J. Liu / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 149 (2015) 48–53
49
et al. reported an excimer-based white phosphorescent OLED with
81% of white rendering index [17]. Piao et al. demonstrated an effi-
cient white OLED with the Commission Internationale de
L’Eclairage (CIE) coordinates of (0.163, 0.325) by combining the
phosphorescent and fluorescent emissions [18]. Hung et al. [16]
obtained a white OLED by using a novel iridium complex as
dopant, the electroluminescent spectrum gave a CIE(x,y) = (0.530,
0.467). However, rare pure white OLEDs based on organic small
molecules were reported [19]. Organic compounds used as white
emissive materials in OLED possess their advantage: purified easily
via thermal sublimated [20], and very stable properties in the
atmosphere. Here, we obtained a pure white OLED based on an
organic small molecule DBIP due to combining the emissions of
DBIP and the low-lying singlet excitons, as was leaded to only a
tri-layer structure’s device being structurally simpler than the
white OLEDs achieved by combining the emissions of red, green,
and blue [21–23].
desorption atmospheric pressure chemical ionization mass spec-
trometry), elemental analysis (EA) was carried out on a Perkin
Elmer 2400 elemental analyzer. Thermogravimetric analysis
(TGA) was conducted under atmospheric conditions using
a
Shimadzu DT-40 system at a heating rate of 10 °C minꢁ1 from 30
to 500 °C. Absorption spectrum was obtained by using a Perkin
Elmer Lambda-35 UV–vis spectrometer. Fluorescence spectra were
recorded by using an FLsp920 fluorescence spectrophotometer
(Xenon lamp) with 2.5 nm of slit for the measurements of excita-
tion and emission. The cyclic voltammetry was performed by using
CHI760 electrochemical workstation under nitrogen. Pt plate was
used as working electrode, Pt wire as auxiliary electrode, standard
calomel electrode as reference electrode and (n-C4H9)4NPF6 as a
supporting electrolyte. Prior to the measurement, nitrogen was
purged into the solution for 5 min. The cyclic voltammogram was
recorded at a scan rate of 50 mV sꢁ1
.
Fabrication and testing of device
Experimental details
The light-emitting diode was fabricated by the following proce-
dure: ITO glass substrate was carefully pre-cleaned and treated
with oxygen plasma for 5 min. Then, all the materials were succes-
sively deposited on the ITO glass substrate by thermal evaporation
under a high vacuum of 1 ꢂ 10ꢁ4 Pa. The active device area was
Synthesis of DBIP
All solvents and chemicals (analytical grade) are commercially
available, and used without further purification. 5.00 g
(0.046 mol) of benzene-1,2-diamine and 3.35 g (0.02 mol) of pyri-
dine-2,6-dicarboxylic acid were added to a 50 mL of porcelain mor-
tar, mixed and ground for 15 min. The mixed material was
transferred into a 25 mL of three-neck round-bottom flask; 10 mL
of 85% (wt) phosphoric acid was added into the flask. The reaction
was conducted under magnetic stirring and 190 °C for 10 h. Then,
the content of the flask was allowed to cool to room temperature,
and then poured into 300 mL of purified water. The resulting mix-
ture was filtered, washed with a 30% (w/w) aqueous sodium
hydroxide and distilled water until the filtrate became neutral
one. The crude product was dried at 80 °C for 12 h, and purified
by using thermal sublimation. A light yellow powder was obtained,
yield: 65% (w/w). 1H NMR (500 MHz, DMSO-d6) ppm 13.01 (s, 2H),
8.36 (dd, J = 7.8, 1.9 Hz, 2H), 8.20 (td, J = 8.1, 2.1 Hz, 1H), 7.78 (s,
4H), 7.34 (s, 4H). m/z: 312.11 ([M+1]+), elemental analysis (EA)
data for DBIP (C19H13N5) was found (calculated) % C = 73.26
(73.30), H = 4.30 (4.21), N = 22..42 (22.49).
0.15 cm2. Resistance of the sheet ITO substrate was 10
X
sqꢁ1
The current density–voltage–brightness data of the electrolumi-
nescent device were obtained with a Keithley 2400 Source meter
and a Keithley 2000 Source multimeter equipped with a calibrated
silicon photodiode. The electroluminescent (EL) spectrum was
measured with a JY SPEX CCD3000 spectrometer. All measure-
ments were carried out under ambient temperature.
Results and discussion
Crystal structure
Fig. 1(a) is a perspective view with atomic labeling scheme.
Fig. 1(b) is the crystal packing drawing. As shown in Fig. 1(a), the
structure of DBIP adopts an anti-anti-configuration, i.e. two N–H
groups and one nitrogen atom are in the same side. There is a
hydrogen bond between H2O and N–H. In addition, some hydrogen
bonds between H2O and H2O or CH3OH are outside of DBIP plane.
The dihedral angles of plane 1 (C27,C28,C29,C30,C31,N1) and
Culture of single crystal and determination of structure
plane
2
(C20,C21,C22,C23,C24,C25), plane
1
and plane
3
(C34,C35,C36,C37,C38,C39), plane
2
and plane 3
are 8.937°,
A single crystal of DBIP was obtained by slowly evaporation
from methanol under ambient temperature. The single crystal with
suitable dimension was chosen for X-ray diffraction testing per-
formed on a BRUCKER SMART APEX-CCD diffractometer equipped
with a graphite-monochromatic MoKa radiation (k = 0.71073 Å)
at 293(2) K. A total of 36,357 reflections were collected in the range
6.628° and 2.322°, respectively. From Fig. 1(b), it can be seen that
all planes of DBIP are parallel to each other, and they constitute a
one-dimensional linear chain linked by the hydrogen bonds among
H2O, CH3OH and DBIP. Interestingly, the CH3OH acting as a bridg-
ing role converts the direction of DBIP once for each two DBIP.
of 1.79 < h < 26.00° by using a
w–x scan mode with 3820 indepen-
dent ones (Rint = 0.0244). The data was corrected by SADABS
program. The parameters of the unit cell were obtained with the
least-squares refinements. The structure was solved by direct
methods with SHELXS-97 and refined by full-matrix least-squares
method on F2 with SHELXL-97. The final refinement gave
R = 0.0514, wR = 0.1360. The crystal structure drawing and the
packing diagram were acquired by diamond 3 software.
Thermal stability of DBIP
As shown in the TGA figure of DBIP (see Fig. 2), the compound
sublimated before reaching the decomposition temperature. The
initial sublimation occurred at 320 °C, and the fastest sublimation
rate appeared at 420 °C.
Photophysical and electrochemical properties
Instrumentations and measurements
The UV–vis absorption and photoluminescent (PL) spectra of
DBIP in CH2Cl2 at 3 ꢂ 10ꢁ6 mol Lꢁ1 and in solid films were illus-
trated in Fig. 3. The absorption spectra of DBIP in CH2Cl2 show a
highly structured peak at 330 nm and a weak shoulder peak at
1H NMR was recorded on a Bruker DRX 500 spectrometer oper-
ating at 500 MHz, in deuterated dimethylsulfoxide solution with
tetramethylsilane as reference. Mass spectrometer (MS) was
obtained by the mass spectrometer of SDAPCI-MS (surface
310 nm, respectively. Both of them are attributed to
p–
p⁄