In an analogous fashion to pentalene, s-indacene distorts into a C 2h symmetry geometry with bond-length alternation to reduce antiaromaticity. Its HOMO and LUMO overlaps and consequently its singlet-triplet gap is positive as shown in Figure . When constrained into D 2h symmetry, pentalene-like HOMO and LUMO are achieved but the Hund's rule violation is between S 1 and T 2 in s-indacene. However, the singlet-triplet gap is inverted without a lower-lying triplet in several of the substituted indacenes.