Highly Strained, Radially π-Conjugated Porphyrinylene Nanohoops

Small π-conjugated nanohoops are difficult to prepare, but offer an excellent platform for studying the interplay between strain and optoelectronic properties, and, increasingly, these shape-persistent macrocycles find uses in host–guest chemistry and self-assembly. We report the synthesis of a new family of radially π-conjugated porphyrinylene/phenylene nanohoops. The strain energy in the smallest nanohoop [2]CPT is approximately 54 kcal mol–1, which results in a narrowed HOMO–LUMO gap and a red shift in the visible part of the absorption spectrum. Because of its high degree of preorganization and a diameter of ca. 13 A, [2]CPT was found to accommodate C60 with a binding affinity exceeding 108 M–1 despite the fullerene not fully entering the cavity of the host (X-ray crystallography). Moreover, the π-extended nanohoops [2]CPTN, [3]CPTN, and [3]CPTA (N for 1,4-naphthyl; A for 9,10-anthracenyl) have been prepared using the same strategy, and [2]CPTN has been shown to bind C70 5 times more strongly than [2]...