To date, alternating co-polymers based on electron-rich and electron-poor units are the most attractive materials to control functionality of organic semiconductor layers in which ultrafast excited-state processes play a key role. We present a computational study of the photoinduced excited-state dynamics of the 4-(2-thienyl)-2,1,3-benzothiadiazole (BT-1T) molecule, which is a common building block in the backbone of π-conjugated polymers used for organic electronics. In contrast to homo-polymer materials, such as oligothiophene, BT-1T has two non-identical units, namely, thiophene and benzothiadiazole, making it attractive for intramolecular charge transfer studies. To gain a thorough understanding of the coupling of excited-state dynamics with nuclear motion, we consider a scenario based on femtosecond time-resolved x-ray absorption spectroscopy using an x-ray free-electron laser in combination with a synchronized ultraviolet femtosecond laser. Using Tully's fewest switches surface hopping approach in combination with excited-state calculations at the level of configuration interaction singles, we calculate the gas-phase x-ray absorption spectrum at the carbon and nitrogen K edges as a function of time after excitation to the lowest electronically excited state. The results of our time-resolved calculations exhibit the charge transfer driven by non-Born-Oppenheimer physics from the benzothiadiazole to thiophene units during relaxation to the ground state. Furthermore, our ab initio molecular dynamics simulations indicate that the excited-state relaxation processes involve bond elongation in the benzothiadiazole unit as well as thiophene ring puckering at a time scale of 100 fs. We show that these dynamical trends can be identified from the time-dependent x-ray absorption spectrum.

中文翻译：

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时间分辨x射线吸收光谱在粒子-孔激发的4-（2-噻吩基）-2,1,3-苯并噻二唑中超快速动力学的模拟

迄今为止，基于富电子和贫电子单元的交替共聚物是控制有机半导体层功能的最有吸引力的材料，其中超快激发态过程起着关键作用。我们目前对4-（2-噻吩基）-2,1,3-苯并噻二唑（BT-1T）分子的光致激发态动力学进行计算研究，该分子是π骨架中的常见结构单元用于有机电子的共轭聚合物。与均聚物材料（如低聚噻吩）相比，BT-1T具有两个不同的单元，即噻吩和苯并噻二唑，因此对分子内电荷转移研究具有吸引力。为了全面了解激发态动力学与核运动的耦合，我们考虑了一种基于飞秒时间分辨x射线吸收光谱的方案，该方案使用x射线自由电子激光器与同步紫外飞秒激光器相结合。使用Tully最少的开关表面跳变方法，并结合配置相互作用单峰一级的激发态计算，我们计算了碳和氮K的气相x射线吸收光谱激发到最低电子激发态后，时间随时间变化的边缘。我们的时间分辨计算结果表明，在弛豫到基态期间，电荷由非Born-Oppenheimer物理学驱动而从苯并噻二唑转化为噻吩单元。此外，我们的从头算分子动力学模拟表明，在100 fs的时间范围内，激发态弛豫过程涉及苯并噻二唑单元中的键延伸以及噻吩环的折叠。我们表明，可以从随时间变化的X射线吸收光谱中识别出这些动态趋势。