Infinitenes as the Most Stable Form of Cycloarenes: The Interplay among π Delocalization, Strain, and π–π Stacking

Abstract

The recent successful preparation of infinitene has sparked widespread attention due to its aesthetic appeal and synthetic challenge. Spectroscopic measurements and follow-up computational investigations suggest that infinitene holds fundamental significance and potential applications in chiroptics, optoelectronics, asymmetric synthesis, and supramolecular chemistry. However, unlike other looped polyarenes enriched with sizes and shapes, the infinitene molecule seems, so far, the only known example of this fascinating new form of nanocarbons, whose further exploitation would be considerably limited because of the lack of molecular diversity. Here, we introduce a whole new family of generalized infinitenes with different sizes and topologies. Three types of infinitene structures are rationally designed by joining two units of coronene, kekulene, or their extended analogs. The constructed molecules of varying sizes, each with a large number of possible topoisomers, are systematically studied by DFT calculations. Comprehensive analysis using a simple energy decomposition model uncovers that the stability of infinitenes is governed by the interplay among π delocalization, steric strain, and π–π stacking. While the first two factors are crucial to the stability of smaller infinitenes, the latter is the primary stabilizing interaction for larger infinitenes. Most importantly, we show that larger-sized infinitenes are actually the energetically most favorable form among all known looped polyarenes; their substantial thermodynamic stability surpassing that of circulenes, various carbon nanobelts, and kekulene-like macrocycles renders them promising targets for synthesis. The simulated 1H NMR, UV–vis, and circular dichroism spectra along with optical rotations for the most stable infinitene species may help their identification in future synthetic efforts.

Enumeration, Nomenclature, and Stability Rules of Carbon Nanobelts

“…If a segment of a CNB comprises more than six rings in a row, the HOMO−LUMO gap is expected to be so narrow that the electronic ground state is most likely an open-shell singlet with a polyradical character. A similar phenomenon has been found in the case of linear oligoacenes, 66 generalized infinitenes, 20 and generalized kekulenes. 21 The extreme case is the zigzag CNBs (n, 0), or the so-called [n]cyclacenes.…”

“…4,8 As a more strict definition, CNBs are considered by many chemists as double-stranded structures with a single layer of benzene rings, 3,5,7,9−15 although multilayered nanobelts, such as the Vogtle belt, 16,17 are also frequently known. 13,18,19 Due to their cylindrical carbon skeleton, CNBs are highly strained molecules, and first-principles calculations 20,21 have shown that they are generally higher in energy than the macrocyclic topoisomers of other types, like the generalized kekulenes, 20−22 clarenes, 20,21 and even the doubly twisted infinitene 23 and its generalized analogues. 20,24,25 become a long-standing synthetic target for ambitious chemists since the first CNB molecule, [n]cyclacene, was theoretically conceived in 1954.…”

“…Due to their cylindrical carbon skeleton, CNBs are highly strained molecules, and first-principles calculations , have shown that they are generally higher in energy than the macrocyclic topoisomers of other types, like the generalized kekulenes, − clarenes, , and even the doubly twisted infinitene and its generalized analogues. ,, Nevertheless, because of their aesthetically pleasing structures, CNBs have become a long-standing synthetic target for ambitious chemists since the first CNB molecule, [ n ]­cyclacene, was theoretically conceived in 1954 . Over the past six decades, a great deal of effort by generations of synthetic chemists has been devoted to the synthesis of these challenging compounds, but all attempts have been unsuccessful.…”

Carbon Nanobelts: Brief History and Perspective

“…X-ray crystal structure analysis has shown that the combined lamellar and herringbone-like packing structure introduced three-dimensional electronic interactions caused by the infinity shape. Since this report, fascinating properties of infinitene have been predicted. − Wu and Isobe et al synthesized a twisted CNB in 2021 and a fused [5]­helicene dimer in 2023 , as a related molecule of infinitene. In addition to aromatic hydrocarbon-based CNBs, heteroatom-embedded CNBs have also been reported. − In 2019, Tanaka et al synthesized the first oxygen-embedded CNB by rhodium-catalyzed alkyne cyclotrimerization .…”

Advancing Evidence-Based Data Interpretation in UV–Vis and Fluorescence Analysis for Nanomaterials: An Analytical Chemistry Perspective

“…As one example, the long-held general practice in assigning UV–vis spectra obtained with dissolved molecular chromophore as an absorbance spectrum is generally invalid for solutions containing nanoscale or larger materials. This is because both absorption and scattering contribute to the UV–vis extinction spectra. − Further complication arises if one considers interference of forward scattered and emitted photons in the UV–vis spectral acquisition. − However, it has been prevalent in current literature to assign the UV–vis spectrum as absorbance or absorption extinction spectrum, − even for samples likely having strong scattering activities. The impact on the scattering activity of numerous nanomaterials has been published extensively in previous works. − ,− …”