Photoluminescence and electroluminescence of deep red iridium(iii) complexes with 2,3-diphenylquinoxaline derivatives and 1,3,4-oxadiazole derivatives ligands

Abstract

Four iridium(III) complexes (Ir1 to Ir4) containing 2,3-diphenylquinoxaline derivatives with or without fluoro-substituted on different positions (L1: 2,3-diphenylquinoxaline; L2: 6,7-difluoro-2,3-diphenylquinoxaline; L3: 2,3-bis(4-fluorophenyl)quinoxaline; L4: 6,7-difluoro-2,3-bis(4-fluorophenyl)quinoxaline) as main ligands and 2-(5-phenyl-1,3,4-oxadiazol-2-yl)phenol (HPOP) as ancillary ligand were synthesized and investigated. All the complexes emit deep red photoluminescence (PL) (Ir1: λ_max: 645 nm; Ir2: λ_max: 650 nm; Ir3: λ_max: 634 nm; Ir4: λ_max: 639 nm). Moreover, the electron mobility of the complexes Ir3 and Ir4 is higher than that of the electron transport material Alq₃ (tris-(8-hydroxyquinoline)aluminium), which is beneficial for their performances in organic light-emitting diodes (OLEDs). The OLEDs with single- or double-emitting layers (EML) were fabricated using Ir3 or Ir4 as the emitter. The double-EML device using Ir4 with the structure of ITO (indium-tin-oxide)/MoO₃ (molybdenum oxide, 5 nm)/TAPC (di-[4-(N,N-ditolyl-amino)-phenyl]cyclohexane, 30 nm)/TcTa (4,4′,4′′-tris(carbazol-9-yl) triphenylamine): Ir4 (2 wt%, 10 nm)/26DCzPPy (2,6-bis(3-(carbazol-9-yl)phenyl)pyridine): Ir4 (2 wt%, 10 nm)/TmPyPB (1,3,5-tri(m-pyrid-3-yl-phenyl)benzene, 40 nm)/LiF (1 nm)/Al (100 nm) displays good electroluminescence (EL) performances with maximum luminance, current efficiency, power efficiency and external quantum efficiency of up to 20 676 cd m⁻², 14.0 cd A⁻¹, 12.0 lm W⁻¹ and 17.8%, respectively, and the efficiency roll-off ratio is mild. The results suggest that the number and position of fluoro-substituents can affect photophysical, electrochemical and electroluminescent properties of the Ir(III) complexes, which are potential deep red phosphorescent materials for specific OLED applications.