New tetraphenylethylene-containing conjugated polymers: Facile synthesis, aggregation-induced emission enhanced characteristics and application as explosive chemsensors and PLEDs

Article abstract of DOI:10.1016/j.polymer.2012.05.035

In this paper, tetraphenylethylene (TPE) units, one of the typical aggregation-induced emission (AIE) moieties, are utilized to construct a new functional polyfluorene (PF) P1, which exhibited the exciting property of the aggregation-induced emission enha

Full text of DOI:10.1016/j.polymer.2012.05.035

Polymer 53 (2012) 3163e3171
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Polymer
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New tetraphenylethylene-containing conjugated polymers: Facile synthesis,
aggregation-induced emission enhanced characteristics and application as
explosive chemsensors and PLEDs
,
a
Wenbo Wu a, Shanghui Ye b, Runli Tang a, Lijin Huang a, Qianqian Li a, Gui Yu b, Yunqi Liu b , Jingui Qin ,
*
,
Zhen Li a
*
a Department of Chemistry, Hubei Key Lab on Organic and Polymeric Opto-Electronic Materials, Luojia Street, Wuhan University, Wuhan 430072, China
b Organic Solids Laboratories, Institute of Chemistry, The Chinese Academy of Sciences, Beijing 100190, China
a r t i c l e i n f o
a b s t r a c t
Article history:
In this paper, tetraphenylethylene (TPE) units, one of the typical aggregation-induced emission (AIE)
moieties, are utilized to construct a new functional polyfluorene (PF) P1, which exhibited the exciting
property of the aggregation-induced emission enhancement (AIEE), instead of the aggregation-caused
quenching (ACQ) of normal PFs, and could probe the explosive with high sensitivity both in the nano-
particles and solid state. Three other TPE-containing polymers, P2eP4, were also successfully prepared,
and demonstrated good performance as explosive chemosensors and light-emitting materials. P3,
bearing carbazole as hole-transporting units showed the best performance with a maximum luminance
efficiency of 1.17 cd/A and a maximum brightness of 3609 cd/m2 at 12.9 V in its light-emitting diode
device.
Received 26 February 2012
Received in revised form
6 May 2012
Accepted 18 May 2012
Available online 26 May 2012
Keywords:
Tetraphenylethylene (TPE)
Aggregation-induced emission
enhancement (AIEE)
Functional polyfluorene
Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction
other advantages. For chemists, the general approach is to modify
the structure at the molecular level.
In the past decades, considerable interests have been attracted
in the development of light-emitting diodes (LED) based on
conjugated polymers, due to their wide-ranging applications [1].
Among them, polyfluorene (PF) and its derivatives are considered
as promising light-emitting materials for their exceptionally high
efficiencies both in photoluminescence (PL) and electrolumines-
cence (EL) [2]. The high PL efficiencies also make some specific
functional PFs widely used in other fields, for example, chemo-
sensors [3]. However, PFs are likely to aggregate and form excimers
in solid states, directly leading to the fluorescence quenching,
namely aggregation-caused quenching (ACQ) [4], hence inhibiting
their prospective utilizations in a large degree. Although many
different approaches including chemical, physical, and engineering
ones have been attempted to hamper these luminophore aggre-
gations, they are still a little far from ideal, because the ACQ effect is
alleviated at the expense of other useful properties [5]. Thus, it is
badly needed to change the ACQ property of PFs, without sacrificing
Tetraphenylethene (TPE) is characterized by its aggregation-
induced emission (AIE) attribute [6], which was first reported by
Tang’s group [7a]: it is virtually nonluminescent when molecularly
dissolved in its good solvents, but emits intensely when aggregated
in its poor solvents or fabricated into thin solid film [7]. This is
exactly opposite to the ACQ effect, and has recently been given
much attention due to the huge potential high-tech applications in
many areas [8]. Thus, if the TPE unit was introduced into the PF
system, perhaps, the original ACQ behavior might be converted to
AIE(E) phenomena. However, the related reports were very scarce.
Inspired by this idea, in this paper, a new functional PF P1 (Chart 1),
containing the TPE moieties in the main chain, was therefore
synthesized through typical Pd-catalytic Suzuki polymerization. As
expected, the light-emitting behavior of P1 was not ACQ any longer,
but aggregation-induced emission enhancement (AIEE). That was
to say, in solution, P1 could emit weak fluorescence, however, the
emission was enhanced largely in solid state including nano-
particles and thin films. And it could be used as chemsensors for the
detection of explosives. Furthermore, other three TPE-containing
polymers, P2eP4 (Chart 2), were successfully prepared, in which
the spirobifluorene, carbazole and 1,3,4-oxadiazole moieties were
* Corresponding authors. Fax: þ86 27 68756757.
E-mail addresses: liuyq@iccas.ac.cn (Y. Liu), lizhen@whu.edu.cn (Z. Li).
0032-3861/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved.
doi:10.1016/j.polymer.2012.05.035

3164
W. Wu et al. / Polymer 53 (2012) 3163e3171
benzophenone (S1) were bought from Alfa-Aesar. All other
reagents were used as received.
Normal PF
n
2.2. Instrumentation
C6H13
C6H13
C6H13
C6H13
1H and 13C NMR spectra were measured on a Varian Mercury300
spectrometer or Varian Mercury600 spectrometer using tetrame-
Aggregation-Caused Quenching
thylsilane (TMS;
d
¼ 0 ppm) as internal standard. The Fourier
transform infrared (FT-IR) spectra were recorded on a Perkin Elmer-
2 spectrometer in the region of 4000-400 cmꢀ1 on KBr pellets.
UVevisible spectra were obtained using a Shimadzu UV-2550
spectrometer. FAB-MS spectra were recorded with a VJ-ZAB-3F-
Mass spectrometer. Elemental analyses were performed by a CAR-
LOERBA-1106 micro-elemental analyzer. Photoluminescence
spectra were performed on a Hitachi F-4500 fluorescence spec-
trophotometer. Gel permeation chromatography (GPC) was used to
determine the molecular weights of polymers. GPC analysis was
performed on a Waters HPLC system equipped with a 2690D
separation module and a 2410 refractive index detector. Poly-
styrene standards were used as calibration standards for GPC. THF
was used as an eluent and the flow rate was 1.0 mL/min. Thermal
analysis was performed on NETZSCH STA449C thermal analyzer at
a heating rate of 10 ꢁC/min in nitrogen at a flow rate of 50 cm3/min
for thermogravimetric analysis (TGA). The thickness of the films
was measured with an Ambios Technology XP-2 profilometer.
Electrochemical Workstation with a Pt disk, a Pt plate, and a Ag/Agþ
electrode as the working electrode, counter electrode and reference
electrode, respectively, in a 0.1 mmol/mL tetrabutylammonium
hexafluorophosphate (Bu4NPF6) acetonitrile solution. Scanning
electron microscopy (SEM) was performed on a QUANTA scanning
electron microscope (FEI Co., Netherlands), and the nanoaggregate
thin films were prepared by its THF/water (1/9 v/v) suspension
(0.02 mg/mL in THF) dropped on the slides and dried at about 60 ꢁC.
Aggregation-Induced Emission
P1
n
C6H13
C6H13
Chart 1. The sketch map from normal PF to TPE-containing P1.
used as comonomers, to further investigate the above idea and
improve the LED performance. All these polymers possessed AIEE
characteristic, and act as good chemosensors and light-emitting
materials. Herein, we would like to present their synthesis, char-
acterization, AIEE properties, sensing behavior, and LED device
performance in detail.
2. Experimental section
2.1. Materials
Tetrahydrofuran (THF) was dried over and distilled from K-Na
alloy under an atmosphere of dry nitrogen. Pyridine was dried over
from CaH2 and distilled under normal pressure before use. 9,9-
Dihexylfluorene-2,7-bis(trimethyleneborate) (S4) was purchased
from Aldrich while the other two bronic esters S5 and S6 were
prepared through the same synthetic routes reported in the liter-
ature [9]. 2,5-Di(4-bromobenzoyl)-1,3,4-oxadiazole (S5) was also
synthesized according to our previous work [1f]. 2-Isopropoxy-
4,4,5,5-tetramethyl-1,3,2-dioxaborolane was purchased from
Aldrich, while n-butyllithium (2.6 M in hexane) and 4-bromo
2.3. Synthesis of 1,2-bis(4-bromophenyl)-1,2-diphenylethene (S2)
To a mixture of Zn (3.90 g, 60.0 mmol) and 40 mL of dry THF was
added TiCl4 (3.3 mL, 30.0 mmol) under an atmosphere of argon at
0 ꢁC. The mixture was stirred for 30 min at room temperature and
another 2 h at 60 ꢁC, then 4-bromobenzophenone (S1) (0.85 mg,
2.5 mmol) in 30 mL of THF with pyridine (1 mL) was added drop-
wise in 1 h at 0 ꢁC. The mixture was refluxed overnight. After
twisted topological structure
P2
n
hole transporting activity
n
P3
n
C H
C H
6 13
P1
N
6
13
electron transporting activity
O
P4
n
N
N
Chart 2. The different chemical structures from P1 to polymers P2eP4.

W. Wu et al. / Polymer 53 (2012) 3163e3171
3165
Fig. 1. The 1H NMR spectra of polymers P1eP4 and monomer S3 conducted in chloroform-d.
cooling to room temperature, 10% K2CO3 (aq) was added and the
mixture was filtrated. The filtrates were extracted with CH2Cl2 and
the extracts were collected, combined, and washed with brine and
dried over anhydrous Na2SO4. After removal of the solvents under
reduced pressure, the crude product was purified by column
chromatography on silica gel using chloroform/petroleum ether (1/
298 K), d (TMS, ppm): 120.6, 126.9, 127.7, 128.0, 130.8, 131.2, 132.8,
140.2, 142.3, 142.7. MS (FAB), m/z [Mþ]: 487.97, calcd: 487.98.
C26H18Br2 (EA) (%, found/calcd): C, 62.71/63.70; H, 3.95/3.70.
2.4. Synthesis of 1,2-diphenyl-1,2-bis(4-(4,4,5,5-tetramethyl-1,3,2-
dioxaborolan-2-yl)phenyl)ethane (S3)
5, V/V) as eluent to afford white solid (1.12 g, 76.2%). IR (KBr),
y
(cmꢀ1): 1584 (C]C). 1H NMR (300 MHz, CDCl3, 298 K),
d
(TMS,
To a solution of S2 (980 mg, 2.0 mmol) in 40 mL dry THF
at ꢀ78 ꢁC, 2.2 mL of n-butyllithium (2.6 M in hexane, 5.7 mmol) was
ppm): 6.87 (d, J ¼ 5.1 Hz, 4H, ArH), 6.99 (s, br, 4H, ArH), 7.10 (s, br,
6H, ArH), 7.20 (d, J ¼ 5.1 Hz, 4H, ArH). 13C NMR (75 MHz, CDCl3,
0.35
0.30
B
A
THF/Water
1/99
1/9
THF/Water
1/99
0.25
P1
10/0
P2
2/8
0.20
3/7
P3
4/6
P4
0.15
5/5
6/4
7/3
0.10
0.05
0.00
8/2
9/1
10/0
400
500
600
400
500
600
700
0
20
40
60
80
100
Wavelength (nm)
Wavelength (nm)
Water Fraction (vol%)
Fig. 2. A: Photoluminescence (PL) spectra of P1 in THF and THF/water mixtures
(2.0 ꢂ 10ꢀ3 mg/mL lex ¼ 377 nm); B: Normalized PL spectra of P1 in the solution and
aggregate states.
Fig. 3. Variations in the fluorescence quantum yields (FF) of P1eP4 with water frac-
tions in the THF/water mixtures. 9,10-diphenylanthracene in cyclohexane (FF ¼ 90%)
as reference.

3166
W. Wu et al. / Polymer 53 (2012) 3163e3171
A
B
60
C
[PA]: µg/mL
0
1 ppm
50
P1
P2
40
OH
50
O2N
NO2
30
20
10
0
NO2
P3
P4
400
500
600
0
10 20 30 40 50 60
[PA]: mg/mL
Wavelength (nm)
Fig. 4. A: PL spectra of P1 in THF/water mixture (1:9 v/v 2.0 ꢂ 10ꢀ3 mg/mL) containing different amounts of picric acid (PA); B: PA concentration effect on the PL peak intensity of P1
in THF/water mixture (1:9 v/v 2.0 ꢂ 10ꢀ3 mg/mL), where I ¼ peak intensity and I0 ¼ peak intensity at [PA] ¼ 0 mg/mL; Inset of B: The structure of PA; C: fluorescence images of
P1eP4 adsorbed in the filter papers before (the left one), after (the middle one) being partially dipped into pure toluene and after (the right one) being partially dipped into
a toluene solution of PA (P1 and P2: 60 mg/mL; P3 and P4: 50 mg/mL).
added dropwise. The mixture was stirred at ꢀ78 ꢁC for 2 h. Then,
2.2 mL of 2-isopropoxy-4,4,5,5,-tetramethyl-1,3,2-dioxaborolane
(9.6 mmol) was added rapidly to the above solution and the
mixture was stirred for 2 h. The resulting mixture was warmed up
to room temperature and stirred for 24 h, then poured into water,
and extracted with chloroform. The combined extracts were
washed with brine, dried over anhydrous Na2SO4, and concen-
trated. The crude product was purified by column chromatography
on silica gel using ethyl acetate/petroleum ether (1:10) as eluent to
40.7, 55.5, 110.4, 114.3, 120.2, 121.3, 126.5, 128.2, 131.1, 131.6, 132.0,
133.3, 139.6, 140.3, 132.0, 151.9.
2.4.2. Synthesis of P2
The synthesis of P2 was similar as P1. S2 (62.1 mg, 0.127 mmol),
bronic ester S5 (72.0 mg, 0.127 mmol), sodium carbonate (134.3 mg,
1.27 mmol), THF (3.9 mL)/water (0.65 mL), and Pd(PPh3)4 (2.0 mg).
The resultant polymer was obtained as pale green powder (57.1 mg,
69.9%). Mw ¼ 5700, Mw/Mn ¼ 1.53 (GPC, polystyrene calibration). IR
afford white powder S3 (438 mg, 37.8%). IR (KBr),
y
(cmꢀ1): 1606
(KBr), y
(cmꢀ1): 1601 (C]C). 1H NMR (300 MHz, CDCl3, 298 K),
(C]C). 1H NMR (300 MHz, CDCl3, 298 K),
d
(TMS, ppm): 1.32 (s, 24H,
d
(TMS, ppm): 6.5e6.7 (ArH), 6.7e7.4 (ArH), 7.4e7.6 (ArH), 7.6e7.8
-CH3), 7.0-7.1 (m, 14H, ArH), 7.53 (d, J ¼ 7.2 Hz, 4H, ArH). 13C NMR
(ArH). 13C NMR (150 MHz, CDCl3, 298 K),
d
(TMS, ppm): 120.2, 120.4,
(75 MHz, CDCl3, 298 K),
d
(TMS, ppm): 25.1, 83.4, 126.7, 127.8, 127.9,
122.5, 124.2, 126.2, 126.3, 126.4, 126.5, 126.6, 126.9, 127.0, 127.3,
127.8, 127.9, 128.8, 130.9, 131.5, 131.6, 131.8, 131.9, 138.6, 140.5, 140.6,
140.8, 141.1, 141.6, 142.0, 142.8, 142.9, 143.8, 148.8, 149.3, 149.5.
130.9, 131.5, 134.2, 124.3, 141.4, 143.5, 146.9. MS (FAB), m/z [Mþ]:
584.35, calcd: 584.33. C38H42B2O4 (EA) (%, found/calcd): C, 78.83/
78.10; H, 7.24/7.24.
2.4.3. Synthesis of P3
2.4.1. Synthesis of P1
The synthesis of P3 was similar as P1. S2 (122.6 mg, 0.25 mmol),
bronic ester S6 (132.8 mg, 0.25 mmol), sodium carbonate (265 mg,
2.5 mmol), THF (7.5 mL)/water (1.25 mL), and Pd(PPh3)4 (4.0 mg).
The resultant polymer was obtained as yellow powder (122.0 mg,
80.3%). Mw ¼ 11,000, Mw/Mn ¼ 1.91 (GPC, polystyrene calibration).
A mixture of 9,9-dihexylfluorene-2,7-bis(trimethyleneborate)
(S4) (125.6 mg, 0.25 mmol), monomer S2 (122.6 mg, 0.25 mmol),
sodium carbonate (265 mg, 2.5 mmol), THF (7.5 mL)/water
(1.25 mL), and Pd(PPh3)4 (4.0 mg) was carefully degassed and
charged with argon. Then the reaction mixture was stirred at
60 ꢁC for 3 days. Then, the trace end groups were capped by
refluxing for 12 h with phenylboronic acid and bromobenzene
sequentially. A lot of methanol was poured into the mixture, then
filtered. The obtained solid was dissolved in THF, and the insol-
uble solid was filtered out. After removal of the solvent, the
residue was further purified by several precipitations from THF
into acetone, and the obtained solid was then washed with a lot
of acetone and dried in a vacuum at 40 ꢁC to a constant weight.
The resultant polymer was obtained as dark green powder
(113.2 mg, 68.4%). Mw ¼ 10,900, Mw/Mn ¼ 1.76 (GPC, polystyrene
IR (KBr),
y
(cmꢀ1): 1601 (C]C). 1H NMR (300 MHz, CDCl3, 298 K),
d
(TMS, ppm): 0.6e1.0 (-CH3), 1.0e1.4 (-CH2-), 1.5e2.1 (-CH2-),
3.8e4.3 (-NCH2-), 6.9e7.2 (ArH), 7.2e7.6 (ArH), 7.9e8.2 (ArH). 13
NMR (150 MHz, CDCl3, 298 K), (TMS, ppm): 11.3, 14.3, 23.3, 24.7,
C
d
29.0, 31.2, 39.6, 47.5, 107.4, 118.7, 120.7, 122.1, 126.8, 127.1, 128.0,
128.1, 129.0, 131.8, 132.2, 138.6, 140.1, 141.0, 142.3, 142.9, 144.2.
2.4.4. Synthesis of P4
The synthesis of P4 was similar as P1. S3 (116.9 mg, 0.20 mmol),
S7 (76.0 mg, 0.20 mmol), potassium carbonate (276 mg, 2.0 mmol),
THF (6 mL)/water (1 mL), and Pd(PPh3)4 (3.2 mg). The resultant
polymer was obtained as yellow powder (65.1 mg, 59.1%).
calibration). IR (KBr),
CDCl3, 298 K), (TMS, ppm): 0.6e1.0 (-CH3), 1.0e1.4 (eCH2e),
1.8e2.2 (eCCH2e), 6.8e7.3 (ArH), 7.3e7.8 (ArH). 13C NMR
(75 MHz, CDCl3, 298 K), (TMS, ppm): 14.3, 22.8, 24.0, 30.0, 31.7,
y
(cmꢀ1): 1604 (C]C). 1H NMR (300 MHz,
d
Mw ¼ 5600, Mw/Mn ¼ 1.66 (GPC, polystyrene calibration). IR (KBr),
(cmꢀ1): 1609 (C]C). 1H NMR (300 MHz, CDCl3, 298 K),
(TMS,
ppm): 7.0e7.3 (ArH), 7.4e7.5 (ArH), 7.6e7.8 (ArH), 8.1e8.3 (ArH). 13C
y
d
d

W. Wu et al. / Polymer 53 (2012) 3163e3171
3167
1.0
0.8
0.6
0.4
0.2
0.0
B
A
P1
1000
100
10
P2
P3
P4
P1
P2
P3
P4
1
400
500
600
4
6
8
10 12 14 16 18
Wavelength (nm)
Voltage (V)
1.2
1.0
0.8
0.6
0.4
0.2
0.0
450
400
350
300
250
200
150
100
50
D
C
P1
P2
P3
P4
P1
P2
P3
P4
0
0
3
6
9
12 15
0
1000 2000 3000 4000
2
Voltage (V)
Luminance (cd/m )
Fig. 5. A: The EL spectra of P1eP4; B: Luminance-voltage characteristics of the PLED devices; C: Current-voltage characteristics of the PLED devices; D: The luminescence efficiency-
current characteristics of PLED devices.
NMR (150 MHz, CDCl3, 298 K),
127.9, 128.1, 131.7, 132.4.
d
(TMS, ppm): 122.7, 126.8, 127.7,
respectively. The active layer was spin-coated from chlorobenzene
solution and TPBI layer was deposited by means of conventional
vacuum deposition onto the ITO coated glass substrates at a pres-
sure of 8 ꢂ 10ꢀ5 Pa. The active area of device was 5 mm2. A quartz
crystal oscillator placed near the substrate was used to monitor the
thickness of each layer, which was calibrated ex situ using an
Ambios Technology XP-2 surface profilometer. UVevis absorption
and fluorescence spectra were collected with a Hitachi U-3010 and
Hitachi F-4500 spectrophotometer, respectively. EL spectra and
chromaticity coordinates were measured with a SpectraScan PR650
photometer. Current density-voltage-luminance (J-V-L) measure-
ments were made simultaneously using a Keithley 4200 semi-
conductor parameter analyzer and a Newport multifunction 2835-
C optical meter, with luminance measured in the forward direction.
All device characterizations were carried out under ambient labo-
ratory air at room temperature.
2.4.5. Fabrication and characterization of PLEDs
In a general procedure, indium-tin oxide (ITO)-coated glass
substrates were etched, patterned, and washed with detergent,
deionized water, acetone, and ethanol sequentially. We fabricated
PLEDs using these polymers P1eP4 as the emissive layer with
a structure of ITO/PEDOT:PSS (25 nm)/Poly-TPD (25 nm)/EML
(32 nm)/TPBI (35 nm)/Cs2CO3 (8 nm):Ag (100 nm), (ITO was
indium-tin oxide; PEDOT was poly(3,4-ethylendioxythiophene);
PSS
was
poly(styrenesulfonate);
TPD
was
N,N0-Bis(3-
methylphenyl)-N,N0-bis(phenyl)-benzidine and TPBI was 1,3,5-
tris(N-phenylbenzimidazol-2-yl)benzene), where PEDOT:PSS,
Poly-TPD and TPBI were used as hole injection, electron-blocking/
hole-transporting and hole-blocking/electron-transporting layer,

3168
W. Wu et al. / Polymer 53 (2012) 3163e3171
3. Results and discussion
The molecular weights of P1eP4, determined by gel permeation
chromatography (GPC) with THF as an eluent and polystyrene
standards as calibration standards, were listed in Table 1 and
experimental section. Here, P2 and P4, only constructed by rigid
aromatic rings, demonstrated lower molecular weights than those
of P1 and P3, possibly due to their relative poor solubility. Actually,
during the polymerization procedure of P4, some insoluble poly-
mer was also yielded, indicating that the soluble fractions with high
molecular weights could not be obtained. Their thermal gravi-
metric analysis (TGA) thermograms were shown in Fig. S10, with
the 5% weight loss temperature (Td) summarized in Table 1. All the
polymers were thermolytically resistant, and the Td values were all
higher than 360 ꢁC. Here, the rigid structure demonstrated the
advantage of good stability, for example, the Td value of P2 was as
high as 473 ꢁC. Also, the glass transition temperature (Tg) were
investigated by using a differential scanning calorimeter (DSC)
technology. It was a pity that the glass transition temperatures of
nearly all the polymers were not obvious in the DSC curve except P1
(143 ꢁC). Since the structure of P2 and P4 were much more rigid
than P1, we could conjecture that their Tg values may be even
higher than 143 ꢁC, while P3 might have a similar Tg value as P1.
3.1. Synthesis and characterizations
As shown in Scheme 1, the bromized TPE S2 was prepared via
McMurry Olefination from 4-bromobenzophenone (S1). The reac-
tion underwent smoothly, and the desired compound was obtained
in the yield of 76.2%. Then, during normal procedure for the prep-
aration of bronic ester [9], S3 was obtained as white solid. Other
comonomers were obtained by following the reported procedures
[1f,9], and the synthetic routes were listed in Scheme S1 in sup-
porting information. The synthetic route of polymers P1eP4 was
shown in Scheme 2, and the polymerization procedure was pro-
ceeded smoothly through the typical Suzuki coupling reaction
using Pd(PPh3)4 as a catalyst, sodium carbonate (aq) as a base, and
THF as the solvent. After three days, the end-capped groups (phe-
nylboronic acid and bromobenzene) were added to react with the
bromo- and bronic ester end groups (which might quench the
fluorescence of the resultant polymers) [l]. P1eP4 were obtained
with satisfied yields (Table 1), and they were readily soluble in
common organic solvents, such as THF, CHCl3, etc.
The molecular structures of these four polymers P1eP4 were
characterized by “wet” spectroscopic techniques, and the detailed
spectral analysis data for them were given in the experimental
section and supporting information. Fig. S1 (in the supporting
information) showed their FT-IR spectra. The TPE-containing
3.2. Optical properties and AIEE effects
As mentioned above, the original goal of the introduction of TPE
moieties to PF, was to change its properties from ACQ to AIE effect.
Thus, we attempted to investigate the photoluminescence (PL)
behaviors of P1eP4 in the solution and aggregate states. The
aggregates were prepared by adding distilled water into their THF
solutions under vigorous stirring. The resultant mixtures were
visually transparent and macroscopically homogenous, suggesting
that the polymer aggregates were nanometer-sized [10]. When
compound S2 exhibited an absorption band at around 1605 cmꢀ1
,
which could be assigned to the stretching vibrations of C]C. The
appearance of the stretching vibration of C]C in the IR spectra of
the polymers indicated that the TPE moieties were successfully
introduced.
Fig. 1 showed the 1H NMR spectra of polymers P1eP4, which
were conducted in the solvent of chloroform-d. All the peaks could
be readily assigned to the resonances of appropriate protons of
these polymers (Scheme 2), and no unexpected peaks appeared.
Except the resonance peaks of TPE unit, there were still some
characteristic peaks assigned to the resonances of protons of the
other comonomers, which were marked in Fig. 1, indicating the
Suzuki polymerization were successful. Taking P3, containing
carbazole as comonomer, as an example: the ArH signals of TPE unit
illuminated with a UV lamp (
l
¼ 365 nm), there was nearly none
fluorescence emitted from the dilute THF solution of P1, while
a very weak sky blue florescence observed from its concentrated
solutions. But the emission became much stronger in its aggregate
states and solid films, and the fluorescence became green light. To
verify the visual observations, the UVevis and PL spectra were
studied. As shown in Fig. S11 (Supporting Information), the UVevis
spectra of P1 in pure THF and THF/water mixtures exhibited similar
profiles. However, the PL behaviors were quite different. Upon
excitation at 377 nm, P1 was a very weak emitter when molecularly
dissolved in pure THF (Fig. 2A). In contrast, when a large amount of
water was added into the solution, an intense PL peak centered at
503 nm was recorded under the same measurement conditions
(Fig. 2A). Meanwhile, its normalized PL spectra in the solution and
aggregate state were showed in Fig. 2B. It was easily seen that in its
PL spectra in aggregate state, there was only one main peak
centered at 503 nm, while in the solution, except this peak, there
were still some small ones. It was understandable. First, we could
confirm that the peak around 503 nm was the emission of the
aggregated particles, since only this peak remained in the aggregate
state and dramatically enhanced in comparison with that in the
solution, similar to the optical properties of TPE. As well known, the
were at about
some new peaks assigned to the protons of ArH in carbazole
(typically at about
¼ 8.3 ppm) and the NH2 protons at about
¼ 4.3 ppm, as well as some CH2 peaks in the range of ¼ 0.8e2.0,
d
¼ 7.0 and 7.5 ppm; except that, there were also
d
d
d
indicating this polymer was derived from TPE moieties and
carbazole moieties. And due to the polymerization, all the peaks
showed an inclination of signal broadening. In their 13C NMR
spectra (see supporting information and experimental section), the
similar phenomenon was also observed. In P3, except the signals of
TPE unit, some alkyl carbon signals and aromatic carbon signals,
which should be derived from carbazole moieties, appeared. In
addition, the peaks at about 25 and 83 ppm, typical signals of the
boronic ester group, disappeared completely, also indicating the
successful polymerization.
O
1. n-BuLi, THF, -78 oC
O
B
O
O
Br
Zn, TiCl4
O
O
B
, -78 o
C
2.
Br
B O
THF, pyridine
reflux
O
Br
S1
S2
S3
Scheme 1. Synthesis of monomers S2 and S3.

W. Wu et al. / Polymer 53 (2012) 3163e3171
3169
Br
O
O
O
OH
B
B
B
Ar
or
Ar=
P1 and S4
B
O
Pd[PPh3]4
Na2CO3
THF/H2O
OH
Br
C6H13
C6H13
+
P1-P3
O
O
O
O
Ar
B
S2
Br
S4-S6
P2 and S5
O
B
O
OH
B
OH
P4
O
Pd[PPh3]4
K2CO3
THF/H2O
+
Br
Br
N
N
N
Br
P3 and S6
S7
S3
B
O
O
Scheme 2. Synthesis of TPE-containing polymers P1eP4.
AIE effect was considered to be induced by the restriction of the
intramolecular rotation (IMR) process of the luminophore
[7a,7b,11]. In the case of TPE, its four phenyl rings underwent an
active IMR process in the solution state, thus, quenched its emis-
sion, while in the aggregate state, the IMR process was impeded,
which blocked the non-radiative decay channel and hence made
TPE emissive. However, in the conjugated polymer of P1, the IMR
process of two of the four phenyl rings in the TPE moieties should
be limited in some degree, and this peak was therefore still present
in the solution state, but very weak. This point, from another side,
confirmed that P1 possessed only AIEE characteristic, not AIE. Thus,
it was also explained that this emission of 503 nm was the
enhanced TPE-centered emission, however, due to the more con-
jugative system, the emission was red-shifted than TPE (465 nm).
On the other hand, just like normal PF, the conjugated system
should give some emission, thus, some blue emissions were
observed in the concentrated solutions. Coupled with some inter-
molecular energy transfer effect, these small peaks in the blue
region were completely disappeared in the aggregate state or solid
state. The same phenomena were also observed in P2eP4
(Figs. S12eS16 in supporting information and Table 2): in their
solutions, there were two types of very weak fluorescence, while
only one peak in their aggregate states accompanying with the
enhanced emission. To confirm the presence of the nanoparticles in
the mixture solvents, SEM picture of P1 and P2 were taken as the
examples. As shown in Fig. S17, in the mixture solvent of THF/water
(1/9 v/v), both of them formed nano-sized particles.
To have a quantitative picture, we estimated the FF values of P1
using 9,10-diphenylanthracene in cyclohexane (FF ¼ 0.9) as refer-
ence. In pure THF, the polymer exhibited negligibly FF value of
9.6 ꢂ 10ꢀ3, while at a water content of 99%, the FF value of the
polymer dramatically increased to 0.194, 21 fold higher than that in
THF solution (Fig. 3 and Table 2). All of the above phenomena
showed that the PL behavior of P1, using TPE units as important
construction block, was changed from ACQ to AIEE active as
expected. Similar to P1, P2eP4 were also AIEE active, and the
fluorescence quantum yields in aggregate states were enlarged in
a large degree in comparison with P1.
Fluorescent stability of the polymers was another important
parameter with regard to real-world applications. Thus, the
annealing experiments of these four polymers were conducted to
investigate this property. Taking P1 as an example: the solid film of
P1 was first baked at 100 ꢁC for 30 min in air, then at 150 ꢁC for
another 30 min, followed by 200 ꢁC and so on. Fig. S18 showed the
PL emission spectra of P1 after annealing in air. Because of the
aggregation effect or keto formation, the stability of normal PF was
poor, and it was reported that thermal treatment of the film of
poly(dihexylfluorene) at 100 ꢁC would be unstable in air [12].
However, once TPE was introduced, P1 was still very stable after
Table 2
Optical properties of polymers in solution and solid state.
a
c
d
e
f
g
No. lAbs,sol lPL,solb (nm)
lPL,agg lPL,fim FF,S
FF,A
FF,A/FF,S
(nm)
(nm)
(nm)
P1 346
P2 366
P3 352
P4 354
408, 432, 501 503
415, 434, 502 506
408, 434, 507 508
504
510
507
515
9.6 ꢂ 10ꢀ3 0.194 21
8.5 ꢂ 10ꢀ3 0.334 39
8.1 ꢂ 10ꢀ3 0.282 35
9.8 ꢂ 10ꢀ3 0.302 31
Table 1
Polymerization results of polymers.
408, 432, 466, 504 506
a
a
a
b
c
No.
Yield (%)
Mw
Mw/Mn
Tg (ꢁC)
Td (ꢁC)
The absorption wavelength of polymer solutions in pure THF, the concentration
was 2 ꢂ 10ꢀ3 mg/mL.
P1
P2
P3
P4
68.4
69.9
80.3
59.1
10900
5700
11000
5600
1.76
1.53
1.91
1.66
143
(ꢀ)d
(ꢀ)d
(ꢀ)d
363
473
403
420
b
The emission wavelength of polymer solutions in pure THF, the concentration
was 2 ꢂ 10ꢀ3 mg/mL.
c
The emission wavelength of polymer solutions in THF/methanol ¼ 1/9 (V/V), the
concentration was 2 ꢂ 10ꢀ3 mg/mL.
a
d
Determined by GPC in THF on the basis of a polystyrene calibration.
The glass transition temperature (Tg) of polymers detected by the DSC analyses
The emission wavelength of polymer in films.
Quantum yields in THF solutions using 9,10-diphenylanthracene in cyclohexane
b
e
at a heating rate of 10 ꢁC/min under nitrogen.
(
FF ¼ 90%) as standard.
c
f
The 5% weight loss temperature of polymers detected by the TGA analyses at
Quantum yields in THF/methanol ¼ 1/9 (V/V) using 9,10-diphenylanthracene in
a heating rate of 10 ꢁC/min under nitrogen.
cyclohexane (FF ¼ 90%) as standard.
d
g
Not obtained.
Ratio of fluorescence quantum yields of aggregate and solution.

3170
W. Wu et al. / Polymer 53 (2012) 3163e3171
baked at 150 ꢁC for half an hour, and the PL spectra were almost the
same as those tested before annealing. To our surprised, the fluo-
rescent stability of P2 and P4 was even better than P1, possibly due
to their much more rigid structure (Figs. S19eS21).
Table 4
Sensing properties of polymers P1eP4 in aggregate state.
Linear rangec (ppm)
R2 d
a
b
No.
cdiscerned (ppm)
cquench (ppm)
P1
P2
P3
P4
1.0
55
52
40
37
<16
<16
<20
<14
0.9855
0.9570
0.9858
0.9662
0.33
0.17
0.17
3.3. Electrochemical characterization
a
The lowest detectable PA concentration.
The lowest PA concentration for the completely quenched fluorescence.
The linear detection range of PA concentration.
The electrochemical properties of the polymers were investi-
gated by using cyclic voltammetry (CV). The onset oxidation
potential of P1 occur at 0.87 V. Based on the onset potential and
according to the equation of EHOMO ¼ ꢀ(Eonset(ox),FOC þ 4.8) eV, the
HOMO energy level was determined to be ꢀ5.67 eV. As the onset
reduction potential could not be observed clearly, the LUMO energy
level was calculated by the equation of ELUMO¼(EHOMO þ Egopt) eV,
where Egopt was the band gap estimated from the absorption spectra
of the polymer film. Thus, the LUMO energy level was estimated to
be ꢀ2.81 eV. The electrochemical data of the other three polymers
were also tested and summarized in Table 3. The different copol-
ymer units resulted in different HOMO and LUMO values, further
confirming that it was feasible to use different comonomers with
different characteristics to adjust the emitting behavior.
b
c
d
The correlation coefficient of the linear regression equation.
polymers reported in the literature, P1eP4 demonstrated higher
sensitivity, which should be ascribed to the “molecular wire effect”
of the conjugated polymers [3,15].
However, for practical applications, using nanoaggregates as
probes was not good enough, and it was more convenient to use in
the solid state. Thus, test strips were prepared by immersing filter
paper into their THF solutions (0.2 mg/mL) and then dried in air, to
investigate if these polymers could be direct used in solid state [16].
The obtained papers were dipped into a toluene solution of PA as
well as pure toluene. As shown in Fig. 4C, the paper film displayed
strong PL upon excitation, however, became nearly nonemissive
3.4. Explosive detection by polymer nanoaggregates
after dipping into the PA solution. As to P3 and P4, only 50 mg/mL of
PA was enough to quench the fluorescence completely. These
demonstrated a prototype device using the AIEE polymers for
detecting explosives in real-world applications.
Photoinduced electron transfer (PET) quenching process was
usually used to detect nitro aromatic explosives, in particular 2,4,6-
trinitrotoluene (TNT) and 2,4,6-trinitrophenol (picric acid, PA), the
common toxic and explosive pollutants [13]. For the sake of anti-
terrorism and homeland-security implications, sensitive sensors
were badly needed. Here, the strong fluorescence of the nano-
aggregates of P1eP4 suspended in the aqueous mixtures made it
possible to be utilized as chemosensors toward TNT and PA. Unlike
2,4-dinitrotoluene (DNT) and TNT, PA was commercial availability,
thus, it could be used as a model compound to evaluate the sensing
property of these polymers.
3.5. Electroluminescence (EL) properties
As mentioned above, the introduction of the TPE unit to PF main
chain was to change the fluorescent behavior of PFs, with aim to
avoid the ACQ effect. As well known, PF and its derivatives were
promising light-emitting materials. Thus, how about the EL prop-
erties of the PLED based on P1eP4 with AIEE characteristics? To
answer this question, we fabricated PLED devices using these
polymers as the emitting layers (EML) according to the “standard
recipe” for screening tests: ITO/PEDOT:PSS (25 nm)/Poly-TPD
(25 nm)/EML (32 nm)/TPBI (35 nm)/Cs2CO3 (8 nm):Ag (100 nm).
The EL data of PLED devices were summarized in Table 5 and Fig. 5.
The efficiency of P1-based PLED was not satisfied with
a maximum luminance efficiency of 0.20 cd/A and a maximum
brightness of 574 cd/m2. If using a twisted structure, spirobi-
fluorene, as comonomer instead of normal fluorene, the efficiency
of PLED device has a large improvement: the turn-on voltage was
1.8 V decreased, and the maximum luminescence was up to
1761 cd/m2. Furthermore, the luminescence efficiency was more
than four times than before. On the other hand, to further improve
the EL efficiency, it was critical to achieve both efficient charge
injection and balanced mobility of both charge carriers inside the
The explosive probe was prepared as nanoaggregates in the
THF/water mixture with 90% water content. After the addition of PA
into the aggregate suspension, the fluorescent intensity of P1
decreased rapidly (Fig. 4A), even at a PA concentration as low as
1.0
mg/mL (1.0 ppm). The fluorescence could be quenched
completely at a PA concentration of 50
m
g/mL (50 ppm), confirming
that the nanoparticle suspension of P1 could detect nitro aromatic
explosives with high sensitivity. The SterneVolmer plot was nearly
linear with a correlation coefficient (R2) of 0.9855 at the PA
concentrations lower than 16 mg/mL (Fig. 4B). Afterward, the curve
deviated from linearity and bent upward, suggesting the occur-
rence of “super-quenching” due to the involvements of self-
absorption and/or energy transfer [14]. Also, P2eP4 could be
used as chemosensors similar to P1, due to their similar AIEE
characteristics. Thanks to their much higher quantum yields than
P1, the quenching efficiencies of P2eP4 were much better (Table 4),
and P3 bearing carbazole moieties possessed the best compre-
hensive performance. In comparison with other TPE-containing
Table 5
EL data of PLEDs devices.
No. lEL (nm) Von (V) Luminc L (cd/m2) CDd J (mA/cm2) CEe (cd/A)
a
b
Table 3
Electrochemical properties of polymers.
P1
P2
P3
P4
494
515
501
509
8.7
6.9
5.7
4.8
574 (16.8)
1761 (15.0)
3609 (12.9)
3109 (12.6)
376
380
434
399
0.20
0.82
1.17
1.14
No.
Egopt (eV)a
Eonset,(ox) (V) vs FOCb
EHOMO (eV)c
ELUMO (eV)d
P1
P2
P3
P4
2.86
2.67
2.63
2.81
0.87
0.75
0.72
0.93
ꢀ5.67
ꢀ5.55
ꢀ5.58
ꢀ5.73
ꢀ2.81
ꢀ2.38
ꢀ2.95
ꢀ2.92
a
The EL spectra of the device.
Turn-on voltage.
Maximum luminescence, while the corresponding operating voltage was given
b
c
a
Band gaps obtained from absorption edge (Eg ¼ 1240/lonset).
EFOC ¼ 0.48 V vs Ag/AgCl.
in the parentheses.
b
c
d
Current density at the maximum luminescence, also the maximum current
EHOMO ¼ ꢀ(Eonset(ox),FOC þ 4.8) eV.
density.
ELUMO ¼ (EHOMO þ Egopt) eV.
Maximum luminescence efficiency.
d
e

W. Wu et al. / Polymer 53 (2012) 3163e3171
(i) Tsai L, Chen Y. J Polym Sci Part A 2007;45:4465;
3171
electroluminescent polymers. Carbazole, which was very famous
for its good hole-transporting and electroluminescent activities,
has been already widely used as important block to construct LED
materials [17]. 1,3,4-Oxadiazole derivatives were the famous elec-
tron deficient materials with good thermal and chemical stabilities
as well as high photoluminescence quantum yields, and generally
used as electron-transport materials in PLEDs [18]. Thus, in this
paper, we also used these two moieties instead of fluorene unit to
construct conjugated polymers P3 and P4 to achieve the balance of
charge transport, and the results were really better than P2 as
expected. For P3-based PLED device, the maximum luminescence
was two folds of P2 with a maximum luminance efficiency of
1.17 cd/A. These results suggested that the incorporation of hole-
transporting carbazole units or electron-transporting oxadiazole
units was also effective in improving EL performance of the AIEE-
based PLED devices, similar to normal PLED devices. However, to
know more information about polymers with AIE or AIEE charac-
teristics, more researches were still needed. Also, it should be
pointed out that the EL devices of these polymers were not opti-
mized, thus, better performance might be achieved. Anyhow, these
preliminary EL results, especially those of P3 and P4, demonstrated
the applicability of AIE or AIEE characteristics polymers.
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In this paper, for the first time, a new series of TPE-containing
conjugated polymers P1eP4 were prepared successfully through
simple palladium-catalyzed Suzuki polycondensation reaction, and
all polymers exhibited good thermal stabilities (Td > 363 ꢁC) and
excellent fluorescent stabilities. As expected, the obtained poly-
mers were AIEE active, thanks to the AIE property of TPE moieties.
Their AIEE features made them acting as explosive chemosensors
both in the aggregate and solid states with high sensitivity. The
PLED devices (ITO/PEDOT:PSS (25 nm)/Poly-TPD (25 nm)/P1eP4
(32 nm)/TPBI (35 nm)/Cs2CO3 (8 nm):Ag (100 nm)) have been
fabricated to investigate their electroluminescent properties.
Interestingly, owing to the introduced hole-transporting or
electron-transporting groups, the EL performance of P3 and P4 was
improved dramatically, with the maximum luminescence of
3609 cd/m2 (P3), and the maximum luminescence efficiency as
high as 1.17 cd/A (P3).
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Acknowledgments
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We are grateful to the National Fundamental Key Research
Program (2011CB932702), and the National Science Foundation of
China (no. 21161160556) for financial support.
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Appendix A. Supplementary material
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Chem Soc Rev 2011;40:1547;
Supplementary data related to this article can be found online at
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