Transformation of single-electron transfer photoproducts into multielectron charge relays: the functions of water-oil two-phase systems and enzyme catalysis

Water-in-oil microemulsions provide an organized environment that effectively controls photosensitized electron transfer processes. Effective charge separation and stabilization of the intermediate photoproducts against back electron transfer processes are achieved by means of hydrophobic and hydrophilic interactions of the photoproducts with the water-oil phases. Water-oil two-phase systems also provide a means for induced disproportionation of a photogenerated one-electron transfer product to the corresponding two-electron charge relay. This induced disproportionation can be achieved by design of opposite solubility properties of the comproportionation products in the two phases. The two-electron charge relay mediates the reduction of meso-1,2-dibromostilbene to trans-stilbene. An alternative route for generating multielectron charge relays involves the enzyme-catalysed production of dihydronicotinamide adenine dinucleotide phosphate (NADPH) using the 4,4′-bipyridinium radical cation as an electron carrier. NADPH is subsequently utilized in the reduction of 2-butanone to (-)-2-butanol in the presence of the enzyme alcohol dehydrogenase.