Achieving non-doped deep-blue OLEDs by applying bipolar imidazole derivatives

Abstract

Abstract In this work, we designed and synthesized two novel bipolar deep-blue emitting materials, 2-(4'-(9H-carbazol-9-yl)-[1,1′-biphenyl]-4-yl)-1 (4(tri-fluoromethyl) phenyl)-1H-phenanthro[9,10-d] imidazole (CzB-FMPPI) and 9-(4'-(4,5-diphenyl-1 (4-(trifluoromethyl) phenyl)-1H-imidazole-2-yl)-[1,1′-biphenyl]-4-yl)-9H-carbazole (CzB-FMPIM). Among them, carbazole and phenanthroimidazole are bridged linkage by the biphenyl ring and act as electron-donating part. The CF3-substituted phenyl ring applies as strong electron-withdrawing moiety. The exhibit highly twisted molecular configuration of two compounds efficiently shorten π-conjugation and inhibit intermolecular interaction, resulting in superior thermal stability and deep blue emission. High decomposition temperature of 431 °C and 414 °C, glass transition temperature of 150 °C and 135 °C for CzB-FMPPI and CzB-FMPIM, respectively, had been achieved. As a consequence of breaking the conjugation of phenanthroimidazole chromophore, CzB-FMPIM achieves an 8 nm blue-shifted emission compared with CzB-FMPPI. CzB-FMPPI exhibits a higher relative fluorescence quantum yield of 92.5% than 83.3% of CzB-FMPIM. Moreover, bipolar property was observed in both compound and homogeneous amorphous films were deposited and applied in the non-doped deep-blue OLEDs. The devices based on two emitters showed maximum luminance of 6667 cd/m2 and 3084 cd/m2, maximum EQE of 4.10% and 3.17%, respectively. Commission International de l'Eclairage (CIE) coordinates of CzB-FMPIM based device achieved (0.15, 0.07) which is extremely close to the NTSC standard blue CIE (0.14, 0.08).