Piezochromic Topology Switch in a Coordination Polymer.

Abrupt color changes coupled to a giant strain in the crystal of coordination polymer CoCl2 bpp (bpp = 1,3-bis(4-pyridyl)propane) mark piezochromic reversible transformations at 1.93 GPa from blue phase α to green phase β and at 2.39 GPa to colorless phase γ. The clearly visible shape and color changes are ideal for calibrating discrete pressure magnitudes associated with these phase transitions. The crystal spectra have been measured and the structures have been determined in situ under pressure in a diamond-anvil cell. In phase α (of monoclinic space group P21 /m) and phase β (orthorhombic space group Pnmm) the tetrahedral Co-coordination is stepwise modified within the 1D chain topology, but in phase γ (triclinic space group P1̅) the Co2+ cations become octahedrally coordinated and the polymer topology transforms to the 2D sheets.