Efficient M-shaped blue emitters having a high conjugation extent with improved roll-off efficiency

Abstract

We have synthesized M-shaped acceptor–donor–acceptor (A–D–A) materials, 4′-(1-(4-(diphenylamino)phenyl)-1H-phenanthro[9,10-d]imidazol-2-yl)-N-(4′-(1-(4-(diphenylamino)phenyl)-1H-phenanthro[9,10-d]imidazol-2-yl)-[1,1′-biphenyl]-4-yl)-N-phenyl-[1,1′-biphenyl]-4-amine (TPA–2PPITPA) and 4′-((E)-2-(1-(4-(diphenylamino)phenyl)-1H-phenanthro[9,10-d]imidazol-2-yl)vinyl)-N-(4′-((E)-2-(1-(4-(diphenylamino)phenyl)-1H-phenanthro[9,10-d]imidazol-2-yl)vinyl)-[1,1′-biphenyl]-4-yl)-N-phenyl-[1,1′-biphenyl]-4-amine (TPA–2SPPITPA) consisting of two identical π-conjugated groups. Both TPA–2SPPITPA (T_d/T_g, °C: 582/219) and TPA–2PPITPA (T_d/T_g, °C: 543/209) exhibit excellent thermal stability and can form stable amorphous films. The highly twisted TPA–2SPPITPA and TPA–2PPITPA exhibit high photoluminescence quantum yields (PLQY_s/f) of 0.90/0.88 and 0.81/0.78, respectively, owing to the suppression of intermolecular stacking. The non-doped blue device with multifunctional TPA–2SPPITPA/TPA–2PPITPA shows an external quantum efficiency (EQE) of 6.13/5.82% with a small efficiency roll-off (1.63/3.0%), current efficiency (CE) of 5.92/5.43 cd A⁻¹, power efficiency (PE) of 5.15/4.98 lm W⁻¹ and commission internationale de l'Eclairage (CIE) of (0.15, 0.07)/(0.15, 0.07). The twisted TPA–2SPPITPA and TPA–2PPITPA with high triplet energy were employed as hosts in PhOLEDs. The green (TPA–2SPPITPA:Ir(ppy)₃)/red device (TPA–2SPPITPA:Ir(MDQ)₂(acac)) exhibits a maximum EQE of 27.23/19.01%, luminance of 76 233/47 690 cd m⁻², CE of 81.87/28.76 cd A⁻¹ and PE of 75.23/24.73 lm W⁻¹ and CIE of (0.33, 0.60)/(0.65, 0.33). The small roll-off efficiency (1.63/3.0%) will be beneficial for the operative stability of devices.