Exploring the World of Double Nanohoops.

Conjugated nanohoops have become highly relevant compounds based on their unique conjugation, structural, optoelectronic, and morphological properties. Recent synthetic efforts have progressed to access more elaborate structures, incorporating two conjugated nanohoops covalently linked by a central linking unit, so-called double nanohoops. Double nanohoops broaden the molecular diversity of strained nanocarbons, and they can show properties exceeding those of single nanohoops. They are often inherently chiral and attractive candidates as chiral-polarized-light emitters, and with two cavities are interesting supramolecular hosts. We herein provide an overview of double nanohoops reported to date, with loops built from only oligo(paraphenylene) units. We categorize their structures into phenylene-linked double nanohoops, double nanohoops with X-linkers, zig-zag double nanohoops, lemniscular double nanohoops, and "other" double nanohoops. We identify four main synthetic pathways that are typically employed, making use of kinked precursors to oligo(paraphenylenes). We individually discuss properties and applications of each double nanohoop. This review shows that creativity and synthetic endurance can furnish unique double nanohoop structures with attractive chiroptical as well as supramolecular properties.