A bibenzimidazole-containing ruthenium(II) complex acting as a cation-driven molecular switch.

Multinuclear ruthenium polypyridyl complexes are currently the subject of extensive investigations, their photophysical properties making them ideal components for photochemically and electrochemically driven molecular devices.1 Among these compounds, mononuclear ruthenium complexes containing biimidazole or bibenzimidazole (bibzimH2) type ligands2 have been shown to be excellent building blocks for the synthesis of heteronuclear complexes of the type {(bpy)nRu[(bibzim)M(bpy)2]3-n} (M ) Ru, Os, Ni, Co).3-5 This behavior makes these complexes prime candidates for application as luminophores for the detection of metal ions. The luminophore approach to the development of sensors has been widely used.6 Ruthenium polypyridyl complexes have been proposed as luminescent sensors for the detection of anions.7 In this contribution, we wish to report the interaction of the nonluminescent compound [Ru(tbbpy)2(bibzim)], 1 (see Figure 1), with a series of metal ions in solution. The results obtained show that the emission of 1 is switched on by the presence of metal ions such as Zn(II), Mg(II), and Cu(I). It is also shown that both the emission intensity and the emission wavelength are dependent on the concentration and the nature of the metal ion. Comparable results regarding the tuning of the emission maxima were recently obtained for substituted phenanthrolines.8 To the best of our knowledge, there is only one other example where the emission intensity of a ruthenium complex is increased by binding of another metal.9 The X-ray structure of 1 and of its protonated analogue [Ru(tbbpy)2(bibzimH2)], 2, are also reported.