Time resolved photoelectron study of excited state dynamics of gas phase thiocytosine

Sulfur substitution of an oxygen atom in the canonical nucleobases disables ultrafast internal conversion pathways back to the ground state and instead triplet states are populated with high quantum yields. The stabilization of electronically excited states with sulfur localized orbital transitions results in easily accessible crossing points to the triplet manifold and intersystem crossing becomes efficient due to strong spin orbit coupling. To date, studies into the photophysics of thiobases have primarily focused on thiocarbonyl compounds. Thiocytosine is a thionated derivative of cytosine that purely exists in its thiol form in the gas phase. The present study employs time-resolved photoelectron spectroscopy to investigate the excited state dynamics of thiocytosine. It shows efficient intersystem crossing despite the absence of the thiocarbonyl group and thereby demonstrates the photophysics of a thiobase from a new tautomer perspective.