Cyclic Tetraindoles with a Cyclooctatetraene Core: Controlling Planarity and Antiaromatic Character via N-Alkylation

Cyclooctatetraene (COT), a representative 4n π-electronic system, typically exhibits nonaromatic character in highly twisted structures, but planarization significantly alters its electronic properties, increasing antiaromaticity. Therefore, investigations on the structure-property relationships of COT derivatives are essential for the development of future functional materials. In this study, we have developed a series of cyclic tetraindoles possessing a central COT ring. The degree of molecular distortion can be controlled by N-alkylation. Specifically, substitution of the N-butyl groups increased the distortion, while the structural fixation by ethylene linkers enhanced its planarity. As planarity increased, a noticeable red-shift in the absorption wavelength, a decrease in oxidation potential, and a reduction in the HOMO-LUMO gap were observed. These results can be attributed to the more effective conjugation of the COT ring with increased planarity, which led to an enhancement of the antiaromaticity.