Phase transformation induced mechanochromism in a platinum salt: a tale of two polymorphs

Red crystals of [Pt(tpy)Cl]NO3 center dot HNO(3)show mechanochromic behaviour turning yellow when pressure is applied. The electronic character and spectroscopic signature of the red and yellow polymorphs change as a result of slipping of the molecular stacking planes in the solid state. The slippage alters the PtMIDLINE HORIZONTAL ELLIPSISPt intermolecular distances from a linear stacked motif with <3.5 angstrom separations in the red polymorph to a less stacked motif of alternating short intradimer and long interdimer interactions in the yellow polymorph.