Light-emitting dyes derived from bifunctional chromophores of diarylamine and oxadiazole: Synthesis, crystal structure, photophysics and electroluminescence

Article abstract of DOI:10.1016/j.dyepig.2010.08.001

The synthesis, structural, photophysical, electrochemical and electroluminescent properties of a novel class of bifunctional molecule are reported in which the hole-transporting triarylamine and electron-transporting oxadiazole components were combined. The strongly luminescent compounds displayed good thermal and morphological stability as well as intense fluorescence both in solution and thin film at room temperature. The effects of the introduction of substituents with different electronic properties upon their absorption and emissive characteristics were correlated with theoretical calculations using density functional theory computations. The photophysics and electrochemistry of such systems were compared to those for the corresponding molecule without an oxadiazole ring. The bipolar compounds could be vacuum-sublimed and applied as emissive dopants for the fabrication of electrofluorescent, organic light-emitting devices with relatively simpler device structures, which can emit tunable colors by varying the aryl ring substituents.

Full text of DOI:10.1016/j.dyepig.2010.08.001

Dyes and Pigments 88 (2011) 333e343
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Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Light-emitting dyes derived from bifunctional chromophores of diarylamine and
oxadiazole: Synthesis, crystal structure, photophysics and electroluminescence
, ,
, ,
,c, ,3
Ze He a, Chi-Wai Kan b 2, Cheuk-Lam Ho a, Wai-Yeung Wong a 1, Chung-Hin Chui b
,
*
*
*
Ka-Lap Tong d, Shu-Kong So d, Tik-Ho Lee a, Louis M. Leung a, Zhenyang Lin e
a Department of Chemistry and Centre for Advanced Luminescence Materials, Hong Kong Baptist University, Waterloo Road, Hong Kong, P.R. China
b Institute of Textiles and Clothing, The Hong Kong Polytechnic University, Hung Hom, Hong Kong, P.R. China
c Department of Medicine and Therapeutics, Li Ka Shing Medical Sciences Building, Prince of Wales Hospital, The Chinese University of Hong Kong, Hong Kong, P.R. China
d Department of Physics, Hong Kong Baptist University, Waterloo Road, Hong Kong, P.R. China
e Department of Chemistry, The Hong Kong University of Science and Technology, Clearwater Bay, Hong Kong, P.R. China
a r t i c l e i n f o
a b s t r a c t
Article history:
The synthesis, structural, photophysical, electrochemical and electroluminescent properties of a novel
class of bifunctional molecule are reported in which the hole-transporting triarylamine and electron-
transporting oxadiazole components were combined. The strongly luminescent compounds displayed
good thermal and morphological stability as well as intense fluorescence both in solution and thin film at
room temperature. The effects of the introduction of substituents with different electronic properties
upon their absorption and emissive characteristics were correlated with theoretical calculations using
density functional theory computations. The photophysics and electrochemistry of such systems were
compared to those for the corresponding molecule without an oxadiazole ring. The bipolar compounds
could be vacuum-sublimed and applied as emissive dopants for the fabrication of electrofluorescent,
organic light-emitting devices with relatively simpler device structures, which can emit tunable colors by
varying the aryl ring substituents.
Received 19 May 2010
Received in revised form
2 August 2010
Accepted 2 August 2010
Available online 10 August 2010
Keywords:
Arylamine
Bipolar molecules
Crystal structures
Electroluminescence
Organic synthesis
Oxadiazole
Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction
interest in OLEDs because they can balance the charge recombi-
nation which is essential for highly efficient devices [6e9]. This
The launch of commercial electronic appliances incorporating
organic light-emitting diodes (OLEDs) as lighting and display units
has illustrated a wide range of applications for these technologies.
OLEDs have the potential to replace both of these areas in the near
future, offering many attractive properties and prospects [1e4].
However, there is certainly room for improvement in order for
OLEDs to compete with other low-cost technologies. Certain issues
including thermal stability, color purity and manufacturing yield
are critical. From the production point of view, reducing the
number of active layers allows a better control of the uniformity of
the device and minimizing interfacial diffusion, which are the main
causes for degradation and poor long-term stability in OLED
operation [5]. Recently, bipolar hosts have aroused considerable
strategy may also negate the possibility of phase segregation and
simplify the device fabrication process relative to those with
multilayer structures [10e13]. However, ways need to be found to
relieve the dilemma between the bipolar transporting properties
and the bandgap of the material, since the electron-donating and
electron-accepting units integrated on bipolar molecules inevitably
lower their bandgaps by intramolecular charge-transfer [9].
Therefore, we are interested in the design and synthesis of some
bipolar dyad materials that also serve as the light-emitter. These
materials are typically composed of a hole-transporting (HT) block
and an electron-transporting (ET) unit which can combine prop-
erties of both functional components to produce new properties
that are hardly possible with either component alone [14,15].
Oxadiazole derivatives are commonly used as ET materials due to
their good thermal and chemical stability as well as their high
photoluminescence (PL) quantum yields, making them ideal for
incorporation into OLED devices [16e19]. They usually exhibit wide
bandgap properties because oxadiazole restricts extension
* Corresponding authors.
E-mail addresses: tccwk@inet.polyu.edu.hk (C.-W. Kan), rwywong@hkbu.edu.hk
(W.-Y. Wong), chchui@graduate.hku.hk (C.-H. Chui).
1
Tel.: þ852 3411 7074; fax: þ852 3411 7348.
2
of
p-conjugation beyond the ring even if the molecule is coplanar
Tel.: þ852 2766 6531; fax: þ852 2773 1432.
3
Tel.: þ852 2632 3120; fax: þ852 2637 5396.
[20,21]. Some bipyridyl oxadiazoles have been reported to serve as
0143-7208/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.dyepig.2010.08.001

334
Z. He et al. / Dyes and Pigments 88 (2011) 333e343
efficient and durable ET and hole-blocking molecular materials
[19]. On the other hand, most HT materials are based on triaryl-
amines since they are a well-known electron donor moiety that can
promote the transport of holes [22,23]. In this article, we present
the synthesis, optical properties and electroluminescent behavior
of a series of new bifunctional pyridyl-based materials containing
oxadiazole and triphenylamine units. Adding different substituents
on the aryl ring of triphenylamine is also shown to be able to tune
the emission wavelength easily.
washed with water (50 mL ꢂ 3), dried over anhydrous MgSO4 and
the solvent removed by rotary evaporation. The product was
purified by column chromatography using CH2Cl2/hexane (1:10,
v/v) as eluent to afford the title compound in 31% yield (1.72 g). 1H
NMR (CDCl3):
d
¼ 7.24e7.18 (m, 2H, Ar), 7.05e6.91 ppm (m, 11H,
Ar); 13C NMR (CDCl3):
d
¼ 160.41, 156.81, 147.85, 143.80, 143.77,
129.16, 125.83, 125.72, 122.70, 122.20, 116.17, 115.83 ppm (Ar); FAB-
MS: m/z 281 (Mþ); Anal. Calcd for C18H13NF2: C, 76.86; H, 4.66; N,
4.98. Found: C, 76.55; H, 4.43; N, 4.67.
2. Experimental
2.2.2. Di(4-tolyl)phenylamine
Yellow viscous liquid from 4-iodotoluene and aniline. Yield: 53%
2.1. General comments
(Eluent: CH2Cl2/hexane (1:1, v/v)). 1H NMR (CDCl3):
d
¼ 7.21e7.15
(m, 2H, Ar), 7.05e6.88 (m, 11H, Ar), 2.29 ppm (s, 6H, CH3); 13C NMR
All reactions were carried out under a nitrogen atmosphere with
the use of standard Schlenk techniques, but no special precautions
were taken to exclude oxygen during work-up. Solvents were
predried and distilled from appropriate drying agents. All chem-
icals, unless otherwise stated, were obtained from commercial
sources and used as received. Preparative TLC was performed
on 0.7 mm silica plates (Merck Kieselgel 60 GF254) prepared in our
laboratory. 1H and 13C NMR spectra were measured in appropriate
solvents on a JEOL EX270 or a Varian INOVA 400 MHz FT-NMR
spectrometer, with the chemical shifts quoted relative to SiMe4.
Fast atom bombardment (FAB) mass spectra were recorded on
a Finnigan MAT SSQ710 mass spectrometer. Electronic absorption
spectra were obtained with a Hewlett Packard 8453 UV-visible
spectrometer. The solution emission spectra and lifetimes were
measured on a Photon Technology International (PTI) Fluorescence
Master Series QM1 spectrophotometer using an xenon lamp and
a N2 laser as the excitation source, respectively. The fluorescence
(CDCl3):
d
¼ 148.11, 145.31, 132.14, 129.70, 128.89, 124.33, 122.84,
121.59 (Ar), 20.87 ppm (CH3); FAB-MS: m/z 273 (Mþ); Anal. Calcd
for C20H19N: C, 87.87; H, 7.01; N, 5.12. Found: C, 87.67; H, 6.88; N,
5.01.
2.2.3. Di(4-methoxyphenyl)phenylamine
Yellow liquid from 1-iodo-4-methoxybenzene and aniline.
Yield: 67% (Eluent: CH2Cl2/hexane (1:1, v/v)). 1H NMR
(CDCl3):
NMR (CDCl3):
d
¼ 7.25e6.79 (m, 13H, Ar), 3.79 ppm (s, 6H, OCH3); 13C
d
¼ 155.52, 148.62, 141.04, 128.80, 126.28, 120.83,
120.47,114.55 (Ar), 55.51 ppm (OCH3); FAB-MS: m/z 305 (Mþ); Anal.
Calcd for C20H19NO2: C, 78.66; H, 6.27; N, 4.59. Found: C, 78.45; H,
6.04; N, 4.33.
2.2.4. N,N-Di(4-fluorophenyl)aminobenzaldehyde
To
a mixture of di(4-fluorophenyl)phenylamine (1.72 g,
6.12 mmol) and DMF (20 mL) at 0 ꢀC, POCl3 (1.03 g, 0.62 mL,
6.73 mmol) was added dropwise with stirring. The resulting
mixture was stirred at 120 ꢀC for 48 h and, after cooling to room
temperature, was poured into ice-water (50 mL), and neutralized
using 4 M aq NaOH solution (50 mL). The mixture was extracted
with CH2Cl2 (50 mL ꢂ 3), washed with water (50 mL ꢂ 3), dried
over anhydrous MgSO4 and the solvent removed by rotary evapo-
ration. The residue was purified by column chromatography using
hexane/ethyl acetate (3:1, v/v) as eluent to afford the target product
quantum yields (
against the anthracene standard (
F
) were determined in CH2Cl2 solutions at 298 K
¼ 0.27) [24]. Thermal analyses
F
were performed with the PerkineElmer TGA6 thermal analyzer at
a heating rate of 20 ꢀC minꢁ1. Electrochemical measurements were
made using a BAS CV-50 W model potentiostat. A conventional
three-electrode configuration consisting of a platinum working
electrode, a Pt-wire counter electrode, and an Ag/AgCl reference
electrode was used. The solvent in all measurements was CH2Cl2,
and the supporting electrolyte was 0.1 M [Bu4N]PF6. Ferrocene was
added as a calibrant after each set of measurements, and all
potentials reported were quoted with reference to the ferrocene-
ferrocenium (Fc/Fcþ) couple at a scan rate of 100 mV sꢁ1. Theo-
retical calculations based on density functional theory (DFT)
approach at the B3LYP level were performed with the use of
Gaussian 03 program [25]. Experimental structures obtained from
X-ray data, whenever available, were used in the calculations. The
basis set used for C, N, O and H atoms was 6-31G [26]. All the MO
plots were made with the use of Molden 3.5 [27]. Compound 2e was
prepared according to the literature method [28,29].
as a yellow solid in 73% yield (1.38 g). 1H NMR (CDCl3):
1H, CHO), 7.68e7.65 (m, 2H, Ar), 7.18e7.16 (m, 4H, Ar), 7.07e7.01
(m, 4H, Ar), 6.94e6.91 ppm (d, 2H, Ar); 13C NMR (CDCl3):
(CHO), 161.65, 158.03, 153.02, 141.76, 141.71, 131.20, 128.83, 127.91,
127.79,118.16,116.74,116.41 ppm (Ar); FAB-MS: m/z 309 (Mþ); Anal.
Calcd for C19H13NF2O: C, 73.78; H, 4.24; N, 4.53. Found: C, 73.67; H,
4.02; N, 4.25.
d
¼ 9.77 (s,
d
¼ 189.97
2.2.5. N,N-Di(4-tolyl)aminobenzaldehyde
Yellow solid from di(4-tolyl)phenylamine. Yield: 64% (Eluent:
CH2Cl2). 1H NMR (CDCl3):
d
¼ 9.71 (s, 1H, CHO), 7.61ꢁ7.58 (d, 3J
(H,H) ¼ 8.1 Hz, 2H, Ar), 7.11e7.01 (m, 8H, Ar), 6.94e6.91 (d, 3J
2.2. General procedures for the preparation of 2a to 2d
(H,H) ¼ 8.1 Hz, 2H, Ar), 2.30 ppm (s, 6H, CH3); 13C NMR
(CDCl3):
d
¼ 189.60 (CHO), 153.17, 143.10, 134.55, 130.91, 130.01,
These compounds were obtained using a procedure similar to
that described below; in each case, a typical example is given for
the fluoro derivative. 4-Tetrazolyltriphenylamine was prepared
according to the literature method [10].
128.08, 126.00, 117.88 (Ar), 20.79 ppm (CH3). FAB-MS: m/z 301
(Mþ); Anal. Calcd for C21H19NO: C, 83.69; H, 6.35; N, 4.65. Found: C,
83.40; H, 6.21; N, 4.55.
2.2.6. N,N-Bis(4-methoxyphenyl)aminobenzaldehyde
2.2.1. Di(4-fluorophenyl)phenylamine
Yellow solid from di(4-methoxyphenyl)phenylamine. Yield: 76%
A mixture of 4-iodofluorobenzene (5 mL, 9.63 g, 43.36 mmol),
aniline (1.83 mL, 20.00 mmol), 1,10-phenanthroline (0.75 g,
4.16 mmol), CuI (0.40 g, 4.04 mmol), KOH (10.0 g, 0.18 mmol) and
xylene (50 mL) was stirred at 140 ꢀC for 72 h. After cooling to room
temperature, the mixture was added to water (200 mL) and CH2Cl2
(100 mL). The mixture was then extracted with CH2Cl2 (50 mL ꢂ 3),
(Eluent: hexane/ethyl acetate (3:1, v/v)).1H NMR (CDCl3):
d
¼ 9.74 (s,
1H, CHO), 7.63e7.60 (m, 2H, Ar), 7.14e7.10 (m, 4H, Ar), 6.90e6.82 (m,
6H, Ar), 3.80 ppm (s, 6H, OCH3); 13C NMR (CDCl3):
d
¼ 190.01 (CHO),
157.10, 153.86, 138.63, 131.26, 127.90, 127.59, 116.60, 114.93 (Ar),
55.46 ppm (OCH3); FAB-MS: m/z 333 (Mþ); Anal. Calcd for
C21H19NO3: C, 75.66; H, 5.74; N, 4.20. Found: C, 75.43; H, 5.44; N, 4.10.

Z. He et al. / Dyes and Pigments 88 (2011) 333e343
335
2.2.7. 4-Cyanophenyl-di(4-fluorophenyl)amine
5.51 mmol) was stirred in DMF (10 mL) and the mixture was heated
under reflux for 90 h. Upon cooling to room temperature, the
mixture was slowly added to 2 M HCl (100 mL). The precipitate was
collected and washed with water. Recrystallization from toluene
gave a pale yellow solid of 4-tetrazolylphenyl-di(4-fluorophenyl)
A mixture of N,N-di(4-fluorophenyl)aminobenzaldehyde (1.38 g,
4.46 mmol), hydroxylamine hydrochloride (0.60 g, 8.63 mmol),
acetic acid (1.2 mL), DMF (8 mL) and pyridine (1 mL) was added to
a reaction flask and the mixture was heated to 130 ꢀC and stirred for
9 h. After cooling to room temperature, the resulting mixture was
added to CH2Cl2 (50 mL), washed with water (20 mL ꢂ 3), dried
over anhydrous MgSO4 and the solvent removed under vacuum.
The residue was purified by column chromatography eluting with
a mixture of CH2Cl2 and hexane (1:1, v/v) to give a yellow solid of
amine (0.76 g, 80%). 1H NMR (d6-acetone):
d
¼ 7.97ꢁ7.93 (m, 2H,
Ar), 7.25e7.12 (m, 8H, Ar), 7.02e6.99 (m, 2H, Ar), 3.23 ppm (s, 1H,
NH); 13C NMR (d6-acetone):
d
¼ 205.96, 162.10, 158.53, 151.22,
143.60, 143.56, 128.90, 128.50, 128.38, 120.74, 117.33, 117.00 ppm
(Ar); FAB-MS: m/z 349 (Mþ); Anal. Calcd for C19H13N5F2: C, 65.33; H,
3.75; N, 20.05. Found: C, 65.10; H, 3.80; N, 19.88.
the title compound (0.84 g, 61%). 1H NMR (CDCl3):
2H, Ar), 7.16e7.01 (m, 8H, Ar), 6.89e6.86 ppm (m, 2H, Ar); 13C NMR
(CDCl3):
d
¼ 7.43e7.39 (m,
d
¼ 161.69, 158.08, 151.34, 141.56, 141.51, 133.10, 127.85,
2.2.11. 4-Tetrazolylphenyl-di(4-tolyl)amine
127.72, 119.38, 118.48, 116.83, 116.49 (Ar), 102.32 ppm (CN); FAB-
MS: m/z 306 (Mþ); Anal. Calcd for C19H12N2F2: C, 74.50; H, 3.95; N,
9.15. Found: C, 74.34; H, 3.77; N, 9.20.
Yellow solid from 4-cyanophenyl-di(4-tolyl)amine. Yield: 94%. 1H
NMR (CDCl3):
d
¼ 7.92e7.89 (m, 2H, Ar), 7.20e7.17 (m, 4H, Ar),
7.07e6.98 (m, 6H, Ar), 2.91 (s, 1H, NH), 2.32 ppm (s, 6H, CH3); 13C
NMR (CDCl3):
d
¼ 144.86, 134.86, 131.12, 130.92, 130.79, 128.72,
2.2.8. 4-Cyanophenyl-di(4-tolyl)amine
127.19,126.52,120.49 (Ar), 20.86 ppm (CH3); FAB-MS: m/z 341 (Mþ);
Anal. Calcd for C21H19N5: C, 73.88; H, 5.61; N, 20.51. Found: C, 73.56;
H, 5.59; N, 20.32.
Yellow solid from N,N-di(4-tolyl)aminobenzaldehyde. Yield: 74%
(Eluent: CH2Cl2/hexane (2:1, v/v)). 1H NMR (CDCl3):
d
¼ 7.38e7.33
(m, 2H, Ar), 7.14e7.11 (m, 4H, Ar), 7.05e7.01 (m, 4H, Ar), 6.90e6.85
(m, 2H, Ar), 2.33 ppm (s, 6H, CH3); 13C NMR (CDCl3):
d
¼ 151.65,
2.2.12. 4-Tetrazolylphenyl-di(4-methoxyphenyl)amine
143.10, 134.86, 132.90, 130.23, 126.08, 119.78, 118.39 (Ar), 101.28
(CN), 20.97 ppm (CH3); FAB-MS: m/z 298 (Mþ); Anal. Calcd for
C21H18N2: C, 84.53; H, 6.08; N, 9.39. Found: C, 84.40; H, 6.16; N, 9.10.
Greenish-yellow powder from 4-cyanophenyl-di(4-methox-
yphenyl)amine. Yield: 24%. 1H NMR (CDCl3):
d
¼ 7.81e7.77 (m, 2H,
Ar), 7.07e7.03 (m, 4H, Ar), 6.89e6.80 (m, 6H, Ar), 3.76 (s, 6H, OCH3),
2.97 ppm (s, 1H, NH); 13C NMR (CDCl3):
d
¼ 156.66, 149.91, 139.28,
2.2.9. 4-Cyanophenyl-di(4-methoxyphenyl)amine
128.17, 127.51, 118.32, 116.38, 114.89, 114.64 (Ar), 55.53 ppm (OCH3);
FAB-MS: m/z 373 (Mþ); Anal. Calcd for C21H19N5O2: C, 67.55; H,
5.13; N, 18.75. Found: C, 67.30; H, 5.22; N, 18.55.
Yellow solid from N,N-di(4-methoxyphenyl)aminobenzaldehyde.
Yield: 76% (Eluent: Hexane/ethyl acetate (3:1, v/v)). 1H NMR
(CDCl3):
Ar), 6.90e6.82 (m, 6H, Ar), 3.80 ppm (s, 6H, OCH3); 13C NMR
(CDCl3):
127.59, 116.60, 114.93 (Ar), 55.46 ppm (OCH3); FAB-MS: m/z 330
(Mþ); Anal. Calcd for C21H18N2O2: C, 76.34; H, 5.49; N, 8.48. Found: C,
76.40; H, 5.30; N, 8.34.
d
¼ 9.74 (s,1H, CHO), 7.63e7.60 (m, 2H, Ar), 7.14e7.10 (m, 4H,
2.2.13. Compound 1a
d
¼ 190.01 (CHO), 157.10, 153.86, 138.63, 131.26, 127.90,
A
mixture of 4-tetrazolylphenyl-di(4-fluorophenyl)amine
(0.76 g, 2.18 mmol), 4-bromobenzoyl chloride (0.60 g, 2.73 mmol)
and dry pyridine (8 mL) was stirred and heated under reflux for 48 h.
After being cooled to room temperature, the mixture was added to
CH2Cl2 (100 mL), washed with water (30 mL ꢂ 3), dried over
anhydrous MgSO4 and the solvent was stripped off under reduced
pressure. The residue was purified by column chromatography
using CH2Cl2 as eluent to afford a yellow solid of 1a (0.77 g, 70%). 1H
2.2.10. 4-Tetrazolylphenyl-di(4-fluorophenyl)amine
A mixture of 4-cyanophenyl-di(4-fluorophenyl)amine (0.84 g,
2.73 mmol), NaN3 (0.36 g, 5.51 mmol) and NH4Cl (0.30 g,
Table 1
X-ray crystal data for selected molecules.
Compound
1c
1d
2a
2b
2c
2e
formula
C28H22N3BrO$CH2Cl2
581.32
Triclinic
P21/c
14.144 (2)
10.145 (1)
18.584 (3)
90
95.938(2)
90
C28H22N3BrO3
528.40
Monoclinic
P21/c
14.1179 (6)
17.8171 (7)
9.5858 (4)
90
90.358(1)
90
C31H20N4F2O
502.51
Triclinic
C31H22N4O
466.53
Triclinic
C31H22N4O
494.58
Monoclinic
P21/c
13.5476 (9)
11.5633 (8)
17.222 (1)
90
98.903 (1)
90
C23H18N2
322.39
Orthorhombic
P212121
10.1593 (8)
12.752 (1)
13.275 (1)
90
formula weight
crystal system
space group
a (Å)
b (Å)
c (Å)
ꢀ
P ı
ꢀ
P ı
9.3204 (7)
11.9915 (9)
12.1553 (9)
73.676 (1)
83.694 (1)
77.613 (1)
1271.7 (2)
2
10.768 (1)
11.761 (2)
19.961 (3)
103.368 (2)
98.022 (2)
101.334 (3)
3836 (2)
4
a
b
g
(deg)
(deg)
(deg)
90
90
V (Å3)
2652.3 (7)
4
1.456
2411.2(2)
4
1.456
2665.3 (3)
4
1.233
1719.9 (2)
4
1.245
Z
Dcalcd (g cmꢁ3
)
1.312
0.092
1.310
0.081
m
(mmꢁ1
)
1.779
1.741
0.076
0.073
F(000)
range (deg)
reflections collected
unique reflections
Rint
observed reflections
no. of parameters
R1, wR2 [I > 2.0
1184
2.20e25.00
10619
4218
0.1139
2164
325
0.0428, 0.0917
0.1224, 0.1158
0.857
1080
1.84e25.00
11799
4238
0.0221
3303
298
0.0304, 0.0561
0.0592, 0.0647
1.039
520
1.80e28.25
7611
5523
0.0142
3078
343
0.0480, 0.1292
0.0932, 0.1586
0.996
976
1.86e25.00
11831
8164
0.0236
4509
244
0.0456, 0.1110
0.1035, 0.1376
0.955
1040
2.13e25.00
12915
4667
0.0171
3719
343
0.0521, 0.1542
0.0643, 0.1708
1.030
680
2.52e25.00
8591
3022
0.0162
2750
q
226
s
(I)] a
0.0294, 0.0728
0.0341, 0.0766
1.024
R1, wR2 (all data)
GoF on F2b
P
P
a
R1 ¼ kFoj ꢁ jFck/SjFoj. wR2 ¼ {S[w(F2o ꢁ Fc2)2]/ [w(Fo2)2]}1/2
.
P
b
GoF ¼ [( wjFoj ꢁ jFcj)2/(Nobs ꢁ Nparam)]1/2
.

336
Z. He et al. / Dyes and Pigments 88 (2011) 333e343
NMR (CDCl3):
d
¼ 7.99e7.90 (m, 4H, Ar), 7.68e7.64 (m, 2H, Ar),
2.34 ppm (s, 6H, CH3); 13C NMR (CDCl3):
d
¼ 164.72, 162.92, 151.21,
7.16e6.98 ppm (m, 10H, Ar); 13C NMR (CDCl3):
d
¼ 164.53, 163.14,
143.80, 134.18, 132.18, 130.11, 128.00, 127.83, 125.85, 125.77, 122.99,
161.42, 157.82, 150.93, 142.32, 142.27, 132.27, 128.07, 127.44, 127.32,
126.03, 122.91, 119.73, 116.72, 116.39, 115.51 ppm (Ar); FAB-MS: m/z
504 (Mþ); Anal. Calcd for C26H16N3BrF2O: C, 61.92; H, 3.20; N, 8.33.
Found: C, 61.78; H, 3.10; N, 8.43.
119.59, 114.60 (Ar), 20.99 ppm (CH3); FAB-MS: m/z 496 (Mþ); Anal.
Calcd for C28H22N3BrO: C, 67.75; H, 4.47; N, 8.46. Found: C, 67.49; H,
4.32; N, 8.50.
2.2.16. Compound 1d
2.2.14. Compound 1b
Yellow solid from 4-tetrazolylphenyl-di(4-methoxyphenyl)
Pale yellow powder from 4-tetrazolyltriphenylamine. Yield: 62%
amine. Yield 65% (Eluent: CH2Cl2/ethyl acetate (10:1, v/v)). 1H NMR
(Eluent: CH2Cl2). 1H NMR (CDCl3):
d
¼ 8.00e7.97 (m, 2H, Ar),
7.94e7.91 (m, 2H, Ar), 7.68e7.65 (m, 2H, Ar), 7.36e7.31 (m, 4H, Ar),
7.19e7.09 ppm (m, 8H, Ar); 13C NMR (CDCl3):
(CDCl3):
(m, 2H, Ar), 7.14e7.09 (m, 4H, Ar), 6.94e6.86 (m, 6H, Ar), 3.81 ppm
(s, 6H, OCH3); 13C NMR (CDCl3):
d
¼ 7.97e7.93 (m, 2H, Ar), 7.87e7.83 (m, 2H, Ar), 7.65e7.61
d
¼ 164.77, 163.23,
d
¼ 164.75, 162.77, 156.68, 151.59,
151.04, 146.53, 132.34, 129.59, 128.14, 128.02, 126.06, 125.71, 124.45,
123.00,120.92,115.64 ppm (Ar); FAB-MS: m/z 468 (Mþ); Anal. Calcd
for C26H18N3BrO: C, 66.68; H, 3.87; N, 8.97. Found: C, 66.65; H, 3.56;
N, 8.77.
139.14, 132.13, 127.94, 127.82, 127.50, 125.75, 122.98, 117.87, 114.83,
113.67 (Ar), 55.44 ppm (OCH3); FAB-MS: m/z 528 (Mþ); Anal. Calcd
for C28H22N3BrO3: C, 63.65; H, 4.20; N, 7.95. Found: C, 63.51; H, 4.10;
N, 7.84.
2.2.15. Compound 1c
2.2.17. Compound 2a
Yellow powder from 4-tetrazolylphenyl-di(4-tolyl)amine. Yield:
A mixture of 2-(tri-n-butylstannyl)pyridine (0.53 g,1.43 mmol), Pd
(PPh3)4 (0.10 g, 0.09 mmol) and 1a (0.46 g, 0.92 mmol) was stirred in
toluene (20 mL) and the mixture was heated to 110 ꢀC for 50 h. Upon
80% (Eluent: CH2Cl2). 1H NMR (CDCl3):
d
¼ 7.98e7.95 (m, 2H, Ar),
7.89e7.86 (m, 2H, Ar), 7.66e7.62 (m, 2H, Ar), 7.14e7.01 (m, 10H, Ar),
R
R
phen/CuCl/KOH
POCl3/DMF
N
+
NH2
xylene
I
R
R
R
NH2OH.HCl/acetic acid
DMF/pyridine
NaN3/NH4Cl
N
CHO
N
CN
DMF
R
R
R
R
O
Cl
N
N
N
SnBu3
Br
N
N
N
N
Br
N
Pd(PPh3)4
toluene
O
NH
pyridine
1a R = F
1b R = H
1c R = Me
1d R = OMe
R
R
R
N
O
N
N
SnBu3
N
N
N
Br
Pd(PPh3)4
toluene
N
N
2e
R
2a R = F
2b R = H
2c R = Me
2d R = OMe
Scheme 1. Synthesis of bifunctional compounds and their bromide precursors.

Z. He et al. / Dyes and Pigments 88 (2011) 333e343
337
cooling to room temperature, the solvent was removed by rotary
evaporation and the residue was purified by column chromatography
using CH2Cl2 as eluent to afford the title compound as a yellow
2.2.19. Compound 2c
Yellow solid from 1c. Yield: 85% (CH2Cl2:ethyl acetate (5:1, v/v)).
1H NMR (CDCl3):
¼ 8.70 (m,1H, Ar), 8.20e8.11 (m, 4H, Ar), 7.91e7.88
(m, 2H, Ar), 7.77e7.73 (m, 2H, Ar), 7.26e7.21 (m, 1H, Ar), 7.13e7.01
(m, 10H, Ar), 2.33 ppm (s, 6H, CH3); 13C NMR (CDCl3):
163.32, 155.62, 150.98, 149.56, 143.72, 141.70, 136.65, 133.98, 129.99,
127.72, 127.14, 126.84, 125.62, 124.14, 122.57, 120.48, 119.50, 114.72
(Ar), 20.89 ppm (CH3); FAB-MS: m/z 495 (Mþ); Anal. Calcd for
C33H26N4O: C, 80.14; H, 5.30; N, 11.33. Found: C, 80.20; H, 5.15; N,
11.16.
d
1
powder (0.28 g, 61%). H NMR (CDCl3):
d
¼ 8.74e8.72 (m, 1H, Ar),
8.23e8.15 (m, 4H, Ar), 7.97e7.93 (m, 2H, Ar), 7.82e7.80 (m, 2H, Ar),
7.32e7.27 (m, 1H, Ar), 7.16e7.11 (m, 4H, Ar), 7.07e7.00 ppm (m, 6H,
d
¼ 164.48,
Ar); 13C NMR (CDCl3):
d
¼ 164.58, 163.81, 160.93, 158.49, 155.97,
150.93, 149.88, 142.46, 142.43, 142.11, 136.94, 128.15, 127.46, 127.42,
127.38, 127.15, 124.27, 122.84, 120.77, 119.87, 116.70, 116.48,
115.80ppm (Ar); FAB-MS: m/z 503 (Mþ); Anal. Calcd for C31H20N4F2O:
C, 74.09; H, 4.01; N, 11.15. Found: C, 73,98; H, 4.08; N, 11.02.
2.2.20. Compound 2d
2.2.18. Compound 2b
Yellow solid from 1d. Yield: 67% (CH2Cl2:ethyl acetate (10:1,
Yellow powder from 1b. Yield: 48% (CH2Cl2:ethyl acetate (5:1,
v/v)). 1H NMR (CDCl3):
d
¼ 8.74e8.72 (m, 1H, Ar), 8.23e8.14 (m, 4H,
v/v)). 1H NMR (CDCl3):
d
¼ 8.75e8.72 (m, 1H, Ar), 8.24e8.15 (m, 4H,
Ar), 7.99e7.94 (m, 2H, Ar), 7.82e7.79 (m, 2H, Ar), 7.36e7.27 (m, 5H,
Ar), 7.19e7.09 ppm (m, 8H, Ar); 13C NMR (CDCl3):
Ar), 7.91e7.88 (m, 2H, Ar), 7.81e7.79 (m, 2H, Ar), 7.31e7.26 (m, 1H,
Ar), 7.15e7.12 (m, 4H, Ar), 6.96e6.87 (m, 6H, Ar), 3.82 ppm (s, 6H,
d
¼ 164.54,
OCH3); 13C NMR (CDCl3):
d
¼ 164.76, 163.42, 156.71, 155.92, 151.58,
163.64, 155.87, 150.85, 149.76, 146.49, 141.98, 136.82, 129.49, 127.95,
127.34, 127.07, 125.59, 124.31, 124.26, 122.73, 120.97, 120.69,
115.88 ppm (Ar); FAB-MS: m/z 467 (Mþ); Anal. Calcd for C31H22N4O:
C, 79.81; H, 4.75; N, 12.01. Found: C, 79.87; H, 4.56; N, 11.89.
149.74, 141.87, 139.32, 136.79, 127.90, 127.53, 127.30, 127.01, 124.38,
122.69, 120.66, 118.06, 114.89, 114.05 (Ar), 55.52 ppm (OCH3); FAB-
MS: m/z 527 (Mþ); Anal. Calcd for C33H26N4O3: C, 75.27; H, 4.98; N,
10.64. Found: C, 75.11; H, 5.08; N, 10.44.
Fig. 1. X-Ray crystal structures of (a) 1c, (b) 1d, (c) 2a, (d) 2b, (e) 2c and (f) 2e.

338
Z. He et al. / Dyes and Pigments 88 (2011) 333e343
Table 2
Table 3
Photophysical data for 2ae2e.
Electrochemical properties and frontier orbital energy levels of 2ae2e.
Compound
Tdecomp (ꢀC)
lmax (nm)a
Emission in
Complex
Eg (eV)
E
(V)a
ox
1/2
HOMO (eV)b
LUMO (eV)c
CH2Cl2 (298 K)
2a
2b
2c
2d
2e
3.00
2.99
2.95
2.90
3.25
þ0.76
ꢁ5.56
ꢁ5.47
ꢁ5.34
ꢁ5.17
ꢁ5.26
ꢁ2.56
ꢁ2.48
ꢁ2.39
ꢁ2.27
ꢁ2.01
lem (nm)b
F
(%)c
þ0.66
þ0.54
2a
2b
2c
2d
466
456
466
421
341
293 (54.1)
364 (66.9)
294 (65.6)
368 (71.2)
296 (50.3)
375 (52.3)
294 (21.7)
375 (22.3)
347 (21.7)
462 (3.74)
81.1
70.0
57.0
40.0
89.2
þ0.37, þ0.98 (i)
þ0.46
472 (4.15)
499 (4.60)
537 (2.71)
429 (3.12)
i ¼ irreversible wave.
a
0.1 M [Bu4N]PF6 in CH2Cl2, versus ferrocene/ferrocenium (Fc/Fcþ) couple.
HOMO level was calculated by using the ferrocene value of 4.8 eV below the
b
vacuum level.
c
Estimated from the HOMO and bandgap energies (LUMO ¼ Eg þ HOMO).
2e
a
Extinction coefficients (104 Mꢁ1 cmꢁ1) are shown in parentheses.
Fluorescence lifetimes (ns) are shown in parentheses.
Fluorescence quantum yields were determined against the anthracene standard
b
c
treatment. The single-layer devices were assembled in the
following sequence: indium tin oxide (ITO) with sheet resistance of
(F
¼ 0.27).
80
U per square on glass substrate (anode), 15 nm of CuPc (hole-
injection layer), 0 or 60 nm of TPD (hole-transport layer), 60 or
80 nm of the emitter, 30 nm of Ca and 80 nm of Al (cathode). The
organic layers were evaporated and laminated in the above
sequence under 4 ꢂ 10ꢁ6 Torr without breaking vacuum between
each vacuum deposition process. The organic and electrode layers
were deposited at a rate of 1 and 10 A per second, respectively. The
thickness of each layer was measured in situ by a quartz crystal
sensor and ex situ by a stylus profilometer (Tencor a-step 500). The
EL spectra were measured by a Larry 2048L CCD system and
current-voltage-luminance characteristics were recorded in air
with a Keithley 236 source measuring unit and International Light
Corporation (Model ILC1400) light meter.
2.3. X-ray crystallography
Single crystals suitable for X-ray crystallographic analyses were
grown by slow evaporation of their respective solutions in CH2Cl2/
hexane at room temperature. Important crystal data pertinent to
individual compounds and the structure refinement results are
assemblied in Table 1. The diffraction experiments were carried out
at 293 K on a Bruker Axs SMART 1000 CCD area-detector diffrac-
ꢁ
tometer using graphite-monochromated Mo-Ka radiation
ꢁ
(l
¼ 0.71073 A). The raw intensity data frames were integrated with
the SAINTþ program using a narrow-frame integration algorithm
[30]. Corrections for Lorentz and polarization effects were also
applied by SAINT. For each analysis, an empirical absorption correc-
tion based on the multiple measurement of equivalent reflections
was applied by using the program SADABS [31]. The structures were
solved by the Direct Methods, and expanded by difference Fourier
syntheses using the software SHELTXL [32]. Structure refinements
were made on F2 by the full-matrix least-squares technique. All non-
hydrogen atoms were refined with anisotropic displacement
parameters. The hydrogen atoms were placed in their ideal positions
but not refined. CCDC numbers 776173e776178.
3. Results and discussion
3.1. Synthesis
Scheme 1 shows the chemical structure and the synthetic
strategies leading to the bifunctional molecules in the present
study. First, the N-arylation of aniline was carried out by the
modified Ullmann condensation with the corresponding
p-iodoarene under the CuCl/phen/KOH catalytic system to give the
substituted triarylamines [33e35]. Formylation of substituted tri-
arylamines with POCl3 in DMF affords diphenylaminobenzaldehyde
which can be derivatized to 4-cyanotriphenylamine using
a hydroxylamine hydrochloride/acetic acid/pyridine mixture and
then to 4-tetrazolyltriphenylamine via the use of NaN3/NH4Cl. The
key bipolar products 2ae2d in this work can be obtained from
2.4. OLED fabrication and measurements
Before device fabrication, the ITO coated glass substrates were
ultrasonically cleaned in organic solvents followed by ozone
1.0
1.0
2a
2a
2b
2b
2c
2d
2c
2d
0.8
0.6
0.4
0.2
0.0
0.8
0.6
0.4
0.2
0.0
250
300
350
400
450
400
500
600
700
Wavelength / nm
Wavelength / nm
Fig. 2. Absorption spectra of 2ae2d at 298 K.
Fig. 3. PL spectra of 2ae2d at 298 K.

Z. He et al. / Dyes and Pigments 88 (2011) 333e343
339
4-tetrazolyltriphenylamine [10] by a two-step reaction. 4-Tetrazo-
lyltriphenylamine reacts with 4-bromobenzoyl chloride to give
1ae1d which can then undergo Stille coupling with 2-(tributyl-
stannyl)pyridine using Pd(PPh3)4 as the coupling catalyst to afford
2ae2d [36]. Likewise, compound 2e was also synthesized from
(4-bromophenyl)diphenylamine for comparative purposes.
the methoxy protons of 2d appear as a singlet at
d
3.82. In the 13C
NMR spectra, a total of 19 carbon resonances for 2b and 20 signals
for both 2c and 2d were observed that are consistent with their
formulations. The spectroscopic data shown in the Experimental
Section are in line with their assigned structures.
1H and 13C NMR analyses clearly indicate that a well-defined
structure has been obtained in each compound. In all cases, 1H NMR
resonances arising from the protons of the organic moieties were
observed. The ortho proton on the pyridyl ring shows a character-
3.2. X-ray crystal structures
The three-dimensional molecular structures of 1c, 1d, 2a, 2b, 2c
and 2e were confirmed by X-ray crystallography. Perspective
drawings of compounds are shown in Fig. 1. Among 2ae2c, each of
them consists of a diarylaminophenyl unit covalently linked to an
oxadiazole ring and a pendant free 2-pyridyl group is attached to
istic downfield resonance (
protons appear in the region
methyl substituents in 2c give rise to a sharp singlet at
d
8.70e8.74). The remaining aromatic
6.8e8.2 with accurate integrals. The
2.33, while
d
d
Fig. 4. Contour plots of the frontier molecular orbitals for 2c.

340
Z. He et al. / Dyes and Pigments 88 (2011) 333e343
Table 4
another phenyl ring. These molecules are notably twisted
which can be attributed to the large space torsion disrupting
Performance of OLED devices AeF.
the
p-conjugation between triarylamine and oxadiazole moieties
Device Vturn-on Luminance
hL
hp
lmax
CIE
(V)
7.2
(cd mꢁ2
)
(cd Aꢁ1
)
(lm Wꢁ1
)
(nm)
for the molecules. The oxadiazole plane makes dihedral angles of
23.8 and 10.9ꢀ (5.0 and 4.7ꢀ; 9.2 and 8.9ꢀ) with the amine-bonded
and pyridyl-bonded phenyl rings, respectively, for 2a (2b; 2c).
A
B
C
D
E
F
896 (13.4 V)a
364 (12.6 V)a
868 (12.8 V)a
728 (10.8 V)a
0.53a
0.50b
0.49c
0.19a
0.17b
0.13c
464
458
480
478
508
512
(0.19, 0.25)
8.0
5.4
5.2
5.4
4.8
0.19a
0.07b
0.17c
0.05a
0.02b
0.05c
(0.18, 0.18)
(0.27, 0.36)
(0.26, 0.36)
(0.25, 0.52)
(0.23, 0.51)
3.3. Thermal and photophysical properties
The thermal properties of 2ae2d were examined by thermog-
ravimetric analysis (TGA). All of the compounds generally exhibit
high thermal stability and their onset decomposition temperatures
(Tdecomp) range from 421 to 466 ꢀC (Table 2). Relative to 2e, incor-
poration of an oxadiazole ring notably increases the thermal
stability of 2ae2d. In addition, they can be easily sublimed at low
pressures (w10ꢁ6 Pa) without thermal decomposition.
The absorption and emission data of 2ae2e are shown in Table 2.
For 2ae2d, two distinct absorption bands were observed at
293e296 and 364e375 nm, which are attributed to the
transitions associated with the NeAr and NeC6H4eX fragments
(Fig. 2). Similar data were observed in the spectra of Ar2NeC6H4eX
species near 300 and 350 nm [37,38]. An analogous ligand system
containing a 2,20-bipyridyl unit also shows absorption features at
316 and 388 nm [39]. While the higher energy absorption peak does
not change with the R groups among 2ae2d, the lower-lying
band is slightly blue-shifted by electron-withdrawing units
0.98a
0.66b
0.51c
0.54a
0.25b
0.15c
0.93a
0.82b
0.59c
0.43a
0.33b
0.19c
3864 (13.6 V)a 2.19a
0.87a
0.81b
0.57c
2.15b
2.00c
pep*
3080 (12.4 V)a 1.63a
0.92a
0.68b
0.45c
1.60b
1.37c
a
b
c
Maximum values of the devices.
At 20 mA cmꢁ2
At 100 mA cmꢁ2
.
.
among 2ae2d, while the strongly electron-donating eOMe group
gives the smallest gap. The HOMO of 2e is mainly related to an
(
lmax ¼ 364 nm) and red-shifted by electron-donating moieties
(
lmax ¼ 375 nm) with respect to 2b (lmax ¼ 368 nm). Due to the
out-of-phase combination between the
(amine)-bonded phenyl rings and the amineeN pp orbital while the
LUMOs are derived mainly from * orbitals of the pyridine ring. The
gap calculated for 2e is greater than those calculated for 2ae2d,
p orbitals of the three N
twisted molecular configuration of 2ae2d, they show inhibited
intramolecular charge-transfer and high bandgaps (Eg) (see Table 3).
Each of 2ae2d is a good light-emitter in both solution and solid
state at 298 K. The photoluminescence (PL) spectrum of 2ae2d is
p
dominated by intraligand 1(
pep*) transitions in the visible region
probably due to the fact that the
delocalized.
p system of 2e is less extensively
and the emissions vary from 462 to 537 nm (Fig. 3). Like the
absorption spectra, for 2ae2d, the emissions are sensitive to the
para-substituents on the Ar2N group and the emission wavelength
3.5. Redox behavior
is affected by the electronic properties of
R in the order:
OMe > Me > H > F. In other words, compound 2a which contains
electron-withdrawing fluoro-substituted triphenylamine shows
a hypsochromic-shifted emission, while those with electron-
releasing substituents (eMe 2c and eOMe 2d) undergo bath-
ochromic-shifted emission with respect to the unsubstituted
congener 2b. This is a clear manifestation of the fact that the
position occupied by the substituents in the arylamine moiety
would dominate the HOMO character so that the electron-donating
and -accepting groups can raise and lower the HOMO levels,
respectively (vide infra).
In order to investigate the electronic effects caused by the
addition of R groups, cyclic voltammetry experiments were carried
out for all compounds. Due to the limited electrochemical window
available in CH2Cl2 and the inability of our instrument to measure
the reduction potentials in the range ꢁ2.7 to ꢁ3.5 V, we obtained
only the oxidation potentials for our compounds and the LUMO
Al (150 nm)
CuPc
TPD
Dye
Ca
Al
3.4. DFT calculations
Ca (1 nm)
Device A (2b) 15 nm 60 nm 60 nm 30 nm 80 nm
Device B (2b) 15 nm 0 nm 80 nm 30 nm 80 nm
Device C (2c) 15 nm 60 nm 60 nm 30 nm 80 nm
Device D (2c) 15 nm 0 nm 80 nm 30 nm 80 nm
Device E (2d) 15 nm 60 nm 60 nm 30 nm 80 nm
Device F (2d) 15 nm 0 nm 80 nm 30 nm 80 nm
2b−2d
TPD (40 nm)
CuPc (40 nm)
ITO
Molecular orbital calculations at the B3LYP level of density
functional theory (DFT) were performed on 2ae2d to study their
electronic structures. The HOMO mainly consists of an out-of-phase
Glass
combination between the
phenyl rings and the amineeN pp orbital while the LUMO is derived
from the orbitals of the oxadiazole ring and the phenylpyridine
p orbitals of the three N(amine)-bonded
N
p
group (Fig. 4). The DFT results indicate that the feature of the
frontier molecular orbitals does not change significantly with
different substituents. The calculated HOMOeLUMO gaps for 2ae2e
follow the order: 2d (315.51 kJ molꢁ1) < 2c (325.15 kJ molꢁ1) < 2b
N
N
N
N
Cu
N
N
N
N
N
(342.52 kJ molꢁ1) < 2a (368.57 kJ molꢁ1) < 2e (383.05 kJ molꢁ1
)
TPD
(1 eV ¼ 96.485 kJ molꢁ1). It was shown that the Ar2N groups
effectively determine the properties of HOMOs. The trend is
expected because the highly electronegative eF substituent tends to
stabilize the HOMO significantly, giving the largest energy gap
CuPc
Fig. 5. The general structure for OLED devices and the molecular structures of the
compounds used. CuPc: copper phthalocyanine; TPD: N,N’-diphenyl-N,N’-bis(3-
methylphenyl)-[1,10-biphenyl]-4,40-diamine; ITO: indium tin oxide.

Z. He et al. / Dyes and Pigments 88 (2011) 333e343
341
levels were estimated from the HOMO and bandgap values (Table
3) [40]. Reversible oxidation waves in the range of 0.37e0.76 V
were observed for 2ae2d that can be assigned to the oxidation of
the triphenylamine moiety. It is noteworthy that 2ae2d reveal
a gentle decrease in the oxidative potentials in the sequence
F > H > Me > OMe, which is a manifestation of the electronic effect
induced by the R substituents (viz. negative effect for eF and
positive effect for both eMe and eOMe). As compared to 2e, the
LUMO level was obviously lowered by oxadiazole addition in
2ae2d, signaling their added ET trait. Generally, the lower the
LUMO level of the material, the better its electron injection prop-
erty is. Such energy levels may provide a closer match to the work
function of Al when it is used as the ET material in OLEDs.
Fig. 5 depicts the general structures for the OLED devices and the
molecular structures of the compounds employed. N,N0-Diphenyl-
N,N0-bis(3-methylphenyl-1,10-biphenyl-4,40-diamine) (TPD) acts as
the HT layer and copper(II) phthalocyanine (CuPc) is a buffer layer
for hole-injection. The device revealed a typical diode behavior, i.e.,
under the forward bias (a positive voltage that was applied to the
ITO electrode), the current increased sharply with the increase of
applied voltage after exceeding the turn-on voltage (Vturn-on).
Moreover, under reverse bias, no obvious increase of current was
observed when the applied voltage was increased. Fig. 6 depicts the
current densityevoltageeluminance (JeVeL) curves and EL spectra
of all the devices.
Devices A, C and E were built using 2b, 2c and 2d, respectively,
and the corresponding EL emission was observed at 464, 480 and
508 nm (Fig. 6), which is consistent with each of their corre-
sponding PL spectra. In other words, the electrofluorescence energy
depends on the nature of R groups and color tuning is made feasible
by appropriate substitution of aryl rings. The EL efficiencies were
3.6. Organic electroluminescence
We also investigated the electroluminescent (EL) behavior of
2be2d in OLED devices and the results are summarized in Table 4.
1000
1000
0.01
1E-7
1.0
device A
device B
device A
device B
800
600
400
200
0
0.8
0.6
0.4
0.2
0.0
300
400
500
600
700
800
0
2
4
6
8
10
12
14
Wavelength / nm
Voltage / V
10000
100
1000
800
600
400
200
0
1.0
0.8
0.6
0.4
0.2
0.0
device C
device D
device C
device D
1
0.01
1E-4
1E-6
0
2
4
6
8
10
12
300
400
500
600
700
800
Voltage / V
Wavelength / nm
4000
3000
2000
1000
0
1.0
0.8
0.6
0.4
0.2
0.0
1000
10
device E
device F
device
device F
E
0.1
1E-3
1E-5
1E-7
0
2
4
6
8
10
12
14
300
400
500
600
700
800
Voltage / V
Wavelength / nm
Fig. 6. JeVeL characteristics and EL spectra of devices (a) A and B, (b) C and D, and (c) E and F.

342
Z. He et al. / Dyes and Pigments 88 (2011) 333e343
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generally shown to be higher for the emitters with electron-
donating substituents than the unsubstituted one. There is no
emission from TPD in each case which implies that transport of
holes through the TPD layer can occur and therefore holes can be
effectively injected into the emissive layer to result in EL emission
in our devices. Also, with the absence of any electron-transporting
layer in these devices, we were still able to get moderate device
performance. All these results clearly indicate that the active layer
consisting of our compounds 2be2d serves as a light-emitter and
an electron-transporting molecule in each case of devices A, C and
E. Interestingly, devices B, D and F containing a sole active layer
between the bilayer anode (ITO/CuPc) and cathode (Ca/Al) can also
give good EL spectra akin to those of devices A, C and E. This implies
that 2be2d are essentially good bipolar small-molecule materials
with both hole- and electron-transporting features. However,
similar device design with the configuration ITO/2be2d/Alq3/Ca/Al
(Alq3: tris(8-hydroxyquinolinato)aluminum) only afforded the Alq3
green emission near 520 nm, suggesting that the bifunctional dye
has a less effective HT characteristics than TPD. Hence, a slightly
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4. Conclusions
In conclusion, the present work reports the synthesis and
characterization of some new light-emitting bifunctional organic
materials incorporating both hole-transporter triarylamine and
electron-transporter oxadiazole units, study of their structural and
photophysical properties and their applications in simple mono-
chromatic OLEDs. Modification of the para-substituents on the NAr2
groups effectively facilitates the fine-tuning of the emission energy
for the compounds and their HOMO/LUMO levels. End-capping of
the molecules with a diarylamino unit has been shown to offer
advantages in terms of lowering the first ionisation potential, which
improves the HT properties. The presence of an electron-deficient
oxadiazole ring can lower the LUMO level which endows these
functional molecules with enhanced ET features. Their possible
multifunctionality as bipolar organic emitters/phosphorescent
hosts and ligand precursors for the generation of new phospho-
rescent metal complexes would render these materials promising
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W.-Y. W thanks the Hong Kong Research Grants Council (HKBU
202709) and the Hong Kong Baptist University (FRG2/08-09/111)
for financial support. C.-W. Kan wishes to acknowledge the finan-
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University (A-PD1G).
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