Electrochemical Behavior of Sequentially Assembled Homo and Heterolayer Molecular Films Based on Dinuclear Ruthenium Complexes

Sequential assembly of layers containing redox-active dinuclear Ru complexes that differ in their redox potential on ITO substrates afforded a series of layered structures with different potential gradients. The Ru complexes Ru-NP and Ru-CP contain tetrapodal phosphonate anchors and bridging 2,3,5,6-tetra(2-pyridyl)pyrazine or 1,2,4,5-tetra(2-pyridyl)benzene ligands, respectively. Oxidation potentials of E1/2 = +0.83 and +1.04 V vs. Fc+/Fc (Ru-NP) and E1/2 = −0.37 and 0.09 V (Ru-CP) were observed for the Ru(II)-Ru(II)/Ru(II)-Ru(III) and Ru(II)-Ru(III)/Ru(III)-Ru(III) couples. As the Ru-NP and Ru-CP units contain several accessible redox-active electronic states, electron transfer (ET) blockage and mediation occurred at the Ru-NP/Ru-CP heterojunction on account of the large potential gradient. An examination of the electrochemical behavior of the multilayer diblock ITO||(Ru-CP)n|(Ru-NP)n (n = 1–5) heterofilms revealed that in ITO||(Ru-CP)4|(Ru-NP)4, the Ru(III)-Ru(III)/Ru(III)-Ru(IV) couple is involved in the electron mediation pathway on the Ru-NP/Ru-CP heterojunction. In the case of the multilayer triblock ITO||(Ru-NP)n|(Ru-CP)n|(Ru-NP)n (n = 4) heterofilms, two catalytic peaks were observed, indicating that the inner and outer Ru-NP/Ru-CP junctions display two different catalytic ET processes. In the alternating Ru-NP/Ru-CP multilayer heterofilms, the ET mediation processes are governed by the sequence order of the Ru-NP/Ru-CP heterojunction attached to the ITO electrode. In multilayer homo- and heterofilms, the dependence of the ET distance on the layer number is discussed on the basis of potential-step chronoamperometry (PSCA) results. In the multilayer Ru-NP and Ru-CP homofilms, relatively low β values were observed, while in the multilayer heterofilms, the ET rate of the catalytic wave was not affected by the number of layers, as the energy gap between the Ru-NP/Ru-CP units is the rate-determining factor. Therefore, the molecular sequence in such multilayer heterofilms, composed of layers containing Ru-NP or Ru-CP complexes, plays an important role.

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