Chromism of Tartrate-Bridged Clamshell-like Platinum(II) Complex: Intramolecular Pt-Pt Interaction-Induced Luminescence Vapochromism and Intermolecular Interactions-Triggered Thermochromism.

A novel luminescent clamshell-like platinum(II) complex with rac-tartrate, rac-[{PtII(4Me2bpy)}2(μ-tart)] (Pt2; 4Me2bpy: 4,4'-dimethyl-2,2'-bipyridine; tartH22-: tartrate), was synthesized, and its vapo-, thermo-, and solvatochromic behavior was studied. The complex was isolated in four crystal forms, namely, Pt2·4H2O (color under ambient light: yellow, λem = 573 nm, intramolecular Pt···Pt separation: 3.55 Å), Pt2·4MeOH (red, 650 nm, 3.33 Å), anhydrate Pt2 (red, no luminescence, 3.70 Å), and Pt2·2H2O (orange, 595 nm, 3.60 Å). Vapo- and heat-responsive crystal-to-crystal phase transitions were observed by time- and/or temperature-dependent powder X-ray diffraction, and the molecular and crystal structures of the above crystal forms were determined by either single-crystal X-ray crystallography or Rietveld refinement of powder X-ray diffraction. Vapor diffusion of MeOH to Pt2·4H2O induced a reversible phase transition to Pt2·4MeOH. This structural transformation significantly changed the intramolecular Pt-Pt interactions, affecting the absorption and emission colors. Accordingly, the luminescence chromism originates from alternation of the luminescent nature between a triplet metal-ligand-to-ligand charge-transfer state (3(MLtart)LbpyCT) of Pt2·4H2O and a triplet metal-metal-to-ligand charge-transfer state (3MMLbpyCT) of Pt2·4MeOH. Heating Pt2·4H2O induced a phase transition to anhydrate Pt2, and under atmospheric conditions, anhydrate Pt2 transformed spontaneously to Pt2·2H2O. Vapor diffusion of H2O to Pt2·2H2O caused reconstruction of Pt2·4H2O. The thermochromism originates from changes in the intermolecular interactions between stacked complexes. The solvatochromism was evaluated using the Kamlet-Taft solvatochromic equation, the acceptor number, and the ET(30) scale for the solvent, and the hydrogen-bond acidity of the solvent contributed significantly to the solvent-dependent absorption properties.