Synthesis, fluorescence properties and theoretical calculations of novel stilbene derivatives based on 1,3,4-oxadiazole bearing anthracene core

Article abstract of DOI:10.1007/s10895-013-1231-9

Two novel stilbene derivatives bearing anthracene core based on 1,3,4-oxadiazole were efficiently synthesized and characterized by ¹H-NMR, mass spectrometry and elemental analysis. The optical properties of the title compounds were investigated

Full text of DOI:10.1007/s10895-013-1231-9

J Fluoresc
DOI 10.1007/s10895-013-1231-9
ORIGINAL PAPER
Synthesis, Fluorescence Properties and Theoretical
Calculations of Novel Stilbene Derivatives Based
on 1,3,4-Oxadiazole Bearing Anthracene Core
Xinwei Li & Huixiong Lu & Daohang He & Chun Luo &
Jianjun Huang
Received: 23 February 2013 /Accepted: 30 April 2013
#
Springer Science+Business Media New York 2013
Abstract Two novel stilbene derivatives bearing anthra-
potentially active components for a wide range of electronic
and optoelectronic devices [1]. As an important class of five-
membered hetero cyclic compounds, 1,3,4-oxadiazoles are
well-known not only for their considerable biological and
medical activities, which include anti-inflammatory, antifun-
gal, anti-HIV, anticancer, and antimicrobial activities [2–5],
but also for their sufficient fluorescence in the visible light
range, large Stokes shifts, high quantum yields [6–8], and so
on. Another feature of the 1,3,4-oxadiazole derivatives is that
the photophysical and spectroscopic properties can be readily
modified by the introduction of substituents in the structure of
the oxadiazole, giving more flexibility to fit well in various
applications [9]. In addition, oxadiazole unit can improve elec-
tron injection and transport properties of the molecules. Thus,
many 1,3,4-oxadiazole derivatives with good hole-transporting
capabilities and high fluorescence emission, have actually been
used as electron-injection materials [10]. Nowadays, com-
pounds with blue luminescence are highly desirable because
of their wide applicability in organic materials to tune the
emission colours in the entire region of visible spectrum [11].
The fluorescent characteristics mainly depend on molecular
structure, such as conjugate system, coplanarity and rigidity.
The longer the conjugate system is, the stronger the fluorescent
intensity and greater emission wavelength will be [12].
Anthracene derivatives have rigid structure, wide energy gaps
and high fluorescent quantum efficiency [13, 14]. Moreover,
anthracene derivatives have been extensively utilized as fluores-
cence sensors for metal ions and biological molecules [15].
Great deals of compounds which contain anthracene core elec-
troluminescent materials have been developed till now.
cene core based on 1,3,4-oxadiazole were efficiently syn-
1
thesized and characterized by H-NMR, mass spectrometry
and elemental analysis. The optical properties of the title
compounds were investigated by UV–vis absorption and
fluorescence emission spectra in different solvents. Chemical
calculations were performed by density functional theory
(DFT) at the (B3LYP)/6-31G* level. The results show the
two compounds exhibit strong green fluorescence emission
ranged from 489–493 nm, and the fluorescence quantum yield
ranged from 0.78–0.92. Their HOMO and LUMO levels are
(−5.44 eV, −2.25 eV) and (−5.45 eV, −2.28 eV), respectively.
The influence of the solvent on the fluorescence intensities
was also discussed.
.
.
.
Keywords Synthesis Stilbene 1,3,4-Oxadiazole
.
.
Anthracene Fluorescence properties DFT
Introduction
Over the past few years, π-conjugated organic materials have
attracted much attention due to the increasing development of
:
:
X. Li H. Lu D. He (*)
School of Chemistry and Chemical Engineering,
South China University of Technology, Guangzhou 510640,
People’s Republic of China
e-mail: daohanghe@yahoo.com.cn
:
:
X. Li C. Luo J. Huang
In continuation of our work in organic fluorescent mate-
rials [16–18], herein, two novel stilbene derivatives bearing
Guangzhou Baiyunshan Hanfang Pharmaceutical Co.Ltd.,
Guangzhou 510970, People’s Republic of China

J Fluoresc
anthracene core based on 1,3,4-oxadiazole were synthesized
by introducing anthracene core and 1,3,4-oxadiazole unit
into the stilbene skeleton. The fluorescence properties of
the title compounds are investigated by UV–vis absorption
and fluorescence emission spectra in dimethylformamide
(DMF), chloroform (CHCl3) and tetrahydrofuran (THF).
Quantum-chemical calculation was performed to obtain
their optimized structures and the electron distribution.
The synthetic route is outlined in Scheme 1.
Intermediates of compounds 1, 2 and 3 were prepared by
the reported methods [17].
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-dimethylformamide (15 mL) under nitrogen atmosphere
was added dropwise a solution of t-BuOK (2 g, 3 %) in
ethanol. The reaction proceeded at room temperature over-
night. Then the resulting mixture was filtered and
washed with ethanol. The residue was recrystallized from
ethanol/DMSO.
Experimental
Instrumentation
2-(anthracen-9-yl)-5-(4-(2,4-dichlorostyryl)phenyl)-
1,3,4-oxadiazole (4a), yellow crystals, yield 92.5 %; m.p.
256–257 °C; H NMR (400 MHz, CDCl3) δ 8.71 (s, 1H,
1
Melting points were determined using RY-1 melting
point apparatus and were uncorrected. H NMR spectra
1
anthracen-H), 8.21 (d, J=8.4 Hz, 2H, C6H4 2,6-H), 8.14–
8.00 (m, 4H, anthracen-H), 7.71 (d, J=8.4 Hz, 2H, C6H4
3,5-H), 7.65 (d, J=8.5 Hz, 1H, C6H3 6-H), 7.61–7.52
(m, 4H, anthracen-H), 7.57 (d, J=16.2 Hz, 1H, CH=CH),
7.44 (d, J=2.1 Hz, 1H, C6H3 3-H), 7.28 (d, J=8.5 Hz, 1H,
C6H3 5-H), 7.12 (d, J=16.3 Hz, 1H, CH=CH); APCI MS:
m/z, 493(M+, 100), 495(M+2, 80), 496(M+3, 20); Anal.
Calcd. for C30H18Cl2N2O (493.2):C, 73.03; H, 3.68; N,
5.68; Found: C, 73.41; H, 3.65; N, 5.72.
were recorded in CDCl3 on a Bruker AVANCE-
400 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 spectrophotometer. Fluorescence
spectra were obtained on a Hitachi F-4500 spectropho-
tometer at room temperature.
2-(anthracen-9-yl)-5-(4-(3,4-dichlorostyryl)phenyl)-
1,3,4-oxadiazole (4b), yellow crystals, yield 91.2 %; m.p.
1
253–254 °C; H NMR (400 MHz, CDCl3) δ 8.71 (s, 1H,
Synthesis
anthracen-H), 8.20 (d, J=8.2 Hz, 2H, C6H4 2,6-H), 8.15–
8.02 (m, 4H, anthracen-H), 7.68 (d, J=8.3 Hz, 2H, C6H4
3,5-H), 7.63 (s, 1H, C6H3 2-H), 7.61–7.51 (m, 4H,
anthracen-H), 7.45 (d, J=8.3 Hz, 1H, C6H3, 5-H), 7.37
(d, J=8.4 Hz, 1H, C6H3 6-H), 7.14 (s, 2H, CH=CH); APCI
MS: m/z, 493(M+, 100), 495(M+2, 70), 496(M+3, 25); Anal.
Calcd. for C30H18Cl2N2O(493.2):C, 73.03; H, 3.68; N, 5.68;
Found: C, 72.99; H, 3.75; N, 5.76.
Synthesis of 2-(anthracen-9-yl)-5-(p-tolyl)-1,3,4-
oxadiazole (1)
Synthesis of 2-(anthracen-9-yl)-5-(4-(bromomethyl)
phenyl)-1,3,4-oxadiazole(2)
Synthesis of 4-(5-(anthracen-9-yl)-1,3,4-oxadiazol-
2-yl)benzylphosphonate(3)
Scheme 1 The synthetic route
of the title compounds

J Fluoresc
Table 1 The fluorescence char-
acteristics of the title compounds
in CHCl3, THF, and DMF
−4
compound
4a
solution
1max(nm)
×10
ε
1ex(nm)
1em(nm)
Stockes shifts
Фx
max
DMF
THF
336
338
335
338
337
337
4.79
5.07
4.43
5.22
6.15
4.92
341
340
343
341
341
342
493
489
489
493
489
489
152
149
146
152
148
147
0.81
0.78
0.88
0.91
0.71
0.92
solution at room temperature
(concentration: 1×10−5 mol/L)
CHCl3
DMF
THF
4b
CHCl3
Results and Discussion
compounds, it was found that their olefinic protons (CH=CH)
from stilbene had different coupling, which indicated the
different substituents on benzene had an effect on olefinic
proton-proton coupling.
1H NMR Spectra
1
In the H NMR spectra of two compounds, 4a showed two
fine doublets corresponding to the olefinic protons
(CH=CH) from stilbene at δ: 7.57 ppm and 7.12 ppm
(J=16.3 Hz). At δ: 7.65, 7.28, and 7.44 ppm, two doublets
and a singlet could be assigned to 6-H, 5-H and 3-H from
aromatic protons (C6H3-protons), respectively. At δ: 8.71,
8.14–8.00, and 7.61–7.52 ppm, two multiplets and a singlet
could be assigned to the protons of anthracen. In addition,
another two downfield doublets were also found at δ: 8.21
and 7.71 ppm which could be assigned to the contribution of
aromatic protons (C6H4-protons). In contrast, the compound
Fluorescence Properties
The data of fluorescence properties for compounds 4a and
4b are summarized in Table 1. It can be seen that com-
pounds 4a and 4b had the similar optical properties such as
the maximum absorption wavelengths, the maximum emis-
sion wavelengths, the Stokes shift and the fluorescence
quantum yields, indicating that the different position Cl
have a limited effect on their fluorescence properties. In
Table 1, compared with 1max and εmax for the same com-
pound in DMF, CHCl3 and THF, the maximum absorption
wavelengths of the two compounds had little change in
different solvents, especially, in DMF and CHCl3, the ab-
sorbency is lower than that in THF. This is because the
dispersion force, permanent dipole and induction of dipole
1
4b in its H NMR spectrum showed the similar resonance
peaks accountable to the protons of anthracen and aromatic
protons (C6H4-protons), respectively, only a singlet was
found corresponding to the olefinic protons (CH=CH) from
1
stilbene at δ: 7.14 ppm . From the H NMR spectra of two
Fig. 1 The fluorescence
emission spectra of the
compounds 4a, 4b in CHCl3,
THF, and DMF solvent at room
temperature (concentration:
1×10−5 mol L−1
)

J Fluoresc
Fig. 2 The fluorescence
emission spectra of the
compounds 1, 4a and 4b
in THF(concentration:
1×10−5 mol L−1
)
in the strongly polar THF with stilbene derivatives are
stronger than that in DMF and CHCl3, which makes 1max
have a little red-shift.
indicated that the solutions molecules had strong coordination
effect and the environment played an important role in deter-
mining the fluorescence intensity of the compounds. As shown
in Fig. 2, compared with the fluorescence characteristic emis-
sion wavelengths of the title compounds in THF solvent, the
maximum fluorescence emission wavelength of compound 4a
and 4b had a greater shift (12 nm) towards higher wavelengths
than the intermediate of compound 1 owing to increasing the
conjugation length of the molecule, which would make the
electron-pair in the highest occupied molecular orbit possess
a lower energy; the electron-pair could be excited easily to
Figure 1 showed fluorescence emission spectra of two stil-
bene derivatives in CHCl3, THF, and DMF (Concentration:
1×10−5 mol/L). The influence of solvent on the fluorescence
intensities of the 4a and 4b was investigated. We could see that
the emission wavelengths and fluorescence intensity of the two
compounds were different in three solvents at the same con-
centration. The fluorescence intensity of the two compounds in
CHCl3 was lower than that in the other solvents. All these
Fig. 3 The fluorescence
spectra of compounds 4a and 4b
at different concentration of
THF(1.0×10−4 mol/L; 1.0×
10−5 mol/L; 1.0×10−6 mol/L)

J Fluoresc
Fig. 4 Optimized geometric
structure and frontier molecular
orbital profiles of 4a and 4b
a
b
HOMO
HOMO
LUMO
LUMO
transit into a higher orbit. The influence of concentration on
fluorescence intensity of the two compounds was also studied
(Fig. 3). It can be seen that the emission intensity of fluores-
cence were greatly decreased with gradual increasing in con-
between anthracene and 1,3,4-oxadiazole is around 134°,
and the dihedral angle in 4a between vinyl and 2,4-dichlo-
robenzene is 21°. Thus the -Cl group attached to the
o-position phenyl seems to affect the planarity of the mole-
cules. In both cases, the HOMO are mainly localized on the
anthracene and 1,3,4-oxadiazole, while the LUMO are
delocalized along the whole molecules, indicating that these
molecules posses intramolecular charge transfer ability. It also
implies that the anthracene core and 1,3,4-oxadiazole unit play
an important part in their emission. Two molecules possess a
high HOMO energy level (−5.44 and −5.46 eV), which could
lead to their better hole-transport properties. Their LUMO
energy level (−2.25 and −2.28 eV) is supposed to facilitate
the acceptance of electrons from the cathode [21]. The band
gaps are 3.19 eV and 3.18 eV, respectively. Two energy gaps
are close, indicating they have the similar the emission spectra
which agree well with the experimental observations.
centration of 4a and 4b in THF. The fluorescence quantum
AsꢀFxꢀn2x ꢀ Φs
yields: Φx ¼
where A is the absorbance at the
AxꢀFsꢀn2s
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, respec-
tively. Rhodamine B in ethanol (Φ=0.89) was taken as the
standard [19]. The two compounds had a large fluorescence
quantum yield (Ф), which was due to the conjugative effect.
The molecular structure containing stilbene, anthracene core
and 1,3,4-oxadiazole unit can increase the conjugation length
and rigid structure of the molecule.
Theoretical Study
The geometries and electronic structures of molecules 4a
and 4b were studied by density function theory (DFT)
calculations at the B3LYP/6-31G* level [20]. The optimized
structures and the frontier molecular orbital profiles of 4a
and 4b are shown in Fig. 4. As expected, molecules 4a and
4b both have non-planar, twisted structures attributed to
steric hindrance imposed by the 1,3,4-oxadiazole, anthra-
cene skeleton, π-bridges, linkages and dihedral angles be-
tween two units. The dihedral angle of two molecules
Conclusions
Two novel stilbene derivatives bearing anthracene core
based on 1,3,4-oxadiazole were synthesized, characterized
by spectral studies, theoretical calculations and their fluo-
rescence spectra were recorded in order to study the changes
in the photophysical properties with different solvents. The
results showed the compounds exhibited strong green

J Fluoresc
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