Key Laboratory of Theoretical and Computational Photochemistry, Ministry of Education, College of Chemistry, Beijing Normal University, Beijing 100875, China.
Phys Chem Chem Phys. 2020 Jun 7;22(21):12120-12128. doi: 10.1039/d0cp01369b. Epub 2020 May 22.
The photophysics of selenium-substituted nucleobases has attracted recent experimental attention because they could serve as potential photosensitizers in photodynamic therapy. Herein, we present a comprehensive MS-CASPT2 study on the spectroscopic and excited-state properties, and photophysics of 2-selenouracil (2SeU), 4-selenouracil (4SeU), and 2,4-selenouracil (24SeU). Relevant minima, conical intersections, crossing points, and excited-state relaxation paths in the lowest five electronic states (i.e., S, S, S, T, and T) are explored. On the basis of these results, their photophysical mechanisms are proposed. Upon photoirradiation to the bright S state, 2SeU quickly relaxes to its S minimum and then moves in an essentially barrierless way to a nearby S/S conical intersection near which the S state is populated. Next, the S system arrives at an S/T/T intersection where a large S/T spin-orbit coupling of 430.8 cm makes the T state populated. In this state, a barrier of 6.8 kcal mol will trap 2SeU for a while. In parallel, for 4SeU or 24SeU, the system first relaxes to the S minimum and then overcomes a small barrier to approach an S/S conical intersection. Once hopping to the S state, there exists an extended region with very close S, T, and T energies. Similarly, a large S/T spin-orbit coupling of 426.8 cm drives the S→ T intersystem crossing process thereby making the T state populated. Similarly, an energy barrier heavily suppresses electronic transition to the S state. The present work manifests that different selenium substitutions on uracil can lead to a certain extent of different vertical and adiabatic excitation energies, excited-state properties, and relaxation pathways. These insights could help understand the photophysics of selenium-substituted nucleobases.
硒取代碱基的光物理性质最近引起了实验关注，因为它们可能作为光动力治疗中的潜在光敏剂。在此，我们通过 MS-CASPT2 研究全面探讨了 2-硒尿嘧啶（2SeU）、4-硒尿嘧啶（4SeU）和 2,4-硒尿嘧啶（24SeU）的光谱和激发态性质及光物理性质。我们研究了最低五个电子态（即 S、S、S、T 和 T）中的相关势能面极小值、锥形交叉点、交叉点和激发态弛豫路径。在此基础上，提出了它们的光物理机制。在受光照射激发到明亮的 S 态后，2SeU 快速弛豫到其 S 能极小值，然后以基本无势垒的方式移动到附近的 S/S 锥形交叉点，S 态在该点上布居。接着，S 体系到达 S/T/T 交叉点，其中 430.8 cm 的大 S/T 自旋轨道耦合使 T 态布居。在该态中，6.8 kcal/mol 的势垒会暂时困住 2SeU。同时，对于 4SeU 或 24SeU，体系首先弛豫到 S 能极小值，然后克服小势垒接近 S/S 锥形交叉点。一旦跃迁至 S 态，存在一个 S、T 和 T 能量非常接近的扩展区域。同样，426.8 cm 的大 S/T 自旋轨道耦合驱动 S→T 系间窜越过程，从而使 T 态布居。同样，一个能量势垒强烈抑制了电子向 S 态的跃迁。本工作表明，在尿嘧啶上进行不同的硒取代可以在一定程度上导致不同的垂直和绝热激发能、激发态性质和弛豫途径。这些见解有助于理解硒取代碱基的光物理性质。
