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Published on the web May 8, 2010
Pure Branch Effect on the Optical Properties of Novel Conjugated Derivatives
Long Yang, Fang Gao,* Jian Liu, Xiaolin Zhong, Hongru Li,* and Shengtao Zhang
College of Chemistry and Chemical Engineering, Chongqing University, Chongqing 400044, P. R. China
(
Received March 3, 2010; CL-100198; E-mail: hrli@cqu.edu.cn)
Pure cooperative branch effects on one- and two-photon
1.0
a
Red Shift
C1
C2
C3
optical properties of conjugated derivatives are presented. AM1
calculations demonstrate that the electron density distribution of
the frontier orbital and the dipole moment changes of the
derivatives are related to the number of branches. The theoretical
results show that not only the energy level of the frontier orbital
could be mediated by the number of branches, but the HOMO
LUMO gap could be regulated.
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.8
0.6
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.4
.2
0.0
2
80
320
360
400
440
Wavelength/nm
Red Shift Red Shift
1
.0
1.0
0.8
0.6
0.4
b
Owing to wide applications such as two-photon fluores-
C1
C2
C3
1
2
0.8
0.6
0.4
cence sensors, two-photon biomarkers, two-photon imaging
3
4
reagents, two-photon photodynamic therapy, and nonlinear
optical materials, development of highly fluorescent and two-
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photon absorption dyes is a significantly important topic. Many
branched compounds characterized with conjugated structures
containing various electron-donating or -accepting groups have
been reported, and one- and two-photon characteristics were
0
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.2
0.2
.0
2
50
300
350
400
450
500
550
Wavelength/nm
6
found to be related to the molecular structures. While to date,
Figure 2. Typical UVvisible absorption and fluorescence
spectroscopy of C1 to C3 in benzene. (a) Real absorption
spectroscopy and (b) normalized absorption and emission
spectroscopy.
pure branch effects on the optical properties of conjugated
derivatives have not been studied well. Such investigation is
very necessary because it could help us in the development of
novel nonlinear optical materials. This letter presents our recent
endeavors on the revealing pure branch effects on optical
properties of organic compounds. For this purpose, three
conjugated derivatives including 4-styryltriphenylamine (C1);
extinction coefficients of C1, C2, and C3 is close to the ratio of
the number of branches, namely 1:2:3. The derivatives exhibit
remarkably strong fluorescence emission in various solvents
with high fluorescent quantum yields. Figure 2b shows a gradual
red-shift for the maximal fluorescence wavelength as C1 <
C2 < C3. This indicates that the order of the extent of internal
charge transfer is C1 < C2 < C3 in the excited state. As
compared with the emission maxima of the derivatives in
benzene, they shift to longer wavelength in ethyl acetate
(EtOAc) which is ascribed to more internal charge transfer
(³, ³*) transition in the excited state in polar solvents.
Figure 3 presents TPA emission of the derivatives under
700 nm Ti:sapphire laser in benzene. Clearly, the maximal TPA
emission exhibits gradual red-shift in the order of C1 ¼
C2 ¼ C3. Furthermore, it shows that the maximal TPA
emission of C2 exhibits red-shift with the increasing polarity
of the solvents (from benzene to EtOAc). TPA cross sections of
4,4¤-distyryltriphenylamine (C2); 4,4¤,4¤¤-tristyryltriphenylamine
(
C3) (Figure 1) are developed. Obviously, no electron-donating
or -accepting groups are located in the branches, thus the
substituent effects could be eliminated.
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C1, C2, and C3 were synthesized via routine routes. C2
and C3 are first reported in this letter. Figure 2a shows that the
UVvis absorption spectroscopy of the derivatives displays
gradual red-shift in the order of C1 < C2 < C3. This implies
that the extent of internal charge transfer is related to the number
of branches in the ground state. Interesting, the optical density of
the derivatives increases with the number of branch. C1, C2, and
C3 are characterized with D³, D(³)2, D(³)3 respectively,
which results in a cooperative effect on the optical properties of
the derivatives. The maximal absorption wavelength of C3 is
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nm longer than that of C2, and 27 nm longer than that of C1 in
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benzene (Table 1). It also suggests the ratio of the molar
the derivatives were determined by fluorescence. The order of
the maximal TPA emission wavelength and TPA cross section of
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the derivatives are the same order as C3 > C2 > C1, clearly
reflecting pure branch effects on two-photon optical properties.
Herein, C2 was used as an example to check the relationship
between TPA emission intensity and the pumped powers, which
follows well the square law in the excitation laser wavelength at
N
N
N
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00 nm. The slope of 1.916 confirms that the derivative has
C1
C2
C3
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excellent two-photon absorption. We shall point out that the
TPA optical parameters of C1 measured in this letter are very
close to reported data. We further determined TPA cross
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Figure 1. Chemical structures of C1, C2, and C3.
Chem. Lett. 2010, 39, 582583
© 2010 The Chemical Society of Japan