Photophysics of methyl-substituted uracils and thymines and their water complexes in the gas phase

We report studies on several methylated uracils and thymines and thymine-water complexes in the gas phase using resonantly enhanced multiphoton ionization (REMPI) and laser-induced fluorescence (LIF) spectroscopy. Results from two different REMPI experiments provided strong evidence that, after photoexcitation to the S-1 state, bare molecules were funneled into and trapped in a dark state via fast internal conversion. Lifetimes of this dark state were determined to be tens to hundreds of nanoseconds, depending on the internal energy and the degree of methyl substitution. The mass spectra of hydrated thymine clusters demonstrated dependence on the excitation wavelength, and the gradual loss of the ion signal with increasing water content across the absorption region of the S-1 state indicated a reduced lifetime of the S-1 state by the water solvent. In addition, the lifetime of the dark state also decreased gradually as thymine became more hydrated. On the basis of these results, we conclude that, in water solutions, the decay from the S-1 state should be essentially fast internal conversion to the ground state, in agreement with studies from the liquid phase. Our work reveals that the photostability is not an intrinsic property of these pyrimidine bases. Rather, it is the water solvent that stabilizes the photophysical and photochemical behavior of these bases under UV irradiation.