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 equivalent to 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 equivalent to 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 equivalent to 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 equivalent to CC6H5)](PF6)(2) (3), where C equivalent to CC6H5 = ethynylbenzene, have also been studied. The multistep syntheses culminate with a Cul-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 phi(em)(rel) of 0.011 (using Ru(bpy)(3)(2+) as a standard with phi(em)(rel) = 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.