PHOTOINDUCED ELECTRON-TRANSFER FROM PORPHYRINS TO QUINONES IN MICELLAR SYSTEMS - AN FT-EPR STUDY

The photoinduced electron transfer from zinc tetrakis(-4-sulfonatophenyl)porphyrin (ZnTPPS) to duroquinone (DQ) in several micellar solutions was investigated with Fourier transform-electron paramagnetic resonance (FT-EPR). The measurements give information on the formation and decay of free radicals, following pulsed-laser excitation of ZnTPPS, with nanosecond time resolution. It was found that the characteristics and time evolution of the EPR spectra depend strongly on surfactant properties. In particular, long-lived [ZnTPPS3- ...DQ-] spin-correlated radical pairs (lifetime > 1-mu-s) were observed in solutions of the cationic surfactant cetyltrimethylammonium chloride (CTAC). By contrast, radical pair lifetimes in anionic and neutral micelles are shorter than 100 ns. The experimental data can be rationalized in terms of the Coulombic and hydrophobic interactions that determine the spatial organization and stability of the different species (reactants and products) in the microheterogeneous systems.