How Should Aromaticity Be Described in Porphyrinoids?

“Porphyrins really are the [18]annulenes of nature”—This is the title of a report dealing with a new macrocycle (see structure; gray C, white H, blue N), which can be considered as either a porphyrin or an [18]annulene. The spectroscopic data prove the aromatic and porphyrinoid character of the new compound, and support the simple concept of a (4n+2)π main conjugation pathway as a key criterion for porphyrinoid Hückel aromaticity.