Modified Imidazole‐Phenol‐Based ESIPT Fluorophores as Self‐Absorption Free Emitters for Efficient Electroluminescent Devices

Excited-state intramolecular proton transfer (ESIPT) molecules are promising fluorophores for various applications including bioimaging, sensing, and optoelectronic devices. Particularly, their self-absorption-free fluorescence properties would make them a perfect choice as emissive materials for organic light-emitting diodes (OLEDs). Nevertheless, to become effective emitters some of their properties need to be altered by structural modifications. Herein, we design and synthesize a series of new ESIPT molecules (2PImBzP, 2ImBzP, and 2FImBzP) by functionalization of imidazole-phenol-based ESIPT cores with electron-deficient benzo[d]thiazole and various ambipolar imidazole moieties (1-phenyl-1H-phenanthro[9,10-d]imidazole (PIm), 1,4,5-triphenyl-1H-imidazole (Im), and (4,5-bis(4-fluorophenyl)-1-phenyl-1H-imidazole (FIm)), respectively. Each molecule displays a complete ESIPT process with intense green emissions from a pure keto form and high solid-state photoluminescence quantum yields (Φ_PL) of 65-80 %. These fluorophores with superior thermal stability and balanced charge carrier mobility are effectively employed as non-doped emitters in OLEDs. The non-doped devices emit greenish lights with high brightness, high current efficiency (CE) (10.95-17.66 cd A^-1), and low turn-on voltages (2.8-2.9 V). The electroluminescence purely originates from the emission of the keto tautomer of the emissive layers. Specifically, the 2PImBzP-based non-doped OLED stands out by achieving a remarkable brightness of 56,220 cd m^-2, a CE of up to 17.66 cd A^-1, and an impressive external quantum efficiency (EQE) of 5.65 % with a slight efficiency roll-off.