Hund’s multiplicity rule manifests that for a given electronic configuration, a higher spin state has a lower energy. Rephrasing this rule for molecular excited states predicts a positive energy gap between singlet and triplet excited states (S–T gap). Although there is general agreement that the S–T gap must be positive, the potential for negative S–T gap in phenalene analogues, such as cycl[3.3.3]azine and heptazine has been discussed during the last two decades. Here we report a fluorescent molecule that disobeys Hund’s rule and possesses a negative S–T gap of –11 meV. The energy inversion of the singlet and triplet excited states results in delayed fluorescence with short time constants of 0.2 ?s, which anomalously decrease with lowering the temperature due to the emissive singlet character of the lowest-energy excited state. Organic light-emitting diodes using this molecule exhibited a fast transient electroluminescence decay with a peak external quantum efficiency of 17%, demonstrating potential implications for optoelectronic devices.