Highly efficient yellow-emitting iridium(III) complexes based on fluorinated 2-(biphenyl-4-yl)-2H-indazole ligands: Syntheses, structures, properties, and density functional theory calculations

Article abstract of DOI:10.1002/jccs.201900141

Two new iridium (III) complexes (Ir1-Ir2) bearing different fluorinated 2-(biphenyl-4-yl)-2H-indazole-based compounds as cyclometalated ligands and Xantphos as an ancillary ligand were synthesized and fully characterized. The ultraviolet (UV)–vis absorpti

Full text of DOI:10.1002/jccs.201900141

Received: 13 April 2019
Revised: 31 May 2019
Accepted: 5 June 2019
DOI: 10.1002/jccs.201900141
A R T I C L E
Highly efficient yellow-emitting iridium(III) complexes based
on fluorinated 2-(biphenyl-4-yl)-2H-indazole ligands: Syntheses,
structures, properties, and density functional theory calculations
Xiao-Han Yang1 | Zi-Cen Zuo1 | Zi-Wen Tao1 | Ding Yuan1 | Yan Chen1
|
Qin Chen1 | Kai Liu1 | Guang-Ying Chen2 | Zheng-Rong Mo2 | Gao-Nan Li1
Zhi-Gang Niu2
|
1Key Laboratory of Electrochemical Energy
Storage and Energy Conversion of Hainan
Abstract
Province, College of Chemistry and
Chemical Engineering, Hainan Normal
University, Haikou, China
2Key Laboratory of Tropical Medicinal
Plant Chemistry of Ministry of Education,
Hainan Normal University, Haikou, China
Two new iridium (III) complexes (Ir1-Ir2) bearing different fluorinated
2-(biphenyl-4-yl)-2H-indazole-based compounds as cyclometalated ligands and
Xantphos as an ancillary ligand were synthesized and fully characterized. The
ultraviolet (UV)–vis absorption, photoluminescence, and electrochemistry proper-
ties were studied. The single crystal structures of Ir1-Ir2 were determined by
X-ray diffraction, showing each adopts the distorted octahedral coordination geom-
etry. To gain insights into the lowest energy electron transitions and the lowest trip-
let excited states, density functional theory calculations were used to further
investigate the origination. Two complexes emit yellow photoluminescence with
quantum yields of 49.7–72.5% in solution at room temperature. Their Commission
Internationale de L'Eclairage color coordinates are (0.42, 0.53) and (0.39, 0.47),
respectively.
Correspondence
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.
Email: ligaonan2008@163.com
Zhi-Gang Niu, Key Laboratory of Tropical
Medicinal Plant Chemistry of Ministry of
Education, Hainan Normal University,
Haikou, China.
K E Y W O R D S
DFT calculations, fluorination, iridium (III) complexes, photoluminescence
Email: niuzhigang1982@126.com
Funding information
Natural Science Foundation of Hainan
Province, Grant/Award Numbers:
218QN236, 219MS043; Program for
Innovative Research Team in University,
Grant/Award Number: IRT-16R19
primary colors (red, green, and blue) of high color purity are
mixed to produce white light. Another approach is that two
1 | INTRODUCTION
White organic light-emitting diodes based on iridium (III)
complexes are currently a major research area of organic
light-emitting diode (OLED) optoelectronic technology, due
to their excellent performance for flat-panel displays and
solid-state lighting.[1–5] In general, there are two pathways
to achieve white emission. One approach is that the three
efficient complementary colors are combined to attain white
light, such as yellow/orange and blue. However, yellow is a
dispensable color in three primary colors, and thus little
research has been done. Since the first-ever yellow emitter
[(bt)2Ir(acac)][6] was reported, only a few yellow-emitting
iridium (III) complexes have been developed, such as
© 2019 The Chemical Society Located in Taipei & Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
J Chin Chem Soc. 2019;1–8.
http://www.jccs.wiley-vch.de
1

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YANG ET AL.
(fbi)2Ir(acac),[7]
(NEP)2Ir(acac),[8]
(MDPP)2Ir(acac),[9]
(3-piq)2Ir(acac),[10] (CzN-Flpy)2Ir(acac),[11] (TPAFlpy)2Ir
(acac),[12] (tpppyp)2Ir(acac),[13] and so on. More recently,
we reported a series of yellow-emitting phenylpyrazole-
based iridium (III) complexes (Ir2-Ir4).[14] To obtain effi-
cient OLEDs, we used fluorine groups to modify the main
ligands in complexes Ir3 and Ir4. Consequently, the phos-
phorescent OLEDs comprising Ir3 and Ir4 as yellow dop-
ants realized device performances with EQEmax values of
9.0 and 10.7%. That is because that the C-F bonds intro-
duced by fluorination can not only decrease the rate of non-
radioactive deactivation and enhance the photoluminescence
efficiency,[15] but also sterically alter the molecular packing
and reduce the self-quenching behavior.[16]
FIGURE 1 ORTEP views of Ir1 (left) and Ir2 (right) with the
atom-numbering scheme at the 50% probability level. Hydrogen atoms
and PF6 anions are omitted for clarity
ORTEP diagrams are depicted in Figure 1. Selected parame-
ters of the molecular structures are collected in Table S1,
and bond lengths and bond angles of each complex are given
in Table S2.
In view of these research foundations, we modified the
phenylpyrazole (ppz) ligand through increasing the π system
and introducing fluorine substituents to get two 2-(biphenyl-
4-yl)-2H-indazole (bpiz)-based main ligands, that is, 2-(20,40-
difluoro-[1,10-biphenyl]-4-yl)-2H-indazole (2,4-dfbpiz, L1)
These complexes exhibit some common structural charac-
teristics: the ^Ir(III) center adopts a distorted-octahedral geome-
try; the C N ligands are in cis-C,C0 and trans-N,N0
configurations. The average Ir–P bond lengths for Ir1
(2.493 Å) are close to those found in Ir2 (2.513 Å). The aver-
age Ir–C bond lengths (2.064 Å for Ir1, 2.051 Å for Ir2) and
Ir–N bond lengths (2.045 Å for Ir1, 2.034 Å for Ir2) are in
agreement with those of previously reported complexes.[17,18]
Obviously, the Ir–Pav distances are longer than the Ir–Cav and
Ir–Nav distances in both complexes. Additionally, in the C^N
main ligand, the coordinated phenyl ring and the
benzopyrazole group are almost coplanar, whereas another
fluoro-substituted phenyl ring is twisted out of planarity from
the aforementioned fragment by 55.02ꢀ (Ir1) and 33.88ꢀ (Ir2).
The hydrogen bonding interactions in the crystal struc-
ture of Ir2 are shown in Figure 2, and the data are
and
2-(30,50-difluoro-[1,10-biphenyl]-4-yl)-2H-indazole
(3,5-dfbpiz, L2). In the meantime, for further enhancing emis-
sion performance, we employed (9,9-dimethyl-9H-xanthene-
4,5-diyl)bis(diphenylphosphane) (Xantphos) with bulky
phosphine groups as an ancillary ligand. Hence, two
corresponding iridium (III) complexes, [(2,4-dfbpiz)2Ir
(Xantphos)] (Ir1) and [(3,5-dfbpiz)2Ir(Xantphos)] (Ir2),
were synthesized (Scheme 1). The photophysical and elec-
trochemical properties of both complexes were studied.
Especially, the luminescence properties were investigated
both in solution (298 and 77 K) and solid state (298 K).
The lowest energy electronic transitions and the lowest
triplet excited states were calculated using density func-
tional theory (DFT) and time-dependent DFT (TD-DFT).
2 | RESULTS AND DISCUSSION
2.1 | Structural description
Single-crystal structures of the two complexes Ir1-Ir2 were
determined through X-ray diffraction studies and the
FIGURE 2 Part of the crystal structure of Ir2, showing selected
noncovalent contacts of the C–Hꢁ ꢁ ꢁN types (dashed red lines). Atoms
involved in hydrogen bonds are shown as balls of arbitrary radii. All
other covalent bonds are represented as sticks
SCHEME 1 Synthetic routes of Ir(III) complexes Ir1-Ir2

YANG ET AL.
3
TABLE 1 Hydrogen bonding
arrangements for Ir2 (Å, ꢀ)
Complex
Ir2
D—Hꢁ ꢁ ꢁA
D—H
0.93
Hꢁ ꢁ ꢁA
2.61
Dꢁ ꢁ ꢁA
D—Hꢁ ꢁ ꢁA
113
C12-H12ꢁ ꢁ ꢁN3
C12-H12ꢁ ꢁ ꢁN4
C72-H72ꢁ ꢁ ꢁN4
3.101(4)
3.087(5)
3.129(5)
0.93
2.56
117
0.93
2.56
120
2.2 | Photophysical properties
The absorption and emission spectra of complexes Ir1-Ir2
in CH2Cl2 solution at room temperature are depicted in
Figure 3, and the corresponding data are recorded in
Table 2. The absorption spectra of both complexes show
strong absorption bands below 300 nm, arising from the
spin-allowed ligand centered π–π* transition. The weaker
absorption bands between 300–400 nm can be assigned to a
mixture of metal-to-ligand charge-transfer transitions
(1MLCT and 3MLCT).[19–21] Clearly, their absorption shapes
are almost identical; however, the absorption onset of Ir2
slightly red-shift compared with that of Ir1, depending on
the different position of F groups in C^N ligands. To further
identify the nature of the low-lying transitions, we have per-
formed DFT calculation studies involving the frontier orbital
analyses and the main transitions.
FIGURE 3 UV–vis absorption and emission spectra of Ir1-Ir2 in
CH2Cl2 solution at room temperature
As for the emission spectra, both complexes exhibit the
emission maxima at 547–551 nm with the shoulder peak at
586–590 nm (Figure 3, right), which indicate that their emis-
sive excited states have 3MLCT and 3LC character.[23,24]
This assumption will be proved by DFT calculations aiming
for the properties of the excited states. It should be noted that
the different substitution positions of fluorine atoms on the
C^N ligands in Ir2 lead to 4 nm bathochromic shifted emis-
sion maximum compared to that of Ir1. The result is consis-
tent with the above absorption analysis. Perhaps, the quite
small change of emission wavelength is attributed to the
large distances between fluoro-substituted position and the Ir
center. A similar behavior was also observed in our previous
study.[25] Based on the photoluminescence spectra of Ir1-
Ir2, their corresponding Commission Internationale de
summarized in Table 1. The conformation of Ir2 allows the
establishment of three intramolecular C–HꢁꢁꢁN hydrogen
bonds, forming two six-membered rings and one five-
membered ring, respectively. Furthermore, there are some
π-π stacking interactions in complexes Ir1-Ir2 (Figure S1).
As for Ir1, the phenyl groups (C45-C46-C47-C48-C49-C50
or C51-C52-C53-C54-C55-C56) connected to the phospho-
rus atoms in the ancillary ligands show intramolecular face-
to-face π-π interactions with the benzene rings (C1-C2-C3-
C4-C5-C6 or C27-C28-C29-C30-C31-C32) and the pyrazole
ring (C5-C6-C7-N2-N1) in C^N ligands. The centroid-
centroid distances between selected two rings are 3.646,
3.642 and 3.643 Å, respectively. Similarly, the short π-π
stacking interactions (3.749, 3.659 and 3.733 Å) are also
observed in Ir2.
TABLE 2 Photophysical data of complexes Ir1-Ir2
Emission
Solution
Absorption
Solid
Complex λabsa (nm)
λema (nm)
Φemb (%) τa (μs) krc (105 s−1) knrc (105 s−1) λemd (nm)
λeme (nm)
CIEx,y
Ir1
Ir2
229, 269(sh), 320, 343, 366 547, 586(sh) 72.5
229, 269(sh), 322, 345, 369 551, 590(sh) 49.7
1.38
1.65
5.25
3.01
1.99
3.05
546, 589(sh) 545, 585(sh) (0.42, 0.53)
550, 594(sh) 549, 587(sh) (0.39, 0.47)
aData were collected from degassed CH2Cl2 solutions at room temperature.
bfac-Ir(ppy)3 as referenced standard (0.4).[22]
cRadiative decay rate (kr) and nonradiative decay rate (knr) were estimated from the measured quantum yields and lifetimes.
dData were collected from degassed CH2Cl2 solutions at 77 K.
eData were collected in the solid state at 298 K.

4
YANG ET AL.
TABLE 3 Electrochemical and theoretical data of complexes
Ir1-Ir2
Eox1 HOMOa Eopt,g LUMOc HOMOd LUMOd
b
Complex (V) (eV)
(eV)
3.39
3.36
(eV)
(eV)
(eV)
Ir1
Ir2
1.13 −5.93
1.12 −5.92
−2.54
−2.56
−6.07
−6.07
−2.29
−2.34
aHOMO energies are deduced from the equation HOMO = − (Eox + 4.8 eV).[29]
bOptical band gaps obtained from the absorption edge.
FIGURE 4 The emission spectra of Ir1-Ir2 in CH2Cl2 solution
at 77 K (left) and in the solid state at 298 K (right)
cLUMO = HOMO + Eopt.g
.
dObtained from theoretical calculations.
L'Eclairage (CIE) color coordinates are (0.42, 0.53) and
(0.39, 0.47), which are not far from the standard yellow
emission in CH2Cl2 solution (Figure S2).
are located on Ir ions (24.15% for Ir1 and 27.51% for Ir2)
and C^N ligands (71.71% for Ir1 and 67.77% for Ir2).
Accordingly, their oxidation processes are assigned to Ir (III)/
Ir (IV) redox and some contributions of C^N ligands. The
HOMO energy levels can be calculated to be −5.93 and
Upon cooling to 77 K, the emission major peaks of
Ir1-Ir2 undergo a weakly hypsochromic shift, while the
shoulder peaks have a bathochromic shift (Figure 4, left).
Note that the emission spectra exhibit a well-resolved
vibronic structure, indicating that the mixing between the
3MLCT and 3LC is so effective that a ligand-centered
emission can clearly be observed in the frozen matrix.[26]
In comparison to the solution state emission, there is a
slightly blue shift by 2 nm in solid state emission at 298 K
(Figure 4, right), as a result of the restriction of intramolecu-
lar rotation. This is a common phenomenon among lumines-
cent iridium(III) complexes.[27]
−5.92 eV based on the equation EHOMO = − (Eox
+
4.8 eV).[29] Subsequently, the corresponding LUMO energy
levels are calculated to be −2.54 and −2.56 eV through the
EHOMO and Eopt,g data. We also find that the HOMO/LUMO
energy levels obtained from CV are a little smaller than those
from DFT, but the variation trends are consistent in both cases
(Table 3).
2.4 | Theoretical calculations
Phosphorescence relative quantum yields (Φem) of Ir1
and Ir2 in dichloromethane solution at room temperature
were determined to be 72.5 and 49.7% with reference to fac-
Ir(ppy)3 (Φ = 40%).[22] As expected, the quantum yields of
two complexes are remarkably improved, resulting from the
existence of the C-F bond of lower vibration frequency.[28]
More importantly, the experimental results reveal that the
quantum yield of 2,4-positions fluorination is higher than
that of 3,5-positions fluorination. From the Φem and τ
values, the radiative decay rate (kr) and the nonradiative
decay rate (knr) were calculated. It is evident that the high
quantum yield of Ir1 correlates with the larger kr value and
the smaller knr value. Compared with phenylpyrazole-based
Ir(III) complexes published in our recent article,[14] the kr
rate constants of Ir1-Ir2 are much larger, indicating that the
complexes have more desirable photophysical properties.
DFT and TD-DFT calculations were performed on com-
plexes Ir1-Ir2 to study the lowest energy electron transitions
and the lowest triplet excited states. The most representative
frontier molecular orbital diagrams of both complexes are
shown in Figure 5. The calculated spin-allowed electron
transitions are listed in Table 4 and compared with the
absorption spectra data measured experimentally. The elec-
tron density distribution data are collected in Table S3.
For Ir1-Ir2, the HOMOs and the HOMO-1s are mainly
located on the metal center and C^N main ligands, while the
LUMOs are primarily dominated on the C^N main ligands
(Figure 5). Theoretical calculations of DFT reveal that the
2.3 | Electrochemical properties
The electrochemical behavior of Ir1-Ir2 was carried out
by cyclic voltammetry to determine the highest occupied
molecular orbital/lowest unoccupied molecular orbital
(HOMO/LUMO) energy levels (Figure S3). The correspond-
ing data are summarized in Table 3. The cyclic voltammetry
(CV) curves in the positive range exhibit sequential irrevers-
ible oxidation peaks. According to DFT calculations, HOMOs
FIGURE 5 Some frontier molecular orbitals of complexes
Ir1-Ir2

YANG ET AL.
5
TABLE 4 Main experimental and calculated optical transitions for Ir1-Ir2
Complex
Ir1
Major assignments
Transition
Character
Oscillation strength Calcd (nm) Exptl (nm)
S1 HOMO-1!LUMO (71%) MLCT/ILCT dπIr/2,4-dfbpiz! π*
0.0495
0.1332
0.0099
0.1776
386
376
389
375
366
2,4-dfbpiz
2,4-dfbpiz
3,5-dfbpiz
3,5-dfbpiz
S2 HOMO!LUMO (73%)
S1 HOMO!LUMO (72%)
MLCT/ILCT dπIr/2,4-dfbpiz! π*
MLCT/ILCT dπIr/3,5-dfbpiz! π*
Ir2
370
S2 HOMO-1!LUMO (69%) MLCT/ILCT dπIr/3,5-dfbpiz! π*
lowest energy spin-allowed transitions of Ir1 and Ir2 come
from HOMO!LUMO and HOMO-1!LUMO transitions
(Table 4). Therefore, it is ascribed to MLCT and intraligand
charge-transfer transition (ILCT), supporting the actual
absorption discussed above.
spectrophotometer. The fluorescence spectra were recorded
on a Hitachi F-7000 spectrophotometer in the degassed
CH2Cl2 solution (298 and 77 K) and the solid state (298 K).
Luminescence lifetime curves were measured using an Edin-
burgh Instruments FLS920P fluorescence spectrometer, and
the data were treated as one-order exponential fitting using
OriginPro 8 software.
The luminescence quantum efficiencies were calculated
by comparison of the fluorescence intensities (integrated
areas) of a standard sample fac-Ir(ppy)3 and the unknown
sample according to the equation.[22,30]
To explore the emission origins of Ir1-Ir2, DFT calcula-
tions are also used to study their lowest triplet excited state
characters, and the relevant information is listed in Table 5.
For the two complexes, the two lowest triplet states (T1 and
T2) are mainly from HOMO!LUMO and HOMO!LUMO
+1 transitions. Based on the electron density distribution
information in Table S3, HOMO are largely located on Ir d-
orbital and C^N main ligands, while LUMO/LUMO+1 are
mainly distributed on the C^N main ligands. Analogous to
the lowest energy electron transitions, their two lowest triplet
ꢀ
ꢁꢀ
ꢁꢀ
ꢁ
2
Iunk
Istd
Astd
Aunk
ηunk
ηstd
Φunk = Φstd
3
3
states (T1 and T2) have MLCT and ILCT characteristics.
The computed results indicate that the difference in the posi-
tion of the two fluorine atoms on the main ligands has no
significant effect on the emission performance.
where Φunk and Φstd are the luminescence quantum yield
values of the unknown sample and fac-Ir(ppy)3 solutions
(Φstd = 0.4),[22] respectively. Iunk and Istd are the integrated
fluorescence intensities of the unknown sample and fac-Ir
(ppy)3 solutions, respectively. Aunk and Astd are the absor-
bance values of the unknown sample and fac-Ir(ppy)3 solu-
tions at their excitation wavelengths, respectively. The ηunk
and ηstd terms represent the refractive indices of the
corresponding solvents (pure solvents were assumed).
Radiative decay constants (kr) were calculated using 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.[31]
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.[32] The structure
was solved and refined using Full-matrix least-squares
based on F2 with program SHELXS-97 and SHELXL-
3 | EXPERIMENTAL
3.1 | General
IrCl3ꢁ3H2O and other chemicals are commercially available
and used without further purification. All solvents were puri-
fied and dried by standard procedures.
1
The H, 31P, and 19F NMR spectra were recorded on a
Bruker AM 400 MHz instrument. Chemical shifts were
reported in ppm relative to Me4Si as internal standard. The
electrospray ionization-mass spectrometry (ESI-MS) spectra
were recorded on an Esquire HCT-Agilent 1200 LC/MS
spectrometer. The electrospray ionization-high resolution
mass spectrometry (ESI-HRMS) spectra were recorded on
an Agilent TOF 6230 LC/MS spectrometer. The UV–vis
spectra were recorded on
a
Hitachi U3900/3900H
TABLE 5 Contribution of triplet
transitions and transition characters for
complexes Ir1-Ir2
Complex
Ir1
Major configuration
Transition
Character
T1
T1
HOMO ! LUMO (37%)
HOMO ! LUMO+1 (29%)
HOMO ! LUMO (39%)
HOMO ! LUMO+1 (25%)
3MLCT/3ILCT
dπIr/2,4-dfbpiz!π*
2,4-dfbpiz
2,4-dfbpiz
3,5-dfbpiz
3,5-dfbpiz
3MLCT/3ILCT
3MLCT/3ILCT
3MLCT/3ILCT
dπIr/2,4-dfbpiz!π*
dπIr/3,5-dfbpiz!π*
dπIr/3,5-dfbpiz!π*
Ir2

6
YANG ET AL.
97[33] within Olex2.[34] All nonhydrogen atoms were
found in alternating difference Fourier syntheses and
least-squares refinement cycles and, during the final
cycles, refined anisotropically. Hydrogen atoms were
placed in calculated positions and refined as riding atoms
with a uniform value of Uiso.
All calculations were carried out with the Gaussian
09 software package.[35] The DFT and TD-DFT were
employed with no symmetry constraints to investigate the
optimized geometries and electron configurations with the
Becke three-parameter Lee-Yang-Parr (B3LYP) hybrid
DFT.[36–38] The LANL2DZ basis set was used to treat the Ir
atom, whereas the 6-31G* basis set was used to treat C,
H, N, O, P, and F atoms. Solvent effects were considered
within the self-consistent reaction field theory using the
polarized continuum model approach to model the interac-
tion with the solvent.[39,40]
and then the reaction mixture was degassed under vacuum
and charged with N2. The mixture was heated at 90ꢀC and
stirred for 10 hr under N2. The mixture was poured into
water and extracted multiple times with ethyl acetate. The
combined organic phases were washed with brine, dried
over Na2SO4. After concentrated, the residue was purified
by column chromatography (petroleum ether: ethyl ace-
tate = 50:1~10:1) to give L1 as a white solid (225 mg, yield:
1
66.9%). H NMR (400 MHz, CDCl3) δ 8.46 (s, 1H), 7.99
(d, J = 7.8 Hz, 2H), 7.80 (d, J = 8.7 Hz, 1H), 7.70 (dd,
J = 22.2, 8.1 Hz, 3H), 7.46 (dd, J = 15.7, 7.6 Hz, 1H), 7.34
(t, J = 7.4 Hz, 1H), 7.13 (t, J = 7.2 Hz, 1H), and 6.90~7.04
(m, 2H). MS (ESI): m/z = 306.2 [M+1]+.
3.2.3 | Synthesis of 2-(30,50-difluoro-[1,10-
biphenyl]-4-yl)-2H-indazole (3,5-dfbpiz, L2)
CV was performed on a CHI 1210B electrochemical
workstation, with a glassy carbon electrode as the working
electrode, a platinum wire as the counter electrode, an
Ag/Ag+ electrode as the reference electrode, and 0.1 M n-
Bu4NClO4 as the supporting electrolyte.
L2 (210 mg, yield: 62.4%) was obtained by using
(3,5-difluorophenyl)boronic
acid
instead
of
(2,4-difluorophenyl)boronic acid by a method similar to that
1
of L1. H NMR (400 MHz, CDCl3) δ 8.45 (s, 1H), 8.01
(d, J = 7.5 Hz, 2H), 7.80 (d, J = 8.7 Hz, 1H), 7.71
(t, J = 7.3 Hz, 3H), 7.34 (t, J = 7.4 Hz, 1H), 7.14
(t, J = 9.4 Hz, 3H), and 6.83 (t, J = 8.7 Hz, 1H). MS (ESI):
m/z = 306.1 [M+1]+.
3.2 | Synthesis of the iridium complexes
3.2.1 | Synthesis of 2-(4-bromophenyl)-2H-
indazole (compound 1)
3.2.4 | Synthesis of (2,4-dfbpiz)2Ir
(Xantphos) (Ir1)
A solution of mixed 4-bromoaniline (1.0 g, 5.81 mmol) and
2-nitrobenzaldehyde (0.9 g, 5.81 mmol) in EtOH (20 mL)
was refluxed for 10 hr under N2. After most of the solvent
evaporated, the solid was filtered and collected to afford the
crude compound N-(4-bromophenyl)-1-(2-nitrophenyl)met-
hanimine, which was used in the next step without further
A mixture of IrCl3ꢁ3H2O (89 mg, 1.0 eq) and L1 (200 mg,
2.2 eq) in 12 mL of ethoxyethanol and H2O (vol:vol = 2:1)
was refluxed at 120ꢀC for 12 hr under N2. After cooling to
room temperature, the solid was collected by filtration and
washed with cooled ether and MeOH. After drying, the
crude product of the chlorine bridge dimer complex
[(2,4-dfbpiz)2Ir(μ-Cl)]2 was used for the next step without
further purification. A mixture of the above dimer complex
[(2,4-dfbpiz)2Ir(μ-Cl)]2(100 mg, 1.0 eq) and Xantphos
ligand (136 mg, 2.5 eq) was dissolved in dry degassed dic-
hloromethane and methanol (6 mL, VDCM: VMeOH = 2:1),
and heated at 45ꢀC and stirred for 15 hr under N2. Then, the
solution was cooled to room temperature, NH4PF6 was
added (76 mg, 5.0 eq) and stirred for another 2 hr. The mix-
ture was dried and purified by column chromatography to
afford a yellow solid of Ir1 (63 mg, yield: 62.40%). 1H
NMR (400 MHz, DMSO) δ 9.42 (s, 2H), 7.93 (d, J = 7.7 Hz,
2H), 7.69 (d, J = 8.5 Hz, 2H), 7.63 (d, J = 8.4 Hz, 2H),
7.24–7.17 (m, 4H), 7.08–6.93 (m, 16H), 6.87–6.81 (m, 6H),
6.52 (dt, J = 21.0, 7.7 Hz, 6H), 6.38 (t, J = 7.3 Hz, 2H),
5.97 (t, J = 8.2 Hz, 4H), 5.90 (s, 2H), and 1.86 (s, 6H). 31P
purification.
N-(4-bromophenyl)-1-(2-nitrophenyl)met-
hanimine (1.1 g, 3.61 mmol) was dissolved in P(OEt)3
(15 mL), and the mixture was heated at 110ꢀC for 12 hr
under N2. The solvent was removed and the residue was
purified by column chromatography (petroleum ether: ethyl
acetate = 100:1–20:1) to afford compound 1 as a white solid
(680 mg, yield: 69.06%). 1H NMR (400 MHz, CDCl3) δ
8.38 (s, 1H), 7.77–7.81 (m, 3H), 7.70 (d, J = 8.4 Hz, 1H),
7.65 (d, J = 8.4 Hz, 2H), 7.34 (dd, J1 = 7.6 Hz, J2 = 6.8 Hz,
1H), and 7.13 (dd, J1 = 8.4 Hz, J2 = 7.6 Hz, 1H); MS (ESI):
m/z = 273.1/275.1 [M+1]+.
3.2.2 | Synthesis of 2-(20,40-difluoro-[1,10-
biphenyl]-4-yl)-2H-indazole (2,4-dfbpiz, L1)
To a degassed mixture of 2-(4-bromophenyl)-2H-indazole
(300 mg, 1.10 mmol), (2,4-difluorophenyl)boronic acid
(209 mg, 1.32 mmol) and K2CO3 (304 mg, 2.20 mmol) in
1,4-dioxane (10 mL), Pd(dppf)2Cl2 (150 mg) was added,
NMR (162 MHz, DMSO)
δ (ppm) −35.72 (s, 2P,
Xantphos), −144.19 (hept, 1P, PF6). 19F NMR (377 MHz,

YANG ET AL.
7
DMSO) δ (ppm) −69.21 (s, 3F, PF6), −71.10 (s, 3F, PF6),
−111.06 (d, 2F, 2,4-dfbpiz), and −113.83 (d, 2F,
2,4-dfbpiz). HRMS (ESI): m/z = 1,381.3376 [M-PF6]+.
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ACKNOWLEDGMENTS
This work was supported by the Natural Science Foundation
of Hainan Province (219MS043, 218QN236) and Program
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How to cite this article: Yang X-H, Zuo Z-C,
Tao Z-W, et al. Highly efficient yellow-emitting
iridium(III) complexes based on fluorinated 2-
(biphenyl-4-yl)-2H-indazole ligands: Syntheses,
structures, properties, and density functional theory
calculations. J Chin Chem Soc. 2019;1–8. https://doi.
org/10.1002/jccs.201900141