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)quino-line-4-carboxylate have been synthesised in two steps from a range of commercially available isatin derivatives. These species are shown to be effective cyclometallating ligands for Ir-III, yielding complexes of the form [Ir((CN)-N-boolean AND)(2)(bipy)]PF6 (where (CN)-N-boolean AND= cyclometallating 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. Intraligand 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 through 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 photosensitisers with upconversion quantum efficiencies of 1.6-6.7 %.

Automated summary

Six substituted ligands based on 2-(naphthalen-1-yl)quinoline-4-carboxylate and 2-(naphthalen-2-yl)quinoline-4-carboxylate were synthesized and found to be effective cyclometallating ligands for Ir-III complexes. X-ray crystallographic studies showed a distorted octahedral geometry of the complexes, with differences in observed triplet lifetimes attributed to the ligand structure. Complexes based on 1-naphthyl derived ligands demonstrated viable photosensitizing properties with upconversion quantum efficiencies of 1.6-6.7%.