Tunable aromaticity in bicalicenes.

The unusual aromatic stability of cyclic bicalicene has been suggested to come from a tetraionic structure, where positive and negative charges are located on the cyclopropene and cyclopentadiene rings, respectively. Energetic, magnetic, geometric and electron delocalization analysis performed on a series of bicalicene derivatives, incorporating different electron donating and withdrawing groups, and electrically perturbed bicalicene structures provide additional proof of the role played by this tetraionic structure in the aromatic stability of bicalicene. In this work the aromatic stabilization is chemically and electrically tuned, enhancing or disrupting the electron delocalization and aromatic stability of the cyclopropene and cyclopentadiene rings by increasing or decreasing their corresponding charges. It is shown how the electron delocalization within these rings is similar to that of cyclopropene cation and cyclopentadiene anion for a perfect polarization of one electron.