Piezochromic Porous Metal–Organic Framework

Pressure changes the color of a new type of metal-organic porous hybrid material CoBbcDabcoH₂O. It is built of Co²⁺ cations linked by 1,4-benzenedicabroxylate (Bdc) anions and 1,4-diazabicyclo[2.2.2]octane (Dabco) molecules into 2-D grid-like sheets, interconnected through OH···O bonds of water molecules to carboxylate H-acceptors. This first piezochromic MOF, stable in air and in many solvents, is an ideal ultraprecise sensor for pressure calibration. The color changes are due to strains generated by pressure in the highly asymmetric crystal field of Co²⁺ octahedral coordination, involving four different ligand types: a Dabco amine (twice), a monodentate carboxylate, a chelating carboxylate, and a water molecule. At 0.33 GPa/296 K and below 225 K/0.1 MPa a phase transition reduces the crystal symmetry from monoclinic to triclinic system and changes the conformation and orientation of linkers.