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Sensitization of Nanocrystalline TiO2 Anchored with Pendant Catechol Functionality Using a New Tetracyanato Ruthenium(II) Polypyridyl Complex

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journal contribution
posted on 03.05.2010, 00:00 by Prasenjit Kar, Sandeep Verma, Anik Sen, Amitava Das, Bishwajit Ganguly, Hirendra Nath Ghosh
We have synthesized a new photoactive ruthenium(II) complex having a pendant catechol functionality (K2[Ru(CN)4(L)] (1) (L is 4-[2-(4′-methyl-2,2′-bipyridinyl-4-yl)vinyl]benzene-1,2-diol) for studying the dynamics of the interfacial electron transfer between nanoparticulate TiO2 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 TiO2 surfaces through the catechol functionality, though 1 has two different types of functionalities (catecholate and cyanato) for binding to TiO2 surfaces. The longer wavelength absorption band tail for 1, bound to TiO2 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 TiO2 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 TiO2 was confirmed by detection of the conduction band electron in TiO2 ([e]TiO2CB) and cation radical of the adsorbed dye (1•+) 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•+ and [e]TiO2CB. This is the first report on ultrafast ET dynamics on TiO2 nanoparticle surface using a solvatochromic sensitizer molecule.