Luminescent PtII(bipyridyl)(diacetylide) chromophores with pendant binding sites as energy donors for sensitised near-infrared emission from lanthanides:: Structures and photophysics of PtII/LnIII assemblies

The complexes [Pt(bipy){CC(4-pyridyl)}(2)] (1) and [Pt(tBu(2)bipy){CC-(4-pyridyl)}(2)] (2) and [Pt(tBu(2)-bipy)(CC-phen)(2)] (3) all contain a Pt(bipy)(diacetylide) core with pendant 4-pyridyl (I and 2) or phenanthroline (3) units which can be coordinated to [Ln(diketonate)(3)} fragments (Ln = a lanthanide) to make covalently-linked Pt-II/Ln(II) polynuclear assemblies in which the Pt-II, chromophore, absorbing in the visible region, can be used to sensitise near-infrared luminescence from the Ln(III) centres. For 1 and 2 one-dimensional coordination polymers [1.Ln(tta)(3)](infinity) and [2(.)Ln(hfaC)(3)](infinity) are formed, whereas 3 forms trinuclear adducts [3(.){Ln(hfac)(3)}2] (tta anion of thenoyi-trifluoroacetone; hfac = anion of hexafluoroacetylacetone). Com-plexes 1-3 show typical Pt-II-based (MLCT)-M-3 luminescence in solution at approximate to 510 nm, but in the coordination polymers [1(.)Ln(tta)(3)](infinity) and [2(.)Ln(hfaC)(3)]infinity the presence of stacked pairs of Pt-II units with short (PtPt)-Pt-... distances means that the chromophores have (MMLCT)-M-3 character and emit at lower energy (approximate to 630 nm). Photophysical studies in solution and in the solid state show that the (MMLCT)-M-3 luminescence in [1.Ln(tta)(3)](infinity) and [2(.)Ln(htaC)(3)](infinity) in the solid state, and the (MLCT)-M-3 emission of [3(.){Ln(hfac)3}2] in solution and the solid state, is quenched by Pt -> Ln energy transfer when the lanthanide has low-energy f-f excited states which can act as energy acceptors (Ln=Yb, Nd, Er, Pr). This results in sensitised near-infrared luminescence from the Ln(III) units. The extent of quenching of the Pt-II-based emission, and the Pt -> Ln energy-transfer rates, can vary over a wide range according to how effective each Ln(III) ion is at acting as an energy acceptor, with Yb-III usually providing the least quenching (slowest Pt -> Ln energy transfer) and either Nd-III or Er-III providing the most (fastest Pt -> Ln energy transfer) according to which one has the best overlap of its f-f absorption manifold with the Pt-II-based luminescence.