Porphyrins and Expanded Porphyrins as Receptors

AbstractThis chapter summarizes recent efforts to use porphyrins and expanded porphyrins as receptors. These unique macrocycles bear resemblance to naturally occurring tetrapyrrolic pigments and have been studied extensively as building blocks for a variety of functional molecular receptors. The introduction discusses the inherent characteristics of porphyrins and their analogs from the perspective of receptor design and is designed to highlight key features. These features include ease of synthesis and functionalization, the hydrogen‐bonding ability of the constituent pyrrolic NH protons, and the fact that the systems in question are often highly colored. This latter feature has afforded an extremely useful means of analyzing host–guest interactions.Because of space limitations, not all of the literature on this topic could be covered in a comprehensive manner. The definition of “receptor” is also limited to species that are thought to be operated via strictly noncovalent‐ or supramolecular‐type interactions. However, this definition is expanded in a few special cases. The goal of this article is not to cite every published article on this topic, but to highlight interesting examples of porphyrin and expanded porphyrins and their utility as receptors. An effort has also been made to describe the analytical techniques used to characterize the resulting host–guest ensembles.This chapter is organized according to the type of guest. One section covers the relatively new area of supramolecular π–π interactions involving porphyrins and nanomaterials, such as fullerenes and nanotubes. Another section discusses porphyrins as receptors for biological entities, including saccharides, peptides, and proteins, as well as DNA. The next section covers porphyrins and expanded porphyrins as anion receptors. This latter treatment includes a discussion of supramolecular electron‐transfer dyads. The use of porphyrins in sensing applications is also presented in summary form. Finally, a perspective on this rapidly growing field of supramolecular chemistry is included in the final remarks.It is hoped that the combination of cited references and listed further readings will allow the interested reader to learn more about this fast‐evolving and fascinating topic.