Chemicals which make not only a dark-red/light-red change of color but also a near-infrared/red change of luminescence via reversible adsorption/desorption of vapor of acetonitrile or ethanol, and therefore permit the observation of on-off change of visually effective luminescence and thus are useful as highly sensitive sensors. The chemicals are binuclear platinum (II) complexes of the following general formula: [Pt2(L)2(bpy)2]X2 or [Pt2(L)2(bpy)2]Y (wherein L is a bridging ligand selected from among 2-pyridinethiol anion, 2-quinolinethiol anion, 8-quinolinethiol anion and so on; bpy is 2,2-bipyridine; X is a monovalent anion; and Y is a bivalent anion), characterized by taking a syn form wherein the two bridging ligands (L) coordinate to one platinum ion at their respective sulfur atoms and to the other platinum ion at their respective nitrogen atoms.
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