Excited singlet state and photoionization of 8-methoxypsoralen. Picosecond transient absorption study.

Time-resolved pico- and nanosecond transient absorption measurements of 8-methoxypsoralen (8-MOP) were performed in different solvents at room temperature. The excited singlet state has two strong absorption bands with maxima at 430 and 630-690 nm. The lifetime of the first excited singlet state, tau(s), ranges from <10 ps up to 2.7 ns, being shorter in low-polar solvents (toluene, THF) and longer in highly polar water, TFE and HFIP. The rate of the singlet depopulation decreases exponentially with increasing solvent polarity as expressed by the Dimroth-Reichardt (ET(N)) or Gutmann solvent acceptor number (AN). In aqueous ethanol solution (H2O: EtOH 10:1), addition of the synthetic nucleobase 2-aminopurine slightly decreased the lifetime tau(s). The long-time (t > 5 ns) absorption spectrum remaining after the complete decay of the S1 state is due to the hydrated electron e(aq) and the radical cation 8-MOPF.+. Ionization is due to both, one and two-quantum excitation. In the intensity range I = 10(6)-10(9) W cm(-2) the quantum yield of electron ejection (ionization) phi(i) does not depend on the intensity of radiation, therefore the ionization is monophotonic, phi(i) = 0.03. At intensities 1> 5 x 10(9) W cm(-2) phi1 becomes larger due to two-quantum excitation followed by ionization.