Photophysical pathways of cytosine in aqueous solution.

The effects of aqueous solvation on the photophysical pathways involving the S(1) excited state in cytosine have been studied with a mean-field QM/MM approach. Two main pathways with small barriers were found previously in isolated cytosine, using multireference configuration interaction (MRCI) methods, that facilitate radiationless decay to the ground state. These pathways are examined in solvated cytosine using a mean-field QM/MM combined with MRCI, and it is found that barriers in each direction increase moderately. The barriers in the presence of the solvent are 0.23 eV and 0.31 eV for the two different pathways compared to 0.15 eV and 0.14 eV in the gas phase, indicating that the aqueous environment does not make one of the two directions much more preferable.