Ligand‐Tuneable, Red‐Emitting Iridium(III) Complexes for Efficient Triplet–Triplet Annihilation Upconversion Performance

Abstract

A series of substituted 2-phenylquinoxaline ligands have been explored to finely tune the visible emission properties of a corresponding set of cationic, cyclometallated iridium(III) complexes. The electronic and redox properties of the complexes were investigated through experimental (including time-resolved luminescence and transient absorption spectroscopy) and theoretical methods. The complexes display absorption and phosphorescent emissions in the visible region that are attributed to metal to ligand charge-transfer transitions. The different substitution patterns of the ligands induce variations in these parameters. Time-dependent DFT studies support these assignments and show that there is likely to be a strong spin-forbidden contribution to the visible absorption bands at λ=500-600 nm. Calculations also reliably predict the magnitude and trends in triplet emitting wavelengths for the series of complexes. The complexes were assessed as potential sensitisers in triplet-triplet annihilation upconversion experiments by using 9,10-diphenylanthracene as the acceptor; the methylated variants performed especially well with impressive upconversion quantum yields of up to 39.3 %.

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

“…A bleach at 350< λ <400 nm is also evident. This general appearance is comparable to that observed previously for Ir III 2‐phenylquinoxaline complexes [18] . Each feature is associated with similar lifetime characteristics which suggests that each peak relates to the same excitation, ISC process and deactivation.…”

“…Ligand centred bands from the various aromatic components are anticipated to contribute to the more intense absorptions between 220–400 nm. Previous studies on related Ir III complexes suggest that spin allowed metal‐to‐ligand charge transfer ( 1 MLCT) absorptions contribute at 350–450 nm with spin forbidden absorptions to 3 MLCT excited states likely to result weaker features >450 nm [18] . The complexes herein demonstrate a similar pattern where the MLCT absorptions tail to around 600 nm, which is attributed to the extended conjugation of the cyclometallating ligands.…”

“…Our own contribution to the area reported a series of cationic Ir III complexes as triplet sensitisers which have shown the highest yet reported TTA‐UC efficiency of 39 % using DPA as the annihilator [18] . These Ir III sensitiser complexes were based upon a 2‐phenylquinoxaline ligand framework, [19,20] which impart good absorption characteristics in the visible region.…”

Targeted cell imaging properties of a deep red luminescent iridium(iii) complex conjugated with a c-Myc signal peptide

“…The absorption spectrum was obtained in water and showed a composite of bands between 200-600 nm; the appearance of the spectrum is reminiscent of closely related [Ir(C^N) 2 (N^N)] + complexes of this type. 26 By inference, ligand-centred transitions are assumed to dominate below 350 nm, with both quinoxaline and bipyridine located p / p* contributing to the spectrum. At 350-450 nm it is likely that spin-allowed metal-to-ligand (5d-to-p*) charge transfers ( 1 MLCT) are present, with a weaker (3 < 1000 M À1 cm À1 ) shoulder feature at 450-600 nm, which is presumably due to a spin forbidden (S 0 / T 1 ) 3 MLCT absorption.…”

Capturing the interplay between spin–orbit coupling and non-Condon effects on the photoabsorption spectra of Ru and Os dyes

“…13 This strategy has only been sparsely employed in the calculation of absorption spectra of metal complexes (see e.g. Refs [17][18][19] ).…”