Deep blue organic light-emitting devices enabled by bipolar phenanthro[9,10-d]imidazole derivatives

Two blue fluorescent phenanthroimidazole derivatives (PhImFD and PhImTD) with a D-π-A structure are synthesized by attaching a hole-transporting dibenzofuran or dibenzothiophene and an electron-transporting phenanthroimidazole moiety and characterized. The nonplanar twisted structures reduce molecular aggregation, which endows both of the compounds with good thermal properties, and film-forming abilities as well as high quantum yields in CH₂Cl₂ and in the solid state. Non-doped organic light emitting diodes (OLEDs) are fabricated by employing the compounds PhImFD and PhImTD as emitters and exhibited promising performances. The devices show a deep blue emission with Commission Internationale de l'Eclairage (CIE) coordinates of (0.15, 0.11) for PhImFD and (0.15, 0.10) for PhImTD. PhImFD and PhImTD, with the desired bipolar-dominant characteristics, render devices with a low driving voltage of 3.6 V. The energy levels of the materials were found to be related to the donor units in the compounds with different substituents. Device B, using PhImTD as the emitting layer (EML), with well fitting energy levels and increased electron transport ability, possesses favorable efficiencies of 1.34 cd m^-2 for CE, 0.82 lm W^-1 for PE and 1.63% for EQE. PhImFD and PhImTD are utilized as blue emitters and the host for a yellow emitter, PO-01, to fabricate white organic light-emitting diodes (WOLEDs) that give a forward-viewing maximum CE of 8.12 cd m^-2 and CIE coordinates of (0.339, 0.330). The results demonstrated not only that the phenanthroimidazole unit is an excellent building block to construct deep blue emission materials, but also that chemical structure modification by the introduction of a suitable electron-donor substituent could influence the performance of devices.