Intermolecular conical intersections: Steering coherent energy transport in functional organic nanostructures

• Applicants: Dr. Antonietta De Sio; Professor Dr. Thomas Frauenheim; Professor Christoph Lienau, Ph.D.; Privatdozentin Dr. Elena Mena-Osteritz, since 4/2024
• Subject Area: Experimental Condensed Matter Physics
• Term: since 2021
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 465141364

The applicants have recently uncovered the first direct signatures of intermolecular conical intersections (CoIns) in aggregated organic thin films (A. De Sio et al., Nature Nano (2020)). Using ultrafast two-dimensional electronic spectroscopy with sub-10-fs time resolution we could follow, in the time domain, the coherent propagation of an optically launched electronic wavepacket on the bright excited state potential energy surface (PES) for a few tens of fs, its reflection-less, highly efficient passage through a CoIn within < 10 fs followed by its dissipation on lower-lying dark states. Our results suggest that such intermolecular CoIns may be a universal feature of aggregated organic thin films and that they may have a profound impact on energy, charge and spin relaxation in those materials. Specifically, they indicate that electronic delocalization in strongly dipole-coupled aggregates resulting in dark and bright states is fundamental for the existence of those CoIns. We propose to test this hypothesis by synthesizing prototypical donor-acceptor type homo- and heterodimers, forming aggregates with well-defined side-by-side and head-to-tail ordering in thin films. Precise control of the aggregate geometry and composition is the key to adjust the relative energetics of the delocalized dark and bright electronic states and, thus, for inducing or suppressing energy or charge transfer via intermolecular CoIns. We will use advanced multi-dimensional ultrafast electronic spectroscopy to explore the photophysical properties of these aggregates and in particular to monitor coherent wavepacket motion on and nonadiabatic transitions between their bright and dark excited state PESs. The experimental studies will be directly related to atomistic nonadiabatic molecular dynamics simulations of vibronic couplings and curve crossing dynamics in such dimer structures. This joint experimental-theoretical approach shall allow us to uncover the existence of intermolecular CoIns in the PESs of these model systems, probe the interplay between electronic ordering and nonadiabatic dynamics and test the potential competition between intermolecular energy relaxation via CoIns and intramolecular charge transfer. In an outlook, we will introduce coupling to dielectric or plasmonic cavities as an approach for tailoring the PES of these aggregates towards manipulating ultrafast nonadiabatic CoIn dynamics, as predicted by recent theoretical work. As such, this project shall explore the yet unknown role of intermolecular CoIns for the fate of optical excitations in organic thin films for photovoltaics. These results shall also provide conceptually new insight into role of strong couplings and non-Born-Oppenheimer dynamics for coherent energy and charge transport in organic materials and nanostructures in general.

• DFG Programme: Research Grants
• International Connection: USA
• Cooperation Partner: Sergei Tretiak, Ph.D.
• Ehemaliger Antragsteller: Professor Dr. Peter Bäuerle, until 3/2024