pH-induced luminescence changes of chromophore-quencher tricarbonylpolypyridylrhenium(I) complexes with 4-pyridinealdazine

The new chromophore-quencher tricarbonylrhenium(I) complexes [Re(4,4′-X2-bpy)(CO)3(PCA)]+, [(4,4′-X2-bpy)(CO)3Re(μ-PCA)Re(CO) 3(4,4′-X2-bpy)]2+, and [(4,4′-X2-bpy)(CO)3ReI(μ-PCA)Ru II(NH3)5]3+ (X = Ph or CO 2Me and PCA = 4-pyridinealdazine) have been synthesized as their PF6- salts and characterized by spectroscopic, electrochemical, and photophysical techniques. In contrast to previously reported species with X = Me or H, these complexes emit at room temperature in CH3CN. The recovery of luminescence can thus be ascribed to the change of energy levels induced by adding electron-withdrawing substituents to the 2,2′-bipyridine ring, since the emissive ReII(X 2bpy-) excited state becomes much lower in energy than the ReII(PCA-) non-emissive excited state. For the mononuclear and symmetric dinuclear rhenium(I) complexes with X = CO 2Me, consecutive protonation of both the pyridinyl and imine N atoms of PCA in aqueous solution leads to unusual bell-shaped curves for both the absorption and the emission intensities vs. pH because of opposite effects on the electronic delocalization of PCA. This latter property can be employed to devise novel luminescent pH sensors of the on-off-on type. The corresponding unsymmetrical dinuclear complexes [(4,4′-X2-bpy)(CO) 3ReI(μ-PCA)RuIII(NH3) 5]4+ have been obtained in situ by oxidizing the [Re I,RuII] precursors in CH3CN solution. Spectral and electrochemical measurements with these complexes lead to the values of the metal-to-metal electronic coupling elements. © Wiley-VCH Verlag GmbH & Co. KGaA, 2007.