Derivatives of the [Ru(bipy)(CN)(4)](2-) chromophore with pendant pyridyl-based binding sites: synthesis, pH dependent-luminescence, and time-resolved infrared spectroscopic studies

Reaction of K-4[Ru(CN)(6)] with 2,2':4',4 " -terpyridine (L-1) or 2,2':3',2 " :6 " ,2'''-quaterpyridine (L-2) in acidic aqueous methanol affords the c omplexes K-2[Ru(L-1)(CN)(4)] and K-2[Ru(L-2)(CN)(4)] respectively, both con taining the {Ru(bipy)(CN)(4)}(2-) chromophore but with pendant pyridyl or b ipyridyl units, respectively. Time-resolved IR analysis of K-2[Ru(CN)(4) (L -2)] in MeCN-D2O showed that the most intense CN stretching vibration shift ed to higher energy by ca. 50 cm(-1) after laser excitation, consistent wit h formation of a Ru(III)/(L-2)(.-) MLCT excited state for which the lifetim e measurement (38 +/- 5 ns, measured by TRIR) agrees reasonably well with t he value measured by luminescence methods (30 +/- 2 ns). Study of the pH de pendence of the absorption and emission spectra of the two complexes reveal ed the presence of two different effects arising from protonation of the pe ndant pyridyl/bipyridyl site (which occurs with pK(a) approximate to 3.1 in each case) and protonation of the cyanide ligands (which occurs with pK(a) approximate to 2 in each case). For K-2[Ru(L-1)(CN)(4)], protonation of th e pendant pyridyl unit results in the (MLCT)-M-1 excited state being lowere d in energy by ca. 1000 cm(-1), whereas at lower pH values (2.5-1), protona tion of the cyanide ligands raises the (MLCT)-M-1 excited state energy by o ver 2000 cm(-1). For K-2[Ru(L-2)(CN)(4)] in contrast, protonation of the pe ndant bipyridyl unit has no detectable effect on the (MLCT)-M-1 energy, as the pendant site is electronically decoupled from the complex core by a sub stantial twist between the free and coordinated bipy components of L-2; pro tonation of the cyanides at lower pH values however destabilises the (MLCT) -M-1 excited state. Protonation of the pendant pyridyl sites results in com plete (for [Ru(L-1)(CN)(4)](2-)) or near-complete (for [Ru(L-2)(CN)(4)](2-) ) quenching of the luminescence; possible reasons for this behaviour are di scussed. The crystal structures of the two related complexes [Ru((t)Bu(2)bi py)(2)(L-1)][PF6](2) and [Cl2Pt(mu -L-2)Ru(bipy)(2)][PF6](2) are also descr ibed to illustrate arguments about the conformations of L-1 and L-2.