A singlet and triplet excited-state dynamics study of the keto and enol tautomers of cytosine

TL;DR

This study uses ab initio surface hopping simulations to explore the excited-state dynamics of keto and enol cytosine tautomers, revealing the roles of singlet and triplet states and matching experimental decay times.

Contribution

It provides a detailed computational analysis of the excited-state relaxation pathways of cytosine tautomers, highlighting the significance of triplet states and conical intersections.

Findings

Triplet states significantly influence keto tautomer relaxation.

Decay times of 7 fs, 270 fs, and 1900 fs match experimental data.

Enol tautomer relaxes via internal conversion through conical intersections.

Abstract

The photoinduced excited-state dynamics of the keto and enol forms of cytosine is investigated using ab initio surface hopping in order to understand the outcome of molecular beam femtosecond pump-probe photoionization spectroscopy experiments. Both singlet and triplet states are included in the dynamics. The results show that triplet states play a significant role in the relaxation of the keto tautomer, while they are less important in the enol tautomer. In both forms, the T$_1$ state minimum is found too low in energy to be detected in standard photoionization spectroscopy experiments and therefore experimental decay times should arise from a simultaneous relaxation to the ground state and additional intersystem crossing followed by internal conversion to the T$_1$ state. In agreement with available experimental lifetimes, we observe three decay constants of 7 fs, 270 fs and 1900 fs - the first two coming from the keto tautomer and the longer one from the enol tautomer. Deactivation of the enol form is due to internal conversion to the ground state via two S$_1$/S$_0$ conical intersections of ethylenic type.