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

Radical cations and dications of [n] cyclo-p-phenylenes ([n]CPPs, n = 5, 6, 10, and 12), which are the models of those of linear oligo-p-phenylenes without a terminus, were synthesized as hexafiuoroantimonate salts by the one- and two-electron chemical oxidation of CPP by NOSbF6 or SbF5. The radical cations, [n]CPP center dot+, and dications, [n]CPP2+, exhibited remarkable bathochromic shifts in their UV-vis-NIR absorption bands, suggesting that [n] CPP center dot+ and larger [n]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 [n]CPP center dot+ and [n]CPP2+, respectively. In [n]CPP center dot+, the spins and charges were equally and fully delocalized over the p-phenylene rings of the CPPs, as noted by ESR. H-1 NMR revealed that the hydrogen of [n]CPP2+ shifted to a high magnetic field from the neutral compounds due to the diamagnetic ring current derived from the in-plane aromaticity of [n]CPP2+. The single resonances observed in all [n]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.