Photoacid Mediated Spin State Switching in Spin-Crossover Systems: The Example of an Fe(II)-Acylhydrazone Complex

Designing molecular objects with switchable spin states at ambient conditions remains a difficult challenge for the development of the next-generation advanced materials. Current approaches often rely on synthetically demanding systems that exhibit unpredictable switchability, random bistability, or lack of reversibility. Here, we present a general, efficient, and easy-to-use method for achieving reversible and optically controlled spin switching of a spin-crossover complex in solution at room temperature. These unprecedented properties are enabled by combining two key components in a solution system: (i) a spin-crossover complex featuring a ligand capable of reversible protonation, which triggers a concerted change in coordination and induces a spin-state change at the metal ion; and (ii) a photoacid that releases a proton upon light irradiation, and undergoes reprotonation in the dark. As a proof of concept, we investigated solutions combining a merocyanine-based photoacid and a spin-crossover Fe(II)-acylhydrazone complex. Upon irradiation at 400-450 nm, the merocyanine photoacid converts to its metastable spiropyran form, releasing a proton that protonates the acylhydrazone ligand. This protonation alters the ligand coordination at the metal center, converting the low-spin Fe(II) state to a high-spin Fe(II) species. In the dark, the system gradually returns to thermodynamic equilibrium, fully restoring both the merocyanine photoacid and the low-spin Fe(II)-acylhydrazone complex. Magnetic and spectroscopic analyses confirm the full reversibility of this spin-state switching over multiple cycles, demonstrating the robustness and potential of this approach.