Ultrafast electron transfer dynamics in ruthenium polypyridyl complexes with a π-conjugated ligand

The excited-state dynamics of two mixed-ligand mononuclear ruthenium(II) complexes, [(bpy)(2)RuL(DQ)](4+) (where bpy = 2,2'-bipyridine, L(DQ) = 1-[4-(4'-methyl)-2,2'-bipyridyl)]-2-[4-(4'-N,N'-tetramethylene-2,2'-bipyridinium]) and [(bpy)(2)RuL](2+) (where L = 1, 2-bis[4-(4'-methyl)-2,2'-bipyridyl)]ethene), were investigated by femtosecond transient absorption spectroscopy. Photoexcitation of the [(bpy)(2)RuL(DQ)](4+) complex at three separate pump wavelengths leads to a common charge-separated state consisting of Ru(3+) and an excited electron delocalized over the extended π-system centered on the ethenyl moiety of the L(DQ) ligand. In [(bpy)(2)RuL](2+), the excited electron is unable to delocalize throughout the π system and remains on the bipyridyl end of ligand L closest to the ruthenium atom. Vibrational cooling in the charge-separated state of [(bpy)(2)RuL(DQ)](4+) indicates that this state is formed faster than excess energy can be dispersed to the solvent and orders of magnitude more rapidly than in previously studied ruthenium-diquat or ruthenium-viologen dyads with nonconjugated linkers.