All Are Aromatic: A 3D Globally Aromatic Cage Containing Five Types of 2D Aromatic Macrocycles

The studies on three-dimensional (3D) aromaticity have been mainly focused on fullerenes, boron-based deltahedrons/clusters, metal clusters and polyhedral hydrocarbons, but there are very limited researches on the fundamental aromaticity rule for 3D fully π-conjugated molecules. Herein, we report a π-conjugated molecular cage in which two aromatic porphyrin units are bridged by four thiophene-based arms. Two-electron chemical oxidation leads to a 3D globally aromatic cage with a C2 symmetry according to X-ray diffraction, NMR, electronic absorption spectra, and theoretical calculations. Detailed magnetic shielding response analysis along different axes reveal that all the possible five types of two-dimensional (2D) macrocycles in the cage skeleton are aromatic and follow Hückel rule. The switch from localized aromaticity to global aromaticity upon chemical oxidation is also observed in a tricyclic model compound. The study indicates that to attain 3D global aromaticity in a molecular cage, all the formally available π-conjugated macrocycles should be 2D aromatic.