The films, [Rux|Coy]n and [Rux|Coy]n[Fe]6, were characterized by UV/Vis spectroscopy (Figure ). A representative UV/Vis spectrum of [Ru40|Co60]3 shows the expected metal-to-ligand charge transfer (MLCT) bands at λmax = 500 nm, corresponding to complex 1 (Figure , orange). Cobalt complex 2 absorbs only weakly in the visible region; therefore, its contribution to the color of the films is nihil (Figure ). The corresponding spectrum of [Ru40|Co60]3[Fe]6 (Figure , purple) was measured after six deposition steps of the iron complex 3 on top of [Ru40|Co60]3. A broad band at λmax = 570 nm was observed for the MLCT band of complex 3. A linear film growth is observed (Figure , inset). The films were further analyzed using transmission electron microscopy (TEM), combined with energy-dispersive X-ray spectroscopy (EDS), to estimate the thickness and to determine the elemental composition and the distribution of the metal complexes. A representative example of a TEM-EDS measurement of [Ru40|Co60]3 is shown in Figure . The samples were coated with a thin layer of carbon and platinum to avoid damage to the organic layer and milled using a focused ion beam (FIB) microscope. Analyses of the resulting cross-sections showed that the films are continuous: no cracks or defects are visible. The estimated thicknesses decrease from ~83 nm for [Ru10|Co90]3 to ~67 nm for [Ru60|Co40]3. Therefore, a fourth deposition cycle was added for these assemblies using solutions containing 70-90% of complex 1. This additional layer resulted in assemblies with thicknesses of ~92 nm for [Ru70|Co30]4 to ~71 nm for [Ru90|Co10]4. We observed an even distribution of various elements (Ru, Co, Pd, N, and Cl), indicating the formation of homogeneous structures. There is no evidence of physical clustering of complexes 1 or 2. A lower amount of complex 1 was observed on the surface than anticipated (~30% instead of 40%); concomitantly, the amount of complex 2 was higher than expected by ~10%. Remarkably, for all [Rux|Coy]n-layers the expected ratios between the metal complexes on the surfaces differ from the ratios used in the feed solutions by 8.6±2.4% (the black and red graphs in Figure ). This effect was confirmed by further analysis of the composition of [Rux|Coy]n using X-ray photoelectron spectroscopy (XPS) for five samples prepared by depositing solutions with increasing percentages of complex 1 from 10 to 50% in steps of 10%. Although XPS is a surface-analytical method, here, the structure of the MAs allowed us to reach a depth of 10-15 nm. The same trend as observed by TEM/EDS was also observed in XPS (the blue graph in Figure ). The data revealed equally lower and higher percentages of complexes 1 and 2, respectively, compared to the expected ratios. To verify that the feed solutions contained both the metal complexes in the expected ratios and to rigorously exclude any experimental errors, we also analyzed drop-casted samples on ITO by XPS. These measurements matched well with the anticipated ratios of complexes 1 and 2 (the green graph in Figure ). The experimentally observed ratios between the two isostructural complexes might reflect the relative rates of the binding of the complexes to the surface with k2 > k1. The origin of the higher reactivity of the cobalt complex (2) involves various factors (facial and meridional isomers, packing, and electronic properties). We have shown previously that these and other polypyridyl complexes can undergo sorting from solution to surface. We wish to better understand the influence of different ratios of the ruthenium (1) and cobalt (2) complexes on the redox chemistry of the top layer consisting of iron complexes (3). Therefore, the compositions of the feeding solutions were varied from 0 to 100% for complex 1 (in steps of 10%), the rest being complex 2. We first analyzed the effect of the electrochemical properties of [Rux|Coy]n on ITO using cyclic voltammetry (CV), with a potential window of 0.2 V to 1.8 V vs Ag/Ag + . Based on their electrochemical behavior, the results were divided into three distinct regions: A = 0-30%, B = 40-60%, and C = 70-100% of complex 1 in the feeding solutions (Scheme 1C). Here we focused on [Ru40|Co60]3 as a representative example of region B (Figure ). The first CV scan of [Ru40|Co60]3 shows an oxidation peak at Eox = 1.25 V with Q1st cy- cle = 0.26 mC cm -2 , which may be associated with the oxidation of both complexes 1 and 2 (Figure ). The reduction peak appeared at Ered = 1.15 V and represents the reduction of complex 1 (Ru 3+/2+ ). The 2 nd scan shows a clear decrease in the oxidation current (Q2nd cycle = 0.11 mC cm -2 , ΔQ = 0.15 mC cm -2 ) (Figure ). This ΔQ indicates charge trapping by cobalt complex 2 after the first cycle; consecutive CVs scans (2-5) show the redox behavior of only complex 1. The amount of charge involved in the oxidation processes during the first CV scan corresponds to one-electron oxidation of both ruthenium (1) and cobalt (2) complexes (M 2+/3+ ). Interestingly, the amount of charge involved for the consecutive cycles correlates well with the oxidation of only complex 1. In support of these assumptions, the ratios between Q2nd cycle (= Ru 2+/3+ ) and ΔQ (= Co 3+ , charge trapped) = 1:1.4 are in good agreement with the experimental ratio between complexes 1 (42%) and 2 (58%) = 1:1.4. Interestingly, a decrease of charges is not seen when measuring the CVs of films consisting of only ruthenium complex 1: [Ru]3. The differences between the first and second CV-cycle with respect to the detected current, shape, and redox potential are negligible (Figure This experiment shows that the differences between the first and second CVs in [Ru40|Co60]3 are due to the involvement of ruthenium complex 1 in the redox processes of the cobalt complexes 2. The UV/Vis measurements of [Ru40|Co60]3 are in agreement with the charge trapping by Co 2+ (Figure ). The absorption band at λ = 500 nm, representing ruthenium complex 1, disappears upon oxidation (light-orange to transparent); it regains its initial intensity when the film is reduced. Concurrently, the intensity of the π-π * transition peak at λ = 370 nm increases upon oxidation. After a full redox cycle, the intensity of this absorption band does not regain its initial intensity, indicating that the cobalt complexes (2) remained in their oxidized state [Co] 3+ .The CV of [Ru40|Co60]3[Fe]6, as representative of region B, shows a significant anodic peak (Q1st cycle = 0.93 mC cm -2 ), and the film becomes transparent (Figure , purple curve). These two observations are in agreement with the oxidation of all three complexes (1-3), and are supported by the electrochemical behavior of [Ru40|Co60]3 shown above (Figure ). Since no direct communication between the top layer of [Fe]6 and the ITO-surface can occur (Figure ), the larger amount of ruthenium complexes (1) facilitates the electron transfer from both the cobalt (2) and iron (3) complexes to the working electrode (Scheme 1C, Region B). The oxidative charge, calculated from the area under the curve, in the first CV cycle, is 4.7 times larger than the oxidative charge observed for the second cycle. The concurrent changes in the optical properties of the films indicate the storage of a positive charge in the top layer consisting of iron complexes (3) as well as the cobalt complexes (2). During the reduction of [Ru40|Co60]3[Fe]6, when scanning from 1.8 V to 0.2 V, only a single cathodic peak appeared at 1.19 V and the film turned light-orange as a result of the reduction of complex 1 (Ru 3+/2+ ) (Figure , purple curve). Continued cycling shows an anodic peak at 1.30 V (Q2nd cycle = 0.25 mC cm -2 ; orange curve). The cathodic charge is smaller by ~24% (Q2nd cycle = 0.19 mC cm -2 ). These anodic and cathodic charges become nearly equal upon increasing the scan rate from 100 mV/s to 300 mV/s. This effect is typical for an electrochemical-chemical (EC) process. Such process does not occur for [Ru40|Co60]3 (Figure ). Therefore, we believe that the EC process for [Ru40|Co60]3[Fe]6 is associated with complex 3. Further support for the occurrence of a chemical reduction pathway is indicated by applying open circuit potential for the oxidized film (1.8 V, 10 s) and measuring the absorbance by UV/Vis spectroscopy. About 10% of the charge was released during the first 20 s, which is about the same duration of the reductive potential during the CV scan. The trapped charges are released in ~20 min (t1/2 = 3.5 min). The film changed color from light orange to purple.