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

Abstract

New chromophore-quencher rhenium(I) tricarbonyls, of formulae [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)-RuII(NH3)5]3+, with X = Ph or CO2Me and PCA = 4-pyridinealdazine, have been synthesized as PF6- salts andcharacterized by spectroscopic, electrochemical and photophysical techniques. Contrasting 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(X2bpy-) excited state becomes much lower in energy than ReII(PCA-) non-emissive excited state. For the mononuclear and the symmetric dinuclear rhenium(I) complexes with X = CO2Me in aqueous solutions, consecutive protonation of both pyridinic and imine N atoms of PCA cause unusual bell-shaped curves of both the absorption and the emission intensities vs. pH, because of opposite effects on the electronic delocalization of PCA. Therefore, the latter property can be employed to devise novel luminescent sensors of acidity of the on-off-on type. The corresponding unsymmetric dinuclear complexes, of formula [(4,4’-X2-bpy)(CO)3ReI(μ- PCA)RuIII(NH3)5]4+, were obtained in situ by oxidizing the [ReI,RuII] precursors in CH3CN solution; from spectral and electrochemical measurements, the values of the metal-to-metal electronic coupling elements have been determined.