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

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• 作者：Eiichi Kayahara ; Takahiko Kouyama ; Tatsuhisa Kato ; Shigeru Yamago
• 刊名：Journal of the American Chemical Society
• 出版年：2016
• 出版时间：January 13, 2016
• 年：2016
• 卷：138
• 期：1
• 页码：338-344
• 全文大小：514K
• ISSN：1520-5126

文摘

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 hexafluoroantimonate salts by the one- and two-electron chemical oxidation of CPP by NOSbF₆ or SbF₅. The radical cations, [n]CPP^?+, and dications, [n]CPP²⁺, exhibited remarkable bathochromic shifts in their UV–vis–NIR absorption bands, suggesting that [n]CPP^?+ 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^?+ and [n]CPP²⁺, respectively. In [n]CPP^?+, the spins and charges were equally and fully delocalized over the p-phenylene rings of the CPPs, as noted by ESR. ¹H 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.