Tian-Yi Li

Syntheses, photoluminescence, and electroluminescence of a series of iridium complexes with trifluoromethyl-substituted 2-phenylpyridine as the main ligands and tetraphenylimidodiphosphinate as the ancillary ligand.

Five bis-cyclometalated iridium complexes with tifluoromethyl-substituted 2-phenylpyridine (ppy) at different positions of its phenyl group as the main ligands and tetraphenylimidodiphosphinate (tpip) as the ancillary ligand, 2-6 (1 is a trifluoromethyl-free complex), were prepared, and their X-ray crystallography, photoluminescence, and electrochemistry were investigated. The number and positions of trifluoromethyl groups at the phenyl ring of ppy greatly affected the emission spectra of Ir(3+) complexes, and their corresponding emission peaks at 533, 502, 524, 480, and 542 nm were observed at room temperature, respectively. Constructed with complexes 2-6 as the emitters, respectively, the organic light-emitting diodes (OLEDs) with the structure of indium-tin oxide/1,1-bis[4-(di-p-tolylamino)phenyl]cyclohexane (30 nm)/Ir (x wt %):bis[3,5-bis(9H-carbazol-9-yl)phenyl]diphenylsilane (15 nm)/1,3,5-tris(1-phenyl-1H-benzo[d]imidazol-2-yl)phenyl (45 nm)/LiF (1 nm)/Al (100 nm) showed good performances. Particularly, device G4 based on 4-trifluoromethyl-substituted complex 4 with x = 8 wt % obtained a maximum luminance of over 39000 cd m(-2) and maximum luminance efficiency (η(L)) and power efficiency (η(p)) of 50.8 cd A(-1) and 29.0 lm W(-1), respectively. The results suggested that all of the complexes 2-6 would have potential applications in OLEDs.

Circularly polarised phosphorescent photoluminescence and electroluminescence of iridium complexes.

Nearly all the neutral iridium complexes widely used as dopants in PhOLEDs are racemic mixtures; however, this study observed that these complexes can be separated into stable optically active Λ and ∆ isomers and that their chirality is an intrinsic property. The circularly polarised phosphorescent photoluminescence (CPPPL) signals of Λ/Δ isomers are perfect mirror images with opposite polarisation and equal intensity exhibiting a “handedness” for the polarisation. For the first time, we applied the Λ/Δ iridium isomers as emitters in OLEDs, and the circularly polarised phosphorescent electroluminescence (CPPEL) spectra reveal completely positive or negative broad peaks consistent with the CPPPL spectra. The results demonstrate that the Λ/Δ isomers have potential application for 3D OLEDs because they can exhibit high efficiency and luminance, and 3D display technology based on circularly polarised light is the most comfortable for the eyes.

N-Heterocyclic Carbenes: Versatile Second Cyclometalated Ligands for Neutral Iridium(III) Heteroleptic Complexes

With 2-(2,4-difluorophenyl)pyridine (dfppy) as the first cyclometalated ligand and different monoanionic N-heterocyclic carbenes (NHCs) as the second cyclometalated ligands, 16 blue or greenish-blue neutral iridium(III) phosphorescent complexes, (dfppy)2Ir(NHC), were synthesized efficiently. The obtained Ir(III) complexes display typical phosphorescence of 455-485 nm with quantum yields up to 0.73. By modifying the phenyl moiety in the NHCs with electron-withdrawing substituents (e.g., -F or -CF3) or replacing it with N-heteroaromatic rings (pyridine or pyrimidine), the HOMO-LUMO gaps are broadened, and the emissions shift to the more blue region accordingly. Furthermore, to extend the application scope of NHCs as the second cyclometalated ligands, five other Ir(III) complexes from blue to red were synthesized with different first cyclometalated ligands. Finally, the organic light-emitting diodes using one blue emitter exhibit a maximum current efficiency of 37.83 cd A(-1), an external quantum efficiency of 10.3%, and a maximum luminance of 8709 cd m(-2). Our results demonstrate that NHCs as the second cyclometalated ligands are good candidates for the achievement of efficient phosphorescent Ir(III) complexes and corresponding devices.

Iridium(III) phosphorescent complexes with dual stereogenic centers: single crystal, electronic circular dichroism evidence and circularly polarized luminescence properties

Iridium complexes with a chiral metal center and chiral carbons, Λ/Δ-(dfppy)2Ir(chty-R) and Λ/Δ-(dfppy)2Ir(chty-S), were synthesized and characterized. These isomers have the same steady-state photophysical properties, and obvious offsets in ECD spectra highlight both the chiral sources. Each enantiomeric couple shows mirror-image CPL bands with a dissymmetry factor in the order of 10-3.