Illuminating spin-crossover octanuclear metal-organic cages

Spin-crossover (SCO) materials that reversibly switch between high- and low-spin states have potential for the storage of spin state-relative information, and have gained much attention incorporating secondary physical properties, such as fluorescence and magneto-optical switching. In this study, we synthesized three octanuclear metal-organic cages (MOCs) using tetra-phenylethylene-based luminophores, aldehydes, and Fe^II salts, by subcomponent self-assembly approach, namely [Fe1]-[Fe3]. By controlling the ligand-field strength and guest encapsulation, we finely tuned their SCO properties. Among them, MOC [Fe2] displayed nearly complete SCO behavior in the solid state, which is rare for high-nuclearity complexes. We also demonstrated the coupling of SCO with fluorescence emission in these MOCs by using isostructural Zn^II complexes ([Zn1]-[Zn3]) as control experiments, for the first time. Theoretical calculations revealed the energy-transfer mechanism between fluorophores and SCO-active centers, which emphasizes the significant contribution of d-d transitions in the interplay between the occurrence of SCO and fluorescence emission.