Synthesis and characterization of green-emitting phosphorescent Ir(III) complexes based on phenyl benzimidazole ligand

Article abstract of DOI:10.1134/S1070363216070343

Several new Ir(III) complexes with 2-(4-bromophenyl)-1H-benzo[d]imidazole or 2-(4-bromophenyl)- 1-methyl-benzo[d]imidazole ligands as cylcometalated ligand and acetylacetonate or picolinate as the ancillary ligand were synthesized and their structures and

Full text of DOI:10.1134/S1070363216070343

ISSN 1070-3632, Russian Journal of General Chemistry, 2016, Vol. 86, No. 7, pp. 1747–1752. © Pleiades Publishing, Ltd., 2016.
Synthesis and Characterization of Green-Emitting
Phosphorescent Ir(III) Complexes Based
1
on Phenyl Benzimidazole Ligand
Meijuan Lin*, Qiang Tang, Huijuan Zeng, Guang Xing, and Qidan Ling**
College of Materials Science and Engineering Fujian Normal University,
Fujian Key Laboratory of Polymer Materials, Fuzhou, 350007 China
e-mail: *mjlin@fjnu.edu.cn; **qdling@fjnu.edu.cn
Received March 30, 2016
Abstract—Several new Ir(III) complexes with 2-(4-bromophenyl)-1H-benzo[d]imidazole or 2-(4-bromo-
phenyl)-1-methyl-benzo[d]imidazole ligands as cylcometalated ligand and acetylacetonate or picolinate as the
ancillary ligand were synthesized and their structures and photophysical properties were characterized. HOMO
and LUMO energy levels and the molecular structures of Ir(III) complexes were scrutinized by DFT calcula-
tions. The complexes exhibited green luminescence at the maximum emission peaks at ca 495–522 nm. The
methyl group substituent and replacing of acetylacetonate with picolinate complex can enhance the complex
thermal stability. HOMO energy levels of the complexes vary from –4.99 to –5.44 eV, the LUMO energy levels
are between –1.52 and –1.97 eV.
Keywords: phenyl benzimidazole, iridium complex, green-emitting phosphorescent, DFT calculation, HOMO
and LUMO energy
DOI: 10.1134/S1070363216070343
Benzimidazole derivatives have recently attracted
attention as the ligands of Ir(III) complexes owing to
their synthetic availability. Photophysical properties of
the cyclometalated iridium complexes can be finely
tuned by systematic control of their structure and the
corresponding ligands [1–8]. Acetylacetonate (acac)
and picolinate (pic) have typically been used as the
ancillary ligands of Ir(III) complexes that are
excellent green phosphorescent organic light-emitting
diodes (OLEDs) [9]. Ir(III) bis(4,6-difluorophenyl-
pyridinato)picolinate (FIrpic) is widely used in blue
phosphorescent OLEDs [10].
EXPERIMENTAL
O-Phenylenediamine (99.5%, Aladdin), 4-bromo-
benzoic acid (99%, J&K Chemical), acetylacetone
99.5%, Aladdin), picolinic acid (99%, J&K
Chemical), iodomethane (99%, J&K Chemical), and
other chemicals were analytical reagent grade and used
without further purification.
(
CHN analysis was performed on a Elementar Vario
EL-III. Melting points were determined on a Reichert
905 melting point meter. FT-IR spectra were recorded
on a Nicolet 380 FT-IR spectrophotometer (KBr
pellets). UV-Vis absorption spectra were recorded on a
Shimadzu UV-2600 spectrophotometer (in CH Cl ). H
7
1
In this communication 2-(4-bromophenyl)-1H-
benzo[d]imidazole and 2-(4-bromophenyl)-1-methyl-
benzo[d]imidazole were used as the cyclometalated
ligands, acetylacetonate and picolinate as the ancillary
ligands in development of four new green phosphore-
scent heteroleptic iridium complexes. Photophysical
properties, HOMO and LUMO energy levels of the
complexes were studied. These also can be used as
reactive monomers in development of polymer lumine-
scent materials due to bromine atom in the structure.
2
2
NMR spectra were measured on a Bruker AVIII-400
NMR spectrometer (in DMSO-d ). Photoluminescent
6
spectra were measured on an Edinburgh FLS920-stm
fluorescent spectrometer at 365 nm excitation wave-
length. Thermogravimetric analysis (TGA) was per-
formed on a Mettler-Toledo TGA/SDTA 851e thermo-
gravimetric analyzer under N at a heating rate of
2
1
0°C/min. DFT calculations were performed
employing the Gaussian 03 software package.
Synthetic routes to the monomers and complexes
are presented in Fig. 1.
1
The text was submitted by the authors in English.
1
747

1
748
MEIJUAN LIN et al.
Br
Br
H N
O
2
K CO , DMF
Polyphosporic acid
2
3
Br
+
o
H C
3
1
75 C, 10 h
CH
3
I, room temperature
HN
N
N
N
OH
H N
2
Br
Br
Br
Cl
.
IrCl3 3H O
Ir
Ir
2
R = H, CH3
2
-Ethoxyethanol/H O = 3 : 1
2
Cl
N
NR
RN
N
RN
N
o
Ar, 120 C, 24 h
2
2
Br
Br
O
O
Br
O
Na
2
CO
3
, 2-ethoxyethanol
Ir
+
Reflux
O
RN
N
Cl
Ir
Ir
Cl
N
NR
RN
N
2
Complex A, R = H
Complex B, R = CH3
2
2
Br
HO
O
+
Na CO , 2-ethoxyethanol
O
N
2
3
O
N
Reflux
Ir
RN
N
2
Complex C, R = H
Complex D, R = CH3
Fig. 1. Synthetic routes of iridium complexes A–D.
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 86 No. 7 2016

SYNTHESIS AND CHARACTERIZATION OF GREEN-EMITTING PHOSPHORESCENT
1749
Synthesis of 2-(4-bromophenyl)-1H-benzo[d]imida-
zole (BrPhBI). O-Phenylenediamine (2.0 g, 18.5 mmol),
down to room temperature small portion of water was
added. A yellow precipitate was filtered off and
washed with ethanol, ether and n-hexane several times.
The yellow solid product was purified by column
chromatography (silica gel, dichloromethane), yield
4
-bromobenzoic acid (4.1g, 20.4 mmol) and poly-
phosphoric acid (25 g, 85%) were added to a three-
necked flask and stirred at 175°C for 10 h. 2-(4-
Bromophenyl)-1H-benzo[d]imidazole as a yellow solid
was collected by suction filtration, washed with
1
47.8%. H NMR spectrum, δ, ppm: 13.42 d.d (J =
32.0, 21.6 Hz, 2H), 8.10–7.39 m (4H), 7.13 d.d (J =
16.6, 8.2 Hz, 4H), 6.89–6.58 m (6H), 5.99–5.93 m
(1H), 2.60 d (J = 63.4 Hz, 3H), 2.09–1.90 m (3H). IR
5
00 mL of distilled water. The residue was neutralized
with 10% aqueous Na CO maintaining pH of 7–8.
2
3
–
1
The crude product was recrystallized from alcohol
spectrum, ν, cm : 3445, 3102, 3051, 2962, 2924,
2848, 1727, 1700, 1629, 1589, 1527, 1456, 1412, 738.
Found, %: C 44.34, H 2.46, N 6.51. C H Br IrN O .
1
(
yield 78%), mp 292.1–292.2°C. H NMR spectrum, δ,
ppm: 13.03 s (1H), 8.13 t (J = 5.4 Hz, 2H), 7.77 t (J =
.4 Hz, 2H), 7.60 d.d (J = 4.9, 3.3 Hz, 2H), 7.21 d.d
3
1
23
2
4
2
5
(
Calculated, %: C 44.56, H 2.77, N 6.71.
1
3
J = 6.0, 3.1 Hz, 2H). C NMR spectrum (101 MHz,
Ir(III) bis{2-(4-bromophenyl)-1-methyl-benzo[d]-
DMSO), δ, ppm: 150.69, 132.45, 129.86, 128.83,
2
imidazole-C ,N}-acetylacetonate [(BrPhMBI) Ir(acac)]
–
1
2
1
2
23.72, 122.80. IR spectrum, ν, cm : 3430, 3051,
909, 2875, 1626, 1446, 1428, 709.
(
complex B). The complex was prepared following the
1
procedure as above (yield 46%). H NMR spectrum, δ,
ppm: 7.88–7.64 m (6H), 7.46–7.32 m (4H), 7.04 d (J =
8.1 Hz, 4H), 6.23 s (1H), 4.37 t (J = 21.0 Hz, 6H), 1.24
s (6H). IR spectrum, ν, cm : C 44.34, H 2.46, N
6.51.:3047, 2920, 1718, 1613, 1577, 1527, 1504, 1461.
Found, %: C 46.03, H 3.28, N 6.21. C H Br IrN O .
2
-(4-Bromophenyl)-1-methyl-benzo[d]imidazole
(
BrPhMBI). 2-(4-Bromophenyl)-1H-benzo[d]imida-
–
1
zole (0.8 g, 2.93 mmol) was dissolved in 25 mL of
dimethyl formamide and 1.25 g of anhydrous potas-
sium carbonate and iodomethane (0.25 mL, 4.0 mmol)
were added at room temperature. The reaction mixture
was stirred for 10 h, then was poured into water
33
27
2
4
2
Calculated, %: C 45.89, H 3.15, N 6.49.
Ir(III) bis{2-(4-bromophenyl)-1H-benzo[d]imida-
(
100 mL) and extracted with ethyl acetate (3 × 30 mL).
2
zole-C ,N}-picolinate [(BrPhBI) Ir(pic)] (complex C).
2
The combined organic layer was washed with
saturated NaCl solution (3 × 100 mL) and 10% NaOH
The complex was prepared following the similar
1
procedure as above (yield 63%). H NMR spectrum, δ,
(
aq) (3 × 100 mL), and dried by anhydrous MgSO4.
ppm: 8.10 s (1H), 8.02 s (1H), 7.80–7.60 m (8H), 7.36–
Upon evaporation of the solvent the product was
purified by column chromatography (silica gel,
petroleum ether : ethyl acetate = 2 : 1) to give a yellow
powder product (0.656 g, yield 78%).
7
.24 m (4H), 7.20 m (4H), 5.52 d (2H). IR spectrum, ν,
–1
cm : 3438, 3089, 2968, 2923, 2854, 1727, 1629, 1590,
1
4
-1
532, 1453, 1301, 1004, 808, 697 cm . Found, %: C
5.02, H 2.53, N 8.21. C H Br IrN O . Calculated,
3
2
20
2
5
2
(
BrPhBI) Ir Cl ·IrCl ·3H O (0.56 g, 1.59 mmol),
%: C 44.77, H 2.35, N 8.16.
4
2
2
3
2
BrPhBI (1.01 g, 3.7 mmol), 2-ethoxyethanol (30 mL),
and water (10 mL) were added into a three-neck flask.
The mixture was refluxed under the atmosphere of Ar
for 24 h and then cooled down to room temperature. A
yellow precipitate was filtered off and washed with
water and ethanol repeatedly (yield 46.7%).
Ir(III) bis{2-(4-bromophenyl)-1-methyl-benzo[d]-
2
imidazole-C ,N}-picolinate
[(BrPhMBI) Ir(pic)]
2
(
complex D). The complex was prepared following the
1
similar procedure as above (yield 61%). H NMR
spectrum, δ, ppm: 7.98 d (J = 8.7 Hz, 4H), 7.88 m
(
6H), 7.70 m (4H), 7.21 m (4H), 4.38 d (J = 7.0 Hz,
–
1
(
BrPhMBI) Ir Cl . The dimeric product was
6H). IR spectrum, ν, cm : 3062, 2970, 2922, 1726,
4
2
2
-
1
prepared according to the procedure applied to the
synthesis of (BrPhBI) Ir Cl using 1.10 mmol BrPhMBI
1654, 1601, 1462, 1358, 1307, 1026, 802, 697 cm .
Found, %: C 46.28, H 2.64, N 7.69. C H Br IrN O .
4
2
2
34 24
2
5
2
and 0.448 mmol IrCl ·3H O (yield 60%).
Calculated, %: C 46.06, H 2.73, N 7.90.
3
2
Ir(III) bis{2-(4-bromophenyl)-1H-benzo[d]imida-
RESULTS AND DISCUSSION
2
zole-C ,N}-acetylacetonate
[(BrPhBI) Ir(acac)]
2
(
(
complex A). A mixture of 0.12 g (0.078 mmol)
BrPhBI) Ir Cl , 0.02 g (0.2 mmol) acetylacetonate,
Photophysical properties. UV-Vis absorption and
photoluminescence (PL) spectra of complexes A–D in
4
2
2
0
.085g sodium carbonate and 15 mL 2-ethoxyethanol
CH Cl solution at room temperature are presented in
2 2
was refluxed under N at 120°C for 24 h. Upon cooling
Fig. 2. The complexes possess similar absorption
2
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 86 No. 7 2016

1
750
MEIJUAN LIN et al.
(
a)
(b)
Wavelength, nm
Wavelength, nm
(d)
(
c)
Wavelength, nm
Wavelength, nm
2 2
Fig. 2. UV absorption and photoluminescence spectra of complexes (a) A, (b) B, (c) C, and (d) D in CH Cl solutions at room
temperature.
bands in the spectral region of 230–320 nm, that are
the benzimidazole have influenced on the emission of
the iridium complexes.
mainly assigned to the spin-allowed π→π* transition
∧
characteristic of the C N ligands. The broad absorption
The CIE values of complexes A–D were calculated
to be (0.301, 0.591), (0.286, 0.544), (0.343, 0.544) and
peaks around 320–400 nm are metal-to-ligand charge-
transfer (MLCT) and ligand-to-ligand charge-transfer
transitions [11]. The long tail absorption from 400 to
(
0.258, 0.573), respectively. As shown in Fig. 3, the
CIE chromaticity coordinate of the complex C is
located in green-yellow light region, and complex A,
B, and D all located in the green region.
5
00 nm were assigned to the mix states containing spin-
3
3
orbit coupling enhanced π→π* and MLCT transitions
12].
[
Thermostability. Thermal durability of com-
plexes is important for OLED devices performance.
Complexes A–D were analyzed by thermogravimetric
As for PL emission spectra, complexes A–D
exhibited intense greenish emission at 505, 508, 522,
and 495 nm, respectively. Variation of the ligands, such
as substitution of methyl group in the benzimidazole in
complexes B and D (cf. complex A and C), the
replacement of acetylacetonate with picolinate in
complexes C and D (cf. complex A and B), resulted in
a shift of the emission peak wavelengths as well as a
slight boost of the intensity of the bands. The data
indicated that the ancillary ligand and modification of
the cyclometalated ligand by adding methyl group to
data under an N atmosphere and the 10% weight loss
2
temperature are 343°, 352°, 422°, and 441°,
respectively. In general, the results indicated that
complexes had an excellent thermal stability even at ca
300°. Compound B was stable up to 352°. The observa-
tions indicated that the Ir(III) complexes thermal
stability has been enhanced by introducing methyl
group substituents and the replace ligand of acetyl-
acetonate with picolinate.
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 86 No. 7 2016

SYNTHESIS AND CHARACTERIZATION OF GREEN-EMITTING PHOSPHORESCENT
1751
y
HOMO energy, meanwhile the LUMO energy could be
readily tuned by modification of the ancillary ligands.
So, we could change the ligands to construct different
energy-gap new Ir(III) complex, design and synthesize
phosphorescent materials for highly efficient OLEDs.
DFT calculation data indicated that the HOMO and
LUMO distributions were mainly focused on
cyclometalated ligands that contributed to the
appropriate selection of ligands and the electron-
donating or -withdrawing groups introduced to the
proper position of ligands thus geting new
phosphorescent materials with excellent electronic and
luminescent properties.
Table summarized the HOMO and LUMO data of
complexes A–D. The HOMO energy levels were in the
range of –4.99 to –5.44 eV. The LUMO energy levels
were between –1.52 and –1.97 eV, while the energy
gap E were very close. The target Ir(III) complexes B
g
and D containing methyl group substituents on the
benzimidazole main ligand promoted both HOMO and
LUMO energies, while the HOMO and LUMO levels
of the complexes containing acetylacetone are higher
than the complexes that used picolinic acid as the
ancillary ligand.
x
Fig. 3. CIE1931 chromaticity coordinates for complexes
A–D.
Theoretical calculations. Calculations of complexes
A–D were performed employing the Gaussian 03
software package. HOMOs of complexes were
assigned to an admixture of the d-orbital of metal and
cyclometalated phenyl ligands, while the LUMOs of
CONCLUSIONS
We report the synthesis and characterization of four
Ir(III) complexes based on 2-(4-bromophenyl)-1H-
benzo[d]imidazole or 2-(4-bromophenyl)-1-methyl-
benzo[d]imidazole as main ligand and the acetyl-
acetone or picolinic acid as ancillary ligand. The
complexes possess similar absorption bands in the
spectral region of 230–320 nm and PL emission
spectra demonstrated maximum peaks of 505, 508,
complexes
A
and
B
were located on the
cyclometalated ligands, complexes C and D were
contributed by the π-orbitals of picolinic acid ancillary
ligands, respectively.
HOMO levels of organic compounds play an
important role in hole-injection/transporting, while
LUMO levels influence the capability of electron-
injection/transporting. In the complexes A–D, electron-
donating/withdrawing were introduced on the phenyl
ring of the phenyl benzimidazole ligand regulating the
5
22, and 495 nm, respectively. DFT calculation results
indicated that Ir(III) complexes containing methyl
group substituents on the benzimidazole main ligand
promoted an increase in both HOMO and LUMO
energies and the HOMO and LUMO levels of the
complexes containing acetylacetone were higher than
those of using picolinic acid as the ancillary ligand.
For the complexes A and B the benzimidazole may
possess hole injection and electronic transmission
bipolar capabilities.
The HOMO and LUMO values for complexes A–D according
to DFT calculations
Complex
HOMO, eV LUMO, eV
g
E , eV
A
B
C
D
–5.09
–4.99
–5.44
–5.36
–1.61
–1.52
–1.97
–1.90
3.48
3.47
3.47
3.46
ACKNOWLEDGMENTS
This work was financially supported by the
National Natural Science Foundation of China
(21374017, 21574021), Natural Science Foundation of
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 86 No. 7 2016

1
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MEIJUAN LIN et al.
Fujian Province (2012J01031), and the Program for
Innovative Research Team in Science and Technology
in Fujian Province (IRTSTFJ).
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