Hund’s multiplicity rule states 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 (∆E ST). Although there is general agreement that ∆E ST must be positive, the potential for negative ∆E ST in phenalene analogues, such as cycl[3.3.3]azine and heptazine has been discussed during the last two decades.1–4 In this presentation, we report a fluorescent heptazine analogue HzTFEX2 that disobeys Hund’s rule, possessing negative ∆E ST of –11 meV (Fig. 1).5 The energy inversion of the singlet and triplet excited states results in delayed fluorescence with short time constants of 0.2 μs, which anomalously decrease with decreasing temperature due to the emissive singlet character of the lowest-energy excited state. Organic light-emitting diodes using this molecule exhibit a fast transient electroluminescence decay with a maximum external quantum efficiency of 17%, demonstrating potential implications for optoelectronic devices.(1) W. Leupin, J. Wirz, J. Am. Chem. Soc. 102, 6068–6075 (1980).(2) W. Leupin, D. Magde, G. Persy, J. Wirz, J. Am. Chem. Soc. 108, 17–22 (1986).(3) J. Ehrmaier, E. J. Rabe, S. R. Pristash, K. L. Corp, C. W. Schlenker, A. L. Sobolewski, W. Domcke, J. Phys. Chem. A 123, 8099–8108 (2019).(4) P. de Silva, J. Phys. Chem. Lett. 10, 5674–5679 (2019).(5) N. Aizawa, Y.-J. Pu, Y. Harabuchi, A. Nihonyanagi, R. Ibuka, H. Inuzuka, B. Dhara, Y. Koyama, K. Nakayama, S. Maeda, F. Araoka, D. Miyajima, Nature 609, 502 (2022). Figure 1