Rational Design of Transition Metal Based Molecular Spin State Switches: Tuning the High Spin to Low Spin Transition Rate by Ligand Substitution

In a systematic study combining experimental and theoretical approaches we investigated the effect of electron withdrawing (EW) and donating (ED) substituents at the axial 4' position on both ground and excited state properties of the [Fe(terpy)2]2+ complex. While these modifications have negligible impact on the ground-state geometries of the complexes, DFT calculations predict systematic changes in the electron density along the molecular axis (Fe and the two axial N atoms) upon substitution, and this is confirmed experimentally by 15N NMR and 57Fe Mössbauer spectroscopy. The predicted variations in the electron density and molecular orbital energies are further corroborated by UV-Vis spectroscopy and cyclic voltammetry measurements. DFT calculations of the excited-state properties relevant for photoswitching applications revealed that the singlet--quintet energy gap is highly sensitive to the ED/EW character of the substituents, whereas the triplet energies are much less affected. Comparison with the quintet lifetimes determined by transient optical absorption spectroscopy (TOAS) experiments show that conventional energy barrier-based theoretical models yield unsatisfactory results for strong substituents. However, inclusion of tunnelling processes between the quintet and singlet states leads to excellent agreement with experiment. The results presented demonstrate the potential of efficient computational screening to guide ligand design for tunable photoactive molecular switches based on abundant 3d transition metals.