Use of Charge-Charge Repulsion to Enhance π-Electron Delocalization into Anti-Aromatic and Aromatic Systems

A series of 9-fluorenyl cations has been studied and it is shown that increasing charge on a heterocyclic substituent group enhances the anti-aromatic character of the carbocation system. Similarly, a series of dibenzosuberenyl cations has been studied and increasing charge on a substituent group is shown to enhance aromatic character in the carbocation system. These studies include the direct observations of dicationic and tricationic species using stable-ion conditions and low temperature NMR. The structures of these ions were further characterized using DFT calculations, confirming that highly charged organic ions may exhibit unusual distributions of π-electrons and delocalization of electrons in 4n or 4n+2 π-systems.