Internal conversion and intersystem crossing pathways in UV excited, isolated uracils and their implications in prebiotic chemistry

TL;DR

This study investigates the ultrafast photodynamics of uracil and related compounds in the gas phase, revealing pathways that influence their photostability and implications for prebiotic chemistry.

Contribution

It provides new insights into the intersystem crossing pathways and excited-state relaxation times of uracil and analogs, highlighting their potential susceptibility to UV damage.

Findings

Femtosecond to picosecond relaxation times for singlet states.

Evidence of intersystem crossing to triplet states.

Triplet state trapping and back-crossing may affect photostability.

Abstract

The photodynamic properties of molecules determine their ability to survive in harsh radiation environments. As such, the photostability of heterocyclic aromatic compounds to electromagnetic radiation is expected to have been one of the selection pressures influencing the prebiotic chemistry on early Earth. In the present study, the gas-phase photodynamics of uracil, 5-methyluracil (thymine) and 2-thiouracil -- three heterocyclic compounds thought to be present during this era -- are assessed in the context of their recently proposed intersystem crossing pathways that compete with internal conversion to the ground state. Specifically, time-resolved photoelectron spectroscopy measurements evidence femtosecond to picosecond timescales for relaxation of the singlet 1$\pi\pi$* and 1n$\pi$* states as well as for intersystem crossing to the triplet manifold. Trapping in the excited triplet state and intersystem crossing back to the ground state are investigated as potential factors contributing to the susceptibility of these molecules to ultraviolet photodamage.