Novel effect of man-made molecular assemblies on photoinduced charge separation. 5. Extremely efficient harvesting of electrons photoliberated from water-soluble zinc porphyrin in the presence of an amphipathic viologen bilayer membrane and a zwitterionic electron mediator

Abstract

Charge separation of photogenerated ion pairs was achieved in nearly quantitative yields by employing zinc tetrakis(N-methylpyridinium)porphyrin as a sensitizer, zwitterionic (sulfonatopropyl)viologen as an electron mediator, and a bilayer membrane of amphipathic viologen as an electron pool. The quantum yield of viologen ion radicals, in the presence of triethanolamine as a sacrificial electron donor, remained high (0.80 +/- 0.02) over a wide range of amphipathic viologen concentration (20 ..mu..M-5mM). This value coincided with the quantum yield of initially generated viologen ion radicals as estimated from laser flash photolysis of the sensitizer-electron mediator binary system. The high quantum yield was attributed to the rapid and very efficient harvesting of the electron-carrying mediator and the successive electron transfer to the surface of the viologen bilayer membrane, which enables temporary storage of the captured electron. Relevance of the present system to chemical conversion and storage of solar energy is discussed. 30 references, 8 figures.