Synthesis, single crystal structure and photo-luminescent property of bipolar compounds containing 1,3,4-oxadiazole and carbazolyl units

Article abstract of DOI:10.1016/j.molstruc.2010.01.016

Two bipolar molecules 1a and 1b, which incorporate both electron-transporting oxadiazole and hole-transporting carbazolyl or N,N′-dimethylphenyl moieties within the conjugation system, were synthesized and characterized by ¹H NMR, ¹³

Full text of DOI:10.1016/j.molstruc.2010.01.016

Journal of Molecular Structure 968 (2010) 32–35
Contents lists available at ScienceDirect
Journal of Molecular Structure
journal homepage: www.elsevier.com/locate/molstruc
Synthesis, single crystal structure and photo-luminescent property
of bipolar compounds containing 1,3,4-oxadiazole and carbazolyl units
*
Jie Han , Yong-Heng Wei
Department of Chemistry, Nankai University, Tianjin 300071, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Two bipolar molecules 1a and 1b, which incorporate both electron-transporting oxadiazole and hole-
transporting carbazolyl or N,N0-dimethylphenyl moieties within the conjugation system, were synthe-
sized and characterized by 1H NMR, 13C NMR, elemental analysis and mass spectrometry. The molecular
structure of compound 1a was further confirmed by single X-ray crystallography. The solutions of both
compounds in CH2Cl2 individually displayed two absorption bands with kmax values at 376–378 nm and
emitted with kmax values at 478–479 nm and quantum yields of 0.61–0.63. The cyclic voltammograms
results showed that the carbazole-containing compound 1a has better hole injection ability than the
N,N0-dimethylphenyl end-capped 1b.
Received 19 October 2009
Received in revised form 31 December 2009
Accepted 7 January 2010
Available online 14 January 2010
Keywords:
1,3,4-Oxadiazole
Carbazole
Bipolar charge transport
Synthesis
Ó 2010 Elsevier B.V. All rights reserved.
1. Introduction
liquid crystals [15] due to their good thermal stability, high pho-
toluminescence quantum yield and the electron-deficient nature
Ever since Tang and Van Slyke reported the first electrolumi-
nescent devices using organic compounds as emitters [1], devel-
opment of organic light-emitting diodes (OLEDs) has been an
active subject of academic and industrial research due to the po-
tential use in displays [2,3]. To achieve high luminescence effi-
ciency, the electron and hole currents in OLEDs should be
balanced. To date, several ways have been successfully adopted
to improve the carrier injection capability [4]. Generally, the most
common method is to dope electron- and hole-transport materi-
als, which results many layers in an OLED and make it sophisti-
cated to make a device [5]. In order to avoid this drawback,
another strategy – to incorporate the electron- and hole-transport
moieties within a single molecule, was first suggested by Murray
et al. who reported the first example of bipolar charge transport
molecule N-(p-tolylamino)phenyl)-N0-(1,2-dimethylpropyl)-1,4,5,-
8-naphthalenetetracarboxylic diimide (TAND) in 1996 [6]. From
then on, there’s an increasing interest in research in this field, be-
cause such materials covalently combining two kinds of func-
tional subunits may reduce the layers in OLED [7–11], and also
avoid the problem of phase separation occasionally encountered
in using doping strategy [12].
of the oxadiazole moiety. In contrast, the triphenylamine deriva-
tives have been proved to be excellent hole-transporting materi-
als, which have shown a wide range of practical applications [16–
18]. On base of the electronic nature of the 1,3,4-oxadiazole and
triphenylamine units, it is possible to develop new bipolar charge
organic compounds by rational combination of these two struc-
tural motifs, and an authoritative review was published by
Hughes and Bryce in 2005 [19]. In recent years, there has been
a growing interest in the development of 1,3,4-oxadiazole- and
triphenylamine-containing compounds due to the potential appli-
cation in OLEDs. Although quite a lot of low molecular weight or-
ganic molecules [20–24] and various organic complexes [25–29]
of such kind of compounds have been reported in literature, the
single X-ray crystal structures of the bipolar charge compounds
are scarce [30,31], which makes it difficult to study the struc-
ture–property relations of this class of materials. Herein, we re-
port the synthesis, single crystal structure and photophysical
property of new bipolar molecules containing N,N0-dimethyl-
phenyl and/or carbazolyl end-capped 2,5-diaryl-1,3,4-oxadiazole
derivatives 1a and 1b, and the synthetic route as well as the reac-
tion conditions is depicted in Scheme 1. In these compounds, the
carbazolyl or N,N0-dimethylphenyl and 1,3,4-oxadiazole moieties
connected with phenyl and ethynyl groups serve as hole- and
electron-transporting functionalities, respectively, which are typ-
ical for the bipolar molecules. In addition, the whole compounds
Among many kinds of organic electron-transporting materials,
2,5-diaryl-1,3,4-oxadiazole derivatives have been intensively
studied in fields of organic light-emitting diodes [13,14] and
form a donor-
molecular wire.
p-acceptor system, which may also be used as
* Corresponding author. Tel.: +86 22 2350 9933.
E-mail address: hanjie@nankai.edu.cn (J. Han).
0022-2860/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.molstruc.2010.01.016

J. Han, Y.-H. Wei / Journal of Molecular Structure 968 (2010) 32–35
33
O
(1)
O
(2)
N
I
N
N
N
N
N
OH
ii
i
O
N
N
(3)
(4)
N
N
1a
O
N
H
N
N
O
N
N
iii
N
N
1b
Scheme 1. Reagents and conditions: (1) 2-methyl-3-butyn-2-ol, CuI, Pd(PPh3)2Cl2, PPh3, triethylamine, tetrahydrofuran, r.t., 7 h; (2) sodium hydroxide, toluene, reflux, 2 h;
(3) 9-(4-bromophenyl)carbazole, CuI, Pd(PPh3)2Cl2, PPh3, triethylamine, tetrahydrofuran, reflux 6 h; (4) CuI, Pd(PPh3)2Cl2, PPh3, triethylamine, tetrahydrofuran, 4-iodo-N,N0-
dimethylaniline, reflux, 6 h.
tively, while the angle between the benzene rings iii and iv is
12.89°. In particular a planar trigonal geometry is observed around
the N amino atom, N1, as reported in other compounds bearing
dialkylamino or diphenylamino groups [36]. The five-membered
ring N1–C1–C6–C7–C12 is inclined by 63.83° to the benzene ring
iii, due to steric repulsion form the phenyl iv and the phenyl groups
in the carbazolyl moiety, which makes the overall molecule of 1a
to adopt a bent T-shape.
2. Result and discussion
2.1. Synthesis and characterization
The starting material 2-(4-iodophenyl)-5-(4-N,N0-dimethyl-
phenyl)-1,3,4-oxadiazole (i) was prepared according to the method
reported by us [32]. 9-(4-Bromophenyl)carbazole was prepared
according to the literature procedures [33]. All of the other chem-
icals were used as purchased from Aldrich. Tetrahydrofuran and
triethylamine were dried over sodium metal and distilled under
a N2 atmosphere prior to use.
2.3. UV–vis absorption and photoluminescence spectra
According to the standard Sonogashira coupling reaction proce-
dures [34], the starting compound (i) reacted with 2-methyl-3-bu-
tyn-2-ol to afford the crude compound (ii), which was directly
used in the following reaction without purification. The key inter-
mediate compound (iii), namely 2-(4-ethynylphenyl)-5-(4-N,N0-
dimethylphenyl)-1,3,4-oxadiazole, was synthesized by refluxing
the crude compound (ii) with sodium hydroxide in toluene. The fi-
nal products 1a and 1b were prepared in good yields according to
the similar procedure to that of compound (ii). The structures and
The UV–vis absorption and photoluminescence spectra of com-
pounds 1a and 1b in dichloromethane are depicted in Fig. 2, and
the photophysical data are summarized in Table 1. Similar absorp-
tion pattern were observed in 1a and 1b, they both displayed a
weak absorption band at kmax = 284–287 nm and a strong absorp-
tion band at kmax = 376–378 nm, which were corresponded to the
p–p
* transitions of the conjugated aromatic rings [37]. As seen in
Fig. 2, both 1a and 1b were emissive at kmax = 478–479 nm with
emission quantum yields of 61% and 63%, respectively. From the
maximum kabs, kem and the emission quantum yields of 1a and
1b, we can see that the terminal N,N-dimethylphenyl and carbaz-
olyl groups influence the photophysical properties of 1a and 1b
slightly. Notably, the Stokes shifts (102 nm for 1a and 101 nm for
1b) are much larger than the analogous 1,3,4-oxadiazole-based
compounds in literatures [38], which may be explained by a
‘‘push–pull effect” created by the strong electron-donating N,N-
dimethylphenyl/carbazolyl groups and the electron-accepting oxa-
diazole moiety [39].
purifications of 1a and 1b were characterized fully by 1H NMR, 13
C
NMR, MS, HRMS and elemental analysis, the molecular structure of
1a was further confirmed by the single X-ray crystallography.
2.2. X-ray crystal structure of compound 1a [35]
In order to investigate the relationship between the structures
and spectroscopic and electrochemical behaviors of the target
compounds, attempts were made to grow single crystals suitable
for X-ray diffraction studies on the final molecules, and yellow
plate-like crystals 1a suitable for single crystal X-ray diffraction
were obtained by slow evaporation of its CH2Cl2 solution at room
temperature. The X-ray molecular structure with atomic number
of 1a is shown in Fig. 1. The dihedral angles between the oxadiaz-
ole ring and the benzene rings i and ii are 5.46°and 5.73°, respec-
2.4. Electrochemical property
The cyclic voltammograms (CVs) of compounds 1a and 1b
were shown in Fig. 3. The resulting data were also summarized
in Table 2. As seen in Fig. 3, both 1a and 1b exhibited two
Fig. 1. Single crystal structure of 1a. Ellipsoids are drawn at 30% probability level.

34
J. Han, Y.-H. Wei / Journal of Molecular Structure 968 (2010) 32–35
1a and 1b, respectively. These reduction waves might be associ-
150
120
90
60
30
0
ated with both the triple bonds and the oxadiazole ring [38].
The presence of the carbazolyl group on the molecular end made
the electrochemical reduction much easier due to the delocaliza-
0.16
0.12
0.08
0.04
0.00
UV- 1a
UV- 1b
Em- 1a
Em- 1b
tion of p-electrons in the carbazole ring system, so we could con-
clude that the carbazole-containing compound 1a may be used to
match the levels of the hole injection and/or transport layers more
closely than 1b.
3. Experimental
3.1. General
300
350
400
450
500
550
600
The nuclear magnetic resonance (NMR) spectra were recorded
on a Bruker AVANCE 300 MHz DRX FT-NMR spectrometer using
CDCl3 as solvent and tetramethylsilane (TMS) as internal standard.
Electron impact (EI) and high-resolution mass spectra (HRMS)
were recorded by a Finnigan MAT 95 mass spectrometer. Elemen-
tal analyses were obtained on Yanaco CHN CORDER MT-3 elemen-
tal analyzer. UV/vis absorption spectra were measured using a
Cary300 absorption spectrometer. Luminescence was measured
using a VARIAN spectrometer. Cyclic voltammetric experiments
were performed on an LK 2005 electrochemical analyzer at room
temperature in nitrogen-purged DMF with 0.1 M TBAPF6 as sup-
porting electrolyte at scanning rate of 100 mV/s. The working elec-
trode was a platinum disk, the assistant electrode was Ag/AgCl, and
a saturated calomel electrode was used as reference electrode, cal-
ibrated against a Fc/Fc+ couple [40]. The onset potential was deter-
mined from the intersection of two tangents drawn at the rising
and background current of the cyclic voltammogram.
Wavelength / nm
Fig. 2. Normalized UV/vis absorption and emission spectra of 1a and 1b in CH2Cl2
at 298 K (concentration = 5 ꢀ 10ꢁ6 mol dmꢁ3).
Table 1
Photophysical data for 1a and 1b in CH2Cl2 at 298 K (concentration = 5 ꢀ 10ꢁ6
mol dmꢁ3).
Compound
kabs/nm (
e
/dm3 molꢁ1 cmꢁ1
)
kem/nm
U
1a
1b
287 (8230), 376 (31,200)
284 (7150), 378 (31,600)
478
479
0.61
0.63
0.02
0.01
3.2. Synthesis of the intermediate compound iii
0.00
A mixture of compound i (452 mg, 1 mmol), 2-methyl-3-butyn-
2-ol (101 mg, 1.2 mmol), bis-(triphenylphosphine) palladium(II)
chloride (0.01 mmol, 7 mg) and copper(I) iodide (0.01 mmol,
2 mg) in a solution of dry THF (20 mL) and triethyl amine (5 mL)
was stirred at room temperature under nitrogen for 12 h. The reac-
tion mixture was evaporated in vacuo and the solid residue was
dissolved with CH2Cl2 (50 mL). The resultant solution was washed
sequentially with aqueous sodium cyanide and brine, and dried
over anhydrous MgSO4, then the solution was filtered and the sol-
vent was removed in vacuo to afford crude solid that was used di-
rectly to prepare the intermediate compound iii. To a methanolic
solution (50 mL) of the as-formed compound ii was added sodium
hydroxide (40 mg, 1 mmol). The reaction mixture was refluxed for
4 h and the solvent was removed by rotary evaporation. The solid
residue was dissolved in CH2Cl2 (50 mL) and the solution was
washed with distilled water and brine and dried over anhydrous
MgSO4. The crude solid was obtained by evaporation of solution,
and then purified by flash column chromatography on silica gel
(eluent: dichloromethane-ethyl acetate, 25: 1 v/v). The total yield
of the two-step reaction was 78%. 1H NMR (CDCl3, 300 MHz): d
8.07 (d, J = 8.57 Hz, 2H), 7.98 (d, J = 9.06 Hz, 2H), 7.62 (d,
J = 8.57 Hz, 2H), 6.76 (d, J = 9.06 Hz, 2H), 3.24 (s, 1H), 3.07 (s, 6H)
ppm; MS-EI (m/z, I%): 289 ([M]+ 86.47); HRMS-EI (m/z): 289.1215
(M+, requies 289.1215). 13C NMR (CDCl3, 75 MHz): 165.65,
163.04, 152.57, 132.79, 128.52, 126.63, 125.08, 124.60, 111.72,
110.82, 93.04, 79.88, 40.19 ppm.
-0.01
-0.02
1a
1b
1.5
1.0
0.5
0.0
E / V
-0.5
-1.0
-1.5
-2.0
Fig. 3. Cyclic voltammograms for 1a and 1b in DMF with 0.1 M TBAPF6 as
supporting electrolyte at scanning rate of 100 mV/s.
Table 2
Electrochemical data for compounds 1a and 1b.
3
2
1
1
2
Compound
E1/2red
E1/2red
E1/2red
E1/2ox
E1/2ox
1a
1b
ꢁ1.99
ꢁ1.77
ꢁ1.80
ꢁ0.93
ꢁ0.93
0.62
0.68
0.86
0.90
ꢁ
irreversible anodic oxidation couples, which could be attributed to
the sequential removal of electrons from the dimethylamino or
arylamine moieties at the molecular end, respectively. These re-
sults suggested that the oxadiazole derivatives containing the
N,N-dimethylphenyl moieties underwent a stepwise oxidation
process, and the second irreversible oxidation step may be due
to the generation of either stable or electro-inactive ionic species
[34]. Since the oxidation process is closely related with the re-
moval of electrons (or the injection of the holes) from the HOMO
of the compounds, the lower oxidation potentials for 1a suggest a
better hole injection ability than 1b. When swept cathodically,
two or three quasi-reversible reduction peaks were observed for
3.3. Synthesis of 2-(4-N,N0-dimethylphenyl)-5-{4-[-(2-(4-N,N0-dimethyl-
amino)phenyl)ethynyl]Phenyl} -1,3,4-oxadiazole 1a
A mixture of compound iii (0.5 mmol), N,N0-dimethyl-4-iodoan-
iline (0.5 mmol), CuI (10 mg), Pd(PPh3)2Cl2 (20 mg) were dissolved

J. Han, Y.-H. Wei / Journal of Molecular Structure 968 (2010) 32–35
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35
in THF (10 ml) and TEA (10 ml). The solution was stirred at room
temperature for 2 h and then heated to reflux for an additional
6 h. The mixture was evaporated to dryness, then the residual yel-
low solid was dissolved in DCM and column chromatographed
(DCM: acetate = 25:1) to yield yellow solid (98 mg, 48%). mp
244–245 °C. 1H NMR (CDCl3, 300 MHz): d 8.06 (d, J = 8.64 Hz,
2H), 7.98 (d, J = 9.08 Hz, 2H), 7.61 (d, J = 8.64 Hz, 2H), 7.47 (d,
J = 8.96 Hz, 2H), 6.76 (d, J = 9.08 Hz, 2H), 6.67 (d, J = 8.96 Hz, 2H),
3.07 (s, 6H), 3.01 (s, 6H) ppm; MS-EI (m/z, I%): 408 (M+, 100);
HRMS-EI (m/z): 408.1950 (M+, requires 408.1950). 13C NMR (CDCl3,
75 MHz): 165.45, 163.44, 152.54, 150.52, 133.07, 131.79, 128.50,
127.40, 126.64, 123.03, 111.95, 111.76, 111.08, 109.54, 93.94,
87.24, 40.31, 40.23 ppm. C26H24N4O (408.50): calcd C, 76.45; H,
5.92; N, 13.72; found: C, 76.74; H, 5.66; N, 13.69.
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Compound 1b was prepared according to the similar proce-
dures as 1a.
2-(4-N,N0-dimethylphenyl)-5-{4-[-(9-phenylcarbazole)ethynyl]-
phenyl}-1,3,4-oxadiazole 1b: Yellow solid; 275 mg, yield 52%; mp
278–280 °C. 1H NMR (CDCl3, 300 MHz) d: 8.15 (m, 4H), 8.00 (d,
J = 9.08 Hz, 2H), 7.79 (d, J = 8.48 Hz, 2H), 7.72 (d, J = 8.52 Hz, 2H),
7.61 (d, J = 8.48 Hz, 2H), 7.44 (m, 4H), 7.31 (m, 4H), 6.78 (d,
J = 9.08 Hz, 2H), 3.08 (s, 6H); MS-EI (m/z, I%): 530 (M+, 100);
HRMS-EI (m/z): 530.2107 (M+, requires 530.2107). 13C NMR (CDCl3,
75 MHz): d 165.66, 163.14, 152.64, 140.68, 138.17, 133.40, 132.34,
128.57, 127.05, 126.79, 126.24, 126.11, 124.17, 123.77, 121.88,
120.54, 120.45, 111.79, 110.92, 109.89, 91.45, 89.80, 40.24 ppm.
C36H26N4O (530.63): calcd C, 81.49; H, 4.94; N, 10.56; found: C,
81.31; H, 5.06; N, 10.35.
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Acknowledgements
This work was supported by a grant from the National Natural
Science Foundation of China (No. 20772064) and the Open Project
of State Key Laboratory of Supramolecular Structure and Materials
(SKLSSM200908). The authors thank Dr. Nianyong Zhu for the sin-
gle X-ray crystallographic measurements.
[35] Crystal data for 1a: C36H26N4O, Mr = 530.61, Orthorhombic, P n a 21. Crystal
size: 0.4 ꢀ 0.25 ꢀ 0.2 mm3, a = 10.841(2) Å, b = 10.504(2) Å, c = 28.378(5) Å,
a
q
= 90°,
b = 90°,
c
l
= 90°,
V = 3231.5(10) Å3,
Z = 4,
T = 301(2) K,
calcd = 1.091 g cmꢁ3
,
Mo = 0.067 mmꢁ1
, F(000) = 1112, total number of
unique reflections 2539 (Rint = 0.0502). Numbers of parameters = 310,
R1 = 0.0997 and wR2 = 0.2683. Crystallographic data (excluding structure
factors) for 1a have been deposited with the Cambridge Crystallographic
Data Center as supplementary publication nos. CCDC: 602983. Copies of the
data can be obtained free of charge on application to CCDC, 12 Union Road,
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