Deep blue phosphorescent iridium(III) cyclometalates with o-carboranes

Article abstract of DOI:10.1016/j.jorganchem.2018.06.010

Heteroleptic iridium(III) cyclometalates, [5-(2-MeCB)dfppy]₂Ir(LX) (2-MeCB = 2-methyl-o-carboran-1-yl; dfppy = 2-(4,6-difluorophenyl)pyridinato-C²,N; LX = picolinate (5a) or tetrakis(1-pyrazolyl)borate (5b)), with o-carborane at the 5-position of the phenyl ring of the dfppy ligand were prepared to investigate the impact of the o-carborane cage on the blue phosphorescence of complexes. X-ray diffraction analysis of 5a revealed a distorted octahedral coordination geometry around the iridium center with the peripheral carborane substitution of the dfppy ligand. The absorption and emission bands of 5a and 5b were hypsochromically shifted relative to those of the sky blue (dfppy)₂Ir(pic) (FIrpic) complex, resulting in deep blue phosphorescence (λ_PL = 459 nm for 5a and 452 nm for 5b vs. 468 nm for FIrpic). The complexes showed high to moderate quantum efficiencies in toluene (Φ_PL = 0.89 for 5a; 0.47 for 5b) and in PMMA film (Φ_PL = 0.45 for 5a; 0.34 for 5b), despite a large decrease in quantum efficiency in THF. A slight decrease in quantum efficiency was observed with increasing doping concentrations of the complexes in PMMA films, indicating suppression of the emission quenching owing to the steric bulkiness of the 2-Me-o-carborane cage. Anodic shifts in the oxidation potentials of 5a and 5b compared with that of FIrpic indicated that the stabilization of the HOMO level by the inductive electron-withdrawing effect of the o-carborane is responsible for the deep blue emission of 5a and 5b.

Full text of DOI:10.1016/j.jorganchem.2018.06.010

Accepted Manuscript
Deep blue phosphorescent iridium(III) cyclometalates with o-carboranes
Minji Kim, Min Hyung Lee
PII:
S0022-328X(18)30470-4
10.1016/j.jorganchem.2018.06.010
JOM 20469
DOI:
Reference:
To appear in: Journal of Organometallic Chemistry
Received Date: 22 April 2018
Revised Date: 29 May 2018
Accepted Date: 10 June 2018
Please cite this article as: M. Kim, M.H. Lee, Deep blue phosphorescent iridium(III) cyclometalates with
o-carboranes, Journal of Organometallic Chemistry (2018), doi: 10.1016/j.jorganchem.2018.06.010.
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ACCEPTED MANUSCRIPT
Deep blue phosphorescent iridium(III) cyclometalates with o-carboranes
Minji Kim and Min Hyung Lee*
Department of Chemistry and EHSRC, University of Ulsan, Ulsan 44610, Republic of Korea
E-mail: lmh74@ulsan.ac.kr
Abstract
Heteroleptic iridium(III) cyclometalates, [5-(2-MeCB)dfppy]2Ir(LX) (2-MeCB = 2-methyl-o-carboran-
1-yl; dfppy = 2-(4,6-difluorophenyl)pyridinato-C2,N; LX = picolinate (5a) or tetrakis(1-pyrazolyl)borate
(5b)), with o-carborane at the 5-position of the phenyl ring of the dfppy ligand were prepared to
investigate the impact of the o-carborane cage on the blue phosphorescence of complexes. X-ray
diffraction analysis of 5a revealed a distorted octahedral coordination geometry around the iridium
center with the peripheral carborane substitution of the dfppy ligand. The absorption and emission bands
of 5a and 5b were hypsochromically shifted relative to those of the sky blue (dfppy)2Ir(pic) (FIrpic)
complex, resulting in deep blue phosphorescence (λPL = 459 nm for 5a and 452 nm for 5b vs. 468 nm
for FIrpic). The complexes showed high to moderate quantum efficiencies in toluene (ΦPL = 0.89 for 5a;
0.47 for 5b) and in PMMA film (ΦPL = 0.45 for 5a; 0.34 for 5b), despite a large decrease in quantum
efficiency in THF. A slight decrease in quantum efficiency was observed with increasing doping
concentrations of the complexes in PMMA films, indicating suppression of the emission quenching
owing to the steric bulkiness of the 2-Me-o-carborane cage. Anodic shifts in the oxidation potentials of
5a and 5b compared with that of FIrpic indicated that the stabilization of the HOMO level by the
inductive electron-withdrawing effect of the o-carborane is responsible for the deep blue emission of 5a
and 5b.
Keywords: Deep blue; Iridium(III) complex; Phosphorescence; o-Carborane
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1. Introduction
Phosphorescent iridium(III) complexes have attracted great attention as efficient emitters in organic
light-emitting diodes (OLEDs) and solid-state lighting [1-15]. In particular, iridium(III) cyclometalates
supported by C^N chelating ligands, such as fac-Ir(ppy)3 (ppy = 2-phenylpyridinato-C2,N), have widely
been used as dopants in phosphorescent OLEDs (PhOLEDs) [16-18] because of their excellent
photophysical properties, high stability, and facile color tunability [16, 19-29]. Although green and red
emitters have been adopted in real applications, blue PhOLEDs lag behind in terms of device efficiency,
stability, and other performance aspects. Thus, the development of new blue phosphorescent emitters
represents a continuing challenge.
To date, various approaches have been made to develop blue emitters, including changes to the type
of C^N chelation and/or to the LX ancillary ligands [30-38]. Among these approaches, electronic
modification of the existing C^N ligand can be considered a viable method to design blue
phosphorescent complexes because the emissive lowest-lying triplet excited states of iridium(III)
cyclometalates are dominated by metal-to-ligand charge transfer (3MLCT) and ligand-centered (3ππ*)
states. For example, bis(2-(4,6-difluorophenyl)pyridinato-C2,N)iridium(III) picolinate (FIrpic), in which
the electron-withdrawing fluorine atoms stabilize the HOMO level, is the most well-known among blue
emitters (Chart 1) [39-42]. However, FIrpic is not a true blue but rather a sky blue emitter (λPL
=
468−471 nm), limiting its use in full-color display applications. To further increase the HOMO–LUMO
band gap to yield pure blue to deep blue emission, the introduction of additional electron-withdrawing
groups such as CF3 [43], CN [44-47], CO2Me, C(O)CF3 [48], P(O)Ph2, or SO2(p-tol) [49] into the 5-
position of the dfppy ligands, or replacing the pic ligand with a stronger ancillary ligand such as Bpz4
(FIr6, λPL = 457−461 nm) [50-52] have been attempted (Chart 1). The foregoing reports have shown that
the electron-withdrawing ability of the additional functional group plays an important role in tuning the
blue emission.
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N
N
N
N
N
N
N
N
N
N
O
N
O
B
Ir
Ir
F
F
6
X
5
4
F
F
2
2
FIr6
X = H (FIrpic), CF3, CN, CO2Me, C(O)CF3, P(O)Ph2, SO2(p-tol)
R
R
6
C
5
C
C
5
C
C
R
N
N
1
C
4
4
2
3
O
O
Ir
Ir
N
N
N
C
C
R
C
II: 5/4-o-RCB
(R = H, Me, iBu)
I: 5/4-o-RCB
(R = H, alkyl, and aryl)
R
C
Chart 1. Examples of (top) blue phosphorescent Ir(III) complexes based on the dfppy ligands and
(bottom) cyclometalated Ir(III) complexes with o-carboranes.
In recent years, our group has demonstrated that o-carborane (1,2-closo-C2B10H12) represents a new
class of electronic and steric auxiliary on the C^N chelating ligand [53-64]. o-Carboranes are well-
known boron clusters that possess unique properties such as steric bulkiness, three-dimensional σ-
aromaticity, and high thermal and chemical stability [65-70]. In particular, due to greater contribution of
the more electronegative carbon atoms to cluster bonding than the boron atoms, the carbon atoms in o-
carborane are electron-deficient and thereby bear electron-withdrawing character [68,71,72]. The
resulting heteroleptic and homoleptic o-carborane-appended iridium(III) cyclometalates have been
successfully employed as phosphorescent dopants in green or red PhOLEDs (I and II in Chart 1).
Depending on the substitution position (4- or 5-position) of o-carborane in the C^N ligand, it can
conjugate with the unsaturated backbone through the σ*(Ccage−Ccage) orbital (4-position) or act as a
strong electron-withdrawing group through C substitution (5-position) [65-70]. The former strategy has
led to bathochromic shift of the emission, whereas the latter has led to hypsochromic shift, relative to
that of the parent complexes [53, 56, 57, 59-61, 64]. Prompted by these findings, we envisaged that
introduction of the o-carborane at the 5-position of the dfppy ligand in FIrpic may stabilize the HOMO
level through the inductive electron-withdrawing effect of o-carborane, thereby achieving deep blue
emission. In this work, we prepared heteroleptic iridium(III) cyclometalates supported by the 5-o-
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carborane-substituted dfppy ligand with a pic (5a) or Bpz4 (5b) ancillary ligand, and explored the
impacts of o-carborane substitution on the blue phosphorescence.
2. Experimental section
2.1. General considerations
All operations were performed under an inert nitrogen atmosphere using standard Schlenk and
glovebox techniques. Anhydrous grade solvents (Aldrich) were dried by passing them through an
activated alumina column and stored over activated molecular sieves (5 Å). Spectrophotometric-grade
solvents (Merck) were used as received. Commercial reagents were used without any further
purification after purchase. 2-(Trimethylstannyl)pyridine [73], 1-bromo-3-ethynyl-2,4-difluorobenzene
(1) (see the Supporting Information for details), potassium tetrakis(1-pyrazolyl)borate (KBpz4) [74], and
FIrpic [75] were synthesized according to the reported procedures. Deuterated solvents from Cambridge
Isotope Laboratories were used. NMR spectra were recorded on a Bruker 300 AM (300.13 MHz for 1H,
13
11
19
75.48 MHz for C, and 96.29 MHz for B) or a Bruker AVANCE III HD 400 (376.46 MHz for F)
spectrometer at ambient temperature. Chemical shifts are given in parts per million (ppm), and are
13
referenced against external Me4Si (1H, C), CFCl3 (19F), and BF3·OEt2 (11B). Elemental analyses were
performed on a Flash 2000 elemental analyzer (Thermo Scientific). Melting (mp) or decomposition
points (dec pt) were measured by an SMP30 Melting Point Apparatus (Stuart Equipment). Mass spectra
were obtained using a JEOL JMS700 high-resolution FAB-mass spectrometer (HR FAB-MS) at the
Korea Basic Science Institute, Daegu, Korea. Cyclic voltammetry experiments were performed using an
Autolab/PGSTAT101 system.
2.2.1. Synthesis of 2
A solution of decaborane (B10H14, 1.1 g, 9.0 mmol), diethyl sulfide (Et2S, 3.0 mL, 28 mmol), and 1
(1.5 g, 6.9 mmol) in toluene (50 mL) was heated to reflux for 3 days under a nitrogen atmosphere. After
cooling down to room temperature, the solvent was evaporated off under reduced pressure and the
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residue was purified by column chromatography on silica gel using n-hexane as an eluent to give a
white powder of 2 (0.92 g, 40%). 1H NMR (CDCl3): δ 7.65 (m, 1H, Ar-H), 6.97 (m, 1H, Ar-H), 4.96 (s,
13
CB-H), 3.7−1.2 (br, 10H, B-H). C NMR (CDCl3): δ 134.6, 114.4, 114.0, 112.8, 106.8, 106.4 (Ar−C),
68.0, 61.3 (CB−C). 11B NMR (CDCl3): δ −1.4, −2.4, −8.6, −9.4, −11.0, −13.1.
2.2.2. Synthesis of 3 (HCBdfppyH)
A mixture of 2 (0.9 g, 2.7 mmol), 2-(trimethylstannyl)pyridine (0.7 g, 2.9 mmol), and Pd(PPh3)4 (0.1
g, 0.09 mmol) in dry toluene (30 mL) was heated to reflux for 12 h under a nitrogen atmosphere. After
cooling down to room temperature, the solvent was evaporated off under reduced pressure and the
residue was purified by column chromatography on silica gel using ethyl acetate/n-hexane (1:5, v/v) as
an eluent. Drying in vacuo afforded a white powder of 3 (0.58 g, 64%). 1H NMR (CDCl3): δ 8.70 (d, J =
4.1 Hz, 1H), 7.89 (m, 1H), 7.76 (td, J = 7.8, 1.8 Hz, 1H), 7.63 (m, 1H), 7.29 (m, 1H), 7.06 (m, 1H), 5.02
13
(s, 1H, CB-H), 3.8−1.2 (br, 10H, B-H). C NMR (CDCl3): δ 151.7, 150.0, 136.6, 133.0, 126.1, 124.6,
124.5, 113.7, 113.4 (Ar−C), 68.7, 61.5 (CB−C). HR FAB-MS: m/z calcd for C13H17F2B10N, 333.2332;
found, 333.2339.
2.2.3. Synthesis of 4 (MeCBdfppyH)
To a solution of 3 (0.15 g, 0.45 mmol) in dry THF (20 mL), lithium diisopropylamide (LDA, 2.0 M in
THF/heptane/ethylbenzene, 0.25 mL, 0.5 mmol) was added slowly at −78 °C. After stirring for 1 h at
this temperature, an excess amount of methyl iodide (0.1 mL, 1.6 mmol) was added and the mixture was
gradually allowed to reach room temperature. After stirring for 1 h, the resulting solution was quenched
with an aqueous NH4Cl solution (10 mL). The mixture was extracted with diethyl ether (30 mL × 3) and
the separated organic portions were dried over MgSO4. After evaporating the solvent under reduced
pressure, the crude product was purified by silica gel column chromatography using ethyl acetate/n-
hexane (1:4, v/v) as an eluent to give a white powder of 4 (0.12 g, 76%). 1H NMR (CDCl3): δ 8.70 (d, J
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= 4.8 Hz, 1H), 8.02 (m, 1H), 7.77 (td, J = 7.7, 1.8 Hz, 1H), 7.67 (m, 1H), 7.30 (m, 1H), 7.11 (m, 1H),
4.10−1.35 (br, 10H, B-H), 1.83 (s, 3H, CB-CH3). 13C NMR (CDCl3): δ 151.8, 150.0, 136.6, 134.9,
124.8, 124.6, 123.1, 114.0, 113.6 (Ar-C), 72.8, 67.5 (CB-C), 23.9 (CB-CH3). 11B NMR (CDCl3): δ −0.5
(bs, 1B), −5.8 (bs, 1B), −9.4 (bs, 8B). HR FAB-MS: m/z calcd for C14H19F2B10N, 347.2496; found,
347.2451.
2.2.4. Synthesis of 5a (MeCBFIrpic)
The ligand 4 (0.7 g, 2.01 mmol) and IrCl3 3H2O (0.31 g, 0.88 mmol) were dissolved in degassed 2-
ethoxyethanol (30 mL) and the mixture was heated at 110 °C for 24 h under nitrogen atmosphere. The
resulting yellow powder was filtered and washed sequentially with small portions of EtOH and diethyl
ether, affording a yellow powder of [(MeCBdfppy)2Ir(µ-Cl)]2 (Yield: 0.72 g). Subsequently, the Ir(III)
dimer (0.72 g, 0.39 mmol) was combined with silver(I) triflate (0.2 g, 0.78 mmol) in degassed CH3CN
(15 mL) and the resulting mixture was stirred for 12 h at room temperature. After concentration of the
solvent under reduced pressure, the resulting yellow solid was dissolved in toluene (15 mL). Picolinic
acid (0.19 g, 1.56 mmol) and triethylamine (0.3 mL, 2.3 mmol) were added to the toluene solution and
the mixture was heated at 110 °C for 12 h. After evaporation of the solvent, the resulting yellow product
was purified by column chromatography on silica gel using MeOH/CH2Cl2 (1:20, v/v) as an eluent,
affording a bright yellow powder of 5a (MeCBFIrpic, 0.72 g, 92%). Single crystals suitable for X-ray
structural determination were obtained by slow evaporation of a MeOH/CH2Cl2 solution of 5a. 1H NMR
(CDCl3): δ 8.73 (dd, J = 4.8, 1.0 Hz, 1H), 8.31 (m, 3H), 8.00 (td, J = 7.7, 1.2 Hz, 1H), 7.86 (t, J = 7.9
Hz, 2H), 7.69 (d, J = 4.8 Hz, 1H), 7.52 (d, J = 1.2 Hz, 1H), 7.46 (d, J = 6.3 Hz, 1H), 7.27 (t, J = 6.3 Hz,
1H), 7.07 (d, J = 6.3 Hz, 1H), 5.97 (d, J = 13.2 Hz, 1H), 5.65 (d, J = 13.2, 1H), 4.0−0.9 (br, 20H, B-H),
1.79 (s, 3H, CB-CH3), 1.77 (s, 3H, CB-CH3). 13C NMR (CDCl3): δ 173.3 (pic−CO), 165.9, 164.4, 157.5,
156.7, 152.2, 149.9, 149.2, 149.0, 140.0, 131.4, 129.9, 125.6, 124.8, 124.5, 118.1, 103.2 (Ar-C), 79.9,
19
75.4 (CB-C), 24.9 (CB-CH3). F NMR (CDCl3): δ −96.4 (s), −97.5 (s), −101.3 (s), −101.8 (s). 11B
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NMR (CDCl3): δ −1.1 (bs, 4B), −6.0 (bs, 4B), −9.5 (bs, 12B). mp = 202 °C. Anal. Calcd for
C34H40B20F4IrN3O2 CH2Cl2: C, 38.49; H, 3.88; N, 3.85. Found: C, 38.89; H, 3.96; N, 3.88.
2.2.5. Synthesis of 5b (MeCBFIr6)
The [(MeCBdfppy)2Ir(µ-Cl)]2 dimer (0.17 g, 0.092 mmol) was treated with silver(I) triflate (0.05 g,
0.19 mmol) in degassed CH3CN (15 mL) and stirred for 12 h at room temperature. After concentration
of the solvent under reduced pressure, the resulting yellow solid was redissolved in CH3CN (15 mL) and
KBpz4 (0.17 g, 0.54 mmol) was added to the solution. The mixture was heated at reflux for 18 h. After
cooling down the yellow solution to room temperature, the colored precipitate was filtered off and
washed with CH2Cl2 (10 mL). The combined filtrate was evaporated under reduced pressure and the
residue was purified by column chromatography on alumina using MeOH/CH2Cl2 (1:30, v/v) as an
eluent. Drying in vacuo afforded a yellow powder of 5b (MeCBFIr6, 0.09 g, 43%). 1H NMR (CDCl3): δ
8.22 (d, J = 9.0 Hz, 2H), 7.71 (dd, J = 9.4, 4.8 Hz, 4H), 7.29 (d, J = 5.3 Hz, 2H), 7.15 (d, J = 2.5 Hz,
2H), 6.84 (t, J = 6.6 Hz, 2H), 6.77 (d, J = 1.5 Hz, 2H), 6.27 (t, J = 2.4 Hz, 2H), 6.16 (d, J = 2.1 Hz, 2H),
13
6.03 (t, J = 1.8 Hz, 2H), 5.67 (d, J = 12.9 Hz, 2H), 3.9−1.0 (br, 20H, B-H), 1.75 (s, 6H, CB-CH3). C
NMR (CDCl3): δ 162.8, 157.9, 150.1, 143.5, 142.4, 139.3, 137.9, 133.5, 130.7, 123.9, 123.8, 123.6,
116.2, 115.9, 107.4, 106.0, 102.3 (Ar-C), 78.6, 74.3 (CB-C), 23.7 (CB-CH3). 19F NMR (CDCl3): δ
−96.5 (s), −101.4 (s). 11B NMR (CDCl3): δ 1.3 (s, 1B), −1.4 (bs, 4B), −6.1 (bs, 4B), −9.4 (bs, 12B). mp
= 220 °C. Anal. Calcd for C40H48B21F4IrN10: C, 41.27; H, 4.16; N, 12.03. Found: C, 41.19; H, 4.41, N,
11.84.
2.3. X-ray crystallography
Single crystals of 5a were coated with Paratone-N oil and the diffraction was measured at 100 K. The
crystallographic measurements were performed on a Bruker SMART Apex II CCD area detector
diffractometer with a graphite-monochromated Mo-Kα radiation (λ = 0.71073 Å). The crystal structures
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were solved by direct methods and refined by full-matrix least-squares refinement with SHELXL-2014
[76]. The positions of all non-hydrogen atoms were refined with anisotropic displacement factors. All
hydrogen atoms were placed using a riding model, and their positions were constrained relative to their
parent atoms using the appropriate HFIX command in SHELXL. Detailed crystallographic data and
selected bond lengths and angles of 5a are given in Tables S1 and S2, respectively.
2.4. Cyclic voltammetry
Cyclic voltammetry measurements were carried out in CH2Cl2 (5 × 10−4 M) with a three-electrode cell
configuration consisting of platinum working and counter electrodes and an Ag/AgNO3 (0.01 M in
CH3CN) reference electrode at room temperature. Tetra-n-butylammonium hexafluorophosphate (0.1 M
in CH2Cl2) was used as the supporting electrolyte. The redox potentials were recorded at a scan rate of
100 mV/s and are reported with reference to the ferrocene/ferrocenium (Fc/Fc+) redox couple.
2.5. Photophysical measurements
UV/vis absorption and PL spectra were recorded on a Varian Cary 100 and a HORIBA FluoroMax-4P
spectrophotometer, respectively. Solution PL spectra were obtained from degassed toluene and THF
solutions. The spectrophotometric-grade solvent was degassed for ca. 30 min and kept in a nitrogen-
filled glovebox. Dilute sample solutions were prepared using 1-cm quartz cuvette in a glovebox and the
cuvette was sealed with a top-closed screw cap. Solution quantum efficiencies were measured with
reference to the quantum efficiency of fac-Ir(ppy)3 (ΦPL = 0.97 in toluene) [77]. The absolute
photoluminescence quantum yields (PLQYs) of film samples were obtained with PMMA matrices
doped with iridium(III) complexes (prepared in THF) on quartz plates. The PLQYs of films were
measured on an absolute PL quantum yield measurement system (Quantaurus-QY C11347-11,
Hamamatsu Photonics) equipped with a 3.3 inch integrating sphere. Emission lifetimes were measured
using the time-correlated single-photon counting (TCSPC) method on an FS5 spectrophotometer
(Edinburgh Instruments) equipped with an EPL-375 picosecond pulsed diode laser at 298 K.
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3. Results and discussion
3.1. Synthesis and characterization
The 5-(2-methyl-o-carboranyl)-substituted dfppyH ligand, MeCBdfppyH (4) was prepared via several
reaction steps starting from 1-bromo-3-ethynyl-2,4-difluorobenzene (1) (see the Supporting Information
and Scheme 1). Reaction of 1 with decaborane (B10H14) in the presence of weak base such as Et2S
produced the o-carborane-substituted compound 2. Subsequent Stille cross-coupling reaction between 2
and 2-(trimethylstannyl)pyridine produced 5-(2-H-o-carboranyl)-substituted dfppyH ligand (3,
HCBdfppyH) [56, 64]. Deprotonation of 3 followed by methylation led to the final ligand 4 in good
yield. Ligand 4 was subject to a cyclometalation reaction with iridium(III) chloride to form the dimeric
complex [(C^N)2Ir(
CH3CN to give the cationic (C^N)2Ir(III) species [78]. The final heteroleptic iridium(III) cyclometalates,
[5-(2-MeCB)dfppy]2Ir(LX) (2-MeCB 2-methyl-o-carboran-1-yl; dfppy 2-(4,6-
µ-Cl)2Ir(C^N)2] (C N = MeCBdfppy), which was further treated with AgOTf in
=
=
difluorophenyl)pyridinato-C2,N; LX = picolinate (5a, MeCBFIrpic) or tetrakis(1-pyrazolyl)borate (5b,
MeCBFIr6)), were prepared from reactions of cationic (C^N)2Ir(III) species with 2-picolinic acid or
potassium tetrakis(1-pyrazolyl)borate (KBpz4) under heating (Scheme 1).
F
C
N
Ir
C
O
N
O
Me
(v)
F
F
Me
C
H
F
N
H
H
Me
F
C
C
C
C
C
C
C
F
F
F
F
F
(i)
(ii)
(iii)
(iv)
5a (MeCBFIrpic)
Br
Br
F
F
F
N
N
1
2
4
3
C
(MeCBdfppyH)
(HCBdfppyH)
N
Ir
C
N
N
N
N
N
N
N
N
Me
(vi)
F
F
B
Me
C
N
C
F
5b (MeCBFIr6)
Scheme 1. Synthesis of [5-(2-MeCB)dfppy]2Ir(LX) (LX = picolinate (5a); tetrakis(1-pyrazolyl)borate
(5b)) complexes. Conditions: (i) B10H14, Et2S, toluene, 110 °C. (ii) 2-(Trimethylstannyl)pyridine,
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Pd(PPh3)4, toluene, 110 °C. (iii) LDA, MeI, THF, −78 °C. (iv) IrCl3 3H2O, 2-ethoxyethanol, 110 °C,
then AgOTf, CH3CN, rt. (v) 2-Picolinic acid, Et3N, toluene, 110 °C. (vi) KBpz4, CH3CN, 80 °C.
The complexes were characterized by means of NMR spectroscopy, elemental analysis, and single
crystal X-ray diffraction. Due to the presence of an unsymmetrical pic ligand, the two MeCBdfppy
ligands of 5a are in different chemical environments, giving rise to a more complicated resonance
pattern in the 1H NMR spectrum of 5a compared to that of 5b (Fig. 1). In fact, the pattern of the proton
resonances of 5a resembles that of FIrpic: the proton signals at the 3-position of the dfppy ligands (Ha in
Fig. 1) in both 5a and FIrpic appear as two doublets due to the H-F coupling. Moreover, the aromatic
proton peaks of 5a are shifted downfield relative to those of FIrpic, indicating an electron-withdrawing
19
effect of o-carborane. A similar feature was also observed in the F{1H} NMR spectra, showing four
different 19F signals for 5a but two for 5b (Fig. S1 and Fig. S2). Broad B−H proton signals in the region
11
of
δ
ca. 1−4 ppm and B signals at
δ
ca. −1 to −10 ppm are consistent with the presence of closo-
carborane in 5a and 5b. In addition, the sharp 11B signal at
δ
ca. 1.3 ppm for 5b can be attributed to the
Bpz4 ancillary ligand.
10

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Fig. 1. 1H NMR spectra of (top) FIrpic, (middle) 5a, and (bottom) 5b († from residual CHCl3; * from pz
groups). Ha denotes the proton at the 3-position of the dfppy ligand.
An X-ray diffraction study unequivocally revealed the molecular structure of 5a (Fig. 2). The iridium
center adopts a distorted octahedral coordination geometry with cis-C,C trans-N,N disposition of the
two C^N chelates, as has usually been observed in (C^N)2Ir(LX) complexes. The carborane cages are
appended at the 5-position of the dfppy ligands. Bond distances and angles around the iridium center are
nearly comparable to those of FIrpic (Table S2) [40], indicating that carborane substitution at the
peripheral position of the dfppy ligand affects little the geometry around the iridium center. Interestingly,
the carboranyl C−C bond distances (1.737 and 1.721 Å) are slightly longer than those of the [5-(2-
MeCB)ppy]2Ir(acac) complex, which lacks fluorine atoms in the ligand (Ccage−Ccage = 1.704 and 1.694
Å) [60]. We tentatively attribute this to the steric congestion between the 4,6-difluorophenyl ring and
the 2-Me-o-carborane cage [56, 60]. As a result, large torsion angles of CPh−CPh−Ccage−Ccage (ψ = 87.9
and 91.3°) are observed for 5a, indicative of the perpendicular orientation of the phenyl rings of the
dfppy ligands to the carboranyl C−C bond axis,. Note that the torsion angles are also greater than those
(ψ = 78.3 and 79.5°) observed for the [5-(2-MeCB)ppy]2Ir(acac) complex [60].
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Fig. 2. X-ray crystal structure of 5a (50% thermal ellipsoids). H atoms and a solvent molecule are
omitted for clarity. Selected bond distances: Ccage–Ccage 1.737(17), 1.721(16) Å;
Ccage)) 87.9°, 91.3°.
ψ ( (CPh–CPh–Ccage–
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3.2. Photophysical properties
To examine the photophysical properties of 5a and 5b, UV/vis absorption and PL experiments were
first carried out in toluene (Fig. 3 and Table 1). Data for FIrpic is included for comparison. While the
intense absorption bands centered at 291 and 285 nm for 5a and 5b, respectively, can be attributed to the
spin-allowed 1ππ* transition of the C^N ligands, the lower energy bands at 350−450 nm can be assigned
to an admixture of 1,3MLCT and 3ππ* (3LC) states (Fig. 3 and Table 1) [16, 28, 29, 39-41, 60, 79]. More
specifically, the absorptions centered at 375 and 367 nm for 5a and 5b, respectively, arise mostly from
1
the spin-allowed MLCT transition, whereas the broad low-energy absorption over 420 nm can be
assigned to the mixture of the spin-forbidden 3MLCT and the 3ππ* (3LC) transition of the C^N ligands.
Although these absorption features are similar to those of the parent FIrpic, the low-energy absorption
bands of 5a and 5b are hypsochromically shifted relative to that of FIrpic and the shift is more
pronounced for 5b than that of 5a. These findings indicate that the introduction of o-carborane into the
5-position of the dfppy ligand can lead to an increase in the transition energy probably due to the
stabilization of the HOMO level by the inductive electron-withdrawing effect of o-carborane [59, 60,
64].
Room temperature PL spectra of 5a and 5b in toluene display structured emission bands similar to
that of FIrpic (Fig. 3). As observed in the absorption, the emission bands are apparently
hypsochromically shifted with respect to those of FIrpic (λPL = 459, 488 nm for 5a and 452, 482 nm for
5b vs. 468 and 495 nm for FIrpic). In contrast to the sky blue emission of FIrpic, the emission of 5a
corresponds to pure blue emission and that of 5b is close to deep blue emission. This result indicates
that the o-carborane substitution into the 5-position of the dfppy ligand could be a viable route to attain
deep blue emission. Furthermore, when compared with reported 5-functionalized FIrpic complexes
(Chart 1), the spectral position of the emission of 5a is comparable to those of the 5-C(O)CF3 and 5-
SO2(p-tol) substituted complexes (λPL = 459 nm) [48, 49], reflecting that o-carborane exerts a strong
inductive
effect.
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Table 1. Photophysical and electrochemical data for 5a and 5b.
Compd
λabs/nm
(ε × 10−3/M−1 cm−1)a
λPL/nm
in toluene
298 Kb/77 Kb
λPL/nm
in THF
298 Kc
λPL/nm
in PMMAd
298 K
ΦPL
τ/µs
Eox
/Vg
toluenee/THFe/PMMAd,f tolueneb
291 (45.0), 375 (6.3), 420 (2.1),
451 (0.48)
459, 488/450, 482
459, 488
459, 488
0.84/0.01/0.45
0.47/0.19/0.34
0.89/0.84/0.89
1.35
1.64
2.77
1.09h
1.47i
0.96i
5a
285 (37.2), 348 (5.4), 367 (4.7),
419 (0.4), 446 (0.2)
452, 482/447, 479
468, 495/458, 491
452, 482
469, 495
453, 484
468, 495
5b
FIrpic
284 (34.0), 381 (5.0), 429 (1.9),
457 (0.38)
a In degassed toluene (5.0 × 10−5 M).
b 2.0 × 10−6 M.
c 1.0 × 10−5 M.
d Spin-coated PMMA film doped with the Ir(III) complex at 2 wt%.
e fac-Ir(ppy)3 as a standard (ΦPL = 0.97 in toluene) [77].
f Absolute PLQY (error: ±0.01).
g In CH2Cl2 (5.0 × 10−4 M, scan rate = 100 mV s−1) with reference to a Fc/Fc+ redox couple.
h Irreversible oxidation (Eonset).
i Reversible oxidation (E1/2).
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The hypsochromic shift of the emission brought about by replacing the pic ligand with Bpz4 is similar to
that observed for FIrpic and FIr6. Emission lifetimes (τ) of 1.35−1.64 µs for 5a and 5b confirm the
phosphorescence origin of the emission. The emission band positions of 5a and 5b were unaffected by
changes in solvent polarity (toluene vs. THF) and strong vibronic progression (Fig. S4) with small
hypsochromic shifts (rigidochromism) of ca. 5−9 nm were observed at 77 K. These findings indicate
that the emitting triplet state (T1) of 5a and 5b represents a 3LC state mixed with a 3MLCT state [39-41,
75, 79-81]. The triplet (T1) energies of 5a and 5b were determined to be 2.75 and 2.77 eV, respectively,
from the phosphorescence at 77 K. These values are consistent with deep blue phosphorescence of 5a
and 5b.
5a
5b
FIrpic
50
40
30
20
10
0
1.0
0.8
0.6
0.4
0.2
0.0
300 350 400 450 500 550 600
λ
/ nm
Fig. 3. UV/vis absorption (5.0 × 10−5 M, left) and PL spectra (2.0 × 10−6 M, right) of 5a, 5b, and FIrpic
in toluene at 298 K.
High to moderate phosphorescence quantum efficiencies (ΦPL) were obtained for 5a and 5b in toluene
(Table 1). The quantum efficiency of 5a (ΦPL = 0.84) is comparable to that (ΦPL = 0.89) of FIrpic
although 5b shows relatively low quantum efficiency (ΦPL = 0.47). The quantum efficiencies of both
complexes were lowered in THF, as was similarly found for carborane substituted complexes [53, 56,
57, 59-62, 64]. This result could be related to the high variation in the Ccage−Ccage bonds in polar media,
leading to facile nonradiative decay of the excited states [59, 60, 62]. However, both 5a and 5b become
substantially emissive in a rigid matrix, exhibiting ΦPL of 0.45 and 34, respectively, in PMMA film.
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This quantum efficiency enhancement can be ascribed to the restricted rotation of o-carborane moieties
in the film state, suppressing the Ccage−Ccage bond variation [59, 60]. Because the quantum efficiency of
the complexes in the rigid state is an important factor for emitters in OLED applications, we further
investigated the quantum efficiency of 5a and 5b in PMMA films at various doping concentration. As
summarized in Table 2, both complexes exhibited gradual but small decreases in quantum efficiency at
doping concentrations above 2 wt%. In particular, the quantum efficiencies of films containing 5b were
quite similar throughout the concentration range of 1−12 wt%. This result suggests that the combined
steric effects of the carborane cages and the Bpz4 ligand suppress the emission quenching that may arise
from undesirable intermolecular interactions [57].
Table 2. Photoluminescence quantum yields (PLQYs) of 5a and 5b in PMMA.
wt% Ir
ΦPL (%)a
5a
5b
29
34
32
29
26
80
60
40
20
0
5a
5b
1
45
45
40
32
28
2
4
8
0
2
4
6
8
10 12 14
wt% of Ir complex in PMMA
12
a Absolute PLQYs of spin-coated PMMA film doped with Ir(III) complexes (error: ±1%).
Finally, to elucidate the reason for the observed deep blue emission, the electrochemical properties of
5a, 5b, and FIrpic were examined by cyclic voltammetry (Fig. 4 and Table 1). All these complexes
underwent oxidations centered on the dπ(Ir) orbitals [53, 56, 57, 60, 75, 82]. Notably, both 5a and 5b
displayed anodic shifts in oxidation (Eox = 1.09 V for 5a and 1.47 V for 5b) relative to the oxidation of
Firpic (Eox = 0.96 V). 5b also showed greater oxidation potential than 5a. These results clearly indicate
that 5a and 5b have deeper HOMO levels than FIrpic. Therefore, the introduction of the o-carborane
into the 5-position of the dfppy ligand can result in the stabilization of the HOMO level by the inductive
electron-withdrawing effect of the o-carborane, leading to a widening of the band gaps. This accounts
for the observed hypsochromic shifts in the absorption and emission bands of 5a and 5b.
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50 µA
5a
5b
FIrpic
0.0
0.4
0.8
1.2
1.6
2.0
Potential (V) / (vs Fc/Fc+)
Fig. 4. Cyclic voltammograms of 5a, 5b, and FIrpic showing oxidation (5 × 10−4 M in CH2Cl2, scan rate
= 100 mV s−1).
4. Conclusion
We prepared and characterized heteroleptic iridium(III) cyclometalates, [5-(2-MeCB)dfppy]2Ir(LX)
(2-MeCB = 2-methyl-o-carboran-1-yl; LX = picolinate (5a) or tetrakis(1-pyrazolyl)borate (5b)), with o-
carborane at the 5-position of the phenyl ring of the dfppy ligand. The phosphorescence wavelengths of
5a and 5b were hypsochromically shifted with respect to that of the sky blue FIrpic complex,
representing deep blue phosphorescence. The complexes showed high to moderate quantum efficiency
in toluene and in PMMA film despite a large decrease in quantum efficiency in THF. The quantum
efficiency of the complexes in PMMA films decreased slightly with increasing doping concentration,
indicating suppression of the emission quenching due to the steric bulkiness of the 2-Me-o-carborane
cage. The oxidation potentials of 5a and 5b underwent substantial anodic shifts compared to that of
FIrpic, indicating that the stabilization of the HOMO level by the inductive electron-withdrawing effect
of the o-carborane is responsible for the deep blue emission of 5a and 5b.
Acknowledgement
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This work was supported by the 2016 Research Fund of University of Ulsan.
Supplementary material:
NMR spectra, UV/vis absorption and PL spectra. CCDC reference number 1836500 for 5a. For
crystallographic data in CIF or other electronic format. See DOI: 10.1039/x0xx00000x. Copies of the
data can be obtained, free of charge, on application to CCDC, 12 Union Road, Cambridge CB2 1EZ,
UK, (fax: +44-(0)1223-336033 or e-mail: deposit@ccdc.cam.ac.Uk).
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Highlights
Deep blue phosphorescent iridium(III) cyclometalates with o-carboranes
Minji Kim and Min Hyung Lee*
o-Carborane-appended heteroleptic Ir(III) cyclometalates [5-(2-MeCB)dfppy]2Ir(LX) were prepared.
The complexes exhibited deep blue phosphorescence.
The stabilization of the HOMO level by o-carborane substitution was responsible for the deep blue
emission.
1