Ultrafast Decay of Electronically Excited Singlet Cytosine via a ð,ð * to nO,ð* State Switch

Recently, one of us determined the singlet fluorescence lifetimes of adenosine, cytidine, guanosine, and thymidine by femtosecond pump-probe spectroscopy.1 The excited states produced by 263 nm light in these nucleosides decay in the subpicosecond range (290-720 fs). Ultrafast radiationless decay to the ground state greatly reduces the probability of photochemical damage, and has motivated the present theoretical study of isolated cytosine, the chromophore of cytidine. The experimental lifetime of 720 fs indicates that there must be an ultrafast decay channel for this species. Here we present an overview of the possible decay channels and approximate energetics, using a valence-bond derived analysis to rationalize the structural details of the paths. The mechanism favored by our calculations and new experimental data1c involves (1) a two-mode decay coordinate composed of initial bond length inversion followed by internal vibrational energy redistributio