Photophysical processes of a fluorene derivative containing 1,3,4-oxadiazole

Article abstract of DOI:10.1016/j.saa.2004.02.019

The photophysical processes of 9,9-bis{4′-[2″-phenyl-5″- (3″′-(methacryl-amido)phenyl)]-1″,3″, 4″-oxadiazolylphenyl}fluorene (F-MAOP) formed by Heck reaction of 9,9-bis(4′-iodophenyl)fluorene (F-IP) and 2-phenyl-5-[3′- (methacrylamido)phenyl]-1,3,4-oxadiazole (OXD) have been carefully studied. The results show that the compound emits blue and blue-violet light and the emission spectra exhibit obvious solvent effect. With the increase of polarity of solvents, the fluorescence spectra change obviously and appear blue shift at room temperature. In addition, the light-emitting can be quenched by both electron donor (N,N-dimethylaniline, DMA) and electron acceptor (C ₆₀). When N,N-dimethylaniline is gradually added into the solution of F-MAOP, the emission intensities of fluorescence are unusually increased. But when the concentration of DMA beyond a certain scope, the emission intensities of fluorescence are gradually decreased. The dimolecular exciplex between F-MAOP and C₆₀ are formed and the quenching effect follows the Stern-Volmer equation. Moreover, interaction between F-MAOP and carbon nanotubes (CNTs) is also studied by fluorescent quenching.

Full text of DOI:10.1016/j.saa.2004.02.019

Spectrochimica Acta Part A 60 (2004) 3029–3032
Photophysical processes of a fluorene derivative
containing 1,3,4-oxadiazole
a
a,∗
b
Liheng Feng , Zhaobin Chen , Fenglian Bai
a
College of Chemistry and Chemical Engineering, University of Shanxi, Taiyuan 030006, PR China
b
Institute of Chemistry, Chinese Academy of Sciences, Beijing 100080, PR China
Received 30 December 2003; accepted 9 February 2004
Abstract
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ꢀꢀ
ꢀꢀ
ꢀꢀꢀ
ꢀꢀ ꢀꢀ ꢀꢀ
The photophysical processes of 9,9-bis{4 -[2 -phenyl-5 -(3 -(methacryl-amido)phenyl)]-1 ,3 ,4 -oxadiazolylphenyl}fluorene (F-MAOP)
ꢀ
ꢀ
formed by Heck reaction of 9,9-bis(4 -iodophenyl)fluorene (F-IP) and 2-phenyl-5-[3 -(methacrylamido)phenyl]-1,3,4-oxadiazole (OXD) have
been carefully studied. The results show that the compound emits blue and blue–violet light and the emission spectra exhibit obvious solvent
effect. With the increase of polarity of solvents, the fluorescence spectra change obviously and appear blue shift at room temperature.
In addition, the light-emitting can be quenched by both electron donor (N,N-dimethylaniline, DMA) and electron acceptor (C60). When
N,N-dimethylaniline is gradually added into the solution of F-MAOP, the emission intensities of fluorescence are unusually increased. But
when the concentration of DMA beyond a certain scope, the emission intensities of fluorescence are gradually decreased. The dimolecular
exciplex between F-MAOP and C60 are formed and the quenching effect follows the Stern–Volmer equation. Moreover, interaction between
F-MAOP and carbon nanotubes (CNTs) is also studied by fluorescent quenching.
©
2004 Elsevier B.V. All rights reserved.
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ꢀ
Keywords: 2-Phenyl-5-[3 -(methacrylamido)phenyl]-1,3,4-oxadiazole; 9,9-Bis(4 -aminophenyl)fluorene; Exciplex; Fullerene (C60); Carbon nanotubes
(CNTs)
1
. Introduction
should facilitate electron injection. To combine 1,3,4-
oxadiazole ring into the structure of light-emitting molecule
and apply it to LEDs will improve the performance
In recent years, organic light-emitting diodes (LEDs) have
ꢀ
intrigued many researchers owing to their potential for appli-
cations in display technology. The use of organic photoemit-
ter has also received a great deal of attention in academia and
optoelectronic industry because of their excellent properties
and easy processability. While the light-emitting of organic
small molecular system reveals numerous advantages, there
are still several critical areas that need to be improved for
using in highly efficient light-emitting devices [1,2].
It is well known that to achieve good performance in
LEDs the injection of electrons and transport of holes
should be balanced. Therefore, it is desirable to prepare
some emissive materials with electron affinity or hole trans-
port ability. The electron-withdrawing 1,3,4-oxadiazole
[
3]. Several 1,3,4-oxadiazole derivatives, such as 2-(4 -
ꢀ
biphenylyl)-5-(4 -tert-butylphenyl)-1,3,4-oxadiazole (PBD)
have been actually used as electron-injection materials to
improve the balance of charge carrier and to increase the
photon/electron quantum efficiency [4–7]. In the paper,
ꢀ
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we have synthesised a novel compound, 9,9-bis{4 -[2 -
ꢀ
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ꢀꢀ ꢀꢀ ꢀꢀ
phenyl-5 -(3 -(methacrylamido)phenyl)]-1 ,3 ,4 -oxadia-
zolylphenyl}fluorene (F-MAOP) from 9,9-bis(4 -iodophe-
nyl)fluorene and 2-phenyl-5-[3 -(methacrylamido)phenyl]-
ꢀ
ꢀ
1,3,4-oxadiazole by Heck reaction. The photophysical
properties are investigated. The photoluminescence (PL)
of F-MAOP are examined in different solvents [8]. More-
over, the molecular interactions of F-MAOP with N,N-
(OXD) unit is believed to have high electron affinity and
dimethylaniline (DMA), fullerene (C ) and carbon nan-
60
otubes (CNTs) are investigated [9,10], in which the for-
mations of exciplex are especially investigated because the
exciplexs are often a middle state of photochemical and
photophysical primary processes.
∗
Corresponding author. Tel.: +86-351-7011492;
fax: +86-351-7011688.
E-mail address: zchen@sxu.edu.cn (Z. Chen).
1
386-1425/$ – see front matter © 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.saa.2004.02.019

3
030
L. Feng et al. / Spectrochimica Acta Part A 60 (2004) 3029–3032
H2N
NH2
I
I
NaNO2/HCl
KI
2
1
CH3
C
I
I
H
N
H C
C
O
N N
2
Pd(OAc)2, P(Ph)3
Heck reaction
+
O
3
2
O
O
C
O
O
H
H
C
C
C
N
N
N
N
C
C
N
N
H
H
CH3
CH3
4
(
F-MAOP)
Scheme 1. The chemical structures of F-MAOP in the study.
2
. Experimental
.1. Materials and measurements
Dichloromethane, THF, ethyl acetate, acetonitrile,
and 2, respectively. It can be seen from Fig. 1 that the max-
imum absorption and emission peaks of F-IP are at 310 and
330 nm, respectively. As shown in Fig. 2 the maximum ab-
sorption and emission peaks of F-MAOP are at 302 and
365 nm, respectively. By comparison with compound F-IP,
the emission spectra of F-MAOP show red-shifted and the
absorption spectra are blue-shifted. These phenomena can
be attributed to their different configuration, greater ␲ sys-
tem conjugation length in F-MAOP than that in F-IP.
2
ethanol, methanol and DMA are analytical grade reagents
and treated according to standard method used before,
fullerene and carbon nanotubes are obtained from Term
Co. in a purify of 99 and 90%, respectively, which are all
applied to measurement of the light-emitting properties.
The measurements of fluorescence are performed on
a Perkin-Elmer LS-50B fluorometer. Both excitation and
emission bands are set at 5 nm on these experimental con-
3.2. Solvent effects on photoluminescence
The F-MAOP has good solubility in common organic
solvents, such as dichloromethane, THF, ethyl acetate,
ethanol, acetonitrile, methanol, etc. The emission spectra
of F-MAOP are investigated in different solvents and all
◦
ditions. All the experiments are carried out at 25 ± 1 C.
2
.2. Synthesis
ꢀ
ꢀ
ꢀꢀ
9
,9-Bis(4 -iodophenyl)fluorene (F-IP) and 9,9-bis{4 -[2 -
ꢀ
ꢀ
ꢀꢀꢀ
ꢀꢀ ꢀꢀ ꢀꢀ
phenyl-5 -(3 -(methacryl-amido)phenyl)]-1 ,3 ,4 -oxadi-
azolylphenyl}fluorene are prepared by our research group
according to the route shown in Scheme 1 [11]. The yield
of F-IP is 81.5%, m.p. 183–184 C; the yield of F-MAOP
is 14.3%, m.p. 155–156 C.
a'
a
120
◦
90
◦
A
6
3
0
0
2
3
. Results and discussion
50
300
350
400
3
.1. Absorption and fluorescence emission spectra
The UV-Vis absorption and fluorescence emission spectra
/nm
Fig. 1. Absorption and emission spectra of F-IP in ethyl acetate. Con-
centration of F-IP, 6.23 × 10 M.
−
5
of F-IP and F-MAOP in ethyl acetate are shown in Figs. 1

L. Feng et al. / Spectrochimica Acta Part A 60 (2004) 3029–3032
3031
6
5
4
3
2
1
00
00
00
00
00
00
1000
b’
b
2
1
3
800
600
0
A
F
7
4
00
00
0
2
2
50
300
350
400
450
3
20
360
400
440
480
/
nm
/
nm
Fig. 2. Absorption and emission spectra of F-MAOP in ethyl acetate.
Fig. 4. Fluorescence spectra of F-MAOP at different concentration of
Concentration of F-MAOP, 4.33 × 10− M.
7
−7
DMA. Concentration of F-MAOP, 2.16×10 M. Concentration of DMA
−
6
−6
−5
;
(
(
mol/l, M), (0) 0.00; (1) 1.35 × 10 ; (2) 2.69 × 10 ; (3) 5.37 × 10
−
5
−4
−4
−4
.
4) 8.64 × 10 ; (5) 1.21 × 10 ; (6) 2.22 × 10 ; (7) 7.92 × 10
results are showed in Fig. 3. It can be seen from Fig. 3
that the emission spectra of F-MAOP is slightly blue shift
with the increase of the solvents polarity. From the exper-
imental results we suppose that this phenomenon may due
to its structure containing carbonyl units and heterocyclic
of F-MAOP. Fig. 4 gives the fluorescence spectra of solu-
tion of F-MAOP in the presence of DMA. ‘0’ is F-MAOP in
−
7
−1
concentration 2.16×10 mol l (M) without DMA, ‘1–7’
are F-MAOP in the presence in different concentration of
DMA. It can be seen that the emission intensity of fluores-
cence are initially increased and then decreased with gradual
increasing in concentration of DMA. Meanwhile, the emis-
sion perks show blue-shift and the peak breadths are nar-
rowed. Based on the experimental facts and references, the
following explanations can be given: 1,3,4-oxadiazole ring
is an electron deficient group and has big electron affinity
while N,N-dimethylaniline is a good electron donor, so the
strong interaction between OXD and DMA happens when
DMA is gradually added into the solution of F-MAOP. The
DMA molecule is associated by OXD unit and the charge of
DMA may then be transferred to OXD, which would lead to
increasing of fluorescence intensity of fluorene unit that are
released from the actions of intramolecular or intermolecu-
lar OXD units. When the concentration of DMA goes be-
yond a certain scope, the excess DMA units would act as
quencher to quench gradually the fluorescence of F-MAOP.
∗
systems, which makes the (n–␲ ) excited state plays more
∗
important role in the F-MAOP than the (␲–␲ ) state. The
inspected emission perk of F-MAOP in solution originates
from the transition of unshared electron-pair on nitrogen
∗
atom of 1,3,4-oxadiazole to the ␲ orbital of carbonyl
group. It is well known that the solvolytic actions between
unshared electron-pair and polar solvent are obviously hap-
pened, which will reduce the energy of the n orbital. Thus,
more energy will be needed for this transition of electron
∗
from the n orbital to the ␲ orbital in a solvent with large
polarity, as compared with the transition in a solvent with
small polarity, which means that the energy gap between
∗
n and ␲ orbital increase with the increasing of solvent
polarity. So the emission spectra of F-MAOP is blue shift
[8]. Further research toward a better understanding of this
effect is currently in progress.
3
.3. Interaction of F-MAOP with N,N-dimethylaniline
3
.4. Interaction between F-MAOP and C60 and formation
The studies of interactions between F-MAOP and DMA
are helpful to understanding the photo–electronic properties
of bimolecular exciplex
Many investigations show that C60 is a good electron ac-
ceptor. It possesses a wide range of physical and chemical
properties. One of the most remarkable properties of C60 is
to induce efficiently a rapid charge separation and a further
slow charge recombination [9]. In the experiment, the inter-
actions of F-MAOP with C60 are examined. The results are
shown in Fig. 5. As we have seen in Fig. 5, on progressive
1
4
1
. acetonitrile
7
6
50
00
2. dichloromethane
3
. THF
4. ethyl acetate
F450
3
1
00
50
increase in concentration of C , the simple molecule flu-
60
orescence of F-MAOP is quenched efficiently. At the same
time, a new emission band appears at a relatively longer
wavelength 487 nm. The emission intensity of the new band
increase with an increase in the concentration of F-MAOP
and the spectra all pass an isostilbic point 432 nm. It indicates
a bimolecular exciplex is formed between F-MAOP and
0
3
00
350
400
450
/
nm
Fig. 3. Emission spectra of F-MAOP solution in different solvents.

3
032
L. Feng et al. / Spectrochimica Acta Part A 60 (2004) 3029–3032
1
000
0
1
6
400
0
5
8
00
3
00
00
00
0
F 600
F
2
4
2
00
00
0
1
3
50
400
450
500
550
350
400
450
500
/
nm
/
nm
Fig. 5. Fluorescence spectra of F-MAOP at different concentration of C60.
Fig. 7. Fluorescence spectra of F-MAOP at different concentration of
−
7
Concentration of F-MAOP, 5.05 × 10 M. Concentration of C60 (mol/l,
−7
CNTs. Concentration of F-MAOP, 1.05×10 M Concentration of CNTs
−
−6
7
−6
−6
M), (0) 0.00; (1) 9.26 × 10 ; (2) 1.39 × 10 ; (3) 1.85 × 10 ; (4)
−7
−7
−6
(
3
mg/ml), (0) 0.00; (1) 4.22 × 10 ; (2) 8.44 × 10 ; (3) 9.27 × 10 ; (4)
−
6
−6
2
.32 × 10 ; (5) 3.71 × 10 ; (6) 5.56 × 10
.6
.
−5
−5
.
.75 × 10 ; (5) 5.83 × 10
1
demonstrates the strong interacting between F-MAOP and
CNTs happens in the excited state. The strong interaction
may be caused by the photoinduced charge transfer and in-
teractions of conjugating ␲–␲ system.
1
.4
0
F
M
F
M
1.2
Acknowledgements
1
.0
The present work is supported by the Natural Science
Foundation of China (30000112); Laboratory of Organic
Solid, Chinese Academy of Sciences and The Returned Stu-
dent Science Foundation of Shanxi Province of China.
0
2
4
6
-6
Concentration (×10 )
0
M
Fig. 6. Dependence of F /FM on concentration of C60. Concentration
of F-MAOP, 5.05 × 10− M.
7
C . The quenching process is conformed to Stern–Volmer
References
6
0
0
equation, F /FM = 1 + KSV[Q], and shown in Fig. 6.
M
The apparent quenching coefficient, Ksv, of C is about
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3
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