A comprehensive photophysical study has been carried out on the two complexes [(bpy)(2)Ru(4,4 ' -PE-bpy)](2+) and [(bpy)2Ru(5,5 ' -PE-bpy)](2+) (44Ru and 55Ru, respectively, where bpy = 2.2 ' -bipyridine and PE = phenyleneethynylene). The objective of this work is to determine the effect of the phenyleneethynylene substituents on the properties of the metal-to-ligand charge-transfer excited state. The complexes have been characterized by using UV-visible absorption, photoluminescence, and UV-visible and infrared transient absorption spectroscopy. The results indicate that the MLCT excited state is localized on the PE-substituted bpy ligands. Moreover, the photophysical data indicate that in the MLCT excited state the excited electron is delocalized into the PE substituents and the manifestations of the electronic delocalization are larger when the substituents are in the 4,4 ' -positions.