Photoinduced energy transfer coupled to charge separation in a Ru(II)-Ru(II)-acceptor triad

The bichromophoric system Ru - Ru-C - PI ([(bpy)(3)Ru - Ph - Ru(dpb)(Metpy- PI)][PF6](3), where bpy is 2,2'-bipyridine, Hdpb is 1,3-di(2-pyridyl)- benzene, Metpy is 4'-methyl- 2,2': 6', 2 ''-terpyridine and PI is pyromellitimide) containing two Ru(II) polypyridyl chromophores with a N-6 and a N5C ligand set, respectively, was synthesized and characterized. Its photophysical properties were investigated and compared to those of the monochromophoric cyclometalated complexes Ru-C - PI ([Ru(dpb)(Metpy-PI)][PF6]), Ru-C -phi- PI ([Ru(dpb)(ttpy- PI)][PF6], ttpy is 4'-p-tolyl- 2,2': 6', 2 ''-terpyridine), Ru-C -phi ([Ru(dpb)(ttpy)][PF6]), and Ru-C ([Ru(dpb)(Metpy)][PF6]). Excitation of the Ru-C unit in the dyads leads to oxidative quenching, forming the Ru-C(III) -phi- PI.- and Ru-C(III) - Pl(.-) charge-separated (CS) states with k(ET)(f) = 7.7 x 10(7) s(-1) (CH3CN, 298 K) in the tolyl-linked Ru-C -phi - PI and k(ET)(f) = 4.4 x 10(9) s(-1) (CH2Cl2, 298 K) in the methylene-linked Ru-C - PI. In the Ru - Ru-C - PI triad, excitation of the Ru-C chromophore leads to dynamics similar to those in the Ru-C - PI dyad, generating the Ru-II - Ru-C(III) - PI.- CS state, whereas excitation of the Ru unit results in an initial energy transfer (k(EnT) = 4.7 x 10(11) s(-1)) to the cyclometalated RuC unit. Subsequent electron transfer to the PI acceptor results in the formation of the same Ru-II - Ru-C(III) - PI.- CS state with k(ET)(f) = 5.6 x 10(9) s(-1) that undergoes rapid recombination with k(ET)(b) = 1 x 10(10) s(-1) (CH2Cl2, 298 K). The fate of the Ru-II - Ru-C(III) - PI.- CS state upon a second photoexcitation was studied by pump - pump - probe experiments in an attempt to detect the fully charge- separated Ru-III - Ru-C(II) - PI.- state.