Effect of micelles on oxygen-quenching processes of triplet-state para-substituted tetraphenylporphyrin photosensitizers

Development of clinically valuable porphyrin drugs for use in photodynamic therapy requires characterization of membrane effects on porphyrin excited-state dynamics and bimolecular quenching with oxygen. This study reports on the quenching of triplet-state tetraphenylporphyrin (TPP), 5-(p-carboxyphenyl)-10,15,20-triphenylporphyrin ((p-COO-)(1)TPPH2), 5-(p-aminophenyl)-10,15,20-triphenylporphyrin ((P-NH2)(1)TPPH2), and 5-(p-hydroxyphenyl)-10,15,20-triphenylporphyrin ((p-OH)(1)TPPH2), by dissolved oxygen in a range of environments, such as cyclohexane solution and aqueous micellar solutions of tetradecyltrimethylammonium bromide (TTAB) and poly(ethyleneglycol)-p-tert-octylphenol (TX-100). The bimolecular quenching rate constants were found to be similar for all environments and porphyrins studied here, but the observed rate constants in aerated solutions varied with solvent and substituent types. These results indicate that location of porphyrins within the micelles was affected by the nature of the detergent headgroup as well as by the porphyrin substituent. They also support the conclusion that an oxygen concentration gradient exists in the micelles with the highest oxygen concentration at the center.