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

The hematoporphyrin-sensitized production of singlet molecular oxygen, O-2((1) Delta(g)), has been investigated in methanol and in aqueous s olution. The quantum yield for formation of O-2((1) Delta(g)) (Phi(Del ta)) has been measured by both steady-state (oxygen consumption) and t ime-resolved (near-infrared luminescence) methods. In methanol, both t echniques indicate that Phi(Delta) = 0.76 and the value remains indepe ndent of sensitizer concentration over a wide range. This finding is c onsistent with the dye persisting in a monomeric form in methanol solu tion. In contrast, Phi(Delta) decreases markedly with increasing sensi tizer 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 eff iciency whereby quenching of the triplet state by O-2 results in gener ation of O-2((1) Delta(g)) is substantially lower for the dimer than f or the corresponding monomer. Because monomer and dimer possess quite different absorption spectral profiles, the efficacy for photodynamic action with hematoporphyrin exhibits a pronounced wavelength dependenc e.