Amethyrin-type expanded porphyrins that display anti-aromatic character upon protonation

The use of protonation to switch nonaromatic expanded porphyrinsto their corresponding anti-aromatic forms has not been widelyexplored. Here, we show that free-base pyriamethyrin and dipyr-iamethyrin display nonaromatic character, as inferred from NMRspectroscopic analyses, their optical properties, and theoreticalcalculations. Addition of two protons extends thep– conjugationof these amethyrin analogues and yields formally anti-aromaticsystems.