Electronic structure and photoreactivity of the triplet states of skin‐sensitizing psoralens

The photoreactive states of skin-sensitizing-carcinogenic psoralens (furocoumarins) are characterized by the configuration analysis method of Baba and coworkers. The S1 (π, π*) states of coumarin and psoralen possess significant charge transfer (CT) character in the CC bond of the pyrone moiety (32% CT to the CC vacant orbital in coumarin and 22% in psoralen) and in the CC bond of the furyl moiety (4% CT to the vacant CC orbital and 14% CT from the occupied CC orbital in psoralen). On the other hand, the T1, (π, π*) state of these molecules shows a substantial localization of the excitation in the pyrone CC bond, thus providing a logical basis for the structure-activity correlation of skin-sensitizing coumarins.