The ultraviolet (UV) photophysics of the natural and modified nucleobases can be surprisingly different. In response to UV radiation, the natural pyrimidine nucleobases undergo ultrafast internal conversion back to the ground state, whereas their thiobase analogues, in which an oxygen has been replaced by sulfur, instead display efficient intersystem crossing to the triplet manifold. Here, the effect of the substituent position is investigated with time-resolved photoelectron spectroscopy on 4-thiouracil, which is contrasted to previous work on 2-thiouracil. Although the photophysical pathway of both structural isomers is similar, i.e., leading from the S₂ (¹ππ^∗) state, via S₁ (¹nπ^∗), to the triplet manifold and subsequently back to the ground state, the intersystem crossing dynamics are strongly influenced by the surrounding intramolecular environment of the sulfur atom.

天然和修饰核碱基的紫外线（UV）光物理特性可能出奇地不同。响应于紫外线辐射，天然嘧啶核苷经历超快内部转化，回到基态，而其硫代碱类似物（其中的氧气已被硫所取代）显示出有效的系统间交叉与三重态歧管。在此，通过时间分辨光电子能谱研究了4-硫尿嘧啶上取代基位置的影响，这与先前对2-硫尿嘧啶的研究形成了对比。虽然这两种结构异构体的光物理途径是相似的，即，从领先₂（¹ ππ ^*）状态下，通过S ₁（¹ Nπ ^*），到三重态歧管并随后回到基态，系统间交叉动力学受到周围硫原子分子内环境的强烈影响。