Thermally Activated Delayed Fluorescence of Aggregates Induced by Strongπ–πInteractions and Reversible Dual-Responsive Luminescence Switching

A reversible dual-responsive luminescent material was introduced by our group to show the simultaneous color and lifetime switching in response to external stimuli.Pristine crystalline powder of(E)-2-(benzo[d]thiazol-2-yl)-3-(pyren-1-yl)acrylonitrile(Py-BZTCN)shows the orderedπ–πstacking with only near-monomer-normal orange-yellow fluorescence,but it exhibits red emission with thermally activated delayed fluorescence(TADF)after grinding,which can be reversibly recovered by heating or fuming treatment.Grinding disturbs the orderedπ–πstacking of pristine powder,leading to the formation of small aggregates with compressed distance and increased overlap ofπ–πstacking between adjacent molecules.The cause of switching was verified by single-crystal X-ray diffraction experiments of two corresponding crystals.This strongπ–πinteraction effectively promotes the excited-state energy splitting and substantially decreases the singlet–triplet energy gap(ΔEST)of aggregates,resulting in the red TADF emission of aggregates through reverse intersystem crossing.This finding proposes a new route to realizing the TADF emission of aggregates through strong intermolecular interactions based on non-TADF monomer,thereby enabling a novel high-contrast dualresponsive luminescence switching.