We carry out a series of substitutions at the LUMO positions, as shown in Figure . Pentalene-1,3,4,6-tetraol has a double-bond delocalized ground-state structure with a negative E(S 1 -T 1 ) = -115 meV (Figure , right), which is very close to that of D 2hconstrained pentalene. Tetra-amino and -methoxy substituted pentalenes yield similar Hund's rule violations, albeit the aminosubstituted case has a lower-lying T 1 state; that is, it constitutes a Hund's rule violation but the singlet-triplet gap is not inverted. When using strong donors, such as amine and dimethylamine, even the disubstituted cases result in inverted gaps. We also tested several electron withdrawing substituents at the 2,5positions where the HOMO coefficients are large, but we found none with the desired bond delocalization nor inverted gaps. The pentalene cores have double-bond delocalized quasi-D 2h symmetry in all the substituted molecules listed in Figure . Still, their symmetry is lower than the D 2h -constrained pentalene. The slightly broken symmetry issuing from substituent geometries does not rule out inverted singlet-triplet gaps as the high local symmetry of the fused-ring core is preserved. However, all the gaps we report here are less negative than the -125 meV gap of the constrained parent pentalene.