Energy Transfer between Transition‐Metal Complexes in Aqueous Solution

Abstract

AbstractElectronic excitation energy transfer between tris(2,2′‐bipyridine)ruthenium(II), [Ru(bipy)3]2+, and chromate(III) complexes such as tri‐oxalatochromate(III), [Cr(ox)3]3‐, trimalonatochromate(III), [Cr(mal)3]3‐, and hexacyanochromate(III), [Cr(CN)6]3‐, was observed in aqueous solution. Kinetics of the energy transfer was studied for a variety of viscosity and ionic strength of the solution. The rate‐determining process of the energy transfer is an intermolecular process between counter complex ions in an ion pair forming in solution but not a diffusion‐controlled encounter process.Main intermolecular path is an energy transfer from the charge‐transfer excited state in the ruthenium counterpart to the chromate(III) ligand‐field excited state in an ion pair. In a rigid solution, in fact, a sensitized phosphorescence of the chromate(III) complex was observed besides the phosphorescence of [Ru(bipy)3]2+ upon excitation of [Ru(bipy)3]2+ in the presence of the chromate(III) complex. Since the excited [Ru(bipy)3]2+ is deactivated before an effective encounter to the chromate(III) complex in such a highly viscous media, the sensitized phosphorescence could not be expected for a diffusion‐controlled process.From a static Stern‐Volmer relationship, the ionic association constants of [Ru(bipy)3]2+ with [Cr(ox)3]3‐ [Cr(mal)3]3‐ and [Cr(CN)6]3‐ at 25°C were obtained being extrapolated to zero ionic strength as 7.82 × 1031.mol−1, 3.26 × 1031.mol−1 and 9.48 × 1031.mol−1.