Switchable colossal anisotropic thermal expansion in a spin crossover framework

Advanced materials with tunable thermal expansion properties have garnered significant attention due to their potential applications in thermomechanical sensing and resistance to thermal stress. Here, switchable colossal anisotropic thermal expansion (ATE) behaviors are realized in a Hofmann-type framework [Fe(bpy-NH₂){Au(CN)₂}₂]·ⁱPrOH (Fe·ⁱPrOH, bpy-NH₂ = [4,4'-bipyridin]-3-amine) through a three-in-one strategy: a vibrational mechanism, an electronic mechanism and molecular motion. Spin crossover (SCO) centers coordinate with dicyanoaurate linkers to form flexible wine-rack frameworks, which exhibit structural deformations driven by host-guest interactions with ⁱPrOH molecules. By means of the vibrational mechanism, a scissor-like motion driven by the rotation of dicyanoaurate is observed within the rhombic grids, resulting in the emergence of colossal ATE in the high temperature region. When the spin transition comes into play, the electronic mechanism is predominant to form reverse ATE behavior, which is associated with host-guest cooperation involving significant molecular motion of the ⁱPrOH guest and adaptive deformation of the host clathrate. A remarkably high negative thermal expansion coefficient up to -7.49 × 10⁵ M K^-1 accompanied by abrupt SCO behavior is observed. As a proof of concept, this study provides a novel perspective for designing dynamic crystal materials with tunable thermomechanical properties by integrating various ATE-related elements into a unified platform.