Luminescence and electroluminescence of bis (2-(benzimidazol-2-yl) quinolinato) zinc. Exciplex formation and energy transfer in mixed film of bis (2-(benzimidazol-2-yl) quinolinato) zinc and N,N′-bis-(1-naphthyl)-N, N′-diphenyl-1,1′-biphenyl-4,4′-diamine

Article abstract of DOI:10.1016/j.molstruc.2004.10.015

The absorption and emission properties of benzimidazol-2-yl-quinoline (BIQ) and bis (2-(benzimidazol-2-yl) quinolinato) zinc (ZnBIQ) a new emitter used for organic light emitting device (OLED) were reported. Exciplexes are observed for ZnBIQ with N,N′-bis-(1-naphthyl)-N,N′-diphenyl-1,1′-biphenyl- 4,4′-diamine (NPB) system, in both electro- and photoluminescent processes. The identification of exciplex emission in co-evaporated and multi-layer ZnBIQ thin film was reported for the first time. The optical formation of the exciplex involves the excitation of a single molecule, followed by the relaxation of that exciton into a lower energy exciplex state. Both BIQ and ZnBIQ possess very high thermal stabilities and can be purified by subliming under the high vacuum condition. Devices consisting of ZnBIQ as the emitting layer have been fabricated, and the emission spectra of ZnBIQ-base devices gave a voltage-dependent spectrum, with the red emission observed (3-7 V), switching over to strong white emission as the bias was raised.

Full text of DOI:10.1016/j.molstruc.2004.10.015

Journal of Molecular Structure 737 (2005) 35–41
www.elsevier.com/locate/molstruc
Luminescence and electroluminescence of bis (2-(benzimidazol-2-yl)
quinolinato) zinc. Exciplex formation and energy transfer in mixed film
of bis (2-(benzimidazol-2-yl) quinolinato) zinc and
0
0
0
0
N,N -bis-(1-naphthyl)-N,N -diphenyl-1,1 -biphenyl-4,4 -diamine
*
Tsun-Ren Chen
Department of Electrical Engineering, Yung-Ta Institute of Technology, Ping-Tung, Taiwan, ROC
Received 9 August 2004; revised 9 October 2004; accepted 14 October 2004
Abstract
The absorption and emission properties of benzimidazol-2-yl-quinoline (BIQ) and bis (2-(benzimidazol-2-yl) quinolinato) zinc (ZnBIQ) a
0
new emitter used for organic light emitting device (OLED) were reported. Exciplexes are observed for ZnBIQ with N,N -bis-(1-naphthyl)-
0
0
0
N,N -diphenyl-1,1 -biphenyl-4,4 -diamine (NPB) system, in both electro- and photoluminescent processes. The identification of exciplex
emission in co-evaporated and multi-layer ZnBIQ thin film was reported for the first time. The optical formation of the exciplex involves the
excitation of a single molecule, followed by the relaxation of that exciton into a lower energy exciplex state. Both BIQ and ZnBIQ possess
very high thermal stabilities and can be purified by subliming under the high vacuum condition. Devices consisting of ZnBIQ as the emitting
layer have been fabricated, and the emission spectra of ZnBIQ-base devices gave a voltage-dependent spectrum, with the red emission
observed (3–7 V), switching over to strong white emission as the bias was raised.
q 2005 Elsevier B.V. All rights reserved.
Keywords: Excimer; Exciplex; Electroluminescence; Imidazole; Device
1
. Introduction
Via the modification of the ligand of metal chelate
compound, the emission color of a metal chelate compound
may be tuned and other properties, such as thermo stability
and carrier mobility, may also be improved upon. Therefore,
the extensive research on the development of new
complexes containing nitrogen and heteroatoms has also
been carried out. In the present work, we reported the
syntheses and luminescent properties of benzimidazol-2-yl-
quinoline (BIQ, 1a) and its zinc complex, bis (2-(benzimi-
dazol-2-yl) quinolinato) zinc (ZnBIQ, 1b), for the first time.
The attachment of the benzimidazol group at 2-position
would allow the ligand to form stable complexes with metal
ions similarly to 8-hydroxyquinoline. In organic emitting
devices the formation of exciplexes appears to play an
important role in the photoluminescent (PL) and electro-
luminescent (EL) characteristics of molecular thin films
Since an organic light emitting diode (OLED) was
reported by Tang and Vanslyke [1], LEDs based on organic
or polymeric materials have generated considerable interest
and enabled the development of low-cost, full-color, flat-
panel displays along with other emissive products [2–5].
Organic and polymer devices provide for advantages over
their inorganic counterparts, such as high luminous
efficiency and fine-pixel formation. These devices can
produce all emission color depending on what kind of
organic emitting materials is selected. The best-known
EL metal chelate compound is Alq3 not only a good
emitter but also a highly efficient electron-transporting
material, where q is the 8-hydroxyquinolinato ligand [6,7].
[8–12]. Exciplex emission has been used in EL devices
in order to tune emission color or to broaden the spectral
width to achieve white light emission [9,10,13,14].
*
Tel.: C886 8 7333632; fax: C886 8 7333632.
E-mail address: lspccj@ms14.hinet.net.
0022-2860/$ - see front matter q 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.molstruc.2004.10.015

36
T.-R. Chen / Journal of Molecular Structure 737 (2005) 35–41
Therefore, the device fabricated with ZnBIQ to investigate
exciplex formation was presented herein.
2.1. Synthesis of benzimidazol-2-yl-quinoline (BIQ)
Quinoline-2-carboxylic acid (20 mmol, 3.4634 g), 1,2-
phenylenediamine (20 mmol, 2.1628 g), and polyphospho-
ric acid (PPA, 20 ml) were added to a flask. The mixture was
heated at 200 8C for 4 h. After cooling to room temperature,
the residue was slowly added to deionized water (500 ml)
with stirring. The solid was collected by suction filtration
and purified by column chromatography using ethyl
acetate/hexane (1:1) as eluent. Colorless crystals of BIQ
2
. Experimental
All starting materials were purchased from Aldrich
1
Chemical Co. H and C NMR spectra were recorded on
13
7
the Bruker 300 MHz NMR spectrometers in DMF-d for
6
BIQ and in DMSO-d for ZnBIQ. Thermogravimetric
1
7
were obtained in 85% yield. H NMR (DMF-d , 223 K),
d ppm: 7.35 (1H, t, JZ7.3 Hz), 7.41 (1H, t, JZ7.5 Hz), 7.77
analyses (TGA) were performed on a Perkin-Elmer
thermogravimeter (Pyris 1) under a dry nitrogen gas flow
at the heating rate of 20 8C/min. Glass transition tempera-
ture (T ) and melting point (T ) of materials were
(
(
2H, m), 7.93 (2H, m), 8.15 (2H, m), 8.67 (2H, m), 13.98
13
1H, s). C NMR (DMF-d , 293 K), d ppm: 151.49, 149.90,
7
g
m
1
1
2
49.32, 144.83, 137.68, 135.62, 124.89, 123.51, 122.29,
21.71, 119.78, 112.40. Calcd for C H N (MWZ
45.28): C, 78.35; H, 4.52; N, 17.13 Found C, 77.96;
determined by differential scanning calorimeter (DSC) of
the Perkin-Elmer differential scanning calorimeter (DSC-7).
The absorption spectra were recorded with the HP-8453A
UV–vis photodiode array spectrophotometer. The EL
spectrum and the Commission Internationale de I’Eclairage
1
6 11 3
H, 4.41; N,16.78.
2.2. Synthesis of bis (2-(benzimidazol-2-yl) quinolinato)
zinc (ZnBIQ)
(
CIE) co-ordinates were measured by Pro-650 Spectro-
scanner, the current-voltage (I–V) characteristics were
measured by Keithley 2400 Source meter. The highest
occupied molecular orbital (HOMO) energy levels of
organic materials were measured form the cyclic voltam-
metry (CV) [15] and the lowest unoccupied molecular
orbital (LUMO) energy levels were determined form the
HOMO energy levels and the optical band gap estimated
from the absorption onset [16]. Photoluminescence (PL)
was measured by Model LS 55 luminescence Spectrometer
Fig. 1a-d.
Diethylzinc (15 wt % in hexane, 2.268 ml, 2 mmol) was
slowly added to 20 ml of THF solution containing
benzimidazol-2-yl-quinoline (0.49 g, 2 mmol) at K10 8C
under N . After the resulting mixture was stirred at room
2
temperature for 1 h, 0.5 ml ethanol was added to quench
reaction. The solvent of mixture was removed under
vacuum condition, and the residual solid was sublimed to
purify the final product. Yellow powder of ZnBIQ was
0
Fig. 1. Molecular structure of (a) benzimidazol-2-yl-quinoline (BIQ), (b) bis (2-(benzimidazol-2-yl) quinolinato) zinc (ZnBIQ), (c) N,N -bis-(1-naphthyl)-
0
0
0
3
N,N -diphenyl-1,1 -biphenyl-4,4 -diamine (NPB), and (d) tris (8-quinnolinolato) aluminum (Alq ).

T.-R. Chen / Journal of Molecular Structure 737 (2005) 35–41
37
1
obtained in 80% yield. H NMR (DMSO-d , 293 K), d ppm:
6
at 264 8C, and no weight loss was observed at lower
temperature. The weight loss of 5 and 10% occurs at 304
and 328 8C, respectively, and the maximum rate of weight
loss takes place at 384 8C. The result of TGA reveals that the
BIQ is quite stable in nitrogen. For ZnBIQ, the onset
temperature for degradation is 362 8C. A 10% weight loss
and maximum rate of weight loss occurs at 450 and 501 8C,
respectively. No weight loss was observed at the tempera-
ture lower than 362 8C. Above 600 8C, there is about 16 wt
% of residue composed of zinc ash. The zinc complex is
reasonably stable upon exposure to air and exhibited a very
high thermal stability in nitrogen, which is attributed to
the fact that the Zn–N (imidazole) bond is highly polarized
7
.28 (2H, m), 7.68 (3H, m), 7.86 (1H, t, JZ8.0 Hz), 8.06
(
1H, d, JZ8.0 Hz), 8.25 (1H, d, JZ8.5 Hz), 8.45 (1H, d,
1
3
6
JZ8.55 Hz), 8.61 (1H, d, JZ8.6 Hz). C NMR (DMSO-d ,
2
93 K), d ppm: 150.95, 147.69, 146.94, 138.87, 131.26,
29.04, 128.76, 128.15, 123.85, 119.33. Calcd for
1
C H N Zn (MWZ553.91): C, 69.39; H, 3.64; N, 15.17.
3
2 20 6
Found C, 69.04; H, 3.53; N, 14.87 MS (FABC): m/z 553.2
C
(
(
(
M , 16.2), 460.2 (4.61), 391.3 (1.75), 307.1 (32.4), 289.1
10.4), 246.1 (7.8), 154.1 (100), 136 (62.2), 107.3 (17), 81
16), 77.6 (13.5).
2
.3. Fabrication of electroluminescent devices
[19,20]. The DSC curves of BIQ and ZnBIQ show that the
glass transition temperature (T ) for BIQ is 106 8C, and the
melting transition (T ) is 226 8C. For ZnBIQ, neither glass
m
The EL devices using ZnBIQ as the emitting layer were
fabricated on the transparent conductive indium–tin oxide
ITO) glass substrates cleaned by detergent solution and
g
(
transition temperature (T ) nor melting transition (T ) was
m
g
deionized water and dried by organic solvent vapor. The
organic layers and the cathode were sequentially deposited
by conventional vacuum vapor deposition in the same
observed up to 420 8C. The DSC result indicate that the
ZnBIQ possesses a very high transition temperature, which
may serve as an advantage for OLED device fabrication
because the use of the materials with high transition
temperature as the active emissive layer or carrier
transporting layer may provide the device with greater
longevity [21,22].
K6
chamber without breaking the vacuum under 3!10 Torr.
The cathode composed of magnesium silver alloy
(
Mg:AgZ10:1) were deposited onto the top layer of organic
materials by co-evaporation of Mg and Ag from different
source. Before the deposition, all of the organic materials
were purified by the train sublimation method. In the present
0
0
0
3.2. Electronic absorption and photoluminescent spectra
wok, the N,N -bis-(1-naphthyl)-N,N -diphenyl-1,1 -biphe-
0
nyl-4,4 -diamine (NPB, 1c) was used as the hole-transport
material (HTM), and tris (8-quinnolinolato) aluminum
K5
Atroomtemperatureandlowconcentration(1!10 M),
the absorption spectral features of ZnBIQ in dichloro-
0
(
Alq , 1d) was employed as the electron-transporting
3
methane and N,N -dimethylformamide (DMF) consist of
three discrete bands (Fig. 2). A strong p–p* short-
material (ETM). The thickness of the organic layers and
cathode were measured by quartz-crystal monitors.
wavelength band centered around 287 nm, and an intense
band centered around 351 nm having the vibrational
K1
separation of 1360 cm
4
with n0,0 transition at 2.84!
0 cm , which possess the reasonably high absorptivity
3
. Results and discussion
K1
1
4
3
K1
K1
(
3w10 dm mol cm ) and a very small bathochromic
3
.1. Synthesis and thermal stabilities
shift with increasing polarity of solvent, which is the typical
p, p* transition [23,24]. The third band is broad in the range
of 370–450 nm undergoing a blue shift with increasing
polarity of solvent, which is assigned to internal charge
Most of imidazoles are good chelating ligands and can
force the resultant metal complexes into special geometries
17]. Therefore, our initial studies have focused on the use
[
of BIQ as a ligand to form the metal complex to investigate
its PL, EL, and application especially with OLED.
According to the line shape analysis of dynamic NMR
spectroscopy, the coalescence temperature of BIQ occurs at
†
3
connecting benzimidazol and quinoline moieties is
20 K. The barrier (DG ) to rotation about the C–C bond
c
K1
1
5.6 kcal mol
at 320 K. This high barrier to rotation
implies that there is restricted rotation about C–C bound [18]
and that the BIQ is a quite rigid molecule. Therefore, the BIQ
possess a high PL quantum yield, F Z0.9 in dichloro-
f
methane solution relative to 3-(2-benzothiazolyl)-7-diethyl-
aminocoumarin (C540). A similar situation has also been
observed in ZnBIQ having a good quantum yield (F Z0.75).
f
The TGA scans under nitrogen for BIQ and ZnBIQ
powder show that the BIQ exhibits an onset for degradation
Fig. 2. Ultraviolet absorption spectra of ZnBIQ and BIQ.

38
T.-R. Chen / Journal of Molecular Structure 737 (2005) 35–41
Fig. 5. PL spectra of the ZnBIQ–NPB mixed films, excited with 323 nm
laser line.
Fig. 3. Ultraviolet absorption spectra for different concentration of ZnBIQ
in DMF and ZnBIQ neat film.
resulting from the relaxation of collision complex into the
lower energy state.
transfer (ICT) [25,26]. The relative intensity of ICT
absorption band is increasing when the concentration of
K5
K3
The PL spectra of the ZnBIQ–NPB mixed films, excited
with 323 nm laser line, were illustrated in Fig. 5. The
emission spectra of the ZnBIQ and NPB neat films were also
shown. Three broad bands are observed, with maxima at
ZnBIQ in DMF increases from 1!10 to 1!10 M, and
has a maximum at 403 nm in the spectrum of the ZnBIQ
neat film (Fig. 3). Comparing the absorption spectra of
ZnBIQ with BIQ (Fig. 2), the fact that the absorption bands
of ZnBIQ are almost identical to BIQ suggests that the
electronic transitions in ZnBIQ are localized on the BIQ
ligand [27].
4
10, 485, and 565 nm, at the spectrum of 10 wt % ZnBIQ.
As the PL spectra of the ZnBIQ solutions and neat film, the
10-nm and 485-nm bands correspond to the ZnBIQ
4
monomer emission and ZnBIQ excimer emission, respect-
ively. However, the 565-nm band is new and does not
correspond to the fluorescence of ZnBIQ or NPB. We have
assigned this broad emission band to the exciplex emission
[8,15,16,28]. As shown in Fig. 5, the proportion of ZnBIQ to
NPB in the matrix dominates the excimer/exciplex ratio.
When the concentration of ZnBIQ increased, the excimer
emission band increased and the exciplex emission band
decreased. Furthermore, when the concentration of ZnBIQ
increased to 25 wt %, the exciplex emission band
disappeared, which indicated that the intermolecular
interaction between ZnBIQ* and ZnBIQ is stronger than
that between ZnBIQ* and NPB.
The photoluminescent (PL) spectra of the ZnBIQ
solutions and neat film, excited with 323 nm laser line,
K6
were illustrated in Fig. 4. At low concentration, 5!10
M
in DMSO, only one emission band is observed with
maximum at 410 nm, corresponding to the relaxation of
ZnBIQ from the excited state of a single molecule into
ground state. Besides the 410-nm band, a new emission
band appeared while the concentration of ZnBIQ increased
K6
K3
from 5!10
to 1!10 M. This new emission band
having a maximum at 485 nm is observed in the spectrum of
the ZnBIQ neat film. We have assigned this new emission
band to the excimer and higher aggregates emission [8,28]
According to the electronic spectra of the ZnBIQ–NPB
mixed films (Fig. 6), the lack of new band in the absorption
spectrum that could be assigned to the collision complex
formation between ZnBIQ and NPB indicates that the
formation of the exciplex take place only in the excited
state. Fig. 7 shows the interpretative diagram of photo-
luminescence for ZnBIQ and the ZnBIQ–NPB mixed film
system, the excimer and exciplex are observed in photo-
luminescent process in which the optical formation of
exciplex involves first the excitation of a single molecule,
followed by relaxation of that exciton into the lower energy
exciplex state [15].
3
.3. Electroluminescent properties
According to the PL spectra of ZnBIQ, the wavelength of
monomer emission is localized in the blue light region,
Fig. 4. Photoluminescent spectra of the ZnBIQ solutions and neat film,
excited with 323 nm laser line.

T.-R. Chen / Journal of Molecular Structure 737 (2005) 35–41
39
Fig. 6. Ultraviolet absorption spectra of ZnBIQ neat film, ZnBIQ–NPB
mixed films, and NPB neat film.
the excimer emission is distributed over the green light
region, and the exciplex emission is extended into the red
light region. Therefore, the device cannot only allow the
emission band to be tuned but also for the white light to be
obtained if the device is fabricated with ZnBIQ as emitter.
As a result, several device structures were prepared in the
present work to investigate the thin film properties for
ZnBIQ. Single and double layer structures without a hole
transporting layer (HTL), Fig. 8a,b, failed to produce any
electroluminescence (EL) because ZnBIQ is a poor hole
injection material (HJM) or a poor hole transporting
material (HTM). The energy gap between the work function
of ITO and the HOMO of ZnBIQ is too high for hole to
transfer; the resultant data will be discussed in detail,
hereinafter.
Fig. 8. The structures of the specimens used.
coordinates when the applied bias is low, and shifted from
red to white region when the bias voltage increases
(
Fig. 10). The observation of above mentioned suggests
that the carrier recombination zone for device C is field
dependent. As the above-mentioned PL spectra of ZnBIQ–
NPB mixed films, the 485-nm and 610-nm bands in EL
spectrum of device C have been assigned to the ZnBIQ
excimer and exciplex emission bands, respectively.
As observed at Fig. 9, the exciplex emission is dominant
in the low applied field, which is resulted from the fact that
the carrier recombination zone is localized at the interface
between HTL (NPB) and emitting layer (ZnBIQ) [29], so
the excited species will have a high probability to collide
with NPB to form excited complex, [ZnBIQ–NPB]*. As the
bias voltage is increased, holes are injected into emitting
Double layer structure with NPB as the HTL and ZnBIQ
as emitting layer, Fig. 8c (device C), resulted in the dramatic
change of spectra as a function of the applied bias. Fig. 9
shows the EL spectra of device C at 5, 10, and 15 V,
respectively. At low bias voltage (5 V), the EL spectrum of
device C possess a broad emission band in the 580–680 nm
region with the maximum at 610 nm and a weak shoulder in
the 450–550 nm region with the maximum at 485 nm. As
the voltage was increased, the 485-nm peak increased in
intensity relative to the 610-nm peak. The color of the
emitting light for device C is in the red region in the CIE
Fig. 7. Interpretative diagram of photoluminescence for ZnBIQ and the
ZnBIQ–NPB mixed film system.
Fig. 9. EL spectra of device C at 5, 10, and 15 V.

40
T.-R. Chen / Journal of Molecular Structure 737 (2005) 35–41
Fig. 12. EL spectra of device D at 5 and 10, respectively.
device D at 5 and 10, respectively. At low bias voltage, the
EL spectra of device D only possess the broad emission
band in the 550–700 nm region with the maximum at
6
6
25 nm. As the voltage was increased, the intensity of
25-nm peak increased, and no other new emission band
was observed. The fact that the EL spectrum of device D is
independent of the bias voltage suggests that the electron
injection and transport is facilitated when the Alq is used.
Fig. 10. CIE coordinate of the emitting light for device C in various applied
bias.
3
layer, and the carrier recombination zone is distributed to
the emitter matrix, so that the excited state of ZnBIQ tend to
form excimer, [ZnBIQ–ZnBIQ]* [30]. Fig. 11 shows the
current-voltage and luminance- voltage characteristics of
device C having a quite low turn on voltage about 2.5 V for
Thus, the carrier recombination zone was confined [31] at
the interface between NPB and ZnBIQ when electrons are
easier to reach the interface between HTL and emitting
layer. Therefore, the exciplex band is the dominant emission
and the color of the emitted light for device D is almost
fixed in the red region in the CIE coordinates of xZ0.54
yZ0.39.
Fig. 13 shows the energy level diagram of the HOMO
and LUMO of the different organic materials and the work
function of cathode and anode. The LUMO energy of
ZnBIQ is 2.7 eV determined form the HOMO energy
current and luminance. This device shows a red light
K2
emission having a brightness of 4 cdm
voltage of 5 V with current density of 10 mA/cm and a
at the driving
2
K2
white light emission having a brightness of 22 cdm at the
2
driving voltage of 12 V with current density of 48 mA/cm .
The fact that the low turn on voltage for device C suggests
that ZnBIQ can be not only an emitter but also an electron-
transporting material (ETM).
(
5.9 eV) obtained from the cyclic voltammetry (CV)
method and the optical band gap estimated from the
absorption onset. Comparing the energy level of ZnBIQ
with NPB, it is clear that the ZnBIQ has the much higher
hole injection barrier than that of NPB; in fact, it is
Triple layer structure device containing HTL (NPB),
ETL (Alq ), and an emitting layer consisting of ZnBIQ,
3
Fig. 8d (device D), was fabricated to compare with double
layer structure device. Fig. 12 shows the EL spectra of
Fig. 13. Energy level diagram of HOMO and LUMO for OLED materials
and work function for ITO and Mg–Ag alloy.
Fig. 11. Current–voltage and luminance–voltage characteristics of device C.

T.-R. Chen / Journal of Molecular Structure 737 (2005) 35–41
41
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3
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density of 30 mA/cm .
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2
[
[
[
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transfer and film-forming properties and has a very high
thermal stability, very important characters for practical
application in the electronic fields, especially for OLED.
The excimer emission resulting from the collision complex
was observed, and this interaction is stronger than that of
exciplex resulting from the collision between ZnBIQ* and
NPB mixed matrix. The devices composed of ZnBIQ as the
emitting layer can tune the emitting color via the controlling
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[
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