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 three-dimensionally 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 (Fe-tpys) 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 (Mw) of polyFe-3D was determined to 3.96 × 105 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) (E1/2 = 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 cm2/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.