Structural Diversity in Expanded Porphyrins

Abstract

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...

Extroverted Confusion—Linus Pauling, Melvin Calvin, and Porphyrin Isomers

“…[43] Next to synthetic studies areas of current interest included the metal coordination chemistry due to the ability of these systems to form carbon-metal bonds, their anion sensing abilities, and their utility as photosensitizers, to name only a few. The whole field of macrocycle modified porphyrins has evolved to include isomeric expanded systems, [44] heteroatom substituted and carbaporphyrins, [45] Möbius aromatics, [46] cyclic oligopyrroles, [18] calixphyrins and -pyrroles, [47] and more. Even the chemical interconversion, i.e.…”

Extrovertierte Verwirrung – Linus Pauling, Melvin Calvin und Porphyrinisomere

“…[43] Dieses Gebiet umfasst unter anderem präparative Untersuchungen, die Metallkoordinationschemie (da diese Systeme zur Bildung von Kohlenstoff-Metall-Bindungen in der Lage sind), Anionensensoren und Photosensibilisatoren. Das ganze Gebiet der Makrocyclus-modifizierten Porphyrine hat sich so weit entwickelt, dass es inzwischen isomere expandierte Systeme, [44] Heteroatom-substituierte Porphyrine, Carbaporphyrine, [45] Möbius-Arene, [46] cyclische Oligopyrrole, [18] Calixphyrine und -pyrrole, [47] usw. umfasst.…”

Synthesis of Tri(4-tritylphenoxy)- and Hexa(4-methylphenoxy)-Substituted Hemihexaphyrazines

“…[3] Moreover like natural tetrapyrrolic macrocycles, porphyrinoids [4][5][6][7] show the light absorption in visible region at high level [8] what stimulates searching of new photoactive materials on their basis. [9][10][11] Immense capabilities of both core and peripheral modification [12][13][14][15] allow to realize directional synthesis of new compounds [16][17][18] with expended coordination cavities which are different in dimensions, composition and number of atoms included.…”