Radical Cation and Dication of Fluorene Fully Annelated with Bicyclo[2.2.2]octene Units:  Importance of the Quinoidal Resonance Structure in the Cationic Fluorene

Fluorene 1 fully annelated with bicyclo[2.2.2]octene units was newly synthesized and oxidized to stable cationic species. The structure of radical cation salt 1^•+SbCl₆^- was determined by X-ray crystallography, while the first fluorene dication 1²⁺ was characterized by ¹H and ¹³C NMR at −80 °C. Combined with the results of theoretical calculations, an important contribution of a quinoidal structure to the resonance hybrid was demonstrated in both 1^•+ and 1²⁺.