A thermally reversible dynamic bond as a magnetic and nonlinear optical switch

In general, flexible metal-organic frameworks (MOFs) change their structures via framework breathing in response to external stimuli (usually guest adsorption and desorption). Here, we show a dynamic coordination bond in a flexible MOF driving a thermo-induced diffuse phase transition occurring in a wide temperature range of 130–270 K (centered at ∼200 K), which enables the switching of magnetism and second-harmonic generation (SHG) responses. In specific, a solvent-free Co^II-based flexible MOF Co₂(TPY)₂(BPTC) (1), bearing biphenyl-3,3′,5,5′-tetracarboxylic acid (H₄BPTC) and 2,2′:6′,2″-terpyridine (TPY) ligands, exhibits a reversible thermo-responsive coordination-mode switching between five- and six-fold. This dynamic bond results in the reversible transformation between a distorted polar framework and a regular nonpolar framework, and therefore leads to the subtle change of the magnetic property and substantial change of the nonlinear optical property of 1.