Porphyrin and Porphyrinoids

The simple porphyrin category includes macrocycles that are accessible synthetically in one or few steps and are often available commercially. In such metallopor-phyrins, one or both axial coordinahon sites of the metal are occupied by ligands whose identity is often unknown and cannot be controlled, which complicates mechanistic interpretation of the electrocatalytic results. Metal complexes of simple porphyrins and porphyrinoids (phthalocyanines, corroles, etc.) have been studied extensively as electrocatalysts for the ORR since the inihal report by Jasinsky on catalysis of O2 reduction in 25% KOH by Co phthalocyanine [Jasinsky, 1964]. Complexes of all hrst-row transition metals and many from the second and third rows have been examined for ORR catalysis. Of aU simple metalloporphyrins, Ir(OEP) (OEP = octaethylporphyrin Fig. 18.9) appears to be the best catalyst, but it has been little studied and its catalytic behavior appears to be quite distinct from that other metaUoporphyrins [CoUman et al., 1994]. Among the first-row transition metals, Fe and Co porphyrins appear to be most active, followed by Mn [Deronzier and Moutet, 2003] and Cr. Because of the importance of hemes in aerobic metabolism, the mechanism of ORR catalysis by Fe porphyrins is probably understood best among all metalloporphyrin catalysts. 

Coordination of metal ions often has a dramatic effect on the n delocalization in porphyrins and porphyrinoids. It has particularly conspicuous influence on the electronic spectra of metalloporphyrins, which show a dependence on the identity of the metal ion, axial ligation, oxidation level, and spin state. In regular porphyrins, metal coordination reduces the number of observed Q bands from four to two, reflecting the higher symmetry of the chromophore relative to the free base. However, detailed quantitative information on the Jt-electron delocalization is more easily accessible from other physical methods. 

Appendix Table 3 H NMR shift data and UV-vis data for selected porphyrins and porphyrinoids ... 

In Mb, heme is located in the heme pocket via multiple noncovalent interactions such as Fe-His coordination, hydrophobic contacts with several nonpolar amino acid residues, and hydrogen bonding between heme propionates and polar amino acids (69). Therefore, the hemin can be easily removed from the heme pocket under acidic conditions to give apomyoglobin (apoMb) (70, 71). Over the past three decades, a variety of artificial iron porphyrins and porphyrinoids have been incorporated into the apoprotein to reconstitute the... 

Similar observations have been made for other porphyrin and porphyrinoid photosensitizers. For instance, several groups have observed that tetra-(4-sulfanatophenyl) porphyrin (TPPS4), a porphyrin of interest as a promising sensitizer for PDT, has some critical solvation issues [47,51,62-65]. In particular, when a typical free-base porphyrin, such as TPPS4, is dissolved as a monomer, the fluorescence lifetime is of the order of 1(C 18 ns, but, when it is aggregated, the fluorescence lifetime can drop to as low as 0.1-0.3 ns... 

The world of supramolecular chemistry is certainly a bigger and better place because of the existence of the porphyrins and porphyrinoids. While the porphyrins have long occupied a special position in the minds of chemists, it is the... 

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