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 pi,pi* 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,pi*) to (1)(pi,pi*) 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 Phi(isc), or short lifetime of the triplet state (tau(T),) caused by the fast T-1 curved right arrow S-0 intersystem crossing.