New blue phosphorescent heteroleptic Ir(iii) complexes with imidazole- and: N -methylimidazole carboxylates as ancillary ligands

Article abstract of DOI:10.1039/d0tc03773g

Imidazole- and N-methylimidazole carboxylate have been introduced to serve as ancillary ligands in heteroleptic Ir(iii) complexes containing 2-phenylpyridine-based main ligands, with consideration of the frontier orbital energy levels. Complexes Ir1-Ir4 were synthesized and found to have LUMO levels higher than that of the corresponding Ir(iii) complex containing pyridyl carboxylate. All the synthesized Ir(iii) complexes exhibited blue phosphorescence emission maxima in the region of 465-467 nm at room temperature. Phosphorescent quantum efficiencies were increased by a factor of 1.8-6.7 by blocking the intermolecular hydrogen bonding between the imidazole carboxylate ligands of neighboring Ir(iii) complexes, which was achieved by N-methylation of the imidazole carboxylate ancillary ligand. The organic light-emitting diodes fabricated using Ir3 and Ir4 as dopant materials in the emissive layers were found to emit blue emission with peaks at 469 and 471 nm and maximum external quantum efficiencies of 20.0 and 22.4%, respectively. Moreover, all the fabricated devices with Ir3 and Ir4 exhibited low efficiency roll-off, indicating excellent stability of the materials.

Full text of DOI:10.1039/d0tc03773g

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Materials Chemistry C
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New blue phosphorescent heteroleptic Ir(III)
complexes with imidazole- and N-methylimidazole
carboxylates as ancillary ligands†
Cite this:DOI: 10.1039/d0tc03773g
a
a
a
a
Hea Jung Park, * Eun Ah Chae, Hye Won Seo, Jae-Ho Jang,
b
b
a
Won Jae Chung, Jun Yeob Lee, * Do-Hoon Hwang
and
a
Ung Chan Yoon*
Imidazole- and N-methylimidazole carboxylate have been introduced to serve as ancillary ligands in
heteroleptic Ir(III) complexes containing 2-phenylpyridine-based main ligands, with consideration of the
frontier orbital energy levels. Complexes Ir1–Ir4 were synthesized and found to have LUMO levels
higher than that of the corresponding Ir(III) complex containing pyridyl carboxylate. All the synthesized
Ir(III) complexes exhibited blue phosphorescence emission maxima in the region of 465–467 nm at room
temperature. Phosphorescent quantum efficiencies were increased by a factor of 1.8–6.7 by blocking
the intermolecular hydrogen bonding between the imidazole carboxylate ligands of neighboring Ir(III)
complexes, which was achieved by N-methylation of the imidazole carboxylate ancillary ligand. The
organic light-emitting diodes fabricated using Ir3 and Ir4 as dopant materials in the emissive layers were
found to emit blue emission with peaks at 469 and 471 nm and maximum external quantum efficiencies
of 20.0 and 22.4%, respectively. Moreover, all the fabricated devices with Ir3 and Ir4 exhibited low
efficiency roll-off, indicating excellent stability of the materials.
Received 8th August 2020,
Accepted 22nd September 2020
DOI: 10.1039/d0tc03773g
rsc.li/materials-c
Designing 2-phenylpyridine-based heteroleptic blue phos-
phorescent Ir(III) complexes by introducing electron-donating
Over the last decade, phosphorescent materials, in particular and electron-withdrawing substituents at appropriate positions
1
. Introduction
1
–5
6
–10
blue-emitting Ir(III) complexes,
have garnered significant on the ligands appeared to be a promising way to achieve a
1
6–18
attention for the construction of highly efficient organic light- large HOMO–LUMO energy band gap.
Earlier molecular
emitting diodes (OLEDs). Recently, control of the molecular orbital calculations of the HOMO and LUMO of the tris-
1
9
2
0
orientation by the transition dipole moments in the emissive (2-phenylpyridinato-N,C )iridium(III) (Ir(ppy)
3
) showed that a
layer has been considered as a way to improve the external d-orbital of Ir(III) metal makes a large contribution (52.8%) to
1
1–13
quantum efficiency of the OLED device.
Heteroleptic Ir(III) the HOMO. In the ligand, the phenyl ring makes a larger
complexes have a horizontal dipole alignment relative to the contribution (38.9%) than the pyridine ring (8.2%) to the
substrate, resulting in higher external quantum efficiencies HOMO of the complex. In contrast to the HOMO, contributions
(
EQEs), whereas homoleptic Ir(III) complexes tend to have an to the LUMO come exclusively from the p-ligand, 2-phenylpyridine,
1
2,14,15
isotropic orientation in films.
The development of new with an orbital from pyridine ring contributing the major portion,
heteroleptic blue phosphorescent Ir(III) complexes, which are 73.5%, and the phenyl ring contributing only 25.9%. Earlier
important components for efficient device performance, is calculations of the HOMO and LUMO energies of some selected
2
0
therefore desirable.
heteroaromatic rings by Tamao indicate that imidazole has a
higher LUMO level (5.35 eV) than pyridine (3.45 eV) (Fig. 1). Based
2
1,22
on energy-level calculations and our observations,
we have
a
designed blue phosphorescent heteroleptic Ir(III) complexes,
Ir1–Ir4, utilizing imidazole- and N-methylimidazole carboxy-
lates with high LUMO levels as ancillary ligands, instead of
Department of Chemistry and Chemistry Institute for Functional Materials,
Pusan National University, Busan 46241, Republic of Korea.
E-mail: heajungp@pusan.ac.kr, ucyoon@pusan.ac.kr
b
School of Chemical Engineering, Sungkyunkwan University, Gyeonggi 16419,
picolinate ((dfpmpy) Ir(pic)) (Fig. 2). In addition, an electron-
2
Republic of Korea. E-mail: leej17@skku.edu
donating methyl group was substituted on the nitrogen atom of
the imidazole moiety to investigate the effect on the emission
wavelength and phosphorescence efficiency.
†
Electronic supplementary information (ESI) available. CCDC 1504882 and
032373. For ESI and crystallographic data in CIF or other electronic format
2
see DOI: 10.1039/d0tc03773g
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over anhydrous sodium sulfate. Removal of the solvent in vacuo
1
gave 3 as an oil (90%). H-NMR (CDCl
3
, 300 MHz) d (ppm) 3.42
(
s, 6H), 3.47 (d, 2H, J = 5.4 Hz), 4.55 (t, 1H, J = 5.4 Hz, 5.4 Hz).
2
2
3
-Imidazole carboxylic acid (4) . N-(2,2-Dimethoxyethyl)tri-
chloroacetamidine (3) (2.00 g, 8.0 mmol) was added to TFA
2 mL) at 0 1C. The solution was stirred at room temperature for
4 h. After the solvent was removed in vacuo, benzene was
(
2
added to the residue and the mixture was evaporated to dryness
1
in vacuo to give impure 2-(trichloromethyl)imidazole. H-NMR
(CD
3
OD, 300 MHz) d (ppm) 7.81 (s).
Water (5 mL) was added to a TFA solution of crude 2-(tri-
Fig. 1 Calculated HOMO and LUMO energies of pyridine and imidazole
heteroaromatic rings.
20
chloromethyl)imidazole and the solution was heated at reflux
temperature for 1 h. After the solvent was removed in vacuo,
benzene was added to the residue and the mixture was evapo-
rated to dryness in vacuo (repeated twice). The solid residue was
crystallized from 2-propanol/THF to give pure 4 (50% yield).
1
m.p. 162–164 1C. H-NMR (DMSO-d
6
, 300 MHz) d (ppm) 7.42
(
s, 1H), 7.66 (s, 1H), 9.07 (COOH).
ꢀ
Synthesis of Ir(III) complexes, Ir1–Ir4
General procedure. A mixture of m-chloro-bridged Ir(III) dimmer
Fig. 2 Molecular structures of (dfpmpy)
Ir(III) complexes, Ir1–Ir4.
2
Ir(pic) and new phosphorescent
0
0
complex, [(2-(2 ,4 -difluorophenyl)-4-methylpyridine)
2
Ir(m-Cl)]
2
(
0
0.15 g, 0.12 mmol), 2-imidazolecarboxylic acid (39 mg,
.35 mmol) as an ancillary ligand, and sodium carbonate
2
. Experimental
Chemicals and instruments
(0.19 g, 1.77 mmol) was heated at 135 1C in 2-ethoxyethanol
(
7 mL) for 24 h under nitrogen. After cooling to room tempera-
Chemical reagents were purchased from Sigma Aldrich or TCI
and used without further purification. 2-Imidazolecarboxylic
acid was prepared by the following literature and 4-imidazole-
ture, the solution was evaporated in vacuo, and water was added
to the residue. The mixture was extracted with dichloromethane,
the organic layer was dried over sodium sulfate, and the filtrate was
evaporated in vacuo. The crude product was subjected to column
chromatography on silica gel and finally purified by recrystalliza-
tion from dichloromethane/hexane to give Ir1 as a yellow solid
2
3
carboxylic acid, 1-methyl-1H-imidazole-2-carboxylic acid, and
1-methyl-1H-imidazole-4-carboxylic acid as ancillary ligands
were purchased from Sigma Aldrich. Phenylpyridine derivatives
as main ligands and m-chloro bridged Ir(III) dimers were
2
1,22,24
(yield 70%). The other new heteroleptic Ir(III) complexes, Ir2–Ir4,
synthesized as previously reported.
Column chromatography was done with silica gel 60G
were also prepared from m-chloro-bridged iridium dimmer complex
and the corresponding ancillary ligands by the similar procedure.
1
13
19
(
particle size 40B63 mm, Merck Co.). H-, C-, and F-NMR
0 0
(2-(2 ,4 -Difluorophenyl)-4-methylpyridine)] Ir(imidazole-2-
2
[
spectra were recorded on CDCl , DMSO-d , and CD OD solu-
3
6
3
1
carboxylate) (Ir1) (70%). H-NMR (CDCl , 300 MHz): d (ppm)
tions and chemical shifts were reported in parts per million
3
1
8
5
5
1
.46 (d, 1H, J = 5.7 Hz), 8.07 (s, 1H), 8.03 (s, 1H), 7.47 (d, 1H, J =
.7 Hz), 7.10 (s, 1H), 6.98 (d, 1H, J = 5.7 Hz), 6.83 (d, 1H, J =
.7 Hz), 6.52 (s, 1H), 6.32–6.45 (m, 2H), 5.79 (dd, 1H, J = 8.7 Hz,
.8 Hz), 5.66 (dd, 1H, J = 8.7 Hz, 1.8 Hz), 2.55 (s, 3H), 2.54
relative to CHCl (7.26 ppm for H-NMR and 77.0 ppm for
3
13
1
C-NMR), DMSO (2.52 ppm for H-NMR), and CH OH (4.78 and
.31 ppm for H-NMR and 49.3 ppm for C-NMR) as internal
3
1
13
3
standards. The absorption and photoluminescence (PL) spectra
were measured using a Jasco V-570 UV-vis spectrometer and a
Hitachi F-4500 fluorescence spectrometer in dichloromethane
1
3
3
(s, 3H). C-NMR (100 MHz, CDCl ): d (ppm) 166.36 (OCQO),
ꢀ
1
1
1
64.54 (d, JCF = 153 Hz), 164.48 (d, JCF = 153 Hz), 164.16
2
2
(
(
d, JCF = 33 Hz), 164.04 (d, JCF = 33 Hz), 162.00, 161.63
at room temperature. Phosphorescence quantum yields (F
p
)
d, 2JCF = 33 Hz), 161.50 (d, 2JCF = 33 Hz), 159.66, 159.36,
were estimated using a dichloromethane solution of FIrpic as a
3
1
25
1
54.26 (d,
J
CF = 6 Hz), 149.00 (d,
J
CF = 164 Hz), 148.82
standard with known value of F = 0.42.
p
1
3
(d,
J
CF = 164 Hz), 147.34 (d,
26.07, 123.91, 123.72, 123.37, 123.19, 121.45, 114.68 (d, JCF
Hz), 114.52 (d, JCF = 4 Hz), 97.85 (dd, JCF = 27 Hz, 27 Hz),
JCF = 6 Hz), 143.15, 128.40,
3
1
4
=
Synthetic procedures
3
2
2
3
2
19
N-(2,2-Dimethoxyethyl)trichloroacetamidine (3) . Aminoa- 97.23 (dd, JCF = 27 Hz, 27 Hz), 21.53, 21.50. F-NMR (564 MHz,
cetaldehyde dimethyl acetal (2) (10.9 mL, 10.5 g, 100 mmol) CDCl ): d (ppm) ꢀ108.76 (dd, J = 17 Hz, 11 Hz, 1F), ꢀ109.03 (dd,
3
+
was added to a solution of trichloroacetonitrile (1) (14.4 g, J = 17 Hz, 11 Hz, 1F), ꢀ110.88 (m, 2F). HRMS (FAB) [M + H] ,
1
00 mmol) in THF (25 mL) at ꢀ35 to ꢀ40 1C (Ar atmosphere). calcd for C H F IrN O 713.1152, found 713.1147.
2
0
8
20
4
4 2
0
The cooling bath was removed when the temperature reached
[(2-(2 ,4 -Difluorophenyl)-4-methylpyridine)]
2
Ir(Imidazole-5-
OD, 300 MHz): d (ppm)
acetate. The organic layer was washed with water and dried 8.46 (d, 1H, J = 5.7 Hz), 8.15 (s, 1H), 8.11 (s, 1H), 7.68 (d, 1H,
1
ambient and the reaction mixture was diluted with ethyl carboxylate) (Ir2) (88%). H-NMR (CD
3
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3
1
3
J = 5.7 Hz), 7.66 (s, 1H), 7.35 (s, 1H), 7.20 (d, 1H, J = 5.7 Hz), 7.09 (d, JCF = 7.5 Hz), 149.01 (d, JCF = 116 Hz), 149.00 (d, JCF
(
=
1
d, 1H, J = 5.7 Hz), 6.53–6.40 (m, 2H), 5.73 (dd, 2H, J = 8.7 Hz, 7.5 Hz), 148.64 (d, JCF = 116 Hz), 137.75, 135.16, 128.45, 124.32,
1
.8 Hz), 5.61 (dd, 2H, J = 8.7 Hz, 1.8 Hz), 2.59 (s, 3H), 2.58 123.94, 123.69, 123.38, 123.11, 114.84, 114.62, 114.46, 114.22,
1
3
2
2
(s, 3H). C-NMR (100 MHz, CD OD): d (ppm) 174.03 (OCQO), 197.63 (dd, J = 27 Hz, 27 Hz), 97.03 (dd, J = 27 Hz, 27 Hz),
3
ꢀ
CF
19
CF
1
2
1
1
1
2
65.92 (d, J = 112 Hz), 163.58, 156.81, 152.54 (d, J = 15 Hz), 34.94, 21.51. F-NMR (564 MHz, CDCl ): d (ppm) ꢀ108.92
CF
CF
3
1
49.91, 149.12, 136.82 (d, JCF = 112 Hz), 125.23, 125.03, 124.84, (dd, J = 17 Hz, 11 Hz, 1F), ꢀ109.21 (dd, J = 17 Hz, 11 Hz, 1F),
2
2
24.69, 123.17, 115.68 (dd, JCF = 17 Hz, 17 Hz), 98.29 (dd, JCF
=
ꢀ110.93 (dd, J = 11 Hz, 11 Hz, 1F), ꢀ111.02 (dd, J = 11 Hz,
1
9
+
3 1 4 4 2
8 Hz, 28 Hz), 21.71. F-NMR (564 MHz, CD OD): d (ppm) 11 Hz, 1F). HRMS (FAB) M , calcd for C29H2 F IrN O 726.1230,
ꢀ
110.65 (dd, J = 17 Hz, 11 Hz, 1F), ꢀ111.04 (dd, J = 17 Hz, 11 Hz, found 726.1234.
1
1
7
F), ꢀ112.41 (dd, J = 11 Hz, 11 Hz, 1F), ꢀ112.51 (dd, J = 11 Hz,
+
1 Hz, 1F). HRMS (FAB) [M + H] , calcd for C H F IrN O
13.1152, found 713.1075.
OLED fabrication and measurements
2
8
20
4
4
2
Poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:
PSS) and 4,4 -cyslohexylidenebis[N,N-bis-(4-methylphenyl)benzen-
amine] (TAPC) were used as the hole injection layer and hole
0
0
[
(2-(2 ,4 -Difluorophenyl)-4-methylpyridine)] Ir(N-methyl-
0
2
1
imidazole-2-carboxylate) (Ir3) (60%). H-NMR (CDCl
d (ppm) 8.54 (d, J = 5.7 Hz, 1H), 8.07 (s, 1H), 8.01 (s, 1H), 7.49
d, 1H, J = 5.7 Hz), 7.01 (d, 1H, J = 5.7 Hz), 6.87 (s, 1H), 6.83 (d, 1H,
J = 5.7 Hz), 6.44 (s, 1H), 6.42–6.30 (m, 2H), 5.80 (dd, 1H, J = 9.0 Hz,
3
, 300 MHz):
transport layer, respectively. 1,3-Bis(N-carbazolyl)benzene (mCP)
(
0
was used as an exciton blocking layer. 3 ,5-Di(9H-carbazol-9-yl)-
0
26
[
1,1 -biphenyl]-3-carbonitrile (mCBP-CN) was a host material
1
2
1
3
1
1
1
1
3
2
.8 Hz), 5.62 (dd, 2H, J = 9.0 Hz, 1.8 Hz), 4.09 (s, 3H), 2.54 (s, 3H),
.53 (s, 3H). C-NMR (100 MHz, CDCl ): d (ppm) 164.89 (OCQO),
64.52 (d, J = 165 Hz), 164.45 (d, J = 165 Hz), 164.15 (d, J =
CF
0 Hz), 164.03 (d, J = 30 Hz), 162.21, 161.62 (d, J = 30 Hz),
61.49 (d, JCF = 30 Hz), 159.64, 159.41, 154.47 (d, JCF = 7 Hz),
48.94 (d, JCF = 149 Hz), 148.68 (d, JCF = 7 Hz), 148.64 (d, JCF
49 Hz), 141.84, 128.56, 128.13, 125.45, 124.81, 123.84, 123.32,
23.04, 114.64 (dd, JCF = 3 Hz, 3 Hz), 114.44 (dd, JCF = 3 Hz,
for blue OLEDs, and the dopants were doped at doping ratios
of 3–15%. Diphenyl(4-(triphenylsilyl)phenyl)phosphineoxide
1
3
3
ꢀ
1
1
2
CF
CF
(
TSPO1) was used as an exciton blocking layer, and 1,3,5-
2
2
CF
CF
tris(1-phenyl-1H-benzo[d]imidazole-2-yl)benzene (TPBi) was
used as the electron injection layer. LiF and Al were deposited
as the electron injection layer and cathode, respectively. The
encapsulation of the devices was carried out using an adhesive
and a glass cover. The device characterization method was voltage
sweep-based current density and luminance measurement
using CS2000 spectroradiometer and Keithley 2400 measurement
system.
2
3
1
3
1
=
3
3
2
2
Hz), 97.70 (dd, JCF = 27 Hz, 27 Hz), 97.01 (dd, JCF = 27 Hz,
19
7 Hz), 34.35, 21.48, 21.45. F-NMR (564 MHz, CDCl ): d (ppm)
3
ꢀ
108.95 (dd, J = 17 Hz, 11 Hz, 1F), ꢀ109.19 (dd, J = 17 Hz, 11 Hz,
1
F), ꢀ110.91 (dd, J = 11 Hz, 11 Hz, 1F), ꢀ110.98 (dd, J = 11 Hz, 11
+
Hz, 1F). HRMS (FAB) M , calcd for C29H2
1 4 4 2
F IrN O 726.1230,
found 726.1233.
3
. Results and discussion
0
0
[
(2-(2 ,4 -Difluorophenyl)-4-methylpyridine)]
2
Ir(N-methyl-
imidazole-5-carboxylate) (Ir4) (92%). H-NMR (CDCl , 300 MHz):
d (ppm) 8.60 (d, 1H, J = 5.7 Hz), 8.07 (s, 1H), 8.02 (s, 1H), 7.55
1
Synthesis and characterization of heteroleptic Ir(III) complexes,
Ir1–Ir4
3
(
(
d, 1H, J = 5.7 Hz), 7.50 (s, 1H), 7.01 (d, 1H, J = 5.7 Hz), 6.95 New Ir(III) complexes, Ir1–Ir4, were synthesized, containing
s, 1H), 6.82 (d, 1H, J = 5.7 Hz), 6.42–6.30 (m, 2H), 5.81 (dd, 1H, 2-(2 ,4 -difluorophenyl)-4-methylpyridine (dfpmpy) as the main
0
0
J = 9.0 Hz, 1.8 Hz), 5.60 (dd, 1H, J = 9.0 Hz, 1.8 Hz), 3.68 (s, 3H), ligand and imidazole- or N-methylimidazole carboxylate as an
1
3
2
1
1
1
3
.55 (s, 3H), 2.54 (s, 3H). C-NMR (75 MHz, CDCl ): d (ppm) ancillary ligand. The derivative in which 2-phenylpyridine
2
1
70.22 (OCQO), 164.60 (d,
J
CF = 29 Hz), 164.46 (d,
J
CF
=
serves as the main ligand was prepared by a previously described
ꢀ
2
1
7,21
16 Hz), 164.42 (d,
JCF = 22 Hz), 164.36 (d, JCF = 116 Hz), method. 2-Imidazolecarboxylic acid, one of the ancillary ligands,
61.29 (d, JCF = 29 Hz), 161.20 (d, JCF = 22 Hz), 159.08, 154.22 was prepared according to the literature (Scheme 1), and the
2
2
23
Scheme 1 Synthesis of new blue phosphorescent Ir(III) complexes, Ir1–Ir4.
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Fig. 3 Single-crystal packing structures of (a) Ir1 and (b) Ir3. (Pink: Ir, blue: N, green: F, dark gray: C, light grey: H.)
other ancillary ligands, 4-imidazolecarboxylic acid and 1-methyl-
The X-ray crystallographic analysis of Ir1 shows two enantio-
2(or -4)-imidazolecarboxylic acid were purchased from commer- mers in the unit cell, each having the coordination geometry of
cially available sources. The synthetic procedure used in preparing the meridional configuration, with intermolecular hydrogen
the new heteroleptic blue phosphorescent Ir(III) complexes involves bonding between the imidazole carboxylate ancillary ligands
2
4
two steps. In the first step, IrCl
3
ꢁ3H
2
O is reacted with an excess of of the two isomers (Fig. 3(a)). Ir3, which containing N-methyl-
the desired C^N main ligand to give a m-chloro-bridged dimer, 5. The substituted imidazole carboxylate, shows the different crystal
m-chloro-bridged dimer can be readily converted to blue emissive, packing structure compared to that of Ir1 by the blocking of the
heteroleptic Ir(III) complexes by replacing the bridging chlorides with intermolecular hydrogen bonding between two adjacent Ir(III)
bidentate imidazole- or N-methylimidazole carboxylate as ancillary complexes which was achieved by N-methylation (Fig. 3(b)).
ligand. All the Ir(III) complexes were synthesized in the manner There was no intermolecular hydrogen bonding between two
described above with high chemical yields of 60–92%.
adjacent Ir(III) complexes in Ir3 and it had a longer distance
Single crystals of Ir1 and Ir3 were obtained by slow diffusion between the two adjacent ancillary ligands than that of Ir1. This
of hexane into chloroform and dichloromethane solution of Ir1 fascinating intermolecular hydrogen bonding between adjacent
and Ir3, respectively, and characterized by X-ray crystallography. Ir(III) complexes has an effect on the phosphorescence efficiencies
Crystallographic data are given in Tables S1 and S2 (ESI†) and of the synthesized Ir(III) complexes, compared with the corres-
Fig. 3. Both in Ir1 and Ir3, the Ir(III) is octahedrally coordinated ponding Ir(III) complexes containing N-substituted imidazole
by three monoanionic bidentate ligands: two main ligands (the carboxylate ancillary ligands. Further details are discussed in
phenylpyridine derivatives) and one ancillary ligand (2-imid- the next section.
azolecarboxylate or N-methylimidazole-2-carboxylate). A heteroleptic
Ir(III) complex of this type, with two bidentate main ligands and an
Photophysical and electrochemical properties
ancillary ligand, has a chiral centre on the Ir atom, and because Absorption and emission spectra of Ir1–Ir4 in dichloromethane
there was no chiral selectivity in the last synthetic step, the final solution are shown in Fig. 4, and the photophysical data are
heteroleptic Ir(III) complex necessarily exists as a racemic mixture.
listed in Table 1. Ir1–Ir4 displayed weak absorption bands in
Fig. 4 (a) Absorption and (b) emission spectra of Ir1–Ir4 in dichloromethane solution.
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Table 1 The photophysical and elecltrochemical properties of Ir1–Ir4
Emission (nm)
Electrochemical
HOMO (eV) LUMO (eV)
a
e
Comp. Absorption
FWHM
(eV) (nm)
E
gap
t (ms) (eV)
a
b
c
d
p
no.
(nm)
l
em (298 K)
l
em (film) (298 K)
l
em (77 K)
T
1
F
Ir1
Ir2
Ir3
Ir4
379, 454
376, 454
378, 452
372, 453
465, 490
467, 490
467, 491
467, 490
465, 490
467, 490
467, 492
468, 493
—
—
459, 491
459, 491
—
—
2.70
2.70
55
56
55
56
0.03
0.10
0.20 2.56
0.18 2.58
—
—
2.77
ꢀ5.51
ꢀ5.51
ꢀ5.45
ꢀ5.47
ꢀ2.74
ꢀ2.74
ꢀ2.69
ꢀ2.71
2.77
2.76
2.76
a
ꢀ5
b
Solution state absorption and emission spectra were measured in CH
2
Cl
2
solutions (1 ꢂ 10 M). Films were prepared by spin coating with
ꢀ5 d
2 2
c
PMMA (5% wt). Measured in 2-methyltetrahydrofuran (MTHF) solution (1 ꢂ 10 M) at 77 K. Measured using FIrpic in CH
Cl solution as a
e
reference. Optical band gap energy was calculated from the absorption edge.
the region of 452–454 nm, corresponding to triplet metal-to- 1.8–6.7 times higher than those of Ir(III) complexes containing
3
ligand charge transfer (MLCT *) transitions, and strong structured imidazolecarboxylates (Ir1 and Ir2). This result indicates that
absorption bands in the region of 372–379 nm, corresponding blocking the intermolecular hydrogen bonding by N-methyla-
1
to singlet metal-to-ligand charge transfer (MLCT *) transitions tion of the imidazole ring induced an increase in the efficiency
(
Fig. 4(a)). All the synthesized Ir(III) complexes showed blue of the phosphorescence process. Additionally, the emission
phosphorescence with emission maxima in the range 465– bands of all of the synthesized Ir(III) complexes had a narrow
67 nm (Fig. 4(b)). Ir1 (lem = 465 nm) showed the same full width at half maximum (FWHM), 55–56 nm, which could
emission maximum in dichloromethane solution as that of contribute to the high purity of the blue light in the OLED
4
2
2,27
(
dfpmpy)
2
Ir(pic)
(lem = 465 nm) which has picolinate (pic) device. Ir1–Ir4 exhibited almost identical solid state emissions
as the ancillary ligand. The positions of the nitrogen atoms in of 5 wt% doped in PMMA films compared to those of solution
the imidazole ring had little effect on the phosphorescence state emissions, indicating there were no significant aggrega-
emission maxima, with shifts of only 1–2 nm. Ir1 showed a tions in film states (Fig. S1–S4, ESI†). Low-temperature photo-
2
nm hypsochromic shift of its peak emission (465 nm) from luminescence spectra and phosphorescence decay for Ir3 and
that of Ir2 (467 nm) in dichloromethane solution (Fig. 4(b)). Ir4 were measured in 2-methyltetrahydrofuran (MTHF) (Fig. 5).
However, the phosphorescence emission wavelength of Ir(III) For these two Ir(III) complexes, the highest-energy emission
complexes containing N-methylated imidazole as ancillary wavelengths at 77 K exhibited 8 nm hypsochromic shifts
ligand was not affected by the positions of the nitrogen atoms relative to those at room temperature. Their triplet energies
in the imidazole ring. Ir3 and Ir4 exhibited identical emission were determined to be 2.70 eV for both Ir3 and Ir4 from the
maxima at 467 nm in solution. Substitution of an electron- emission lmax at 77 K (Fig. 5(a)). The phosphorescence lifetimes
donating methyl group on a nitrogen atom of the imidazole of Ir3 and Ir4 were found to be 2.56 ms and 2.58 ms, respectively
ring (N-methylation) had virtually no effect on the phosphor- (Fig. 5(b)).
escence emission wavelengths but there were dramatic changes
The oxidation potentials of Ir1–Ir4 were obtained by cyclic
in the phosphorescence quantum efficiencies (Table 1). Ir(III) voltammetry (CV), whereas their reduction potentials were not
complexes containing N-methylated imidazolecarboxylates detectable (Fig. S5, ESI† and Table 1). The HOMO energy level
(
Ir3 and Ir4) had phosphorescence quantum efficiencies was determined from the onset point of the oxidation potential,
Fig. 5 (a) Low temperature PL spectra and (b) phosphorescence decay profiles of Ir3 and Ir4 in 1 ꢂ 10ꢀ5 M MTHF at 77 K (Phosphorescence lifetimes
were monitored at 480 nm with excitation at 355 nm.)
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+
and the peak was referenced to the Fc/Fc couple in dichloro- decomposition above 300 1C and suitable for device applications.
methane solution as ꢀ4.8 eV (0.60 V vs. Ag/AgCl). The LUMO Ir4 showed higher thermal stability than Ir3, which was expected
energy level was estimated from the HOMO energy level and the that the Ir4-based device will be more stable than the Ir3-based
optical band gap (E ). Ir1–Ir4 had HOMO and LUMO energies device during the device operation.
g
in the ranges ꢀ5.45 to ꢀ5.51 eV and ꢀ2.76 to ꢀ2.77 eV,
PHOLED performances of Ir3 and Ir4
respectively. As expected, the replacement of a pyridine ring
by an imidazole ring on the ancillary ligand raised the LUMO Among the four synthesized Ir(III) complexes, Ir3 and Ir4 were
energy level. Both Ir1 and Ir2, which contain an imidazole ring selected to evaluate their electroluminescence (EL) perfor-
as ancillary ligand, had the same LUMO of ꢀ2.74 eV, which is mance as blue phosphorescent dopant materials in OLEDs
higher than that of (dfpmpy)
2
Ir(pic) (ꢀ2.77 eV). However, the because of their higher phosphorescent quantum yields than
maximum emission wavelengths of Ir1 and Ir2 were virtually those of Ir1 and Ir2. The Ir(III) complexes were thermally
unchanged compared to that of (dfpmpy) Ir(pic) because evaporated as an emissive layer (EML) on indium tin oxide
2
the HOMO energy was also increased by the replacement of (ITO) in the following device configurations: ITO/PEDOT:PSS
pyridine by imidazole. Substitution of an electron donating (60 nm)/TAPC (20 nm)/mCP (10 nm)/mCBP-CN: x wt% Ir
methyl group on a nitrogen (N-methylation) of the imidazole dopant (3, 5, 10, or 15%) (25 nm)/TSPO1 (5 nm)/TPBi (20 nm)/
ring raised both the LUMO (0.03–0.1 eV) and HOMO (0.04–0.09 eV) LiF(1.5 nm)/Al (200 nm) (Fig. 6). The reference device was
energy levels of the Ir(III) complexes; Ir3 and Ir4 had higher LUMO fabricated using FIrpic as a blue dopant material to compare
and HOMO energies than those of non-N-methylated Ir1 and Ir2, the device performances (Fig. S9 and S10, ESI†).
respectively.
The PHOLED performances were evaluated at various
The thermal stability of synthesized Ir(III) complexes is an dopant doping ratios from 3% to 15%, and showed the best
important parameter for their purification by sublimation, and results at 5% dopant concentration for both Ir3 and Ir4
also for device stability. The thermal stabilities of Ir3 and Ir4 (Table 2). The electroluminescence (EL) spectra of Ir3 and Ir4
were measured by thermogravimetric analysis (TGA) (Fig. S6, at 5% dopant concentration show emission maxima at 469 and
ESI†). The TGA thermograms of Ir3 and Ir4 revealed that 5% 471 nm, respectively, which are red-shifted about 2 nm compared
weight loss occurs on heating at ca. 347 1C and 384 1C, to the solution photoluminescence (PL) (Fig. 7). The increase
respectively, indicating that they are resistant to thermal of dopant concentration from 3% to 15% resulted in 2 nm
Fig. 6 (a) Chemical structures and (b) energy diagram of the device structures.
Table 2 Blue PHOLED performance of Ir3 and Ir4 based devices
a
d
ꢀ2
b
c
c
ꢀ1
c
ꢀ1
lmax/FWHM (nm)
t
V /V
(V)
Lmax (cd m
)
CIE (x,y)
EQE (%)
P.E. (lm W
)
C.E. (cd A
)
Ir3 (3%)
Ir3 (5%)
Ir3 (10%)
Ir3 (15%)
Ir4 (3%)
469/56
469/56
470/57
471/56
470/56
471/58
472/58
473/58
469/50
470/50
3.1/5.0
3.1/5.0
3.1/5.0
3.1/5.0
3.1/4.8
3.1/4.8
3.1/4.8
3.1/4.9
3.1/4.9
3.1/4.9
14 624 @ 11.0 V
17 951 @ 10.5 V
19 190 @ 11.0 V
19 273 @ 11.0 V
17 846 @ 10.5 V
22 758 @ 10.5 V
25 327 @ 11.0 V
24 617 @ 11.0 V
12 359 @ 11.0 V
14 766 @ 10.5 V
0.14, 0.31
0.14, 0.31
0.14, 0.33
0.14, 0.32
0.14, 0.32
0.14, 0.33
0.14, 0.34
0.15, 0.35
0.14, 0.28
0.14, 0.29
19.6/18.3
20.0/19.1
18.7/17.5
17.7/17.3
21.9/21.3
22.4/22.0
20.8/20.6
16.6/16.6
19.6/18.6
20.7/19.9
32.3/22.3
32.9/23.4
30.9/22.1
28.8/21.7
35.3/27.9
36.7/29.3
32.8/27.8
26.0/22.1
30.2/21.4
32.5/23.4
38.1/35.7
39.2/37.5
37.7/35.3
35.6/34.6
43.4/42.3
45.5/44.8
43.0/42.5
34.7/34.7
34.7/33.3
37.6/36.4
Ir4 (5%)
Ir4 (10%)
Ir4 (15%)
FIrpic (3%)
FIrpic (5%)
a
ꢀ2
b
ꢀ2
c
ꢀ2
Turn-on voltage/driving voltage at 1000 cd m
.
Value measured at 1000 cd m
.
Value maximum/value measured at 1000 cd m
.
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Fig. 7 Electroluminescence spectra of blue dopants, (a) Ir3 and (b) Ir4 with 3%, 5%, 10%, and 15% ratio to mCBP-CN host.
Fig. 8 I–V–L curves of (a) Ir3 and (b) Ir4 at different doping ratio.
purity compared with other phosphorescent Ir(III) dopants
2
8–33
which have similar emission wavelengths.
Fig. 8 shows
the current-density–voltage–luminance (J–V–L) characteristics
of the Ir3 and Ir4 devices at different doping ratios. All devices
exhibited a low turn-on voltage of 3.1 V and a driving voltage of
4
.8–5.0 V. The device doped with Ir4 showed a lower driving
ꢀ2
voltage of 4.8 V at 1000 cd m than the Ir3 device (5.0 V),
suggesting better charge balance and carrier hopping, with
consequent higher device efficiency. Fig. 9 shows the external
quantum efficiencies (EQEs) and power efficiencies (PEs) of the
devices as a function of the luminance at 5% doping ratio. The
maximum EQEs of the optimized devices doped with Ir3
and Ir4 were 20.0% and 22.4%, respectively. Furthermore, the
EQEs of Ir3 and Ir4 devices remained at 19.1% and 22.0%
at 1000 cd m , indicating a small efficiency roll-off of both
Ir3- and Ir4-based devices (Fig. 9 and 10). Ir4 exhibited higher
device performances than FIrpic in the same device conditions,
which indicated that Ir4 can be a good candidate for the blue
Fig. 9 External quantum efficiency versus luminance (solid) and power
efficiency versus luminance (open) curves of Ir3, Ir4, and FIrpic at 5%
doping ratio.
ꢀ2
red-shifted EL maxima for both Ir3 and Ir4 devices, as a result of phosphorescent emitter of OLEDs. While high EQEs were
molecular aggregation at high dopant concentration. The CIE color obtained in Ir3 and Ir4 devices, the EQEs were considered to
coordinates of the Ir3 and Ir4 devices at the optimized doping be higher given the PLQY of the Ir3 (58.8%) and Ir4 (67.5%)
ꢀ2
concentration for a brightness of 1000 cd m were (0.14, 0.31) and emitters in the solid films. These high EQEs of Ir3 and Ir4
0.14, 0.33), respectively. FIrpic showed slightly deeper blue devices are assumed to be due to the anisotropic dipole
(
emission of (0.14, 0.29) at 5% doping concentration compared orientation obtained by the heteroleptic molecular structure
1
1,13
to Ir3 and Ir4. However, Ir3 and Ir4 still showed high color design, based on previously reported studies.
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Fig. 10 Current efficiency versus luminance curves of (a) Ir3, (b) Ir4, and (c) FIrpic at different doping ratio.
4
. Conclusions
4 M. J. Jurow, C. Mayr, T. D. Schmidt, T. Lampe, P. I.
Djurovich, W. Br u¨ tting and M. E. Thompson, Nat. Mater.,
We successfully developed novel blue phosphorescent hetero-
leptic Ir(III) complexes, Ir1–Ir4, with imidazole-2(or -5)-carboxylate
or N-methylimidazole-2(or 5)-carboxylate as an ancillary ligand,
which can be easily synthesized in good chemical yields.
At room temperature, Ir1–Ir4 exhibited phosphorescence emis-
sion in the blue region at 465–467 nm. Using an imidazole- or
N-methylimidazole ring in the ancillary ligand in place of a
pyridine ring led to an increase in LUMO levels, but because of
an accompanying increase in HOMO levels, there were no
significant changes in the emission wavelengths. N-Methy-
lation on the imidazole ring of the ancillary ligand resulted
in a 1.8–6.7-fold improvement in phosphorescence quantum
efficiency by blocking the intermolecular hydrogen bonding
between the imidazole carboxylates of neighboring Ir(III) com-
plexes. The OLED devices doped with Ir3 and Ir4 as blue
dopants showed high EQE and low efficiency roll-off. Ir4
showed better device performances than Ir3, with a maximum
2
016, 15, 85–91.
5
6
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2
1 K.-H. Kim and J.-J. Kim, Adv. Mater., 2018, 30, 175600.
2 H. Shin, Y. H. Ha, H.-G. Kim, R. Kim, S.-K. Kwon, Y.-H. Kim
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7
8
9
1
015, 44, 14613–14624.
1
1
1
ꢀ
1
EQE of 22.4%, maximum PE of 36.7 lm W , and maximum CE
ꢀ1
of 45.5 cd A . The different positions of the nitrogen atoms in
the imidazole ring of the ancillary ligand had little effect on the
optical bandgap but affected the device performances.
1
4 C. Mayr, T. D. Schmidt and W. Br u¨ tting, Appl. Phys. Lett.,
2
014, 105, 183304.
1
1
5 C. Mayr and W. Brutting, Chem. Mater., 2015, 27, 2759–2762.
6 J.-B. Kim, S.-H. Han, K. Yang, S.-K. Kwon, J.-J. Kim and
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Conflicts of interest
1
1
1
There are no conflicts to declare.
Acknowledgements
This study was supported by Basic Science Research Program
through the National Research Foundation of Korea (NRF) funded
by the Ministry of Education (NRF-2018R1A6A3A11040462 and 20 K. Tamao, M. Uchida, T. Izumizawa, K. Furukawa and
2
020R1I1A1A01053129).
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Cho and U. C. Yoon, J. Org. Chem., 2013, 78, 8054–8064.
2 H. J. Choi, M. H. Hyun, H. J. Park and U. C. Yoon, J. Lumin.,
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