A Green Processing Strategy for the Formation of Electrochromic Metal-Organic Assemblies

A key challenge in the development of electrochromic coatings is establishing fabrication methods that utilize sustainable materials and green solvents. This must be achieved while also optimizing multiple parameters, including color contrast, stability, switching time, and optical transmittance. In this study, we report water-soluble, redox-active iron-polypyridyl complexes as building blocks for electrochromic coatings and devices fabricated by layer-by-layer spin coating and automated spray coating. The solubility of these complexes in polar and nonpolar media was modulated by tuning the counterions (PF₆^-, Cl^-, and SO₄^2-), without affecting the electrochromic performance of their assemblies. Furthermore, the use of two water-soluble palladium complexes, [PdCl₄]^2- and trans-[Pd(3-SO₃H-py)₂Cl₂], allowed us to demonstrate that the resulting molecular assemblies can be formed using aqueous solutions. Overall, the molecular assemblies exhibited excellent optical and electrochromic properties with switching times between 0.9 and 1.8 s and up to 95% charge retention even after 2000 cycles. The practicality of our approach was demonstrated by fabricating laminated devices that retain the physicochemical properties of the original molecular assemblies.