Long-range resonance energy transfer to [Ru(bpy)₃]²⁺

We report the first experimental evidence of long-range nonradiative singlet-singlet resonance energy transfer from excited coumarin 460 molecules to [Ru(bpy)₃]²⁺ acceptors (bpy is 2,2′-bipyridine). The molecules were randomly dispersed in fluid solutions of both high (glycerol) and low (CH₃CN) viscosity at room temperature. The frequency-domain donor (coumarin 460) fluorescence decays were satisfactorily analyzed in terms of Förster theory incorporating a term for translational diffusion. Two different [Ru(bpy)₃]²⁺ (quencher) concentrations in each solvent were used, and the coumarin 460 emission was measured and analyzed both independently and globally. In glycerol, diffusion does not play a significant role in the energy transfer process, whereas in CH₃CN, diffusion is able to enhance the energy transfer reaction. In both solvents, mutual diffusion coefficients were determined from the experimental data and χ²_R error surfaces were constructed to judge their statistical uncertainty. The results demonstrate that metal-to-ligand charge transfer compounds can serve as intermolecular energy transfer acceptors in schemes utilizing long-range Förster-type processes.