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

Abstract

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 %.

An energetics assessment of benzo[a]tetracene and benzo[a]pyrene as triplet–triplet annihilation emitters

“…6,[89][90][91] Pentacene and its derivatives are quintessential SF materials, 24,[92][93][94] whereas anthracene and its derivatives are known TTA emitters. 6,9,15,95 This is because the E loss TTA,S exhibits an inverse dependence on the length of the acene backbone. 14,24,96,97 As shown in Fig.…”

Sensitization-initiated electron transfer via upconversion: mechanism and photocatalytic applications

“…Among multi-component systems for two-photon mechanisms, sensitized triplet-triplet annihilation upconversion (sTTA-UC) is currently one of the most intensively pursued approaches, 1 enabling photoreactions to proceed with NIR or red light instead of UV or blue excitation. 2,[11][12][13][14][15][16][17][18] In sTTA-UC the sensitizer is photo-excited, and then transfers its excitation energy to a co-catalyst with an energetically lower lying triplet excited state, the so-called annihilator. [19][20][21] Subsequent triplettriplet annihilation leads to upconversion, populating the uorescent singlet excited state of the annihilator.…”

Polysubstituted Ligand Framework for Color Tuning Phosphorescent Iridium(III) Complexes

“…Our own studies have investigated Ir­(III) 2-pqx species by considering different substituents on the phenyl part of the ligand . Further tuning of the emission properties can be achieved through substituents on the quinoxaline ring, and this has led to application in triplet–triplet annihilation energy upconversion (TTA-UC), wherein the Ir­(III) complex acts as the photosensitizer. Targeted bioimaging studies which make use of the low energy red emission of a peptide conjugated Ir­(III) 2-pqx complex have also been successfully demonstrated …”