Controlling Second Coordination Sphere Effects in Luminescent Ruthenium Complexes by External Pressure

Two luminescent heteroleptic Ru(II) complexes with a 2,2’ - biimidazole (biimH 2) ligand form doubly hyrogen-bonded salt bridges to 4-sulfobenzoate anions in single crystals. The structure of one of these cation-anion adducts shows that the biimH 2 ligand is deprotonated. Its 3MLCT luminescence bands does not shift significantly under the influence of an external hydrostatic pressure, a behavior typical for these electronic transitions. In contrast, hydrostatic pressure on the other crystalline cation-anion adduct induces a shift of proton density from the peripheral N-H groups of biimH 2 towards benzoate, leading to a pronounced red- shift of the 3MLCT luminescence band. Such a significant and pressure-tuneable influence from an interaction in the second coordination sphere is unprecedented in artificial small molecule-based systems.