Influence of the Diphosphine Coordination Mode on the Structural and Optical Properties of Cyclometalated Platinum(II) Complexes: An Experimental and Theoretical Study on Intramolecular Pt···Pt and π···π Interactions

Abstract

The reaction of [Pt(C^N)(CFCO)(SMe)] (1), in which C^N is either benzo[ h]quinolinate (bhq), 1a, or 2-phenylpyridinate (ppy), 1b, with 1 equiv of bis(diphenylphosphino)methane (dppm) gave the bischelate complexes [Pt(C^N)dppm]CFCO (2). The binuclear complexes [Pt(C^N)(CFCO)(μ-dppm)] (3) were prepared, using an unusual reaction pathway, by the addition of equimolar amount of complexes 1 and 2, through the ring opening of the chelating dppm ligand and coordination of the CFCO anion to the platinum center. The proposed reaction pathway and effect of the solvent polarity were investigated by density functional theory (DFT) calculations. The crystal structure of 3a shows considerable intramolecular Pt···Pt and π···π interactions. The crystal structure and formation pathway toward 3 were compared with the similar analogue [Pt(bhq)(Cl)(μ-dppm)] (5). All complexes were fully characterized using multinuclear NMR spectroscopy and elemental analysis. Furthermore, the crystal structures of some complexes including 1b, 2a, 2b, 3a, and 5 were confirmed by X-ray crystallography. The effect of dimerization via a change in the coordination mode of dppm, from a chelate mode in complex 2 to a bridge mode in complexes 3 and 5, upon the excited states of the studied compounds was investigated in their distinguished absorption and emission profiles. The appearance of a remarkably low energy band in the absorption spectra of 3, which was assigned to a metal-metal to ligand charge transfer [MMLCT; dσ*(Pt) → π*(C^N)] transition showing negative solvatochromism, is important evidence for the Pt···Pt intramolecular interaction. The vibronically resolved and long-lifetime emission of 2a in poly(methyl methacrylate) media and powder states at 77 and 300 K, along with time-dependent DFT calculations, suggested that the triplet ligand-centered (LC) emission was mixed with some MLCT character. Unstructured and short-lifetime emission in 3 refers to the phosphorescenceMMLCT [dσ*(Pt) → π*(C^N)] transition. Although complex 5 is a binuclear compound, the long distance of the Pt···Pt interaction caused the occurrence of the MMLCT transition to fade and act as a mononuclear unit, and the emission originated mostly from theMLCT transition. As a result, more metal participation leads to more red-shifted absorption and emission spectra of the studied complexes upon going from LC to MLCT to MMLCT transitions (λ and λ: 3a > 3b > 5 > 2a > 2b).

Tetradentate Pt(II) Complexes for Spectrum‐Stable Deep‐Blue and White Electroluminescence

“…[ 25,26 ] Generally, Pt…Pt interaction leads to a new metal‐metal‐to‐ligand charge transfer (MMLCT) transition state involving a filled Pt…Pt antibonding orbital and a high‐lying π* orbital on the ligand. [ 27,28 ] Their semiconducting and long‐wavelength luminescent properties can be utilized for fabricating high‐performance light‐emitting field‐effect transistors and near‐infrared OLEDs. [ 5,29 ] Molecular stacking via π‐π and hydrogen bonds could also form excimer, giving new emissions in lower energy band.…”

Dual-Emissive Bis(diphenylphosphino)amine Platinum Complexes: Structural, Reactivity, Photophysical, and Theoretical Investigations

“…The basic strategy is the reaction of 1a , b with 0.5 equiv of dppa that resulted in the formation of 3a , b . The second route was proposed according to the successful synthesis of analogous dppm complexes from bis-chelate dppm complexes . Therefore, treatment of 1a , b with 2a , b for 1 h at room temperature gave 3a , b .…”

Synthesis, structural characterization, and luminescence properties of mono- and di-nuclear platinum(II) complexes containing 2-(2-pyridyl)-benzimidazole

“…We thank the Iran National Science Foundation (Grant No. 97012633), Shiraz University, the Islamic Azad University (Shiraz Branch) and 30 Department of Chemistry and Biochemistry at UCSB for financial support.…”

“…Among the reported emissive Pt(II) compounds, those with the [Pt(C^N)R] moiety (C^N= cyclometalated ligand and R= alkyl, aryl or halide) are very attractive due to their ease of preparation [22,25,27,28]. These cyclometalated Pt(II) complexes have been synthesized through a C-H activation of ligands C^N (such as ppy, bpy and bhq) with a suitable Pt(II) precursor complex [19,[29][30][31].…”