Piezochromic Porous Metal–Organic Framework

Abstract

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