Photophysical properties of pyrazinopsoralen, a new monofunctional psoralen

Pyrazinopsoralen (PzPs), a new monofunctional psoralen, has a UV absorption spectrum similar to other psoralens except that it absorbs more strongly in the longUVA 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 Φ(isc), or short lifetime of the triplet state (τ(T)) caused by the fast T₁ S₀ intersystem crossing.