Photoinduced electron-transfer in supramolecular complex of zinc porphyrin with poly(amido amine) dendrimer donor

We report on electron-transfer reactions triggered by photoexcitation of zinc meso-tetrakis(N-methylpyridinium-4-yl)porphyrin (ZnTMPyP4+) complexed with poly(amido amine) dendrimers (PAMAMs). In aqueous medium, cationic ZnTMPyP4+ associates to negatively charged PAMAMs with a binding constant of 1.3 × 105 M−1 determined for generation 2.5 dendrimer. Electron-transfer from tertiary amines of the PAMAMs to excited singlet-state of ZnTMPyP4+ induces weak fluorescence quenching without appreciable effect on the porphyrinʹs photostability in the presence of oxygen. However, under deaerated conditions ZnTMPyP4+ is efficiently reduced to a chlorin, and further on to tetrahydroporphyrin, when exposed to light. A mechanism is proposed for the successive photoreduction of ZnTMPyP4+ based on the spectral identification of reaction intermediates and products. The addition of methyl viologen (Mv2+) 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 ZnTMPyP4+ and methyl viologen.