EFFECT OF AGGREGATION ON THE HEMATOPORPHYRIN-SENSITIZED PRODUCTION OF SINGLET MOLECULAR-OXYGEN

The hematoporphyrin-sensitized production of singlet molecular oxygen, O-2((1) Delta(g)), has been investigated in methanol and in aqueous solution. The quantum yield for formation of O-2((1) Delta(g)) (Phi(Delta)) has been measured by both steady-state (oxygen consumption) and time-resolved (near-infrared luminescence) methods. In methanol, both techniques indicate that Phi(Delta) = 0.76 and the value remains independent of sensitizer concentration over a wide range. This finding is consistent with the dye persisting in a monomeric form in methanol solution. In contrast, Phi(Delta) decreases markedly with increasing sensitizer concentration in water due to dimerization of the dye. Analysis of the steady-state data indicates Phi(Delta) 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) Delta(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.