Electron transfer between excited states of some sulfonated phtha-locyanines and tyrosine as well as trptophan in homogeneous aqueous solution and aqueous micellar media

Abstract

Rate constants for electron transfer between excited states of several tetrasulfonated phthalocyanines (MTSPC, M = H2, Zn, ClAl, ClGa) and tyrosine or trptophan have been measured in homogeneous aqueous and aqueous micellar media. Cationic micelles formed by surfactant cetyl trimethyl ammonium chloride (CTAC) promote the electron transfer reaction, whereas neutral micelles formed by Triton X-100 depress this process. The calculated free energy change shows that phthalocyanines act as electron donors in the electron transfer reaction of its excited singlet states with tyrosine or trptophan (Type Is), whereas they act as electron acceptors in the reaction of its excited triplet states with tyrosine or trptophan (type IT). The two different electron transfer processes involving singlet and triplet of sensitizer respectively compete with each other and form different intermediates which may induce the formation of different products. Factors that govern the importance of Type Is in the whole reaction include not only the excited singlet and triplet lifetimes but also the rate constants of the electron transfer processes and substrate concentration. In the case of ZnTSPC as sensitizer and trptophan as substrate, the contribution of S1 (singlet state) to electron transfer reaction (Type I mechanism) is always much larger than that of T1 (triplet state).