Insights into the Deactivation of 5-Bromouracil after UV Excitation

TL;DR

This study investigates the deactivation mechanisms of 5-bromouracil after UV excitation using quantum chemistry and nonadiabatic dynamics, revealing that ground state relaxation is likely the main pathway in the gas phase.

Contribution

It combines high-level quantum chemistry with nonadiabatic dynamics simulations to elucidate the primary deactivation pathways of 5-bromouracil post-UV irradiation.

Findings

Ground state relaxation is the predominant pathway.

Intersystem crossing involves 1nOpi* and 3pipi* states.

C-Br photodissociation is a possible but less likely pathway.

Abstract

5-Bromouracil is a nucleobase analogue that can replace thymine in DNA strands and acts as a strong radiosensitizer, with potential applications in molecular biology and cancer therapy. Here, the deactivation of 5-bromouracil after UV irradiation is investigated in the singlet and triplet manifold by accurate quantum chemistry calculations and nonadiabatic dynamics simulations. It is found that after irradiation to the bright 1pipi* state, three main relaxation pathways are in principle possible: relaxation back to the ground state, intersystem crossing, and C-Br photodissociation. Based on accurate MS-CASPT2 optimizations, we propose that ground state relaxation should be the predominant deactivation pathway in gas phase. We then employ different electronic structure methods to assess their suitability to carry out excited-state dynamics simulations. MRCIS was used in surface hopping simulations to compute the ultrafast intersystem crossing dynamics, which mostly involves the 1nOpi* and 3pipi* states.