Thermal Bistability of Magnetic Susceptibility, Light Absorption, Second Harmonic Generation, and Dielectric Properties in a Polar Spin‐Crossover Iron–Rhenium Chain Material

Abstract The bistability of multiple physical properties driven by external stimuli in a solid is a desired prerequisite for its application in memory devices with convenient data readout. We present a pathway for thermal bistability detectable in four physical properties: magnetic, light absorption, second‐harmonic generation (SHG), and dielectric. We report a novel heterometallic (TBA){[Fe II (phIN) 4 ][Re V (CN) 8 ]} ⋅ (phIN) ( 1 ) (TBA=tetrabutylammonium cation, phIN=phenyl isonicotinate) cyanido‐bridged chain material. Owing to an appropriate {N 6 } coordination sphere of Fe(II) centers, 1 reveals a thermal spin crossover (SCO) effect which is complete and cooperative providing a distinct thermal hysteresis loop in magnetic measurements. Moreover, it exhibits simultaneous thermal bistability in (a) visible‐light absorption due to the presence of efficient d‐d electronic transitions in the low‐spin (LS) state, (b) SHG activity as it crystallizes in a polar C c space group due to the bulky substituent on phIN ligands, and (c) dielectric parameters, including dielectric constant, which can be correlated with subtle changes in polarity between LS and HS (high spin) phases. Thus, we present a remarkable thermally controlled hysteretic behavior in four physical functionalities realized by properly functionalizing an SCO‐active coordination compound.