Cyclometalated heteronuclear Pt/Ag and Pt/Tl complexes: a structural and photophysical study

Abstract

To investigate the factors influencing the luminescent properties of polymetallic cycloplatinated complexes a detailed study of the photophysical and structural properties of the heteronuclear complexes [Pt₂Me₂(bhq)₂(μ-dppy)₂Ag₂(μ-acetone)](BF₄)₂, 2, [PtMe(bhq)(dppy)Tl]PF₆, 3, and [Pt₂Me₂(bhq)₂(dppy)₂Tl]PF₆, 4, [bhq = benzo[h]quinoline, dppy = 2-(diphenylphosphino)pyridine] was conducted. Complexes 3 and 4 synthesized by the reaction of [PtMe(bhq)(dppy)], 1, with TlPF₆ (1 or 1/2 equiv.) and stabilized by unsupported Pt–Tl bonds as revealed by multinuclear NMR spectroscopy and confirmed by X-ray crystallography for 3. DFT calculations for the previously reported butterfly Pt₂Ag₂ cluster 2 reveal that in the optimized geometry the bridging acetone molecule is removed and the metal core displays a planar-shaped geometry in which according to a QTAIM calculation and natural bond orbital (NBO) analysis the Ag⋯Ag metallophilic interaction is strengthened. In contrast to the precursor 1, which is only emissive in glassy solutions (³MLCT 485 nm), all 2–4 heteropolynuclear complexes display intense emissions in the solid state and in glassy solutions. Time-dependent density functional theory (TD-DFT) is used to elucidate the origin of the electronic transitions in the heteronuclear complexes 2 and 3. The low energy absorption and intense orange emission for cluster 2 (solid 77 K and glass) are attributed to metal–metal to ligand charge transfer (MM′LCT) with a minor L′LCT contribution. For 3 and 4 two different bands are developed: the high energy band (602–630 nm) observed for 4 at 77 K (solid, glass) and in diluted glasses for 3 is ascribed to emission from discrete Pt₂Tl units of mixed ³L′LCT/³LM′CT origin. However, the low energy band (670–690 nm) observed at room temperature (solid) for both complexes and also in concentrated glasses for 3 is assigned to ³ππ excited states arising from intermolecular interactions.