Key Laboratory of Theoretical and Computational Photochemistry, Ministry of Education College of Chemistry, Beijing Normal University, Beijing 100875, P. R. China.
College of Chemical Engineering, Zhejiang University of Technology, Hangzhou 310014, Zhejiang, P. R. China.
Phys Chem Chem Phys. 2021 Jun 2;23(21):12421-12430. doi: 10.1039/d1cp01142a.
Sulfur-substituted nucleobases are highly promising photosensitizers that are widely used in photodynamic therapy, and there are numerous studies exploring their unique photophysical behaviors. However, relevant photophysical investigations on selenium and tellurium substitutions are still rare. Herein, the high-level multistate complete-active-space second-order perturbation (MS-CASPT2) method was performed for the first time to explore the excited-state relaxation processes of tellurium-substituted guanine (TeG) and cytosine (TeC). Based on the electronic state properties in the Franck-Condon (FC) region, we found that the lowest five (S0, S1, S2, T1, and T2) and six (S0, S1, S2, T1, T2 and T3) states will participate in the nonadiabatic transition processes of TeG and TeC systems, respectively. In these electronic states, two kinds of minimum and intersection structures (i.e., planar and twisted structures) were obtained for both TeG and TeC systems. The linearly interpolated internal coordinate (LIIC) paths and spin-orbit coupling (SOC) constants revealed several possible planar and twisted excited-state decay channels, which could lead the systems to the lowest reactive triplet state of T1. Small energy barriers in the T1 state will trap the TeG and TeC systems for a while before they finally populate to the ground state. Although tellurium substitution would further redshift the absorption wavelength and enhance the intersystem crossing (ISC) rate to the T1 state compared with sulfur and selenium substitutions, the rapid ISC process of T1 → S0 may make it a less effective photosensitizer to sensitize the molecular oxygen. We believe our present work will provide important mechanistic insights into the photophysics of tellurium-substituted nucleobases.
硫代碱基是一种很有前途的光敏剂，广泛应用于光动力疗法，有大量研究探索其独特的光物理行为。然而，有关硒和碲取代的光物理研究仍然很少。本文首次采用高精度多组态完全活性空间二级微扰（MS-CASPT2）方法研究了碲取代鸟嘌呤（TeG）和胞嘧啶（TeC）的激发态弛豫过程。根据 Franck-Condon（FC）区域的电子态性质，我们发现最低的五个（S0、S1、S2、T1 和 T2）和六个（S0、S1、S2、T1、T2 和 T3）态将参与 TeG 和 TeC 体系的非绝热跃迁过程。在这些电子态中，我们为 TeG 和 TeC 体系都得到了两种最低和交叉结构（即平面和扭曲结构）。线性内坐标（LIIC）路径和自旋轨道耦合（SOC）常数揭示了几种可能的平面和扭曲激发态衰变通道，这些通道可能导致体系到达最低的反应三重态 T1。T1 态中的小能垒会使 TeG 和 TeC 体系暂时困住，然后它们最终会落入基态。尽管与硫和硒取代相比，碲取代会进一步红移吸收波长并提高到 T1 态的系间窜越（ISC）速率，但 T1→S0 的快速 ISC 过程可能使其成为一种不太有效的光敏剂，无法敏化分子氧。我们相信，我们目前的工作将为碲取代碱基的光物理提供重要的机制见解。
