Transparent Ion-Exchange Membrane Exhibiting Intense Emission under a Specific pH Condition Based on Polypyridyl Ruthenium(II) Complex with Two Imidazophenanthroline Groups.

The development and the photophysical behavior of a transparent ion-exchange membrane based on a pH-sensitive polypyridyl ruthenium(II) complex, [(bpy)Ru(Hbpib)Ru(bpy)](ClO) (bpy = 2,2'-bipyridine, Hbpib = 1,4-bis([1,10]phenanthroline[5,6-]-imidazol-2-yl)benzene), are experimentally and theoretically reported. The emission spectra of [(bpy)Ru(Hbpib)Ru(bpy)]@Nafion film were observed between pH 2 and pH 11 and showed the highest relative emission intensity at pH 5 ( = 594.4 nm). The relative emission intensity of the film significantly decreased down to 75% at pH 2 and 11 compared to that of pH 5. The quantum yields () and lifetimes () showed similar correlations with respect to pH, = 0.13 and = 1237 ns at pH 5, and = 0.087 and = 1014 ns and = 0.069 and = 954 ns at pH 2 and pH 11, respectively. These photophysical data are overall considerably superior to those of the solution, with the radiative- () and non-radiative rate constants () at pH 5 estimated to be = 1.06 × 10 s and = 7.03 × 10 s. Density functional theory calculations suggested the contribution of ligand-to-ligand- and intraligand charge transfer to the imidazolium moiety in Ru-Hbpib species, implying that the positive charge on the Hbpib ligand works as a quencher. The Ru-Hbpib species seems to enhance non-radiative deactivation by reducing the energy of the upper-lying metal-centered excited state. These would be responsible for the pH-dependent "" emission behavior.