Tryptophan quenching of zinc-phthalocyanine and porphycene fluorescence in micellar CTAC

Singlet excited state deactivation of a zinc phthalocyanine (ZnPc), porphycene (Po) and tetrapropyl-porphycene (PrPo) by anionic tryptophan (Trp(-)) were investigated in cetyltrimethylammonium chloride (CTAC) micelles at pH 9.2 +/- 0.1, regulated by a Tris buffer. Data obtained from steady-state experiments over a wide range of detergent and added NaCl concentrations were analyzed by using a pseudophase ion-exchange model (Abuin et at, J. Phys. Chem. 87, 5166-5172, 1983), The model was applied to derive singlet quenching rate constants for ZnPc and the porphycenes by Trp(-) and the selectivity coefficient for Trp(-)/Cl- exchange at the micellar surface. The results point to an electron transfer quenching. Neutral tryptophan also quenches efficiently ZnPc fluorescence in CTAC without added buffer and Trp(-) does not deactivate the triplet state of these dyes. By flash photolysis, only the absorption of the triplet species was detected.