Aromaticity of Osmaacenes in Their Lowest-Lying Singlet and Triplet States

Thearomatic character of a series of osmaacenes in their lowest-lyingsinglet and triplet states was thoroughly examined by means of themagnetically induced current densities and multicentre delocalizationindices (MCIs). Both employed approaches agree that the osmabenzenemolecule (OsB) in the S-0 state exhibits dominant pi-Hu''ckel-type aromatic character, with a small but nonnegligibleamount of pi-Craig-Mo''bius aromaticity. Contraryto benzene, which is antiaromatic in the T-1 state, OsB preserves some of its aromaticity in the T-1 state. In higher members of the osmaacene series in the S-0 and T-1 states, the central Os-containing ring becomesnonaromatic, acting as a barrier between the two side polyacenic unitswhich, on the other hand, exhibit a significant extent of pi-electrondelocalization. Thearomatic character of osmaacenes in their lowest-lyingsinglet and triplet states was examined by means of the magneticallyinduced current densities and multicentre delocalization indices.Contrary to benzene, which is antiaromatic in the T-1 state,osmabenzene molecule preserves some of its aromaticity in the T-1 state. In higher osmaacenes in the S-0 and T-1 states, the central Os-containing ring becomes nonaromatic,acting as a barrier between the two aromatic polyacenic units.