First Principles Simulations of Soft X-ray Spectroscopy of Molecular Charge and Energy Transfer Processes

这个项目的首要目标是发展和数值实现描述分子能量和电荷转移的软X射线光谱的理论方法。在用于光催化剂或有机光伏的新型太阳能材料中，涉及价激发的能量和电荷转移是普遍存在的过程。在这里，未来的性能改进将受益于对分子尺度的深入机理理解。此外，通过利用非平凡的量子效应作为工作原理，可以克服由于经典物理而造成的性能限制。使用光学检测的这些过程的共同特征通常面临拥塞光谱的挑战，这很难明确地分配。相比之下，X射线光谱学是特定于元素的，因此可以通过分子络合物的单个原子的吸收边来确定它们的地址。在这个快速发展的领域，大型设备和台式软X射线源正变得越来越可用，这需要足够的理论发展。虽然用光学泵浦和X射线探头等技术研究含有单一过渡金属中心的化合物已成为当今的标准，但对具有多个金属中心的体系的单独研究仍然很少。特别是，第一原理理论在涉及到重要的L边时面临着它的局限性，因为它们的光谱是由多组态和自旋-轨道耦合效应形成的。这个项目将遵循一种新的战略来应对这种情况。利用X射线吸收的局域性质，多发色络合物将用相互作用的碎片来描述。这个想法是从Frenkel激子理论中借来的，该理论在价激发的背景下得到了很好的确立。应用于多发色络合物的价态和核心激发态，将为描述光触发价态动力学的X射线光谱提供一个统一的框架。两种类型的申请将被考虑。首先，在原则证明研究中将考虑双金属化合物，从而扩大了最初提案的范围。其次，将结合实验研究有机光伏模型体系中的能量传递和界面电荷分离。将特别强调揭示激子波包相干性和离域化的特征，这将给这一领域正在进行的辩论带来新的转折。

The overarching goal of this project is the development and numerical implementation of theoretical methods for the description of the soft X-ray spectroscopy of molecular energy and charge transfer. Energy and charge transfer involving valence excitations are ubiquitous processes in novel solar energy materials used as photocatalysts or in organic photovoltaics. Here, future performance improvements will benefit from an in-depth mechanistic understanding on the molecular scale. Moreover, performance limitations due to classical Physics might be overcome by making use of nontrivial quantum effects as a working principle. Common characterization of these processes employing optical detection often faces the challenge of congested spectra, which are difficult to assign unambiguously. In contrast, X-ray spectroscopy is element specific such that individual atoms of a molecular complex can be addressed by means of their absorption edges. Large scale facilities but also table top soft X-ray sources are becoming increasingly available in this rapidly developing field, demanding for adequate theoretical developments. While the investigation of compounds containing a single transition metal center with, for instance, optical pump and X-ray probe techniques is becoming standard nowadays, respective studies of systems with multiple metal centers are still rare. In particular first principles theory faces its limits when it comes to the important L-edges since their spectra are shaped by multiconfigurational and spin-orbit coupling effects. This project will follow a new strategy to cope with this situation. Making use of the local nature of X-ray absorption, multichromophoric complexes will be described in terms of interacting fragments. The idea is borrowed from Frenkel exciton theory, which is well-established in the context of valence excitations. Application to valence and core excited states of multichromophoric complexes will provide a unifying frame for the description of the X-ray spectroscopy of optically triggered valence state dynamics. Two types of applications will be considered. First, in a proof-of-principle study bimetallic compounds will be considered, thus extending the scope of the original proposal. Second, energy transfer and interfacial charge separation in model systems for organic photovoltaics will be investigated in close cooperation with experiment. Particular emphasis will be paid to unravel the signatures of exciton wave packet coherence and delocalization, which shall give a new twist to an ongoing debate in this field.