Room temperature functional photoswitches: remote control of metallic electronic and spin changes exploiting ligand dynamics

• Applicant: Dr. Katharina Kubicek
• Subject Area: Physical Chemistry of Molecules, Liquids and Interfaces, Biophysical Chemistry
• Term: since 2022
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 497431350

This research project aims to understand the elementary reaction steps following photo-induced electronic excitation and the associated atomic structural dynamics in solvated molecular compounds and larger assemblies relevant for functional switches, but also towards photocatalysis, light-harvesting, and related processes. Understanding and controlling ultrafast photoinduced molecular switching reactions is a crucial goal of physics, chemistry, and biology with applications ranging from light-driven molecular manipulation, logical devices, actuators and engines to data storage and information technology.This proposal concentrates on new spin-change coordination complexes with earth-abundant metal centers and photochromic ligands. They can accomplish reversible switching between metal high spin (HS) and low spin (LS) states with room temperature stability via light-triggered isomerization of the ligand. This goes beyond traditional spin crossover complexes which transform between metal LS and HS states by direct metal-to-ligand charge transfer excitation, and fail to operate reliably as long-term stable spin switches at room temperature. Our approach departs from merely acting on the photophysics of the central metal, but triggers its desired function remotely by photochemical changes on the ligand. This gives us ample flexibility to unpack the chemical toolbox using varying ligand molecules to chemically optimize the function. A promising starting candidate system is solvated [Fe(H2B(pz)2)2phen*] (pz = 1-pyrazolyl), featuring a photocyclizable diarylethene-derived ligand phen*. It exhibits an exceptionally large 40% metal HS -> LS photoconversion yield, and resides in its photoexcited LS state for nearly 3 weeks at room temperature. It belongs to the larger class of so-called Ligand-Driven Light-Induced Spin Change (LD-LISC) systems, which we will investigate with (ultra)fast time-resolved techniques.A suite of advanced pump-probe methods enables us to identify and trace the ensuing excited state dynamics with pico- to femtosecond time resolution. Next to conventional laboratory-based optical laser spectroscopic tools we use x-ray spectroscopy and scattering techniques at synchrotrons and free-electron lasers to extract new information about the changes in the electronic and geometric structure next to the actual spin state of the reacting system. We exploit both chemical- and photo-steering of reaction pathways to enhance the stability and switching efficiency at room temperature.Synthesis and theoretical modeling of suitable complexes will be supported by the experiments revealing details about the excited state manifold. This sequence enables new strategies for the detailed control and rational targeted development of new photoswitches. Using the indirect (ligand-driven) switching technique holds promise to enter the room temperature range for functional, efficient and long-term stable molecular devices.

• DFG Programme: Research Grants