INTRAMOLECULAR, PHOTOINDUCED ELECTRON-TRANSFER IN RUTHENIUM(II) BIPYRIDINE-QUINONE COMPLEXES

Intramolecular electron transfer quenching following metal-to-ligand charge transfer (MLCT) excitation of complexes of Ru-II containing the ligand N-((4'-methyl-2,2'-bipyridyl)-4-methyl)-9,10-anthraquinone-2-carboxamide (bpy-AQ) have been investigated by transient emission and absorption measurements. For [Ru-II(bpy)(2)(bpy-AQ)](2+) (bpy is 2,2'-bipyridine) in acetonitrile at room temperature, partial quenching of the initial (Ru-II(bpy)(2)) excited state occurs as evidenced by decreased emission quantum yields (0.062 vs 0.011) and lifetimes (980 vs 158 ns) relative to [Ru-III(bpy(.-))(bpy)(dmb)](2+*) (dmb is 4,4'-dimethyl-2,2'-bipyridine). In [Ru-II(tmb)(2)(bpy-AQ)](2+)(tmb is 4,4',5,5'-tetramethyl-2,2'-bipyridine), electron transfer quenching is nearly complete (Phi(cm) = 0.002, tau = 6 ns). In neither case is there evidence for AQ(.-) (lambda(max) = 590 nm) by transient absorption measurements, and it is concluded that the expected redox-separated states, e.g., [Ru-III(bpy)(2)(bpy-AQ.(-))](2+), undergo rapid back electron transfer to return to the ground states. MLCT excitation of the donor-chromophore-acceptor complex [Ru-II(bpy-AQ)(2)(bpy-PTZ)](2+) [bpy-PTZ is {10- [(4'-methyl-2,2'-bipyridin-4-yl)methyl] phenothiazine}] leads to formation of the redox-separated state [Ru-II(bpy-AQ)(bpy-AQ(.-))(bpy-PTZ(.+))](2+) (lambda(max)(-PTZ(.+)) = 510 nm) in approximately 40% yield. It undergoes back-electron-transfer to give the ground state with k (25 degrees C, CH3CN) = 6.7 x 10(6) s(-1).