Chromism of Tartrate-Bridged Clamshell-like Platinum(II) Complex: Intramolecular Pt-Pt Interaction-Induced...

A novel luminescent clamshell-like platinum(II) complex with rac-tartrate, rac-[{Pt^II(⁴Me₂bpy)}₂(μ-tart)] (Pt₂; ⁴Me₂bpy: 4,4'-dimethyl-2,2'-bipyridine; tartH₂^2-: 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, λ_em = 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(ML_tart)L_bpyCT) of Pt₂·4H₂O and a triplet metal-metal-to-ligand charge-transfer state (³MML_bpyCT) 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 E_T(30) scale for the solvent, and the hydrogen-bond acidity of the solvent contributed significantly to the solvent-dependent absorption properties.