Tryptanthrin from indigo: Synthesis, excited state deactivation routes and efficient singlet oxygen sensitization

The microwave-assisted synthesis of tryptanthrin from indigo in mild oxidation conditions, and a comprehensive study of the excited state properties of this compound in a variety of solvents with different polarity and viscosity values at room and low temperatures are reported. In contrast with indigo, emission of the triplet state of tryptanthrin is observed with a very efficient singlet oxygen sensitization quantum yield, indicating that the triplet state is efficiently populated. From time-resolved fluorescence and femtosecond transient absorption data, further supported with time-dependent density functional theory (TDDFT) calculations, two species, with S₁ states with locally excited (LE) of π,π* nature and a charge transfer (CT) of n,π* characteristics, originated from an initially populated Frank-Condon S₂ state (π,π*), are observed. The two electronically independent species are energetically nearly degenerate and inter-conversion is predicted (and rate constants determined) to occur between LE (S₁) and CT (S₁) species. Due to the low value of the fluorescence quantum yield (~10⁻³) and high triplet state yield (ϕ_T≥ϕ_Δ), the high stability of this compound is associated to the high efficiency of the radiationless deactivation processes which involve the formation of the CT state which efficiently converts, through S₁ ~~> T_n intersystem crossing, to the T₁ triplet state.