TRIPLET ELECTRONIC STRUCTURE AND PHOTOREACTIVITY OF 8‐METHOXYPSORALEN

Psoralen and its derivatives are implicated in a variety of photobiological processes including skin‐sensitization in mammals, the experimental photochemotherapy of psoriasis, and photomutagenesis in bacteria. Although the various derivatives differ markedly in photoactivity, their excited triplet states as characterized by conventional luminescence spectroscopy are very similar and closely resemble that of the parent compound, coumarin, which is inactive as a skin sensitizer. Employing a more sensitive probe of triplet electronic structure, we have utilized optical detection of magnetic resonance to measure the zero‐field splittings in the triplet state of several psoralens and find a striking variation among the derivatives. D*, taken as a composite measure of the dipole‐dipole interaction of the unpaired electrons in the triplet state was found to be anomalously large, greater than 0.140 cm^‐1, for the very active compounds, psoralen and 8‐methoxypsoralen, while D*= 0.124cm^‐1 for the inactive, though smaller, coumarin molecule. Although 8‐hydroxypsoralen has a large D* value its inactivity as a skin‐sensitizer may be explained by the dissociation of the hydroxyl proton. The resulting (excited) triplet state anion has D*= 0.120 cm^‐1.