Photophysical pathways of 2,4-dithiothymine in microhydrated clusters: insights from theory into UVA activity

DNA nucleobases have evolved to be inherently photostable, dissipating UV energy through ultrafast internal conversion. In contrast, sulfur substitution at the C2 and C4 positions of thymine (2,4-dithiothymine, 2,4dtThy) dramatically alters this behavior, shifting a photoprotective structure toward efficient UVA absorption. Here, we employ TD-DFT calculations at the B3LYP-D3(BJ)/def2-TZVP level to investigate the electronic and structural factors underlying this transformation in both isolated and microsolvated environments. Our results reproduce the experimentally observed ~100 nm redshift of the absorption maximum into the UVA region (356 nm vs. 363 nm expt.). Analysis of aromaticity indices reveals that the enhanced triplet yield and longevity of 2,4dtThy arise from a transition between antiaromatic character in thymine and a stabilized Baird-aromatic state in the triplet manifold. Moreover, the photophysical pathway S 1 (nπ*) to T 1 (π*), with a gap of only ~0.5 eV, remains largely unaffected by microsolvation. These findings provide a fundamental electronic rationale for the ultrafast (~180 fs) intersystem crossing observed experimentally. • HOMER index quantifies the aromatic stabilization of T1 for 2,4-dithiothymine. • Microhydrated clusters preserve the S1-T1 gap (~0.5 eV). • QTAIM reveals weaker C S⋯H bonds that maintain the sulfur-centered excitation. • N H⋯O bonds anchor the ring without quenching photosensitivity. • The sulfur-centered nπ excitation remains active under specific microhydration.