We have also applied this strategy on a range of derivatives of the parent azulene, which has a small positive singlet-triplet gap of E(S 1 -T 1 )=+55 meV. Although we find Hund's rule violations with the S 1 state, the MO order changes and a lower-lying triplet state means that E(S 1 -T 1 ) remains positive (Figure ). However, it is clear that the Heilbronner strategy can be applied to tune the excited states of azulene, which may furthermore be relevant to controlling its well-studied anti-Kasha emission. Next, we test the Heilbronner substituent strategy on molecules which are already known to possess negative singlet-triplet gaps. Isopyrene has a negative singlet-triplet gap of E(S 1 -T 1 )=-21 meV (Figure ). Although the singlet-triplet gap is already inverted in the parent system, we find several derivatives with enhanced negative gaps when substituted with acceptors at HOMO coefficient positions, or with donors at LUMO coefficient positions. Most notably, the 5,10-bis(trifluoromethyl) and 5,10dicarbonitrile derivatives achieve E(S 1 -T 1 ) as low as -51 meV and E(S 1 -T 1 )=-63 meV, respectively. As in azupyrene, there are several cases where another triplet state becomes T 1 . These cases of Hund's rule violations are listed in Table .