Two-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'-dicarboxy-2,2'-bipyridine), by anionic oxidation of 4,4'-dimethyl-2',2-bipyridine (dmb) using molecular oxygen (4 atm), and of the sensitizer precursor 4, tris(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) are reported. The anionic sensitizer Ru(dCb)(3)(4-) (5) was shown to bind to vesicles of lecithin when these were made positively charged by cationic bipyridinium electron acceptors. With cetylmethylviologen (CMV2+) as quencher, the time-resolved decay of the Ru(dcb)(3)(4-) emission followed a model for diffusion-controlled quenching in two dimensions. 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 the water region at the vesicle surface. The quantum yield of primary charge separation was 0.06 +/- 0.02, which is significant, if not high, despite the large elecrostatic attraction between the reactants. Attempts 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 covalent Linking of a donor to the sensitizer.