PHOTOINDUCED ELECTRON-TRANSFER PROCESSES USING ORGANIZED REDOX-FUNCTIONALIZED BIPYRIDINIUM POLYETHYLENIMINE TIO2 COLLOIDS AND PARTICULATE ASSEMBLIES

Polyethylenimine, PEI, acts as an effective supporting polymer for the stabilization of semiconductor colloids of TiO2. Chemical modification of PEI by N-(2-carboxyethyl)-N'-methyl-4,4'-bipyridinium, MVP2+ (2), generates a redox-functionalized polymer, PEI-MVP2+, that also stabilizes TiO2 colloids. Photoreduction of bipyridinium units, which are covalently linked to the polymer backbone within the TiO2-PEI-MVP2+ assembly, proceeds effectively upon excitation of the semiconductor colloid. At pH = 8.9, photoreduction of bipyridinium units of the TiO2-PEI-MVP2+ assembly is ca. 54-fold and 13-fold faster than the reduction of N,N-dimethyl-4,4'-bipyridinium, MV2+, or N,N'-bis(3-sulfonatopropyl)-4,4'-bipyridinium, PVS0, by TiO2-PEI, respectively. At pH = 5.1, photoreduction of PEI-MVP2+ is ca. 92-fold and 12-fold faster than that of MV2+ and PVS0 by TiO2-PEI, respectively. The enhanced yield of reduction of the bipyridinium relay units of the TiO2-PEI-MVP2+ assembly is attributed to the control of electron-transfer reactions at the semiconductor-solution interface. The redox polymer stabilized colloid, TiO2-PEI-MVP2+, concentrates the electron relay units at the semiconductor interface. Consequently, conduction band electrons formed upon photoexcitation of the TiO2 are effectively trapped by the redox relay units on the polymer. This electron trapping competes with the degradative 'electron-hole' recombination. Time-resolved laser photolysis studies reveal that the interfacial electron transfer in the TiO2-PEI-MVP2+ assembly proceeds within the laser pulse time constant (<0.5 ns). On the other hand, reduction of N,N'-dimethyl-4,4'-bipyridinium, MV2+, by TiO2-PEI through a diffusional mechanism is slow, k(et) - 750 +/- 80 s-1. The photoinduced electron-transfer process in TiO2-PEI-MVP2+ was, coupled to the biocatalyzed reduction of nitrate (NO3-) to nitrite (NO2-) by electrostatic association of the enzyme nitrate reductase (E.C. 1.6.6.2) with the polymer backbone. This photo-biocatalyzed electron transfer was also accomplished by applying rigid, functionalized semiconductor particles. In this photosystem, nitrate reductase is covalently linked to the redox polymer PEI-MVP2+-TiO2.