One-Step Synthesis of a Three-Dimensionally Hyperbranched Fe(II)-Based Metallo-Supramolecular Polymer Using an Asymmetrical Ditopic Ligand for Durable Electrochromic Films with Wide Absorption, Large Optical Contrast, and High Coloration Efficiency

Abstract

For the development of durable electrochromic (EC) materials with wide absorption, large optical contrast, and high coloration efficiency, a threedimensionally hyperbranched Fe(II)-based metallo-supramolecular polymer (polyFe-3D) was one-step synthesized by the complexation of an Fe(II) salt and an asymmetrical ditopic ligand (L) containing terpyridine (tpy) and phenanthroline (phen) moieties, according to the formation of a 1:2 complex of Fe(II) and tpys (Fetpys) and a 1:3 complex of Fe(II) and phens (Fe-phens). The formation of Fe-tpys and Fe-phens was confirmed by the appearance of two metal-to-ligand charge transfer (MLCT) absorptions in the visible area during the titration experiment. The molecular weight (M w ) of polyFe-3D was determined to 3.96 × 10 5 Da by a size-exclusion chromatography−viscometry measurement. PolyFe-3D with the two different Fe(II) complexes (Fe-typs and Fe-phens) exhibited a pair of redox waves for Fe(II/III) (E 1/2 = 0.74 V vs Ag/Ag + ) reversibly in the cyclic voltammograms. The linear relationship between the peak current and the root of the scan rate during redox suggested a diffusion-controlled redox reaction. The spin-coated polymer film showed reversible EC changes between magenta and pale green, caused by the redox of Fe(II/III). A wide visible region from 430 to 640 nm was covered with the two MLCT absorptions of the Fe-tpys and Fe-phens moieties. The broad MLCT absorption disappeared upon applying 1.2 V vs Ag/Ag + and reappeared at 0 V vs Ag/Ag + . The EC changes were revealed to have both a large transmittance change (ΔT) with 70% at 573 nm and a very high coloration efficiency (η) with 689 cm 2 /C. The coloration efficiency exceeded those of the reported linear (one-dimensional), two-dimensional nanosheeted, and three-dimensional hyperbranched Fe(II)-based metallo-supramolecular polymers. A polyFe-3D film showed high durability for the EC changes of more than 1000 cycles. A solid-state simple device fabricated using polyFe-3D exhibited reversible EC changes of more than 1000 cycles without degradation.

Nanocomposites of Fe(II)-Based Metallo-Supramolecular Polymer and a Layered Inorganic–Organic Hybrid for Improved Electrochromic Materials

“…Interestingly, the MLCT absorptions (the intensities of normalized absorbance) were enhanced by combining with LIIm about polyFe/LIIm composites-3/1 and 3/0.1 ( Figure 3 a,c). The absorbance of the MLCT absorption was comparable with that of the π–π* absorption for polyFe/LIIm composites-3/1, although the MLCT absorptions are significantly weaker than π–π* absorption in many cases for Fe-based complexes [ 17 ] and polyFe–clay hybrids [ 27 ]. The absorption close to 385 nm is attributed to the metal (Fe(II)) d–d transition of the polyFe part [ 15 , 27 ].…”

“…Electrochromic (EC) materials have received considerable attention as an important material used in electrochemical glass tinting, i.e., smart windows and antiglare rearview mirrors in cars [ 1 , 2 , 3 , 4 , 5 , 6 , 7 , 8 , 9 , 10 , 11 , 12 , 13 , 14 , 15 , 16 , 17 , 18 , 19 , 20 , 21 , 22 , 23 , 24 , 25 , 26 , 27 , 28 , 29 ]. Many types of EC materials have been reported and are often categorized as follows.…”

“…Metal oxides, such as tungsten oxide, are the first generation of EC materials; transition metal complexes, such as Prussian blue, are the second generation of EC materials; and organic molecules and π-conjugated polymers are the third generation of EC materials [ 3 , 4 , 7 , 8 , 9 , 10 , 11 , 12 ]. Recently, metallo-supramolecular polymers (MSPs), composed of metal ions with multi-topic organic ligands, such as bis-terpyridine, were reported as the fourth generation of EC materials [ 13 , 14 , 15 , 16 , 17 , 18 , 19 ]. MSPs demonstrate absorption in the visible region because of the metal-to-ligand charge transfer (MLCT) (d → π*) from the metal ion center to the organic ligand.…”

“…The appearance/disappearance of MLCT absorption is switched by the electrochemical redox of the metal ion in MSPs because the enhanced d–d splitting width is too large to allow the MLCT in the visible region for oxidized MSPs. The colors and redox potential of MSPs are tunable by changing metal ions (such as Fe(II), Ru(II), and Os(II)) or the substituents and spacer of organic ligands [ 13 , 14 , 15 , 16 , 17 , 18 , 19 ]. Three-dimensional hyperbranched MSPs show better EC properties than linear MSPs [ 17 ].…”

“…The colors and redox potential of MSPs are tunable by changing metal ions (such as Fe(II), Ru(II), and Os(II)) or the substituents and spacer of organic ligands [ 13 , 14 , 15 , 16 , 17 , 18 , 19 ]. Three-dimensional hyperbranched MSPs show better EC properties than linear MSPs [ 17 ]. The high molar extinction coefficient of MLCT absorption in the complex moieties and abovementioned tunability on the bandgap of MLCT have enabled fine EC properties with high optical contrast and abundant color variation.…”