Iridium(III) 2-Phenylbenzimidazole Complexes: Synthesis, Structure, Optical Properties, and Applications in Dye-Sensitized Solar Cells

Article abstract of DOI:10.1002/ejic.201501068

A series of bis-cyclometalated iridium(III) complexes, [Ir(LH)₂(H₂dcbpy)][PF₆] (1), [Ir(LMe)₂(H₂dcbpy)][PF₆] (2), and [Ir(LPh)₂(H₂dcbpy)][PF₆] (3), where LH

Full text of DOI:10.1002/ejic.201501068

DOI: 10.1002/ejic.201501068
Full Paper
Dye-Sensitized Solar Cells
Iridium(III) 2-Phenylbenzimidazole Complexes: Synthesis,
Structure, Optical Properties, and Applications in Dye-
Sensitized Solar Cells
Stanislav I. Bezzubov,*[a] Yuri M. Kiselev,[b] Andrey V. Churakov,[a] Sergey A. Kozyukhin,[a]
Alexey A. Sadovnikov,[a] Vitaly A. Grinberg,[c] Viktor V. Emets,[c] and Vladimir D. Doljenko[b]
Abstract: A series of bis-cyclometalated iridium(III) complexes, absorb light up to 550 nm with molar absorptivities of 1500–
[Ir(LH)2(H2dcbpy)][PF6] (1), [Ir(LMe)2(H2dcbpy)][PF6] (2), and 2000
–1 cm–1. Complexes 1 and 2 possess very similar optical
M
[Ir(LPh)2(H2dcbpy)][PF6] (3), where LH = 1-H-2-phenylbenzimid- properties, whereas the introduction of the phenyl ring (com-
azole, LMe = 1-methyl-2-phenylbenzimidazole, LPh = 1,2-di- plex 3) causes a hypsochromic shift (≈ 30 nm) of the lumines-
phenylbenzimidazole, and H2dcbpy = 2,2′-bipyridine-4,4′-di- cent maximum as well as resulting in an almost 50 % increase
carboxylic acid, has been synthesized and fully characterized by in the extinction coefficient at 490 nm compared with 1 and 2.
1
elemental analysis, H and 31P NMR spectroscopy, mass spec- A dye-sensitized solar cell (DSSC) based on complex 3 exhibits
trometry, and single-crystal X-ray analysis. The complexes show a short-circuit photocurrent of 2.8 mA cm–2, an open-circuit
strong luminescence in the yellow–orange region in ethanol at photovoltage of 0.44 V, and a power conversion efficiency of
room temperature (quantum yield is up to 22 %), and 0.7 %.
Introduction
Since the publication of the first efficient dye-sensitized solar
cell (DSSC) in 1991,[1] hundreds of transition-metal complexes,
organic dyes, quantum dots, and other substances have been
tested as photosensitizers in these devices.[2] The most widely
used dyes are ruthenium(II) polypyridine complexes, for
example, N3 and “black dye” (Figure 1, a, b) because of their
intense metal-to-ligand charge-transfer bands (MLCT) in the
visible range with molar absorptivities of approximately
15000
M
–1 cm–1.[3] However, the main drawback of sensitizers
like N3 and “black dye” is the presence of two (or three) labile
isothiocyanate groups, which cause poor long-term stability of
the complexes.[4] Cyclometalated ruthenium(II) complexes are
considered to be more stable, and they have been intensively
studied in recent years,[5] but their synthesis is often accompa-
nied by difficulties and can be carried out only with a limited
number of ligands.
[a] Institute of General and Inorganic Chemistry,
Russian Academy of Sciences,
Figure 1. RuII and IrIII photosensitizers: (a) N3, (b) “black dye” (TBA = tetra-
butylammonium cation), (c) the best IrIII dye.[7]
Leninskii Prosp. 31, Moscow 119991, Russian Federation
E-mail: stas.bezzubov@gmail.com
http://www.igic.ras.ru/
[b] Department of Chemistry, M. V. Lomonosov Moscow State University,
Leninskie Gory 1/3, Moscow 119991, Russian Federation
[c] Frumkin Institute of Physical Chemistry and Electrochemistry,
Russian Academy of Sciences,
In contrast, iridium(III) easily forms bis- or tris-cyclometalated
complexes that demonstrate high thermal and chemical stabil-
ity, long excited-state lifetimes, and emission color tunability by
cyclometalated (C N) ligands variations.[6] Unfortunately, only a
relatively low device performance (2.2 %) has been achieved by
Leninskii pr. 31, Moscow 119071, Russia
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/ejic.201501068.
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using the best iridium dye to date (Figure 1, c), which seems to ing the molecular and electronic structures as well as optical
be due to weak light absorbance in the range 500–600 nm.[7]
properties of the corresponding IrIII complexes. Thus, we report
a joint experimental and theoretical study of a series of cyclo-
metalated IrIII complexes with 1-R-2-phenylbenzimidazoles (R =
H, CH3, C6H5) and 2,2′-bipyridine-4,4′-dicarboxylic acid as a li-
gand with “anchoring” groups, namely, [Ir(LH)2(H2dcbpy)]-
[PF6] (1), [Ir(LMe)2(H2dcbpy)][PF6] (2), and [Ir(LPh)2(H2dcbpy)]-
[PF6] (3), where LH = 1-H-2-phenylbenzimidazole, LMe = 1-
methyl-2-phenylbenzimidazole, LPh = 1,2-diphenylbenzimid-
azole, and H2dcbpy = 2,2′-bipyridine-4,4′-dicarboxylic acid.
Relationships between the ligand structure and luminescent
and electrochemical properties of IrIII complexes have been suc-
cessfully explored by several groups.[8] As a result, emission
maxima as well as values of oxidation potentials of IrIII com-
plexes can be finely tuned by varying the HOMO and LUMO
energy levels. Decreasing the HOMO–LUMO gap of the com-
plexes can be achieved by introduction of electron-donor
groups (EDG) into C^N ligands, whereas electron-withdrawing
groups (EWG) result in the opposite effect. Recently, a simple
model based on Hammett constants that allows the influence
of EDG or EWG on the luminescent and redox properties of IrIII
complexes to be quantitatively estimated has been devel-
oped.[9] However, the effect of EDG on the electronic absorption
spectra of IrIII compounds is not so clear as it might be ex-
pected.
Results and Discussion
Synthesis
Complexes 2 and 3 were prepared in very good yields (> 85 %),
whereas the synthesis of 1 was less successful (63 %) as addi-
tional purification steps were required (Figure 2). According to
the literature procedure,[18] an excess of C N ligand was elimi-
nated by washing the product of the cyclometalation stage
with alcohol and ether because usually C N ligands are much
more soluble in these solvents than the corresponding dimers.
In contrast, LH and the corresponding IrIII dimer demonstrate
comparable solubility in most organic solvents. Thus, residual
LH was removed after the reaction of the dimer with H2dcbpy
by converting the desired complex to a water-soluble sodium
salt followed by simple filtration.
Up to now, there have been several efforts to improve the
light-harvesting properties of iridium-based dyes by modifica-
tion of the C^N ligands with EDG.[8] This method works well for
organic (metal-free) dyes,[10] but it can give rise to unexpected
results when used for metal complexes. The introduction of CF3
groups into C^N ligands caused a small redshift of the long-
wavelength absorption band in a series of 2-phenylbenzothi-
azole-based IrIII complexes, whereas introduction of NMe2
groups resulted in the opposite effect.[11] The second and third
methoxy groups in the phenyl unit of 3-methyl-2-phenylpyr-
idine also gave rise to hypsochromic shifts of the absorption
maxima of the corresponding IrIII complexes instead of the ex-
pected bathochromic shifts.[12] The addition of the second CF3
group in the phenyl unit of 2-[2-(trifluoromethyl)phenyl]pyr-
idine caused a small redshift of the absorption maxima, while
increasing the HOMO–LUMO gap of the complex.[13]
These examples clearly demonstrate that new experimental
and theoretical data concerning the electronic structure of IrIII
complexes are demanded. In particular, we need information
about the influence of EDG not only on the HOMO but also on
the HOMO–1, HOMO–2, and other occupied molecular orbitals
(MOs), because the absorption spectra of IrIII complexes are usu-
ally formed by many bands originating from different electronic
transitions (not only the HOMO → LUMO transition). To obtain
more information, one needs to comprehensively study at least
several series of IrIII complexes by varying the electron-donor/
withdrawing properties of the ligands over wide ranges. In this
case, using such C N aromatic donors like 2-arylbenzimidazoles
would be completely justified because these ligands could be
easily modified by conventional chemical reactions in ways that
immediately affect the electronic environment of iridium
through the highly conjugated heteroaromatic system.[14,15]
Figure 2. (i) 1-R-2-phenylbenzimidazole (R = H, CH3, Ph), 2-ethoxyethanol/
H2O (3:1), Ar, 135 °C, 24 h; (ii) 2,2′-bipyridine-4,4′-dicarboxylic acid (H2dcbpy),
CH2Cl2/MeOH (2:1), Ar, darkness, 65 °C, 5–24 h; (iii) NH4PF6, 30 min.
Crystal Structures
The molecular structures of 1–3 as well as their packing features
were determined by X-ray analysis. Complexes 1–3 crystallize
as neutral molecules with a general formula [Ir(C N)2(Hdcbpy)]
owing to deprotonation of one of the carboxy groups in
H2dcbpy. Similar zwitterionic molecules are known for some
closely related IrIII compounds.[19] There are two independent
molecules with slightly different geometries in the crystals of 1
and 2 (Figure S1 in the Supporting Information). The crystal of
3, in turn, contains only one independent molecule.
Recently, we have studied two series of IrIII complexes with
2-aryl-1-phenylbenzimidazoles in which the aryl unit was modi-
fied by electron-donating/withdrawing groups.[16,17] The prob-
ability of electronic transitions from donor to acceptor ligands
was shown to depend dramatically on the contribution of the
In all three structures, the central ion has a coordination
Ir d orbitals to the occupied MOs of the complexes. The incor- number (CN) = 6 and possesses a distorted octahedral coordi-
poration of different substituents into the benzimidazole unit nation environment (Figure 3) formed by two bidentate C N
of similar C N ligands is very likely to be useful for further vary- ligands (two C and two N atoms) and bidentate Hdcbpy (two
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Figure 3. Molecular structures of 1–3 (one of the two independent molecules for 1 and 2). Displacement ellipsoids are shown at the 50 % probability level.
Minor components of disordered COOH groups and their parts are drawn with open lines. All hydrogen atoms except those for COOH and N–H groups are
omitted for clarity.
N atoms). Two nitrogen atoms (from the C N ligands) adopt a cis-angles N–Ir–N vary within a wide range [84.4(2)–100.9(2)°].
trans-configuration to each other in this octahedron, which is The N and C atoms that engage in trans-positions to each
very common for similar IrIII complexes. All bond lengths and
angles formed by the central iridium ions have typical values
for such compounds according to the Cambridge Structural Da-
tabase (CSD; ver. 5.36, Feb 2015). The Ir–C lengths vary within
the ranges 2.019(6)–2.031(5) Å, Ir–N(C N) within 2.027(5)–
other form angles C–Ir–N and N–Ir–N, lying within 170.0(2)–
176.0(2)°.
Both C N ligands and Hdcbpy are slightly nonplanar in all
the complexes. Surprisingly, the dihedral angles between the
benzimidazole planes and phenyl rings in C N ligands vary over
a wide range from 2.5(4) to 14.4(2)°. In coordinated Hdcbpy, the
p-carboxypyridine rings are inclined to each other with dihedral
angles varying between 5.6(5)–11.3(4)°. These values are com-
mon for similar IrIII complexes according to the CSD. The planes
of the C N ligands are almost orthogonal to the plane of
Hdcbpy for complex 2 [89.4(2) and 81.2(1)° for the two C N
ligands], whereas for 1 [81.4(1) and 79.3(1)°], and 3 [80.6(1) and
76.4(1)°] these angles are noticeably less. In the structure of 3,
the N-phenyl rings are inclined to the planes of the C N ligands
[89.8(2) and 68.7(2)°].
2.055(4) Å, and Ir–N(Hdcbpy) within 2.112(5)–2.128(4)
Å
(Table 1). The Ir–N(Hdcbpy) lengths are significantly longer than
Ir–N(C N) because of the trans-effect of the C donor atoms in
the C N ligands.[20] The intraligand C–Ir–N and N–Ir–N angles
are noticeably less than 90°, lying in the ranges 78.9(2)–80.0(2)
and 76.7(2)–77.5(2)°, respectively. The interligand cis-angles C–
Ir–C for 2 [85.1(2)°] and 3 [87.2(2) and 87.3(2)°] are less than
90°, whereas for 1 they slightly exceed 90° [91.5(2) and 92.1(2)°].
In turn, the interligand cis-angles N–Ir–C for 1–3 are all more
than 90°, lying within the range 92.6(2)–99.1(2)°, whereas the
Table 1. Selected bond lengths [Å] and angles [°] for 1–3.
Compound
1 (two independent molecules)
2 (two independent molecules)
3
Ir(1)–C(1)
Ir(1)–C(14, 15, 20)
Ir(1)–N(1)
Ir(1)–N(3)
Ir(1)–N(5)
2.020(5), 2.018(6)
2.025(5), 2.025(5)
2.044(4), 2.044(4)
2.045(4), 2.055(4)
2.113(4), 2.117(5)
2.114(5), 2.120(4)
79.8(2), 80.0(2)
79.7(2), 79.6(2)
77.2(2), 77.1(2)
92.1(2), 91.5(2)
94.9(2), 94.0(2)
96.8(2), 94.2(2)
173.7(2), 170.1(2)
95.2(2), 94.7(2)
94.0(2), 97.4(2)
174.0(2), 170.0(2)
98.3(2), 94.8(2)
90.9(2), 88.1(2)
172.3(2), 171.8(2)
87.7(2), 94.8(2)
95.2(2), 98.0(2)
2.024(6), 2.034(6)
2.031(5), 2.031(6)
2.029(5), 2.027(5)
2.032(5), 2.027(5)
2.114(5), 2.112(5)
2.125(5), 2.118(4)
79.7(2), 79.4(2)
78.9(2), 80.0(2)
76.9(2), 77.5(2)
87.2(2), 87.3(2)
95.9(2), 95.5(2)
98.2(2), 99.1(2)
176.0(2), 175.6(2)
93.6(2), 92.7(2)
96.9(2), 97.0(2)
173.6(2), 174.5(2)
99.8(2), 100.2(2)
88.8(2), 88.3(2)
172.1(2), 171.0(2)
85.0(2), 85.9(2)
98.5(2), 99.5(2)
2.019(6)
2.026(6)
2.033(4)
2.039(4)
2.127(4)
2.128(4)
79.4(2)
79.1(2)
76.7(2)
85.1(2)
96.1(2)
99.1(2)
175.8(2)
95.6(2)
99.1(2)
175.7(2)
100.9(2)
85.7(2)
173.3(2)
84.4(2)
99.6(2)
Ir(1)–N(6)
C(1)–Ir(1)–N(1)
C(14, 15, 20)–Ir(1)–N(3)
N(5)–Ir(1)–N(6)
C(1)–Ir(1)–C(14, 15, 20)
C(1)–Ir(1)–N(3)
C(1)–Ir(1)–N(6)
C(1)–Ir(1)–N(5)
C(14, 15, 20)–Ir(1)–N(1)
C(14, 15, 20)–Ir(1)–N(5)
C(14, 15, 20)–Ir(1)–N(6)
N(1)–Ir(1)–N(5)
N(1)–Ir(1)–N(6)
N(1)–Ir(1)–N(3)
N(3)–Ir(1)–N(5)
N(3)–Ir(1)–N(6)
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Molecules of 1 form a 3D packing arrangement with strong the long-wavelength bands (> 450 nm) as the studied com-
intermolecular hydrogen bonds between the carboxy groups. plexes were considered as potential photosensitizers in solar
There are wide channels passing along the c axis, which are cells, and hence the visible spectral range was more interesting.
filled by solvent molecules (Figure S2). The asymmetric unit in In addition, the MOs of 1–3 that participate in the correspond-
the structure of 1 contains ten molecules of DMSO (four of ing electronic transitions were also determined (Figure 5, Fig-
them are disordered over two adjacent sites) and four molecu- ure S3).
Based on the statistical analysis of residual square sums,[23]
the optimum number of Gaussian components for deconvolu-
tion of the absorption spectra of 1–3 was determined to be
four (Figure S4, Table S1). In turn, time-dependent (TD)-DFT cal-
culations also predicted four intensive absorption bands (oscil-
lator strength, f > 0.02) in the studied spectral range (Table S2).
These data allowed us to assign G1–G4 to electronic transitions
between certain MOs.
les of water. All water and six DMSO molecules form moderate
and weak hydrogen bonds among themselves as well as with
the N–H fragment of the coordinated C N ligands and the
carboxy groups. Similar packing is seen for molecules of 3, ex-
cept for the hydrogen bonding participation of the solvent mol-
ecules. In contrast, the crystal of 2 contains molecular layers
passing along the b axis, which are formed by strong hydrogen
bonds between the carboxy groups of Hdcbpy.
The HOMO → LUMO electronic transition makes only a negli-
gible contribution to the absorption spectra of 1–3. The ob-
served bands G1–G4 correspond to the HOMO–1 → LUMO,
Optical Properties
Complexes 1–3 demonstrate strong light absorbance in the
HOMO–3 → LUMO,
HOMO–1 → LUMO+1,
and
HOMO–
range 250–300 nm (ε ≈ 70000
tronic transitions located on the ligands (Figure 4). The broad
bands at 350–400 nm (ε ≈ 15000–20000
–1 cm–1) as well as
the bands in the visible range (> 400 nm) are likely to originate
from MLCT and ligand-to-ligand charge transfers (LLCT), which
are common for similar IrIII compounds.[14]
M
–1 cm–1) owing to π → π* elec-
2 → LUMO+1 transitions, respectively, which are of a combined
nature, mostly MLCT and LLCT. As Ir d orbitals link the π-sys-
tems of the C N and N N ligands, their contribution to the occu-
pied MOs of 1–3 immediately affects the intensity of LLCT.[16]
The greater contribution of the Ir d orbitals, the more intensive
LLCT and, clearly, MLCT bands. For example, εmax of G1 is much
lower than those of G2–G4, especially for complex 3, where
the contribution of the Ir d orbitals in HOMO–1 is 13 %
(Table S3).
M
The parameters of G1–G4, that is, λmax, full width at half max-
imum (FWHM), and εmax, are very similar for the absorption
spectra of 1 and 2 (Table 2). The CH3 group seems to have a
negligible effect on the electronic structure of 2. In contrast, G2
and G3 are more intensive and significantly blueshifted (25–
30 nm) in the spectrum of 3 compared with 1 and 2. This exper-
imental evidence is likely to be caused by the slight electron-
withdrawing effect of the N-phenyl ring in combination with
its weak conjugation with the benzimidazole unit of the LPh
ligand.
Figure 4. UV/Vis absorption spectra of 1–3 in DMSO solution at room temper-
ature.
The π-system of the N-phenyl ring also influences the lumi-
nescent properties of 3 (Figure S5, Table 2), shifting hypsochro-
mically (≈ 30 nm) the emission maximum compared with 1 and
To gain insight into the electronic structures of 1–3, the ex- 2, which exhibit almost the same emission maxima: 610 and
perimental absorption bands were deconvoluted into their 615 nm, respectively. The luminescence quantum yield in-
Gaussian components.[21,22] This procedure was applied only to creases in the series of complexes as follows: 2 < 1 ≈ 3 (Table 2).
Table 2. Optical properties of 1–3 and their oxidation potentials.[a]
Compound
1
λabs [nm] (FWHW [cm–1], ε·[10–3
M
–1 cm–1])
λexc [nm]
λem [nm]
QI[b] [%]
21.4
Eox [V]
306 (64), 370 (19),
408
610
1.5[c]
430 (2000, 2.70), 486 (1800, 1.40),
526 (2100, 0.85), 570 (1800, 0.15)
307 (77), 370 (17),
430 (2000, 2.90), 485 (2000, 1.20),
524 (1800, 0.90), 564 (2100, 0.25)
304 (69), 370 (18),
2
3
387
380
615
585
8.7
1.5[c]
22.1
1.55[16]
423 (2000, 3.20), 460 (1500, 2.11),
496 (2100, 2.14), 560 (1700, 0.08)
[a] Measured in DMSO at room temp. FWHM = full width at half maximum, QI = quantum yield of photoluminescence, Eox = oxidation potential. Estimated
errors: 1 nm for λmax 5 % for QI; and 20 mV for Eox. The UV bands (304–307 and 370 nm) were not decomposed into
20 cm–1 for FWHM; 1 % for εmax
Gaussians. [b] Measured relative to rhodamine 6G. [c] Irreversible process.
;
;
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Figure 5. Absorption spectra of 1 and 2 (DMSO, room temp.) with deconvolution into their Gaussian components (λmax, FWHM, εmax). FWHM: full width at
half maximum. Black solid line: experimental spectra, red dotted line: sum of Gaussian components. Dominant monoexcitations (and their contributions, [%])
for G1–G4 with plots of molecular orbitals participating in the corresponding electronic transitions are presented. Hydrogen atoms are omitted for clarity.
Estimated errors: 1 nm for λmax
;
20 cm–1 for FWHM; and 1 % for εmax
.
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Photovoltaic Performance
TD-DFT approach. Complex 3 has been successfully tested in a
DSSC as a photosensitizer.
In our previous paper,[16] IrIII 2-arylbenzimidazole complexes,
particularly 3, were shown to be promising candidates for appli-
cations in DSSC as dyes owing to their suitable redox potentials
of the ground and excited states and good optical characteris-
tics. Unfortunately, 1 and 2 demonstrated irreversible redox be-
havior and poor solubility in most organic solvents (except
DMSO). As 3 exhibited better light-harvesting properties in the
visible range than both 1 and 2, it was tested in a DSSC. The
current–voltage characteristics of the DSSC with photoanodes
made with 3 as the photosensitizer are presented in Figure 6.
Experimental Section
Materials and Methods: All commercially available reagents were
of at least reagent grade and used without further purification. Sol-
vents were distilled and dried according to standard procedures.
Preparation of all iridium complexes was carried out under dry ar-
gon. Purification and other manipulations with complexes were per-
formed in air.
1H and 31P NMR spectra were acquired at 25 °C with a Bruker Av-
ance 400 instrument and chemical shifts were reported in ppm ref-
erenced to residual solvent signals. Matrix-assisted laser desorption
ionization (MALDI) mass spectra were measured with a Bruker Auto-
Flex II apparatus. Electronic absorption spectra were measured with
an OKB Spectr SF-2000 spectrophotometer. Luminescence spectra
were recorded by using a Perkin–Elmer LS-55 spectrometer. An
Econix-Expert Ltd Ecotest-VA polarograph was used for the electro-
chemical measurements, with a glassy carbon working electrode,
platinum counter electrode, and saturated Ag/AgCl reference elec-
trode. Alternating-current polarographic curves were recorded in
Ar-saturated acetonitrile with 0.1
M (nBu4N)PF6 at a scan rate of
50 mV s–1, frequency of 25 Hz, and amplitude of 30 mV. Ferrocene
was used as an external standard prior to measurements. Elemental
analyses were performed by using an Elementar VarioMicroCube
CHN analyzer.
Synthesis
Figure 6. Current–voltage characteristics of the DSSC with photoanodes made
with 3 as a sensitizer.
2,2′-Bipyridine-4,4′-dicarboxylic acid (H2dcbpy),[24] 1-H-2-phenyl-
benzimidazole (LH), 1,2-diphenylbenzimidazole (LPh),[25] and 1-
methyl-2-phenylbenzimidazole (LMe)[26] were prepared according
to published procedures. All the complexes were synthesized by
a two-stage method including a modified version of Nonoyama's
synthesis[18] for the preparation of the cyclometalated μ-chloro-
bridged iridium(III) dimers [(C N)2Ir(μ-Cl)2Ir(C N)2] followed by their
reaction with H2dcbpy, and finally with NH4PF6 to produce the de-
sired complexes.[16]
Using complex 3 as a photosensitizer gave rise to the follow-
ing parameters of the corresponding DSSC: a short-circuit pho-
tocurrent of 2.8 mA cm–2, an open-circuit photovoltage of
0.44 V, a fill factor of 50 %, and a power conversion efficiency
of 0.7 %. These low characteristics are mainly caused by insuffi-
cient εin the range 500–600 nm as well as poor cell assembling
(a low fill factor value).
There are two possible ways to enhance Ir-based DSSC per-
formance. 1) Increasing the fill factor by optimizing the proper-
ties of other cell components (for example, varying electrolytes
and additives, redox mediators, or different methods of TiO2
surface treatment). 2) Improving light-harvesting properties of
iridium complexes by changing the substituents on the C N li-
gands. The latter way should be based on a systematic study
of the electronic structure of the complexes by using both ex-
perimental and computational results.
After washing the intermediate iridium(III) dimers with water, eth-
anol, and ether (to eliminate unreacted starting material), they were
used in the second step without characterization or further purifica-
tion.
[Ir(LMe)2(H2dcbpy)][PF6] (2[PF6]): Yield 94 %, orange–red powder.
1H NMR ([D6]DMSO): δ = 9.16 (s, 2 H), 8.20 (d, J = 5.3 Hz, 2 H), 8.05–
8.09 (m, 4 H), 7.79 (d, J = 8.3 Hz, 2 H), 7.27 (t, J = 7.8 Hz, 2 H), 7.07
(t, J = 7.7 Hz, 2 H), 6.96 (t, J = 7.9 Hz, 2 H), 6.85 (t, J = 7.4 Hz, 2 H),
6.21 (d, J = 7.6 Hz, 2 H), 5.64 (d, J = 8.3 Hz, 2 H), 4.35 (s, 6 H) ppm.
31P NMR ([D6]DMSO): δ= –144.23 (sept, J = 709 Hz) ppm. MS (MALDI
TOF): m/z = 850.8 (calcd. 850.9 for [Ir(LMe)2(H2dcbpy)]+). Elemental
analysis calcd. (%) for C40H30N6O4F6PIr (995.89): C 48.24, H 3.04, N
8.44; found: C 48.29, H 2.98, N 8.41.
Conclusion
[Ir(LPh)2(H2dcbpy)][PF6] (3[PF6]): Yield 86 %, red powder. Data of
We have presented the synthesis, crystal structures, and optical
properties of three iridium(III) complexes with 2-phenylbenz-
imidazole-based cyclometalated ligands and 2,2′-bipyridine-
4,4′-dicarboxylic acid as an ancillary ligand. Although the CH3
substituent (complex 2) causes little variation in the photophys-
ical properties compared with unsubstituted 1, introduction of
the phenyl ring (complex 3) results in an almost 50 % increase
in the molar absorptivity at 490 nm compared with 1 and 2.
analyses are consistent with reference[16]
.
[Ir(LH)2(H2dcbpy)][PF6] (1[PF6]): This compound was prepared ac-
cording to a modified literature procedure.[16] The crude product of
the cyclometalation stage was combined with an excess of H2dcbpy
in a mixture of CH2Cl2/CH3OH (2:1) without any purification and
heated at reflux for 12 h in the dark under argon. After evaporation
of the solvents, the red solid was treated with an excess of 1
and then filtered to remove undissolved colorless LH. The dark-red
HCl to pH 2. The brown
M KOH
The nature of the electronic transitions has been studied by a basic solution was then acidified with 2
M
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precipitate was separated and thoroughly washed with water by
centrifugation. Next, it was dissolved in a minimal amount of
CH3OH and filtered to remove excess colorless H2dcbpy. The filtrate
was treated with a 10-fold excess of NH4PF6 and the solvents evapo-
rated to dryness. After flash column chromatography (SiO2, eluent
CH2Cl2/CH3OH from 100:0 to 95:5), the desired complex was ob-
located at the inversion center. The other CH2Cl2 molecule also en-
gages a partially (< 50 %) occupied site. Both carboxy groups of 3
form short intermolecular contacts with their own symmetry equiv-
alents, and one of the groups is disordered over two sites. Owing
to the electroneutrality of the complex, one component of the dis-
order as well as the second COOH group were refined in protonated
forms with half-occupied hydrogen atoms.
1
tained, yield 63 %, brown powder. H NMR ([D6]DMSO): δ= 9.15 (s,
2 H), 8.22 (d, J = 5.5 Hz, 2 H), 8.02 (d, J = 5.7 Hz, 2 H), 7.91 (d, J =
7.5 Hz, 2 H), 7.57 (d, J = 8.1 Hz, 2 H), 7.15 (t, J = 7.7 Hz, 2 H), 7.03
(t, J = 7.5 Hz, 2 H), 6.83 (m, 4 H), 6.19 (d, J = 7.7 Hz, 2 H), 5.59 (d,
J = 8.3 Hz, 2 H) ppm. 31P NMR ([D6]DMSO): δ= –144.21 (sept, J =
711 Hz) ppm. MS (MALDI TOF): m/z = 822.9 (calcd. 822.9 for
CCDC CCDC 1423988 (for 1), 1423989 (for 2), and 1424070 (for 3)
contain the supplementary crystallographic data for this paper.
These data can be obtained free of charge from The Cambridge
Crystallographic Data Centre.
[Ir(LH)2(H2dcbpy)]+).
Elemental
analysis
calcd.
(%)
for
Computational Details: Density functional theory (DFT) calcula-
tions were performed with the Firefly QC package,[29] which is par-
tially based on the GAMESS (US)[30] source code, by using the B3LYP
functional,[31] Stuttgart–Dresden ECP for iridium, and the 6-31G*
basis sets for all other atoms.[32,33] On a basis of the optimized
geometries, natural bond orbital (NBO) theory was employed to
analyze molecular orbital compositions (%). Time-dependent DFT
(TD-DFT) calculations were carried out at the ground-state geome-
tries to obtain vertical excitation energies, oscillator strengths of
the electronic transitions, and theoretical absorption spectra. The
lowest 30 singlet–singlet excitations were computed.
C38H26N6O4F6PIr (967.83): C 47.16, H 2.71, N 8.68; found: C 47.12, H
2.77, N 8.74.
X-ray Structure Determination
Upon standing a DMSO/H2O solution of 1 for a month, crystals
suitable for X-ray analysis were grown. Single crystals of 2 and 3
were obtained by the slow diffusion of diethyl ether vapors into
solutions of the complexes in CH2Cl2 (Table 3). Crystallographic data
were collected at 150 K with a Bruker SMART APEX II diffractometer
using graphite-monochromatized Mo-Kα radiation (λ = 0.71073 Å)
and using a ω-scan mode. Absorption corrections based on meas-
urements of equivalent reflections were applied.[27] The structures
were solved by direct methods and refined by full-matrix least-
Photovoltaic Cell Fabrication
The photoanodes were assembled as follows. TCO22-15 fluorinated
squares on F2 with anisotropic thermal parameters for all non-hy- tin oxide coated 2.9 × 2.9 cm glasses (Solaronix®, specific surface
drogen atoms. In all the structures, hydrogen atoms were placed in
calculated positions and refined by using a riding model. In the
resistivity ca. 15 Ω sq–1) were purified by aging in sulfochromic mix-
ture followed by ultrasonication in organic solvents (2-propanol and
structure of 2, solvent dichloromethane and ether molecules were acetone) and distilled water, and then dried at 50 °C in air. Applica-
not located and their contribution was suppressed by the SQUEEZE
tion of Ti–nanoxide paste (D/SP, Solaronix®), comprising titania pow-
der and α-terpineol as binding agent, was performed by the stand-
procedure.[28] One of the carboxy groups in complex 2 is disordered
over two sites with approximately equal occupancies. In the struc- ard “doctor blade” technique by using a stencil with a 2.0 × 2.0 cm
ture of 3, one of the CH2Cl2 molecules lies in a half-occupied posi-
tion spatially overlapping with the half-occupied ether molecule
(ca. 90 μm depth) square hole. After application of the paste, the
raw photoanodes were dried at 50 °C in air, and then calcined in a
Table 3. Details of the X-ray crystal data collection and structure refinement for 1–3.
1
2
3
Empirical formula
C38H25N6O4Ir·
·5(CH3)2SO·2H2O
1248.51
C40H29N6O4Ir
C50H33N6O4Ir·
·0.5(C2H5)2O·0.94CH2Cl2
1091.16
Mw
849.89
Temperature [K]
150(2)
150(2)
150(2)
Size [mm]
Cryst. system
Space group
a [Å]
b [Å]
c [Å]
0.25 × 0.25 × 0.20
triclinic
0.25 × 0.20 × 0.05
triclinic
0.30 × 0.03 × 0.01
triclinic
P1
P1
P1
¯
¯
¯
14.7319(10)
18.3576(12)
20.3636(14)
89.6180(10)
76.9440(10)
89.1120(10)
5364.1(6)
12.5405(13)
15.6278(16)
23.688(3)
13.9296(16)
14.0013(16)
15.1785(17)
100.010(2)
98.657(2)
α[°]
ꢀ[°]
88.093(2)
85.910(2)
80.759(2)
4569.2(8)
γ[°]
118.195(2)
2475.3(5)
2
1.464
2.850
V [Å3]
Z
4
4
ρcald [g cm–3
]
1.546
2.746
1.235
2.961
Absorption coefficient [mm–1
]
F(000)
2536
1680
1089
θrange [°]
1.81 < θ< 29.00
58955/28302
99.3
2.15 < θ< 28.00
46474/21861
99.0
2.20 < θ< 26.00
21900/9715
99.8
Number of collected/unique reflections
Completeness to θ[%]
Number of data/restraints/parameters
28302/371/1304
1.021
21861/32/923
1.081
9715/55/658
1.026
Goodness of fit on F2
Final R indices [I > 2σ(I)]
R1 = 0.0463, wR2 = 0.1198
R1 = 0.0682, wR2 = 0.1326
2.524/–2.461
R1 = 0.0552, wR2 = 0.1474
R1 = 0.0671, wR2 = 0.1521
4.587/–2.134
R1 = 0.0414, wR2 = 0.1003
R1 = 0.0549, wR2 = 0.1050
1.791/–1.069
R indices (all data)
Largest diff peak/hole [e/Å3]
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muffle furnace at 450 °C for 1 h (heating rate 3 °C min–1, in air).
The thickness of obtained titanium dioxide film was about 15 μm,
determined by using an Alpha-Step D-100 profilometer (KLA-Ten-
cor). Sensibilization of titanium dioxide was performed by soaking
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M)
A three-electrode photoelectrochemical cell PECC-2 (Zahner) was
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was used as the reference electrode. The voltammetric measure-
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1.5 global one sun of illumination (100 mW cm–2) provided by a
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distances was determined with a Nova apparatus (OPHIR-SPIRICON
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irradiation and in the dark. The photoanode area was 1.0 cm2. The
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Acknowledgments
This work was supported by the Russian Foundation for Basic
Research (RFBR) (project numbers 13-03-00972 a, 13-03-12415),
the Russian Science Foundation (RSCF) (project number 14-23-
00176), and the Tomsk State University Competitiveness Im-
provement Program. Dr. Ivan Vatsouro and Dr. Kirill Puchnin are
acknowledged for assistance concerning NMR measurements.
Keywords: Iridium · N ligands · Dyes/pigments · UV/Vis
spectroscopy · Luminescence · Density functional calculations
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Received: September 17, 2015
Published Online: December 21, 2015
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© 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim