Syntheses, crystal structure and photophysical property of iridium complexes with 1,3,4-oxadiazole and 1,3,4-thiadiazole derivatives as ancillary ligands

Article abstract of DOI:10.1016/j.jorganchem.2015.02.025

Six iridium complexes with 2-(2-trifluoromethyl)pyrimidine-pyridine as the main ligand and 2-(5-phenyl-1,3,4-oxadiazol-2-yl)-phenol or 2-(5-phenyl-1,3,4-thiadiazol-2-yl)-phenol derivatives as ancillary ligands were synthesized. The crystal structures of the complexes adopted pseudo-octahedral coordination geometry with the conventional trans-N, cis-C arrangement of main ligand and the ancillary ligand was connected to iridium center by an N atom from 1,3,4-oxadiazole or 1,3,4-thiadiazole group and an O atom from phenol moiety. Electrochemical study confirmed the ancillary ligand variations have effects on the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels. The density functional theory (DFT) calculations suggested that the frontier orbitals and the electronic properties of the complexes can be manipulated by introducing different ancillary ligands. The compositions of LUMO on the 1,3,4-thiadiazole ancillary ligands are higher than that of 1,3,4-oxadiazole derivatives, and the HOMO - LUMO gaps are also decreased. Therefore, the emissions of the complexes with 1,3,4-thiadiazole ancillary ligands are shift from green to red. The organic light-emitting diodes with Ir3 and Ir6 as emitters show maximum current efficiencies of 41.08 and 50.92 cd/A, respectively, with mild efficiency roll-off. This work provides a way to tune the emission and device efficiency of Ir^III complexes by introducing of 1,3,4-thiadiazole group in ancillary ligand.

Full text of DOI:10.1016/j.jorganchem.2015.02.025

Journal of Organometallic Chemistry 785 (2015) 11e18
Contents lists available at ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Syntheses, crystal structure and photophysical property of iridium
complexes with 1,3,4-oxadiazole and 1,3,4-thiadiazole derivatives as
ancillary ligands
Xuan Liu, Song Zhang, Yi-Ming Jin, Guang-Zhao Lu, Liang Jiang, Xiao Liang, Qiu-Lei Xu*,
You-Xuan Zheng*
State Key Laboratory of Coordination Chemistry, Collaborative Innovation Center of Advanced Microstructures, School of Chemistry and Chemical
Engineering, Nanjing University, Nanjing 210093, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Six iridium complexes with 2-(2-trifluoromethyl)pyrimidine-pyridine as the main ligand and 2-(5-
phenyl-1,3,4-oxadiazol-2-yl)-phenol or 2-(5-phenyl-1,3,4-thiadiazol-2-yl)-phenol derivatives as ancil-
lary ligands were synthesized. The crystal structures of the complexes adopted pseudo-octahedral co-
ordination geometry with the conventional trans-N, cis-C arrangement of main ligand and the ancillary
ligand was connected to iridium center by an N atom from 1,3,4-oxadiazole or 1,3,4-thiadiazole group
and an O atom from phenol moiety. Electrochemical study confirmed the ancillary ligand variations have
effects on the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital
(LUMO) energy levels. The density functional theory (DFT) calculations suggested that the frontier or-
bitals and the electronic properties of the complexes can be manipulated by introducing different
ancillary ligands. The compositions of LUMO on the 1,3,4-thiadiazole ancillary ligands are higher than
that of 1,3,4-oxadiazole derivatives, and the HOMO - LUMO gaps are also decreased. Therefore, the
emissions of the complexes with 1,3,4-thiadiazole ancillary ligands are shift from green to red. The
organic light-emitting diodes with Ir3 and Ir6 as emitters show maximum current efficiencies of 41.08
and 50.92 cd/A, respectively, with mild efficiency roll-off. This work provides a way to tune the emission
and device efficiency of IrIII complexes by introducing of 1,3,4-thiadiazole group in ancillary ligand.
© 2015 Elsevier B.V. All rights reserved.
Received 10 November 2014
Received in revised form
8 February 2015
Accepted 15 February 2015
Available online 27 February 2015
Keywords:
Iridium complex
2-(2-Trifluoromethyl)pyrimidine-pyridine
1,3,4-Oxadiazole
1,3,4-Thiadiazole
Organic light-emitting diodes
Introduction
metal while the LUMO is primarily localized on the heterocyclic
rings of the cyclometalated ligands. Therefor some popular strate-
Phosphorescent iridium complexes are the favorable choices for
organic light-emitting diodes (OLEDs) due to the high efficiency
and stability [1e4]. With active design of ligands, phosphorescent
IrIII complexes could exhibit a wide emission range from near ul-
traviolet to red [5e7]. Recently, many efficient single-color and
white OLEDs employing IrIII phosphors as emissive dopants have
been reported [5e10]. The frontier molecular orbitals (FMOs),
especially the highest occupied molecular orbital (HOMO) and
lowest unoccupied molecular orbital (LUMO), are crucial for the
photophysical properties [11]. According to the density functional
theory calculations, the HOMO is basically centered on the IrIII
gies aiming to change the HOMO - LUMO gap are adopted to tune
the emission color by changing the substitutions of main ligands
[12e17].
Although most ancillary ligands do not make contribution to the
lowest excited state directly, but some ancillary ligands with strong
fields can lower the HOMO energies of cyclometalated IrIII com-
plexes and tune the emissions. For example, the highly efficient
phosphorescent
blue
emitters
of
iridium(III)bis(4,6-
difluorophenylpyridinato)picolate (FIrpic) [18] and iridium(III)
bis(4,6-difluorophenylpyridinato)tetrakis(1-pyrazolyl)-borate
(Fir6) [19] used picolate and tetrakis(1-pyrazolyl)-borate as ancil-
lary ligands, respectively. In addition, some ancillary ligands also
can alter the energy of the excited state by modifying the electron
density at the metal center and affect the carrier mobility of the
complexes. Thus, the photophysical, electrochemical, and
* Corresponding authors. Tel.: þ86 25 83596775; fax: þ86 25 83314502.
E-mail addresses: qiuleixu@163.com (Q.-L. Xu), yxzheng@nju.edu.cn
(Y.-X. Zheng).
http://dx.doi.org/10.1016/j.jorganchem.2015.02.025
0022-328X/© 2015 Elsevier B.V. All rights reserved.

12
X. Liu et al. / Journal of Organometallic Chemistry 785 (2015) 11e18
electroluminescent properties of IrIII complexes can be tuned
through controlling both the cyclometalated and ancillary ligands.
In our group, we have fabricated some efficient devices using
tetraphenylimidodiphosphinate and 1,3,4-oxadiazole derivatives as
ancillary ligands, which may improve the electron mobility of the
IrIII complexes and benefit their OLEDs performances [20,21]. But
because these ancillary ligands have no effect on the HOMO/LUMO
states of the IrIII complexes, the emission colors were decided by
the cyclometalated ligands. In this study, using the novel 2-(2-
trifluoromethyl)pyrimidine-pyridine as the main cyclometalated
ligand and 1,3,4-oxadiazole derivatives as ancillary ligands we
prepared three green IrIII complexes. When the 1,3,4-oxadiazole
derivatives were replaced by the 1,3,4-thiadiazole derivatives, the
emission colors were tuned from green to red. The results of the
density functional theory (DFT) calculation suggested that the
frontier orbitals and electronic properties of the complexes can be
manipulated by introducing 1,3,4-thiadiazole derivatives.
the highly redundant data sets. Data were collected using a narrow-
frame method with scan width of 0.30ꢂ in
and an exposure time
u
of 5 s/frameꢁ1. Absorption corrections were applied using SADABS
[25] supplied by Bruker. The structures were solved by Patterson
methods and refined by full-matrix least-squares on F2 using the
program SHELXS-97 [26]. The positions of metal atoms and their
first coordination spheres were located from direct-methods E-
maps; other non-hydrogen atoms were found in alternating
difference.
OLED fabrication and measurement
All OLEDs with the emission area of 0.1 cm2 were fabricated on
the pre-patterned ITO-coated glass substrate with a sheet resis-
tance of 15
U/sq. All chemicals used for devices were sublimed in
vacuum (2.2 10ꢁ4 Pa) prior to use. The deposition rate for organic
compounds is 1e2 Å/s. The phosphor and host were co-evaporated
from two separate sources. The cathode consisting of LiF/Al was
deposited by evaporation of LiF with a deposition rate of 0.1 Å/s and
then by evaporation of Al metal with a rate of 3 Å/s. The effective
area of the emitting diode is 0.1 cm2. The characteristics of the
devices were measured with a computer controlled KEITHLEY 2400
source meter with a calibrated silicon diode in air without device
encapsulation. On the basis of the uncorrected PL and EL spectra,
the CIE coordinates were calculated using a test program of the
Spectra scan PR650 spectrophotometer.
Experimental section
General information
All chemicals were commercial purchased without further pu-
rification. All the reactions were carried out under nitrogen atmo-
sphere. 1H NMR spectra were measured on a Bruker AM 500
spectrometer. Electrospray ionization mass spectra (ESI-MS) were
obtained with ESI-MS (LCQ Fleet, Thermo Fisher Scientific). The
high resolution mass spectra (HR EI-MS) were recorded on an
Agilent 6540 UHD Accurate-Mass Q-TOF LC/MS. Cyclic voltammetry
(CV) curves were measured at room temperature in deaerated
mixed solution (CH2Cl2: CH3CN ¼ 1: 1), using a three-electrode cell
equipped with a polished Pt plate as working electrode, a Pt wire as
counter electrode, and a AgNO3 (0.1 mol/L)/Ag in CH3CN as refer-
ence electrode on a CHI600E electrochemical workstation using
tetra-n-butylammomiumperchlorate (0.1 M) as the supporting
electrolyte with a potential scan rate of 0.05 V/s. UVevis absorption
and photoluminescence (PL) spectra were measured on a Shimadzu
UV-3100 and a Hitachi F-4600 spectrophotometer at room tem-
perature, respectively.
Syntheses
The syntheses routes of the cyclometalated and ancillary li-
gands, the corresponding IrIII complexes are shown in Scheme 1.
The 2-(5-phenyl-1,3,4-oxadiazol-2-yl)-phenol derivatives (L1-L3)
were reported in our former publication [27].
Syntheses of 2-(2-trifluoromethyl)pyrimidine-pyridine (TPP) ligand
[28]
4.4 mmol (1.00 g) 5-Bromo-2-(trifluoromethyl)pyrimidine and
0.22 mmol (0.25 g) tetrakis(triphenylphosphine)palladium(0) were
dissolved in 50 mL anhydrous methylbenzene and 4.2 mmol
(1.38 g) 2-(tributylstannyl)pyridine was added dropwise, and the
solution was refluxed for 24 h in 120 ꢂC. Silica column chroma-
tography purification (petroleum ether: EtOAc ¼ 5:1 as eluent) gave
white powder (0.67 g, 2.98 mmol) in 71% yield.
The luminescence quantum efficiencies were calculated by a
comparison of the emission intensities (integrated areas) of a
standard sample fac-Ir(ppy)3 and the unknown samples in dea-
erated CH2Cl2 solutions of 5 ꢀ 10ꢁ5 mol/L according to the equation
[22,23].
Syntheses of [(TPP)2Ir(m-Cl)2]2
ꢀ
ꢁꢀ
ꢁꢀ
ꢁ
IrCl3$3H2O (0.49 g,1.44 mmol) and TPP (0.67 g, 2.98 mmol) were
dissolved in 20 mL 2-ethoxyethanol at 60 ꢂC, and then 7 mL H2O
was added. The solution was heated at 100 ꢂC for 14 h. After cooling
to room temperature, 20 mL H2O was added to give yellow pre-
cipitate, which was filtered and washed with petroleum ether in
73% yield (0.71 g, 0.55 mmol).
2
Iunk
Istd
Astd
Aunk
hunk
hstd
Funk ¼ Fstd
where Funk and Fstd are the luminescence quantum yields of the
unknown samples and fac-Ir(ppy)3 solutions (Fstd ¼ 40% [22]),
respectively. Iunk and Istd are the integrated emission intensities of
the unknown samples and fac-Ir(ppy)3 solutions, respectively. Aunk
and Astd are the absorbance values of the unknown samples and
fac-Ir(ppy)3 solutions at their excitation wavelengths, respectively.
hunk and hstd terms represent the refractive indices of the corre-
sponding acetonitrile (1.345, pure solvent was assumed).
General syntheses of 2-(5-phenyl-1,3,4-thiadiazol-2-yl)-phenol
derivatives (L4 - L6)
2-Methoxybenzohydrazide (10 mmol) was dissolved in 50 mL
CHCl3, and benzoyl chloride (10 mmol) was added dropwise for 1 h
at room temperature. Then, the solvent was removed under vac-
uum gave benzoyl-2-methoxybenzohydrazine as pale yellow solid
with high yield. Benzoyl-2-methoxybenzohydrazine (3.7 mmol)
and phosphorus pentasulfide (4.4 mmol) were stirred in 40 mL
dimethylbenzene at 140 ꢂC for 3 h. Water (about 20 mL) was added
till no more gas generated. The mixture was washed with water
(40 mL ꢀ 3 times) and then dried over anhydrous sodium sulfate.
Silica column chromatography purification (petroleum ether:
EtOAc ¼ 4: 1 as eluent) gave white powder. The powder was
X-ray crystallography
Crystallographic measurements of the single crystals were car-
ried out on a Bruker SMART CCD diffractometer (Bruker Daltonic
Inc.) using monochromated Mo K
room temperature. Cell parameters were retrieved using SMART
software and refined using SAINT [24] program in order to reduce
a
radiation (
l
¼ 0.71073 Å) at

X. Liu et al. / Journal of Organometallic Chemistry 785 (2015) 11e18
13
Scheme 1. Synthetic routes of the ligands and complexes.
dissolved in CH2Cl2 followed by adding BBr3 carefully at ꢁ78 ꢂC and
stirred at room temperature for 20 h. Colorless crystal was
recrystallized from ethanol. In order to be easier for further re-
actions, the ligand was reacted with equivalent KOH to give KLX (LX
represents the ancillary ligand).
J ¼ 5.1 Hz, 1H), 7.91 (dddd, J ¼ 9.7, 9.3, 8.4, 4.6 Hz, 6H), 7.76 (dd,
J ¼ 8.1, 1.8 Hz, 1H), 7.61e7.42 (m, 3H), 7.38e7.30 (m, 1H),
7.30e7.28 (m, 1H), 7.27e7.21 (m, 1H), 6.90e6.82 (m, 1H), 6.58
(ddd, J ¼ 8.0, 6.9,1.0 Hz,1H). 13C NMR (300 MHz, DCCl3)
d 186.40,
182.78, 168.08, 150.28, 149.93, 146.63, 146.10, 138.21, 138.14,
134.37, 129.22, 129.15, 127.39, 125.43, 124.33, 124.14, 119.94,
119.43, 116.66, 116.48, 114.47, 106.70. HR EI-MS Calcd for [Mþ
H]þ: m/z 879.1243 found: m/z 879.1239.
L4: (yield: 81%) 1H NMR (300 MHz, DMSO-d6, ppm)
d 11.32 (s,
1H), 8.28 (dd, J ¼ 7.9, 1.5 Hz, 1H), 8.15e7.96 (m, 2H), 7.58 (dd,
J ¼ 8.9, 5.4 Hz, 3H), 7.50e7.33 (m, 1H), 7.12 (d, J ¼ 8.2 Hz, 1H),
7.07e6.90 (m,1H).MS(ESI) Calcd for [MꢁH]ꢁ: m/z 254.04, found:
m/z 254.08.
Ir2 (yield: 51%): 1H NMR (300 MHz, CDCl3, ppm)
d 8.88 (d,
J ¼ 5.5 Hz, 1H), 8.63 (s, 1H), 8.56 (d, J ¼ 5.5 Hz, 1H), 8.32 (d,
J ¼ 5.3 Hz, 1H), 8.08e7.78 (m, 6H), 7.73 (dd, J ¼ 8.1, 1.9 Hz, 1H),
7.39e7.31 (m, 1H), 7.30e7.28 (m, 1H), 7.27e7.23 (m, 1H), 7.18 (t,
L5: (yield: 84%) 1H NMR (300 MHz, DMSO-d6, ppm)
d 11.31 (s,
J ¼ 8.6 Hz, 2H), 6.85 (d, J ¼ 8.0 Hz, 1H), 6.57 (t, J ¼ 7.5 Hz, 1H). 13
C
1H), 8.27 (d, J ¼ 7.5 Hz, 1H), 8.11 (dd, J ¼ 8.1, 5.6 Hz, 2H), 7.41 (t,
J ¼ 8.7 Hz, 3H), 7.10 (d, J ¼ 8.2 Hz, 1H), 7.02 (t, J ¼ 7.5 Hz, 1H).
MS(ESI) Calcd for [MꢁH]ꢁ: m/z 271.04, found: m/z 271.17.
NMR (300 MHz, DCCl3)
d 186.40, 182.78, 168.08, 162.48, 162.04,
160.18, 150.28, 149.93, 146.63, 146.10, 138.21, 134.37, 129.22,
127.39, 125.43, 124.33, 124.14, 119.43, 116.66, 116.48, 114.47,
106.70. HR EI-MS Calcd for [Mþ H]þ: m/z 897.1148 found: m/z
897.1146.
L6: (yield: 84%) 1H NMR (300 MHz, CDCl3, ppm)
d 11.39 (s, 1H),
8.14 (t, J ¼ 8.4 Hz, 2H), 7.80 (t, J ¼ 8.0 Hz, 2H), 7.58 (dd, J ¼ 7.8,
1.2 Hz, 1H), 7.51e7.37 (m, 1H), 7.17 (d, J ¼ 8.4 Hz, 1H), 6.99 (dd,
J ¼ 20.2, 12.5 Hz, 1H). MS(ESI) Calcd for [MꢁH]ꢁ: m/z 321.03,
found: m/z 321.33.
Ir3 (yield: 48%): 1H NMR (300 MHz, CDCl3, ppm)
d 8.88 (d,
J ¼ 5.7 Hz, 1H), 8.60 (d, J ¼ 14.7 Hz, 2H), 8.31 (d, J ¼ 5.8 Hz, 1H),
8.07e7.82 (m, 6H), 7.75 (d, J ¼ 8.2 Hz, 3H), 7.34 (ddd, J ¼ 7.3, 6.3,
2.5 Hz, 2H), 7.25 (d, J ¼ 6.0 Hz, 1H), 6.86 (d, J ¼ 8.7 Hz, 1H), 6.59
(t, J ¼ 7.4 Hz, 1H). 13C NMR (300 MHz, DCCl3)
d 185.65, 184.50,
General syntheses of complexes (Ir1eIr6)
166.71,165.81, 162.03, 161.94, 150.16,v149.76,v146.48, 145.83,
138.20,138.06 134.25, 131.53,v127.23, 126.19, 125.66, 124.11,
123.95, 119.80, 119.37, 115.41. HR EI-MS Calcd for [Mþ H]þ: m/z
947.1116 found: m/z 947.1116.
KLX (0.5 mmol) and [(TPP)2Ir(m-Cl)2]2 (0.24 mmol) were dis-
solved in 10 mL of 2-ethoxyethanol and heated at 120 ꢂC for 14 h.
The crude compound was purified by silica column chromatog-
raphy (petroleum ether: acetone ¼ 3: 1 as eluent) and got bright
yellow powder.
Ir4 (yield: 58%): 1H NMR (300 MHz, CDCl3, ppm)
d 8.97 (d,
J ¼ 5.1 Hz, 1H), 8.63e8.56 (m, 2H), 8.23 (d, J ¼ 5.2 Hz, 1H),
8.02e7.93 (m, 3H), 7.93e7.81 (m, 3H), 7.45 (d, J ¼ 4.5 Hz, 2H),
7.37 (ddd, J ¼ 5.6, 3.3, 1.6 Hz, 3H), 7.21 (dt, J ¼ 7.1, 3.9 Hz, 2H),
Ir1 (yield: 45%): 1H NMR (300 MHz, CDCl3, ppm)
J ¼ 5.1 Hz, 1H), 8.60 (d, J ¼ 14.5 Hz, 1H), 8.57 (s, 1H), 8.33 (d,
d 8.89 (d,

14
X. Liu et al. / Journal of Organometallic Chemistry 785 (2015) 11e18
Fig. 1. The molecular structures of complexes Ir1, Ir3, Ir4 and Ir6 in the crystals with thermal ellipsoids shown at 50% probability level. The hydrogen atoms and solvent molecules
are omitted for clarity.
6.83 (t, J ¼ 9.8 Hz, 1H), 6.56e6.47 (m, 1H). 13C NMR (300 MHz,
Table 2
DCCl3)
d 186.24, 184.83, 166.07, 165.66, 164.17, 162.19, 162.19,
Selected bond lengths (Å), angles (ꢂ) of complexes Ir1, Ir3, Ir4 and Ir6.
162.07, 150.41, 149.85, 146.49, 145.85, 138.21, 138.05, 134.01,
131.62, 129.30, 127.09, 124.17, 123.69, 119.81,119.41,115.30. HR EI-
MS Calcd for [Mþ H]þ: m/z 895.1014 found: m/z 895.1012.
Bond length (Å)
Ir1
Ir3
Ir4
Ir6
IreN(1)
IreN(2)
IreN(3)
IreO(1)
IreC(1)
IreC(2)
Angle (ꢂ)
O(1)-Ir-N(3)
O(1)eIr-N(1)
N(1)eIr-N(2)
2.058(1)
2.037(1)
2.140(2)
2.151(2)
1.964(2)
1.996(1)
Ir1
2.039(3)
2.053(3)
2.130(2)
2.154(4)
1.961(3)
1.975(3)
Ir3
2.055(9)
2.053(9)
2.160(9)
2.126(7)
1.958(10)
1.960(10)
Ir4
2.038(4)
2.052(4)
2.134(2)
2.120(3)
1.973(5)
1.992(4)
Ir6
Ir5 (yield: 55%): 1H NMR (300 MHz, CDCl3 ppm)
d 8.96 (d,
J ¼ 5.7 Hz, 1H), 8.60 (d, J ¼ 6.2 Hz, 2H), 8.22 (d, J ¼ 5.6 Hz, 1H),
7.98 (dd, J ¼ 7.8, 3.6 Hz, 2H), 7.93e7.81 (m, 2H), 7.52e7.41 (m,
2H), 7.41e7.31 (m, 2H), 7.22 (dt, J ¼ 12.0, 5.6 Hz, 2H), 7.09 (t,
J ¼ 8.5 Hz, 2H), 6.82 (d, J ¼ 7.8 Hz, 1H), 6.52 (t, J ¼ 6.9 Hz, 1H). 13
C
83.267(4)
93.596(4)
174.442(7)
85.166(8)
94.363(9)
171.893(10)
83.519(3)
94.915(3)
173.783(3)
86.658(11)
94.843(13)
174.186(14)
NMR (300 MHz, DCCl3)
d 150.35, 149.86, 146.51, 145.84, 138.24,
138.24, 138.09, 134.09, 131.58, 129.19, 129.12, 125.54, 124.18,
124.01, 119.84, 119.42, 116.67, 116.41, 115.37. HR EI-MS Calcd for
[Mþ H]þ: m/z 913.0920 found: m/z 913.0918.
Ir6 (yield: 61%): 1H NMR (300 MHz, CDCl3, ppm)
d 8.96 (d,
diffusion of MeOH into CH2Cl2 solution at room temperature. The
ORTEP diagrams of these complexes are shown in Fig.1; the cor-
responding crystallographic data are summarized in Table 1.
Important bond lengths and angles are listed in Table 2.
J ¼ 5.1 Hz, 1H), 8.60 (d, J ¼ 7.2 Hz, 2H), 8.21 (d, J ¼ 5.2 Hz, 1H),
7.99 (t, J ¼ 6.9 Hz, 2H), 7.94e7.84 (m, 2H), 7.66 (d, J ¼ 8.4 Hz, 2H),
7.58 (d, J ¼ 8.2 Hz, 2H), 7.42e7.30 (m, 2H), 7.27e7.17 (m, 2H),
6.82 (d, J ¼ 7.8 Hz, 1H), 6.54 (dd, J ¼ 10.8, 4.0 Hz, 1H). 13C NMR
The crystal structure diagrams indicate that the complexes have
distorted octahedral coordination geometry. It is obvious that
iridium center is surrounded with two C^N cyclometalated ligands
and one N^O ancillary ligand. It is clear that the ancillary ligand was
connected to iridium center by an N atom from 1,3,4-oxadiazole (or
1,3,4-thiadiazole) group and an O atom from phenol moiety. The
dihedral angle between planes [Ir1eC1eN1] and [Ir1eN3eO1] is
86.7ꢂ in complex Ir1 and that is 87.9ꢂ between planes [Ir1eC1eN1]
and [Ir1eN3eO1] in Ir3, which suggest that the introduction of
fluorine atoms in ancillary ligand will cause larger molecular
deformation. However, the dihedral angle between planes
[Ir1eC1eN1] and [Ir1eN3eO1] is 85.8ꢂ in complex Ir4, which is
smaller than that in Ir1 and Ir3 revealing that thiadiazole group has
(300 MHz, DCCl3) d 187.52, 183.32, 167.92, 161.01, 159.62, 151.81,
149.19, 147.55, 147.12, 143.78, 143.70, 139.43, 134.37, 129.70,
129.58, 128.43, 125.32, 121.32, 121.09, 117.18, 116.88, 114.42,
107.20. HR EI-MS Calcd for [Mþ H]þ: m/z 963.0888 found: m/z
963.0086.
Result and discussion
X-ray crystallography
Single crystals of Ir1, Ir4 and Ir6 were obtained from vacuum
sublimation and single crystal of Ir3 was grown by using solvent
Table 1
Parameters associated with the single crystal diffraction data collection for Ir1, Ir3, Ir4 and Ir6.
Ir1
Ir3
Ir4
Ir6
Formula
C34H19F6IrN8O2
877.77
Triclinic
C35H18F9Ir1N8O2C1H4O1
977.82
Monoclinic
C2/c
31.0782(10)
14,8485(5)
20.9395(12)
90
131.32
90
7257.6(5)
8
C34H19F6IrN8O1S1
893.83
Triclinic
C35H18F9IrN8O1S1
961.83
Triclinic
Formula weight
Crystal system
Space group
a [Å]
b [Å]
c [Å]
p1
p1
p1
12.7323(9)
14.4154(11)
20.3414(17)
90.874(2)
108.2240(10)
108.2620(10)
3341.0(4)
4
12.5346(12)
14.1831(14)
20.820(2)
91.125(2)
107.080(2)
106.271(2)
3375.3(6)
4
10.5658(7)
11.9292(8)
14.3172(10)
86.1120(10)
70.4240(10)
87.3240(10)
1695.8(2)
2
a
b
g
[ꢂ]
[ꢂ]
[ꢂ]
V/[A3]
Z
F(000)
1704
3808
1736
932
Reflns collected
11317
25692
22711
11616
Unique(Rint
)
8653(0.0208)
0.0505, 0.1589
1.037
8725(0.0491)
0.0218, 0.0537
1.011
8546(0.0118)
0.0573, 0.1538
0.978
7427(0.0203)
0.0297, 0.0730
1.026
R1, wR2 [I > 2s(I)]
GOF on F2
CCDC number
1028448
1028450
1028451
1028452
R1 ¼ SkFoj ꢁ jFck=SFoj: wRb2 ¼ ½SwðFo2 ꢁ Fc2Þ2=SwðFo2Þꢃ1=2
GOF ¼ ½ðSwjFoj ꢁ jFcjÞ2=ðNobs ꢁ NparamÞꢃ1=2
:

X. Liu et al. / Journal of Organometallic Chemistry 785 (2015) 11e18
15
Table 4
Percentage distributions of HOMO and LUMO in the complexes Ir1eIr6.
Complex
Orbital
Energy/eV
Egap/eV
Composition (%)
(calculated)
(calculated)
Ir
TPP
LX
Ir1
Ir2
Ir3
Ir4
Ir5
Ir6
HOMO
LUMO
HOMO
LUMO
HOMO
LUMO
HOMO
LUMO
HOMO
LUMO
HOMO
LUMO
ꢁ5.389
ꢁ2.116
ꢁ5.401
ꢁ2.222
ꢁ5.446
ꢁ2.235
ꢁ5.400
ꢁ2.130
ꢁ5.406
ꢁ2.228
ꢁ5.449
ꢁ2.318
3.273
3.179
3.211
3.270
3.178
3.131
14.62
2.60
14.81
2.67
15.05
2.68
14.22
2.29
14.97
2.88
15.27
1.03
5.55
96.04
5.73
98.18
5.57
96.04
6.60
65.35
6.68
91.55
6.66
79.82
1.35
79.47
1.15
79.19
1.28
79.18
32.36
78.35
5.57
78.07
91.03
7.94
voltammetry in deaerated solution (CH2Cl2: CH3CN ¼ 1: 1) (Fig. 2).
The HOMOs were calculated from the oxidation (Eox) peak poten-
tials and the bandgaps (Eg) were calculated from the UVevis ab-
sorption edges [29], respectively. Then the LUMOs were
determined according to the equation LUMO ¼ HOMO þ Eg. The
electrochemical data are collected in Table 3.
Fig. 2. Cyclic voltammograms of Ir1eIr6 in CH2Cl2: CH3CN ¼ 1:1 solutions (10ꢁ4 M).
Table 3
Electrochemical data of the complexes Ir1eIr6.
Complex
Eox [V]a
Ered[V]
HOMO[eV]b
LUMO[eV]
ECgV[eV]
The cyclic voltammograms of the complexes in the positive
range show strong oxidation peaks, while the reduction peaks are
not obvious, demons trating that the redox process of the com-
plexes is not reversible completely, which is also observed in
related IrIII complexes containing oxadiazole units [30e33]. In the
negative potential scan rang all complexes exhibit quasi-reversible
oxidation and reduction process due to the 1,3,4-oxadiazol/1,3,4-
thiadiazole substituents suggesting the electron-transporting and
electron trapping characteristics. From Fig. 2 and Table 3 it is can be
observed that the vary of the ancillary ligands has effects on the
electrochemical properties, and the 1,3,4-thiadiazole units caused
the lower HOMO/LUMO levels and the LOMO-LUMO gaps. But there
are only small potential changes among Ir1eIr5 which are 0.88,
0.88, 0.87, 0.90 and 0.87 V, respectively. And the differences of the
HOMO/LUMO values (ꢁ5.53 to ꢁ5.57 eV for HOMO, ꢁ2.95
to ꢁ3.08 eV for LUMO) of the complexes Ir1eIr5 are neglectable.
The results suggested that the introduction of the F/CF3 groups has
little effects on the HOMO/LUMO levels. However, the CF3
substituted 1,3,4-thiadiazole makes the oxidation peak of Ir6
moving to the negative range and the HOMO/LUMO levels and
Ir1
Ir2
Ir3
Ir4
Ir5
Ir6
0.88
0.88
0.87
0.90
0.87
0.65
ꢁ1.46
ꢁ1.46
ꢁ1.45
ꢁ1.47
ꢁ1.46
ꢁ1.37
ꢁ5.55
ꢁ5.55
ꢁ5.53
ꢁ5.57
ꢁ5.54
ꢁ5.32
ꢁ2.95
ꢁ2.98
ꢁ2.93
ꢁ3.08
ꢁ3.02
ꢁ2.89
2.60
2.57
2.60
2.49
2.52
2.43
a
Oxidation potential measured in solution at 50 mV/s.
The HOMþO energy levels were calculated from the equation: HOMO
b
(eV) ¼ Eox ꢁ EFc/Fc þ 4.8.
less influence in structure distortion than that of oxadiazole group.
The IreN bond lengths between iridium and ancillary ligand range
from 2.13 to 2.16 Å and the lengths between iridium and C^N
cyclometalated ligands range from 2.04 to 2.06 Å. These differences
might be caused by the heteropolarity of O, S, F and the steric
hindrance effects of ligands.
Electrochemistry
In order to determine the HOMO and LUMO energy levels, the
electrochemical properties of Ir1eIr6 were measured by cyclic
Fig. 3. Contour plots of HOMOs (bottom) and LUMOs (top) of Ir1eIr6 with theoretical (black) and experimental (red, determined by cyclic voltammetry) energy levels. (For
interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

16
X. Liu et al. / Journal of Organometallic Chemistry 785 (2015) 11e18
Fig. 4. Normalized absorption (left) and emission (right) spectra of Ir1eIr6 in aerated/deaerated CH2Cl2 solutions (5.0 ꢀ 10ꢁ5 mol/L).
LOMO-LUMO gap were decreased to ꢁ5.32/ꢁ2.89 eV and 2.43 eV,
respectively. The trend corresponds to the DFT calculation results
very well.
Photophysical property
The UVevis absorption spectra of the complexes Ir1eIr6 in
CH2Cl2 are shown in Fig. 4 and the data are collected in Table 5. The
intense bands below 350 nm with extinction coefficients on the
order of 104 Mꢁ1 cmꢁ1 are assigned to the spin-allowed intraligand
(*) transition of cyclometalated main ligands and ancillary ligands.
The lower-energy absorption bands in the range 350e500 nm are
primarily due to the mixing of the spin-allowed singlet metal-to-
ligand charge-transfer (1MLCT) and triplet metal-to-ligand
charge-transfer (3MLCT) states, or LLCT (ligand-to-ligand charge-
transfer) transition [44e48]. A strong spineorbit coupling occurs
because of the presence of the heavy iridium center, these singlet to
triplet transitions become partially allowed and obtain massive
intensity by mixing with the 1MLCT transitions. But, compared with
Ir1eIr3 with 2-(5-phenyl-1,3,4-oxadiazol-2-yl)-phenol derivatives
as ancillary ligands, the absorption bands of Ir4eIr6 containing 2-
(5-phenyl-1,3,4-thiadiazole-2-yl)-phenol derivatives have an
obvious red shift, which agrees with the sequence of the HOMO-
LUMO gaps.
For providing further study of the electronic structures of the
complexes, the theoretical calculation was performed on optimized
geometries in CH2Cl2. The calculations on the ground electronic
states of the complexes were carried out using density functional
theory (DFT) [34,35] and time-dependent DFT (TD-DFT) at the
B3LYP [36,37] level. The basis set used for C, H, N, O, F and S atoms
was 6e31G(d, p) [38] while the LanL2DZ [39] basis set was used for
Ir atoms. The solvent effect of CH2Cl2 was taken into consideration
using conductor-like polarizable continuum model (C-PCM) [40].
All these calculations were performed with Gaussian 09 [41].
QMForge program was used to give accurate percentage data of
FMOs. Contour plots of the frontier molecular orbitals (FMOs) are
shown in Fig. 3. The energies and % composition of ligand and metal
orbitals are shown in Table 4.
According to the results, it is clear that the HOMO is mainly
locate on a mixture of the d orbitals of iridium (14.62e15.27%) and
the
p orbitals of the phenyl ring of the ancillary ligand
(78.07e79.82%). The oxidation processes occur with metal centered
orbitals and a contribution from the phenyl ring of ancillary ligand
[42,43]. While the locations of the LUMO are vary in different
complexes. The LUMOs of Ir1eIr3 and Ir5 are mainly located on C^N
ligands and the compositions are 96.04%, 98.18%, 96.04% and
91.55%, respectively, and the composition of LUMO of Ir4 in C^N
ligands is 65.35%. It is interesting that for complex Ir6, the
composition of LUMO on the TPP ligand is only low as 7.94%, but the
ratio on ancillary ligand is increased to 91.03%, which are much
higher than that of other complexes. The electronic cloud overlaps
in the phenyl ring of the ancillary ligand result in a small oxidation
potential (0.65 V, Fig. 2 and Table 3), lowering HOMO/LUMO levels
and HOMO - LUMO gaps. It can be seen that the frontier orbitals and
the electronic properties of the complexes can be manipulated by
introducing 1,3,4-thiadiazole derivatives.
Photoluminescence measurements were conducted in dea-
erated CH2Cl2 solutions at room temperature (Fig. 4, Table 5). Under
the excitation of 370 nm, the complexes Ir1eIr4 emit intense green
phosphorescence in CH2Cl2 at room temperature with peaks
maxima in the range of 515e523 nm. Under the excitation of
470 nm, the complexes Ir5eIr6 emit intense orange and red
phosphorescence peaked at 583 and 601 nm, respectively. From
Table 5
Absorption and emission data of the complexes Ir1e Ir6.
Complexes
Absorption [
l
, nm (ε,103 Mꢁ1 cmꢁ1)]
Emission (lmax, nm)
298 K
Fem(%)
Ir1
Ir2
Ir3
Ir4
Ir5
Ir6
235(60), 263(51), 397(1.0)
237(50), 263(46), 400(1.1)
234(46), 265(38), 412(1.1)
235(40), 273(40), 430(1.0)
235(38), 274(38), 444(1.2)
238(40), 270(31), 454(1.4)
517
516
525
519
580
601
25
26
27
18
14
13
Fig. 5. Energy level diagram of HOMO and LUMO levels (relative to vacuum level) for
materials used in this work and their molecular structures.

X. Liu et al. / Journal of Organometallic Chemistry 785 (2015) 11e18
17
Fig. 4 it can be observed that when the 2-(5-phenyl-1,3,4-
oxadiazol-2-yl)-phenol derivatives were replaced by 2-(5-phenyl-
1,3,4-thiadiazole-2-yl)-phenol derivatives, the emissions of Ir4eIr6
are red shift by comparing with those of Ir1eIr3, which also agrees
the sequence of the HOMO - LUMO gaps and absorption spectra.
The results suggest the introduction of 1,3,4-thiadiazole into
ancillary ligands can affect the luminescence properties of the IrIII
complexes.
the host exciton to the phosphor is nearly complete upon electrical
excitation. The CIE color coordination are x ¼ 0.33, y ¼ 0.59 for D1,
and x ¼ 0.30, y ¼ 0.47 for D2, corresponding to the green and
orange-red region, respectively.
For devices D1 and D2, the turn-on voltages (Vturn-on) are 3.1 and
3.3 V, respectively. The maximum current and power efficiencies of
D1 are 41.08 cd/A and 31.46 lm/W, respectively. The driving volt-
ages needed to obtain the practical brightness of 100 and 1000 cd/
m2 are 4.5 and 5.4 V, respectively. For D2, the maximum current
and power efficiencies are 50.92 cd/A and 40.31 lm/W, respectively.
The driving voltages for 100 and 1000 cd/m2 are 4.3 and 5.2 V,
respectively. The results suggest the replacement of the ancillary
ligands not only changed the emission color, also increased the
efficiency of the device. Furthermore, for these devices, the effi-
ciency roll-off ratios among the practical brightness of 100 and
1000 cd/m2 are mild, which are useful for the real application.
OLED performance
In order to evaluate the electroluminescence (EL) performances
of Ir(III) complexes, we chose 1,3,4-oxadiazole contained Ir3 and
1,3,4-thiadiazole contained Ir6 as examples fabricated the OLEDs.
The devices with the structure of ITO/TAPC (1,1- Bis[4-[N,N-di(p-
tolyl)amino]phenyl]cyclohexane, 40 nm)/Ir3 or Ir6 (10 wt%): mCP
(1,3-bis(carbazol-9-yl)benzene, 20 nm)/TmPyPB (1,3,5-tri[(3-
pyridyl)-phen-3-yl]benzene, 50 nm)/LiF (1 nm)/Al (100 nm)
named D1 and D2, respectively. TAPC and TmPyPB were employed
as the hole and electron transport layers, respectively. mCP was
used as a host material. The optimized Ir(III) complexes with 10 wt%
doped concentration in mCP was used as the emitting layer. The
energy diagram of devices, as well as the molecular structure of
these materials, is shown in Fig. 5. Apparently, the HOMO/LUMO
levels of Ir(III) complexes are all in the range of the HOMO and
LUMO levels of host material. So, good carrier trapping is expected,
holes and electrons will be well confined within the light emitting
layer.
Conclusion
We reported the syntheses and characterization of a series of
new bis-cyclometalated iridium complexes Ir1eIr6 with the 2-(2-
trifluoromethyl)pyrimidine-pyridine main ligand and 2-(5-
phenyl-1,3,4-oxadiazol-2-yl)-phenol
or
2-(5-phenyl-1,3,4-
thiadiazol-2-yl)-phenol derivatives as ancillary ligands. The crys-
tal structures of the complexes revealed that all the complexes
possess pseudo-octahedral coordination geometry. The DFT calcu-
lations suggested that the frontier orbitals and the electronic
properties of the complexes can be manipulated by introducing
1,3,4-thiadiazole derivatives. The compositions of LUMO on the
1,3,4-thiadiazole ancillary ligands are higher than that of 1,3,4-
oxadiazole derivatives, and the HOMOeLUMO gaps are also
decreased. Therefore, the emissions of the complexes with 1,3,4-
thiadiazole contained ancillary ligands are shift from green to red.
The devices with Ir3 and Ir6 as emitters show maximum current
efficiencies of 41.08 and 50.92 cd/A, respectively, with mild effi-
ciency roll-off. Thus, our research provides a new way to tune the
The EL spectra, current densityevoltageeluminance (IeVeL),
current
efficiencyeluminance
(h
ceL)
and
power
effi-
ciencyeluminance (
h
ceL) characteristics of each device are shown
in Fig. 6. Two devices show typical emission peaks of complexes Ir3
and Ir6 in CH2Cl2 at 523 and 600 nm, respectively. There is no
emission from host in D1 and the emission of D2 has very only a
weak peak in 369 nm which belongs to a little residual emission of
mCP. The results suggest that energy and/or charge transfer from
Fig. 6. Characteristics of devices (D1 and D2) with configuration ITO/TAPC (40 nm)/Ir3 or Ir6 (10 wt%): mCP (20 nm)/TmPyPB (50 nm)/LiF (1 nm)/Al (100 nm): (a) EL spectra at 8 V;
(b) current densityevoltageeluminance (IeVeL); (c) current efficiencyeluminance ( ceL); (d) power efficiencyeluminance ( ceL) curves.
h
h

18
X. Liu et al. / Journal of Organometallic Chemistry 785 (2015) 11e18
emission and device efficiency of IrIII complexes by introducing of
1,3,4-thiadiazole group.
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Acknowledgments
This work was supported by the National Natural Science
Foundation of China (21371093, 91433113), the Major State Basic
Research Development Program (2011CB808704, 2013CB922101)
and the Natural Science Foundation of Jiangsu Province
(BK20130054).
€
€
€
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1997.
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Appendix A. Supplementary material
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CCDC 1028448; 1028450; 1028451; and 1028452 contain the
supplementary crystallographic data for this paper. These data can
be obtained free of charge from The Cambridge Crystallographic
Data Center via www.ccdc.cam.ac.uk/data_request/cif.
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