A long-lived transient resulting from flash photolysis of hematoporphyrin in aqueous solution

A long-lived transient with a lifetime of several hundred microseconds was observed following the flash photolysis of aqueous solutions of hematoporphyrin buffered at pH 7.5. The transient-ground state difference absorption spectrum was determined 500 microseconds after flash photolysis. The yield of this species was found to increase with increasing hematoporphyrin concentration and it was also found to depend on the excitation wavelength. The lifetime of the species is not significantly affected by the presence of oxygen. Because the triplet state of hematoporphyrin is not the only long-lived species produced by flash photolysis of aqueous hematoporphyrin solutions, the observed triplet state extinction coefficients will be lower than the true value and hence the triplet state yields of hematoporphyrin determined by the flash photolysis, "complete conversion" technique, are only upper limits. The formation of the long-lived species is discussed in terms of electron transfer between the monomer partners in hematoporphyrin dimer and aggregates which are present in aqueous solutions of hematoporphyrin, particularly in concentrated solutions.