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 Å, [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]­CPT. Our failed synthesis of [2]­CPTA highlights a limitation of the experimental approach most commonly
used to prepare strained nanohoops, because in this particular case
the sum of aromatization energies no longer outweighs the buildup
of ring strain in the final reaction step (DFT calculations). These
results indicate that forcing ring strain onto organic semiconductors
is a viable strategy to fundamentally influence both optoelectronic
and supramolecular properties.