Protonation-triggered conformational changes of meso- octakis(pentafluorophenyl) octaphyrin and 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 octaphyrin and to Hückel aromatic conformation for 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  and  octaphyrins in conjunction with their Möbius or Hückel aromaticity.
All Science Journal Classification (ASJC) codes
- Colloid and Surface Chemistry