pubs.acs.org/IC Published on Web 03/29/2010 r 2010 American Chemical Society Inorg. Chem. 2010, 49, 4167–4174 4167 DOI: 10.1021/ic902533x Sensitization of Nanocrystalline TiO 2 Anchored with Pendant Catechol Functionality Using a New Tetracyanato Ruthenium(II) Polypyridyl Complex Prasenjit Kar, ‡ Sandeep Verma, † Anik Sen, ‡ Amitava Das,* ,‡ Bishwajit Ganguly,* ,‡ and Hirendra Nath Ghosh* ,† † Radiation and Photo Chemistry Division, Bhabha Atomic Research Center, Mumbai, India, and ‡ Central Salt and Marine Chemicals Research Institute (CSIR), Bhavnagar 364002, Gujarat, India Received December 19, 2009 We have synthesized a new photoactive ruthenium(II) complex having a pendant catechol functionality (K 2 [Ru(CN) 4 (L)] (1)(L is 4-[2-(4 0 -methyl-2,2 0 -bipyridinyl-4-yl )vinyl ]benzene-1,2-diol) for studying the dynamics of the interfacial electron transfer between nanoparticulate TiO 2 and the photoexcited states of this Ru(II ) complex using femtosecond transient absorption spectroscopy. Steady-state absorption and emission studies revealed that the complex 1 showed a strong solvatochromic behavior in solvents or solvent mixtures of varying polarity. Our steady-state absorption studies further revealed that 1 is bound to TiO 2 surfaces through the catechol functionality, though 1 has two different types of functionalities (catecholate and cyanato) for binding to TiO 2 surfaces. The longer wavelength absorption band tail for 1, bound to TiO 2 through the proposed catecholate functionality, could also be explained on the basis of the DFT calculations. Dynamics of the interfacial electron transfer between 1 and TiO 2 nanoparticles was investigated by studying kinetics at various wavelengths in the visible and near-infrared region. Electron injection to the conduction band of the nanoparticulate TiO 2 was confirmed by detection of the conduction band electron in TiO 2 ([e - ] TiO 2 CB ) and cation radical of the adsorbed dye (1 •þ ) in real time as monitored by transient absorption spectroscopy. A single exponential and pulse-width limited (<100 fs) electron injection was observed. Back electron transfer dynamics was determined by monitoring the decay kinetics of 1 •þ and [e - ] TiO 2 CB . This is the first report on ultrafast ET dynamics on TiO 2 nanoparticle surface using a solvatochromic sensitizer molecule. 1. Introduction Dye-sensitized solar cells (DSSC) based on mesoporous nanocrystalline semiconductor surface have attracted signif- icant interest because of their potential for developing low- cost devices. 1-3 More recently, significant efforts in this area of research have been directed toward developing a better insight in understanding the factors that actually control the dynamics of the interfacial electron transfer spectroscopy. These are believed to be crucial for the development of alternative to solid-state photovoltaic devices for solar-to- electrical energy conversion. 1-3 Arguably the most essential process, which has a direct relevance for achieving better efficiency of DSSC, is the efficiency of the electron injection from the photoexcited state of the sensitizer dye to the conduction band of the semiconductor and slow back elec- tron transfer reaction from semiconductor to parent dye cation. 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