Solvent effect on ultrafast decay of uracil studied by femtosecond transient absorption spectroscopy

As one of the building blocks in RNA chain, uracil and its derivatives have attracted a great deal of interest since its ultrafast dynamics is closely related to mutagenic and carcinogenic effects. In this study, the solvent effect on the ultrafast decay of excited uracil is studied by femtosecond transient absorption spectroscopy in the UV region. The uracil molecule is populated to the (1)(pi, pi*) state (i.e. S-2 state) with a pump pulse at 264 nm. Broad-band white light continuum in the UV region from 280 to 360 nm is used as the probe. With a detail analysis of the measured transient spectra, two decay time constants, i.e. 9.8 ps and > 1000 ps, are directly obtained at 300 nm in the solvent of acetonitrile. Compared with our previous experiments, where no obvious triplet population is observed in water, triplet population is found to play an important role in acetonitrile. A comparison of excited-state dynamics among different solvents is also carried out. It reveals that the decay from the (1)(n, pi*) state (i.e., S-1 state) to the T-1 state shows a clear dependence on the H bonding of the solvents. With stronger H bonding, the (1)(n, pi*) excited state decays faster and has less chance to transfer to the triplet state. These results suggest that only when the (1)(n, pi*) state has excess vibrational energy can it transit to the triplet state through the intersystem crossing process. With this new information obtained in the present measurement, the decay dynamics of uracil on the S-2 excited state can be further understood.