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 (ΦPLs) 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 ΦPLs 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 ΦPLs 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 (ηexts) 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 ΦPLs and long fluorescence lifetimes in solid state but also unlocks
the huge potential of ESIPT emitters in realizing high-efficiency
OLEDs.