Excited-state coordination chemistry: excited-state basicity of...

The proton dependencies of the absorption and emission spectra of bis(2,2'-bipyridyl)(2,3-bis(2-pyridyl)pyrazine)ruthenium(II), (bpy)(2)Ru(dpp)(2+) indicate that population of the dpp-localized MLCT state increases the basicity of dpp peripheral nitrogens. NMR spectra reveal the protonation of the peripheral dpp pyridine in the ground state, pK(a) of 1.12 +/- 0.03, occurs intermediate between the changes evident in the absorption and emission spectra. As a result, the emissivity of aqueous solutions of (bpy)(2)Ru(dpp)(2+) as a function of [H(+)] derives from two emissive species: the unprotonated complex and the monoprotonated complex [(bpy)(2)Ru(dppH(py))](3+) with the proton attached to the peripheral dpp pyridine. Although protonation in the MLCT state generally quenches the emission, the emissivity of the monoprotonated complex, albeit weak, is attributed to the asymmetric distribution of the charge in the MLCT state. The majority of the transferred charge resides at the peripheral pyrazinyl nitrogen, and excited-state acid-base chemistry occurs predominantly at this site. Nonetheless, ground-state protonation of the peripheral dpp pyridine dramatically increases the nonradiative decay rate and significantly influences subsequent excited-state protonation processes. Protonation of the excited state changes from a bimolecular process to a combination of inter- and intramolecular processes where the proton transfers from the dpp pyridyl nitrogen to the dpp pyrazinyl nitrogen and from the surrounding aqueous solvent shell. Energetically, changes in the absorption spectra originally attributed to the first protonation of the complex and from which the DeltapK(a) of the excited state have been estimated, in fact, correspond to the second protonation of the complex.