Quenching of porphyrin triplet and singlet oxygen by stable nitroxide radicals: Importance of steric hindrance

To study the nature of quenching, we used stable nitroxide radicals (most o f which contain piperidine or pyrrolidine rings) as quenchers of triplet he matoporphyrin as well as of singlet molecular oxygen. A characteristic feat ure of quenching triplet porphyrin is a near-diffusion-limited rate constan t, whereas the rate constant for quenching singlet oxygen is about three or ders of magnitude lower. Accessibility of the nitroxide moiety in radicals was characterized quantitatively based on semi-empirical calculations (at A MI level), with the use of van tier Waals radii of the species. While varia tion in the rate constant values for quenching triplet porphyrin can be ful ly explained by the steric hindrance of the neighbouring groups of the nitr oxide radical. no much effect can be observed in quenching singlet oxygen.