Solvent effect on the singlet excited state lifetimes of nucleic acid bases: a computational study of 5-fluorouracil and uracil in acetonitrile and water

The first comprehensive quantum mechanical study of solvent effect on the behavior of the two lowest energy excited states of uracil derivatives is presented. The absorption and emission spectra of uracil and 5-fluorouracil in acetonitrile and water solution have been computed at the TD-DFT level, using the Polarizable Continuum Model (PCM) to treat bulk solvent effect. The computed spectra and the solvent shifts provided by our method are close to their experimental counterpart. S0/S1 conical intersection, located in the presence of hydrogen bonded solvent molecules by CASSCF (8/8) calculations, indicates that the mechanism of ground state recovery, involving out of plane motion of the 5 substituent, does not depend on the nature of the solvent. Extensive explorations of the excited state surfaces in the FC region show that solvent can modulate the accessibility of an additional decay channel, involving a dark n/Ĉ* excited state, providing a unifying explanation for the experimental trend of 5-fluorouracil excited state lifetime in different solvents. The microscopic mechanisms underlying solvent effect on the excited state behavior of nucleobases are discussed.