This lecture covers the properties and structures of xanthene dyes, including fluoresceine and tetrachloro fluoresceine, as well as their applications in photochemistry. It also discusses spin-orbit coupling, the energy-gap law, and the quenching of emission through the Stern-Volmer relation. Furthermore, it explores intermolecular electronic energy transfer mechanisms, such as Förster resonance energy transfer (FRET) and Dexter electron exchange, and their impact on fluorescence quenching.