Thermally Activated Delayed Fluorescence of Aggregates Induced by Strong π-π Interactions and Reversible Dual-Responsive Luminescence Switching

Abstract

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 ordered π-π stacking 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 ordered π-π stacking of pristine powder, leading to the formation of small aggregates with compressed distance and increased overlap of π-π stacking between adjacent molecules. The cause of switching was verified by single-crystal X-ray diffraction experiments of two corresponding crystals. This strong π-π interaction 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 dual-responsive luminescence switching.