Predicting Inverted Singlet–Triplet Gaps with XYG3-Type Doubly Hybrid Functionals

Molecules exhibiting an inverted energetic ordering of the lowest singlet and triplet excited states, commonly referred to as having an inverted singlet–triplet gap (ΔEST), have attracted significant interest due to their potential in enabling next-generation organic light-emitting diode (OLED) materials. Conventional time-dependent density functional theory (TD-DFT) methods, however, often fail to predict such inversions because they cannot adequately account for double excitations. Doubly hybrid functionals (DHs), which incorporate double excitations in a perturbative way, offer a promising alternative. In this work, we demonstrate that XYG3-type doubly hybrid functionals (xDHs) are able to correctly predict the inverted ΔEST. These functionals provide a cost-effective approach compared to highly accurate but prohibitively expensive coupled-cluster-based methods. Among the xDHs benchmarked, the nonempirical XYG2 functional exhibits the best performance and is strongly recommended for predicting the inverted ΔEST. It is anticipated that xDHs hold considerable promise for future use in the design and discovery of advanced OLED materials.