Richard E. Sassoon

Separation of photoredox products by local potential fields

Abstract Polyelectrolytes, ion exchangers and charged colloids can be used to control the rates of photo-induced chemical reactions. In particular, polyelectrolytes possess several additional features which make them attractive as suitable microassemblies in such systems. These include a well-defined structure whose properties of size, shape, charge density etc. may be modified by conventional synthetic techniques, the possibility of attaching specific molecular chromophores chemically to a polyelectrolyte such that they possess the desired properties (e.g. light absorption, redox potential, solubility etc.) and their possible adsorption to colloidal catalysts and electrodes. Special emphasis on charge separation and the inhibition of photochemical back reactions by polyelectrolytes is therefore made. Charge separation effects are critically reviewed. Large effects on the quantum yields of photochemical electron transfer products are reported. The reactions of zwitterionic quenchers with excited photosensitizers in the presence of polyelectrolytes or charged colloids are also described in which there seems to be a lack of microenvironmental effect on the quantum yields of charge separation. The possible reasons for this are discussed. In general, back reactions are expected to be inhibited in the presence of polyelectrolytes when one of the reacting species lies in the polymer field and the other is an ion bearing the same charge as the polyion. However, the inhibition effect is usually found to be only of one or two orders of magnitude. On the basis of pulse radiolytic measurements of the rate of bimolecular reactions of polyelectrolyte radicals, it can be concluded that polyelectrolytes may lead to the inhibition of back reactions by up to five or six orders of magnitude if the two photochemical transient species are bound to different polyelectrolyte molecules carrying the same electric charge. A photochemical system which fulfils the conditions is described, and ways in which to apply such systems to the photochemical cleavage of water are discussed.