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

An NN PtII bis(acetylide) complex was prepared with the rhodamine fluorophore [Pt–Rho, in which NN is dbbpy = 4,4′‐bis(tert‐butyl‐2,2′‐bipyridine)]. Complex Pt–Rho shows intense absorption in the visible region (ϵ = 185800 M–1 cm–1 at 556 nm) and a fluorescence emission of the ligand (λem = 580 nm, ΦL = 41.1 %, τL = 2.50 ns). The long‐lived rhodamine‐localised intraligand triplet excited state (3IL, τT = 83.0 μs) 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 (ϵ = 14700 M–1 cm–1 at 424 nm) and a short‐lived metal‐to‐ligand charge‐transfer excited state (3MLCT) (τT = 1.36 μs). 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 3IL excited state of the organic chromophore by metallation with PtII will be useful for the preparation of transition metal complexes that show intense absorption of visible light and long‐lived triplet excited states.