1. Synthesis and chemical characterization
In order to tune the HOMO and LUMO energy levels of the
polymers to facilitate carrier injection from the electrode and thus
the carrier balance in the emissive layer (EML), various electron-
donating substituents, such as carbazole (6), 3,6-di-tert-butyl
carbazole (7), and bis(9,9-dioctyl-9H-fluoren-2-yl)amine groups (3)
wereintroduced ontothe pendent fluorene as the side chain for LEPs
of P1 and P2 based on TFT and TFB, respectively. In addition, the
bulky pendant of G2 carbazole dendrimer (8) was also introduced
onto the pendent fluorene of TFT to give P1d. Although great effort
was made to introduce 3 and 8 onto TFA, it was proved to be futile
finally. It was suspected that the electron-donating property of TFA
is unfavorable to cuprous iodide or copper powder catalyzed Ull-
mann coupling reaction and palladium-catalyzed Buchwald-Hart-
wig CeN coupling reaction. The reactivity of palladium-catalyzed
CeN coupling reaction of the electrically neutral TFB is higher
than that of the electron-donating TFA, but less than that of the
electron-withdrawing TFT. Thus, unfortunately, the monomers and
the corresponding polymers based onTFB with pendant of 8 and TFA
with pendant of 3 or 8 were not obtained. The structures and the
synthetic routes of the TFT- and TFB-based monomers and the cor-
responding polymers are outlined in Schemes 1 and 2, respectively.
2.1.12.7. P2c. 1H NMR (300 MHz, CDCl3),
d (ppm): 8.00e7.50 (m,
31H), 7.45e7.28 (br, 10H), 2.20e1.80 (m, 24H), 1.30e1.00 (m, 120H),
0.90e0.50 (m, 60H). Anal. Calcd. for [C180H245N]n: C, 89.23; H,10.19;
N, 0.58. Found: C, 86.48; H, 10.66; N, 0.70.
2.2. Measurement and characterization
1H NMR and 13C NMR spectra were recorded on a Bruker AV-300
(300 MHz) in deuterated chloroform (CDCl3) with tetramethylsi-
lane as an internal reference. Number-average molecular weight
(Mn) and weight-average molecular weight (Mw) were determined
by a Waters GPC 2410 in THF using a calibration curve with stan-
dard polystyrene as a reference. Elemental analysis was performed
on a Vario EL elemental analysis instrument (Elementar Co.).
Differential scan calorimetry (DSC) measurements were performed
on a Netzsch DSC 204 under N2 flow at a heating rate of 10 ꢁC minꢀ1
and a cooling rate of 20 ꢁC minꢀ1. Thermogravimetric analysis
(TGA) measurements were performed on a Netzsch TG 209 under

166
H. Ye et al. / Polymer 54 (2013) 162e173
Scheme 1. Synthetic routes of the monomers based on 2,4,6-trifluoren-2-yltriazine (TFT) (9ae9d) and 1,3,5-trifluoren-2-ylbenzene (TFB) (12ae12c). Also shown the molecular
structures of the monomers based on trifluoren-2-ylamine (TFA) (13ae13c). Conditions: i: Pd2(dba)3, DPPF, NaOtBu, toluene, reflux; ii: 1) CF3SO3H, CHCl3, 0 ꢁC; 2) 0 ꢁC w r.t.; iii: CuI,
K2CO3, 18-crown-6, o-dichlorobenzene, 180 ꢁC; iv: CH3COCl, anhydrous AlCl3, CH2Cl2; v: SiCl4, EtOH.

H. Ye et al. / Polymer 54 (2013) 162e173
167
Scheme 2. Synthetic routes of the polymers based on 2,4,6-trifluoren-2-yltriazine (TFT) (P1aeP1d), 1,3,5-trifluoren-2-ylbenzene (TFB) (P2aeP2c), and trifluoren-2-ylamine (TFA)
(P3aeP3b). Conditions: Pd(OAc)2/tricyclohexyl phosphine (P(Cy)3), toluene, Et4NOH, deionized water, 90e100 ꢁC.
The polymers were synthesized by palladium-catalyzed Suzuki
cross-coupling polycondensation of an equi-molecular mixture of
two monomers of TFT derivatives (9ae9d) or TFB derivatives (12ae
12c) or TFA derivatives (13ae13b) and 14 to give P1aeP1d, P2ae
P2c, and P3aeP3b, respectively. The chemical structures of the
polymers were verified by 1H NMR and elemental analysis.
The developed polymers are soluble in conventional organic
solvents, like toluene, chloroform, and tetrahydrofuran (THF).
Molecular weights of the polymers were determined by gel perme-
ation chromatography (GPC) utilizing THF as a solvent and poly-
styrene as a standard. As shown in Table 1, except for P1c, Mn of the
polymers are from 20.9 to 52.9 kDa with polydispersity index
(PDI ¼ Mw/Mn) between 1.16 and2.16. The C, H, and N contents for the
polymers are close to those calculated from the feed compositions.
were achieved, respectively. The pendant of 3,6-di-tert-butyl
carbazole (7) endows the polymers higher Tg in comparison to
those of the polymers with carbazole (6) pendant due to the bulky
tert-butyl groups. For P1d, which possesses more bulky pendant of
G2 carbazole dendrimer (8), Tg up to 94 ꢁC was achieved, which is
15 ꢁC higher than that of P1a with the smallest carbazole pendant.
For P3a and P3b, Tg up to w90 ꢁC was achieved, which is much
higher than those of P1aeP1b and P2aeP2b with the same side
chain, respectively. Considering that the above-mentioned three
types of polymers possess the identical pendants, the only differ-
ence is the structure of the main chain, exactly, the distinct junction
of the side chain to the backbone, i.e. electron-donating amine,
electron-withdrawing triazine, and electrically neutral benzene.
Wherefore, the different Tg of these polymers could be attributed to
the difference in rigidity and polarity of the main chains, and thus
the unequal molecular interaction. Owing to the much more flex-
ible alkyl chains of bis(9,9-dioctyl-9H-fluoren-2-yl)amine (3) in P1c
and P2c, these two polymers exhibit the lowest Tg of 56 and 63 ꢁC,
respectively, among all the developed polymers.
3.2. Thermal properties
As shown in Table 1, all the polymers exhibit good thermal
stability with 5% weight-loss temperatures (Td) over 400 ꢁC due to
the introduction of the bulky TFT, TFB, and TFA building blocks into
the main chains, which impart the chains with rigidity and non-
coplanarity. With the change of the substituents at the C-7 posi-
tion of the pendent fluorene of TFT and TFB, relatively high glass
transition temperatures (Tg) ranging from 56 to 94 ꢁC, 63e79 ꢁC
3.3. Optical properties
In toluene solution, the polymers P2a and P2b based on the
electrically neutral TFB exhibit one distinct absorption band at
about 360 nm, which can be attributed to the pep* transition of the
conjugated polymer backbone (Fig. 1) [24]. Besides, they also
exhibit one peak combined with a slight absorption shoulder in the
range of 280e320 nm that can be attributed to the pendent car-
bazolyl. Compared to the absorption bands of P2a and P2b, bath-
ochromic shift of about 10 nm was observed for the absorption
bands of P2c. In our previous work, it was reported that the
absorption bands of the polymers containing TFA as a building
block of main chain peak at w400 nm in toluene solution [18].
Therefore, this bathochromic shift can be attributed to the electron-
donating component of TFA that acts as the side chain of P2c and
Table 1
Molecular weights and thermal properties of the polymers.
Polymer
Mn (kDa)
Mw (kDa)
PDI
Td (ꢁC)
Tg (ꢁC)
P1a
P1b
P1c
P1d
P2a
P2b
P2c
P3a
P3b
36.3
52.9
4.95
22.3
40.1
32.2
48.7
26.3
20.9
71.7
114
1.97
2.16
1.14
1.51
2.11
1.16
2.11
1.67
1.80
424
420
427
422
424
405
429
416
423
79
82
56
94
66
79
63
89
90
5.67
33.8
84.3
37.2
103
43.8
37.5
expands the
p-conjugation system compared with the carbazolyl
group. For the films, besides the absorption bands at w360 nm,

168
H. Ye et al. / Polymer 54 (2013) 162e173
observed in the range of 400e470 nm. A slight absorption band was
found in the range of 280e320 nm and can be also ascribed to the
* transition of the pendent carbazolyl for P1a and P1b. Since
P1a
P1b
P1c
P1d
P1a
P1b
P1c
P1d
pep
there are more carbazole groups in 8 that acts as the pendant in
P1d, the absorption band in the range of 280e320 nm is much
more intense than those of P1a and P1b, and it is consistent with
the absorption in films. Similarly, besides the absorption bands at
360 and 390 nm, another intense band of 215 nm was also observed
with a shoulder at 240 nm (Fig. 2) for the films, which can be
P2a
P2b
P2c
P2a
P2b
P2c
attributed to the ne
For the polymers based on the electron-donating TFA, P3a and
P3b, the distinct absorption band of the * transition of the
p* transitions of fluorene.
pep
conjugated polymer backbone is at about 400 nm in both toluene
solution and solid film, and a broad absorption band of the pendent
carbazolyl was observed in the range of 300e350 nm (Fig. 1) and
280e320 nm (Fig. 2), in toluene solution and solid film, respec-
tively. Compared with the polymers based on the electron-
withdrawing TFT and the electrically neutral TFB, the difference
of the distinct absorption band can be ascribed to the disparity of
P3a
P3b
P3a
P3b
the
p-conjugation in the polymer backbone.
280 320 360 400 440 480 520 560 600 350 400 450 500 550 600 650 700
Wavelength (nm) Wavelength (nm)
Similar bathochromic shifts were also observed for their PL
spectra recorded in both toluene solution and thin film. For the
toluene solutions of the polymers based on TFB, PL spectra peak at
410 and 449 nm, 410 and 439 nm, 425 and 449 nm for P2a, P2b, and
P2c, respectively (Fig. 1). For the films, PL spectra peak at 416 nm
with a shoulder at 440 nm for P2a and P2b. In comparison, PL
spectrum of P2c peaks at 429 nm with a shoulder at 450 nm due to
Fig. 1. Normalized UVevis absorption (Left) and PL (Right) spectra of the polymers in
dilute toluene solutions.
another intense band was observed at w215 nm (Fig. 2), which can
be attributed to the nep* transitions of fluorene. These absorption
bands are absent for the solutions since they are overlapped by the
absorption of toluene.
the electron-donating pendant of TFA that expands the
p-conju-
gation. For the polymers based on the electron-withdrawing TFT in
toluene solution, PL spectra peak at w430 and 460 nm for P1a and
P1d, 430 and 450 nm for P1b. In comparison, PL spectrum of P1c
shifts to the longer wavelength of 480 nm with a slight PL band at
430 nm, which can be attributed to the emissions according to the
ICT interaction formed between the electron-withdrawing main
chain and the electron-donating side chain and the polymer
backbone, respectively. Different from most of ever reported LEPs
whose emission colors were tuned by incorporation of low-energy
chromophore and thus energy transfer from the high energy
chromophore backbone to the low-energy chromophore unit,
emission color of P1c was tuned by the ICT interaction between the
electron-withdrawing main chain and the electron-donating side
chain although either of them emits blue light. Moreover, the PL
band according to the ICT interaction dominates the emission
spectrum even in a dilute solution. In contrast, for most of the LEPs
achieved by incorporation of low-energy chromophore, the PL
bands according to the high energy chromophore backbone
generally dominates the emission spectra in a dilute solution due to
the incomplete energy transfer. For the films, PL spectra peak at
w436 nm with a shoulder at 460 nm for P2a, P2b, and P1d,
respectively. In comparison, PL spectrum of P1c in solid film shifts
to the longer wavelength and peaks at 482 nm, and the slight
emission band at 430 nm according to the polymer backbone
shown in the dilute solution disappeared completely in the solid
film. Compared with the PL spectra of the polymers based on the
electrically neutral TFB in both toluene solution and thin film, all
the PL spectra of the polymers based on the electron-withdrawing
TFT shift to the longer wavelength, and it can be explained by the
strong electron affinity of TFT component and thus the ICT inter-
action between the electron-donating pendant and the electron-
withdrawing backbone. PL spectra of P3a and P3b also shift to
the longer wavelength in comparison to the polymers based on the
electrically neutral TFB, P2a and P2b, and the electron-
withdrawing TFT, P1a and P1b, in both toluene solution and thin
film. As compared with the PL spectra of the polymers in toluene
solution, the PL spectra of the polymers in thin film slightly shift to
For the polymers based on the electron-withdrawing TFT in
toluene solution, the absorption band of the pep* transition of the
conjugated polymer backbone situates at 390 nm except for P1c at
360 nm, which shifts to the shorter wavelength for 30 nm may be
attributed to the retardation of electronic delocalization caused by
the twist between the main chains and the side chains of TFA [25].
Noting that due to the intense ICT interaction formed between the
electron-withdrawing segment of TFT and the electron-donating
component of 3, a broad and intense absorption band was
P1a
P1b
P1c
P1d
P1a
P1b
P1c
P1d
P2a
P2b
P2c
P2a
P2b
P2c
P3a
P3b
P3a
P3b
200 250 300 350 400 450 500 550 600 360 400 440 480 520 560 600 640 680
Wavelength (nm)
Wavelength (nm)
Fig. 2. Normalized UVevis absorption (Left) and PL (Right) spectra of the polymers in
thin films.

H. Ye et al. / Polymer 54 (2013) 162e173
169
the longer wavelength (Fig. 2), which can be probably ascribed to
the aggregation and dielectric environment of the solid films
[26,27]. The PL quantum efficiencies are listed in Table 2. It was
found that polymers based on TFT and TFB showed much higher
efficiencies than those of polymers based on TFA, indicating that
polymers contained TFT and TFB could more effectively restrain the
aggregation of the molecular chains relative to the polymers based
on TFA.
PL spectra of the polymers P2 and P3 also exhibit sol-
vatochromism nature in different solvents (Figs. S1 and S2).
However, the maximum emission wavelengths change slightly in
solvents with different polarities to give broader FWHM values and
obscure vibronic structure in a polar environment. In comparison to
the absorption process, absorption bands of P2 and P3 are almost
consistent in these solvents, suggesting that absorption involves
a FranckeCondon transition from the non-dipolar ground state.
This is quite different from the excited state that may be possesses
a charge-transfer component, which is stabilized by high dielectric
constant solvents and thus red-shifts.
To gain more insights into the photophysical properties of the
polymers based on TFT, TFB and TFA, UVevis absorption and PL
spectra in solvents with varying polarity, such as toluene (dielectric
constant Dk ¼ 2.38), dichloromethane (DCM) (Dk ¼ 9.1), and THF
(Dk ¼ 7.5) were measured [20]. For P1 containing electron-
withdrawing backbone and electron-donating pendants, their
photophysical properties in solution are strongly dependent on the
solvent polarity. As depicted in Fig. 3, the absorption profile and
maxima of P1a, P1b and P1d are almost consistent in varying
solvents, suggesting their solvent polarity independent ground
state electronic structures and small dipole moments associated
with the ICT transitions [28]. It is worth pointing out that the
absorption spectrum of P1c displays a bathochromic shifted peak at
379 nm with an emerging shoulder at 440 nm in THF solution
compared with that in DCM and toluene solutions, indicating
a quite different chain conformation in THF due to the intense ICT
interaction. PL spectra of P1a, P1b and P1d in toluene solution
show a well-resolved vibronic structure with two emission peaks.
In comparison, only one emission peak with broadened emission
spectra and obscure vibronic structure was observed in THF and
DCM solutions. In addition, their PL spectra in THF and DCM solu-
tions exhibit obvious spectral red-shifts in emission and broader
full width at half-maximum (FWHM) compared with that recorded
in toluene solution. Note that those are good solvents for all the
polymers and the observed red-shift of the PL spectra occurs in
rather dilute solutions (w1 ꢂ 10ꢀ5 M on the basis of the molecular
weight of the polymer repeat unit), suggesting that aggregation
(inter-molecular) effects are not responsible for the red-shift. This
phenomenon could be interpreted that the more polar excited state
resulting from a charge-transfer should be stabilized relative to the
less polar ground state in a solvent of greater polarity [29]. This
excited state stability should result in a red-shift of the emission
spectra of the polymers in more polar solvents. Interestingly, PL
spectrum of P1c in DCM solution displays two obvious emission
peaks at 417 and 560 nm. The emission peak with higher-energy
can be attributed to the emission of the polymer backbone, and
the emission peak with lower energy can be attributed to the ICT
interaction. Different from the toluene and THF solutions, the
emission according to the polymer backbone dominates the PL
spectrum, indicating higher stability of the excited state of polymer
backbone.
3.4. Electrochemical properties
The electrochemical properties of the polymers were investi-
gated by CV. Under the same experimental condition, the redox
potential (E1/2) of ferrocene/ferrocenium (Fc/Fcþ) was measured to
be 0.28 V to the Ag/Agþ reference electrode. Note that the redox
potential of Fc/Fcþ has an absolute energy level of ꢀ4.80 eV to
vacuum [30,31]. Thus, HOMO energy levels are calculated according
to the following equations: HOMO ¼ ꢀe (Eox þ 4.52) (eV), where Eox
is the onset oxidation potential vs Ag/Agþ [32]. LUMO energy levels
of the polymers are estimated from the HOMO energy levels and
the optical energy band gaps (Egopt). The optical energy band gaps of
the polymers are estimated from the absorption edges of the UVe
vis spectra of the thin films (Table 2). HOMO energy levels of all the
TFB-based polymers are about ꢀ5.7 eV. With introducing two tert-
butyl groups at the 3- and 6-positions of the carbazole component
in P2a, slightly reduced HOMO and LUMO energy levels were
achieved for P2b. This phenomenon was also confirmed for the
TFA- and TFT-based polymers. The decreased HOMO and LUMO
energy levels can be attributed to the tert-butyl groups that may
break the
pep stacking of carbazoles and fluorenes. Since the
electron-donating ability of 3 is stronger than that of carbazole,
HOMO energy levels of the polymers containing pendant 3 is
higher-lying in comparison to those of the corresponding polymers
with carbazole pendant. Due to the electron-withdrawing TFT
segments introduced into the skeleton of the polymers P1, LUMO
energy levels of the polymers P1 were reduced to ꢀ2.8 eV, which
are 0.1 eV lower-lying in comparison to those of the polymers P2
based on the electrically neutral TFB. For P1d, which possesses
bulky G2 carbazole dendrimer pendant of 8, its HOMO energy level
rises to ꢀ5.69 eV, which is 0.1 eV higher-lying relative to that of
P1b. It is worth noting that LUMO energy level of P1c was greatly
reduced to ꢀ3.09 eV, which could be attributed to the strong
electron affinity of TFT component and thus the ICT interaction
between the electron-donating pendant 3 and the electron-
withdrawing unit TFT, rendering the narrowest optical energy
band gap of 2.58 eV. In comparison, for P3a and P3b, owing to the
introduction of the electron-donor TFA into the main chain and
carbazole as the pendant, HOMO and LUMO energy levels ascend to
ꢀ5.4 and ꢀ2.6 eV, respectively, which are higher-lying than the
electrically neutral TFB-based polymers P2a and P2b, and the
electron-withdrawing TFT-based polymers P1a and P1b. Further-
more, according to the absorption edges of the solid films, their
optical energy band gaps are estimated to be 2.80 eV, which is
narrower compared to those of P2a-P2b and P1a-P1b.
Table 2
Optical energy band gaps (Egopt), PL quantum efficiencies (QEPL) and electrochemical
properties of the polymer films.
a
Polymer
Egopt (eV)
Eox (V)
EHOMOb (eV)
ELUMOc (eV)
QEPL (%)
P1a
P1b
P1c
P1d
P2a
P2b
P2c
P3a
P3b
2.91
2.91
2.58
2.88
3.00
3.00
2.98
2.80
2.80
1.25
1.27
1.15
1.17
1.20
1.22
1.18
0.95
1.04
ꢀ5.77
ꢀ5.79
ꢀ5.67
ꢀ5.69
ꢀ5.72
ꢀ5.74
ꢀ5.70
ꢀ5.35
ꢀ5.44
ꢀ2.86
ꢀ2.88
ꢀ3.09
ꢀ2.81
ꢀ2.72
ꢀ2.74
ꢀ2.72
ꢀ2.55
ꢀ2.64
27.8
23.6
9.2
21.5
33.3
26.7
21.0
2.7
3.5. Density functional theory calculations
To get further insights into the molecular orbitals of polymers,
density functional theory (DFT) calculations of the polymer units
based on pendants of carbazole and 3 were performed using the
Gaussian suite of programs (Gaussian 03W). To obtain their
HOMO and LUMO energy levels, molecular structure optimization
10.5
a
Estimated with the absorption edges of the solid films.
Calculated according to HOMO ¼ ꢀe(Eox þ 4.52).
b
c
Calculated from the HOMO energy level and Egopt
.

170
H. Ye et al. / Polymer 54 (2013) 162e173
P1a
P1a
P1b
P1c
P1d
P1b
P1c
P1d
280 320 360 400 440 480 520 560 600 350 400 450 500 550 600 650 700 750
Wavelength (nm)
Wavelength (nm)
Fig. 3. Normalized UVevis absorption (Left) and PL (Right) spectra of P1aeP1d in dilute toluene (B), DCM (C) and THF (,) solutions.
(halogen and alkyl chain were omitted) and single-point energy
calculations were performed at the B3LYP/6-31G(d) and B3LYP/6-
311 þ G(d,p) levels, respectively. Besides 9a, 9c, 12a, 12c, and 13a,
calculation of 13c was also performed for contradistinction, although
it could not be obtained as above-mentioned. As shown in Fig. 4,
HOMOs of the polymer units based onTFTand TFB are mainly located
at the pendant of carbazole or 3. In comparison, their LUMOs reside
along the central arylene skeleton of TFT and TFB, respectively. For
Fig. 4. Calculated spatial distributions (DFT, Gaussian 03W) of LUMOs (Top) and HOMOs (Bottom) of the monomers. Halogen and alkyl chain were omitted.

H. Ye et al. / Polymer 54 (2013) 162e173
171
the polymer units based on TFA, both of the HOMOs and LUMOs are
delocalized over the whole -conjugation system since the whole
a
5000
4000
3000
2000
1000
0
p
103
101
molecule is electron-rich. Note that the difference in the HOMO/
LUMO localization is especially pronounced for 9c, in accordance
with the intense electron-accepting behavior of the TFT moiety and
stronger electron-donor of 3. Under the assumption that the long
wavelength transitions are predominantly determined by the
HOMO-LUMO transition, the excitation leads to an ICT interaction
from the electron-donor of 3 to the electron-acceptor of TFT moiety
[33]. These calculations verify our initial assumption that the
electron-rich 3 acts as an electron-donor and the electron-poor TFT
unit as an electron-acceptor in the ICT complex. The calculated
HOMO and LUMO levels are demonstrated in Table S1. It can be
clearly seen that lower-lying LUMO energy level can be obtained by
introducing electron-withdrawing moiety of TFT instead of electri-
cally neutral TFB and electron-donor TFA. Moreover, different energy
levels can be obtained by the change of the electron-donating
moiety. Although the experimental energy levels are somewhat
different from the calculated ones, the same trend was ascertained.
10-1
10-3
10-5
10-7
0
2
4
6
8
10
12
14
Voltage (V)
b
5
4
3
2
1
0
P1a
P1b
P1c
P1d
P2a
P2b
P2c
P3a
P3b
3.6. Electroluminescence
To evaluate their electroluminescence properties, double-layer
devices with a configuration of ITO/PEDOT:PSS (40 nm)/EML (70e
80 nm)/TPBI (30 nm)/CsF (1.5 nm)/Al were fabricated, where the
developed polymers were used as an EML and TPBI (2,20,200-
(1,3,5-benzenetriyl)-tris-(1-phenyl-1H-benzimidazole)) was used
as an electron-transport and hole-block layer. As shown in Scheme
3, due to the lower-lying LUMO and HOMO energy levels of TPBI,
electron-injection from the cathode can be enhanced and carrier
recombination can be well confined within the EML.
As shown in Fig. 5(a), current densities of the device based on
P1a, the polymer based on the electron-withdrawing TFT and
carbazole pendant, are higher than those of the device based on
P2a, the polymer based on the same pendant and the electrically
neutral TFB. It can be attributed to its relatively lower-lying LUMO
energy level that facilitates electron-injection from the electron-
transport layer of TPBI. Similar phenomenon was also observed
for the devices based on the polymers P1b and P2b with 3,6-di-
tert-butyl carbazole pendant. Current densities of the device based
on P3a, the polymer based on the electron-donating TFA and
carbazole pendant, are equal to or even higher than those of the
device based on P1a, although its LUMO energy level is 0.3 eV
higher-lying that that of P1a. The increased current density can be
attributed to the higher-lying HOMO energy level of P3a that
facilitates hole-injection from the anode. As shown in Table 3, turn-
on voltages (Von) for electroluminescence (a luminance of 1 cd mꢀ2
was detected) of the devices are between 2.9 and 5.6 V, and the
0
50
100
150
200
Current density (mA/cm2)
Fig. 5. (a) Current density and luminance versus voltage and (b) current efficiency
versus current density characteristics of the devices with the configuration of ITO/
PEDOT:PSS (40 nm)/EML (70e80 nm)/TPBI (30 nm)/CsF (1.5 nm)/Al. EML: P1aeP1d,
P2aeP2c, and P3aeP3b.
device based on P3a exhibits the lowest Von due to the highest
HOMO energy level that facilitates hole-injection and transport
from the anode. The turn-on voltages of the devices based on P1ae
P1c are lower than those of the devices based on P2aeP2c,
respectively, due to their lower-lying LUMO energy levels are
more close to the LUMO energy levels of the electron-transport
layer TPBI and thus facilitate electron-injection into the EML.
Current efficiency (CE) versus current density characteristics of
the devices are shown as Fig. 5(b). The maximum CE (CEmax) of the
devices based on P2a, P2b, and P2c are 0.85, 0.44, and 1.34 cd Aꢀ1
,
-
e
Table 3
-
-
Summary of the electroluminescent performance of the devices with the configu-
ration of ITO/PEDOT:PSS (40 nm)/EML (70e80 nm)/TPBI (30 nm)/CsF (1.5 nm)/Al.
e
e
2.7
EML Von (V) Lmax (cd mꢀ2
)
CEmax (cd Aꢀ1
)
EQEmax (%) CIE (x, y)
a
4.1
P1a
P1b
P1c
P1d
P2a
P2b
P2c
P3a
P3b
3.2
4.3
3.3
4.8
5.0
5.6
3.8
2.9
3.2
138
99
3420
1030
1020
910
2560
2090
1650
0.12
0.08
4.10
1.26
1.50
0.85
1.93
2.44
1.70
0.07
0.1
(0.23, 0.28)
(0.22, 0.28)
(0.24, 0.51)
(0.16, 0.14)
(0.21, 0.17)
(0.21, 0.18)
(0.21, 0.16)
(0.16, 0.12)
(0.18, 0.19)
EML
4.7
+
Al/CsF TPBI
ITO
2.45
1.27
1.44
0.63
1.26
3.00
1.53
5.2
+
+
h
h
h
x
6.2
PEDOT
:PSS
a
Scheme 3. Energy diagram of the devices with the configuration of ITO/PEDOT:PSS
(40 nm)/EML (70e80 nm)/TPBI (30 nm)/CsF (1.5 nm)/Al.
Turn-on voltage for electroluminescence (a luminance of
1
cd mꢀ2 was
detected).

172
H. Ye et al. / Polymer 54 (2013) 162e173
respectively. The relatively low efficiency may be attributed to the
low carrier balance achieved in the device since these polymers are
comprised of only electron-donors of carbazole, 3,6-di-tert-butyl
carbazole, and 3 as the pendants. In comparison, the device based
on P1c exhibits a maximum CE of 4.10 cd Aꢀ1, corresponding to
external quantum efficiency (EQE) of 2.45%, which is the highest
one among all the devices. Since TFA pendant is a well-known hole
transporter and TFT is a well-known electron transporter, the
introduction of 3 onto TFT as the pendant of the polymer may
facilitate hole and electron-transport into the EML to give an
improved current density and reduced driving voltage. In addition,
carrier balance can be also improved to give an improved CE. Note
that the devices based on P1b, P2b, and P3b show lower CE in
comparison to the devices based on P1a, P2a, and P3a, respectively.
The decreased efficiency can be attributed to the reduced carrier
transport property and thus the carrier balance due to the bulky
tert-butyl groups combined with carbazole. It was found that the
efficiency of the device based on P1d is much higher than that of
P1a and P1b, which can be attributed to more carbazole units in the
pendant 8 of P1d to give an improved charge carrier balance and
higher CE. Aside from the improved efficiency, the improved hole
transport also induces the carrier recombination zone apart from
the interface of EML and anode for the device, restraining excimer
formation at the interface of EML and anode to give an improved
color purity with CIE coordinates of (0.16, 0.14).
the same type of excitons [34]. However, a long tail was found in the
EL spectra at the wavelengths longer than 500 nm for the devices
based on P2a and P2b, which is absent in the corresponding PL
spectra. It is anticipated that the carrier recombination zone in these
devices may be close to TPBI because of the disappointing electron-
transport capacity of TFB and the long tail found for the devices
based on P2a and P2b may be according to the excimer formed at the
interface of EML and TPBI. The EL spectra of the devices based on P1a
and P1b peak at 464 and 466 nm, respectively, with larger FWHM
values compared with the EL spectra of the devices based on P3ae
P3b and P2aeP2b. It is suspected that the wide EL spectra may be
generated from the carrier recombination zone that is close to
PEDOT:PSS because of the relativelygood electron-transport capacity
of TFT. Similar to the PL spectrum, bathochromic shift was also
observed in the EL spectrum for the device based on P1c, and it can be
attributed to the strong electron affinity of the electron-acceptor TFT
thatinduces the ICTinteractionwiththe pendantofelectron-donor 3.
Due to the ICT interaction, the emission colors of the devices based on
the TFT-based polymers P1 could be effectively tuned in the region of
deep-blue with CIE coordinates of (0.16, 0.14) and light-green with
CIE coordinates of (0.24, 0.51) for the polymers based on TFT by
introducing various substituents onto the pendent fluorene as the
pendants (Table 3). In addition, efficient deep-bluelightemissionwas
achieved for the device based on P3a as an EML, that shows
a maximum CE of 2.44 cd Aꢀ1, corresponding to EQE of 3.00%, with
CIE coordinates of (0.16, 0.12). Moreover, the CIE coordinates of the
deep-blue device is close to the National Television System
Committee (NTSC) and High Definition Television (HDTV) standard
blue with CIE coordinates of (0.16, 0.07) and (0.15, 0.06), respectively.
EL spectra of the devices based on P3a and P3b peak at 446 and
448 nm, respectively, with a subsidiary peak at 472 nm (Fig. 6), which
is consistent with their PL spectra in solid film state, indicating that
both EL and PL emissions mostly arise from the same excited state or
4. Conclusions
P1a
P1b
P1c
P1d
In summary, a series of LEPs based on the building blocks of
electron-donating TFA (P3aeP3b), electrically neutral TFB (P2ae
P2c), and electron-withdrawing TFT (P1aeP1d) were successfully
synthesized via palladium-catalyzed Suzuki cross-coupling poly-
condensation. It was found that the photophysical properties of P1
in solution are strongly dependent on the solvent polarity, and all of
the polymers displayed positive solvatochromism nature in the
emission spectra in different solvents. Moreover, their emission
color could be effectively tuned by changing the structure of the
main chain, from the electron-donating TFA to the electrically
neutral TFB and the electron-withdrawing TFT. Aside from the main
chain, their emission color could also be effectively tuned by
changing the substituent at the pendent fluorene of TFT, TFB, and
TFA. Deep-blue light emission was achieved for the device based on
P1d as an EML, giving a maximum CE of 1.26 cd Aꢀ1, corresponding
to EQE of 1.27%, with CIE coordinates of (0.16, 0.14). Due to the
intense ICT interaction, light-green emission was achieved for the
device based on P1c that shows the highest maximum CE of
4.10 cd Aꢀ1, corresponding to EQE of 2.45%, with CIE coordinates of
(0.24, 0.51). The current results provide a promising approach to
design LEPs by introducing various electron-acceptors and
electron-donors as building blocks of main chain or side chain for
tuning their emission color as well as energy level.
P2a
P2b
P2c
P3a
P3b
Acknowledgments
The authors greatly appreciate the financial support from
the National Natural Science Foundation of China (51073057),
the Ministry of Science and Technology (2009CB930604 and
2011AA03A110), the Ministry of Education (NCET-11-0159), and
the Fundamental Research Funds for the Central Universities
(2011ZZ0002).
300
400
500
600
700
800
Wavelength (nm)
Fig. 6. EL spectra of the devices based on the polymers as an EML with the configu-
ration of ITO/PEDOT:PSS (40 nm)/EML (70e80 nm)/TPBI (30 nm)/CsF (1.5 nm)/Al. EML:
P1aeP1d, P2aeP2c, and P3aeP3b.

H. Ye et al. / Polymer 54 (2013) 162e173
173
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[17] Pu YJ, Higashidate M, Nakayama KI, Kido JJ.
J Mater Chem 2008;18:
Supplementary data related to this article can be found at http://
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