Characterization and optical properties of novel unsymmetrical stilbene-based 1,3,4-oxadiazole derivatives

Article abstract of DOI:10.1016/j.saa.2012.04.013

A series of novel unsymmetrical stilbene-based 1,3,4-oxadiazole derivatives were efficiently synthesized by a four-pots reaction sequence. All of the title compounds were characterized by MS, ¹H-NMR and elemental analysis. UV-Vis absorption and fluorescence emission spectra of the compounds were investigated in dilute chloroform solution; the compounds exhibit strong blue-green fluorescence ranged from 397 to 499 nm and show potential for application in organic optical materials.

Full text of DOI:10.1016/j.saa.2012.04.013

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 95 (2012) 184–187
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Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
Characterization and optical properties of novel unsymmetrical stilbene-based
1,3,4-oxadiazole derivatives
a
b
b
Dao-Hang He a, , Xin-Wei Li , Chong Yang , Jun Yang
⇑
a School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou 510640, China
b Testing Center, South China Agricultural University, Guangzhou 510642, China
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of novel unsymmetrical stilbene-based 1,3,4-oxadiazole derivatives were efficiently synthesized
by a four-pots reaction sequence. All of the title compounds were characterized by MS, 1H-NMR and
elemental analysis. UV–Vis absorption and fluorescence emission spectra of the compounds were
investigated in dilute chloroform solution; the compounds exhibit strong blue-green fluorescence ranged
from 397 to 499 nm and show potential for application in organic optical materials.
Ó 2012 Elsevier B.V. All rights reserved.
Received 22 February 2012
Received in revised form 12 March 2012
Accepted 3 April 2012
Available online 23 April 2012
Keywords:
Synthesis
1,3,4-Oxadiazole
Stilbene
Fluorescence
Introduction
capability, durability and thermal stability, especially high fluores-
cent quantum yields, it is envisioned that the introduction of the
Stilbene derivatives are well known not only for their consider-
able biological and medical activities, such as anti-microbial and
insecticidal effects, vasodilation action, and insect baculovirus
synergists [1–5], but also for their interesting features of the
stilbene derivatives which are their effects on light and witness
applications in main chain liquid crystalline polymers [6–8], optical
brighteners [9–11] and PPV-type electroluminescent copolymer
[12]. Moreover, stilbene derivatives show distinctive blue fluores-
cence emission properties to be used in organic electroluminescent
(EL) materials [13–15].
1,3,4-oxadiazole unit into the stilbene skeleton can improve their
photo-electric properties, durability, and thermal stability
[27–28]. In this paper, we report the synthesis of six novel 1,3,4-
oxadiazole-based stilbene derivatives via the Wittig-Horner
reaction with good overall yield. All of the title compounds were
characterized on the basis of MS, 1H-NMR and elemental analysis.
The photophysical processes of the title compounds are investi-
gated by UV–Vis absorption and fluorescence emission spectra in
chloroform solution. The synthetic route is outlined in Scheme 1
(see the Supplementary material).
1,3,4-Oxadiazoles are a class of heterocycles with a broad
spectrum of biological activities, which have attracted significant
interest in medicinal and pesticide chemistry [16–20], and the
development of materials for organic electroluminescent (EL)
devices since these compounds possess high electron-accepting
properties and display strong fluorescence with high quantum yield
[21–25].
Experimental
Chemical and instrument
Melting points were determined using RY-1 melting point
apparatus and were uncorrected. 1H-NMR spectra were recorded
in CDCl3 on a Bruker AVANCE-400 MHz NMR spectrometer using
TMS as an internal standard. Mass spectra were obtained with a
HPLC/MS LCQDECA spectrometer (APCI). Elemental analyses were
performed on a Vario EL I CHN elemental analyzer. UV–Vis absorp-
tion spectra were recorded on a Hitachi UV-3010 spectrophotome-
ter. Fluorescence spectra were obtained on a Hitachi F-4500
spectrophotometer at room temperature. The purity of the com-
pounds was confirmed by TLC on silica gel ‘G’-coated glass plates.
Recently, there is great interest to increase the structural or
spatial dimensions of
p-conjugated molecules in order to tune
and acquire more favorable physical properties [26]. Based on
the fact that the 1,3,4-oxadiazole unit has good hole-transporting
⇑
Corresponding author. Tel./fax: +86 20 87110234.
E-mail address: daohanghe@yahoo.com.cn (D.-H. He).
1386-1425/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.saa.2012.04.013

D.-H. He et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 95 (2012) 184–187
185
Scheme 1. The synthetic route of the title compounds.
Synthesis
stilbene derivatives. The mass spectra of compounds 4a–f show
the intense molecular ion peaks and the characteristic ion peaks.
These molecular ion peaks were consistent with their molecular
formula.
Typical procedure for the synthesis of compounds 1, 2 and 3
Intermediates of compounds 1, 2 and 3 were prepared by the
reported methods [29].
Optical properties
Typical procedure for the synthesis of compounds 4
To a stirred solution of aromatic aldehydes (1.7 mmol) and the
intermediate 3 (0.9 g, 1.7 mmol) in anhydrous N,N-dimethylform-
amide (15 mL) under nitrogen atmosphere was added dropwise a
solution of t-BuOK (2 g, 3%) in ethanol. The reaction proceeded at
room temperature overnight. Then the resulting mixture was fil-
tered and washed with ethanol. The residue was recrystallized
from ethanol/DMSO, giving compounds 4a–g. Analytical and spec-
tral data were obtained from all compounds (see the Supplemen-
tary material).
Table 1 shows the UV–Vis absorption and fluorescence emission
spectra in dilutechloroform solution. The UV–Visabsorptionspectra
and fluorescence spectra of the compounds are shown in Fig. 1 and
Fig. 2 (in 10ꢀ5 mol Lꢀ1 chloroform solution). The absorption spectra
of compounds 4a–h show maximum absorption (kmax) at 309–
343 nm. The maximum absorption peaks (kabs), which can be attrib-
uted to the charge-transfer type
p–p
ꢁ electronic transitions, situate
in 311–342 nm in solution. The charge-transfer absorption band of
the compounds strongly depends on the conjugation length of
molecular structure. Compared with the fluorescence characteristic
emission wavelengths of the title compounds, the maximum fluo-
rescence emission wavelength of compounds 4e and 4h had a great-
er shift toward higher wavelengths than the other compounds
owing to increasing the conjugation length of the molecule, which
would make the electron-pair in the highest occupied molecular or-
bit possess a lower energy; the electron-pair could be excited easily
to transit into a higher orbit. Compounds 4a, 4b, 4c and 4f had the
different maximum emission wavelength peaks, indicating that
the halogen substituents have a great effect on their electronic en-
ergy levels. The fluorescence quantum yields Ux ¼ ðAs ꢂ Fx ꢂ n2xꢂ
UsÞ=ðAx ꢂ Fs ꢂ n2s Þ where A is the absorbance at the excitation wave-
length, F the area under the fluorescence curve and n the refraction
index. Subscripts s and x refer to the standard and to the sample of
unknown quantum yield, respectively. Rhodamine B in ethanol
Results and discussion
The synthesis and characterization
A series of novel unsymmetrical stilbene-based 1,3,4-oxadiazole
derivatives were synthesized by a four-pots reaction of the oxidative
cyclization, bromination, esterification, and Wittig-Horner reaction.
The title compoundswere characterized with spectral and analytical
methods. 1,3,4-Oxadiazole is the electron-accepting five-membered
hetero-aromatic unit containing two imine –C@N– groups, because
of the electron-withdrawing ability of the imine –C@N– group. The
1H-NMR spectra of compounds 4a–h show that the proton chemical
shifts of phenyl ring, naphthalene nucleus and anthracene nucleus
ranged from d 7.31 to d 8.32 ppm, d 7.50 to d 8.23 ppm and d 7.45
to d 8.45, respectively. A doublet of CH@CH was obtained between
d 7.15 and d 8.10 ppm. According to the chemical shifts and coupling
constant (16.0–16.6 Hz), it was determined that two protons had
different chemical environments because of the effect of substitu-
ents belonging to the trans-structure, which could stably exist in
(U = 0.89) was taken as the standard [30]. It can be seen that com-
pounds 4d and 4g had a greater fluorescence quantum yield, which
can be explained by the conjugative effect. The molecular structure
containing naphthalene nucleus can increase the conjugation length
and rigid structure of the molecule. As can be seen in Table 1,

186
D.-H. He et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 95 (2012) 184–187
Table 1
Absorption and fluorescence characteristics of the title compounds in dilute chloroform solution at room temperature (concentration: 1 ꢂ 10ꢀ5 mol Lꢀ1).
a
Compound
kmax (nm)
ꢂ10Eꢀ4
e
max(L molꢀ1 cmꢀ1
)
kex (nm)
kem (nm)
Stokes shifts (nm)
Ux
4a
4b
4c
4d
4e
4f
343
333
333
342
314
342
342
309
6.40
5.74
4.53
3.21
3.15
5.53
3.09
3.68
344
346
348
358
317
350
364
322
397
398
402
436
498
403
438
499
53
54
54
78
181
53
0.82
0.78
0.67
0.74
0.23
0.69
0.73
0.21
4g
4h
74
177
a
Fluorescence quantum yield.
Fig. 1. The UV–Vis absorption spectra of target compounds (1 ꢂ 10ꢀ5 mol Lꢀ1 in
Fig. 3. The fluorescence emission spectra of the compounds 4a and 4b in
chloroform, THF and DMF at room temperature (concentration: 1.0 ꢂ 10ꢀ5 mol Lꢀ1).
chloroform).
Fig. 4. Fluorescence spectra of 4a and 4b at different concentration of THF;
1,1.0 ꢂ 10ꢀ4 mol Lꢀ1; 2, 1.0 ꢂ 10ꢀ5 mol Lꢀ1; 3, 1.0 ꢂ 10ꢀ6 mol Lꢀ1
.
Fig. 2. The fluorescence emission spectra of target compounds (1 ꢂ 10ꢀ5 mol Lꢀ1 in
chloroform).
could see that the emission wavelengths and fluorescence intensity
of the two compounds were different in three solutions at the same
concentration. All these indicated that the solution molecules had
strong coordination effect and the environment played an important
role in determining the fluorescence intensity of the compounds.
The influence of concentration on fluorescence intensity of the two
compounds was also studied (Fig. 4). It can be seen that the emission
intensity of fluorescence was greatly decreased with gradual
increasing in concentration of 4a and 4b in THF.
compounds 4e and 4h had a smaller fluorescence quantum yield,
which maybe attributed to the difference in the steric hindrance
imposed by the 1,3,4-oxadiazole, anthracene skeleton,
p-bridges,
linkages and dihedral angles between two units that seem to affect
the planarity of the molecules. The influence of solution on the fluo-
rescence intensities of the compounds 4a and 4b was investigated.
Fig. 3 showed fluorescence emission spectra of the two compounds
in chloroform, THF and DMF (concentration: 1 ꢂ 10ꢀ5 mol Lꢀ1). We

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187
[4] G.R. Pettit, M.D. Minardi, H.J. Rosenberg, E. Hamel, M.C. Bibby, S.W. Martin,
M.K. Jung, R.K. Pettit, T.J. Cuthbertson, J.C. Chapuis, J. Nat. Prod. 68 (2005)
1450–1458.
[5] J.R. Ioset, A. Marston, M.P. Gupta, K. Hostettmann, J. Nat. Prod. 64 (2001)
710–715.
[6] E. Wolarz, T. Fischer, J. Stumpe, Thin Solid Films 424 (2003) 179–185.
[7] C.H. Wan, J.F. Kuo, C.Y. Chen, Liq. Cryst. 27 (2000) 523–532.
[8] A. Ravikrishnan, P. Sudhakara, P. Kannan, J. Mater. Sci. 45 (2010) 435–442.
[9] S.I. Um, Y.H. Kang, J.K. Lee, Dyes Pigment. 75 (2007) 681–686.
[10] J.K. Lee, S.I. Um, Y. Kang, D.J. Baek, Dyes Pigment. 64 (2005) 25–30.
[11] D.H. He, Y.C. Zhu, Z.R. Yang, A.X. Hu, Unit, J. Chin. Chem. Soc. 56 (2009)
268–270.
[12] A.M. Machado, J.D.D. Neto, R.F. Cossiello, T.D.Z. Atvars, L. Ding, F.E. Karasz, L.
Akcelrud, Polymer 46 (2005) 2452–2460.
[13] M. Strukelj, R.H. Jordan, A. Dodabalapur, J. Am. Chem. Soc. 118 (1996)
1213–1214.
[14] J.Y. Li, D. Liu, Y.Q. Li, C.S. Lee, H.L. Kwong, S.T. Lee, Chem. Mater. 17 (2005)
1208–1212.
[15] Y.C. Zhu, D.H. He, Z.R. Yang, Spectrochim Acta A Mol. Biomol. Spectrosc. 72
(2009) 417–420.
Conclusion
A series of novel unsymmetrical stilbene-based 1,3,4-oxadiazole
derivatives were synthesized, characterized by spectral studies and
their fluorescence spectra were recorded in order to study the
changes in the photophysical properties with structural modifica-
tions. The results showed that the compounds exhibited strong blue
fluorescence emission. It can be concluded that the absorption and
fluorescence characteristic of the new target compounds show a
significant dependence on their molecular structure. The influence
of solution on the fluorescence intensities of the compounds
indicated that the solutions and the environment played an impor-
tant role in determining the fluorescence intensity of the
compounds.
[16] H. Rajak, P. Singour, M.D. Kharya, P. Mishra, Chem. Biol. Drug Des. 77 (2011)
152–158.
Acknowledgements
[17] Z.K. Abd el-Samii, J. Chem. Technol. Biotechnol. 53 (1992) 143–146.
[18] A.O. Maslat, M. Abussaud, H. Tashtoush, M. Al-Talib, Pol. J. Pharmacol. 54
(2001) 55–59.
[19] S. Cao, X.H. Qian, G.H. Song, Q.C. Huang, J. Fluoresc. Chem. 117 (2002) 63–66.
[20] R. Franski, Asian J. Chem. 17 (2005) 2063–2075.
[21] G. Hughes, M.R. Bryce, J. Mater. Chem. 15 (2005) 94–107.
[22] Y. Shirota, H. Kageyama, Chem. Rev. 107 (2007) 953–1010.
[23] Y.T. Tao, Q. Wang, Y. Shang, C.L. Yang, L. Ao, J.G. Qin, D.G. Ma, Z.G. Shuai, Chem.
Commun. (2009) 77–79.
The authors thank the Fundamental Research Funds for the
Central Universities (No. 2012ZM0035) and Guangdong Provincial
Natural Science Foundation of China (No. 04300531) for the finan-
cial assistance.
Appendix A. Supplementary material
[24] Y.T. Tao, Q. Wang, C.L. Yang, Z.Q. Zhang, T.T. Zou, J.G. Qin, D.G. Ma, Angew.
Chem. Int. Ed. 47 (2008) 8104–8107.
[25] A.P. Kulkarni, C.J. Tonzola, A. Babel, S.A. Jenekhe, Chem. Mater. 16 (2004)
4556–4573.
[26] F. Yang, X.L. Zhang, K. Sun, M.J. Xiong, P.F. Xia, Z.J. Cao, Z.H. Li, Synth. Met. 158
(2008) 988–992.
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.saa.2012.04.013.
[27] J.X. Zhang, Y.P. Cui, M.L. Wang, C.X. Xu, Y. Zhong, J.Z. Liu, Chem. Lett. (2001)
824–825.
References
[28] T.L. Zhang, H.H. Li, Z.L. Yuan, Chin. J. Org. Chem. 25 (2005) 997–1000.
[29] X. Li, D. He, New 1,3,4-oxadiazole-containing tolane derivative used in
resisting growth of insects, particularly Lepidoptera insects including beet
armyworm and cabbage looper, CN Patent 10058109 (2011-03-11).
[30] X. Yang, Z.T. Pan, Y. Ma, J. Anal. Sci. 19 (2003) 588–599.
[1] V. Cardile, L. Lombardo, C. Spatafora, C. Tringali, Bioorg. Chem. 33 (2005)
22–33.
[2] R. Argauer, M. Shapiro, J. Econ. Entomol. 90 (1997) 416–420.
[3] M. Shapiro, R. Argauer, J. Econ. Entomol. 90 (1997) 899–904.