DNA in living beings is constantly damaged by exogenous and endogenous agents. However, in some cases, DNA photodamage can have interesting applications, as it happens in photodynamic therapy. In this work, the current knowledge on the photophysics of 4-thiouracil has been extended by further quantum-chemistry studies to improve the agreement between theory and experiments, to better understand the differences with 2-thiouracil, and, last but not least, to verify its usefulness as a photosensitizer for photodynamic therapy. This study has been carried out by determining the most favorable deactivation paths of UV–vis photoexcited 4-thiouracil by means of the photochemical reaction path approach and an efficient combination of the complete-active-space second-order perturbation theory//complete-active-space self-consistent field (CASPT2//CASSCF), (CASPT2//CASPT2), time-dependent density functional theory (TDDFT), and spin-flip TDDFT (SF-TDDFT) methodologies. By comparing the data computed herein for both 4-thiouracil and 2-thiouracil, a rationale is provided on the relatively higher yields of intersystem crossing, triplet lifetime and singlet oxygen production of 4-thiouracil, and the relatively higher yield of phosphorescence of 2-thiouracil.

中文翻译：

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4-硫氧嘧啶光物理性质的量子化学研究及其与2-硫氧嘧啶的比较

生物体中的 DNA 不断受到外源性和内源性因素的损害。然而，在某些情况下，DNA 光损伤可以有有趣的应用，就像光动力疗法中发生的那样。在这项工作中，通过进一步的量子化学研究扩展了目前关于 4-硫氧嘧啶光物理学的知识，以提高理论与实验之间的一致性，更好地理解与 2-硫氧嘧啶的差异，最后但并非最不重要的一点是，验证其作为光动力疗法的光敏剂的有用性。本研究通过光化学反应路径方法和完全活性空间二阶微扰理论//完全活性的有效结合，确定了紫外可见光激发4-硫氧嘧啶的最有利失活路径。 -空间自洽场（CASPT2//CASSCF）、（CASPT2//CASPT2）、时间相关密度泛函理论（TDDFT）和自旋翻转TDDFT（SF-TDDFT）方法。通过比较本文计算的 4-硫氧嘧啶和 2-硫氧嘧啶的数据，提供了 4-硫氧嘧啶的系间窜越、三线态寿命和单线态氧产生的相对较高产率以及 2-硫氧嘧啶的磷光相对较高产率的基本原理。硫氧嘧啶。