Singlet and triplet state properties of substituted flavothiones

The properties of the singlet and triplet excited states of some substituted flavothiones in different solvents are presented. Steady-state and pulsed techniques were used to characterize the emitting S2 state (π → π* in nature, kF ≈ 107 s−1) in terms of fluorescence quantum yields (∼10−4) and lifetimes (in the picosecond range). As for other flavothiones, no emission was observed from the lowest excited S1 state of n,π* nature. Information on the state order and on the oscillator strengths of singlet-singlet transitions were obtained by semiempirical calculations. The lowest excited triplet states (T1) were characterized by laser flash photolysis in terms of absorption spectra, formation quantum yields, decay lifetimes and molar absorption coefficients. The interaction of T1 with molecular oxygen leads to singlet oxygen with quantum yield generally high (ϕΔ∼0.55). The study of triplet properties allowed information on the mechanism of intersystem crossing and on the triplet nature of the various flavothiones to be obtained.