|Porphyrins are organic pigments, of both natural and synthetic origin, all of which contain the porphyrin ring (Figure 1) as part of their structure.
In addition, porphyrin chemistry deals with various analogues and derivatives of porphyrins and, particularly, with their metal complexes.
The porphyrin molecule is usually described as a union of four pyrrolic rings linked by four methine bridges to form a macrocycle. (The term macrocycle refers to cyclic molecules large enough to accomodate a metal ion in the interior).
Numbering of the porphyrin nucleus runs along the twenty peripheral carbon atoms as shown in Figure 2. The inner nitrogens are given locants 21-24.
Porphyrins are in general substituted on the periphery. There are 12 positions that can be substituted. The eight β positions are those on the pyrrolic rings and the four meso positions correspond to the methine bridges. The unsubstituted porphyrin has been named porphine.
The two most frequenly encountered substitution patterns are provided by β-octaalkylporphyrins and meso-tetraarylporphyrins (Figure 3). Naturally occurring porphyrins are β-substituted in an unsymmetrical fashion.
Fig 3. Favourite substitution patterns: beta-octaethylporphyrin (left), meso-tetraphenylporphyrin (center), protoporphyrin IX (right).
Fig. 2. Numbering of the porphyrin molecule. The meso and beta positions are shown in color.