Quantum chemical study on the population of the lowest triplet state of psoralen

The efficient population of the low-lying triplet ππ* state of psoralen is studied with the quantum chemical CASPT2 method. Minima, singlet–triplet crossings, conical intersections, and reaction paths on the low-lying singlet and triplet states hypersurfaces of the system have been computed together with electronic energy gaps and spin–orbit coupling terms. A mechanism is proposed, favorable in the gas phase, for efficient deactivation of the initially populated singlet excited ππ* state, starting with an intersystem crossing with an nπ* triplet state and evolving via a conical intersection toward the final lowest-lying ππ* triplet state, protagonist of the reactivity of psoralen.