Measurement of ultrafast photoinduced electron transfer from chemically anchored Ru-dye molecules into empty electronic states in a colloidal anatase TiO2 film

Electron transfer from the excited electronic singlet state of chemisorbed ruthenium(II) cis-di(isothiocyanato)bis(2,2'-bipyridyl-4,4'-dicarboxylate) into empty electronic states in a colloidal anatase TiO2 film was measured as a transient absorption signal of the injected hot electrons with a rise time <25 fs. Optical absorption of the anchored dye molecules led to the excited singlet state of the dye with a small admixture of charge transfer states. The electron transfer reaction reported here did not involve redistribution of vibrational excitation energy and was thus completely different from the well-known Marcus-Levich-Jortner-Gerischer type of electron transfer in the case of weak electronic interaction. It was also not a direct optical charge transfer transition from the donor to the acceptor level but rather an electron transfer reaction with an ultrashort but finite reaction time.