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

Two luminescent heteroleptic RuII complexes with a 2,2′‐biimidazole (biimH2) ligand form doubly hydrogen‐bonded salt bridges to 4‐sulfobenzoate anions in single crystals. The structure of one of these cation‐anion adducts shows that the biimH2 ligand is deprotonated. Its 3MLCT luminescence band 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 biimH2 towards benzoate, leading to a pronounced redshift of the 3MLCT luminescence band. Such a significant and pressure‐tunable influence from an interaction in the second coordination sphere is unprecedented in artificial small‐molecule‐based systems. Cation–anion adducts are squeezed together by hydrostatic pressure, leading to delocalization of protons in the hydrogen bonds between these cation–anion adducts and changes in the luminescence of RuII complexes.