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Inspired by the chemistry of porphyrins, in the last decade, a new research area where porphyrin analogues such as expanded, isomeric, and contracted porphyrins have been synthesized, and their chemistry has been exploited extensively. Expanded porphyrins are macrocyclic compounds where pyrrole or heterocyclic rings are connected to each other through meso carbon bridges. Depending on the number of pyrrole rings in conjugation or the number of double bonds linking the four pyrrole rings expanded porphyrins containing up to 64 pi electrons are reported in the literature. The interest in these systems lies in their potential applications as anion binding agents, as photosensitizers for photodynamic therapy (PDT), in antisensing applications, as MRI contrasting agents, and more recently, as material for nonlinear optical application. Expanded porphyrins containing more than four pyrrole or heterocyclic rings, such as sapphyrin (five pyrrole), rubyrin (six pyrrole), heptaphyrin (seven pyrrole), and octaphyrin (eight pyrrole), are reported in the literature. Furthermore, substituents on expanded porphyrins can be attached either at the meso carbons or at beta-pyrrole positions. beta-substituted expanded porphyrins generally adopt normal structure where all the pyrrole nitrogens point inward in the cavity 1, while the meso-substituted expanded porphyrins exhibit normal 2, inverted 3, fused 4, confused 5, and figure eight 6 conformations. The conformation of expanded porphyrin is dependent on the nature of the linkage of the heterocyclic rings, the nature and the number of the heteroatoms present in the cavity, and the state of protonation. It is possible to change one conformation to another by varying temperature or by simple chemical modification, such as protonation by acids. An understanding of the structure-function correlation in expanded porphyrins is an important step for designing these molecules for their potential applications. In this context, even though several meso aryl expanded porphyrins are reported in literature, there is no comprehensive understanding of structural diversity exhibited by them. In this Account, an attempt has been made to provide a systematic understanding of the conditions and circumstances that lead to various conformations and structures. Specifically, the structural diversities exhibited by five pyrrolic macrocycles to ten pyrrolic macrocycles are covered in this Account. In pentapyrrolic systems, sapphyrins, N-fused, and N-confused pentaphyrins are described. It has been shown that the positions of the heteroatom affect the conformation and in turn the aromaticity. In hexapyrrolic systems, rubyrins and hexaphyrins are covered. The conformation of core-modified rubyrins was found to be dependent on the number and nature of the heteroatom present inside the core. Further, in the hexapyrrolic systems, an increase in the number of meso carbons from four (rubyrin) to six (hexaphyrin) increases the conformational flexibility, where different types of conformations are observed upon go...

The synthesis, spectral and structural characterization of meso-aryl sapphyrins and rubyrins containing heteroatoms such as S, O, Se in addition to pyrrole nitrogens are reported. The synthesis of the desired expanded porphyrins has been achieved using a single precursor, the modified tripyrranes containing heteroatoms, through an unprecedented oxidative coupling reaction in moderately good yields. The product distribution and the isolated yields were found to be dependent on the nature of the acid catalyst and its concentration. Use of 0.1 equiv of acid exclusively gave 26π rubyrins while a higher concentration of acid gave a mixture of 18π porphyrin, 22π sapphyrin, and 26π rubyrin. Two additional products, 22π oxasmaragdyrin and 18π oxacorrole, were isolated in the reaction of oxatripyrrane. All of the sapphyrins and rubyrins exhibit well-defined intense Soret and Q-bands in the visible region, and the intensity and the position of the absorption maxima were dependent on the number and the nature of the heteroatoms present in the cavity. The solid-state structures of sapphyrins 8 and 9 show small deviations from planarity with formation of supramolecular ladders stabilized by weak C−H···S, C−H···Se, and C−H···N hydrogen bonds. 1H NMR studies reveal retainment of supramolecular arrays in solution. The TFA adduct of 8 shows unusual binding in which both the hydroxyl oxygen and the carbonyl oxygen participate, which is reminiscent of metal carboxylate binding and in total contrast to that observed for β-substituted sapphyrins. 1H NMR studies on rubyrins indicate rapid rotation of heterocyclic rings at room temperature, and protonation leads to a decrease in rate of rotation at room temperature. 1H NMR spectra of 10 and 17 in its free base form recorded at −50 °C reveal that the heterocyclic rings are inverted and protonation leads to dramatic ring flipping. However, 11 shows normal structure in the solution. The single-crystal X-ray structures of 10, 11, and 17 show that the heterocyclic rings, thiophene in 10, selenophene in 11, and furan and thiophene in 17, are inverted in the solid state.