Synthesis, characterization, and optoelectronic properties of heteroleptic iridium complexes containing substituted 1,3,4-oxadiazole and β-diketone as ligands

Article abstract of DOI:10.1080/00958972.2012.654784

The new heteroleptic iridium(III) complexes (BuOXD)₂Ir(tta) and (BuOXD)₂Ir(tmd) [BuOXD=2-(4-butyloxyphenyl)-5-phenyl[1,3,4] oxadiazolato-N4,C2, tta=1,1,1-trifluoro-4-thienylbutane-2,4-dionato, tmd=2,2,6,6-tetramethylheptane-3,5-diona

Full text of DOI:10.1080/00958972.2012.654784

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Synthesis, characterization, and
optoelectronic properties of
heteroleptic iridium complexes
containing substituted 1,3,4-oxadiazole
and β-diketone as ligands
Amit Kumar a b , Ritu Srivastava a , Partap S. Kadyan b ,
Modeeparampil N. Kamalasanan a & Ishwar Singh b
a Center for Organic Electronics (OLED Lab.) , National Physical
Laboratory (Council of Scientific and Industrial Research) , Dr.
K.S. Krishnan Road, New Delhi-110012 , India
b Department of Chemistry , Maharshi Dayanand University ,
Rohtak , Haryana-124001 , India
Published online: 31 Jan 2012.
To cite this article: Amit Kumar , Ritu Srivastava , Partap S. Kadyan , Modeeparampil N.
Kamalasanan & Ishwar Singh (2012) Synthesis, characterization, and optoelectronic properties of
heteroleptic iridium complexes containing substituted 1,3,4-oxadiazole and β-diketone as ligands,
Journal of Coordination Chemistry, 65:3, 453-462, DOI: 10.1080/00958972.2012.654784
To link to this article: http://dx.doi.org/10.1080/00958972.2012.654784
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Journal of Coordination Chemistry
Vol. 65, No. 3, 10 February 2012, 453–462
Synthesis, characterization, and optoelectronic properties
of heteroleptic iridium complexes containing substituted
1,3,4-oxadiazole and b-diketone as ligands
AMIT KUMARyz, RITU SRIVASTAVA*y, PARTAP S. KADYANz,
MODEEPARAMPIL N. KAMALASANANy and ISHWAR SINGHz
yCenter for Organic Electronics (OLED Lab.), National Physical Laboratory
(Council of Scientific and Industrial Research),
Dr. K.S. Krishnan Road, New Delhi-110012, India
zDepartment of Chemistry, Maharshi Dayanand University,
Rohtak, Haryana-124001, India
(Received 4 October 2011; in final form 6 December 2011)
The new heteroleptic iridium(III) complexes (BuOXD)2Ir(tta) and (BuOXD)2Ir(tmd)
[BuOXD ¼ 2-(4-butyloxyphenyl)-5-phenyl[1,3,4]oxadiazolato-N4,C2,
tta ¼ 1,1,1-trifluoro-
4-thienylbutane-2,4-dionato, tmd ¼ 2,2,6,6-tetramethylheptane-3,5-dionato] have been synthe-
sized and characterized. These complexes have two cyclometalated ligands (C^N) and a
bidentate diketone ligand (X) [C^N)2Ir(X)], where X is a ꢀ-diketone with trifluoromethyl,
theonyl or t-butyl groups. The color tuning with the change in electronegativity of substituents
in the ꢀ-diketones has been studied. Photoluminescence spectra of the complexes showed peak
emissions at 523 and 549 nm, respectively. The electroluminescent properties of these complexes
have been studied by fabricating multi layer devices with device structure ITO/ꢁ-NPD/8%
iridium complex doped CBP/BCP/Alq3/LiF/Al. The electroluminescence spectra also showed
peak emissions at 526 and 570 nm for (BuOXD)2Ir(tta) and (BuOXD)2Ir(tmd), respectively.
These metal complexes showed good thermal stability in air to 340ꢀC.
Keywords: Heteroleptic iridium complex; ꢀ-Diketones; Electroluminescence; Color tuning
1. Introduction
Cyclometalated or oligopyridyl complexes of group 10 metals have unique photo-
physical properties [1–4] and focus on iridium(III) complexes has been given for
fabricating organic light emitting devices (OLEDs) because of their good quantum
yield, thermal stability, and high electroluminescence (EL) efficiency [5–11]. 1,3,4-
Oxadiazole derivatives such as 2-(4-tert-butylphenyl)-5-(4-biphenyl)-[1,3,4]-oxadiazole
(PBD) are widely used as electron-injecting/hole-blocking and excellent electron-
transport material in multilayer OLEDs [12–16]. High electron affinity makes them
good candidates for electron injection and transportation [17–19]. Recently some
fluorinated 1,3,4-oxadiazole based heteroleptic iridium(III) complexes were reported
*Corresponding author. Email: ritu@mail.nplindia.ernet.in
Journal of Coordination Chemistry
ISSN 0095-8972 print/ISSN 1029-0389 online ß 2012 Taylor & Francis
http://dx.doi.org/10.1080/00958972.2012.654784

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A. Kumar et al.
where the emission was tuned to blue by dithiolate ancillary ligands [20]. There are
also reports of mixed ligand complexes of iridium(III) using different ꢀ-diketones and
1,3,4-oxadiazoles for tuning the emission color [21]. Lamansky et al. [22] have shown
that if triplet-state energy of the ꢀ-diketone (X) is lower than the C^N3(ꢂ–ꢂ*) or
3MLCT levels, the triplet X level will be the lowest energy excited state and a switch
of emission from ‘‘C^N2Ir’’ to X will be seen in a series of complexes prepared with
different ꢀ-diketones (i.e., acetylacetone, 1,1,5,5-tetramethylacetylacetone, benzyolace-
tonate, etc.) and its influence on color tuning of iridium complexes. In this way color is
tuned in mixed ligand iridium complexes using different ꢀ-diketones with the same
oxadiazole.
We report the synthesis and characterization of the new heteroleptic iridium(III)
complexes (BuOXD)2Ir(tta) and (BuOXD)2Ir(tmd) [BuOXD ¼ 2-(4-butyloxyphenyl)-
5-phenyl[1,3,4]oxadiazolato-N4,C2, tta ¼ 1,1,1-trifluoro-4-thienylbutane-2,4-dionato,
tmd ¼ 2,2,6,6-tetramethylheptane-3,5-dionato]. These complexes have two cyclometa-
lated ligands (C^N) and one bidentate diketone ligand (X) represented as [(C^N)2Ir(X)],
where X is chosen to be different ꢀ-diketones with trifluoromethyl, thionyl, or t-butyl
groups. Substitutions on the ꢀ-diketone lead to the alteration of electronic properties
of the complexes as well as their emission color.
2. Experimental
2.1. Materials
The various chemicals used to synthesize ligand and metal complexes were purchased
from Fluka. Solvents were used as supplied.
2.2. Synthesis
2.2.1. Synthesis of 1,3,4-oxadiazole ligand. The alkyl-oxy substituted oxadiazole was
synthesized in a stepwise manner starting from p-hydroxymethylbenzoate as shown
in scheme 1 of figure 1. p-Hydroxymethylbenzoate (32 mmol), bromobutane (40 mmol),
and K2CO3 (72 mmol) were added to 100 mL dry DMF. The reaction mixture was
refluxed at 80–90ꢀC. Progress of the reaction was monitored by thin layer chroma-
tography. After completion of reaction, the compound was extracted in chloroform.
The solid product obtained after removal of solvent was further purified by column
chromatography using 5% ethyl acetate in hexane as eluent giving pure
p-butyloxymethylbenzoate.
p-Butyloxymethylbenzoate (24 mmol) and hydrazine hydrate (200 mmol) were taken
in 50 mL methanol and refluxed overnight. Solid p-butyloxybenzylhydrazine obtained
after the evaporation of excess methanol was washed several times with water. Further
p-butyloxybenzylhydrazine (29 mmol), benzoyl chloride (35 mmol), LiCl (35 mmol),
and triethylamine (43 mmol) were refluxed at 80ꢀC in dry DMF producing
1-(p-butyloxybenzylcarbonyl)-2-benzylcarbonylhydrazine (BuAXD) which was finally
refluxed in POCl3 overnight to give 2-(4-butyloxyphenyl)-5-phenyl[1,3,4]oxadiazole
(BuOXD). After every step products were purified by column chromatography
(scheme 1 of figure 1).

Heteroleptic iridium complexes
455
Figure 1. Scheme 1: Synthetic route of BuOXD; Scheme 2: Synthetic route of binuclear and mononuclear
iridium complexes.
(BuAXD) Yield 78%, FT-IR (neat) ꢃmax: 3194, 1578 (N–H); 2956 (C–H); 1626 (C¼O
from amide); 1126 (C–O–C); 840, 804, 741. Anal. Calcd for C18H20N2O3 (%): C, 69.23;
H, 6.41; N, 8.97. Found (%): C, 69.12; H, 6.28; N, 8.68.
(BuOXD) Yield 82%, FT-IR (neat) ꢃmax: 2954(C–H); 1608 (C¼N); 1582, 1551
(C¼C); 1308, 1254 (C–O); 1121, 1068 (C–O–C); 686, 736. Anal. Calcd for C18H18N2O2
(%): C, 73.47; H, 6.12; N, 9.52. Found (%): C, 73.24; H, 5.97; N, 9.35.

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A. Kumar et al.
2.2.2. Synthesis of binuclear iridium-oxadiazole complex. Tetrakis{2-(4-butyloxyphe-
nyl)-5-phenyl[1,3,4]oxadiazolato-N4, C2}di(ꢄ-chloro) diiridium(III) [(BuOXD)4Ir2
(ꢄ-Cl)2]. Iridium trichloride hydrate (0.5 mmol) and BuOXD (1.25 mmol) were added
to 40 mL mixture of 2-ethoxyethanol and water (v/v ¼ 3 : 1). The mixture was refluxed
for 24 h and cooled to room temperature. The resulting solid was collected by filtration,
washed with ethanol, water, and hexane, and pumped dry to give the crude chloro-
bridged dimer.
(BuOXD)4Ir2(ꢄ-Cl)2 Yield 56%, FT-IR (neat) ꢃmax: 2954(C–H); 1608 (C¼N); 1582,
1551 (C¼C); 1308, 1254 (C–O); 686, 736.
2.2.3. Synthesis of mononuclear iridium-oxadiazole complexes. (BuOXD)2Ir(tta) and
(BuOXD)2Ir(tmd). Without further purification, the dimer was added to a mixture of
Na2CO3, diketone (tta or tmd) in 2-ethoxyethanol (30 mL). After reflux for 24 h, the
reaction mixture was poured into water. The light brown precipitate was filtered off and
washed with water, hexane, and ether (scheme 2 of figure 1).
(BuOXD)2Ir(tta). Yield 82%, Photoluminescence (in ethanol) ¼ 523 nm, FT-IR
(neat) ꢃmax: 2954(C–H); 1608 (C¼N); 1583, 1552 (C¼C); 1300, 1253 (C–O); 1126,
1040 (C–O–C); 687, 737. Anal. Calcd for C44H39N4O6SF3Ir (%): C, 52.8; H, 3.9; N, 5.6.
Found (%): C, 52.42; H, 3.48; N, 5.29.
(BuOXD)2Ir(tmd). Yield 84%, Photoluminescence (in ethanol) ¼ 550 nm, FT-IR
(neat) ꢃmax: 2953(C–H); 1609 (C¼N); 1583, 1551 (C¼C); 1301, 1253 (C–O); 1126, 1070
(C–O–C); 686, 736. Anal. Calcd for C47H54N4O6Ir (%): C, 58.63; H, 5.61; N, 5.82.
Found (%): C, 58.47; H, 5.36; N, 5.35.
2.3. Instrumentation
C, H, and N analyses of the complexes were done by an Elemental Analyzer Perkin-
Elmer 2400 CHN. Perkin-Elmer 2000 FTIR was used to record infrared (IR) spectra in
KBr pellets. The UV-Vis spectra of complexes were recorded in CH2Cl2 solution using a
UV-Vis spectrophotometer (Shimadzu 2401 PC). Photoluminescence (PL) spectra of
the complexes were studied using a spectrofluorometer (Fluorolog Jobin Yvon-Horiba,
model-3-11) at room temperature. The EL spectrum was recorded with a high
resolution spectrometer (Ocean Optics HR-2000CG UV-NIR). The current–voltage–
luminescence (I–V–L) characteristics was measured with a luminance meter (LMT
l-1009) interfaced with a Keithley 2400 programmable voltage–current digital source
meter. All the measurements were performed at room temperature and under ambient
atmosphere, without any encapsulation.
2.4. OLED device fabrication and characterizations
Indium-tin oxide (ITO) coated glass substrate with a sheet resistance 20 ꢀ cmꢁ2 was
used as anode which was patterned and cleaned using deionized water, acetone,
trichloroethylene, and isopropyl alcohol sequentially for 20 min using an ultrasonic
bath and dried in a vacuum oven. The hole transporting layer and the emitting layers
were deposited on the substrate sequentially under high vacuum (1 ꢂ 10ꢁ5 torr) at a
deposition rate of 0.1 Asꢁ1. The thickness of the deposited film was monitored in-situ by
˚

Heteroleptic iridium complexes
457
˚
quartz crystal. 300 A of N,N-diphenyl-N,N-bis(1-naphthyl)-1,1-biphenyl-4,4-diamine
˚
(ꢁ-NPD) as the hole transporting layer; 350 A of the (8%) complex (BuOXD)2Ir(tta) or
(BuOXD)2Ir(tmd) doped in 4,4’-biscarbazolylbiphenyl (CBP) as the emitting layer;
˚
60 A of 2,9-dimethyl 4,7-diphenyl-1,10-phenanthroline (BCP) as a hole and exciton
˚
blocking layer (HBL), 280 A of tris(8-hydroxyquinolinato)aluminum (Alq3) as electron
˚
˚
transport layer, and a cathode comprising 10 A lithium fluoride and 1000 A aluminum
were sequentially deposited onto the substrate to complete the device structure.
3. Results and discussion
3.1. Material synthesis and characterization
We synthesized new iridium complexes bearing 1,3,4-oxadiazole and ꢀ-diketone based
ligands with procedures previously reported by Dedeian et al. [23–25]. The syntheses
of the ligands and iridium complexes are depicted in schemes 1 and 2 of figure 1. The
synthetic method used to prepare these complexes involved two steps. In the first step,
IrCl3 ꢃ 3H2O was allowed to react with an excess of the cyclometalated ligand (2.5 times)
to give a chloro-bridged dinuclear complex, i.e. (BuOXD)4Ir2(ꢄ-Cl)2. The chloro-
bridged dinuclear complex could readily be converted to mononuclear complexes
(BuOXD)2Ir(X) by replacing the two bridging chlorides with a bidentate ꢀ-diketone,
finally producing iridium(III) octahedrally coordinated by three chelating ligands. The
coordination geometry of the ‘‘(BuOXD)2Ir’’ fragment in the mononuclear complex
is the same as that for the dinuclear complexes. Thermal studies showed that all the
mononuclear iridium complexes with different ꢀ-diketones were thermally stable to
340ꢀC. As indicated by the NMR data, the maximum high-field chemical shift
is observed for the proton to the ortho-metalated carbon that experiences the largest
shielding of any of the ligand protons. The (BuOXD)2Ir(X) complexes are stable in air
and could be sublimed in vacuum without decomposition before device fabrication.
3.2. UV-Vis absorption and PL spectra
UV-Vis absorption and PL spectra for the oxadiazole ligand and iridium metal
complexes were measured in CH2Cl2. The relevant spectral characteristics are given
in figure 2. Oxadiazole derivatives exhibited strong absorption bands at 250–350 nm
from ꢂ–ꢂ* electronic transitions focusing on the conjugated oxadiazole moieties [26].
As shown in curve A of figure 2(a), the peak at 296 nm is due to UV-absorption of the
ꢂ–ꢂ* electronic transitions of oxadiazole ligand. Curves B and C in figure 2(a) show
absorption spectra of free tta and (BuOXD)2Ir(tta) complex, respectively. Figure 2(a)
curves D and E show absorption spectra of the free tmd and (BuOXD)2Ir(tmd),
respectively. By comparing absorption spectra of free ligands, the intense absorptions
in the ultraviolet region of the spectra, between 250 and 350 nm, can be assigned to
1
ligand-centered, spin-allowed ꢂ–ꢂ* electronic transitions. Relatively weaker absorp-
tions at 350–400 nm are well-resolved, ascribed to a spin-allowed, metal-to-ligand
charge transfer (1MLCT) transition. The long tail extended to lower energies (400–
500 nm), likely associated with both 3MLCT and 3ꢂ–ꢂ* transitions, which gain
considerable intensity by mixing with the 1MLCT transition through spin–orbit

458
A. Kumar et al.
Figure 2. (a) Curve A: absorption spectrum of BuOXD; curve B: absorption spectrum of tta; curve C:
absorption spectrum of (BuOXD)2Ir(tta); curve D: absorption spectrum of tmd; and curve E: absorption
spectrum of (BuOXD)2Ir(tmd). (b) Curve A: PL spectra of ligand in solution; curve B: PL spectra of ligand
in film. (c) Curve A: PL spectra of (BuOXD)4Ir2(ꢄ-Cl)2; curve B: PL spectra of (BuOXD)2Ir(tta); and
curve C: PL spectra of (BuOXD)2Ir(tmd) in CH2Cl2.
coupling [27]. By comparing curves B and C, the peak at 376 nm of curve C is due to the
MLCT band of (BuOXD)2Ir(tta), which is of lowest energy and hence the emission in
the (BuOXD)2Ir(tta) complex is Ir–CN centered. Also by comparing curves D and E
the peak at 393 nm of curve D is due to a tmd centered n–ꢂ* electronic transition. The
energy of these transitions is lower than the MLCT of (BuOXD)2Ir(tmd), and hence
in (BuOXD)2Ir(tmd) the emission switched from Ir–CN center to ligand (tmd) center.
Curves A and B of figure 2(b) show the PL spectra of BuOXD in CH2Cl2 and in film,
respectively. In comparison to the PL spectrum in solution, the PL spectrum of the
ligand in film shows bathochromic shift and broader emission. This may be attributed
to stronger intermolecular interaction in the solid state, including hydrogen bonding
and ꢂ–ꢂ* interactions between aromatic ligands of oxadiazole [28]. Figure 2(c) shows
the PL emission spectra for dinuclear and mononuclear ([C^N)4Ir2(ꢄ-Cl)2],
[C^N)2Ir(X)]) complexes in CH2Cl2 at room temperature. All these complexes show
strong luminescence from 500 to 600 nm. Emission bands from MLCT states are
generally broad and featureless [29]. It is observed that changing the ꢀ-diketone in
[C^N)2Ir(X)] typically has an effect on the maximum emission wavelength in the
spectrum. Curve A in figure 2(c) is the PL spectrum of the (BuOXD)4Ir2(ꢄ-Cl)2
complex showing ꢅmax at 540 nm. Curves B and C of figure 2(c) show the PL of
(BuOXD)2Ir(tta) and (BuOXD)2Ir(tmd) with ꢅmax at 523 and 550 nm, respectively.
This blue and red shift in ꢅmax of (BuOXD)2Ir(tta) and (BuOXD)2Ir(tmd), respectively,
with respect to (BuOXD)4Ir2(ꢄ-Cl)2 may be because of the difference in electroneg-
ativity of the groups attached to the ꢀ-diketone.
3.3. Electroluminescent characterization
1,3,4-Oxadiazoles generally emit light in the blue region of the electromagnetic
spectrum. To study the EL of ligand and iridium complexes, electroluminescent devices
were fabricated using the ligand or metal complexes as emissive layer materials.
Eight percent metal complex doped CBP was used as emissive layer to avoid the
concentration quenching of Ir–Ir metal. The device structure and thickness of the layers
are as follows:
˚
Device 1. ITO/ꢁ-NPD (300 A)/BuOXD (350 A)/BCP (60 A)/Alq3 (280 A)/LiF (10 A)/Al
˚
˚
˚
˚
˚
(1000 A).

Heteroleptic iridium complexes
459
Figure 3. (a) EL spectra of BuOXD. (b) Curve A: normalized EL spectra of (BuOXD)2Ir(tta) and
curve B: normalized EL spectra of (BuOXD)2Ir(tmd). (c) EL spectra of (BuOXD)2Ir(tta) at various voltages.
(d) EL spectra of (BuOXD)2Ir(tmd) at various voltages.
˚
Device 2. ITO/ꢁ-NPD (300 A)/8% (BuOXD)2Ir(tta) doped CBP (350 A)/BCP (60 A)/
˚
˚
˚
˚
Alq3 (280 A)/LiF (10 A)/Al (1000 A).
˚
˚
Device 3. ITO/ꢁ-NPD (300 A)/8% (BuOXD)2Ir(tmd) doped CBP (350 A)/BCP (60 A)/
˚
˚
˚
Alq3 (280 A)/LiF (10 A)/Al (1000 A).
˚
˚
EL spectra of BuOXD, normalized EL spectra of (BuOXD)2Ir(tta) and
(BuOXD)2Ir(tmd), EL spectra of (BuOXD)2Ir(tta) at different voltage and EL spectra
of (BuOXD)2Ir(tmd) at different voltages are shown in figure 3(a)–(d), respectively. The
BuOXD, (BuOXD)2Ir(tta) and (BuOXD)2Ir(tmd) show EL at 405, 526, and 570 nm,
respectively. As shown in figure 3(b), device 2 having fluoro substituted dopant exhibits
the expected blue shift as compared to that of device 3, which is consistent with the PL
investigation. The EL emission spectra of the devices matched the PL emission spectra
of the complexes in solution, which indicated that the EL emission is from the triplet
excited states of the metal complexes. Figure 4 shows the schematic energy level
diagram of the device structure used in this study. The LUMO of Alq3 (3.0 eV) is almost
matching with the LUMO of BCP (2.9 eV) and hence there is no barrier for electron
transport. The barrier of 0.6 eV between BCP and the emissive layer leads to the

460
A. Kumar et al.
Figure 4. Schematic energy level diagram of the device used in this study.
Figure 5. (a) Curve A, J–V and curve B, V–L characteristics of device using 8% (BuOXD)2Ir(tta) doped
CBP as emissive material. (b) Curve A, J–V and curve B, V–L characteristics of device using 8%
(BuOXD)2Ir(tmd) doped CBP as emissive material.
accumulation of electrons at the interface of BCP and emissive layer. Likewise, there
is no barrier for holes coming from ITO to HTL and the emissive layer. But HOMO of
BCP (6.5 eV) lies much below the HOMO of the emissive layer and therefore the
emissive layer/BCP interface also acts as a barrier for holes coming from ITO resulting
in limiting the recombination zone in the emissive layer. Furthermore, no emission
from CBP was observed, indicating a complete energy transfer from the host exciton to
the Ir-dopant. Meanwhile, there is no exciton decay in the Alq3 layer due to the
hole blocking action of the BCP layer. Hence Alq3 and BCP do not contribute to the EL
spectrum and act as electron transport and hole blocking layers, respectively.
Figure 5(a) shows the J–V–L characteristics of (BuOXD)2Ir(tta). Curve A shows
the current–voltage and curve B shows the voltage–luminescence characteristics. From
this figure it can be concluded that the device showed maximum luminescence of
2172 cdmꢁ2 at 15 V. Figure 5(b) shows the J–V–L characteristics of (BuOXD)2Ir(tmd),

Heteroleptic iridium complexes
461
Figure 6. Curve A, CE–L and curve B, PE–L characteristics of device using 8% (BuOXD)2Ir(tta) doped
CBP as emissive material. Inset: Curve A, CE–L and curve B, PE–L characteristics of device using 8%
(BuOXD)2Ir(tmd) doped CBP as emissive material.
where curve A shows the current–voltage and curve B shows the voltage–luminescence
characteristics. This figure shows that the device had a maximum luminescence of
210 cdmꢁ2 at 19 V. Figure 6 shows EL efficiency–brightness characteristics of the
iridium complexes. Curves A and B show current efficiency and power efficiency with
brightness of the device having (ButOXD)2Ir(tta) as emissive material. The device
shows maximum current efficiency 0.34 cdAꢁ1 at 657 cdmꢁ2. The inset of figure 6
shows the efficiencies of the device having (ButOXD)2Ir(tmd) as emissive material. The
efficiencies of the device having (ButOXD)2Ir(tmd) are less than the device having
(ButOXD)2Ir(tta) as emissive material. This reduction in efficiencies can be attributed
to switching of emission center from ‘‘C^N2Ir’’ to X in (ButOXD)2Ir(tmd).
4. Conclusion
Substitution on ꢀ-diketone with functional groups of different electronegativity alters
the electronic properties of the complex, resulting in tuning the emission color. The
present complexes are stable in air and can be deposited by thermal evaporation during
device fabrication. Electroluminescent characteristics of these metal complexes show
that these materials can be used as good emitting materials for OLED applications.
This study provides a convenient way to tune the emission color by changing the
secondary ligand.
Acknowledgments
The authors thank the Director, National Physical Laboratory, New Delhi and Head,
Department of Chemistry, M. D. University, Rohtak (Haryana) for providing the

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A. Kumar et al.
necessary facilities. The authors also gratefully recognize the financial support from the
Department of Science and Technology (DST) and Council of Scientific and Industrial
Research (CSIR), New Delhi, India.
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