Biological Activities of Phthalocyanines—vi. Photooxidation of L‐tryptophan by Selectively Sulfonated Gallium Phthalocyanines: Singlet Oxygen Yields and Effect of Aggregation

The photosensitized oxidation of L‐tryptophan by gallium phthalocyanines sulfonated to different degrees is studied as a function of both substrate and sensitizer concentrations in water and 50% MeOH‐H 2 O solutions. The maximum quantum yield of singlet oxygen was found to be nearly 0.5 for all sulfonated gallium complexes. The effect of adding sulfonate groups in the phthalocyanine backbone is to change the tendency of dye molecules to dimerize or aggregate in a particular solvent. A shift in the chemical equilibrium away from the monomeric state, which occurs at high dye concentrations and at lower degrees of dye sulfonation, results in a reduced photochemical yield. The variation of quantum yields in different solvent systems and at several wavelengths is similarly accounted for by the fraction of light absorbed by the productive monomer state.