T.-R. Chen et al. / Journal of Organometallic Chemistry 691 (2006) 1998–2004
2003
Fig. 7c shows a three-layer OLED with the configura-
tion ITO/NPB/NPB + complex 1 (80 wt%)/Alq3/Mg–Ag
(device C). Fig. 9 shows the EL spectra of this device hav-
ing an emission maximum at 596 nm. Comparing to device
A and B, the emission band of this device was apparently
shifted to a shorter wavelength, and an orange OLED
(coordinates x = 0.52 and y = 0.48) was obtained.
Fig. 7d shows a three-layer OLED with the configura-
tion ITO/NPB/NPB + complex 1 (10 wt%)/Alq3/Mg–Ag
(device D). Fig. 9 shows the EL spectra of this device hav-
ing an emission maximum at 526 nm. Interestingly, the
emission wavelength of this device became much shorter,
and a yellow OLED (coordinates x = 0.47 and y = 0.50)
was obtained.
Fig. 7e shows a three-layer OLED with the structure
ITO/NPB/2/Alq3/Mg–Ag (device E). According to the
energy diagram, Fig. 10, the LUMO level of 2 is lower than
that of Alq3, which suggests that the electron will be easy to
transfer from Alq3 to emitter, so the recombination zone
will not be confined at the Alq3/emitter interface, but be
located in the emitting layer; therefore, an emission band
covering the whole visible region from 400 to 750 nm with
the maximum brightness of 320 cd/m2 was observed, and a
perfect white light OLED (CIE = 0.33, 0.37) was obtained.
teristics were measured by Keithley 2400 Source meter. The
highest occupied molecular orbital (HOMO) energy levels
of organic materials were measured from the cyclic voltam-
metry (CV) [27] and the lowest unoccupied molecular orbi-
tal (LUMO) energy levels were determined from the
HOMO energy levels and the optical band gap estimated
from the absorption onset [28]. Photoluminescence (PL)
was measured by Model LS 55 luminescence Spectrometer.
4.2. Synthesis
Ligands, 2-(2-quinolyl)naphtho[b]imidazole (QNI) and
2-(2-quinolyl)benzimidazole (QBI), were prepared as previ-
ously reported [29,30]. Complexes 1 and 2 were prepared in
dry THF by reacting, in a 1:1.05 molar ratio, triphenylbo-
ron with QNI or QBI. The solutions were refluxed for 12 h
under nitrogen. After the solvent was removed by vacuum,
the residues were purified by a train sublimation method.
Selected analytical data concerning complexes 1 and 2 are
the following.
4.2.1. BPh2(2-(2-quinolyl)naphtho[b]imidazolato) (1)
1
Pale yellow solid. 35% yield. M.p. >200 ꢁC. H NMR
(CDCl3, 293 K), d ppm: 8.76 (d, 1H, J = 8.7 Hz, quinolyl),
8.70 (d, 1H, J = 8.7 Hz, quinolyl), 8.40 (s, 1H, imidazole),
8.37 (d, 1H, J = 8.7 Hz, quinolyl), 8.03 (dd, 1H, J = 8.0,
1.2 Hz, quinolyl), 7.96 (m, 1H, imidazole), 7.80 (m, 2H,
imidazole and quinolyl), 7.67 (m, 2H, imidazole), 7.43
(m, 4H, ph), 7.32 (m, 2H, quinolyl), 7.24 (m, 6H, ph).
MS data: 459.2 (35.13%), 382.1 (100%), 332.2 (61.62%),
295.2 (27.27%), 281.2 (20.64%), 267.2 (15.13%), 245.2
(11.05%), 229.5 (5.67%), 191.6 (19.73%), 152.8 (9.32%),
121.1 (14.23%), 104.1 (12.16%), 91.1 (9.51%), 73.1
(13.99%), 57.1 (14.51%). Anal. Calc. for C32H21N3B
(MW = 459.2): C, 83.70; H, 4.83; N, 9.15. Found C,
83.64; H, 4.87; N, 9.12%.
3. Concluding remarks
A new family of emitter for OLED, BPh2(2-(2-quinolyl)-
naphtho[b]imidazolato) (1) and BPh2(2-(2-quinolyl)benz-
imidazolato) (2), have been successfully synthesized and
investigated. It has been shown that the novel ligands
QNI and QBI are capable of chelating to B(III) centers
and the resulting compounds possess appreciable photolu-
minescent efficiency and very high thermal stabilities. This
study further indicates that the emission band of the
devices could be modified by changing the composition
of emitting layer and therefore, OLEDs with different col-
ors could be obtained.
4.2.2. BPh2(2-(2-quinolyl)benzimidazolato) (2)
1
Pale yellow solid. 30% yield. M.p. >200 ꢁC. H NMR
4. Experimental
(CDCl3, 293 K),d ppm: 9.2 (d, 1H, J = 6.3 Hz, quinolyl),
8.67 (d, 1H, J = 6 Hz, imidazole), 8.35 (dd, 1H, J = 6.3,
1.2 Hz, quinolyl), 8.06 (d, 1H, J = 6.6 Hz, quinolyl), 7.82
(m, 2H, imidazole), 7.76 (m, 1H, imidazole), 7.28 (m, 3H,
quinolyl), 7.20 (m, 10H, ph). MS data: 409.2 (13.36%),
382.5 (3.64%), 332.5 (100%), 331.5 (25.58%), 331 (1.57%),
295.4 (3.31%), 245.4 (2.72%), 205.2 (1.27%), 166.7
(1.25%), 151.3 (1.89%), 128.2 (2.41%), 77.1 (3.52%), 51.1
(1.52%). Anal. Calc. for C28H20N3B (MW = 409.2): C,
82.20; H, 4.89; N, 10.26. Found C, 82.14; H, 4.97; N,
10.26%.
4.1. General procedure
All starting materials were purchased from Aldrich
Chemical Co. 1H and 13C NMR spectra were recorded
on the Bruker 300 MHz NMR spectrometers in CDCl3.
Thermogravimetric analyses (TGA) were performed on a
Perkin–Elmer thermogravimeter (Pyris 1) under a dry
nitrogen gas flow at the heating rate of 20 ꢁC/min. Glass
transition temperature (Tg) and melting point (Tm) of
materials were determined by differential scanning calori-
meter (DSC) of the Perkin–Elmer differential scanning cal-
orimeter (DSC-7). The absorption spectra were recorded
with the HP-8453A UV–Vis photodiode array spectropho-
tometer. The EL spectrum and the Commission Internatio-
nale de I’Eclairage (CIE) co-ordinates were measured by
Pro-650 Spectroscanner, the current–voltage (I–V) charac-
4.2.3. Determination of the crystal structure
The diffraction data of complex 1 was collected on a
Bruker AXS P4 diffractometer, which equipped with
graphite-monochromated Mo Ka radiation (k =
˚
0.71073 A). Structure refinements were carried out using
the SHELXTL software package [31]. A pale yellow crystal