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.