Giant Porphyrin Goes Big On Aromaticity

By increasing the number of pyrrole groups in a porphyrin ring from the usual four to 12, an international team of chemists has succeeded in making the largest aromatic molecule known to date (Chem. Eur. J. 2015, DOI: 10.1002/chem.201500650). Expanding the realm of aromaticity is nothing new for Dongho Kim of South Korea’s Yonsei University and Atsuhiro Osuka of Japan’s Kyoto University. Their groups have been collaborating for a few years to make big aromatic porphyrins in the quest to study anion and metal binding and electrochromic effects. In their latest effort, the researchers used a tripyrrole building block to construct a [52π]dodecaphyrin, a porphyrin ring consisting of a dozen connected pyrrole groups and adorned with pentafluorophenyl groups. This conjugated molecule has 52 π electrons and is nonaromatic—it doesn’t obey Hückel’s rule of possessing 4n + 2 π electrons. To meet the criterion, the team oxidized the molecule with a benzoquinone to form a [50π]dodecaphyrin, which bests the former aromaticity record holder, a [46π]decaphyrin the groups made last year. Subsequently protonating the [50π]dodecaphyrin using methanesulfonic acid helped flatten the molecule and improve its electron delocalization, thereby enhancing its aromaticity further.