Helical Nanographenes Containing an Azulene Unit: Synthesis, Crystal Structures, and Properties. (3rd February 2020) - Ma, Ji
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. Abstract : Verdreht und ungerade : Helikale Nanographene mit eingebetteter Azuleneinheit und globaler Aromatizität wurden über eine Scholl‐Cyclisierung synthetisiert. Röntgenkristallographische Analysen bestätigen die Bildung der Azuleneinheit im helikalen π‐System. Der eingebettete Azulenkern nimmt eine stark verzerrte Konformation an und ist weniger aromatisch als ein Azulen‐Molekül.