Photosensitization of nanoparticulate TiO₂ using a Re(i)-polypyridyl complex: studies on interfacial electron transfer in the ultrafast time domain

Abstract

We have synthesized a new photoactive rhenium(I)-complex having a pendant catechol functionality [Re(CO)₃Cl(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 Re(I)-complex using femtosecond transient absorption spectroscopy. Our steady state absorption studies revealed that complex 1 can bind strongly to TiO₂ surfaces through the catechol functionality with the formation of a charge transfer (CT) complex, which has been confirmed by the appearance of a new red-shifted CT band. The longer wavelength absorption band for 1, bound to TiO₂ through the proposed catecholate functionality, could also be explained based on 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 regions. Electron injection into the conduction band of the nanoparticulate TiO₂ was confirmed by detection of the conduction band electron in TiO₂ ([e^-]_TiO2^CB) and the 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.