Light-Driven Tyrosine Radical Formation in a Ruthenium-Tyrosine Complex Attached to Nanoparticle TiO 2 Raed Ghanem, † Yunhua Xu, ‡ Jie Pan, † Tobias Hoffmann, ‡ Johan Andersson, † Toma ´s ˇ Polı ´vka, † Torbjo 1 rn Pascher, † Stenbjo 1 rn Styring, § Licheng Sun,* ,‡ and Villy Sundstro 1 m* ,† Departments of Chemical Physics and Biochemistry, Lund UniVersity, P.O. Box 124, S-22100 Lund, Sweden, and Department of Organic Chemistry, Stockholm UniVersity, S-10691 Stockholm, Sweden Received July 22, 2002 We demonstrate a possibility of multistep electron transfer in a supramolecular complex adsorbed on the surface of nanocrystalline TiO 2 . The complex mimics the function of the tyrosine Z and chlorophyll unit P 680 in natural photosystem II (PSII). A ruthenium(II) tris(bipyridyl) complex covalently linked to a L-tyrosine ethyl ester through an amide bond was attached to the surface of nanocrystalline TiO 2 via carboxylic acid groups linked to the bpy ligands. Synthesis and characterization of this complex are described. Excitation (450 nm) of the complex promotes an electron to a metal-to-ligand charge-transfer (MLCT) excited state, from which the electron is injected into TiO 2 . The photogeneration of Ru(III) is followed by an intramolecular electron transfer from tyrosine to Ru(III), regenerating the photosensitizer Ru(II) and forming the tyrosyl radical. The tyrosyl radical is formed in less than 5 µs with a yield of 15%. This rather low yield is a result of a fast back electron transfer reaction from the nanocrystalline TiO 2 to the photogenerated Ru(III). Introduction The mechanism of photosynthetic water oxidation into molecular oxygen in photosystem II (PSII) has been exten- sively studied. 1,2 Light is absorbed by the primary donor chlorophyll unit P 680 , and then, an electron is transferred from the excited state chlorophyll, via several steps, to quinone on the acceptor side of PSII, leading to the oxidation of P 680 . Photogenerated P 680 + recaptures an electron by oxidizing a nearby tyrosine (Tyr Z ), located approximately 8 Å away from P 680 , into a neutral tyrosyl radical (Tyr Z • ). 1 This radical in its turn oxidizes a tetranuclear Mn-cluster bound to PSII, and after four consecutive turnovers, two water molecules are oxidized into molecular oxygen. Moreover, recent results proposed that the Tyr Z itself catalyzes water oxidation by abstracting a hydrogen atom from a water molecule coor- dinated to the manganese cluster Mn 4 . 3 This shows that the Tyr Z plays a crucial role in this process whatever the exact mechanism of photosynthetic water oxidation is. 3 Many efforts have been made in recent years to design and synthesize supramolecular complexes to mimic the important parts of the light-driven process in PSII in order to construct an artificial photosynthetic system for fuel production using reducing equivalents from water. 4 Because photophysical properties and electrochemical behavior of ruthenium(II) tris(bipyridyl) complexes are well studied, 5 and because they are photochemically stable and easy to func- tionalize with a long-lived excited state, they are suitable * Corresponding authors. E-mail: villy.sundstrom@chemphys.lu.se (V.S.); licheng.sun@organ.su.se (L.S.). Fax: +46-46-2224119 (V.S.); fax +46- 8-154908 (L.S.). † Department of Chemical Physics, Lund University. ‡ Stockholm University. § Department of Biochemistry, Lund University. (1) (a) Tommos, C.; Tang, X. S.; Warncke, K.; Hoganson, C. W.; Styring, S.; McCracken, J.; Diner, B. A.; Babcock, G. T. J. Am. Chem. Soc. 1995, 117, 10325. (b) Hoganson, C. W.; Lydakis-Simantiris, N.; Tang, X. S.; Tommos, C.; Warncke, K.; Babcock, G. T.; Diner, B. A.; McCracken, J.; Styring, S. Photosynth. Res. 1995, 46, 177. (c) Vermaas, W.; Styring, S.; Schro ¨der, W.; Anderson, B. Photosynth. Res. 1993, 38, 249. (d) For recent reviews on photosynthetic water oxidation, see: Biochim. Biophys. Acta 2001, 1503 (1-2), 5. (2) Yachandra, V. K.; De Rose, V. J.; Latimer, M. J.; Mukerji, I.; Sauer, K.; Klein, M. P. Science 1993, 260, 675. (3) (a) Hoganson, C. W.; Babcock, G. T. Science 1997, 277, 1953. (b) Yagi, M.; Kaneko, M. Chem. ReV. 2001, 101, 21. (4) (a) Sun, L.; Hammarstro ¨m, L.; A ° kermark, B.; Styring, S. Chem. Soc. ReV. 2001, 30 (1), 36. (b) Hammarstro ¨m, L.; Sun, L.; A ° kermark, B.; Styring, S. Biochim. Biophys. Acta 1998, 1365 (1-2), 193. (5) (a) Kalyanasundaram, K. Photochemistry of Polypyridine and Por- phyrin Complexes; Academic Press: London, 1992; Chapter 6. (b) Meyer, T. J. Acc. Chem. Res. 1989, 22, 163. (c) Juris, A.; Balzani, V.; Barigelletti, F.; Campagna, S.; Belser, P.; Vonzelewsky, A. Coord. Chem. ReV. 1988, 84, 85. Inorg. Chem. 2002, 41, 6258-6266 6258 Inorganic Chemistry, Vol. 41, No. 24, 2002 10.1021/ic020472+ CCC: $22.00 © 2002 American Chemical Society Published on Web 10/29/2002