Synthesis and characterization of highly efficient blue Ir(III) complexes by tailoring β-diketonate ancillary ligand for highly efficient PhOLED applications

Article abstract of DOI:10.1016/j.orgel.2016.09.030

We synthesized two new heteroleptic blue-emitting Ir(III) complexes tBuCN-FIrmMes and tBuCN-FIrdMes by using systematically tailored ancillary ligands 1-mesitylbutane-1,3-dione (mMes) and 1,3-dimesitylpropane-1,3-dione (dMes), respectively. Near-unity photoluminescence quantum yield (PLQY: 96?±?3%) was achieved for tBuCN-FIrmMes in doped films with short phosphorescence lifetime and high horizontal dipole ratio (Θ: 76%) than the di-mesityl complex, tBuCN-FIrdMes (PLQY: 75?±?3%, Θ: 71%). Here, introduction of mMes reduces the singlet-triplet splitting energy (ΔE_ST), permanent dipole moment (μ), and the symmetry of tBuCN-FIrmMes as compared with tBuCN-FIrdMes, these parameters can highly affect the PLQY and Θ values. As a result, the blue phosphorescent organic light-emitting diode (PhOLED) of tBuCN-FIrmMes exhibits high external quantum efficiency (EQE) of?≥?20% throughout a wide luminance range 1000–10000?cd/m² with a very low efficiency roll-off, which is crucial for solid-state lighting applications.

Full text of DOI:10.1016/j.orgel.2016.09.030

Organic Electronics 39 (2016) 91e99
Contents lists available at ScienceDirect
Organic Electronics
journal homepage: www.elsevier.com/locate/orgel
Synthesis and characterization of highly efficient blue Ir(III) complexes
by tailoring
applications
b-diketonate ancillary ligand for highly efficient PhOLED
,
,
Ganguri Sarada a 1, Bomi Sim b 1, Chang-Ki Moon b, Woosum Cho a, Kwon-Hyeon Kim b,
Vijaya Gopalan Sree a, Eunju Park a, Jang-Joo Kim b **, Sung-Ho Jin a
,
, *
a Department of Chemistry Education, Graduate Department of Chemical Materials, and Institute for Plastic Information and Energy Materials, Pusan
National University, Busan, 609-735, Republic of Korea
b Department of Materials Science and Engineering, Seoul National University, Seoul, 151-744, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history:
We synthesized two new heteroleptic blue-emitting Ir(III) complexes tBuCN-FIrmMes and tBuCN-
FIrdMes by using systematically tailored ancillary ligands 1-mesitylbutane-1,3-dione (mMes) and 1,3-
dimesitylpropane-1,3-dione (dMes), respectively. Near-unity photoluminescence quantum yield (PLQY:
96 3%) was achieved for tBuCN-FIrmMes in doped films with short phosphorescence lifetime and high
Received 23 May 2016
Received in revised form
16 August 2016
Accepted 25 September 2016
horizontal dipole ratio (
Here, introduction of mMes reduces the singlet-triplet splitting energy (
moment ( ), and the symmetry of tBuCN-FIrmMes as compared with tBuCN-FIrdMes, these parameters
can highly affect the PLQY and values. As a result, the blue phosphorescent organic light-emitting
Q
: 76%) than the di-mesityl complex, tBuCN-FIrdMes (PLQY: 75 3%,
Q: 71%).
D
EST), permanent dipole
m
Keywords:
Blue PhOLEDs
Q
diode (PhOLED) of tBuCN-FIrmMes exhibits high external quantum efficiency (EQE) of
ꢀ
20%
b
-diketonate ancillary ligands
throughout a wide luminance range 1000e10000 cd/m2 with a very low efficiency roll-off, which is
crucial for solid-state lighting applications.
Low efficiency roll-off
High photoluminescence quantum yields
© 2016 Elsevier B.V. All rights reserved.
1. Introduction
the phosphorescent emitter can play an important role in mini-
mizing triplet-triplet annihilation (TTA), which is the main reason
The performance of blue phosphorescent organic light-emitting
diodes (PhOLEDs) is still a bottleneck though high efficiencies and
long operating lifetimes have been realized in green and red
emitting PhOLEDs [1e4]. In most of the applications, OLEDs are
usually needed to operate at a luminance higher than 1000 cd/m2,
especially 5000e10000 cd/m2 for solid-state lighting [5,6]. Effi-
ciency roll-off is a usual problem in PhOLEDs at such a high lumi-
nance level, particularly for blue PhOLEDs. The external quantum
efficiency (EQE) value drops substantially when the luminance
crosses 1000 cd/m2, although EQE more than 20% can be accom-
plished at a low luminance [7e11]. The use of a mixed-host system
signifies a promising technique to realize a low efficiency roll-off
and low driving voltages [12]. Besides, triplet exciton lifetime of
for efficiency roll-off in PhOLEDs [13e15]. Sometimes, the low
thermal stability of the materials (host/emitter) used in device
fabrication may also initiate the efficiency roll-off [4].
Bis[2-(4,6-difluorophenyl)pyridinato- C2,N](picolinato)iridiu-
m(III) (FIrpic) has been widely used as emitter for blue PhOLEDs,
but it is limited to sky blue color with y-coordinate > 0.3 [16e18].
Therefore, development of blue PhOLEDs possessing high color
purity with at least y-coordinate <0.2, along with high EQE and low
roll-off in EQE at high luminance (>1000 cd/m2) remains a great
challenge [19,20]. This requires the design of new blue emitting
Ir(III) complexes with narrow spectral width and/or reduced longer
wavelength shoulder peaks, high photoluminescence quantum
yields (PLQYs) and short phosphorescence lifetimes (t). Recently,
several reports reveal that emitters possessing preferential hori-
zontal emitting dipoles are beneficial to optical out-coupling or
light extraction of OLEDs without further out-coupling techniques
while extraction of the internally generated light in OLEDs has been
a critical issue for achieving high EQEs [21e29]. In this regard,
* Corresponding author.
** Corresponding author.
E-mail addresses: jjkim@snu.ac.kr (J.-J. Kim), shjin@pusan.ac.kr (S.-H. Jin).
G. Sarada and B. Sim made equal contribution for this article.
1
http://dx.doi.org/10.1016/j.orgel.2016.09.030
1566-1199/© 2016 Elsevier B.V. All rights reserved.

92
G. Sarada et al. / Organic Electronics 39 (2016) 91e99
heteroleptic Ir(III) complexes are one of the most prominent
phosphorescent emitters to have preferential horizontal dipole
orientation. Interestingly, the emitting dipole orientation and the
PLQY were largely affected by the substituents on the main ligands
and the molecular structure of the ancillary ligand [30,31].
With the aim to develop ideal blue emitters for display and
solid-state lighting applications, we designed and synthesized two
heteroleptic Ir(III) complexes with a new class of ancillary ligands,
1-mesitylbutane-1,3-dione (mMes) or 1,3-dimesitylpropane-1,3-
dione (dMes), consisting of bulky mesityl ring(s). Here, alteration
of the ancillary ligand with a mesityl and a methyl group (mMes)
rather than two symmetrically placed mesityl groups (dMes) at the
developed previously [30]. Ancillary ligand, mMes, was purchased
from TCI Co., LTD. The other ancillary ligand, dMes, was synthesized
as per the literature procedure [34]. 1H and 13C NMR spectra were
recorded on a Varian Mercury Plus 300 MHz spectrometer in CDCl3
using tetramethylsilane as an internal reference. The chemical
shifts were reported in ppm relative to the singlet of CDCl3 at 7.26
and 77 ppm for the 1H and 13C NMR, respectively. UVevisible and
the emission spectra were recorded with a JASCO V-570 and Hitachi
F-4500 fluorescence spectrophotometers at room temperature.
Transient PL measurements were carried out using compact fluo-
rescence lifetime spectrometer C11367 at room temperature.
Thermal analyses were carried out on a Mettler Toledo TGA/SDTA
peripheries of the
short , thus, high radiative decay rate constant (Kr), iii) high color
purity, and iv) high horizontal dipole ratio ( ) for the mMes based
b
-diketonate framework results; i) high PLQY, ii)
851eanalyzer under N2 atmosphere at a heating rate of 10 ꢁC minꢂ1
.
t
Cyclic voltammetry (CV) studies were carried out with a CHI 600C
potentiostat (CH Instruments) at a scan rate of 100 mV sꢂ1 in
anhydrous dichloromethane (CH2Cl2) solvent with 0.1 M TBAClO4
as supporting electrolyte. A platinum wire was used as the counter
electrode and an Ag/AgCl electrode was used as the reference
electrode. The potentials were referenced to the ferrocene/ferro-
cenium redox couple (Fc/Fcþ). Density functional theory (DFT) and
time-dependent-DFT (TD-DFT) calculations were carried out using
Gaussian09 program [35]. The geometrical features of the Ir(III)
complexes in solid-state were characterized by single crystal X-ray
analysis using Bruker SMART CCD area-detector diffractometer. The
single crystals of tBuCN-FIrmMes and tBuCN-FIrdMes were grown
in dimethylformamide (DMF) solvent by slow evaporation and the
structures were solved using a PLATON (version release date 21/06/
2015) and further refined by full-matrix least-squares calculations
on F2. All hydrogen atoms were omitted to reduce complexity.
Q
complex (tBuCN-FIrmMes) relative to the dMes based analogue
(tBuCN-FIrdMes). As a result, the blue PhOLED of tBuCN-FIrmMes
exhibited one of the best performances up to a very high lumi-
nance range of 1000e10000 cd/m2 with EQEs of 22.4% and 19.8% at
1000 cd/m2 and 10000 cd/m2, respectively, representing
a
remarkably low roll-off, which is crucial for practical display/solid-
state lighting applications [32,33].
2. Experimental section
2.1. General information
Synthetic route of the Ir(III) complexes, tBuCN-FIrmMes and
tBuCN-FIrdMes, is provided in Scheme 1. The cyclometalating
ligand and its dimer were synthesized using the procedure we have
Scheme 1. Synthetic route of tBuCN-FIrmMes and tBuCN-FIrdMes.

G. Sarada et al. / Organic Electronics 39 (2016) 91e99
93
2.2. Synthesis of bis{3-[4-(tert-butyl)pyridin-2-yl]-2,6-
difluorobenzonitrile}(4-mesitylbut-3-en-2-ketonate)iridium(III)
[tBuCN-FIrmMes]
J ¼ 6.3 Hz, 2H, Ar H), 8.35 (s, 2H, Ar H), 7.35 (d, J ¼ 5.4 Hz, 2H, Ar H),
6.70 (s, 4H, mes-Ar H), 6.00 (d, J ¼ 9 Hz, 2H, Ar H), 5.49 (s, 1H,
acacH), 2.18e1.88 (m, 18H, mes-CH3), 1.50 (s, 18H, tert-butyl); 13C
NMR (75 MHz, CDCl3):
d (ppm) 185.56, 164.61, 163.123, 161.51,
Dimer (0.8 g, 0.52 mmol), ancillary ligand-mMes (1.06 g,
5.2 mmol), and anhydrous K2CO3 (0.72 g, 5.2 mmol) were taken in
1,4-dioxane (25 mL) and refluxed under N2 atmosphere over 6 h
period. After that, HCl (1 N, 15 mL) was added to the reaction
mixture and extracted using CH2Cl2. Organic layer was dried over
anhydrous Na2SO4 and concentrated under vacuum to get the
crude product, which was purified by column chromatography on
silica gel (EtOAc:Hexane, 2:8 v/v as eluent) to obtain the blue light-
emitting yellow solid tBuCN-FIrmMes (0.15 g, yield 30%). 1H NMR
161.04, 159.78, 157.93, 147.96, 138.43, 137.95, 132.90, 130.14, 128.43,
120.60, 120.14, 116.91, 111.06, 106.14, 85.23, 35.63, 30.62, 21.0, 18.75.
Calcd for C53H49F4IrN4O2: C, 61.08; H, 4.74; N, 5.38. Found: C, 61.32;
H, 4.58; N, 5.54. Finally, both the Ir(III) complexes were purified by
train sublimation technique, after the column purifications, to
obtain highly pure materials.
3. Results and discussion
(300 MHz, CDCl3):
d
(ppm) 8.49 (d, J ¼ 6 Hz, 1H, Ar H), 8.40e8.33
The new Ir(III) complexes were thoroughly characterized by 1H,
13C NMR spectroscopy (Figs. S1 and S2), and the single crystal XRD
analysis (Table S1). Fig. 1 shows the chemical structures and crystal
ORTEP diagrams of tBuCN-FIrmMes (CCDC 1419975) and tBuCN-
FIrdMes (CCDC 1419976). tBuCN-FIrdMes consists of two-fold
rotational symmetry e.g. passing through the Ir and C17 atoms
(Fig. 1) and the asymmetric unit contains only half a molecule
indicating its symmetry, whereas, the asymmetric unit of tBuCN-
FIrmMes contains three molecules by representing no such sym-
metry (Fig. S3) [36]. From the thermogravimetric analysis shown in
Fig. 2a, both the Ir(III) complexes exhibit excellent thermal stabil-
ities possessing the high decomposition temperatures (Td, at 5%
weight loss) of 411 ꢁC for tBuCN-FIrmMes and 416 ꢁC for tBuCN-
FIrdMes, pertaining to the bulky nature of the mesityl group(s)
(Td of the standard sky-blue emitter, FIrpic: 356 ꢁC).
(m, 2H, Ar H), 8.30e8.26 (s, 1H, Ar H), 7.39 (d, J ¼ 4.5 Hz, 1H, Ar H),
7.30 (d, J ¼ 7.5 Hz, 1H, Ar H), 6.70 (s, 2H, mes-Ar H), 6.03 (d,
J ¼ 8.1 Hz, 1H, Ar H), 5.76 (d, J ¼ 8.7 Hz, 1H, Ar H), 5.37 (s, 1H, acacH),
2.18 (s, 3H, CH3),1.90e1.75 (m, 9H, mes-CH3),1.47 (d, J ¼ 7.2 Hz,18H,
tert-butyl); 13C NMR (75 MHz, CDCl3):
d (ppm) 186.80, 184.42,
164.32, 164.06, 163.12, 162.86, 161.30, 161.04, 159.72, 157.84, 147.94,
147.47, 138.45, 137.74, 133.02, 130.07, 128.30, 120.95, 120.69, 120.49,
116.59, 111.04, 103.581, 35.54, 30.60, 29.0, 18.64. Calcd for
C
45H41F4IrN4O2: C, 57.62; H, 4.41; N, 5.97. Found: C, 57.94; H, 4.56;
N, 5.68.
2.3. Synthesis of bis{3-[4-(tert-butyl)pyridin-2-yl]-2,6-
difluorobenzonitrile}(1,3-dimesitylpropane-1,3-diketonate)
iridium(III) [tBuCN-FIrdMes]
Fig. 2b represents the UVevisible absorption and the photo-
luminescence (PL) spectra measured in chloroform (CHCl3) solution
(1 ꢃ 10ꢂ5 M), and the PL spectra measured for the neat films, spin-
coated using chlorobenzene solution (10 mg of Ir(III) complex in
Same procedure was followed using the ancillary ligand-dMes
and obtained the titled product as yellow solid tBuCN-FIrdMes
(0.21 g, yield 39%). 1H NMR (300 MHz, CDCl3):
d (ppm) 8.65 (d,
Fig. 1. Chemical structures and the crystal ORTEP diagrams of tBuCN-FIrmMes and tBuCN-FIrdMes. [Hydrogen atoms are omitted for clarity].

94
G. Sarada et al. / Organic Electronics 39 (2016) 91e99
conjugation by the twisted orientation of the mesityl group(s) as
shown by the dihedral angles C3-C4-C5-C6 (102.81ꢁ) and C17-C18-
C-19-C20 (96.74ꢁ), respectively. However, the vibronic shoulder
peak and the PL spectral width are notably reduced for tBuCN-
FIrmMes relative to tBuCN-FIrdMes in both solution and neat film
state, as tBuCN-FIrdMes exhibits multiple intermolecular
p …. p
stacking (parallel-offset) interactions between the mesityl and the
functionalized phenyl ring (C^N ligand) [Fig. S4]. tBuCN-FIrmMes
displayed intense blue light in CHCl3 solution with the maximum
PL intensity (PLmax) at 462 nm and an unresolved vibronic peak at
489 nm with
a full width at half maximum (FWHM) of
2 ꢃ 105 cmꢂ1, whereas, tBuCN-FIrdMes emitted blue light with
PLmax at 460 nm and a well resolved vibronic peak at 489 nm with a
FWHM of 1.85 ꢃ 105 cmꢂ1. Interestingly, tBuCN-FIrmMes main-
tained high color purity even in the neat film showing completely
suppressed shoulder peak with FWHM of 1.69 ꢃ 105 cmꢂ1, although
tBuCN-FIrdMes exhibited high intensity shoulder peak with FWHM
of 1.33 ꢃ 105 cmꢂ1, (Fig. 2b).
The relative PLQYs of tBuCN-FIrmMes and tBuCN-FIrdMes
measured in CH2Cl2 solution (1 ꢃ 10ꢂ5 M) using FIrpic (¼42% in
CHCl3) [37] as standard are 89% and 71% ( 5%), respectively
(Table 1). The relative PLQYs were also measured for the thin films
prepared by spin-coating a CHCl3 solution of poly (methyl meth-
acrylate) [PMMA] and the target Ir(III) complex (10% w/w relative to
PMMA) using Coumarin-102 (¼46% in PMMA film) [38,39] as
standard. The results follow the trend of the absolute PLQYs of the
deposited films, discussed below (see Tables 1 and 2). Moreover,
the
t
of tBuCN-FIrmMes measured in both CH2Cl2 solution and the
of tBuCN-
doped PMMA film are lower than the corresponding
t
FIrdMes, as shown by the decay profiles (Fig. S5), this gives high
Kr for tBuCN-FIrmMes (Table 1). The absolute PLQYs were
measured for the films of 9-(4-(9H-pyrido [2,3-b]indol-9-yl)
phenyl)-9H-3,90-bicarbazole (pBCb2Cz):diphenylphosphine oxide-
4-(triphenylsilyl)phenyl (TSPO1):10% Ir(III) complexes, the same
host-guest combination was used for the emitting layer (EML) of
the PhOLEDs, using an integrating sphere. The PLQYs of tBuCN-
FIrmMes and tBuCN-FIrdMes were found to be 96
3% and
Fig. 2. (a) TGA curves and (b) UVevisible absorption and PL spectra in CHCl3
(1 ꢃ 10ꢂ5 M), and PL spectra of the neat films of the Ir(III) complexes measured at room
temperature. [Here, s ¼ solution and f ¼ film].
75 3%, respectively (Table 2). Here, the substitution of one mesityl
ring with a methyl group in the ancillary ligand, mMes, resulted in
22% increment in PLQY of tBuCN-FIrmMes as compared with
tBuCN-FIrdMes. Because both the Ir(III) complexes exhibit same
lowest triplet (T1) energy of 2.72 eV, measured from PL analysis at
77 K (Fig. S6), that is lower than the mixed-host materials; the low
PLQY of tBuCN-FIrdMes in the doped film is due to the complex
itself. When both the mesityl rings are replaced with methyl groups
0.5 mL solvent), of tBuCN-FIrmMes and tBuCN-FIrdMes at room
temperature. The structural variation of the ancillary ligands has
not affected the peak positions of the absorption and the PL spectra
of tBuCN-FIrmMes and tBuCN-FIrdMes due to the restricted
p-
Table 1
Thermal, photophysical, and electronic properties.
Complex
Td (ꢁC)
PLmax (nm)a
T1 (eV)b
Measured in CH2Cl2
Measured in PMMA films
HOMO, LUMO (eV)k
f,g
f,j
PLQY (%)c,d
t
(
m
s)e
Kr, Knr (105 sꢂ1
)
PLQY (%)c,h
t
(
m
s)i
Kr, Knr (105 sꢂ1
)
tBuCN-FIrmMes
tBuCN-FIrdMes
411
416
462, 488
460, 489
2.72
2.72
89
71
0.240
0.392
37.1, 4.58
18.1, 7.40
94
80
0.93
1.15
10.1, 0.645
6.96, 1.74
ꢂ5.86, ꢂ3.1
ꢂ5.88, ꢂ3.14
a
Measured in CHCl3 solution at room temperature (1 ꢃ 10ꢂ5 M).
Measured at 77 K in CHCl3.
b
c
Relative PLQY.
d
e
f
Measured in CH2Cl2 solution (1 ꢃ 10ꢂ5 M) using FIrpic (¼42% in CHCl3) as standard (error 5%).
Phosphorescence lifetimes measured in CH2Cl2 solution (1 ꢃ 10ꢂ3 M) after N2 purging.
Kr ¼ PLQY/
t
, Knr ¼ (1ꢂPLQY)/
t.
g
h
Calculated from the solution PLQY and phosphorescence lifetimes.
Measured for the thin films by spin-coating a CHCl3 solution of PMMA and the target Ir(III) complex (10% w/w relative to PMMA) using Couamrin-102 (PMMA film ¼ 46%)
as standard (error 5%).
i
Phosphorescence lifetimes measured for the above films (PMMA).
Calculated from the film PLQY and phosphorescence lifetimes.
j
k
HOMO is calculated from the onset oxidation potentials from CV, LUMO ¼ Eg (optical) þ HOMO.

G. Sarada et al. / Organic Electronics 39 (2016) 91e99
95
Table 2
Photophysical properties of the Ir(III) complexes in mixed-host films (deposited), and the DFT & TD-DFT data.
d,e
Complex
PLQY (%)a,b
t
(m
s)b,c
Kr, Knr (105 sꢂ1
)
Q
(%)b,f
m
(D)g
S1 (eV)h
T1 (eV)i
D
EST (eV)j
HOMO, LUMO (eV)k
tBuCN-FIrmMes
tBuCN-FIrdMes
96
75
3
3
1.1
1.35
8.73, 0.36
5.55, 1.85
76
71
11.25
13.17
2.95
2.83
2.64
2.45
0.31
0.38
ꢂ5.79, ꢂ1.87
ꢂ5.30, ꢂ2.19
a
Absolute PLQY.
b
Measured for the thin films of pBCb2Cz:TSPO1:10 wt% Ir(III) complexes.
Phosphorescence lifetimes.
c
d
e
f
Kr ¼ PLQY/
t
, Knr ¼ (1ꢂPLQY)/
t.
Calculated using the PLQY and lifetimes of the films [pBCb2Cz:TSPO1:10 wt% Ir(III) complexes].
Horizontal dipole ratio.
g
h
i
Permanent dipole moment calculated from DFT.
Lowest singlet excited state energy obtained from TD-DFT.
Lowest triplet excited state energy obtained from TD-DFT.
j
Singlet-triplet splitting energy,
D
EST ¼ S1ꢂT1.
k
HOMO and LUMO energy calculated from DFT.
Fig. 3. (a) HOMO/LUMO distributions obtained by DFT and (b) Spin density distribution of T1 state using TD-DFT (UB3LYP) for tBuCN-FIrmMes and tBuCN-FIrdMes.

96
G. Sarada et al. / Organic Electronics 39 (2016) 91e99
in the ancillary ligand, acetylacetonate (acac) [30], the PLQY is
reduced (90 3%) relative to tBuCN-FIrmMes. The beneficial effects
of mesityl group attached to phenylpyridine backbone on PLQY of
the blue heteroleptic Ir(III) complexes and on performance of the
complexes in T1 state were obtained from TD-DFT and the results
(Fig. 3b) show that the T1 states of tBuCN-FIrmMes and tBuCN-
FIrdMes are entirely contributed by the ancillary ligand and the Ir
atom. In addition, from the TD-DFT calculations, the lowest singlet
(S1) and T1 states are found to be dominated by the HOMO to LUMO
transition for the two Ir(III) complexes. To understand the basis for
the superior photophysical properties of tBuCN-FIrmMes as
compared with tBuCN-FIrdMes, we calculated the permanent
PhOLEDs were demonstrated in literature reports [40,41]. The
t
measured by the transient PL for the same doped film
(pBCb2Cz:TSPO1:10 wt% Ir(III) complex) containing tBuCN-
FIrmMes (1.1 ms) is shorter than tBuCN-FIrdMes (1.35 ms) result-
ing in enhanced Kr for tBuCN-FIrmMes. For instance,
Kr ¼ 8.73 ꢃ 105 sꢂ1 and non-radiative decay rate constant
(Knr) ¼ 0.36 ꢃ 105 sꢂ1 for tBuCN-FIrmMes, and Kr ¼ 5.55 ꢃ 105 sꢂ1
and Knr ¼ 1.85 ꢃ 105 sꢂ1 for tBuCN-FIrdMes. Hence, it is worth
highlighting that simple tailoring of the ancillary ligand structure
dipole moment (
using DFT and the TD-DFT. The resultant
0.31 eV and 11.253 D for tBuCN-FIrmMes and 0.38 eV and 13.166 D
for tBuCN-FIrdMes, respectively. In general, a minimal EST is
beneficial for increasing the ISC rates [42e45], resulting to an
enhanced Kr (theoretically, Kr is inversely proportional to EST
[45]. Here, the calculated EST of tBuCN-FIrmMes is less than
m
) and the singlet-triplet splitting energy (
DEST)
D
EST and the values are
m
D
significantly improved the emission properties (PLQY, Kr,
t, and
D
)
FWHM) of tBuCN-FIrmMes relative to tBuCN-FIrdMes.
D
Fig. 3a shows the highest occupied molecular orbital (HOMO)
and lowest unoccupied molecular orbital (LUMO) distributions of
tBuCN-FIrmMes and tBuCN-FIrdMes, calculated from DFT using
Gaussian09 program. Ground state geometry optimization for the
two molecules was performed using B3LYP exchange-correlation
functional, LANL2DZ basis set for Ir atom, and 6-31G(d) basis set
for all other atoms. For tBuCN-FIrmMes, the HOMO is located
mostly on Ir atom and functionalized phenyl moiety and slightly on
pyridyl rings; whereas LUMO is mainly located on pyridyl rings and
a phenyl group. In case of tBuCN-FIrdMes, the HOMO is distributed
on Ir atom and phenylpyridine part; whereas LUMO is located on
pyridyl and mesityl rings. The calculated HOMO, LUMO and the T1
energies are given in Table 2. Spin density distributions of the Ir(III)
tBuCN-FIrdMes and many other reported heteroleptic Ir(III) com-
plexes [42,43]. Thus, the high Kr and PLQY of tBuCN-FIrmMes were
closely related to its small DEST. Hence, it is rational to conclude that
the structural variation between tBuCN-FIrmMes and tBuCN-
FIrdMes (mesityl-methyl instead of mesityl-mesityl) is the reason
for their distinct photophysical properties. The higher
m value of
tBuCN-FIrdMes as compared with tBuCN-FIrmMes is the evidence
for its broadening of PL spectral width [21,22].
The emitting dipole orientations of the thin films of
pBCb2Cz:TSPO1:10% Ir(III) complexes deposited by thermal evap-
oration on fused silica substrate are preferentially horizontal with
Q
of 76% for tBuCN-FIrmMes and the
(Fig. 4). This high value of tBuCN-FIrmMes enhances optical
out-coupling or light extraction of the PhOLEDs. The low of
Q of 71% for tBuCN-FIrdMes
Q
Q
tBuCN-FIrdMes is tentatively correlated with its high molecular
symmetry due to the two symmetrically placed mesityl groups [31],
Fig. 4. (a) Comparison of the experimentally measured angle-dependent p-polarized
PL profiles at maxima of each dopants of the thin films composed of
pBCb2Cz:TSPO1:10% Ir(III) complex on a fused silica substrate (open symbols) and
simulated profiles (solid lines) and (b) Chemical structures of pBCb2Cz and TSPO1.
Fig. 5. (a) Energy-level diagram and (b) J-V-L characteristics of the optimized PhO-
LEDs. [ETL ¼ TSPO1: 1 wt% Rb2CO3].

G. Sarada et al. / Organic Electronics 39 (2016) 91e99
97
TSPO1:1 wt% Rb2CO3 (35 nm)/LiF(1 nm)/Al (100 nm). Before
fabrication of OLEDs, device structure was optimized using the
optical simulation based on classical dipole model to maximize the
out-coupling efficiency [28]. The theoretically achievable
maximum EQEs of the PhOLEDs with the measured
Q and PLQYs of
the Ir(III) complexes are 25.6% with tBuCN-FIrmMes and 19.1% with
tBuCN-FIrdMes. Experimentally, we approached the theoretical
maximum EQEs using the 1:1 M ratio of the mixed-host and 10 wt%
of the dopant in the EML due to a good charge balance. Therefore, a
1:1 M ratio of pBCb2Cz:TSPO1 as the mixed-host and the 10 wt%
Ir(III) complex as dopant were used as the EML. The pBCb2Cz and
TSPO1 (or) TSPO1:1 wt% Rb2CO3 were used as the hole transporting
layer (HTL) and the electron transporting layer (ETL), respectively.
Fig. 5a shows the energy level diagram of the blue PhOLEDs.
Figs. 5b and 6a show the current density-voltage-luminance (J-
V-L) and the EQE-L characteristics of the PhOLEDs containing
TSPO1:1 wt% Rb2CO3 as ETL. The data of the PhOLEDs without
electrical doping (Rb2CO3) in ETL were given in Fig. S8 and Table S2.
Upon doping Rb2CO3 (1 wt%) in ETL, the performance of the PhO-
LEDs is improved representing the betterment in charge balance. In
particular, the doped-ETL devices showed the maximum EQEs of
22.4% and 17.8% for tBuCN-FIrmMes and tBuCN-FIrdMes, respec-
tively, with the turn on voltage (Von) of 3.0 V. Interestingly, the
overall performance of tBuCN-FIrmMes based devices is highly
superior to the ones with tBuCN-FIrdMes. For example, the PhOLED
(doped-ETL) with tBuCN-FIrmMes showed an excellent maximum
luminance (Lmax) of 26000 cd/m2 at 7.8 V, whereas the one with
tBuCN-FIrdMes showed 16000 cd/m2 at 8 V. Similarly, the PhOLED
of tBuCN-FIrmMes displayed one of the best stability with EQEs of
22.4% at 1000 cd/m2, 19.8% at 10000 cd/m2, and 16.5% at 20000 cd/
m2 compared with the PhOLED of tBuCN-FIrdMes, where the EQEs
are 17.8% at 1000 cd/m2 and 14.9% at 10000 cd/m2. This represents
that the maximum EQE has retained up to 88% at 10000 cd/m2 and
74% at 20000 cd/m2 for tBuCN-FIrmMes based device, which is very
rare, but essential for solid-state lighting. Especially, the remark-
ably high Lmax and the low roll-off in EQE up to the high brightness
level (10000 cd/m2) for tBuCN-FIrmMes are far better than the
other reported blue Ir(III) complex dopants [7e11]. As discussed
above, the structural modification i.e replacement of a mesityl ring
Fig. 6. (a) EQE-L characteristics and (b) CIE coordinates at 10 mA/cm2 of the optimized
PhOLEDs. [ETL ¼ TSPO1: 1 wt% Rb2CO3].
with a methyl group brings about high PLQY and
Q for tBuCN-
FIrmMes, this is the main reason for its superior device perfor-
mance than tBuCN-FIrdMes. The low roll-off in EQE is mainly
as explained before. The HOMO energy levels were deduced from
the onset oxidation potentials, obtained by CV analysis (Fig. S7), and
were found to be ꢂ5.86 and ꢂ5.88 eV for tBuCN-FIrmMes and
tBuCN-FIrdMes, respectively. The LUMO levels were calculated
using the HOMO values and the optical band gaps, obtained from
the absorption edges, and were found to be ꢂ3.10 and ꢂ3.14 eV for
tBuCN-FIrmMes and tBuCN-FIrdMes, respectively. The HOMO/
LUMO of the two Ir(III) complexes were not varied as like the peak
positions in the absorption and the PL spectra.
To evaluate the performance of tBuCN-FIrmMes and tBuCN-
FIrdMes as dopants, we fabricated blue PhOLEDs with the
following simple configuration: ITO (70 nm)/ReO3 (1 nm)/pBCb2Cz
(40 nm)/pBCb2Cz:TSPO1:10 wt% Ir(III) complex (30 nm)/TSPO1 (or)
attributed to the short
t
(1.1
ms) of tBuCN-FIrmMes as compared
with tBuCN-FIrdMes (1.35
ms) and also credited to the mixed-host
system [13e15].
The electroluminescence (EL) spectra do not change with
increasing current density (Fig. S9), indicating that the exciton
formation zone in the EML did not shift with the driving voltage.
Although the maximum EL intensity (ELmax) shows the same
wavelength (464 nm) for the devices of both the Ir(III) complexes,
the CIE coordinates (Fig. 6b) are slightly blue shifted for tBuCN-
FIrmMes (0.140, 0.206) relative to tBuCN-FIrdMes (0.147, 0.215).
The data of the PhOLEDs are listed in Table 3.
Table 3
Data of the blue PhOLEDs (ETL ¼ TSPO1: 1 wt% Rb2CO3) with the new Ir(III) complexes.
Dopant
Von (V)a
Lmax (cd/m2)
EQE (%)b
PE (lm/W)b
ELmax/CIE (nm)/(x, y)c
tBuCN-FIrmMes
tBuCN-FIrdMes
3.0
3.0
26000
16000
22.4/22.4/19.8/16.5
17.8/17.8/14.9/-
31.9/24.9/15.3/11
31.7/21.1/11.4/-
464/(0.140, 0.206)
464/(0.147, 0.215)
a
Turn on voltage (at 1 cd/m2).
b
c
In the order of maximum, then values at 1000, 10000, and 20000 cd/m2.
Measured at 10 mA/cm2.

98
G. Sarada et al. / Organic Electronics 39 (2016) 91e99
EQE of 30%, low drive voltage, and low efficiency roll-off at high brightness in
4. Conclusion
blue phosphorescent OLEDs, Adv. Opt. Mater. 4 (2016) 86.
[13] Y. Zhang, S.R. Forrest, Triplet diffusion leads to tripletetriplet annihilation in
organic phosphorescent emitters, Chem. Phys. Lett. 590 (2013) 106.
[14] S. Reineke, K. Walzer, K. Leo, Triplet-exciton quenching in organic phospho-
rescent light-emitting diodes with Ir-based emitters, Phys. Rev. B 75 (2007)
125328.
In summary, two blue Ir(III) complexes, tBuCN-FIrmMes and
tBuCN-FIrdMes, with the newly introduced ancillary ligands 1-
mesitylbutane-1,3-dione (mMes) and 1,3-dimesitylpropane-1,3-
dione (dMes), respectively, were synthesized. Here, we shown
that the simple structural alteration of the ancillary ligand (mMes)
viz. replacement of one of the mesityl rings by a methyl group
brings high PLQY (96%) and short
doped films by minimization of
tBuCN-FIrdMes (75%, 1.35
further advantage for the light out-coupling of the PhOLEDs,
accredited to the symmetry reduction of tBuCN-FIrmMes due to the
structural alteration. Owing to these collective reasons, the blue
PhOLED of tBuCN-FIrmMes showed the high Lmax of 26000 cd/m2,
and the EQEs of 22.4% (max.) at 1000 cd/m2 and 19.8% at 10000 cd/
m2. To the best of our knowledge, the high Lmax and the low EQE
roll-off, up to a wide luminance range 1000e10000 cd mꢂ2, for
tBuCN-FIrmMes are among the best data for blue PhOLEDs that
exhibit y-coordinate ꢄ 0.21.
[15] C. Murawski, K. Leo, M.C. Gather, Efficiency roll-off in organic light-emitting
diodes, Adv. Mater. 25 (2013) 6801.
[16] M. Kim, S.K. Jeon, S.-H. Hwang, J.Y. Lee, Molecular design of triazine and
carbazole based host materials for blue phosphorescent organic emitting di-
odes, Phys. Chem. Chem. Phys. 17 (2015) 13553.
[17] H. Fu, Y.-M. Cheng, P.-T. Chou, Y. Chi, Feeling blue? Blue phosphors for OLEDs,
Mater. Today 14 (2011) 472.
t
D
(1.1
EST and
of tBuCN-FIrmMes is the
m
s) for tBuCN-FIrmMes in
m
as compared with
m
s). The high
Q
[18] E. Baranoff, B.F.E. Curchod, FIrpic: archetypal blue phosphorescent emitter for
electroluminescence, Dalton Trans. 44 (2015) 8318.
[19] S.O. Jeon, S.E. Jang, H.S. Son, J.Y. Lee, External quantum efficiency above 20% in
deep blue phosphorescent organic light-emitting diodes, Adv. Mater. 23
(2011) 1436.
[20] S.H. Kim, J. Jang, S.J. Lee, J.Y. Lee, Deep blue phosphorescent organic light-
emitting diodes using a Si based wide bandgap host and an Ir dopant with
electron withdrawing substituents, Thin Solid Films 517 (2008) 722.
[21] M.J. Jurow, C. Mayr, T.D. Schmidt, T. Lampe, P.-I. Djurovich, W. Brütting,
M.E. Thompson, Understanding and predicting the orientation of heteroleptic
phosphors in organic light-emitting materials, Nat. Mater. 15 (2016) 85.
[22] A. Graf, P. Liehm, C. Murawski, S. Hofmann, K. Leo, M.C. Gather, Correlating the
transition dipole moment orientation of phosphorescent emitter molecules in
OLEDs with basic material properties, J. Mater. Chem. C 2 (2014) 10298.
[23] C.-Y. Lu, M. Jiao, W.-K. Lee, C.-Y. Chen, W.-L. Tsai, C.-Y. Lin, C.-C. Wu, Achieving
above 60% external quantum efficiency in organic light-emitting devicesu
ITO-free low-index transparent electrode and emitters with preferential
horizontal emitting dipoles, Adv. Funct. Mater. (2016), http://dx.doi.org/
10.1002/adfm.201505312.
Acknowledgements
This work was supported by grant fund from the National
Research
Foundation
(NRF)
(2011e0028320,
2014R1A2A1A01002030) and the Pioneer Research Center Program
through the NRF (2013M3C1A3065522) by the Ministry of Science,
ICT & Future Planning (MSIP) of Korea.
[24] K.-H. Kim, J.-L. Liao, S.W. Lee, B. Sim, C.-K. Moon, G.-H. Lee, H.J. Kim, Y. Chi, J.-
J. Kim, Crystal organic light-emitting diodes with perfectly oriented non-
doped Pt-based emitting layer, Adv. Mater. 28 (2016) 2526.
[25] T. Lampe, T.D. Schmidt, M.J. Jurow, P.I. Djurovich, M.E. Thompson, W. Brütting,
Dependence of phosphorescent emitter orientation on deposition technique
in doped organic films, Chem. Mater. 28 (2016) 712.
[26] D. Yokoyama, A. Sakaguchi, M. Suzuki, C. Adachi, Horizontal orientation of
linear-shaped organic molecules having bulky substituents in neat and doped
vacuum-deposited amorphous films, Org. Electron. 10 (2009) 127.
[27] J. Frischeisen, D. Yokoyama, C. Adachi, W. Brütting, Determination of molec-
ular dipole orientation in doped fluorescent organic thin films by photo-
luminescence measurements, Appl. Phys. Lett. 96 (2010) 073302.
[28] S.-Y. Kim, W.-I. Jeong, C. Mayr, Y.-S. Park, K.-H. Kim, J.-H. Lee, C.-K. Moon,
W. Brütting, J.-J. Kim, Organic light-emitting diodes with 30% external quan-
tum efficiency based on a horizontally oriented emitter, Adv. Funct. Mater. 23
(2013) 3896.
[29] H.A. Al-Attar, G.C. Griffiths, T.N. Moore, M. Tavasli, M.A. Fox, M.R. Bryce,
A.P. Monkman, Highly efficient, solution-processed, single-layer, electro-
phosphorescent diodes and the effect of molecular dipole moment, Adv.
Funct. Mater. 21 (2011) 2376.
[30] J.-H. Lee, G. Sarada, C.-K. Moon, W. Cho, K.-H. Kim, Y.G. Park, J.Y. Lee, S.-H. Jin,
J.-J. Kim, Finely tuned blue iridium complexes with varying horizontal emis-
sion dipole ratios and quantum yields for phosphorescent organic light-
emitting diodes, Adv. Opt. Mater. 3 (2015) 211.
Appendix A. Supplementary data
Supplementary data related to this article can be found at http://
dx.doi.org/10.1016/j.orgel.2016.09.030.
References
[1] H. Fukagawa, T. Shimizu, H. Hanashima, Y. Osada, M. Suzuki, H. Fujikake,
Highly efficient and stable red phosphorescent organic light-emitting diodes
using platinum complexes, Adv. Mater. 24 (2012) 5099.
[2] T. Giridhar, C. Saravanan, W. Cho, Y.G. Park, J.Y. Lee, S.-H. Jin, An electron
transporting unit linked multifunctional Ir(III) complex: a promising strategy
to improve the performance of solution-processed phosphorescent organic
light-emitting diodes, Chem. Commun. 50 (2014) 4000.
[3] H.-Y. Li, L. Zhou, M.-Y. Teng, Q.-L. Xu, C. Lin, Y.-X. Zheng, J.-L. Zuo, H.-J. Zhang,
X.-Z. You, Highly efficient green phosphorescent OLEDs based on a novel
iridium complex, J. Mater. Chem. C 1 (2013) 560.
[31] K.-H. Kim, C.-K. Moon, J.-H. Lee, S.-Y. Kim, J.-J. Kim, Highly efficient organic
light-emitting diodes with phosphorescent emitters having high quantum
yield and horizontal orientation of transition dipole moments, Adv. Mater. 26
(2014) 3844.
[32] V. Sivasubramaniam, F. Brodkorb, S. Hanning, H.P. Loebl, V. Elsbergen,
H. Boerner, U. Scherf, M. Kreyenschmidt, Investigation of FIrpic in PhOLEDs via
LC/MS technique, Cent. Eur. J. Chem. 7 (2009) 836.
[33] R. Seifert, I.R. Moraes, S. Scholz, M.C. Gather, B. Lüssem, K. Leo, Chemical
degradation mechanisms of highly efficient blue phosphorescent emitters
used for organic light emitting diodes, Org. Electron 14 (2013) 115.
[34] C. Zhang, P. Yang, Y. Yang, X. Huang, X.-J. Yang, B. Wu, High-yield synthesis of
[4] C. Xiang, X. Fu, W. Wei, R. Liu, Y. Zhang, V. Balema, B. Nelson, F. So, Efficiency
roll-off in blue emitting phosphorescent organic light emitting diodes with
carbazole host materials, Adv. Funct. Mater. 26 (2016) 1463.
[5] Y. Feng, P. Li, X. Zhuang, K. Ye, T. Peng, Y. Liu, Y. Wang, A novel bipolar
phosphorescent host for highly efficient deep-red OLEDs at a wide luminance
range of 1000e10000 cd mꢂ2, Chem. Commun. 51 (2015) 12544.
[6] J. Lee, H.-F. Chen, T. Batagoda, C. Coburn, P.I. Djurovich, M.E. Thompson,
S.R. Forrest, Deep blue phosphorescent organic light-emitting diodes with
very high brightness and efficiency, Nat. Mater. 15 (2016) 92.
[7] S. Lee, S.-O. Kim, H. Shin, H.-J. Yun, K. Yang, S.-K. Kwon, J.-J. Kim, Y.-H. Kim,
Deep-blue phosphorescence from perfluoro carbonyl-substituted iridium
complexes, J. Am. Chem. Soc. 135 (2013) 14321.
[8] J.-B. Kim, S.-H. Han, K. Yang, S.-K. Kwon, J.-J. Kim, Y.-H. Kim, Highly efficient
deep-blue phosphorescence from heptafluoropropyl-substituted iridium
complexes, Chem. Commun. 51 (2015) 58.
[9] T. Duan, T.-K. Chang, Y. Chi, J.-Y. Wang, Z.-N. Chen, W.-Y. Hung, C.-H. Chen, G.-
H. Lee, Blue-emitting heteroleptic Ir(III) phosphors with functional 2,30-
bipyridine or 2-(pyrimidin-5-yl)pyridine cyclometalates, Dalton Trans. 44
(2015) 14613.
[10] Y.-C. Chiu, J.-Y. Hung, Y. Chi, C.-C. Chen, C.-H. Chang, C.-C. Wu, Y.-M. Cheng, Y.-
C. Yu, G.-H. Lee, P.-T. Chou, En route to high external quantum efficiency
(~12%), organic true-blue-light-emitting diodes employing novel design of
iridium (III) phosphors, Adv. Mater. 21 (2009) 2221.
[11] C.-H. Hsieh, F.-I. Wu, C.-H. Fan, M.-J. Huang, K.-Y. Lu, P.-Y. Chou, Y.-H.O. Yang,
S.-H. Wu, I.-C. Chen, S.-H. Chou, K.-T. Wong, C.-H. Cheng, Design and synthesis
of iridium bis(carbene) complexes for efficient blue electrophosphorescence,
Chem. Eur. J. 17 (2011) 9180.
1,3-dimesityl-propane-1,3-dione: isolation of its aluminum complex as
stable intermediate, Syn. Commun. 38 (2008) 2349.
a
[35] M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M. A. Robb, J. R.
Cheeseman, G. Scalmani, V. Barone, B. Mennucci, G. A. Petersson, H. Nakatsuji,
M. Caricato, X. Li, H. P. Hratchian, A. F. Izmaylov, J. Bloino, G. Zheng, J. L.
Sonnenberg, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T.
Nakajima, Y. Honda, O. Kitao, H. Nakai, T. Vreven, J. A. Montgomery Jr, J. E.
Peralta, F. Ogliaro, M. Bearpark, J. J. Heyd, E. Brothers, K. N. Kudin, V. N.
Staroverov, R. Kobayashi, J. Normand, K. Raghavachari, A. Rendell, J. C. Burant,
S. S. Iyengar, J. Tomasi, M. Cossi, N. Rega, J. M. Millam, M. Klene, J. E. Knox, J. B.
Cross, V. Bakken, C. Adamo, J. Jaramillo, R. Gomperts, R. E. Stratmann, O.
Yazyev, A. J. Austin, R. Cammi, C. Pomelli, J. W. Ochterski, R. L. Martin, K.
Morokuma, V. G. Zakrzewski, G. A. Voth, P. Salvador, J. J. Dannenberg, S.
Dapprich, A. D. Daniels, O. Farkas, J. B. Foresman, J. V. Ortiz, J. Cioslowski, D. J.
Fox, Gaussian, Inc., Wallingford CT, 2009.
[36] W. Massa, Crystal structure Determination, second ed., ©Springer-Verlag,
Berlin Heidelberg, 2004 http://dx.doi.org/10.1007/978-3-662-06431-3. ISBN
[12] K. Udagawa, H. Sasabe, F. Igarashi, J. Kido, Simultaneous Realization of High

G. Sarada et al. / Organic Electronics 39 (2016) 91e99
solution processed OLEDs, J. Mater. Chem. C 4 (2016) 3726.
99
978-3-642-05841-7.
[37] T.H. Noh, H. Lee, J. Jang, O.-S. Jung, Organization and energy transfer of fused
aromatic hydrocarbon guests within anion-confining nanochannel MOFs,
Angew. Chem. Int. Ed. 54 (2015) 9284.
[42] X. Shang, D. Han, Q. Zhan, D. Zhou, G. Zhang, Theoretical investigation of the
effects of N-substitution on the photophysical properties of two series of
iridium(III) complexes, New J. Chem. 39 (2015) 2588.
[38] A.P. Green, A.R. Buckley, Solid state concentration quenching of organic flu-
orophores in PMMA, Phys. Chem. Chem. Phys. 17 (2015) 1435.
[39] S. Liu, P. Müller, M.K. Takase, T.M. Swager, “Click” synthesis of heteroleptic
tris-cyclometalated iridium(III) complexes: Cu(I) triazolide intermediates as
transmetalating reagents, Inorg. Chem. 50 (2011) 7598.
[43] Y. Si, S. Zhang, G. Gahungu, J. Yang, Z. Wu, Modification of the emission colour
and quantum efficiency for oxazoline- and thiazoline-containing iridium
complexes via different N^O ligands, RSC Adv. 5 (2015) 18464.
[44] G. Godefroid, S. Juanjuan, Q. Xiaochun, L. Yuqi, S. Yanling, S. Xiaohong,
W. Zhijian, Shedding light on unusual photophysical properties of bis-
[40] V.N. Kozhevnikov, Y. Zheng, M.A. Clough, H.A. Al-Attar, G.C. Griffiths,
K. Abdullah, S. Raisys, V. Jankus, M.R. Bryce, A.P. Monkman, Cyclometalated
Ir(III) complexes for high-efficiency solution-processable Blue PhOLEDs,
Chem. Mater. 25 (2013) 2352.
[41] A.F. Henwood, A.K. Bansal, D.B. Cordes, A.M.Z. Slawin, I.D.W. Samuel,
E. Zysman-Colman, Solubilised bright blue-emitting iridium complexes for
cyclometalated
iridium(III)
complexes
containing
2,5-diaryl-1,3,4-
oxadiazole-based and acetylacetonate ligands, Dalton Trans. 41 (2012) 10228.
[45] X. Qu, Y. Liu, Y. Si, X. Wu, Z. Wu, A theoretical study on supramolecularly-
caged positively charged iridium(III) 2-pyridyl azolate derivatives as blue
emitters for light-emitting electrochemical cells,, Dalton Trans. 43 (2014)
1246.