Protonated [4n] and [4n+2]π octaphyrins choose their möbius/hückel aromatic topology

Protonation-triggered conformational changes of meso- octakis(pentafluorophenyl) [36]octaphyrin and [38]octaphyrin have been investigated. The X-ray crystal structures and ¹H NMR analyses revealed that the protonation process cuts off intramolecular hydrogen bonds between aminic and iminic pyrrole units and, at the same time, produces intermolecular hydrogen-bond network between aminic pyrrole unit and counter-anions. Such a replacement induces some pyrrole inversion, leading to Möbius aromatic conformation for [36]octaphyrin and to Hückel aromatic conformation for [38]octaphyrin. These protonated octaphyrins show similar structures only with a subtle difference in tilted pyrrole angles, which results in their different topologies. This feature strongly suggests that the macrocycles control their topologies by pyrrole rotation to gain [4n]π Möbius or [4n+2]π Hückel aromatic stabilization, depending on the number of π-electrons. Detailed photophysical properties such as absorption/fluorescence, excited singlet/triplet state lifetimes, and two-photon absorption cross-section values have been presented for both protonated [36] and [38] octaphyrins in conjunction with their Möbius or Hückel aromaticity.