Inverted singlet–triplet emitters for organic light-emitting diodes

Theoretical and experimental studies since the 1980s have pointed to the existence of organic molecules that violate Hund's rule of maximum multiplicity, with the lowest singlet excited state having lower energy than the lowest triplet excited state. With the rising prevalence of organic light-emitting diodes (OLEDs) in display technology, these types of molecules are being investigated as a new class of organic emitters. The singlet–triplet inversion implies that thermal activation is not necessary to achieve fast triplet harvesting, providing potential benefits over conventional thermally activated delayed fluorescence emitters. Here, we overview prominent studies regarding inverted singlet and triplet excited states in the context of OLEDs.