Octagon‐Embedded Carbohelicene as a Chiral Motif for Circularly Polarized Luminescence Emission of Saddle‐Helix Nanographenes

• Published in: Angewandte Chemie 2021-03, Vol.133 (11), p.6159-6165
• Main Authors: Medel, Miguel A., Tapia, Rubén, Blanco, Victor, Miguel, Delia, Morcillo, Sara P., Campaña, Araceli G.
• Format: Article
• Language: English
• Subjects: Chemistry; chirality; Circular polarization; circularly polarized luminescence; Crystallography; Fjords; helicenes; Isomerization; nanographenes; octagonal carbocycle; Racemization; Twisting
• ISSN: 0044-8249; 1521-3757
• DOI: 10.1002/ange.202015368

We report a new family of hexa‐peri‐hexabenzocoronene (HBC)‐based helical nanographenes incorporating π‐extended carbo[5]helicenes bearing an octagonal carbocycle. This family represents a new kind of highly distorted saddle‐helix hybrid nanographenes. For the first time, the eight‐membered ring becomes a constituent of both a carbo[5]helicene and a HBC and thus, the negative curvature is responsible for twisting both units. This novel chiral motif, namely, oct‐[5]helicene results in the largest torsion angle recorded so far for a carbo[5]helicene (θ=79.5°), as it has been suggested by DFT‐calculations and confirmed by X‐ray crystallography. Consequently, the barriers of isomerization become exceptionally high for a [5]helicene unsubstituted in the fjord region since neither racemization nor decomposition were observed at 200 °C for 1 or 3 during 5 h. Therefore, racemic resolutions allowed subsequent chiroptical studies showing the ECD and CPL responses of this novel family of chiral nanographenes. Using π‐extended oct‐[5]helicene as novel helical unit, a new family of saddle‐helix hybrid nanographenes is prepared. They are chiral and emissive compounds that become part of the still exclusive family of circularly polarized luminescence (CPL)‐emissive nanographenes.