Efficient yellow electroluminescence of four iridium(iii) complexes with benzo[d]thiazole derivatives as main ligands

Abstract

Four bis-cyclometalated iridium(III) complexes, (bt)₂Ir(tpip), (fbt)₂Ir(tpip), (cf₃bt)₂Ir(tpip) and (dfbt)₂Ir(tpip) (bt = 2-phenylbenzo[d]thiazole, fbt = 6-fluoro-2-phenylbenzo[d]thiazole, cf₃bt = 2-phenyl-6-(trifluoromethyl)benzo[d]thiazole, dfbt = 5,7-difluoro-2-phenylbenzo[d]thiazole, tpip = tetraphenylimidodiphosphinate) have been synthesized and investigated. All complexes emit yellow light peaks at 564–574 nm with quantum efficiencies (Φ_em) of 27.1–48.4% and excited state lifetimes of 2.40–2.81 μs in degassed CH₂Cl₂ solution at room temperature. Correspondingly, organic light-emitting diodes (OLEDs) using these complexes as emitters achieve yellow electrophosphorescence with good device characteristics. Due to its highest photoluminescence quantum yield (48.4%), the device based on (dfbt)₂Ir(tpip) displays the best device performance with a maximum current efficiency (η_{c, max}) of up to 69.8 cd A⁻¹ and a maximum external quantum efficiency (EQE_max) of up to 24.3%. Furthermore, all devices showed low efficiency roll-off ratios. The EQE could still be retained at 17.7%, 16.4%, 18.3% and 20.6% for four devices at a luminance of 1000 cd m⁻². These results suggest that these materials have potential application in efficient OLEDs.