Triple-chromic (photo-, thermo-, and mechano-chromic) metal complexes containing N-salicylideneaminopyridine ligands

Abstract

N-Salicylideneaminopyridine (SAP) is a well-known organic chromic compound that shows a reversible colour change upon UV light irradiation (photochromism) and upon cooling (thermochromism). Herein, we report novel multi-chromic metal complexes containing SAP derivatives as ligands, viz. [Ni(NCS)₂(3,5-t-Bu-SAP)₄] (Ni1) and [Co(NCS)₂(3,5-t-Bu-SAP)₄] (Co1). The Ni1 crystals exhibited both photo- and thermochromism with new colour variations, which were due to the light absorption of the Ni(II) ions and chromic properties of the SAP ligands. The Co1 crystal also exhibited photo- and thermochromism originating from the SAP ligands. Furthermore, the Co1 crystal exhibited mechanochromism induced by grinding with a mortar, which was considered to be attributable to the change of the Co1 coordination structure. Such a triple-chromic material is rare and very fascinating for the applications of multiple sensors, memory devices, and functional inks.