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

Abstract

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, τ_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 (E_T^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 τ_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-MOP^•+. Ionization is due to both, one and two-quantum excitation. In the intensity range I = 10⁶–10⁹ W cm⁻² the quantum yield of electron ejection (ionization) ϕ_i does not depend on the intensity of radiation, therefore the ionization is monophotonic, ϕ_i = 0.03. At intensities I > 5 × 10⁹ W cm⁻²ϕ_i becomes larger due to two-quantum excitation followed by ionization.