Among the most promising approaches is the development of materials with inverted singlet-triplet energy gaps (INVEST), where the first excited singlet state (S 1 ) lies energetically below the lowest triplet state (T 1 ), breaking Hund's rule. This energetic ordering significantly enhances reverse intersystem crossing rates, enabling efficient conversion of triplet to singlet excitons and thus potentially achieving 100% internal quantum efficiency. While several heptazine-based INVEST emitters have been identified, systematic design principles that connect molecular structure to the magnitude of the inverted gap remain elusive.