Electrostatic interactions, control photoproducts

We have examined two types of organized media that effectively control the charge separation and back reactions of the intermediate photoproducts. These include, (a) charged colloids i.e. SiC>2 and ZrC>2 colloids that introduce electrostatic interactions between the photoproducts and interface (7-10), and (b) water-in-oil microemulsions that provide aqueous-oil two phase systems capable of controlling the reactions by proper design of the hydrophobic-hydrophilic balance of the photoproducts ( 6). 

Ru(bpy) +, is diffusion controlled. In the presence of the SiC>2 colloid this back electron transfer process is substantially retarded and ca. 200-fold slower than in the homogeneous phase. The functions of the Si02 colloid in charge separation and retardation of back reactions are attributed to electrostatic interactions of the photoproducts and the charged colloid interface (Figure 3). 

Fig. 8a, b. Control of photoinduced ET reactions in organized microenvironments a) application of charged interfaces to effect charge separation and retard recombination processes by means of electrostatic interactions b) application of water-oil two phase systems in charge separation and stabilization of photoproducts against back reactions by means of hydrophobic-hydrophilic interactions... 

Charged colloids and water-in-oil microemulsions provide organized environments that control photosensitized electron transfer reactions. Effective charge separation of the primary encounter cage complex, and subsequent stabilization of the photoproducts against back electron transfer reactions is achieved by means of electrostatic and hydrophobic interactions of the photoproducts and the organized media. 

A related system that uses electrostatic and hydrophobic interactions as a means to retard back ET reaction and consequently enhance H2-evolution has been exemplified by Gratzel and his colleagues. Photoreduction of JV-tetradecyl-JV -methyl-4,4 -bipyridinium, C14MV2+, (16), using Ru(bpy)2 + as photosensitizer in the presence of a sacrificial electron donor, has been controlled (17,138] in a micellar medium composed of positively charged micelles. The reduced photoproduct... 

---