Development of Novel Stabilization Method for Unstable Cationic π-Electron Systems and Its Application to the Investigation of their Electronic Structures

Abstract

Novel structural modification, i.e., annelation of bicyclo [2.2.2] octene units, for stabilizing unstable cationic π-electron systems has been developed. The rigid alkyl substituent stabilizes the cationic species due to cumulative weak thermodynamic effects and steric protection, and therefore it does not distort the intrinsic electronic structure of the π-systems. By applying this structural modification, the first isolation and X-ray structural analysis of various radical cation and dication salts and the generation and first NMR observation of quite unstable cationic π-conjugated systems were realized. Combined with the results of theoretical calculations, the electronic properties of these cationic species were investigated. These involve aromaticity of cationic π-conjugated systems including sulfur and silicon atom (s), relationship between the molecular orbital and structural changes upon one electron oxidation, and electronic structure in oxidized states of π-conjugated oligomers.