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

Abstract

A novel luminescent clamshell-like platinum(II) complex with rac-tartrate, rac-[{Pt II ( 4 Me 2 bpy)} 2 (μ-tart)] (Pt 2 ; 4 Me 2 bpy: 4,4′-dimethyl-2,2′-bipyridine; tartH 2 2− : tartrate), was synthesized, and its vapo-, thermo-, and solvatochromic behavior was studied. The complex was isolated in four crystal forms, namely, Pt 2 •4H 2 O (color under ambient light: yellow, λ em = 573 nm, intramolecular Pt•••Pt separation: 3.55 Å), Pt 2 • 4MeOH (red, 650 nm, 3.33 Å), anhydrate Pt 2 (red, no luminescence, 3.70 Å), and Pt 2 •2H 2 O (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 Pt 2 •4H 2 O induced a reversible phase transition to Pt 2 •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 metalligand-to-ligand charge-transfer state ( 3 (ML tart )L bpy CT) of Pt 2 •4H 2 O and a triplet metal-metal-to-ligand charge-transfer state ( 3 MML bpy CT) of Pt 2 •4MeOH. Heating Pt 2 •4H 2 O induced a phase transition to anhydrate Pt 2 , and under atmospheric conditions, anhydrate Pt 2 transformed spontaneously to Pt 2 •2H 2 O. Vapor diffusion of H 2 O to Pt 2 •2H 2 O caused reconstruction of Pt 2 •4H 2 O. 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.

Chromic Platinum Complexes Containing Multidentate Ligands

“…Complex 24 (Figure 12d), for example, is yellow in its hydrated form but changes color to red when heated at 95 °C (Figure 12e). This is explained by the evaporation of water, causing the rearrangement of 24 and the reinforcement of metal⋅⋅⋅metal interactions [58] . This behavior is typically observed in vapochromic complexes, which show chromism over vapor uptake/heating cycles [58,94] …”

“…The family of bipyridine‐based vapochromic complexes increased in number with the incorporation of other ancillary ligands, such as chlorides, [52] thioglucose, [53] acetylides, [54–56] phenylpyridine, [57] and tartrate [58] . All of these species display a chromic behavior originating from a vapor‐induced molecular rearrangement and the disruption of Pt⋅⋅⋅Pt interactions.…”

Reversible stimuli-responsive chromism of a cyclometallated platinum(ii) complex

“…We confirmed the reversible formation of non-covalent dimers in both in the solid state and solution. Reversible formation of a bridged [35][36][37][38] cationic [2-Pt 2 ] + dimer was confirmed in polar solvents. The extent of formation of [2-Pt 2 ] + is highly solvent dependent, and strongly affects the absorption spectra due to metal-metal-to-ligand charge transfer (at 570 nm absorbance), resulting in strong solvatochromic behaviour.…”

“…Alternatively, bridging ligands have previously been employed to generate ''butterfly complexes'' where two platinum(II) centres are held in close proximity in an intermolecular fashion, allowing for precise control of metal-metal distance, and thus photophysical properties. [35][36][37][38] It is desirable to combine features of responsive supramolecular systems with the precision of covalently linked systems to develop stimuli-responsive, precise, reversible control 12 over the assembly of cyclometalated platinum(II) complexes into discrete supramolecular structures, 19 which may ultimately be exploited in photophysical applications.…”

Thermo-responsive light-emitting metal complexes and related materials

“…[1][2][3][4] In particular, thermo-responsive functional materials with specific luminescence changes have attracted wide attention due to their potential applications in temperature sensing, [5][6][7][8][9][10][11][12] biological imaging, [13][14][15][16][17][18][19][20] optoelectronic devices, [21][22][23][24][25] and information processing. [26][27][28] To date, tremendous progress has been made in the development of thermo-responsive light-emitting materials, including various organic dyes and polymers, [29][30][31][32][33][34][35][36][37] metal complexes, [38][39][40][41][42][43][44] metal clusters and metal-organic frameworks (MOFs), [45][46][47][48]…”