Aspects of artificial photosynthesis. Photosensitized electron transfer and charge separation in cationic surfactant vesicles

Abstract

Electron transfer and charge separation have been investigated in cationic dioctadecyldimethylammonium chloride (DODAC) vesicles by laser spectroscopy. h-Methylphenothiazine (MPTH) and its long-chain analogue, N-dodecylphenothiazine (DPTH), were used as electron donors, while a surfactant derivative of tris(2,2'-bipyridine)ruthenium perchlorate, RuC/sub 18/(bpy)/sub 3//sup 2 +/, acted as the photoactive electron acceptor, DODAC vesicles organized these donor and acceptor molecules. RuC/sub 18/(bpy)/sub 3//sup 2 +/ molecules were anchored onto the surface of the vesicles, while MPTH molecules were distributed among the hydrophobic bilayers of the vesicles. The metal to ligand charge transfer excited state of the ruthenium complex, RuC/sub 18/(bpy)/sub 3//sup 2 +/*, readily accepted an electron from MPTH to give MPTH/sup +/ and RuC/sub 18/(bpy)/sub 3//sup +/. First, there is a rapid geminate recombination at the very site of the generation of the cation radical. Second, some of the MPTH/sup +/. Escapes into the vesicle entrapped water pools and, owing to spatial confinement, the combination occurs at the inner surface of the vesicles. Finally, a part of MPTH/sup +/. Escapes into the bulk aqueous solution where it survives for extended periods (>>1 ms). MPTH and NaCl affect the outer surface charge density of the vesicles. The amounts of MPTH/sup +/. produced and thatmore » expelled into the bulk aqueous solution were maximized in the presence of 1.0 x 10/sup -3/ M NaCl. Under this condition there was still a sufficient electrostatic repulsion between MPTH/sup +/. and the charged surface of the vesicles to slow down considerably the undesirable charge recombination reactions. Carrying out the electron transfer using DPTH resulted only in short-lived DPTH/sup +/. in low concentrations. The long hydrocarbon chain on this molecule prevented the expulsion of this cation radical from the vesicle. Relevance of this study to light-induced photochemical energy conversion is discussed.« less