Reduction of the porphyrin ligand

The porphyrin macrocycle contains conjugated double bonds that form an 18-membered Tr-electron system. This Tr-system can accept several electrons in a stepwise manner. The addition of one electron forms a porphyrin Tr-radical anion. Such Tr-radical anions have been prepared from metalloporphyrins and from metal free porphyrins and related compounds by irradiation in various environments. 

Most studies have been carried out in aqueous or aqueous-alcohol solutions and the intermediate and final products have been followed by spectrophotometry. All porphyrins react with the hydrated electron with diffusion-controlled rate constants (k 10 L mol s ) to form the Tr-radical anions. 

The rate constants in these cases depend on the reduction potential of the porphyrin and the radical and can vary from 10 to 10 L mol s. The radical anions formed in these reactions generally exhibit intense absorption 

The radical anions are stable in aprotic media in the absence of oxidants but decay rapidly in protic solvents. The stability of porphyrin radical anions in aqueous solutions was found to depend very strongly upon the nature of the substituents on the ring and the nature of the metal center. 

A dramatic difference was observed in the behavior of porphyrin radical anions that contain phenyl v . pyridinium substituents at the meso positions. It was suggested that the radical anions decay either via protonation, to form a neutral radical, or disproportionation, to form a dianion, which then protonates very rapidly. The dianion can take-up a proton at a meso site to form a phlorin anion or take up two protons at a pyrrole ring to form a chlorin. 

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