Effects of structural modification on the electronic excited-state dynamics of cytosine studied by time-resolved photoelectron spectroscopy

The photophysical properties of the canonical nucleobases have been studied extensively due to their biological relevance as chromophores in DNA. These studies have now been extended to modified nucleobases to investigate the effect of small structural changes on their UV photostability. Gas-phase spectroscopic techniques, such as time-resolved photoelectron spectroscopy (TRPES), in combination with ab initio calculations and dynamics simulations are particularly powerful in unraveling the mechanistic details of the electronic population dynamics in these molecules. To date, most gas-phase TRPES studies have focused on thiouracils, which undergo highly efficient intersystem crossing, and the role of the thiocarbonyl group in promoting these dynamics. Cytosine and its derivatives, which exist as multiple structural isomers in the gas phase, provide an ideal platform to extend this earlier work. The present TRPES study focuses on the photodynamics of enol and keto cytosine derivatives from a new tautomer perspective. Specifically, the effects of thionation (thiocytosine), switching of exocyclic groups (isocytosine), and a ring-substitution (azacytosine) are investigated.