Theoretical Study of the Mechanism of Aggregation-Caused Quenching in Near-Infrared Thermally Activated Delayed Fluorescence Molecules: Hydrogen-Bond Effect

Aggregation-caused quenching (ACQ) has long been a problem that inhibits the application of organic light-emitting materials in organic light-emitting diodes, especially near-infrared (NIR) materials. Figuring out the reasons that induce ACQ is important for the quantum efficiency enhancement of NIR materials. In this paper, an NIR molecule (TPA-QCN) with thermally activated delayed fluorescence (TADF) is studied based on first-principles calculations and excited-state dynamics investigation in both toluene and in the aggregation state. Our calculation indicates that aggregation can induce a smaller energy gap between the first singlet excited state and the first triplet excited state, which is favorable for TADF. Both the decreased fluorescent rate and the increased nonradiative rate will induce emission quenching in the aggregation state. Based on detailed analyses of the reorganization energy and intermolecular interaction, we find that the hydrogen bond will induce enhanced contribution to the reorganization energy from C–H stretching vibration modes and thus a larger nonradiative rate in the aggregation state than in toluene. A new mechanism of ACQ is proposed, and it could help in the design of new types of NIR-TADF molecules with enhanced fluorescence efficiency.