Long-lived thermally activated delayed fluorescence by oxygen defect-assisted exciton recombination in boric oxide-based composites incorporated with benzimidazole-derived carbon dots.

Organic thermally activated delayed fluorescence (TADF) materials, capable of harvesting both singlet and triplet excitons to achieve a theoretic internal quantum yield of 100%, have exhibited promising applications in a wide-range of fields. However, most of them suffer from a large singlet-triplet energy gap (ΔEST) and a slow reverse intersystem coupling rate. Herein, using two N-heterocyclic benzimidazole derivatives as carbon source, two binary composites (R1-CDs@B2O3 and R2-CDs@B2O3) with uniformly dispersed N-doped carbon dots (N-CDs) in a hydroxyl-functionalized boric oxide matrix, were successfully fabricated. Benefiting from sufficiently small ΔEST (0.07 and 0.15 eV) and oxygen defect-assisted exciton recombination, these materials emit intense deep-blue TADF emission under ambient conditions, featuring ultralong lifetimes exceeding 2.53 s, high photoluminescence quantum yields (PLQY) up to 70.7% and durable afterglow lasting more than 27 s. R2-CDs@B2O3 with more carbonyl groups anchored on the carbon core achieves improved lifetime and PLQY via enhanced spin-orbit coupling. A high-efficiency white-light-emitting ternary composite was constructed through singlet-singlet energy transfer. Owing to their high PLQY, tunable emission and naked eye-visible afterglow, these materials were intriguingly used in dynamic display of polychrome three-dimensional artworks and multi-level information anti-counterfeiting and encryption. In particular, the developed epoxy matrix encapsulation strategy for improving the long-term stability of the composites significantly overcomes the practical limitations arising from the hygroscopic nature of the B2O3 matrix.