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 modificatio n of PEI by N-(2-carboxyethyl)-N'-methyl-4,4'-bipyridinium, MVP2+ (2), generates a redox-functionalized polymer, PEI-MVP2+, that also stabil izes TiO2 colloids. Photoreduction of bipyridinium units, which are co valently linked to the polymer backbone within the TiO2-PEI-MVP2+ asse mbly, proceeds effectively upon excitation of the semiconductor colloi d. At pH = 8.9, photoreduction of bipyridinium units of the TiO2-PEI-M VP2+ 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, photor eduction of PEI-MVP2+ is ca. 92-fold and 12-fold faster than that of M V2+ and PVS0 by TiO2-PEI, respectively. The enhanced yield of reductio n of the bipyridinium relay units of the TiO2-PEI-MVP2+ assembly is at tributed to the control of electron-transfer reactions at the semicond uctor-solution interface. The redox polymer stabilized colloid, TiO2-P EI-MVP2+, concentrates the electron relay units at the semiconductor i nterface. Consequently, conduction band electrons formed upon photoexc itation of the TiO2 are effectively trapped by the redox relay units o n the polymer. This electron trapping competes with the degradative 'e lectron-hole' recombination. Time-resolved laser photolysis studies re veal that the interfacial electron transfer in the TiO2-PEI-MVP2+ asse mbly proceeds within the laser pulse time constant (<0.5 ns). On the o ther 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, coup led 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 tran sfer was also accomplished by applying rigid, functionalized semicondu ctor particles. In this photosystem, nitrate reductase is covalently l inked to the redox polymer PEI-MVP2+-TiO2.