Photophysical Behavior of Angelicins and Thioangelicins: Semiempirical Calculations and Experimental Study

The decay processes of the lowest excited singlet and triplet states of fire methylated angelicins (4,6,4'-trimethylaragelicin, MA, and four methylated thioangelicins, MTA; see Scheme 1) were investigated in five solvents by stationary and pulsed fluorometric and flash photolytic techniques. In particular, the solvent effects on absorption, fluorescence, 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 carried out to obtain information (transition probabilities and nature) on the lowest excited singlet and triplet states. The quantum mechanical calculations and the solvent effect on the photophysical properties showed 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 proximity effect. In fact, for MA in cyclohexane, only the S-1 --> S-0 internal conversion is operative, while in acetonitrile and ethanol, where the n,pi: state is shifted to higher energy, the efficiencies of fluorescence and ISC increase significantly. The energy gap between S1 and St increases in MTA, where the furanic oxygen is replaced by a sulfur atom. Consequently, the solvent effect on the photophysical parameters of MTA is less marked than for MA; e.g. fluorescence and triplet-triplet absorption are: also detectable in the nonpolar cyclohexane. The lowest excited singlet state of molecular oxygen O-2((1)Delta(g)) was produced efficiently in polar solvents by energy transfer from the T-1 state of MA and MTA.