Binaphthalene bridged bipolar transporting materials for blue electroluminescence: toward high EL efficiency via molecular tuning

Two isomeric bipolar transporting molecules containing arylamine and benzimidazole moieties linked by 1,1′-binaphthalene bridge have been synthesized and used for blue light-emitting diodes. The highly twisted binaphthalene bridge is beneficial for amorphous morphology, good solubility, high thermal stability and high photoluminescence quantum efficiency (Φ_PL). The charge transfer bands of these compounds exhibit interesting solvent-polarity dependent fluorescence properties. The physical properties of the compounds were tunable upon binding of the benzimidazole with binaphthalene group via C- (BINAPC) or N- (BINAPN) linkage. Three-layered blue-emitting OLEDs using BINAPC or BINAPN as the emitting layer, NPB as the hole transporting layer, and TPBI as the electron transporting layer as well as hole-blocking layer exhibit good performance. External quantum efficiencies of 2.49% with color coordinates of (0.15, 0.11) and 2.67% with color coordinates of (0.16, 0.16) were achieved for BINAPC and BINAPN, respectively.