Electron Transfer from the Singlet and Triplet Excited States of Ru(dcbpy)₂(NCS)₂ into\nNanocrystalline TiO₂ Thin Films

Abstract

Time-resolved absorption spectroscopy was used to study the femtosecond and picosecond time scale electron\ninjection from the excited singlet and triplet states of Ru(dcbpy)₂(NCS)₂ (RuN3) into titanium dioxide (TiO₂)\nnanocrystalline particle film in acetonitrile. The fastest resolved time constant of ∼30 fs was shown to reflect\na sum of two parallel ultrafast processes, nonergodic electron transfer (ET) from the initially excited singlet\nstate of RuN3 to the conduction band of TiO₂ and intersystem crossing (ISC). The branching ratio of 1.5\nbetween the two competing processes gives rate constants of 1/50 fs^-1 for ET and 1/75 fs^-1 for ISC. Following\nthe ultrafast processes, a minor part of the electron injection (40%) occurs from the thermalized triplet state\nof RuN3 on the picosecond time scale. The kinetics of this slower phase of electron injection is nonexponential\nand can be fitted with time constants ranging from ∼1 to ∼60 ps.