Sensitization of nanocrystalline TiO₂ anchored with pendant catechol functionality using a new tetracyanato ruthenium (II) polypyridyl complex

Abstract

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^-]_TiO2^CB) 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^-]_TiO2^CB. This is the first report on ultrafast ET dynamics on TiO₂ nanoparticle surface using a solvatochromic sensitizer molecule