ENVIRONMENTAL EFFECTS ON PHOTOINDUCED ELECTRON TRANSFER REACTIONS

Photoinduced electron transfer reactions between an electronically excited photosensitizer molecule and a reversible electron acceptor capable of hydrogen production are currently being studied to determine the possible utility of these reactions in solar energy conversion systems. For efficient operation of these reaction systems, the back-reaction between initial photoproducts must be minimized. Charged macromolecular systems, e.g., colloidal silica sols and polyelectrolytes, have been utilized in the past to reduce the back-reaction rate through electrostatic interactions between the charged macroions and oppositely charged photoproducts.; The influence of charged macromolecules on the forward and back electron-transfer reaction between oppositely charged initial photoproducts, the photosensitizer Zinc tetra(4-N-methyl pyridinium) poryphyrin (ZnP('+4)) and the neutral electron acceptor, propyl viologen sulfonate (PVS('0)), has been investigated using continuous photolysis and flash photolysis techniques.; Polyelectrolytes have been shown to have a profound influence on interionic reaction rates. Polyelectrolyte addition increases the rate of reaction between similarly charged species and decreases the rate of reaction between oppositely charged species. Flash photolysis experiments have been performed to study the effect of anionic polyelectrolyte poly(styrene sulfonate) (PSS) on the photoreaction between ZnP('4) and PVS('0). These experiments show that all bimolecular rate constants involving PSS-bound ZnP('+4) are decreased by a factor of 40-100 relative to solutions containing no polyelectrolyte. This result can be understood in terms of a hydrophobic "envelopment" of ZnP('+4) by PSS. Steric blockage of the approach of reactants by the polyelectrolyte suppresses all bimolecular reactions involving PSS-bound species.; Addition of the cationic polyelectrolyte poly(N,N-dimethyl-3,5-dimethylene piperidinium chloride) instead of PSS resulted in a selective decrease in back-reaction rate by a factor of (DBLTURN)2. This observation is consistent with the primary salt effect mechanism of the influence of polyelectrolytes on interionic reactions.; The influence of negatively charged colloidal silica particles on the photoreaction between ZnP('+4) and PVS('0) is well known. The strong electric field surrounding the charged silica particle acts to prevent back reaction between particle-bound ZnP('+5-)(.) and PVS('-)(.). A serious limitation in the use of these colloids has been the instability of the silica particles in neutral and acidic media. PVS('-)(.) is thermodynamically incapable of water reduction to hydrogen in high pH media. (Abstract shortened with permission of author.)...