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In organic chemistry, a chlorin is a large heterocyclic aromatic ring consisting, at the core, of 3 pyrroles and one reduced pyrrole coupled through 4 methine linkages. Unlike a porphyrin, a chlorin is therefore largely aromatic but not aromatic through the entire circumference of the ring.

Magnesium-containing chlorins are called chlorophylls, and are the central photosensitive pigment in chloroplasts. A related compound, with 2 reduced pyrroles, is called a bacteriochlorin.

Because of their photosensitivity, chlorins are in active use as photosensitizing agents in experimental laser cancer therapies.

See also: corrin