A Stable All-thiophene-based Core-modified [38]Octaphyrin Diradicaloid: Conformation and Aromaticity Switch at Different Oxidation States.

A soluble and stable core-modified [38]octaphyrin, MC-T8, containing eight thiophene rings was synthesized by Yamamoto coupling followed by oxidative dehydrogenation. X-ray crystallographic analysis revealed a nearly planar backbone, and the molecule is globally aromatic with a dominant 38 pi conjugation pathway. The dication MC-T8(2+) is antiaromatic, and the backbone is distorted, with a different orientation of the thiophene rings. The tetracation MC-T8(4+) becomes aromatic again, with a shallow-bowl-shaped geometry. Both the neutral compound and the dication demonstrated open-shell diradical character with a small singlet-triplet energy gap (-2.70 kcal mol(-1) for MC-T8 and -3.78 kcal mol(-1) for MC-T8(2+)), and they are stable owing to effective spin delocalization.