The effect of methylation on the excited state dynamics of 2-thiouracil

Thiouracils, where an oxygen atom in uracil is replaced by sulfur, have been observed to undergo fast and efficient intersystem crossing (ISC) from the lowest singlet excited state, S₁(np*), to the triplet manifold along a sulfur-out-of-plane (op-S) coordinate. Our recent time-resolved photoelectron spectroscopy (TRPES) studies on a series of thiouracils have shown that minor changes such as the position and degree of thionation profoundly alter the subsequent dynamics on the lowest triplet state, T₁. These differences are attributed to intricate details of the potential energy topography, specifically the presence and relative energies of two minima with boat-like (T_{1 boat}) and sulfur-out-of-plane (T_{1 op-S}) geometries. In the case of 2-thiouracil, the excited state population equilibrates between both minima. ISC back to the ground state (GS) depends on spin-orbit couplings (SOC) and accessibility of the T₁/GS crossing points at boat-like and op-S geometries with T_{1 op-S}/GS clearly favored.

The TRPES study presented here interrogates the equilibration and ISC dynamics of the lowest triplet state of 2-thiothymine. Methylation provides a sensitive probe of barriers along a pathway that involves displacements of the methyl substituent and will manifest itself in slower triplet dynamics. As such, it provides insight into the role of the boat-like minimum in the excited state deactivation mechanism.