Spin state modulation and kinetic control of thermal contraction in a [FeCo] discrete Prussian blue analogue.

Stimuli-responsive switchable molecules represent an important category of magnetic materials with significant potential for functional devices. However, engineering complexes with controlled switchability remains challenging due to their sensitivity to lattice interactions. Herein, we report a [FeCo] square complex [FeTp(CN)][Co{(F5-Ph)Py}]·2ClO·4CHOH·2HO {1·4CHOH·2HO; (F5-Ph)Py = ()-'-(perfluorophenyl) picolinimidamide and Tp = hydrotris(1-pyrazolyl)borate}, tailored with hydrogen bonding (HB) donor and acceptor moieties for effective lattice interactions. The alteration in HB interactions in crystal phases obtained single-crystal-to-single-crystal (SC-SC) transformation (, 1·4CHOH·2HO ↔ 1·2HO ↔ 1) led to a remarkable change in magnetic properties. Complexes 1·4CHOH·2HO and 1 possess [Fe (μ-CN)Co ] and [Fe (μ-CN)Co ] configurations, respectively. Meanwhile, 1·2HO demonstrates multi-responsive (thermo-, photo- and pressure) reversible two-step electron transfer coupled spin transition (ETCST) accompanied by thermal contraction and expansion. The complete diamagnetic [Fe (μ-CN)Co ] configuration in 1·2HO was obtained at an intermediate temperature accompanied by thermal contraction, an unusual behaviour observed for the first time in cyanide-bridged systems. Additionally, 1·2HO displayed temperature-induced excited spin state trapping (TIESST) of ∼70% of the paramagnetic [Fe (μ-CN)Co ] configuration at low temperatures. The isothermal relaxation of the thermally trapped paramagnetic state shows a much faster and complete conversion to a diamagnetic state at ∼140 K, compared to the relaxation observed at other temperatures (100-190 K), corroborating the observed unique magnetic behaviour. Hence, this result provides valuable insight into the strategic design of complexes with enhanced and controlled switchability for potential applications such as actuators and sensors.