Photoinduced Energy Transfer Coupled to Charge Separation in a Ru(II)−Ru(II)−Acceptor Triad

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Abstract
The bichromophoric system Ru−RuC−PI ([(bpy)3Ru−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 N6 and a N5C ligand set, respectively, was synthesized and characterized. Its photophysical properties were investigated and compared to those of the monochromophoric cyclometalated complexes RuC−PI ([Ru(dpb)(Metpy-PI)][PF6]), RuC−φ−PI ([Ru(dpb)(ttpy-PI)][PF6], ttpy is 4‘-p-tolyl-2,2‘:6‘,2‘ ‘-terpyridine), RuC−φ ([Ru(dpb)(ttpy)][PF6]), and RuC ([Ru(dpb)(Metpy)][PF6]). Excitation of the RuC unit in the dyads leads to oxidative quenching, forming the RuCIII−φ−PI•- and RuCIII−Pl·- charge-separated (CS) states with kfET = 7.7 × 107 s-1 (CH3CN, 298 K) in the tolyl-linked RuC−φ−PI and kfET = 4.4 × 109 s-1 (CH2Cl2, 298 K) in the methylene-linked RuC−PI. In the Ru−RuC−PI triad, excitation of the RuC chromophore leads to dynamics similar to those in the RuC−PI dyad, generating the RuII−RuCIII−PI•- CS state, whereas excitation of the Ru unit results in an initial energy transfer (kEnT = 4.7 × 1011 s-1) to the cyclometalated RuC unit. Subsequent electron transfer to the PI acceptor results in the formation of the same RuII−RuCIII−PI•- CS state with kfET = 5.6 × 109 s-1 that undergoes rapid recombination with kbET = 1 × 1010 s-1 (CH2Cl2, 298 K). The fate of the RuII−RuCIII−PI•- CS state upon a second photoexcitation was studied by pump−pump−probe experiments in an attempt to detect the fully charge-separated RuIII−RuCII−PI•- state.

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