Synthesis, Properties, Calculations and Applications of Small Molecular Host Materials Containing Oxadiazole Units with Different Nitrogen and Oxygen Atom Orientations for Solution-Processable Blue Phosphorescent OLEDs

Abstract

A series of new small molecules based on symmetric electron-acceptor of 1,3,4-oxadiazole moiety or its asymmetric isomer of 1,2,4-oxadiazole unit were successfully synthesized and applied to solution-processable blue phosphorescent organic light-emitting diodes for the first time, and their thermal, photophysical, electrochemical properties and density functional theory calculations were studied thoroughly. Due to the high triplet energy levels (ET, 2.82–2.85 eV), the energy from phosphorescent emitter of iridium(III) bis[(4,6-difluorophenyl)-pyridinate-N,C2′]picolinate (FIrpic) transfer to the host molecules could be effectively suppressed and thus assuring the emission of devices was all from FIrpic. In comparison with the para-mode conjugation in substitution of five-membered 1,3,4-oxadiazole in 134OXD, the meta-linkages of 1,2,4-isomer appending with two phenyl rings cause the worse conjugation degree and the electron delocalization as well as the lower electron-withdrawing ability for the other 1,2,4-oxadiazole-based materials. Noting that the solution-processed device based on 134OXD containing 1,3,4-oxadiazole units without extra vacuum thermal-deposited hole/exciton-blocking layer and electron-transporting layer showed the highest maximum current efficiency (CEmax) of 8.75 cd/A due to the excellent charge transporting ability of 134OXD, which far surpassed the similar devices based on other host materials containing 1,2,4-oxadiazole units. Moreover, the device based on 134OXD presented small efficiency roll-off with current efficiency (CE) of 6.26 cd/A at high brightness up to 100 cd/m2. This work demonstrates different nitrogen and oxygen atom orientations of the oxadiazole-based host materials produce major impact on the optoelectronic characteristics of the solution-processable devices.