Blue-emitting Ir(III) complexes using fluorinated bipyridyl as main ligand and 1,2,4-triazol as ancillary ligand: syntheses, photophysical properties and performances in devices

Article abstract of DOI:10.1016/j.tet.2016.11.012

The blue-light-emitting Ir(III) complexes using 2-(3-(trifluoromethyl) -1H-1,2,4-triazol-5-yl)pyridine and fluorinated 2-phenylpyridine as ligands were synthesized and characterized in details. Their molecular structures were confirmed by¹H NMR

Full text of DOI:10.1016/j.tet.2016.11.012

Tetrahedron 72 (2016) 8335e8341
Contents lists available at ScienceDirect
Tetrahedron
journal homepage: www.elsevier.com/locate/tet
Blue-emitting Ir(III) complexes using fluorinated bipyridyl as main
ligand and 1,2,4-triazol as ancillary ligand: syntheses, photophysical
properties and performances in devices
,
,
,
,
, b, *
Peng Sun a b, Kexiang Wang a b, Bo Zhao a b, Tingting Yang a b, Huixia Xu a
,
Yanqin Miao a b, Hua Wang a b, Bingshe Xu a
a Key Laboratory of Interface Science and Engineering in Advanced Materials, Ministry of Education, Taiyuan University of Technology, Taiyuan 030024, PR
,
,
, b
China
b Shanxi Research Center of Advanced Materials Science and 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:
The blue-light-emitting Ir(III) complexes using 2-(3-(trifluoromethyl) -1H-1,2,4-triazol-5-yl)pyridine and
fluorinated 2-phenylpyridine as ligands were synthesized and characterized in details. Their molecular
structures were confirmed by 1H NMR, FT-IR and elemental analyzer. The emission peaks of complex
(F2,4ppy)2Ir(tfmptz) using 2-(2,4-difluorophenyl)pyridine as main ligand are located at 457 and 481 nm.
Increasing the number of F atoms leads to a lower the lowest unoccupied molecular orbital energy level.
Phosphorescent organic light-emitting devices doped the synthesized Ir(III) complexes into 4,40-N,N0-
dicarbazolebiphenyl as emitting layers were fabricated. The based-(F3,4,5ppy)2Ir(tfmptz) device shows
the maximum current efficiency of 9.9 cd/A with the electroluminescent peaks at 471 and 502 nm.
© 2016 Elsevier Ltd. All rights reserved.
Received 2 August 2016
Received in revised form
24 October 2016
Accepted 2 November 2016
Available online 4 November 2016
Keywords:
Blue-emitting Ir(III) complexes
1,2,4-triazol
Fluorinated bipyridyl
1. Introduction
ancillary ligand of picolinate or using functionalized 2-
phenylpyridine (ppy) as main ligands in FIrpic have been
Phosphorescent organic light-emitting devices (PhOLEDs) are
considered to be the promising candidates for the next-generation
lighting and display applications.1e3 Phosphorescent light-emitting
materials of heavy metal complex, especially iridium metal, have
the excellent performances in PhOLEDs because of the advantages
of a theoretical internal quantum efficiency of 100%.4,5 However,
the blue-emitting phosphorescent complex remains inferior to that
of red and green counterparts, such as color purity and luminance
efficiency.6,7 The well-known blue Ir(III) emitter of bis[2-(40,60-
difluoro)phenylpyridinato-N,C ]iridium(III)picolinate (FIrpic) ex-
hibits the moderate external quantum efficiency (EQE) of 5.7% with
the 1931 Commission Internationale de L'Eclairage coordinates
(CIE) coordinates of (0.16, 0.29).8,9 But FIrpic cannot be regarded as
a true-blue emitter due to its relatively low triplet energy level (ET)
of 2.62 eV and sky-blue emission colors with CIEy > 0.2.10 Recently,
a variety of blue-emitting Ir(III) complexes by changing the
reported.11e13 The former can alter the energy of the excited state
by replacing the picolinate with the other ligands, such as tetra-
kis(1-pyrazolyl)borate (FIr6) with CIE coordinates of (0.16,
0.26),14,15 phosphite ligands with CIE coordinates of (0.16, 0.21),12
and (pyri-din-2-yl)-1H-tetrazolate (FIrN4) with CIE coordinates of
(0.15, 0.24)11 et al. Meanwhile, The latter can decrease the highest
occupied molecular orbital (HOMO) energy level by introducing
electron-withdrawing groups such as eOCF3, eCOCF2CF2CF3,16
cyan,17 P]O, S]O18 or nitrogen atom19 into the 5-position of
difluorophenyl ring. Therefore, the fluorinated ppy is an effective
way to tune the emission color of Ir(III) complexes. The conversion
of CeH bonds in these complexes to CeF bonds may have several
benefits, such as reduction of irradiative exciton decay, enhance-
ment of photoluminescence (PL) efficiency, and electron
mobility.20,21
2
0
In addition, the functionalized 1, 2, 4-triazole is another ligand
of choice for giving blue emission in Ir(III) complexes.3,22 In this
work, we successfully synthesized two blue Ir(III) complexes con-
sisting of main ligands 2-(40, 60-difluoro)phenylpyridinato-
* Corresponding author. Key Laboratory of Interface Science and Engineering in
Advanced Materials, Ministry of Education, Taiyuan University of Technology,
Taiyuan 030024, PR China.
2
0
N,C (F2,4ppy) [(F2,4ppy)2Ir(tfmptz)] and 2-(3,4,5-trifluorophenyl)
pyridine (F3,4,5ppy) [(F3,4,5ppy)2Ir(tfmptz)], and ancillary ligand of
E-mail address: xuhuixia@tyut.edu.cn (H. Xu).
http://dx.doi.org/10.1016/j.tet.2016.11.012
0040-4020/© 2016 Elsevier Ltd. All rights reserved.

8336
P. Sun et al. / Tetrahedron 72 (2016) 8335e8341
2-(5-(trifluoromethyl)-4H-1,2,4-triazol-3-yl)pyridine. Both Ir(III)
complexes were thermally stable and easily sublimed with good
yields with the higher ET than that of FIrpic. The chemical structures
of complexes FIrpic, (F2,4ppy)2Ir(tfmptz) and (F3,4,5ppy)2Ir(tfmptz)
are shown in Scheme 1. The synthesized procedure are shown in
Scheme S1 and S2.
J ¼ 7.9 Hz), 7.97 (t, 1H, J ¼ 7.8 Hz), 7.53 (d, J ¼ 7.7 Hz, 1H). FT-IR (KBr,
cmꢀ1): 3137, 1606, 1494, 1461, 1428, 1217, 1189, 1009, 765, 714. Anal.
Calcd for C8H16N5F3: C, 40.17; N, 29.29; H, 6.69. Found: C, 40.55; N,
28.87; H, 7.13.
1.2. (F2,4 ppy)2Ir(tfmptz)
1.1. 2-(3, 4, 5-trifluorophenyl)pyridine (F3,4,5ppy)
A mixture solution of 2-(2,4-difluorophenyl)pyridine (0.48 g,
2.5 mmol), IrCl3$3H2O (0.35 g, 1 mmol), 2-ethoxyethanol (15 ml)
and H2O (5 ml) was refluxed at 120 ꢁC for 24 h under nitrogen
atmosphere. After cooling to room temperature, a lot of water was
added and then precipitate was filtered, washed by water, ethanol
and diethyl ether to obtain the intermediate chloro-bridged dimer
A mixture of 2-bromopyridine (1.6 g, 10 mmol), (3, 4, 5-
trifluorophenyl)boronic acid (1.9 g, 12 mmol), tetrakis(-
triphenylphosphine)palladium(0) (Pd(PPh3)4) (170 mg, 0.15 mmol),
Na2CO3 (2 M, 15 mL, 30 mmol) and tetrahydrofuran (THF) (40 mL)
was heated to reflux for 24 h under nitrogen atmosphere. After
cooling to room temperature, the water (50 mL) was poured into
mixture and extracted with CH2Cl2. The organic extracts were
washed using water and dried over using anhydrous MgSO4. The
crude product was purified with silica gel column chromatography
using a 10:1 mixture of petroleum ether and ethyl acetate as the
eluent. The white crystals of 2-(3,4,5-trifluorophenyl)pyridine
(F2,4ppy)2Ir(
Without further purification, a mixture of the (F2,4ppy)2Ir(
Cl)2Ir(F2,4ppy)2 (0.37 g, 0.3 mmol), Na2CO3 (0.64 g, 6 mmol), and 2-
(3-(trifluoromethyl)-1H-1,2,4-triazol-5-yl)pyridine (64.2 mg,
m-Cl)2Ir(F2,4ppy)2.
m-
0.3 mmol) was added into 2-ethoxyethanol (20 ml) and heated to
refluxed at 140 ꢁC for 24 h under a nitrogen atmosphere, then
extracted with CH2Cl2 and dried over anhydrous Na2SO4. The crude
product was subjected to silica gel column chromatography using a
10:1 mixture of CH2Cl2 and ethanol as the eluent. The yellow solid
of (F2,4ppy)2Ir(tfmptz) were obtained. Yield: 40%. 1H NMR
(1.7 g) were obtained. Yield: 81%, 1H NMR (600 MHz, CDCl3,
(d, 1H, J ¼ 4.8 Hz), 7.81e7.75 (m, 1H), 7.70e7.64 (m, 3H), 7.29 (d, 1H,
d): 8.68
J
¼
7.5 Hz). 19F NMR (377 MHz, CDCl3,
d
): ꢀ133.94 (d,
J ¼ 20.9 Hz), ꢀ160.01 (t, J ¼ 20.8 Hz). FT-IR (KBr, cmꢀ1): 3640, 3146,
1617, 1572, 1531, 1475, 1148, 1416, 1359, 1258, 1202, 1036, 858, 778.
Anal. Calcd for C24H24O2N: C, 78.26; N, 3.80; H, 6.52. Found: C,
78.92; N, 3.74; H, 6.306.
(600 MHz, CDCl3,
d
): 8.31e8.23 (m, 3H), 7.91 (t, 1H, J ¼ 7.8 Hz), 7.74
(d, 3H, J ¼ 10.2 Hz), 7.42 (d, 1H, J ¼ 5.8 Hz), 7.00 (t, 1H, J ¼ 6.5 Hz),
6.90 (t, 1H, J ¼ 6.5 Hz), 6.52 (s, 1H), 6.45 (s, 1H), 5.77 (d, 1H,
J ¼ 8.4 Hz), 5.67 (d, 1H, J ¼ 8.7 Hz). 19F NMR (377 MHz, CDCl3,
d): ꢀ63.32, ꢀ106.66 (d,
J
¼
10.9 Hz), ꢀ107.26 (d,
1.1.1. 2-(3-(trifluoromethyl)-1H-1,2,4-triazol-5-yl)pyridine (tfmptz)
a. A mixture solution of picolinonitrile (1.0 g, 10 mmol), sodium
methoxide (0.05 g, 1 mmol), and methanol (20 mL) was stirred for
10 h at room temperature under nitrogen atmosphere. Then NH4Cl
(0.5 g, 10 mmol) be added and reacted other 20 h at room tem-
perature. The mixture was heated to reflux for 4 h. After cooling to
room temperature, the mixture was added a lot of diethyl ether and
then filtered. The white crystals of pyridine-2-carboximidamide
hydrochloride were obtained.
b. A mixture solution of ethyl trifluoroacetate (2.0 g, 14 mmol),
hydrazine hydrate (1.0 g, 21 mmol), and THF (20 mL) was stirred for
12 h at room temperature under nitrogen atmosphere. Then after
NaOH (0.5 g, 14 mmol) and pyridine-2-carboximidamide hydro-
chloride (2.0 g, 15 mmol) were added into the mixture and heated
to reflux for 12 h. After cooling to room temperature, the saturated
NaHCO3 solution was added and then extracted with ethyl acetate.
The organic extract was with water and dried over anhydrous
MgSO4. The crude product was subjected to silica gel column
chromatography using a 3:1 mixture of petroleum ether and ethyl
acetate as the eluent. The white crystals of 2-(3-(trifluoromethyl)-
1H-1,2,4 -triazol-5-yl)pyridine were obtained. Yield: 82%. 1H NMR
J ¼ 10.9 Hz), ꢀ109.29 (d, J ¼ 11.4 Hz), ꢀ109.91 (d, J ¼ 11.5 Hz). FT-IR
(in KBr, cmꢀ1): 3616, 3083, 1602, 1478, 1405, 1294, 1243, 1173, 1132,
987, 820, 758. MALDI-TOF: calcd for IrC30H16N6F7, 785.51; found,
785.84; Anal. Calcd for IrC30H16N6F7: C, 45.86; N, 10.7; H, 2.03.
Found: C, 46.04; N, 10.09; H, 2.31.
1.3. (F3,4,5ppy)2Ir(tfmptz)
The procedure is similar with the synthesis of
(F2,4ppy)2Ir(tfmptz) by using F3,4,5ppy (0.52 g, 2.5 mmol) as main
ligand and tfmptz as ancillary ligand. Overall yields: 44%.1H NMR
(600 MHz, CDCl3,
d
): 8.26e8.20 (m, 3H), 8.15 (t, 1H, J ¼ 7.8 Hz), 8.07
(d, 2H, J ¼ 36.7 Hz), 7.90 (t, 2H, J ¼ 7.8 Hz), 7.85 (d, 1H, J ¼ 5.6 Hz),
7.54 (d, 1H, J ¼ 5.9 Hz), 7.50 (d, 1H, J ¼ 7.3 Hz), 7.38e7.34 (m, 1H),
7.18 (d, 1H, J ¼ 7.3 Hz), 7.11 (d, 1H, J ¼ 7.4 Hz). 19F NMR (377 MHz,
CDCl3,
d
): ꢀ63.41, ꢀ127.51 (dd, J ¼ 24.1, 5.2 Hz), ꢀ128.89 (dd,
J ¼ 23.3, 5.1 Hz), ꢀ142.38 (dd, J ¼ 19.6, 5.3 Hz), ꢀ143.81 (dd, J ¼ 19.8,
5.2 Hz), ꢀ157.18 (dd, J ¼ 24.4, 19.3 Hz), ꢀ157.75 (dd, J ¼ 23.9,
19.6 Hz). FT-IR (KBr, cmꢀ1): 3670, 3310, 2925,1610,1474,1409,1350,
1300, 1202, 1140, 1040, 814, 758. MALDI-TOF: calcd for
IrC30H14N6F9, 821.49; found, 821.94; Anal. Calcd for IrC30H14N6F9: C,
43.85; N, 10.20; H, 1.70. Found: C, 44.52; N, 10.04; H, 1.869. The 1H
(600 MHz, CDCl3,
d): 13.40 (s, 1H), 8.82e8.77 (m, 1H), 8.32 (d, 1H,
N
N
N
N
N
N
N
Ir
Ir
Ir
F
F
N
N
O
O
N
2
N
CF3
F
F
2
CF3
2
F
F
F
(F3,4,5ppy)2Ir(tfmptz)
(F2,4ppy)2Ir(tfmptz)
FIrpic
Scheme 1. Chemical structures of FIrpic, (F2,4ppy)2Ir(tfmptz) and (F3,4,5ppy)2Ir(tfmptz).

P. Sun et al. / Tetrahedron 72 (2016) 8335e8341
8337
NMR, 19F NMR and MS spectra of all compounds are all shown in
supporting information.
1.4. General information
The NMR spectra were recorded with a Bruker spectrometer at
600 or 377 MHz (600 for 1 H, 377 MHz for 19 F) at ambient tem-
perature. Chemical shifts are given in ppm Mass spectra were ob-
tained on SHIMADZU matrix-assisted laser desorption/ionization
time-of-flight mass spectrometer (MALDI-TOF-MASS). C, H, and N
microanalysis were carried out with an Elemental Vario EL
Elemental analyzer. FT-IR spectra were measured with a Nicolet
7199B spectrometer in KBr pellets in the range of 4000e400 cmꢀ1
.
UVevis absorption spectra were measured using Lambda Bio 40.
The Photoluminescence (PL) spectra were recorded by HORIBA
FluouoMax-4 spectrophotometer. The low-temperature phospho-
rescence spectra at 77 K in 2-methyltetrahydrofuran (2-MeTHF)
and excited-state lifetime in film were measured on an Edinburgh
F-980 spectrometer. The absolute quantum yields of the complexes
were determined through an absolute method by employing an
integrating sphere. Thermogravimetric analysis curves (TGA) were
undertaken using a Netzsch TG 209F3 under dry nitrogen atmo-
sphere heating at a rate of 10 ꢁC/min.
Fig. 1. TGA curves of all title complexes.
characterized by thermal gravimetric analysis (TGA), CV, UVevi-
sible and PL spectroscopy.
Theoretical calculations were performed using the Gaussian 03
package. Geometry optimization was performed on the ground
state of each complex by density functional theory (DFT) in B3LYP/
6-31G(d) basis sets for N, H, C and LANL2DZ basis sets for Ir atom.
The molecular orbital distributions, involving of the highest occu-
pied molecular orbitals (HOMO) and the lowest unoccupied mo-
lecular orbitals (LUMO) were analyzed.
2.2. Thermal stability
The thermal properties of complexes were analyzed by TGA and
the curves are shown in Fig. 1. The onset of decomposition tem-
perature (Td, the temperature at which 5% weight loss) was 388 and
391 ꢁC for (F2,4ppy)2Ir(tfmptz), and (F3,4,5ppy)2Ir(tfmptz), respec-
tively. This guaranteed the fast train sublimation with good yield
without much residue at higher temperatures.
Cyclic voltammetry (CV) was measured out in film by drop-
coating with chromatographic purity at room temperature using
a CHI 660E voltammetry analyzer. The platinum wire is used as
working electrode. The platinum electrode is the counter and a
calomel electrode is the reference. The scan rate for CV curves is
100 mV sꢀ1. The HOMO levels were calculated according to the
equation EHOMO ¼ ꢀ4.8-eEcox, where Eocx was the first oxidation peaks
measured from CV curves. On the other hand, the LUMO levels were
calculated based on the equation LUMO ¼ HOMO þ Eg, and Eg is
from the intersection of absorption and emission spectra.
PhOLEDs with area of 3 ꢂ 3 mm2 were fabricated by vacuum
deposition onto indium tin oxide (ITO) glass substrate. ITO glass
substrate was cleaned with deionized water, acetone and ethanol in
turn. The electroluminescent (EL) spectra and CIE coordinates were
measured by PR-655 spectrophotometer. The current density -
voltage - luminance (J-V-L) characteristics of PhOLEDs were
recorded using Keithley 2400 Source Meter and ST-900M Spot
Brightness Meter.
2.3. Photophysical properties
Fig. 2 shows the steady-state UVevis absorption and PL spectra
of (F2,4ppy)2Ir(tfmptz) and (F3,4,5ppy)2Ir(tfmptz) in CH2Cl2 solution
and the corresponding spectra data are summarized in Table 1. The
absorption spectra at room temperature feature characteristic
transition involving strong ligand-centered (LC) spin-allowed p-p*
transition around 253 and 256 nm for (F2,4ppy)2Ir(tfmptz) and
(F3,4,5ppy)2Ir(tfmptz), respectively. The singlet metal-to-ligand
charge transfer (1MLCT) transitions are observed in the ranges of
363e384 and 362e382 nm. The calculations indicated that these
2. Results and discussion
2.1. Synthesis
Both complexes F2,4ppy)2Ir(tfmptz) and (F3,4,5ppy)2Ir(tfmptz)
were synthesized according to the previously reported standard
procedures, in which the ligand exchange with corresponding
ancillary ligands afforded the titled Ir(III) complexes in Scheme 1.
The ligands F2, 4 ppy and F3,4,5ppy were reacted with iridium tri-
chloride hydrate (IrCl3$3H2O) to yield the cyclometalated Ir(III)
m-chloride bridged dimer with the quantitative yield. The dimer
was converted to the heteroleptic Ir(III) complexes
(F2,4ppy)2Ir(tfmptz) and (F3,4,5ppy)2Ir(tfmptz) by replacing the two
bridge chlorides with 2-(3-(trifluoromethyl)-1H-1,2,4-triazol-5-yl)
pyridine. The complexes were confirmed by 1H NMR, elemental
analysis and FT-IR. The photophysical properties were
Fig. 2. UVevis absorption, emission spectra in CH2Cl2 solution at room temperature
and phosphorescent spectra in 2-MeTHF at 77 K.

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P. Sun et al. / Tetrahedron 72 (2016) 8335e8341
Table 1
Photophysical, and electrochemical properties of complexes (F2,4ppy)2Ir(tfmptz) and (F3,4,5ppy)2Ir(tfmptz).
b
d
e
HOMOc (eV)
Eg (eV)
FPL
%
te
ms
a
a
Complexes
T1 (eV)
Td (ꢁC)
labs (nm)
lem (nm)
(F2,4ppy)2Ir(tfmptz)
(F3,4,5ppy)2Ir(tfmptz)
372, 400
371, 402
457, 481
469, 502
2.74
2.67
388
391
ꢀ5.51
ꢀ5.54
2.9
2.8
20.1
24.3
2.48
3.23
a
Recorded in CH2Cl2 solution at room temperature.
Triplet level (T1) calculated based on low-temperature PL spectra at 77 K in 2-MeTHF solution.
Measure in film by drop-coating.
b
c
d
e
Measured in the film.
Band gap (Eg) was estimated from the intersection of UVevis and emission spectra.
absorptions correspond to the transitions from HOMO-1 to LUMO
(89%) with the oscillator strength (f) of 0.0698 and 0.0762. The spin-
forbidden MLCT (3MLCT) transition appeared at 400 and 402 nm
with the weak absorption, are mainly ascribed to the transition
from HOMO to LUMO. The two complexes have very similar ab-
sorption, suggesting that the introduction of a fluorine atom at the
main ligands did not greatly affect the ground state.
The emission spectra of titled complexes are also displayed in
Fig. 2. The vibronic structure of emission bands indicate a large
amount of 3LC character, and the excited state is believed to be a
mixture of 3LC and 3MLCT.23,24 The complex of (F2,4ppy)2Ir(tfmptz)
shows the emission maximum peaks at 457 and 481 nm, and the
emission bands of (F3,4,5ppy)2Ir(tfmptz) appears at 469 nm with a
vibronic peak at 502 nm with the higher absolute quantum yields
(FPL) of 24.3% in film. This result indicates that the fluorine atoms
on the main ligand have obvious effects on the excited state of the
Ir(III) complexes. The band gaps (Eg) of 2.92 and 2.82 eV were
attained by using the intersection of absorption and PL spectra at
room temperature in CH2Cl2 solution. The excited-state lifetimes
Fig. 3. Phosphorescent spectra in 2-MeTHF at 77 K.
(t) of (F2,4ppy)2Ir(tfmptz) and (F3,4,5ppy)2Ir(tfmptz) in film are 2.48
Fig. 4. Optimized molecular structures, HOMO and LUMO of complexes (F2,4ppy)2Ir(tfmptz) and (F3,4,5ppy)2Ir(tfmptz).

P. Sun et al. / Tetrahedron 72 (2016) 8335e8341
8339
functional theory (DFT) calculations were employed. The DFT cal-
culations show that the HOMO of (F2,4ppy)2Ir(tfmptz) is populated
on the one of the two F2,4ppy (45.2%), 1,2,4-trizole moieties (4.2%)
and d metal orbitals of Ir (50.8%). While the LUMO is mainly
localized on the other F2,4ppy moieties (55.9%), mainly localized on
the pyridyl fragment, ancillary ligand of tfmptz (42.0%) and litter
Ir orbital (3.2%),27 as shown in Fig. 4. The complexes of
(F3,4,5ppy)2Ir(tfmptz) and (F2,4ppy)2Ir(tfmptz) have the similar
molecular orbitals. The percentage of the Ir atom to HOMO and
LUMO are 53.2% and 3.0%.
2.4. Electrochemical properties
To better understand the nature of the electronic structures,
cyclic voltammetry (CV) measurements were performed. The
electrochemical data were listed in Table 1. The oxidization of
Fig. 5. Cyclic voltammograms of (F2,4ppy)2Ir(tfmptz) and (F3,4,5ppy)2Ir(tfmptz).
and 3.23 ms, respectively.
The low-temperature emission spectra in 2-MeTHF at
77 K display the well-resolved vibrational features with blue-
shifted by 10 nm with respect to the spectra at ambient tempe-
rature (Fig. 3), giving further evidence for the 3MLCT character of
the excited state.24,25 The triplet energies (ET) of complexes
(F2,4ppy)2Ir(tfmptz) and (F3,4,5ppy)2Ir(tfmptz) were estimated to be
2.74 and 2.67 eV. These values are higher than that of FIrpic
(ET ¼ 2.62 eV)26 owing to the strong
s-donor and weak p-acceptor
of ancillary ligand.
To study the effects of the number of F on excited state, density
Fig. 7. Current density-voltage-luminance curves of devices.
Fig. 6. The energy level diagram (a) and EL spectra (b) of Devices 1 and 2 (mCP:(F2,4ppy)2Ir(tfmptz) for Device 1; CBP: (F3,4,5ppy)2Ir(tfmptz) for Device 2).
Table 2
Device performances of devices 1 and 2.
b
e
f
Lmaxc (cd/m2)
hcd (cd/A)
hp (lm/W)
CIE (x, y)
hEQE; max (%)
a
Device
Vturnꢀon (V)
lmax (nm)
Device 1
Device 2
460, 490
471, 502
4.0
3.7
4937
6881
5.2
9.9
3.0
5.2
(0.17, 0.23)
(0.21, 0.39)
3.0
4.4
a
The maximum peaks at EL spectra.
Voltage at a luminance of 1 cd/m2.
The maximum luminance.
The maximum current efficiency.
The maximum power efficiency.
b
c
d
e
f
The maximum external quantum efficiency.

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P. Sun et al. / Tetrahedron 72 (2016) 8335e8341
Fig. 8. Current efficiency-current density (a) and Current efficiency-power efficiency (b) curves of PhOLEDs.
complexes (F2,4ppy)2Ir(tfmptz) and (F3,4,5ppy)2Ir(tfmptz) are Ecox
¼
turn-on voltage at 4.0 V (defined as voltage at a luminance of 1 cd/
m2) and a maximum luminance of 4937 cd/m2. The peak current
and luminescent efficiencies, without any device optimization, are
5.2 cd/A and 3.0 lm/W, respectively, in Fig. 8. Similarly, the Device
2 shows a maximum luminance (Lmax) of 6881 cd/m2 with the
maximum current efficiencies (hc) and maximum power efficiency
0.71 and 0.74 V, as shown in Fig. 5. The HOMO energy levels were
calculated to be ꢀ5.51 and ꢀ5.54 eV. Therefore, the LUMO energy
levels of ꢀ2.61 and ꢀ2.74 eV were obtained by
LUMO ¼ Eg ꢀ HOMO. While, the calculated HOMO/LUMO energy
levels are ꢀ5.71/ꢀ1.84 and ꢀ5.66/ꢀ2.02 eV for (F2,4ppy)2Ir(tfmptz)
and (F3,4,5ppy)2Ir(tfmptz), respectively. The LUMO energy levels
were regulated by introduction of the more electron-withdrawing
fluorine atom.
(hp) of 9.9 cd/A and 5.5 lm/W, respectively. The maximum external
quantum efficiency (hEQE) reached 3% and 4.4% for Devices 1
and 2.
It is difficult to select suitable carrier-injection and transport
materials for based-(F2,4ppy)2Ir(tfmptz) blue PhOLEDs due to the
higher T1. So the performances of based-(F2,4ppy)2Ir(tfmptz) de-
vices are inferior to that of the (F3,4,5ppy)2Ir(tfmptz).
2.5. Performances in PhOLEDs
Based on the properties data in solution, two complexes were
applied to construct PhOLEDs. The compound of 1,3-bis(N-carba-
zolyl)benzene (mCP) with T1 of 2.90 eV was selected as the host in
based-(F2,4ppy)2Ir(tfmptz) device (Device 1). In based-
(F3,4,5ppy)2Ir(tfmptz) device, 4, 40-N, N0-dicarbazolebiphenyl (CBP,
T1 ¼ 2.56 eV) were employed as host (Device 2). The devices were
fabricated by the vacuum sublimation with the structures of ITO/
MoO3 (3 nm)/TCTA (25 nm)/mCP: (F2, 4ppy)2Ir(tfmptz) 4.5 wt%
(15 nm)/TPBi (35 nm)/LiF (1 nm)/Al (200 nm) (Device 1) and ITO/
NPB (30 nm)/TCTA (10 nm)/CBP: (F3,4,5ppy)2Ir(tfmptz) 8%wt
(30 nm)/TPBi (35 nm)/LiF (1 nm)/Al (200 nm) (Device 2). N0-bis(-
naphthalene)-N,N0 -bis(phenyl)-benzidine (NPB) was used as hole-
transport layer; and 1,3,5-tris(2-nhenylbenzimidazolyl)-benzene
(TPBi) with T1 ¼ 2.6 eV was used as electron-transport materials.
Tris(4-carbazoyl-9-ylphenyl)amine (TCTA) with T1 ¼ 2.85 eV can
reduce hole injection barrier. MoO3 and LiF (lithium fluoride)
served as the hole- and electron-injecting layers, respectively. The
energy level diagrams of device 1 and 2 are displayed in Fig. 6a. The
performances of devices were measured in the air at room
temperature.
3. Conclusions
In conclusion, two blue-emitting Ir(III) complexes based on 2-
(3-(trifluoromethyl)-1H-1,2,4-triazol-5-yl)pyridine as ancillary
ligand, 2-(3, 4, 5-trifluorophenyl)pyridine (F3,4,5ppy) and 2-(2, 4-
difluorophenyl)pyridine (F2,4 ppy) as main ligands have been re-
ported and their photophysical properties have been investigated.
A blue emitter of (F2,4ppy)2Ir(tfmptz) was obtained with the
maximum emission peaks at 457 and 481 nm. The conversion of
CeF to CeH bonds of 2-position at 2-phenylpyridine results in a red
shift of emission. The LUMO is mainly localized on fluorinated
2-phenylpyridine moieties, ancillary ligand with minor contribu-
tion of the d Ir(III) orbitals. The (F2,4ppy)2Ir(tfmptz)-based devices
have the superior performances with the current efficiency of
5.2 cd/A.
Acknowledgments
The EL spectra and representative performance data of Ir(III)
complexes are depicted in Fig. 6b and Table 2, respectively. The EL
spectra show a dominant emission from the Ir(III) complex
without any other residual emission from host or adjacent layers,
thus revealing effective energy transfer from the host to Ir
dopants. The CIE of Device 1 and 2 calculated from the EL spectra
at the voltage of 8 V are (0.17, 0.23) and (0.21, 0.39), respectively.
This work was financially supported by the International
Science and Technology Cooperation Program of China
(2012DFR50460); National Natural Scientific Foundation of China
(21071108, 60976018, 61274056, 61205179, 61605138); Natural
Science Foundation of Shanxi Province (2015021070); Program for
Changjiang Scholar and Innovation Research Team in University
(IRT0972); and Shanxi Provincial Key Innovative Research Team in
Science and Technology (201513002-10).
The
CIE
coordinates
of
the
device
incorporating
(F2,4ppy)2Ir(tfmptz) indicate purer and bluer emission than the
device doped with (F3,4,5ppy)2Ir(tfmptz). Additionally, the EL
spectra of two devices barely change and CIE values remain almost
constant.
Appendix A. Supplementary data
The J-V-L characteristics, efficiency-versus-current density
curves of two devices are shown in Fig. 7. The Device 1 exhibits a
Supplementary data related to this article can be found at http://
dx.doi.org/10.1016/j.tet.2016.11.012.

P. Sun et al. / Tetrahedron 72 (2016) 8335e8341
8341
2016;181:56.
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