Control of spin switchability and electron transfer in Prussian blue analogues: Insights into secondary interactions and chemical modifications

The research progress on 3D Prussian blue analogues (PBAs) and their successful downsizing using capping ligands to achieve 2D, 1D, and even discrete molecular analogues has greatly enhanced our understanding of the switchable phenomena such as spin crossover (SCO) and electron transfer coupled spin transition (ETCST) in these systems. These molecules exhibit diverse optical and magnetic properties, making them highly attractive for next-generation compact smart devices. However, better control over switchability is necessary to fulfil specific purposes. While substantial work over the past three decades has elucidated the rationale behind ETCST occurrence in some complexes but not in others, parallel investigations into the peculiar ETCST behaviour in response to subtle changes in ligand field, reaction conditions, and guest molecule size have provided improved control over parameters like H-bonding and π⋅⋅⋅π interactions. Efforts to increase the operational temperature range for chemical, thermal, piezo, and photo-responsive switchability of these molecules to practical levels and preserve their pristine properties after integration into devices require further exploration to unlock the optimisation possibilities. This review discusses factors affecting various stimuli-responsive ETCST behaviour in both solid (ligand field strength, lattice solvents, counter-ions and crystal packing) and solution phases (solvent polarity and protonation/deprotonation) with representative examples. We envisage that a better understanding of the factors controlling ETCST properties will enable precise design and targeted synthesis of these molecules.