Visible light-harvesting cyclometalated Ir(III) complexes with pyreno[4,5-d]imidazole C^N ligands as triplet photosensitizers for triplet–triplet annihilation upconversion

Cyclometalated Ir(III) complexes with pyrenyl-fused imidazole ligands were prepared. The complexes show strong absorption of visible light and long-lived triplet excited state and were used as triplet photosensitizers for triplet–triplet annihilation (TTA) upconversion. Pyreno[4,5-d]imidazole C^N ligand was used to access the long-lived T1 excited state (Ir-1, bpy = 2,2′-bipyridine as the N^N ligand. τT = 56.1 μs). In order to enhance the absorption in visible range, a coumarin derived N^N ligand was used (Ir-2, ɛ = 51,500 M−1 cm−1 at 466 nm, τT = 73.9 μs). The complexes show room temperature phosphorescence in the red. The T1 excited states of Ir-1 and Ir-2 were identified as mainly intraligand (3IL) states, vs. the metal-to-ligand-charge-transfer (3MLCT) state for the model complex (Ir-0), proved by steady state emission, transient absorption, 77 K emission spectra and DFT calculations. The complexes were used as triplet photosensitizers for TTA upconversion and upconversion quantum yield up to 23.7% was observed.

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► New pyrenyl-fused imidazole C^N cyclometalation ligand for cyclometalated Ir(III) complexes is described.
► The new complexes show strong visible light-harvesting ability.
► The lifetime of the triplet excited state of the Ir(III) complexes is up to 73.9 μs.
► The Ir(III) complexes were used as triplet photosensitizers for TTA upconversion and quantum yield up to 23.7% was observed.
► The time-resolved transient absorption of the Ir(III) complexes were predicted with DFT calculations.