Monodentate Benzo[d]imidazole-Based Iridium(III) Complexes and Their Dual Fluorescent and Phosphorescent Emissions

Article abstract of DOI:10.1002/bkcs.11949

We herein reported the preparation of three heteroleptic iridium complexes possessing the monodentate benzo[d]imidazole ligand, namely IrBzH (bis[2-(4,6-difluorophenyl)pyridinato-C^N](1-pyridin-2-yl)-1H-benzo[d]imidazolate)iridium(III)), IrBzCN (bis[2-(4,6-difluorophenyl)pyridinato-C^N](2-(1H-benzo[d]imidazolate-1-yl)isonicotinonitrile)iridium(III)), and IrBzBr (bis[2-(4,6-difluorophenyl)pyridinato-C′N](1-(4-bromopyridin-2-yl)-1H-benzo[d]imidazolate)iridium(III)). The successful preparation of these complexes was confirmed by multinuclear NMR spectroscopy and elemental analysis, while the molecular structures were determined by X-ray diffraction. The photoluminescence spectra of all three complexes in toluene at ambient temperature exhibited a dual-emissive pattern in the high- and low-energy regions. For IrBzCN and IrBzBr, the emissions were simultaneous in accordance with the excitation energies (λ_em = 342 and 370 nm), indicating that exhibiting both fluorescent (in the high-energy region) and phosphorescent (in the low-energy region) emissions is an inherent property of these complexes. Time-dependent density functional theory calculations verified that each fluorescent emission of the Ir complexes is associated with a ligand-to-ligand charge transfer transition, and the phosphorescent emission can be attributed to typical triplet metal-to-ligand charge transfer transitions between Ir³⁺ and the difluorophenylpyridine ligand.

Full text of DOI:10.1002/bkcs.11949

Article
DOI: 10.1002/bkcs.11949
BULLETIN OF THE
C. H. Ryu et al.
KOREAN CHEMICAL SOCIETY
Monodentate Benzo[d]imidazole-Based Iridium(III) Complexes and
Their Dual Fluorescent and Phosphorescent Emissions
†
†
†
‡
‡
‡
Chan Hee Ryu, Mingi Kim, Changho Sohn, Ju Hyun Hong, Sehee Im, So Yeon Kim, and
†,‡,
*
Kang Mun Lee
†Department of Chemistry, Institute for Molecular Science and Fusion Technology, Kangwon National
University, Chuncheon, Gangwon, 24341, Republic of Korea
‡
Department of Chemistry, Division of Chemistry and Biochemistry, Kangwon National University,
Chuncheon, Gangwon, 24341, Republic of Korea. *E-mail: kangmunlee@kangwon.ac.kr
Received November 18, 2019, Accepted December 10, 2019
We herein reported the preparation of three heteroleptic iridium complexes possessing the monodentate
benzo[d]imidazole ligand, namely IrBzH (bis[2-(4,6-difluorophenyl)pyridinato-C ^N ](1-pyridin-2-yl)-1H-
benzo[d]imidazolate)iridium(III)), IrBzCN (bis[2-(4,6-difluorophenyl)pyridinato-C ^N ](2-(1H-benzo[d]
0
imidazolate-1-yl)isonicotinonitrile)iridium(III)), and IrBzBr (bis[2-(4,6-difluorophenyl)pyridinato-C N]
(1-(4-bromopyridin-2-yl)-1H-benzo[d]imidazolate)iridium(III)). The successful preparation of these com-
plexes was confirmed by multinuclear NMR spectroscopy and elemental analysis, while the molecular
structures were determined by X-ray diffraction. The photoluminescence spectra of all three complexes in
toluene at ambient temperature exhibited a dual-emissive pattern in the high- and low-energy regions. For
IrBzCN and IrBzBr, the emissions were simultaneous in accordance with the excitation energies
(
λem = 342 and 370 nm), indicating that exhibiting both fluorescent (in the high-energy region) and
phosphorescent (in the low-energy region) emissions is an inherent property of these complexes. Time-
dependent density functional theory calculations verified that each fluorescent emission of the Ir
complexes is associated with a ligand-to-ligand charge transfer transition, and the phosphorescent emis-
3+
sion can be attributed to typical triplet metal-to-ligand charge transfer transitions between Ir and the
difluorophenylpyridine ligand.
Keywords: Benzo[d]imidazole, Charge transfer, Fluorescence, Iridium, Phosphorescence
Introduction
of such groups into the ligands as a molecular scaffold for
Ir(III) complexes can evoke fascinating luminescent char-
acteristics, such as fine-tuning of the emission color, and
modifications to alter the phosphorescence emission effi-
ciency, thereby expanding their use as outstanding lumi-
nescent materials.
Organometallic complexes possessing luminescent proper-
ties have been extensively examined as outstanding func-
tional materials for use in optoelectronic devices.
1
–6
In
particular, iridium(III)-based organometallic complexes
have been widely studied as potential and prominent phos-
phorescent luminophores for organic light-emitting diode
devices due to their superior properties, such as high quan-
tum efficiencies, good color purities, and high thermal and
In line with the exploration of a novel class of Ir(III)-
based luminophores, including new types of ligands, we
systematically designed 1-(pyridin-2-yl)-1H-benzo[d]imid-
azole derivative ligands and introduced them into dfphpy
ligand-based Ir(III) complexes as an ancillary ligand. Inter-
estingly, these benzo[d]imidazole ligands bind to the Ir(III)
center in a N-monodentate fashion, and furthermore, the
obtained Ir complexes simultaneously exhibited both fluo-
rescent and phosphorescent emissions. In fact, there have
been nearly previously reported studies on monomeric
Ir(III) complexes that can display such dual emissions. We
therefore investigated the synthesis, characterization, struc-
tural analysis, and fundamental nature of the photophysical
properties for these novel benzo[d]imidazole-based Ir(III)
chelating complexes (i.e., IrBzH, IrBzCN, and IrBzBr) in
detail.
6–22
electrochemical stabilities.
The specific properties of
these Ir(III)-chelating complexes can be finely tuned by the
systematic formation of ligand frameworks, since the elec-
tronic characteristics for the complexes that correspond to
the emission based on lowest-lying singlet or triplet excited
states (metal-to-ligand charge transfer ( MLCT or MLCT)
or ligand-to-ligand charge transfer (LL’CT)) are modified
through the introduction of various functional groups on
the C ^N chelate ligands (e.g., 2-phenylpyridine (ppy) and
1
3
23–41
2
-(2,4-difluorophenyl)pyridine (dfphpy)).
The introduction
Chan Hee Ryu, Mingi Kim, and Changho Sohn contributed
equally to this work.
Bull. Korean Chem. Soc. 2020
© 2020 Korean Chemical Society, Seoul & Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Wiley Online Library
1

Article
BULLETIN OF THE
ISSN (Print) 0253-2964 | (Online) 1229-5949
KOREAN CHEMICAL SOCIETY
Experimental
114.31, 112.64. Anal. Calcd for C H N : C, 73.83; H,
1
2 9 3
4
.65; N, 21.52. Found: C, 73.61; H, 4.55; N, 21.40.
General Considerations. All experiments were performed
Data for BzCN. 1H-Benzo[d]imidazole (13.0 g,
under an inert atmosphere (N2 gas) based on standard
110 mmol), 2-bromoisonicotinonitrile (18.3 g, 100 mmol),
potassium tert-butoxide (15.6 g, 140 mmol), CuI (0.96 g,
5 mmol), 1H-benzo[d][1,2,3]triazole (1.20 g, 10.0 mmol),
4
2,43
Schlenk and glove box techniques.
Anhydrous grade
solvents obtained from Sigma-Aldrich were dried through
an activated alumina column and were stored with activated
molecular sieves (5 Å). Spectrophotometric-grade solvents
and DMSO (60 mL) afforded BzCN as a pale yellow solid.
1
Yield = 11.2 g, 51%. H NMR (CDCl ): δ 8.80 (dd,
3
(
toluene, ethyl acetate, n-hexane, dichloromethane, and tet-
J = 5.0, 0.7 Hz, 1H), 8.63 (s, 1H), 8.09 (dd, J = 7.5,
1.6 Hz, 1H), 7.90 (dd, J = 8.2, 1.3 Hz, 1H), 7.84 (s, 1H),
7.53 (dd, J = 5.0, 1.2 Hz, 1H), 7.44 (td, J = 7.3, 1.3 Hz,
rahydrofuran (THF)) were used from Alfa Aesar. The
following commercial reagents were obtained from Sigma-
Aldrich and were used without further purification:
copper(I) iodide, potassium carbonate, sodium carbonate,
and triethylamine. Similarly, the following reagents were
obtained from TCI Chemicals and were used as received:
13
2H). C NMR (CDCl ): δ 150.73, 150.66, 144.77, 140.84,
3
131.60, 124.94, 124.12, 123.24, 122.76, 121.05, 115.76,
115.41, 112.74. Anal. Calcd for C H N : C, 70.90; H,
13 8 4
3.66; N, 25.44. Found: C, 70.73; H, 3.42; N, 25.40.
Data for BzBr. 1H-Benzo[d]imidazole (13.0 g,
110 mmol), 4-bromo-2-iodopyridine (28.4 g, 100 mmol),
potassium tert-butoxide (15.6 g, 140 mmol), CuI (0.96 g,
5 mmol), 1H-benzo[d][1,2,3]triazole (1.20 g, 10.0 mmol),
2
2
-bromopyridine, 2-bromo-4-cyanopyridine, and 4-bromo-
-iodopyridine. In addition, 1H-benzo[d]imidazole, 1H-
benzo[d][1,2,3]triazole, 2,4-difluorobenzeneboronic acid,
and potassium tert-butoxide were purchased from Alfa
Aesar and were used without further purification. Deute-
rium solvents (CDCl and tetrahydrofuran‑d , Cambridge
and DMSO (60 mL) afforded BzBr as a pale yellow solid.
1
Yield = 12.9 g, 47%. H NMR (CDCl ): δ 8.63 (dd,
3
8
3
Isotope Laboratories) were used after drying with activated
molecular sieves (5 Å). NMR spectra were examined by
using a Bruker Avance 400 spectrometer (400.13 MHz for
J = 2.4, 1.1 Hz, 1H), 8.09 (s, 1H), 7.93–7.87 (m, 1H),
7.78–7.71 (m, 2H), 7.50–7.46 (m, 1H), 7.42–7.36 (m, 2H).
13
C NMR (CDCl ): δ 145.30, 144.14, 141.53, 140.92,
3
1
13
H and 100.62 MHz for C) at ambient temperature
298 K). Chemical shifts are referenced to external tetra-
133.72, 133.26, 132.65, 129.27, 124.49, 123.54, 121.11,
109.75. Anal. Calcd for C H BrN : C, 52.58; H, 2.94; N,
(
12
8
3
1
13
methylsilane (Me Si) for H and C NMR) and are given
15.33. Found: C, 52.33; H, 2.84; N, 15.15.
4
in ppm. Elemental analysis was carried out on an EA1110
General Synthesis of the Iridium Complexes. Toluene
(10 mL) was added via cannula to a mixture of [(dfphpy)2Ir
(μ-Cl)] , NEt , and 1-(pyridin-2-yl)-1H-benzo[d]imidazole
44
analyzer (Fisons Instruments). The dfphpy ligand and
45
[
(dfphpy) Ir(μ-Cl)]2 were prepared according to literature
2
2
3
procedures.
at room temperature. The reaction mixture was then slowly
ꢀ
General Synthesis of 1-(Pyridin-2-yl)-1H-benzo[d]imid-
azole Derivatives. All 1-(pyridin-2-yl)-1H-benzo[d]imid-
azole derivative ligands (BzH, BzCN, and BzBr) were
synthesized using the materials and amounts described
later. All reagents were combined in DMSO and refluxed
heated to 110 C and refluxed for 12 h. After cooling to
ambient temperature, deionized water (50 mL) was slowly
poured into the reaction mixture, which was subsequently
extracted with dichloromethane (3 × 50 mL). The com-
bined organic layers were dried over anhydrous MgSO , fil-
4
ꢀ
for 14 h at 110 C. After cooling to room temperature,
tered, and concentrated to dryness, affording a dark yellow
residue. The crude product was purified via column chro-
matography on silica gel (eluent: ethyl acetate/n-hexane =
1/5, v/v) to yield the iridium complex as a yellow solid.
Data for IrBzH. [(dfphpy) Ir(μ-Cl)] (0.17 g, 0.14 mmol),
deionized water (50 mL) was slowly poured into the reac-
tion mixture and the mixture was washed with ethyl acetate
(3 × 50 mL). The organic layers were separated, combined,
dried over anhydrous MgSO , filtered, and concentrated to
4
2
2
dryness, affording a dark yellow residue. The crude product
was purified via column chromatography on silica gel (elu-
ent: dichloromethane/ethyl acetate = 4/1, v/v) to yield the
product as a yellowish solid.
NEt3 (0.050 mL, 0.36 mmol), BzH (0.07 g, 0.36 mmol),
and toluene (10 mL) afforded IrBzH as a yellow solid.
1
Yield = 0.23 g, 19%. H NMR (CDCl ): δ 9.96 (s, 2H),
3
8.62 (d, J = 3.7 Hz, 1H), 8.32 (t, J = 7.5 Hz, 2H), 8.15 (d,
J = 8.3 Hz, 1H), 8.08 (d, J = 8.2 Hz, 1H), 7.97–7.89 (m,
1H), 7.80–7.73 (m, 1H), 7.69 (t, J = 7.7 Hz, 1H), 7.58 (d,
J = 8.1 Hz, 1H), 7.37 (dd, J = 7.2, 5.0 Hz, 1H), 7.32
(t, J = 7.3 Hz, 1H), 7.25–7.20 (m, 1H), 7.08 (dt, J = 14.0,
8.1 Hz, 2H), 6.95 (t, J = 6.3 Hz, 1H), 6.42 (dd, J = 11.7,
2.4 Hz, 1H), 6.35 (dd, J = 11.8, 2.3 Hz, 1H), 6.06 (dd,
Data for BzH. 1H-Benzo[d]imidazole (13.0 g, 110 mmol),
2-bromopyridine (9.8 mL, 100 mmol), potassium tert-
butoxide (15.6 g, 140 mmol), CuI (0.96 g, 5 mmol), 1H-
benzo[d][1,2,3]triazole (1.20 g, 10.0 mmol), and DMSO
(
60 mL) afforded BzH as a yellow solid. Yield = 9.5 g,
1
4
8
1
1
9%. H NMR (CDCl ): δ 8.63 (dd, J = 4.9, 1.1 Hz, 1H),
3
13
.59 (s, 1H), 8.10–8.05 (m, 1H), 7.93 (td, J = 7.8, 1.9 Hz,
H), 7.88 (dd, J = 6.8, 1.8 Hz, 1H), 7.60 (d, J = 8.2 Hz,
H), 7.43–7.35 (m, 2H), 7.32 (dd, J = 7.4, 4.9 Hz, 1H).
J = 8.7, 1.9 Hz, 1H), 5.65 (dd, J = 8.7, 2.1 Hz, 1H).
C
NMR (CDCl ): δ 165.04, 150.47, 149.49, 149.14, 144.62,
3
142.25, 139.32, 137.81, 137.56, 132.49, 125.02, 124.44,
123.27, 123.08, 123.00, 122.43, 122.27, 121.98, 118.91,
115.51, 113.91, 97.77, 97.50. Anal. Calcd for
1
3
C NMR (CDCl ): δ 149.91, 149.49, 144.73, 141.34,
3
138.95, 132.14, 124.21, 123.29, 121.84, 120.68, 114.33,
Bull. Korean Chem. Soc. 2020
© 2020 Korean Chemical Society, Seoul & Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.bkcs.wiley-vch.de
2

BULLETIN OF THE
Article
Monodentate Benzo[d]imidazole-Based Iridium(III) Complexes
KOREAN CHEMICAL SOCIETY
C H ClF IrN : C, 50.84; H, 2.64; N, 8.72. Found: C,
atmosphere using a HORIBA FluoroMax-4P spectropho-
tometer. The absolute quantum yields for the photo-
luminescence of IrBzH, IrBzCN, and IrBzBr in toluene
(3.0 × 10− M) were obtained using a Fluoromax-4P spec-
trophotometer (HORIBA) equipped on a 3.2 in. integrat-
ing sphere (FM-sphere, HORIBA) at ambient
temperature. The fluorescence and phosphorescence
decay lifetimes for IrBzH, IrBzCN, and IrBzBr in tolu-
ene (3.0 × 10− M) were detected using a time-correlated
single-photon counting spectrometer (FLS920, TCSPC
spectrometer form EDINBURGH Instruments) including
an semiconductor diode laser (EPL-375 ps pulsed excita-
tion source) and a detector of a microchannel plate pho-
tomultiplier tube (MCP-PMT, 200–850 nm) at 298 K.
Cyclic voltammetry (CV) measurements using an
AUTOLAB/PGSTAT12 system were carried out for all
iridium complexes in acetonitrile (0.5 mM) using a three-
electrode cell configuration (platinum working and car-
bon counter electrodes, and a silver/silver nitrate (0.1 M
in acetonitrile) reference electrode) at ambient tempera-
ture. An acetonitrile solution of Bu NꢁPF (0.1 M) was
3
4
21
4
5
50.66; H, 2.50; N, 8.67.
Data for IrBzCN. [(dfphpy) Ir(μ-Cl)] (0.17 g, 0.14 mmol),
2
2
5
NEt3 (0.050 mL, 0.36 mmol), BzCN (0.08 g, 0.36 mmol),
and toluene (10 mL) afforded IrBzCN as a yellow solid.
1
Yield = 0.30 g, 24%. H NMR (CDCl ): δ 10.08 (s, 1H),
3
9.97 (d, J = 5.4 Hz, 1H), 8.81 (d, J = 4.8 Hz, 1H), 8.35 (d,
J = 8.6 Hz, 1H), 8.26 (d, J = 5.5 Hz, 1H), 8.14–8.07 (m, 2H),
5
7
1
.83 (s, 1H), 7.78 (t, J = 7.7 Hz, 1H), 7.72 (t, J = 7.9 Hz,
H), 7.59 (d, J = 4.9 Hz, 1H), 7.38 (t, J = 7.7 Hz, 1H), 7.28
(d, J = 6.1 Hz, 1H), 7.18–7.13 (m, 1H), 7.09 (d, J = 8.3 Hz,
1
6
H), 6.96 (t, J = 6.3 Hz, 1H), 6.44 (dd, J = 11.8, 2.3 Hz, 1H),
.37 (dd, J = 12.0, 2.3 Hz, 1H), 6.10–6.02 (m, 1H), 5.63 (dd,
13
J = 8.7, 2.0 Hz, 1H). C NMR (THF-d ): δ 164.81, 150.61,
8
150.41, 145.23, 142.45, 137.94, 137.50, 132.35, 128.68,
127.92, 124.92, 124.35, 124.21, 123.48, 122.65, 122.45,
122.35, 121.98, 118.77, 117.27, 115.56, 114.14, 96.68, 96.52.
Anal. Calcd for C H ClF IrN : C, 50.76; H, 2.43; N, 10.15.
35
20
4
6
Found: C, 50.58; H, 2.35; N, 10.00.
Data for IrBzBr. [(dfphpy) Ir(μ-Cl)]
(0.17 g,
2
2
0
.14 mmol), NEt (0.050 mL, 0.36 mmol), BzBr (0.10 g,
3
4
6
0
.36 mmol), and toluene (10 mL) afforded IrBzBr as a yel-
used as a supporting electrolyte. Each potential (oxida-
tion and reduction) was investigated at a scan rate of
100 mV s− an+d measured with reference to the ferro-
cene/ferrocene redox couple.
1
low solid. Yield = 0.23 g, 19%. H NMR (CDCl ): δ 9.85
3
1
(d, J = 5.6 Hz, 1H), 9.51 (s, 1H), 8.46 (d, J = 2.4 Hz, 1H),
8
1
7
2
.25 (t, J = 8.4 Hz, 2H), 8.04–7.98 (m, 1H), 7.74–7.67 (m,
H), 7.64 (t, J = 8.2 Hz, 3H), 7.33 (d, J = 3.6 Hz, 1H),
.18–7.13 (m, 1H), 7.10–7.05 (m, 2H), 6.90 (td, J = 8.24,
.4 Hz, 1H), 6.37–6.32 (m, 3H), 6.04–5.98 (m, 1H), 5.55
1
3
(dd, J = 8.4, 1.9 Hz, 1H). C NMR (THF-d ): δ 165.13,
8
ꢀ
1
1
1
9
4
50.37, 145.57, 144.97, 142.23, 137.90, 137.66, 134.25,
33.45, 131.39, 129.46, 125.29, 124.75, 123.34, 123.14,
22.51, 122.31, 121.95, 119.43, 115.35, 114.66, 110.59,
7.84, 97.59. Anal. Calcd for C H BrClF IrN : C,
Table 1. Selected bond lengths (Å) and angles ( ) for IrBzH,
IrBzCN, and IrBzBr.
Compound
IrBzH
IrBzCN
IrBzBr
3
4
20
4
5
Lengths
In-Cl
In-N1
In-N4
In-C13
6.29; H, 2.29; N, 7.94. Found: C, 45.99; H, 2.13; N, 7.82.
2.5024(11)
2.197(3)
2.048(4)
2.001(4)
2.041(4)
1.998(4)
2.5003(8)
2.188(3)
2.040(3)
1.991(4)
2.041(3)
1.991(4)
2.4709(11)
2.188(4)
2.046(4)
2.016(5)
2.054(4)
1.992(5)
X-Ray Crystallographic Analysis for Single Crystals of
the Iridium Complexes. Single crystals of the iridium
complexes IrBzH, IrBzCN, and IrBzBr were protected by
coating with Paratone oil and attached to glass capillaries.
Next, crystallographic measurements were performed on a
Bruker SMART Apex II CCD area detector diffractometer
In-N5
In-C24
Angles
using
graphite-monochromated
Mo-Kα
radiation
Cl-Ir-N1
Cl-Ir-N4
Cl-Ir-C13
Cl-Ir-N5
Cl-Ir-C24
N1-Ir-N4
N1-Ir-N5
N1-Ir-C13
N1-Ir-C24
N4-Ir-N5
N4-Ir-C13
N4-Ir-C24
N5-Ir-C13
N5-Ir-C24
C13-Ir-C24
86.93(10)
95.12(11)
175.23(13)
88.53(10)
93.42(13)
86.85(13)
96.45(13)
94.28(14)
176.92(16)
175.22(14)
80.36(17)
96.17(15)
95.91(16)
80.50(15)
85.62(16)
87.18(8)
88.46(9)
90.10(10)
96.59(9)
85.43(10)
87.95(11)
93.92(12)
95.59(12)
175.78(14)
97.61(15)
84.40(15)
177.43(16)
92.85(17)
176.07(14)
79.87(18)
96.10(18)
98.14(18)
80.40(18)
87.97(18)
(
λ = 0.71073 Å). Each structure was solved by a straight-
forward way, and all non-hydrogen atoms deal with aniso-
tropic refinement by the SHELXTL/PC package system,
giving the X-ray crystallographic data of IrBzH, IrBzCN,
and IrBzBr in CIF format (CCDC 1964005–1964007).
Hydrogen atoms have been refined based on the
corresponding carbon atoms with geometrically calculated
positions and isotropic thermal parameters. Detailed crystal-
lographic data and selected bond lengths and angles are dis-
played in Table S1 and Table 1, respectively.
175.08(11)
98.01(12)
85.15(12)
176.86(13)
96.31(13)
174.19(12)
80.32(14)
94.47(14)
96.74(14)
80.30(15)
86.50(14)
Photophysical Measurements. Ultraviolet–visible (UV–
Vis) absorption spectra were obtained in THF (3.0 ×
−5
10
M) under an inert atmosphere using a Varian Cary
1
00 spectrophotometer. Photoluminescence (PL) spectra
−
5
were obtained in toluene (3.0 × 10 M) under an inert
Bull. Korean Chem. Soc. 2020
© 2020 Korean Chemical Society, Seoul & Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.bkcs.wiley-vch.de
3

BULLETIN OF THE
Article
Monodentate Benzo[d]imidazole-Based Iridium(III) Complexes
KOREAN CHEMICAL SOCIETY
ꢀ
Scheme 1. Synthetic routes for IrBzH, IrBzCN, and IrBzBr. Reagents and conditions: (i) 1H-Benzo[d]imidazole, CuI, DMSO, 110 C,
ꢀ
1
4 h. (ii) [Ir(dfphpy) Cl] , toluene, NEt , 110 C, 12 h.
2
2
3
Theoretical Calculations. Each structure at the ground
S ) and first singlet (S ) and triplet (T ) excited states of
subsequent reaction of [(dfphpy) Ir(μ-Cl)] with each benzo
2
2
(
[d]imidazole ligand and a weak base (NEt ) in toluene
3
0
1
1
IrBzH was optimized via density functional theory (DFT)
calculations based on the B3LYP functional. All calcula-
tions assumed a toluene solution and solvent effects were
reflected with the CPCM (conductor-like polarizable con-
afforded the iridium complexes IrBzH, IrBzCN, and
IrBzBr in moderate yields (19% for IrBzH, 24% for
IrBzCN, and 19% for IrBzBr). The iridium complexes
1
13
were then characterized using H and C NMR spectros-
copy (Figures S1–S6) and elemental analysis. In addition,
the structures of the iridium complexes were fully charac-
terized by X-ray diffraction (Figure 1). Investigation into
46,47
48
tinuum) model.
for the iridium atom, and the 6-31G(d) basis set was uti-
The LANL2DZ basis set was utilized
4
9
5
0
lized for all other atoms. TD-DFT calculations based on
the B3LYP functional (TD-B3LYP) were utilized for the
electronic transition energies, which also accounted for
electron correlation. The calculations were carried out using
the GAUSSIAN 09 program. The percent contribution of
each molecular orbital for the complex was estimated by
3+
their structural features showed that the Ir centers are in a
distorted octahedral environment with the bond between
−
3+
Cl and Ir in the equatorial position. Interestingly, each
5
1
benzo[d]imidazole ligand is bound at the equatorial plane
3+
of the Ir center, which is composed of the C atoms from
−
5
2
each dfphpy ligand and Cl , as a monodentate terminal
the GaussSum 3.0 program.
ligand using the N1 atom of the benzo[d]imidazole moiety.
As a result, the N1 atom clearly exhibited a meridional for-
mation with the two axial N atoms (N4 and N5) of the
dfphpy ligands.
Results and Discussion
Synthesis and Characterization. The synthetic route to
the monodentate benzo[d]imidazole ligand-based Ir com-
plexes IrBzH, IrBzCN, and IrBzBr is shown in Scheme 1.
Optical and Electrochemical Properties. UV–Vis absorp-
tion and PL measurements were performed in degassed
organic solvent to estimate the optical properties of the Ir
complexes (Figure 2 and Table 2). Although the PL spectra
could be intensively obtained in toluene, the absorption spec-
tra were carried out in THF because those could not be cor-
rectly measured due to absorption bands of toluene itself in
The
1-(pyridin-2-yl)-1H-benzo[d]imidazole
derivative
ligands (BzH, BzCN, and BzBr) were obtained by the
copper(I)-catalyzed coupling of 1H-benzo[d]imidazole with
2-bromopyridine (for BzH), 2-bromoisonicotinonitrile (for
BzCN), or 4-bromo-2-iodopyridine (for BzBr). The
Figure 1. X-ray crystal structures of IrBzH (left), IrBzCN (center), and IrBzBr (right) (50% thermal ellipsoids). The H atoms and sol-
vents in the unit cell are omitted for clarity.
Bull. Korean Chem. Soc. 2020
© 2020 Korean Chemical Society, Seoul & Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.bkcs.wiley-vch.de
4

BULLETIN OF THE
Article
Monodentate Benzo[d]imidazole-Based Iridium(III) Complexes
KOREAN CHEMICAL SOCIETY
−
5
Figure 2. (a) UV–Vis absorption spectra in THF (3.0 × 10 M)
−5
and (b) PL spectra in toluene (3.0 × 10 M) at 298 K for IrBzH,
IrBzCN, and IrBzBr.
the high-energy region below ~310 nm. All three complexes
displayed apparent low-energy absorption bands in THF at
1
λabs = 381 nm, which could be assignable to MLCT transi-
−
1
−1
−1
−1
tions (ε = 8200 M cm for IrBzH, 12 300 M cm for
−1
−1
IrBzCN, and 8100 M cm for IrBzBr). The shallow
absorption traces for all complexes at λabs = 430 nm, which
3
tailed off to 450 nm, were attributed to MLCT transitions
1
−1
−1
−1
(
ε = 4000 M− cm
for IrBzH, 5100 M
cm
for
−1
−1
IrBzCN, and 4000 M cm for IrBzBr). The fact that all
complexes showed significantly similar absorption properties
in the low-energy region suggests that the origin of the
MLCT transitions is the same in all complexes.
The emission characteristics of the three Ir complexes
were then examined by PL (Figure 2(b) and Table 2). Inter-
estingly, the PL spectra of all three complexes in degassed
toluene at ambient temperature clearly showed separate
high- (λem = ca. 380 nm) and low- (ca. 475 nm) energy
emissions (in the case of IrBzH) or simultaneously
exhibited a dual emissive pattern in the high- and low-
energy regions (IrBzCN and IrBzBr) in accordance with
the excitation energies (λem = 342 nm and 370 nm). The
Bull. Korean Chem. Soc. 2020
© 2020 Korean Chemical Society, Seoul & Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.bkcs.wiley-vch.de
5

BULLETIN OF THE
Article
Monodentate Benzo[d]imidazole-Based Iridium(III) Complexes
KOREAN CHEMICAL SOCIETY
Figure 3. (a) Oxidative and (b) reductive CV curves for IrBzH, IrBzCN, and IrBzBr (0.5 mM in acetonitrile, scan rate = 100 mV s−1).
Figure 4. Frontier molecular orbitals for IrBzH in S
0
, S
1
, and T
1
states with their relative energies from DFT calculations (iso-
value = 0.02 a.u.).
emission decay lifetimes (τobs, Table 2 and Figures S7 and
S8) of each high- and low-energy region for the Ir com-
plexes were 2.8–2.9 ns and 0.8–1.1 μs, respectively,
indicating that the high-energy emissions represent fluores-
cence and the low-energy emissions are the typical phos-
phorescence of Ir complexes. Consequently, the three Ir
complexes present an inherent property of exhibiting both
fluorescent and phosphorescent emissions. Each fluorescent
emission of the Ir complexes is associated with the LL’CT
transition, while the phosphorescence can be attributed to
typical MLCT transitions between Ir and the dfphpy
ligand (see the electronic transition results from the TD-
DFT calculations later). Furthermore, the absolute quantum
Table 3. Major low-energy electronic transition in toluene for
IrBzH in S , S , and T states calculated via the TD-B3LYP
0
1
1
a
method with 6-31G(d) basis sets .
3
3+
b
State
λcalc/nm
421.13
fcalc
Assignment
IrBzH
S
0
0.0365
0.0789
0.0231
0
H ! L (91.9%)
H ! L + 1 (91.4%)
H ! L (97.7%)
H ! L (89.8%)
388.75
efficiencies (Φ , Table 2) for all Ir complexes were moder-
PL
S1
T1
387.94
480.02
ate at 5–9% and were not significantly altered by the excita-
tion energies.
a
The electrochemical stabilities and potentials for all three
Ir complexes were obtained by CV measurements (Figure 3
and Table 2). All three Ir complexes were subjected to a
Singlet energies for the vertical transition calculated at the optimized
geometries.
H = HOMO, L = LUMO.
S
0
b
Bull. Korean Chem. Soc. 2020
© 2020 Korean Chemical Society, Seoul & Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.bkcs.wiley-vch.de
6

BULLETIN OF THE
Article
Monodentate Benzo[d]imidazole-Based Iridium(III) Complexes
KOREAN CHEMICAL SOCIETY
reversible and consistent oxidation cycles during a continu-
ous processes, suggesting strong electrochemical stability.
photoluminescence spectra of all three complexes in the
solution state at 298 K revealed both fluorescent and
phosphorescent emissions, which could be assigned to
Moreover, the oxidation (E , Table 2) and reduction onset
ox
3
(
0
Ered) potentials for these complexes were determined to be
.57 to 0.63 eV and − 1.95 to −2.02 eV, respectively, indi-
LL’CT (ligand-to-ligand charge transfer) and MLCT
transitions, respectively. The electronic transition analysis
was supported by temperature-dependent density func-
tional theory calculations. Relevant further studies into
iridium complexes are currently underway in our labora-
tory with the aim of expanding the scope of ligands that
exhibit this dual emission, including various benzo[d]
imidazole derivatives.
cating that the band gaps (Eox − Ered) of these Ir complexes
are similar at ~2.59 eV. Consequently, the functional
groups introduced into the benzo[d]imidazole ligand did
not affect its photophysical properties.
Molecular Orbital Analysis Based on Computational
Calculations. To elucidate the origin of the electronic
intramolecular transitions and photophysical properties of
the Ir complexes, TD-DFT calculations for the ground
Acknowledgments.
This study was supported by
state (S ), first singlet (S ), and triplet (T ) excited states
0
1
1
Supporting Business for College Innovation from Kangwon
National University.
4
2,43
in toluene
were carried out on IrBzH as a representa-
tive complex based on the B3LYP functional and the
6
-31G(d) basis set (Figure 4 and Table 3). The S0-
Supporting Information. Additional supporting informa-
tion may be found online in the Supporting Information
section at the end of the article.
optimized geometry of IrBzH demonstrates that the low-
est energy absorption mainly correlates to the HOMO !
LUMO and LUMO+1 transition (>91%, Table 3). Each
LUMO level of IrBzH is primarily delocalized over the
different dfphpy ligands (90.2% on the dfphpy1 ligand for
the LUMO and 89.5% on the dfphpy2 ligand for the
LUMO+1, Table S3), while the HOMOs receive a major
contribution from the iridium center (49.0%). These
results suggest that the absorption spectra of these iridium
complexes could be primarily attributed to the MLCT
from the metal center to the dfphpy ligands. Conversely,
the major transitions for both the S - and T -optimized
References
1. J.-H. Jou, S. Kumar, A. Agrawal, T. -H. Li, S. Sahoo,
J. Mater. Chem. C 2015, 3, 2974.
2
3
. R. Visbal, M. Gimeno, Chem. Soc. Rev. 2014, 43, 3551.
. V. W. -W. Yam, K. M. -C. Wong, Chem. Commun. 2011, 47,
1
1579.
4
. H. Yersin, A. F. Rausch, R. Czerwieniec, T. Hofbeck,
T. Fischer, Coord. Chem. Rev. 2011, 255, 2622.
1
1
geometries of IrBzH are HOMO ! LUMO transitions
5
. D. Wróbel, A. Graja, Coord. Chem. Rev. 2011, 255, 2555.
(
λ
calc
= 387.94 nm for the S state and 480.02 nm for the
1
6. Y. Chi, P. -T. Chou, Chem. Soc. Rev. 2010, 39, 638.
7. Y.-I. Kim, S.-J. Yun, D. Kim, S. K. Kang, Bull. Kor. Chem.
Soc. 2018, 39, 133.
T1 state Table 3). The electronic intramolecular transitions
of the S and T states for IrBzH are in good agreement
1
1
with the experimental results for fluorescent and phospho-
rescent emissions. Although the LUMO levels of both
states are mainly localized on the dfphpy ligand (>93%,
Table S3), the major contribution of each HOMO is sig-
8. M. Kim, J. Kim, K.-M. Park, Y. Kang, Bull. Kor. Chem. Soc.
2018, 39, 703.
9
. M. Kim, J. Park, H. Oh, Y. Ryu, Y. Kang, Bull. Kor. Chem.
Soc. 2018, 39, 24.
1
1
1
1
1
0. H. Y. Shin, S. Jeong, S. H. Kim, J.-I. Hong, Bull. Kor. Chem.
Soc. 2017, 38, 830.
1. X. Yang, G. Zhou, W. -Y. Wong, Chem. Soc. Rev. 2015,
nificantly different, as that in the S state is dominant at
1
another dfphpy ligand (68%), and that in the T state is
1
mainly over the Ir center (45%). These results strongly
indicate that the high-energy fluorescence below 400 nm
for the Ir complexes is due to LL’CT transitions between
the two dfphpy ligands, and the low-energy phosphores-
cence above 420 nm is due to MLCT transitions.
4
4, 8484.
2. N. Jung, E. Lee, J. Kim, H. Park, K.-M. Park, Y. Kang, Bull.
Kor. Chem. Soc. 2012, 33, 183.
3. E. Baranoff, I. Jung, R. Scopelliti, E. Solari, M. Grätzel,
M. K. Nazeeruddin, Dalton Trans. 2011, 40, 6860.
4. G. Zhou, W. Y. Wong, X. Yang, Chem. Asian J. 2011,
6, 1706.
Conclusions
1
1
5. Y. You, S. Y. Park, Dalton Trans. 2009, 1267.
6. L. Flamigni, A. Barbieri, C. Sabatini, B. Ventura,
F. Barigelletti, Top. Curr. Chem. 2007, 281, 143.
7. P. T. Chou, Y. Chi, Chem. Eur. J. 2007, 13, 380.
8. E. Holder, B. M. W. Langeveld, U. S. Schubert, Adv. Mater.
We successfully synthesized and fully characterized
monodentate benzo[d]imidazole ligand-based heteroleptic
iridium complexes IrBzH, IrBzCN, and IrBzBr, and
confirmed their solid-state structures by X-ray diffraction
measurements of their single crystals. All three com-
plexes displayed apparent low-energy absorption bands in
1
1
2
005, 17, 1109.
1
2
9. H. Yersin, Top. Curr. Chem. 2004, 241, 1.
0. C. Adachi, M. A. Baldo, S. R. Forrest, M. E. Thompson,
Appl. Phys. Lett. 2000, 77, 904.
1
tetrahydrofuran, which were assigned to MLCT (metal-
to-ligand charge transfer) transitions, in addition to weak
absorption traces attributed to MLCT transitions. The
21. M. A. Baldo, M. E. Thompson, S. R. Forrest, Nature 2000,
403, 750.
3
Bull. Korean Chem. Soc. 2020
© 2020 Korean Chemical Society, Seoul & Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.bkcs.wiley-vch.de
7

BULLETIN OF THE
Article
Monodentate Benzo[d]imidazole-Based Iridium(III) Complexes
KOREAN CHEMICAL SOCIETY
2
2. A. Tsuboyama, H. Iwawaki, M. Furugori, T. Mukaide,
J. Kamatani, S. Igawa, T. Moriyama, S. Miura, T. Takiguchi,
S. Okada, M. Hoshino, K. Ueno, J. Am. Chem. Soc. 2003,
38. X. Yang, X. Xu, G. Zhou, J. Mater. Chem. C 2015, 3, 913.
39. R. Seifert, I. Rabelo de Moraes, S. Scholz, M. C. Gather,
B. Lüssem, K. Leo, Org. Electron. 2013, 14, 115.
1
25, 12971.
40. C. -W. Lee, J. -Y. Lee, Adv. Mater. 2013, 25, 5450.
41. E. Baranoff, B. F. E. Curchod, J. Frey, R. Scopelliti,
F. Kessler, I. Tavernelli, U. Rothlisberger, M. Grätzel,
M. K. Nazeeruddin, Inorg. Chem. 2012, 51, 215.
2
3. A. B. Tamayo, B. D. Alleyne, P. I. Djurovich, S. Lamansky,
I. Tsyba, N. N. Ho, R. Bau, M. E. Thompson, J. Am. Chem.
Soc. 2003, 125, 7377.
2
4. V. V. Grushin, N. Herron, D. D. LeCloux, W. J. Marshall,
V. A. Petrov, Y. Wang, Chem. Commun. 2001, 1494.
5. S. Lamansky, P. Djurovich, D. Murphy, F. Abdel-Razzaq,
R. Kwong, I. Tsyba, M. Bortz, B. Mui, R. Bau,
M. E. Thompson, Inorg. Chem. 2001, 40, 1704.
42. D. F. Shriver, M. A. Drezdzon, The Manipulation of Air-
Sensitive Compounds, 2nd ed., Wiley, New York, NY, 1986.
43. G. S. Girolami, T. B. Rauchfuss, R. J. Angelici, Synthesis and
Technique in Inorganic Chemistry, 3rd ed., University Sci-
ence Books, Sausalito, CA, 1999.
2
2
6. S. Lamansky, P. Djurovich, D. Murphy, F. Abdel-Razzaq,
H. E. Lee, C. Adachi, P. E. Burrows, S. R. Forrest,
M. E. Thompson, J. Am. Chem. Soc. 2001, 123, 4304.
7. T. Karatsu, M. Takahashi, S. Yagai, A. Kitamura, Inorg.
Chem. 2013, 52, 12338.
44. Y. Cudre, F. F. de Carvalho, G. R. Burgess, L. Male,
S. J. A. Pope, I. Tavernelli, E. Baranoff, Inorg. Chem. 2017,
56, 11565.
45. Y. You, S. Y. Park, J. Am. Chem. Soc. 2005, 127, 12438.
46. M. Cossi, N. Rega, G. Scalmani, V. Barone, J. Comput.
Chem. 2003, 24, 669.
47. V. Barone, M. Cossi, J. Phys. Chem. A 1998, 102, 1995.
48. J. S. Binkley, J. A. Pople, W. J. Hehre, J. Am. Chem. Soc.
1980, 102, 939.
2
2
8. K. Beydoun, M. Zaarour, J. A. G. Williams, T. Roisnel,
V. Dorcet, A. Planchat, A. Boucekkine, D. Jacquemin,
H. Doucet, V. Guerchais, Inorg. Chem. 2013, 52, 12416.
9. S. Lee, S. O. Kim, H. Shin, H. J. Yun, K. Yang,
S. K. Kwon, J. J. Kim, Y. H. Kim, J. Am. Chem. Soc. 2013,
2
49. P. J. Hay, W. R. Wadt, J. Phys. Chem. 1985, 82, 270.
50. E. Runge, E. K. U. Gross, Phys. Rev. Lett. 1984, 52, 997.
51. M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria,
M. A. Robb, J. R. Cheeseman, G. Scalmani, V. Barone,
B. Mennucci, G. A. Petersson, H. Nakatsuji, M. Caricato,
X. Li, H. P. Hratchian, A. F. Izmaylov, J. Bloino, G. Zheng,
J. L. Sonnenberg, M. Hada, M. Ehara, K. Toyota, R. Fukuda,
J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao,
H. Nakai, T. Vreven, J. A. Montgomery Jr., J. E. Peralta,
F. Ogliaro, M. Bearpark, J. J. Heyd, E. Brothers, K. N. Kudin,
V. N. Staroverov, T. Keith, R. Kobayashi, J. Normand,
K. Raghavachari, A. Rendell, J. C. Burant, S. S. Iyengar,
J. Tomasi, M. Cossi, N. Rega, J. M. Millam, M. Klene,
J. E. Knox, J. B. Cross, V. Bakken, C. Adamo, J. Jaramillo,
R. Gomperts, R. E. Stratmann, O. Yazyev, A. J. Austin,
R. Cammi, C. Pomelli, J. W. Ochterski, R. L. Martin,
K. Morokuma, V. G. Zakrzewski, G. A. Voth, P. Salvador,
J. J. Dannenberg, S. Dapprich, A. D. Daniels, O. Farkas,
J. B. Foresman, J. V. Ortiz, J. Cioslowski, D. J. Fox, Gaussian
09 Revision D, Gaussian. Inc., Wallingford, CT, 2013.
52. N. M. O’Boyle, A. L. Tenderholt, K. M. Langner, J. Comp.
Chem. 2008, 29, 839.
1
35, 14321.
3
0. R. M. Edkins, K. Fucke, M. J. G. Peach, A. G. Crawford,
T. B. Marder, A. Beeby, Inorg. Chem. 2013, 52, 9842.
1. F. Monti, F. Kessler, M. Delgado, J. Frey, F. Bazzanini,
G. Accorsi, N. Armaroli, H. J. Bolink, E. Ortí, R. Scopelliti,
M. K. Nazeeruddin, E. Baranoff, Inorg. Chem. 2013, 52, 10292.
2. 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. 2013, 52, 4916.
3. B. Wiegmann, P. G. Jones, G. Wagenblast, C. Lennartz,
I. Munster, S. Metz, W. Kowalsky, H. H. Johannes, Organo-
metallics 2012, 31, 5223.
4. C. -L. Ho, W. -Y. Wong, New J. Chem. 2013, 37, 1665.
5. X. Li, J. Zhang, Z. Zhao, L. Wang, H. Yang, Q. Chang,
N. Jiang, Z. Liu, Z. Bian, W. Liu, Z. Lu, C. Huang, Adv.
Mater. 2018, 30, 1705005.
3
3
3
3
3
3
6. H. -H. Kuo, Y. -T. Chen, L. R. Devereux, C. -C. Wu,
M. A. Fox, C. -Y. Kuei, Y. Chi, G. -H. Lee, Adv. Mater.
2
017, 29, 1702464.
3
7. Z. Chen, L. Wang, S. Su, X. Zheng, N. Zhu, C. -L. Ho, S. Chen,
W. -Y. Wong, ACS Appl. Mater. Interfaces 2017, 9, 40497.
Bull. Korean Chem. Soc. 2020
© 2020 Korean Chemical Society, Seoul & Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.bkcs.wiley-vch.de
8