Intramolecular Electron Transfer in Unsymmetric Azohetero-aromatic Radical Bridged Diruthenium(III) Set Up

The article demonstrated the stabilization of MLCT excited state in structurally characterized unsymmetric N,N/N,O⁻ donating azoheteroaromatic radical bridged (acac)₂Ru^III(μ-L3^•−)Ru^III(acac)₂ (2, S=1/2) [L3= deprotonated 5-(diethylamino)-2-(6-methybenzo[d] thiazol-2-yl)diazenylphenol, acac= electron rich acetylacetonate, N−N(azo) distance: 1.379(4) Å] via intramolecular electron transfer (IET) at the metal-azo interface. On the contrary, similar IET driven stabilization of MLCT excited state was failed to take place in N,N donating HL3 derived mononuclear counterpart (acac)₂Ru^II(HL3) (1, S=0) with unperturbed azo function (N=N distance: 1.3361(18) Å) in spite of the fact of facile Ru^II/Ru^III oxidation (0.28 V versus SCE) and azo reduction (−1.05 V versus SCE), presumably due to the influence of hydrogen bonding interaction at the back face of coordinated HL3. On the other hand, the use of electron poor bpy (2,2′-bipyridine) as a co-ligand facilitated exclusive generation of the mononuclear [(bpy)₂Ru^IIL3]ClO₄ ([3]ClO₄, S=0) encompassing N,O⁻-donating L3 with neutral azo group (N=N distance: 1.317(6) Å), unlike N,N donating HL3 in 1 due to the impact of σ-donating acac versus π-accepting bpy, which also devoided of IET step. DFT calculations further suggested the origin of multiple redox steps and absorption profiles of 1, 2, 3⁺.