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 ( OsB ) in the S 0 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 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 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.