Visible light-harvesting cyclometalated Ir(III) complexes with pyreno[4,5-d] imidazole (CN)-N-boolean AND 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 (CN)-N-boolean AND ligand was used to access the long-lived T-1 excited state (Ir-1, bpy = 2,2'-bipyridine as the (NN)-N-boolean AND ligand. tau(T)= 56.1 mu s). In order to enhance the absorption in visible range, a coumarin derived (NN)-N-boolean AND ligand was used (Ir-2, epsilon = 51,500 M-1 cm(-1) at 466 nm, tau(T) = 73.9 mu s). The complexes show room temperature phosphorescence in the red. The T-1 excited states of Ir-1 and Ir-2 were identified as mainly intraligand ((IL)-I-3) states, vs. the metal-to-ligand-charge-transfer ((MLCT)-M-3) 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. (C) 2012 Elsevier Ltd. All rights reserved.