PHOTOPHYSICAL BEHAVIOR OF ANGELICINS AND THIOANGELICINS - SEMIEMPIRICAL CALCULATIONS AND EXPERIMENTAL-STUDY

The decay processes of the lowest excited singlet and triplet states o f fire methylated angelicins (4,6,4'-trimethylaragelicin, MA, and four methylated thioangelicins, MTA; see Scheme 1) were investigated in fi ve solvents by stationary and pulsed fluorometric and flash photolytic techniques. In particular, the solvent effects on absorption, fluores cence, quantum yields of fluorescence (phi(F)) and triplet formation ( phi(T)), lifetimes of fluorescence (tau(F)) and the triplet state (tau (T)) and the quantum yields of singlet oxygen production (phi(Delta)) were investigated, Semiempirical (ZINDO/S-CI) calculations were carrie d out to obtain information (transition probabilities and nature) on t he lowest excited singlet and triplet states. The quantum mechanical c alculations and the solvent effect on the photophysical properties sho wed that the lowest excited singlet state (S-1) is a partially allowed pi,pi state, while the close-lying S-2 state is n,pi* in nature. The efficiencies of fluorescence, S-1 --> T-1 intersystem crossing (ISC) and S-1 --> S-0 internal conversion (IC) strongly depend on the energy gap between S1 and S2 and are explained in terms of the so-called pro ximity effect. In fact, for MA in cyclohexane, only the S-1 --> S-0 in ternal conversion is operative, while in acetonitrile and ethanol, whe re the n,pi: state is shifted to higher energy, the efficiencies of fl uorescence and ISC increase significantly. The energy gap between S1 a nd St increases in MTA, where the furanic oxygen is replaced by a sulf ur atom. Consequently, the solvent effect on the photophysical paramet ers of MTA is less marked than for MA; e.g. fluorescence and triplet-t riplet absorption are: also detectable in the nonpolar cyclohexane. Th e lowest excited singlet state of molecular oxygen O-2((1)Delta(g)) wa s produced efficiently in polar solvents by energy transfer from the T -1 state of MA and MTA.