Stimuli-Responsive Spin-State Switching in Supramolecular Helicates Bridging Solid–Solution Bistability

The design of molecular systems that exhibit bistability in both solid and solution phases remains a formidable challenge in coordination chemistry and materials science. Spin-crossover systems have been widely studied in crystalline form, yet examples that preserve spin-state switching in solution are exceedingly rare and of considerable current interest for processable and functional materials. Here, we report a family of metallosupramolecular [Fe₂L₃] helicates assembled from an in situ generated, flexible bis-bidentate imidazolyl ligand and characterized by synchrotron single-crystal X-ray diffraction. Both helicates (M1 and M2) exhibit temperature-dependent spin-state switching, as confirmed by solid-state magnetic measurements and the solution-phase variable-temperature Evans ¹H NMR method, establishing a rare case of bistability across the solid-solution divide. The NMR studies further reveal dynamic spin-state equilibria in solution, providing unprecedented molecular-level insight into the switching process. The spin-state switching is governed by short intramolecular contacts and exhibits pronounced sensitivity toward crystalline solvent molecules, highlighting the intricate interplay between the supramolecular environment, cooperativity, structural matrix, and spin dynamics. These findings position [Fe₂L₃] helicates as a novel class of multifunctional, solution-accessible bistable systems, opening opportunities for the development of switchable molecular devices that extend beyond the crystalline state.