Interfacial Electron Transfer Dynamics Involving a New Bis-Thiocyanate Ruthenium(II)-Polypyridyl Complex, Coupled Strongly to Nanocrystalline TiO2, through a Pendant Catecholate Functionality

Ruthenium(II)-polypyridyl bis-thiocyanate complex (1) with pendant catecholate functionality was synthesized and characterized by various spectroscopic and analytical techniques. Optical absorption studies show that this molecule absorbs in the red region of the solar spectrum. Further, this molecule could couple very strongly with TiO2 nanoparticles through pendant catecholate functionality and sensitize efficiently. Interfacial electron transfer dynamics between 1 and TiO2 nanoparticles was investigated by using femtosecond transient absorption spectroscopy and following kinetics at various wavelengths in the visible and near-infrared region. Electron injection to the conduction band of the nanoparticulate TiO2 was confirmed by direct detection of an electron in the conduction band, the cation radical of the adsorbed dye (1(.+)), and a bleach of the dye in real time as monitored by transient absorption spectroscopy. A single exponential and pulse width limited (< 100 fs) electron injection was observed. Presumably, this originates from the nonthermalized excited states of 1 and tends to suggest that electron injection competes with the thermalization of the photoexcited states due to large coupling elements for the forward ET reaction. Back electron transfer dynamics was determined by monitoring the decay kinetics of 1(.+) and the injected electron and also from recovery kinetics of the bleach of the adsorbed dye. Interfacial electron transfer dynamics was also carried out on TiO2 thin film with complex 1 as a sensitizer and compared with that of nanoparticles.