Systematic mapping of the excited quintet state potentials in model Fe-based molecular photoswitches

The rational design of efficient light-activated functional molecules requires the understanding of their potential energy surfaces (PES), but the ones obtained with quantum chemical (QC) tools are hard to verify with available spectroscopy tools. The combination of X-ray spectroscopy and scattering has been shown to reveal intricate details of the PES, and this is the approach we wish to exploit here. We propose to study modified Fe(II)-polypyridine complexes, model systems for light-switchable molecular magnets, for which QC calculations suggest the feasibility of altering the excited state PES by ligand substitution. Transient optical absorption shows that the lifetime of the excited quintet state changes upon substitution, suggesting variation of the energy landscape. The proposed experiment will test the quintet-singlet energy gap (beyond the molecular structures). Such confirmation of the systematically varied PES shall help us to design molecules with optimized functionality.