In recent years numerous structural variants of porphyrins have been synthesized which are not only interesting as 18it arenes. but also because of their diverse applications in medicine, and as catalysts and chromophores. In most of these porphyrinoids. including porphyrin isomers, por-phyrin analogs, and expanded porphyrins, either a pyrrole ring has been formally replaced by a unit containing a nitrogen or a different heteroatom. or the arrangement of pyrroles and CH groups has been changed. Butfew porphyrinoids with less than four heteroatoms in the inner cycle have been reported that can bridge the gap to the formally related C-annulenes. which are prepared by completely different synthetic strategies and display strongly contrasting spectroscopic and physical properties.
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