This manuscript highlights a beginner-to-intermediate level scope of electrochemical impedance spectroscopy (EIS), which is an indispensable, non-destructive electrochemical technique that can be applied for materials characterization.

In this study, we present a range of efficient highly durable electrochromic materials that demonstrate excellent redox and lifetime stability, sufficient coloration contrast ratios, and the best-in-class electron-transfer constants. The materials were formed by anchoring as little as a monolayer of predefined iron complexes on a surface-enhanced conductive solid support. The thickness of the substrate was optimized to maximize the change in optical density. We demonstrate that even a slight change in molecular sterics and electronics results in materials with sufficiently different properties. Thus, minor changes in the ligand design give access to materials with a wide range of color variations, including green, purple, and brown. Moreover, ligand architecture dictates either orthogonal or parallel alignment of corresponding metal complexes on the surface due to mono- or bis-quaternization. We demonstrate that monoquaternization of the complexes during anchoring to the surface-bound template layer results in redshifts of the photoabsorption peak. The results of in-solution bis-methylation supported by density functional theory calculations show that the second quaternization may lead to an opposite blueshift (in comparison with monomethylated analogs), depending on the ligand electronics and the environmental change. It is shown that the variations of the photoabsorption peak position for different ligands upon attachment to the surface can be related to the calculated charge distribution and excitation-induced redistribution. Overall, the work demonstrates a well-defined method of electrochromic material color tuning via manipulation of sterics and electronics of terpyridine-based ligands.

The article describes novel electrochromic materials (ECMs) that are based on a monolayer consisting of two or three isostructural metal complexes of 4′-(pyridin-4-yl)-2,2’:6′,2’’-terpyridine simultaneously deposited on surface-enhanced support. The support was made by screen printing of indium tin oxide (ITO) nanoparticles on ITO-glass and has a surface area sufficient for a monolayer to give color visible to the naked eye. The ability to separately electrochemically address the oxidation state of the metal centers on the surface (i.e., Co2+/Co3+, Os2+/Os3+, and Fe2+/Fe3+) provides an opportunity to achieve several distinct color-to-color transitions, thus opening the door for constructing monolayer-based multicolor ECMs.

Electrochromic (EC) materials that change their color under applied voltage are a rapidly growing segment of "smart" materials. Recent and potential applications of EC materials include "smart" windows, a range of optoelectronic, energy conversion, and indication devices that require miniaturization, easy integration, and sustainable development. This can be achieved by forming just a monolayer of EC molecules on a conductive support with a large surface area (i.e., surface-enhanced conductive support). In this study, we have developed a range of supports by screen printing two commercial indium tin oxide (ITO-30 and ITO-50) nanoparticles and synthesized fluorine-doped tin oxide (FTO) nanoparticles on ITO/glass and FTO/ glass substrates. We have discovered the influence of spacer conjugation (single, double, and triple bond) in the terpyridine-based iron complexes anchored as EC monolayers, on the properties of EC materials. Resulting materials demonstrate fast charge transfer kinetics and a significant color difference that depends on both the nature of the ligand and substrate. Solid-state EC devices (ECDs) demonstrate a noticeable optical difference in colored and bleached states (ΔOD), enhanced spectroelectrochemical stability, and exceptional coloration efficiency. Electron mobility and EC memory are heavily impacted by the substrate. Moreover, sufficient values of pseudocapacitance and the ability to power up an LED suggest potential applications of these materials in dual-function EC supercapacitors. Density functional theory (DFT) and time-dependent density functional theory (TDDFT) calculations have been employed to support the experimental findings in terms of geometries, electronic structure, interpretation of photospectra, charge distributions, and transfer, revealing significant variations between the ligands.

Novel electrochromic (EC) materials were developed and formed by a two-step chemical deposition process. First, a self-assembled monolayer (SAM) of 2,2':6',2″-terpyridin-4'-ylphosphonic acid, L, was deposited on the surface of a nanostructured conductive indium-tin oxide (ITO) screen-printed support by simple submerging of the support into an aqueous solution of L. Further reaction of the SAM with Fe or Ru ions results in the formation of a monolayer of the redox-active metal complex covalently bound to the ITO support (Fe-L/ITO and Ru-L/ITO, respectively). These novel light-reflective EC materials demonstrate a high color difference, significant durability, and fast switching speed. The Fe-based material shows an excellent change of optical density and coloration efficiency. The results of thermogravimetric analysis suggest high thermal stability of the materials. Indeed, the EC characteristics do not change significantly after heating of Fe-L/ITO at 100 °C for 1 week, confirming the excellent stability and high EC reversibility. The proposed fabrication approach that utilizes interparticle porosity of the support and requires as low as a monolayer of EC active molecule benefits from the significant molecular economy when compared with traditional polymer-based EC devices and is significantly less time-consuming than layer-by-layer growth of coordination-based molecular assemblies.