Probing the Impact of Solvation on Photoexcited Spin Crossover Complexes with High-Precision X-ray Transient...

Investigating the photoinduced electronic and structural response of bistable molecular building blocks incorporating transition metals in solution phase constitutes a necessary stepping stone for steering their properties toward applications and performance optimizations. This work presents a detailed X-ray transient absorption (XTA) spectroscopy study of a prototypical spin crossover (SCO) complex [Fe^II(mbpy)₃]²⁺ (where mbpy = 4,4'-dimethyl-2,2'-bipyridine) with an [Fe^IIN₆] first coordination shell in water (H₂O) and acetonitrile (CH₃CN). The unprecedented data quality of the XTA spectra together with the direct fitting of the difference spectra in k space using a large number of scattering paths enables resolving the subtle difference in the photoexcited structures of an Fe^II complex in two solvents for the first time. Compared to the low spin (LS) ¹A₁ state, the average Fe-N bond elongations for the photoinduced high spin (HS) ⁵T₂ state are found to be 0.181 ± 0.003 Å in H₂O and 0.199 ± 0.003 Å in CH₃CN. This difference in structural response is attributed to ligand-solvent interactions that are stronger in H₂O than in CH₃CN for the HS excited state. Our studies demonstrate that, although the metal center of [Fe^II(mbpy)₃]²⁺ could have been expected to be rather shielded by the three bidentate ligands with quasi-octahedral coordination, the ligand field strength in the HS excited state is nevertheless indirectly affected by solvation effects that modifies the charge distribution within the Fe-N covalent bonds. More generally, this work highlights the importance of including solvation dynamics in order to develop a generalized understanding of the spin-state switching at the atomic level.