Spacer Conjugation and Surface Support Effects in Monolayer Electrochromic Materials

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.