New mono- and dinuclear rhenium(I) tricarbonyls, of formulas [Re(bpy)( CO)(3)(PCA)](+) (1), [(bpy)(CO) 3ReI(PCA) Re'- CO)(3)(bpy)](2+) (2), and [(bpy)(CO)(3)Re-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=Ns NdC- chain of PCA, as indicated by the p K a values of the ground states, obtained by absorption measurements, which are ca. 3 orders of magnitude lower than the p K 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 (pKa*) 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) Re-3(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.