On Diphenalenes Comprising Seven Hexagonal Rings

Benzenoid hydrocarbons with two phenalene moieties, diphenalenes, comprising seven benzene rings, form three groups, with the seventh benzene ring fused to (i) zethrene, (ii) uthrene, or (iii) it links phenalene subunits. The DFT/cc-pVTZ calculations indicate that four molecules in the former group are more stable in the singlet state, four in the next in the triplet, whereas in the latter group, three are more stable as singlets and two as triplets. The singlet-triplet gap usually exceeds | ± 10| kcal/mol, but for two molecules of the (iii) group, it is close to 5 kcal/mol. We show also that the diphenalenes' Gibbs free energy differences can be presented as a multivariate function of the number of border Bay, Cove, and Fjord motifs. Diphenalenes, including [4]-helicene moiety, are inherently nonplanar. Although the calculations implied a slight skewness of other diphenalenes, it appeared apparent as their lowest harmonic frequencies overcame the barrier between the left and right skewed forms. For the rings at the phenalene junction, the HOMA and INICS aromaticity indices reveal a substantial decrease of aromaticity in the stable singlet state molecules. All molecules in the triplet state, and those in an excited singlet state, exhibit only a slight decline, if any. Unexpectedly, the HOMA averaged over all rings indicated the more stable singlet state to be less aromatic than the less stable triplet one, which contradicts the fairly common assumption that the greater aromaticity, the greater stability. The alternating partial spins pattern in the triplet state diphenalenes differs between the Kekuléan and non-Kekuléan ones at the phenalenes junction bond. It can be utilized to mnemotechnically predict whether an assembly of two phenalenes will have a singlet or triplet ground state.