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

• Applicant: Professor Dr. Oliver Kühn
• Subject Area: Theoretical Chemistry: Electronic Structure, Dynamics, Simulation ; Physical Chemistry of Molecules, Liquids and Interfaces, Biophysical Chemistry
• Term: since 2014
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 252105138

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.

• DFG Programme: Research Grants