Tuning the photophysical and electrochemical properties of iridium(III) 2-aryl-1-phenylbenzimidazole complexes

Article abstract of DOI:10.1016/j.ica.2014.02.024

A series of heteroleptic bis-cyclometalated iridium(III) complexes, [Ir(cpbi)₂(H₂dcbpy)][PF₆] (1), [Ir(pbi) ₂(H₂dcbpy)][PF₆] (2), and [Ir(mpbi) ₂(H₂dcbpy)][PF₆] (3), where pbi = 1,2-diphenylbenzimidazole, cpbi = 2-(4-chlorophenyl)-1-phenylbenzimidazole, mpbi = 2-(3,4-dimethoxyphenyl)-1-phenylbenzimidazole, and H₂dcbpy = 2,2′-bipyridine-4,4′-dicarboxylic acid has been synthesized and characterized by elemental analysis, ¹H, ³¹P NMR, and high resolution mass-spectra. Molecular structure of complex 3 has been determined from single-crystal X-ray analysis. The complexes exhibit absorption up to 550 nm with molar absorptivities of 2500 M^-1 cm^-1. They have strong luminescence in broad yellow-to-red region in solutions at room temperature. While chloro-substituent (complex 1) causes a little hypsochromic shift of the absorption maxima compared to unsubstituted 2, introduction of two methoxy-groups (complex 3) gives rise to a bathochromic shift of about 100 nm. Alternating current voltammetry studies of the complexes indicates reversible oxidation and reduction potentials. Calculated excited state oxidation potentials for 1-3 are negative enough to efficiently inject electrons into the conduction band of TiO₂ (E_F). 2014 Elsevier B.V. All rights reserved.

Full text of DOI:10.1016/j.ica.2014.02.024

Inorganica Chimica Acta 415 (2014) 22–30
Contents lists available at ScienceDirect
Inorganica Chimica Acta
journal homepage: www.elsevier.com/locate/ica
Tuning the photophysical and electrochemical properties of iridium(III)
2-aryl-1-phenylbenzimidazole complexes
⇑
Stanislav I. Bezzubov , Vladimir D. Doljenko, Sergey I. Troyanov, Yuri M. Kiselev
Department of Chemistry, Lomonosov Moscow State University, Moscow 119991, Russian Federation
a r t i c l e i n f o
a b s t r a c t
Article history:
A
series of heteroleptic bis-cyclometalated iridium(III) complexes, [Ir(cpbi)2(H2dcbpy)][PF6] (1),
Received 9 December 2013
Accepted 11 February 2014
Available online 27 February 2014
[Ir(pbi)2(H2dcbpy)][PF6] (2), and [Ir(mpbi)2(H2dcbpy)][PF6] (3), where pbi = 1,2-diphenylbenzimidazole,
cpbi = 2-(4-chlorophenyl)-1-phenylbenzimidazole, mpbi = 2-(3,4-dimethoxyphenyl)-1-phenylbenzimi-
dazole, and H2dcbpy = 2,20-bipyridine-4,40-dicarboxylic acid has been synthesized and characterized by
elemental analysis, 1H, 31P NMR, and high resolution mass-spectra. Molecular structure of complex 3
has been determined from single-crystal X-ray analysis. The complexes exhibit absorption up to
Keywords:
Iridium(III) complexes
Benzimidazole
550 nm with molar absorptivities of 2500 Mꢀ1 cmꢀ1
. They have strong luminescence in broad
yellow-to-red region in solutions at room temperature. While chloro-substituent (complex 1) causes a
little hypsochromic shift of the absorption maxima compared to unsubstituted 2, introduction of two
methoxy-groups (complex 3) gives rise to a bathochromic shift of about 100 nm. Alternating current
voltammetry studies of the complexes indicates reversible oxidation and reduction potentials. Calculated
excited state oxidation potentials for 1–3 are negative enough to efficiently inject electrons into the
conduction band of TiO2 (EF ꢁ ꢀ0.5 V). Thus, from the electrochemical point of view, all the complexes,
especially 3, are good candidates for operation in DSSC as dyes.
Cyclometalation
Electrochemical studies
DFT calculation
Absorption spectra
Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction
onto the semiconductor surface. Photochemical and thermal
stability of the dye as well as its ability to sustain repeated
Cyclometalated iridium(III) complexes are of great interest for
application in OLEDs [1–6], chemosensors [7–9], photocatalytic
hydrogen systems [10,11], and biomarkers [12,13] because of their
high thermal and chemical stability, long excited-state lifetimes,
and emission colour tunability by cyclometalated ligands varia-
tions. Recently, some of these complexes have been demonstrated
to be suitable for solar energy harvesting [14–16].
oxidation/reduction cycles are also required. The most widely used
sensitizers are ruthenium(II) polypyridine complexes. For example,
N3 ([Ru(H2dcbpy)2(NCS)2], where H2dcbpy = 2,20-bipyridine-4,40-
dicarboxylic acid; Fig. 1) absorbs a great part of solar spectrum
due to intense red-shifted metal-to-ligand charge transfer band
(MLCT) centered at ca. 530 nm with molar absorptivity of ca.
15000 Mꢀ1 cmꢀ1 [15] and gives rise to 10% efficiency (
g) at air
In dye-sensitized solar cells (DSSCs) [17] a photosensitizer (dye)
acts as light absorber and charge separator, along with a semicon-
ductor. The photosensitizer choice is strictly controlled by several
requirements [17c]: the absorption spectrum of the sensitizer
should cover visible light region as much as possible with high
extinction coefficients. The LUMO level of the dye should be higher
in energy than Fermi level of the semiconductor used (ꢀ0.5 V for
TiO2); for efficient dye regeneration, the HOMO level must be at
more positive potential than that of the redox pair (for Iꢀ3 /Iꢀ ꢁ 0.4 V
versus NHE). Anchoring groups (–COOH, –H2PO3, –SO3H, etc.) in
the sensitizer structure are essential for strong binding the dye
mass (AM) 1.5 irradiance [18]. Still, the main drawback of sensitiz-
ers like N3 is the presence of two labile isothiocyanate-groups,
which possess poor long-term stability [19]. Cyclometalated ruthe-
nium(II) complexes are considered to be more stable, and they are
intensively studied in recent years [17b], but the efficiencies of the
devices based on these dyes are less than 10%. In contrast, irid-
ium(III) complexes usually contain two or three cyclometalated
ligands and possess high thermal stability and light-harvesting
properties [16]. Surprisingly, there are just several reports on
application of iridium dyes in DSSC. 1-Phenylpyrazole [20],
2-phenylpyridine (ppy), 2-phenylquinoline (pq) (Fig. 1),
benzo[h]quinoline [21,22] as cyclometalated ligands and H2dcbpy,
H2dcbq (dcbq = 2,20-biquinoline-4,40-dicarboxylic acid), and
pyridine-2,5-dicarboxylic acid as auxiliary ligand have been used
to produce simple sensitizers with efficiencies less than 1%.
⇑
Corresponding author. Tel.: +7 (495) 9394549.
E-mail address: Stas.Bezzubov@gmail.com (S.I. Bezzubov).
http://dx.doi.org/10.1016/j.ica.2014.02.024
0020-1693/Ó 2014 Elsevier B.V. All rights reserved.

S.I. Bezzubov et al. / Inorganica Chimica Acta 415 (2014) 22–30
23
OH
O
Purification and other manipulations with complexes were per-
formed in air.
+
OH
1H, 13C, and 31P NMR spectra were acquired at 25 °C on a Bruker
Avance 400 instrument and chemical shifts are reported in ppm
referenced to residual solvent signals. High resolution mass spectra
(HRMS) were measured using electrospray ionization (ESI) and
time-of-flight (TOF) mass analyzer. The measurements were done
in a positive ion mode (interface capillary voltage ꢀ4500 V); mass
range from m/z 50 to m/z 3000 Da. Matrix-assisted laser desorption
ionization (MALDI) mass spectra were run on a Bruker AutoFlex II
apparatus. Electronic absorption spectra were measured on a OKB
Spectr SF-2000 spectrophotometer. Luminescence spectra were re-
corded using a Perkin Elmer LS-55 spectrometer. An Econix-Expert
Ltd Ecotest-VA polarograph was used for electrochemical measure-
ments with glassy carbon working electrode, platinum counter
electrode, and saturated Ag/AgCl reference electrode. Elemental
analyses were performed using an Elementar VarioMicroCube
CHN analyzer.
OH
O
O
O
O
O
N
N
N
HO
Ir
N
N
N
N
Ru
N
C
2
S
N
C
S
OH
OH
Irppy
N 3
+
+
OH
OH
O
O
N
N
N
Ir
Ir
N
N
N
N
O
O
2
2.2. Synthesis of ligands
2
OH
OH
Benzimidazole derivatives (abbreviated as bi) 1,2-diphenyl-
benzimidazole (pbi), 2-(3,4-dimethoxyphenyl)-1-phenylbenzimi-
dazole (mpbi), 2-(4-chlorophenyl)-1-phenylbenzimidazole (cpbi)
were synthesized by reaction of N-phenyl-o-phenylenediamine
with sodium bisulfite adducts of corresponding benzaldehydes fol-
lowing similar procedures [30].
Irppz
Irpq
Fig. 1. Ruthenium(II) and iridium(III) dyes.
Sophisticated derivatives of ppy and H2dcbpy have been applied
but efficiencies of the dyes of about 1% have been achieved [23].
The same authors have developed neutral sensitizers bearing
acetylacetone (acac) and carboxylate-functionalized ppy and pq
ligands, that has increased the efficiency to 2.2% [24]. Tridentate
iridium(III) complexes with 2,6-diphenylpyridine derivatives have
2.2.1. Synthesis of pbi
N-Phenyl-o-phenylenediamine (1.56 g, 8.5 mmol) was dis-
solved in ethanol (25 ml). To this solution benzaldehyde sodium
bisulfite adduct (2.23 g, 13.9 mmol) and ethanol (5 ml) were
added. The mixture was refluxed for 3 h and cooled. Distilled water
(20 ml) was added and the resulting precipitate was filtered and
washed sufficiently with water. After two recrystallizations from
aqueous ethanol the solid was dried in vacuo at 50 °C during
10 h. Yield: 1.62 g, 70%, colorless needles. 1H NMR (DMSO-d6,
ppm): d 7.79 (d, 1H, J = 7.4 Hz), 7.49–7.58 (m, 5H), 7.23–7.42 (m,
7H), 7.17 (d, 1H, J = 7.5 Hz). 13C NMR (DMSO-d6, ppm): 152.3,
143.0, 137.5, 136.9, 130.5, 130.3, 130.0, 129.6, 129.3, 128.8,
128.0, 123.8, 123.2, 119.9, 110.9. MS (MALDI-TOF): m/z = 271.2
(Calc. 271.3 for [M+H]+).
been reported to have high molar absorptivities (
e
ꢁ 9000 Mꢀ1
cmꢀ1 at 515 nm) but low DSSC efficiencies (2.2%) which have been
explained by poor cell construction (
of CF3-substituted benzothiazoles have been also used but again
the maximal of 1.4% have been achieved [14]. The main disad-
g = 3.3% for N3) [25]. A series
g
vantages of the above iridium sensitizers are narrow absorption
spectra and low molar absorptivities in visible light range [23]. Re-
cently, two panchromic iridium(III) complexes bearing bis(arylimi-
no)acenaphtene ligands and absorbing light up to 800 nm with
e
= 2500 at 630 nm have been reported [15]. Unfortunately, these
dyes have no anchoring groups and are not suitable for DSSC oper-
ation from the energetic point of view.
2.2.2. mpbi
Yield: 66%, colorless plates. 1H NMR (CDCl3, ppm): d 7.86 (d, 1H,
J = 8.0 Hz), 7.42–7.51 (m, 3H), 7.27–7.32 (m, 3H), 7.13–7.23 (m,
3H), 7.09 (dd, 1H, J1 = 8.3 Hz, J2 = 2.0 Hz), 6.73 (d, 1H, J = 8.3 Hz),
3.82 (s, 3H), 3.68 (s, 3H). 13C NMR (CDCl3, ppm): 151.7, 149.7,
148.1, 142.3, 136.9, 136.8, 129.5, 128.2, 127.1, 122.7, 122.6,
122.1, 121.8, 119.0, 111.9, 110.3, 109.9, 55.4, 55.3. MS (MALDI-
TOF): m/z = 331.2 (Calc. 331.4 for [M+H]+).
In contrast to other N^C aromatic donors used in iridium(III)
chemistry, 2-aryl-1-phenylbenzimidazole ligands can be easily
modified by conventional chemical reactions, that affects
immediately the electronic environment of iridium through highly
conjugated heteroaromatic system [26,27]. Thus, we present the
synthesis, photophysical, and electrochemical studies of a series
of iridium(III) complexes with 2-aryl-1-phenylbenzimidazoles,
where the aryl units are phenyl ring substituted with electron-
withdrawing (Cl) or electron-donating (OMe) groups at their
para- (and meta-) positions.
2.2.3. cpbi
Yield: 43%, light brown powder. 1H NMR (CDCl3, ppm): d 8.00
(d, 1H, J = 8.0 Hz), 7.54–7.61 (m, 5H), 7.43 (t, 1H, J = 7.3 Hz),
7.31–7.37 (m, 5H), 7.27 (m, 1H). 13C NMR (CDCl3, ppm): 150.7,
142.1, 136.7, 136.2, 135.4, 130.3, 129.6, 128.5, 128.3, 127.8,
127.0, 123.3, 122.9, 119.4, 110.1. MS (MALDI-TOF): m/z = 305.6
(Calc. 305.8 for [M+H]+).
2. Experimental
2.1. Materials and methods
All commercially available reagents were at least reagent grade
and used without further purification. Solvents were purified and
dried according to standard procedures. 2,20-Bipyridine-4,40-dicar-
boxylic acid (H2dcbpy) [28], sodium bisulfite adducts of aldehydes
[29] were prepared according to published procedures. Preparation
of all iridium complexes was carried out under dry argon.
2.3. Synthesis of iridium complexes
Cyclometalated
l-chloro-bridge iridium dimers [(bi)2Ir(l-Cl)2-
Ir(bi)2] were synthesized by Nonoyama’s method [31] and used
in the next step without purification because of their poor

24
S.I. Bezzubov et al. / Inorganica Chimica Acta 415 (2014) 22–30
solubility in most common organic solvents. Cationic complexes
[Ir(bi)2(H2dcbpy)][PF6] were prepared by similar procedures.
one of the methoxy groups is orientationaly disordered. Solvent
molecules are also disordered.
2.3.1. Preparation of l-chloro-bridge iridium dimer
2.5. Computational details
To a 25 ml round-bottomed flask containing IrCl3ꢂ3H2O (80 mg,
0.25 mmol) and mpbi (316 mg, 0.96 mmol) a mixture of 2-ethoxy-
ethanol and water (3:1 v/v, 10 ml) was added. The mixture was re-
fluxed for 20 h under argon atmosphere and cooled to room
temperature. Distilled water (3 ml) was added and the precipitate
formed was collected by filtration, washed several times with
water, ethanol, acetone, and dried in vacuo at 60 °C during 12 h.
Density functional theory (DFT) calculations were performed
with Firefly QC package [33], which is partially based on the
GAMESS (US) [34] source code, using the B3LYP functional [35],
Stuttgart-Dresden ECP for iridium and 6-31G⁄ basis sets for all
other atoms [36,37]. On the basis of the optimized geometries,
the natural bond orbital (NBO) was employed to analyze the
molecular orbital compositions (%). Time-dependent DFT (TDDFT)
calculations were carried out at the ground state geometries to ob-
tain vertical excitation energies and theoretical absorption spectra.
The lowest 30 singlet-singlet excitations were computed.
2.3.2. Synthesis of complex [Ir(mpbi)2(H2dcbpy)][PF6] (3)
The
crude
l
-chloro-bridge
iridium
dimer
(70 mg,
0.0395 mmol), H2dcbpy (19.3 mg, 0.079 mmol) in a mixture of
CH2Cl2 and methanol (2:1 v/v, 15 ml) were refluxed under Ar
atmosphere in dark for 7 h and cooled to room temperature. A
solution of NH4PF6 (124 mg, 0.76 mmol) in methanol (3 ml) was
added and the mixture was stirred for 30 min. The solid formed
was filtered off, and the solution was concentrated to dryness.
The dark red solid was extracted with CH2Cl2, the extract was fil-
tered, and evaporated to dryness. After flash chromatography
(SiO2, eluent CH2Cl2/MeOH 3/1), the product was recrystallized
from a mixture of chloroform/hexane (3/1) to give dark red pow-
der. Yield: 41.4 mg, 42%. 1H NMR (CDCl3, ppm): d 9.00 (s, 2H),
8.35 (d, 2H, J = 5.4 Hz), 8.06 (d, 2H, J = 5.2 Hz), 7.58–7.72 (m, 6H),
7.53 (d, 2H, J = 7.1 Hz), 7.29 (m, 2H), 7.10 (t, 2H, J = 7.6 Hz), 7.00
(d, 2H, J = 8.0 Hz), 6.89 (t, 2H, J = 7.7 Hz), 6.12 (s, 2H), 5.70 (s,
2H), 5.66 (d, 2H, J = 8.1 Hz), 3.85 (s, 6H), 3.92 (s, 6H). 31P NMR
(CDCl3, ppm) d ꢀ144.51 (septet, J = 712 Hz). HRMS (ESI): Calc. for
3. Results and discussion
1-Phenylbenzimidazole derivatives were prepared by reaction
of N-phenyl-o-phenylenediamine with appropriate benzaldehyde
sodium bisulfite adducts [30] (Fig. 2a). N-Phenyl substituted
ligands have been chosen because of their better solubility in most
organic solvents than that of 1-H-benzimidazoles [26]. Cationic
bis-cyclometalated iridium(III) complexes were synthesized in
two steps: treatment of IrCl3ꢂ3H2O with 3.8–4.0 eq. of benzimid-
azole ligands gave
l-chloro-bridge dimers [(bi)2Ir(l-Cl)2Ir(bi)2]
(bi – cyclometalated ligand); further reaction of the dimers with
bidentate 2,20-bipyridine-4,40-dicarbꢀoxylic acid (H2dcbpy) fol-
lowed by replacement of Clꢀ with PF6 produced the desired com-
plexes [Ir(bi)2(H2dcbpy)][PF6] (Fig. 2b). The complexes possessed
limited solubility in CHCl3 and MeOH, and an extended solubility
in DMSO. Low yield (40–55%) of the complexes might be due to
poor solubility of H2dcbpy in CH2Cl2 and MeOH. The use of more
soluble diester of H2dcbpy [38] would scarecely provide better to-
tal yield as additional hydrolysis and protonated complexes purifi-
cation steps would be required.
The dimethoxy complex appeared as pure 3 isomer rather than
30. In the aromatic region of 1H NMR spectrum of the sample two
singlets at 5.70 and 6.12 ppm (see Fig. 3) were assigned to protons
H1 and H2, while in the case of 30 two aromatic protons of dime-
thoxy benzene ring must produce a more complicated AB spectral
pattern. Three characteristic low-field signals (singlet and two
dublets with small spin–spin coupling constants) that remained
nearly unaffected by benzimidazole ligand change were assigned
to three protons of 2,20-bipyridine-4,40-dicarboxylic acid coordi-
nated to Ir3+ [39]. All spectral data were consistent with the struc-
tures of complexes 1–3 presented in Fig. 2b.
C
54H42N6O8Ir (M+): 1095.2692. Found: 1095.2701 (M+). Anal. Calc.
for C54H42N6O8F6PIr (MW 1240.13): C, 52.30; H, 3.41; N, 6.78.
Found: C, 52.47; H, 3.34; N, 6.79%.
2.3.3. [Ir(pbi)2(H2dcbpy)][PF6] (2)
Yield: 53%, orange red powder. 1H NMR (DMSO-d6, ppm): d 9.31
(s, 2H), 8.37 (d, 2H, J = 5.6 Hz), 8.21 (d, 2H, J = 5.4 Hz), 7.79 (m, 8H),
7.68 (m, 2H), 7.25 (t, 2H, J = 7.8 Hz), 7.02–7.12 (m, 4H), 6.85 (t, 2H,
J = 7.4 Hz), 6.75 (t, 2H, J = 7.5 Hz), 6.57 (d, 2H, J = 7.8 Hz), 6.39 (d,
2H, J = 6.3), 5.75 (d, 2H, J = 8.2 Hz). 31P NMR (DMSO-d6, ppm) d
ꢀ144.24 (septet, J = 709 Hz). MS (MALDI-TOF): m/z = 975.1 (Calc.
975.1 for [Ir(pbi)2(H2dcbpy)]+), m/z = 731.2 (Calc. 730.9 for
[Ir(pbi)2]+). Anal. Calc. for C50H34N6O4F6PIr (MW 1120.02): C,
53.62; H, 3.06; N, 7.50. Found: C, 53.55; H, 3.09; N, 7.62%.
2.3.4. [Ir(cpbi)2(H2dcbpy)][PF6] (1)
Yield: 39%, yellow orange powder. 1H NMR (DMSO-d6, ppm): d
8.96 (s, 2H), 8.19 (d, 2H, J = 5.1 Hz), 8.03 (d, 2H, J = 5.4 Hz), 7.79 (br
s, 8H), 7.59 (br s, 2H), 7.24 (t, 2H, J = 7.0 Hz), 7.10 (d, 2H, J = 8.2 Hz),
7.02 (br s, 2H), 6.90 (d, 2H, J = 8.4 Hz), 6.56 (d, 2H, J = 8.6 Hz), 6.25
(s, 2H), 5.72 (m, 2H). 31P NMR (DMSO-d6, ppm) d ꢀ144.21 (septet,
J = 711 Hz). MS (MALDI-TOF): m/z = 1043.7 (Calc. 1043.9 for
[Ir(cpbi)2(H2dcbpy)]+), m/z = 799.2 (Calc. 799.7 for [Ir(cpbi)2]+).
Anal. Calc. for C50H32N6O4Cl2F6PIr (MW 1188.91): C, 50.51; H,
2.71; N, 7.07. Found: C, 50.39; H, 2.54; N, 6.94%.
Upon recrystallization of 3 from DMSO/H2O by slow cooling the
solution from 100 °C to room temperature, X-ray suitable crystals
of [Ir(mpbi)2(Hdcbpy)]ꢂ3DMSOꢂH2O (3-solvate) were collected.
The molecular structure of the complex is shown in Fig. 4, selected
crystallographic data are presented in Tables 1 and 2.
SO3Na
OH
NH2
NH
N
N
i)
Ar
+
Ar
Ar =
bi
2.4. X-ray crystallography
Cl
O
CH
CH
3
3
OH
HO
Data collection for [Ir(mpbi)2(Hdcbpy)]ꢂ3DMSOꢂH2O (3 solvate)
O
CH3
H3C
O
O
was performed at 100 K on an IPDS diffractometer (Stoe) using
ii)
iii)
monochromatized Mo K
a radiation (k = 0.71073 Å). Numerical
H2dcbpy
N
N
N
N
absorption correction was applied with Tmax/Tmin = 0.8223/
0.7147. The structure was solved and anisotropically refined with
SHELX package [32]. All the hydrogen atoms were included into
refinement in geometrically calculated positions. In the structure,
Fig. 2a. Preparation of ligands: benzimidazoles (bi) and 2,20-bipyridine-4,40-dicar-
boxylic acid (H2dcbpy): (i) ethanol, reflux, 3-5 h; (ii) K2Cr2O7, conc. H2SO4, 75 °C,
1 h; (iii) 50% HNO3, reflux, 2 h.

S.I. Bezzubov et al. / Inorganica Chimica Acta 415 (2014) 22–30
25
+
Ph
Ph
N
N
OH
R
R
N
N
O
O
N
N
ii)
i)
Ir
Ir
IrCl3* 3H2O
iii)
Cl
N
N
R
R
N
N
OH
Ph
Cl
Ph
2
1 - 3
CH3
O
O
R
1
2
3
CH3
Fig. 2b. Two-step synthesis of iridium cationic complexes: (i) 2-ethoxyethanol/H2O
(3/1, v/v), argon, reflux, 24 h; (ii) H2dcbpy, CH2Cl2/MeOH (2/1, v/v), argon, darkness,
reflux, 5–24 h; (iii) NH4PF6, 30 min.
In complex 3-solvate, iridium ion resides in a distorted octahe-
dral environment of two C and two N atoms from mpbi, and two N
atoms from Hdcbpy. N atoms from C^N ligands adopt trans-config-
uration, which is common for many bis-cyclometalated iridium
complexes [38,40]. The average Ir–C and Ir–N (Hdcbpy) distances
(2.025 and 2.126 Å, respectively) are in good agreement with those
of [Ir(ppy)2(Hdcbpy)] (2.019 and 2.125 Å, respectively, ppy =
2-phenylpyridine) [39]. The distances between Ir and N of Hdcbpy
(2.118 and 2.134 Å) are larger than those between Ir and N in benz-
imidazole ligands (2.043 and 2.047 Å) because of trans-effect of the
C donor in mpbi [39]. Benzimidazolyl and dimethoxyphenyl rings
in cyclometalated ligands are more coplanar than two pyridyl rings
of Hdcbpy (average dihedral angles are 3.6 and 0.8° in two mpbi
and 8.6° in dcbpy). These values are in good agreement with those
of [Ir(tpy)2(Hdcbpy)] (tpy = 2-(p-tolyl)pyridine): 3.8 and 0.5° in
two tpy and 11.5° in Hdcbpy [38]. The plane of N-phenyl ring of
benzimidazole ligand is inclined to the plane of benzimidazolyl
at the angle of 80.1°.
Fig. 4. Molecular structure of [Ir(mpbi)2(Hdcbpy)] (hydrogen atoms and solvate
molecules are omitted for clarity).
counterion was found in the structure of 3-solvate, one of the
carboxylic groups of 3 appeared to be deprotonated during crystal-
lization to produce neutral molecule. Indeed, the distances be-
tween C and two O in deprotonated carboxylic group of 3-solvate
are very close to each other (1.245 and 1.246 Å), while in proton-
ated –COOH group, C–O (OH) distance (1.311 Å) is notably larger
than C@O distance (1.213 Å).
Absorption spectra of ligands and complexes 1–3 are presented
in Figs. 5 and 6. All benzimidazoles possess strong absorption band
in UV region due to
molar absorptivities of 20000 Mꢀ1 cmꢀ1. Complexes have greater
extinction coefficients in range
= 45000–60000 Mꢀ1 cmꢀ1
250–300 nm due to
p⁄ transitions located on benzimidazoles
p–
p⁄ transition centered at ca. 300 nm with
(e
)
Deprotonation of H2dcbpy in related iridium(III) compounds
leading to neutral complexes has been reported [39]. As no
p
–
Fig. 3. Aromatic region of 1H NMR spectrum of 3 (CDCl3).

26
S.I. Bezzubov et al. / Inorganica Chimica Acta 415 (2014) 22–30
Table 1
Selected
crystallographic
data
for
[Ir(mpbi)2(Hdcbpy)]⁄3DMSO⁄H2O.
Formula
C60H61IrN6O12S3
1346.57
Mr (g molꢀ1
T (K)
)
100
ꢀ
Space group
P1
a (Å)
b (Å)
c (Å)
12.4800(3)
13.7434(4)
18.5908(5)
99.292(2)
97.331(2)
111.455(2)
2867.63
1.560
a
(°)
b (°)
(°)
c
V (Å)
Dc (g cmꢀ3
)
Z
2
lMo (mmꢀ1
)
2.507
‘T’min,
0.715, 0.822
58.36
14188
max
Fig. 5. UV–Vis absorption (left) and emission (right) spectra of ligands in CH2Cl2
solution at room temperature.
2hmax
Nref
R1
0.0410
wR2
S
0.0922
1.037
In DSSC, the absorption spectrum of the dye should cover visible
light region as much as possible with high extinction coefficients.
While absorption maxima of 1–3 in visible light region can be eas-
ily tuned by 2-aryl substituents change, the molar absorptivity is
just slightly varied in range 3100–2600 Mꢀ1 cmꢀ1. These values
are high among other iridium(III) complexes in this spectral range,
but are smaller than those of simple ruthenium(II) dyes (for N3,
Table 2
Selected bond lengths (Å) and angles (°) in the coordination environment of Ir atom in
[Ir(mpbi)2(Hdcbpy)].
Ir1–C9
Ir1–C30
Ir1–N2
Ir1–N4
Ir1–N5
Ir1–N6
2.024(4)
2.025(4)
2.043(3)
2.047(3)
2.134(4)
2.118(3)
C9–Ir–C30
N5–Ir1–N6
C30–Ir1–N2
C9–Ir1–N4
N2–Ir1–N4
89.17(16)
76.29(13)
79.40(15)
79.40(16)
169.56(13)
e
= 15000 Mꢀ1 cmꢀ1 at 530 nm [14]). Thus, further modification
of the cyclometalated ligands is needed to expand their conjuga-
tive system.
Luminescent spectra were measured in CH2Cl2 solutions at
room temperature. Emission maxima of the ligands are centered
at ꢃ365 nm and are slightly influenced by substituents (Fig. 5),
and H2dcbpy. The broad bands at 350–425 nm
27000 Mꢀ1 cmꢀ1) in spectra of 1–3 could be assigned to spin-
allowed metal-to-ligand charge transfer (1MLCT) [d (Ir) – p⁄
(H2dcbpy)] and, to a greater extent, to ligand-to-ligand charge
transfer (1LLCT) [ (bi) – p⁄ (H2dcbpy)], which are common for
(e = 15000–
Table 3
p
Photophysical properties of ligands, 1–3, and reference complexes in solutions.
a
Substance
k
abs, nm (
e
ꢂ10ꢀ3, Mꢀ1 cmꢀ1
)
kex (nm)a
kem (nm)a
p
iridium(III) compounds [26,27,38–40]. Low energy bands of com-
plexes are likely to have mixed spin-allowed and spin-forbidden
1MLCT–3MLCT and 1LLCT–3LLCT nature due to strong spin-orbital
coupling of iridium(III) [26,27] and are very sensitive to electron-
donor (withdrawing) properties of substituents in 2-phenyl ring
of benzimidazoles: changing chlorine to two methoxy-groups
gives rise to nearly 100 nm bathochromic shift of the bands
(Fig. 6). Additionally, low energy absorption bands of 1–3 are sig-
nificantly red shifted compared to related iridium(III) complexes
Irppy, Irpq [21], and Irppz [20] (see Table 3). This can be explained
cpbi
pbi
mpbi
1
2
3
297(20)
293(18)
304(22)
250
296
313
421
582
506
627
368
363
367
556
680
648
760
580b
553b
604c
307(47), 365(15), 462(3.1)
306(44), 369(18), 465(3.5), 490(3.1)
306(58), 373(27), 530(2.6)
Irppy
Irpq
Irppz
291(24), 379(6.3)b
321(14), 346(11), 436(1.9)b
455(0.7)c
a
b
c
Recorded in air-equilibrated CH2Cl2 at room temperature.
EtOH [21].
[20].
by stronger
p-donor properties of benzimidazolyl than those of
pyridyl, quinolynyl, and pyrazolyl [41].
Fig. 6. UV–Vis absorption spectra of 1–3 in CH2Cl2 solution at room temperature.

S.I. Bezzubov et al. / Inorganica Chimica Acta 415 (2014) 22–30
27
Fig. 7. Normalized emission spectra of complexes 1–3 in CH2Cl2 solutions at room temperature.
while complexes 1–3 exhibit luminescence in broad yellow-to-red
region (Fig. 7). Changing the aryl substituents in the complexes
causes blue- and red-shifts of emission in the same way as in
absorption spectra of 1–3. Still, the emission maximum for 3 de-
pends on the excitation wavelength (Fig. 1⁄). The ratio of the inten-
sities for resulting two emission bands at 648 and 760 nm is as 2:1.
Benzimidazole-based iridium(III) complexes are known to pos-
sess intensive phosphorescence [26,27]. The relative contribution
of 3MLCT versus ligand-based 3
cent emission of 1–3 may affect the Stokes shifts between 3MLCT
and emission bands of complexes: ligand-based 3p p⁄ transitions
p–
p⁄ transitions to the phosphores-
–
gives rise to a larger Stokes shift [42]. Since Stokes shifts for 1
(94 nm) and 3 (118 nm, first emission band) are significantly smal-
ler than those for 2 (190 nm) and 3 (230 nm, second emission
band), phosphorescence of 1 appears to have predominantly
3MLCT character, while phosphorescence of 2 is likely to result
from 3LC transitions. Emission of 3 seems to have contribution
from both 3MLCT and 3LC transitions, with emission maxima being
controlled by the excitation wavelength. The values of E0–0 (energy
difference between ground and excited states, both taken at their
zero vibrational levels [43]) were estimated from the onset of
emission spectra at ca. 10% intensity (Table 4). They are decreased
by increasing donor properties of the cyclometalated ligands.
Electrochemical behavior of 1–3 was studied by alternating cur-
rent voltammetry (ACV) in acetonitrile using ferrocene as an exter-
nal standard at the beginning of the experiment (Fig. 8). The ACV
Fig. 8. AC voltammogramms of 1–3 (V vs. NHE) recorded at room temperature at
scan rate of 50 mV/s, frequency 25 Hz, and amplitude 30 mV in Ar-saturated
acetonitrile with 0.1 M (nBu4N)ClO4 as supporting electrolyte.
method was applied to achieve higher sensitivity and to measure
more accurate values of redox potentials. All the complexes have
reversible ground state oxidation (Eox) and reduction (Ered) poten-
tials (Table 4). Complexes 1–3 are strong oxidants with Eox > 1.1 V
versus NHE, but they are sufficiently stable (due to iridium(III)
inertness and high Ir–C bonds strength) to sustain repeated oxida-
tion/reduction cycles, that has been shown by their reversible elec-
trochemical behavior. In addition, Eox values of 1–3 are positive
enough for the oxidized complexes to be spontaneously reduced
not only by an Iꢀ3 /Iꢀ but also by a Brꢀ3 /Brꢀ redox system (E ꢁ
1.1 V versus NHE). The latter redox pair yields a significant increase
of open-circuit voltage (Voc), that has enhanced DSSC efficiency
[44]. So, complexes 1–3 combined with Brꢀ3 /Brꢀ pair may produce
Table 4
Electrochemical properties of 1–3 and reference complexes in solutions.
very stable DSSC with high Voc
.
Eox values of 1–3 are strongly dependent on the cyclometalated
ligand used. Introduction of methoxy-substituents (complex 3)
destabilizes the HOMO level and reduces Eox compared to the
complex of unsubstituted 1,2-diphenylbenzimidazole (2), while
chloro-substituent (complex 1) stabilizes HOMO level and
increases oxidation potential. Changing the substituents causes a
little variation of Ered values. It is more negative for 3 than for 1
Complex
E1/2, Va vs. NHE (
Ered, V
D
Ep, mV)
D
E, V
E0–0, eV
E(S+/S⁄), Vb
Eox, V
1
2
3
ꢀ0.86(46)
ꢀ0.88(61)
ꢀ0.94(44)
1.68(58)
1.55(50)
1.19(55)
1.59c
2.54
2.43
2.13
2.47
ꢀ0.79
ꢀ0.54
ꢀ0.58
ꢀ0.87
2.09
1.77d
2.46e
Irppz
N3
ꢀ0.89f
1.13f
2.02
and 2 because of strong
p-donor properties of mpbi. The great
Recorded in Ar-saturated acetonitrile with 0.1 M (nBu4N)ClO4 at scan rate of
50 mV/s, frequency 25 Hz, and amplitude 30 mV. Ferrocene was used as an external
standard prior to measurements. The values are measured against NHE. E = Eox-
a
Eox (ꢁ500 mV) and little Ered (ꢁ80 mV) variation is likely to indi-
cate that HOMOs of the complexes are largely localized on the
cyclometalated ligands, while LUMOs are mainly localized on the
strongly electron-withdrawing auxiliary ligand (H2dcbpy).
The Eox and Ered values of 3 are very close to that of N3 [45] with
D
ꢀEred. Estimated error: 20 mV for Eox and Ered
.
Calculated from E(S+/S⁄) = EoxꢀE0–0, where E0–0 is estimated from the onset of
b
the emission spectrum at ca. 10% intensity.
Determined from CV measurements in acetonitrile with 0.1 M (nBu4N)PF6.
c
d
the electrochemical gap (2.13 V) being slightly larger than
DE for
Second emission band for 3 (maximum at 760 nm) was used.
Estimated from the intersection of absorption and emission spectra [20].
MeOH [40].
e
N3 (2.02 V). Additionally, calculated excited state oxidation poten-
f
tials for 1–3, E(S+/S⁄) are sufficiently negative to efficiently inject

28
S.I. Bezzubov et al. / Inorganica Chimica Acta 415 (2014) 22–30
Fig. 9. Energy diagram and plot of the frontier orbitals for 1–3 (hydrogen atoms are omited for clarity).
electrons into the conduction band of TiO2 (EF ꢁ ꢀ0.5 V). Thus,
from the electrochemical point of view, all the complexes, espe-
cially 3, are good candidates for operation in DSSC as dyes.
To further understand electronic and photophysical properties
of 1–3 DFT studies were performed. Ground state geometries of
1–3 were optimized in the gas phase at B3LYP level with the Stutt-
gart-Dresden effective-core potential (ECP) for iridium and 6-31G⁄
basis sets for all other atoms. For each complex, geometries and
SCF-energies of three possible isomers were computed, and the
isomers with N atoms of C^N ligands adopting trans-configuration
were found to be more stable by approximately 45 kJ/mol (Fig. 2⁄).
This fact correlates well with results of X-ray crystal structure
analysis of 3. For time-dependent DFT (TDDFT) calculations ground
state geometries of these isomers were used. Plot of the frontier
molecular orbitals is shown in Fig. 9. Molecular orbitals composi-
tions for 1–3 are presented in Table 5. HOMOs of complexes are
Table 5
Frontier molecular orbitals compositions (%) for 1–3.
Complex
1
Location
HOMO ꢀ 2
51.0
4.6
44.4
50.0
3.9
46.1
47.2
3.6
HOMO ꢀ 1
22.2
5.0
72.8
13.0
2.6
84.4
5.9
HOMO
LUMO
LUMO+1
LUMO+2
Ir
49.2
3.0
47.8
42.9
2.4
54.7
20.8
1.4
10.0
83.9
6.1
9.0
84.2
6.8
10.2
83.4
6.4
4.1
95.0
0.9
3.7
95.2
1.1
4.2
94.8
1.0
3.6
93.4
3.0
3.7
92.5
3.8
3.5
93.6
2.9
H2dcbpy
cpbi
Ir
H2dcbpy
pbi
Ir
2
3
H2dcbpy
mpbi
1.9
92.2
49.2
77.8

S.I. Bezzubov et al. / Inorganica Chimica Acta 415 (2014) 22–30
29
mainly localized on Ir atom and benzimidazole ligands. Contribu-
tion of iridium orbitals to HOMOs is significantly decreased from
1 to 3, that seems to increase immediately HOMOs energy levels.
Iridium orbitals contribution to HOMOs ꢀ 1 is decreased from 1
to 3 producing similar HOMOs ꢀ 1 energies change. From HOMO-
s ꢀ 2 to HOMOs ꢀ 4, electron density is nearly equally localized
both on Ir center and C^N ligands with very small decreasing irid-
ium orbitals contribution from 1 to 3, that results in close orbital
energies for all complexes (purple frames on Fig. 9). Additionally,
N-phenyl rings of benzimidazoles have very small contribution to
occupied orbitals, that reflects their negligible participation in
the cojugated system of ligands.
LUMOs of 1–3 are largely localized on H2dcbpy with small con-
tributions from iridium and C^N ligands orbitals, that well corre-
lates with our conclusions from electrochemical data. LUMOs + 1
and LUMO + 2 are nearly entirely located on H2dcbpy, and orbital
energies have little variations from 1 to 3. Our results are in good
agreement with the ability of independent tuning HOMO and
LUMO energy levels known for many iridium(III) complexes
[4–6]. In addition, electron density of lower unoccupied orbitals
is significantly resides on carboxy-groups, that is extremely needed
for efficient electron injection in DSSC [17c]. HOMO–LUMO gaps
correlate well with E0–0 values obtained from spectral data and also
reproduce experimental absorption maxima change: bathochromic
shift of low-energy bands from 1 to 3. Still, HOMO–LUMO gaps give
overestimated, especially for 3, values of transition energies; these
transitions were not observed in experimental spectra. To gain in-
sight into the optical properties of the complexes, TDDFT approach
was applied.
The most intensive transitions and TDDFT electronic spectra
(overlaid with experimental spectra) for 1–3 are summarized in
Table 6 and Fig. 10. Calculated absorption spectra are in good
agreement with experimental data. Two transitions (at 496 nm
for 1 and at 511 nm for 2) are slightly red-shifted compared to
experimental bands that may be due to neglecting the solvatation
effect. In UV range intensive electronic transitions originate mainly
in Ir-bi ? bi and bi ? bi excitations (MLCT/ILCT), that correlates
well with our assignment based on the literature data. Surpris-
ingly, in low-energy region (k > 420 nm) transitions to S1 state
(HOMO ? LUMO excitations) have very small oscillator strengths
(f < 0.001) for all the complexes. Still, there are two similar inten-
sive transitions, which imply charge transfers from Ir-bi moiety
to H2dcbpy unit (MLCT/LLCT). Differences between complexes 1,
2, and 3 appear when the origin of these transitions is analyzed.
For 1 and 2, they result mainly from HOMO ꢀ 1 ? LUMO (496
Fig. 10. Experimental (red) and calculated TDDFT (blue) electronic spectra of 1–3.
(For interpretation of the references to colour in this figure legend, the reader is
referred to the web version of this article.)
and 511 nm) and HOMO ꢀ 3 ? LUMO (432 and 456 nm) excita-
tions, while for 3 – from HOMO ꢀ 3 ? LUMO (528 nm) and
HOMO ꢀ 3 ? LUMO + 1 (431 nm) excitations. That is, absorption
of all the complexes in visible range is mainly described by
electronic transitions from occupied orbitals with close energies
to the lowest unoccupied orbitals. So, bathochromic shift of the
lowest-energy band due to strong electron-donor properties of
Table 6
TDDFT singlet excited states for 1–3.a
b
Compound
State
k (nm)
f
Dominant monoexcitation
Nature
1
S1
S2
S6
S7
634
496
432
414
0.001
0.03
0.03
0.02
H ? L (0.96)
Ir-cpbi ? H2dcbpy
Ir-cpbi ? H2dcbpy
Ir-cpbi ? H2dcbpy
Ir-cpbi ? H2dcbpy
H-1 ? L (0.98)
H-3 ? L (0.54)
H-1 ? L+1 (0.94)
2
3
S1
S3
S5
S8
681
511
456
420
0.001
0.04
0.03
0.03
H ? L (0.98)
Ir-pbi ? H2dcbpy
Ir-pbi ? H2dcbpy
Ir-pbi ? H2dcbpy
Ir-pbi ? H2dcbpy
H-1 ? L (0.98)
H-3 ? L (0.88)
H-1 ? L+1 (0.60)
S1
S8
S15
930
528
431
0.0003
0.05
0.03
H ? L (0.98)
H-3 ? L (0.94)
H-3 ? L+1 (0.74)
Ir-mpbi ? H2dcbpy
Ir-mpbi ? H2dcbpy
Ir-mpbi ? H2dcbpy
a
Vertical excitation wavelengths (k), oscillator strengths (f), dominant monoexcitations (with their contributions within parentheses), and nature of the electronic
transitions are presented. H = HOMO, L = LUMO, H-1 = HOMO ꢀ 1, L+1 = LUMO + 1, etc.
b
Only the singlet states with f > 0.02 and H ? L excitations have been included.

30
S.I. Bezzubov et al. / Inorganica Chimica Acta 415 (2014) 22–30
[9] M.M.-S. Toro, J.F. Fernandez-Sanchez, E. Baranoff, M. Grätzel, M.K.
Nazeeruddin, A. Fernandez-Gutierrez, Talanta 82 (2010) 620.
[10] J.I. Goldsmith, W.R. Hudson, M.S. Lowry, T.H. Anderson, S. Bernhard, J. Am.
Chem. Soc. 127 (2005) 7502.
[11] E.D. Cline, S.E. Adamson, S. Bernhard, Inorg. Chem. 47 (2008) 10378.
[12] K.K.-W. Lo, W.-K. Hui, C.-K. Chung, K.H.-K. Tsang, T.K.-M. Lee, Coord. Chem.
Rev. 250 (2006) 1724.
[13] K.K.-W. Lo, K.H.-K. Tsang, K.-S. Sze, C.-K. Chung, T.K.-M. Lee, K.Y. Zhang, W.-K.
Hui, C.-K. Li, J.S.-Y. Lau, D.C.-M. Ng, N. Zhu, Coord. Chem. Rev. 251 (2007) 2292.
[14] Y.-J. Yuan, J.-Y. Zhang, Z.-T. Yu, J.-Y. Feng, W.-J. Luo, J.-H. Ye, Z.-G. Zou, Inorg.
Chem. 51 (2012) 4123.
[15] K. Hasan, E. Zysman-Colman, Inorg. Chem. 51 (2012) 12560.
[16] Y.-J. Yuan, Z.-T. Yu, H.-L. Gao, Z.-G. Zou, C. Zheng, W. Huang, Chem. Eur. J. 19
(2013) 6340.
dimethoxy-groups is significantly smaller than it might be ex-
pected from simple analysis of HOMO–LUMO gaps. This conclusion
is confirmed by the experimental spectra change from 1 to 3.
Hence, the effect of donor substituents in benzimidazole ligands
on expanding absorption spectra of the complexes into red region
seems to be too small to provide panchromatic properties required
for DSSC sensitizer. Extending conjugated system of the ligands is
likely to result in enhancing light-harvesting properties of irid-
ium(III) complexes.
4. Conclusions
[17] For recent reviews on DSSCs, see: (a) C.A. Bignozzi, R. Argazzi, R. Boaretto, E.
Busatto, S. Carli, F. Ronconi, S. Caramori, Coord. Chem. Rev. 257 (2013) 1472;
(b) P.G. Bomben, K.C.D. Robson, B.D. Koivisto, C.P. Berlinguette, Coord. Chem.
Rev. 256 (2012) 1438;
In summary, we have presented the synthesis, crystal structure,
spectroscopic and electrochemical properties of three new irid-
ium(III) complexes with 2-arylbenzimidazole-based cyclometalat-
ed ligands in which electron-donor/withdrawing properties of the
substituents have been varied, and 2,20-bipyridine-4,40-dicarbox-
ylic acid as ancillary ligand. The complexes absorb visible light
(c) A. Hagfeldt, G. Boschloo, L. Sun, L. Kloo, H. Pettersson, Chem. Rev. 110
(2010) 6595;
(d) P.D. Frischmann, K. Mahata, F. Wurthner, Chem. Soc. Rev. 42 (2013) 1847.
}
[18] M.K. Nazeeruddin, A. Kay, I. Rodicio, R. Humphry-Baker, E. Muller, P. Liska, N.
Vlachopoulos, M. Grätzel, J. Am. Chem. Soc. 115 (1993) 6382.
[19] M. Grätzel, Acc. Chem. Res. 42 (2009) 1788.
[20] E.I. Mayo, K. Kilsa, T. Tirrell, P.J. Djurovich, H.B. Gray, Photochem. Photobiol.
Sci. 5 (2006) 871.
[21] C. Dragonetti, A. Valore, A. Colombo, S. Righetto, V. Trifiletti, Inorg. Chim. Acta
388 (2012) 163.
[22] Z. Ning, Q. Zhang, W. Wu, H. Tian, J. Organomet. Chem. 694 (2009) 2705.
[23] E. Baranoff, J.-H. Yum, M. Grätzel, M.K. Nazeeruddin, J. Organomet. Chem. 694
(2009) 2661.
[24] E. Baranoff, J.-H. Yum, I. Jung, R. Vulcano, M. Grätzel, M.K. Nazeeruddin, Chem.
Asian J. 5 (2010) 496.
[25] Y. Shinpuku, F. Inui, M. Nakai, Y. Nakabayashi, J. Photochem. Photobiol. A:
Chem. 222 (2011) 203.
significantly (e
= 1000–3000 Mꢀ1 cmꢀ1) up to 600 nm. The nature
of electronic transitions has been studied by TDDFT approach.
Reversible oxidation potentials of 1–3 are high enough (Eox > 1.2 V
versus NHE) for the oxidized complexes to be spontaneously
reduced not only by a common Iꢀ3 /Iꢀ redox mediator but also by
a Brꢀ3 /Brꢀ redox couple. Thus, all the complexes, especially 3, are
good candidates for operation in DSSC as dyes.
Acknowledgements
[26] W.-S. Huang, J.T. Lin, C.-H. Chien, Y.-T. Tao, Sh.-S.h. Sun, Y.-S.h. Wen, Chem.
Mater. 16 (2004) 2480.
[27] G.-G. Shan, H.-B. Li, H.-Z. Sun, H.-T. Cao, D.-X. Zhu, Z.-M. Su, Dalton Trans. 42
(2013) 11056.
The authors thank the Russian Foundation for Basic Research
(project 13-03-00972) for financial support. Dr. Ivan Vatsouro
and Dr. Kirill Puchnin are acknowledged for assistance concerning
NMR measurements.
[28] A.M. Oki, R.J. Morgan, Synth. Commun. 25 (1995) 4093.
[29] A.R. Ronzio, T.D. Waugh, Org. Synth. 3 (1955) 438.
[30] H.F. Ridley, R.G.W. Spickett, G.M. Timmis, J. Heterocycl. Chem. 2 (1965) 453.
[31] M. Nonoyama, Bull. Chem. Soc. Jpn. 47 (1974) 767.
[32] G.M. Sheldrick, Acta Crystallogr., Sect. A 64 (2008) 112.
[33] A.A. Granovsky, Firefly version 7.1.G, http://www.classic.chem.msu.su/gran/
firefly/index.html.
Appendix A. Supplementary material
[34] M.W. Schmidt, K.K. Baldridge, J.A. Boatz, S.T. Elbert, M.S. Gordon, J.H. Jensen, S.
Koseki, N. Matsunaga, K.A. Nguyen, S. Su, T.L. Windus, M. Dupuis, J.A.
Montgomery, J. Comput. Chem. 14 (1993) 1347.
[35] P.J. Stephens, F.J. Devlin, C.F. Chabalowski, M.J. Frisch, J. Phys. Chem. 98 (1994)
11623.
[36] D. Andrae, U. Haussermann, M. Dolg, H. Stoll, H. Preuss, Theor. Chim. Acta 77
(1990) 123.
[37] R. Krishnan, J.S. Binkley, R. Seeger, J.A. Pople, J. Chem. Phys. 72 (1980) 650.
[38] D. Hanss, J.C. Freys, G. Bernardinelli, O.S. Wenger, Eur. J. Inorg. Chem. (2009)
4850.
CCDC 974801 contains the supplementary crystallographic data
for the structure. These data can be obtained free of charge from
The Cambridge Crystallographic Data Centre via www.ccdc.cam.
ac.uk/data_request/cif. Supplementary data associated with this
article can be found, in the online version, at http://dx.doi.org/
10.1016/j.ica.2014.02.024.
[39] W. Jiang, Y. Gao, Y. Sun, F. Ding, Y. Xu, Z. Bian, F. Li, J. Bian, C. Huang, Inorg.
Chem. 49 (2010) 3252.
References
[40] Q. Zhao, S.J. Liu, M. Shi, C.M. Wang, M.X. Yu, L. Li, F.J. Li, T. Yi, C.H. Huang, Inorg.
Chem. 45 (2006) 6152.
[41] A.R. Katritzky, A.F. Pozharskii (Eds.), Handbook of heterocyclic chemistry, 2nd
ed., Pergamon, 2000, p. 748.
[42] S. Lamansky, P. Djurovich, D. Murphy, F. Abdel-Razzaq, H.-E. Lee, C. Adachi, P.E.
Burrows, S.R. Forrest, M.E. Thompson, J. Am. Chem. Soc. 123 (2001) 4304.
[43] P. Ceroni (Ed.), The exploration of supramolecular systems and nanostructures
by photochemical techniques, Springer, 2012, p. 232.
[44] C. Teng, X. Yang, S. Li, M. Cheng, A. Hagfeldt, L.-Z. Wu, L. Sun, Chem. Eur. J. 16
(2010) 13127.
[1] V. Bulovic, G. Gu, P.E. Burrows, S.R. Forrest, M.E. Thompson, Nature 380 (1996)
29.
[2] S. Kappaun, C. Slugovc, E.J.W. List, Int. J. Mol. Sci. 9 (2008) 1527.
[3] E. Holder, B.M.W. Langeveld, U.S. Schubert, Adv. Mater. 17 (2005) 1109.
}
[4] K. Mullen, U. Scherf (Eds.), Organic light emitting devices. Synthesis, properties
and applications, WILEY-VCH, Weinheim, 2006, p. 410.
[5] W.-Y. Wong, C.-L. Ho, Coord. Chem. Rev. 253 (2009) 1709.
[6] R.C. Evans, P. Douglas, C.J. Winscom, Coord. Chem. Rev. 250 (2006) 2093.
[7] G. Di Marco, M. Lanza, A. Mamo, I. Stefio, C. Di Pietro, G. Romeo, S. Campagna,
Anal. Chem. 70 (1998) 5019.
[45] H.C. Zhao, J.P. Harney, Y.-T. Huang, J.-H. Yum, M.K. Nazeeruddin, M. Grätzel,
M.-K. Tsai, J. Rochford, Inorg. Chem. 51 (2012) 1.
}
[8] M.E. Kose, R.J. Crutchley, M.C. DeRosa, J.R. Reynolds, K.S. Schanze, Langmuir 21
(2005) 8255.