Global Aromaticity and Antiaromaticity in Porphyrin Nanoring Anions

Doping, through oxidation or reduction, is often used to modify the properties of π-conjugated oligomers. In most cases, the resulting charge distribution is difficult to determine. If the oligomer is cyclic and doping establishes global aromaticity or antiaromaticity, then it is certain that the charge is fully delocalized over the entire perimeter of the ring. Here we show that reduction of a six-porphyrin nanoring using decamethylcobaltocene results in global aromaticity (in the 6– state; [90 π]) and antiaromaticity (in the 4– state; [88 π]), consistent with Hückel’s rules. Aromaticity is assigned by NMR spectroscopy and density-functional theory calculations.