Synthesis and optical properties of novel anthracene-based stilbene derivatives containing an 1,3,4-oxadiazole unit

Article abstract of DOI:10.1016/j.dyepig.2011.10.004

A series of anthracene-based stilbene derivatives containing an 1,3,4-oxadiazole unit were efficiently synthesized by a four-pots reaction sequence. All of the target compounds were characterized using, FT-IR, ¹H NMR, ¹³C NMR, mass s

Full text of DOI:10.1016/j.dyepig.2011.10.004

Dyes and Pigments 93 (2012) 1422e1427
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Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Synthesis and optical properties of novel anthracene-based stilbene derivatives
containing an 1,3,4-oxadiazole unit
*
Xinwei Li, Daohang He
School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou 510640, 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 anthracene-based stilbene derivatives containing an 1,3,4-oxadiazole unit were efficiently
synthesized by a four-pots reaction sequence. All of the target compounds were characterized using, FT-
IR, 1H NMR, 13C NMR, mass spectrometry and elemental analysis. For the samples, the UVevisible
absorption coefficient, maximum wavelength, fluorescence excitation wavelength, fluorescence emission
wavelength and fluorescence quantum yield, were measured in dilute tetrahydrofuran solution. Cyclic
voltammetry studies revealed that the molecules have low-lying LUMO and HOMO energy levels sug-
gesting these compounds may possess good electron-transporting or hole-blocking properties.
Ó 2011 Elsevier Ltd. All rights reserved.
Received 8 July 2011
Received in revised form
1 October 2011
Accepted 5 October 2011
Available online 19 October 2011
Keywords:
Synthesis
1,3,4-Oxadiazole
Stilbene
Anthracene
Fluorescence
Optical property
1. Introduction
molecular assembly. There is presently great interest to increase the
structural or spatial dimensions of p-conjugated molecules in order
Over the past decades, the development of organic light-
emitting diode (OLED) materials has attracted much attention,
owing to their potential application in flat panel displays and solid-
state light sources. Stilbene derivatives are well known not only for
their considerable biological and medical activities [1e3], but also
for their fluorescence in the visible light range, large Stokes shifts
and high quantum yields [4e6]. Moreover, stilbene derivatives have
been extensively investigated for various potential applications
including optical brighteners, PPV-type electroluminescent copol-
ymer and organic electroluminescent (EL) materials [7e13]. Fluo-
rescent heterocyclic compounds are of interest in many areas such
as emitters for electroluminescence devices, molecular probes for
biochemical research, in traditional textile and polymer fields,
fluorescent whitening agents and photoconducting materials
[14e18]. 1,3,4-Oxadiazoles are an important class of heterocyclic
compounds with broad spectrum of biological activities,
compounds and have been developed as materials for EL devices
since these compounds possess high electron-accepting properties
and display strong fluorescence with a high quantum yield [19e22].
Fluorescent characteristics rely largely on molecular structure and
to tune and acquire more favorable physical properties [23].
Anthracene derivatives have a rigid structure, wide energy gaps
and high fluorescent quantum efficiency [24]. A great deal of
anthracene-based electroluminescent materials have been devel-
oped [25,26].
Based on these ideas, we recently synthesized a series of novel
anthracene-based stilbene derivatives containing
a
1,3,4-
oxadiazole unit. It is envisioned that the introduction of the 1,3,4-
oxadiazole unit and anthracene core into the stilbene skeleton
can improve their photo-electric properties, by extending conju-
gated styryl units. As expected, the emission color of these
compounds can be easily tuned from blue to green by increasing
conjugation length. Particularly, all of the compounds exhibited
desirable HOMO levels (ꢀ5.51 to ꢀ6.06 eV), which have promising
potential for application in OLEDs.
2. Experimental
2.1. Chemicals and instruments
All chemicals and reagents were commercially available and
used without further purification. All solvents were carefully dried
and freshly distilled according to common laboratory techniques.
* Corresponding author. Tel./fax: þ86 20 87110234.
E-mail address: daohanghe@yahoo.com.cn (D. He).
0143-7208/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.dyepig.2011.10.004

X. Li, D. He / Dyes and Pigments 93 (2012) 1422e1427
1423
Melting points were recorded on Electrothermal digital RY-1
2.2.3. Synthesis of 4-(5-(anthracen-9-yl)-1,3,4-oxadiazol-2-yl)
benzylphosphonate 3
melting point apparatus and were uncorrected. FT-IR spectra
were measured as KBr pellets on a Bruker Vector 33 in the region of
A mixture of compound 2 (8.15 g, 20 mmol) and triethyl phos-
phite (4.8 mL, 28.1 mmol) was heated under reflux for 5 h. The
excess triethyl phosphate was evaporated under reduced pressure,
and then filtered by addition of hexane. The residue was recrys-
tallized from THF/hexane giving 8.96 g compound 3 as yellow solid,
4000e400 cmꢀ1 1H NMR and 13C 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 per-
formed on a Vario EL Z CHN elemental analyzer. UVevis absorp-
yield 95.1%; m.p. 138-129 ꢂC; 1H NMR (400 MHz, CDCl3)
d
¼ 8.70 (s,
tion spectra were recorded on
a
Hitachi UV-3010
1H, anthracen-H), 8.16 (d, J ¼ 8.0 Hz, 2H, C6H4 2,6-H), 8.12e8.09 (m,
2H, anthracen-H), 8.07e8.02 (m, 2H, anthracen-H), 7.59e7.53 (m,
4H, anthracen-H), 7.50 (dd, J ¼ 8.3 Hz, 2.2 Hz, 2H, C6H4 3,5-H),
4.10e4.03 (m, 4H, OCH2), 3.25 (d, J ¼ 22.1 Hz, 2H, CH2), 1.28 (t,
J ¼ 7.1 Hz, 6H, CH3); APCI MS: m/z, 473(M þ 1, 100), 474(M þ 2, 30).
spectrophotometer. Fluorescence spectra were obtained on a Hita-
chi F-4500 spectrophotometer at room temperature. The purity of
the compounds was confirmed by TLC on silica gel ‘G’-coated glass
plates. The cyclic voltammograms reported here were recorded
with a CHI830B electrochemical workstation at a constant scan rate
of 50 mV sꢀ1. The measurements were performed in 1 mmol Lꢀ1
dichloromethane solutions of the samples with tetrabutylammo-
nium hexafluorophosphate solution (0.1 mol Lꢀ1 in dichloro-
methane) as supporting electrolyte at room temperature. The cyclic
voltammetric system was constructed using a three-electrode
undivided electrochemical cell consisted of a Pt working elec-
trode, a Pt-wire counter electrode, and a Ag/AgNO3 reference
electrode (0.1 mol Lꢀ1 in CH2Cl2) under protection of nitrogen, and
each measurement was calibrated with an internal standard,
ferrocene/ferrocenium redox system [27,28]. The fluorescence
quantum yields Fx ¼ (As ꢁ Fx ꢁ n2x ꢁ Fs)/(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,
2.2.4. Typical procedure for the synthesis of compounds 4aeh
To a stirred solution of the aromatic aldehyde (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 overnight. Then the
resulting mixture was filtered and washed with ethanol. The
residue was recrystallized from ethanol/DMSO.
2.2.4.1. 2-(anthracen-9-yl)-5-(4-styrylphenyl)-1,3,4-oxadiazole
(4a). Yield 83.5%; m.p. 203e204 ꢂC; FT-IR (KBr, cmꢀ1): 3053, 3028,
1626, 1606, 1569, 1545, 1497, 1447, 1420, 1362, 1200, 1184, 1085,
1005, 961, 891, 754, 726; 1H NMR (400 MHz, CDCl3)
d 8.70 (s, 1H,
anthracen-H), 8.19 (d, J ¼ 8.2 Hz, 2H, C6H4eH), 8.14e7.99 (m, 4H,
anthracen-H), 7.69 (d, J ¼ 8.3 Hz, 2H, C6H4eH), 7.60e7.52 (m, 6H,
anthracen-H C6H5 2,6-H), 7.39 (t, J ¼ 7.5 Hz, 2H, C6H5 3,5-H), 7.30 (t,
J ¼ 7.5 Hz, 1H, C6H5 4-H), 7.26 (d, J ¼ 16.4 Hz, 2H, CH]CH), 7.16 (d,
respectively. Rhodamine B in ethanol (
standard [29].
F
¼ 0.89) was taken as the
2.2. Synthesis
J ¼ 16.3 Hz, 2H, CH]CH); 13C NMR (101 MHz, CDCl3)
d 165.72,
163.00, 140.97, 136.72, 131.48, 131.17, 131.06, 128.83, 128.79, 128.32,
127.76, 127.49, 127.42, 127.14, 126.82, 125.74, 125.12, 122.61; APCI
MS: m/z, 425(M þ 1, 100), 426(M þ 2, 35); Anal. Calcd. for
C30H20N2O(424.5): C, 84.88; H, 4.75; N, 6.60; Found: C, 83.76; H,
4.90; N, 6.63.
2.2.1. Synthesis of 2-(anthracen-9-yl)-5-(p-tolyl)-1,3,4-
oxadiazole 1
4-Methylbenzohydrazide (15 g, 0.1 mol) and anthracene-9-
carbaldehyde (21 g, 0.1 mol) were dissolved in ethanol (500 mL).
The mixture was heated under reflux for 1 h until the disappear-
ance of the starting material by TLC to give a clear yellow solution.
Chloramine-T (27 g, 0.3 mol) was poured into the mixture and
stirred for 4 h at 76 ꢂC. The excess ethanol was evaporated under
reduced pressure, and then the mixture was washed with water
and filtered. The residue was recrystallized from acetone giving
27.3 g of compound 1 as yellow crystals, yield 75.4%; m.p.
2.2.4.2. 2-(anthracen-9-yl)-5-(4-(2-(naphthalen-1-yl)vinyl)phenyl)-
1,3,4-oxadiazole (4b). Yield 86.1%; m.p. 241e242 ꢂC; FT-IR (KBr,
cmꢀ1): 3053, 1626, 1605, 1572, 1544, 1494, 1445, 1420, 1362, 1201,
1183, 1086, 1006, 962, 889, 758, 770, 743, 725; 1H NMR (400 MHz,
CDCl3)
d
8.71 (s, 1H, anthracen-H), 8.25 (d, J ¼ 4.8 Hz, 1H,
naphthalene-H), 8.23 (d, J ¼ 8.3 Hz, 2H, C6H4 2,6-H), 8.14e8.06 (m,
4H, anthracen-H), 8.05 (d, J ¼ 16.1 Hz, 1H, CH]CH), 7.90 (d,
J ¼ 7.7 Hz, 1H, naphthalene-H), 7.85 (d, J ¼ 8.2 Hz, 1H, naphthalene-
H), 7.80 (d, J ¼ 6.0 Hz, 1H, naphthalene-H), 7.79 (d, J ¼ 8.1 Hz, 2H,
C6H4 3,5-H), 7.62e7.54 (m, 4H, anthracen-H), 7.57e7.49 (m, 3H,
naphthalene-H), 7.22 (d, J ¼ 16.0 Hz, 1H, CH]CH); 13C NMR
192e193 ꢂC; 1H NMR (400 MHz, CDCl3)
d 8.67 (s, 1H, anthracen-H),
8.08 (d, J ¼ 8.2 Hz, 2H, C6H4 2,6-H), 8.10e8.05 (m, 4H, anthracen-H),
7.58e7.50 (m, 4H, anthracen-H), 7.35 (d, J ¼ 8.0 Hz, 1H, C6H4 3,5-H),
2.45 (s, 3H, CH3); 13C NMR (101 MHz, CDCl3):
d 165.97, 162.82,
142.55, 131.46, 131.41, 131.04, 129.91, 128.77, 127.70, 127.10, 125.71,
125.13, 121.16, 117.40, 21.71; APCI MS: m/z, 337(M þ 1, 100),
338(M þ 2, 24).
(101 MHz, CDCl3)
d 165.71, 163.04, 141.19, 134.35, 133.77, 131.49,
131.38, 131.06, 130.42, 128.80, 128.74, 128.70, 128.22, 127.77, 127.56,
127.32, 126.36, 126.01, 125.75, 125.69, 125.13, 123.92, 123.60, 122.77,
117.29; APCI MS: m/z, 475(M þ 1, 100), 476(M þ 2, 40); Anal. Calcd.
for C34H22N2O (474.5): C, 86.05; H, 4.67; N, 5.90; Found: C, 85.78; H,
4.38; N, 6.23.
2.2.2. Synthesis of 2-(anthracen-9-yl)-5-(4-(bromomethyl)phenyl)-
1,3,4-oxadiazole 2
To a stirred solution of 1 (8.8 g, 26 mmol) in carbon tetrachloride
(300 mL) was added N-Bromosuccinimide (NBS) (5.5 g, 31 mmol),
the reaction was heated under reflux for 8 h and then the excess
cooled solvent. The resulting mixture was filtered, washed with
ethanol. The residue was recrystallized from THF/ethanol giving
8.34 g of compound 2 as solid, yield 77.5%; m.p. 185e186 ꢂC; 1H
2.2.4.3. 2-(anthracen-9-yl)-5-(4-(2-(anthracen-9-yl)vinyl)phenyl)-
1,3,4-oxadiazole (4c). Yield 87.8%; m.p. >300 ꢂC; FT-IR (KBr, cmꢀ1):
3048, 3026, 1626, 1608, 1573, 1545, 1493, 1447, 1421, 1365, 1202,
1183,1089,1008, 960, 890, 849, 784, 732; 1H NMR (400 MHz, CDCl3)
NMR (400 MHz, CDCl3)
d
¼ 8.71 (s, 1H, anthracen-H), 8.19 (d,
d 8.71 (s, 1H, anthracen-H), 8.44 (s, 1H, anthracen-H), 8.38e8.33 (m,
J ¼ 8.4 Hz, 2H, C6H4 2,6-H), 8.15e8.08 (m, 2H, anthracen-H),
8.06e7.98 (m, 2H, anthracen-H), 7.56 (d, J ¼ 8.4 Hz, 2H, C6H4 3,5-
H), 7.57e7.55 (m, 4H, anthracen-H), 4.55 (s, 2H, CH2); APCI MS:
m/z, 415(M þ 2, 100), 418(M þ 3, 25).
2H, anthracen-H), 8.28 (d, J ¼ 8.3 Hz, 2H, C6H4 2,6-H), 8.16e8.07 (m,
4H, anthracen-H), 8.10 (d, J ¼ 16.3 Hz, 1H, CH]CH) 8.06e7.99 (m,
2H, anthracen-H), 7.85 (d, J ¼ 8.3 Hz, 2H, C6H4 3,5-H), 7.82e7.63 (m,
4H, anthracen-H), 7.53e7.43 (m, 4H, anthracen-H), 7.04 (d,

1424
X. Li, D. He / Dyes and Pigments 93 (2012) 1422e1427
J ¼ 16.6 Hz, 1H, CH]CH); 13C NMR (101 MHz, CDCl3)
d
165.70,
d 165.65, 163.04, 140.60, 135.24, 133.95, 131.51, 131.47, 131.06,
163.12, 140.82, 136.16, 131.95, 131.54, 131.51, 131.08, 129.72, 128.82,
127.80, 127.64, 127.60, 127.27, 127.02, 125.80, 125.76, 125.29, 125.13,
123.07, 117.27; APCI MS: m/z, 525(M þ 1, 100), 526(M þ 2, 40); Anal.
Calcd. for C38H24N2O (524.2): C, 87.00; H, 4.61; N, 5.34; Found: C,
87.17; H, 4.77; N, 5.63.
129.79, 129.02, 128.80, 128.01, 127.96, 127.77, 127.53, 127.18, 125.75,
125.10,122.84; APCI MS: m/z, 459(M þ 1,100), 461(M þ 3, 45); Anal.
Calcd. for C30H19ClN2O(458.1):C, 78.51; H, 4.17; N, 6.10; Found: C,
78.67; H, 4.74; N, 6.60.
2.2.4.6. 2-(anthracen-9-yl)-5-(4-(4-bromostyryl)phenyl)-1,3,4-
oxadiazole (4f). Yield 90.5%; m.p. 281e282 ꢂC; FT-IR (KBr, cmꢀ1):
3053, 3025, 1626, 1605, 1569, 1545, 1495, 1445, 1421, 1361, 1201,
1185, 1086, 1073, 1007, 959, 891, 840, 727; 1H NMR (400 MHz,
2.2.4.4. 2-(anthracen-9-yl)-5-(4-(4-fluorostyryl)phenyl)-1,3,4-
oxadiazole (4d). Yield 82.5%; m.p. 240e241 ꢂC; FT-IR (KBr,
cmꢀ1):3050, 3027, 1626, 1598, 1578, 1541, 1509, 1446, 1421, 1364,
1234, 1215, 964, 837, 732; 1H NMR (400 MHz, CDCl3)
d
8.70 (s, 1H,
CDCl3)
d
8.71 (s, 1H, anthracen-H), 8.19 (d, J ¼ 8.2 Hz, 2H, C6H4 2,6-
anthracen-H), 8.19 (d, J ¼ 8.2 Hz, 2H, C6H4 2,6-H), 8.14e8.01 (m, 4H,
anthracen-H), 7.67 (d, J ¼ 8.2 Hz, 2H, C6H4 3,5-H), 7.60e7.47 (m, 6H,
anthracen-H C6H4 2,6-H), 7.22 (d, J ¼ 16.3 Hz, 2H, CH]CH), 7.08 (t,
J ¼ 8.4 Hz 2H, C6H4 3,5-H), 7.07 (d, J ¼ 16.1 Hz, 2H, CH]CH); 13C NMR
H), 8.13e8.04 (m, 4H, anthracen-H), 7.68 (d, J ¼ 8.2 Hz, 2H, C6H4 3,5-
H), 7.60e7.53 (m, 4H, anthracen-H), 7.51 (d, J ¼ 8.4 Hz, 2H, C6H4 2,6-
H), 7.42 (d, J ¼ 8.4 Hz, 2H, C6H4 3,5-H), 7.19 (d, J ¼ 16.5 Hz, 1H, CH]
CH), 7.14 (d, J ¼ 16.5 Hz, 1H, CH]CH); APCI MS: m/z, 503(Mþ, 85),
(101 MHz, CDCl3)
d
165.68, 163.01, 140.80, 132.92, 131.49, 131.06,
504(M
þ
2, 100), 506(M
þ
3, 25); Anal. Calcd. for
129.91, 128.79, 128.41, 128.33, 127.76, 127.52, 127.22, 127.07, 125.74,
125.11, 122.66, 117.28, 115.94, 115.72; APCI MS: m/z, 443(M þ 1, 100),
444(M þ 2, 35); Anal. Calcd. for C30H19FN2O(442.5):C, 81.43; H, 4.33;
N, 6.33; Found: C, 81.72; H, 4.95; N, 6.80.
C30H19BrN2O(503.4):C, 71.58; H, 3.80; N, 5.56; Found: C, 72.02; H,
3.92; N, 6.06.
2.2.4.7. 2-(anthracen-9-yl)-5-(4-(2-methoxystyryl)phenyl)-1,3,4-
oxadiazole (4g). Yield 83.6%; m.p. 216e217 ꢂC; FT-IR (KBr, cmꢀ1):
3049, 3006, 2933, 2831, 1627, 1604, 1573, 1543, 1496, 1463, 1421,
1362, 1334, 1247, 1201, 1183, 1085, 1007, 962, 890, 745, 725; 1H NMR
2.2.4.5. 2-(anthracen-9-yl)-5-(4-(4-chlorostyryl)phenyl)-1,3,4-
oxadiazole (4e). Yield 93.3%; m.p. 277e278 ꢂC; FT-IR (KBr, cmꢀ1):
3053, 3026, 1626, 1605, 1567, 1544, 1495, 1445, 1421, 1362, 1202,
1185, 1087, 1007, 959, 891, 841, 726; 1H NMR (400 MHz, CDCl3)
(400 MHz, CDCl3)
d
8.70 (s, 1H, anthracen-H), 8.17 (d, J ¼ 8.4 Hz, 2H,
C6H4 2,6-H), 8.13e8.05 (m, 4H, anthracen-H), 7.70 (d, J ¼ 8.4 Hz, 2H,
C6H4 3,5-H), 7.64 (d, J ¼ 16.6 Hz, 1H, CH]CH), 7.63 (d, J ¼ 7.5 Hz, 1H,
C6H4 6-H), 7.59e7.53 (m, 4H, anthracen-H), 7.29 (t, J ¼ 8.5 Hz, 1H,
C6H4 4-H), 7.18 (d, J ¼ 16.5 Hz, 1H, CH]CH), 6.99 (t, J ¼ 7.5 Hz, 1H,
C6H4 5-H), 6.93 (d, J ¼ 8.2 Hz, 1H, C6H4 3-H), 3.92 (s, 1H, CH3); 13C
d
8.71 (s, 1H, anthracen-H), 8.19 (d, J ¼ 8.2 Hz, 2H, C6H4 2,6-H),
8.14e8.01 (m, 4H, anthracen-H), 7.68 (d, J ¼ 8.2 Hz, 2H, C6H4-3,5-H),
7.61e7.52 (m, 4H, anthracen-H), 7.48 (d, J ¼ 8.4 Hz, 2H, C6H4 2,6-H),
7.36 (d, J ¼ 8.4 Hz, 2H, C6H4 3,5-H), 7.20 (d, J ¼ 16.3 Hz,1H, CH]CH),
7.13 (d, J ¼ 16.3 Hz, 1H, CH]CH); 13C NMR (101 MHz, CDCl3)
NMR (101 MHz, CDCl3)
d 165.80, 162.92, 157.18, 141.68, 131.47,
Fig. 1. The synthetic route of new target compounds.

X. Li, D. He / Dyes and Pigments 93 (2012) 1422e1427
1425
Table 1
UVevis, fluorescent and electrochemical parameters of target compounds.
a
c
Compound
lmax(nm)a
ꢁ10Eꢀ4
3
max(L molꢀ1 cmꢀ1
)
lex(nm)a
lem(nm)a
Stokes shifts(nm)b
Fx
Eg(eV)d
Eox(V)e
Ered(V)e
EHOMO/LUMO(eV)f
4a
4b
4c
4d
4e
4f
334
351
388
335
337
338
349
368
5.00
4.03
3.12
4.69
4.99
6.03
6.03
5.40
337
368
396
337
339
341
353
339
488
488
496
488
487
488
489
488
151
120
100
151
148
147
136
149
0.78
0.66
0.43
0.78
0.77
0.74
0.77
0.09
2.54
2.37
2.05
2.61
2.67
2.55
2.60
eg
1.14
1.01
0.70
1.16
1.26
1.13
1.17
e
ꢀ1.39
ꢀ1.35
ꢀ1.34
ꢀ1.44
ꢀ1.41
ꢀ1.42
ꢀ1.42
e
ꢀ5.94/ꢀ3.40
ꢀ5.81/ꢀ3.44
ꢀ5.50/ꢀ3.45
ꢀ5.96/ꢀ3.35
ꢀ6.06/ꢀ3.39
ꢀ5.93/ꢀ3.38
ꢀ5.97/ꢀ3.37
e
4g
4h
a
Solution in 10ꢀ5 mol Lꢀ1 THF.
b
c
d
e
f
Stokes shifts ¼ lem
ꢀ
lex.
The fluorescence quantum yields were measured in THF using Rhodamine B in ethanol (
F
¼ 0.89) as the standard.
Eg ¼ (EoxꢀEred).
Eox and Ered are measured vs. ferrocene/ferrocenium.
EHOMO ¼ (ꢀ4.8ꢀEox), ELUMO ¼ (ꢀ4.8ꢀEred).
Not determined.
g
131.44, 131.05, 129.40, 128.77, 127.78, 127.41, 127.15, 126.70, 126.06,
125.78, 125.73, 125.14, 122.28, 120.82, 117.35, 111.01, 55.54; APCI
MS: m/z, 455(M þ 1, 100), 456(M þ 2, 38); Anal. Calcd. for
C31H22N2O2 (454.5):C, 81.92; H, 4.88; N, 6.16; Found: C, 81.77; H,
5.09; N, 6.55.
melting points data of target compounds indicate that most of
them have high melting points of >200 ꢂC.
3.2. Optical properties
The optical properties for all new target compounds are
summarized in Table 1. The UVevis absorption spectra and fluo-
rescence spectra of the compounds are shown in Figs. 2 and 3 (in
10ꢀ5 mol Lꢀ1 tetrahydrofuran solution). It can be observed that the
eight compounds 4aeh have quite similar UVevis absorption
characteristics. The maximum absorption peaks (labs), which can
2.2.4.8. 2-(anthracen-9-yl)-5-(4-(4-nitrostyryl)phenyl)-1,3,4-
oxadiazole (4h). Yield 89.6%; m.p. 287e288 ꢂC; FT-IR (KBr, cmꢀ1):
3050, 3030, 1627, 1592, 1567, 1542, 1512, 1497, 1444, 1422, 1341,
1264, 1201, 1180, 1110, 1086, 1007, 960, 889, 852, 752, 726; 1H NMR
(400 MHz, CDCl3)
d
8.72 (s, 1H, anthracen-H), 8.25 (d, J ¼ 8.6 Hz, 2H,
C6H4 3,5-H), 8.23 (d, J ¼ 8.2 Hz, 2H, C6H4 2,6-H), 8.15e8.03 (m, 4H,
anthracen-H), 7.73 (d, J ¼ 8.2 Hz, 2H, C6H4 2,6-H), 7.68 (d, J ¼ 8.6 Hz,
2H, C6H4 3,5-H), 7.61e7.53 (m, 4H, anthracen-H), 7.33 (d,
J ¼ 16.5 Hz, 1H, CH]CH), 7.28 (d, J ¼ 16.2 Hz, 1H, CH]CH); APCI
MS: m/z, 470(M þ 1, 100), 471(M þ 2, 45); Anal. Calcd. for
C30H19N3O3(469.5):C, 76.75; H, 4.08; N, 8.95; Found: C, 76.91; H,
4.60; N, 9.14.
be attributed to the charge-transfer (CT) type
pep* electronic
transitions, situate in 334e388 nm in solution. The CT absorption
band of the compounds strongly depends on the conjugation
length. As can be seen in Fig. 2, the largest red-shift of the CT band
was observed at 388 nm for compound 4c, which possesses the
longest conjugation length, relative to that of the other compounds.
Whereas, compound 4a, having a shorter conjugation length
(compared to 4b) exhibits a blue shifted absorption maximized at
334 nm. The introduction of two anthracene groups extends the
conjugation length of the molecular structure. The longer wave-
3. Results and discussion
length absorption band of 4c can be attributed to the
p-p* transi-
3.1. The synthesis and characterization
tion in the 1,3,4-oxadiazole, stilbene and anthracene. Compared
with the maximum absorption peak of compound 4a (334 nm),
compound 4g (349 nm) had a greater shift toward higher wave-
lengths in accord with the stronger electron donating methoxy
The synthetic pathway and the structures of the target
compounds are shown in Fig. 1. A series of novel anthracene-based
stilbene derivatives containing an 1,3,4-oxadiazole unit were
synthesized in good yield by the oxidative cyclization of
hydrazones derived from aromatic aldehydes and 4-
methylbenzohydrazide by chloramine-T, bromination of N-Bro-
mosuccinimide (NBS), esterification, and WittigeHorner reaction.
The key stage in the synthesis of these target compounds is the
preparation of intermediate 1, in which chloramine-T was used as
oxidative cyclization agent under mild reaction conditions. The
chemical structures of the target compounds are confirmed by
FT-IR,1H NMR, 13C NMR, MS and elemental analysis, which were
shown detailed in experiment section. 1,3,4-Oxadiazole is the
electron-accepting five-membered hetero-aromatic unit contain-
ing two imine eC]Ne groups. Because of the electron-
withdrawing ability of the imine eC]Ne group, the 13C NMR
data of target compounds show the chemical shifts of two C atoms
of the imine eC]Ne group around 165.0 and 163.0 ppm. According
to the chemical shifts and coupling constant (16.0e16.7 Hz) of
doublet of CH]CH in the 1H NMR spectra of target compounds, it
was indicated that two protons had different chemical environ-
ments owing to the effect of different substituents belonging to the
trans-structure, which could confirm the existence of stilbene. The
Fig. 2. The absorption spectra of target compounds (1 ꢁ 10ꢀ5 mol Lꢀ1 in THF).

1426
X. Li, D. He / Dyes and Pigments 93 (2012) 1422e1427
4. Conclusion
The approach reported here for the anthracene-based stilbene
derivatives containing an 1,3,4-oxadiazole unit could be an efficient
method, especially for industry, as the reaction conditions are mild,
the yield is good and the product is easily purified. It can be
concluded that the absorption and fluorescence characteristic of
the new target compounds show a significant dependence on their
molecular structure. These compounds exhibit high fluorescence
quantum yield (Fx), bright green emission with a large Stokes shift.
These observations indicate that the high
p-conjugated molecule
structure enhances the light absorption and the fluorescence
emission ability of the target compounds due to the contribution of
anthracene. These compounds could possess a medium strong
fluorescence-emitting ability with Fx values in the region of
0.34e0.78. Electrochemical measurements indicate that the target
compounds have good electron affinities of 2.05e2.67 eV, implying
that they may have good electron injection capabilities.
Fig. 3. The emission spectra of target compounds (1 ꢁ 10ꢀ5 mol Lꢀ1 in THF).
Acknowledgments
group. Compounds 4d, 4e and 4f had the similar maximum
absorption peaks, indicating that the halogen substituents have
a limited effect on their electronic energy levels. As can be seen in
Table 1 and Fig. 3, these compounds were highly fluorescent and
The authors thank National Natural Science Foundation of China
(No.10874047) and Guangdong Provincial Natural Science Foun-
dation of China (No.04300531) for the financial assistance
exhibit strong blue-green fluorescence. The efficient p-conjugation
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