Phase Transformation Induced Mechanochromism in a Platinum Salt: A Tale of Two Polymorphs

Red crystals of [Pt(tpy)Cl]NO3•HNO3 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 Pt•••Pt intermolecular distances from a linear stacked motif with <3.5 A separations in the red polymorph to a stacked motif of alternating short intradimer and long interdimer interactions in the yellow polymorph.