Photoelectron Spectra of 2-Thiouracil, 4-Thiouracil and 2,4-Dithiouracil

TL;DR

This study simulates UV photoelectron spectra of thiouracils, revealing spectral features and excited-state behaviors, with implications for monitoring photodynamics and intersystem crossing in these molecules.

Contribution

It provides the first theoretical spectra for 4-thiouracil and 2,4-dithiouracil, and analyzes their excited-state photoelectron features using advanced multireference calculations.

Findings

Calculated spectra match experimental data for 2-thiouracil after a shift.

Distinct spectral bands correspond to sulfur, oxygen lone pairs, and pyrimidine π system.

Excited-state spectra enable monitoring of excited populations but complicate intersystem crossing observation.

Abstract

Ground- and excited-state UV photoelectron spectra of thiouracils (2-thiouracil, 4-thiouracil and 2,4-dithiouracil) have been simulated using multireference configuration interaction calculations and Dyson norms as measure for the photoionization intensity. Except for a constant shift, the calculated spectrum of 2-thiouracil agrees very well with experiment, while no experimental spectra are available for the two other compounds. For all three molecules, the photoelectron spectra show distinct bands due to ionization of the sulphur and oxygen lone pairs and the pyrimidine $\pi$ system. The excited-state photoelectron spectra of 2-thiouracil show bands at much lower energies than in the ground state spectrum, allowing to monitor the excited-state population in time-resolved UV photoelectron spectroscopy (TRPES) experiments. However, the results also reveal that single-photon ionization probe schemes alone will not allow monitoring all photodynamic processes existing in 2-thiouracil. Especially, due to overlapping bands of singlet and triplet states the clear observation of intersystem crossing will be hampered.