Delocalization-to-Localization Charge Transition inDivinylthiophene-Bridged Biruthenium Complexes as a Function of Lengthof the Bridging Chain by Both IR Spectroscopy and Theoretical Investigations

A series of biruthenium complexes linked by oligothiophene bridges of variable lengths (4a–4d) have been prepared to probe intramolecular electronic delocalization as a function of the chain length of the bridging ligand. In going from monothiophene to quaterthiophene-bridged biruthenium-vinyl complex radical cations (4a+–4d+), the transition from a positive charge delocalized state to a localized state is clearly displayed by IR spectroscopy and density functional theory calculations and the bridge contributing to the charge and the unpaired spin density exhibits good linear correlation with the length of the bridging chain. The shortest member 4a+ displays a charge delocalized mixed-valence property as a result of the strong electronic coupling between two ruthenium termini. In the longest member 4d+, the charge is nearly fully localized at the bridge and the mixed-valence property disappears. Therefore, for even longer systems, the charge should be exclusively localized on the bridge.