Covalently Linked Free-Base and Metallo-Bis-Porphyrins: Chemistry and Diversity

Porphyrins are important macrocyclic compounds that exist in nature, particularly in their metallated forms. Their unique properties have led the chemical community, since the earliest studies, to embrace the challenge of synthesizing porphyrins. In the quest to produce and manipulate the periphery of these macrocyclic systems for more useful and versatile applications, various strategies have been implemented. Among these, the synthesis of macromolecular systems based on covalently linked porphyrins is one of the most interesting. It is generally believed from the large number of studies conducted that inter-porphyrin interactions occur when there is a strong overlap of the macrocyclic π orbitals mediated by the linkage. While nature uses diverse covalent linkages and self-assembled interactions, our mastery of these is still in its infancy. Attempts to imitate and explore nature sometimes provide chemists with a fertile field for synthetic and mechanistic studies. Synthetic bis-porphyrin prepared for a wide variety of purposes have generally utilized covalent linkers to maintain desired geometry and to control the electronic communication between the two porphyrin moieties involved. This review aims to highlight recent synthetic strategies and the various routes employed to design and synthesize these macrocyclic systems as free-base or metallated forms, with linkages of varied nature, to fine tune the overall system properties for particular applications. Because of the extent of this subject in terms of macromolecular porphyrin systems, in this review we have focused only on bis-porphyrin systems, and have ignored coverage of macromolecular and supramolecular porphyrin based systems that involve more than two porphyrin cores.