Abstract
Abstract We report on electron-transfer reactions triggered by photoexcitation of zinc meso -tetrakis(N-methylpyridinium-4-yl)porphyrin (ZnTMPyP 4+ ) complexed with poly(amido amine) dendrimers (PAMAMs). In aqueous medium, cationic ZnTMPyP 4+ associates to negatively charged PAMAMs with a binding constant of 1.3 × 10 5 M −1 determined for generation 2.5 dendrimer. Electron-transfer from tertiary amines of the PAMAMs to excited singlet-state of ZnTMPyP 4+ induces weak fluorescence quenching without appreciable effect on the porphyrin's photostability in the presence of oxygen. However, under deaerated conditions ZnTMPyP 4+ is efficiently reduced to a chlorin, and further on to tetrahydroporphyrin, when exposed to light. A mechanism is proposed for the successive photoreduction of ZnTMPyP 4+ based on the spectral identification of reaction intermediates and products. The addition of methyl viologen (Mv 2+ ) inhibits this process by acting as an electron acceptor towards the radical ion ZnTMPyP ●,3+ that is formed in the primary electron-transfer step. Laser flash photolysis technique provided insight on electron-transfer reactions between the triplet state of ZnTMPyP 4+ and methyl viologen.
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