Synthesis and characterization of 1,3,4-oxadiazole derivatives containing alkoxy chains with different lengths

Article abstract of DOI:10.1016/j.molstruc.2007.01.032

The synthesis, optical properties, electrochemical properties, electronic structures and applications in electroluminescent device of three series of 1,3,4-oxadiazole derivatives, 1,4-bis[(4-methylphenyl)-1,3,4-oxadiazolyl]phenylene (OXD₁), 5,5′-di-(4-methyl)-2,2′-p-(2,5-bisalkoxyphenylene)-bis-1,3,4 -oxadiazole (OXD_2-n) and 1,4-bis[(4-alkoxyphenyl)-1,3,4-oxadiazolyl]phenylene (OXD_3-n) are reported. The molecular structures of the oxadiazole compounds were confirmed by FT-IR, ¹H NMR spectroscopy and elemental analysis. The optical and electrochemical properties of the compounds were investigated by UV-vis absorption and photoluminescence spectroscopy as well as cyclic voltammetry. The results show that introduction of two alkoxy groups whose electron-donating ability is stronger than that of methyl groups increases the electron density of the conjugated segment of OXD_2-n (with side-on alkoxy substituents) and OXD_3-n (with end-on alkoxy substituents), and thus leads to the absorption maximum bathochromic-shift compared to that of OXD₁. The HOMO and LUMO energy levels of the compounds studied are in the range of -2.78 to -2.89 and -5.75 to -6.20 eV. Calculations on the representative compounds by the Dmol³ package of MS Modeling 3.0 revealed that the increase of energy levels in both OXD_2-n and OXD_3-n was due to the change of the frontier molecular orbital distribution in the central benzene ring. The light-emitting devices have been fabricated using blends of MEH-PPV and these compounds as emissive layers, among which, maximum brightness up to 11810 cd m^-2 (8.5 V) has been observed, which is 40 times brighter than that with MEH-PPV. The result of the devices suggested that oxadiazole derivatives studied function well as electron-transporting materials and can be used in LEDs, and thus to enhance the efficiency of LEDs.

Full text of DOI:10.1016/j.molstruc.2007.01.032

Available online at www.sciencedirect.com
Journal of Molecular Structure 846 (2007) 55–64
www.elsevier.com/locate/molstruc
Synthesis and characterization of 1,3,4-oxadiazole
derivatives containing alkoxy chains with different lengths
a
a
b
a,
b
Xiao-bing Zhang , Ben-chen Tang , Peng Zhang , Min Li *, Wen-jing Tian
a
Key Laboratory for Automobile Materials, Ministry of Education, PRC and Department of Material Science and Engineering, Jilin University,
Changchun 130012, China
b
Key Laboratory for Supramolecular Structure and Materials of Ministry of Education, Jilin University, Changchun 130012, China
Received 19 October 2006; received in revised form 22 December 2006; accepted 8 January 2007
Available online 25 January 2007
Abstract
The synthesis, optical properties, electrochemical properties, electronic structures and applications in electroluminescent device of
three series of 1,3,4-oxadiazole derivatives, 1,4-bis[(4-methylphenyl)-1,3,4-oxadiazolyl]phenylene (OXD1), 5,50-di-(4-methyl)-2,20-p-
(2,5-bisalkoxyphenylene)-bis-1,3,4-oxadiazole (OXD2–n) and 1,4-bis[(4-alkoxyphenyl)-1,3,4-oxadiazolyl]phenylene (OXD3–n) are report-
ed. The molecular structures of the oxadiazole compounds were confirmed by FT-IR, 1H NMR spectroscopy and elemental analysis. The
optical and electrochemical properties of the compounds were investigated by UV–vis absorption and photoluminescence spectroscopy
as well as cyclic voltammetry. The results show that introduction of two alkoxy groups whose electron-donating ability is stronger than
that of methyl groups increases the electron density of the conjugated segment of OXD2–n (with side-on alkoxy substituents) and OXD3–n
(with end-on alkoxy substituents), and thus leads to the absorption maximum bathochromic-shift compared to that of OXD1. The
HOMO and LUMO energy levels of the compounds studied are in the range of ꢀ2.78 to ꢀ2.89 and ꢀ5.75 to ꢀ6.20 eV. Calculations
on the representative compounds by the Dmol3 package of MS Modeling 3.0 revealed that the increase of energy levels in both
OXD2–n and OXD3–n was due to the change of the frontier molecular orbital distribution in the central benzene ring. The light-emitting
devices have been fabricated using blends of MEH-PPV and these compounds as emissive layers, among which, maximum brightness up
to 11810 cd mꢀ2 (8.5 V) has been observed, which is 40 times brighter than that with MEH-PPV. The result of the devices suggested that
oxadiazole derivatives studied function well as electron-transporting materials and can be used in LEDs, and thus to enhance the effi-
ciency of LEDs.
ꢀ 2007 Elsevier B.V. All rights reserved.
Keywords: Oxadiazole derivatives; Photoluminescent; Electron-transporting; Energy level
1. Introduction
[2]. However, most of the EL materials are predominantly
hole-transporting, it is in most of cases easier to inject and
transport holes rather than electrons, and consequently,
the devices exhibit relatively low efficiency [3]. In order to
fabricate more efficient and high performance device, new
materials with high electron-transporting capability are
demanded. Typical electron-transporting materials usually
contain a p-electron deficient heterocyclic moiety, such as
oxadiazoles [4], triazoles [5], triazines [6], pyridines [7],
pyrimidines [8], quinolines [9], quinoxalines [10], etc., which
can effectively improve the electron affinity. Among these
heterocyclics, oxadiazoles is one of the most widely used
materials for OLED. The first oxadiazole that had been
Since the discovery of electroluminescence in Poly (1,4-
phenylene vinylene) (PPV) by Burroughes et al. in 1990
[1], organic and polymeric light-emitting diodes (LEDs)
have gained considerable attention. Much progress has
been made in research and development of organic electro-
luminescent (EL) materials and devices in the past decade
*
Corresponding author. Tel.: +86 431 85168254; fax: +86 431
85168444.
E-mail address: minli@mail.jlu.edu.cn (M. Li).
0022-2860/$ - see front matter ꢀ 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.molstruc.2007.01.032

56
X. Zhang et al. / Journal of Molecular Structure 846 (2007) 55–64
O
C
O
successfully applied in OLED is 2-(-biphenyl)-5-(4-tert-
butylphenyl)-1,3,4-oxadiazole (PBD) [4a]. Various kinds
of oxadiazole molecules have been used to obtain high
EL performances, such as bisoxadiazoles [4b,4c], branched
or dendrimeric oxadiazoles [4d], polymers containing oxa-
diazole units either in the main chain [4e–h] or side chain
[4i–k]. In our previous paper, we reported the electrochem-
ical properties and the electron structure of a series of pen-
tameric oxadiazole–phenylene derivatives with alkoxy side
chain attached to the metaposition of the central benzene
[11]. We report here the synthesis, optical properties, elec-
trochemical properties and electronic structures of different
types of symmetrical oxadiazole derivatives in which alk-
oxy chains with different lengths were linked either side-
on or end-on to the conjugated rods.
H3C CH2
CH2 CH3
O
C O
1A
i
O
C
O
H2N HN
C
NH NH2
1B
ii
O
C
O
C
O
C
O
H3C
NH NH
NH NH
C
CH3
1C
2. Experimental
iii
2.1. General
N
N
N
N
CH3
H3C
All reactions were magnetically stirred. Commercially
available reagents were used without further purification.
All reactions were monitored by thin layer chromatogra-
phy (TLC). Compounds were visualized with UV light at
254 and 365 nm. Melting points were measured on a
WRX-1S instrument. Infrared (IR) spectra were recorded
with a Perkin-Elmer spectrum one B spectrometer. 1H
NMR spectra were recorded on a Varian-Unity spectrom-
eter at 500 MHz using tetramethylsilane (TMS) as an inter-
nal standard. UV–vis absorption spectra were recorded on
a Shimazu UV-160 spectrometer. Photoluminescence (PL)
was measured by Shimadzu RF-5301 PC or Perkin-Elmer
LS 55 spectrometer. Current–Voltage and luminescence–
Voltage characteristics were measured by Keithley 2400
Sour-Measure Unit connected with PR 650 spectrometer.
Cyclic voltammetry (CV) was performed on a BAS
100BW electrochemical workstation. All CV measurements
were carried out in anhydrous dimethylformamide (DMF)
containing 0.1 M tetrabutylammonium perchlorate
(TBAP) as a supporting electrolyte, purging with nitrogen
prior to conduct the experiment. Platinum wire (MF-2013)
was used as a working electrode, Ag/Ag+ as a reference
electrode, and another platinum wire (MF-1032) as a
counter electrode. Differential scanning calorimetry
(DSC) analyses were performed on a DSC-821e instrument.
The rate of heating and cooling was 10 ꢁC minꢀ1, the
weight of the sample was about 2 mg.
O
O
OXD1
Scheme 1. (i) Hydrazine monohydrate, ethanol, reflux, 24h; (ii) p-toluoy
chloride, pyridine, reflux 8h; (iii) POCl3, reflux 24 h.
OH
OH
O
C
O
C
O
C
O
C
i
HO
OH
H3C CH2
CH2 CH3
O
O
HO
HO
2A
2B
O
CH2 CH3
n-1
O
O
ii
H3C CH2
CH2 CH3
C
O
O
C
H3C CH2
O
n-1
2Cn
O
CH2 CH3
n-1
O
O
iii
C
H2N HN
C
NH NH2
H3C CH2
O
n-1
2Dn
iv
O
CH2 CH3
n-1
O
C
O
C
O
O
C
H3C
NH NH
NH NH
CH3
C
H3C CH2
O
n-1
2En
2.2. Synthesis of compounds
O
CH2 CH3
n-1
N
N
N
N
Three types of oxadiazole derivatives named as
OXD1, OXD2–n (n = 7 and 16), OXD3–n (n = 7, 10
and 16) were synthesized by cyclocondensation of dihyd-
razides, the synthetic routes are shown in Schemes 1–3.
The chemical structures of the oxadiazoles were con-
v
CH3
H3C
O
O
H3C CH2 n-1O
OXD2-n
n=7, 16
Scheme 2. (i) Ethanol, H2SO4 (98%), reflux 18 h; (ii) RBr (R = CnH2n + 1),
acetone, K2CO3, reflux 40 h; (iii) hydrazine monohydrate, ethanol, reflux,
40 h; (iv) p-toluoy chloride, pyridine, reflux 8 h; (v) POCl3, reflux 40 h.
1
firmed by FT-IR, H NMR spectroscopy and elemental
analysis.

X. Zhang et al. / Journal of Molecular Structure 846 (2007) 55–64
57
O
O
i
HO
CH2 CH3
H3C CH2
O
CH2 CH3
C O
C O
n-1
3Bn
3Cn
3An
O
ii
H3C CH2
O
C NH NH2
n-1
iii
O
O
O
C
O
C
H3C CH2
O
C
NH NH
C
NH NH
O
CH2 CH3
n-1
n-1
3Dn
iv
N
O
N
N
N
O
CH2 CH3
H3C CH2
O
n-1
n-1
O
OXD3-n
n=7, 10, 16
Scheme 3. (i) RBr (R = CnH2n + 1), acetone, K2CO3, reflux 40 h; (ii) hydrazine monohydrate, ethonal, reflux, 40 h; (iii) terephthaloyl chloride, THF/
pyridine, r.t. 8 h; (iv) POCl3, reflux, 40 h.
2.2.1. Synthesis of OXD1 (see Scheme 1)
phosphorous oxychloride (POCl3) and refluxed for about
24 h. After cooling to room temperature, the mixture was
poured into water. And the precipitate was filtered, washed
thoroughly with water, dried completely, and recrystallized
from DMSO to give 0.68 g white powder of OXD1 (yield
74%, mp > 300 ꢁC).
2.2.1.1. Terephthalic dihydrazide (1B). Diethyl terephthal-
ate (5 g, 22.5 mmol) was dissolved in 50 mL ethanol and
hydrazine monohydrate (10 mL) was added. The reaction
mixture was refluxed for about 24 h. The product precipi-
tated from the mixture. After cooling to room temperature,
the precipitate was filtered, washed with water thoroughly
and dried completely to give 4.27 g white powder (yield
81%, mp > 300 ꢁC).
1H NMR (DMSO ppm from TMS, 500 MHz) 8.38 (s,
4H), 8.09 (d, 4H, J = 2.5 Hz), 7.48 (d, 4H, J = 5.0 Hz),
2.39 (s, 6H).
1H NMR (dimethyl sulphoxide (DMSO) ppm from
TMS, 500 MHz) 9.89 (s, 2H), 7.87 (s, 4H), 4.59 (s, 4H).
FTIR (KBr disc cmꢀ1) 3326, 3035, 1630, 1607, 1543,
1492, 1342, 1292, 1103, 1016, 928, 889, 738, 714, 641, 497.
FTIR (KBr disc cmꢀ1) 3025, 2966, 2921, 2856, 1611,
1572, 1554, 1495, 1456, 1409, 1310, 1270, 1182, 1108,
1075, 1016, 963, 852, 823, 730, 707, 624, 497.
Anal. Calcd for C24H18N4O2: C, 73.08%; H, 4.60%; N,
14.20%. Found: C, 73.25%; H, 4.74%; N, 14.41%.
2.2.1.2. 1,4-Bis[(4-methylphenyl)hydrozide]phenylene (1C).
Terephthalic dihydrazide (1 g, 5.15 mmol) was added into
100 mL pyridine and p-toluoy chloride (1.75 g, 11.3 mmol)
was added. The mixture was refluxed for about 8 h, filtered,
washed with water thoroughly and dried completely to give
1.98 g white solid (yield 89%, mp > 300 ꢁC).
2.2.2. Synthesis of OXD2–n (see Scheme 2)
Synthesis of side-on oxadiazoles with different length
of alkyl chains were performed through the similar proce-
dure. A representative procedure for 5,50-di-(4-methyl)-
2,20-p-(2,5-bisheptyloxyphenylene)-bis-1,3,4-oxadiazole
(OXD2–7) was given below.
1H NMR (DMSO ppm from TMS, 500 MHz) 10.65 (s,
2H), 10.51 (s, 2H), 8.05 (s, 4H), 7.85 (d, 4H, J = 5.0 Hz),
7.34 (d, 4H, J = 5.0 Hz), 2.39 (s, 6H).
2.2.2.1. Diethyl 2,5-dihydroxyterephthalate (2B). 2,5-
Dihydroxyterphthalate acid (1.5 g, 7.58 mmol) was
dissolved in 60 mL ethanol, sulfuric acid (98%) 6 mL
was added and refluxed for about 18 h. The reaction mix-
ture was cooled, the product precipitated, the precipitate
was filtered and washed thoroughly with water and dried
completely to give 1.6 g green needle-like crystal (yield
83%, mp 133–135 ꢁC).
FTIR (KBr disc cmꢀ1) 3197, 3027, 2921, 2850, 1598,
1568, 1514, 1462, 1270, 1177, 1116, 1013, 866, 837, 785,
747, 723, 661, 637, 602, 541, 474, 442.
2.2.1.3. 1,4-Bis[(4-methylphenyl)-1,3,4-oxadiazolyl]pheny-
lene (OXD1). 1,4-Bis[(4-methylphenyl)hydrozide]pheny-
lene (1C) (1 g, 2.33 mmol) was added into 70 mL

58
X. Zhang et al. / Journal of Molecular Structure 846 (2007) 55–64
1H NMR (CDCl3 ppm from TMS, 500 MHz) 10.16 (s,
1.71–1.76 (m, 4H), 1.37–1.42 (m, 4H), 1.27–1.35 (m,
2H), 7.49 (s, 2H), 4.40–4.45 (q, 4H), 1.43 (t, 6H,
J = 7.5 Hz).
12H), 0.87 (t, 6H, J = 5.0 Hz).
FTIR (KBr disc cmꢀ1) 3326, 3280, 2919, 2848, 1652,
1619, 1537, 1493, 1468, 1416, 1385, 1276, 1216, 1096,
1058, 997, 886, 719, 591.
FTIR (KBr disc cmꢀ1) 3295, 2991, 2941, 2908, 2876,
1688, 1496, 1470, 1403, 1371, 1321, 1288, 1217, 1199,
1179, 1100, 1020, 888, 879, 803, 793, 660, 512.
2.2.3.5. 1,4-Bis[(4-methylphenyl)hydrozide]-2,5-bisheptyl-
oxy benzene (2E7). 2,5-Bisheptyloxyterephthalic dihydraz-
ide (0.35 g, 0.83 mmol) was dissolved in 20 mL pyridine,
p-toluoy chloride (0.26 g, 1.17 mmol) was added and the
product precipitated out during reaction. The mixture
was stirred at room temperature for about 8 h. The
crude product was filtered and recrystallized from chloro-
form to give 0.47 g white powder (yield 82%, mp 232–
234 ꢁC).
2.2.3. Synthesis of diethyl 2,5-dialkoxyterephthalates (2C7,
2C16)
2.2.3.1. 2,5-Bisheptyloxyterephthalic acid diethyl ester (2C7).
Diethyl 2,5-dihydroxyterephthalate (0.5 g, 1.97 mmol) was
dissolved in 40 mL of dry acetone. 1-Bromoheptane (2 g,
11.17 mmol), anhydrous potassium carbonate (2.00 g)
and KI (0.04 g) were added. The reaction mixture was
refluxed for about 40 h. The hot reaction mixture was fil-
tered and the residue thoroughly washed twice with ace-
tone. Solvent was removed by evaporation in a rotary
evaporator. The crude product was recrystallized from
methanol to give 0.66 g white powder (yield 75%, mp 36–
37 ꢁC).
1H NMR (CDCl3 ppm from TMS, 500 MHz) 11.37 (t,
2H, J = 10 Hz), 9.47 (t, 2H, J = 10 Hz), 7.90 (s, 2H), 7.79
(d, 4H, J = 2.5 Hz), 7.30 (d, 4H, J = 2.5 Hz), 4.27 (t, 4H,
J = 5.0 Hz), 2.43 (s, 6H), 2.07–2.10 (m, 4H), 1.53–1.55
(m, 4H), 1.43–1.44 (m, 4H), 1.30–1.33 (m, 8H), 0.89 (t,
6H, J = 7.5 Hz).
1H NMR (CDCl3 ppm from TMS, 500 MHz) 7.34 (s,
2H), 4.35–4.40 (q, 4H), 4.00 (t, 4H, J = 5.0 Hz), 1.78–
1.81 (m, 4H), 1.44–1.48 (m, 4H), 1.29–1.40 (m, 18H),
0.89 (t, 6H, J = 5.0 Hz).
FTIR (KBr disc cmꢀ1) 3224, 3191, 3030, 2957, 2912,
2852, 1595, 1568, 1514, 1471, 1461, 1410, 1383, 1288,
1245, 1211, 834, 742.
FTIR (KBr disc cmꢀ1) 2954, 2931, 2856, 1695, 1505,
1468, 1425, 1385, 1306, 1234, 1214, 1110, 1063, 1024,
1007, 958, 887, 786, 725.
2.2.3.6. 1,4-Bis[(4-methylphenyl)hydrozide]-2,5-biscetyloxy
benzene (2E16). (yield 80%, mp 183–185 ꢁC).
1H NMR (CDCl3 ppm from TMS, 500 MHz) 11.36 (t,
2H, J = 5.0 Hz), 9.40 (t, 2H, J = 10 Hz), 7.92 (s, 2H),
7.79 (d, 4H, J = 7.5 Hz), 7.30 (d, 4H, J = 7.5 Hz), 4.29 (t,
4H, J = 7.5 Hz), 2.43 (s, 6H), 2.09–2.11 (m, 4H), 1.53–
1.55 (m, 4H), 1.42–1.44 (m, 4H), 1.29–1.34 (m, 8H), 0.88
(t, 6H, J = 7.5 Hz).
2.2.3.2. 2,5-Biscetyloxyterephthalic acid diethyl ester (2C16).
(yield 76%, mp 62–63 ꢁC).
1H NMR (CDCl3 ppm from TMS, 500 MHz) 7.34 (s,
2H), 4.35–4.39 (q, 4H), 3.99 (t, 4H, J = 10 Hz), 1.78–1.81
(m, 4H), 1.45–1.48 (m, 4H), 1.26–1.40 (m, 54H), 0.89 (t,
6H, J = 5.0 Hz).
FTIR (KBr disc cmꢀ1) 3195, 3028, 2951, 2923, 2848,
1598, 1568, 1514, 1465, 1458, 1401, 1308, 1209, 831, 742.
FTIR (KBr disc cmꢀ1) 2955, 2920, 2849, 1697, 1504,
1468, 1426, 1385, 1305, 1234, 1216, 1110, 1065, 1024,
959, 881, 787, 721.
2.2.3.7. 1,4-Bis[(4-methyloxyphenyl)-1,3,4oxadiazolyl]-2,5-
bisheptyloxyphenylene (OXD2–7). 2E7 (0.40 g, 0.61 mmol)
was dissolved in 20 mL phosphorous oxychloride and
refluxed for about 40 h. After cooling to room tempera-
ture, the mixture was poured into 500 mL water and prod-
uct was collected through filtration and dried completely.
The product was recrystallized from DMSO to give
0.31 g white cotton-like solid of OXD2–7 (yield 82%, mp
170–172 ꢁC).
2.2.3.3. 2,5-Bisheptyloxyterephthalic dihydrazide (2D7). 2C7
(0.47 g, 1.04 mmol) and hydrazine monohydrate (4 mL)
were added into 30 mL ethanol (99.5%) and refluxed for
about 40 h. The reaction mixture was cooled to room tem-
perature, and the product precipitated. The product was fil-
tered and recrystallized from ethanol to give 0.39 g white
cotton-like solid. (yield 86%, mp 114–115 ꢁC).
1H NMR (DMSO ppm from TMS, 500 MHz) 9.21 (s,
2H), 7.38 (s, 2H), 4.59 (s, 4H), 4.06 (t, 4H, J = 7.5 Hz),
1.71–1.76 (m, 4H), 1.37–1.42 (m, 4H), 1.28–1.34 (m,
12H), 0.87 (t, 6H, J = 5.0 Hz).
1H NMR (CDCl3 ppm from TMS, 500 MHz) 8.04 (d,
4H, J = 5.0 Hz), 7.83 (s, 2H), 7.35 (d, 4H, J = 2.5 Hz),
4.19 (t, 4H, J = 5.0 Hz), 2.46 (s, 6H), 1.91–1.94 (m, 4H),
1.55–1.58 (m, 4H), 1.37–1.40 (m, 4H), 1.26–1.29 (m, 8H),
0.87 (t, 6H, J = 5.0 Hz).
FTIR (KBr disc cmꢀ1) 3383, 3305, 3218, 2952, 2922,
2850, 1650, 1622, 1469, 1384, 1206, 1036, 1016, 975, 903,
806, 592.
FTIR (KBr disc cmꢀ1) 2956, 2927, 2856, 1617, 1584,
1551, 1497, 1483, 1461, 1446, 1392, 1333, 1292, 1245,
1228, 1181, 1117, 1079, 1011, 974, 873, 823, 748, 725,
507.
Anal. Calcd for C38H46N4O4: C, 73.28%; H,
7.44%; N, 9.00%. Found: C, 73.45%; H, 7.43%; N,
8.63%.
2.2.3.4. 2,5-Biscetyloxyterephthalic dihydrazide (2D16).
(yield 87%, mp 111–113 ꢁC).
1H NMR (DMSO ppm from TMS, 500 MHz) 9.21 (s,
2H), 7.38 (s, 2H), 4.59 (s, 4H), 4.06 (t, 4H, J = 7.5 Hz),

X. Zhang et al. / Journal of Molecular Structure 846 (2007) 55–64
59
2.2.3.8. 1,4-Bis[(4-methyloxyphenyl)-1,3,4-oxadiazolyl]-
2,5-biscetyloxyphenylene (OXD2–16). (yield 84%, mp 138–
139 ꢁC).
2.2.4.4. 4-Cetyloxybenzoic hydrazide (3C16). 3B16 (4.1 g,
10.5 mmol) and hydrazine monohydrate (10 mL) were add-
ed into 40 mL ethanol (99.5%) and the mixture was
refluxed for about 40 h. The reaction mixture was cooled
and the product precipitated. The product was filtered
and recrystallized from ethanol to give 3.34 g white solid
(yield 85%, mp 106–108 ꢁC).
1H NMR (CDCl3 ppm from TMS, 500 MHz) 8.04 (d,
4H, J = 5.0 Hz), 7.83 (s, 2H), 7.34 (d, 4H, J = 5.0 Hz),
4.19 (t, 4H, J = 7.5 Hz), 2.45 (s, 6H), 1.89–1.93 (m, 4H),
1.54–1.58 (m, 4H), 1.35–1.39 (m, 4H), 1.25–1.31 (m,
44H), 0.88 (t, 6H, J = 7.5 Hz).
1H NMR (DMSO ppm from TMS, 500 MHz) 9.59 (s,
1H), 7.77 (d, 2H, J = 5.0 Hz), 6.95 (d, 2H, J = 5.0 Hz),
4.39–4.40 (s, 2H), 4.00 (t, 2H, J = 7.5 Hz), 1.69–1.72 (m,
2H), 1.38–1.40 (m, 2H), 1.24–1.31 (m, 24H), 0.85 (t, 3H,
J = 5.0 Hz).
FTIR (KBr disc cmꢀ1) 2953, 2919, 2850, 1617, 1584,
1553, 1497, 1480, 1456, 1446, 1390, 1336, 1290, 1226,
1079, 1067, 1018, 874, 823, 746, 722, 506.
Anal. Calcd for C56H82N4O4: C, 76.84%; H, 9.44%; N,
6.40%. Found: C, 76.88%; H, 9.38%; N, 6.02%.
FTIR (KBr disc cmꢀ1) 3333, 3272, 3073, 3065, 2956,
2919, 2850, 1660, 1604, 1571, 1547, 1505, 1472, 1416,
1393, 1339, 1303, 1251, 1188, 1170, 1111, 1046, 1025,
1000, 946, 846, 812, 718, 650.
2.2.4. Synthesis of OXD3–n (see Scheme 3)
The synthesis of end-on oxadiazoles with different
lengths of alkyl chains were performed through the similar
procedure. A representative procedure for OXD3–16 was
given below.
2.2.4.5. 4-Heptyloxy-benzoic hydrazide (3C7). (yield 82%,
mp 91–92 ꢁC).
1H NMR (DMSO ppm from TMS, 500 MHz) 9.59 (s,
1H), 7.78 (d, 2H, J = 2.5 Hz), 6.96 (d, 2H, J = 2.5 Hz),
4.43 (s, 2H), 4.00 (t, 2H, J = 5.0 Hz), 1.68–1.72 (m, 2H),
1.37–1.42 (m, 2H), 1.28–1.35 (m, 6H), 0.87 (t, 3H,
J = 7.5 Hz).
2.2.4.1. Ethyl-4-cetyloxy benzoate (3B16). Ethyl-4-hydroxy
benzoate (2 g, 1.2 mmol) was dissolved in 40 mL dry ace-
tone, cetyl bromide (5 mL), K2CO3 (2 g), KI (0.04 g), were
added. The reaction mixture was refluxed for about 40 h.
The hot reaction mixture was filtered and the residue was
thoroughly washed twice with acetone. The solvent was
removed from the combined filtrates in a rotary evaporator
and the crude product was recrystallized from ethanol to
give 4.1 g waxy solid (yield 88%, mp 48–49 ꢁC).
FTIR (KBr disc cmꢀ1) 3321, 3181, 3065, 3022, 2953,
2935, 2918, 2856, 1647, 1618, 1576, 1508, 1477, 1395,
1352, 1304, 1252, 1187, 1172, 1114, 1041, 1016, 985, 887,
834, 758, 717, 651.
1H NMR (CDCl3 ppm from TMS, 500 MHz) 7.98 (d,
2H, J = 5.0 Hz), 6.90 (d, 2H, J = 2.5 Hz), 4.32–4.36 (q,
2H), 4.00 (t, 2H, J = 5.0 Hz), 1.78–1.81 (m, 2H), 1.42–
1.45 (m, 2H), 1.26–1.39 (m, 27H), 0.88 (t, 3H, J = 5.0 Hz).
FTIR (KBr disc cmꢀ1) 3065, 2956, 2919, 2850, 1715,
1607, 1580, 1510, 1470, 1393, 1366, 1315, 1284, 1251,
1170, 1129, 1107, 1129, 1107, 1025, 851, 769, 719, 697,
646, 516.
2.2.4.6. 4-Decyloxy-benzoic hydrazide (3C10). (yield 80%,
mp 89–91 ꢁC).
1H NMR (DMSO ppm from TMS, 500 MHz) 9.57 (s,
1H), 7.78 (d, 2H, J = 2.5 Hz), 6.96 (d, 2H, J = 2.5 Hz),
4.40 (s, 2H), 4.00 (t, 2H, J = 7.5 Hz), 1.69–1.72 (m, 2H),
1.38–1.41 (m, 2H), 1.25–1.31 (m, 12H), 0.86 (t, 3H,
J = 7.5 Hz).
FTIR (KBr disc cmꢀ1) 3322, 3185, 3065, 3022, 2957,
2935, 2917, 2852, 1646, 1618, 1574, 1507, 1471, 1394,
1352, 1304, 1254, 1188, 1173, 1114, 1051, 1030, 987, 888,
835, 757, 717, 652.
2.2.4.2. 4-Heptyloxy-benzoic acid ethyl ester (3B7). Yellow
oil (yield 76%).
1H NMR (CDCl3 ppm from TMS, 500 MHz) 7.98 (d,
2H, J = 5.0 Hz), 6.90 (d, 2H, J = 5.0 Hz), 4.32–4.36 (q,
2H), 4.00 (t, 2H, J = 5.0 Hz), 1.77–1.81 (m, 2H), 1.43–
1.47 (m, 2H), 1.30–1.39 (m, 9H), 0.90 (t, 3H, J = 7.5 Hz).
FTIR (liquid film cmꢀ1) 3078, 2931, 2858, 1714, 1607,
1580, 1511, 1467, 1421, 1390, 1366, 1314, 1275, 1254,
1168, 1103, 1023, 847, 771, 725, 696, 646, 513.
2.2.4.7. 1,4-Bis[(4-cetyloxyphenyl)hydrozide]phenylene
(3D16). 3C16 (3 g, 8 mmol) and terephthaloyl chloride
(0.81 g, 4 mmol) were dissolved in 70 mL tetrahydrofuran
(THF) and 20 mL pyridine was added, the mixture was
stirred at room temperature for about 8 h. The product
was filtered and washed with water and dried completely,
led to 3.08 g white powder (yield 87%, mp 272–274 ꢁC).
1H NMR (DMSO ppm from TMS, 500 MHz) 10.60 (s,
2H), 10.41 (s, 2H), 8.02 (s, 4H), 7.90 (d, 4H, J = 2.5 Hz),
7.05 (d, 4H, J = 2.5 Hz), 4.02–4.06 (m, 4H), 1.72–1.75
(m, 4H), 1.39–1.42 (m, 4H), 1.24–1.35 (m, 48H), 0.85 (t,
6H, J = 5.0 Hz).
2.2.4.3. 4-Decyloxy-benzoic acid ethyl ester (3B10). Yellow
oil (yield 81%).
1H NMR (CDCl3 ppm from TMS, 500 MHz) 7.99 (d,
2H, J = 7.5 Hz), 6.90 (d, 2H, J = 2.5 Hz), 4.32–4.36 (q,
2H), 4.00 (t, 2H, J = 7.5 Hz), 1.78–1.81 (m, 2H), 1.44–
1.47 (m, 2H), 1.27–1.39 (m, 15H), 0.89 (t, 3H, J = 7.5 Hz).
FTIR (liquid film cmꢀ1) 3078, 2927, 2855, 1716, 1607,
1580, 1511, 1467, 1421, 1392, 1367, 1314, 1276, 1254,
1168, 1103, 1023, 847, 770, 696, 647, 513.
FTIR (KBr disc cmꢀ1) 3210, 3015, 2956, 2920, 2871,
2851, 1599, 1573, 1506, 1462, 1397, 1307, 1251, 1169,
1112, 1049, 1026, 863, 843, 807, 758, 720, 696, 660, 629,
543, 496.

60
X. Zhang et al. / Journal of Molecular Structure 846 (2007) 55–64
2.2.4.8. 1,4-Bis[(4-heptyloxyphenyl)hydrozide]phenylene
(3D7). (yield 92%, mp 297–299 ꢁC).
1H NMR (CDCl3 ppm from TMS, 500 MHz) 8.30 (s,
4H), 8.10 (d, 4H, J = 5.0 Hz), 7.05 (d, 4H, J = 2.5 Hz),
4.05 (t, 4H, J = 5.0 Hz), 1.82–1.85 (m, 4H), 1.49 (m, 4H),
1.25–1.37 (m, 12H), 0.89 (t, 6H, J = 2.5 Hz).
1H NMR (DMSO ppm from TMS, 500 MHz) 10.61 (s,
2H), 10.42 (s, 2H), 8.04 (s, 4H), 7.91 (d, 4H, J = 5.0 Hz),
7.05 (d, 4H, J = 5.0 Hz), 4.05 (t, 4H, J = 5.0 Hz), 1.73–
1.75 (m, 4H), 1.41–1.44 (m, 4H), 1.29–1.36 (m, 12H),
0.87 (t, 6H, J = 5.0 Hz).
FTIR (KBr disc cmꢀ1) 2957, 2946, 2919, 2851, 1609,
1492, 1476, 1414, 1400, 1302, 1258, 1177, 1047, 1071,
1013, 833, 738, 705, 663, 512.
FTIR (KBr disc cmꢀ1) 3209, 3014, 2956, 2924, 2870,
2854, 1598, 1572, 1506, 1462, 1397, 1306, 1251, 1169,
1113, 1041, 1014, 864, 844, 807, 758, 724, 660, 630, 542,
450.
Anal. Calcd for C42H54N4O4: C, 74.30%; H, 8.20%; N,
8.25%. Found: C, 74.41%; H, 7.95%; N, 7.96%.
2.3. Devices fabrication
To investigate the electron-transporting properties of
these oxadiazole derivatives, devices using blends of
MEH-PPV and these compounds as emissive players were
fabricated according to the following procedure. Blends of
MEH-PPV and OXD1, MEH-PPV and OXD2–7, MEH-
PPV and OXD3–7 (weight ratios of MEH-PPV and the oxa-
diazole derivatives were 5:1, 10:1 and 20:1, respectively)
were dissolved in chloroform with concentration of
5 mg mLꢀ1. Single-layer electroluminescent devices were
fabricated by spin-coating the above solutions onto ITO-
coated glass at a rate of 2000 rpm (the thickness of the film
is approximately 50–70 nm) and depositing LiF (1 nm) and
Al (250 nm) orderly (as the cathode) in vapor vacuum of
4 · 10ꢀ5 torr. The emitting area was 2 · 2.5 mm2.
2.2.4.9. 1,4-Bis[(4-decyloxyphenyl)hydrozide]phenylene
(3D10). (yield 94%, mp 295–297 ꢁC).
1H NMR (DMSO ppm from TMS, 500 MHz) 10.60 (s,
2H), 10.40 (s, 2H), 8.04 (s, 4H), 7.91 (d, 4H, J = 2.5 Hz),
7.05 (d, 4H, J = 5.0 Hz), 4.04 (t, 4H, J = 10 Hz), 1.72–
1.75 (m, 4H), 1.41–1.42 (m, 4H), 1.27–1.39 (m, 24H),
0.86 (t, 6H, J = 5.0 Hz).
FTIR (KBr disc cmꢀ1) 3210, 3016, 2953, 2921, 2870,
2852, 1598, 1571, 1504, 1460, 1397, 1307, 1251, 1169,
1113, 1026, 864, 843, 806, 758, 722, 696, 660, 630, 542,
503.
2.2.4.10. 1,4-Bis[(4-cetyloxyphenyl)-1,3,4-oxadiazolyl]phenyl-
ene (OXD3-16). 3D16 (1.56 g, 1.77 mmol) was dissolved in
40 mL phosphorous oxychloride and refluxed for about
40 h. The mixture was poured into 500 mL water and the
precipitate was collected by filtration and dried completely.
The product was recrystallized from DMSO to give 0.90 g
white powder of OXD3–16 (yield 60%, mp 214–215 ꢁC).
1H NMR (CDCl3 ppm from TMS, 500 MHz) 8.30 (s,
4H), 8.10 (d, 4H, J = 5.0 Hz), 7.04 (d, 4H, J = 5.0 Hz),
4.06 (t, 4H, J= 7.5 Hz), 1.82–1.85 (m, 4H), 1.47–1.49 (m,
4H), 1.26–1.38 (m, 48H), 0.88 (t, 6H, J = 5.0 Hz).
FTIR (KBr disc cmꢀ1) 2957, 2918, 2850, 1610, 1498,
1472, 1414, 1396, 1304, 1254, 1173, 1074, 1024, 838, 739,
708, 664, 514.
3. Results and discussion
3.1. Optical properties
We examined the absorption spectra for each oligomer
in dilute chloroform solution. The photophysical proper-
ties of the oligomers are summarized in Table 1. The lowest
energy absorption bands are from the p–p* transitions by
virtue of their large molar extinction coefficients. Two
absorption bands were observed for OXD2–n, while only
one was observed for OXD1 and OXD3–n, respectively.
The coil length has no effect on the maximum absorption
Anal. Calcd for C54H78N4O4: C, 76.55%; H, 9.28%; N,
6.61%. Found: C, 76.75%; H, 9.17%; N, 6.37%.
wavelength (kmax) of both end-on oxadiazoles (OXD3–n
)
and side-on oxadiazoles (OXD2–n). OXD2–n shows
red-shifted kmax compared to that of the analogous
pentaphenylene [12]. The bathochromic-shift of OXD2–n
is presumably due to the intramolecular charge transfer
with a donor–acceptor alternating arrangement in the con-
2.2.4.11. 1,4-Bis[(4-heptyloxyphenyl)-1,3,4-oxadiazolyl]phe-
nylene (OXD3–7). White cotton-like solid (yield 63%, mp
246–247 ꢁC).
1H NMR (CDCl3 ppm from TMS, 500 MHz) 8.29 (s,
4H), 8.10 (d, 4H, J = 2.5 Hz), 7.04 (d, 4H, J = 5.0 Hz),
4.05 (t, 4H, J = 5.0 Hz), 1.82–1.85 (m, 4H), 1.47–1.49 (m,
4H), 1.32–1.40 (m, 12H), 0.91 (t, 6H, J = 7.5 Hz).
FTIR (KBr disc cmꢀ1) 2946, 2924, 2868, 1609, 1492,
1477, 1414, 1399, 1301, 1256, 1176, 1073, 1030, 1012,
848, 830, 738, 705, 663, 512.
Table 1
Properties of the oxadiazole compounds
Compounds kmax of
emission in
kmax of
emission in
kmax of
absorbance in
Tm
(ꢁC)
chloroform
(nm)
thin film (nm) chloroform
(nm)
Anal. Calcd for C36H42N4O4: C, 72.70%; H, 7.12%; N,
9.42%. Found: C, 72.85%; H, 7.08%; N, 9.05%.
OXD1
379,396
397
410
419
420
416
440
442
318,328
329,339
329,339
329,339
307,367
307,367
>300
243.6
232.0
214.9
177.4
139.8
OXD3–7
OXD3–10
OXD3–16
OXD2–7
OXD2–16
400
401
425
426
2.2.4.12. 1,4-Bis[(4-decyloxyphenyl)-1,3,4-oxadiazolyl]phen-
ylene (OXD3–10). White cotton-like solid (yield 66%, mp
231–232 ꢁC).

X. Zhang et al. / Journal of Molecular Structure 846 (2007) 55–64
61
jugated framework of the oligomers, which significantly
3.3. Electrochemical properties
reduces the band gaps [13]. The kmax of the lowest energy
absorption bands of side-on substituted OXD2–7 and
OXD2–16 red shifted about 40 nm compared to that of
OXD1. Introduction of two alkoxy groups whose elec-
tron-donating ability is stronger than that of methyl groups
increases the electron density of the conjugated segment of
OXD3–n, and thus leads to the absorption maximum bath-
ochromic-shift of 10 nm compared to that of OXD1
(328 nm). OXD2–n and OXD3–n exhibited strong blue fluo-
rescence in dilute solution with nearly identical maximum
emission wavelength at about 425 and 400 nm, respectively.
The maximum emission wavelength of OXD1 centered at
379 and 396 nm. The maximum emissions of the com-
pounds in solid films bathochromic-shifted about 15–
20 nm compared to those in chloroform. This may be
due to, in the solid state, the intrachain and/or interchain
mobility of the excitons and excimers generated in the com-
pounds. The absorption and fluorescence spectra of OXD2–7
in chloroform and as thin film which was prepared by spin-
coating the solution onto quartz plate, are displayed in
Fig. 1.
The electrochemical properties of the oligomers were
investigated by cyclic voltammetry (CV) in DMF. The
CV curve of OXD1 is shown in Fig. 2, and Table 2 summa-
rizes electrochemical parameters calculated from CV and
band gaps ðEogptÞ which is estimated from the maximum
absorption wavelength (kmax). The LUMOs and HOMOs
of OXD1, OXD3–7, OXD3–10 and OXD3–16 are ꢀ2.89,
ꢀ2.82, ꢀ2.84, ꢀ2.82 eV and ꢀ6.20, ꢀ6.02, ꢀ6.05,
ꢀ6.06 eV, respectively. Introduction of two alkoxy groups
whose electron-donating ability is stronger than that of
methyl groups (on OXD1) to the end of the oxadiazole
rod lowered the electron affinity, because the electron-with-
drawing effect of the oxadiazole unit is slightly compensat-
ed by the two electron-donating alkoxy groups. Similarly,
lateral attachment of alkoxy groups to the oxadiazole rod
(OXD2–7, OXD2–16) also makes both the HOMOs and
LUMOs higher than that of OXD1. It can be seen that
LUMOs of OXD1, OXD2–n and OXD3–n are in the range
ꢀ2.78 to ꢀ2.89 eV. The HOMO energy levels of OXD3–n
and OXD2–n are 0.14–0.18 and 0.37–0.45 eV higher than
that of OXD1. The above results showed that position of
alkoxy substituents in the oxadiazole derivatives remark-
ably affects the electron distribution of the HOMOs. The
LUMO and HOMO energy levels of the compounds stud-
ied are in the range of ꢀ2.78 to ꢀ2.89 and ꢀ5.75 to
ꢀ6.20 eV, which means that they should show good perfor-
mance as electron-transporting/hole-blocking materials.
3.2. Thermal properties
Thermal properties of OXD2–n and OXD3–n were inves-
tigated by DSC analysis, which indicated that there were
several phase transitions before the crystalline sample com-
pletely melted. OXD2–7 and OXD2–16 melt at 177.4 and
139.8 ꢁC, while OXD3–7, OXD3–10, and OXD3–16 with ter-
minal alkoxy groups exhibit higher melting temperatures at
243.6, 232.0 and 214.9 ꢁC, respectively.
3.4. Electronic structure
Two points should be noted (1) introduction of alkoxy
groups to the rigid segment lowered the melting tempera-
ture (Tm, see Table 1), the longer the alkoxy groups, the
lower the melting points. (2) Lateral attachment of flexible
groups to the rigid segment makes Tm much lower than
those with end-on flexible groups.
Calculations of the electronic structure of OXD1,
OXD2–7 and OXD3–7 were performed by the Dmol3 pack-
age of MS Modeling 3.0 through the geometry optimiza-
tion using GGA method with PW91 function. The
corresponding electron density diagrams of the HOMO
and LUMO are depicted in Fig. 3, and the frontier
Fig. 1. UV–vis absorption and fluorescence spectra of OXD2–7 in
chloroform and as solid film.
Fig. 2. Cyclic voltammetry curve of OXD1 (in DMF/supporting electro-
lyte: TBAP, scan rate: 50 mV sꢀ1).

62
X. Zhang et al. / Journal of Molecular Structure 846 (2007) 55–64
Table 2
The electrochemical parameter, energy level parameters of OXD1, OXD2–n and OXD3–n
Compounds
Ered (V)
Energy (eV)-Exp.a
Energy (eV)-Cal.c
EHOMO (IP)b
ELUMO (EA)
Eogpt
EHOMO
ELUMO
Eg
OXD1
ꢀ1.95
ꢀ2.03
ꢀ2.01
ꢀ2.03
ꢀ2.03
ꢀ2.07
ꢀ6.20
ꢀ6.02
ꢀ6.05
ꢀ6.06
ꢀ5.83
ꢀ5.75
ꢀ2.89
ꢀ2.82
ꢀ2.84
ꢀ2.82
ꢀ2.82
ꢀ2.78
3.31
3.20
3.21
3.24
3.01
2.97
ꢀ5.80
ꢀ5.52
ꢀ2.75
ꢀ2.53
3.05
2.99
OXD3–7
OXD3–10
OXD3–16
OXD2–7
OXD2–16
a
ꢀ5.48
ꢀ2.63
2.85
Energy level determined from the below formula: EHOMO = eEox + 4.5 eV and ELUMO = ꢀeEred ꢀ 4.5 eV [11], where Eox and Ered are the oxidation
and reduction potentials in volts vs. the normal hydrogen electrode potential. Electrochemical band gap Eg = ELUMO–EHOMO. Optical band gap Egopt
calculated from the optical absorption spectra.
b
For OXD1, OXD2–n and OXD3–n, the HOMO energies are estimated according to the Eogpt and the LUMO energies from electrochemical experimental
results.
c
Calculated from Dmol3 package of MS Modeling 3.0 through the geometry optimization using GGA method with PW91 function.
Fig. 3. The density diagram plots of the frontier molecular orbitals of OXD1, OXD3–7 and OXD2–7
.
molecular orbital energies are summarized in Table 2. The
calculations showed that both LUMOs and HOMOs distri-
bution for OXD3–7 are similar to that of OXD1. The
HOMO of OXD1 is the p and p* type orbital localized
on the oxadiazole and phenyl groups, while the LUMO
mainly localized on central phenyl and the two oxadiazole
rings. The HOMO of OXD2–7 is remarkably different from
that of OXD1 in that the symmetry matching 2p(O) orbit
participates in the composition, resulting in the HOMO
energy higher than that of OXD1 (0.32 eV). Meanwhile,
it can be seen that the LUMO density diagrams of
OXD1, OXD3–7 and OXD2–7 are almost identical and they
are in the similar energy levels. As a result, the HOMO–
LUMO energy gaps decrease on going from OXD1 to
OXD2–7. This result is consistent with the fact that the low-
er energy absorption of OXD2–7 (at 367 nm) red-shifted
compared with that of OXD1 (at 328 nm).
The higher HOMO energy levels of OXD2–7 (ꢀ5.48 eV)
and OXD3–7 (ꢀ5.52 eV) than that of OXD1 (ꢀ5.80 eV)
should lead OXD2–n and OXD3–n much easier to be oxi-
dized than OXD1. Alkoxy substituents affect the electron-
donating and -accepting abilities of the species through

X. Zhang et al. / Journal of Molecular Structure 846 (2007) 55–64
63
Table 3
The electroluminescent device properties of oxadiazoles
Emissive layer
Turn-on voltage V
Maximum brightness cd mꢀ2 (V)a
Current efficiency cd Aꢀ1
MEH-PPV
MEH-PPV + OXD1
2.5
2.5
2.5
2.5
3.0
2.5
3.0
3.5
3.0
3.0
295(8.5)
6066(9.0)
480(7.5)
4997(8.5)
11810(8.5)
2471(8.5)
5243(9.0)
1721(8.5)
3624(11.0)
1688(8.5)
0.14
0.89
0.11
0.56
1.10
0.53
0.39
0.34
0.68
0.64
5:1b
10:1
20:1
5:1
10:1
20:1
5:1
MEH-PPV + OXD2–7
MEH-PPV + OXD3–7
10:1
20:1
a
Bias voltage at maximum brightness.
Weight ratio of MEH-PPV to OXD derivatives.
b
the perturbation to the frontier molecular orbital. Conclu-
sively, the electron-transporting characteristics as an
OLED material are effectively improved.
improved compared to that of the MEH-PPV single-layer
device. The maximum brightness and current efficiency of
device (II), in which the weight ratio of MEH-PPV:
OXD2–7 was 5:1, are 11810 cd mꢀ2 (8.5 V) and 1.10 cd Aꢀ1
,
which are 40 times brighter and 6.9 times more efficient
than that of MEH-PPV. Luminance–voltage (L–V) and
current efficiency–voltage (g–V) characteristics of devices
3.5. Electroluminescent properties
The EL properties of the single-layer LED were evaluated,
using the blends of poly (2-methyoxy-5-(20-ethyl-hexyloxy)-
1,4-phenylene vinylene) (MEH-PPV) and oxadiazoles as the
emissive layer. Typical device structures were (I) ITO/
PEDOT/MEH-PPV + OXD1/LiF/Al, (II) ITO/PEDOT/
MEH-PPV + OXD2–7/LiF/Al, (III) ITO/PEDOT/MEH-
PPV + OXD3–7/LiF/Al.
The EL properties of the LEDs are listed in Table 3. The
turn-on voltages of the devices were 2.5–3.5 V. The maxi-
mum brightness of the LEDs was in the range of
480 cd mꢀ2 for device (I) (MEH-PPV: OXD1, 10:1, weight
ratio) and 11810 cd mꢀ2 for device (II) (MEH-PPV:
OXD2–7, 5:1, weight ratio). Fig. 4 shows the EL and PL
spectrum of device (II), using blends of MEH-PPV and
OXD2–7 (5:1) as emissive layer. It can be seen that both
brightness and current efficiency of the devices were
Fig. 5. Luminance–voltage curves (a) and current efficiency–voltage
curves (b) for device (II), using blend of MEH-PPV and OXD2–7 as
emissive layer.
Fig. 4. EL and PL spectra of device (II), using blend of MEH-PPV and
OXD2–7 (5:1, weight ratio) as emissive layer.

64
X. Zhang et al. / Journal of Molecular Structure 846 (2007) 55–64
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layers are shown in Fig. 5.
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derivatives studied function well as electron-transporting
materials and can be used in LEDs to balance hole/electron
injection and transport, and thus to improve the efficiency
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We have successfully prepared and characterized new
symmetrical oxadiazole derivatives. The PL maxima of
the oxadiazole compounds in the solid state located at
420–450 nm. All the compounds studied show quasi-revers-
ible redox behavior, the LUMO and HOMO energy levels
are in the range of ꢀ2.78 to ꢀ2.89 and ꢀ5.75 to ꢀ6.20 eV.
Both HOMO and LUMO energy levels of OXD2–n and
OXD3–n increased compared to that of OXD1, which is
due to the introduction of alkoxy chains. The HOMO ener-
gy level of OXD2–n with side-on alkoxy chains is higher
than that of OXD3–n with end-on alkoxy tail, while the cor-
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Crystallographic data for the compounds reported in
this paper have been deposited with the Cambridge Crys-
tallographic Data Centre as supplementary materials
(CCDC 245025 (OXD1), CCDC 248090 (OXD2–7), CCDC
248091 (OXD3–7)). These data can be obtained free of char-
ge via www.ccdc.cam.ac.uk/conts/retrieving.html, or from
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
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This work was supported by the National Science Foun-
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gram for New Century Excellent Talents in Universities
of China Ministry of Education, and Special Foundation
for PhD Program in Universities of China Ministry of Edu-
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