Helical Nanographenes Containing an Azulene Unit: Synthesis, Crystal Structures, and Properties

Abstract

Three unprecedented helical nanographenes (1, 2, and 3) containing an azulene unit are synthesized. The resultant helical structures are unambiguously confirmed by X‐ray crystallographic analysis. The embedded azulene unit in 2 possesses a record‐high twisting degree (16.1°) as a result of the contiguous steric repulsion at the helical inner rim. Structural analysis in combination with theoretical calculations reveals that these helical nanographenes manifest a global aromatic structure, while the inner azulene unit exhibits weak antiaromatic character. Furthermore, UV/Vis‐spectral measurements reveal that superhelicenes 2 and 3 possess narrow energy gaps (2: 1.88 eV; 3: 2.03 eV), as corroborated by cyclic voltammetry and supported by density functional theory (DFT) calculations. The stable oxidized and reduced states of 2 and 3 are characterized by in‐situ EPR/Vis–NIR spectroelectrochemistry. Our study provides a novel synthetic strategy for helical nanographenes containing azulene units as well as their associated structures and physical properties.

Helical Nanographenes Embedded with Contiguous Azulene Units

“…[24][25][26] However, the bottom-up synthesis of nanocarbons with multiple azulene units is still limited because of the absence of reliable methods, unlike that of other nanocarbons composed only of hexagonal rings and isolated nonhexagonal ring(s). [27][28][29][30] Peña, Pascual and co-workers 31 and Müllen, Fasel, Feng and co-workers 32,33 addressed this issue by conducting on-surface dehydrocyclization of rationally designed precursors to incorporate azulene units into nanocarbons. Mastalerz, 34 Zhang, 35 and Chi 36 also achieved in-solution construction of highly fused azulene units via Scholl cyclization of ingeniously tailored precursors.…”

Multi‐Redox Active Carbons and Hydrocarbons: Control of their Redox Properties and Potential Applications

“…To date, various multi‐redox active C x H y molecules have been explored. These include, but are not limited to, nanographenes, [16–24] radicaloids (mostly based on diindenoarenes), [25–34] macrocycles, [35–39] fullerene fragments (including buckybowls), [40–45] nanographene‐buckybowl hybrids, [46] pentalene derivatives, [47] ethynylenes, [48] spiro‐compounds, [14] cumulenes, [49] other C x H y molecules with either the azulene, [50] barrelene, [51] fluoranthene, [52] fluorene, [53] diphenalene, [54] phenylene, [55] or Schwarzite framework, [56] and hydrofullerenes [57–61] . The redox properties of dissolved C x H y molecules are summarized in Table 1, where C x H y molecules that show at least six redox reactions (Scheme 1) are listed.…”

“…Ma et al. synthesized a similar azulene‐embedded helical nanographene 1 , [18] which, unlike 127 , underwent eight redox reactions (two reduction and six oxidation reactions). Fernández and Cruz et al.…”

NIR‐Absorbing π‐Extended Azulene: Non‐Alternant Isomer of Terrylene Bisimide

“…However, the unique electronic properties of azulene are usually not preserved when embedded into a larger π‐scaffold. The aromaticity of such azulene moieties is typically lost due to structural distortions . But even if planarity prevails, such PAHs usually possess non‐ or anti‐aromatic “formal azulene“ units because of the dominant aromaticity of the surrounding benzenoid rings.…”