Aryl-crowding in carborane cages: aromatic characteristics of spherical aromatic units crowned with aromatic rings†

The inclusion of multiple aromatic units in a single molecular entity offers further understanding of their collective behavior. Herein, we evaluated the aromatic characteristics of a consecutive exo-aryl functionalization over the prototypical spherical aromatic ortho-carborane cage, which contained up to four phenyl rings crowning its C–C region, resulting in an aryl-crowding. Results showed that the multiple planar–spherical aromatic contacts yielded a collective interaction, enhancing the shielding regions. In the studied species, each connected aromatic unit retained the respective Clar's sextet and spherical aromatic states, showing independent shielding cones. The interplay of the enabled shielding cone from phenyl rings was dependent on the orientation of the external field owing to the planar aromatic characteristic of phenyl rings. This was in contrast to the spherical aromatic carborane feature, where the shielding cone resulted from different orientations of the field. Thus, the resulting behavior served as a representative case to further rationalize the overall aromatic characteristics for scenarios involving multiple aromatic units in a non-symmetrical fashion, as can be obtained in multiple aromatic arrays via covalent and non-covalent interactions. We expect that our results may encourage further evaluation of non-classical aromatic aggregates, providing an understanding of the interplay between different neighbor aromatic states in extended architectures built up from aromatic units.