Analyzing the Intermolecular Dynamics of Excited States in Molecular Semiconductors

• Applicant: Dr. Sebastian Hammer
• Subject Area: Experimental Condensed Matter Physics
• Term: from 2021 to 2024
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 490894053

Despite intense fundamental as well as application driven research, some important photophysical phenomena observed in molecular materials and their composites are not fully understood, yet. One important aspect, often neglected at first order, is the interaction of photoexcited states in molecular aggregates with the surrounding lattice and its phononic excitations. Hence, the proposed project aims to observe the lattice dynamics directly, i.e. the adiabatic reorganization as well as the coherent and non-coherent generation of phonons, accompanying and supporting the evolution of excitonic states in molecular crystals. For this purpose, ultrafast electron diffraction (UED) as well as ultrafast electron diffuse scattering (UEDS) will be used to allow for insights into the mechanisms of the exciton-lattice coupling. By directly observing the nuclear dynamics via UED and the time resolved phonon population across the entire Brillouin zone via UEDS, already existing models based on indirect spectroscopic evidences in combination with density functional theory calculations and molecular dynamics simulations will be complemented. For this purpose, two bi- /multimolecular excitations, the excimer and the charge transfer (CT) state, which couple strongly to the surrounding lattice and its dynamics, will be investigated on distinct model systems. The pump-probe experiments to determine the time dependent structural response upon photoexcitation will be carried out on structurally highly defined molecular single crystals. The lattice dynamics during the formation of an excimer state will be studied on the model system pyrene. By performing UED and UEDS on pyrene single crystals it is expected that the understanding of the lattice reorganization and phonon generation during excimer formation can be deepened as well as that non-radiative phonon-assisted decay channels can be identified. Both processes will be analyzed as function of the dimer constituents’ molecular masses utilizing perdeuterated pyrene as reference due to its larger mass but similar electronic and structural properties. In the second part of the project, the lattice dynamics as well as phonon-assisted non-radiative decay channels upon photoexcitation of the CT-complex tetracene:F4TCNQ (2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane) will be studied. The intended UED and UEDS studies will offer the unique opportunities (i) to deduce the time dependent electrostatic potential and, hence, the charge (re)distribution in the excited states from the low index Bragg reflections, (ii) to analyze the changes in the intermolecular geometry upon photo excitation and (iii) to identify phonon modes acting as non-radiative loss channels.These results complement the models on photophysical processes in molecular systems and are relevant for future optoelectronic applications exceeding the examined model systems.

• DFG Programme: WBP Fellowship
• International Connection: Canada
• Host: Professor Dr. Bradley Siwick