Quantum dynamics of the ultrafast pi pi*/n pi* population transfer in uracil and 5-fluoro-uracil in water and acetonitrile.

We present the first fully quantum-mechanical dynamical study of the pi pi* --> n pi* decay in photoexcited uracil derivatives in solution. The dynamics of this process for uracil (U) and 5-fluoro-uracil (5FU) in acetonitrile and aqueous solution is investigated, by treating both electrons and nuclei at the quantum-mechanical level, and solving the time-dependent Schrodinger equation for the evolution of the photoexcited system. The potential energy surfaces along the most relevant nuclear degrees of freedom have been obtained at the time-dependent density functional theory level, accounting for the solvent effect by mixed atomistic/continuum models. The obtained results nicely agree with experimental evidence. For U, we predict an ultrafast (<50 fs) pi pi* --> n pi* decay with 10-25% yields, weakly dependent on the solvent. This finding strongly indicates that the npi* state is responsible for the decay channel observed in experiments for U in polar solvents, yielding to a final recovery of the ground state in tens of picoseconds. At variance, our dynamical calculations predict a remarkable solvent effect for 5FU, showing that the pi pi* --> n pi* decay channel is open in acetonitrile and closed in water, in agreement with the much faster decay observed in experiments in the first solvent. The analysis of our theoretical simulations, also including explicitly the laser excitation step, allows us to point out a number of interesting features for the decay dynamics of photoexcited molecules with close-lying electronic states, whose general interest goes beyond the specific system under investigation.