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

The aromatic character of a series of osmaacenes in their lowest-lying singlet and triplet states was thoroughly examined by means of the magnetically induced current densities and multicentre delocalization indices (MCIs). Both employed approaches agree that the osmabenzene molecule () in the S state exhibits dominant π-Hückel-type aromatic character, with a small but nonnegligible amount of π-Craig-Möbius aromaticity. Contrary to benzene, which is antiaromatic in the T state, preserves some of its aromaticity in the T state. In higher members of the osmaacene series in the S and T states, the central Os-containing ring becomes nonaromatic, acting as a barrier between the two side polyacenic units which, on the other hand, exhibit a significant extent of π-electron delocalization.