Effect of Aggregation on the Hematoporphyrin‐sensitized Production of Singlet Molecular Oxygen

The hematoporphyrin‐sensitized production of singlet molecular oxygen, O 2 ( 1 Δ g ), has been investigated in methanol and in aqueous solution. The quantum yield for formation of O 2 ( 1 Δ g ) (Φ Δ ) has been measured by both steady‐state (oxygen consumption) and time‐resolved (near‐infrared luminescence) methods. In methanol, both techniques indicate that Φ Δ = 0.76 and the value remains independent of sensitizer concentration over a wide range. This finding is consistent with the dye persisting in a monomelic form in methanol solution. In contrast, Φ Δ decreases markedly with increasing sensitizer concentration in water due to dimerization of the dye. Analysis of the steady‐state data indicates Φ Δ values of 0.74 and 0.12, respectively, for monomer and dimer. It is further shown that the efficiency whereby quenching of the triplet state by O 2 results in generation of O 2 ( 1 Δ g ) is substantially lower for the dimer than for the corresponding monomer. Because monomer and dimer possess quite different absorption spectral profiles, the efficacy for photodynamic action with hematoporphyrin exhibits a pronounced wavelength dependence.