Achieving Stimuli-responsive Spin-state Switching in Potassiuminserted Molecular Replica of Prussian Blue

Diving into the age-old three-dimensional Prussian blue network, we opted for a “complex-as-a-ligand” strategy to design and synthesise a soluble molecular replica of alkali-inserted Fe8 cube, {K ⊂ [FeIIILS(Ttp)(CN)3]4[FeIILS(POPy3)]4}2.(BF4)9.[FeIIILS(Ttp)(CN)3], referred to as K⊂Fe8. This mixed-valence cyanide-bridged molecular cube, hosting a potassium cation, was structurally characterised by temperature-dependent X-ray diffraction. Insertion of the potassium cation and its strong interaction with the cyanide bridges impart additional stability to the cationic cubic motif. Magnetic measurements on the crystal in mother liquor suggest thermally induced spin state switching, partially converting the {FeIIILS−CN−FeIILS} pairs into {FeIIILS−CN−FeIIHS} pairs, a feature that is lesser-known for alkali-inserted molecular Prussian blue so far. Additionally, K⊂Fe8 displays a reversible photoinduced LIESST effect under light irradiation (ON: 808 nm laser light and OFF: 532 nm laser light), confirmed by (photo)magnetic measurements. The photoinduced metastable state relaxes back to the ground state at 50 K (TLIESST = 50 K). Temperature-dependent Mössbauer spectroscopy further confirms the thermo-induced switching of {FeIIILS−CN-FeIILS}  {FeIIILS−CN-FeIIHS} pairs.