Porphyrins are a group of
heterocyclic macrocycle organic compounds, composed of four modified
pyrrole subunits interconnected at their
α carbon atoms via
methine bridges (=CH−). The parent porphyrin is
porphin, and substituted porphines are called porphyrins. The porphyrin ring structure is
aromatic, with a total of 26 electrons in the
conjugated system. Various analyses indicate that not all atoms of the ring are involved equally in the conjugation or that the molecule's overall nature is substantially based on several smaller conjugated systems. One result of the large conjugated system is that porphyrin molecules typically have very intense absorption bands in the visible region and may be deeply colored; the name "porphyrin" comes from the
Greek word πορφύρα (
porphyra), meaning
purple.