Pt(II) Complexes with a Novel Pincer N^C^N Ligand: Synthesis, Characterization, and Photophysics

Abstract

A series of new platinum square planar complexes [Pt(NCN)L]+/0 with the pincer N^C^N cyclometallated ligand (NC(H)N = 1,3-bis(1-phenyl-1H-phenanthro[9,10-d]imidazol-2-yl)benzene) containing the following L: Cl−, acetonitrile, pyridine, dimethylaminopyridine, 2,6-dimethylphenylisocyanide, has been synthesized. Application of bridging acetate ion as L ligand allowed obtaining a binuclear [Pt(NCN)]2OOCCH3 complex. The bulky and rigid structure of N^C^N-ligand provokes instability of its pincer coordination that makes possible transformation of the molecular architecture to give a heteronuclear complex with the Pt-Ag-Pt coordination core. The composition and structure of the obtained compounds were characterized in solution and in the solid state using ESI mass-spectrometry, NMR spectroscopy, elemental analysis, and single-crystal XRD crystallography. The complexes luminesce in solid state, solution, and in polymeric matrix demonstrating moderate to bright emission at ca. 550 nm with quantum yields up to 22% and lifetime of excited state up to 22 µs. TD DFT computational approach together with analysis of the photophysical properties in different media reveals the predominant ligand-centered 3IL nature of the radiative excited state localized at the N^C^N-ligand. The ancillary ligand L demonstrates a minor influence on the energy of emission but affects dramatically emission efficiency and lifetime. The chloride complex displays dual (fluorescence and phosphorescent) luminescence due to labile coordination of an N-coordinated functionality that produces a dangling aromatic fragment, which gives emission from a singlet excited state.