Proton-induced tuning of electrochemical and photophysical properties in mononuclear and dinuclear ruthenium complexes containing 2,2'-bis(benzimidazol-2-yl)-4,4'-bipyridine: Synthesis, molecular structure, and mixed-valence state and excited-state properties

A series of mono- and dinuclear Ru(bpy)(2) complexes (bpy = 2,2'-bipyridine) containing 2,2'-bis(benzimidazol-2-yl)-4,4'-bipyridine (bbbpyH(2)) were prepared. The mononuclear complex [Ru(bpy)2(bbbpyH(2))](ClO4)(2) . CH3OH . 4H(2)O was characterized by an X-ray structure determination. Crystal data are as follows: triclinic, space group <P(1)over bar>, a = 14.443(4) Angstrom, b = 15.392(4) Angstrom, c = 11.675(2) Angstrom, alpha = 101.44(2)degrees, beta = 107.85(2)degrees, gamma = 96.36(2)degrees V = 2380(1) Angstrom(3), Z = 2. The coordination geometry of the ruthenium(II) ion is approximately octahedral. The dihedral angle between the two pyridyl rings in bbbpyH(2) is 9.4(3)degrees, which is close to coplanar, in the complex. Mono- and dinuclear complexes exhibit broad charge-transfer absorption bands at 420-520 nm and emission at 660-720 nm in CH3CN solution with lifetimes of 200-800 ns at room temperature. Transient difference absorption spectra and resonance Raman (rR) spectra were used to assign the charge-transfer bands in the 420-520 nm region and to identify the lowest excited states. Both absorption and emission spectra are sensitive to solvent and solution pH. Deprotonation of the dinuclear complex raises the energies of the pi* orbitals of the bbbpyH(2) ligand, so that they become closer in energy to the pi* orbitals of bpy. The intervalence band of [(bpy)(2)Ru(bbbpyH(2))Ru(bpy)(2)](5+) is observed at 1200 nm (epsilon 170 M(-1) cm(-1)) in CH3CN. The value of the electronic coupling matrix element, H-AB, was determined as 120 cm(-1). Upon deprotonation, the IT band was not observed. It is therefore concluded that a superexchange pathway occurs predominantly via the Ru(II) d pi-bbbpyH(2) pi* interaction, since deprotonation decreases the interaction. The role of the intervening fragments in the bridging ligand is discussed from the viewpoint of orbital energies and their orbital mixing with Ru d pi orbitals.