A novel blue emission PLED material: Synthesis, photophysical and electrochemical properties

Article abstract of DOI:10.1016/j.dyepig.2015.02.016

A novel π-conjugated polymer (PCQH) containing N-benzylcarbazole and 8-methoxyquinoline units has been successfully synthesized and characterized. The polymer possesses good thermal stability with the decomposition temperature (Td) of 295 °C and the glass transition temperature (Tg) of 250 °C. The absorption and emission spectra show that the polymer can emit blue light with higher fluorescence quantum yield (81%) in CHCl₃. With the increase of solvent polarity, the emission peaks of PCQH display an obvious red-shifted. The study of the interactions of PCQH with electron donor (DMA) and electron acceptor (DMTP) indicates that the polymer has excellent electron transfer property. Furthermore, the interactions of PCQH with carbon materials (C₆₀ and CNTs) also have been investigated. The electrochemical properties and energy levels of PCQH were calculated by the Gaussian 09 software, which are correspond to the experiment detection with cyclic voltammetry method.

Full text of DOI:10.1016/j.dyepig.2015.02.016

Dyes and Pigments 117 (2015) 116e121
Contents lists available at ScienceDirect
Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
A novel blue emission PLED material: Synthesis, photophysical and
electrochemical properties
*
Ninghua Yin, Liheng Feng
School of Chemistry and Chemical Engineering, Shanxi University, Taiyuan 030006, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 6 January 2015
Received in revised form
A novel
p-conjugated polymer (PCQH) containing N-benzylcarbazole and 8-methoxyquinoline units has
been successfully synthesized and characterized. The polymer possesses good thermal stability with the
decomposition temperature (Td) of 295 C and the glass transition temperature (Tg) of 250 C. The
absorption and emission spectra show that the polymer can emit blue light with higher fluorescence
3
quantum yield (81%) in CHCl . With the increase of solvent polarity, the emission peaks of PCQH display
ꢀ
ꢀ
17 February 2015
Accepted 18 February 2015
Available online 25 February 2015
an obvious red-shifted. The study of the interactions of PCQH with electron donor (DMA) and electron
acceptor (DMTP) indicates that the polymer has excellent electron transfer property. Furthermore, the
interactions of PCQH with carbon materials (C60 and CNTs) also have been investigated. The electro-
chemical properties and energy levels of PCQH were calculated by the Gaussian 09 software, which are
correspond to the experiment detection with cyclic voltammetry method.
Keywords:
Conjugated polymer
Carbazole
Quinoline
Blue emission
Material
© 2015 Elsevier Ltd. All rights reserved.
Suzuki coupling
1. Introduction
most important chelators for metal ions have been applied in a
variety of investigations involving metal complexes [16]. They are
Polymer light-emitting devices (PLEDs) have drawn immense
attention because of their large -conjugated backbones, delo-
also applied in manufacturing dyes, food colorants, pH indicators
and other organic compounds [17e20]. In the light-emitting de-
vices field, 8-hydroxyquinoline aluminum has been widely used as
the emissive and electron transporting material. It is well known
that the balance of the injection of electron and hole transport is
crucial for obtaining good performance LEDs. So, it is a significative
work to combine the hole-transfer group, electron-transfer group
and luminescence group in the same polymer molecule [21].
Based on the above thought, herein, we designed and syn-
p
calized electronic structures, low cost, ease of modification of
properties by appropriate substitution, and of solvent processabil-
ity [1e3]. Among various PLEDs, the blue emitting material is scarce
and desired for the development of current PLEDs [4]. Compared
with organic small molecule light-emitting materials, the most
obvious advantage of the polymer light-emitting materials lies in
allowing adjustment of their properties by the facile design and
modification [5e7].
thesized a novel
p-conjugated polymer (PCQH) containing N-
Carbazole is a typical hole-transfer material, and these dyes
containing carbazole units have excellent photo-conductivity and
relatively intense luminescence. Through introducing carbazole
units to the core structure of PLED materials, the thermal stability
and glass state durability of these materials can be greatly
improved [8e10]. Conjugated polymers with carbazole units
display strong brightness and high stability. Due to carbazole with
high triplet energy level and low oxidation potential, its derivations
have been widely applied as reasonable host materials in OLED and
PLED [11e15]. Hydroxyquinoline and its derivations as one of the
benzylcarbazole and 8-methoxy-quinoline units as a PLEDs ma-
terial. In the polymer, N-benzylcarbazole is a hole-transfer unit;
8-methoxyquinoline acts as both electron-transfer and lumines-
cence roles. The absorption and emission spectra, electro-
chemical properties of the polymer PCQH have been studied. And
the interactions of PCQH with N, N-dimethylaniline (DMA) and
dimethylterephthalate (DMTP) also been investigated by fluo-
rescence technique. The results indicate that the designed poly-
mer has not only excellent electronic transmission stability
which comes from the 8-methoxyquinoline group, but also the
hole-transfer ability that belongs to N-benzylcarbazole group.
The obtained results suggest that the
PCQH) may a desired blue-emitting material in polymer light-
emitting diodes (PLEDs).
p-conjugated polymer
(
*
Corresponding author. Tel.: þ86 351 7010904; fax: þ86 351 7011688.
E-mail address: lhfeng@sxu.edu.cn (L. Feng).
http://dx.doi.org/10.1016/j.dyepig.2015.02.016
143-7208/© 2015 Elsevier Ltd. All rights reserved.
0

N. Yin, L. Feng / Dyes and Pigments 117 (2015) 116e121
117
2
. Experimental
J ¼ 5.6, 2.0 Hz, 2H), 5.52 (s, 2H), 1.38 (s, 24H); 13C NMR (CDCl
3
,
1
00 MHz) d 24.99, 46.51, 83.49, 108.42, 119.39, 123.12, 126.34,
2
.1. Materials and instruments
127.49, 128.11, 128.76, 132.28, 136.77, 142.85; EI-MS (C31
09.25, m/z) 510 [M þ 1], 509 [M], 384 [M ꢁ 125].
37 2 4
H B NO ,
5
Unless otherwise stated, all chemical reagents were obtained
from commercial suppliers and used without further purification.
Solvents used were purified and dried by standard methods prior to
2.2.3. Polymer (PCQH)
A mixture with 0.317 g monomer 1 (1.0 mmol) and 0.509 g
monomer 2 (1.0 mmol) in toluene (10.0 mL), 2.0 M Na CO (5.0 mL)
was added and stirred for 30 min at room temperature under ni-
trogen atmosphere. Then, 50 mg Pd(PPh catalyst was quickly
added to the suspension and heated to 110 C for 48 h under ni-
trogen atmosphere. After the suspension was concentrated to
2e3 mL, 50 mL chloroform was added to the suspension. The ob-
tained mixture was washed with water for three times. The com-
use. Bis(pinacolato)diboron, Pd(dppf)Cl
formamide, dimethyl sulfate, C60, carbon nanotubes CNTs, dime-
thylterephthalate, 1,4-Dicyanobenzene, 5,7-dibromo-8-
2
,
Pd(PPh
3
)
4
,
dimethyl
2
3
3 4
)
ꢀ
hydroxyquinoline and N-benzyl-3,6-dibromo-carbazole were pur-
chased from Aldrich (Steinheim, Germany). Other solvents
including cyclohexane, chloroform, ethyl acetate, dichloromethane,
acetone and methanol of analytical-grade were purchased from
1
13
Beijing Chemical Plant (Beijing, China). H NMR and C NMR were
measured on a Bruker ARX400 spectrometer with chemical shifts
reported as ppm (TMS as an internal standard). Gel permeation
4
bined organic layer was dried over anhydrous MgSO . After
removal of a mass of chloroform, the residue was precipitated with
methanol and the precipitate was collected by centrifugation. The
crude polymer was purified by precipitation for twice from chlo-
roform into methanol. The final yellow solid product was obtained
chromatography (GPC) measurements were performed on
a
Waters-410 system against polystyrene standard with tetrahydro-
furan (THF) as eluant. UVeVis absorption and fluorescence emis-
sion spectra were taken on a JASCO V-550 spectrophotometer.
Fluorescence spectra were acquired with a Varian Cary Eclipse
fluorescence spectrophotometer, the excitation and emission slit
widths were both 5.0 nm. High-resolution mass spectra (HRMS)
were acquired on an Agilent 6510 Q-TOF LC/MS instrument (Agilent
Technologies, Palo Alto, CA) equipped with an electrospray ioni-
zation (ESI) source. The glass transition temperature of polymer
was determined by DSC using a DSC-Q10 instrument under a ni-
trogen atmosphere. The decomposition temperature correspond-
ing to 5% weight loss was detected using a PerkineElmer Pyris 1
TGA thermal analyzer. Cyclic voltammetry (CV) measurement was
determined on a three-electrode AUTOLAB (model PGSTAT30)
1
through centrifugation and drying under vacuum in 39.7% yield. H
1
NMR (400 MHz, CDCl
3
, ppm): H NMR (d-DMSO, 400 MHz, ppm)
d
8.98 (s, 1H), 8.61e7.76 (m, 2H), 7.93e7.74 (m, 2H), 7.68e7.55 (m,
13
5H), 7.33e7.26 (m, 5H), 5.79 (s, 2H), 4.12 (s, 3H); C NMR (d-DMSO,
100 MHz) 46.43, 62.18, 110.18, 115.79, 122.56, 123.09, 128.95,
129.26, 130.71, 131.99, 132.46, 133.74, 136.40, 138.19, 143.29, 151.92,
152.37; M ¼ 35,692, PDI ¼ 1.21.
¼ 29,715, M
d
n
w
3. Results and discussion
3.1. Synthesis and photophysical properties of conjugated polymer
PCQH
workstation in a solution of Bu
4
NClO
4
(0.1 M) in acetonitrile at a
The p-conjugated polymer (PCHQ) containing hole transport
scan rate of 50 mV/s at room temperature.
(carbazole) and luminophore (8-methoxyquinoline) units was ob-
tained via Suzuki coupling of monomers 1 and 2 within yield of
39.7% (Scheme 1). The UVevis absorption and fluorescence emis-
2
2
.2. Synthesis of monomer 1, 2 and polymer (PCQH)
ꢁ
6
sion spectra of conjugated polymer (PCHQ, 1.0 ꢂ 10 mol/L) were
.2.1. 5,7-dibromo-8-methoxyquinoline (monomer 1)
determined in CHCl and showed in Fig.1. As can be seen from Fig.1,
3
A solution of 3.02 g 5,7-dibromo-8-hydroxyquinoline (0.01 mol)
and 8.28 g anhydrous potassium carbonate (0.06 mol) in 100 mL
acetone was heated and refluxed for 30 min under stirring. Then,
2.6 g fresh dimethyl sulfate (0.1 mol) generally added by dropping
funnel to the mixture. The whole mixture was refluxed for 24 h.
Next, the reaction mixture was cooled and filtered the resulting
solid. The organic solution was concentrated to 20 mL and poured
the maximum absorption peak of polymer PCHQ is about 300 nm
with two major absorption bands at 230e270 nm and
280e330 nm, which mainly comes from the characteristic peaks of
carbazole group. And, the corresponding optical energy band gap of
PCHQ is 4.13 eV by calculating from the absorption band edge of the
absorption spectrum. The fluorescence spectrum of PCQH exhibits
that it has a strong fluorescence emission band at 370e590 nm
1
into 300 mL iced water under stirred. The product was obtained by
with an emission maximum (
with ideal blue emitting materials. The fluorescent quantum yield
) of PCQH in CHCl was 0.81% based on quinine sulfate in 0.10 M
lem) at 435 nm, which is matched
1
filtering and drying under vacuum in 95.6% yield. H NMR (CDCl
3
,
4
1
00 MHz)
d
8.99 (d, J ¼ 4.0 Hz, 1H), 8.52 (d, J ¼ 8.8 Hz, 1H), 8.00 (s,
(F
3
13
H), 7.56 (t, J ¼ 4.4, 4.4 Hz, 1H), 4.18 (s, 3H); C NMR (CDCl
3
,
sulfuric acid as the reference [22].
1
1
[
00 MHz)
43.58, 150.81, 153.51; EI-MS (C10
M þ 1], 418.9 [M þ 2], 414.9 [M ꢁ 2].
d
62.31, 116.44, 116.52, 122.55, 128.15, 133.71, 136.28,
Next, the photophysical properties of PCQH in different solvents
were observed by fluorescence instrument. Fig. 2 gives the photo-
luminescence (PL) spectra of PCQH in different organic solvents
including methanol, acetone, ethyl acetate, chloroform, dichloro-
methane, n-hexane. The PL spectra vary from 420 nm to 460 nm
with the increase of solvent polarity. In addition, the emission
bands display good normal Gaussian shape and their half band-
H
7
2
Br NO, 316.98, m/z) 317.9
0 0 0 0 0
.2.2. N-benzyl-3,6- di(4 ,4 ,5 ,5'-tetramethyl-1 ,3 ,2'-dioxaborolan)
2
carbazole (monomer 2)
To solution of 4.15
0.01 mol), 6.09 bis(pinacolato)diboron (0.024 mol),
.29 g Pd(dppf)Cl
a
g N-benzyl-3,6-dibromocarbazole
(
g
widths (lem½) increases significantly with the increase of the sol-
0
2
(8 mol %) and 5.88 g potassium acetate
vent polarity. The Stokes shift for PCQH is prominent in polar
solvents such as dichloromethane, acetone and ethanol. The
obvious solvent effect can be explained as following: the polymer
ꢀ
(
0.06 mol) in 70 mL DMF was stirred at 90 C for 12 h under ni-
trogen atmosphere. The reaction mixture was cooled to room
temperature, poured into the 300 mL ice water, filtrated and then
purified by column chromatography on silica gel with ethyl acetate/
has a unique De
donor (carbazole unit) and the electron receptor (quinoline unit).
Based on the De eA structure, the intramolecular charge transfer
peA structure, which comes from the electron
petroleum ether (1/20) as the eluant to afford a white power in
p
1
7
3.1% yield. H NMR (CDCl
3
, 400 MHz, ppm)
d
8.67 (s, 2H), 7.88 (d,
(ICT) action of the polymer takes place with the increase of polar
solvents and the extent of the action depends on the size of the
J ¼ 8.4 Hz, 2H), 7.34 (d, J ¼ 8.4 Hz, 2H), 7.26e7.21 (m, 3H), 7.11 (dd,

118
N. Yin, L. Feng / Dyes and Pigments 117 (2015) 116e121
Scheme 1. The synthesis routes of monomers and polymer PCQH.
solvent polarity. So the emission spectra are obvious red-shifted
with the increase of solvent polarity.
an electronic storage body and its electron can transfer freely on
conjugated system, which is desired for the PLED materials.
p-
3.2. The interactions of PCQH with electron donor and acceptor
3.3. The interactions of PCQH with C60 and CNTs
In order to study the mechanism of molecular interaction, the
Due to their remarkable electron transfer property, C60 and CNTs
have been widely utilized as photo-electron materials in chemistry,
physical and materials fields. Herein, we investigated the in-
teractions of polymer PCQH with C60 and CNTs in static solution,
fluorescence quenching technique was adopted on many experi-
ments. Due to DMA acting as a typical electron donor and DMTP as
a typical electron acceptor, the electron transfer property of PCQH
was investigated by the interactions of it with DMA and DMTP. The
interactions of PCQH with DMA or DMTP were showed in Fig. S1
and Fig. S2. It can be seen from these figures that the fluores-
cence of PCQH can be quenched and the quenching processes
respectively. Fig. 4 gives the interaction processes of PCQH and C60
.
As we have seen in Fig. 4, with the gradual increasement of C60, the
fluorescence of PCQH was quenched efficiently and the process was
also following the SterneVolmer equation. The apparent quenching
4
ꢁ1
follow the SterneVolmer equation F
and F are the emission intensities of PCQH in the absence and
0
/F ¼ 1 þ KSV[Q], where the F
o
constant was 3.79 ꢂ 10
M , which indicates that the strong
interaction between PCQH and C60 happens in the excited state.
Similarly, the electron transfer process between PCQH and CNTs
also can be observed (Fig. S3). The results can be explained by two
presence of DMA or DMTP, [Q] is the concentration of DMA or
DMTP. Accordingly, the quenching coefficients,
K
SV
,
are
.76 ꢂ 103 M (DMA) and 7.9 ꢂ 103 M (DMTP), respectively
Fig. 3). These results display that the conjugated polymer PCQH is
ꢁ
1
ꢁ1
8
(
aspects. Firstly, the large p-conjugated system of C60 and CNTs may
Fig. 2. Fluorescence spectra of conjugated polymer PCQH in different solvents.
ꢁ
3
Determined method: the stock solution (PCQH, 1.0 ꢂ 10 mol/L in CHCl
3
was added to
Fig. 1. The UVevis absorption and fluorescence emission spectra of polymer PCQH
the corresponding solvents respectively, and the final solutions were all
ꢁ
6
ꢁ6
(
1.0 ꢂ 10 mol/L) in CHCl
3
.
1.0 ꢂ 10 mol/L).

N. Yin, L. Feng / Dyes and Pigments 117 (2015) 116e121
119
Fig. 3. The interaction fluorescence spectra of PCQH (1.0 ꢂ 10ꢁ mol/L) with DMA or DMTP in CHCl3, respectively.
6
change the configuration of PCQH by
to the rapid photo-induced charge transfer from excited PCQH to
60, charge transfer of conjugated system may be dramatically
modified and distorted [23].
p
-
p
interaction. Secondly, due
electron density mainly distributes on the carbazole moiety. The
LUMO energy level of PCQH is ꢁ1.06 eV and electron density dis-
tribution mainly distributes on the quinoline moiety. In addition,
complete localization of the HOMO and LUMO is essential for
efficient hole and electron transport and for the prevention of
reverse energy transfer as well [25].
C
3.4. Thermal properties of conjugated polymer PCQH
3.6. Electrochemical properties
Thermal property is one of the most parameters for evaluating
the PLED materials. The thermal properties of PCQH were evaluated
by thermal gravimetric analysis (TGA) with a heating rate of 5 C/
ꢀ
The electrochemical properties of PCQH were studied by CV
with saturated calomel electrode as the reference electrode. Based
on the supporting electrolyte (0.10 tetrabutylammonium
min and differential scanning calorimetry (DSC) under the nitrogen
atmosphere. The polymer exhibits high thermal stability. The
M
ꢀ
perchlorate) under nitrogen atmosphere, the electrochemical ex-
periments were investigated on 1.0 mM PCQH in acetonitrile. As
can been seen from Fig. 7, one peak and one reversible reduction
peak were observed within the entire electrochemical window. The
reversible oxidation peak is around þ0.5 eV, which mainly comes
from the oxidation of the carbazole groups. The reversible reduc-
tion peak is around ꢁ1.1 eV are due to electron injection into the
vacant p-orbital of quinolone. The reduction potential (ꢁ1.1 eV) of
conjugated polymer suggests that the electron distribution of
LUMO lies in the quinolone moiety. The HOMO energy level
is ꢁ5.25 eV, which is calculated by the empirical equation
decomposition temperatures (Td, 5% weight loss) is 295 C (Fig. 5),
which might be attributed to its large molecular mass. And the
ꢀ
glass transition temperatures (Tg) of the polymer is 250 C. The
result indicates that PCQH has good thermal stability and high glass
transition temperature, which would be facilitated for the opto-
electronic device applications.
3.5. Theoretical calculation
Theoretical calculation for its energy levels is an effective
method for obtaining a reasonable qualitative indication of the
excitation and emission properties of a conjugated polymer [24].
Based on density functional theory (DFT) and taking one repeat unit
as a model, the geometries and electron density distributions of the
HOMO and LUMO energy levels of PCQH were obtained in Fig. 6. As
can be seen Fig. 6, the HOMO energy level of PCQH is ꢁ5.14 eV and
[
HOMO ¼ ꢁ(Eox þ 4.5 þ 0.24) eV]. The HOMO energy level suggests
that the electron distribution mainly on the carbazole moiety,
which indicates the PCQH can be used as a good hole-transport
material. The energy gap (Eg) of PCQH is 4.13 eV, which is calcu-
lated from CV data well agreed with the UVevis onset result. Thus,
the LUMO energy level is ꢁ1.12 eV, which is calculated by the
HOMO and Eg. The HOMO, LUMO and Eg are all very close to the
theoretical values.
ꢁ
6
Fig. 4. The interaction fluorescence spectra of PCQH (1.0 ꢂ 10 mol/L) with C60 in
ꢁ
6
ꢁ6
ꢁ6
CHCl
3
. The concentrations of C60 are 1.0 ꢂ 10 mol/L, 3.5 ꢂ 10 mol/L, 8.5 ꢂ 10 mol/
ꢁ5 ꢁ5 ꢁ5 ꢁ5
ꢀ
L, 1.25 ꢂ 10
mol/L, 2.1 ꢂ 10
mol/L, 3.1 ꢂ 10
mol/L, 4.6
ꢂ
10
mol/L,
Fig. 5. TGA thermogram of conjugated polymer PCQH with a heating rate of 5 C/min
.2 ꢂ 10ꢁ mol/L, 8.6 ꢂ 10 mol/L, respectively.
5
ꢁ5
under nitrogen atmosphere.
6

120
N. Yin, L. Feng / Dyes and Pigments 117 (2015) 116e121
Fig. 6. Selected computed (Gaussian03) orbital plot for PCQH.
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