I. Willner et al., PHOTOINDUCED ELECTRON-TRANSFER PROCESSES USING ORGANIZED REDOX-FUNCTIONALIZED BIPYRIDINIUM POLYETHYLENIMINE TIO2 COLLOIDS AND PARTICULATE ASSEMBLIES, Journal of physical chemistry, 97(28), 1993, pp. 7264-7271
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.