Ruthenium(II) Charge-Transfer Sensitizers Containing 4,4′-Dicarboxy-2,2′-bipyridine. Synthesis, Properties, and Bonding Mode of Coordinated Thio- and Selenocyanates

Article abstract of DOI:10.1021/ic9515665

The synthesis and properties of several complexes of Ru(II) containing 4,4′-dicarboxy-2,2′-bipyridine (dcbpyH₂), 2,6-bis(1-methylbenzimidazol-2-yl)pyridine (bmipy), or 2,6-bis(1-methylbenzimidazol-2-yl)-4-phenylpyridine (phbmipy), and monodentate ligands (X^- = Cl^-, I^-, NCS^-, NCSe^-, CN^-) are reported. The introduction of the ambident ligands X^- = NCS^-, NCSe^-, and CN^- into the coordination sphere of [Ru(bmipy)(dcbpy)I]^- and cis-Ru(dcbpyH₂)₂Cl₂ has been studied in situ via ¹H and ¹³C NMR spectroscopy using ¹³C-enriched ligands X^-. Introduction of thiocyanate and selenocyanate initially yields the two possible linkage isomers in comparable amounts; prolonged reaction time converts the S-bound isomer and the Se-bound isomer to the N-bound isomers. The isoselenocyanate complex decomposes rapidly, yielding the cyano complex under loss of Se. The N-bound isothiocyanato complex K[Ru(bmipy)(dcbpy)(NCS)] was found to be an efficient sensitizer for nanocrystalline TiO₂; the incident monochromatic photon-to-current efficiency (IPCE) is nearly quantitative at 520 nm. Introduction of a phenyl group in the 4-position of the 2,6-bis(1-methylbenzimidazol-2-yl)pyridine ligand gives a red-shifted absorption maximum for the corresponding phenylated K[Ru(ph-bmipy)(dcbpy)(NCS)] complex with an increased molar absorption coefficient for the MLCT maximum at 508 nm. At longer wavelengths above 620 nm, phenyl substitution does not enhance the absorption coefficients of the complex. Compared to that of K[Ru(bmipy)(dcbpy)(NCS)], the performance of the phenylated complex is reduced in a solar cell due to lower IPCE values. The visible spectra of the halide complexes K[Ru(bmipy)(dcbpy)X] (X^- = Cl^-, I^-) show enhanced red response, but the complexes exhibit strongly reduced overall IPCE values. A comparison of the complexes to cis-Ru(dcbpyH₂)₂(NCS)₂ is presented. Possible strategies for the design of more efficient sensitizers are discussed.

Full text of DOI:10.1021/ic9515665

Inorg. Chem. 1996, 35, 4779-4787
4779
Ruthenium(II) Charge-Transfer Sensitizers Containing 4,4′-Dicarboxy-2,2′-bipyridine.
Synthesis, Properties, and Bonding Mode of Coordinated Thio- and Selenocyanates
Oliver Kohle,* Stefan Ruile, and Michael Gra1tzel
Institut de Chimie Physique II, Ecole Polytechnique Fe´de´rale de Lausanne,
CH-1015 Lausanne, Switzerland
ReceiVed December 7, 1995X
The synthesis and properties of several complexes of Ru(II) containing 4,4′-dicarboxy-2,2′-bipyridine (dcbpyH2),
2,6-bis(1-methylbenzimidazol-2-yl)pyridine (bmipy), or 2,6-bis(1-methylbenzimidazol-2-yl)-4-phenylpyridine (ph-
bmipy), and monodentate ligands (X- ) Cl-, I-, NCS-, NCSe-, CN-) are reported. The introduction of the
ambident ligands X- ) NCS-, NCSe-, and CN- into the coordination sphere of [Ru(bmipy)(dcbpy)I]- and
cis-Ru(dcbpyH2)2Cl2 has been studied in situ via 1H and 13C NMR spectroscopy using 13C-enriched ligands X-.
Introduction of thiocyanate and selenocyanate initially yields the two possible linkage isomers in comparable
amounts; prolonged reaction time converts the S-bound isomer and the Se-bound isomer to the N-bound isomers.
The isoselenocyanate complex decomposes rapidly, yielding the cyano complex under loss of Se. The N-bound
isothiocyanato complex K[Ru(bmipy)(dcbpy)(NCS)] was found to be an efficient sensitizer for nanocrystalline
TiO2; the incident monochromatic photon-to-current efficiency (IPCE) is nearly quantitative at 520 nm. Introduction
of a phenyl group in the 4-position of the 2,6-bis(1-methylbenzimidazol-2-yl)pyridine ligand gives a red-shifted
absorption maximum for the corresponding phenylated K[Ru(ph-bmipy)(dcbpy)(NCS)] complex with an increased
molar absorption coefficient for the MLCT maximum at 508 nm. At longer wavelengths above 620 nm, phenyl
substitution does not enhance the absorption coefficients of the complex. Compared to that of K[Ru(bmipy)-
(dcbpy)(NCS)], the performance of the phenylated complex is reduced in a solar cell due to lower IPCE values.
The visible spectra of the halide complexes K[Ru(bmipy)(dcbpy)X] (X- ) Cl-, I-) show enhanced red response,
but the complexes exhibit strongly reduced overall IPCE values. A comparison of the complexes to cis-Ru-
(dcbpyH2)2(NCS)2 is presented. Possible strategies for the design of more efficient sensitizers are discussed.
Introduction
of the complex from the reaction mixture by adding a less polar
solvent such as diethyl ether. NMR data or spectra are not
There has been an increasing interest in Ru(II) complexes
containing 4,4′-dicarboxy-2,2′-bipyridine as an attaching group
for the sensitization of nanocrystalline TiO2.1-11 Because of
the carboxyl group, there are differences in synthesis and
chemical properties between ruthenium(II) 4,4′-dicarboxy-2,2′-
bipyridines and the analogous ruthenium(II) 2,2′-bipyridine
complexes. It is difficult to obtain crystalline products of
ruthenium(II) 4,4′-dicarboxy-2,2′-bipyridine complexes. Usu-
ally, the complexes are isolated by adding an acid to the aqueous
solution of the complex anions in order to precipitate the
complex at the isoelectric point. Another method is precipitation
reported.
The thiocyanate ligand in cis-RuII(dcbpyH2)2(NCS)2 was
found to be N-bound, determined by infrared spectroscopy.8 In
general, a variety of physical methods have been utilized to
determine the bonding mode of coordinated thiocyanate in
transition metal complexes.12,13 Thio- and selenocyanate
ligands in Ru complexes are known in the N-, S-, and Se-bound
modes and as bridging ligands for dimer complexes.14-30
(12) Kargol, J. A.; Crecely, R. W.; Burmeister, J. L. Inorg. Chem. 1979,
18, 2532.
(13) Kargol, J. A.; Crecely, R. W.; Burmeister, J. L. Inorg. Chim. Acta
1977, 25, L109.
(14) Kakoti, M.; Chaudhury, S.; Deb, A. K.; Goswami, S. Polyhedron 1993,
12, 783.
(15) Dwyer, F. P.; Goodwin, H. A.; Gyarfas, E. C. Aust. J. Chem. 1963,
16, 42.
(16) Lin, S. W.; Schreiner, A. F. Inorg. Chim. Acta 1971, 5, 290.
(17) De Haas, K. S. J. Inorg. Nucl. Chem. 1973, 35, 3231.
(18) Abbadi, A. R. M.; Shehadeh, M. A.; Kingston, J. V. J. Inorg. Nucl.
Chem. 1974, 36, 1173.
(19) Schwerdtfeger, H. J.; Preetz, W. Angew. Chem., Int. Ed. Engl. 1977,
16, 108.
(20) Wajda, S.; Rachlewicz, K. Inorg. Chim. Acta 1978, 31, 35.
(21) Durham, B.; Walsh, J. L.; Carter, C. L.; Meyer, T. J. Inorg. Chem.
1980, 19, 860.
X Abstract published in AdVance ACS Abstracts, June 15, 1996.
(1) Desilvestro, J.; Gra¨tzel, M.; Kavan, L.; Moser, J.; Augustynski, J. J.
Am. Chem. Soc. 1985, 107, 2988.
(2) Vlachopoulos, N.; Liska, P.; Augustynski, J.; Gra¨tzel, M. J. Am. Chem.
Soc. 1988, 110, 1216.
(3) Liska, P.; Vlachopoulos, N.; Nazeeruddin, M. K.; Comte, P.; Gra¨tzel,
M. J. Am. Chem. Soc. 1988, 110, 3686.
(4) Amadelli, R.; Argazzi, R.; Bignozzi, C. A.; Scandola, F. J. Am. Chem.
Soc. 1990, 112, 7099.
(5) Nazeeruddin, M. K.; Liska, P.; Moser, J.; Vlachopoulos, N.; Gra¨tzel,
M. HelV. Chim. Acta 1990, 73, 1788.
(6) O’Regan, B.; Moser, J.; Anderson, M.; Gra¨tzel, M. J. Phys. Chem.
1990, 94, 8720.
(7) O’Regan, B.; Gra¨tzel, M. Nature 1991, 353, 737.
(8) Nazeeruddin, M. K.; Kay, A.; Rodicio, I.; Humphry-Baker, R.; Mu¨ller,
E.; Liska, P.; Vlachopoulos, N.; Gra¨tzel, M. J. Am. Chem. Soc. 1993,
115, 6382.
(9) Heimer, T. A.; Bignozzi, C. A.; Meyer, G. J. J. Phys. Chem. 1993,
97, 11987.
(22) Poddar, R. K.; Parashad, R.; Agarwala, U. J. Inorg. Nucl. Chem. 1980,
42, 837.
(23) Hoggard, P. E.; Porter, G. B. J. Inorg. Nucl. Chem. 1981, 43, 185.
(24) Fricke, H. H.; Preetz, W. Z. Naturforsch. 1983, 38B, 917.
(25) Palaniappan, V.; Agarwala, U. C. Inorg. Chem. 1986, 25, 4064.
(26) Palaniappan, V.; Agarwala, U. C. Inorg. Chem. 1988, 27, 3568.
(27) Palaniappan, V.; Sathaiah, S.; Bist, H. D.; Agarwala, U. C. J. Am.
Chem. Soc. 1988, 110, 6403.
(10) Argazzi, R.; Bignozzi, C. A.; Heimer, T. A.; Castellano, F. N.; Meyer,
G. J. Inorg. Chem. 1994, 33, 5741.
(11) Bignozzi, C. A.; Argazzi, R.; Indelli, M. T.; Scandola, F. Sol. Energy
Mater. Sol. Cells 1994, 32, 229.
(28) Herber, R. H.; Nan, G.; Potenza, J. A.; Schugar, H. J.; Bino, A. Inorg.
Chem. 1989, 28, 938.
S0020-1669(95)01566-7 CCC: $12.00 © 1996 American Chemical Society

4780 Inorganic Chemistry, Vol. 35, No. 16, 1996
Kohle et al.
Glaser AG, respectively. cis-Ru(dcbpyH2)2(NCS)2, nanocrystalline TiO2
electrodes (prepared according to a sol-gel procedure described
previously7), and electrolyte (34% N-methyloxazolidin-2-one, 32%
acetonitrile, 34% diethyl ketone, 0.5 M 1-hexyl-3-methylimidazolium
iodide, and 40 mM iodine) for solar cell measurements were obtained
from Solaronix SA. 13C-enriched potassium selenocyanate was syn-
thesized according to a published procedure by using 13C-enriched
potassium cyanide and selenium.41 The crude product was dissolved
in methanol, the mixture was filtered, the filtrate was evaporated to
dryness, and the residue was used as such. In the 13C NMR spectrum
of the colorless crystalline product, cyanide was not detectable.
Sephadex LH-20 (Pharmacia) was used for the chromatographic
purification of the Ru complexes. The ligand 2,6-bis(1-methylbenz-
imidazol-2-yl)pyridine was obtained in two steps according to a
published procedure.42,43 4,4′-Dicarboxy-2,2′-bipyridine44 containing
4-carboxy-4′-methylbipyridine was purified as follows. The crude
product containing 8% of the monomethyl species was dissolved in
sulfuric acid (96%) and precipitated by diluting the acid carefully with
water to a concentration of 20%. The product was filtered off and
washed with water. Repeating this procedure three times yielded a
pure product. The 4,4′-dicarboxy-2,2′-bipyridine was dissolved in
diluted base, precipitated by using an excess of 20% HCl, filtered off,
and dried under vaccum. cis-Ru(dcbpyH2)2Cl2 was synthesized by a
procedure similar to that described earlier8 and purified on a Sephadex
LH-20 column using methanol.
Synthesis of the Ligand and Complexes. (a) Ru(bmipy)Cl3. A
500 mg (1.47 mmol) sample of 2,6-bis(1-methylbenzimidazol-2-yl)-
pyridine was dissolved in 200 mL of ethanol. To the stirred solution
of the ligand was added an equimolar solution of ruthenium(III) chloride
(352 mg, 42.34% Ru) in 30 mL of ethanol, and the mixture was refluxed
in the dark under argon for 1 h. After cooling at 25 °C, the precipitate
was filtered off. Washing with ethanol and drying of the product in
vacuo yielded a brown product. Yield: 92%. Anal. Calcd for
Ru(bmipy)Cl3‚2.5H2O: C, 42.62; H, 3.75; N, 11.83. Found: C, 42.80;
H, 3.14; N, 11.83.
(b) Ru(ph-bmipy)Cl3. The complex was obtained in the same way
as described above by using 500 mg (1.20 mmol) of 2,6-bis(1-
methylbenzimidazol-2-yl)-4-phenylpyridine instead of 2,6-bis(1-meth-
ylbenzimidazol-2-yl)pyridine and 287 mg (1.20 mmol) of ruthenium(III)
chloride. Anal. Calcd for Ru(ph-bmipy)Cl3‚H2O: C, 50.60; H, 3.62;
N, 10.93. Found: C, 50.63; H, 3.76; N, 10.69.
An important goal in recent publications on Ru sensitizers
has been the structural variation of the complexes in order to
obtain sensitizers showing increased absorption coefficients for
enhanced light harvesting. Several attempts have been made.
Low π*-level ligands31 such as 5,5′-dicarboxy-2,2′-bipyridine
(π*-level tuning) resulted in a red-shifted spectrum and increased
light absorption at longer wavelengths.10 Unfortunately, the use
of low π*-level ligands was accompanied by strongly reduced
injection yields in the conduction band of TiO2. Polynuclear
complexes exhibiting an antenna effect have been employed in
order to increase absorption coefficients.4,5,11 However, the
antenna does not enhance the light response efficiently at longer
wavelengths, where absorption coefficients and the IPCE of Ru
charge-transfer sensitizers decrease strongly. Moreover, these
bulky sensitizers require more space on the TiO2 surface and
penetrate less easily in the small cavities of the nanocrystalline
TiO2 than the mononuclear complexes. Hence, for polynuclear
complexes, the increased absorption coefficients in solution do
not necessarily lead to enhanced light absorption on the TiO2
electrode because of the reduced surface concentration of the
bulkier sensitizer molecules on the nanoporous TiO2.
Phenyl groups introduced in a suitable position of a polypy-
ridyl ligand in a Ru complex increase the absorption coefficients
of the metal to ligand charge transfer (MLCT) maxima.32-39
Visible spectra of Ru sensitizers containing the 4,4′-bis(p-
carboxyphenyl)-2,2′-bipyridine show this effect.37 The intro-
duction of phenyl groups between the peripheral carboxyl group
and bipyridine caused a red shift and increased absorption
coefficients of the MLCT maxima. However, the sensitization
of TiO2 was found to be inefficient. In general, Ru polypyridyl
sensitizers already show suitable absorption coefficients in the
wavelength domain between 400 and 600 nm. In this work,
we focus our attention on longer wavelengths, where insufficient
light absorption of Ru sensitizers limits its performance in a
solar cell.
Due to the high mass of ruthenium, its complexes exhibit
significant spin-orbit coupling.40 We attempted to enhance the
red response by using the heavy iodide ligand in order to
increase the spin-orbit coupling constant of the complex and
the absorption coefficient of the spin-forbidden 3MLCT transi-
tion.
(c) Ru(bmipy)(dcbpyH)Cl. To 925 mg (1.56 mmol) of Ru(bmipy)-
Cl3‚2.5H2O and 400 mg (1.64 mmol) of 4,4′-dicarboxy-2,2′-bipyridine
in 250 mL of DMF was added 3 mL of triethylamine. The mixture
was refluxed for 3 h in the dark under argon. During this time, the
color of the solution turned violet and a solid precipitated. The solution
was allowed to cool at room temperature. The precipitate was filtered
off, washed with DMF and a small amount of ethanol, and dried.
Yield: 83%. Anal. Calcd for Ru(bmipy)(dcbpyH)Cl‚3H2O: C, 48.95;
H, 3.86; N, 12.11. Found: C, 48.45; H, 4.07; N, 12.60.
(d) Ru(ph-bmipy)(dcbpyH)Cl. The complex was synthesized by
reacting 400 mg (1.64 mmol) of 4,4′-dicarboxy-2,2′-bipyridine with
1.0 g (1.56 mmol) of Ru(ph-bmipy)Cl3‚H2O as described above. Anal.
Calcd for Ru(ph-bmipy)(dcbpyH)Cl‚H2O: C, 57.60; H, 3.72; N, 12.06;
Cl, 4.36; Ru, 12.43. Found: C, 56.74; H, 4.29; N, 12.08; Cl, 4.23;
Ru, 12.28.
Experimental Section
Materials. Reagents and solvents were purchased from Fluka AG
and used without further purification, unless otherwise stated. 13C-
enriched potassium cyanide (92% 13C) and 13C-enriched potassium
thiocyanate (99% 13C) are products from The British Oxygen Co. and
(29) Chakravarty, B.; Adhikari, S. Indian J. Chem. 1991, 30A, 692.
(30) Pechy, P.; Rotzinger, F. P.; Nazeeruddin, M. K.; Kohle, O.; Zakeer-
uddin, S. M.; Humphry-Baker, R.; Gra¨tzel, M. J. Chem. Soc., Chem.
Commun. 1995, 65.
(e) Na[Ru(bmipy)(dcbpy)I]. A 320 mg (0.41 mmol) sample of
Ru(bmipy)(dcbpyH)Cl‚3H2O was dissolved in 150 mL of methanol and
1 mL of triethylamine. A 1.8 g (120 mmol) amount of sodium iodide
was added, and the mixture was refluxed for 3 h in the dark under
argon. After evaporation to dryness, the solid was redissolved in a
minimum amount of methanol, and the solution was added to the top
of a LH-20 column (3 × 20 cm). Chromatography was carried out
under reduced light by first using a solution of methanol containing
40 g/L sodium iodide. When the front of the product band passed
half-way through the column, elution was continued with pure methanol.
(31) Anderson, P. A.; Strouse, G. F.; Treadway, J. A.; Keene, R. K.; Meyer,
T. J. Inorg. Chem. 1994, 33, 3863.
(32) Lin, C.-T.; Boettcher, W.; Chou, M.; Creutz, C.; Sutin, N. J. Am. Chem.
Soc. 1976, 98, 6536.
(33) Stone, M. L.; Crosby, G. A. Chem. Phys. Lett. 1981, 79, 169.
(34) Cook, M. J.; Thomson, A. J. Chem. Br. 1984, 914.
(35) Cook, M. J.; Lewis, A. P.; McAuliffe, G. S. G.; Skardar, V.; Thomson,
A. J.; Glasper, J. L.; Robbins, D. J. J. Chem. Soc., Perkin Trans. 1984,
2, 1293.
(36) Phifer, C. C.; McMillin, D. R. Inorg. Chem. 1986, 25, 1329.
(37) Kalyanasundaram, K.; Nazeeruddin, M. K.; Gra¨tzel, M.; Viscardi, G.;
Savarino, P.; Barni, E. Inorg. Chim. Acta 1992, 198-200, 831.
(38) Constable, E. C.; Cargill Thompson, A. M. W.; Tocher, D. A.; Daniels,
M. A. M. New J. Chem. 1992, 16, 855.
(41) Waitkins, G. R.; Shutt, R. Inorg. Synth. 1946, 2, 186.
(42) Addison, A. W.; Burke, P. J. J. Heterocycl. Chem. 1981, 18, 803.
(43) Piguet, C.; Bocquet, B.; Mu¨ller, E.; Williams, A. F. HelV. Chim. Acta
1989, 72, 323.
(44) Bos, K. D.; Kraaijkamp, J. G.; Noltes, J. G. Synth. Commun. 1979, 9,
497.
(39) Maestri, M.; Armaroli, N.; Balzani, V.; Constable, E. C.; Cargill
Thompson, A.M. W. Inorg. Chem. 1995, 34, 2759.
(40) (a) Kober, E. M.; Meyer, T. J. Inorg. Chem. 1982, 21, 3967. (b) Juris,
A.; Balzani, V.; Barigelletti, F.; Campagna, S.; Belser, P.; von
Zelewsky, A. Coord. Chem. ReV. 1988, 84, 85.

Ru(II) Charge-Transfer Sensitizers
Inorganic Chemistry, Vol. 35, No. 16, 1996 4781
The product band was collected and the solvent evaporated to a volume
of 30-40 mL. Diethyl ether was slowly added until the product
precipitated. The solid was filtered off, washed with a small amount
of a 2:1 mixture of diethyl ether/methanol, and dried in vacuo. Yield:
79%. The product contained about 3% sodium iodide. Anal. Calcd
for Na[Ru(bmipy)(dcbpy)I]‚3H2O, containing 3.3% sodium iodide: C,
43.24; H, 3.19; N, 10.70; I, 16.61. Found: C, 43.38; H, 3.01; N, 10.65;
I, 16.75.
(f) Na[Ru(ph-bmipy)(dcbpy)I]. The complex was synthesized and
purified in the same way as described above by using 320 mg (0.39
mmol) of Na[RuII(ph-bmipy)(dcbpyH)Cl]‚H2O as starting material. The
material contained between 3 and 5% sodium iodide. Elemental
analysis gave a C:N ratio of 4.71; calcd 4.77.
(g) K[Ru(bmipy)(dcbpy)(NCS)]. A 200 mg (0.22 mmol) sample
of Na[Ru(bmipy)(dcbpy)I] was dissolved in 25 mL of methanol, and
1.0 g (10.3 mmol) of potassium thiocyanate was added. The mixture
was refluxed for 6 h in the dark under argon, and the solution was
allowed to cool at room temperature. The N-bound K[Ru(bmipy)-
(dcbpy)(NCS)] complex crystallized slowly from the reaction mixture.
The dark needles were filtered off, washed with a small amount of
cold methanol, and dried in air. The product contained small amounts
of S-bound isomer. Anal. Calcd for K[Ru(bmipy)(dcbpy)(NCS)]‚
4H2O: C, 47.94; H, 3.67; N, 13.15; K, 4.59; S, 3.76. Found: C, 47.93;
H, 3.63; N, 12.73; K, 4.89; S, 3.95.
Figure 1. Structure of Ru(bmipy)(dcbpyH)I.
8452A diode array spectrophotometer at room temperature using quartz
cells of 1 cm path length. Cyclic voltammetry was performed with an
EG&G 273A potentiostat in electrochemical cells with a volume of 5
mL. A three-electrode setup was composed of a glassy carbon working
electrode (surface 0.07 cm2, embedded in Teflon), a glassy carbon
counter electrode separated from the working electrode compartment
by a bridge containing the same electrolyte as the test solution, and a
silver/silver chloride wire as a quasi reference electrode. The quasi
reference electrode was calibrated with a Ag/AgCl/saturated KCl
electrode. If not stated otherwise, all potentials were measured in
1-methyl-2-pyrrolidone (Fluka) dried over molecular sieves. In all
experiments, 0.1 M tetrabutylammonium trifluoromethanesulfonate
(Fluka) was used as electrolyte. Scan rates were between 100 and 300
mV/s. Action spectra were measured according to a procedure
described elsewhere.8 The IPCE values reported are overall yields
which are uncorrected for losses due to light absorption and reflection
by the conducting glass support. Coating of electrodes was carried
out by soaking the TiO2 films in a solution of the Ru complex in ethanol
until the layer was intensely colored. After adsorption of the complexes,
the layers were treated for a few seconds with acetic acid in order to
control the degree of protonation on the TiO2 surface. The treatment
with acetic acid resulted in increased IPCE values and a good
reproducibility of the measurements.
The mother liquor was evaporated and the resulting solid redissolved
in 10 mL of water. The solution was acidified dropwise with 5% HCl
to precipitate the product at the isoelectric point. The precipitated solid
contained a mixture of S-bound and N-bound complex. It was washed
with water and dried in vacuo. Working in this manner yielded a
product containing about 80% N-bound and 20% S-bound complex.
Anal. Calcd for Ru(bmipy)(dcbpyH)(NCS)‚2.5H2O: C, 51.90; N,
14.24; H, 3.72; S, 4.07. Found: C, 52.10; N, 14.20; H, 3.35; S, 3.97.
(h) K[Ru(ph-bmipy)(dcbpy)(NCS)]. The complex was synthesized
as described above by using Na[Ru(ph-bmipy)(dcbpy)I] as starting
material. Dark crystals of the N-bound complex were obtained;
1
S-bound complex was not detectable by H NMR spectroscopy. The
mother liquor was enriched with S-bound complex. Anal. Calcd for
K[Ru(ph-bmipy)(dcbpy)(NCS)]‚H2O: C, 54.97; N, 12.82; H, 3.34; S,
3.67. Found: C, 55.11; N, 12.95; H, 3.43; S, 3.45.
Results and Discussion
(i) 2,6-Bis(1-methylbenzimidazol-2-yl)-4-phenylpyridine. The
ligand was synthesized in six steps according to the preparation of a
similar ligand reported in the literature.45 The product was recrystallized
from methanol. Mp: 251 °C. C27H21N5. Anal. Calcd: C, 78.05; H,
5.09; N, 16.86 Found: C, 78.00; H, 5.08; N, 16.93. 13C NMR data
(CDCl3): a 15-line spectrum with peaks at 150.46, 150.27, 150.01,
142.52, 137.12, 136.93, 129.53, 129.00, 127.22, 123.53, 122.92, 122.77,
120.10, 109.85, 32.48 ppm. 1H NMR data (CDCl3) (multiplicity,
integral): 8.67 (s, 2), 7.93-7.83 (m, 4), 7.58-7.28 (m, 9), 4.26 ppm
(s, 6).
Methods. 1H- and proton-decoupled 13C NMR spectra were
recorded with a Bruker AC-P 200 spectrometer. Chemical shifts are
given in ppm, relative to TMS. The substitution reaction of Na[Ru-
(bmipy)(dcbpy)I] and cis-Ru(dcbpyH2)2Cl2 with nucleophilic agents
(potassium salts of CN-, NCS-, and NCSe-) were followed by reacting
a few milligrams of the precursor complex with about 8 mg of the
potassium salt of the pseudohalide in methanol-d4 solution in an NMR
tube. The reaction was carried out by heating the NMR tube in an oil
bath in the dark; the bath temperature was between 50 and 75 °C. NMR
spectra were recorded after cooling the NMR tube to room temperature.
Ligand exchange is slow at room temperature; the substitution reaction
can be stopped when desired. Repeated heating, cooling, and recording
of the NMR spectra resulted in the presented figures. Exact conditions
(reaction time, temperature, and concentration) are reported under
Results and Discussion. All thermal reactions and measurements were
carried out under exclusion of light. Pulse repetition time for the 13C
NMR measurements was 3 s.
The nucleophilic substitution of coordinated iodide in
[Ru(bmipy)(dcbpy)I]- by X- ) CN-, NCS-, and NCSe- and
of the chloride in cis-Ru(dcbpyH2)2Cl2 by NCS- was studied
1
in methanol-d4 by H and 13C NMR spectroscopy using 13C-
enriched ligands X-:
[Ru(bmipy)(dcbpy)I]- + X- f
[Ru(bmipy)(dcbpy)X]- + I- (1)
cis-Ru(dcbpyH2)2Cl2 + 2NCS- f
cis-Ru(dcbpyH2)2(NCS)2 + 2Cl- (2)
Na[Ru(bmipy)(dcbpy)I] was a suitable precursor for these
studies. Chromatography yielded a product that was pure by
NMR criteria. It showed a sufficient solubility in methanol-
d4, which was found to be a useful solvent for studying the
substitution reactions. Figure 1 shows the structure of the Ru-
(bmipy)(dcbpyH)I complex.
1
Spectra a-d of Figure 2 monitor portions of the H NMR
spectra recorded during the reaction of the iodo complex with
cyanide. Only the signal of the 6H doublet of the complex is
shown. The chemical shift of the 6H doublet is strongly
influenced when the iodide ligand is replaced by other nucleo-
philes; other signals superpose more or less. Spectrum a of
Figure 2 corresponds to a solution of Na[Ru(bmipy)(dcbpy)I]
and 13C-enriched potassium cyanide in methanol-d4. When the
mixture was heated, the iodo complex disappeared quickly and
two new doublets appeared. In the 13C NMR spectrum, apart
Infrared spectra were obtained from KBr pellets with a Perkin-Elmer
Paragon 1000 spectrophotometer. Electronic spectra in the UV-vis
range were measured in methanol solution with a Hewlett Packard
(45) Bochet, C. G.; Piguet, C.; Williams, A. F. HelV. Chim. Acta 1993,
76, 372.

4782 Inorganic Chemistry, Vol. 35, No. 16, 1996
Table 1. 1H NMR Data (in Methanol-d4)
Kohle et al.
complex
1′
2′
3′
4′
5′
6′
7′
o
m/p
Li[Ru(bmipy)(dcbpy)Cl]
8.26
8.74
d
8.75
d
8.78
d
8.84
s
4.54
s
4.53
s
4.57
s
4.62
s
7.71
7.42
ddd
7.42
ddd
7.45
ddd
7.43
ddd
7.46
ddd
7.10
ddd
7.10
ddd
7.14
ddd
7.13
ddd
7.15
ddd
6.28
d
6.33
d
6.25
d
6.36
d
t
8.27
t
d
Na[Ru(bmipy)(dcbpy)I]
7.70
d
K[Ru(bmipy)(dcbpy)(NCS)]
Na[Ru(ph-bmipy)(dcbpy)I]
K[Ru(ph-bmipy)(dcbpy)(NCS)]
8.34
t
7.74
d
7.70-7.80
8.23
d
8.26
d
7.70-7.80
m
7.75
d
m
7.70-7.80
m
8.87
s
4.64
s
6.28
d
complex
1
2
3
4
5
6
Li[Ru(bmipy)(dcbpy)Cl]
7.50
7.35
8.78
d
8.74
d
8.80
d
8.76
d
9.26
d
9.23
d
9.26
d
9.24
d
8.50
10.72
d
11.27
d
10.23
d
11.29
d
d
dd
dd
8.41
dd
Na[Ru(bmipy)(dcbpy)I]
7.35-7.40
7.35-7.40
m
7.47
d
m
7.39
dd
K[Ru(bmipy)(dcbpy)(NCS)]
Na[Ru(ph-bmipy)(dcbpy)I]
K[Ru(ph-bmipy)(dcbpy)(NCS)]
8.53
dd
8.42
dd
8.55
dd
7.45
m
7.56
d
7.40-7.45
m
7.41
dd
8.87
d
9.27
d
10.25
d
1
Figure 2. Substitution reaction of 3 mg of Na[Ru(bmipy)(dcbpy)I]
with 10 mg of KCN in 0.6 mL of methanol-d4 followed by H NMR
spectroscopy: (a) before heating; (b) +1 h at 70 °C; (c) +1 h at 70
°C; (d) +10 h at 70 °C.
Figure 3. Portions of the H NMR spectra for the reaction of 13C-
enriched potassium thiocyanate (7 mg) with Na[Ru(bmipy)(dcbpy)I]
(2 mg) in 0.6 mL of methanol-d4: (a) before heating; (b) +30 min at
70 °C; (c) +30 min at 70 °C; (d) +1 h at 70 °C; (e) +2 h at 70 °C; (f)
+24 h at 70 °C.
1
from the signal at 163.5 ppm (free cyanide), only one intense
peak at 154.4 ppm showing coordinated cyanide was detectable.
The second species appearing as an intermediate product in
spectrum b of Figure 2 at 10.20 ppm corresponds to the hydroxo
complex. Iodide is easily substituted by water, yielding an aquo
complex. The neutral aquo complex is not soluble in methanol
and hence escapes detection by NMR. Cyanide acts as a base
and deprotonates the aquo complex, forming the hydroxo
complex. In spectrum d of Figure 2, the iodo and the hydroxo
complex have completely reacted with the strong nucleophile
cyanide, yielding the C-bound cyano complex (doublet at 10.60
ppm).46,47
trum a of Figure 3 presents the reaction mixture before the tube
was heated. After 30 min of heating, two new doublets
indicated two products (spectrum b). The corresponding 13C
NMR spectra (Figure 4) exhibited two new signals which are
attributed to two differently coordinated thiocyanates. The
signal of ionic thiocyanate was found at 133.6 ppm, that of the
N-bound at 134.4 ppm, and that of the S-bound at 123.4 ppm.
Weaker signals appearing in the 13C NMR spectrum cor-
responded to the carbon atoms of the organic complex ligands.
The weak downfield shift of 0.8 ppm for the N-bound isomer
and the relatively strong upfield shift of the S-bound isomer
compared to free thiocyanate are in agreement with literature
data for transition metal complexes of thiocyanate.12,13 After
1 h of heating, the N-:S-bound ratio in the reaction mixture
was 2:1; prolonged heating for 24 h converted most of the
S-bound isomer to the N-bound. In Figure 3, the doublet at
10.24 ppm is attributed to the N-bound isomer and the doublet
The substitution of iodide by thiocyanate gave a different
1
result. Figures 3 and 4 illustrate the reaction followed by H
(6H doublet) and 13C NMR spectroscopy, respectively. Spec-
(46) Pesek, J. J.; Mason, W. R. Inorg. Chem. 1979, 18, 924.
(47) Jackson, W. G.; Rahman, A. F. M. M. Inorg. Chem. 1990, 29, 3247
and references cited therein.

Ru(II) Charge-Transfer Sensitizers
Inorganic Chemistry, Vol. 35, No. 16, 1996 4783
Figure 4. 13C NMR spectra corresponding to the H NMR spectra
presented in Figure 3: (a) spectrum c of Figure 3; (b) spectrum e of
Figure 3.
1
Figure 5. Portion of the IR spectrum in KBr of (a) N-bound K[Ru-
(bmipy)(dcbpy)(NCS)], (νCN ) 2110 cm-1) and (b) a sample containing
21% S- and 79% N-bound forms of the same complex (S-bound
complex: νCN ) 2056 cm-1).
at 10.56 ppm to the S-bound isomer. Conversion of the S-bound
to the thermodynamically more stable N-bound isomer was not
complete even after 24 h of reflux (spectrum f), and about 5%
S-bound isomer remained. The binding constants of the N- and
S-bound thiocyanates appear to be similar. The proportion of
the isomers was determined by integrating the NMR signals of
the 6H doublets.
Illuminating an NMR tube containing the iodo complex and
potassium thiocyanate in methanol-d4 for 1 h at room temper-
ature by a xenon lamp equipped with a 420 nm cutoff UV filter
gave the same result of a 2:1 ratio of the linkage isomers. The
N-bound isothiocyanato complex was found to be quite insoluble
in methanol and crystallized slowly (concentration of the
precursor complex should be about 4-8 mg/mL of methanol)
from the supersaturated reaction mixture. Form and size of the
crystals depended on the excess of potassium thiocyanate used
in the reaction mixture. The reaction with sodium thiocyanate
instead of potassium thiocyanate also yielded crystals. After
crystallization, the mother liquor was enriched with the S-bound
isomer. We could not observe the formation of a dimeric
species containing bridging thiocyanate when the iodo complex
was reacted with an equimolar amount of isothiocyanato
complex in methanol-d4 without potassium thiocyanate. The
reaction did not yield any NMR-detectable product.
Our results on the linkage isomers are supported by infrared
spectroscopy. Figure 5 shows a portion of the IR spectrum of
the crystalline isothiocyanato complex and a sample containing
both isomers with an N:S ratio of 79%:21%, determined by 1H
NMR spectroscopy. In agreement with the literature,16 the νCN
band of the S-bound isomer is shifted to lower energies, relative
to those of the N-bound species. The band of a dimer Ru
complex containing a bridged thiocyanate should be shifted to
higher energies.27 The value of the S-bound complex νCN band
is within the region commonly associated with ionic thiocyanate.
The use of 13C NMR chemical shifts of coordinated pseudoha-
lides was demonstrated to be a powerful diagnostic tool for
bonding mode determination of these groups.12,13 The NMR
results clearly point out the advantage of 13C NMR measure-
ments. The 13C NMR chemical shift reveals that the ion is
coordinated and S-bound.12,13
The phenylated [Ru(ph-bmipy)(dcbpy)I]- complex behaved
in a similar way, forming N- and S-bound linkage isomers in a
2:1 ratio, respectively. The N-bound complex also yielded dark
crystals. The additional phenyl group did not affect the shift
of the 13C NMR peaks of the coordinated N- and S-bound
thiocyanates significantly in comparison to the 13C NMR
spectrum of [Ru(bmipy)(dcbpy)(NCS)]-. When the solutions
of K[Ru(bmipy)(dcbpy)(NCS)] and K[Ru(ph-bmipy)(dcbpy)-
(NCS)] in methanol were exposed to intense visible light, we
could observe decomposition and formation of the cyano
complexes.
The selenocyanate anion behaved in a more complex fashion
than the thiocyanate anion when introduced into the complex.
Figures 6 and 7 monitor the reaction of the [Ru(bmipy)(dcbpy)I]-
with selenocyanate, followed by 1H and 13C NMR spectroscopy.
When the reaction mixture was heated, the isomeric N- and
Se-bound complexes appeared in nearly equal amounts. First,
the disappearance of the 6H doublet of the iodo complex was
accompanied by the appearance of three doublets (spectrum b
of Figure 6). Thereafter, in spectrum c of Figure 6, a fourth
doublet was visible. 13C NMR spectroscopy allowed us to
assign the different species. The line at 119.8 ppm in the 13C
NMR spectrum is attributed to the free selenocyanate anion,
and the two weaker lines at 122.5 and 117.1 ppm are assigned
to free NC77Se-. The signal shifted 12.1 ppm upfield at 107.6
ppm represents an Se-bound ligand, while the signal downfield
shifted at 121.7 ppm is attributed to N-bound selenocyanate.
The line strongly downfield shifted at 154.7 ppm indicates
decomposition via loss of Se from coordinated selenocyanate
and the formation of the cyano complex. After a reaction time
of 2 h under reflux, most of the iodo complex had been
1
converted. Monitoring the H NMR spectrum of the sample
left at room temperature in the dark showed the instability of
the isoselenocyanato complex. Spectrum e shows a decreased
intensity for the doublet at 10.22 ppm (isoselenocyanato
complex) and an increased intensity for the doublet at 10.58
ppm (cyano complex) after a period of 48 h at room temperature.
Further heating of the NMR tube transformed the Se-bound

4784 Inorganic Chemistry, Vol. 35, No. 16, 1996
Kohle et al.
Figure 6. Reaction of 3.5 mg of Na[Ru(bmipy)(dcbpy)I] with 6 mg
of KN13CSe in 0.6 mL of methanol-d4 followed by 1H NMR
spectroscopy: (a) before heating; (b) +30 min at 75 °C; (c) + 30 min
at 75 °C; (d) + 1 h at 75 °C; (e) +48 h at room temperature; (f) +3.5
h at 75 °C; (g) +14 h at 75 °C.
Figure 8. Reaction route to cis-Ru(dcbpyH2)2(NCS)2 from cis-
Ru(dcbpyH2)2Cl2 and potassium thiocyanate in methanol.
The formation of S-bound isomers was also detected with
cis-Ru(dcbpyH2)2(NCS)2 synthesized as previously described.8
1
The H NMR spectrum of the product always indicated the
presence of byproducts. These weak additional NMR signals
were assigned to the mixed N-/S-bound cis-Ru(dcbpyH2)2-
(NCS)(SCN) complex. For a complex containing two ligand
sites for substitution, several isomeric species are possible.
During the reaction of the precursor complex cis-Ru(dcbpyH2)2-
Cl2 with thiocyanate, all possible isomers, cis-Ru(dcbpyH2)2-
(NCS)2, cis-Ru(dcbpyH2)2(NCS)(SCN), and cis-Ru(dcbpyH2)2-
(SCN)2, as well the two monochloro intermediate species cis-
Ru(dcbpyH2)2Cl(NCS) and cis-Ru(dcbpyH2)2Cl(SCN), were
identified on the basis of in situ 1H and 13C NMR data and the
trends observed for the reaction of K[Ru(bmipy)(dcbpy)I] with
Figure 7. 13C NMR spectra of the substitution reaction described in
Figure 6. Corresponding spectra: (a) spectrum c of Figure 6; (b)
spectrum d of Figure 6; (c) spectrum g of Figure 6.
isomer to the N-bound, and decomposition of the latter yielded
cyano complex. Decomposition was nearly complete after 20
h of heating (spectrum g of Figure 6). Instability was found
only for coordinated selenocyanate; free selenocyanate did not
decompose to cyanide. In the 13C NMR spectrum, no signal at
163.5 ppm corresponding to free cyanide was detectable.
During our 13C NMR measurement, we could not observe a
signal for N-bound cyanide which should be formed after loss
of Se. This can be explained by the low stability and the fast
isomerization of the N-bound cyano complex to the C-bound
species. The additional doublet visible in spectrum b of Figure
6 at 10.20 ppm arises from the hydroxo complex, indicating
basic conditions in the reaction mixture.
potassium thiocyanate. Simultaneously measuring 1H and 13
C
NMR spectra during reaction with 13C-enriched thiocyanate,
monitoring the appearance and disappearance of the signals,
and comparing the intensities allowed us to correlate the signals.
Figure 8 illustrates the reaction route in detail. Figure 9 shows
1
the change in the H NMR spectrum of the 6H protons of the
two 4,4′-dicarboxy-2,2′-bipyridine ligands during the reaction
of cis-Ru(dcbpyH2)2Cl2 with thiocyanate; Figure 10, the change
in the 13C NMR spectrum. In the 1H NMR spectrum, a complex
containing two different types of monodentate ligands gives two
doublets of the same intensity for the 6H protons, while for a
complex containing two identical ligands, only one doublet is

Ru(II) Charge-Transfer Sensitizers
Inorganic Chemistry, Vol. 35, No. 16, 1996 4785
Figure 10. 13C NMR spectra of the substitution reaction described in
Figure 9. Corresponding spectra: (a) spectrum c of Figure 9; (b)
spectrum e of Figure 9; (c) spectrum f of Figure 9.
complex and the mixed chloro/thiocyanato complex. The 13C
NMR spectrum shows an additional signal at 125.0 ppm for
the S-bound thiocyanate; the signal of the chloro/isothiocyanato
complex could be covered by the signal of the bis(isothiocy-
anato) complex. The signals of the monochloro complexes
disappeared quickly due to the fast exchange of chloride by
thiocyanate. Spectrum g of Figure 9 represents the final state
of the reaction after additional refluxing for 16 h. It should be
noted that most of the reaction time for the synthesis of cis-
Ru(dcbpyH2)2(NCS)2 is required for the isomerization of
S-bound thiocyanate, the chloro ligand being substituted within
a period of 1 h by thiocyanate under reflux in higher boiling
point solvents. We could not obtain cis-Ru(dcbpyH2)2(NCS)2
completely free of the mixed N-/S-bound complex even by using
higher boiling point solvents such as DMF/water mixtures.
Nevertheless, it was possible to reduce the amount of the mixed
N-/S-bound isomer to about 2% in higher boiling solvents under
prolonged refluxing.
Figure 9. Reaction of cis-Ru(dcbpyH2)2Cl2 with 13C-enriched thiocy-
anate followed by 1H NMR: (a) before heating; (b) +1 h at 55 °C; (c)
+1 h at 55 °C; (d) +12 h at room temperature; (e) +2 h at 55 °C; (f)
+2 h at 55 °C; (g) +16 h at 75 °C. Figure 10 shows the corresponding
13C NMR spectra.
obtained. As expected, the five intermediate and final species
shown in Figure 8 gave eight new doublets for the 6H protons
1
in H NMR spectra b and c of Figure 9. According to Figure
8, three different complexes containing S-bound thiocyanate
were visible in the domain between 124 and 125 ppm of the
corresponding 13C NMR spectrum, spectrum a of Figure 10.
Spectrum a of Figure 9 corresponds to the dichloro complex
(doublet at 10.20 ppm), contaminated with small amounts of
the mixed aquo/chloro complex (doublets at 10.27 and 9.44
ppm). The chloro ligand reacted easily with water in methanol
solution; substitution of chloride by traces of water occurred
slowly in the dark even at room temperature. Spectra f and g
of Figure 9 represent the final products: the bis(isothiocyanato)
complex (doublet at 9.65 ppm) and the mixed N-/S-bound
complex (two smaller doublets of the same intensity at 9.98
and 9.59 ppm). The corresponding 13C NMR spectrum,
spectrum c of Figure 10, shows four signals: a broad signal at
133.6 (free thiocyanate), a strong signal at 135.4 (bis(isothio-
cyanato) complex), and two weaker ones at 136.0 and 124.2
ppm (mixed N-/S-bound complex). The additional doublet in
spectrum e of Figure 9 at 9.84 ppm corresponds to the
bisthiocyanato complex, containing two S-bound ligands. The
corresponding 13C NMR spectrum, spectrum b of Figure 10,
shows a further peak at 124.1 ppm, indicating S-bound
thiocyanate. Spectra b-d of Figure 9 show four additional
doublets, corresponding to the mixed chloro/isothiocyanato
In general, the relation between the mode of binding in mixed-
ligand ruthenium thiocyanate complexes and the role of the
coligands is still not very clear. Electronic, steric, and solvent
factors largely influence the bonding mode of pseudohalides.
In the case of bipyridine as a coligand, the S-bound mode has
been ruled out by earlier workers.14,20,28 The appearance of
small amounts of S- and Se-bound linkage isomers as byproducts
was observed for the complexes Ru(bipy)2ClY (Y ) NCS-,
NCSe-; bipy ) bipyridine).26,27 In contrast to the behavior of
cis-Ru(bipy)2(NCS)2 reported in the literature,14,20,28 we have
evidence for isomers containing S-bound thiocyanate in the case
of the carboxylated complex.
Our findings on the 13C chemical shifts of the N-, S-, and
Se-bound isomers are in good agreement with literature.12,13
N-bound thio- and selenocyanates are weakly shifted downfield
and S- and Se-bound upfield, compared to ionic thio/seleno-
cyanates. For the mixed complex cis-Na4[Ru(dcbpy)2(NCS)-
(SCN)] in D2O/NaOD solution, we could observe a weakly
upfield shifted signal at 132.8 ppm for N-bound thiocyanate
(ionic: 134.2 ppm) and a signal at 126.8 ppm for the S-bound
1
thiocyanate. cis-Na4[Ru(dcbpy)2(NCS)2]: H NMR data (D2O/
0.05 M NaOD) 9.50 (d, 6H, 6 Hz), 8.89 (d, 3H, 1.5 Hz), 8.72
(d, 3′H, 1.5 Hz), 8.16 (dd, 5H, 6 Hz, 1.5 Hz), 7.77 (d, 6′H, 6

4786 Inorganic Chemistry, Vol. 35, No. 16, 1996
Kohle et al.
Table 2. Electrochemical and Absorption Data
Ea/V
abs maxb/nm (ꢀ/103 M-1 cm-1
)
complex
oxidn
redn
K[Ru(bmipy(dcbpy)(NCS)]
K[Ru(ph-bmipy)(dcbpy)(NCS)]
Li[Ru(bmipy)(dcbpy)Cl]
Na[Ru(bmipy)(dcbpy)I]
0.83
0.99d
0.69c
0.70
0.79
-1.25
-1.16
-1.28c
-1.35
-1.20
308 (45.8)
308 (62.4)
310 (42.7)
310 (48.6)
310 (59.5)
346 (30.8)
348 (30.7)
344 (29.2)
346 (31.6)
350 (30.3)
360 (40.0)
364 (38.1)
360 (38.8)
362 (33.4)
362 (31.3)
500 (14.0)
508 (18.0)
512 (14.0)
514 (14.0)
522 (17.5)
Na[Ru(ph-bmipy)(dcbpy)I]
a Potentials versus Ag/AgCl/satd KCl. b Measured in methanol. c Measured in DMSO. d Irreversible.
Figure 12. Photoaction spectra of Na[Ru(bmipy)(dcbpy)I] (- - - -),
K[Ru(bmipy)(dcbpy)(NCS)] (- - -), K[Ru(ph-bmipy)(dcbpy)(NCS)]
(-‚-‚-), and cis-Ru(dcbpyH2)2(NCS)2 (s). Spectra are not corrected for
losses of light due to the conductive glass support.
Figure 11. Visible spectra of Na[Ru(bmipy)(dcbpy)I] (s), Li[Ru-
(bmipy)(dcbpy)Cl] (- - -), K[Ru(bmipy)(dcbpy)(NCS)] (- - -), K[Ru-
(ph-bmipy)(dcbpy)(NCS)] (-‚-‚-), and cis-Ru(dcbpyH2)2(NCS)2
(- - - -) in methanol.
is defined by eq I.8 The IPCE values for cis-Ru(dcbpyH2)2-
Hz), 7.47 ppm (dd, 5′H, 6 Hz, 1.5 Hz); 13C NMR data (D2O/
0.05 M NaOD) 172.6 and 172.2 (carboxyl carbons), 159.7 and
156.6 (2, 2′), 154.1 and 153.2 (6, 6′), 145.4 and 144.8 (4, 4′),
132.8 (coord. NCS-), 126.5 and 125.4 (5, 5′), 123.0 and 122.7
ppm (3, 3′). The 6H doublets of cis-Na4[Ru(dcbpy)2(NCS)-
(SCN)] in D2O/0.05 M NaOD were detected at 9.68 (d, 6 Hz)
and 9.48 ppm (d, 6 Hz). Other signals are more or less covered
by the signals of cis-Na4[Ru(dcbpy)2(NCS)2].
IPCE (%) )
(1.25 × 103) × photocurrent density (µA/cm2)
× 100% (I)
wavelength (nm) × photon flux (W/cm2)
(NCS)2 of nearly 80% in the wavelength range between 400
and 600 nm are impressive. Taking into account the losses due
to the conducting glass support, the conversion of the photon
flux into electrical current in the external circuit is practically
quantitative in this wavelength domain. Reduced IPCE values
in the wavelength domain beetween 400 and 520 nm reported
in the literature8,10 are due to the light absorption by the triiodide
anion. Reduced absorption coefficients of the Ru complex limit
light absorption at longer wavelengths above 600 nm; the IPCE
values depend on the thickness of the TiO2 layer. A 8 µm thick
layer gave an IPCE value at 700 nm between 45% and 50%.
For the Ru(bmipy)(dcbpyH)(NCS) sensitizer, we could obtain
an IPCE of about 70% at 520 nm, in agreement with earlier
preliminary investigations.48 In particular, the IPCE values are
reduced at longer and at shorter wavelengths compared to those
of cis-Ru(dcbpyH2)2(NCS)2 which can be explained by a
decreased packing density on the TiO2 surface and by the
absorption spectrum. It is blue-shifted and shows reduced
absorption coefficients at shorter (400-450 nm) and longer
wavelengths above 530 nm. As a consequence, the sensitizer
is not as performant as cis-Ru(dcbpyH2)2(NCS)2, but for
Figure 11 and Table 2 report the visible absorption features
of Li[Ru(bmipy)(dcbpy)Cl], Na[Ru(bmipy)(dcbpy)I], K[Ru-
(bmipy)(dcbpy)(NCS)], K[Ru(ph-bmipy)(dcbpy)(NCS)], and
cis-Ru(dcbpyH2)2(NCS)2 in methanol. The chloro and the iodo
complexes show very similar spectra. The maximum of the
iodo complex is slightly red-shifted; both spectra nearly
superpose at longer wavelengths. We could not observe a
significantly stronger 3MLCT transition in the visible spectrum
for the iodo complex due to a possible heavy-atom effect of
the iodide ligand; the effect is too small. The weaker donor
strength of the thiocyanate results in a blue shift of the spectrum,
compared to those of the halide complexes. The MLCT
maximum of the phenylated K[Ru(ph-bmipy)(dcbpy)(NCS)]
exhibits a red shift and a higher molar absorption coefficient
when compared to that of the unsubstituted isothiocyanato
complex. Unfortunately, the phenylation does not enhance light
absorption at longer wavelengths in the domain between 620
and 750 nm. The spectrum of cis-Ru(dcbpyH2)2(NCS)2 exhibits
a strong red shift and a second MLCT maximum in the near-
UV which is advantageous for efficient light harvesting.
Figure 12 shows a comparison of action spectra. The incident
monochromatic photon-to-current conversion efficiency (IPCE)
(48) Mu¨ller, E.; Nazeeruddin, M. K.; Kohle, O.; Gra¨tzel, M. Unpublished
work.

Ru(II) Charge-Transfer Sensitizers
Inorganic Chemistry, Vol. 35, No. 16, 1996 4787
regenerative cells under simulated AM 1.5 solar light, we could
obtain impressive currents of 12 mA/cm2 and a solar to electric
energy conversion efficiency exceeding 6%.
The absorption spectrum exhibits two maxima for light absorp-
tion in the visible domain; absorption coefficients are relatively
high due to the two bipyridines carrying four carboxyl groups.
The molecule is relatively small and adsorbs efficiently on TiO2,
favoring a dense loading on the surface of porous TiO2 and
efficient light absorption of the TiO2/dye electrode.
The iodo and chloro complexes Na[Ru(bmipy)(dcbpy)X] (X-
) Cl-, I-) showed strongly reduced overall injection efficien-
cies. Compared to that of the thiocyanate ligand, the stronger
electron donating effect of the halide ligands causes an increased
energetic destabilization of the Ru(t2g) level.21 Shifting the Ru-
(t2g) level too strongly may lead to problems with regeneration
of the complex by iodide after electron injection into TiO2 due
to an insufficient driving force for the redox reaction.7,8
However, the difference in the oxidation potentials (Table 2)
of the isothiocyanato and the iodo complex is too small to
explain the strongly decreased IPCE values of the halide
complexes. The halide ligands are very labile and easily
replaced by water. When heating the dye solution or exposing
it to light during absorption on the TiO2 layers, we observed
increased IPCE values due to the formation of aquo complex.
The aquo complex injects electrons more efficiently into TiO2
than the halide complexes.
Conclusions
The introduction of thiocyanate and selenocyanate into Ru-
(II) complexes containing 4,4′-dicarboxy-2,2′-bipyridine resulted
in the formation of linkage isomers in ratios of 2:1 (N:S) and
1:1 (N:Se), respectively. The S- and the Se-bound isomers were
transformed to the N-bound isomers upon heating. In methanol
solution, the isoselenocyanato complex decomposed readily
under loss of Se. 13C NMR spectroscopy is an excellent method
for determining the bonding mode of ambidentate ligands such
as thiocyanate and selenocyanate in Ru polypyridyl complexes.
In particular, by using 13C-enriched ligands, even barely soluble
complexes can be studied. Reacting the precursor complex with
1
a ligand in an NMR tube and combining 13C with H NMR
The phenylated complex Ru(ph-bmipy)(dcbpyH)(NCS) ex-
hibits an IPCE value of 60% at 520 nm. Injection into TiO2 is
slightly reduced due to the phenylation but still quite efficient.
The phenyl group does not influence electron injection as
strongly as was observed for the Ru-4,4′-bis(p-carboxyphenyl)-
2,2′-bipyridine complexes.37 This confirms that the phenyl
group acted as an insulating barrier (distance effect) for the
excited electron when introduced between the bipyridine and
the carboxyl group.
Ru(bmipy)(dcbpyH)(NCS) containing 20% S-bound complex
gave about 10-20% lower IPCE values than the pure N-bound
complex. Apparently, electron injection is influenced by the
bonding mode of thiocyanate.
There are several reasons for the still unmatched performance
of cis-Ru(dcbpyH2)2(NCS)2 as a sensitizer for solar cells. The
π*-level system allows quantitative charge injection into TiO2.
Two thiocyanate ligands shift the absorption spectrum strongly
to longer wavelengths, without shifting the Ru(t2g) level too
strongly for iodide regeneration. A better tuning of the Ru(t2g)
level and the π* level appears very difficult. N-bound thiocy-
anate as a ligand supports quantitative charge injection; replacing
thiocyanate in a Ru sensitizer by stronger donor ligands such
as iodide or chloride influences electron injection negatively.
spectroscopy give more detailed information: Thermodynami-
cally unstable side products, in the case of the Ru complexes
of the S- and Se-bound isomers, can be detected and identified.
The variation of the structure of the model sensitizer K[Ru-
(bmipy)(dcbpy)(NCS)] by introducing a phenyl group or replac-
ing the thiocyanate by iodide resulted in a less performant
sensitizer. The bis(isothiocyanato) complex cis-Ru(dcbpyH2)2-
(NCS)2 is an impressively powerful sensitizer for nanocrystalline
TiO2 solar cells. As discussed in the present paper, it appears
very difficult to obtain a more performant Ru sensitizer. The
synthesis of organic sensitizers is more promising. Absorption
coefficients increased by an order of magnitude and reduced
losses by singlet-triplet relaxation compared to those of Ru
sensitizers promise a further quantum step in developing a more
performant dye solar cell.
Acknowledgment. We thank Peter Hones, Dr. Peter Pe´chy,
Dr. Robin Humphry-Baker, Stefanie Maier, and Dr. Francois
Rotzinger for experimental help and Tobias and Andreas Meyer
from Solaronix SA (Aubonne, Switzerland) for a sample of cis-
Ru(dcbpyH2)2(NCS)2, TiO2 electrodes, electrolyte, and experi-
mental help.
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