| Abstract: | Abstract— Although psoralen and many substituted psoralens are potent skin-photosensitizing agents, hydroxypsoralens are not. A satisfactory molecular interpretation of this structural specificity has been given in terms of dissociation of the hydroxyl group of 5- and 8-hydroxypsoralens in their excited states. The dissociation process in the S1 state effectively competes with S1→T1 intersystem crossing, thus reducing the photoreactive T1 population. The T1 states of the anions are more delocalized than those of neutral psoralens so that they are less reactive toward photocycloaddition with pyrimidine bases of DNA. The lack of significant phosphorescence of hydroxypsoralens in ionizing solvent or in the presence of base at low temperatures (14–77 K.) indicates ineffective S1→T1 and/or effective T1→S0 intersystem crossing. These factors make hydroxypsoralens unreactive, electronically and kinetically, as skin photosensitizers, which are known to react with DNA. In correlation with the hydroxypsoralens' spectroscopic characterization, they are also found to be ineffective or less effective photosensitizers in Bacillus subtilis. |
