Porphyrins have been popular building blocks for the creation of supramolecular structures for a variety of reasons. Biological interest centres around the role of arrays of chlorophyll molecules in photosynthesis. The bacterial photosynthetic reaction centre is known to contain a ‘special pair’ of bacteriochlorophyll molecules bound within a protein,35,36 and the light harvesting antenna complex LH2 consists of a cyclic array of 27 bacteriochlorophylls.37 Chemists have tried to prepare porphyrin arrays as models of these naturally occurring arrays to aid in the understanding of their function.
Porphyrins also possess properties such as an intense chromophore and delocalized p system that may make them suitable building blocks for new materials. Specific examples relating to conductivity, optical and magnetic properties will be presented in the following sections.
The porphyrin molecule can be considered as an approximately flat square core which is readily substituted with functional groups at angles of 90° or 180° apart. Many metals may be coordinated to the pyrrolic nitrogen atoms, and this offers the possibility of coordination of additional axial ligands which may lie on one or both sides of the porphyrin plane. The axial ligand binding strength, selectivity and kinetics and the ease of porphyrin metallation/demetallation is strongly dependent on the nature of the metal and its oxidation state, thus providing a diverse range of coordination chemistry with which to assemble porphyrin containing structures. The reader is referred to a recent book for a comprehensive review.38
The spectroscopic properties of porphyrins also make them attractive as supramolecular synthons. The absorption and emission spectra are modulated by the metallation state, binding of axial ligands and porphyrin-porphyrin association, making UV/visible and fluorescence spectroscopies useful tools for monitoring metallation and assembly processes. In the case of diamagnetic porphyrins, NMR spectroscopy is an invaluable characterization tool. The aromatic ring current leads to substantial upfield shifts of protons located over the face of the p system, and such shifts are diagnostic of axial ligand binding to metalloporphyrins. In contrast, protons around the periphery of the p system experience a downfield shift.
The following sections present a brief review of the literature concerning the assembly of multiporphyrin structures using coordination chemistry and hydrogen bonds. Covalently linked porphyrin oligomers, although an active area of research, are not covered and the interested reader is referred to recent papers and reviews from groups working in this field.39-45