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] 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′-dimethyl2,2′-bipyridine, and [Ru(4-methyl-4′-(2-hydroxyethylpyrenyl)-2,2′-bipyridine)3](PF6)2 exhibit visible emission characteristic of the [Ru(bpy)3] 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(II) complex resulting in the observation of sensitized MLCT-based emission. On account of the energetic proximity between the 3MLCT states and the 3pyrene states, a long-lived 3MLCT 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 μs, whereas the tetrad complex exhibits a lifetime of 9.0 μs. The results are indicative of excited-state equilibrium between the 3MLCT and 3pyrene 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.