Solution-processable high-efficiency bis(trifluoromethyl)phenyl functionalized phosphorescent neutral iridium(III) complex for greenish yellow electroluminescence

Article abstract of DOI:10.1016/j.tetlet.2018.03.073

A novel and highly efficient bis(trifluoromethyl)phenyl functionalized iridium(III) complex is designed and synthesized. The complex shows intensive greenish yellow phosphorescence (525 nm with 563 nm as shoulder), high photoluminescence efficiency (0.90) and moderate full width at half maximum (72 nm). The bulky bis(trifluoromethyl)phenyl moiety introduced into the complex provides the excellent solubility and effective steric hindrance for solution-processed organic light-emitting diodes. The maximum power efficiency and current efficiency of electroluminescence are 4.13 lm/W and 9.54 cd/A, respectively.

Full text of DOI:10.1016/j.tetlet.2018.03.073

Tetrahedron Letters xxx (2018) xxx–xxx
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Solution-processable high-efficiency bis(trifluoromethyl)phenyl
functionalized phosphorescent neutral iridium(III) complex for greenish
yellow electroluminescence
a
a
⇑
Hongxin Li , Peng Tao , Yanan Xu, Xinwen Zhang, Shujuan Liu, Qiang Zhao
Key Laboratory for Organic Electronics and Information Displays and Institute of Advanced Materials (IAM), Jiangsu National Synergetic Innovation Center for Advanced
Materials (SICAM), Nanjing University of Posts and Telecommunications (NUPT), Nanjing 210023, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
A novel and highly efficient bis(trifluoromethyl)phenyl functionalized iridium(III) complex is designed
and synthesized. The complex shows intensive greenish yellow phosphorescence (525 nm with 563
nm as shoulder), high photoluminescence efficiency (0.90) and moderate full width at half maximum
(72 nm). The bulky bis(trifluoromethyl)phenyl moiety introduced into the complex provides the excel-
lent solubility and effective steric hindrance for solution-processed organic light-emitting diodes. The
maximum power efficiency and current efficiency of electroluminescence are 4.13 lm/W and 9.54 cd/A,
respectively.
Received 11 February 2018
Revised 20 March 2018
Accepted 23 March 2018
Available online xxxx
Keywords:
Greenish yellow phosphorescence
Iridium(III) complexes
Organic light-emitting diodes
Steric hindrance
Ó 2018 Elsevier Ltd. All rights reserved.
Neutral iridium(III) complexes are considered as the excellent
candidates for the triplet emitters for phosphorescent organic
light-emitting diodes (OLEDs) among various kinds of phosphores-
cent metal complexes.1 The greenish yellow emission is the essen-
tial chromaticity component for solid-state lighting.2 The
incorporation of complementary greenish yellow color could effec-
tively simplify the device structures of the white electrolumines-
cence. Compared to the phosphors with three primary colors, the
developments of phosphors showing complementary greenish
yellow emission are less than that of the three primary colors.
The mostly used cyclometalated ligand for the design of greenish
yellow iridium(III) complexes is 2-aryl-benzothiazole.3–5 Liu et al.
reported benzothiazole-based iridium(III) complexes by incorpo-
rating main group elements.3 Li et al. reported a series of 2-aryl-
benzothiazole-based iridium(III) complexes with excellent device
performance.4 Recently, we also realized the highly efficient yellow
emission with extremely broad full width at half maximum
(FWHM) based on fluorine functionalized 2-phenylquinoline irid-
ium(III) phosphors.1c The above mentioned cyclometalated
2-phenylpyridine could be another choice for the realization of
greenish yellow emission. For instance, Wong et al. designed
and synthesized high-efficiency greenish yellow iridium(III)
complexes by using 2-phenylpyridine containing aromatic
sulfonyl group.6 Based on the above consideration, we select the
2-phenylpyridine derivative as the cyclometalated ligand for the
realization of solution-processable greenish yellow-emitting irid-
ium(III) phosphor.
In this letter, we report a novel bis(trifluoromethyl)phenyl func-
tionalized greenish yellow neutral iridium(III) complex Ir1 by
modifying the 2-phenylpyridine with bulky bis(trifluoromethyl)
phenyl moiety. The rigid bis(trifluoromethyl)phenyl moiety could
not only guarantee the excellent solubility but also give rise to
effective steric hindrance for eliminating phosphorescence
quenching. The 2-(3-(trifluoromethyl)-1H-1,2,4-triazol-5-yl)pyri-
dine was served as the ancillary ligand. The phosphor designed
shows bright greenish yellow emission with quite high photolumi-
nescence efficiency (UPL) and moderate FWHM suitable for solid-
state lighting. The power efficiency (PE) and current efficiency
(CE) of solution-processed greenish yellow OLEDs based on this
phosphor are 4.13 lm/W and 9.54 cd/A, respectively.
ligands with relatively large
p conjugation degrees often suffer
from the lower solubility in organic solvents. To meet the
requirements of solution-processed organic light-emitting diodes,
The bis(trifluoromethyl)phenyl functionalized 2-phenylpyri-
dine cyclometalated ligand was synthesized by two-step Suzuki
coupling reactions. The target complex was prepared as glassy yel-
low crystals in high yields by the facile method (Scheme 1) and
fully confirmed by nuclear magnetic resonance (NMR), mass
⇑
Corresponding author.
E-mail address: iamqzhao@njupt.edu.cn (Q. Zhao).
a
H. Li and P. Tao contributed equally to this work.
https://doi.org/10.1016/j.tetlet.2018.03.073
0040-4039/Ó 2018 Elsevier Ltd. All rights reserved.
Please cite this article in press as: Li H., et al. Tetrahedron Lett. (2018), https://doi.org/10.1016/j.tetlet.2018.03.073

2
H. Li et al. / Tetrahedron Letters xxx (2018) xxx–xxx
ORTEP of Ir1
Ir1
Scheme 1. Synthetic route, chemical structure and ORTEP plot of Ir1.
spectra and X-ray single crystal diffraction (Supporting Informa-
tion).2a In the fluorine NMR, three singlets, corresponding to the
three kinds of CF3 groups, originated from the symmetry loss
induced by the ancillary ligand. The fluorine NMR signals of the
two CF3 in bis(trifluoromethyl)phenyl moiety were exactly
overlapped, implying that the bulky bis(trifluoromethyl)phenyl
moiety could rotate freely in solution at room temperature.
Single crystals are easily formed from the mixture solvents of
methanol and chloroform. Table S1 summarized the
crystallographic refinement parameters of complex. Scheme 1
displayed ORTEP plot of the complex, clearly revealing the
symmetry loss in Ir1 as shown in fluorine NMR spectra. Iridium
atom adopted slightly distorted octahedral coordination
geometry with cis-metalated carbon and trans-nitrogen atoms,
similar to previously reported results.1d In crystals, two kinds of
partially enlarged p conjugation through the r bond connection
of bis(trifluoromethyl)phenyl ring and phenylpyridine, the other
is the blue-shift of triazole-type ancillary ligand. The UPL of the
complex is quite high up to 0.90, and the lifetime of the complex
is 7.29 ls in degassed CH2Cl2. The emission exhibited moderate
FWHM (72 nm), which is comparable with the reported greenish
yellow iridium(III) phosphors.3–5,8 We noted that the lifetime
(7.29 ls) of the complex is much longer than that of the commonly
iridium(III) phosphors in solution, which may have negative effect
on the performance of electroluminescence caused by the annihi-
lation of triplet excitons.9 Fig. 2a depicted the emission spectrum
at the temperature of 77 K. The spectrum with fine structures of
vibronic bands shows slightly blue-shift compared to that at room
temperature, the rigidochromic shift of complex is 9 nm. Then, the
triplet energy (T1) of complex was estimated to be 2.40 eV from the
highest-energy vibronic sub-band of 77 K emission spectrum.2a
The cyclic voltammetry in CH2Cl2 was carried out to investigate
the electrochemical property of complex. As depicted in Fig. 2b, the
complex shows reversible oxidation wave and the oxidation
potential was calculated to be 0.81 V. The Ir centered oxidation
should be responsible for this positive oxidation potential.1d The
energy levels of the highest occupied molecular orbital (HOMO)
and the lowest unoccupied molecular orbitals (LUMO) of
complex based on the cyclic voltammetry are estimated to be
ꢀ5.61 and ꢀ3.25 eV, respectively.
weak interactions could be observed (Fig. 1), one is,
p-p
interactions between phenylpyridine of adjacent complexes
beneficial for the phosphorescent emission in aggregation states
by the formation of triplet metal-to-ligand-ligand charge-transfer
transition, another is F-
p interactions between fluorine of bis
(trifluoromethyl)phenyl moiety and triazole ring.7 As shown in
Fig. S1, the distance between adjacent iridium centers in the crystal
of Ir1 (11.320 Å in Ir1) is significantly longer than that of model
complex (7.919 Å in model complex),1f supporting the effective
steric hindrance in Ir1.
The photophysical properties of complex were explored in
To evaluate the electroluminescence (EL) performance, the
greenish yellow phosphor was further used to prepare the solu-
tion-processed OLEDs. As depicted in Fig. S2, the greenish yellow
OLEDs were prepared with the structures of ITO/PEDOT:PSS (30
nm)/emission layer (40 nm)/TmPyPb (60 nm)/LiF (0.8 nm)/Al
(100 nm). The indium tin oxide (ITO) acts as the anode, and poly
(3,4-ethylenedioxythiophene) doped with poly(styrenesulfonate)
(PEDOT:PSS) acts as hole injection layer. The phosphor was doped
into a 1:4 wt mixture of hole-transporting host material 4,40,400-tris
(N-3-methylphenyl-N-phenylamino)triphenylamine (m-MTDATA)
and bipolar host material 2,6-bis(3-(9H-carbazol-9-yl)phenyl)pyr-
dichloromethane and exhibited in Fig. 2a Table 1. The 1p
?
p⁄
transition of cyclometalated ligand in complex corresponds to
the strong absorption band in the wavelength of 250–350 nm.
The weak absorption in the wavelength of 350–450 nm is probably
attributed to the triplet and singlet transitions from metal-to-
ligand charge-transfer.1d The complex shows intensive greenish
yellow emission at the wavelength of 525 nm with the 563 nm
shoulder emission in dichloromethane under the excitation of
395 nm. The emission energy of the complex is governed by the
two opposite effects, one is the red-shift of the emission by
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H. Li et al. / Tetrahedron Letters xxx (2018) xxx–xxx
3
a)
a)
1.0
1.0
0.8
0.6
0.4
0.2
0.0
298 K
77 K
0.8
0.6
0.4
0.2
0.0
@298 K
300
400
500
600
700
Wavelength (nm)
100
10
b)
1.5
1.0
b)
1
0.5
0.0
-0.5
-1.0
0.1
0.01
0.0
0.3
0.6
0.9
1.2
+
E/V (vs Fc/Fc
)
0
2000 4000
6000
8000
10000
τ (delaytime) / ns
Fig. 2. (a) UV–visible absorption, photoluminescence spectra of Ir1 at 298 K (in
degassed CH2Cl2) and at 77 K (in 2-MeTHF); (b) phosphorescent decay of Ir1 in
degassed CH2Cl2 at 298 K. Inset: luminescence photograph in CH2Cl2 (up) and cyclic
voltammogram under a scan rate of 100 mV/s in CH2Cl2 (down).
Fig. 1. (a) The intermolecular packing of Ir1 by
p-p interactions (a) and F-p
4.1–4.6 V and show intensive greenish yellow phosphorescence
with wavelength at 525 nm and 563 nm (shoulder). As depicted
in Fig. 4, the normalized ELspectra are overlapped quite well,
implying that the electroluminescence is independent of the dop-
ing concentrations and the phosphorescence originates from the
unimolecular iridium(III) complex owing to the bulky bis(trifluo-
romethyl)phenyl moiety of complexes. Notably, there exists the
evident difference between the EL and photoluminescence (PL)
spectra, as shown in Fig. S3, the intensity of the shoulder emission
of EL is distinctly lower than that of PL in both dichloromethane
and film (Ir1 doped m-MTDATA:26Dczppy). This shoulder emis-
sion loss in EL spectra probably originated from weak microcavity
effect of the fabricated OLEDs.11 The 1931 Commission Interna-
tionale de L’Eclairage (CIE) coordinates of (0.37, 0.60) are corre-
sponding to the greenish yellow region. The L-V-J curves of the
greenish yellow-emitting devices can be found in Fig. S4. As listed
interactions (b) between adjacent molecules in crystals with thermal ellipsoids
drawn at the 30% probability level and solvent molecules and H atoms removed for
clarity.
idine (26Dczppy) to form the emission layer. The doping concen-
trations of 4%, 6% and 10% in weight were selected to investigate
the concentration-dependent device performance respectively.
The energy levels and the illustration of device structure of the
greenish yellow OLEDs are depicted in Fig. S2. According to
the energy level of OLED (Fig. S2a), the HOMO/LUMO levels of
the phosphor are all within those of the m-MTDATA and 26Dczppy
host.10 Thus, the dominant EL mechanism should be attributed to
good carrier trapping in this host-guest system. The performance
of the devices were shown in Fig. 3 and Table 2. Under various dop-
ing concentrations, the turn-on voltages of these devices in the
Table 1
Photophysical and electrochemical properties for Ir1.
ox
b
c
d
Complex
Emissiona
kem [nm]
E
[eV]
Eg [eV]
T1 [eV]
HOMO/LUMO [eV]c
onset
FWHM [nm]
72
s
[
l
s]
UPL
Ir1
525, 563 (sh)
7.29
0.90
0.81
2.36
2.40
ꢀ5.61/ꢀ3.25
a
b
c
At a concentration of 1.0 ꢁ 10ꢀ5 mol/L in degassed CH2Cl2, kex = 395 nm.
In CH2Cl2.
ox
HOMO (eV) = ꢀe(E
+ 4.8), Eg = 1240/k, LUMO (eV) = Eg + HOMO.
onset
d
The triplet energy (T1) was estimated from the highest-energy vibronic sub-band of the phosphorescence spectrum at a concentration of 1.0 ꢁ 10ꢀ5 mol/L in 2-MeTHF 77
K, kex = 395 nm.
Please cite this article in press as: Li H., et al. Tetrahedron Lett. (2018), https://doi.org/10.1016/j.tetlet.2018.03.073

4
H. Li et al. / Tetrahedron Letters xxx (2018) xxx–xxx
in Table 2, the maximum luminance almost kept the same
(a)
(ꢂ11,000 cd/m2) in the doping concentrations from 4% to 10%.
Device with 4% doping concentration gives the best efficiencies,
and the CE and PE of greenish yellow OLED based on novel phos-
phor are 9.54 cd/A and 4.13 lm/W, respectively. The relatively
low electroluminescence efficiencies could be caused by the quite
10
8
4
6
10
6
long excited state lifetime (7.29 ls) of the phosphor. The long life-
time of the phosphor could easily lead to annihilation of triplet
excitons formed in the emission layer.9 These results obtained
demonstrated that the excited state lifetime of phosphor is another
important factor in determining the performance of light-emitting
devices.
To summarize, a novel phenylpyridine-based greenish yellow
neutral iridium(III) complex with bulky bis(trifluoromethyl)phe-
nyl moiety was designed and synthesized. Owing to the effective
steric hindrance originated from the bulky bis(trifluoromethyl)
phenyl moiety, the complex shows bright greenish yellow emis-
sion at wavelength of 525 nm and 563 nm (shoulder) with high
UPL up to 0.90, and the FWHM of complex is moderate (72 nm),
which is suitable for the solution-processed white electrolumi-
nescence. The relatively low efficiencies (9.54 cd/A) of this solu-
tion-processed greenish yellow device could be due to the quite
4
2
0
0
2500
5000
7500
10000
12500
Luminance (cd/m2)
(b)
4
3
2
1
0
4
6
10
long excite state lifetime (7.29 ls) of the complex. The further
works on exploring more suitable bis(trifluoromethyl)phenyl
functionalized phenylpyridine-based iridium(III) phosphors
toward high-efficiency electroluminescence are going on in our
laboratory.
Acknowledgments
0
2500
5000
7500
10000
12500
H. Li and P. Tao contributed equally to this work. The authors
acknowledge National Program for Support of Top-Notch Young
Professionals, Scientific and Technological Innovation Teams of
Colleges and Universities in Jiangsu Province (TJ215006), Priority
Academic Program Development of Jiangsu Higher Education Insti-
tutions (YX03001) for financial support.
Luminance (cd/m2)
Fig. 3. The CE-L curves (a), PE-L curves (b) of the greenish yellow-emitting devices.
Table 2
EL data for the greenish yellow-emitting devices.
X% kEL/nm
CIE(x, y)a
VONb/V Lmax
/
CEmax
/
PEmax
/
FWHM/ nm
cdꢃmꢀ2 cdꢃAꢀ1 lmꢃWꢀ1
A. Supplementary data
4
6
528, 564 (sh) (0.37, 0.60) 4.1
528, 564 (sh) (0.37, 0.60) 4.1
10 528, 564 (sh) (0.37, 0.60) 4.6
11,064 9.54
11,453 8.40
10,979 8.30
4.13
3.03
3.00
70
70
70
Supplementary data (details of the complex synthesis, NMR,
mass spectra, X-ray crystallography analysis and the device fabri-
cation and testing) associated with this article can be found, in
the online version, at https://doi.org/10.1016/j.tetlet.2018.03.073.
These data include MOL files and InChiKeys of the most important
compounds described in this article.
Current density is 100 mAꢃcmꢀ2
.
a
Luminance is 1 cdꢃmꢀ2
.
b
1.0
4
0.8
6
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