Chemically driven magnetic responsivity to multiple physical stimuli in a spin-crossover layered iron(ii)–rhenium(v) framework

Abstract

Achieving the responsivity of a magnetic molecular material to physical and chemical stimuli paves the way for a new generation of switchable materials for memory devices and sensing applications. Here we present a two-dimensional spin-crossover {[Fe^II(4-phpy)₄]₃[Re^V(CN)₈]₂}·H₂O·MeOH (1^sol, 4-phpy = 4-phenylpyridine) coordination polymer, which enables the switching of magnetic properties using three physical stimuli, i.e., temperature, light, and pressure. Its responsivity is chemically modulated, as upon drying in the air, it undergoes a reversible single-crystal-to-single-crystal (SCSC) transformation to the air-stable phase of {[Fe^II(4-phpy)₄]₃[Re^V(CN)₈]₂}·2H₂O (1^air) of different magnetic responsivity than the original phase. The 1^sol phase exhibits a weakly cooperative, gradual thermal Fe(II) SCO effect in the 80–160 K range, while the 1^air phase shows a two-step thermal SCO phenomenon with the distinct thermal hysteresis loop for the higher-temperature step in the 150–210 K region. Moreover, the thermal SCO effect of 1^air can be modulated by relative humidity. The thermal hysteresis loop of 1^air is modified by external pressure, resulting in a magnetic memory effect at room temperature. The 1^sol and 1^air phases also exhibit a pronounced thermally reversible photomagnetic effect under 520 and 638 nm irradiation, distinctly stronger for 1^sol, indicating a more effective light-induced excited spin-state trapping (LIESST) for the MeOH-containing system. Interestingly, for 1^air, the 808 nm irradiation leads to a thermally reversible decrease of the magnetization (reverse-LIESST) while the standard yet weakened LIESST is observed under such conditions for 1^sol. This work demonstrates the ability of the layered iron(II)–octacyanidorhenate(V) framework to effectively integrate chemical and physical stimuli for multi-switching of the magnetic signal through the spin crossover effect of modifiable characteristics.