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

Abstract

The decay processes of the lowest excited singlet and triplet states of five methylated angelicins (4,6,4'-trimethylangelicin, 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 (+F) and triplet formation (&), lifetimes of fluorescence ( T~) and the triplet state ( T~) and the quantum yields of singlet oxygen production (+,J were investigated. Semiempirical (ZINDOK-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,) is a partially allowed n,n* state, while the close-lying S2 state is n,n* in nature. The efficiencies of fluorescence, S, + TI intersystem crossing (ISC) and S1 + So 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 S1 + So internal conversion is operthrough S, -+ So IC only, while in acetonitrile and ethanol, fluorescence and ISC increase significantly. For these compounds there is strong evidence that a '(n,.rr*) state lies just

Photophysical Properties of the Lowest Excited Singlet and Triplet States of Thio- and Seleno-psoralens

“…The ratio F / F gives the fluorescence rate constant k F , reported in the same table. These values, of the order of 10 7 s Ϫ1 , are comparable with those of angelicins (23) and thioangelicins (30); this is in agreement with a fluorescence emission in TFE from a partially allowed ,* singlet state.…”

“…Unfortunately, in the present study, the quantum yields and lifetimes, important parameters for the characterization of the lowest excited singlet and triplet states, could be measured only for 1, 2 and 3 in the polar/protic solvent TFE and, therefore, it is difficult to have a complete picture of the photophysical behavior of these HPS. For compounds 1, 2 and 3, k F values were obtained in the range 1-6 ϫ 10 7 s Ϫ1 (Table 2), comparable with those of angelicins (23) and thioangelicins (30). This gives a clear indication that the S 1 → S 0 transition is allowed, the lowest excited singlet state is ,* in nature, followed by the n,* one at higher energy.…”