发表机构
Deutsches Elektronen-Synchrotron DESY; Department of Chemistry, Kyungpook National University; CNR-Istituto Officina dei Materiali (IOM); James R. Macdonald Laboratory, Physics Department, Kansas State University; Elettra-Sincrotrone Trieste S.C.p.A.; Aix-Marseille Université, CNRS, ICR; Institute of Physical Chemistry, Universität Hamburg（德国电子同步辐射中心; 庆北国立大学化学系; 意大利国家研究委员会材料研究所; 堪萨斯州立大学詹姆斯·R·麦克唐纳物理实验室; 的里雅斯特电 synchrotron 公司; 艾克斯-马赛大学，法国国家科学研究中心，化学反应研究所; 汉堡大学物理化学研究所）
机构由 AI 辅助整理，请以论文原文为准。AI 中文总结
本研究通过时间分辨多中心X射线光电子能谱揭示胸腺嘧啶光动力学中暗态为单重态与三线态混合，并阐明甲基的双重作用，为解析核碱基稳定性提供新方法。
AI 中文摘要
DNA核碱基的光稳定性依赖于紫外线激发后的超快非辐射弛豫。在胸腺嘧啶中，内部转换（IC）与系间窜越（ISC）之间的竞争决定了超快能量耗散与潜在长寿命光化学之间的平衡。然而，连接早期单重态电子和结构演化与长寿命三线态暗态形成的精确分子机制仍不清楚。在此，我们利用时间分辨多中心X射线光电子能谱（tr-mc-XPS）并结合最先进的从头算量子计算，研究了胸腺嘧啶的气相光动力学。通过同时探测多个碳1s核心能级，我们追踪了位点特异性的电子和核动力学。我们发现，长寿命暗态最初并非纯三线态；相反，在前240皮秒内它以单重态和三线态的混合态存在，之后才完全转变为$^3\pi\pi^*$态。在飞秒时间尺度上，我们解析了原子特异的振动相干性，频率约为730 cm$^{-1}$（一种周期为46飞秒的环呼吸模式），该相干性在初始的$^1\pi\pi^*$到$^1n\pi^*$内部转换后仍然存在。此外，我们确定了甲基基团（C9H$_3$）的关键双重作用。在单重态弛豫过程中，甲基基团作为惯性旁观者，其质量阻碍了直接内部转换到基态。然而，在$^3\pi\pi^*$态形成后，态依赖的超共轭将甲基基团与嘧啶环耦合，使其成为ISC的高度敏感电子报告器。这些发现确立了tr-mc-XPS作为一种强大方法，用于解析控制核碱基稳定性的复杂、多时间尺度光动力学。
英文摘要
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.
