Phosphorescence color tuning of cyclometalated iridium complexes by o-carborane substitution

Article abstract of DOI:10.1021/ic3015699

Heteroleptic (C^a?N)₂Ir(acac) (C ^a?N = 4-CBppy (1); 5-CBppy (2), 4-fppy (4) CB = ortho-methylcarborane; ppy = 2-phenylpyridinato-C²,N, 4-fppy = 2-(4-fluorophenyl)pyridinato-C²,N, acac = acetylacetonate) complexes were prepared and characterized. While 1 exhibits a phosphorescence band centered at 531 nm, which is red-shifted compared to that of unsubstituted (ppy)₂Ir(acac) (3) (λ_em = 516 nm), the emission spectrum of 2 shows a blue-shifted band at 503 nm. Comparison with the emission band for the 4-fluoro-substituted 4 (λ_em = 493 nm) indicates a substantial bathochromic shift in 1. Electrochemical and theoretical studies suggest that while carborane substitution on the 4-position of the phenyl ring lowers the ³MLCT energy by a large contribution to lowest unoccupied molecular orbital (LUMO) delocalization, which in turn assigns the lowest triplet state of 1 as [d_π(Ir)→π*(C ^a?N)] ³MLCT in character, the substitution on the 5-position raises the ³MLCT energy by the effective stabilization of the highest occupied molecular orbital (HOMO) level because of the strong inductive effect of carborane. An electroluminescent device incorporating 1 as an emitter displayed overall good performance in terms of external quantum efficiency (6.6%) and power efficiency (10.7 lm/W) with green phosphorescence.

Full text of DOI:10.1021/ic3015699

Article
pubs.acs.org/IC
Phosphorescence Color Tuning of Cyclometalated Iridium
Complexes by o‑Carborane Substitution
Taewon Kim,† Hyungjun Kim,‡ Kang Mun Lee,‡ Yoon Sup Lee,*,‡ and Min Hyung Lee*,†
†Department of Chemistry and EHSRC, University of Ulsan, Ulsan 680-749, Republic of Korea
‡Department of Chemistry, KAIST, Daejeon 305-701, Republic of Korea
S
* Supporting Information
ABSTRACT: Heteroleptic (C∧N)2Ir(acac) (C∧N = 4-CBppy
(1); 5-CBppy (2), 4-fppy (4) CB = ortho-methylcarborane; ppy
= 2-phenylpyridinato-C2,N, 4-fppy = 2-(4-fluorophenyl)-
pyridinato-C2,N, acac = acetylacetonate) complexes were
prepared and characterized. While 1 exhibits a phosphor-
escence band centered at 531 nm, which is red-shifted
compared to that of unsubstituted (ppy)2Ir(acac) (3) (λem
=
516 nm), the emission spectrum of 2 shows a blue-shifted
band at 503 nm. Comparison with the emission band for the 4-
fluoro-substituted 4 (λem = 493 nm) indicates a substantial
bathochromic shift in 1. Electrochemical and theoretical
studies suggest that while carborane substitution on the 4-
3
position of the phenyl ring lowers the MLCT energy by a
large contribution to lowest unoccupied molecular orbital (LUMO) delocalization, which in turn assigns the lowest triplet state
3
3
of 1 as [dπ(Ir)→π*(C∧N)] MLCT in character, the substitution on the 5-position raises the MLCT energy by the effective
stabilization of the highest occupied molecular orbital (HOMO) level because of the strong inductive effect of carborane. An
electroluminescent device incorporating 1 as an emitter displayed overall good performance in terms of external quantum
efficiency (6.6%) and power efficiency (10.7 lm/W) with green phosphorescence.
of the phenyl ring of the ppy ligand together with the metal t2g
manifold (dπ orbitals) and (ii) the π* orbital of the pyridyl ring
of the ppy ligand governs the lowest unoccupied molecular
orbital (LUMO) level. Consequently, the introduction of an
electron-withdrawing group such as a fluorine atom into the
phenyl ring of the ligand usually gives rise to an increase in the
HOMO−LUMO band gap by lowering the HOMO level, while
introduction into the pyridyl ring narrows the band gap by
lowering the LUMO level. As a result, the energy level of the
triplet state formed from the HOMO−LUMO transition can be
fine-tuned in accordance with the change in the band gap.
Moreover, the orbital analyses further suggest that the position
of the substituent on each ring of ligand is also important for
tuning the emission wavelength.9 For example, the electron-
withdrawing effect of a fluorine atom attached on the 4- and/or
6-carbon positions of the phenyl ring gives a large blue shift by
the greater HOMO stabilization than LUMO, but substitution
at the 5-position offsets the electron-withdrawing effect of the
fluorine atom by weak π-donation from fluorine into the
electron density of the HOMO at this position, thereby
reducing the band gap.
INTRODUCTION
■
Phosphorescent heavy metal complexes have attracted a great
deal of interest as emitting materials in organic light-emitting
diodes (OLEDs) because of their excellent properties such as
good color purity, high quantum efficiency, relatively short
phosphorescence lifetime, and high photo- and thermal
stability.1,2 In particular, modification of the cyclometalating
ligand (C∧N ligand) has enabled control of emission color over
the entire visible region that can be beneficial for realizing full-
color and white-light displays.3 The tuning of phosphorescence
color has usually been achieved by the variation of the
substituent on the C∧N ligand, for example, ppy (2-phenyl-
pyridinato-C2,N), since the emissive lowest-energy excited
3
3
states such as MLCT and π−π* (3LC) are largely involved
with the electronic structure of the ligand.4−8
Although these approaches to phosphorescence color tuning
by use of the electron-donating or -withdrawing substituents
Experimental and theoretical studies reported to date have
established the basic principles for the emission color control as
follows:6−10 (i) the highest occupied molecular orbital
(HOMO) level of the complex is dominated by the π orbital
Received: July 18, 2012
Published: December 20, 2012
© 2012 American Chemical Society
160
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Inorganic Chemistry
Article
methane and toluene (Aldrich) were used as received. Commercial
reagents were used without any further purification after purchasing
from Aldrich (2-bromopyridine, n-BuLi (2.5 M solution in n-hexanes),
1-bromo-4-iodobenzene, 1-bromo-3-iodobenzene, ethynyl(trimethyl)-
silane, KOH, diethyl sulfide, lithium diisopropylamide (2.0 M in
THF), MeI, 2,4-pentanedione (acacH), Na2CO3, quinine sulfate),
Strem (iridium(III) chloride hydrate, Pd(PPh3)4), TCI (Me3SnCl),
Katchem (Decaborane), and Lumtec (fac-Ir(ppy)3, >99%, sublimed
grade). 3-Bromophenylacetylene,15 2-(trimethylstannyl)pyridine,16 1-
(4-bromophenyl)-2-methyl-1,2-closo-carborane (1a),14 (ppy)2Ir-
have been well established, it is still highly desirable to develop
phosphors whose emitting properties can be easily tuned by
modification of the electronic structure of the ligand. In an
effort to design a novel family of color-tunable phosphors, we
have been interested in 1,2-closo-C2B10H12, so-called o-
carborane as a substituent since it possesses unique properties
such as a highly polarizable σ-aromatic character, electron-
deficient nature, and good thermal and chemical stability.11
Owing to these interesting properties, o-carborane has recently
received attention in the field of fluorescent materials.12 In
particular, we have recently demonstrated that when incorpo-
rated into an organic π-system such as triarylborane, o-
carborane significantly stabilizes a LUMO level by direct
contribution to LUMO delocalization, as well as by a strong
inductive electron-withdrawing effect.13,14 The bulkiness of o-
carborane may also hinder the intermolecular interactions
between phosphorescent emitter molecules and thereby
suppress triplet−triplet annihilation and concentration quench-
ing in solid state. These properties of carborane led us to
consider whether the substitution of an o-carborane into the
C∧N ligand could exert an effect on the emission color tuning
of an Ir(III) complex.
17
(acac),7 and [(4-fppy)2Ir(μ-Cl)]2 were synthesized according to
the reported procedures. Deuterated solvents from Cambridge Isotope
Laboratories were used. NMR spectra were recorded on a Bruker 300
1
a.m. spectrometer (300.13 MHz for H, 75.48 MHz for 13C, 96.29
MHz for 11B) at ambient temperature. Chemical shifts are given in
parts per million (ppm), and are referenced against external Me4Si
(1H, 13C) and BF3·OEt2 (11B). Elemental analyses were performed on
an EA1110 (FISONS Instruments) by the Environmental Analysis
Laboratory at KAIST. HR EI−MS measurement (JEOL JMS700) was
carried out at Korea Basic Science Institute. UV−vis absorption and
PL spectra were recorded on a Varian Cary 100 and a HORIBA
FluoroMax-4P spectrophotometer, respectively. Lifetime measure-
ments were carried out with an Edinburgh FLS 920 spectropho-
tometer equipped with Xenon microsecond flashlamp. Cyclic
voltammetry experiment was performed using an AUTOLAB/
PGSTAT101 system.
In this report, we introduced an o-carborane into the 4- or 5-
position of the phenyl ring of the ppy ligand and investigated
the photophysical properties of Ir(III) complexes (1 and 2 in
Chart 1). We find that a simple change in the position of
Synthesis of 1-(3-BrPh)-2-Me-1,2-closo-C2B10H10 (2a). This
compound was prepared in a manner analogous to the synthesis of
1a.14 To a toluene solution (50 mL) of decaborane (B10H14, 2.47 g,
13.6 mmol) and 3-bromophenylacetylene (2.00 g, 16.3 mmol) was
added an excess amount of Et2S (5 equiv) at room temperature. The
reaction mixture was stirred at reflux for 3 d. The solvent was removed,
and the resulting solid residue was purified by column chromatography
on alumina, affording 1-(3-BrPh)-1,2-closo-C2B10H11 as a white solid.
Yield: 2.41 g, 59%. 1H NMR (CDCl3): δ 7.52 (t, J = 2.0 Hz, 1H, Ph−
CH), 7.51 (dt, J = 8.1, 2.0 Hz, 1H, Ph−CH), 7.40 (dt, J = 8.1, 2.0 Hz,
1H, Ph−CH), 7.18 (t, J = 8.1 Hz, 1H, Ph−CH), 3.93 (s, 1H, CH),
1.3−3.5 (br, 10H, B−H). 13C NMR (CDCl3): δ 135.6, 133.3, 130.8,
130.5, 126.5, 123.1, 75.2, 60.2 (C2B10H10). 11B NMR (CDCl3): −2.0,
−4.1, −8.9, −11.0, −12.7. HR EI-MS: m/z calcd for C8H15B10Br,
300.1288; found, 300.1291.
Chart 1
Next, the solution of 1-(3-BrPh)-1,2-closo-C2B10H11 (2.23 g, 7.64
mmol) in tetrahydrofuran (THF) was treated with lithium
diisopropylamide (4.20 mL, 8.40 mmol) at 0 °C. After stirring for
an additional 1 h, an excess amount of MeI (3 equiv, 25.2 mmol) was
added into the mixture at 0 °C. The reaction mixture was allowed
slowly to reach room temperature and was stirred for 2 h. After
quenching the reaction with water (30 mL), the aqueous layer was
extracted with ether (3 × 30 mL). The organic layers were separated,
dried over MgSO4, and concentrated under reduced pressure.
Purification by column chromatography on alumina afforded 1-(3-
BrPh)-2-Me-1,2-closo-C2B10H10 (2a) as a white solid. Yield: 1.79 g,
75%. 1H NMR (CDCl3): δ 7.77 (t, J = 2.0 Hz, 1H, Ph−CH), 7.59 (dt,
J = 8.1, 2.0 Hz, 2H, Ph−CH), 7.26 (t, 1H, J = 8.1 Hz Ph−CH), 1.67
(s, 3H, B−CH3), 1.3−3.5 (br, 10H, B−H). 13C NMR (CDCl3): δ
134.2, 134.0, 133.2, 130.5, 129.9, 123.2 (Ph-C), 80.8, 77.5 (C2B10H10),
23.4 (B-CH3). 11B NMR (CDCl3): −2.8, −4.5, −10.1. HR EI-MS: m/z
calcd for C9H17B10Br, 314.1444; found, 314.1448.
Synthesis of 2-{p-(2-Me-1,2-carboran-1-yl)phenyl}pyridine
(4-CBppyH, 1b). 1a (1.75 g, 5.57 mmol), 2-(trimethylstannyl)-
pyridine (1.48 g, 6.13 mmol), and Pd(PPh3)4 (0.13 g, 2.0 mol %) were
dissolved in toluene (70 mL), and the solution was heated to reflux
overnight. After cooling to room temperature, the reaction mixture
was quenched by addition of saturated aqueous NH4Cl. The organic
phase was separated and the aqueous layer was further extracted with
Et2O (30 mL). The combined organic layers were dried over MgSO4,
and concentrated under reduced pressure. The residue was purified by
flash column chromatography using CH2Cl2/toluene (1/1, v/v) as an
eluent. Drying in vacuo afforded 1b as a white solid (1.03 g, 54%). 1H
NMR (CDCl3): δ 8.70 (dq, 1H, J = 4.8, 0.9 Hz), 7.99 (dt, 2H, J = 8.7,
carborane substitution on the phenyl ring leads to facile tuning
of emission wavelength from red to blue shift with respect to
that of the unsubstituted (ppy)2Ir(acac) complex (3). Synthesis,
characterization, and photophysical and electroluminescent
properties of carborane-substituted heteroleptic Ir(III) com-
plexes 1 and 2 are described with theoretical calculations.
EXPERIMENTAL SECTION
■
Chemical and Instrumentation. 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 dichloro-
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Inorganic Chemistry
Article
Table 1. Photophysical and Electrochemical Data for 1−4
a
λem/nm
c
c
compound
Eox/V
Ered/V
a
a
a,b
(C∧N)
λabs/nm (ε × 10−3/M−1 cm−1
)
298 K 77 K τ/μs
Φem
(ΔEp/mV)
(ΔEp/mV)
e
f
f
1 (4-CBppy) 261 (38.3), 279 (38.1), 353 (5.4), 390 (3.4), 415 (2.7), 474 (2.0), 509 (0.5) 531
524
493
3.0
0.6
0.02
0.15
0.51 (90)
−2.12 (270)
−2.19 (360)
e
2 (5-CBppy) 256 (48.4), 274 (43.1), 299 (33.4), 342 (10.1), 378 (4.8), 401 (3.9),
503
0.55 (140)
451 (2.4), 481 (1.0)
d
d
g
g
3 (ppy)
260 (47.1), 339 (8.4), 405 (3.5), 460 (2.4), 491 (0.6)
516
493
502
483
1.6
1.5
0.34
0.40
0.41
−2.60
e
4 (4-fppy)
259 (40.9), 287 (29.5), 331 (11.3), 393 (5.0), 444 (2.5), 474 (0.8)
0.46 (100)
h
a
b
c
Measured in degassed CH2Cl2 (∼ 10−5 M). Quinine sulfate (0.5 M H2SO4) as a standard (Φ = 0.55).19 Measured in DMF (1 mM, scan rate =100
d
e
f
g
h
mV/s) with reference to a Fc/Fc+ redox couple. Data from ref 7. Reversible oxidation. Quasi-reversible reduction. Data from ref 31. Not
observed.
2.1 Hz), 7.70−7.80 (m, 4H), 7.27 (ddd, 1H, J = 6.9, 4.8, 1.5 Hz), 1.70
(s, 3H, B−CH3), 1.50−3.70 (br, 10H, B−H). 13C NMR (CDCl3): δ
155.7, 149.9, 141.5, 137.1, 131.5, 131.3, 127.3, 123.0, 120.8 (ppy-C),
81.8, 77.4 (C2B10H10), 23.2 (B-CH3). 11B NMR (CDCl3): −3.0, −4.4,
−10.0. HR EI-MS: m/z calcd for C14H21B10N, 313.2605; found,
313.2606.
Synthesis of 2-{m-(2-Me-1,2-carboran-1-yl)phenyl}pyridine
(5-CBppyH, 2b). This compound was prepared in a manner
analogous to the synthesis of 1b using 2a (1.79 g, 5.72 mmol), 2-
(trimethylstannyl)pyridine (1.66 g, 6.87 mmol), and Pd(PPh3)4 (0.13
g, 2.0 mol %). Yield: 1.24 g, 70%. 1H NMR (CDCl3): δ 8.71 (dq, 1H, J
= 4.8, 1.8 Hz), 8.30 (t, 1H, J = 1.8 Hz), 8.03 (dt, 1H, J = 7.8, 1.2 Hz),
7.81 (td, 1H, J = 7.8, 1.8 Hz), 7.70 (tt, 2H, J = 7.8, 1.2 Hz), 7.48 (t,
1H, J = 7.8 Hz), 7.27 (dd, 1H, J = 4.8, 1.2 Hz), 1.72 (s, 3H, B−CH3),
1.50−3.70 (br, 10H, B−H). 13C NMR (CDCl3): δ 156.1, 150.2, 140.5,
137.2, 131.7, 131.5, 130.0, 129.5, 129.1, 123.0, 120.9 (ppy-C), 82.3
77.5 (C2B10H10), 23.5 (B-CH3). 11B NMR (CDCl3): δ −3.0, −4.5,
−10.0 . HR EI-MS: m/z calcd for C14H21B10N, 313.2605; found,
313.2605.
Synthesis of [(4-CBppy)2Ir(μ-Cl)]2 (1c). 1b (1.03 g, 3.29 mmol)
and IrCl3·3H2O (0.51 g, 1.43 mmol) were dissolved in the mixed
solvent of 2-ethoxyethanol (40 mL) and distilled water (20 mL). The
reaction mixture was heated to 110 °C and stirred for 1 d. After
cooling to room temperature, 30 mL of distilled water was added to
precipitate out the solid materials. The precipitate was filtered and
washed with small portions of ethanol (2 × 10 mL). The crude solid
was extracted with CH2Cl2, and the solution was dried over MgSO4.
Filtration followed by drying in vacuo gave a light yellow solid of 1c.
Yield: 1.07 g, 88%. 1H NMR (CDCl3): δ 9.31 (d, 4H, J = 5.7 Hz), 7.92
(m, 8H), 7.44 (d, 4H, J = 8.4 Hz), 7.01 (m, 8H), 6.01 (d, 4H, J = 1.8
Hz), 1.33 (s, 12H, B−CH3), 1.20−3.50 (br, 40H, B−H). 13C NMR
(CDCl3): δ 167.1, 151.7, 145.9, 144.1, 137.4, 132.7, 130.6, 124.4,
123.4, 123.2, 120.0, 82.0, 71.5 (C2B10H10), 22.9 (B-CH3). 11B NMR
(CDCl3): δ −4.3, −9.5. Anal. Calcd for C56H80B40Cl2Ir2N4: C, 39.63;
H, 4.75; N, 3.30. Found: C, 39.63; H, 4.81; N, 3.27.
diffraction study were obtained from slow evaporation of an acetone/
MeOH solution of 1. H NMR (CDCl3): δ 8.50 (d, 2H, J = 6.0 Hz),
1
7.87 (m, 4H), 7.42 (d, 2H, J = 8.4 Hz), 7.30 (td, 2H, J = 6.0, 2.7 Hz),
6.94 (dd, 2H, J = 8.4, 2.1 Hz), 6.22 (d, 2H, J = 2.1 Hz), 5.31 (s, 1H,
acac-CH), 1.83 (s, 6H, acac-CH3), 1.14 (s, 6H, B−CH3), 1.10−3.00
(br, 20H, B−H). 13C NMR (CD2Cl2): δ 185.7 (acac-CO), 167.2,
149.2, 147.9, 147.8, 138.5, 135.0, 130.4, 124.5, 123.8, 123.6, 120.0,
101.2 (acac-CH), 83.2, 77.9 (C2B10H10), 28.8 (acac-CH3), 22.9 (B-
CH3). 11B NMR (CDCl3): δ −4.8 (br, 4B), −10.4 (br, 6B). Anal.
Calcd for C33H47B20IrN2O2: C, 43.45; H, 5.19; N, 3.07. Found: C,
43.38; H, 5.19; N, 3.08.
Synthesis of (5-CBppy)2Ir(acac) (2). This compound was
prepared in a manner analogous to the synthesis of 1 using 2c (1.37
g, 0.81 mmol), 2,4-pentanedione (0.24 g, 2.43 mmol), and sodium
carbonate (0.86 g, 8.09 mmol). Yield: 1.40 g, 95%. 1H NMR (CDCl3):
δ 8.48 (d, 2H, J = 5.4 Hz), 7.92 (d, 2H, J = 8.1 Hz), 7.83 (dt, 2H, J =
8.4, 1.5 Hz), 7.71 (d, 2H, J = 2.1 Hz), 7.23 (m, 2H), 6.87 (dd, 2H, J =
8.1, 2.1 Hz), 6.22 (d, 2H, J = 8.1 Hz), 5.23 (s, 1H, acac-CH), 1.79 (s,
6H, acac-CH3), 1.57 (s, 6H, B−CH3), 1.10−3.10 (br, 20H, B−H). 13
C
NMR (CD2Cl2): δ 185.4 (acac-CO), 167.0, 152.5, 148.7, 146.5, 138.1,
134.0, 131.2, 126.2, 123.7, 123.2, 119.4, 100.9 (acac-CH), 84.1, 78.1
(C2B10H10), 28.5 (acac-CH3), 23.3 (B-CH3). 11B NMR (CDCl3): δ
−4.5 (br, 4B), −10.1 (br, 6B). Anal. Calcd for C33H47B20IrN2O2: C,
43.45; H, 5.19; N, 3.07. Found: C, 43.56; H, 5.21; N, 3.06.
Synthesis of (4-fppy)2Ir(acac) (4). This compound was prepared
in a manner analogous to the synthesis of 1 using [(4-fppy)2Ir(μ-Cl)]2
(0.176 g, 0.154 mmol), 2,4-pentanedione (0.039 g, 0.385 mmol), and
sodium carbonate (0.163 g, 1.54 mmol). Yield: 0.122 g, 62%. 1H NMR
(CD2Cl2): δ 8.43 (dt, 2H, J = 4.2 Hz), 7.80 (m, 4H), 7.59 (dd, 2H, J =
8.7, 5.7 Hz), 7.21 (dt, 2H, J = 6.0, 2.1 Hz), 6.58 (dt, 2H, J = 8.7, 2.7
Hz), 5.87 (dd, 2H, J = 9.9, 2.7 Hz), 5.30 (s, 1H), 1.80 (s, 6H). 13C
NMR (CD2Cl2): 185.5 (acac-CO), 148.7, 138.1, 126.1, 126.0, 122.4,
119.6, 119.3, 119.2, 108.7, 108.4, 101.0 (acac-CH), 28.8 (acac-CH3).
Anal. Calcd for C27H21F2IrN2O2: C, 51.01; H, 3.33; N, 4.41. Found: C,
50.99; H, 3.62; N, 4.21.
X-ray Crystallography. A specimen of suitable size and quality
was coated with Paratone oil and mounted onto a glass capillary. The
crystallographic measurement was performed using a Bruker Apex II-
CCD area detector diffractometer, with graphite-monochromated
MoKα radiation (λ = 0.71073 Å). The structure was solved by direct
methods, and all nonhydrogen atoms were subjected to anisotropic
refinement by full-matrix least-squares on F2 by using the SHELXTL/
PC package.18 Hydrogen atoms were placed at their geometrically
calculated positions and were refined riding on the corresponding
carbon atoms with isotropic thermal parameters. The detailed
crystallographic data are given in Supporting Information, Table S1.
UV−vis Absorption and PL Measurements. UV−visible
absorption and photoluminescence (PL) measurements were
performed in degassed CH2Cl2 with a 1-cm quartz cuvette. PL
quantum efficiencies were measured with reference to that of quinine
sulfate in 0.5 M H2SO4 (Φ = 0.55)19 and were calculated according to
the following equation, which includes refractive index correction:
Synthesis of [(5-CBppy)2Ir(μ-Cl)]2 (2c). This compound was
prepared in a manner analogous to the synthesis of 1c using 2b (1.24
g, 3.99 mmol) and IrCl3·3H2O (0.61 g, 1.73 mmol). Yield: 1.46 g,
1
93%. H NMR (CDCl3): δ 9.08 (d, 4H, J = 5.7 Hz), 7.88 (m, 8H),
7.69 (s, 4H), 6.88 (t, 4H, J = 6.6 Hz), 6.79 (d, 4H, J = 8.1 Hz), 5.90 (d,
4H, J = 8.1 Hz), 1.53 (s, 12H, B−CH3), 1.20−3.50 (br, 40H, B−H).
13C NMR (acetone-d6): δ 167.9, 153.1, 151.2, 147.1, 139.8, 132.5,
132.4, 127.3, 125.9, 125.4, 121.5, 85.5, 79.7 (C2B10H10), 22.9 (B-CH3).
11B NMR (CDCl3): δ −4.4, −10.1. Anal. Calcd for C56H80B40Cl2Ir2N4:
C, 39.63; H, 4.75; N, 3.30. Found: C, 39.38; H, 4.37; N, 3.09.
Synthesis of (4-CBppy)2Ir(acac) (1). Into the flask containing the
dimeric iridium(III) complex 1c (1.07 g, 0.63 mmol), 2,4-
pentanedione (0.19 g, 1.90 mmol), and Na2CO3 (0.67 g, 6.32
mmol) was added acetonitrile (50 mL). The reaction mixture was
heated to 80 °C and stirred for 2 d. After cooling to room temperature,
the orange precipitate formed was collected by filtration and washed
with acetonitrile (10 mL). The crude solid was extracted with CH2Cl2
to remove remaining salts. The solution was dried over MgSO4 and
filtered. Evaporation of solvent followed by drying in vacuo afforded
orange solid of 1. Yield: 0.95 g, 83%. Single crystals suitable for X-ray
162
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Inorganic Chemistry
Article
a
Scheme 1 .
a
Conditions: (i) Pd(PPh3)4, toluene, 110 °C, 54% (1b) and 70% (2b). (ii) IrCl3·3H2O, 2-ethoxyethanol/H2O, 110 °C, 88% (1c) and 93% (2c). (iii)
2,4-Pentanedione, Na2CO3, acetonitrile, 80 °C, 83% (1) and 95% (2).
2
⎛
⎝
⎞
⎠
LANL2DZ effective core potentials (ECPs) and corresponding basis
sets.24 All calculations described here were carried out using the
GAUSSIAN 09 program.25
η ArIs
s
⎜
⎜
⎟
⎟
Φ = Φ
s
r
η 2A I
s r
r
where Φ is the quantum efficiency; η is the refractive index of the
solvent; A is absorbance at the excitation wavelength; I is the
integrated area under the emission spectrum; and the subscripts s and r
refer to the sample and reference, respectively. The η values of 1.424
for CH2Cl2 and 1.346 for 0.5 M H2SO420 were used in the calculation.
The detailed conditions for absorption and PL measurements are
given in the figure captions and Table 1.
RESULTS AND DISCUSSION
Synthesis and Characterization. The ortho-methylcarbor-
ane (CB) substituted 2-phenylpyridine ligands, 4-CBppyH (1b)
■
Cyclic Voltammetry. Cyclic voltammetry measurements were
carried out in dimethylformamide (DMF, 1 mM) with a three-
electrode cell configuration consisting of platinum working and
counter electrodes and a Ag/AgNO3 (0.01 M in CH3CN) reference
electrode at room temperature. Tetra-n-butylammonium hexafluor-
ophosphate (0.1 M) 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.
Fabrication of Electroluminescent Devices. The configuration
of EL devices is as follows: ITO/NPB (40 nm)/CBP:emitter (8 wt %)
(30 nm)/BCP (10 nm)/Alq3 (40 nm)/LiF (1 nm)/Al (100 nm),
where emitter is 1 or fac-Ir(ppy)3; NPB is N,N′-bis(naphthalene-1-yl)-
N,N′-bis(phenyl)-benzidine (EM Index, a local company in Korea,
>99%, sublimed grade); CBP is 4,4′-bis(9-carbazolyl)-biphenyl
(Nichem, >99%, sublimed grade); BCP is bathocuproine (Lumtec,
>99.8%, sublimed grade); Alq3 is tris(8-hydroxyquinolinolato)
aluminum (EM Index, >99%, sublimed grade). Onto a glass substrate
precoated with ITO, a hole-transporting NPB layer was deposited
using the vacuum evaporation method (∼10−7 Torr). A layer of CBP
host was subsequently codeposited with an emitter material under
high vacuum. The film thickness of the EML was determined with a
TENCOR alpha-step 500 profiler. Hole-blocking BCP, electron
transporting Alq3 layers, and a cathode layer of LiF and Al were
successively deposited on top of the EML (deposition rates: BCP, Alq3
= 1 Å/s, LiF = 0.1 Å/s, Al electrode = 3 Å/s). EL spectra were
obtained with a PR-650 spectrometer. Current−voltage (J−V) and
luminance−voltage (L−V) characteristics were recorded on a
current−voltage source (Keithley 237) and a luminescence detector
(PR-650). All EL measurements were carried out in an N2-filled
glovebox at room temperature.
Figure 1. Crystal structure of 1 (40% thermal ellipsoids). H-atoms are
omitted for clarity. Selected bond lengths (Å) and angles (deg): Ir−N
2.047(4), Ir−C(4) 1.987(5), Ir−O 2.161(3), C(1)−C(2) 1.514(7),
C(1)−C(13) 1.697(8), N−Ir−N′ 171.9(2), O−Ir−O′ 87.0(2), N−
Ir−C(4) 80.5(2).
and 5-CBppyH (2b) were prepared in moderate yield from
Stille coupling reactions between 2-(trimethylstannyl)pyridine
and para- or meta-carborane substituted bromobenzene (1a
and 2a) (Scheme 1). The starting 1a and 2a can easily be
accessed by the reaction of arylalkyne with B10H14 in the
presence of weak base such as Et2S, MeCN, and N,N-
dimethylaniline,26 followed by alkylation of the resulting 1-Ar-
2-H-1,2-closo-carborane using alkyl halides (Supporting In-
formation, Scheme S1).14,27 Because of susceptibility of
carborane toward nucleophilic anions such as hydroxide at
elevated temperature, the normal Suzuki−Miyaura coupling
reaction under aqueous basic conditions could not be applied
Theoretical Calculations. The geometries of the ground (S0) and
lowest-lying triplet excited (T1) states of 1 were optimized using the
density functional theory (DFT) method. The electronic transition
energies including electron correlation effects were computed by the
time dependent density functional theory (TD-DFT)21 method using
the B3LYP22 functional (TD-B3LYP). The 6-31G(d) basis set23 was
used for all atoms except for the iridium atom which was treated with
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Inorganic Chemistry
Article
Figure 4. HOMO and LUMO of 1 at the ground state (S0) (left) and
lowest triplet excited state (T1) (right) optimized geometries (isovalue
= 0.04).
Table 2. Energy Levels and Orbital Analyses of the Excited
States for 1 from TD-DFT Calculations
a
Figure 2. (a) UV−vis absorption and (b) PL spectra of 1−4 in
degassed CH2Cl2 at room temperature. The inset shows the enlarged
absorption spectra at the lower energy region.
states
λcalc
f
assignment
b
T1
S1
S2
S3
S4
525.5
451.5
442.4
399.7
399.4
0
HOMO→LUMO (90.5%)
HOMO→LUMO (96.0%)
HOMO→LUMO+1 (96.0%)
HOMO-1→LUMO+1 (95.5%)
HOMO-1→LUMO (95.0%)
0.0385
0.0014
0.0225
0.0024
a
Singlet energies for the vertical transition calculated at the optimized
b
S0 geometries. Triplet energy for the adiabatic transition correspond-
ing to the 0−0 phosphorescence.
Figure 3. Cyclic voltammograms of 1, 2, and 4 (1 mM in DMF, scan
rate = 100 mV/s).
to the synthesis of the ligands. The cyclometalation reactions of
iridium(III) chloride with 1b and 2b afforded the chloro-
bridged dimeric iridium(III) complexes, [(C∧N)2Ir(μ-Cl)2Ir-
(C∧N)2] (C∧N = 4-CBppy (1c); 5-CBppy (2c)) in high yield.
Since the cyclometalation reaction is highly sensitive to steric
effects,28 one can expect the regioselective cyclometalation of
the ligand 2b with Ir(III) at the 2-position of the phenyl ring.
Spectroscopic data also reveal the formation of a single
regioisomer 2c. Treatment of 1c and 2c with acetylacetone
(acacH) under mild basic conditions29 cleanly led to the
heteroleptic Ir(III) complexes, (4-CBppy)2Ir(acac) (1) and (5-
CBppy)2Ir(acac) (2) in high yield. For comparison purpose, the
4-fluorine substituted Ir(III) complex, (4-fppy)2Ir(acac) (4)
was analogously prepared.
Figure 5. EL spectra of device (D1) fabricated with CBP host doped
with 1 (8 wt %) as an emitter with respect to the current density. Inset
shows a photograph of the working device (cell area: 2 × 2 mm2).
signals centered at δ −5 and −10 ppm in a 4:6 ratio confirm the
presence of closo-carborane. An X-ray diffraction study revealed
the molecular structure of 1. As shown in Figure 1, complex 1
which crystallizes in space group C2/c adopts a Λ-
configuration. The molecule possesses a C2-axis connecting Ir
and C17 atoms (Supporting Information, Figure S1). The
chelating 4-CBppy ligands are in a trans disposition of the
pyridine rings, similar to those observed in usual heteroleptic
Ir(III) complexes1,7,8,30 Geometrical parameters such as bond
Complexes 1 and 2 have been characterized by NMR
spectroscopy, elemental analysis, and X-ray diffraction method.
1H and 13C NMR spectra show the expected resonances
corresponding to the (C∧N)2Ir(acac). Two broad 11B NMR
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Inorganic Chemistry
Article
complexes.31,33,34 Despite the electron-deficient nature of
carborane, the large difference of about 40 nm (1450 cm−1)
between the emission wavelengths of 1 and 4 is quite unusual.
The blue shift of 13 nm (501 cm−1) observed for 2 in
comparison with the emission wavelength of 3 is also
pronounced. These results indicate that a change in the
position of the carborane substituent on the phenyl ring can
lead to phosphorescence color tuning. The phosphorescence
spectra of 1 and 2, which lack vibronic structure, closely
resemble the spectra of 3 and 4, suggesting that the lowest
3
energy excited state of 1 and 2 is mainly MLCT in character.
The small Stokes shifts between λmax for the 3MLCT absorption
and emission bands for 1 (57 nm, ∼2260 cm−1) and 2 (52 nm,
∼2290 cm−1) further support this assignment, as similarly
found in ppy-ligand containing Ir(III) complexes.8 Inspection of
the phosphorescence quantum efficiencies indicates that the 4-
substituted 1 is much less emissive than the 5-substituted 2
(Table 1). The reason for this is not understood, but might be
partly related to the small HOMO−LUMO band gap of 1,
which may facilitate the nonradiative decay processes according
to the energy-gap law.35 In addition, we observe from TD-DFT
calculation that the triplet state of 1 is significantly stabilized by
a major contribution from the carborane (see below). This
might retard the radiative decay process, as consistent with the
relatively long emission lifetime of 1.
Electrochemistry. The electrochemical properties of 1 and
2 were examined by cyclic voltammetry (Table 1 and Figure 3).
Compounds 1 and 2 undergo reversible oxidation at 0.51 and
0.55 V, respectively. Interestingly, these oxidation potentials are
anodically shifted in comparison with those of 3 (0.41 V)31 and
4 (0.46 V), indicating that the HOMO of 1 and 2 is lower in
energy than 3 and 4, probably because of the effective
stabilization of the HOMO by the inductive effect of carborane
(see also DFT results). On the other hand, 1 and 2 display
reduction processes that are chemically reversible but electro-
chemically quasi-reversible. Since it is known that aryl
substituted-o-carboranes typically undergo a reduction in a
range similar to that observed for 1 and 2,13,36 this reduction
feature could be a consequence of the involvement of the
carborane in the pyridyl reduction, which, in turn, implies that
the carborane may affect the LUMO level. Although the
reduction process in 4 was not observed in this study, one may
assume that the reduction would occur in the range of −2.4 ∼
−2.6 V because the (4,6-dfppy)2Ir(acac) complex is reported to
undergo reduction at −2.44 V.31 Thus, comparison of the
reduction potentials indicates that reduction of 1 is the most
facile among the complexes, reflecting significant LUMO
stabilization in 1. Considering that the reduction process
mainly involves the π* orbital of the pyridyl ring fragment in
(C∧N)2Ir(acac) complexes, such a LUMO stabilization by
carborane substitution on the phenyl ring is remarkable.
Although less than 1, complex 2 similarly shows a low
reduction potential, indicating that the carborane at the 5-
Figure 6. External quantum efficiency−luminance (ηEQE−L) and
power efficiency−luminance (ηPE−L) curves of device D1. Inset shows
the current density−applied voltage−luminance (J−V−L) character-
istics of D1.
lengths and angles around the Ir atom are also in a similar range
reported for other (C∧N)2Ir(acac) complexes.5,7,31
Photophysical Properties. To examine the photophysical
properties, UV−vis absorption and PL experiments were
carried out with 1 and 2 in degassed CH2Cl2 (Figure 2 and
Table 1). For comparison, the spectra of 3 and 4 were also
obtained. Compound 1 features an intense absorption band in
the region of 250−350 nm, which can be assigned to the spin-
1
allowed π−π* transition of the 4-CBppy ligand (1LC). The
structured band shape and band energy, as well as the high
extinction coefficient, are consistent with those observed for the
free ligand, 4-CBppyH (1b) (Supporting Information, Figure
1
S2). A similar π−π* transition band with a slight blue shift is
also observed for 2. While the lower energy band of 1 at 415
nm results from spin-allowed metal-to-ligand charge transfer
(1MLCT), the band at 474 nm, which tails to over 500 nm, can
be majorly assigned to a mixture of spin-forbidden MLCT
3
(3MLCT) and the π−π* transition of the 4-CBppy ligand
(3LC).8,32 It is notable that the MLCT bands for 1 are
significantly red-shifted compared to those of 4 bearing a 4-
fluorine atom (393 and 444 nm) and are even lower in energy
than those for 3 (405 and 460 nm). In contrast, the lowest-
energy bands for 2 are observed at the apparently blue-shifted
region (401 and 451 nm) compared to those for 1 and 3. These
findings indicate that carborane substitution on the 4-position
of the phenyl ring lowers the excited state energy, while the
same substitution on the 5-position raises it (vide infra).
An identical trend in the excited state energy is also found in
the PL spectra of complexes. While 1 exhibits an emission band
centered at 531 nm which is red-shifted compared to that for 3
(λem = 516 nm), the emission spectrum of 2 shows a blue-
shifted band at 503 nm (Figure 2). The emission lifetimes of
3.0 and 0.6 μs for 1 and 2, respectively, confirm the
phosphorescence origin of the emission. The phosphorescence
band of 4 appears at a high energy region (493 nm), similar to
that observed for other fluoro-substituted (C∧N)2Ir(acac)
a
Table 3. Device Performance of PhOLEDs.
b
c
d
e
f
device
emitter
CIE (x, y)
Vturn‑on (V)
Lmax (cd/m2)
EQEmax (%)
PEmax (lm/W)
D1
D2
1
(0.392, 0.585)
(0.330, 0.612)
4.6
3.4
13300
40100
6.6
10.7
25.0
fac-Ir(ppy)3
10.9
a
b
Device structure: ITO/NPB (40 nm)/CBP:emitter (8 wt %) (30 nm)/BCP (10 nm)/Alq3 (40 nm)/LiF (1 nm)/Al (100 nm). Measured at 10
c
d
e
f
mA/cm2-current density. Applied voltage at the luminance of 1 cd/m2. Maximum luminance. Maximum external quantum efficiency. Maximum
power efficiency.
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position of phenyl ring also contributes to LUMO stabilization.
This reduction behavior in 1 and 2 may be related to the strong
inductive electron-withdrawing effect of carborane, as well as to
the participation of carborane in the LUMO delocalization.
Theoretical Calculations. To elucidate the photophysical
and electrochemical properties of 1 and 2 discussed above, TD-
DFT calculations on both the ground state (S0) and lowest
triplet excited state (T1) optimized structures of 1 were
performed at the B3LYP/LANL2DZ level (Figure 4 and Table
2). We find that the lowest energy absorption is mainly
characterized by a HOMO−LUMO transition. While the
HOMO resides on both the π orbital on the phenyl ring of the
ligand (32.8%) and Ir(dπ) (53.5%), the LUMO is located on
the C∧N ligand and has the following contributions: pyridyl
ring (52.8%), phenyl ring (30.2%), and carboranyl carbon
atoms (12.6%). This suggests that the lowest energy absorption
is mainly MLCT in character with substantial contribution from
ligand-centered π−π* transition, and the LUMO could be
significantly stabilized by the contribution from the carboranyl
carbon atoms. The calculated LUMO level for 1 (−2.04 eV) is
much lower than that of the reported value for 3 (−1.57 eV) at
the same level of theory.9 This feature is essentially the same as
that found in the triarylboranes bearing o-carborane.13,14 In
contrast, the HOMO in 1 has little contribution from the 4-
position of the phenyl ring, indicating that there might be no
additional stabilization by delocalization through the carborane.
Nevertheless, the calculated HOMO level of −5.51 eV is still
lower than that of 3 (−5.08 eV), pointing to the presence of a
strong inductive electron-withdrawing effect of carborane.
Indeed, these results are consistent with the electrochemical
data of 1. Although the extent of the inductive effect exerted by
the carborane on the HOMO and LUMO could not be exactly
estimated, the resulting HOMO−LUMO band gap of 1 is
narrowed by 0.04 eV because of more LUMO stabilization than
that of 3. This result may correlate with the observed red shift
(10 nm, 595 cm−1) of the lowest singlet absorption for 1. In the
case of the 4-fluoro-substituted 4, while the strong inductive
effect of the fluorine atom substantially lowers the HOMO
level, the stabilization of the LUMO level does not appear
pronounced, owing to a weak π donation from the p-orbital of
fluorine to the LUMO through the 4-position. The reported
calculation data for 4 is also consistent with this hypothesis.9
Consequently, the increased band gap may lead to blue shift of
the absorption band, as shown in Figure 2.
The blue shift observed in the absorption band of 5-
carborane substituted 2 could be similarly explained by the
orbital analyses described for 1. While the HOMO electron
density at the 5-position of the phenyl ring would be effectively
stabilized by the inductive effect of carborane, the absence of a
LUMO contribution from carborane at the 5-position could
hardly lead to the lowering of the LUMO level by as much as in
1. Accordingly, the band gap is increased to afford a blue shift
of the absorption band. In fact, this feature is in good
agreement with the electrochemical data of 1 and 2, that is, 2
has slightly higher oxidation and reduction potentials than does
1. This 5-substitution effect of carborane is in contrast to the 5-
fluorine substituted Ir(III)34 and Pt(II)6 complexes in which a
marginal blue shift or even a red shift upon 5-fluoro-
substitution is observed owing to the offset of the inductive
effect of the fluorine atom by π donation into the HOMO.
Next, the TD-DFT calculations at the T1 optimized
geometry for 1 show that the lowest energy triplet state is
also dominated by a HOMO−LUMO transition. As shown in
Figure 4 (right), the HOMO is localized on both the phenyl
ring of the ligand and the Ir atom, with similar contributions as
observed in the ground state structure. However, it can be seen
that the LUMO bears a large contribution from the carborane
(54.3%) with full delocalization over the ligand. As a result, the
LUMO energy level is significantly stabilized (−2.76 eV vs
−2.04 eV at the S0 state), while the HOMO level is changed
little (−5.43 eV vs −5.51 eV at the S0) in accordance with the
red-shifted phosphorescence band of 1. Furthermore, this
feature indicates that the lowest triplet state (T1) is governed
3
by the carborane and thus has [dπ(Ir)→π*(C∧N)] MLCT
3
character with some contribution from LC. The calculated T1
→ S0 transition is also in good agreement with the observed
phosphorescence band (Table 2).
Electroluminescent Properties. To explore the possibility
of using 1 and 2 as an emitting material in phosphorescent
OLEDs (PhOLEDs), devices based on emissive layers
containing a 9,9′-(1,1′-biphenyl)-4,4′-diylbis-9H-carbazole
(CBP) (ET = 2.56 eV)37,38 host doped with 1 (ET = 2.37 eV
from 0−0 phosphorescence at 77 K) or 2 (ET = 2.51 eV) were
fabricated by vacuum deposition with the following config-
uration: ITO/NPB (40 nm)/CBP:emitter (8 wt %) (30 nm)/
BCP (10 nm)/Alq3 (40 nm)/LiF (1 nm)/Al (100 nm), where
NPB is N,N′-bis(naphthalene-1-yl)-N,N′-bis(phenyl)-benzi-
dine,39 CBP is 4,4′-bis(9-carbazolyl)-biphenyl, BCP is bath-
ocuproine,40 and Alq3 is tris(8-hydroxyquinolinolato)-
aluminum. Unfortunately, it was found that 2 undergoes
thermal decomposition during the course of vacuum
sublimation, thus only a device incorporating 1 (D1) was
tested. A reference device D2 with the well-known fac-Ir(ppy)3
emitter was also fabricated in the identical device structure.38
As shown in Figure 5, D1 emits green light (λem = 536 nm),
which is very similar to the PL spectrum of 1, indicating that
light emission originates only from phosphorescent dopant 1.
The electroluminescence (EL) spectra obtained at various
current densities ranging from 10 to 100 mA/cm2 do not
exhibit much change except for a gradual increase in the
emission intensity, indicating stable light emission. According
to the EL characteristics (Figure 6 and Table 3), the turn-on
voltage of D1 (4.6 V) was somewhat higher than that of device
D2 (3.4 V). With the increasing applied voltage, D1 showed a
high level of current density and luminance with a maximum
brightness (Lmax) of 13,300 cd/m2 at 12.0 V. The lifetime tests
for D1 and D2 under the constant current yielding the initial
luminance of 200 cd/m2 indicate that the devices based on 1
exhibit high level of stability comparable to that of fac-Ir(ppy)3-
based devices (Supporting Information, Figure S5). Although
D1 displayed overall good performance in terms of external
quantum efficiency (ηEQE, 6.6%) and power efficiency (ηPE, 10.7
lm/W), these were still inferior to the performance of the D2
device (Table 3 and Supporting Information, Figure S6 for EL
characteristics for D2). As seen from the low PL quantum
efficiency of 1, the stabilization of triplet excitons or charge
carrier (electron) trapping by the carborane is likely to cause an
imbalance between electrons and holes, resulting in the
observed lower device performance.
CONCLUSION
■
The introduction of an o-carborane to the 4- and 5-position of
the phenyl ring of a ppy ligand in heteroleptic (C∧N)2Ir(acac)
complexes gave rise to red and blue shifts of the
phosphorescence band, respectively, compared to that of
(ppy)2Ir(acac). We find from electrochemical and theoretical
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Article
Y.-S.; Kang, J.-W.; Kang, D. M.; Park, J.-W.; Kim, Y.-H.; Kwon, S.-K.;
Kim, J.-J. Adv. Mater. 2008, 20, 1957−1961. Yang, X.; Wang, Z.;
Madakuni, S.; Li, J.; Jabbour, G. E. Adv. Mater. 2008, 20, 2405−2409.
Williams, E. L.; Haavisto, K.; Li, J.; Jabbour, G. E. Adv. Mater. 2007,
19, 197−202. Zhao, Q.; Liu, S.; Shi, M.; Wang, C.; Yu, M.; Li, L.; Li,
F.; Yi, T.; Huang, C. Inorg. Chem. 2006, 45, 6152−6160. Yu, X.-M.;
Kwok, H.-S.; Wong, W.-Y.; Zhou, G.-J. Chem. Mater. 2006, 18, 5097−
5103. Sajoto, T.; Djurovich, P. I.; Tamayo, A.; Yousufuddin, M.; Bau,
R.; Thompson, M. E.; Holmes, R. J.; Forrest, S. R. Inorg. Chem. 2005,
44, 7992−8003. Li, J.; Djurovich, P. I.; Alleyne, B. D.; Yousufuddin,
M.; Ho, N. N.; Thomas, J. C.; Peters, J. C.; Bau, R.; Thompson, M. E.
Inorg. Chem. 2005, 44, 1713−1727. D’Andrade, B. W.; Forrest, S. R.
Adv. Mater. 2004, 16, 1585−1595. D’Andrade, B. W.; Holmes, R. J.;
Forrest, S. R. Adv. Mater. 2004, 16, 624−628. Furuta, P. T.; Deng, L.;
studies that a carborane substitution on the 4-position of the
phenyl ring lowers the MLCT energy by LUMO stabilization
3
via the contribution of the carborane to LUMO delocalization,
3
while substitution on the 5-position raises the MLCT energy
by the stabilization of the HOMO level because of the strong
inductive electron-withdrawing effect of carborane. An EL
device incorporating the 4-carborane substituted Ir(III)
complex displayed overall good performance with green
phosphorescence, suggesting that this type of complex could
potentially be useful as phosphorescent emitter in PhOLEDs.
ASSOCIATED CONTENT
* Supporting Information
■
S
́
Garon, S.; Thompson, M. E.; Frechet, J. M. J. J. Am. Chem. Soc. 2004,
Crystallographic data for 1 in cif format, additional UV−vis, PL,
and EL data, and computational details. This material is
available free of charge via the Internet at http://pubs.acs.org.
126, 15388−15389.
(4) Kui, S. C. F.; Hung, F.-F.; Lai, S.-L.; Yuen, M.-Y.; Kwok, C.-C.;
Low, K.-H.; Chui, S. S.-Y.; Che, C.-M. Chem.Eur. J. 2012, 18, 96−
109. Lin, C.-H.; Chi, Y.; Chung, M.-W.; Chen, Y.-J.; Wang, K.-W.; Lee,
G.-H.; Chou, P.-T.; Hung, W.-Y.; Chiu, H.-C. Dalton Trans. 2011, 40,
1132−1143. Chiu, Y.-C.; Hung, J.-Y.; Chi, Y.; Chen, C.-C.; Chang, C.-
H.; Wu, C.-C.; Cheng, Y.-M.; Yu, Y.-C.; Lee, G.-H.; Chou, P.-T. Adv.
Mater. 2009, 21, 2221−2225. Chiu, Y.-C.; Lin, C.-H.; Hung, J.-Y.; Chi,
Y.; Cheng, Y.-M.; Wang, K.-W.; Chung, M.-W.; Lee, G.-H.; Chou, P.-
T. Inorg. Chem. 2009, 48, 8164−8172. Xu, M.; Wang, G.; Zhou, R.;
An, Z.; Zhou, Q.; Li, W. Inorg. Chim. Acta 2007, 360, 3149−3154. You,
Y.; An, C.-G.; Kim, J.-J.; Park, S. Y. J. Org. Chem. 2007, 72, 6241−6246.
Tamayo, A. B.; Alleyne, B. D.; Djurovich, P. I.; Lamansky, S.; Tsyba, I.;
Ho, N. N.; Bau, R.; Thompson, M. E. J. Am. Chem. Soc. 2003, 125,
7377−7387. Tsuboyama, A.; Iwawaki, H.; Furugori, M.; Mukaide, T.;
Kamatani, J.; Igawa, S.; Moriyama, T.; Miura, S.; Takiguchi, T.; Okada,
S.; Hoshino, M.; Ueno, K. J. Am. Chem. Soc. 2003, 125, 12971−12979.
Grushin, V. V.; Herron, N.; LeCloux, D. D.; Marshall, W. J.; Petrov, V.
A.; Wang, Y. Chem. Commun. 2001, 1494−1495.
AUTHOR INFORMATION
Corresponding Author
*E-mail: YoonSupLee@kaist.ac.kr (Y.S.L.), lmh74@ulsan.ac.kr
(M.H.L.).
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by the Basic Science Research
Program (No. 2012039773 for M.H.L. and No. 2007-0056341
for Y.S.L.) and Priority Research Centers Program (No. 2009-
0093818 for M.H.L.) through the National Research
Foundation of Korea (NRF) funded by the Ministry of
Education, Science and Technology. Computational resources
for this work were provided by KISTI (KSC-2011-C2-18). We
thank Dongwoo FineChem Co. for assisting with the EL
measurements.
(5) Edkins, R. M.; Wriglesworth, A.; Fucke, K.; Bettington, S. L.;
Beeby, A. Dalton Trans. 2011, 40, 9672−9678.
(6) Brooks, J.; Babayan, Y.; Lamansky, S.; Djurovich, P. I.; Tsyba, I.;
Bau, R.; Thompson, M. E. Inorg. Chem. 2002, 41, 3055−3066.
(7) Lamansky, S.; Djurovich, P.; Murphy, D.; Abdel-Razzaq, F.;
Kwong, R.; Tsyba, I.; Bortz, M.; Mui, B.; Bau, R.; Thompson, M. E.
Inorg. Chem. 2001, 40, 1704−1711.
(8) Lamansky, S.; Djurovich, P.; Murphy, D.; Abdel-Razzaq, F.; Lee,
H. E.; Adachi, C.; Burrows, P. E.; Forrest, S. R.; Thompson, M. E. J.
Am. Chem. Soc. 2001, 123, 4304−4312.
(9) Liu, T.; Xia, B.-H.; Zhou, X.; Zheng, Q.-C.; Pan, Q.-J.; Zhang, H.-
X. Theoret. Chim. Acta. 2008, 121, 155−164.
(10) Takizawa, S.-Y.; Nishida, J.-I.; Tsuzuki, T.; Tokito, S.;
Yamashita, Y. Inorg. Chem. 2007, 46, 4308−4319. Djurovich, P. I.;
Murphy, D.; Thompson, M. E.; Hernandez, B.; Gao, R.; Hunt, P. L.;
Selke, M. Dalton Trans. 2007, 3763−3770. Zhao, Q.; Cao, T.; Li, F.;
Li, X.; Jing, H.; Yi, T.; Huang, C. Organometallics 2007, 26, 2077−
2081. Park, N. G.; Choi, G. C.; Lee, Y. H.; Kim, Y. S. Curr. Appl. Phys.
2006, 6, 620−626. Jung, S. O.; Kang, Y.; Kim, H.-S.; Kim, Y.-H.; Yang,
K.; Kwon, S.-K. Bull. Korean Chem. Soc. 2003, 24, 1521−1524. Hay, P.
J. J. Phys. Chem. A 2002, 106, 1634−1641.
(11) Chen, Z.; King, R. B. Chem. Rev. 2005, 105, 3613−3642. King,
R. B. Chem. Rev. 2001, 101, 1119−1152. Hawthorne, M. F. Advances in
Boron Chemistry: Special Publication No. 201; Royal Society of
Chemistry: London, U.K., 1997; Vol. 82, p 261; Williams, R. E.
Chem. Rev. 1992, 92, 177−207.
(12) Hosmane, N. S. Boron Science: New Technologies and
Applications; CRC Press: New York, 2012; Dash, B. P.; Satapathy,
R.; Gaillard, E. R.; Norton, K. M.; Maguire, J. A.; Chug, N.; Hosmane,
N. S. Inorg. Chem. 2011, 50, 5485−5493. Kokado, K.; Chujo, Y. Dalton
Trans. 2011, 40, 1919−1923. Park, M. H.; Lee, K. M.; Kim, T.; Do, Y.;
Lee, M. H. Chem. Asian J. 2011, 6, 1362−1366. Kokado, K.; Chujo, Y.
J. Org. Chem. 2011, 76, 316−319. Dash, B. P.; Satapathy, R.; Gaillard,
E. R.; Maguire, J. A.; Hosmane, N. S. J. Am. Chem. Soc. 2010, 132,
6578−6587. Kokado, K.; Tokoro, Y.; Chujo, Y. Macromolecules 2009,
REFERENCES
■
(1) Baranoff, E.; Jung, I.; Scopelliti, R.; Solari, E.; Gratzel, M.;
Nazeeruddin, M. K. Dalton Trans. 2011, 40, 6860−6867.
̈
(2) Chi, Y.; Chou, P.-T. Chem. Soc. Rev. 2010, 39, 638−655. You, Y.;
Park, S. Y. Dalton Trans. 2009, 1267−1282. Flamigni, L.; Barbieri, A.;
Sabatini, C.; Ventura, B.; Barigelletti, F. Top. Curr. Chem. 2007, 281,
143−203. Chou, P.-T.; Chi, Y. Chem.Eur. J. 2007, 13, 380−395. Ma,
B.; Djurovich, P. I.; Thompson, M. E. Coord. Chem. Rev. 2005, 249,
1501−1510. Holder, E.; Langeveld, B. M. W.; Schubert, U. S. Adv.
Mater. 2005, 17, 1109−1121. Yersin, H. Top. Curr. Chem. 2004, 241,
1−26. Adachi, C.; Baldo, M. A.; Forrest, S. R.; Thompson, M. E. Appl.
Phys. Lett. 2000, 77, 904−906. Baldo, M. A.; Thompson, M. E.;
Forrest, S. R. Nature 2000, 403, 750−753.
(3) Yang, X.; Zhao, Y.; Zhang, X.; Li, R.; Dang, J.; Li, Y.; Zhou, G.;
Wu, Z.; Ma, D.; Wong, W.-Y.; Zhao, X.; Ren, A.; Wang, L.; Hou, X. J.
Mater. Chem. 2012, 22, 7136−7148. Zhu, M.; Li, Y.; Hu, S.; Li, C. g.;
Yang, C.; Wu, H.; Qin, J.; Cao, Y. Chem. Commun. 2012, 48, 2695−
2697. Kuwabara, J.; Namekawa, T.; Haga, M.-a.; Kanbara, T. Dalton
Trans. 2012, 41, 44−46. Lu, K.-Y.; Chou, H.-H.; Hsieh, C.-H.; Yang,
Y.-H. O.; Tsai, H.-R.; Tsai, H.-Y.; Hsu, L.-C.; Chen, C.-Y.; Chen, I. C.;
Cheng, C.-H. Adv. Mater. 2011, 23, 4933−4937. Chen, S.; Tan, G.;
Wong, W.-Y.; Kwok, H.-S. Adv. Funct. Mater. 2011, 21, 3785−3793.
Hudson, Z. M.; Helander, M. G.; Lu, Z.-H.; Wang, S. Chem. Commun.
2011, 47, 755−757. Bolink, H. J.; De Angelis, F.; Baranoff, E.; Klein,
C.; Fantacci, S.; Coronado, E.; Sessolo, M.; Kalyanasundaram, K.;
Gratzel, M.; Nazeeruddin, M. K. Chem. Commun. 2009, 4672−4674.
̈
Sajoto, T.; Djurovich, P. I.; Tamayo, A. B.; Oxgaard, J.; Goddard, W.
A.; Thompson, M. E. J. Am. Chem. Soc. 2009, 131, 9813−9822. Zhou,
G.; Ho, C.-L.; Wong, W.-Y.; Wang, Q.; Ma, D.; Wang, L.; Lin, Z.;
Marder, T. B.; Beeby, A. Adv. Funct. Mater. 2008, 18, 499−511. Park,
167
dx.doi.org/10.1021/ic3015699 | Inorg. Chem. 2013, 52, 160−168

Inorganic Chemistry
Article
42, 9238−9242. Peterson, J. J.; Werre, M.; Simon, Y. C.; Coughlin, E.
B.; Carter, K. R. Macromolecules 2009, 42, 8594−8598. Kokado, K.;
Tokoro, Y.; Chujo, Y. Macromolecules 2009, 42, 2925−2930. Peterson,
J. J.; Simon, Y. C.; Coughlin, E. B.; Carter, K. R. Chem. Commun. 2009,
4950−4952. Kokado, K.; Chujo, Y. Macromolecules 2009, 42, 1418−
1420.
(29) Evans, N. R.; Devi, L. S.; Mak, C. S. K.; Watkins, S. E.; Pascu, S.
I.; Kohler, A.; Friend, R. H.; Williams, C. K.; Holmes, A. B. J. Am.
̈
Chem. Soc. 2006, 128, 6647−6656.
(30) Vadavi, R. S.; Kim, H.; Lee, K. M.; Kim, T.; Lee, J.; Lee, Y. S.;
Lee, M. H. Organometallics 2012, 31, 31−34. Rai, V. K.; Nishiura, M.;
Takimoto, M.; Hou, Z. Chem. Commun. 2011, 47, 5726−5728. Shin,
C. H.; Huh, J. O.; Lee, M. H.; Do, Y. Dalton Trans. 2009, 6476−6479.
Zhao, Q.; Li, L.; Li, F.; Yu, M.; Liu, Z.; Yi, T.; Huang, C. Chem.
Commun. 2008, 685−687. Sie, W.-S.; Jian, J.-Y.; Su, T.-C.; Lee, G.-H.;
Lee, H. M.; Shiu, K.-B. J. Organomet. Chem. 2008, 693, 1510−1517.
(13) Lee, K. M.; Huh, J. O.; Kim, T.; Do, Y.; Lee, M. H. Dalton
Trans. 2011, 40, 11758−11764.
(14) Huh, J. O.; Kim, H.; Lee, K. M.; Lee, Y. S.; Do, Y.; Lee, M. H.
Chem. Commun. 2010, 46, 1138−1140.
(15) Malamas, M. S.; Barnes, K.; Johnson, M.; Hui, Y.; Zhou, P.;
Turner, J.; Hu, Y.; Wagner, E.; Fan, K.; Chopra, R.; Olland, A.; Bard,
J.; Pangalos, M.; Reinhart, P.; Robichaud, A. J. Bioorg. Med. Chem.
2010, 18, 630−639.
(16) Schwab, P. F. H.; Fleischer, F.; Michl, J. J. Org. Chem. 2002, 67,
443−449. Zhang, B.; Breslow, R. J. Am. Chem. Soc. 1997, 119, 1676−
1681.
Orselli, E.; Kottas, G. S.; Konradsson, A. E.; Coppo, P.; Frohlich, R.;
̈
De Cola, L.; van Dijken, A.; Buchel, M.; Borner, H. Inorg. Chem. 2007,
46, 11082−11093.
̈
̈
(31) Baranoff, E.; Curchod, B. F. E.; Frey, J.; Scopelliti, R.; Kessler,
F.; Tavernelli, I.; Rothlisberger, U.; Gratzel, M.; Nazeeruddin, M. K.
̈
Inorg. Chem. 2012, 51, 215−224.
(32) Schmid, B.; Garces, F. O.; Watts, R. J. Inorg. Chem. 1994, 33, 9−
14. Colombo, M. G.; Hauser, A.; Gudel, H. U. Top. Curr. Chem. 1994,
171, 143−171. Colombo, M. G.; Hauser, A.; Guedel, H. U. Inorg.
̈
(17) Shavaleev, N. M.; Monti, F.; Costa, R. D.; Scopelliti, R.; Bolink,
H. J.; Ortí, E.; Accorsi, G.; Armaroli, N.; Baranoff, E.; Gratzel, M.;
̈
Chem. 1993, 32, 3088−3092.
Nazeeruddin, M. K. Inorg. Chem. 2012, 51, 2263−2271. Cline, E. D.;
Adamson, S. E.; Bernhard, S. Inorg. Chem. 2008, 47, 10378−10388.
(18) Sheldrick, G. M. SHELXL-93: Program for the refinement of
(33) Rausch, A. F.; Thompson, M. E.; Yersin, H. J. Phys. Chem. A
2009, 113, 5927−5932. Adachi, C.; Kwong, R. C.; Djurovich, P.;
Adamovich, V.; Baldo, M. A.; Thompson, M. E.; Forrest, S. R. Appl.
Phys. Lett. 2001, 79, 2082−2084.
(34) Ragni, R.; Plummer, E. A.; Brunner, K.; Hofstraat, J. W.;
Babudri, F.; Farinola, G. M.; Naso, F.; De Cola, L. J. Mater. Chem.
2006, 16, 1161−1170.
(35) Lakowicz, J. R. Principles of Fluorescence Spectroscopy, 3rd ed.;
Springer: New York, 2006; pp 687−688.
crystal structures; University of Gottingen: Gottingen, Germany, 1993.
̈
̈
(19) Brouwer, A. M. Pure Appl. Chem. 2011, 83, 2213−2228.
Melhuish, W. H. J. Phys. Chem. 1961, 65, 229−235.
(20) Ahire, J. H.; Wang, Q.; Coxon, P. R.; Malhotra, G.; Brydson, R.;
Chen, R.; Chao, Y. ACS Appl. Mater. Interfaces 2012, 4, 3285−3292.
(21) Runge, E.; Gross, E. K. U. Phys. Rev. Lett. 1984, 52, 997−1000.
(22) Stephens, P. J.; Devlin, F. J.; Chabalowski, C. F.; Frisch, M. J. J.
Phys. Chem. 1994, 98, 11623−11627. Lee, C.; Yang, W.; Parr, R. G.
Phys. Rev. B 1988, 37, 785−789.
(36) Tricas, H.; Colon, M.; Ellis, D.; Macgregor, S. A.; McKay, D.;
Rosair, G. M.; Welch, A. J.; Glukhov, I. V.; Rossi, F.; Laschi, F.;
Zanello, P. Dalton Trans. 2011, 40, 4200−4211. Fox, M. A.; Nervi, C.;
Crivello, A.; Batsanov, A. S.; Howard, J. A. K.; Wade, K.; Low, P. J. J.
Solid State Electron. 2009, 13, 1483−1495. Fox, M. A.; Nervi, C.;
Crivello, A.; Low, P. J. Chem. Commun. 2007, 2372−2374.
(23) Francl, M. M.; Pietro, W. J.; Hehre, W. J.; Binkley, J. S.; Gordon,
M. S.; DeFrees, D. J.; Pople, J. A. J. Chem. Phys. 1982, 77, 3654−3665.
Hariharan, P. C.; Pople, J. A. Theoret. Chim. Acta. 1973, 28, 213−222.
Hehre, W. J.; Ditchfield, R.; Pople, J. A. J. Chem. Phys. 1972, 56, 2257−
2261.
(37) Brunner, K.; van Dijken, A.; Borner, H.; Bastiaansen, J. J. A. M.;
̈
Kiggen, N. M. M.; Langeveld, B. M. W. J. Am. Chem. Soc. 2004, 126,
6035−6042.
(24) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 270−283.
(25) Frisch, M. J. T., G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M.
A.; Cheeseman, J. R.; Montgomery, J. A. Jr.; Vreven, T.; Kudin, K. N.;
Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.;
Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.;
Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa,
J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene,
M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Bakken, V.;
Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.;
Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.;
Morokuma, K.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.;
Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.;
Farkas, O.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman,
J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.; Cioslowski, J.;
Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.;
Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.;
Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen,
W.; Wong, M. W.; Gonzalez, C.; Pople, J. A. Gaussian 09, Revision
A.02; Gaussian, Inc.: Wallingford, CT, 2009.
(38) Baldo, M. A.; Lamansky, S.; Burrows, P. E.; Thompson, M. E.;
Forrest, S. R. Appl. Phys. Lett. 1999, 75, 4−6.
(39) Van Slyke, S. A.; Chen, C. H.; Tang, C. W. Appl. Phys. Lett.
1996, 69, 2160−2162.
(40) O’Brien, D. F.; Baldo, M. A.; Thompson, M. E.; Forrest, S. R.
Appl. Phys. Lett. 1999, 74, 442−444.
(26) Jiang, W.; Knobler, C. B.; Hawthorne, M. F. Inorg. Chem. 1996,
35, 3056−3058. Hawthorne, M. F.; Berry, T. E.; Wegner, P. A. J. Am.
Chem. Soc. 1965, 87, 4746−4750. Heying, T. L.; Ager, J. W.; Clark, S.
L.; Mangold, D. J.; Goldstein, H. L.; Hillman, M.; Polak, R. J.;
Szymanski, J. W. Inorg. Chem. 1963, 2, 1089−1092.
(27) Songkram, C.; Takaishi, K.; Yamaguchi, K.; Kagechika, H.;
Endo, Y. Tetrahedron Lett. 2001, 42, 6365−6368. Lamrani, M.;
Hamasaki, R.; Mitsuishi, M.; Miyashita, T.; Yamamoto, Y. Chem.
Commun. 2000, 1595−1596. Kabalka, G. W.; Reddy, N. K.; Narayana,
C. Tetrahedron Lett. 1992, 33, 7687−7688.
(28) Li, L.; Brennessel, W. W.; Jones, W. D. Organometallics 2009,
28, 3492−3500.
168
dx.doi.org/10.1021/ic3015699 | Inorg. Chem. 2013, 52, 160−168