# Ab Initio Molecular Dynamics Relaxation and Intersystem Crossing Mechanisms of 5-Azacytosine

## TL;DR

This study uses advanced molecular dynamics simulations to explore the relaxation and intersystem crossing mechanisms of photoexcited 5-azacytosine, revealing ultrafast ground state relaxation and low intersystem crossing yield due to small spin-orbit couplings.

## Contribution

It provides detailed mechanistic insights into 5-azacytosine's relaxation pathways and explains the low intersystem crossing yield based on electronic state character and spin-orbit coupling differences.

## Key findings

- Ultrafast relaxation to ground state within 1 ps.
- Identification of two conical intersections as relaxation funnels.
- Low intersystem crossing yield explained by small spin-orbit couplings.

## Abstract

The gas phase relaxation dynamics of photoexcited 5-azacytosine has been investigated by means of SHARC (surface-hopping including arbitrary couplings) molecular dynamics, based on accurate multireference electronic structure computations. Both singlet and triplet states were included in the simulations in order to investigate the different internal conversion and intersystem crossing pathways of this molecule. It was found that after excitation, 5-azacytosine undergoes ultrafast relaxation to the electronic ground state with a time constant of about 1~picosecond. Two important conical intersections have been identified as the funnel responsible for this deactivation mechanism. The very low intersystem crossing yield of 5-azacytosine has been explained by the size of the relevant spin-orbit coupling matrix elements, which are significantly smaller than in related molecules like cytosine or 6-azauracil. This difference is due to the fact that in 5-azacytosine the lowest singlet state is of $n_\mathrm{N}\pi^*$ nature, whereas in cytosine and 6-azauracil it is of $n_\mathrm{O}\pi^*$ character.