There are thousands of porphyrins and metalloporphyrins, and hundreds of new derivatives appear each year. This variety arises because the cyclic conjugated tetrapyrrole nucleus (Figure 1 ) can have different substituents at the eight β-pyrrole positions, at the four meso [5,10,15,20] carbon atoms, and N-alkyl groups can be added to the four central nitrogen atoms. Since its synthesis in 1972, over 8000 literature references have appeared on 5,10,15, 20-tetrakis(N-methyl-4-pyridyl)porphyrin compounds, and a similar number on its precursor, 5,10,15,20-tetrakis(4-pyridyl) porphyrin and its derivatives. Most metals and metalloids in the periodic table form metalloporphyrins, and iron porphyrins have been prepared in oxidation states ranging from 0 to +5. In addition, the porphyrin ring itself can be oxidized, reduced, and cleaved.

5 , 10 , 15 and 20 ) positions. H ₂ -TPP: meso-tetraphenylporphyrin; H ₂ -TPyP(4), meso-tetrakis(4-pyridyl)porphyrin; H ₂ -TMPyP(X): the ortho ( 2 ), meta ( 3 ) and para ( 4 ) isomers of meso-tetrakis(N-methyl-X-pyridyl)porphyrin; H ₂ -TPPC ₄ , meso-tetrakis(4-carboxyphenyl)porphyrin; H ₂ -TPPS ₄ : meso-tetrakis(4-sulfonatophenyl)porphyrin; H ₂ -TAPP, meso-tetrakis(4-N,N,N-trimethylanilinium)porphyrin. A fuller explanation of the nomenclature of porphyrins can be found in Chapter 2 .