Hysteretic magnetism–dielectricity switching in 2D Hofmann type spin-crossover compounds

Abstract

Manipulations of hysteretic magnetism–dielectricity switching in molecular materials are essential for future switching and memory devices. While examples showing simultaneous and wide hysteretic magnetism–dielectricity transitions are still rare, in this work, we present two 2D Hofmann type compounds {Fe(3,4-bpt)₂[M(CN)₄]} (3,4-bpt = 3,4-bis(4-pyridyl)thiophene; M = Pt (1), Pd (2)) showing reversible switching in magnetism, dielectricity, and thermochromism actuated by the spin transition of Fe(II). Magnetic susceptibility measurement analyses unveiled that 1 and 2 experienced a thermo-induced spin state transition characterized by a thermal hysteresis of 20 K. Their dielectric properties show good consistency with magnetic properties, revealing a well-defined bistable state within the same temperature range of 100 to 220 K for 1 and 2. Both of them exhibited a 20 K-wide hysteresis in temperature-dependent permittivity and magnetization profiles. This simultaneous magnetism–dielectricity switching was demonstrated by variable-temperature X-ray diffraction studies, showing that the key crystallographic parameters underwent hysteretic transitions in the same temperature range. Detailed structural analyses revealed that the intermolecular interactions between the thiophene ring of 3,4-bpt and the terminal nitrogen atom of the cyanide for 1 and 2 displayed prominent cooperativity during the spin crossover process. The X-ray structural analyses revealed that the variations in the Fe(II) coordination sphere result in changes in the local electric dipoles for the asymmetry between the HT phase and the LT phase, contributing to the dielectric transition. Moreover, 1 and 2 displayed high sensitivity to external light irradiation, which can be reversibly switched between HS and LS states under alternating irradiation with lasers at 532 and 808 nm. These results demonstrate that 1 and 2 provide a new platform for multi-channel switches actuated by the spin crossover of Fe^II.