Compositionally Controlled Electron Transfer in Metallo-Organics

We demonstrate here the assembly of a nanolayer of electrochromic iron complexes on the top of composite layers of cobalt and ruthenium complexes. Depending on the ratio of the latter two complexes, we can tailor materials that show different electron transport pathways, redox activities, and color transitions. No redox activity of the top layer, consisting of iron complexes, is observable when the relative amount of the ruthenium complexes is low in the underlying composite layer because of the insulating properties of the isostructural cobalt complexes. Increasing the amount of ruthenium complexes opens an electron transport channel, resulting in charge storage in both the cobalt and iron complexes. The trapped charges can be chemically released by redox-active ferrocyanide complexes at the film–water interface.

UV/Vis Spectroscopy. UV/Vis spectra were recorded on a Cary 100 spectrophotometer. The absorbance was measured using the Cary Win UV−Scan application program, version 3.00 (182) by Varian (350−800 nm). The transmittance was measured using the Cary WinUV−Kinetics application program, version 3.00 (182) by Varian. Bare substrates were used to compensate for the background absorption.

Electrochemical Characterization.
Electrochemical experiments were carried out using a CHI760E electrochemical workstation. The following configuration of the electrochemical cell was used: ITO/PET (0.5 cm × 1.5 cm) served as the working electrode, Ag/Ag + was used as the reference electrode, and a Pt wire was used as the counter electrode. Tetrabutylammonium hexafluorophosphate (TBAPF6) in ACN (0.1 M) was used as the supporting electrolyte. Spectroelectrochemistry measurements were performed in a N2-filled glovebox for Region C.
Focused Ion Beam (FIB) Microscopy. The lamella was prepared using a dual beam FIB-SEM Helios 600. The images were taken at cross-sections that were generated by milling the sample with a 30 keV Ga + FIB. The sample was first locally coated with a 150−200 nm-thick layer of platinum using electron beam-assisted deposition, which was followed by the ion beam-assisted deposition of a 500−600 nmthick layer of platinum. This coating protects the molecular assembly from ion-beam damage, providing a clean edge of the cross-section. High angle annular dark field (HAADF) STEM imaging and EDS measurements were carried out using a Thermo Fisher Scientific Themis Z TEM, which is double aberration-corrected and equipped 3 S with a Super-X large solid angle X-ray detector for EDS. The measurements were carried out using 200 kV acceleration voltage.

X-ray photoelectron spectroscopy (XPS).
The measurements were carried out with a Kratos AXIS ULTRA system using a monochromatic Al Ka X-ray source (hn = 1486.6 eV) at 75 W and detection pass energies ranging between 20 and 80 eV. Curve fitting analysis was based on linear or Shirley background subtraction and application of Gaussian-Lorenzian line shapes.  Figure S9C). Note that as the relative amount of complex 1 is higher, the thickness of the film is lower for the same number of deposition steps, allowing direct communication between iron complex 3 and the electrode surface ( Figure S2). This effect is manifested by the additional reduction peak at ~1.0 V. Therefore an additional deposition step of complexes 1 and 2 was applied. There is a neglectable contribution of the cobalt complex (2) to the oxidative current ( Figure S9C). Thus, the CVs resemble the electrochemical data observed for a layer consisting exclusively of ruthenium complex 1. Upon oxidation of complex 1 (Vox = 1.25 V), the thin film becomes transparent and regains its initial orange color during the reduction process (Vred = 1.15 V). Continued cycling resulted in repetitive CV curves resembling the second cycles, each by current, shape, and redox potential for regions B and C.
After adding a layer of iron complex 3 (six deposition cycles), the following results were observed. For [Ru90|Co10]4[Fe]6, CV measurement from 0.2 V to 1.8 V showed two peaks at 1.2 V and 1.4 V, associated with the oxidation of both the ruthenium (1) and iron complexes (3). For the reduction, only one peak was observed, corresponding to the formation of divalent 1 (Ru 3+ à 2+ ), whereas positive charges remained trapped as Fe 3+ in the top layer (Figure S9C'). The area beneath the first oxidation peak (Q1st cycle = 2.20 mC cm -2 ) corresponds to the oxidation of both