Photophysical processes of a copolymer containing naphthalene and carbazole rings.

Article abstract of DOI:10.1016/S1386-1425(00)00243-2

The photophysical processes of copolymer formed by copolymerization of beta-naphthyl-methacrylate (NMA) with vinylcarbazole (VCZ) were studied. The results show that when the solution of copolymer (NMA-VCZ) in THF is located in a low concentration range (about 10(-8) mol/l), the fluorescence emission is in good agreement with that of NMA monomer and the excimer is formed with gradual increase in concentration of copolymer (NMA-VCZ). The fluorescence of copolymer (NMA-VCZ) can be efficiently quenched both by electron donors and acceptors where the quenching effects follow the Stern-Volmer equation. The dimolecular exciplex between copolymer (NMA-VCZ) and N,N-dimethylaniline (DMA) is formed and the triple exciplex is also observed in the same system.

Full text of DOI:10.1016/S1386-1425(00)00243-2

Spectrochimica Acta Part A 56 (2000) 1869–1873
www.elsevier.nl/locate/saa
Photophysical processes of a copolymer containing
naphthalene and carbazole rings
Xiaoying Yuan a, Zhaobing Chen b,*, Jinli Qiao c, Jian Chai d
a Test Center of Shanxi Uni6ersity, Taiyuan 030006, PR China
b Department of Chemistry, Shanxi Uni6ersity, Taiyuan 030006, PR China
c Taiyuan Uni6ersity of Technology, Taiyuan 030021, PR China
d Department of Physics, Shanxi Uni6ersity, Taiyuan 030006, PR China
Received 3 December 1999; received in revised form 11 February 2000; accepted 11 February 2000
Abstract
The photophysical processes of copolymer formed by copolymerization of b-naphthyl-methacrylate (NMA) with
vinylcarbazole (VCZ) were studied. The results show that when the solution of copolymer (NMA–VCZ) in THF is
located in a low concentration range (about 10−8 mol/l), the fluorescence emission is in good agreement with that of
NMA monomer and the excimer is formed with gradual increase in concentration of copolymer (NMA–VCZ). The
fluorescence of copolymer (NMA–VCZ) can be efficiently quenched both by electron donors and acceptors where the
quenching effects follow the Stern–Volmer equation. The dimolecular exciplex between copolymer (NMA–VCZ) and
N,N-dimethylaniline (DMA) is formed and the triple exciplex is also observed in the same system. © 2000 Elsevier
Science B.V. All rights reserved.
Keywords: Copolymer; b-naphthyl-methacrylate; Vinylcarbazole; Excimer; Triple exciplex
1. Introduction
cence properties have also been investigated, the
studies on synthesis and photophysical processes
of new polymer still remain to be a challenge in
this area [2]. We have synthesised a novel copoly-
mer consisting of naphthalene moiety, which
emits blue fluorescence with high quantum yield
and polyvinylcarbazole (PVCZ) moiety, which
can form high-quality thin films with excellent
hole-transporting function. In this paper, we re-
port the results of study on the photophysical
processes of copolymer b-naphthyl-methacrylate–
Recently, polymeric light-charge materials have
attracted much attention for their potential appli-
cation in light-emitting devices [1]. Although a
great number of polymers have been synthesised
and the photoluminescence and electrolumines-
* Corresponding author. Tel.: +86-351-7011492; fax: +86-
351-7011688.
E-mail address: common@mail.sxu.edu.cn (Z. Chen).
1386-1425/00/$ - see front matter © 2000 Elsevier Science B.V. All rights reserved.
PII: S1386-1425(00)00243-2

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X. Yuan et al. / Spectrochimica Acta Part A 56 (2000) 1869–1873
Scheme 1.
vinylcarbazole (NMA–VCZ) using steady fluores-
cence quenching technique etc. in which the for-
mations of exciplex are especially investigated
since the exciplexes are often a middle state of
photochemical, photophysical and photobiologi-
cal primary processes.
3. Results and discussion
3.1. Luminous properties of copolymer
(NMA–VCZ) and formation of excimer
The fluorescence spectra of copolymer (NMA–
VCZ) are shown in Fig. 1a. It can be seen when
concentration of copolymer (NMA–VCZ) is
5.6×10−9 mol/l, the maximal fluorescence emis-
sion is at 310 nm which is in good agreement with
that of NMA monomer (see Fig. 1b). This means
2. Experimental
NMA was synthesised by esterification between
b-naphthol and methacryloyl chloride in the pres-
ence of triethylamine. Then copolymer (NMA–
VCZ) was synthesised by polymerisation of NMA
(2%) with VCZ (98%) in the presence of 2,2%-azo-
bisisobutyronitrile (AIBN) as initiator. The syn-
thesis
and
composition
of
copolymer
(NMA–VCZ) are outlined in Scheme 1. The
molecular weight of copolymer (NMA–VCZ) is
12 800.
N,N-dimethyl-aniline (DMA) and THF were
both analytical grade reagents and treated accord-
ing to standard methods used before.
The measurements of fluorescence were per-
formed on a Perkin-Elmer LS-50B fluorometer
with an excitation wavelength of 260 nm. Both
excitation and emission bands were set at 3.5 nm
on these experimental conditions. All the experi-
ments were carried out at 2591°C, about which
temperature control was maintained by a model
501 superthermostat circulating water bath.
Fig. 1. Excitation and emission fluorescence spectra of copoly-
mer (NMA–VCZ) (a) and NMA (b). Concentration of (a),
5.6×10−9 mol/l; and (b), 1.48×10−5 mol/l. Excitation and
emission wavelength are 260 and 310 nm, respectively.

X. Yuan et al. / Spectrochimica Acta Part A 56 (2000) 1869–1873
1871
tration of copolymer (NMA–VCZ) itself (Fig. 2,
1–3), and at a relatively longer wavelength a new
emission band appeared at 370 nm and the spectra
passed an isostilbic point of 345 nm. Evidently, the
fluorescence emission at 370 nm contributed to the
excimer formed between copolymer (NMA–VCZ)
molecules [3].
3.2. Formation of bimolecular exciplex
Fig. 3 gives the fluorescence spectra of copolymer
(NMA–VCZ) in the presence of DMA. It can be
seen that the fluorescence of copolymer (NMA–
VCZ) was also efficiently quenched with increasing
concentration of DMA. At the same time, a new
structureless emission band appeared at a relatively
longer wavelength of 345 nm and all the spectra
passed an isostilbic point of 320 nm. The fluores-
cence quenching pattern was similar to that of
copolymer (NMA–VCZ) concentration quenching
described above, which indicated that a bimolecu-
lar exciplex was formed between copolymer
(NMA–VCZ) and DMA.
Fig. 2. Fluorescence spectra of copolymer (NMA–VCZ) and
its excimer. Concentration of copolymer (NMA–VCZ) (mol/
l), 0, 5.6×10−9; 1, 1.13×10−8; 2, 2.24×10−8; 3, 4.49×
10−8. Excitation wavelength, 260 nm.
The ratio of the fluorescence intensity of exciplex,
FE, to that of copolymer (NMA–VCZ), FM, was
plotted as a function of [DMA], and a straight line
was obtained with a good correlation (k\0.998).
From the slope of the line, an estimate of the
probability of exciplex formation, Ke [4], could be
made as 6.32×105. This suggested that the degree
of exciplex formation between copolymer (NMA–
VCZ) and DMA was stronger. Plot of F0M/FM, i.e.
the ratio of fluorescence intensity versus [DMA]
also gave a straight line which conformed to
Stern–Volmer equation F0M/FM=1+Ksv[DMA],
so it could be estimated that Ksv, the apparent
quenching coefficient, was 7.04×104 M−1 [5]. The
result suggested that the quenching effect mainly
came from the stronger charge-transfer process of
NMA with DMA, among which the exciplex was
dominant.
Fig. 3. Fluorescence spectra of copolymer (NMA–VCZ) and
exciplex. Concentration of copolymer (NMA–VCZ), 4.1×
10−9 mol/l. Concentration of DMA (mol/l), 0, 0.00; 1, 6.06×
10−6; 2, 1.21×10−5 3, 1.82×10−5; 4, 4.24×10−5
.
that the copolymer (NMA–VCZ) mainly manifests
the part of luminous behaviour of NMA containing
naphthalene ring in low concentration range. Un-
der this condition, when the concentration of
copolymer (NMA–VCZ) was fixed at 5.6×10−9
mol/l (Fig. 2, 0), the fluorescence was quenched
efficiently with the progressive increase in concen-
3.3. Formation of triple exciplex
As shown in Fig. 4, when concentration of
copolymer (NMA–VCZ) and DMA is fixed at
4.1×10−9 and 4.2×10−5 mol/l, respectively, the
emission intensity of bimolecular exciplex formed

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X. Yuan et al. / Spectrochimica Acta Part A 56 (2000) 1869–1873
by copolymer (NMA–VCZ) with DMA is max-
imised. On the basis of the maximal emission
spectrum, the fluorescence of bimolecular exciplex
is again quenched with addition of copolymer
(NMA–VCZ), meantime, a new broad structure-
less emission band appears at a relative further
long wavelength of 400 nm, and similarly all the
spectra pass an new isostilbic point of 362 nm.
These experimental results show that trimolecular
exciplex (triple exciplex), i.e. AAD is formed. The
fluorescence peak at 400 nm arose from the triple
exciplex [6,7]. As described earlier, plot of FT/FE
versus concentration of copolymer (NMA–VCZ)
is also linear and KT can be obtained as 7.55×
104.
The whole photophysical processes can be sum-
marised as follows:
A“hw Aꢀ
Aꢀ“1 A+hwM
(light-excitation)
(1)
K
(fluorescence emission of single molecule)
(2)
(3)
K
Aꢀ“2 A K
(decay of non-radiation)
Aꢀ+D“3 (AD)ꢀ
(formation of bimolecular exciplex)
(4)
K
(AD)ꢀ“4 A+D+hwE
(fluorescence emission of biomolecular exciplex)
(5)
K
(AD)ꢀ“5 A+D
(decay of non-radiation)
(6)
(7)
(8)
K
(AD)ꢀ+A“6 (AAD)ꢀ
Fig. 4. Fluorescence spectra of a triple exciplex. Concentration
of copolymer (NMA–VCZ)(mol/l), 1, 4.1×10−9; 2, 7.2×
10−8; 3, 1.41×10−7; 4, 2.11×10−7; 5, 2.80×10−7. Concen-
tration of DMA 4.24×10−5 mol/l.
(formation of triple exciplex)
(AAD)ꢀ“7 AAD+hwT
K
(fluorescence emission of triple exciplex)
(AAD)ꢀ“8 AD+A
K
(decay of non-radiation)
(9)
According to the steady-state equilibrium, the
following equation can be obtained through Eqs.
(7)–(9):
(
d(AAD)ꢀ
K6(AD)ꢀ(A)−(K7+K8)(AAD)ꢀ=0
(10)
(
dt
hence the Eq. (11) is obtained as follows:
(AAD)ꢀ
(AD)ꢀ
K6
=
(A)
(11)
K7+K8
This means that the ratio of fluorescence inten-
sity of triple exciplex to that of bimolecular exci-
plex is proportional to the concentration of
Fig. 5. Dependence of FT/FE on concentration of copolymer
(NMA–VCZ). Concentration of DMA, 4.24×10−5 mol/l.

X. Yuan et al. / Spectrochimica Acta Part A 56 (2000) 1869–1873
1873
copolymer (NMA–VCZ). The above result is plot-
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primary [7].
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Acknowledgements
This work was supported by NSF of Shanxi
Province and Science Foundation for Returned
Students Studying Abroad of Shanxi Province.
.