Synthesis, Structure, Characterization, and Photophysical Studies of a New Platinum Terpyridyl-Based Triad with Covalently Linked Donor and Acceptor Groups

A new terpyridyl-containing Pt triad [Pt(pytpy)(p-C⋮C−C6H4−NH−CO−C6H2(OMe)3)](PF6)2 (4), where pytpy = 4‘-(4-pyridin-1-ylmethylphenyl)-[2,2‘;6‘,2‘ ‘]terpyridine and p-C⋮C−C6H4−NH−CO−C6H2(OMe)3 = N-(4-ethynylphenyl)-3,4,5-trimethoxybenzamide, has been synthesized and structurally characterized. The related donor−chromophore dyad [Pt(ttpy)(p-C⋮C−C6H4−NH−CO−C6H2(OMe)3)]PF6 (2), where ttpy = 4‘-p-tolyl-[2,2‘;6‘,2‘ ‘]terpyridine, and the chromophore−acceptor dyad [Pt(pytpy)(C⋮CC6H5)](PF6)2 (3), where C⋮CC6H5 = ethynylbenzene, have also been studied. The multistep syntheses culminate with a CuI-catalyzed coupling reaction of the respective acetylene with either [Pt(ttpy)Cl]PF6 or [Pt(pytpy)Cl](PF6)2. X-ray and spectroscopic studies support assignment of a distorted square planar environment around the Pt(II) ion with three of its coordination sites occupied by the terpyridyl N-donors and the fourth coordination site occupied by the acetylenic carbon. Although the parent compound [Pt(ttpy)(C⋮CC6H5)]PF6 (1) is brightly luminescent in fluid solution at 298 K, dyad 2 as well as triad 4 exhibit complete quenching of the emission. The chromophore−acceptor (C−A) dyad 3 displays weak solution luminescence at room temperature with a φrel em of 0.011 (using Ru(bpy)3 2+ as a standard with φrel em = 0.062). Electrochemically, the donor−chromophore (D−C) dyad and the donor−chromophore−acceptor (D−C−A) triad exhibit both metal-based and donor ligand-based oxidations, whereas the triad and the C−A dyad show the expected pyridinium- and terpyridine-based reductions. Transient absorption studies of the dyad and triad systems indicate that although the trimethoxybenzene group acts as a reductive donor, in the present system, the pyridinium group fails to act as an acceptor.