The heterocycle chlorin is a macrocyclic tetrapyrrole derivative. It is found in the photosynthetic center either as the uncoordinated phaeophytin or as its magnesium complex, which acts as a primary acceptor in chlorophyll.

In chlorin (2,3-dihydroporphyrin), unlike in porphyrin, the ring system is partially hydrogenated. In phaeophytin an additional ring is condensed onto the macrocycle.