Photochemical electron transfer across surfactant vesicle bilayers

Abstract

Diethyl (BTDE) and dibutyl (BTDB) esters of 2,1,3-benzothiadiazole-4,7,- dicarboxylic acid are effective as combined chromophores and electron transfer catalysts in the photochemical transfer of electrons from suitable donors to anthraquinones. The less electron withdrawing nature of the ester groups than of -CN has improved the redox potential and by altering the nature of the ester R group, we can tailor other properties into the molecule such as increasing solubility in the lipophilic environment of the micellar core or vesicle bilayer by increasing the length of the R group. The stability of the radical anions obtained by reduction decreases in the order BTDN⁻ >BTDE⁻ >BTDB ⁻; correlating inversely with their reducing power. The more transient nature of BTDB⁻ is compensated for by its more favourable redox, micellepartition and light absorption properties. The rate of onward electron transfer from all the radical anions is sufficient for it to dominate over the radical decomposition. The rate of electron transfer is the same for each chromophore and this is interpreted in terms of charge compensating diffusion of OH⁻ or H+ across the vesicle as being the overall rate determining process. Mechanistic studies have highlighted that transmembrane electron transfer is affected by the diffusion of the radical anion. Attempts will be made to couple the transmembrane electron transfer with the production of H₂. Kinetic studies carried out on the electron transfer from MES to AQDS in a micellar system mediated by BTDB show that the reaction is first order in [BTDB], light intensity (with saturation at high I) and [MESH] at low concentration. It tends to zero order at higher [MESH] and is zero order in [AQDS] and [H+] between pH 6.5 and 10. A mechanism is proposed in which the BTDB acts as a chromophore and electron transfer catalyst and the rate determining step is transfer of the electron from BTDB⁻ to AQDS. The system can be modified to allow transfer of electrons across a vesicle bilaycr constructed from simple surfactant molecules (DODAB). It is shown that the initial rate of electron transfer depends upon the overall surface area of the vesicle present in solution but that the yield is determined by the availability of MES⁻ within the inner water pools of the vesicles. For unilamellar vesicles, all of the MES⁻ present within them is available for reaction but for multilamellar vesicles only the MES⁻ within the outer bilayer is available. Kinetic studies upon the vesicular system indicates similar results to those obtained in the micelles except that at higher BTDB concentration the reaction is zero order in all reagents except H+. This suggests that some process other than electron transfer must be rate determining. Since the rate increases with [H+], we assume that charge compensating flow of H+ across the vesicle bilayer is rate determining. Attempts to couple the transbilayer electron transfer to H₂ production were unsuccessful. Studies were also carried out on benzofurazan-4,7-dicarbonitrile (BFDN) but it proved to be a less effective electron transfer agent in the micelle system than BTDN or BTDB and was ineffective for electron transfer across the vesicles. Some possible explanations for this behaviour are discussed.