# Electronic Relaxation Dynamics of 6‑Azauracil: The Effect of Ring Substitution on Intersystem Crossing

## TL;DR

This study explores how replacing a carbon with nitrogen in 6-azauracil affects its electronic relaxation and increases intersystem crossing efficiency.

## Contribution

The paper reveals how ring substitution alters deactivation pathways and enhances intersystem crossing in 6-azauracil.

## Key findings

- 6-azauracil shows restricted access to ethylenic conical intersections due to nitrogen substitution.
- Intersystem crossing becomes highly efficient in 6-azauracil compared to uracil.
- Time-resolved photoelectron spectroscopy highlights the role of the lowest singlet excited state in this process.

## Abstract

In an effort to obtain a fundamental understanding of
substitution
effects on the photodynamic response of the canonical nucleobases,
recent spectroscopic studies have focused on derivatives with single
atom substitutions. Uracil undergoes ultrafast internal conversion
to the ground state facilitated by various ring distortions at the
ethylenic bond. In 6-azauracil, site-specific nitrogen substitution
at this carbon double-bond restricts access to ethylenic conical intersections
that lead back to the ground state. Instead, intersystem crossing
into the triplet manifold becomes highly efficient. This study uses
time-resolved photoelectron spectroscopy to investigate the photodynamics
of 6-azauracil with particular focus on the role of the lowest singlet
excited state and hindrance of ethylenic deactivation coordinates
in promoting intersystem crossing.