π-Bridge mediated coupling between inter- and intra-molecular charge transfer in aggregates for highly efficient near-infrared emission

Intermolecular charge transfer (inter-CT) is commonly considered to quench luminescence in molecular aggregates, especially for near-infrared (NIR) emission. Herein, by elaborate comparison of π-bridge effects in donor/acceptor (D/A) molecules, it is disclosed that a π-bridge is essential in D/A molecule to involve inter-CT in aggregates for inducing desired thermally activated delayed fluorescence (TADF) and largely suppressing non-radiative decays, and importantly, electron-donating π-bridge is critical to maximize radiative decay for inter-CT dominated emission by effective electronic coupling with bright intramolecular charge transfer (intra-CT) for high-efficiency NIR emission. As a proof-of-concept, TPATAP with thienyl as π-bridge realized prominent photoluminescence quantum yields of 18.9% at 788 nm in solid films, and achieved record-high maximum external quantum efficiencies of 4.53% at 785 nm in devices. These findings provide fresh insight into interplay between inter-CT and intra-CT in molecular aggregates and open a new avenue to attenuate the limitation of energy gap law for developing highly efficient NIR emitters and improving the luminescent efficiency of various inter-CT systems, such as organic photovoltaic, organic long persistent luminescence, etc.