Abstract
<jats:p>Redox-active films of Fe(II) polypyridyl complexes are attractive materials for electrochromic devices, molecular electronics, and energy storage, yet the influence of the deposition pathway and of the substrate on film formation and function remains incompletely understood. Starting from a single aminofunctionalized Fe(II) bis(terpyridine) precursor, we compare two electrodeposition routes, oxidative electropolymerization of the amine (Fe-poly) and electroreduction of the corresponding diazonium salt (Fe-diazo), on ITO-coated glass and flexible ITO-coated PET, using complementary electrochemical and spectroscopic techniques. Both routes offer well-defined multilayer films with distinct growth behaviour: Fe-poly films are predominantly physisorbed and reach higher loadings (≈1800 μC cm-2 after 20 deposition cycles), whereas the covalently anchored Fe-diazo films exhibit improved charge transport on PET-ITO. Efficient diazonium-based deposition is shown to require the processes associated with two interdependent cathodic pre-peaks, revealing a more complex growth mechanism than commonly assumed. Both materials display highly reversible Fe(III)/Fe(II) redox activity, strong resistance to overoxidation, and stable cycling over 1000 cycles. Operando UV-Vis spectroelectrochemistry demonstrates reversible modulation of the metal-to-ligand charge-transfer (MLCT) band and reproducible electrochromic switching on both rigid and flexible substrates. Finally, photoelectrochemically assisted oxidation of films deposited on photoactive ZnO nanowires affords a potential gain of approximately 1 V, independently of the anchoring chemistry, extending these coatings towards light-assisted flexible devices.</jats:p>