Photophysical properties of some methyl-substituted angelicins: Fluorometric and flash photolytic studies

This paper describes the results of a study of the photophysical properties of various methyl-angelicins (MA) in solvents of different polarity and proticity. The behavior of their excited singlet and triplet states was investigated by fluorometry and nanosecond laser flash photolysis. On the basis of semiempirical (ZIN-DO/S-CI) calculations and the solvent effect on the absorption and fluorescence properties, the lowest excited singlet state (S-1) is assigned to a partially allowed pi,pi(*) state. The close lying S-2 state is n,pi(*) in nature. The efficiency of the decay pathways of S-1 (fluorescence, intersystem crossing and internal conversion) strongly depends on the energy gap between the S-1 and S-2 states consistent with the manifestation of ''proximity effect.'' Thus, MA in cyclohexane decay only through S-1 --> S-0 internal conversion, while in acetonitrile and ethanol, where the n,pi(*) state is located at higher energy, their fluorescence and intersystem crossing increase significantly. The lowest excited triplet states (T-1) were characterized in terms of their absorption spectra, decay kinetics, molar absorption coefficients and formation quantum yields. The interaction of T-1 MA with molecular oxygen leads to an efficient formation of singlet oxygen, as evidenced by the appearance of characteristic IR phosphorescence centered at 1269 nm.