Molecular-Level Approaches to Photosensitive Nanostructured Materials: A Combined Theoretical and Experimental Study of Ultrafast Energy and Charge Transfer

• Applicant: Professorin Dr. Irene Burghardt
• Subject Area: Theoretical Chemistry: Electronic Structure, Dynamics, Simulation ; Physical Chemistry of Solids and Surfaces, Material Characterisation
• Term: from 2012 to 2022
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 209000113

This project is concerned with the molecular-level investigation of photoinduced charge generation in perylene diimide (PDI)-based donor-acceptor (DA) block co-oligomer species forming highly ordered lamellar mesophases, which show promise for applications in organic photovoltaics. These DA systems represent the most recent generation of a new class of materials which have been spectroscopically and theoretically investigated during the initial funding period of this project, in the framework of a joint DFG/ANR project with the group of S. Haacke at Strasbourg University. Notably, it was shown in these previous investigations that (i) the first-generation materials show ultrafast charge separation but high recombination rates, which could be explained by the specific molecular packing properties, (ii) the second-generation materials can be chemically tuned, such as to obtain charge transfer states with lifetimes up to a few nanoseconds, and low recombination rates in solution. Against this background, the present follow-up project is devoted to the detailed modeling of the second-generation materials in a highly ordered thin-film morphology. Based upon available structural data as well as new time-resolved spectroscopic studies that are currently underway, we will use a combination of electronic structure and microelectrostatics calculations, quantum dynamical calculations, molecular dynamics simulations, and Kinetic Monte Carlo (KMC) simulations to obtain a detailed picture of the process, from the initial charge separation to long-range carrier transport. A first-principles parametrized lattice Hamiltonian will be constructed using information from electronic structure calculations for suitable fragments, along with a microelectrostatics treatment that accurately describes the local electrostatics including polarization effects. This lattice Hamiltonian provides a unified representation for both quantum dynamical simulations (on the femtosecond to picosecond scale) and KMC simulations (on the picosecond to microsecond scale), and permits a consistent embedding of the dynamical modelling in a multi-scale setting. In this context, high-dimensional quantum dynamical calculations using the Multi-Layer Multiconfiguration Time-Dependent Hartree (ML-MCTDH) method will be typically carried out for more than one hundred electronic states and vibrational modes, to cover a time scale up to tens of picoseconds. A special focus will be placed upon the role of molecular packing, inducing exciton and charge delocalization, along with additional transfer pathways. In particular, PDI stacking is expected to lead to a complex photochemistry of charge separated species, which could tangibly influence the charge generation and transport. The present analysis is expected to quantify the combined effects of chemical composition and molecular packing, providing essential ingredients for the rational design of next-generation DA materials.

• DFG Programme: Research Grants
• International Connection: France
• Cooperation Partner: Professor Dr. Stefan Haacke