Cyclometalated iridium(III) complexes based on phenyl-imidazole ligand

Article abstract of DOI:10.1021/ic901834v

Phenyl-imidazole-based ligands with various substitution patterns have been used as the main ligand for heteroleptic bis-cyclometalated iridium complexes. Two series of complexes have been prepared and their photophysical and electrochemical properties we

Full text of DOI:10.1021/ic901834v

Inorg. Chem. 2011, 50, 451–462 451
DOI: 10.1021/ic901834v
Cyclometalated Iridium(III) Complexes Based on Phenyl-Imidazole Ligand
†
†
Etienne Baranoff,*,† Simona Fantacci,‡,§ Filippo De Angelis,‡ Xianxi Zhang,†,# Rosario Scopelliti, Michael Gratzel, and
€
Md. Khaja Nazeeruddin*,†
†Laboratory for Photonics and Interfaces, Institute of Chemical Sciences and Engineering,
School of Basic Sciences, Swiss Federal Institute of Technology, CH-1015 Lausanne, Switzerland,
‡
ꢀ
Istituto CNR di Scienze e Tecnologie Molecolari, c/o Dipartimento di Chimica, Universita di Perugia,
via Elce di Sotto 8, I-06123 Perugia, Italy, §Italian Institute of Technology (IIT), Center for Biomolecular
Nanotechnologies, I-73010 Arnesano, Lecce, Italy, and #School of Chemistry and Chemical Engineering,
Liaocheng University, Liaocheng 252059, P. R. China
Received September 17, 2009
Phenyl-imidazole-based ligands with various substitution patterns have been used as the main ligand for heteroleptic
bis-cyclometalated iridium complexes. Two series of complexes have been prepared and their photophysical and
electrochemical properties were studied. The phosphorescence emission maxima range from about 490 to 590 nm,
that is, from greenish-blue to orange. The first series is of the form Ir(L)2(acac) (L: a phenyl-imidazole based ligand;
acac: acetylacetonate). In the first complex, 1a, L is 1,4,5-trimethyl-2-phenyl-1H-imidazole. Then, methyl groups
are replaced with phenyl groups and chlorines are grafted on the cyclometalated phenyl ring. The second series is
of the form Ir(4,5-dimethyl-1,2-diphenyl-1H-imidazole)2(La) (La: ancillary ligand being acetylacetonate, acac, N,N-
dimethylamino-picolinate, NPic, picolinate, Pic, or 2-(diphenylphosphino)acetic acid, P). These series show that
modifying the substitution pattern on the ligands can alter the photophysical and electrochemical properties of the
complexes. Overall, we show that compared to complexes containing phenyl-pyridine ligands, highest occupied mole-
cular orbitals (HOMOs) and lowest unoccupied molecular orbitals (LUMOs) are more delocalized over the entire main
ligand in complexes containing phenyl-imidazole. Contrary to expectations, when chlorine atoms are used as strong
acceptor substituents on the orthometalated phenyl, a red shift of the emission is observed. This behavior has been
rationalized using theoretical calculations on the excited state of the chloro-substituted complex 3a compared to the
model 1a.
Introduction
possibility to easily tune the color of emission from deep blue
to red.7-12
Cyclometalated iridium(III) complexes are often con-
sidered as the most promising family of triplet emitters
for organic light-emitting diodes (OLEDs).1-6 This is due
to their excellent phosphorescent quantum yield and the
The most commonly used cyclometalated ligands are
based on 2-phenyl-pyridine (ppy). Simple variation of the
substitution pattern with donor and acceptor groups allows
the tuning of the photophysical properties.13-18 Another
approach is to directly modify the skeleton of the main ligand
*To whom correspondence should be addressed. E-mail: etienne.baranoff@
epfl.ch (E.B.); mdkhaja.nazeeruddin@epfl.ch (M.K.N.).
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2010 American Chemical Society
Published on Web 12/14/2010
pubs.acs.org/IC

452 Inorganic Chemistry, Vol. 50, No. 2, 2011
Scheme 1. Molecular Structures of Complexes
Baranoff et al.
by using various heterocyclic rings instead of pyridine.19-25
A commonly used heterocyclic ring for undertaking such tun-
ing is 2-phenyl-benzimidazole.26-32 Surprisingly, it appears that
cyclometalated iridium complexes using a 2-phenyl-imidazole
ligand without the fused benzene ring (i.e., not based on benzo-
[d]imidazole motif) are hardly reported in the literature.33-35
Recently, we reported an iridium complex without a fused
benzene ring, acetylacetonato bis(1-methyl-2-phenylimidazole)
iridium(III), hereafter labeled N966.36,37 The N966 complex
exhibits unique photophysical behavior as the lowest unoc-
cupied molecular orbitals (LUMOs) of the main ligand and
of the ancillary ligand are almost degenerate, which leads
to broad photo- and electro-phosphorescence. This makes
this dopant suitable for white organic light-emitting diode
(WOLED) based on a single emitting center. To gain insight
into this class of molecules, we prepared complexes related to
N966, with various substitution patterns. In this manuscript,
we report our initial results on this family of iridium complexes
with a focus on heteroleptic complexes (Scheme 1, Chart 1).
It includes synthesis and electrochemical and photophysical
characterizations. Finally, we focus on the unexpected red-
shifting effect of the chloro substitution with theoretical
calculations to rationalize this counterintuitive experimental
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Article
Inorganic Chemistry, Vol. 50, No. 2, 2011 453
Chart 1. Substitution Patterns for Ligands, Dimers, and Complexes
set for Ir (N, C, H), including scalar-relativistic corrections as
implemented in the ADF program.44 At the optimized geome-
tries, we performed ground and excited state DFT/TDDFT
calculations at the B3LYP/LANL2DZ level of theory45,46 in
CH2Cl2 solution by means of the PCM solvation model,47 as
implemented in the Gaussian 03 program package.48 The
nonequilibrium49 implementation of the PCM algorithm was
used for TDDFT calculations. Both singlet-singlet and singlet-
triplet TDDFT excitations have been computed.
Zinc powder was activated as follows: the zinc powder is
stirred in 1 N HCl solution for 20 min. The powder is filtered
and washed successively with water, acetone, and Et2O. The
powder is then dried in an oven at 80 ꢀC for 2 h, cooled down,
and used as is.
L1, 1,4,5-Trimethyl-2-phenylimidazole. Benzaldehyde (5.3 g,
0.049 mol), 40% aq. methylamine (3.90 g, 0.049 mol), and 2,3-
butanedione monoxime (4.95 g, 0.049 mol) were added to glacial
acetic acid (60 mL). After being refluxed for 2 h, the orange
solution was cooled, and zinc (15 g, activated granules) was
added. The resulting mixture was refluxed for 1 h and then kept
overnight at room temperature. The suspension was filtered off
and washed with water. Treatment of the filtrate with ammo-
nium hydroxide resulted in the separation of a yellow oil, which
was extracted with toluene (3 ꢀ 100 mL). The combined organic
extracts were dried with anhydrous magnesium sulfate, filtered,
and then concentrated under a vacuum to give a brown-orange
viscous oil, which was purified by column chromatography
(SiO2, CH2Cl2/MeOH 5%) to give a viscous yellow orange oil
which crystallized on standing (1.8 g, y: 20%). 1H NMR
(CDCl3, 400 MHz): δ 7.59-7.54 (m, 2H); 7.45-7.29 (m, 3H);
3.54 (s, 3H); 2.22 (s, 3H); 2.19 (s, 3H). 13C NMR (CDCl3,
100 MHz): δ 146.02, 133.20, 131.37, 128.90, 128.64, 128.43,
124.36, 32.12, 12.84, 9.40. TOF HRMS ES: MHþ m/z: calc.
187.1235 found: 187.1238.
Sigma-Aldrich and used as received. Hydrated iridium trichlor-
ide was used as received from Heraeus. UV-visible spectra were
recorded in a 1 cm path length quartz cell on a Cary 5 spectro-
photometer. Emission spectra were recorded on a Fluorolog
3-22 using a 90ꢀ optical geometry. The emission spectra were
photometrically corrected using a NBS calibrated 200 W tung-
sten lamp as a reference source. Lifetime of excited state
measurements using a FL-1061PC TCSPC and 371 and 406 nm
Nanoled as the excitation source. Voltammetric measurements
employed a PC controlled AutoLab PSTAT10 electrochemical
workstation. Cyclic voltammograms (CV) were obtained at
a scan rate of 100 mV/s using 0.1 M TBAPF6 as supporting
electrolyte in dimethylformamide (DMF). Glassy carbon, sput-
tered platinum, and platinum wire were employed as working,
counter, and reference electrodes, respectively. At the end of
each measurement, the ferrocenium/ferrocene (Fcþ/Fc) poten-
tial was measured and used as an internal reference. NMR
spectra were measured with a Bruker AV-400 spectrometer,
and the reported chemical shifts were referenced to tetramethyl-
silane SiMe4. The data collections from X-ray crystallography
were performed at low temperature using Mo KR radiation. A
Bruker APEX II CCD, having kappa geometry, was employed for
1a and 2b, while 4a was measured on a marμX system. The data
of 1a and 2b were reduced by means of EvalCCD38 and then
corrected for absorption,39 whereas automar40 was used for the
treatment of the data belonging to compound 4a. The solutions
and refinements were performed by SHELX.41 The structures were
refined using full-matrix least-squares based on F2 with all non-
hydrogen atoms anisotropically defined. Hydrogen atoms were
placed in calculated positions by means of the “riding” model.
The geometries of the investigated complexes were opti-
mized under C2 symmetry constraints using the BP86 exchange-
correlation functional,42,43 together with a TZP (DZP) basis
L2, 4,5-Dimethyl-1,2-diphenylimidazole. Benzaldehyde (5.3 g,
0.049 mol), aniline (4.65 g, 0.05 mol), and 2,3-butanedione
monoxime (4.95 g, 0.049 mol) were added to glacial acetic acid
(60 mL). After being refluxed for 2 h, the orange solution was
cooled, and zinc (20 g, activated granules) was added. The
resulting mixture was refluxed for 3 h and then kept overnight at
room temperature. The suspension was filtered off and washed
with water. Treatment of the filtrate with ammonium hydroxide
resulted in the separation of a yellow oil, which was extracted
with toluene (3 ꢀ 100 mL). The combined organic extracts were
dried with anhydrous magnesium sulfate, filtered, and then con-
centrated under a vacuum to give a brown-orange viscous
oil, which was purified with column chromatography (SiO2,
EtOAc/Et2O 50%) to give a beige solid (4.9 g, y: 40%). 1H NMR
(CDCl3, 400 MHz): δ 7.46-7.40 (m, 3H); 7.33-7.28 (m, 2H);
7.21-13 (m, 5H); 2.19 (s, 3H); 2.01 (s, 3H). 13C NMR (CDCl3,
100 MHz): δ 145.28, 137.81, 133.33, 130.39, 129.77, 128.81,
128.49, 128.29, 128.15, 128.13, 125.64, 12.50, 9.74. TOF HRMS
ES: MHþ m/z: calc. 249.1392 found: 249.1400.
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H. P.; Cross, J. B.; 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.;
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454 Inorganic Chemistry, Vol. 50, No. 2, 2011
Baranoff et al.
L3, 2-(2,4-Dichlorophenyl)-1,4,5-trimethylimidazole. 2,4-Di-
chloro-benzaldehyde (2.15 g, 0.012 mol), 40% aq. methylamine
(0.96 g, 0.012 mol), and 2,3-butanedione monoxime (1.24 g,
0.012 mol) were added to glacial acetic acid (20 mL). After being
refluxed for 2.5 h, the orange solution was cooled, and zinc (12 g,
activated granules) was added. The resulting mixture was re-
fluxed for 3 h and then kept overnight at room temperature. The
suspension was filtered off and washed with water. Treatment of
the filtrate with ammonium hydroxide resulted in the separation
of a yellow oil, which was extracted with toluene (3ꢀ100 mL).
The combined organic extracts were dried with anhydrous
magnesium sulfate, filtered, and then concentrated under a
vacuum to give a brown-orange viscous oil, which was purified
with column chromatography (SiO2, CH2Cl2/MeOH 0-10%)
to give a beige solid (1.5 g, y: 48%). 1H NMR (CDCl3,
400 MHz): δ 7.68 (d, 1H, J = 8.4 Hz); 7.66 (d, 1H, J =
2.0 Hz); 7.51 (dd, 1H, J = 8.4 Hz, J = 2 Hz); 3.48 (s, 3H);
2.36 (s, 3H); 2.34 (s, 3H). 13C NMR (CDCl3, 100 MHz):
δ 152.45, 137.59, 136.27, 134.73, 130.29, 128.09, 126. 68,
123.03, 121.52, 31.73, 9.28, 7.52. TOF HRMS ES: MHþ m/z:
calc. 255.0456 found: 255.0452.
L6, 1-Methyl-2,5-diphenylimidazole. Benzaldehyde (4.6 g,
0.043 mol), 40% aq. methylamine (3.37 g, 0.043 mol), and
2-isonitrosoacetophenone (6.44 g, 0.043 mol) were added to
glacial acetic acid (60 mL). After being refluxed for 3 h, the dark
orange solution was cooled, and zinc (20 g, activated 10 μm dust)
was added. The resulting mixture was refluxed for 1.5 h and then
kept overnight at room temperature. The suspension was fil-
tered off and washed with water. After treatment of the filtrate
with ammonium hydroxide, the filtrate was extracted with
toluene (3 ꢀ 100 mL). The combined organic extracts were
washed with water and brine, dried with anhydrous magnesium
sulfate, filtered, and then adsorbed on silica gel. Chromatog-
raphy column purification was performed using silica and
CH2Cl2/MeOH 1% as the eluent to yield L6 as a white solid
(4.11 g, y: 40%). 1H NMR (CDCl3, 400 MHz): δ 7.69 (m, 2H);
7.51-7.32 (m, 8H); 7.19 (s, 1H); 3.67 (s, 3H). 13C NMR (CDCl3,
100 MHz): δ 149.64, 135.67, 131.18, 130.52, 129.05, 129.02,
128.93, 128.90, 128.80, 128.13, 127.80, 34.02. TOF HRMS ES:
MHþ m/z: calc. 235.1235 found: 235.1225.
General Method for Synthesis of Chloro-Bridged Dimers.
IrCl3 xH2O was dissolved in a mixture of 2-ethoxyethanol/H2O
3
3:1 (about 20 mL for 100 mg) by heating under argon around
60 ꢀC for 30 min. The ligand in 2-ethoxyethanol (2.5 eq., 5-
15 mL) was added and the solution was heated at 120 ꢀC
overnight. After cooling to RT, water was added and the
precipitate was filtered, washed with water and ether and dried,
and used without further purification.
L4, 1,4-Dimethyl-2,5-diphenylimidazole. Benzaldehyde (4.08 g,
0.038 mol), 40% aq. methylamine (3.00 g, 0.038 mol), and
1-phenyl-1,2-propanedione-2-oxime (6.27 g, 0.038 mol) were
added to glacial acetic acid (70 mL). After being refluxed for 5 h,
the yellow solution was cooled, and zinc (20 g, activated
granules) was added. The resulting mixture was refluxed for
2 h and then kept overnight at room temperature. The suspension
was filtered off and washed with water. After treatment of the
filtrate with ammonium hydroxide, the filtrate was extracted with
CH2Cl2 (3 ꢀ 100 mL). The combined organic extracts were
washed with water and brine, dried with anhydrous magnesium
sulfate, filtered, and then concentrated under a vacuum to give
a viscous oil, which was purified with column chromatography
(SiO2, EtOAc/Et2O 10/90 to 100/0) to give a viscous colorless oil
which crystallized upon addition of a few drops of toluene. After
drying, the compound is obtained as a white solid (2.6 g, y: 27%).
No isomer is detected in the crude.50 1H NMR (CDCl3,
400 MHz): δ 7.69-7.65 (m, 2H); 7.50-7.34 (m, 8H); 3.54
(s, 3H); 2.29 (s, 3H). 13C NMR (CDCl3, 100 MHz): δ 147.59,
135.36, 131.23, 130.92, 130.82, 130.24, 129.04, 128.86, 128.72,
127.91, 33.88, 13.64. TOF HRMS ES: MHþ m/z: calc. 249.1392
found: 249.1400.
1a: The dimeric iridium(III) complex 1 (94 mg; 0.078 mM)
was dissolved in 100 mL of dichloromethane solvent under
argon. To this solution was added acetylacetone ligand (30 mg,
0.30 mM) and tetrabutylammonium hydroxide (159 mg, 0.199 mM).
The reaction mixture was refluxed under argon overnight. After
being cooled to RT, the solvents evaporated to dryness. The
crude obtained was purified with column chromatography
(SiO2, CH2Cl2) followed by further purification on SiO2 TLC,
CH2Cl2/MeOH 3%. The major yellow band was collected to
afford 1a as a pale yellow solid (18 mg, 17%). 1H NMR (CDCl3,
400 MHz): δ 7.34 (dd, 2H, J = 1.2 Hz, J = 7.6 Hz); 6.69 (dt, 2H,
J = 1.2 Hz, J = 7.6 Hz); 6.56 (dt, 2H, J = 1.2 Hz, J = 7.6 Hz);
6.39 (dd, 2H, J = 1.2 Hz, J = 7.6 Hz); 5.19 (s, 1H); 3.90 (s, 6H);
2.24 (s, 6H); 2.14 (s, 6H); 1.67 (s, 6H). TOF MS ES: MHþ m/z:
calc. 662.2233 found: 662.2218 Anal. Calcd. for C29H33Ir-
N4O2 CH2Cl2: C, 48.25; H, 4.72; N, 7.50. Found: C, 48.18; H,
4.52; N, 7.19.
3
L5, 4-Methyl-1,2,5-triphenylimidazole. Benzaldehyde (4.07 g,
0.038 mol), aniline (3.57 g, 0.038 mol), and 1-phenyl-1,2-propa-
nedione-2-oxime (6.24 g, 0.038 mol) were added to glacial acetic
acid (60 mL). After being refluxed for 3 h, the dark orange
solution was cooled, and zinc (20 g, activated 10 μm dust) was
added. The resulting mixture was refluxed for 1.5 h and then
kept overnight at room temperature. The suspension was fil-
tered off and washed with water. After treatment of the filtrate
with ammonium hydroxide, the filtrate was extracted with
CH2Cl2 (3 ꢀ 100 mL). The combined organic extracts were
washed with water and brine, dried with anhydrous magnesium
sulfate, filtered, and then concentrated under a vacuum until
a white precipitate appeared. EtOH was added and the white
precipitate was filtered off. Filtrate was evaporated, dissolved in
a minimum of CH2Cl2, and Et2O was added. The slightly beige
precipitate was filtered off. After drying, the compound was
obtained as beige a solid (10.48 g, y: 88%). 1H NMR (CDCl3,
400 MHz): δ 7.30-7.14 (m, 11H); 7.08-7.01 (m, 4H); 2.23
(s, 3H). 13C NMR (CDCl3, 100 MHz): δ 144.44, 135.63, 131.40,
130.53, 130.37, 129.91, 129.64, 129.44, 128.94, 128.88, 128.77,
128.69, 128.19, 127.85, 120.08, 11.74. TOF HRMS ES: MHþ m/z:
calc. 311.1548 found: 311.1544.
1b: The dimeric iridium(III) complex 1 (102 mg; 0.085 mM)
was dissolved in 100 mL of dichloromethane solvent under
argon. To this solution was added methyl 4-(dimethylamino)-
pyridine-2-carboxylate ligand (35 mg, 0.194 mM) and tetrabu-
tylammonium hydroxide (270 mg, 0.337 mM). The reaction
mixture was refluxed under argon for 9 h. After being cooled to
RT, water was added and the mixture was extracted with
dichloromethane, and the organic layer was washed with water
and brine, dried over MgSO4 and the solvents were evaporated
to dryness. The crude obtained was purified with column chro-
matography (SiO2, CH2Cl2/MeOH 5%) followed by further
purification on SiO2 TLC, CH2Cl2/MeOH 7%. The major
yellow band was collected to afford 1b as a yellow solid
1
(23 mg, 19%). H NMR (CDCl3, 400 MHz): δ 7.50 (d, 1H,
J = 2.8 Hz); 7.43-7.37 (m, 2H); 7.24 (d, 1H, J = 3.2 Hz); 6.80
(dt, 1H, J = 1.6 Hz, J = 6.8 Hz); 6.74 (dt, 1H, J = 1.6 Hz, J =
8.0 Hz); 6.70-6.60 (m, 3H); 6.36-6.30 (m, 2H); 3.90 (s, 3H);
3.86 (s, 3H); 3.01 (s, 6H); 2.15 (s, 3H); 2.12 (s, 3H); 1.53 (s, 3H);
1.39 (s, 3H). TOF MS ES: MHþ m/z: calc. 728.2451 found:
728.2449 Anal. Calcd. for C32H35IrN6O2: C, 52.80; H, 4.85; N,
11.55. Found: C, 52.64; H, 4.52; N, 11.47.
2a: The dimeric iridium(III) complex 2 (117 mg; 0.081 mM)
was dissolved in 100 mL of dichloromethane solvent under argon.
To this solution was added acetylacetone ligand (30 mg, 0.30 mM)
and tetrabutylammonium hydroxide (220 mg, 0.275 mM).
(50) Lantos, I.; Zhang, W.-Y.; Shui, X.; Eggleston, D. S. J. Org. Chem.
1993, 58, 7092.

Article
Inorganic Chemistry, Vol. 50, No. 2, 2011 455
The reaction mixture was refluxed under argon overnight. After
being cooled to RT, the solvents were evaporated to dryness.
Ethanol (2 mL) was used to dissolve the crude product obtained
and water (50 mL) was added. The precipitate was filtrated
and washed with water and Et2O. The solid obtained was
dissolved in MeOH and purified on Sephadex LH-20 using
MeOH as a mobile phase to afford 2a as a brownish solid (32 mg,
38%). 1H NMR (CDCl3, 400 MHz): δ 7.62-7.52 (m, 6H);
7.47-7.44 (m, 2H); 7.38-7.35 (m, 2H); 6.55 (dt, 2H, J = 1.2 Hz,
J = 7.6 Hz); 6.50 (dd, 2H, J = 1.2 Hz, J = 7.6 Hz); 6.36 (dt, 2H,
J = 1.6 Hz, J = 7.2 Hz); 6.08 (dd, 2H, J = 1.2 Hz, J = 8.0 Hz);
5.30 (s, 1H); 2.22 (s, 6H); 2.02 (s, 6H); 1.79 (s, 6H). TOF MS ES:
MHþ m/z: calc. 786.2546 found: 786.2548 Anal. Calcd. for
C39H37IrN4O2: C, 59.60; H, 4.75; N, 7.13. Found: C, 59.48; H,
4.53; N, 6.89.
2b: The dimeric iridium(III) complex 2 (206 mg; 0.142 mM)
was dissolved in 200 mL of dichloromethane solvent under argon.
To this solution was added methyl 4-(dimethylamino)pyridine-
2-carboxylate ligand (92 mg, 0.51 mM) and tetrabutyl-ammo-
nium hydroxide (450 mg, 0.562 mM). The reaction mixture was
refluxed under argon overnight. After being cooled to RT, the
solvents were evaporated to dryness. MeOH (2 mL) was used to
dissolve the crude product obtained and water (50 mL) was
added. The precipitate was filtrated and washed with water and
Et2O. The solid obtained was dissolved in MeOH and purified
on Sephadex-LH20 using MeOH as a mobile phase to afford
2b as an orange solid (47 mg, 29%). 1H NMR (CDCl3, 400 MHz):
δ 7.65-7.47 (m, 8H); 7.38-7.31 (m, 4H); 6.73 (d, 1H,
J = 7.2 Hz); 6.60 (m, 2H); 6.48-6.38 (m, 4H); 6.17 (d, 1H,
J = 7.6 Hz); 6.10 (d, 1H, J=7.6 Hz); 3.06 (s, 6H); 2.20 (s, 3H);
1.93 (s, 3H); 1.89 (s, 3H); 1.46 (s, 3H). TOF MS ES: MHþ m/z:
(td, 1H, J = 1.2 Hz, J = 7.4 Hz); 6.63 (tt, 1H, J = 1.4 Hz, J =
7.4 Hz); 6.57 (t, 1H, J = 7.4 Hz); 6.45 (td, 1H, J = 1.2 Hz, J =
7.2 Hz); 6.34 (t, 1H, J = 7.2 Hz); 6.21 (dd, 1H, J = 1.4 Hz, J =
8.0 Hz); 6.09 (d, 1H, J = 8.0 Hz); 4.06 (m, 1H); 3.54 (m, 1H);
1.91 (s, 3H); 1.86 (s, 3H); 1.73 (s, 3H); 1.63 (s, 3H). TOF MS ES:
MHþ m/z: calc. 930.2675 found: 930.2648 Anal. Calcd. for
C48H42IrN4O2P CH3OH: C, 61.17; H, 4.82; N, 5.82. Found:
C, 61.02; H, 4.26; N, 5.67.
3
3a: The dimeric iridium(III) complex 3 (180 mg; 0.122 mM)
was dissolved in 200 mL of dichloromethane solvent under argon.
To this solution was added acetylacetone ligand (40 mg, 0.40 mM)
and tetrabutylammonium hydroxide (359 mg, 0.448 mM). The
reaction mixture was refluxed under argon overnight. After
being cooled to RT, the solvents were evaporated to dryness.
The crude obtained was purified with column chromatography
(SiO2, CH2Cl2) followed by further purification on SiO2 TLC,
CH2Cl2/petroleum ether 40%. The major yellow band was col-
1
lected to afford 3a as a yellow solid (22 mg, 22%). H NMR
(CDCl3, 400 MHz): δ 6.81 (d, 2H, J = 2.0 Hz); 5.93 (d, 2H, J =
2.0 Hz); 5.23 (s, 1H); 3.90 (s, 6H); 2.27 (s, 6H); 2.08 (s, 6H); 1.71
(s, 6H). TOF MS ES: MHþ m/z: calc. 798.0674 found: 798.0666
Anal. Calcd. for C29H29Cl4IrN4O2: C, 43.56; H, 3.66; N, 7.01.
Found: C, 43.44; H, 3.53; N, 6.81.
4a: The dimeric iridium(III) complex 4 (60 mg; 0.042 mM)
was dissolved in 80 mL of dichloromethane under argon. To
this solution was added acetylacetone (20 mg, 0.20 mM) and
tetrabutylammonium hydroxide (186 mg, 0.248 mM). The
reaction mixture was refluxed under argon overnight. After
being cooled to RT, the solvents were evaporated to dryness.
The crude obtained was purified with a short column chroma-
tography (SiO2, CH2Cl2). The major yellow band was collected
to afford 4a as a yellow solid (52 mg, 60%). 1H NMR (CDCl3,
400 MHz): δ 7.54-7.49 (m, 4H); 7.47-7.41 (m, 8H); 6.77
(dt, 2H, J = 7.2 Hz, J = 1.6 Hz); 6.65 (dt, 2H, J = 7.2 Hz,
J = 1.6 Hz); 6.52 (dd, 2H, J = 7.2 Hz, J = 1.6 Hz); 5.23 (s, 1H);
3.90 (s, 6H); 2.21 (s, 6H); 1.69 (s, 6H). TOF MS ES: MHþ m/z:
calc. 786.2546 found: 786.2534 Anal. Calcd. for C39H37Ir-
N4O2.CH2Cl2: C, 55.17; H, 4.51; N, 6.43. Found: C, 54.97;
H, 4.43; N, 6.38.
calc. 852.2764 found: 852.2748 Anal. Calcd. for C42H39IrN6O2
MeOH: C, 59.21; H, 4.61; N, 9.86. Found: C, 58.98; H, 4.56;
N, 9.77.
3
2c: The dimeric iridium(III) complex 2 (124 mg; 0.086 mM)
was dissolved in 100 mL of dichloromethane solvent under argon.
To this solution was added 2-picolinic acid ligand (41 mg, 0.33 mM)
and tetrabutylammonium hydroxide (522 mg, 0.652 mM). The
reaction mixture was refluxed under argon for 10 h. After being
cooled to RT, the solvents were evaporated to dryness. Ethanol
(2 mL) was used to dissolve the crude product obtained and
water (50 mL) was added. The precipitate was filtrated and
washed with water and Et2O. The solid obtained was dissolved
in MeOH and purified on Sephadex-LH-20 using MeOH as
a mobile phase to afford 2c as a brownish solid (49 mg, 50%).
1H NMR (CDCl3, 400 MHz): δ 8.28 (d, 1H, J = 7.6 Hz); 7.86
(dt, 1H, J = 1.6 Hz, J = 7.6 Hz); 7.80 (d, 1H, J = 7.6 Hz);
7.65-7.45 (m, 8H); 7.37-7.27 (m, 3H); 6.73 (d, 1H, J = 7.2 Hz);
6.63 (dq, 2H, J = 1.2 Hz, J = 7.6 Hz); 6.50 (t, 1H, J = 8.0 Hz);
6.41 (t, 2H, J = 7.6 Hz); 6.17 (d, 1H, J = 7.6 Hz); 6.13 (d, 1H,
J = 7.6 Hz); 2.19 (s, 3H); 1.94 (s, 3H); 1.88 (s, 3H); 1.33 (s, 3H).
TOF MS ES: MHþ m/z: calc. 809.2343 found: 809.2334 Anal.
5a: The dimeric iridium(III) complex 5 (75 mg; 0.038 mM)
was dissolved in 80 mL of dichloromethane under argon. To
this solution was added acetylacetone (20 mg, 0.20 mM) and
tetrabutylammonium hydroxide (162 mg, 0.21 mM). The
reaction mixture was refluxed under argon overnight. After
being cooled to RT, the solvents were evaporated to dryness.
The crude obtained was purified with a short column chroma-
tography (SiO2, CH2Cl2). The major yellow band was collected
1
to afford 5a as yellow solid (47 mg, 48%). H NMR (CDCl3,
400 MHz): δ 7.47-7.38 (m, 6H); 7.38-7.32 (m, 4H); 7.28-7.15
(m, 10H); 6.63 (m, 4H); 6.41 (m, 2H); 6.18 (d, 2H, J = 2.7 Hz);
5.29 (s, 1H); 2.33 (s, 6H); 1.80 (s, 6H). TOF MS ES: MHþ m/z:
calc. 910.2859 found: 910.2839 Anal. Calcd. for C49H41Ir-
N4O2: C, 64.67; H, 4.54; N, 6.16. Found: C, 64.57; H, 4.42;
N, 6.09.
Calcd. for C40H34IrN5O2 CH2Cl2: C, 55.09; H, 4.06; N, 7.83.
3
Found: C, 55.08; H, 4.12; N, 7.69.
2d: The dimeric iridium(III) complex 2 (76 mg; 0.052 mM)
was dissolved in 120 mL of dichloromethane solvent under
argon. To this solution was added 2-(diphenylphosphino)-
ethanoic acid ligand (45 mg, 0.184 mM) and tetrabutylammo-
nium hydroxide (300 mg, 0.375 mM). The reaction mixture was
refluxed under argon overnight. After being cooled to RT, the
solvents were evaporated to dryness. The crude obtained was
dissolved in a minimum of methanol and water was added. The
precipitate was filtrated and washed with water and Et2O. The
solid was purified with column chromatography (SiO2, CH2Cl2/
MeOH 0-5%) followed by further purification on SiO2 TLC,
CH2Cl2/MeOH 5%. The major band was collected to afford
2d as a colorless solid (16 mg, 25%). 1H NMR (CDCl3,
400 MHz): δ 7.69-7.46 (m, 7H); 7.40 (m, 1H); 7.32-7.22
(m, 4H); 7.19-7.07 (m, 8H); 6.93 (d, 1H, J = 7.2 Hz); 6.70
6a: The dimeric iridium(III) complex 6 (110 mg; 0.079 mM)
was dissolved in 150 mL of dichloromethane under argon. To
this solution was added acetylacetone (60 mg, 0.60 mM) and
tetrabutylammonium hydroxide (330 mg, 0.428 mM). The
reaction mixture was refluxed under argon overnight. After
being cooled to RT, the solvents were evaporated to dryness.
The crude obtained was purified with a short column chroma-
tography (SiO2, CH2Cl2). The major yellow band was collected
1
to afford 6a as yellow solid (79 mg, 65%). H NMR (CDCl3,
400 MHz): δ 7.55-7.42 (m, 12H); 7.09 (s, 2H); 6.78 (dt, 2H, J =
7.2 Hz, J = 1.6 Hz); 6.68 (dt, 2H, J = 7.2 Hz, J = 1.6 Hz); 6.54
(dd, 2H, J = 7.2 Hz, J = 1.6 Hz); 5.30 (s, 1H); 4.10 (s, 6H); 1.78
(s, 6H). TOF MS ES: MHþ m/z: calc. 758.2233 found: 758.2199
Anal. Calcd. for C37H33IrN4O2: C, 58.64; H, 4.39; N, 7.39.
Found: C, 58.45; H, 4.30; N, 7.28.

456 Inorganic Chemistry, Vol. 50, No. 2, 2011
Scheme 2. Synthetic Strategy for the Ligands
Baranoff et al.
Results and Discussion
The various ligands have been synthesized following a
one-pot condensation strategy to form the imidazole ring.51
Instead of the more commonly utilized combination of
diketone and ammonium acetate, the present syntheses are
based on the use of a dione monoxime as starting material
(Scheme 2).
Figure 1. Absorption spectra of acetylacetone-based complexes in
CH2Cl2 solution at room temperature.
This approach is particularly useful when a nonsymmetric
substitution pattern is needed with R1 different from R2
(Chart 1). Indeed, in this case no isomer is found in the crude
product, which makes the purification process much easier.
After condensation, the imidazole oxide intermediate is not
isolated but directly reduced using metallic zinc. Zinc has
been used as granules or 10 μm powder. In both cases, zinc
has been used directly as such or activated with hydrochloric
acid before use. Unsurprisingly, it has been found that the
most effective form of zinc to achieve the reduction is the
activated 10 μm powder. In this case, the yields of reaction are
much higher than with nonactivated granules. The prepara-
tion of the iridium complexes follows the usual two-step
strategy. First, 2.5 equiv of ligand are refluxed overnight with
IrCl3 xH2O in a mixture of 2-ethoxyethanol and water to
3
afford the chloro-bridged iridium dimer complex.52 This
dimer is then reacted with an excess of ancillary ligand and
tetrabutylammonium hydroxide as a base in dichloromethane
as a low boiling solvent to give the desired mononuclear
iridium complex. Purification was achieved either by column
chromatography on silica gel, Sephadex LH-20, recrystalli-
zation, or a combination of these techniques. It should be
noted that yields of synthesis for all complexes are low to very
low, particularly compared to yields usually obtained during
synthesis of complexes based on a phenyl-pyridine ligand.
This might be due to the increased basicity of imidazole
compared to pyridine making the purification over silica gel
column unsuitable. In general, better yields and higher purity
can be obtained when the crude reaction is directly filtered on
a very short plug of silica gel, eluting with dichloromethane
containing 0-1% of methanol if needed (see 4a, 5a, and 6a).
The main yellow band is collected and the solvent is removed
under reduced pressure. The crude product is dissolved in
the minimum of dichloromethane and poured into a large
amount of hexane to isolate the complex.
Figure 2. Absorption spectra of nonacetylacetone-based complexes in
CH2Cl2 solution at room temperature.
exposed to light and oxygen both in solution and in the solid
state as the color turns brownish within a couple of days.
All complexes display strong absorption bands in the UV
region with a molar extinction coefficient between 50000 and
1
60 000 L mol-1 cm-1. Those bands are assigned to π-π*
transitions on the imidazole ligand. 2d is an exception as no
clear band is observed but a broad shoulder is seen at wave-
length lower than 320 nm. Compared to acac, the ancillary
ligand (diphenylphosphino)ethanoic acid P is expected to
stabilize the highest occupied molecular orbitals (HOMO) of
the complex due to the d-orbital contribution of the
phosphorus atom, much more than the LUMO orbital.
This should ultimately increase the HOMO-LUMO energy
gap and a blue shift of the emission should be observed
(see below).53,54 It appears that complexes with additional
phenyl rings on the imidazole show similar absorption
intensity in the UV part of the spectra. This is attributed to
the presence of methyl groups in proximity to the phenyl
substituents. The steric hindrance forces the phenyls to be
orthogonal to the imidazole ring (see X-ray crystal structures
below), which effectively decreases the electronic coupling
between the two chromophores.55 The extinction coefficients
of the multichromophoric materials are then considered as
additive.56,57 The extinction coefficient of benzene is about
These syntheses led to two series of complexes. One con-
sists of acetylacetone (acac) as the ancillary ligand. The pur-
pose is to study the influence on the photo- and electroche-
mical properties of the complexes of the positioning of the
methyl and phenyl groups on the imidazole ring. The second
series consists of complexes with various ancillary ligands.
The absorption spectra of the complexes in dichloro-
methane solution are shown in Figure 1 for the acac-based
series and in Figure 2 for the nonacac-based series. It should
be noted that complexes are apparently poorly stable when
(53) Chiu, Y.-C.; Chi, Y.; Hung, J.-Y.; Cheng, Y.-M.; Yu, Y.-C.; Chung,
M.-W.; Lee, G.-H.; Chou, P.-T.; Chen, C.-C.; Wu, C.-C.; Hsieh, H.-Y. ACS
Appl. Mater. Inter 2009, 1, 433.
(54) 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.
(55) Beaven, G. H.; Johnson, E. A. Spectrochim. Acta 1959, 14, 67.
(56) Flamigni, F.; Baranoff, E.; Collin, J.-P.; Sauvage, J.-P. Chem.;Eur.
J. 2006, 12, 6592.
(51) Lousame, M.; Fernandez, A.; Lopez-Torres, M.; Vazquez-Garcia,
D.; Vila, J. M.; Suarez, A.; Ortigueira, J. M.; Fernandez, J. J. Eur. J. Inorg.
Chem. 2000, 2055.
(52) Nonoyama, M. Bull. Chem. Soc. Jpn. 1974, 47, 767.

Article
Inorganic Chemistry, Vol. 50, No. 2, 2011 457
Table 1. Photophysicala and Electrochemical Properties of Complexesb
c
c
λ
abs/nm
λem/nm
τ/ns
Φp E1/2red E1/2ox
1a
1b
2a
2b
2c
2d
3a
4a
5a
6a
280, 350, 386, 412 487, 522
145 0.047 -3.29i
0.12
0.20
0.19
0.26
0.35
0.28
0.52
0.19
0.25
0.19
0.17
278, 346, 404
282, 350, 416
280, 352, 406
278, 342, 400
330, 366
485, 520
510sh, 540 740 0.43
539
591
538
517 0.11
-2.87
-2.59i
-2.84
1635 0.22
92 0.038 -2.55
24 0.002 -2.95i
290, 364, 398, 418 570
286, 354, 396, 416 547
1057 0.25
1108 0.40
-2.72i
-2.48
-2.94
-2.53
300, 356, 396, 422 517sh, 551 1890 0.95
262, 356, 390, 422 557 662 0.32
N966d 260, 308, 348, 440 570
29 0.015 -3.30
a In degassed dichloromethane solution at room temperature. b Poten-
tials are given to ferrocene as internal reference. c Conditions: working
electrode glassy carbon, counter electrode sputtered Pt; reference elec-
trode Pt wire. DMF, NBu4PF6 0.1 M. i: irreversible, wave peak given.
d From ref 36; redox potential measured in DMSO.
Figure 4. Emission spectra of non-acetylacetone-based complexes in
CH2Cl2 solution at room temperature.
replaced with the less electron donating phenyl group, which
resulted in an additional red shift of the emission maximum
of 11 nm. Finally, in 6a where the last methyl group is
replaced by hydrogen, the emission is further red-shifted by
6 nm. In phenyl-pyridine based iridium complexes, it is known
that the addition of donor groups on the pyridine ring leads
to a blue shift of the emission mainly through destabilization
of the LUMO energy level of the complex.13-18 From our
results, the same trend is observed using an imidazole ring
instead of pyridine. Therefore, as a first approximation, the
trend in emission maxima can be rationalized by saying that
the imidazole ring has a significant LUMO character. There-
fore, a decrease of the number of donor substituents on the
imidazole ring stabilizes the LUMO more than the HOMO,
leading to a reduced HOMO-LUMO gap, that is, a red shift
of emission.
It is particularly interesting to compare 2a and 4a as they
are isomers relative to the positions of the methyl and phenyl
groups on the R2 and R3 substituents, with R2 = Me and
R3 = Ph for 2a and R2 = Ph and R3 = Me for 4a. This
exchange between the methyl and phenyl positions induces a
7 nm red shift of the emission. This shows that the donor
influence of the methyl group is stronger whenattachedto the
nitrogen atom. Therefore, it can be deduced that the nitrogen
atom is likely to be more directly involved in the LUMO
orbital than in the HOMO orbital.
Complex 3a is worth discussing more in detail as it exhibits
counterintuitive emission maximum. The complex 3a is the
same as 1a with two chloro-substituents in 2 and 4 positions
of the orthometalated phenyl ring. Chlorine acts as an
acceptor group based on its Hammett parameter,60 and
therefore a blue shift of the emission maximum could have
been expected by analogy with widely used fluoro substitu-
ents. Indeed, it is generally accepted that acceptor substitu-
ents on the cyclometalated phenyl ring tend to stabilize the
HOMO of the complex more than they stabilize the LUMO
of the complex. It is well established in the case of ppy-based
complexes. This isbecause usually the HOMOofthe complex
is mostly localized on the cyclometalated phenyl ring with a
large contribution of the central metal atom.61 Surprisingly,
the chloro substitution in 3a induces an emission maximum
red shift of more than 50 nm when compared to 1a.
Figure 3. Emission spectra of acetylacetone-based complexes in CH2Cl2
solution at room temperature.
200 L mol-1 cm-1 at 254 nm,58,59 which is negligible com-
pared to the ε value (∼60 000 L mol-1 cm-1) of 1a. In the
near-UV and visible range, weaker absorption bands with
molar extinction coefficients around 10 000 L mol-1 cm-1 are
assigned to metal-to-ligand charge transfer (MLCT) transi-
tions. Once again 2d stands out as an exception with the
lowest energy absorption band being at 366 nm, with almost
no tail extending in the visible part of the spectrum, which
makes the complex colorless.
When the complexes in dichloromethane solution are
excited at room temperature in the MLCT band (400 nm),
they emit from bluish-green to orange (Table 1). The emis-
sion spectra of acac-based complexes are shown in Figure 3
and the emission spectra for the complexes having different
ancillary ligands are shown in Figure 4. Using only methyl
and phenyl groups and by modifying simply the substitution
pattern on the imidazole ring, the emission maxima can be
tuned from 520 to 591 nm. In the case of 1a, which has the
imidazole ring fully substituted with methyl groups, the
emission maximum is observed at 522 nm with a higher
energy band of lower intensity at 488 nm. Replacing the
methyl group on the nitrogen by a phenyl (i.e., complex 2a)
shifts the emission maximum to 540 nm. In addition, the
shape of the spectrum becomes less structured, suggesting an
increased MLCT character. In 5a, a second methyl group is
As the emission spectra of 1a and 3a show very different
shapes while the onsets of emission in dichloromethane
may appear similar, a simple explanation could be emission
(57) Flamigni, F.; Ventura, B.; Baranoff, E.; Collin, J.-P.; Sauvage, J.-P.
Eur. J. Inorg. Chem. 2007, 5189.
(58) Du, H.; Fuh, R. A.; Li, J.; Corkan, A.; Lindsey, J. S. Photochem.
Photobiol. 1998, 68, 141.
(59) Inagaki, T. J. Chem. Phys. 1972, 57, 2526 and references therein.
(60) Hansch, C.; Leo, A.; Taft, R. W. Chem. Rev. 1991, 91, 165.
(61) Hay, P. J. J. Phys. Chem. A 2002, 106, 1634.

458 Inorganic Chemistry, Vol. 50, No. 2, 2011
Baranoff et al.
They are attributed to internal ligand modes other than
vibrations of carbon-carbon double bond.63 It should be
noted that at low temperature the separation between
the emission maxima of 1a and 3a is 1100 cm-1, while it is
3000 cm-1 at room temperature. This suggests that signifi-
cant stabilization occurs in the excited state of 3a at room
temperature leading to a low energy triplet excited state. This
is further supported by theoretical calculations performed on
1a and 3a (see below).
In the second series, the use of different ancillary ligands
allows further tuning of the band shape and emission max-
imum. In the case of 1b, the emission maximum is nonsigni-
ficantly blue-shifted compared to 1a. The main change is a
broadening of the emission spectrum by 640 cm-1. The use of
picolinic acid as ancillary ligand in 2c leads to unstructured
orange emission centered at 591 nm. As complexes 2a, 2b,
and 2d have very similar emission maxima, the result of 2c
can be explained by emission from the ancillary ligand Pic,
somehow related to an interligand energy transfer.64-66
Indeed, the energy level of the LUMO of Pic is much lower
than the dimethylamino substituted pic, NPic, and that of
acac and P. In the case of 2d, the similar emission maximum
to 2a came as a surprise. As expected, the HOMO is indeed
stabilized as shown by the oxidation potentials (see below).
Thismeans that the LUMOorbital is affectedby the ancillary
ligand pointing to a significant LUMO character on the
phenyl. Combined with the above-discussed substituent
effect on the imidazole ring, this suggests that the LUMO is
more delocalized on the entire ligand in contrast to the
phenyl-pyridine case.
Figure 5. Emission spectra of 3a in various solvents. DCM-hexane is a
1:4 mixture of dichloromethane and hexane, respectively.
Photoluminescence quantum yields Φp range from very
poor for 2d to very high for 5a. It can be noted that the
quantum yields tend to increase with the number of phenyl
substituents on the imidazole. As the phenyls are orthogonal
to the imidazole, effectively cutting off the electronic com-
munication between the two groups, they are not expected to
have a significant electronic impact on the quantum yields,
except perhaps for stabilizing the LUMO. This could increase
the energy gap with the deactivating MC state. However, in
this case, 6a could have shown a better quantum yield.
Therefore, the phenyl rings could simply isolate the complex
from the surrounding solvent, effectively decreasing the
nonradiative deactivation through solvent vibrations. The
low quantum yield of 2d is likely due to the flexibility of P.54
In 2c, the low quantum yield is attributed to emission from
Pic and not from the main ligand similar to what has been
observed with ppy.67 Radiative lifetimes τrad are in the
microsecond range as expected for cyclometalated iridium-
(III) complexes. Different lifetimes are observed for the two
positional isomers 2a and 4a and 2a shows the shorter
lifetime. This points toward differences inthe emitting excited
state. These differences obviously originate from the different
positions of the phenyl and methyl groups on the imidazole.
Oxidation and reduction potentials of the complexes have
been obtained by cyclic voltammetry in DMF. Oxidations
Figure 6. Emission spectra at 77 K of 1a in dichloromethane and 3a in
dichloromethane, acetonitrile, and tetrahydrofuran.
solvatochromism due to an increase of the MLCT character
in 3a. To verify this hypothesis, we measured the emission
of 3a in various solvents with different polarities (Figure 5).
A red shift of the emission maximum is observed in more polar
solvents as expected for an emission with significant MLCT
character. However, the emission shape does not change
much and the shift of emission maximum is limited to 20 nm
from DCM-hexane to acetonitrile, which is much smaller
than the 50 nm red-shift observed from 1a to 3a. Therefore,
a solvent effect is unlikely to be the cause for the observed red
shift in 3a compared to 1a. To gain further insight into the
emission properties of 1a and 3a, we recorded the emission
spectra at 77 K (Figure 6). The spectra of 3a show more
structure with a vibronic progression and peaks separated by
about 1450 cm-1. Compared to the emission maximum at
room temperature, the emission maximum at 77 K is sig-
nificantly blue-shifted to about 507 nm for all solvents. On
the other hand, the solvatochromic shift is reduced to only
5 nm.62 Again, this shows that the influence of the solvent is
very unlikely to be the cause of the red-shifted emission of 3a
compared to 1a. The spectrum of 1a in dichloromethane at
77 K is highly structured with bands at 479, 516, 560, 610, and
about 670 nm. All peaks in this main vibronic series are
separated as well by about 1450 cm-1. Few additional
shoulders of lower intensity are observed in the spectrum.
(63) Rausch, A. F.; Homeier, H. H. H.; Yersin, H. Top. Organomet.
Chem. 2010, 29, 193.
(64) You, Y.; Park, S. Y. J. Am. Chem. Soc. 2005, 127, 12439.
(65) You, Y.; Kim, K. S.; Ahn, T. K.; Kim, D.; Park, S. Y. J. Phys. Chem.
C 2007, 111, 4052.
(66) You, Y.; Seo, J.; Kim, S. H.; Kim, K. S.; Ahn, T. K.; Kim, D.; Park,
S. Y. Inorg. Chem. 2008, 47, 1476.
(67) Xu, M. L.; Zhou, R.; Wang, G. Y.; Xiao, Q.; Du, W. S.; Che, G. B.
Inorg. Chim. Acta 2008, 361, 2407.
(62) The solvent mixture DCM-hexane gave emission spectrum with lot
of defects, likely coming from low quality glass. Therefore, it is not
reproduced in the figure. It is essentially the same as with other solvents.

Article
Inorganic Chemistry, Vol. 50, No. 2, 2011 459
Figure 7. ORTEP drawings of 1a, 2b, and 4a.
are reversible and their values are influenced by the substit-
uents on the imidazole ring. Indeed, 1a, 2a, 4a, and 5a are
respectively oxidized at 0.12, 0.19, 0.19, and 0.25 V vs
ferrocene. In this case, the isomers 2a and 4a exhibit no
measurable difference in oxidation potential. Replacing the
methyl by phenyl groups increases the oxidation potential
(i.e., the HOMO of the complex is stabilized). As the sub-
stituents on the imidazole ring can significantly influence the
HOMO energy level, this suggests that the imidazole pos-
sesses significant HOMO character. The presence of chlorine
in 3a induces, as expected, a strong stabilization of the
HOMO level resulting in an oxidation potential of 0.52 V vs
ferrocene. The donor character of the dimethylamino group
can be observed when 2b, which has NPic as ancillary ligand,
and 2c, which has Pic as ancillary ligand, are compared.
Indeed, the oxidation potential of 2b is lower, indicating a
destabilization of the HOMO energy by the donor group on
the ancillary ligand.68-71 The reduction potentials are much
less conclusive for general trends as mostly irreversible waves
are observed. The disparity of the remaining reversible
reductions points to a variety of ground state LUMO
localization. Besides 2c where the LUMO of the complex can
be reasonably anticipated to be mainly on the picolinate
ancillary ligand, other complexes seem to be highly depen-
dent on the substitution pattern of the imidazole ring. This is
expected as the imidazole has a significant LUMO character.
On the other hand, as the LUMO localized on the main and
ancillary ligands are close in energy, precise assignment based
solely on redox potentials is not a simple task.
ligands. In addition, while the imidazole ring is mostly of
LUMO character and the HOMO is mostly located on the
phenyl and the iridium center (similar to phenyl-pyridine
based complexes), these molecular orbitals are more deloca-
lized over the entire main ligand compared to the case of
phenyl-pyridine where they are fairly well localized. This is
consistent with previous observations.36
The X-ray crystal structures of 1a, 2b, and 4a are shown in
Figure 7, and their crystallographic data are shown in Table 2.
Single crystals were grown by slow diffusion of hexane in a
dichloromethane solution of the complex. The geometry
around the iridium center is a slightly distorted octahedron
with the methyl group in the R position of the nitrogen coor-
dinated to the iridium likely inducing most of the distortion
as it is pointing into the ancillary ligand. As expected for these
complexes prepared at low temperature, the two imidazole
rings are coordinated in the trans configuration in contrast to
the high temperature case.72
To provide insight into the structural, electronic, and optical
propertiesof theinvestigated complexes, weperformed DFT/
TDDFT calculations on the selected compounds 1a and 3a,
for which counterintuitive spectroscopic data have been
obtained.
Geometry optimizations were performed on both the
ground singlet and lowest triplet excited states of 1a and 3a,
the latter corresponding to the lowest SCF triplet states.
The calculated optimized structural parameters for the
ground and excited states of the investigated complexes are
reported in Table 3 and compared to the X-ray geometry of
1a. The agreement between ground state calculated structural
data and available experimental parameters is good, and
minor differences in the metal coordination sphere are
computed between the two complexes. We notice, however,
that the steric hindrance exerted by the Cl-substituent in 3a,
leads to an out-of-plane displacement of the Cl atoms by ca.
10ꢀ; see the —ClCCC dihedral angle in Table 3 and Figure 8,
while in 1a the corresponding H atom essentially lies in the
ligand plane. Moving to the excited state geometries, both
Overall, one can conclude that, based on electrochemical
results, the HOMO and LUMO energy levels of imidazole-
based complexes lie at higher energy levels compared to
related complexes based on phenyl-pyridine cyclometalated
(68) De Angelis, F.; Fantacci, S.; Evans, N.; Klein, C.; Zakeeruddin,
€
S. M.; Moser, J.-E.; Kalyanasundaram, K.; Bolink, H. J.; Gratzel, M.;
Nazeeruddin, M. K. Inorg. Chem. 2007, 46, 5989.
(69) Bolink, H. J.; Coronado, E.; Santamaria, S. G.; Sessolo, M.; Evans, N.;
Klein, C.; Baranoff, E.; Kalyanasundaram, K.; Graetzel, M.; Nazeeruddin,
M. K. Chem. Commun. 2007, 3276.
ꢀ
(72) Baranoff, E.; Suarez, S.; Bugnon, P.; Barolo, C.; Buscaino, R.;
(70) Minaev, B.; Minaeva, V.; Agren, H. J. Phys. Chem. A 2009, 113, 726.
(71) Zhang, M.; Li, Z.-S.; Li, Y.; Liu, J.; Sun, J.-Z. Int. J. Quantum Chem.
2009, 109, 1167.
Scopelliti, R.; Zuppiroli, L.; Graetzel, M.; Nazeeruddin, M. K. Inorg. Chem.
2008, 47, 6575.

460 Inorganic Chemistry, Vol. 50, No. 2, 2011
Baranoff et al.
Table 2. Crystallographic Data for 1a, 2b, and 4a
1a 1.7CH2Cl2
3
2b 2CH2Cl2
3
4a
empirical formula
formula weight
temperature, K
C30.7H36.4Cl3.4IrN4O2
C44H43Cl4IrN6O2
C39H37IrN4O2
785.93
140(2)
0.71073
monoclinic
I2/a
806.17
100(2)
0.71073
triclinic
P1
1021.84
100(2)
0.71073
triclinic
P1
˚
wavelength (A)
crystal system
space group
unit cell dimensions
˚
a (A)
9.2362(17)
13.6956(15)
14.136(2)
69.463(10)
83.415(14)
77.185(9)
1631.5(4)
2
1.641
4.404
799
10.0882(16)
11.9006(6)
19.0655(17)
76.990(8)
84.823(9)
67.972(9)
2067.3(4)
2
1.642
3.534
1020
16.517(3)
10.650(2)
19.517(4)
90
103.68(3)
90
3335.6(11)
4
1.565
4.043
1568
˚
b (A)
˚
c (A)
R (deg)
β (deg)
γ (deg)
3
˚
volume (A )
Z
density, calcd (g/cm3)
absorption coefficient (mm-1
F(000)
)
crystal size (mm3)
0.49 ꢀ 0.39 ꢀ 0.31
3.03-27.50
27355
0.36 ꢀ 0.29 ꢀ 0.29
3.10-27.5
38788
0.40 ꢀ 0.32 ꢀ 0.20
2.54-27.58
10194
θ range for data collection (deg)
reflections collected
independent reflections
7401
9426
3730
[R(int) = 0.0584]
full-matrix least-squares on F2
7401/54/407
1.209
R1 = 0.0473
wR2 = 0.1242
R1 = 0.0565
wR2 = 0.1291
[R(int) = 0.0406]
full-matrix least-squares on F2
9426/0/514
1.135
R1 = 0.0246
wR2 = 0.0579
R1 = 0.0308
wR2 = 0.0618
[R(int) = 0.0662]
full-matrix least-squares on F2
3730/168/260
1.109
R1 = 0.0602
wR2 = 0.1406
R1 = 0.0633
wR2 = 0.1427
refinement method
data/restraints/parameters
goodness-of-fit on F2
final R indices [I > 2σ(I)]
R indices (all data)
Table 3. Comparison between Calculated Geometrical Structures and Average of
Available Experimental Data for 1a and 3a Complexesa
1a
3a
parameter
<exp>
S0
T1
S0
T1
Ir-N
2.057
2.007
2.156
1.402
172.1
88.3
79.8
98.4
1.61
1.65
2.049
2.003
2.172
1.442
2.044
1.998
2.159
1.418
2.039
1.995
2.162
1.458
2.037
1.979
2.170
1.420
Ir-C
Ir-O
C-C
—N-Ir-N
—O-Ir-O
—N-Ir-C
—N-Ir-O
—CNCC
—H(Cl)CCC
171.8
172.2
173.1
173.6
88.4
79.8
97.8
2.5
87.7
80.5
97.2
1.6
88.4
79.0
99.5
3.2
86.6
80.2
99.1
15.4
16.1
Figure 8. Optimized molecular structures of 3a in its ground and excited
state.
1.6
0.2
10.3
Figure 9. A common feature of the investigated cyclometa-
lated complexes is a HOMO made of the Ir dxy orbital
combined in an antibonding fashion with π orbitals of the
cyclometalated ligand. The LUMOs are π* combinations
localized on the cyclometalated and acac ligands. A similar
electronic structure is found for 1a and 3a, even though a
sizable stabilization of both occupied and unoccupied mo-
lecular orbitals is predicted for the latter, with an associated
slight reduction of the HOMO-LUMO gap; see Figure 9.
Moreover, while in1a the acac-based orbital is the LUMOþ1,
in 3a the same orbital is the LUMOþ2. Our calculated
electronic structure is fully consistent with electrochemical
data obtained in the context of the present investigation for
1a and 3a. This shows a strong positive shift of both the
oxidation and reduction potentials by 0.40 and 0.57 V,
respectively, going from 1a to 3a. This is paralleled by
a calculated positive HOMO and LUMO shift of 0.52 and
0.48 eV, respectively. It should be noted that the experimental
value of the positive shift of the reduction potential going
from 1a to 3a may not be quantitatively exact as both waves
are irreversible. Moving from S0 to T1 optimized geometries,
a —CNCC refers to the dihedral angle between the methyl-imidazole
and the phenyl moieties of the cyclometalated ligand in 1a and 3a.
—H(Cl)CCC refers to the dihedral angle between the H(Cl) of the
phenyl moiety and the imidazole carbon in the cyclometalated ligand
for 1a (3a).
complexes exhibit similar structural variations compared to
the ground state, except the dihedral angle characterizing the
relative arrangement of the imidazole and phenyl moieties of
the cyclometalated ligand (—CNCC in Table 3). While this
parameter is close to zero in the ground state of 1a and 3a, the
—CNCC dihedral angle substantially increases in the triplet
excited state of 3a (3.2 vs 15.4ꢀ, in S0 and T1, respectively),
while it remains essentially constant in 1a (2.5 vs 1.6ꢀ). A
similar behavior is observed also for the —ClCCC dihedral
angle. Thus, a considerable distortion from planarity is
calculated for the excited state of 3a, which leads to an
additional stabilization of the excited state for this complex;
see below.
A molecular orbital energy diagram calculated for the
singlet-optimized geometries for 1a and 3a is reported in

Article
Inorganic Chemistry, Vol. 50, No. 2, 2011 461
Figure 9. Molecular orbital energydiagram for 1a and 3a and isodensity
plots of selected orbitals for 1a.
Table 4. Calculated Lowest Excitation Energies (nm) at Both the Singlet and
Triplet Geometries for 1a and 3aa
1a
3a
S0
T1
S0
T1
S0 f T1
S0 f T2
S0 f T3
S0 f S1
S0 f S2
S0 f S3
446
440
438
393 (0.005)
387 (0.093)
379 (0.002)
485
476
456
413 (0.004)
412 (0.100)
400 (0.002)
460
457
426
397 (0.092)
380 (0.04)
367 (0.024)
528
519
429
430 (0.111)
409 (0.007)
386 (0.026)
Figure 10. Upper panel: charge density difference between the lowest
excited triplet TDDFT state and the ground state (contour value 0.001).
Lower panel: spin density of the unrestricted triplet state (contour
value 0.002).
factors related to the overlap between vibrational wave func-
tions for the ground and excited states, which we are not
including in our description.
a For singlet-singlet transitions oscillator strengths are also reported.
we observe a reduction of the HOMO-LUMO gap from
3.95 (3.89) to 3.74 (3.57) eV for 1a (3a). The larger HOMO
-LUMO gap reduction calculated for 3a (0.21 vs 0.32 eV) is
related to the excited state structural distortion discussed
above. It is also interesting to notice that for the analogous
N966 complex we calculated three almost degenerate LU-
MOs of mixed character, distributed among the acac and
cyclometalated ligands. In contrast, for 1a the three orbitals
are essentially fully localized on either ligand. The peculiar
LUMOs mixing in N966 was related to the broad and
unstructured emission exhibited by that complex, as opposed
to the narrower and the well-resolved emission found for 1a;
see Figure 3.
We now move to the descriptions of the excited states of 1a
and 3a, which are summarized in Table 4. In line with the
electronic structure picture discussed above, the lowest tran-
sition energies in 3a are red-shifted compared to those cal-
culated for 1a. This behavior is particularly evident if one
considers the excited states related to the emission process,
that is, the S0 f T1 transitions calculated at the T1 optimized
geometries; in this case, a red-shift from 485 to 528 nm is
calculated going from 1a to 3a, which is perfectly paralleled
by the HOMO-LUMO gap decrease due to the excited state
structural distortion.
For 1a a clear vibronic progression can be distinguished
with features at 487 and 522 nm, and an associated peak
separation of ca. 1380 cm-1, typical of the aromatic C-C
stretching of the cyclometalated ligands. Our calculated
transition at 485 nm nicely compares with the blue end of
the emission spectrum. For 3a a broad emission is found,
peaking at 570 nm. Converting into energy units and adding
1380 cm-1 to the emission maximum wavenumber, we can
estimate a transition at ca. 530 nm, which nicely compares to
our calculated data of 528 nm. As anticipated from experi-
mental data, it should be noted that phenyl-imidazole and
phenyl-pyridine based complexes differ rather fundamentally
in HOMO and LUMO localization. While in the case of
phenyl-pyridine, it is well accepted that the HOMO is mostly
localized on the coordinated phenyl and the LUMO mostly
localized on the pyridine ring, in the case of phenyl-imidazole
both HOMO and LUMO are mostly delocalized over the entire
cyclometalated ligand. This could explain why the LUMO
energy level of 3a is significantly affected by the distortion
induced by the chloride substituents.
It is finally interesting to provide a visual representation of
the charge flow associated to the emission process. The most
rigorous way to approach this issue is that of plotting the
charge density difference between the ground and excited
state, as derived by a ground state DFT and an excited state
TDDFT calculation, respectively. Alternatively, one might
look at the spin density distribution in the lowest triplet state,
as obtained by an unrestricted DFT calculation, as represen-
tative of the excited state electron distribution. Both types of
plots are reported in Figure 10.
A comparison between calculated transition energies and
the experimental emission spectra at room temperature is not
straightforward, since 1a and 3a have rather different emis-
sion profiles, with 1a (3a) showing a structured (unstructured)
emission. We recall indeed that for a given electronic transi-
tion, the emission profile is determined by Franck-Condon

462 Inorganic Chemistry, Vol. 50, No. 2, 2011
Baranoff et al.
As it can be noticed, both graphical representations pro-
vide a similar picture of the excited states for 1a and 3a,
essentially related to charge reorganization involving the
metal and the cyclometalated ligand, as might be expected
from the nature of the HOMO and LUMO.
be due to a significant geometrical and electronic relaxation
accompanying the formation of the emitting excited state in
this complex.
We are now investigating this family of complexes more
in detail in order to fully understand the parameters control-
ling the broadness of the emission spectrum. We hope then
to be able to improve the photophysical properties of
N966 for single emitting center WOLED with improved
performances.
Conclusion
Two series of heteroleptic complexes based on a phenyl-
imidazole as the main ligand have been synthesized and char-
acterized electrochemically and photophysically. We have
shown that emission wavelength can be tuned by applying
known strategies used with complexes based on phenyl
pyridine as main ligand. Differences are however observed as
the HOMO and LUMO orbitals of imidazole based com-
plexes are lying at higher energy and are less localized on the
cyclometalated ring and the imidazole ring, respectively, than
with phenyl-pyridine ligands. Interestingly, by using chloro
substituents as strong acceptor groups a red shift of emission
is observed instead of the expected blue-shift. This appears to
Acknowledgment. We acknowledge financial support
for this work by Solvay S.A., DCTRP New Business
Development. Z.X. also acknowledges financial support
from the National Natural Science Foundation of China
(No. 20501011).
Supporting Information Available: Crystallographic informa-
tion files. This material is available free of charge via the Internet
at http://pubs.acs.org.