Instituto Federal de Minas Gerais, Campus Ouro Preto, Ouro Preto 35400-000, MG, Brazil.
Istituto di Fotonica e Nanotecnologie-Consiglio Nazionale delle Ricerche (IFN-CNR), Piazza Leonardo da Vinci 32, I-20133 Milano, Italy.
Molecules. 2022 Feb 10;27(4):1200. doi: 10.3390/molecules27041200.
Understanding the primary steps following UV photoexcitation in sulphur-substituted DNA bases (thiobases) is fundamental for developing new phototherapeutic drugs. However, the investigation of the excited-state dynamics in sub-100 fs time scales has been elusive until now due to technical challenges. Here, we track the ultrafast decay mechanisms that lead to the electron trapping in the triplet manifold for 6-thioguanine in an aqueous solution, using broadband transient absorption spectroscopy with a sub-20 fs temporal resolution. We obtain experimental evidence of the fast internal conversion from the S(ππ*) to the S(nπ*) states, which takes place in about 80 fs and demonstrates that the S(nπ*) state acts as a doorway to the triplet population in 522 fs. Our results are supported by MS-CASPT2 calculations, predicting a planar S(ππ*) pseudo-minimum in agreement with the stimulated emission signal observed in the experiment.
了解硫代 DNA 碱基（硫碱基）中紫外线光激发后的主要步骤对于开发新型光疗药物至关重要。然而，由于技术挑战，直到现在，人们仍难以在亚 100 飞秒时间尺度内研究激发态动力学。在这里，我们使用具有亚 20 飞秒时间分辨率的宽带瞬态吸收光谱法，追踪导致电子在三重态中捕获的超快衰减机制，研究了水溶液中 6-硫鸟嘌呤的情况。我们获得了实验证据，证明 S(ππ*)到 S(nπ*)态的快速内转换发生在大约 80 飞秒内，表明 S(nπ*)态在 522 飞秒内充当三重态的入口。我们的结果得到了 MS-CASPT2 计算的支持，该计算预测 S(ππ*)伪最小平面与实验中观察到的受激发射信号一致。
