New yellow-emitting iridium(III) complexes containing 2-phenyl-2H-indazole-based ligands for high efficient OLEDs with EQE over 25%

Article abstract of DOI:10.1016/j.ica.2020.119704

Six new pidz-based bis-cyclometalated Ir(III) complexes (Ir1-Ir6) have been synthesized and characterized. These complexes contain 2-phenyl-2H-indazole (pidz, 2a), 2-(4-fluorophenyl)–2H-indazole (fpidz, 2b), 2-(p-tolyl)–2H-indazole (ch₃pidz, 2c), 2-(4-(trifluoromethyl)phenyl)–2H-indazole (cf₃pidz, 2d), 2-(2,4-difluorophenyl)–2H-indazole (2,4-f₂pidz, 2e) and 2-(3,5-difluorophenyl)–2H-indazole (3,5-f₂pidz, 2f) as cyclometalated (C^∧N) ligands, and acetylacetone (acacH) as ancillary ligand. The crystal structure of Ir2 has been determined by X-ray analysis. Different substituents of C^∧N ligand in Ir2-Ir6 induce either a bathochromic or hypsochromic shift in the absorption spectra relative to the parent complex Ir1. The phenomenon is further well explained by DFT calculation and electrochemical study. All of the iridium(III) complexes are yellow emissive with quantum yields of 13.1–32.3% and lifetimes of 1.32–1.77 μs in solution at room temperature. We demonstrate that their emission originates from a hybrid ³MLCT/³ILCT/³LLCT excited state on the basis of the experimental and theoretical investigation. The corresponding yellow-emitting devices based on complexes Ir1, Ir2 and Ir4 can produce efficient electrophosphorescence with a luminance efficiency of 35.1–52.2 cd·A^-1, a power efficiency of 20.8–32.1 lm·W^?1 and an external quantum efficiency of up to 25.6%. All these EL data definitely suggest the bulky –CF₃ skeleton in the doped materials could benefit the fabrication of high-efficiency phosphorescent OLEDs.

Full text of DOI:10.1016/j.ica.2020.119704

InorganicaChimicaActa509(2020)119704
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Research paper
New yellow-emitting iridium(III) complexes containing 2-phenyl-2H-
indazole-based ligands for high efficient OLEDs with EQE over 25%
Zhuo Liu, Rui-Lian Yang, Xiao-Han Yang, Jing-Lan Cao, Xu-Ru Fan, Shui-Xing Wu, Zhi-Gang Niu⁎,
Gao-Nan Li⁎
Key Laboratory of Electrochemical Energy Storage and Energy Conversion of Hainan Province, College of Chemistry and Chemical Engineering, Hainan Normal University,
Haikou 571158, China
A R T I C L E I N F O
A B S T R A C T
Keywords:
Six new pidz-based bis-cyclometalated Ir(III) complexes (Ir1-Ir6) have been synthesized and characterized.
These complexes contain 2-phenyl-2H-indazole (pidz, 2a), 2-(4-fluorophenyl)–2H-indazole (fpidz, 2b), 2-(p-
tolyl)–2H-indazole (ch3pidz, 2c), 2-(4-(trifluoromethyl)phenyl)–2H-indazole (cf3pidz, 2d), 2-(2,4-
difluorophenyl)–2H-indazole (2,4-f2pidz, 2e) and 2-(3,5-difluorophenyl)–2H-indazole (3,5-f2pidz, 2f) as cyclo-
metalated (C∧N) ligands, and acetylacetone (acacH) as ancillary ligand. The crystal structure of Ir2 has been
determined by X-ray analysis. Different substituents of C∧N ligand in Ir2-Ir6 induce either a bathochromic or
hypsochromic shift in the absorption spectra relative to the parent complex Ir1. The phenomenon is further well
explained by DFT calculation and electrochemical study. All of the iridium(III) complexes are yellow emissive
with quantum yields of 13.1–32.3% and lifetimes of 1.32–1.77 μs in solution at room temperature. We de-
monstrate that their emission originates from a hybrid 3MLCT/3ILCT/3LLCT excited state on the basis of the
experimental and theoretical investigation. The corresponding yellow-emitting devices based on complexes Ir1,
Ir2 and Ir4 can produce efficient electrophosphorescence with a luminance efficiency of 35.1–52.2 cd·A-1, a
power efficiency of 20.8–32.1 lm·W−1 and an external quantum efficiency of up to 25.6%. All these EL data
definitely suggest the bulky –CF3 skeleton in the doped materials could benefit the fabrication of high-efficiency
phosphorescent OLEDs.
Iridium (III) complexes
Yellow photoluminescence
Organic light-emitting diodes
DFT calculations
1. Introduction
benzoimidazole-based cyclometalated ligands have been extensively
studied [23–31], because this type of ligand framework can offer an
Cyclometalated iridium(III) complexes have been widely employed
to fabricate organic light emitting diodes (OLEDs) on account of their
relatively short phosphorescence lifetimes, high quantum efficiencies,
good thermal stability, and excellent color tunability [1–3]. In parti-
cular, yellow iridium(III)-based phosphorescent emitters play an im-
portant role in the full color display applications and the high-efficiency
two-color (light blue, yellow/orange) WOLEDs (white organic light
emitting diodes) [4–6]. To successfully achieve yellow emissions, the
cyclometalated ligands should possess a medium-sized π system, be-
cause smaller or larger π systems can result in blue or red emissions [7].
Therefore, researchers designed different sizes of hetero-atoms ligands,
including C,N-heterocyclic rings [8–11], N,N-heterocyclic rings
[12–17], S,N-heterocyclic rings [18–20] and O,N-heterocyclic rings
[8,21]. Among these, a kind of commonly used ligand is N,N-hetero-
cyclic compound, such as pyrazine [12], benzoimidazole [13–15],
imidazopyridine [16], pyrimidine [17], and phthalazine [22]. To date,
easy manipulation of donors, substituents and chain length for many
specific applications [32]. As the isomer of benzoimidazole, benzoin-
dazole should also has these similar advantages. Surprisingly, it appears
that cyclometalated Ir(III) complexes using the benzoindazole-based
ligands are rarely reported in the literature, especially the yellow-
emissive ones.
In 2018, our group firstly reported a series of novel 2-phenyl-2H-
indazole-based Ir(III) complexes, (h-1-pidz)2Ir(tpip), (f-1-pidz)2Ir(tpip)
and (cf3-1-pidz)2Ir(tpip), emitting in the yellow region with a peak at
556–576 nm [33]. Unfortunately, using complexes (f-1-pidz)2Ir(tpip)
and (cf3-1-pidz)2Ir(tpip) as dopants, both phosphorescent OLEDs ex-
hibited device performances with EQEmax values of only 9.0 and 10.7%.
To improve the efficiency of this class of complexes, in this work, we
further design three new pidz-based derivatives (2c, 2e and 2f) by in-
troducing methyl group and fluorine group to the parent ligand (2a). In
the meanwhile, we employ the other two reported ligands (2b and 2d)
⁎ Corresponding authors.
E-mail addresses: niuzhigang1982@126.com (Z.-G. Niu), ligaonan2008@163.com (G.-N. Li).
https://doi.org/10.1016/j.ica.2020.119704
Received 28 January 2020; Received in revised form 25 April 2020; Accepted 25 April 2020
Availableonline28April2020
0020-1693/©2020ElsevierB.V.Allrightsreserved.

Z. Liu, et al.
InorganicaChimicaActa509(2020)119704
Scheme 1. Synthetic routes of Ir(III) complexes Ir1-Ir6.
Fig. 1. Chemical structures of complexes Ir1-Ir6.
Fig. 2. (a) Perspective diagram of complex Ir2 with
thermal ellipsoids shown at the 50% probability
level. Hydrogen atoms and solvent molecules are
omitted for clarity; (b) Selected non-covalent
CeH···O contacts (dashed red lines). Atoms involved
in hydrogen bonds are shown as balls of arbitrary
radii. All other atoms and covalent bonds are re-
presented as wires or sticks. Symmetry codes: x, 1/2-
y, −1/2 + z; 1/2-x, 1-y, −0.5 + z.
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Z. Liu, et al.
InorganicaChimicaActa509(2020)119704
as the cyclometalated ligands and acetylacetone (acacH) as the ancil-
lary ligand to synthesize a series of iridium(III) complexes (Ir1-Ir6)
(Scheme 1). Their photophysical and electrochemical properties have
been studied systematically. The lowest energy electronic transitions
and the lowest triplet excited states have been calculated using density
functional theory (DFT) and time-dependent DFT (TD-DFT). All these Ir
(III) complexes produce the expected yellow emission in CH2Cl2 solu-
tion (Fig. 1). Cyclometalated iridium(III) complexes (Ir1, Ir2 and Ir4)
have been employed as emitters to construct yellow-emitting OLEDs
(D1, D2 and D4), which display good EL performance with an external
quantum efficiency of up to 25.6%.
patterns on cyclometalated ligands. For example, Ir2 (420 nm) and Ir4-
Ir6 (424 nm, 418 nm, 420 nm) exhibit a relatively blue-shifted ab-
sorption band than Ir1 (427 nm), which is due to the electron-with-
drawing effect exerted by the fluorine substituent. On the contrary, Ir3
(429 nm) with electron-donating –CH3 group gives rise to a slightly red-
shifted absorption band compared to Ir1 (427 nm).
All of the iridium complexes reported here are yellow-emitting in
solution at room temperature. Their emission spectra display structured
profiles comprising 3MLCT transitions with maxima at 554–570 nm,
and the vibronic progression with shoulder peaks at 588–608 nm
[35–36]. For the purpose of comparison, we take Ir1 (564 nm) as the
parent complex. Changing the substituents (-F, –CH3, -CF3) on cyclo-
metalated ligands in Ir2-Ir4 (554 nm, 558 nm, 570 nm) have a mod-
erate impact on the emission wavelength. However, complexes Ir5
(565 nm) and Ir6 (566 nm) exhibit nearly identical emission spectra,
suggesting that substitutions at the 2,4- and 3,5-positions have a
minimal influence on the luminescence. The different behavior in-
dicates that the substituents on the phenyl of C∧N ligands play an im-
portant role in positioning the emission profile.
Remarkably, the Φem of fluorinated complexes Ir2 and Ir4-Ir6 are
higher than that of un-fluorinated analogues Ir1 and Ir3 (Table 1). This
is dependent on the fact that the presence of the C − F bonds could lead
to the reduced radiationless deactivation rate in comparison with that
seen with C e H bonds [33,37]. Subsequently, their radiative decay rate
(kr) and the nonradiative decay rate (knr) were estimated from the Φem
and τ values. It is noteworthy that Ir4 bearing –CF3 group on the C∧N
ligands, has the largest kr and the smallest knr, resulting in the highest
quantum yield in the series.
2. Results and discussion
2.1. Structural description
The single crystal structure of Ir2 was obtained through X-ray dif-
fraction analysis, and the ORTEP diagram is shown in Fig. 2a. The
crystallographic data and structure refinement details are listed in
Table S1, with selected bond lengths and bond angles collected in Table
S2. Single crystal structure of Ir2 was deposited to Cambridge Crys-
tallographic Data Centre (CCDC) with deposition number of 1967686.
As shown in Fig. 2a, the iridium metal center adopts a distorted
octahedral coordination geometry with the C and N atoms of the cy-
clometalated ligands and the two O atoms of the ancillary ligand. The
nitrogen atoms in the cyclometalated ligands are trans conformation
with respect to one another, while the two carbon atoms are cis dis-
positions. All of the Ir-C (1.963(8)
− 2.036(8)Å), Ir − N
(2.040(7) − 2.053(7) Å) and Ir − O (2.128(6) − 2.136(5) Å) bond
lengths are close to those in previously reported iridium complexes
[34]. The bite angle of O1–Ir1–O2 (86.6(2)°) in a six-membered chelate
ring is much larger than that of C13–Ir1–N1 (80.0(3)°) or C26–Ir1–N3
(80.0(3)°) in a five-membered chelate ring. In addition, the adjacent
non-covalent contacts in the crystal structure are presented in Fig. 2b,
including intramolecular hydrogen bond (C2–H2···O2, 2.48 Å) and in-
termolecular hydrogen bond (C9–H9···O2, 2.42 Å; C20–H20···O1,
2.57 Å).
2.3. Theoretical calculations
In order to further understand the lowest-energy electronic transi-
tions, density functional theory (DFT) and time-dependent DFT
(TDDFT) calculations were performed on Ir1-Ir6. The calculated energy
levels and the most representative molecular frontier orbital diagrams
for these complexes are presented in Fig. 4 and Fig. S1, respectively.
The calculated spin-allowed electronic transitions are provided in
Table 2. The electron density distributions are summarized in Table S3.
For all the complexes, the calculations clearly show that the HOMO
is located more on the iridium center (47.92 ~ 51.68%) and the phenyl
ring (41.21 ~ 45.88%) from C∧N ligands. The LUMO and LUMO + 1
are very similar, dominated by the whole C∧N ligands (96.18 ~ 96.97%
and 96.20 ~ 96.83%). Hence, the low-energy absorption transitions
(HOMO → LUMO, HOMO → LUMO + 1) of Ir1-Ir6 can be ascribed as
the 1MLCT transition from the d orbital of iridium to π* orbital of the
C^N ligand and the 1ILCT transition from π → π* orbital of the C^N
ligand, supporting the experimental assignment of the absorption tails.
Upon the introduction of electron-donating methyl group on the C^N
ligand from Ir1 to Ir3, the HOMO and LUMO are destabilized by 0.058
and 0.042 eV. In contrast, fluorinating the pidz ligand from Ir1 to Ir2,
the HOMO and LUMO are stabilized by 0.154 and 0.032 eV. Changing
2.2. Photophysical properties
The UV–vis absorption and photoluminescence (PL) emission
spectra of all complexes in CH2Cl2 solution at room temperature are
depicted in Fig. 3, and the corresponding photophysical data are pro-
vided in Table 1. The strong absorption bands in the range of
220–320 nm match those of the free ligands (Fig. S3), and thus can be
attributed to the spin-allowed ligand-centered (1LC) π → π* transitions.
The weaker absorption bands at ca. 320–500 nm are ascribed to a
mixture of metal to ligand (Ir to C∧N) charge transfer (1MLCT) and
intraligand (C∧N to C∧N) charge transfer (1ILCT) transitions on the basis
of TD-DFT analysis. With the same ancillary ligand, the UV–vis ab-
sorption spectra of these complexes are perturbed by the substituent
Fig. 3. UV–vis absorption (a) and PL emission (b) spectra of complexes Ir1-Ir6 in CH2Cl2 solution at room temperature.
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InorganicaChimicaActa509(2020)119704
Table 1
Photophysical data of complexes Ir1-Ir6.
Complex
Absorptionaλabs (nm)ɛ (104 cm−1·M−1
)
Emission
λema(nm)
Φemb(%)
τa(μs)
krc(105 s−1
)
knrc(105 s−1
)
Ir1
Ir2
Ir3
Ir4
Ir5
Ir6
251(2.20), 304(1.58), 315(1.47), 355(0.57), 427(0.38)
564, 597(sh)
554, 588(sh)
558, 593(sh)
570, 608(sh)
565, 603(sh)
566, 605(sh)
40.8
58.6
33.1
74.2
64.7
68.8
1.57
1.57
1.32
1.77
1.68
1.69
2.60
3.73
2.51
4.19
3.85
4.07
3.77
2.64
5.07
1.46
2.10
1.85
248(2.32), 303(1.86), 311(1.86), 348(0.89), 387(0.69), 420(0.31)
255(2.23), 309(1.85), 319(1.79), 353(0.84), 394(0.60), 429(0.35)
246(2.86), 303(1.98), 314(1.92), 358(0.74), 395(0.63), 424(0.37)
241(3.53), 302(2.49), 313(2.45), 349(1.18), 386(0.86), 418(0.43)
241(3.48), 303(2.54), 312(2.50), 353(1.23), 389(0.92), 420(0.37)
a
Data were collected from degassed CH2Cl2 solutions at 298 K. b Absolute emission quantum yields measured in degassed CH2Cl2 solution. c Radiative decay rate
(kr) and nonradiative decay rate (knr) were estimated from the measured quantum yields and lifetimes.
the –F to –CF3, 2,4-F2 and 3,5-F2 substituents, the HOMOs of Ir4-Ir6 are
further lowered with similar values of −5.387 ~ −5.549 eV. However,
the energetic difference between the LUMO orbitals of Ir4-Ir6 is very
large, from −1.709 to −1.866 eV. In the case of the energy gap, the
order is Ir3 < Ir1 < Ir6 < Ir4 < Ir2 < Ir5, correlating well with
the experimental low-energy absorption bands. The finding indicates
the electron-donating group (–CH3) can shrink the energy gap, while
the electron-withdrawing group can enlarge it, especially 2,4-F2 sub-
stituents.
To understand the nature of the transitions in the emission profile,
the TD-DFT calculations were also carried out to study their lowest
triplet excited state (T1) characters, and the relevant information is
presented in Table 2. For complexes Ir1-Ir6, their T1 are ascribed to
spin–orbit-induced mixings of several transitions, but with major con-
tributions from HOMO-1 → LUMO + 1 (Ir1-Ir3, Ir5 and Ir6) and
HOMO-1 → LUMO (Ir4). Base on the electron density distribution in-
formation in Table S3, HOMO-1 are located on Ir d-orbital, C∧N ligands
and ancillary ligands, while LUMO and LUMO + 1 are largely domi-
nated on C∧N ligands. Consequently, their (T1) states have
3MLCT/3ILCT/3LLCT characteristics, which is in line with the vi-
bronically structured bands observed in the emission spectra.
In order to gain the HOMO energy levels, we hence focused on the
first oxidation waves of Ir1-Ir6. From Table 3, the onset oxidation
potentials (Eonsetox) of these iridium complexes are 0.67–0.80 V (vs Fc/
Fc+). Three complexes exhibit quasi-reversible oxidation processes
(Ir2, ΔEp = 103 mV, ipc/ipa = 0.55; Ir5, ΔEp = 106 mV, ipc/ipa = 0.17;
Ir6, ΔEp = 122 mV, ipc/ipa = 1.73). Apparently, with the electron-
withdrawing fluorine-substituted ligands, Ir2 and Ir4-Ir6 show more
positive oxidation potentials relative to Ir1, resulting in the stabiliza-
tion of the HOMO orbitals. Nevertheless, Ir3 consisting of the electron-
donating –CH3 ligands displays the opposite situation, which in turn
leads to much higher HOMO energy levels. According to the equation of
ox
HOMO = − [Eonset + 4.8] eV [39], the HOMO energy levels are
deduced to be −5.47 ~ −5.60 eV. The calculated results reveal the
above order is Ir2, Ir4-Ir6 < Ir1 < Ir3, which was consistent with
the DFT-calculated values. Therefore, these electrochemical features
provide evidence that the chemical modification of cyclometalated li-
gands affects the energy levels of HOMO. Additionally, the energy le-
vels of the LUMO are obtained from the HOMO and Eopt,g values. From
Table 3, it can be observed that the experimental LUMO energy levels
are lower than the theoretical ones, but they have the same order, that
is Ir4 < Ir6 < Ir5 < Ir2 < Ir1 < Ir3.
2.4. Electrochemical properties
2.5. Electroluminescence devices
The electrochemical properties of all the complexes were measured
by cyclic voltammetry in CH2Cl2 solution (0.1 M n-Bu4NClO4) and the
electrochemical waves are given in Fig. 5. On the basis of their oxida-
tion potentials and absorption spectra, the HOMO and LUMO energy
levels are estimated and included in Table 3, as well as compared with
the theoretical data. All complexes exhibit complicated quasi-reversible
or irreversible oxidation processes, which is presumably caused by the
reaction or decay products resulting from multiple oxidation processes
centered on iridium and cyclometalated ligands [38]. However, no re-
duction process due to the ligands was observed within the solvent
cathodic potential limit.
To illustrate electroluminescence (EL) properties of Ir1, Ir2 and Ir4
as the phosphorescent emitter, the corresponding monochrome devices
D1, D2 and D4 were fabricated with the following configurations via
vacuum thermal evaporation: ITO/MoO3 (5 nm) / TAPC (30 nm) / 2,6-
DCzPPy : Ir complex (8 wt%, 10 nm) / TmPyPB (40 nm) / LiF (1 nm) /
Al (100 nm). The schematic energy level diagrams of electropho-
sphorescent devices and the molecular structures of materials used in
these devices are depicted in Fig. 6. MoO3 acts as a hole-injection layer
(HIL), TPAC [4,4′-(cyclohexane-1,1-diyl)bis(N,N-di-p-tolylaniline)] as a
hole-transport layer (HTL), TmPyPb [1,3,5-tri[(3-pyridyl)-phen-3-yl]
benzene] as an electron-transport layer (ETL), and LiF as an electron-
Fig. 4. Calculated energy levels and energy gaps for complexes Ir1-Ir6. The molecular orbital diagrams of complex Ir1 are plotted as representative.
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InorganicaChimicaActa509(2020)119704
Table 2
Contributions, oscillator strengths and transition characters of singlet and triplet transitions for complexes Ir1-Ir6.
Complex
Ir1
Major assignments
Transition
Character
Oscillation Strength
S1
T1
S1
T1
S1
T1
S1
T1
S1
T1
S1
T1
HOMO → LUMO + 1
HOMO-1 → LUMO + 1
HOMO → LUMO + 1
HOMO-1 → LUMO + 1
HOMO → LUMO + 1
HOMO-1 → LUMO + 1
HOMO → LUMO
1MLCT/1ILCT
dπIr/pidz → π*pidz
0.1048
0
0.1104
0
0.1130
0
0.0946
0
0.0855
0
3MLCT/3ILCT/3LLCT
1MLCT/1ILCT
dπIr/pidz/acac → π*pidz
dπIr/fpidz → π*fpidz
Ir2
3MLCT/3ILCT/3LLCT
1MLCT/1ILCT
dπIr/fpidz/acac → π*fpidz
dπIr/ch3pidz → π*ch3pidz
Ir3
3MLCT/3ILCT/3LLCT
1MLCT/1ILCT
dπIr/ch3pidz/acac → π*ch3pidz
dπIr/cf3pidz → π*cf3pidz
Ir4
HOMO-1 → LUMO
3MLCT/3ILCT/3LLCT
1MLCT/1ILCT
dπIr/cf3pidz/acac → π*cf3pidz
dπIr/2,4-f2pidz → π*2,4-f2pidz
dπIr/2,4-f2pidz/acac → π*2,4-f2pidz
dπIr/3,5-f2pidz → π*3,5-f2pidz
dπIr/3,5-f2pidz/acac → π*3,5-f2pidz
Ir5
HOMO → LUMO + 1
HOMO-1 → LUMO + 1
HOMO → LUMO + 1
HOMO-1 → LUMO + 1
3MLCT/3ILCT/3LLCT
1MLCT/1ILCT
Ir6
0.0915
0
3MLCT/3ILCT/3LLCT
injection layer (EIL). In addition, 2,6-DCzPPy layers doped with Ir(III)
complexes act as the emissive layer (EML).
unsubstituted or substituted with electron-donating groups (–CH3)/
electron-withdrawing groups (-F, -CF3, 2,4-F2 and 3,5-F2). The influ-
ence of these substituents on the structure–property relationships of
iridium complexes was systematically investigated through photo-
physical and electrochemical studies, as well as theoretical calculations.
The yellow OLEDs by using the complexes Ir1, Ir2 and Ir4 as triplet
emitters achieved good device performance with the external quantum
efficiencies of 17.3–25.6%. This work not only can represent a new
approach to develop high-efficient pidz-based iridium(III) complexes,
but also provide some important information in designing two-color
(light blue, yellow/orange) WOLEDs.
The EL spectra, current density–luminance–voltage (J-L-V), current
efficiency–luminance (ηc-L) and external quantum efficiency–lumi-
nance (EQE-L) characteristics of devices (8 wt% doping level) are
shown in Fig. 7. The key device performance data are summarized in
Table 4. At a driving voltage of 8 V, the normalized EL spectra of these
devices D1, D2 and D4 show the maximum emission peak at
556–570 nm, together with a shoulder peak at 590–610 nm (Fig. 7a).
The Commission Internationale de L’Eclairage (CIE) color coordinates
are (0.47, 0.49) for D1, (0.47, 0.47) for D2 and (0.47, 0.47) for D4. All
of these values correspond to the yellow region of the CIE chromaticity
diagram. In each case, the close resemblance between the EL and PL
spectra indicates that the triplet excitons of the phosphors are the same
type and the energy can be transferred from 2,6-DCzPPy to the emitters
in the EML. For all devices, the turn on voltages (Vturn-on) are 3.9–4.0 V
4. Experimental
4.1. General
and the maximum luminance (Lmax
)
are 8508–15295 cd·m−2 at
All reactions were performed under nitrogen atmosphere. All of the
reagents were purchased from commercial sources and used without
further purification, unless stated otherwise. 2-Phenyl-2H-indazole
(pidz, 2a), 2-(4-fluorophenyl)–2H-indazole (fpidz, 2b) and 2-(4-(tri-
fluoromethyl)phenyl)–2H-indazole (cf3pidz, 2d) were synthesized ac-
cording to our previous paper [34]. 1H NMR spectra were recorded on a
Bruker AM 400 MHz instrument, and chemical shifts were reported in
ppm relative to Me4Si as internal standard. MALDI-TOF-MS spectra
were recorded on a Bruker Autoflex IITM TOF/TOF instrument. The
elemental analyses were measured on a Vario EL Cube Analyzer system.
UV–vis spectra were recorded on a Hitachi U3900/3900H spectro-
photometer. Fluorescence spectra were carried out on a Hitachi F-7000
spectrophotometer in degassed CH2Cl2 solutions at room temperature
and corrected for the detector response. Luminescence lifetimes were
measured on a HORIBA FL-3 fluorescence spectrometer and the data
were treated as one-order exponential fitting using OriginPro 8 soft-
ware. The absolute photoluminescence quantum yield (Φem) was mea-
sured on a HORIBA FL-3 fluorescence spectrometer equipped with an
integrating sphere. Radiative decay constants (kr) were calculated using
12.6–13.9 V (Fig. 7b). In the series, device D4 exhibits the best device
performance with a luminance efficiency (ηc,max) of 52.2 cd·A-1, a
power efficiency (ηp,max) of 32.1 lm·W−1 and a maximum external
quantum efficiency (EQEmax) of 25.6%. The two other devices exhibit a
ηc,max of 35.1 cd·A-1, a ηp,max of 20.8 lm·W−1 and EQEmax of 17.3% for
D1, and a ηc,max of 40.3 cd·A-1, a ηp,max of 26.9 lm·W−1 and EQEmax of
19.8% for D2 (Fig. 7c-d). These results reveal that the overall efficiency
parameters are D4 > D2 > D1, which may be attributed to a higher
PL triplet yield of the former as compared to the latter series [40].
Moreover, it should be highlighted that the bulky –CF3 groups of dopant
Ir4 in the EML can restrain the self-quenching of luminescence, thus
improving device performance.
3. Conclusion
A novel series of bis-cyclometalated pidz-based iridium complexes
along with acacH as the ancillary ligand were synthesized and char-
acterized. In the cyclometalated ligands, the phenyl ring was either
Fig. 5. Cyclic voltammograms for complexes Ir1-Ir6 in CH2Cl2 solution (0.1 M n-Bu4NClO4). The scan rate was 100 mV·s−1 and ferrocene was used as a reference.
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InorganicaChimicaActa509(2020)119704
Table 3
Electrochemical and theoretical data of complexes Ir1-Ir6.
ox
c
Complex
Eonset a(V)
ΔEpb(mV)
ipc/ipa
Eopt,gd(eV)
EHOMO/ELUMOe(eV)
EHOMO/ELUMOf(eV)
g
g
Ir1
Ir2
Ir3
Ir4
Ir5
Ir6
0.69
0.74
0.67
0.77
0.80
0.79
-
-
2.90
2.95
2.89
2.92
2.97
2.95
−5.49/−2.59
−5.54/−2.59
−5.47/−2.58
−5.57/−2.65
−5.60/−2.63
−5.59/−2.64
−5.23/−1.64
−5.38/−1.67
−5.17/−1.60
−5.49/−1.87
−5.55/−1.71
−5.39/−1.78
103
0.55
g
g
-
-
-
-
g
g
106
122
0.17
1.73
a
b
c
d
The first oxidation processes, and values are reported to Fc/Fc+
.
The peak separation. The peak current ratios. Calculated from the UV–vis absorption edges
e
ox
f
(Eopt,g = 1240/λonset). Deduced from the equation EHOMO = -[Eonset + 4.8] eV and LUMO = HOMO + Eopt,g, respectively. Obtained from theoretical calcu-
lations. Irreversible.
g
the equation Φem / τ, where τ is the excited-state lifetime of the sample,
and subsequently, nonradiative decay constants (knr) were calculated
using the relationship knr = (1 - Φem) / τ [41].
4.5. OLED fabrication and measurement
All OLEDs were fabricated on the pre-patterned ITO-coated glass
substrate with a sheet resistance of 10 Ω·sq-1. The deposition rate for
organic compounds is 1–2 Å·s−1. The phosphor and the host 2,6DCzPPy
were co-evaporated to form emitting layer 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−1 and then by evaporation of Al metal
with a rate of 3 Å·s−1. The characteristic curves of the devices were
measured with a computer which controlled KEITHLEY 2400 source
meter with a calibrated silicon diode in air without device encapsula-
tion. On the basis of the uncorrected PL and EL spectra, the Commission
Internationale de l’Eclairage (CIE) coordinates were calculated using a
test program of the Spectra scan PR650 spectrophotometer.
4.2. X-ray crystallography
X-ray diffraction data were collected with an Agilent Technologies
Gemini A Ultra diffractometer equipped with graphite-monochromated
Mo-Kα radiation (λ = 0.7107 Å) at room temperature. Data collection
and reduction were processed with CrysAlisPro software [42]. The
structure was solved using Superflip [43] and refined using
SHELXL − 2014 [44] within Olex2 [45].
4.6. Synthesis
4.3. Theoretical calculations
4.6.1. 2-(p-tolyl)–2H-indazole (ch3pidz, 2c)
All calculations were carried out with Gaussian 09 software package
[46]. The density functional theory (DFT) and time-dependent DFT
(TDDFT) were employed with no symmetry constraints to investigate
the optimized geometries and electron configurations with the Becke
three-parameter Lee-Yang-Parr (B3LYP) hybrid density functional
theory [47–49].
Aminotoluene (500 mg, 4.67 mmol) and 2-nitrobenzaldehyde
(715 mg, 4.67 mmol) were added in EtOH (15 mL) and refluxed for 5 h.
After the solvent was removed in vacuo, the obtained yellow compound
1c was dissolved in P(OEt)3 (10 mL) and heated at 110 °C for 8 h. The
solvent was removed, and the resulting residue was purified by column
chromatography on silica gel, eluted with EtOAc / hexanes (1:100, v/v)
to give compound 2c (585 mg, yield: 60.2%) as white solid. 1H NMR
(400 MHz, CDCl3) δ 8.37 (s, 1H), 7.74 ~ 7.92 (m, 3H), 7.70 (dt,
J = 8.4, 0.8 Hz, 1H), 7.28 ~ 7.38 (m, 3H), 7.11 (ddd, J = 8.4, 6.4,
0.8 Hz, 1H), 2.42 (s, 3H).
4.4. Electrochemistry
Cyclic voltammetry (CV) was performed on a CHI 1210B electro-
chemical workstation, with a glassy carbon electrode as the working
electrode, a platinum wire as the counter electrode, an Ag/AgCl elec-
trode as the reference electrode, and 0.1 M n-Bu4NClO4 as the sup-
porting electrolyte. The redox potentials are reported relative to an
external ferrocene/ferrocenium (Fc/Fc+) reference.
Compounds 2e and 2f were obtained in good yields by following the
procedure described for 2c.
4.6.2. 2-(2,4-difluorophenyl)–2H-indazole (2,4-f2pidz, 2e)
1H NMR (400 MHz, CDCl3) δ 8.37 (s, 1H), 7.75 (d, J = 8.6 Hz, 1H),
Fig. 6. Energy level diagram of phosphorescent OLEDs and their molecular structures of used materials.
6

Z. Liu, et al.
InorganicaChimicaActa509(2020)119704
Fig. 7. Characteristics of devices D1, D2 and D4: (a) normalized EL spectra at 8 V; (b) luminance–voltage–current density curves; (c) current efficiency–luminance
curves; (d) external quantum efficiency–luminance curves.
Table 4
EL performances of single-EML devices D1, D2 and D4.
Device
Emitter
Vturn-ona(V)
Lmaxb[cd·m2(V)]
ηc,maxc(cd A1)
ηp,maxc(lm W1)
EQEmaxc(%)
ELd(nm)
CIEe(x, y)
D1
D2
D4
Ir1
Ir2
Ir4
3.9
4.0
3.9
8508(12.6)
9916(13.4)
15295(13.9)
35.1
40.3
52.2
20.8
26.9
32.1
17.3
19.8
25.6
562, 600(sh)
556, 590(sh)
570, 610(sh)
(0.47, 0.49)
(0.47, 0.47)
(0.47, 0.47)
a
b
c
Turn-on voltage recorded at a luminance of 1 cd·m−2
.
Maximum luminance. Data at maximum and 1000 cd⋅m−2 for current efficiency (ηc,max), power
d
e
efficiency (ηp,max) and EQEmax, respectively. Values were collected at 8 V. CIE (Commission Internationale de l'Eclairage) coordinates at 8 V.
7.68 (d, J = 8.5 Hz, 1H), 7.46 ~ 7.59 (m, 2H), 7.33 ~ 7.40 (m, 1H),
7.12 (dd, J = 8.3, 6.8 Hz, 1H), 6.84 (tt, J = 8.6, 2.2 Hz, 1H).
(400 MHz, CDCl3) δ 8.52 (2 s, 2H), 7.75 (d, J = 8.4 Hz, 2H), 7.61 (d,
J = 8.8 Hz, 2H), 7.33 ~ 7.39 (m, 4H), 7.24 (t, J = 6.8 Hz, 2H), 6.58
(dt, J1 = 2.8 Hz, J2 = 8.4 Hz, 2H), 5.74 (dd, J1 = 2.8 Hz, J2 = 9.2 Hz,
2H), 5.28 (s, 1H), 1.81 (s, 6H). MALDI-TOF calcd for C31H23F2IrN4O2:
714.142 ([M + H]+), found: 714.231, 613.025 (M−acac). Anal. calcd
for C31H23F2IrN4O2: C, 52.17; H, 3.25; N, 7.85%; Found: C, 52.18; H,
3.27; N, 7.86%.
4.6.3. 2-(3,5-difluorophenyl)–2H-indazole (3,5-f2pidz, 2f)
1H NMR (400 MHz, CDCl3) δ 8.90 (s, 1H), 8.25 (d, J = 7.8 Hz, 1H),
8.10 (d, J = 8.0 Hz, 1H), 7.76 (t, J = 7.5 Hz, 1H), 7.66 (t, J = 7.8 Hz,
1H), 6.67 ~ 6.85 (m, 3H).
(ch3pidz)2Ir(acac) (Ir3) (72 mg, 43.6%), yellow solid. 1H NMR
(400 MHz, CDCl3) δ 8.54 (s, 2H), 7.75 (d, J = 8.4 Hz, 2H), 7.64 (d,
J = 8.7 Hz, 2H), 7.27 ~ 7.35 (m, 4H), 7.15 ~ 7.25 (m, 2H), 6.66 (dd,
J = 8.0, 1.2 Hz, 2H), 5.94 (d, J = 1.0 Hz, 2H), 1.95 (s, 6H), 1.78 (s,
6H). MALDI-TOF calcd for C33H29IrN4O2: 706.192 ([M + H]+), found:
706.328, 605.141 (M−acac). Anal. calcd for C33H29IrN4O2: C, 56.15;
H, 4.14; N, 7.94%; Found: C, 56.17; H, 4.13; N, 7.97%.
5. General procedure for the Ir(III) complexes Ir1-Ir6.
The cyclometalated Ir(III) dichloro-bridged dimmers [(C^N)2Ir(μ-
Cl)]2 were synthesized according to the method reported by Nonoyama
[50]. And then the complexes Ir1-Ir6 were prepared by the reaction of
Ir(III) dichloro-bridged dimmers, 8.0 equiv of acetylacetone (acacH)
and 5.0 equiv of sodium carbonate in 2-ethoxyethanol at 120 °C. After
the reaction was completed, the mixture was diluted with water and
extracted multiple times with DCM. The combined organic phase was
concentrated and the residue was purified by column chromatography
to provide the desired complexes Ir1-Ir6.
(cf3pidz)2Ir(acac) (Ir4) (62 mg, 40.6%), yellow solid. 1H NMR
(400 MHz, CDCl3) δ 8.55 (2 s, 2H), 7.76 (d, J = 8.8 Hz, 2H), 7.61 (d,
J = 8.8 Hz, 2H), 7.34 ~ 7.42 (m, 4H), 7.24 ~ 7.28 (m, 2H), 6.75 (dd,
J1 = 1.2 Hz, J2 = 8.8 Hz, 2H), 5.84 (d, J = 1.2 Hz, 2H), 5.27 (s, 1H),
1.81 (s, 6H). MALDI-TOF calcd for
([M + H]+), found: 814.316, 713.177 (M−acac). Anal. calcd for
33H23F6IrN4O2: C, 48.71; H, 2.85; N, 6.88%; Found: C, 48.69; H, 2.86;
N, 6.86%.
(2,4-f2pidz)2Ir(acac) (Ir5) (69 mg, 42.9%), yellow solid. 1H NMR
C33H23F6IrN4O2: 814.135
(pidz)2Ir(acac) (Ir1) (76 mg, 45.3%), yellow solid. 1H NMR
(400 MHz, CDCl3) δ 8.57 (2 s, 2H), 7.75 (d, J = 8.4 Hz, 2H), 7.67 (dd,
J1 = 0.8 Hz J2 = 8.8 Hz, 2H), 7.41 (dd, J1 = 0.8 Hz J2 = 8.0 Hz, 2H),
7.30 ~ 7.35 (m, 2H), 7.22 ~ 7.25 (m, 2H), 6.84 ~ 7.88 (m, 2H), 6.59
(dt, J1 = 1.2 Hz, J2 = 7.6 Hz, 2H), 6.13 (dd, J1 = 1.2 Hz, J2 = 7.6 Hz,
2H), 5.26 (s, 1H), 1.94 (s, 6H). MALDI-TOF calcd for C31H25IrN4O2:
678.161 ([M + H]+), found: 678.300, 577.070 (M−acac). Anal. calcd
for C31H25IrN4O2: C, 54.94; H, 3.72; N, 8.27%; Found: C, 54.95; H,
3.73; N, 8.28%.
C
(400 MHz, CDCl3) δ 8.55 (s, 2H), 7.77 (d, J = 8.4 Hz, 2H), 7.61 (d,
J = 8.8 Hz, 2H), 7.41 (d, J = 8.8 Hz, 2H), 7.34 ~ 7.38 (m, 2H), 6.75
(dd, J = 8.4, 1.6 Hz, 2H), 5.84 (d, J = 1.6 Hz, 2H), 5.27 (s, 1H), 1.81
(s, 2H). MALDI-TOF calcd for C31H21F4IrN4O2: 750.123 ([M + H]+),
found: 750.251, 649.024 (M−acac). Anal. calcd for C31H21F4IrN4O2: C,
49.66; H, 2.82; N, 7.47%; Found: C, 49.67; H, 2.81; N, 7.48%.
(fpidz)2Ir(acac) (Ir2) (65 mg, 38.3%), yellow solid. 1H NMR
7

Z. Liu, et al.
InorganicaChimicaActa509(2020)119704
(3,5-f2pidz)2Ir(acac) (Ir6) (78 mg, 48.1%), yellow solid. 1H NMR
11 (2010) 1511–1515.
[12] G. Ge, J. He, H. Guo, F. Wang, D. Zou, J. Organomet. Chem. 694 (2009) 3050–3057.
[13] E. Baranoff, S. Fantacci, F. Angelis, X. Zhang, R. Scopelliti, M. Grätzel,
M.K. Nazeeruddin, Inorg. Chem. 50 (2011) 451–462.
[14] G. Zhang, F.-I. Wu, X. Jiang, P. Sun, C.-H. Cheng, Synth. Met. 160 (2010)
1906–1911.
[15] S. Wang, Y. Duan, C.-X. Du, Y.-J. Wu, Inorg. Chem. Commun. 14 (2011)
1393–1395.
[16] S. Takizawa, J. Nishida, T. Tsuzuki, S. Tokito, Y. Yamashita, Inorg. Chem. 46 (2007)
(400 MHz, CDCl3) δ 8.90 (s, 2H), 7.79 (d, J = 8.5 Hz, 2H), 7.59 (d,
J = 8.8 Hz, 2H), 7.38 (t, J = 7.7 Hz, 2H), 7.24 ~ 7.28 (m, 2H),
6.46 ~ 6.51 (m, 2H), 5.54 (dd, J = 8.4, 2.2 Hz, 2H), 5.28 (s, 1H), 1.82
(s, 6H). MALDI-TOF calcd for C31H21F4IrN4O2: 750.123 ([M + H]+),
found: 750.246, 649.032 (M−acac). Anal. calcd for C31H21F4IrN4O2: C,
49.66; H, 2.82; N, 7.47%; Found: C, 49.69; H, 2.80; N, 7.49%.
4308–4319.
[17] Z.-Q. Wang, C. Xu, X.-M. Dong, Y.-P. Zhang, X.-Q. Hao, J.-F. Gong, M.-P. Song, B.-
M. Ji, Inorg. Chem. Commun. 14 (2011) 316–319.
CRediT authorship contribution statement
[18] M. Xu, R. Zhou, G. Wang, J. Yu, Inorg. Chim. Acta. 362 (2009) 515–518.
[19] Y.J. Kim, Y.H. Son, J.H. Kwon, J. Inf. Disp. 14 (2013) 109–113.
[20] Y.H. Son, Y.J. Kim, M.J. Park, H.Y. Oh, J.S. Park, J.-H. Yang, M.C. Suh, J.H. Kwon,
J. Mater. Chem. C 1 (2013) 5008–5014.
[21] T.-R. Chen, J. Organomet. Chem. 693 (2008) 3117–3130.
[22] B. Tong, Q. Mei, S. Wang, Y. Fang, Y. Meng, B. Wang, J. Mater. Chem. 18 (2007)
1636–1639.
[23] X. Shen, H. Huo, C. Wang, B. Zhang, K. Harms, E. Meggers, Chem. Eur. J. 21 (2015)
9720–9726.
[24] H. Huo, C. Fu, K. Harms, E. Meggers, J. Am. Chem. Soc. 136 (2014) 2990–2993.
[25] T.-B. Gao, R.-Q. Yan, A.J. Metherell, D.-K. Cao, D.-J. Ye, M.D. Ward, Dalton Trans.
46 (2017) 16787–16791.
Zhuo Liu: Conceptualization, Investigation, Writing - original draft.
Rui-Lian Yang: Investigation, Visualization. Xiao-Han Yang: Formal
analysis, Data curation. Jing-Lan Cao: Investigation, Validation. Xu-Ru
Fan: Investigation, Validation. Shui-Xing Wu: Software, Formal ana-
lysis, Funding acquisition. Zhi-Gang Niu: Methodology, Resources,
Supervision, Funding acquisition. Gao-Nan Li: Writing - review &
editing, Project administration, Funding acquisition.
Declaration of Competing Interest
[26] L. Han, D. Zhang, Y. Zhou, Y. Yang, H.Y. Woo, F. Bai, R. Yang, Dyes Pigm. 99 (2013)
1010–1015.
[27] E. Baranoff, S. Fantacci, F. De Angelis, X. Zhang, R. Scopelliti, M. Grätzel,
M.K. Nazeeruddin, Inorg. Chem. 50 (2011) 451–462.
[28] J. Yellol, S.A. Pérez, G. Yellol, J. Zajac, A. Donaire, G. Vigueras, V. Novohradsky,
C. Janiak, V. Brabec, J. Ruiz, Chem. Commun. 52 (2016) 14165–14168.
[29] G.S. Yellol, A. Donaire, J.G. Yellol, V. Vasylyeva, C. Janiak, J. Ruiz, Chem.
Commun. 49 (2013) 11533–11535.
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influ-
ence the work reported in this paper.
Acknowledgements
[30] Y. Hori, N. Otomura, A. Nishida, M. Nishiura, M. Umeno, I. Suetake, K. Kikuchi, J.
Am. Chem. Soc. 140 (2018) 1686–1690.
[31] H. Wang, Z. Feng, S.J. Del Signore, A.A. Rodal, B. Xu, J. Am. Chem. Soc. 140 (2018)
3505–3509.
[32] P. Laha, U. De, F. Chandra, N. Dehury, S. Khullar, H.S. Kimb, S. Patra, Dalton Trans.
47 (2018) 15873–15881.
[33] Z.-G. Niu, H.-B. Han, M. Li, Z. Zhao, G.-Y. Chen, Y.-X. Zheng, G.-N. Li, J.-L. Zuo,
Organometallics 37 (2018) 3154–3164.
This work was supported by the Natural Science Foundation of
Hainan Province (No. 219MS043), the National Natural Science
Foundation of China (Nos. 21663011 and 21501037) and the 2019
National Innovation Experiment Program for University Students.
[34] Z.-G. Niu, T. Zheng, Y.-H. Su, P.-J. Wang, X.-Y. Li, F. Cui, J. Liang, G.-N. Li, New J.
Appendix A. Supplementary data
Chem. 39 (2015) 6025–6033.
[35] A. Bossi, A.F. Rausch, M.J. Leitl, R. Czerwieniec, M.T. Whited, P.I. Djurovich,
H. Yersin, M.E. Thompson, Inorg. Chem. 52 (2013) 12403–12415.
[36] Q.-L. Xu, C.-C. Wang, T.-Y. Li, M.-Y. Teng, S. Zhang, Y.-M. Jing, X. Yang, W.-N. Li,
C. Lin, Y.-X. Zheng, J.-L. Zuo, X.-Z. You, Inorg. Chem. 52 (2013) 4916–4925.
[37] C.-L. Ho, W.-Y. Wong, Q. Wang, D. Ma, L. Wang, Z. Lin, Adv. Funct. Mater. 18
(2018) 928–937.
Supplementary data to this article can be found online at https://
doi.org/10.1016/j.ica.2020.119703.
References
[38] K. Sebastian, R. Michael, M. Eric, B. Frank, W.R. Peter, Organometallics 38 (2019)
1721–1732.
[39] W. Lee, T.H. Kwon, J. Kwon, J. Kim, C. Lee, J.-I. Hong, New J. Chem. 35 (2011)
2557–2563.
[40] C.-L. Ho, W.-Y. Wong, G.-J. Zhou, B. Yao, Z. Xie, L. Wang, Adv. Funct. Mater. 17
(2007) 2925–2936.
[1] X. Yang, X. Xu, J. Dang, G. Zhou, C.L. Ho, W.Y. Wong, Inorg. Chem. 55 (2016)
1720–1727.
[2] Y. Tao, Y. Yang, J. Qin, Chem. Soc. Rev. 40 (2011) 2943–2970.
[3] G.-N. Li, Y.-P. Zeng, K.-X. Li, H.-H. Chen, H. Xie, F.-L. Zhang, G.-Y. Chen, Z.-G. Niu,
J. Fluoresc. 26 (2006) 323–331.
[41] J.A. Porras, I.N. Mills, W.J. Transue, S. Bernhard, J. Am. Chem. Soc. 138 (2016)
9460.
[4] B.-Y. Ren, R.-D. Guo, D.-K. Zhong, C.-J. Ou, G. Xiong, X.-H. Zhao, W. Huang, Inorg.
[42] CrysAlisPro Version 1.171.36.21. Agilent Technologies Inc 2012 Santa Clara, CA,
USA.
[43] L. Palatinus, G.J. Chapuis, Appl. Crystallogr. 40 (2007) 786–790.
[44] G.M. Sheldrick, Acta Crystallogr. Sect. A 64 (2008) 112–122.
[45] O.V. Dolomanov, L.J. Bourhis, R.J. Gildea, J.A.K. Howard, H.J. Puschmann, Appl.
Crystallogr. 42 (2009) 339–341.
[46] M.J. Frisch G.W. Trucks H.B. Schlegel et al. Gaussian 09, Revision A.01, Gaussian
2009 Wallingford, Con, USA.
[47] C. Lee, W. Yang, R.G. Parr, Phys. Rev. B 37 (1988) 785–789.
[48] B. Miehlich, A. Savin, H. Stoll, H. Preuss, Chem. Phys. Lett. 157 (1989) 200–206.
[49] A.D. Becke, J. Chem. Phys. 98 (1993) 5648–5652.
[50] M. Nonoyama. Bull, Chem. Soc. Jpn. 47 (1974) 767−768.
Chem. 56 (2017) 8397–8407.
[5] S. Reineke, F. Lindner, G. Schwartz, N. Seidler, K. Walzer, B. Lüssem, K. Leo, Nature
459 (2009) 234–238.
[6] S. Gong, Y. Chen, C. Yang, C. Zhong, J. Qin, D. Ma, Adv. Mater. 22 (2010)
5370–5373.
[7] C. Fan, C. Yang, Chem. Soc. Rev. 43 (2014) 6439–6469.
[8] S. Lamansky, P. Djurovich, D. Murphy, F. Abdel-Razzaq, H.E. Lee, C. Adachi,
M.E. Thompson, J. Am. Chem. Soc. 123 (2001) 4304–4312.
[9] C.-L. Li, Y.-J. Su, Y.-T. Tao, P.-T. Chou, C.-H. Chien, C.-C. Cheng, R.-S. Liu, Adv.
Funct. Mater. 15 (2005) 387–395.
[10] A. Tsuboyama, H. Iwawaki, M. Furugori, T. Mukaide, J. Kamatani, S. Igawa,
K. Ueno, J. Am. Chem. Soc. 125 (2003) 12971–12979.
[11] S.-L. Lai, S.-L. Tao, M.-Y. Chan, T.W. Ng, M.F. Lo, C.S. Lee, S.T. Lee, Org. Electron.
8