Molecular receptors based on expanded porphyrins

Studies on the design and synthesis of receptor molecules capable of select ively binding and transporting substrates (neutral, anionic and cationic) i s currently being pursued to develop artificial membranes permeable to the bound species. Expanded porphyrin systems by virtue of increased cavity siz e and aromatic nature are capable of binding a variety of substrates depend ing 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 receptor s for various substrates. Specifically, expanded porphyrins such as sapphyr ins and rubyrins in their protonated form bind a variety of anionic and neu tral substrates and it has been shown that they act as carriers for transpo rting different ionic and neutral species. Additionally, expanded porphyrin s find their application as MRI contrasting agents and as sensitizers for p hotodynamic therapy. In this review, an attempt has been made to discuss mo lecular receptor properties in the solid and solution phases of two expande d porphyrins, sapphyrin and rubyrin which are 22 pi and 26 pi electron syst ems respectively. Furthermore, the synthesis, binding and transport propert ies of core modified expanded porphyrin systems are also highlighted.