Dependence of Field Switched Ordered Arrays of Dinuclear Mixed-Valence Complexes on the Distance between the Redox Centers and the Size of the Counterions

trans-[(H₂NCH₂CH₂C⋮N)(dppe)₂Ru(C⋮C)₆Ru(dppe)₂(N⋮CCH₂CH₂NH₂)][PF₆]₂, 2[PF₆]₂, a derivative of trans-[Cl(dppe)₂Ru(C⋮C)₆Ru(dppe)₂Cl] functionalized for binding to a silicon substrate, has been prepared and characterized spectroscopically, electrochemically, and with a solid state, single-crystal structure determination. Covalent binding via reaction of one amine group to a boron-doped, smooth Si−Cl substrate is verified by XPS measurements and surface electrochemistry. Vertical orientation is demonstrated by film thickness measurements. Synthesis of the 2[PF₆]₃ mixed-valence complex on the surface is established by electrochemical techniques. Measurement of the ac capacitance of the film at 1 MHz as a function of voltage across the film with a pulse−counter pulse technique demonstrates controlled electric field generation of the two stable mixed-valence forms differing in the spatial location of one electron, that is, switching. As compared to [trans-Ru(dppm)₂(C⋮CFc)(NCCH₂CH₂NH₂)][PF₆][Cl], 1[PF₆][Cl], the magnitude of the capacitance signal per complex observed on switching is shown to increase with increasing distance between the metal centers. Additional experiments on 1[X][Cl] show that the potential for switching 1[X][Cl] increases in the order [X]^- = [SO₃CF₃]^- < [PF₆]^- < [Cl]^-. A simple electrostatic model suggests that the smaller is the counterion, the greater is the perturbation of the metal sites and the larger is the barrier for switching.