Nature Materials, Published online: 05 September 2025; doi:10.1038/s41563-025-02352-1A simple physical model based on electronic interactions between excited configurations enables the quick and reliable prediction of singlet–triplet energy gaps in polycyclic heteroaromatic emitters. Guided by this model, organic emitters with small singlet–triplet energy gaps can be designed for applications in organic light-emitting diodes with high efficiency and colour purity.