Iridium(III) Sensitisers and Energy Upconversion: The Influence of Ligand Structure upon TTA‐UC Performance

• Published in: Chemistry : a European journal 2021-02, Vol.27 (10), p.3427-3439
• Main Authors: Elgar, Christopher E., Otaif, Haleema Y., Zhang, Xue, Zhao, Jianzhang, Horton, Peter N., Coles, Simon J., Beames, Joseph M., Pope, Simon J. A.
• Format: Article
• Language: English
• Subjects: Chemistry; Computer applications; Coordination compounds; Crystallography; density functional theory; Iridium; iridium complexes; Ligands; phosphorescent species; Photoluminescence; Photons; Quinoline; spectroscopy; Upconversion
• ISSN: 0947-6539; 1521-3765
• DOI: 10.1002/chem.202004146

Six substituted ligands based upon 2‐(naphthalen‐1‐yl)quinoline‐4‐carboxylate and 2‐(naphthalen‐2‐yl)quinoline‐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 IrIII, yielding complexes of the form [Ir(C^N)2(bipy)]PF6 (where C^N=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 IrIII 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 %. Red, red shine: A series of substituted naphthylquinolines have been synthesised and investigated as cylcometallating ligands for IrIII. The resultant complexes were shown to be emissive in the deep‐red region and several were identified as viable photosensitisers for triplet–triplet annihilation upconversion.