Structural deformations and bond length alternation in porphyrin pi-cation radicals.

This review discusses the structural changes that occur when the porphyrin ring of metalloporphyrin complexes is oxidized to form a pi-cation radical species. Although various differences in core conformation between the pi-cation derivative and the unoxidized homologue have been observed, there does not appear to be a general pattern of change. A frequently observed feature in pi-cation derivatives is the appearance of an alternating bond distance pattern in the inner ring of the porphyrin consistent with a localized structure rather the delocalized structure usually seen. The pattern, first seen in cofacial dimeric structures, has now been seen in monomeric systems as well. The nature and frequency of the observation and possible explanation are given.