Molecular Engineering of Photosensitizers for Nanocrystalline Solar Cells: Synthesis and Characterization of Ru Dyes Based on Phosphonated Terpyridines S. M. Zakeeruddin, M. K. Nazeeruddin, P. Pechy, F. P. Rotzinger, R. Humphry-Baker, K. Kalyanasundaram,* and M. Gra 1 tzel* Laboratory for Photonics and Interfaces, Institute of Physical Chemistry, Swiss Federal Institute of Technology, 1015 Lausanne, Switzerland V. Shklover and T. Haibach Laboratory of Crystallography, Swiss Federal Instiute of Technology, 8092 Zurich, Switzerland ReceiVed January 3, 1997 X We report the results of an investigation on the preparation, spectral, and photoelectrochemical properties of Ru(II)-polypyridyl complexes containing a new phosphonated terpyridine (P-terpy) ligand: [Ru(H 2 P-terpy) 2 ] and [Ru(HP-terpy)(Me 2 bpy)(NCS)]. Resonance Raman spectral and luminescence studies allow probing into the nature of the low-energy MLCT transitions observed in these complexes. The crystal and molecular structure of the mixed-ligand complex [Ru(HP-terpy)(Me 2 bpy)(NCS)] based on X-ray diffraction study is reported. This complex appears to be a promising candidate as a photosensitizer in dye-sensitized photoelectrochemical cells based on nanocrystalline films of TiO 2 . Introduction Polypyridine complexes of Ru(II) have been the testing ground to probe many of fundamental processes governing the photoreactivity of transition metal complexes. 1,2 By varying the nature of the polypyridine ligand and introduction of different donor/acceptor/functional groups, it has been possible to “fine tune” the spectral and redox properties of theses complexes. Thus complexes have been synthesized that serve as electron donors or electron acceptors or energy transfer agents in light-induced electron transfer processes. Unlike Ru(bpy) 3 2+ , the lowest excited state of the analogous complex with the tridentate terpyridine ligand, Ru(terpy) 2 2+ , is practically non- luminescent and very short lived. It is desirable to identify complexes of terpyridine derivatives with better emission properties. Luminescence, though not a prerequisite for a complex to serve as a photosensitizer, helps enormously the direct measurements of excited-state properties and reactions. Of particular relevance to the present work are the recent efforts of Constable, Sauvage, and others to develop a series of oligopyridines and their derivatives (such as substituted terpy- ridines and quarterpyridines) as potential multidentate ligands. 3-5 One type of application of polypyridine complexes that we have been interested for a number of years is their use as photosensitizers in photoelectrochemical solar cells. 6-8 Of particular interest are their utility in dye-sensitized solar cells based on nanocrystalline films of TiO 2 . 9-11 Optical excitation using visible light of polypyridyl complexes incorporated in these nanosized membrane films leads to rapid transfer of electron from the excited state of the dye to the conduction band of the semiconductor. The oxidized form of the dye is rapidly reduced back to its ground state by redox mediators (such as iodide) present in the electrolyte separating the illuminated and counter electrode. The injected electrons diffuse through the semiconductor particles to arrive at the back-contact and then flow through the external circuit to arrive at the counter- electrode. Regeneration of the depleted redox relay leads to net conversion of visible light to electricity. Design of efficient photosensitizers for this type of application involves considerable molecular engineering. First, the spectral absorption properties must be tuned to have maximum visible light response. Second, the redox properties of the metal complex should be tuned for the complex in the excited state to have sufficient driving force to participate in electron transfer reactions. Polypyridine complexes of transition metals have been materials of first choice in view of the extensive knowledge available on this series of complexes. Metal-to-ligand charge transfer transitions dominate their visible light absorption and much of their photophysical and redox behavior. A third important factor to consider in dye design is the role of functional groups on the dye that allow efficient adsorption X Abstract published in AdVance ACS Abstracts, November 1, 1997. (1) (a) Kalyanasundaram, K. Photochemistry of Polypyridine and Por- phyrin Complexes; Academic Press: London, 1992. (b) Roundhill, D. M. Photophysics and Photochemistry of Coordination Compounds; Wiley: New York, 1994. (c) Balzani, V.; Scandola, F. Supramolecular Photochemistry; Wiley: Chichester, U.K., 1991. (d) Juris, A.; Cam- pagna, S.; Balzani, V.; Belser, P.; von Zelewsky, A. Coord. Chem. ReV. 1988, 84, 85. (2) Kalyanasundaram. K., Gra ¨tzel, M., Eds. Photosensitization and Photocatalysis Using Inorganic and Organometallic Compounds; Kluwer Academic Publishers: Dordrecht, The Netherlands, 1993. (3) Constable, E. C. Pure Appl. Chem. 1996, 68, 253; Prog. Inorg. Chem. 1994, 42, 67; Tetrahedron 1992, 48, 10013 and references cited therein. (4) Sauvage, J.-P.; Collin, J.-P.; Chambron, J.-C.; Guillerez, S.; Coudret, V.; Balzani, V.; Barigelletti, F.; De Cola, L.; Flamigni, L. Chem. ReV. 1994, 94, 993 and references cited therein. (5) Maestri, M.; Armaroli, N.; Balzani, V.; Constable, E. C.; Cargill Thompson, A. M. W. Inorg. Chem. 1995, 34, 2759. Barigelletti, F.; Flamigni, L.; Guardigli, M.; Sauvage, J.-P.; Collin, P.-P.; Sour, A. J. Chem. Soc., Chem. Commun. 1996, 1329. (6) DeSilvestro, J.; Gra ¨tzel, M.; Kavan, L.; Augustynski, J. J. Am. Chem. Soc. 1985, 107, 2988. (7) Vlachopoulos, N.; Liska, P.; Augustynski, J.; Gra ¨tzel, M. J. Am. Chem. Soc. 1988, 110, 1216. Liska, P.; Vlachopoulos, V.; Nazeeruddin, Md. K.; Comte, P.; Gra ¨tzel, M. J. Am. Chem. Soc. 1988, 110, 3686. (8) Nazeeruddin, Md. K.; Liska, P.; Moser, J.; Vlachopoulos, V.; Gra ¨tzel, M. HelV. Chim. Acta 1990, 73, 1788. 5937 Inorg. Chem. 1997, 36, 5937-5946 S0020-1669(97)00008-6 CCC: $14.00 © 1997 American Chemical Society