Ultrafast internal conversion of excited cytosine via the lowest ππ* electronic singlet state

Computational evidence at the CASPT2 level supports that the lowest excited state ππ* contributes to the S1/S0 crossing responsible for the ultrafast decay of singlet excited cytosine. The computed radiative lifetime, 33 ns, is consistent with the experimentally derived value, 40 ns. The n0π* state does not play a direct role in the rapid repopulation of the ground state; it is involved in a S2/S1 crossing. Alternative mechanisms through excited states πσ* or nNπ* are not competitive in cytosine. Copyright © 2003 American Chemical Society.