Self-healing ferroelastic metal–organic framework sensing guests, pressure and chemical environment

Abstract

Strong strain shuttering ferroelastic crystals that self-heal after releasing the strain have been revealed for a new metal–organic framework [Cd(BDC)(AZPY)]_n (BDC = terephthalic acid; AZPY = 4,4′-azobispyridine), with the Cd(II) cation hepta-coordinated. It has been obtained in the form of single crystals without and with guests, acetonitrile or DMF (dimethylformamide). This pleochroic porous complex, hereafter referred to as AMU3, is built of two interwoven frameworks with the planar AZPY linkers (C_2h symmetric) orientationally disordered on the D_2h sites. The disorder of AZPY linkers persists down to 100 K at least, but it is eliminated by high pressure. The onset of AZPY ordering in the 0.2–0.9 GPa range depends both on the guest and on the hydrostatic fluid. The mechanism of AMU3 ferroelasticity involves the AZPY disordering in prototypic orthorhombic phase α of space-group Cmce. The AZPY ordering reduces the symmetry of phase β to monoclinic subgroup P2₁/n. The shear strain in the bulk of ferroelastic orientational-state domains is accommodated by the tilts and conformational changes of the ordered AZPY linkers, while along the boundaries between domains the Cd–N bonds break. On releasing the pressure the Cd–N bonds are restored and the signs of the cracks disappear.