The final molecule where we identify inverted S 1 -T 1 gaps is indeno[1,2,3-e f ]heptalene, which consists of a fused heptalene and indene as shown in Figure . The unsubstituted molecule distorts into a C s geometry with bond-length alternation and a positive singlet-triplet gap. When the molecule is constrained into the double-bond delocalized C 2v structure a Hund's rule violation between S 1 and T 2 is achieved. The key frontier MOs involved in the lowest excitations are heptalene-like and it is at the heptalene unit that substitution has the most significant effect. As with s-indacene, the singlet-triplet gap can become inverted upon substitution. As shown in Figure , we identify several in-denoheptalene derivatives with a negative E(S 1 -T 1 ). Donors at the 3,6 positions (R 1 in Figure ) provide the most negative gaps, but even disubstitution with hydroxyls at the six-membered ring (R 4 ) can cause the molecule to attain the necessary double-bond delocalization. Notably, we also succeed in finding an acceptorsubstituted inverted gap molecule. In indeno[1,2,3-e f ]heptalene, substituting with electron-withdrawing nitrile substituents at the 2,7-positions (R 2 ) has the same effect as donors at the LUMO positions. To the best of our knowledge, indeno[1,2,3-e f ]heptalenes are yet to be synthesized.