Deciphering the internal conversion and triplet formation in thymine via time-resolved multi-center X-ray photoelectron spectroscopy
Abstract
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 ∼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 (C9H3). 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.
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BibTeX
@misc{wang2026deciphering,
title = {{Deciphering the internal conversion and triplet formation in thymine via time-resolved multi-center X-ray photoelectron spectroscopy}},
author = {Xiaojun Wang and Woojin Park and Dennis Mayer and Fabiano Lever and Giovanni Cirmi and Michele Di Fraia and Benjamin Erk and Marion Kuhlmann and Zane Phelps and Oksana Plekan and Federico Pressacco and Sebastian Schulz and Cheol Ho Choi and Miquel Huix-Rotllant and Markus Gühr},
howpublished = {arXiv},
year = {2026},
month = oct,
doi = {10.48550/arxiv.2610.01419},
eprint = {2610.01419},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2610.01419},
}
RIS
TY - UNPB
TI - Deciphering the internal conversion and triplet formation in thymine via time-resolved multi-center X-ray photoelectron spectroscopy
AU - Xiaojun Wang
AU - Woojin Park
AU - Dennis Mayer
AU - Fabiano Lever
AU - Giovanni Cirmi
AU - Michele Di Fraia
AU - Benjamin Erk
AU - Marion Kuhlmann
AU - Zane Phelps
AU - Oksana Plekan
AU - Federico Pressacco
AU - Sebastian Schulz
AU - Cheol Ho Choi
AU - Miquel Huix-Rotllant
AU - Markus Gühr
PY - 2026
DA - 2026/10/01
PB - arXiv
DO - 10.48550/arxiv.2610.01419
UR - https://arxiv.org/abs/2610.01419
AB - 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 ∼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.
ER -