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Abstract
Combining a sterically bulky, electron-deficient 2-(2,4-difluorophenyl)-4-(2,4,6-trimethylphenyl)pyridine (dFMesppy) cyclometalating C∧N ligand with an electron rich, highly rigidified 1,1'-(α,α'-o-xylylene)-2,2'-biimidazole (o-xylbiim) N∧N ligand gives an iridium complex, [Ir(dFMesppy)2(o-xylbiim)](PF6), that achieves extraordinarily bright blue emission (ΦPL = 90%; λmax = 459 nm in MeCN) for a cationic iridium complex. This complex is compared with two reference complexes bearing 4,4'-di-tert-butyl-2,2'-bipyridine, and solution-processed organic light emitting diodes (OLEDs) have been fabricated from these materials.
Highlights
The world’s ever increasing demand for energy necessitates an overhaul in virtually all areas of technology
Exceptional performance red2b and green[8] organic light emitting diodes (OLEDs) based on iridium have been reported, a corresponding deep blue emitter remains elusive.2a This is due in no small part to the host–guest configuration of the emissive layers in OLEDs – the requirement being that the high triplet energies required for blue emitters (42.8 eV) necessitate even higher triplet energies for the host materials
Even devices that show deep blue emission using iridium complexes show poorer efficiencies compared to their red- and green-emitting counterparts (B30% external quantum efficiency (EQE)); a recent review on blue emitters in OLEDs identified a champion true blue device based on iridium as having Commission International de L’Eclairage (CIE) coordinates of (0.14, 0.10), with an EQE of 7.6%,2a while a recent report[9] outlined the use of a triscyclometalated NHC iridium complex that achieved CIE coordinates of (0.16, 0.09) at an EQE of 10.1% – a different story from the higher efficiencies (420%) reported for sky-blue emitters.2a,10
Summary
The world’s ever increasing demand for energy necessitates an overhaul in virtually all areas of technology. In our own research efforts, we have identified, among other motifs, nitrogen-rich heterocyclic ancillary ligands as promising candidates in this endeavour.[11] In particular, we have shown biimidazole-type N4N ancillary ligands to be effective motifs for invoking significant destabilisation of the LUMO energies of these complexes (Fig. 1) Using these scaffolds we demonstrated that very bright blue emission could be realised through alkylation of the distal nitrogen atoms with a rigid o-xylylene linker.11b Efficient non-radiative decay processes observed for the protonated ([Ir(dFppy)2(H2biim)](PF6), where dFppy is 2-(4,6difluorophenyl)pyridinato and H2biim is 1H,10H-2,20-biimidazole) and especially the methylated ([Ir(dFppy)2(dMebiim)](PF6), where dMebiim is 1,10-dimethyl-2,20-biimidazole) analogues could be strongly suppressed in ([Ir(dFppy)2(o-xylbiim)](PF6), where o-xylbiim is 1,10-(a,a0-o-xylylene)-2,20-biimidazole), to give exceptionally bright emission in MeOH solution (FPL = 68%, lem = 450 nm).
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