Hydroxy-Tetraphenylimidazole Derivatives as Efficient Blue Emissive Materials for Electroluminescent Devices

Herein, three hydroxy-tetraphenylimidazole (HPI)-based fluorophores (HPI-TPA, HPI-PCz, and HPI-CzP) are designed and synthesized by disubstituted HPI core with arylamine units of triphenylamine (TPA), phenyl carbazole (PCz), and carbazole phenyl (CzP) at 3,5-positions of the N-phenyl ring of HPI, respectively. Their photophysical properties are theoretically and experimentally examined. HPI-TPA shows a hybridized local and charge transfer (HLCT) excited state characteristic and emits deep blue color via an HLCT mechanism, while both HPI-PCz and HPI-CzP exhibit excited-state intramolecular proton transfer (ESIPT) property and display pure keto form emissions. They possess high thermal stability and are successfully fabricated as emitters in organic light-emitting diodes (OLEDs). All devices exhibit intense blue color emissions with low turn-on voltages (3.5-3.7 V). Particularly, HPI-TPA-based OLED emits light in the deep-blue region with a high maximum external quantum efficiency (EQE_max ) of 3.77% and a decent efficiency roll-off.