Excited-state intramolecular proton-transfer solid-state fluorophores with aggregation-induced emission as efficient emitters for electroluminescent devices†

Excited-state intramolecular proton transfer (ESIPT) materials have emerged as highly promising candidates for emitters in organic light-emitting diodes (OLEDs), attributed to their distinctive photophysical properties. These materials have garnered considerable attention due to their pronounced Stokes shifts, exceptional color purity, and potential for achieving high efficiency and stability in OLED applications. To develop highly luminescent solids with large Stokes shifts, two imidazole-based ESIPT-AIE emitters, PPy-HPI and PPy-HPIC, have been synthesized following the strategy of combining excited-state intramolecular proton transfer (ESIPT) and aggregation-induced emission (AIE) systems. These emitters integrate the ESIPT cores of 2-(2-hydroxy-5-methylphenyl)-1,4,5-triphenylimidazole (HPI) and 2-(2-hydroxy-5-methylphenyl)-1-phenylphenanthroimidazole (HPIC) with the AIE-active luminogen 4-(1-phenylvinyl)pyridine (PPy). The ESIPT and AIE properties were confirmed through both theoretical and experimental studies. The emitters demonstrated ESIPT and AIE characteristics, featuring a large Stokes shift and strong yellow-green to yellow emission in the solid state, which originated from the pure keto form. A non-doped OLED utilizing PPy-HPIC demonstrated yellow emission with CIE coordinates of (0.47, 0.51), a maximum external quantum efficiency (EQE_max) of 3.13% and a maximum current efficiency (CE_max) of 9.48 cd A⁻¹. Notably, the non-doped OLED incorporating PPy-HPI exhibited yellow-green emission with CIE coordinates of (0.15, 0.10), an EQE_max of 3.61%, and CE_max of 10.56 cd A⁻¹, with minimal efficiency roll-off. These results signify a major breakthrough in creating ESIPT molecules for use as non-doped emitters in fluorescent OLEDs.