Molecules in which the first excited singlet state (S1) is lower in energy than the lowest triplet state (T1) are poised to make significant contributions to the field of organic electronic materials. Such a class of inverted singlet-triplet gap materials may overcome existing limitations stemming from spin statistics in, among other applications, organic light-emitting diodes (OLEDs), photocatalysts, and lasers. Inverted gap molecules are a specific case of Hund's rule of maximum spin multiplicity being violated in the lowest excited states (red in Figure ). This inversion was demonstrated in a heptazine-based OLED device by Aizawa et al. However, violations of Hund's rule can exist among higher excited states (blue in Figure ), although such cases may not be optically useful. At this point, two types of compounds with inverted singlet-triplet gaps have been classified, namely azaphenalenes and non-alternant polycyclic hydrocarbons (Figure ).