Intramolecular singlet and triplet energy transfer in a ruthenium(II) diimine complex containing multiple pyrenyl chromophores

We report the synthesis and the photophysical properties of UV light-harvesting arrays constructed around a [Ru(bpy)(3)](2+) core (bpy = 2,2'-bipyridine) bearing one and three pyrenyl units in the periphery. The free ligand containing the pyrenyl unit, 4-methyl-4'-(2-hydroxyethylpyrenyl)-2,2'-bipyridine, displays an intense emission band centered near 400 nm with a lifetime of 264 ns, characteristic of singlet pyrene emission.;The complexes [Ru(dmb)(2)(4-methyl-4'-(2-hydroxyethylpyrenyl)-2,2'-bipyridine)](PF6)(2), where dmb is 4,4'-dimethyl-2,2'-bipyridine, and [Ru(4-methyl-4'- (2-hydroxyethylpyrenyl) -2,2'-bipyridine)(3)] (PF6)(2) exhibit visible emission characteristic of the [Ru(bpy)(3)](2+) unit, regardless of excitation wavelength. The singlet emission from the pyrene chromophores is almost quantitatively quenched by the metal-to-ligand charge transfer (MLCT) states of each respective Ru(TI) complex resulting in the observation of sensitized MLCT-based emission. On account of the energetic proximity between the (MLCT)-M-3 states and the (3)pyrene states, a long-lived (MLCT)-M-3 emission is observed which decays with the same first-order Lifetime as the pyrene triplet states in deaerated CH3CN. In deaerated CH3CN, the bichromophoric system displays a lifetime of 2.96 mu s, whereas the tetrad complex exhibits a lifetime of 9.0 mu s. The results are indicative of excited-state equilibrium between the (MLCT)-M-3 and (3)pyrene states. Our findings demonstrate rapid and efficient singlet-singlet energy transfer through the antenna effect whereas the reversible triplet-triplet energy transfer processes help sustain long-lived MLCT excited states.