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Photophysical Properties of Pyrazinopsoralen, A New Monofunctional Psoralen *

Abstract— Pyrazinopsoralen (PzPs), a new monofunctional psoralen, has a UV absorption spectrum similar to other psoralens except that it absorbs more strongly in the long‐UVA than 8‐methoxypsoralen. The solvent effects on the UV absorption and fluorescence emission spectra indicate that the lowest excited singlet state is the π,π* state like other psoralen derivatives. It shows a much lower fluorescence quantum yield (0.0008 in ethanol at room temperature) than the other psoralens as expected by the increased proximity effect (vibronic perturbation) due to close 1(n,π*) to 1(π,π*) states. The fluorescence lifetime was 1.05 ns in methylcyclohexane with a single exponential decay, while more than two components were observed in other solvents with the short‐lived component being the major (>95%). The triplet state of PzPs could not be detected by phosphorescence, laser flash excitation (T‐T absorption) and singlet oxygen formation probably due to very low φisc, or short lifetime of the triplet state (τT) caused by the fast T1→ S0 intersystem crossing.