Environmentally Benign Solvents for the Formation of Electrochromic Metal-Organic Assemblies

The development of electrochromic coatings requires optimization of multiple parameters, including color contrast, stability, switching time, and optical transmittance. Within this optimization space a key challenge is establishing fabrication methods that utilize sustainable materials and green solvents. 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 counterion (PF6 − , Cl − , SO4 2−), without affecting their electrochromic performance. Although the iron-polypyridyl complexes contain large hydrophobic moieties, the counterions compensate for the lattice energy resulting in water-soluble materials. Furthermore, the use of two water-soluble palladium complexes, [PdCl4] 2− and trans-[(HO₃SPy)₂PdCl₂], allowed us to demonstrate that the resulting molecular assemblies can be formed entirely in water. Overall, the molecular assemblies (MAs) exhibited excellent optical and electrochromic properties with switching times between 0.9-1.8 seconds 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 MAs, which is further discussed in this report.