# Disruptive Model That Explains for the Long-Lived Triplet States Observed for 2-Thiocytosine upon UVA Radiation

## TL;DR

This paper explains how 2-thiocytosine forms long-lived triplet states under UVA light through radical reactions, without needing complex spin transitions.

## Contribution

A novel theoretical explanation for the formation of triplet states in 2-thiocytosine via radical recombination pathways.

## Key findings

- Radical fragments from the NH group rupture under UVA radiation lead to stable triplet tautomers.
- The amino-thiol form of 2-thiocytosine forms via a barrierless singlet recombination pathway.
- Cytosine does not form triplet states under UVA due to insufficient energy for N–H bond breaking.

## Abstract

The possible role
of radical species in the formation
of the long-lived
triplet states observed for 2-thiocytosine upon UV irradiation was
theoretically investigated. It is predicted that the radical fragments
arising from the homolytic rupture of the NH group of the thiobase
can be yielded upon ultraviolet-A radiation. Recombination of the
radicals through the most favorable singlet channel yields the lowest-lying
tautomer of the 2-thiocytosine (the amino-thiol form) through a barrierless
pathway. The rebounding of the radical fragments along the triplet
channels that emerge from the attack of the hydrogen to the nitrogen
atoms next to the C–S bond leads to stable structures for the
amino-thion-N1H and amino-thion-N3H tautomers.
These results allow for the rationalization of the near-unity triplet
yields observed when this pure light-atom organic molecule is exposed
to UV irradiation, without invoking intersystem crossings between
the electronic states of different spin-multiplicities. A similar
study for cytosine showed that the energy required to induce the homolytic
breaking of the N–H bond of the nucleobase is not attainable
under UVA radiation. This result is consistent with the experimental
fact that no triplet states are observed when this molecule is exposed
to that light.