High-Pressure Structural Investigation of Multi-Switchable Spin Crossover Compounds with Redox-Active Ligands

Spin crossover (SCO) compounds are attractive for sensors, actuators, and memory devices due to their reversible spin-state switching. We have developed a family of Hofmann-type materials, {Fe(R-pbpy⁺)₂[µ²-Pt(CN)₄]₂}, featuring redox-active bipyridinium ligands that tune electronic properties and SCO behavior. Remarkably, some derivatives (R = Me, Br) show an unprecedented HS→LS→HS→LS sequence under pressure, as indicated by Raman spectroscopy, but the structural origin of this re-entrant HS state is unknown. We propose high-pressure single-crystal X-ray diffraction at ESRF ID15B to track Fe–N bond distances and crystal packing up to 4 GPa. This will establish whether multiple spin states are general, clarify ligand-dependent electron transfer, and test Marcus theory predictions, providing design rules for multi-switchable SCO materials.