Cycloparaphenylene-Phenalenyl Radical and Its Dimeric Double Nanohoop
We report the first example of a neutral spin-delocalized carbon-nanoring radical, achieved by integration of an open-shell graphene fragment phenalenyl into cycloparaphenylene (CPP). We show that spin distribution in this hydrocarbon partially extends from the phenalenyl onto the CPP segment as an interplay of steric and electronic effects. The resulting geometry is reminiscent of a diamond ring, with pseudo-perpendicular arrangement of the radial and the planar π-surface. Remarkably, this geometry gives rise to a steric effect that governs a highly selective dimerization pathway, yielding a giant double nanohoop. Its π-framework made of 158 sp2-carbon atoms was unambiguously elucidated by single crystal X-ray diffraction, which revealed a three-segment CPP–peropyrene–CPP structure. This nanocarbon shows a fluorescence profile characteristic of peropyrene, regardless of which segment gets excited. These results in conjunction with DFT suggest that adjustment of the size of the CPP segments in this double nanohoop could deliver true donor–acceptor systems.