Photophysical properties of the lowest excited singlet and triplet states of thio- and seleno-psoralens

The decay processes of the lowest excited singlet and triplet states of fiv e heteropsoralens (HPS) were investigated by steady-state and shift-phase f luorometry and by laser-flash photolysis in different solvents. The emissio n spectra of WS are detectable only in trifluoroethanol (TFE, where fluores cence Lifetimes (tau(F)) and quantum yields (phi(F)) were measured. The tri plet lifetimes (tau(T)), triplet (phi(T)) and singlet-oxygen production (ph i(Delta)) quantum yields were determined in benzene, ethanol and TFE by las er-hash photolysis. Semiempirical (INDO/1-CI) calculations allowed the natu re of the lowest excited singlet and triplet states and transition probabil ities to he obtained. Theoretical and experimental results indicate that th e two lowest excited singlet states S-1 and S-2 Of HPS are close-lying and different in nature (pi,pi* and n,pi*), The "proximity effect" between thes e two states controls the photophysical properties of HPS as it does for th e other furocoumarins. Ho However, HPS have a peculiar behavior with respec t to the related compounds because they are fluorescent and have, in three cases, detectable intersystem crossing only in TFE, This behavior can be te ntatively explained by a different energy gap and/or order between the S-1 and S-2 states,