Photophysical properties of hydroxy-substituted flavothiones

Flavothione and a number of synthesized hydroxy- (mono- and di-) substitute d flavothiones have been thoroughly examined, particularly regarding their absorption, emission, photophysical (triplet yields and lifetimes), and oxy gen-photosensitizing characteristics. These were all studied as a function of the nature of the solvent (four), which was particularly critical in ter ms of aiding in determining the energy and configurational nature of the lo west triplet state as well as the mechanism of intersystem crossing. Theore tical calculations were also performed. Both the location and number of hyd roxyl groups have a substantial impact on the nature of the lowest excited triplet state as well as on the relative location of the two lowest excited singlet and triplet states. These in turn affect the magnitude and even th e existence of triplet-state occupation as well as the ability to sensitize oxygen (to singlet oxygen). Three groups of compounds exist as characteriz ed by the configurational nature of the triplet and the mechanism of inters ystem crossing, or the essential absence of intersystem crossing altogether . The quantum yield of singlet oxygen formation is high for one group where the T(pi, pi*) state is lowest and generally high in another group where t he T(n, pi*) state is lowest, except in ethanol where competitive H-atom ab straction occurs. The potential of all hydroxy compounds as photosensitizer s is evaluated.