Synthesis and characterization of novel boron-containing hole-blocking materials for phosphorescent organic light emitting diodes

Abstract

Four-coordinate boron compounds of $Ar_2B\cdot1$ (1·H = 2-(2-pyridyl)phenol; Ar = Ph; 2, $C_6F_5$ ; 3) containing 2-(2-pyridyl)phenolate (1) as a bidentate [N,O] chelating ligand was prepared and tested as hole-blocking layer (HBL) materials in phosphorescent OLEDs. The crystal structure of 2 reveals the pseudo-tetrahedral geometry around the boron center with bidentate [N,O] chelation by 1. Both compounds possess the identical HOMO-LUMO energy gap of 3.1 eV appropriate for hole-blocking materials for phosphorescent OLEDs. The devices incorporating 2 and 3 as HBL materials displayed stable green phosphorescence of $Ir(ppy)_3$ (ppy = 2-phenylpyridine), indicating that 2 and 3 function as HBL materials. Although the device based on 3 exhibits lowering of device performance at high current density (> $10 mA/cm^2$), the device based on 2 displays better performances in terms of luminance, power and luminance efficiency, and external quantum efficiency in a wide range of current densities (0.1 ~ $100 mA/cm^2$) than the reference device incorporating BAlq as HBL materials. Various substituted dioxaborole and diazaborole compounds were synthesized, and their photophysical and electrochemical properties were studied. The increase in molecular weight of the compounds leads to the increase in thermal stability of the compounds. Moreover, diazaborole derivatives show higher thermal stabilities than dioxaborole ones with similar molecular weights. All the compounds exhibit blue or violet-blue luminescent properties and possess large HOMO-LUMO energy gaps in a range of 3.4 ~ 3.8 eV with low lying HOMO levels in a range of 5.3 ~ 5.7 eV. On the basis of previous studies in boron compounds, bis(naphto-dioxaborole) and bis(diazaborole) compounds was prepared and introduced as HBL materials in phosphorescent OLEDs. They show high thermal stability with glass transition temperatures of 107℃ and proper HOMO-LUMO energy levels for HBL materials. Furthermore, their molecular ...