Synthesis and optical properties of two novel stilbene derivatives containing 1,3,4-oxadiazole moiety

Article abstract of DOI:10.1016/j.saa.2008.10.011

Two novel stilbene derivatives containing 1,3,4-oxadiazole moiety were synthesized and characterized by elemental analyses, ¹H NMR, MS. The photophysical processes of the title compounds were investigated by UV-vis absorption and fluorescence e

Full text of DOI:10.1016/j.saa.2008.10.011

Spectrochimica Acta Part A 72 (2009) 417–420
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
Synthesis and optical properties of two novel stilbene derivatives containing
1,3,4-oxadiazole moiety
Yong-Chuang Zhu, Dao-Hang He∗, Zhuo-Ru Yang
School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou 510640, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 28 May 2008
Received in revised form 16 August 2008
Accepted 9 October 2008
Two novel stilbene derivatives containing 1,3,4-oxadiazole moiety were synthesized and characterized by
elemental analyses, 1H NMR, MS. The photophysical processes of the title compounds were investigated
by UV–vis absorption and fluorescence emission spectra in different solutions. The fluorescence quantum
yield (˚) of 1a and 1b in THF is 0.65 and 0.69, respectively. The influence of the solution on the fluorescence
intensities was also discussed. Under ultraviolet light excitation, the two compounds exhibit strong blue
fluorescence emission. They may serve as potential applications in organic electroluminescent materials.
Crown Copyright © 2008 Published by Elsevier B.V. All rights reserved.
Keywords:
Stilbene
1,3,4-Oxadiazole moiety
Synthesis
Fluorescence properties
1. Introduction
thermal stability, especially high fluorescence quantum yields, have
actually been used as electron-injection materials to improve the
It is well known that stilbene derivatives have considerable
biological and medical activities, such as anti-microbial and insecti-
cidal effects, vasodilation action, and insect baculovirus synergists
[1–5]. In addition, interesting features of the stilbene derivatives are
their effects on light and witness applications in main chain liquid
crystalline polymers [6,7], PPV-type electroluminescent copoly-
mer [8], optical brighteeners and photochemically crosslinkable
polymers. The stilbene skeleton could be an excellent choice for a
central chromophore with which to construct new photoresponsive
materials [9]. Moreover, stilbene derivatives show distinctive blue
fluorescence emission properties to be used in organic electrolu-
minescent (EL) materials [10,11]. However, there still remain many
important and fundamental challenges. A blue light-emitting mate-
rial with high efficiency deep blue color, better thermal stability and
long operational lifetime is till in demand, especially the emission of
the light at the blue end of the visible spectrum, which is crucial for
many applications [12]. To obtain stable EL devices and high fluo-
rescence quantum yield, a well-balanced injection of positive (hole)
and negative (electron) charge carrier into an emitting layer is con-
sidered to be prerequisite for high luminous efficiency. Recently
considerable research efforts have been carried out to enhance per-
formance suitable for practical use [13,14]. Many 1,3,4-oxadiazole
derivatives with good hole-transporting capabilities, durability and
balance of charge carrier and to increase the proton/electron quan-
tum efficiency [15,16]. Furthermore, oxadiazole rings can increase
the degree of conjugation of the stilbene derivative. In this paper, we
report the synthesis of a four-step synthesis of stilbene derivatives
containing 1,3,4-oxadiazole moiety, which may serve as potential
intramolecular charge transfer compounds, the requisite 2,5-di-p-
tolyl-1,3,4-oxadiazoleframework is availableviathedirectcoupling
of p-toluic acid with hydrazine hydrate promoted by polyphos-
phoric acid. The photophysical processes of the title compounds
are investigated by UV–vis absorption and fluorescence emission
spectra in chloroform, THF, acetone and DMF. The results show the
two compounds exhibit strong blue fluorescence emission, which
may serve as potential application in organic electroluminescent
materials. The synthetic route is outlined in Scheme 1.
2. Experimental
2.1. Instrumentation
Melting points were determined using RY-1 melting point appa-
ratus and were uncorrected. 1H NMR spectra were recorded in
CDCl3 on a Bruker AVANCE-600 MHz NMR spectrometer using
TMS as internal standard. Mass spectra were obtained with a
HPLC/MS LCQDECA spectrometer (APCI). Elemental analyses were
performed on a Vario EL Ш CHN elemental analyzer. UV–vis
absorption spectra were recorded on a Hitachi UV-3010 spec-
trophotometer. Fluorescence spectra were obtained on a Hitachi
∗
Corresponding author. Tel.: +86 20 87110234; fax: +86 20 87110234.
E-mail address: daohanghe@yahoo.com.cn (D.-H. He).
1386-1425/$ – see front matter. Crown Copyright © 2008 Published by Elsevier B.V. All rights reserved.
doi:10.1016/j.saa.2008.10.011

418
Y.-C. Zhu et al. / Spectrochimica Acta Part A 72 (2009) 417–420
Scheme 1. The synthetic route of the title compounds.
F-4500 spectrophotometer at room temperature. The fluorescence
quantum yields ˚x = (As × Fx × n2x × ˚s)/(Ax × Fs × ns2) where A is
the absorbance at the excitation wavelength, 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 (˚ = 0.89) was taken as the
standard [17].
4.02–4.08 (m, 8H, OCH2), 7.49 (dd, J = 2.4 Hz, J = 8.4 Hz, 4H, C6H4
3,5-H), 8.09 (d, J = 8.4 Hz, 4H, C6H4 2,6-H).
2.2.4. Typical procedure for the synthesis of compounds 1
To
a stirred solution of aromatic aldehydes (3.4 mmol)
and the intermediate 2 (0.9 g, 1.7 mmol) in anhydrous N, N-
dimethylformamide (15 mL) under nitrogen atmosphere was added
dropwise a solution of t-BuOK (2 g, 3%) in ethanol. The reaction pro-
ceeded at room temperature overnight. Then the resulting mixture
was filtered and washed with ethanol. The residue was recrystal-
lized from ethanol/DMSO giving compounds 1.
2.2. Synthesis
2.2.1. Synthesis of 2,5-di-p-tolyl-1,3,4-oxadiazole (4)
p-Toluic acid (17 g, 125 mmol) and 80% hydrazine hydrate
(4 mL, 66 mmol) were added to 60 mL of stirred phosphoric acid,
respectively, the reaction proceeded at 150 ◦C for 15 h until the
disappearance of the starting material by TLC. The cool mixture
was poured into cold water, neutralized by 5% Na2CO3 and filtered,
the residue was recrystallized from CHCl3/methanol giving 9.4 g of
compound 4 as white crystals, yield 60%, m.p. 176–177 ◦C; 1H NMR
(CDCl3): ı 2.43 (s, 6H, CH3), 7.32 (d, J = 8.4 Hz, 4H, C6H4 3,5-H), 8.01
(d, J = 8.4 Hz, 4H, C6H4 2,6-H); APCI MS: m/z, 251 (M+1, 100), 252
(M+2, 18).
2,5-Bis(4-(2,4-dichlorostyryl)phenyl)-1,3,4-oxadiazole (1a),
yield 92%, m.p. 269–270 ◦C; 1H NMR (CDCl3): ı 7.11 (d, J = 16.2 Hz,
2H, CH=CH), 7.28 (d, J = 8.4 Hz, 2H, C6H3 5-H), 7.45 (s, 2H, C6H3
3-H), 7.56 (d, J = 16.2 Hz, 2H, CH=CH), 7.65 (d, J = 8.4 Hz, 2H, C6H3
6-H), 7.70 (d, J = 7.8 Hz, 4H, C6H4 3,5-H), 8.16 (d, J = 7.8 Hz, 4H, C6H4
2,6-H); APCI MS: m/z, 565 (M+1, 100), 567 (M+3, 54); Anal. calcd.
for C30H18 Cl4N2O (564.3): C, 63.85; H, 3.22; N, 4.96; Found: C,
62.85; H, 3.28; N, 5.06.
2,5-Bis(4-(3,4-dichlorostyryl)phenyl)-1,3,4-oxadiazole (1b),
yield 93%, m.p. 279–280 ◦C; 1H NMR (CDCl3): ı 7.13 (s, 4H, C6H3
5,6-H), 7.36 (d, J = 8.4 Hz, 2H, CH=CH), 7.45 (d, J = 8.4 Hz, 2H, CH=CH),
7.64 (s, 2H, C6H3 2-H), 7.66 (d, J = 8.4 Hz, 4H, C6H4 3,5-H), 8.15 (d,
4H, J = 8.4 Hz, C6H4 2,6-H); APCI MS: m/z, 565 (M+1, 100), 567 (M+3,
50); Anal. calcd. for C30H18 Cl4N2O (564.3): C, 63.85; H, 3.22; N, 4.96;
Found: C, 63.29; H, 3.25; N, 5.01.
2.2.2. Synthesis of
2,5-bis(4-(bromomethyl)phenyl)-1,3,4-oxadiazole (3)
To a stirred solution of 4 (5 g, 20 mmol) in carbon dichloride
(70 mL) was added DBDMH (4 g, 20 mmol), the reaction proceeded
with refluxing for 15 h and then the excess solvent was removed.
The resulting mixture was filtered, washed with ethanol. The
residue was recrystallized from THF/ethanol giving 6 g of com-
pound 3 as white crystals, yield 75%, m.p. 227–228 ◦C; 1H NMR
(CDCl3): ı 4.55 (s, 4H, CH2Br), 7.57 (d, J = 8.4 Hz, 4H, C6H4 3,5-H),
8.13 (d, J = 8.4 Hz, 4H, C6H4 2,6-H); APCI MS: m/z, 409 (M+1, 100).
3. Results and discussion
3.1. 1H NMR spectra
In the 1H NMR spectra of two compounds, 1a showed two fine
doublets corresponding to the olefinic protons (CH=CH) from stil-
bene at ı: 7.11 ppm and 7.56 ppm (J = 16.2 Hz). At ı: 7.28, 7.65, and
7.45 ppm, two doublets and a singlet could be assigned to 5-H, 6-
H and 3-H from aromatic protons (C6H3-protons), respectively. In
addition, another two downfield doublets were also found at ı: 7.70
and 8.16 ppm which could be assigned to the contribution of aro-
matic protons (C6H4-protons). In contrast, the compound 1b in its
1H NMR spectra showed the similar resonance peaks accountable to
the olefinic protons and aromatic protons (C6H4-protons), respec-
tively, only a singlet was found due to 5,6-H from the benzene ring
2.2.3. Synthesis of diethyl
4-(5-(4-(diethoxyphosphino)methyl)phenyl)-1,3,4-oxadiazol-2-
yl)-benzyl-phosphonate
(2)
A mixture of compound 3 (5 g, 14.7 mmol) and triethyl phos-
phite (13 mL, 76.6 mmol) was refluxed for 5 h. The excess triethyl
phosphate was evaporated under reduced pressure, then filtered by
addition of hexane. The residue was recrystallized from THF/hexane
giving 6.2 g of compound 2, yield 80%, m.p. 111–112 ◦C; 1H NMR
(CDCl3): ı 1.27 (t, J = 7.2 Hz, 12H, CH3), 3.43 (d, J = 22.2 Hz, 4H, CH2),

Y.-C. Zhu et al. / Spectrochimica Acta Part A 72 (2009) 417–420
419
Fig. 1. UV–vis absorption spectra of the compounds 1a, 1b in DMF, acetone,
Fig. 2. Fluorescence emission spectra of the compounds 1a, 1b in chloroform, THF,
acetone and DMF at room temperature (concentration: 1.0 × 10−5 mol/L).
THF compared to compound
4 in THF at room temperature (concentration:
1 × 10−5 mol/L).
emission wavelengths of the 1a was different in four solutions and
the fluorescence intensity order in different solutions (at the same
concentration) was THF (curve 2) > acetone (curve 3) > DMF (curve
6) > chloroform (curve 8). In contrast, the influence of the solution
on the fluorescence intensities of 1b was also studied. The fluo-
rescence intensity in other solutions was also lower than that in
THF (curve 1). The fluorescence intensity in chloroform was lower
than those in other solutions. All these indicated that the solutions
molecules had strong coordination effect and the environment
played an important role in determining the fluorescence intensity
of the compounds. Compared with the fluorescence characteristic
emission wavelengths in four solutions, as given in Fig. 2, the two
compounds had the similar emission wavelengths. In addition, the
shape of emission spectra was lightly affected by the intermolecu-
lar interaction. Compared to the emission spectra of compound 4
in THF (Fig. 3), a remarkable bathochromic of the emission max-
imum of 1a and 1b was observed. 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
were greatly decreased with gradual increasing in concentration of
1a and 1b in THF.
(C6H3-protons). From the 1H NMR spectra of two compounds, it was
found that their olefinic protons (CH=CH) from stilbene had differ-
ent coupling constant, which indicated the different substituents
on benzene had an effect on proton–proton coupling.
3.2. UV–vis absorption spectra
UV–vis absorption spectra of the compounds 1a, 1b in DMF,
acetone and THF were given in Fig. 1 at a concentration of
1.0 × 10−5 mol/L. The results are summarized in Table 1 compared
to compound 4 in THF. As seen in Fig. 1 and Table 1, it was also
observed that the UV–vis absorption spectra of the two compounds
did not show regular variation with the polarity of solutions. The
two compounds had practically the same UV–vis spectrum with
only one intense and relatively narrow absorption band centred
at 351–361 nm in acetone, THF and DMF, corresponding to elec-
tronic transitions of oxadiazole ring in the molecule, even if there
was a shoulder in DMF for 1a and 1b (Fig. 1, curves 3 and 5).
The maximum molar extinction coefficients of 1a and 1b in THF
are 1.043 × 105 L mol−1 cm−1 and 1.114 × 105 L mol−1 cm−1, respec-
tively. By Comparison the absorption curve 7 and the other curves
1–6, it was evident that ꢀmax of compounds 1a and 1b in THF had a
greatly shift towards higher wavelength compared to compound 4
in THF due to the increase the conjugation length of the molecule.
Table 2 shows the fluorescence spectra data of the two com-
pounds 1a and 1b in solutions and the solid state. It is easily seen
3.3. Fluorescence properties
Fig. 2 showed fluorescence emission spectra of two stilbene
derivatives in chloroform, THF, acetone and DMF (concentration:
1 × 10−5 mol/L). The influence of solution on the fluorescence inten-
sities of the 1a and 1b was investigated. We could see that the
Table 1
UV–vis absorption spectra property of 1a, 1b, and 4 in solution.
Compound
Solution
ꢀmax (nm)
×10−4 εmax (L mol−1 cm−1
)
DMF(3)
Acetone(4)
THF(2)
356
351
354
9.96
8.37
10.43
1a
DMF(5)
Acetone(6)
THF(1)
361
355
359
8.04
4.83
11.14
1b
4
Fig. 3. Fluorescence emission spectra of the compounds 1a, 1b and 4 in THF (con-
centration: 1.0 × 10−5 mol/L).
THF(7)
288
6.35

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Y.-C. Zhu et al. / Spectrochimica Acta Part A 72 (2009) 417–420
Table 2
The fluorescence spectra data of 1a, 1b, and 4.
Compound
Solution
ꢀex (nm)
ꢀem (nm)
RFIa
˚
Solid state
DMF(6)
Acetone(3)
THF(2)
389
366
360
367
362
452
3748
–
419, 400 s
414, 394 s
417, 396 s
416, 394 s
2813, 2383 s
3194, 2795 s
3409, 3097 s
2699, 2529 s
0.34
0.42
0.65
0.50
1a
CHCl3(8)
Solid state
DMF(4)
Acetone(5)
THF(1)
430
364
358
368
367
469
2139
–
418, 398 s
412, 391 s
415, 394 s
418, 397 s
2996, 2473 s
2893, 2493 s
3700, 3203 s
2762, 2681 s
0.40
0.45
0.69
0.55
1b
CHCl3(7)
4
THF
280
340, 354 s
540, 513
–
a
RFI is relative fluorescence intensity; s: shoulder.
anism. The decrease of the fluorescence quantum yield in polar
solvents was attributed to the deactivation of the excited state by
TICT, larger the solution polarity, more obvious the solution for the
non-emissive “TICT” state.
4. Conclusions
Two novel stilbene derivatives were successfully synthesized,
the structures of two compounds were also confirmed on the
basis of MS, 1H NMR, and elemental analyses. The photophysical
properties had also been carried out. The results showed the two
compounds exhibited strong blue fluorescence emission. The fluo-
rescence quantum yield (˚) of 1a and 1b in THF is the biggest. The
influence of solution on the fluorescence intensities of the com-
pounds indicated that the solutions and the environment played
an important role in determining the fluorescence intensity of the
compounds.
Acknowledgements
The authors thank Chinese Postdoctoral Science Foundation
(No.2003033425) and Guangdong Provincial Natural Science Foun-
dation of China (No.04300531) for the financial assistance.
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that the fluorescence quantum yield (˚) in THF is the biggest for 1a
and 1b, with the increase of the solution polarity, the fluorescence
quantum yield (˚) of the two stilbene derivatives decreases. This
can be explained by a twisted internal charge transfer (TICT) mech-