A porphyrin is a heterocyclic macrocycle derived from four pyrroline subunits interconnected via their α carbon atoms via methine bridges (=CH-). Porphyrins are aromatic and they obey Huckel's rule for aromaticity in that they posess 4n+2 pi electrons which are delocalized over the macrocycle. The macrocycle, therefore, is a highly-conjugated system, and, as a consequence, is deeply coloured - the name porphyrin comes from a Greek word for purple. The macrocycle has 22 pi electrons. The parent porphyrin is porphine, and substituted porphines are called porphyrins. Many porphyrins occur in nature, such as in green leaves and red blood cells, and in bio-inspired synthetic catalysts and devices.