Proton-Induced Luminescence of Mono- and Dinuclear Rhenium(I) Tricarbonyl Complexes Containing 4-Pyridinealdazine

New mono- and dinuclear rhenium(I) tricarbonyls, of formulas [Re(bpy)(CO)₃(PCA)]⁺ (1), [(bpy)(CO)₃Re^I(PCA)Re^I(CO)₃(bpy)]²⁺ (2), and [(bpy)(CO)₃Re^I(PCA)Ru^II(NH₃)₅]³⁺ (3) (bpy = 2,2‘-bipyridine, PCA = 4-pyridinecarboxaldehydeazine), have been synthesized as PF₆^- salts and characterized by spectroscopic, electrochemical, and photophysical techniques. These species do not emit at room temperature in CH₃CN; however, in aqueous solutions, a decrease in pH induces luminescence in all of them, due to protonation of one of the N atoms of the −CNNC− chain of PCA, as indicated by the pK_a values of the ground states, obtained by absorption measurements, which are ca. 3 orders of magnitude lower than the pK_a value of the pyridine N of PCA in complex 1. On the other hand, the values of pK_a* of the excited states, obtained by emission measurements, of complexes 1 and 2 are similar (pK_a* = 2.7 ± 0.1 at I = 0.1 M) and higher than those of the corresponding ground states. At low values of pH, chemical decomposition takes place rapidly in complex 3, but not in 1 and 2, supporting the possible use of these latter species as luminescent sensors of pH. The heterodinuclear complex, of formula [(bpy)(CO)₃Re^I(PCA)Ru^III(NH₃)₅]⁴⁺, was obtained by bromine oxidation of the [Re^I, Ru^II] precursor in CH₃CN solution; from spectral and electrochemical measurements, the recombination charge-transfer reaction [Re^II, Ru^II ] → [Re^I, Ru^III], which occurs after photoexcitation, is predicted to lie in the Marcus inverted region.