Flash Photolysis Studies of Hemato‐and Copro‐porphyrins in Homogeneous and Microheterogeneous Aqueous Dispersions

This paper describes an experimental study of the photo properties of the triplet (T,) states of hematoporphyrin (HP) and coproporphyrin (CP), particularly in relation to their medium dependence and reactivity towards oxygen. Triplet‐triplet absorption spectra of HP and CP have been determined in aqueous buffer at pH = 7.4 and in water‐methanol and water‐formamide mixtures. The spectra corrected for ground state contributions show major absorption peaks near 400 nm and lesser peaks near 500 nm. Extinction coefficient measurements have been made and their dependences on solvent composition investigated. Natural lifetimes of the T 1 states of HP and CP and the bimolecular quenching constants with oxygen have been determined. The quantum yields of T 1 formation are ca. 0.6 in buffer rising to 0.8 and higher in predominantly organic media. Incorporation of the porphyrins into micellar phases similarly causes φ T , to increase. Quantum efficiencies of O − 2 and O 2 ( l Δ g ) formation have been determined for HP in buffer, some binary mixtures and micellar dispersions. Superoxide yields are low and may result from photo‐ionization processes. O 2 ( l Δ g ) yields are large but appear to have an unexpected dependence on the medium.