Fluorescent probes are vital molecular tools for DNA biognostics $-$ often aiding in the detection of malignant tumors. Biomimetic nucleobase analogues are amongst the most widely used probes due to their versatility and biocompatibility. Thieno[3,4-d]pyrimidin-4-amine (TPA) and isothiazolo[4,3-d]pyrimidin-7-amine (iTPA) are two recently developed sulphur-containing heteroaromatic fluorescent nucleobase analogues. Their inherent structures give rise to superior biomimetic properties that ensure their optimal interaction with double-helix DNA. Using high-level electronic structure methods, we simulate the absorption and emission spectra in order to better understand the origins of the strong absorption and emission profiles observed experimentally. Radiationless decay pathways are also computed and compared with those of the canonical DNA nucleobases, in order to unravel the geometric and electronic properties that make the thio-analogues fluorescent.

荧光探针是DNA生物诊断的重要分子工具 $--$通常有助于检测恶性肿瘤。仿生核碱基类似物因其多功能性和生物相容性而成为最广泛使用的探针。噻吩并[3,4- d ]嘧啶-4-胺（TPA）和异噻唑并[4,3- d] pyrimidin-7-amine（iTPA）是两个最近开发的含硫杂芳族荧光核碱基类似物。它们的固有结构带来了卓越的仿生特性，从而确保了它们与双螺旋DNA的最佳相互作用。使用高级电子结构方法，我们模拟了吸收和发射光谱，以便更好地了解实验观察到的强吸收和发射轮廓的起源。还计算了无辐射衰变途径并将其与规范DNA核碱基的衰变途径进行比较，以阐明使硫代类似物发荧光的几何和电子性质。