Synthesis and Characterization of [<i>n</i>]CPP (<i>n</i> = 5, 6, 8, 10, and 12) Radical Cation and Dications: Size-Dependent Absorption, Spin, and Charge Delocalization

Radical cations and dications of [<i>n</i>]­cyclo-<i>p</i>-phenylenes ([<i>n</i>]­CPPs, <i>n</i> = 5, 6, 10, and 12), which are the models of those of linear oligo-<i>p</i>-phenylenes without a terminus, were synthesized as hexafluoro­antimonate salts by the one- and two-electron chemical oxidation of CPP by NOSbF₆ or SbF₅. The radical cations, [<i>n</i>]­CPP^•+, and dications, [<i>n</i>]­CPP²⁺, exhibited remarkable batho­chromic shifts in their UV–vis–NIR absorption bands, suggesting that [<i>n</i>]­CPP^•+ and larger [<i>n</i>]­CPP²⁺ exhibit longer polyene character than the shorter analogues. The larger batho­chromic shift was consistent with the narrower HOMO–SOMO and HOMO–LUMO gaps in larger [<i>n</i>]­CPP^•+ and [<i>n</i>]­CPP²⁺, respectively. In [<i>n</i>]­CPP^•+, the spins and charges were equally and fully delocalized over the <i>p</i>-phenylene rings of the CPPs, as noted by ESR. ¹H NMR revealed that the hydrogen of [<i>n</i>]­CPP²⁺ shifted to a high magnetic field from the neutral compounds due to the diamagnetic ring current derived from the in-plane aromaticity of [<i>n</i>]­CPP²⁺. The single resonances observed in all [<i>n</i>]­CPP²⁺ strongly suggest the complete delocalization of the charges over the CPPs. Furthermore, the contribution of biradical character was clarified for [10]- and [12]­CPP by VT-NMR experiment and theoretical calculation