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Analyzing the Intermolecular Dynamics of Excited States in Molecular Semiconductors

Subject Area Experimental Condensed Matter Physics
Term from 2021 to 2024
Project identifier Deutsche Forschungsgemeinschaft (DFG) - Project number 490894053
 
Final Report Year 2024

Final Report Abstract

Molecular semiconductors are materials comprising van der Waals bonded units of individual molecules. In a static picture the optical and electronic properties of these materials are mainly determined by the properties of the molecular units and their orientation to each other due to the weak intermolecular interaction. The soft van der Waals bonds allow for large amplitudes of lattice vibrations which can dynamically change the respective molecular positions on time scales of electronic excitations. Even static lattice distortions are possible. While the electronic part of these processes can be observed well using spectroscopic techniques, the nuclear part, i.e. the dynamics of the molecular units, is often only accessible indirectly by means of quantum chemical simulations which, due to the size of the unit cell, come with great computational effort. This project aims to directly observe the lattice dynamics and distortions that accompany optically excited states using ultrafast electron diffraction. The analysis is carried out using a software package developed as part of the project. One of the material systems investigated is the molecular semiconductor zinc phthalocyanine (ZnPc) in its crystallographic ɑ-polymorph. This material is, for example, being investigated as a donor in organic solar cells, and is a prototypical excimer material. During excimer formation, a molecular exciton is energetically stabilized by a lattice distortion and transformed into a long-living state. The diffraction experiments performed show that excimer formation in ZnPc occurs in two steps. A fast (~400 fs) decrease of the intermolecular distance between two neighboring molecules is followed by a shearing motion aligning their π-systems (~15 ps), which corresponds to the final excimer geometry that is stable over 300 ps. Thanks to the new experimental insights, the parameter space for quantum chemical calculations could be precisely restricted so that further insights into the exact geometry could be gained in a reasonable amount of time and computational effort. For the first time, the structural dynamics of excimer formation could be observed directly. Since measurement and analysis methods can be transferred directly to other systems, technology-relevant systems can now be investigated directly and, for example, loss channels can be identified and modified.

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