Novel Br-DPQ blue light-emitting phosphors for OLED

Article abstract of DOI:10.1002/bio.2750

A new series of blue light-emitting 2,4-diphenylquinoline (DPQ) substituted blue light-emitting organic phosphors namely, 2-(4-methoxy-phenyl)-4-phenyl-quinoline (OMe-DPQ), 2-(4-methyl-phenyl)-4-phenylquinoline (M-DPQ), and 2-(4-bromo-phenyl)-4-phenylquin

Full text of DOI:10.1002/bio.2750

Research article
Received: 2 March 2014,
Revised: 19 June 2014,
Accepted: 2 July 2014
Published online in Wiley Online Library
(wileyonlinelibrary.com) DOI 10.1002/bio.2750
Novel Br-DPQ blue light-emitting phosphors
for OLED
a
b
c
H. K. Dahule, N. Thejokalyani and S. J. Dhoble
ABSTRACT: A new series of blue light-emitting 2,4-diphenylquinoline (DPQ) substituted blue light-emitting organic phosphors
namely, 2-(4-methoxy-phenyl)-4-phenyl-quinoline (OMe–DPQ), 2-(4-methyl-phenyl)-4-phenylquinoline (M-DPQ), and 2-(4-bromo-
phenyl)-4-phenylquinoline (Br-DPQ) were synthesized by substituting methoxy, methyl and bromine at the 2-para position of
DPQ, respectively by Friedländer condensation of 2-aminobenzophenone and corresponding acetophenone. The synthesized
phosphors were characterized by different techniques, e.g., Fourier transform infra-red (FTIR), differential scanning calorimeter
(
DSC), UV-visible absorption and photoluminescence spectra. FTIR spectra confirms the presence of chemical groups such as
C=O, NH, or OH in all the three synthesized chromophores. DSC studies show that these complexes have good thermal stability.
Although they are low-molecular-weight organic compounds, they have the potential to improve the stability and operating life-
time of a device made out of these complexes. The synthesized polymeric compounds demonstrate a bright emission in the blue
region in the wavelength range of 405–450 nm in solid state. Thus the attachment of methyl, methoxy and bromine substituents
to the diphenyl quinoline ring in these phosphors results in colour tuning of the phosphorescence. An electroluminescence (EL) cell
of Br-DPQ phosphor was made and its EL behaviour was studied. A brightness–voltage characteristics curve of Br-DPQ cell revealed
that EL begins at 400 V and then the brightness increases exponentially with applied AC voltage, while current–voltage (I–V)
characteristics revealed that the turn on voltage of the fabricated EL cell was 11 V. Hence this phosphor can be used as a promising
blue light material for electroluminescent devices. Copyright © 2014 John Wiley & Sons, Ltd.
Keywords: DPQ; electroluminescence cell; phosphor; quinoline; OLED
methoxy-phenyl)-4-phenyl-quinoline (OMe-DPQ), 2-(4-methyl-
phenyl)-4-phenylquinoline (M-DPQ), and 2-(4-bromo-phenyl)-
Introduction
Recently, much attention has been paid in the field to white
organic lighting emitting diodes (WOLEDs) as a light source for
the next-generation general illumination due to the high
brightness, long lifetime, low power consumption, and environ-
mentally friendly characteristics promised by solid-state lighting
4-phenylquinoline (Br-DPQ) phosphors and the fabrication
and characterization of blue-light-emitting electrolumines-
cence (EL) cells of Br-DPQ phosphors.
(1,2). To generate white light, the most common and simple
Materials and methods
method is to combine tri-colour red–green–blue (RGB) phosphor
or bi-colour yellow–blue phosphor as luminescent materials for
applications in flat panel displays (3–5), Hg-free lamps and
solid-state lighting. The basic units of modern electronic
appliances can be made from both traditional inorganic
semiconductors and organic semiconductors, i.e. hydrocarbon
molecules that combine semiconducting properties with some
mechanical properties such as easy processing ability and
flexibility (6). Pyrazoloquinoline derivatives (7), distyrylenes,
anthracene derivative, spirofluoreres are demonstrated to be
blue emitting materials for fabricating blue organic lighting
emitting diodes (OLEDs). Incorporating deep-blue phosphors
as emissive material in the effective device architectures of
cOLEDS is one of the key challenges as they usually require large
triplet energy hosts to prevent the reverse energy transfer from
dopants to hosts (8). π-conjugated rigid polyquinolines have
been extensively investigated as thermally stable, photocon-
ductive, photo luminescent, and nonlinear polymeric materials
All reagents and solvents were used as received without further
purification. All reactions were performed under an argon
atmosphere. A BRUKER Fourier transform infra-red (FTIR) spec-
trometer was used to confirm the packing arrangements, chain
conformational properties of OMe-DPQ, M-DPQ and Br-DPQ
chromophores over the range 4000–600/cm by averaging 500
scans at a maximum resolution of 20/cm. Differential scanning
calorimeter (DSC) analysis was carried out under nitrogen
environment using the Mettler Tolledo System. The optical
*
Correspondence to: S.J. Dhoble, Department of Physics, RTM Nagpur Uni-
versity, Nagpur 440033, India. E-mail: sjdhoble@rediffmail.com
a
Department of Physics Shivaji Science College, Nagpur-440012, India
b
Department of Applied Physics, Laxminarayan Institute of Technology,
Nagpur, India
(
9–13). Of the hetero-aromatic polymers, quinoline-based
c
Department of Physics, RTM Nagpur University, Nagpur 440033, India
polymers are of interest as these are reported to exhibit n-type
onductivity upon doping and possess excellent thermal as well
as oxidative stability (14–17). The present work deals with
synthesis and characterization of blue-light-emitting 2-(4-
Abbreviations: DSC, differential scanning calorimeter; EL, electrolumines-
cence; FTIR, Fourier transform infra-red; RGB, red–green–blue; UV, ultra-violet;
WOLED, white organic lighting emitting diodes.
Luminescence 2014
Copyright © 2014 John Wiley & Sons, Ltd.

H. K. Dahule et al.
–
1
absorption spectrum of complex in dichloromethane was ob-
tained on an Analytik Specord-50 spectrophotometer. The
photoluminescence (PL) spectrum wass obtained by a SHIMADZU
RF 5301 spectrofluorometer.
cm as shown in Fig. 1(a), (b) and (c). This may be due to
scattering resulting from the crystalline nature of the phosphor
and hence not recorded. In OMe-DPQ, a broad peak in the
range of 3590–3000 reveals the presence of O–H bonding,
while small stretching peaks in the range 2300–2100 confirms
the presence of a methoxy (O–CH ) group. Aromatic CC stretch
3
Synthesis of DPQ derivatives ((OMe-DPQ), (M-DPQ), (Br-DPQ))
bands were from the carbon–carbon bonds in the aromatic
ring and due to the imine (C=N) group in the range of the
quinoline ring. This was found to be a characteristic of the
quinoline ring (19).
The quinoline-derived ligand (OMe-DPQ) is synthesized according
to Scheme 1(a) as shown in Fig. 1(a) from the condensation of 2-
aminobenzophenone reaction with 4-methoxy acetophenone
using the acid-catalyzed Friedländer reaction (18,19).
The 2-aminobenzophenone reacts with the corresponding 4-
methxyacetophenone, in the presence of diphenyl phosphate as
catalyst at 140°C in an argon atmosphere, water vapour is pro-
duced and finally undergoes crystallization giving the polymer
OMe-DPQ. The crude product is then washed with hexane (25
Differential scanning calorimetry (DSC)
Thermal properties of OMe-DPQ, Br-DPQ and Br-DPQ were
evaluated by DSC measurements. The DSC trace of these
polymeric compounds containing quinoline showed no crystal-
lization but only melting and glass transitions peaks. This
clearly indicates that the incorporation of the rigid quinoline
unit into the phenyl backbone reduces the segmental mobility
and effectively suppresses the tendency of the polymer chains
to densely pack. This improved thermal resistance of the copol-
ymers bodes well for stable blue emission from light-emitting
diodes (LEDs) made from them. Thermally induced phase tran-
sitions were observed, these typical thermal characteristics are
similar to the previous reports (22). These results show that
these compounds have good thermal stability, although they
are low-molecular-weight organic compounds, they in turn
improve the stability and operating lifetime of the device.
Figure 2 (a) indicates that OMe-DPQ undergoes a glass transi-
tion at 41°C, followed by crystallization at 88°C and crystalline
ml × fives times) to produce a crystalline solid (C22H17NO), mol.
wt. = 311.242 g; yield: 73%. Similarly, M-DPQ and Br-DPQ were syn-
thesized by reacting 2-aminobenzophenone with the correspond-
ing 4-methylacetophenone or 4-bromo acetophenone in the
presence of diphenyl phosphate as catalyst at 140°C in the same
atmosphere according to Scheme 1(b) and 1(c), respectively, with
molecular formulae C H NO and C H N, mol. wt. = 295.226
22
17
22 17
and 295.242 g offering the yields as 71% and 72%, respectively.
Results and Discussion
Fourier transform infra-red (FTIR) characterization
FTIR spectra of OMe-DPQ, M-DPQ and Br-DPQ display a broad
background with some asymmetric peaks observed below 600
Scheme 1. Synthesis of (a) OMe-DPQ; (b) M-DPQ; and (c) Br-DPQ organic phosphors.
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Novel Br-DPQ phosphors for OLED
a
b
c
Figure 1. FTIR spectra of (a) OMe-DPQ, (b) M-DPQ, and (c) Br-DPQ organic
–
1
phosphors. In the wave number range less than 1000 cm , phenyl group bending
can be observed. While in the characteristic spectra of M-DPQ and Br-DPQ, a broad
–1
absorption peak below 3000 cm disappeared and the sharp peaks shifted to-
Figure 2. DSC scans of (a) OMe-DPQ, (b) M-DPQ and (c) Br-DPQ organic phos-
phors. Figure 2(c) indicates that Br-DPQ undergoes a glass transition at 51°C,
followed by crystallization at 131°C and crystalline melting point at 314°C, proving
their efficiency in their use as blue light-emitting phosphors in the fabrication of
blue OLEDs and displays for solid-state lighting.
wards lower wave number. The absorption bands in the finger print region
–
1
(
1600–1350 cm ) are generally due to intra-molecular phenomena, and are highly
specific for each material. They predicted aromatic ring stretching, revealing the
–1
presence of a C=C group. A strong peak at 1352 cm predicts aromatic ring
stretching and the presence of nitro compounds. The peaks between the range
–
1
1
000–600 cm revealed the bending of a phenyl group. Two prominent peaks in the
–1
lower range between 766 and 698 cm are due to C–H alkaline bonding. These spectra
confirm the formation of the desired phosphors.
two absorption peaks in the UV region, one at 270 nm and the
other at 326 nm due to π→π* and n→π* transition, respectively.
DPQ exhibits two absorption peaks in the UV region, one at 254
nm and the other at 327 nm due to π→π* and n→π* transition,
respectively. OMe-DPQ exhibits two absorption peaks in the UV
region, one at 272 nm and the other at 333 nm due to π→π*
and n→π* transition, respectively. Peak positions of both the
bands lies at the same wavelength for all the compounds synthe-
sized, indicating the role of DPQ in the complex. Thus the synthe-
sized phosphors have good solubility in DCM.
melting point at 425°C. Figure 2(b) indicates that M-DPQ
undergoes a glass transition at 41°C, followed by crystallization
at 108°C and crystalline melting point at 303°C.
Absorption spectra
–
4
The UV-visible light absorption spectra of 10 M solutions of
OMe-DPQ, M-DPQ and Br-DPQ, pure DPQ in dichloromethane
(
DCM) at room temperature are shown in Fig. 3 (a), (b), (c)
Photoluminescence spectra
and (d), respectively. M-DPQ exhibits two absorption peaks in
the UV region, one at 268 nm and the other at 345 nm due
to π→π* and n→π* transition, respectively. Br-DPQ exhibits
Upon excitation of OMe-DPQ, M-DPQ or Br-DPQ main chains in
solid state at 378 nm, the emission spectrum displays two
Luminescence 2014
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H. K. Dahule et al.
Figure 3. Absorption spectra of (a) M-DPQ, (b) Br-DPQ , (c) DPQ and (d) OMe-DPQ, organic phosphors in DCM solution.
unresolved bands at 428 and 405 nm with some vibronic
structure as shown in Fig. 4(a). M-DPQ also displays two
unresolved bands around 435 and 415 nm as shown in Fig. 4
tuning of phosphorescence. Hence these phosphors can be
used as a promising blue light material for electrolumines-
cent devices.
(
b), while Br-DPQ PL spectra display a sharp peak at 450 nm
and a weak shoulder at 431 nm as shown in Fig. 4(c). Thus
the synthesized polymeric compounds demonstrate a bright
emission in blue region in the wavelength range of 405–450
nm in solid state. Attachment of methyl and methoxy
substituents to the diphenyl quinoline ring results in colour
Development and characterization of EL cells
In order to study the EL characteristics of the synthesized mate-
rials EL cells have been prepared by placing Br-DPQ phosphor
between two electrodes as shown in Fig. 5. One of the
Figure 4. Photoluminescence (PL) spectra of (a) OMe-DPQ, (b) M-DPQ, and (c) Br-DPQ organic phosphors.
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Novel Br-DPQ phosphors for OLED
Figure 5. Anatomy of fabricated EL cell.
electrodes is transparent, made up of a thin conducting layer of
SnO coated on glass plates and the other electrode is simply
2
aluminum foil. The emission of light can be observed through
the transparent electrode under applied AC voltage, measured
by a photomultiplier tube. In the present investigations, we have
restricted our study to the EL measurements brightness versus
voltage and current versus voltage.
Voltage–brightness characteristics. To obtain the voltage–
brightness characteristics of a given EL cell, the frequency of
function generator was fixed at 600 Hz and then AC voltage of
fixed frequency across the cell was increased and the
corresponding brightness was recorded with the help of
photomultiplier tube by means of a pico ammeter. The
brightness–voltage characteristics curve of Br-DPQ cell revealed
that EL begins at 400 V and then after brightness increases
exponentially with applied AC voltage as shown in Fig. 6. By
increasing the applied electric field, injection of electrons and
holes from cathode and anode are enhanced, they recombine
immediately and emit more photons and hence the EL intensity
or brightness of blue colour increases with electric field.
Figure 7. Current–voltage characteristics for the Br-DPQ sample.
voltage of the fabricated EL cell was found to be 11 V. By adding
additional hole and electron transport and injection layers, the
efficiency of the cell can be enhanced.
Conclusion
DPQ-substituted blue emitting organic phosphor materials
OMe-DPQ, M-DPQ and Br-DPQ have been synthesized in nearly
quantitative yields by Friedländer condensation of 2- amino-
benzophenone and corresponding acetophenone. FTIR spectra
confirm the formation of the synthesized DPQ-substituted blue
light-emitting organic complexes. DSC studies show that
M-DPQ has good thermal stability, even though they are low-
molecular-weight organic compounds, which may in turn
improve the stability and operating lifetime of the device.
The UV–vis absorption spectra in dichloromethane provided
information on the electronic structures of the polymeric com-
pound. The synthesized polymeric compounds demonstrate a
bright emission in blue region in the wavelength range of
Voltage–current characteristics. In order to obtain voltage–
current characteristics of a given EL cell, again AC voltage of
fixed frequency across the cell was increased and the corre-
sponding current through the EL cell was recorded by a micro
ammeter. Figure 7. clearly shows a linear relation between
current and voltage, indicating the ohmic nature of the device.
Such ohmic behaviour can be attributed to the ‘hoping’ conduc-
tivity of electrons through a fine emissive layer. The turn-on
405–450 nm in solid state. Thus the attachment of methyl
and methoxy substituents to the diphenyl quinoline ring in
these phosphors results in colour tuning of phosphorescence.
Brightness–voltage characteristics curve of the Br-DPQ EL cell
revealed that EL begins at 400 V and then the brightness
increases exponentially with applied AC voltage. The turn-on
voltage of the fabricated EL cell was found to be 11 V. Hence
this phosphor can be used as a promising blue light material
for electroluminescent devices.
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