A blue emission polymer: Synthesis, photophysical and electrochemical properties

Article abstract of DOI:10.1016/j.saa.2013.04.109

(Figure Presented) A novel π-conjugated polymer (PCPyrene) containing N-benzylcarbazole and pyrene units has been synthesized and characterized. The polymer possesses high thermal stability with the decomposition temperature of 440°C. It shows higher fluorescence quantum yields of in solution and solid state, respectively. PCPyrene can emit bright blue-lights both in different organic solutions (440-460 nm) and in the solid state (492 nm). Compared the emission spectra of PCPyrene in solutions with in solid state, the solid state emission of PCPyrene is significantly red-shifted. Additionally, it is not obvious changes of the solid emission spectra even after being annealed at 150°C under nitrogen for 24 h. The electrochemical properties and energy levels of PCPyrene were also investigated by cyclic voltammetry. Furthermore, in order to provide a basis forecasting the structure-physical property relationships, the photophysical properties of PCPyrene have been carefully investigated by fluorescence emission and UV-vis absorption spectra.

Full text of DOI:10.1016/j.saa.2013.04.109

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 113 (2013) 159–163
Contents lists available at SciVerse ScienceDirect
Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
A blue emission polymer: Synthesis, photophysical and electrochemical
properties
⇑
Caihong Zhang, Lifeng Li, Yue Wang, Liheng Feng
School of Chemistry and Chemical Engineering, Shanxi University, Taiyuan 030006, PR China
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
ꢀ
ꢀ
ꢀ
ꢀ
A novel p-conjugated polymer with
blue light emission was established.
The polymer is comprised of
carbazole and pyrene groups.
The polymer shows high thermal
stability under 440 °C.
n
The polymer demonstrates high
energy level of the HOMO (about
ꢁ5.2 eV).
*
N
ꢀ
The polymer possesses high
fluorescent quantum yields.
a r t i c l e i n f o
a b s t r a c t
Article history:
A novel p-conjugated polymer (PCPyrene) containing N-benzylcarbazole and pyrene units has been syn-
Received 30 January 2013
Received in revised form 15 April 2013
Accepted 24 April 2013
thesized and characterized. The polymer possesses high thermal stability with the decomposition tem-
perature of 440 °C. It shows higher fluorescence quantum yields of in solution and solid state,
respectively. PCPyrene can emit bright blue-lights both in different organic solutions (440–460 nm)
and in the solid state (492 nm). Compared the emission spectra of PCPyrene in solutions with in solid
state, the solid state emission of PCPyrene is significantly red-shifted. Additionally, it is not obvious
changes of the solid emission spectra even after being annealed at 150 °C under nitrogen for 24 h. The
electrochemical properties and energy levels of PCPyrene were also investigated by cyclic voltammetry.
Furthermore, in order to provide a basis forecasting the structure–physical property relationships, the
photophysical properties of PCPyrene have been carefully investigated by fluorescence emission and
UV–vis absorption spectra.
Available online 9 May 2013
Keywords:
Polymer
Pyrene
Carbazole
Photoluminescence
PLEDs
Ó 2013 Elsevier B.V. All rights reserved.
Introduction
cessability by solution coating or inkjetting and good thermal sta-
bility [2–14].
Since the report on electroluminescence of poly(p-phenylene-
vinylene) (PPV) by the Cambridge group [1], electroluminescent
conjugated polymers have been attracting much attention due to
their potential applications in flat-panel displays, solid-state light-
ing sources and backlights for liquid–crystal displays. As a kind of
functional materials, polymer and its derivatives have several
advantages such as tunable color by molecular design, good pro-
Among these p-conjugated polymer materials, carbazole and its
derivatives play an important role in the ongoing challenge to de-
velop electroluminescence materials because of their intense lumi-
nescence and structure stabilities. They are widely used in polymer
light-emitting diodes (PLEDs) as emitters, and their reversible re-
dox processes make them as the suitable hole-carriers. Further-
more, their molecular and optical properties can be tuned by
structurally modifying 2,3,6,7 or 9H positions of carbazole group
[15–17]. The advantages of carbazole and its derivatives make
them one of the major classes of advanced materials and have been
⇑
Corresponding author. Tel.: +86 351 7018842; fax: +86 351 7018613.
E-mail address: lhfeng@sxu.edu.cn (L. Feng).
1386-1425/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.saa.2013.04.109

160
C. Zhang et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 113 (2013) 159–163
actively pursued for applications in PLEDs and other optoelectronic
devices [18–21]. Fluorescent characteristics rely on molecular
structure and molecular assembly. There is presently great interest
recrystallized by petroleum ether to give white solid 10.2 g (yield
78.1%).
to increase the structural or spatial dimensions of
molecules in order to tune and acquire more favorable physical
p-conjugated
N-benzyl-3,6-dibromocarbazole 2
A solution of 13.33 g (0.0831 mol) Br2 in 20 mL glacial acetic
acid was generally added by dropping funnel to another solution
of 10.0 g (0.0389 mol) N-benzyl-9H-carbazole in 60 mL glacial ace-
tic acid. The mixture was stirred for 24 h at room temperature. The
mixture solution was poured into 500 mL water and the resulting
solid was filtered. The crude products were purified by washing
(100 mL petroleum ether) and recrystallizing (ethyl acetate:petro-
leum ether = 1:4) to yield product as white needles (12.2 g, yield
properties [22]. Due to the large planar conjugated aromatic char-
acteristic, pyrene has strong
p-electron delocalization energy and
efficient fluorescence property, which makes its derivatives widely
used as fluorescence probes in recognized DNA labels and highly
efficient emitters in PLEDs [23–26]. So it is worth looking forward
to obtain the functional polymer with pyrene and carbazole units.
Based on the thought, herein, we synthesized a novel p-conju-
gated polymer containing N-benzylcarbazole and pyrene units as
PLEDs material. Its absorption and emission spectra, electrochem-
ical properties and the interaction with N,N-dimethylaniline
(DMA) or dimethylterephthalate (DMTP) have been studied. The
results of this report provide a clear evidence that the designed
polymer had not only significantly improved the thermal stability
of large planar pyrene rings, but also improved the hole injection
ability compared with the conjugated carbazole derivatives. The
combination of N-benzylcarbazole and pyrene is expected to ex-
pand the variety of photophysical and photochemical properties.
The basic data suggest that the polymer may exhibit the designed
polymer had not as blue-emitting material in polymer light-emit-
ting diodes (PLEDs).
74.1%). 1H NMR (CDCl3, 400 MHz)
d 8.13 (s, 2H), 7.50 (d,
J = 8.4 Hz, 2H), 7.27–7.18 (m, 5H), 7.05 (d, J = 6.8 Hz, 2H), 5.42 (s,
2H); 13C NMR (CDCl3, 100 MHz) d 46.77, 110.66, 112.48, 123.34,
123.65, 126.24, 127.81, 128.95, 129.30, 136.21, 139.54. EI-MS
(C19H13Br2, 415.12, m/z) 417 [M + 2], 415 [M], 413 [M ꢁ 2].
N-benzyl-3,6-di(40,40,50,50-tetramethyl-10,30,20-dioxaborolan)carbazole
3
A mixture with 4.15 g (0.01 mol) N-benzyl-3,6-dibromocarbaz-
ole, 6.09 g (0.024 mol) bis(pinacolato)diboron, 0.29 g (8 mol%)
Pd(dppf)Cl2 and 5.88 g (0.06 mol) potassium acetate in 70 mL
DMF was stirred at 80 °C for 24 h under nitrogen atmosphere.
The reaction mixture was cooled to room temperature, poured into
the 300 mL ice water, filtrated and then purified by column chro-
matography on silica gel with ethyl acetate/petroleum ether (1/
20) as the eluant to afford a white power (3.51 g, 68.9%). 1H NMR
(CDCl3, 400 MHz) d 8.69 (s, 2H), 7.87 (d, J = 8.4 Hz, 2H), 7.35 (d,
J = 8.4 Hz, 2H), 7.25–7.21 (m, 3H), 7.09 (dd, J = 5.6, 2.0 Hz, 2H),
5.53 (s, 2H), 1.38 (s, 24H); 13C NMR (CDCl3, 100 MHz) d 24.97,
46.53, 83.58, 108.40, 119.37, 123.02, 126.33, 127.49, 128.09,
128.79, 132.28, 136.76, 142.87; EI-MS (C31H37B2NO4, 509.25, m/
z) 510 [M + 1], 509 [M], 384 [M ꢁ 125].
Experimental
Materials and instruments
Carbazole, bis(pinacolato)diboron, Pd(dppf)Cl2, Pd(PPh3)4 and
1,6-dibromopyrene were purchased from Aldrich (Steinheim, Ger-
many). Other solvents including cyclohexane, diethyl ether, ethyl
acetate, dichloromethane, acetonitrile, methanol and ethanol of
analytical-grade were purchased from Beijing Chemical Plant (Bei-
jing, China). All chemicals and reagents were used as received un-
less otherwise stated.
All UV absorption spectra were recorded on a Shimadzu UV-265
UV–vis absorption spectrophotometer (Tokyo, Japan). Fluorescence
spectra were taken on a Hitachi F4500 spectrofluorometer (Tokyo,
Japan). Both excitation and emission slits were set at 5 nm. All the
experiments are carried out at 20 1 °C. The fluorescence lifetime
measurements were performed on an Edinburgh FLS920 fluores-
cence lifetime spectrometer (Livingston, UK) operating in the
time-correlated single photon counting mode. The excitation
source was a hydrogen lamp (Edinburgh Instruments). The lifetime
values were obtained from the re-convolution fit analysis using a
double-exponential decay function of the decay profiles with
F900 analysis software. The goodness of fit was evaluated by the
Polymer PCPyrene 4
To 100 mL three-neck flask, equipped with mechanical stirrer
was added 0.509 g (1.0 mmol) N-benzyl-3,6-di(40,40,50,50-tetrame-
thy-10,30,20-dioxaborolan)carbazole, 0.36 g (1.0 mmol) 1,6-dibro-
mopyrene, 0.588 g (6.0 mmol) potassium acetate, 60 mL DMSO
and 0.2 g Pd(PPh3)4 under nitrogen atmosphere. The mixture was
stirred and heated at 80 °C for 48 h. The black mixture was poured
into 150 mL methanol. The solid was collected by filtrating and
redissolved by 50 mL CHCl3. The crude produce was obtained by
removing undissolved substance and solvent. The crude polymer
was purified by precipitation from chloroform into methanol twice
and dried under vacuum to give yellow solids 0.143 (yield 31.4%).
1H NMR (CDCl3, 400 MHz) d 8.47–8.31 (m, 4H), 8.19–8.04 (broad,
6H), 7.79–7.58 (broad, 5H), 7.49–7.32 (broad, 4H), 5.75 (s, 2H).
GPC: Mn = 10,500, Mw = 15,400, PDI = 1.46.
reduced v2 value (close to 1 in all case). Cyclic voltammetric mea-
surements were carried out on a CHI 660C Electrochemical work-
station (Shanghai, China).
Fluorescence quantum yields
The fluorescence quantum yields were obtained using quinine
bisulfate in 0.05 M sulfuric acid as standard and calculated on
the basis of Eq. (1) [27]:
Synthesis of polymer PCPyrene (Scheme 1)
N-benzylcarbazole 1
FuAsn2u
FsAun2s
A mixture of 8.35 g (0.005 mol) carbazole, 4.21 g (0.075 mol)
potassium hydroxide and 150 mL THF was stirred and refluxed
for 2 h. Next, a solution of 10.48 g (0.0825 mol) benzyl chloride
was dropped slowly to the above mixture and refluxed for 24 h.
The solvent was removed by reducing pressure to 20 mL, the solu-
tion was poured into 300 mL water and the suspension was ob-
tained vacuum filtered, and the retained solid was dissolved by
100 mL petroleum ether. The undissolved solid was collected and
/uf ¼ /sf
ð1Þ
where F represents the corrected fluorescence peak area, A is the
absorbance at the excitation wavelength, n is the refractive index
of the solvent used, /f is the fluorescence quantum efficiency and
the superscripts and subscripts ‘s’ and ‘u’ refer to standard and un-
known, respectively.

C. Zhang et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 113 (2013) 159–163
161
Scheme 1. The synthesis route of PCPyrene.
454 nm. By comparison with monomers, the emission spectrum
of PCPyrene shows an obvious red-shift. These phenomena can
Results and discussion
be attributed to their different configuration, greater
polymer.
p-system in
Photophysical properties
The UV–vis absorption and photoluminescence (PL) spectra of
the polymer (PCPyrene) were measured in different organic sol-
vents including cyclohexane, ethyl acetate, diethyl ether, dichloro-
methane, methanol, ethanol and acetonitrile. The corresponding
data were summarized in Table 2. The absorption spectra of PCPy-
rene (Fig. 1) exhibited the characteristic vibronic pattern of the
conjugated substituted pyrene backbones (kmax ꢂ 370 nm). The
additional absorption bands (kmax ꢂ 250 nm, 280 nm) can be as-
signed to the substituents at the 1- and 6-positions of the central
pyrene core. The absorption maxima of the polymer in the solvents
did not vary much with the change of the solvent polarity. Upon
excitation of the PCPyrene at 370 nm, the emission spectra display
maxima at 440–460 nm in different organic solution and as shown
in Fig. 2. The kmax of PCPyrene is obviously red-shifted with the sol-
vent polarity increased. In addition, the emission bands display
good normal Gaussian shape and their half bandwidths (kem½) in-
creases significantly with the increase in solvent polarity. The
Stokes shift for PCPyrene is very pronounced in polar solvents such
as dichloromethane, acetonitrile and ethanol as displayed in Ta-
ble 2. For PCPyrene, carbazole unit serves as an excellent electron
donor, pyrene unit is electron receptor. The link of carbazole and
The UV–vis absorption spectra and fluorescence emission spec-
tra of monomer 1, monomer 2 and polymer PCPyrene in dichloro-
methane were shown Table 1. From the Table 1, the monomer 1
(with carbazole groups) shows two peaks at about 241 and
270 nm. Monomer 2 has four main peaks at 242, 280, 335 and
360 nm. The absorption peak at 360 nm is the characteristic vib-
ronic pattern of the conjugated pyrene backbones. The additional
absorption bands can be assigned to the substituent at the 1-
and 6-positions of the central pyrene core. In contrast, the polymer
combination of the two monomers, the absorption spectra of PCPy-
rene exhibit the characteristic vibronic pattern of the conjugated
pyrene backbones and with an obvious blue shift of the peak
absorption at 372 nm. The fluorescence emission spectra of mono-
mer 1, monomer 2 and polymer PCPyrene also was determined in
dichloromethane. As can be seen from Table 1, the emission spec-
trum of monomer 1 shows two peaks at 345 and 360 nm, the emis-
sion spectrum of monomer 2 shows two peaks at 380 and 410 nm,
whereas the emission spectra of the PCPyrene are located at
Table 1
pyrene units may form a D–p–A structure. As a result, this provides
a good basis for PCPyrene to possess intramolecular charge transfer
The UV–vis absorption and fluorescence emission spectra of monomers and polymer
PCPyrene.
(ICT) character especially in the polar solvents.
Compounds
Abs. k (nm)
PL k (nm)
It can be found that the absorption and PL spectra of PCPyrene
show obvious red-shift compared to those of pyrene, which can be
attributed to the increasing of conjugation length. The PL quantum
yields of the material in different solutions were determined (Ta-
Monomer 1
Monomer 2
Polymer
241
242
247
270
280
286
345
380
454
360
410
335
372
360

162
C. Zhang et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 113 (2013) 159–163
Table 2
Electrochemical properties
The photophysical properties of polymer PCPyrene.
The electrochemical behaviors of PCPyrene were investigated
by cyclic voltammetry at room temperature as shown in Fig. 3.
The CVs were applied to the Pt working electrode in 0.10 mol Lꢁ1
tetrabutylammonium perchlorate (Bu4NClO4) acetonitrile support-
ing electrolyte. The electrolytic cell was a conventional three-elec-
trode cell consisting of a Pt working electrode, a Pt wire counter
electrode and Ag/AgCl reference electrode. PCPyrene exhibits
reversible oxidation process with half-wave oxidation potentials
(E1/2ox) of 0.95 V (vs. Ag/AgCl). The HOMO energy level was
ꢁ5.28 eV and was calculated by using the energy level value of
ꢁ4.8 eV for ferrocene (Fc) with respect to zero vacuum level [29–
31]. The LUMO energy level was ꢁ1.93 eV and was estimated
according to the HOMO energy level values in combination with
the band gap. The band gap (Eg) of the polymer can be estimated
Solvent
Abs. kmax (nm)
PL kmax kem½
Stoke
shift
(nm)
U %
(nm)
(nm)
n-Hexane
Diethyl ether
Ethyl acetate
250 286 373 444
251 285 372 448
256 284 368 452
61
61
69
70
69
71
73
69
71
76
84
82
85
86
86
0.171
0.775
0.813
0.870
0.104
0.040
0.086
0.416
Dichloromethane 247 286 372 454
Acetonitrile
Methanol
Ethanol
Film
248 285 375 460
250 287 375 461
248 287 376 461
492
1. Acetonitrile
0.8
0.6
0.4
0.2
0.0
2. Dichloromethane
3. Diethyl ether
4. Methanol
1
6
from the onset absorption (UVonset
)
with Eg (eV) = hc/k
(h = 6.626 ꢃ 10ꢁ34 Js, c = 3 ꢃ 10ꢁ17 nm/s, 1 eV = 1.602 ꢃ 10ꢁ19 J).
The band gap of the polymer is 2.87 eV.
5. Ethanol
6. n-Hexane
7. Ethyl acetate
The interactions of PCPyrene with electron donor and accepter
7
The fluorescence quenching technique is an effective method
for study of the mechanism of molecular interaction. N,N-dimeth-
ylaniline (DMA) is a typical electron donor, dimethylterephthalate
(DMTP) are typical electron accepter. The studies of interactions
between PCPyrene and DMA and DMTP are helpful to understand
the optical electronic properties of PCPyrene. Fig. 4 showed the
emission spectra of PCPyrene in chloroform with different concen-
trations of DMTP. It can be seen from Fig. 4 that the fluorescence of
PCPyrene is quenched and the quenching process follows the
Stern–Volmer equation F0/F = 1 + KSV[Q], where the F0 and F are
the emission intensity of PCPyrene in the absence and presence
of DMTP, [Q] is the concentration of DMTP. The quenching coeffi-
cient, KSV, is 282.43 Mꢁ1. The fluorescence intensity of PCPyrene
emission spectra were not changed when the electron donor
DMA were added. These results suggest that the intramolecular
charge transfer (ICT) state of the conjugated polymer PCPyrene is
more stable. The intermolecular charge transfer was slightly oc-
curred in the presence of stronger electron accepter.
200
250
300
350
400
450
500
Wavelength/nm
Fig. 1. UV–vis absorption spectrum of PCPyrene in n-cyclohexane, diethyl ether,
ethyl acetate, dichloromethane, acetonitrile, methanol and ethanol. The concentra-
tion of polymer is 2.6 mg/ml.
1
8
1.0
0.8
0.6
0.4
0.2
0.0
1. n-Hexane
2. Diethyl ether
3. Ethyl acetate
4. Dichloromethane
5. Acetonitrile
6. Methanol
7. Ethanol
Thermal properties
8. Film
The thermal properties of PCPyrene were investigated using
thermal gravimetric analysis (TGA) with a heating rate of 5 °C/
min under the nitrogen atmosphere. The polymer exhibits high
0.000006
0.000004
0.000002
0.000000
-0.000002
-0.000004
350
400
450
500
550
600
650
wavelength/nm
Fig. 2. Normalized fluorescence spectra of PCPyrene in n-cyclohexane, diethyl
ether, ethyl acetate, dichloromethane, acetonitrile, methanol, ethanol and film. The
concentration of polymer is 1.2 mg/ml. The excitation wavelength is 370 nm.
ble 2) by using quinine bisulfate as the reference standard. The
fluorescence quantum yields of PCPyrene were found to be gener-
ally higher in dichloromethane, diethyl ether and ethyl acetate
than that in other solvents [28].
The film PL of PCPyrene was 492 nm, constituting a 30–50 nm
red-shift from the solution (Fig. 2). The solid state PL quantum
yields were measured to be 0.416. No obvious change in the emis-
sion film was observed even after annealing at 150 °C for 24 h un-
der nitrogen, which indicates that we have successfully improved
the thermal and luminescent stability of pyrene by incorporating
the N-benzylcarbazole group onto the pyrene ring.
-0.4
0.0
0.4
0.8
1.2
1.6
Potential(V vs Ag/AgCl)
Fig. 3. Cyclic voltammogram of PCPyrene. The supporting electrolyte is 0.10 M
Bu4NClO4 in acetonitrile and the scan rate is 0.10 V/s.

C. Zhang et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 113 (2013) 159–163
163
Youth Science Foundation of Shanxi Province (2009021006-1 and
2012011007-2).
1.25
1.20
1.15
1.10
1.05
1.00
2000
1600
1200
800
400
0
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y = 281.92 x + 0.9957
R = 0.9982
K = 282.43
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Conclusion
A new polymer PCPyrene containing N-benzylcarbazole and
pyrene has been successfully synthesized. The absorption maxi-
mum of the polymer is about 370 nm and the photoluminescence
maximum of the polymer displays larger changes in different sol-
vent polarity and has obvious ICT character. The solid state emis-
sion is significantly red-shifted, the quantum yield of
photoluminescence is 0.87 in dichloromethane and the polymer
was stable beyond 400 °C. It is a blue emitting material with the
band gap of 2.87 eV estimated from the onset absorption. Further-
more, the interactions between PCPyrene and quencher DMA and
DMTP were investigated. These data indicate its potential use as
blue-emitting material in polymer light-emitting diodes (PLEDs).
Acknowledgements
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The work described in this paper was supported by the National
Nature Science Foundation (Nos. 20802042 and 21105060) and the