LUMINESCENCE QUENCHING IN SUPRAMOLECULAR SYSTEMS - A COMPARISON OF DNA-MEDIATED AND SDS MICELLE-MEDIATED PHOTOINDUCED ELECTRON-TRANSFER BETWEEN METAL-COMPLEXES

Photoinduced electron transfer reactions between photoexcited Ru(phen) (2)dppz(2+) (phen = 1,10-phenanthroline, dppz = dipyridophenazine) and accepters Rh(phi)(2)bpy(3+) and Rh(phen)(2)phi(3+) (phi = 9,10-phenan threnequinone diimine, bpy = 2,2'-bipyridine) are compared in micelles and DNA. Both microheterogeneous environments contain a negatively ch arged surface and hydrophobic interior and the cationic complexes asso ciate strongly with each. However, reactions between molecules bound t o DNA or to micelles show striking differences which can be correlated with the unique character of the highly ordered, pi-stacked basepairs in DNA compared to the disordered, aliphatic chains in the micelles. In DNA, Rh(phi)(2)bpy(3+) quenches Ru(phen)(2)dppz(2+) on a fast time scale (unimolecular rate constant greater than or equal to 10(8) s(-1 )), whereas no detectable quenching of Ru(II) emission by Rh(phen)(2) phi(3+) is observed. In contrast, both complexes quench equally well i n SDS micelles. Although static quenching on the nanosecond time scale is observed for Rh(phi)(2)bpy(3+) in DNA, reactions in SDS occur dyna mically by intramicellar diffusion, with a bimolecular rate constant o f 1.1 x 10(8) M(-1) s(-1) for Rh(phi)(2)bpy(3+) and 1.2 x 10(8) M(-1) s(-1) for Rh(phen)(2)phi(3+). Reactions on DNA are also shown to be DN A-mediated in that no solvent-isotope effects are observed in the quen ching. In addition, there is enantioselectivity seen in reactions on t he right-handed DNA helix but not in the achiral micelle, indicating t hat quenching is sensitive to the geometry of intercalation. Efficient electron transfer quenching in DNA compared to SDS micelles therefore provides evidence against the cooperative association of molecules on DNA and for the importance of intercalative stacking of the donor and acceptor for fast electron transfer through the DNA pi-stack.