Investigating Hydrogen Bonding in Quinoxaline-Based Thermally Activated Delayed Fluorescent Materials

In recent years, hydrogen bonding (H bonding) as an intramolecular locking strategy has been proposed to enhance photoluminescence, color purity, and photostability in thermally activated delayed fluorescence (TADF) materials. Rigidification as a design strategy is particularly relevant when using electron-deficient N-heterocycles as electron acceptors, because these materials often suffer from poor performance as orange to near-infrared emitters as a result of the energy gap law. To critically evaluate the presence of H bonding in such materials, two TADF-active donor-acceptor dyads, ACR-DQ and ACR-PQ, were synthesized. Despite their potential sites for intramolecular H bonding and emissions spanning yellow to deep red, computational analyses (including frequency, natural bond orbital, non-covalent interaction, and potential energy surface assessments) and crystal structure examinations collectively suggest the absence of H bonding in these materials. Our results indicate that invoking intramolecular H bonding should be done with caution in the design of rigidified TADF materials.