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Octagon-embedded carbohelicene as chiral motif for CPL emission of saddle-helix nanographenes.

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 = 79.5º), 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 ºC 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.