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.