Phosphorescent organoplatinum(II) D₂A₂ metallacycles: Synthesis, self-assembly, and photophysical properties

A pair of organoplatinum(II) metallacycles, M1 and M2, was self-assembled by combining one of two donor molecules, cis-[Pt(dhim)₂(C≡CC₅H₄N)₂] (D1; dhim = 1,3-dihexyl-2-H-imidazole-2-yelidene) and Pt(tbbpy)(C≡CC₅H₄N)₂ (D2; tbbpy = 4,4′-di-tert-butyl-2,2′-bipyridine), with an acceptor precursor, Pt(tbbpy)(OTf)₂ (A1; OTf = CF₃SO₃ ⁻), respectively. Both donor molecules exhibit an idealized 90° angle between the coordination vectors of their ethynylpyridine moieties. When mixed with A1, coordination-driven self-assembly occurs at 50 °C (4 days in acetonitrile for M1 and 18 h in CH₂Cl₂ for M2). Both metallacycles were characterized by ¹H NMR, FT-IR, and FT-ICR-MS techniques that support D₂A₂ self-assembly of molecular squares. The structure of building block D1 was determined by X-ray diffraction, confirming the expected square coordination geometry and 90° orientation of the pyridyl coordination vectors. Photophysical studies of M1 and M2 reveal that the metallacycles display triplet emission bands at 466 and 469 nm, respectively, that originate from transitions localized on building blocks D1 and D2. This phosphorescent behavior is assigned to ³ILCT and ³LLCT (π_C≡C* → π_C≡C; ILCT = intraligand charge transfer, LLCT = ligand-to-ligand charge transfer) transitions based on previous studies of phenyl analogs to D1 and D2 that indicate that the ethynyl moieties dominate in their contributions to the molecular orbitals involved in absorption and emission.