Stable Cationic Nanobelts Synthesized by Chemical Oxidation of Methylene-bridged [6]Cycloparaphenylene

Cationic molecules have distinct properties that differ from those of electronically neutral molecules. They have been extensively studied and utilized as reaction intermediates, organic salts, and dyes. Among them, cationic arenes are of particular interest because they can act as charge carrier intermediates in p-type organic semiconductors. Although heteroatom-containing cationic arenes have been reported, those without heteroatoms, namely cationic aromatic hydrocarbons, are still rare and relatively unexplored due to their instability in air. Nanobelts are cyclic arenes that only consist of annulated structures. Recently, various types of nanobelts have been synthesized, and several unique properties of nanobelts have been unveiled. However, cationic nanobelts without heteroatoms have never been synthesized, and their properties are of significant interest from both fundamental and application perspectives. Herein, we report the first synthesis of cationic hydrocarbon nanobelts (radical cation and dication) obtained via the chemical oxidation of methylene-bridged [6]cycloparaphenylene ([6]MCPP). These cationic species turned out to be remarkably stable in air, which made it possible to measure and uncover their structural and electronic properties. Notably, the [6]MCPP dicationic salt has sharp absorption and fluorescence bands at much longer wavelengths than those of neutral [6]MCPP, close to the near-infrared region. From both experimental and theoretical investigation, the existence of a strong diatropic belt current in [6]MCPP dication has been indicated. In addition, a longer lifetime was observed for the hexamethyl[6]MCPP dicationic salt than for the [6]MCPP dicationic salt in solution. These findings regarding cationic hydrocarbon nanobelts contribute to a novel molecular design for stable cationic aromatic hydrocarbons.