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

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 [FeII (mbpy)3 ]2+ (where mbpy = 4,4'-dimethyl-2,2'-bipyridine) with an [FeII N6 ] first coordination shell in water (H2 O) and acetonitrile (CH3 CN). 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 FeII complex in two solvents for the first time. Compared to the low spin (LS) 1 A1 state, the average Fe-N bond elongations for the photoinduced high spin (HS) 5 T2 state are found to be 0.181 ± 0.003 Å in H2 O and 0.199 ± 0.003 Å in CH3 CN. This difference in structural response is attributed to ligand-solvent interactions that are stronger in H2 O than in CH3 CN for the HS excited state. Our studies demonstrate that, although the metal center of [FeII (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.