Synthesis and characterization of blue-to-green electrophosphorescence emitter based on pyrazole iridium complexes

Article abstract of DOI:10.1016/j.dyepig.2013.04.022

To overcome the roll-off efficiency of electrophosphorescent organic light-emitting devices, Ir(III) complexes of (ppz)₂Ir(N?N) were synthesized and named for (ppz)₂Ir(tfmpptz), (ppz) ₂Ir(fpptz), (ppz)₂Ir(tfmptz) and (ppz)₂Ir(pybi) (ppz: 1-phenylpyrazolato; N?N: N, N′-heteroaromatic). Their molecular structures, photophysical properties and cyclic voltammetric data were reported. The crystals of (ppz)₂Ir(tfmpptz), (ppz) ₂Ir(fpptz) are monoclinic, space group P2(1)/n, and (ppz) ₂Ir(tfmptz) is triclinic, space group P-1. The emission maximum peaks of (ppz)₂Ir(tfmpptz), (ppz)₂Ir(fpptz), (ppz) ₂Ir(tfmptz) and (ppz)₂Ir(pybi) at room temperature appear at 486, 497, 473 and 530 nm, respectively. The devices with architecture of ITO/NPB (30 nm)/CBP: (ppz)₂Ir(N?N) (6%, 30 nm)/BAlq(10 nm)/Alq₃(30 nm)/LiF(1 nm)/Al (100 nm) were fabricated by thermal evaporation. The results indicate that the larger steric hindrance of ancillary ligands efficiently suppressed the roll-off of efficiency at high current density.

Full text of DOI:10.1016/j.dyepig.2013.04.022

Dyes and Pigments 99 (2013) 67e73
Contents lists available at SciVerse ScienceDirect
Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Synthesis and characterization of blue-to-green
electrophosphorescence emitter based on pyrazole iridium complexes
,
b
,
c
c
c
a
,
,b
Xu Huixia a , Yue Yan , Qu Litao , Hao Yuying , Wang Hua b, Chen Liuqing a
,
*
Xu Bingshe a
,
b
a Key Laboratory of Interface Science and Engineering in Advanced Materials, Ministry of Education, Taiyuan University of Technology,
Taiyuan 030024, PR China
b Shanxi Research Center of Advanced Materials Science and Technology, Taiyuan 030024, PR China
c Department of Physics and Optoelectronics, Taiyuan University of Technology, Taiyuan 030024, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
To overcome the roll-off efficiency of electrophosphorescent organic light-emitting devices, Ir(III) com-
plexes of (ppz)2Ir(N^N) were synthesized and named for (ppz)2Ir(tfmpptz), (ppz)2Ir(fpptz), (ppz)2Ir
(tfmptz) and (ppz)2Ir(pybi) (ppz: 1-phenylpyrazolato; N^N: N, N0-heteroaromatic). Their molecular
structures, photophysical properties and cyclic voltammetric data were reported. The crystals of
(ppz)2Ir(tfmpptz), (ppz)2Ir(fpptz) are monoclinic, space group P2(1)/n, and (ppz)2Ir(tfmptz) is triclinic,
space group P-1. The emission maximum peaks of (ppz)2Ir(tfmpptz), (ppz)2Ir(fpptz), (ppz)2Ir(tfmptz) and
(ppz)2Ir(pybi) at room temperature appear at 486, 497, 473 and 530 nm, respectively. The devices with
architecture of ITO/NPB (30 nm)/CBP: (ppz)2Ir(N^N) (6%, 30 nm)/BAlq(10 nm)/Alq3(30 nm)/LiF(1 nm)/Al
(100 nm) were fabricated by thermal evaporation. The results indicate that the larger steric hindrance of
ancillary ligands efficiently suppressed the roll-off of efficiency at high current density.
Received 2 January 2013
Received in revised form
16 April 2013
Accepted 17 April 2013
Available online 25 April 2013
Keywords:
Iridium complex
Triazol
Pyrazole
Crown Copyright Ó 2013 Published by Elsevier Ltd. All rights reserved.
Electrophosphorescent
Blue-light emitting
Roll-off efficiency
1. Introduction
emission (ppy)2Ir(acac) (bis(2-phenylpyridine) (acetylacetonate)-
iridium(III)) by the introduction of electron-withdrawing fluoro
Phosphorescent organic light-emitting devices (PhOLEDs) have
shown great potential in digital terminal displays and planar solid-
state lighting. However, it is difficult to develop high-efficiency blue
PhOLEDs because of the roll-off in efficiency at high current density
[1e3]. The primary importance in achieving high-efficiency
PhOLED is that the density of triplet excitons should be right low
for the purpose of minimizing tripletetriplet (TeT) exciton anni-
hilation. This intention can be realized by synthesizing Ir(III)
complex with larger steric hindrance to distribute the triplet exci-
tons in the bulk emission layer [4,5]. The well-known blue-light
Ir(III) complex, FIrpic (iridium(III) bis(4,6-difluorophenylpyri
dinato-N, C20) picolinate) was designed based on the green
groups to the phenyl ring and the use of pic ligand as the ancillary
ligand 4. The efficiencies of FIrpic-based OLEDs have been improved
over the years, but a few critical problems need to be resolved, such
as inferior color purity [6,7]. The true-blue fac-Ir(ppz)3 (fac-tris(1-
phenylpyrazolato)iridium(III)) was reported with a lmax414 nm at
77 K, while it is non-emissive at room temperature as a result of the
small activation energy to a non-emissive dd excited state [8]. To
obtain room-temperature blue-light phosphorescent emitter, FIrtaz
and FIrN4 were reported by substituting pic with triazolate or tet-
razolate ligand [9]. However, the efficiency decrease sharply from
7.2 to 1.7 lm/W of device based-on FIrN4 at 100 mA/cm2 of current
density. Thus, it is
a desirable approach to realize room-
temperature blue phosphorescent emitter and avoid the effi-
ciency roll-off by inserting larger steric hindrance N, N0-hetero-
aromatic (N^N) ligands [3,10].
Therefore, in this manuscript, the design and synthesis of a se-
ries of (ppz)2Ir(N^N) phosphorescent heteroleptic iridium com-
plexes were described. Their chemical structure and synthetic routs
are displayed in Scheme 1.
* Corresponding author. Key Laboratory of Interface Science and Engineering in
Advanced Materials, Ministry of Education, Taiyuan University of Technology,
Taiyuan 030024, PR China. Tel.: þ86 03516014852; fax: þ86 0351 6010311.
E-mail address: xuhuixia0824@163.com (X. Huixia).
0143-7208/$ e see front matter Crown Copyright Ó 2013 Published by Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.dyepig.2013.04.022

68
X. Huixia et al. / Dyes and Pigments 99 (2013) 67e73
Scheme 1. The synthetic routs of (ppz)2Ir(N^N) complexes.
2. Experimental section
X-ray single-crystal diffractions of all complexes were per-
formed on Bruker SMART APEX II diffractometer with Mo Ka ra-
ꢀ
2.1. General information
diation (
l
¼ 0.710 73 A). The structures were solved with direct
methods (SHELX-97) and refined with full-matrix least-squares
technique. All non-hydrogen atoms were refined anistropically
and hydrogen atoms of organic ligands were geometrically
placed.
1H NMR data were recorded with Switzerland Bruker DR ꢀ 600
NMR spectrometers. FT-IR spectra were measured with a Nicolet
7199B spectrometer in KBr pellets in the range of 4000e400 cmꢁ1
.
UVevis absorption spectra were recorded by Lambda Bio 40,
American PE Co. The electroluminescent (EL) spectra were
measured by PR-655 spectrophotometer. Photoluminescent (PL)
spectra were examined by Cary Eclipse fluorescence spectropho-
tometer in CH2Cl2 solution. Phosphorescent spectra at 77 K in 2-
methyltetrahydrofuran were measured on Hitachi Model F-7000
spectrophotometer upon excitating at the absorption maxima.
Cyclic voltammetry was performed with Autolab/PG STAT302 in a
one-compartment electrolysis cell using two platinum wires as
working electrode and counter, a calomel electrode as reference.
TBAPF6 was used as supporting electrolyte (0.1 M). Cyclic voltam-
metric behaviors were monitored at scan rate 50 mV/s.
To evaluate the electroluminescent properties of these com-
plexes, several electroluminescent devices were fabricated by us-
ing (ppz)2Ir(N^N) complexes as dopant emitters. Organic layers
were fabricated by high-vacuum (3 ꢀ 10ꢁ4 Pa) thermal evaporation
onto a glass substrate precoated with an indium tin oxide (ITO)
layer. The device structure was ITO/NPB (30 nm)/CBP: (ppz)2Ir(N^N)
complexes (6%, 30 nm)/BAlq(10 nm)/Alq3(30 m)/LiF(1 nm)/Al
(100 nm). Materials used for the device were NPB (N, N0-Bis
(naphthalen)-N, N0-bis (phenyl)-benzidine) for hole-transport
layer, (ppz)2Ir(N^N) (6%) in CBP (4,40-N,N0-dicarbazolebiphenyl) for
light-emitting layer, BAlq (aluminum (III) bis (2-methyl-8-
quninolinato)-4-phenylphenolate) for hole-blocking layer, and
Alq3 (tris(8-hydroxyquinoline) aluminum) for electron-transport
layer. The voltageecurrent density (V-J) characteristics of PhO-
LEDs were recorded on Keithley 2400 Source Meter and L-2188
Spot Brightness Meter.
2.2. Theoretical methodology
All calculations were performed using Gaussian 03 package by
employing experimental parameters determined by X-ray single-
crystal diffraction as input file. The geometry optimization of
ground state were carried out using B3LYP functional with 6-
31G(d) basis sets [11,12], except for LANL2DZ basis sets for Ir atom.
The theoretical absorption spectra were calculated by time-
dependent density functional theory (TD-DFT) method [13,14]
based on the optimized ground-state structures with solvent ef-
fects take into account by the polarizable continuum model (PCM)
[15]. Dichloromethane (CH2Cl2) was chosen as to be consistent with
experimental conditions.
3. Synthesis of ligand
Procedure for the synthesis of 2-(5-(4-(trifluoromethyl)phenyl)-
2H-1,2,4-triazol-3-yl)pyridine (Htfmpptz): A mixture of picolino-
nitrile (50 mmol), hydrazine hydrate (50 mmol) and ethanol
(25 ml) was reacted for 8 h under nitrogen air. The redundant
ethanol was removed at room temperature by vacuum. The res-
idue was washed using ether. A white crystal of (2-pyridine)
amidrazone was filtered and dried. Then, (2-pyridine)amidrazone
(30 mmol), Na2CO3 (30 mmol) and 4-(trifluoromethyl)benzoyl
chloride (30 mmol) in THF solution were reacted for 6 h at room
temperature under nitrogen air. The resulting residue was
recrystallized in ethanol. The needle-like crystal of 2-(5-(4-

X. Huixia et al. / Dyes and Pigments 99 (2013) 67e73
69
Fig. 1. ORTEP diagrams of (ppz)2Ir(tfmpptz) (a) (ppz)2Ir(fpptz) (b) and (ppz)2Ir(tfmptz) (c) with thermal ellipsoid show at 30% probability level; (Selected bond distances of
ꢀ
(ppz)2Ir(tfmpptz): Ir(1)-N(1) ¼ 2.176(5), Ir(1)-N(2) ¼ 2.131(5), Ir(1)-N(5) ¼ 2.016(5), Ir(1)-N(7) ¼ 2.042(5), Ir(1)-(C32) ¼ 1.997(6), Ir(1)-(C23) ¼ 2.010(6) A; Selected bond distances
ꢀ
for (ppz)2Ir(fpptz): Ir(1)-N(1) ¼ 2.163(5), Ir(1)-N(2) ¼ 2.125(6), Ir(1)-N(5) ¼ 2.015(5), Ir(1)-N(7) ¼ 2.021(5), Ir(1)-C(18) ¼ 1.964(8), Ir(1)-C(27) ¼ 2.035(6) A; Selected bond distances
ꢀ
for (ppz)2Ir(tfmptz): Ir(1)-N(1) ¼ 2.145(4), Ir(1)-N(2) ¼ 2.108(4), Ir(1)-N(5) ¼ 2.016(4), Ir(1)-N(7) ¼ 2.026(5), Ir(1)-C(18) ¼ 2.013(5), Ir(1)-C(9) ¼ 2.015(5) A.).
(trifluoromethyl)phenyl)-2H-1,2,4-triazol-3-yl)pyridine was ob-
tained. Yield: 80%. 1H NMR (600 MHz, DMSO):
15.08(1H, s),
8.76(1H, d), 8.31(2H, d), 8.20(1H, d), 8.05(1H, t), 7.88(2H, d),
7.58(1H, s); FT-IR (KBr) cmꢁ1: 3083, 1631, 1470, 1329, 1171, 1113,
1063, 1017, 847, 731.
Ethyl trifluoroacetate and hydrazine hydrate (1:1.2) were added in
anhydrous THF (30 ml) and the mixture was stirred at room temper-
ature for 12 h. The equiv of NaOH with 2-iminazole pyridine hydro-
chloride (1:1) were added in above-mentioned solution and heated to
reflux for 12 h. Sodium carbonate was used to adjust pH value. The
solution was extracted into ethyl acetate, which was combined, dried
over MgSO4. The residue was subjected to silica gel column chroma-
tography using a 1:3 mixture of ethyl acetate and hexane as the eluent
and crystallized in ethyl acetate. The white crystals were obtained.
d
The synthesis process of 2-(5-(4-fluorophenyl)-2H-1, 2, 4-
thiazol-3-yl)pyridine (Hfpptz) was similar with that of Htfmpptz.
Yield: 80%. 1H NMR (600 MHz, DMSO):
d 14.87(1H, s), 8.74(1H, d),
8.18(1H, d), 8.14e8.12(2H, m), 8.04(1H, t), 7.57(1H, dt), 7.34(2H,
ddd); FT-IR (KBr) cmꢁ1: 3075, 1606, 1465, 1420, 1212, 1158, 1017,
856, 756, 619.
Yield: 80%. 1H NMR (600 MHz, CDCl3):
d 14.05(1H, dr), 8.84(1H, d),
8.35(1H, d), 7.99(1H, dt), 7.55(1H, ddd); FT-IR (KBr) cmꢁ1: 3137 1606,
1494, 1461, 1428, 1217, 1189, 1009, 765, 714.
Procedure for the synthesis of 2-(5-(trifluoromethyl)-2H-1, 2, 4-
triazol-3-yl)pyridine (Htfmptz): Picolinonitrile and sodium meth-
oxide (10:1) resolved in methanol was stirred under nitrogen at
room temperature for 12 h. Then, NH4Cl was added. After stirred at
room temperature for 16 h, the mixture was heated to reflux for 4 h.
The solvent was removed at room temperature by vacuum. The
product was washed in ether for many times in order to remove
residual NH4Cl. 2-Iminazole pyridine hydrochloride were obtained.
Procedure for the synthesis of 2-(pyridin-2-yl)-1H-benzo[d]imid-
azole (Hpybi): Benzene-1, 2-diamine and picolinic acid were heated
to 220 ꢂC using polyphosphoric acid (PPA) as catalyst, as described
in Ref. [16]. 1H NMR (600 MHz, DMSO):
d 13.09(1H, s), 8.74(1H, d),
8.33(1H, dd), 8.01(1H, dt), 7.71(1H, d), 7.52e7.55 (2H, m), 7.26e
7.20(2H, m); FT-IR (KBr) cmꢁ1: 3322, 3245, 3054, 1593, 1492, 1417,
1319, 1259, 1130, 1037, 840, 798, 734, 522, 468.

70
X. Huixia et al. / Dyes and Pigments 99 (2013) 67e73
Fig. 2. The absorption and PL spectra of all complexes at room temperature in CH2Cl2 (a) and phosphorescence spectra of (ppz)2Ir(fpptz) and (ppz)2Ir(tfmptz) at 77 K in
2-methyltetrahydrofuran (b).
4. Synthesis of complexes
g
¼
108.957(4)ꢂ,
Z
¼
2, rc
¼
1.640 mg/cm3, Crystal
size ¼ 0.29 ꢀ 0.27 ꢀ 0.22 mm3, F(0 0 0) ¼ 668, GOF ¼ 1.031, 8972
independent reflections collected with R(int) ¼ 0.0304, Final R
Iridium trichloride hydrate in 2-ethoxyethanol was treated with
2, 5-dihydro-1-phenyl-1H-pyrazole (Hppz) to give the chloride-
bridged dimer. Further reaction of the dimmer with the N^N li-
gands of Htfmpptz, Hfpptz, Htfmptz and Hpybi afforded complexes
(ppz)2Ir(tfmpptz), (ppz)2Ir(fpptz), (ppz)2Ir(tfmptz) and (ppz)2Ir(-
pybi), respectively [17].
[I > 2
s
(I)] ¼ 0.0708, wR2 (all data) ¼ 0.0728.
Spectra data for (ppz)2Ir(pybi): 1H NMR (600 MHz, DMSO):
8.80(1H, s), 8.76(1H, s), 8.45(1H, d), 8.14(1H, t), 7.92 (1H, d),
d
7.64(2H t), 7.60(1H, d), 7.48(1H, s), 7.09-6.99 (4H, m), 6.83e6.80
(4H, m), 6.59(2H, t), 6.32(1H, d), 6.28(1H, d), 6.19(1H, d); FT-IR (KBr)
cmꢁ1:2054, 1640, 1482, 1125, 748, 578.
Spectra data for (ppz)2Ir(tfmpptz): 1H NMR (600 MHz, DMSO):
d
8.81(1H, d), 8.77(1H, d), 8.17(1H, d), 8.15(1H, s), 8.13(1H, s),
8.05(1H, dt), 7.83(1H, d), 7.570(1H, s), 7.69(1H, s), 7.61(1H, d),
7.58(1H, d),7.38(1H, ddd), 7.05(1H, d), 6.97(1H, dt), 6.91 (1H, dt),
6.88(1H, d), 6.82(1H, dt), 6.70(1H, dt), 6.64(1H, t), 6.60(1H, t),
6.23(1H, dd), 6.17(1H, dd); FT-IR (KBr) cmꢁ1: 2029, 1615,1481, 1416,
1333, 1171, 1121, 1067, 860, 756.
5. Results and discussion
5.1. Electronegativity of ligands
The electron withdrawing abilities of the ligands were
Selected single crystal data of (ppz)2Ir(tfmpptz): C32H22F3IrN8,
measured by the electronegativity (c), which was estimated from
M ¼ 767.78, Monoclinic, space group P2(1)/n, T ¼ 296(2) K,
Table 1
¼ 90ꢂ,
ꢀ
a ¼ 12.062(7),ꢂ b ¼ 16.755(10), c ¼ 17.647(11) A,
a
¼
g
The calculated excited energies, dominant orbital excitations and oscillator strength
from time-dependent DFT of all complexes.
3
b
¼ 102.682(6) , V ¼ 3479(4) A , Z ¼ 4, rc ¼ 1.466 mg/cm3, Crystal
ꢀ
size ¼ 0.26 ꢀ 0.23 ꢀ 0.21 mm3, F(0 0 0) ¼ 1496, GOF ¼ 0.984, 19579
Molecular
States
S
lcal/nm
f
Assignments
independent reflections collected with R(int) ¼ 0.0675, Final R
(ppz)2Ir(tfmpptz)
336
0.254
HOMO-2 / LUMO (90%)
HOMO-3 / LUMO (3%)
HOMO-2 / LUMOþ1(84%)
HOMO-3 / LUMO (2%)
HOMO-2 / LUMOþ1(23%)
HOMO / LUMOþ1(26%)
HOMO-2 / LUMOþ3(10%)
HOMO-2 / LUMO (92%)
HOMO-2 / LUMOþ3(26%)
HOMO / LUMOþ4(56%)
HOMO-2 / LUMOþ2(7%)
HOMO-2 / LUMO(35%)
HOMO / LUMO (13%)
HOMO-3 / LUMO (11%)
HOMO / LUMOþ1(15%)
HOMO / LUMOþ2(77%)
HOMO-2 / LUMOþ3(75%)
HOMO-4 / LUMOþ2(4%)
HOMO / LUMOþ4(5%)
HOMO / LUMO (78%)
HOMO-1 / LUMO (13%)
HOMO-1 / LUMO (10%)
HOMO / LUMO (81%)
HOMO-5 / LUMO (67%)
HOMO-6 / LUMO (18%)
HOMO-3 / LUMOþ1(6%)
HOMO-3 / LUMO (41%)
HOMO-2 / LUMO (19%)
HOMO-1 / LUMO (39%)
[I > 2
s
(I)] ¼ 0.0732, wR2 (all data) ¼ 0.0788.
Spectra data for (ppz)2Ir(fpptz): 1H NMR (600 MHz, DMSO):
289
398
0.1968
0
d
8.82(1H, d), 8.78(1H, d), 8.15(1H, d), 8.05(1H, dt), 7.98e7.96
(2H, m), 7.83(1H, d), 7.62(1H, d), 7.59(1H, d), 7.37 (1H, ddd), 7.17
(2H, t), 7.05(1H, d), 6.99(1H, dt), 6.92(1H, dt), 6.88(1H, d),
6.81(1H, dt), 6.71(1H, dt), 6.65(1H, t), 6.61(1H, t), 6.24(1H, dd),
6.18(1H, dd); FT-IR (KBr) cmꢁ1: 2029, 1615, 1511, 1482, 1407, 1055,
760, 632.
T
S
(ppz)2Ir(fpptz)
(ppz)2Ir(tfmptz)
(ppz)2Ir(pybi)
342
281
0.156
0.2274
Selected single crystal data of (ppz)2Ir(fpptz): C31H22FIrN8,
T
S
387
0
M ¼ 717.77, Monoclinic, space group P2(1)/n, T ¼ 296(2) K,
ꢀ
a ¼ 12.079(5)ꢂ, b ¼ 16.556(7), c ¼ 16.459(7) A,
a
¼
g
¼ 90,
3
ꢀ
b
¼ 102.163(4) , V ¼ 3218(2) A , Z ¼ 4, rc ¼ 1.482 mg/cm3, Crystal
332
266
0.0498
0.1265
size ¼ 0.32 ꢀ 0.30 ꢀ 0.21 mm3, F(0 0 0) ¼ 1400, GOF ¼ 1.096, 19014
independent reflections collected with R(int) ¼ 0.0602, Final R
[I > 2
s(I)] ¼ 0.0728, wR2 (all data) ¼ 0.0772.
Spectra data for (ppz)2Ir(tfmptz): 1H NMR (600 MHz, DMSO):
8.81(1H, dd), 8.79(1H, dd), 8.19(1H, d), 8.09(1H, dt), 7.85 (1H, d),
T
S
396
394
294
0
d
7.62(1H, dd), 7.58(1H, dd), 7.48(1H, ddd), 7.08 (1H, d), 6.99 (1H, dt),
6.91(2H, dt), 6.84(1H, d), 6.81(1H, dt), 6.69(1H, dt), 6.65 (1H, t),
6.63(1H, t), 6.23(1H, dd), 6.14(1H, dd); FT-IR (KBr) cmꢁ1: 3129, 1617,
1478, 1420, 1283, 1200, 1121, 1034, 756.
0.0387
0.0377
Selected single crystal data of (ppz)2Ir(tfmptz): C26H18F3IrN8,
T
451
0
M ¼ 691.68, Triclinic, space group P-1, T ¼ 293(2) K, a ¼ 9.259(5),
ꢀ
b ¼ 11.956(6), c ¼ 13.730(7) A,
a
¼ 95.760(4),
b
¼ 99.131(4),

X. Huixia et al. / Dyes and Pigments 99 (2013) 67e73
71
Fig. 3. The molecular orbitals characterization of complexes (ppz)2Ir(N^N) from DFT.
ꢀ
ꢀ
the average value of the HOMO (highest occupied molecular
orbital) and LUMO (lowest unoccupied molecular orbital) energies:
2.003 A) <(ppz)2Ir(tfmptz) (av. 2.014 A), indicating that the stron-
ger electron-withdrawing abilities of ancillary ligand resulted in a
stronger coordinate field. The bond lengths of the Ir-N (N^N) in three
complexes were longer than those of the Ir(ppz)3. The interaction
between Ir3þ and N of ppz became stronger relative to Ir(ppz)3 [8].
Additionally, the bond energy of Ir-N became weaker by increasing
the conjugation of the ancillary ligand.
c
¼ ꢁ(EHOMO þ ELUMO)/2 [18,19]. By optimizing the ligands, the
gradually increasing calculated values were Hpybi < Hfpptz
c
< Htfmpptz < Htfmptz, indicating that the electron-withdrawing
abilities of the ligands increased along opposite order.
5.2. Molecular structures
5.3. Photophysical properties
Single crystals of (ppz)2Ir(tfmpptz), (ppz)2Ir(fpptz) and
(ppz)2Ir(tfmptz) were grown from methanol or dichloromethane
solution and characterized using X-ray crystallography. Structures
are shown in Fig. 1 and selected crystallographic data are provided
in experimental section. Three complexes had deformed octahedral
coordination geometry around the iridium center with cis- C, C and
trans- N, N chelate configuration. The CeC and CeN intraligand
bond lengths and angles were within normal ranges. The electron-
withdrawing trifluoromethyl group of (ppz)2Ir(tfmpptz) existed as
disorder structure and each fluoro occupied 50%, as shown in
Fig. 1a. The average bond lengths of Ir-C were increasing in the
The absorption and emission spectra of these complexes were
recorded in CH2Cl2 solution at room temperature and were dis-
played in Fig. 2a. The intense absorption bands at 250e280 nm,
were attributed to the ligand-centered (
pep
*)1 transitions of N^N
ligands. The (pep
*)1 bands were accompanied by weaker, lower
energy features extending into the visible region from 290 to
350 nm, which was likely associated with single metal-to-ligand
charge-transfer (1MLCT) transitions [20]. The absorption bands at
about 400 nm were assigned to the triplet MLCT transitions
involving the ppz and N^N ligands and mainly corresponding to the
transition from HOMO to LUMO on the basis of time-dependent
DFT calculations. The calculated excited energies, dominant
orbital excitations and oscillator strength from time-dependent
DFT were presented in Table 1.
ꢀ
order of (ppz)2Ir(fpptz) (av. 2.000 A) < (ppz)2Ir(tfmpptz)(av.
A close comparison of the emission wavelengths with the
structures of these iridium complexes revealed the key feature of
these complexes. As shown in Fig. 2a, the emission maximum of
(ppz)2Ir(tfmpptz), (ppz)2Ir(fpptz), (ppz)2Ir(tfmptz) and (ppz)2Ir(-
pybi) at room temperature appeared at 486, 497, 473 and 530 nm,
respectively. The emission spectra of complexes (ppz)2Ir(tfmpptz)
and (ppz)2Ir(fpptz) with blue-green light emission had red-shifts of
13 and 24 nm relative to (ppz)2Ir(tfmptz). While, bright-green light
was observed for complex (ppz)2Ir(pybi). The emission colors of
these complexes with the same cyclometalating ligands and
different N^N ligands changed from blue to green, and the
Table 2
The performances of devices AeC.
Device Von(V) Lmax(cd/m2)
hc (cd/A) hp(lm/W) lmax (nm) CIE
A
B
C
6.4
5.9
7
26 487 (11 V) 6.25
22 278 (13 V) 5.7
12 142 (13 V) 7.3
1.63
1.53
2.1
480
453 483
453 505
(0.23, 0.31)
(0.22, 0.29)
(0.25, 0.30)
Fig. 4. The EL spectra of devices AeC by using (ppz)2Ir(N^N) as dopants (at voltage of
8 V).

72
X. Huixia et al. / Dyes and Pigments 99 (2013) 67e73
(ppz)2Ir(tfmpptz) and (ppz)2Ir(fpptz). So the emission of
(ppz)2Ir(tfmptz) exhibited better blue phosphorescent. In the case
of complex (ppz)2Ir(pybi), the HOMO and LUMO all appeared on
the N^N of pybi ligand, suggesting that only N^N e ligand-based
charge transitions had contribution to the excited state (in Fig. 3).
This also provides rationale for explaining the observed large red
shift of the emission spectra of complex (ppz)2Ir(pybi).
5.4. Electrochemical behaviors
The electrochemical behaviors of these iridium complexes were
investigated by cyclic voltammetry. The measured onset oxidation
(E0ox) and reduction potentials (E0red) of each complex were used
to calculate the HOMO and LUMO levels. The HOMO/LUMO levels
of complexes (ppz)2Ir(tfmpptz), (ppz)2Ir(fpptz), (ppz)2Ir(tfmptz)
and (ppz)2Ir(pybi) are ꢁ6.01/ꢁ3.17, ꢁ6.05/ꢁ3.26, ꢁ6.59/ꢁ3.26
and ꢁ6.00/ꢁ3.34 eV, respectively.
The stronger-electronegativity ancillary ligand Htfmptz resulted
in a slightly stabilized HOMO level for complex (ppz)2Ir(tfmptz).
However, one of the key factors affecting the LUMO level in these
systems was the degree of conjugation. More effective conjugation
of Hpybi stabilized the LUMO for complex (ppz)2Ir(pybi). The sta-
bilized LUMO level facilitated the electron injection. The energy
gaps of complexes (ppz)2Ir(tfmpptz), (ppz)2Ir(fpptz), (ppz)2Ir
(tfmptz) and (ppz)2Ir(pybi) were 2.84, 2.79, 3.33 and 2.66 eV,
respectively.
Fig. 5. LeVeJ curves for devices A, B and C.
wavelengths varied by 57 nm. This result provides that the N^N li-
gands play a key factor in the emission color of these iridium
complexes.
Phosphorescent spectra of (ppz)2Ir(fpptz) and (ppz)2Ir(tfmptz)
at 77 K were displayed in Fig. 2b. The complex (ppz)2Ir(tfmptz)
show phosphorescent peaks at 426, 452 nm and a shoulder peak at
479 nm. Compared with PL spectra, phosphorescent spectra have
obvious blue shift and without fine vibronic progressions. There-
fore, the lowest excited triplet states of these complexes are likely
to be dominantly the 3MLCT excited state [8]. The triplet levels (ET)
of (ppz)2Ir(fpptz) and (ppz)2Ir(tfmptz), which can be inferred from
phosphorescent spectra, are 2.76 and 2.91 eV.
To gain insight into the photophysical behavior of all complexes,
DFT was applied to molecular orbital studies. Calculations show
that the LUMOs of these complexes were all located on the ancillary
N^N ligands and Iridium atoms, as displayed in Fig. 3. Thus, it would
get easy to tune emission color and obtain pure-blue phosphores-
cent by modification N^N ligand. On the other hand, HOMOs had the
different distributions. The HOMOs of complexes (ppz)2Ir(tfmpptz)
and (ppz)2Ir(fpptz) had similar plots, which populated on the N^N
and ppz ligands, especially on the N^N ligand. However, the HOMO
and LUMO of (ppz)2Ir(tfmptz) were located on N^N and ppz ligands,
respectively. The charge transitions became more difficult than
5.5. Electroluminescent properties
To character the electroluminescent properties of these com-
plexes, the devices AeC were fabricated by using complex-
es(ppz)2Ir(tfmpptz), (ppz)2Ir(fpptz) and (ppz)2Ir(tfmptz) as the
dopant emitter, respectively. The device A-C structures were ITO/
NPB (30 nm)/CBP: (ppz)2Ir(N^N) (ppz)2Ir(N^N) (6%, 30 nm)/
BAlq(10 nm)/Alq3(30 nm)/LiF(1 nm)/Al (100 nm). Therein, the
(ppz)2Ir(N^N) is (ppz)2Ir(tfmpptz) (for device A), (ppz)2Ir(fpptz) (for
device B) and (ppz)2Ir(tfmptz) (for device C), respectively.
The EL spectra of device A was originated from (ppz)2Ir(tfmpptz)
with maximum peak at 480 nm, as can be seen in Fig. 4, indicative
of efficient energy transfer from host of CBP to (ppz)2Ir(tfmpptz).
The EL emissions of NPB and CBP are at the range of 400e430 nm
[21]. The maximum emission peaks of device B located at 453 and
483 nm and at 505 nm for device C. Therefore, EL spectra of device B
Fig. 6. The J-hc(a) and J- hp(b) curves for devices A, B and C.

X. Huixia et al. / Dyes and Pigments 99 (2013) 67e73
73
and C indicate efficient energy transfer from host to corresponding
Acknowledgments
iridium complexes.
The turn-on voltages (the voltage at a brightness of 1 cd/m2)
were 6.4 V for device A, 5.9 V for device B and 7 V for device C. The
higher turn-on voltages may be resulted from the high energy
barrier between NPB and CBP. Device A had maximum luminance
(Lmax) of 26 487 cd/m2, maximum current efficiency (hc) of 5.5 cd/A,
maximum power efficiency (hp) of 1.6 lm/W with CIE coordinates of
(0.23, 0.31), as listed in Table 2 and displayed in Fig. 5 and Fig. 6.
Both devices A and B showed lower maximumhcthan device C. But
the efficiencies of device C were dramatically decreased with
increasing current density. A measure of the roll-off can be defined
as the percentage drop in hEL at a given field (F) or current density
This work was financially supported by National Natural Sci-
entific Foundation of China (21071108, 60976018, 21101111,
61205179); Natural Science Foundation of Shanxi Province
(2008011008, 2010021023-2); Program for Changjiang Scholar and
Innovation Research Team in University (IRT0972).
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6. Conclusion
In summary, four pyrazole complexes that gave blue, green-blue
and green phosphorescence were reported by the choice of different
ancillary ligand. The stronger electronegativity of ancillary ligand
led to a lager energy gap, consequently, blue-shift emission spectra.
The PhOLEDs doped with complex (ppz)2Ir(fpptz) had a better per-
formance with maximum luminance of 26487 cd/m2 and a rather
lower efficiency roll-off value. Our investigations show that the
subtle change in ligand overcame the roll-off of efficiency. The per-
formances were achieved without optimization for OLED energy
level structure. The efficiency of PhOLED doped these Ir(III) com-
plexes could be further improved by optimizing device structure.
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