Transition metal complexes such as ruthenium complexes, having metal-to-ligand charge transfer (MLCT) states, are extensively used in solar energy conversion and electron transfer in biological systems and at interfaces. The dynamics of metal-to-ligand charge transfer and subsequent intermolecular, intramolecular, and interfacial electron transfer processes can be highly complex and inhomogeneous, especially when molecules are involved in interactions and
perturbations from heterogeneous local environments and gated by conformation fluctuations. We have employed single-molecule
spectroscopy, a powerful approach for studying inhomogeneous systems, to study the electron transfer dynamics of ruthenium complexes. We have applied a range of statistical analysis methods to reveal nonclassical photon emission behaviors of single ruthenium complexes, e.g., photon antibunching and photophysical ground-state recovering dynamics on a microsecond time-scale. The use of photon antibunching to measure phosphorescence lifetimes and single-molecule electron transfer dynamics at room temperature is demonstrated, which is a novel way of probing ground state regeneration in back electron transfer processes.