Ruthenium(II) diimines, which have been extensively studied for their remarkable photo physical properties, also have a long history as luminescent sensors. Interest in this laboratory focuses on compounds with peripheral base sites, such as (bis (2,2'-bipyridine)(2,3-bis(2-pyridyl)pyrazine) ruthenium(II), [(bpy)₂Ru(dpp)]²⁺. Excitation populates a metal-to-ligand charge transfer state localized on the dpp ligand, which leads to an incredible increase in basicity. In the ground state, the peripheral dpp pyridine is the strongest base, pK_a=1.62, whereas in the excited state, the pyrazyl nitrogen is the strongest base. This inversion of basicity on excitation raises the possibility of intramolecular proton transfer, which complicates the detailed characterization of excited state proton transfer dynamics, but raises potential applications as luminescent sensing device.

To gain a better understanding of the dynamics of the excited state protonation and particularly its thermodynamics, the complex (bis(2,2'-bipyridine) (2-(2-Pyridyl) pyrazine) ruthenium(II), [(bpy)₂Ru(pypz)]²⁺ has been synthesized. Here we describe the excited- state acid-base properties of this complex.