Room-Temperature Long-Lived 3IL Excited State of Rhodamine in an N∧N PtII Bis(acetylide) Complex with Intense Visible-Light Absorption

An N boolean AND N Pt-II bis(acetylide) complex was prepared with the rhodamine fluorophore [ Pt-Rho, in which N boolean AND N is dbbpy = 4,4'-bis(tert-butyl-2,2'-bipyridine)]. Complex Pt-Rho shows intense absorption in the visible region (epsilon = 185800 M(-1)cm(-1) at 556 nm) and a fluorescence emission of the ligand (lambda(em) = 580 nm, Phi(L) = 41.1%, tau(L) = 2.50 ns). The long-lived rhodamine-localised intraligand triplet excited state ((IL)-I-3, tau(T) = 83.0 mu s) was proposed to be populated upon excitation of the rhodamine ligand, proved by nanosecond time-resolved transient absorption and spin density analysis; the triplet excited state was studied by time-dependent DFT calculations. In comparison, the model complex (dbbpy) PtII bis(phenylacetylide) (Pt-Ph) shows weak absorption in the visible region (epsilon = 14700 M(-1)cm(-1) at 424 nm) and a short-lived metal-to-ligand charge-transfer excited state ((MLCT)-M-3) (tau(T) = 1.36 mu s). Complex Pt-Rho was used as triplet sensitiser for upconversion based on triplet-triplet annihilation. An upconversion quantum yield of 11.2% was observed. Our strategy to access the long-lived (IL)-I-3 excited state of the organic chromophore by metallation with Pt-II will be useful for the preparation of transition metal complexes that show intense absorption of visible light and long-lived triplet excited states.