Influence of the protonic state of an imidazole-containing ligand on the electrochemical and photophysical properties of a ruthenium(II)-polypyridine-type complex

The synthesis and characterisation of Ru(bpy)2(PhenImHPh)]2+ where PhenImHPh represents the 2-(3,5-di-tert-butylphenyl)imidazo4,5-f]1,10]phenanthroline ligand are described. The compounds issued from the three different protonic states of the imidazole ring Ru(bpy)2(PhenImPh)]+ (I), Ru(bpy)2(PhenImHPh)]2+ (II) and Ru(bpy)2(PhenImH2Ph)]3+ (III) were isolated and spectroscopically characterised. The X-ray structures of Ru(bpy)2(PhenImPh)](PF6)H2O.6 MeOH, Ru(bpy)2(PhenImHPh)](NO3)2H2O.3 MeOH and Ru(bpy)2(PhenImH2Ph)](PF6)3 5 H2O are reported. Electrochemical data obtained on these complexes indicate almost no potential shift for the Ru(III/II) redox couple. Therefore a Coulombic effect between the imidazole ring and the metal centre can be ruled out. The monooxidised forms of I and II have been characterised by EPR spectroscopy and are reminiscent of the presence of a radical species. The emission properties of the parent compound Ru(bpy)2(PhenImHPh)]2+ were studied as a function of pH and both the lifetimes and intensities decreased upon deprotonation. Photophysical properties, investigated in the absence and presence of an electron acceptor (methylviologen), were distinctly different for the three compounds. Transient absorption features indicate that unique excited states are involved. Theoretical data obtained from DFT calculations in water on the three protonic forms are presented and discussed in the light of the experimental results.