Singlet and triplet state properties of substituted flavothiones

Abstract

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, k_F ≈ 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 (ϕ_Δ∼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.