Iridium(III) sensitisers and energy upconversion: the influence of ligand structure upon TTA-UC performance

Six substituted ligands based upon 2‐(naphthalen‐1‐yl)quinoline‐4‐carboxylate and 2‐(naphthalen‐2‐yl)quinoline‐4‐carboxylate have been synthesized in two steps from a range of commercially available isatin derivatives. These species are demonstrated to be effective cyclometalating ligands for Ir(III) yielding complexes of the form [Ir(C^N)2(bipy)]PF6 (where C^N = cyclometalating ligand; bipy = 2,2’‐bipyridine). X‐ray crystallographic studies on three examples demonstrate that the complexes adopt a distorted octahedral geometry wherein a cis‐C,C and trans‐N,N coordination mode is observed. Intra‐ligand torsional distortions are evident in all cases. The Ir(III) complexes display photoluminescence in the red part of the visible region (668‐693 nm) which is modestly tuneable via the ligand structure. The triplet lifetimes of the complexes are clearly influenced by the precise structure of the ligand in each case. Supporting computational (DFT) studies suggest that the differences in observed triplet lifetime are likely due to differing admixtures of ligand‐centred versus MLCT character instilled by the facets of the ligand structure. Triplet‐triplet annihilation upconversion (TTA‐UC) measurements demonstrate that the complexes based upon the 1‐naphthyl derived ligands are viable photosensitizers with upconversion quantum efficiencies of 1.6‐6.7%.