Imidazole-based solid-state fluorophores with combined ESIPT and AIE features as self-absorption-free non-doped emitters for electroluminescent devices

Excited-state intramolecular proton transfer (ESIPT) fluorophores with an eye-catching aspect of a large Stokes shift in optical properties have been exceptionally considered as prime candidates for numerous applications. Nevertheless, as non-doped emitters for electroluminescent devices, the device performance is still far behind the traditional fluorescence emitters. Herein, two imidazole-based ESIPT-aggregate induced emission (AIE) fluorophores (HITPE and HPITPE) are designed and synthesized by covalently linked the ESIPT cores of 2-(2-hydroxyphenyl)-1,4,5-triphenylimidazole (HI) and 2-(2-hydroxylphenyl)-1-phenylphenanthroimidazole (HPI) with AIE-active luminogen of tetraphenylethene (TPE) at 5-position of the 2-hydroxyphenyl unit. The ESIPT, AIE, and photophysical properties are theoretically and experimentally studied. Both molecules display ESIPT and AIE characters with intense sky-blue/green-blue color emissions from a pure keto form in the solid-state and photoluminescence quantum yields of 57–64%. They possess high thermal and electrochemical stabilities. They are successfully fabricated as non-doped emitters in organic light-emitting diodes (OLED), and all devices exhibit strong keto-form emissions with low turn-on voltages (3.0–3.2 V). Especially, HPITPE-based OLED achieves a high luminance of 10680 cd m−2, LE value of 3.67 cd A−1, and external quantum efficiency (EQE) of 3.26% with a slight efficiency roll-off. Importantly, this represents an advance in the development of ESIPT molecules as a non-doped emitter for fluorescent OLEDs.