Energy transfer between transition-metal complexes in double complex salts

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Abstract
In a double complex salt consisting of cationic complex tris (2, 2′‐bipyridine) ruthenium (II) [Ru(bipy)3]2+ and anionic chromium (III) complex [CrL6]3−, an excitation energy transfer from the excited ruthenium (II) complex to the chromium (III) complex is observed. When the 4T2g state of [CrL6]3− is lower than the ``metal to ligand'' charge‐transfer singlet excited state 1CT of [Ru(bipy)3]2+, the phosphorescence of [Ru(bipy)3]2+ is completely quenched and the counterpart complex [CrL6]3− emits an enhanced phosphorescence even if it is irradiated at a wavenumber which excites exclusively [Ru(bipy)3]2+ but not [CrL6]3−. An efficient path of the excitation energy transfer in the double complex salt is the one from the 1CT state of [Ru(bipy)3]2+ to the 4T2g state of [CrL6]3−. When the 4T2g state of [CrL6]3− is higher than the 1C T state of [Ru(bipy)3]2+, the excitation transfer is competitive with emission and both [Ru(bipy)3]2+ and [CrL6]3− in the double complex salt phosphoresce Even such a chromium (III) complex behaves as an efficient quencher to excited [Rh(bipy)3]3+ in the double complex salt and emits a strong phosphorescence upon excitation of the [Rh(bipy)3]3+. Since the 4T2g state of such a chromium (III) complex is lower than the lowest singlet excited state of [Rh(bipy)3]3+, an excitation transfer takes place from the excited [Rh(bipy)3]3+ to [CrL6]3−.