Deciphering the internal conversion and triplet formation in thymine via time-resolved multi-center X-ray photoelectron spectroscopy
The photostability of DNA nucleobases relies on ultrafast nonradiative relaxation following ultraviolet excitation. In thymine, the competition between internal conversion (IC) and intersystem crossing (ISC) determines the balance between ultrafast energy dissipation and potentially long-lived photochemistry. However, the precise molecular mechanisms linking early singlet-state electronic and structural evolution to the formation of long-lived triplet dark states remain unresolved. Here, we investigate the gas-phase photodynamics of thymine using time-resolved multi-center X-ray photoelectron spectroscopy (tr-mc-XPS) supported by state-of-the-art ab initio quantum calculations. By simultaneously probing multiple carbon 1s core levels, we track site-specific electronic and nuclear dynamics. We discover that the long-lived dark state is not initially a pure triplet; rather, it exists as a mixture of singlet and triplet states for the first 240 ps before transitioning fully to the $^3ππ^*$ state. On the femtosecond timescale, we resolve atom-specific vibrational coherence at $\sim$730 cm$^{-1}$ (a ring-breathing mode with a period of 46 fs) that survives the initial $^1ππ^*$ to $^1nπ^*$ IC. Furthermore, we identify a critical dual role for the methyl group (C9H$_3$). During singlet relaxation, the methyl group acts as an inertia spectator, and its mass hinders direct internal conversion to the ground state. Upon $^3ππ^*$ state formation, however, state-dependent hyperconjugation couples the methyl group to the pyrimidine ring, turning it into a highly sensitive electronic reporter of ISC. These findings establish tr-mc-XPS as a powerful approach for disentangling the complex, multi-timescale photodynamics governing nucleobase stability.