Interpreting solvent effects on complex dihydrogenphosphate binding using amidobenzimidazole functionalised emissive ruthenium diimine complexes

The photophysical behaviour of two amidobenzimidazole complexes, [Ru(bpy)₂(bbiab)](PF₆)₂ and [Ru(phen)₂(bbiab)](PF₆)₂ (where bbiab = 4,4'-bis(<i>N</i>-1<i>H</i>-benzimidazol-2-yl-carboxamidyl)-2,2'-bipyridine) in a variety of solvents suggest that the ³MLCT state is sensitive to the introduction of small aliquots of solvent (dilution) resulting in distinct incremental changes to the absorption spectra, and quenching in emission. This solvent dependency is more pronounced in polar media and is proposed to be a consequence of "dimerization" of these complexes, which is supported by ¹H-DOSY-NMR spectroscopy and gas phase DFT studies on the free ligands. The pH behaviour of these complexes indicates that protonation of the imidazole results in changing the emissive state from the bpy and phen ligands to bbiab. Significant quenching in both complexes is seen on the introduction of acetate, which ¹H-NMR spectroscopy suggests is association with the bbiab imidazole groups. On the introduction of HSO₄^- and H₂PO₄^- salts, the photophysical response suggests several distinct interactions are present as anion concentration increases, with both the amide and imidazole groups able to act as hydrogen-bond donors and acceptors. These effects are solvent dependent implying a complex set of equilibria, ranging from protonation of the imidazole in non-protic solvents, dissociation of the complex dimer, a simple one-to-one species, and a combined bis(dihydrogenphosphate)-complex anion, with speciation reliant on complex charge, associated ligand hydrophobicity, and solvent.