2-DIMENSIONAL EMISSION QUENCHING AND CHARGE SEPARATION USING A RU(II)-PHOTOSENSITIZER ASSEMBLED WITH MEMBRANE-BOUND ACCEPTORS

Novel syntheses of the bipyridine ligand 1, dcHb (dcHb = 4,4'-dicarbox y-2,2'-bipyridine), by anionic oxidation of 4,4'-dimethyl-2',2-bipyrid ine (dmb) using molecular oxygen (4 atm), and of the sensitizer precur sor 4, ,4'-diethoxycarbonyl-2,2'-bipyridine)ruthenium(II) bis(triflate ), from a chloride-free Ru(II) precursor 3b, Ru-II(DMSO)(4)(triflate)( 2-n)(EtOH), (n = 0-2, DMSO = dimethyl sulfoxide, triflate = OSO2CF3) a re reported. The anionic sensitizer Ru(dCb)(3)(4-) (5) was shown to bi nd to vesicles of lecithin when these were made positively charged by cationic bipyridinium electron acceptors. With cetylmethylviologen (CM V2+) as quencher, the time-resolved decay of the Ru(dcb)(3)(4-) emissi on followed a model for diffusion-controlled quenching in two dimensio ns. However, the diffusion coefficient obtained from a fit to the data was very small, (6 +/- 2) x 10(-11) m(2) s(-1), comparable to values for amphiphiles in bilayers, even though Ru(dcb)(3)(4-) diffuses in th e water region at the vesicle surface. The quantum yield of primary ch arge separation was 0.06 +/- 0.02, which is significant, if not high, despite the large elecrostatic attraction between the reactants. Attem pts were made to increase the charge separation yield by the use of a monocationic acceptor. Possible extensions of the system are discussed , such as charge separation across the vesicle membrane and the covale nt Linking of a donor to the sensitizer.