Cyclometalated iridium(III) complexes containing 2-phenyl-2H-indazole ligand: Synthesis, photophysical studies, and DFT calculations

Article abstract of DOI:10.1002/jccs.202000271

Heteroleptic cyclometalated iridium(III) complexes (Ir1–Ir5) featuring piz-based ligands and acetylacetone ancillary ligand are synthesized and characterized. Their photophysical and electrochemical properties were studied, and DFT calculations were used to further support the experiment results. All the complexes emit yellow color with quantum yields of 12.2–56.5% in dichloromethane solution at room temperature, and the emission originates from a hybrid ³MLCT/³ILCT/³LLCT excited state.

Full text of DOI:10.1002/jccs.202000271

Received: 29 June 2020
Revised: 28 July 2020
Accepted: 30 July 2020
DOI: 10.1002/jccs.202000271
A R T I C L E
Cyclometalated iridium(III) complexes containing
2-phenyl-2H-indazole ligand: Synthesis, photophysical
studies, and DFT calculations
Shao-Bin Dou1 | Xing-Liang Jia1 | Zhi-Jun Liu1 | Ya-Qi Meng1
|
Zheng-Rong Mo2 | Gao-Nan Li1
| Zhi-Gang Niu2
1Key Laboratory of Electrochemical
Abstract
Energy Storage and Energy Conversion of
Hainan Province, College of Chemistry
and Chemical Engineering, Hainan
Normal University, Haikou, China
Heteroleptic cyclometalated iridium(III) complexes (Ir1–Ir5) featuring piz-
based ligands and acetylacetone ancillary ligand are synthesized and character-
ized. Their photophysical and electrochemical properties were studied, and
DFT calculations were used to further support the experiment results. All the
complexes emit yellow color with quantum yields of 12.2–56.5% in dic-
hloromethane solution at room temperature, and the emission originates from
a hybrid 3MLCT/3ILCT/3LLCT excited state.
2Key Laboratory of Tropical Medicinal
Plant Chemistry of Ministry of Education,
Hainan Normal University, Haikou,
China
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, China.
K E Y W O R D S
DFT calculation, fluorination, iridium(III) complexes, yellow photoluminescence
Email: ligaonan2008@163.com
Zhi-Gang Niu, Key Laboratory of Tropical
Medicinal Plant Chemistry of Ministry of
Education, Hainan Normal University,
Haikou, China.
Email: niuzhigang1982@126.com
Funding information
2019 Hainan Provincial Innovation
Experiment Program for University
Students; National Natural Science
Foundation of China, Grant/Award
Number: 21501037; Natural Science
Foundation of Hainan Province, Grant/
Award Number: 219MS043
1 | INTRODUCTION
chemical sensors.1–4 In particular, the yellow phosphores-
cent iridium complexes are indispensable components to ful-
Due to unique photophysical properties, luminescent irid-
ium complexes have exhibited enormous potential for the
development of modern optoelectronic technologies, such as
organic light-emitting diodes (OLEDs), light-emitting elec-
trochemical cells (LEECs), biological labeling reagents, and
fill the requirements of full-color displays and WOLEDs. In
order to obtain yellow emission, N,N-heterocyclic com-
pounds are widely used as the N-coordinated structural frag-
ments of cyclometalated ligands, for example, pyrimidine,5
pyrazine,6 benzoimidazole,7–9 imidazopyridine,10 and
J Chin Chem Soc. 2020;1–7.
http://www.jccs.wiley-vch.de
© 2020 The Chemical Society Located in Taipei & Wiley-VCH GmbH
1

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DOU ET AL.
phthalazine.11 As the isomer of benzoimidazole, however,
benzopyrazole is rarely reported in the literature.
and the relevant data are collected in Table 1. In
Figure 2a, the intense bands below 350 nm are assigned
to the spin-allowed π–π* transitions of the ligands. The
1
Recently, we succeeded in the preparation of several
yellow-emitting iridium(III) complexes based on
2-phenyl-2H-indazole (piz) ligands.12–14 Through these
research results, we can come to the following conclu-
sions. First, fluorinated complexes always display the
expected high-efficiency OLEDs because C F bonds
introduced by fluorination can not only decrease the rate
of nonradiative deactivation and enhance the photo-
luminescence efficiency, but also sterically alter the
molecular packing and reduce the self-quenching behav-
ior.13 Second, acetylacetone (acacH) has the highest trip-
let energy by theoretical calculation and thus has little
effect on the emission energy in iridium(III) complexes.
Therefore, in this investigation, we seek to improve
the efficiency of the piz-based iridium(III) complexes by
introducing the fluorine group while maintaining the yel-
low emission of these complexes by using acacH as the
ancillary ligand. As a result, a series of corresponding
weaker bands at about 400–450 nm can be ascribed to
both metal-to-ligand (MLCT) and ligand-to-ligand
(LLCT) transitions.12 With the same ancillary ligand, the
UV–vis absorption spectra of these complexes are
perturbed by the substituent patterns on cyclometalated
ligands. In comparison with parent complex Ir1
(405 nm), the low-energy MLCT bands of Ir3 (404 nm)
and Ir4 (402 nm) are blue-shifted, whereas that of Ir2
(408 nm) and Ir5 (410 nm) are red-shifted. The results
imply that
F/2,4-F2 groups generate electron-
withdrawing effect; on the contrary,
groups have an electron-donating effect.
OCF3/3,5-F2
Under light excitation conditions, all complexes emit
yellow light (Figure 1 and CIE data) with maxima at
564–570 nm, accompanied by
a shoulder peak at
597–600 nm (Figure 2b). The structured spectral feature
suggests that the phosphorescence originates primarily
3
iridium(III)
complexes,
(4-hbpiz)2Ir(acac)
(Ir1),
from the MLCT state, together with some contribution
3
(4-ocf3bpiz)2Ir(acac) (Ir2), (4-fbpiz)2Ir(acac) (Ir3),
(2,4-dfbpiz)2Ir(acac) (Ir4), and (3,5-dfbpiz)2Ir(acac) (Ir5),
was designed and synthesized (Scheme 1). All these
Ir(III) complexes produce the expected yellow emission
in CH2Cl2 solution (Figure 1). Their photophysical and
electrochemical properties were studied. The lowest-
energy electronic transitions and the lowest triplet
excited states have been calculated using density func-
tional theory (DFT) and time-dependent DFT (TD-DFT).
from the LC excited states.15 In consistent with their
absorption spectra, the substituents on the C˄N ligands
also have a similar influence on the emission wavelength.
Using the DFT calculations, we confirmed that the char-
ges of the coordinating carbons in complexes Ir5 are less
electron deficient, whereas those of complexes Ir2–Ir4
are electron deficient, when compared to complex Ir1
(Figure 3). The relatively less/more electron deficiency
causes a raising/lowering of the σ donation from the
cyclometalated ligand, thus leading to an energy
decrease/increase of 3MLCT emitting level.16 In addition,
we observed that the shift of emission wavelength for
complexes Ir1–Ir5 is quite small, probably because of the
large distances between the substituted position and the
Ir center. Indeed, their HOMO/LUMO energy levels
show very little change (Figure 4), further supporting the
above deduction. This kind of behavior was also observed
in our previous study.13,17
2 | RESULTS AND DISCUSSION
2.1 | Photophysical properties
The UV–vis absorption and photoluminescence
(PL) spectra of iridium(III) complexes Ir1–Ir5 in CH2Cl2
solution at room temperature are depicted in Figure 2,
SCHEME 1 Synthetic routes of complexes Ir1–Ir5

DOU ET AL.
3
FIGURE 1 Chemical
structures of complexes Ir1–Ir5
FIGURE 2 UV–vis absorption (a) and PL emission (b) spectra of complexes Ir1–Ir5 in CH2Cl2 solution at room temperature
TABLE 1 Photophysical and electrochemical data of complexes Ir1–Ir5
Complex Absorptiona λabs (nm) Emissiona λem (nm) Φem (%)b CIE (x,y)
Eox (V)a HOMOc (eV) HOMOd (eV)
Ir1
Ir2
Ir3
Ir4
Ir5
228, 264, 319, 366, 405
229, 261, 316, 365, 406
229, 266, 319, 366, 404
229, 263, 315, 360, 402
229, 265, 317, 366, 407
569, 599sh
569, 599sh
565, 598sh
564, 597sh
570, 600sh
12.2
28.2
42.1
56.5
51.7
(0.51, 0.48) 0.72
(0.52, 0.48) 0.61
(0.51, 0.49) 0.66
(0.50, 0.50) 0.69
(0.52, 0.48) 0.66
−5.52
−5.41
−5.46
−5.49
−5.46
−5.24
−5.30
−5.26
−5.27
−5.33
aData were collected from degassed CH2Cl2 solutions at room temperature.
bfac-Ir(ppy)3 as referenced standard (40%).
cHOMO energies are deduced from the equation HOMO = − (Eox + 4.8 eV).
dObtained from theoretical calculations.

4
DOU ET AL.
FIGURE 3 The charges of the coordinating carbons and nitrogens in complexes Ir1–Ir5
are some changes between the CV experimental and the-
oretical data, perhaps owing to different oxidation of the
piz-Ir(III) environment.
2.3 | Theoretical calculations
To understand in depth the photophysical properties
of Ir1–Ir5, the ground and excited states have been
studied through DFT and TD-DFT calculations. The
frontier molecular orbital energy levels and the energy
gaps are presented in Figure 4. The calculated singlet
and triplet transitions are provided in Table 2. The
electron density distributions are summarized in
Table S1. The HOMOs/HOMO-2s of Ir1–Ir5 are pri-
marily composed of iridium d orbitals and two
FIGURE 4 Calculated energy levels and energy gaps for
complexes Ir1–Ir5
cyclometalated ligand
π
orbitals. However, the
HOMO-1s are delocalized over the entire complexes.
On the other hand, the LUMOs/LUMO+1s are mainly
located on the π* orbitals of the two cyclometalated
ligands. Consequently, the lowest-energy electron
transitions can be attributed to the MLCT/ILCT transi-
tions, and the lowest triplet excited states have
3MLCT/3ILCT/3LLCT characteristics. From Figure 4,
it appears that the frontier orbital energy levels of all
complexes are approximate values, so their actual
absorption and emission spectra do not change much.
The spin-density distributions of complexes Ir1–Ir5
are also calculated and depicted in Figure 5. Generally, the
electronic structure of triplet states is modeled using a
combination of HOMOs and LUMOs, where each orbital
contains an unpaired electron.18 From these figures, it can
be seen that complexes Ir1 and Ir3–Ir5 display analogous
spin density, which is localized mainly on the Ir(III) ion
and one cyclometalated ligand. In the case of complex Ir2,
the spin density resides on the iridium core and two
cyclometalated ligands. The results indicate that the HOM-
O!LUMO electron transfers are from the iridium center
to C˄N ligand (3MLCT) and C˄N ligand to C˄N ligand
(3ILCT), which are in good agreement with the structured
bands observed in experiment photoluminescence.
To investigate the effect of fluorination on the C˄N
ligands, the photoluminescent relative quantum yields
(Φem) of all complexes were measured at room temperature
(Table 1). Clearly, the Φem of fluorinated complexes (Ir2–
Ir5) is much higher than that of the unfluorinated complex
(Ir1). In particular, the Φem of difluorinated complexes (Ir4
and Ir5) are the largest ones in the family of complexes.
Besides, the Φem of Ir1 and Ir2 are significantly lower than
those of other complexes. This is demonstrated by the fact
that the presence of the C-F bonds on phenyl rings could
lead to the reduced radiationless deactivation rate.12
2.2 | Electrochemical properties
Cyclic voltammograms of all complexes were recorded in
CH2Cl2 solution containing 0.1 M n-Bu4NClO4 and mea-
sured relative to a ferrocenium/ferrocene reference (Fc+/
Fc) (Figure S1). Each of the complexes shows a
quasireversible oxidation peak (Eox) in the 0.61–0.72 V
region (Table 1). The HOMO energy levels derived from
Eox are −5.41 to −5.52 eV, which are all less than those
obtained in DFT calculations (Table 1). Moreover, there

DOU ET AL.
5
TABLE 2 Contributions, oscillator strengths, and transition characters of singlet and triplet transitions for complexes Ir1–Ir5
Complex
Ir1
Major assignments
Transition
Oscillation strength
S1
HOMO ! LUMO+1
MLCT/ILCT
0.13
T1
HOMO-2 ! LUMO (27%)
HOMO-1 ! LUMO+1 (33%)
HOMO ! LUMO (15%)
HOMO ! LUMO
HOMO-2 ! LUMO (29%)
HOMO-1 ! LUMO (26%)
HOMO ! LUMO (13%)
HOMO ! LUMO+1
HOMO-2 ! LUMO (26%)
HOMO-1 ! LUMO+1 (34%)
HOMO ! LUMO (15%)
HOMO ! LUMO+1
HOMO-2 ! LUMO (29%)
HOMO-1 ! LUMO+1 (34%)
HOMO ! LUMO (13%)
HOMO ! LUMO+1
3MLCT/3ILCT
3MLCT/3ILCT/3LLCT
3MLCT/3ILCT
MLCT/ILCT
3MLCT/3ILCT
3MLCT/3ILCT/3LLCT
3MLCT/3ILCT
MLCT/ILCT
3MLCT/3ILCT
3MLCT/3ILCT/3LLCT
3MLCT/3ILCT
MLCT/ILCT
3MLCT/3ILCT
3MLCT/3ILCT/3LLCT
3MLCT/3ILCT
MLCT/ILCT
0
0
0
Ir2
Ir3
Ir4
Ir5
S1
0.1189
T1
0
0
0
S1
0.13
T1
0
0
0
S1
0.1204
T1
0
0
0
S1
0.1217
T1
HOMO-2 ! LUMO (28%)
HOMO-1 ! LUMO+1 (33%)
HOMO ! LUMO (14%)
3MLCT/3ILCT
3MLCT/3ILCT/3LLCT
3MLCT/3ILCT
0
0
0
FIGURE 5 Triplet spin-density distributions of complexes Ir1–Ir5 from DFT calculations. Contours are isovalued at 0.0004
1
3 | EXPERIMENTAL
3.1 | Synthesis of ligands
compound 1 (2.9 g, yield: 73.8%). H NMR (400 MHz,
CDCl3) δ 8.35 (s, 1H), 7.77–7.79 (m, 3H), 7.68 (d,
J = 7.8 Hz, 1H), 7.63 (d, J = 7.8 Hz, 2H), 7.33 (t,
J = 7.4 Hz, 1H), 7.12 (t, J = 7.3 Hz, 1H).
3.1.1 | Synthesis of 2-(4-bromophenyl)-
2H-indazole (compound 1)
3.1.2 | Synthesis of 2-([1,10-biphenyl]-
4-yl)-2H-indazole (4-hbpiz, L1)
4-Bromoaniline
(2.5
g,
14.6 mmol)
and
2-nitrobenzaldehyde (2.2 g, 14.6 mmol) in EtOH (50 ml)
was refluxed for 5 hr in the presence of N2. After the sol-
vent was removed, the obtained yellow solid was directly
used and dissolved in P(OEt)3 (50 ml). The reaction mix-
ture was heated at 110ꢀC for 10 hr in the presence of N2.
The crude product was purified by a silica gel column
(petroleum ether: ethyl acetate = 50:1) to obtain
Compound 1 (490 mg, 1.79 mmol) and phenylboronic
acid (328 mg, 2.69 mmol) in DMF (5 ml) was stirred for
10 min in the presence of N2, and then, K2CO3 (496 mg,
3.59 mmol) and Pd(dppf)Cl2 (51 mg, 0.07 mmol) were
added. The reaction mixture was heated at 100ꢀC for
8 hr. The crude product was purified by a silica gel

6
DOU ET AL.
column to obtain L1 (310 mg, yield: 66.9%). 1H NMR
(400 MHz, CDCl3) δ 8.47 (s, 1H), 7.99 (d, J = 7.9 Hz, 2H),
7.69–7.85 (m, 4H), 7.65 (d, J = 7.1 Hz, 2H), 7.48 (t, J = 7.0 Hz,
2H), 7.32–7.39 (m, 2H), 7.13 (t, J = 8.0 Hz, 1H).
L2–L3 were obtained in excellent yields by following
the procedure described for L1.
(400 MHz, CDCl3) δ 8.56 (s, 2H), 7.74 (dd, J1 = 8.5,
J2 = 3.5 Hz, 4H), 7.42 (d, J = 8.2 Hz, 2H), 7.33–7.38 (m,
2H), 7.13–7.20 (m, 12H), 7.05 (dd, J1 = 8.2, J2 = 1.8 Hz,
2H), 6.34 (d, J = 1.8 Hz, 2H), 5.29 (s, 1H), 1.82 (s, 6H).
HRMS calcd for C43H33IrN4O2: 830.220 ([M]+). Found:
830.226. Anal. calcd for C43H33IrN4O2: C, 62.23; H,
4.01; N, 6.75. Found: C, 62.22; H, 4.02; N, 6.77.
Ir2–Ir5 were obtained in good to excellent yields by
following the same procedure described for Ir1.
3.1.3 | Synthesis of 2-(40-
(trifluoromethoxy)-[1,10-biphenyl]-4-yl)-2H-
indazole (4-ocf3bpiz, L2)
3.2.3 | Synthesis of (4-ocf3bpiz)2Ir
(acac) (Ir2)
1
L2 H NMR (400 MHz, CDCl3) δ 8.47 (s, 1H), 8.01 (d,
J = 7.6 Hz, 2H), 7.80 (d, J = 8.8 Hz, 1H), 7.71–7.73 (m,
3H), 7.66 (d, J = 7.6 Hz, 2H), 7.32–7.36 (m, 3H), 7.13 (t,
J = 7.3 Hz, 1H).
1
Ir2 (48 mg, yield: 62.8%). H NMR (400 MHz, CDCl3) δ
8.59 (s, 2H), 7.76 (d, J = 8.8 Hz, 2H), 7.74 (d, J = 8.6 Hz,
2H), 7.45 (d, J = 8.2 Hz, 2H), 7.36 (t, J = 7.6 Hz, 2H),
7.26–7.28 (m, 2H), 7.12–7.14 (m, 4H), 7.01–7.04 (m, 6H),
6.29 (s, 2H), 5.29 (s, 1H), 1.83 (s, 6H). HRMS calcd for
C45H31IrF6N4O4: 998.188 ([M + H]+). Found: 999.286.
Anal. calcd for C45H31IrF6N4O4: C, 54.16; H, 3.13; N,
5.61. Found: C, 54.15; H, 3.14; N, 5.60.
3.1.4 | Synthesis of 2-(40-fluoro-[1,10-
biphenyl]-4-yl)-2H-indazole (4-fbpiz, L3)
1
L3 H NMR (400 MHz, CDCl3) δ 8.44 (s, 1H), 7.97 (d,
J = 7.6 Hz, 2H), 7.80 (d, J = 8.6 Hz, 1H), 7.68–7.73 (m,
3H), 7.54–7.63 (m, 2H), 7.34 (t, J = 7.4 Hz, 1H),
7.11–7.18 (m, 3H).
3.2.4 | Synthesis of (4-fbpiz)2Ir
(acac) (Ir3)
L4 and L5 were synthesized according to method pre-
viously published.13
1
Ir3 (39 mg, yield: 53.6%). H NMR (400 MHz, CDCl3) δ
8.59 (s, 2H), 7.71–7.77 (m, 4H), 7.45 (d, J = 8.3 Hz, 2H),
7.36 (t, J = 7.5 Hz, 2H), 7.23–7.25 (m, 2H), 6.99–7.11 (m,
6H), 6.87 (t, J = 8.1 Hz, 4H), 6.26 (s, 2H), 5.28 (s, 1H), 1.82
(s, 6H). HRMS calcd for C43H31IrF2N4O2: 866.201 ([M
+ H]+). Found: 867.370. Anal. calcd for C43H31IrF2N4O2: C,
59.64; H, 3.61; N, 6.47. Found: C, 59.61; H, 3.62; N, 6.48.
3.2 | Synthesis of complexes
3.2.1 | Synthesis of Ir-Dimer
The reaction mixture of IrCl3ꢁ3H2O (88 mg, 0.30 mmol) and
L1 (200 mg, 0.74 mmol) in 2-ethoxymethanol (8 ml) and
H2O (4 ml) was refluxed for 10 hr in the presence of N2.
After cooling to room temperature, the resulting precipitate
was collected by filtration and washed with water to give the
yellow solid Ir-Dimer (232 mg, yield: 78.9%), which was
used in the next step without further purification.
3.2.5 | Synthesis of (2,4-dfbpiz)2Ir
(acac) (Ir4)
1
Ir4 (47 mg, yield: 60.6%). H NMR (400 MHz, CDCl3) δ
8.58 (s, 2H), 7.69–7.76 (m, 4H), 7.46 (d, J = 7.5 Hz, 2H),
7.31–7.38 (m, 2H), 7.23–7.26 (m, 2H), 6.95–7.05 (m, 4H),
6.68–6.71 (m, 4H), 6.23 (s, 2H), 5.27 (s, 1H), 1.82 (s, 6H).
HRMS calcd for C43H29IrF4N4O2: 902.190 ([M + H]+).
Found: 903.215. Anal. calcd for C43H29IrF4N4O2: C,
57.26; H, 3.24; N, 6.21. Found: C, 57.24; H, 3.26; N, 6.23.
3.2.2 | Synthesis of (4-hbpiz)2Ir
(acac) (Ir1)
To a mixture of Ir-Dimer (1.0 eq) and acetylacetone
(8.0 eq) in five volumes of 2-ethoxyethanol was added
Na2CO3 (5.0 eq), and this mixture was refluxed for 12 hr
in the presence of N2. After cooling to room temperature,
the reaction mixture was poured into water. The
resulting precipitate was collected by filtration and
washed with water and purified by flash column chroma-
tography to afford Ir1 (69 mg, yield: 84.9%). 1H NMR
3.2.6 | Synthesis of (3,5-dfbpiz)2Ir
(acac) (Ir5)
1
Ir5 (50 mg, yield: 64.3%). H NMR (400 MHz, CDCl3) δ
8.62 (s, 2H), 7.78 (d, J = 8.3 Hz, 2H), 7.72 (d, J = 8.7 Hz,

DOU ET AL.
7
T. W. Rees, L. N. Ji, H. Chao, Coord. Chem. Rev. 2019,
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2H), 7.45 (d, J = 8.5 Hz, 2H), 7.39 (t, J = 7.3 Hz, 2H), 7.29
(d, J = 7.3 Hz, 2H), 7.01 (d, J = 7.8 Hz, 2H), 6.55–6.62 (m,
6H), 6.24 (s, 2H), 5.29 (s, 1H), 1.83 (s, 6H). HRMS calcd
for C43H29IrF4N4O2: 902.19 ([M + H]+). Found: 903.19.
Anal. calcd for C43H29IrF4N4O2: C, 57.26; H, 3.24; N,
6.21. Found: C, 57.28; H, 3.21; N, 6.23.
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4 | CONCLUSIONS
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In conclusion, a new series of piz-based iridium(III)
complexes with acacH as the ancillary ligand have
been synthesized and characterized. Their photo-
physical properties and electrochemical behaviors have
been investigated, as well as complemented by DFT
and TD-DFT calculations. The calculated results
showed that the lowest-lying absorptions are all attrib-
uted to MLCT/ILCT transitions, and the lowest-lying
triplet excited states have a mixed 3MLCT/3ILCT/
3LLCT character.
ACKNOWLEDGMENTS
This work was supported by the Natural Science Founda-
tion of Hainan Province (219MS043), the National Natu-
ral Science Foundation of China (21501037), and the
2019 Hainan Provincial Innovation Experiment Program
for University Students.
[16] P. Coppo, E. A. Plummerab, L. De Cola, Chem. Commun.
2004, 15, 1774.
[17] Z. G. Niu, T. Zheng, Y. H. Su, P. J. Wang, X. Y. Li, F. Cui, J.
Liang, G. N. Li, New. J. Chem. 2015, 39(8), 6025.
[18] A. Bossi, A. F. Rausch, M. J. Leitl, R. Czerwieniec, M. T.
Whited, P. I. Djurovich, H. Yersin, M. E. Thompson, Inorg.
Chem. 2013, 52, 12403.
ORCID
Gao-Nan Li https://orcid.org/0000-0001-6408-2672
REFERENCES
SUPPORTING INFORMATION
Additional supporting information may be found online
in the Supporting Information section at the end of this
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