A Kinetic Description of Ultrafast Excitation, Relaxation, and Charge Transfer in Ru Dye-Semiconductor Systems*

We describe a predictive kinetic framework for ultrafast photophysics of ruthenium polypyridyl dyes in solution and on solid surfaces to probe how kinetic processes such as absorption, relaxation, and intersystem crossing affect excited state lifetimes. Employing a form of kinetic Monte Carlo that produces an absolute time base, we compute transient absorption (TA) signals and find excellent agreement with experimental spectroscopic data. We compare dye photophysics in solution to that of sensitized metal oxide films, where charge injection from the excited states may occur. Dye molecules have similar excitation and decay kinetics in solution and on ZrO₂ films where there is no charge transfer. In contrast, charge transfer to the semiconductor competes with intramolecular transitions for dyes bound to TiO₂. Comparison of simulated TA spectra to experiment allow rate coefficients for charge injection to be estimated. The kinetic framework is readily integrated into multiscale models for dye-sensitized light harvesting systems.