Rhodium-Catalyzed Synthesis, Crystal Structures, and Photophysical Properties of [6]Cycloparaphenylene Tetracarboxylates

The synthesis of C2-symmetrical [6]­cycloparaphenylene (CPP) tetracarboxylates has been achieved via macrocyclization by the rhodium-catalyzed intermolecular stepwise cross-alkyne cyclotrimerization and subsequent reductive aromatization. The 1H NMR spectra of the thus-obtained C2-symmetrical [6]­CPP-tetracarboxylates revealed that the rotation of unsubstituted benzene rings is slow at room temperature. These [6]­CPPs formed columnar packing structures, and their absorption maxima were significantly blue-shifted compared to that of nonsubstituted [6]­CPP due to the presence of four electron-withdrawing ester moieties.