Aix Marseille Univ, CNRS, ICR, Marseille, France.
Phys Chem Chem Phys. 2018 Jun 20;20(24):16428-16436. doi: 10.1039/c8cp02306a.
The decay of the triplet state of photosensitizers is essential to their performance in singlet-oxygen generation. Experiments have shown that in thionucleosides, this decay is enhanced compared to that in the corresponding thionucleobases. In this work, we applied quantum-chemical methods and chemical-kinetic modeling to investigate the effects of the sugar substituent on the triplet decay of thionucleosides. The computed rates for the energetically favored conformers of thiothymidine, thiouridine, and thioguanosine (and the respective thionucleobases) show a remarkable quantitative agreement with the experimental results. We additionally show that the triplet decay enhancement is caused by the repulsion interaction between the sugar group and the sulfur atom, which reduces the activation energy for intersystem crossing by destabilizing the T1 minimum. In some instances, an intramolecular hydrogen bond stabilizes the energy of the T1/S0 crossing point, also reducing the activation energy. This molecular understanding of the mechanism of enhanced triplet decay provides a guideline to control the triplet decay rate, which was tested in new thiothymidine derivatives.
激发态三重态的衰减对于它们在单线态氧生成中的性能至关重要。实验表明，在硫核苷中，与相应的硫核苷碱基相比，这种衰减得到了增强。在这项工作中，我们应用量子化学方法和化学动力学建模来研究糖取代基对硫核苷三重态衰减的影响。计算得到的硫代胸腺嘧啶、硫代尿嘧啶和硫鸟嘌呤（以及相应的硫核苷碱基）的高能构象的速率与实验结果具有显著的定量一致性。我们还表明，三重态衰减的增强是由于糖基和硫原子之间的排斥相互作用引起的，这种相互作用通过使 T1 能垒不稳定来降低系间窜越的活化能。在某些情况下，分子内氢键稳定了 T1/S0 交叉点的能量，也降低了活化能。这种对增强三重态衰减机制的分子理解为控制三重态衰减速率提供了指导，我们在新的硫代胸腺嘧啶衍生物中进行了测试。
