Singlet-Triplet Inversion

The inversion of singlet and triplet states is a rare phenomenon, where, in opposition to Hund's first rule, singlet electronic states are stabilized relative to their triplet counterparts. The recent discovery of organic molecules exhibiting this inversion presents exciting new technological opportunities, such as addressing stability issues in organic light-emitting diodes (OLEDs). In this review, we describe fundamental molecular properties that can yield singlet-triplet inversion, generally ascribed to a phenomenon known as dynamic spin polarization. We discuss the systems in which singlet-triplet inversion was theoretically proposed, experimentally verified, and first implemented in an OLED device. We highlight key insights from the extensive computational work being carried out to understand the intricacies of these systems. Finally, we consider the outlook for future inverted singlet-triplet (IST) emitters.