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At present, cyclometalated iridium (Ir)(iii) complexes are the most promising emitters for OLEDs. Contrary to well-developed Ir(iii)-based red and green phosphorescent complexes, the efficient blue emitters are limited and the performances of blue OLEDs are still not satisfactory. Inspired by this, we designed two novel blue Ir(iii) complexes employing 2-(2,4-difluoropyridyl)pyridine (dfpypy) and 2,6-difluoro-3-(pyridin-2-yl)benzonitrile (FCN) as the main ligands, respectively, and phenyl(pyridin-2-yl)phosphinate (ppp) as the ancillary ligand. The rational design of the molecular structure makes the two complexes achieve blue emission and possess good electron mobility simultaneously. The devices based on the Ir(iii) phosphors exhibited good electroluminescence performances with low turn-on voltages, a peak current efficiency of 24.51 cd A -1 , a maximum external quantum efficiency of 12.4%, a peak power efficiency of 21.99 lm W -1 and low efficiency roll-off. These results provide an effective strategy for the future molecular design of blue Ir(iii) complexes with good electron transport property for OLEDs.

Aiming to balance the injection and transport of electrons and holes, nitrogen heterocycle and 1,3,4-oxadiazole derivatives were introduced in iridium(iii) complexes to obtain organic light-emitting diodes (OLEDs) with high performances. Thus, two novel Ir(iii) complexes (Ir(tfmphpm)2(pop) and Ir(tfmppm)2(pop)) with green emissions using 2-(3,5-bis(trifluoromethyl)phenyl)pyrimidine (tfmphpm) and 2-(2,6-bis(trifluoromethyl)pyridin-4-yl)pyrimidine (tfmppm) as cyclometalating ligands, and 2-(5-phenyl-1,3,4-oxadiazol-2-yl)phenol (pop) as an ancillary ligand were synthesized. Both emitters show high photoluminescence efficiencies up to 94% and good electron mobility. The devices using two emitters with the structure of ITO (indium-tin-oxide)/MoO3 (molybdenum oxide, 5 nm)/TAPC (di-[4-(N,N-ditolyl-amino)-phenyl]cyclohexane, 30 nm)/mCP (1,3-bis(9H-carbazol-9-yl)benzene, 5 nm)/Ir(iii) complexes (6 wt%) : PPO21 (3-(diphenylphosphoryl)-9-(4-(diphenylphosphoryl)phenyl)-9H-carbazole, 10 nm)/TmPyPB (1,3,5-tri(m-pyrid-3-yl-phenyl) benzene, 40 nm)/LiF (1 nm)/Al (100 nm) display good electroluminescence performances with a maximum luminance of 48 981 cd m-2, a maximum current efficiency of 92.79 cd A-1 and a maximum external quantum efficiency up to 31.8%, respectively, and the efficiency roll-off ratio is low, suggesting that they have potential application in OLEDs.

Two novel iridium(iii) complexes Ir(BTBP)2mepzpy and Ir(BTBP)2phpzpy were successfully synthesized, in which 2',6'-bis(trifluoromethyl)-2,4'-bipyridine (BTBP) was used as the main ligand, and 2-(3-methyl-1H-pyrazol-5-yl)pyridine (mepzpy) and 2-(3-2-(3-phenyl-1H-pyrazol-5-yl))pyridine (phpzpy) were introduced as the ancillary ligands, respectively. Both Ir(iii) complexes displayed bluish green emission peaks at 486 and 487 nm with high quantum efficiencies of 0.73 and 0.69, respectively. The organic light-emitting diodes (OLEDs) with the structure of ITO/HATCN (hexaazatriphenylenehexacarbonitrile, 5 nm)/TAPC (bis[4-(N,N-ditolylamino)-phenyl]cyclohexane, 40 nm)/Ir(BTBP)2mepzpy or Ir(BTBP)2phpzpy (10 wt%): 2,6-DCzPPy (2,6-bis(3-(9H-carbazol-9-yl)phenyl)pyridine, 10 nm)/TmPyPB (1,3,5-tri[(3-pyridyl)-phen-3-yl]benzene, 30 nm)/LiF (1 nm)/Al (100 nm) exhibited high efficiencies with low efficiency roll-off. Especially, the device based on the Ir(BTBP)2mepzpy complex achieved a maximum current efficiency of 58.17 cd A-1 and a maximum external quantum efficiency of 25.33% with Commission Internationale de 1'Eclairage coordinates of (0.19, 0.43). These results indicate that novel cyclometalated Ir(iii) complexes for high efficiency OLEDs with low efficiency roll-off were obtained by our rational design.

Four novel chiral iridium(iii) complexes with triplex stereogenic centers were synthesized by introducing chiral carbon atoms into cyclometalated and ancillary ligands, and separated into eight isomers, which coincide with the old Chinese philosophy Eight Trigrams. The electron circular dichroism and circularly polarized luminescence spectra of four pairs of isomers show perfect mirror images with a dissymmetry factor (g lum factor) of around 0.003.

Two novel iridium(iii) complexes, Ir(tfmpiq) 2 (acac) (tfmpiq = 1-(2,6-bis(trifluoromethyl)pyridin-4-yl)isoquinoline, acac = acetylacetone) and Ir(tfmpqz) 2 (acac) (tfmpqz = 4-(2,6-bis(trifluoromethyl)pyridin-4-yl)quinazoline), were synthesized and thoroughly investigated. Both complexes emit orange-red photoluminescence with high quantum yields (Ir(tfmpiq) 2 (acac): λ max : 587 nm, η PL : 42%; Ir(tfmpqz) 2 (acac): λ max : 588 nm, η PL : 91%). Furthermore, the complex containing quinazoline shows higher electron mobility than that with isoquinoline. The organic light-emitting diodes (OLEDs) with single- or double-emitting layers (EML) were fabricated using two new emitters. The double-EML device using Ir(tfmpqz) 2 (acac) with the structure of ITO (indium-tin-oxide)/MoO 3 (molybdenum oxide, 3 nm)/TAPC (di-[4-(N,N-ditolyl-amino)-phenyl]cyclohexane, 50 nm)/Ir(tfmpqz) 2 (acac) (1 wt%):TcTa (4,4',4''-tris(carbazole-9-yl)triphenylamine, 10 nm)/Ir(tfmpqz) 2 (acac) (1 wt%):2,6DCzPPy (2,6-bis(3-(carbazol-9-yl)phenyl)pyridine, 10 nm)/TmPyPB (1,3,5-tri(m-pyrid-3-yl-phenyl)benzene, 50 nm)/LiF (1 nm)/Al (100 nm) displays good electroluminescence performances with a maximum luminance of 96 609 cd m -2 , a maximum current efficiency of 59.09 cd A -1 , a maximum external quantum efficiency of 20.2%, a maximum power efficiency of 53.61 lm W -1 , and the efficiency roll-off ratio is mild.