Negatively Curved Octagon-Incorporated Aza-nanographene and its Assembly with Fullerenes

A negatively curved aza-nanographene (NG) containing two octagons was synthesized by a regioselective and stepwise cyclodehydrogenation procedure, in which a double aza[7]helicene was simultaneously formed as an intermediate. Their saddle-shaped structures with negative curvature were unambiguously confirmed by X-ray crystallography, thereby enabling the exploration of the structure-property relationship by photophysical, electrochemical and conformational studies. Moreover, the assembly of the octagon-embedded aza-NG with fullerenes was probed by fluorescence spectral titration, with record-high binding constants (K_a=9.5×10³ M^-1 with C₆₀, K_a=3.7×10⁴ M^-1 with C₇₀) found among reported negatively curved polycyclic aromatic compounds. The tight association of aza-NG with C₆₀ was further elucidated by X-ray diffraction analysis of their co-crystal, which showed the formation of a 1 : 1 complex with substantial concave-convex interactions.