Sánchez-Rodríguez José A, Mohamadzade Abed, Mai Sebastian, Ashwood Brennan, Pollum Marvin, Marquetand Philipp, González Leticia, Crespo-Hernández Carlos E, Ullrich Susanne
Department of Physics and Astronomy, University of Georgia, Athens, GA 30602, USA.
Phys Chem Chem Phys. 2017 Aug 2;19(30):19756-19766. doi: 10.1039/c7cp02258a.
Single-atom substitution within a natural nucleobase-such as replacing oxygen by sulfur in uracil-can result in drastic changes in the relaxation dynamics after UV excitation. While the photodynamics of natural nucleobases like uracil are dominated by pathways along singlet excited states, the photodynamics of thiobases like 2-thiouracil populate the triplet manifold with near unity quantum yield. In the present study, a synergistic approach based on time-resolved photoelectron spectroscopy (TRPES), time-resolved absorption spectroscopy (TRAS), and ab initio computations has been particularly successful at unraveling the underlying photophysical principles and describing the dissimilarities between the natural and substituted nucleobases. Specifically, we find that varying the excitation wavelength leads to differences between gas-phase and condensed-phase experimental results. Systematic trends are observed in the intersystem crossing time constants with varying excitation wavelength, which can be readily interpreted in the context of ab initio calculations performed both in vacuum and including solvent effects. Thus, the combination of TRPES and TRAS experiments with high-level computational techniques allows us to characterize the topology of the potential energy surfaces defining the relaxation dynamics of 2-thiouracil in both gas and condensed phases, as well as investigate the accessibility of conical intersections and crossings, and potential energy barriers along the associated relaxation coordinates.
在天然核碱基内进行单原子取代——比如在尿嘧啶中用硫取代氧——会导致紫外激发后弛豫动力学发生剧烈变化。虽然像尿嘧啶这样的天然核碱基的光动力学主要由单重激发态的途径主导，但像2-硫尿嘧啶这样的硫代碱基的光动力学以接近单位量子产率填充三重态。在本研究中，一种基于时间分辨光电子能谱（TRPES）、时间分辨吸收光谱（TRAS）和从头算计算的协同方法在揭示潜在的光物理原理以及描述天然和取代核碱基之间的差异方面特别成功。具体而言，我们发现改变激发波长会导致气相和凝聚相实验结果之间存在差异。在不同激发波长下的系间窜越时间常数中观察到了系统趋势，这可以在真空以及包括溶剂效应的情况下进行的从头算计算的背景下轻松解释。因此，TRPES和TRAS实验与高级计算技术的结合使我们能够表征定义气相和凝聚相中2-硫尿嘧啶弛豫动力学的势能面的拓扑结构，以及研究锥形交叉点和交叉的可达性，以及沿着相关弛豫坐标的势能垒。
