Intersystem crossing in 5-azacytosine: time-resolved photoelectron and quantum-chemical insights into the effect of aza substitution on cytosine

Abstract

Single-atom substitutions in nucleobases can cause drastic changes in their photophysical response to ultraviolet radiation, in particular their propensity for intersystem crossing. Azabases are formed by replacing an endocyclic carbon atom with a nitrogen atom. 5-Azacytosine (5AC), an aza-derivative of cytosine with substitution at the C5 position of the ring, exists in both keto and enol tautomeric forms in the gas phase. However, most prior investigations have focused exclusively on the keto tautomer. In the present study, time-resolved photoelectron spectroscopy (TRPES) and extended multistate complete active space second-order perturbation theory, with double ξ-basis sets (XMS-CASPT2/cc-pVDZ) are employed to investigate the photophysics of both keto and enol forms of 5AC. The study finds that both tautomers preferentially undergo internal conversion (IC) to the ground state, with a measurable contribution from an intersystem crossing (ISC) pathway only in the keto form.