Martensitic Transition Associated with Spin-Crossover Enabling Crystal Jumping and Pyroelectric Response

Integrating electronic switching with molecular machinery remains a central challenge in the development of multifunctional stimuli-responsive materials. Herein, we demonstrate a synergistic coupling of spin crossover (SCO) and molecular motion within a hydrogen-bonded supramolecular architecture constructed from spin-active [Fe(3-bpp)₂]²⁺ complexes and flexible bpa molecules (3-bpp = 2,6-bis(pyrazol-3-yl)pyridine; bpa = 1,2-bis(4-pyridyl)ethane). In this crystal, temperature-induced spin conversion of the Fe(II) centers is mechanically associated with a large-amplitude pedal-like conformational change of the bpa linkers. This cooperative evolution of electronic and molecular structures drives a diffusionless martensitic phase transition accompanied by rapid anisotropic lattice deformation. As a result, the material exhibits a thermosalient effect, manifested as the macroscopic jumping of single crystals. Moreover, the conformational change of bpa induces a concomitant displacement of the AsF₆⁻ counteranions along the crystallographic polar axis, leading to a substantial modulation of lattice polarization and a distinct pyroelectric response. These findings establish a rational design strategy for bridging electronic switching and molecular mechanics, paving the way for sophisticated multifunctional dynamic crystals.