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.