Solid-State Fluorophores with Synergic Excited State Intramolecular Proton Transfer (ESIPT) and Aggregation-Induced Emission (AIE) Properties as Effective Non-Doped Emitters for Electroluminescent Devices

Abstract

Here, we present the concept of combining excited-state intramolecular proton transfer (ESIPT) and aggregation-induced emission (AIE) features into a single molecule as a strategy to generate high-performance ESIPT-based non-doped organic light-emitting diodes (OLEDs). Two ESIPT-AIE-type green fluorophores, TBzHPI and TBzHI, are meticulously designed and synthesized by incorporating a conventional AIE moiety of triphenylethylene (TPE) with specific ESIPT cores of 2-(benzo[d]thiazol-2-yl)-6-(1-phenyl-1H-phenanthro[9,10-d]imidazol-2-yl)phenol (BzHPI) and 2-(benzo[d]thiazol-2-yl)-6-(1,4,5-triphenyl-1H-imidazol-2-yl)phenol (BzHI), respectively. The ESIPT and AIE properties are thoroughly validated through both theoretical calculations and experimental investigations. Both TBzHPI and TBzHI exhibit large Stokes shifted emissions (135-146 nm) and strong green emission of the pure keto form in the solid state owing to the collective effects of ESIPT and AIE properties in molecules. Their uses as non-doped emitters in OLEDs have been accomplished, with all devices showing strong keto-form emissions and low turn-on voltages of 2.8 V. Particularly, TBzHPI-based device demonstrates a remarkably high luminance of 54,825 cd m2, a current efficiency (CE) of 18.42 cd A-1, and an external quantum efficiency (EQE) of 5.76%, with only a slight decrease in efficiency. This finding is significant as it represents the highest EQE reported so far for ESIPT molecules used as non-doped emitters in fluorescent OLEDs.