Molecular Receptors Based on Expanded Porphyrins

Studies on the design and synthesis of receptor molecules capable of selectively binding and transporting substrates (neutral, anionic and cationic) is currently being pursued to develop artificial membranes permeable to the bound species. Expanded porphyrin systems by virtue of increased cavity size and aromatic nature are capable of binding a variety of substrates depending on the nature of the porphyrin and the cavity size. Recently there are a number of reports on the use of expanded porphyrins as molecular receptors for various substrates. Specifically, expanded porphyrins such as sapphyrins and rubyrins in their protonated form bind a variety of anionic and neutral substrates and it has been shown that they act as carriers for transporting different ionic and neutral species. Additionally, expanded porphyrins find their application as MRI contrasting agents and as sensitizers for photodynamic therapy. In this review, an attempt has been made to discuss molecular receptor properties in the solid and solution phases of two expanded porphyrins, sapphyrin and rubyrin which are 22 and 26 electron systems respectively. Furthermore, the synthesis, binding and transport properties of core modified expanded porphyrin systems are also highlighted.