Pentacosacyclenes: cruciform molecular nanocarbons based on cyclooctatetraene

Abstract

Pentacosacyclene (PC) and pentacosacyclene tetraimide (PCTI) were obtained in concise syntheses involving radial extension of tridecacyclene. PC is an electron-rich hydrocarbon with a C₈₈ π-conjugated framework, whereas PCTI is electron-deficient and contains a C₉₆N₄ core. PC and PCTI both have non-planar saddle-shaped conformations, and PC was found to self-assemble with C₆₀ to produce a uniquely structured supramolecular crystalline phase. In solution, PCTI undergoes eight single-electron reductions, while PC exhibits two reversible oxidations and three reversible reduction events. Chemically generated anions of PC and PCTI showcase extended near-infrared to infrared absorptions, with the lowest energy bands observed at >3200 nm for the PCTI monoanion and ca. 2800 nm for the PCTI dianion. The electronic and redox properties of pentacosacyclenes can be explained using molecular orbital and valence bond theories as originating from changes in the local aromaticity of five- and eight-membered rings.