De Novo Design of Excited-State Intramolecular Proton Transfer Emitters via a Thermally Activated Delayed Fluorescence Channel

Developing excited-state intramolecular proton transfer (ESIPT) emitters with high photoluminescence quantum yields (Φ PL s) and long fluorescence lifetimes in solid state remains a formidable challenge. In this study, we integrated the molecular design tactics of thermally activated delayed fluorescence (TADF) into ESIPT molecules with the goals of improving their Φ PL s and increasing their fluorescence lifetimes. Two proof-of-concept molecules, PXZPDO and DMACPDO, were developed by adopting symmetric D−π–A−π–D molecular architectures (where D and A represent donors and acceptors, respectively) featuring electron-donating phenoxazine or a 9,9-dimethyl-9,10-dihydroacridine moiety, an ESIPT core β-diketone, and phenylene π-bridges. Both molecules exhibited sole enol-type forms stabilized by intramolecular hydrogen bonds and exhibited a unique and dynamic ESIPT character that was verified by transient absorption analyses. Endowed with distinct TADF features, PXZPDO and DMACPDO showed high Φ PL s of 68% and 86% in the film state, coupled with notable delayed fluorescence lifetimes of 1.33 and 1.94 μs, respectively. Employing these ESIPT emitters successfully achieved maximum external quantum efficiencies (η ext s) of 18.8% and 23.9% for yellow and green organic light-emitting diodes (OLEDs), respectively, which represent the state-of-the-art device performances for ESIPT emitters. This study not only opens a new avenue for designing efficient ESIPT emitters with high Φ PL s and long fluorescence lifetimes in solid state but also unlocks the huge potential of ESIPT emitters in realizing high-efficiency OLEDs.