Photophysics of Angelicin─The Simplest Angular Furocoumarin

Angelicin (ANG) is the simplest angular furocoumarin. It undergoes photoinduced DNA monoadduct formation and does not form interstrand cross-links, resulting in photobiological behavior distinct from that of its linear analogues (psoralens). Here, the photophysics of ANG in the absence of DNA was investigated by steady-state and time-resolved spectroscopy. The nature of the solvent (water, acetonitrile, tetrahydrofuran, toluene, and cyclohexane) strongly affects the excited-state behavior. In water, the first excited singlet state S1 has a lifetime of ∼1 ns. Its decay is accompanied by the population of the triplet state. In contrast, in all aprotic solvents, the excited singlet state persists only for a few picoseconds and relaxes to the ground state without detectable triplet formation. Quantum mechanical/molecular mechanical (QM/MM) computations were used to explicitly account for solvation effects. Electronic excitation energies were obtained using a combination of density functional theory and multireference configuration interaction (DFT/MRCI) calculations, revealing that all relevant excited states are of ππ* character. Thus, the solvent effects are not due to energetic upshifts of nπ* states in protic solvents. The rapid S1 decay in aprotic solvents is tentatively ascribed to a transient opening of the α-pyrone ring.