Aggregation-enhanced TADF in deep-red emitters for high-performance OLEDs

When isolated molecules undergo aggregation, their intermolecular interactions increase, potentially altering their electronic structures and affecting photophysical properties such as fluorescence lifetime. The extent of this change largely depends on the molecular structure. For thermally activated delayed fluorescent (TADF) materials, their luminescence mechanism in terms of the scale of increased lifetime from single molecules to aggregates and how that influences their optoelectronic device performance remain largely unexplored. In this study, we report a series of deep-red TADF emitters that are designed from D-A and D2-A types of molecular structures, with quinoxaline-6,7-dicarbonitrile (QCN) as the electron acceptor (A) and naphthalene-substituted N,N-diphenylamine (ND) as the electron donor (D). The D2-A type emitter (αND)2-QCN exhibits a pronounced aggregation-enhanced TADF (AE-TADF) effect compared to the D-A type emitters. This enhancement results from a significant reduction in the singlet-triplet energy gap (ΔEST) upon aggregation, driven by the formation of intermolecular hydrogen bonds in their J-aggregates. The AE-TADF effect facilitates more efficient reverse intersystem crossing (RISC), enabling the high performance of an (αND)2-QCN-based deep-red OLED (λEL,max = 622 nm) with a maximum external quantum efficiency (EQEmax) of 14.3%.