Spin State Modulation and Kinetic Control of Thermal Contraction in a [Fe2Co2] Discrete Prussian Blue Analogue

Stimuli-responsive switchable molecules represent an important category of magnetic materials with significant potential for functional devices. However, engineering of the complexes with controlled switchability remains challenging due to their sensitivity to lattice interactions. Herein, we have reported a [Fe2Co2] square complex tailored with hydrogen bonding (HB) donor/acceptor moieties for effective lattice interactions. The alteration in HB interactions within differently solvated crystal phases (1•4CH3OH•2H2O, 1•2H2O, 1) leads to a remarkable change in magnetic properties. 1•4CH3OH•2H2O and 1 exhibits [FeIILS(CN)CoIIILS] and [FeIIILS(CN)CoIIHS] configurations, respectively. While, 1•2H2O demonstrates a multi-responsive (thermo-, photo- and pressure) reversible electron transfer coupled spin transition (ETCST) with unusual stability of the diamagnetic state at an intermediate temperature (~140 K), which is accompanied by two-step thermal contraction and expansion. A faster relaxation dynamics to the diamagnetic state with a rate constant of 4×10−4 s−1, at 140 K compared to other temperatures, corroborates the unusual magnetic behaviour. Our result provides valuable insights into the strategic design of complexes with enhanced and controlled switchability for potential applications such as actuators and sensors.