A METAL-TO-LIGAND CHARGE-TRANSFER EXCITED-STATE OF A BIRUTHENIUM(II) COMPOUND BRIDGED BY 2,6-BIS(2-PYRIDYL)BENZODIIMIDAZOLE

A charge-transfer (CT) excited state of a binuclear ruthenium(II) compound with the tetradentate bridging ligand 2,6-bis(2-pyridyl)benzodiimidazole (dpimbH2) was studied by means of emission and transient absorption spectroscopy at 77-300 K. Absorption spectra of the lowest excited states of a mononuclear compound, Ru(bpy)2(dpimbH2)2+, and a binuclear compound, [Ru(bpy)2]2(dpimbH2)4+, were quantitatively determined. The lowest excited states of the Ru(II) compounds were assigned as Ru-to-dpimbH2 CT, and their spectra were compared with the absorption spectra of both the oxidized species (Ru(bpy)2(dpimbH3)4+) and the reduced species ([Ru(bpy)2](dpimbH2)3+). The 58 meV lower phosphorescence energy of [Ru(bpY)2]2(dpimbH2)4+ compared with that of the mononuclear compound was accounted for in terms of stabilization of the d(pi)-orbital (Ru(III)) and the pi*-orbital (dpimbH2). The extent of electronic interaction between ruthenium(III) and ruthenium(II) ions was estimated for the CT states of [Ru(bpY)2]2(dpimbH2)4+ and a mixed-valence compound of [Ru(bPY)2]2(dpimbH2)5+ by using emission spectroscopy, Ru(II)-to-Ru(III) CT absorption spectroscopy, and differential voltammetry.