Porphyrins are a group of organic compounds that occur in nature, specially as the pigment in red blood cells. They are cyclic macromolecules called macrocycles that are characterized by the presence of four modified pyrrole subunits interconnected at their a carbon atoms via methine bridges (=CH-). Porphyrins are aromatic, and they obey Hückel's rule for aromaticity in that they possess 4n+2 pi electrons that are delocalized over the macrocycle. The macrocycles, therefore, are highly-conjugated system and consequently are deeply colored - the name porphyrin comes from a Greek word for purple. The macrocycle has 26 pi electrons. The parent porphyrin is porphine, and substituted porphines are called porphyrins.