We demonstrate that differential intramolecular CT values between S 1 and T 1 states strongly correlate with ∆E ST (R 2 > 0.94), providing a direct structure-property relationship. Our analysis reveals that strongly electron-donating groups like -N(CH 3 ) 2 and -NH 2 minimize the magnitude of inverted energy gaps in monosubstituted heptazines, while the parent heptazine (-H) and phenyl-substituted variants exhibit the most negative ∆E ST values. Paradoxically, tri-NH 2 substitution produces the most negative ∆E ST (-0.27 eV) among trisubstituted derivatives, highlighting a complex interplay between resonance effects, inductive stabilization, and excited-state aromaticity.