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Photophysical Properties of the Lowest Excited Singlet and Triplet States of Thio‐ and Seleno‐psoralens
Author(s) -
Aloisi G. G.,
Elisei F.,
Moro S.,
Miolo G.,
Dall'Acqua F.
Publication year - 2000
Publication title -
photochemistry and photobiology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.818
H-Index - 131
eISSN - 1751-1097
pISSN - 0031-8655
DOI - 10.1562/0031-8655(2000)0710506ppotle2.0.co2
Subject(s) - intersystem crossing , excited state , chemistry , flash photolysis , singlet fission , singlet state , photochemistry , triplet state , fluorescence , atomic physics , physics , reaction rate constant , kinetics , quantum mechanics
The decay processes of the lowest excited singlet and triplet states of five heteropsoralens (HPS) were investigated by steady‐state and shift‐phase fluorometry and by laser‐flash photolysis in different solvents. The emission spectra of HPS are detectable only in trifluoroethanol (TFE), where fluorescence lifetimes (τ F ) and quantum yields (φ F ) were measured. The triplet lifetimes (τ T ), triplet (φ T ) and singlet‐oxygen production (φ Δ ) quantum yields were determined in benzene, ethanol and TFE by laser‐flash photolysis. Semiempirical (INDO/1‐CI) calculations allowed the nature of the lowest excited singlet and triplet states and transition probabilities to be obtained. Theoretical and experimental results indicate that the two lowest excited singlet states S 1 and S 2 of HPS are close‐lying and different in nature (π,π* and n,π*). The “proximity effect” between these two states controls the photophysical properties of HPS as it does for the other furocoumarins. However, HPS have a peculiar behavior with respect to the related compounds because they are fluorescent and have, in three cases, detectable intersystem crossing only in TFE. This behavior can be tentatively explained by a different energy gap and/or order between the S 1 and S 2 states.