Pyridyl-based Metal-supramolecular Electrochromic Materials And Devices

Electrochromic devices（ECD）can reversibly and persistently change their optical properties by applying an alternating small voltage.It has broad application prospects due to their low power consumption and color tunability,such as electronic labels,anti-dazzle rearview mirrors,military camouflage,and other frontier fields.The performance of ECD is largely determined by electrochromic materials.After decades of development,researchers have developed various types of materials such as transition metal oxides,small organic molecules,conducting polymers,and metal-supramolecules.In particular,metal-supramolecules have received extensive attention due to their ease of processing and color tunability of organic materials and the stability of inorganic materials.In recent years,terpyridyl-based metal-supramolecules have been extensively studied due to their short switching time and high cycling stability.At present,the synthesis methods of metal-supramolecules are liquid-liquid interface method and liquid-air interface method.Due to the limitation of container volume,the preparation of large-area material films is difficult,and the shape of the films is irregular.In addition,the adhesion between the film and the substrate after secondary transfer is relatively poor,which seriously affects the preparation and cycle stability of large-area electrochromic devices.In this paper,two small molecule terpyridyl-based ligands were synthesized by Suzuki coupling reaction using（4-（[2,2’:6’,2’’-terpyridyl]-4’-yl）phenyl）boronic acid as the intermediate.It is further coordinated with ferrous acetate under nitrogen protection to form supramolecular materials withπ-π,π-d conjugation and intramolecular pore structure.In order to maximize the retention of the inherent carrier transport properties and charge transfer properties of electroactive materials,the two supramolecular materials were deposited on the conductive substrate by electrostatic spraying,and the prepared films were subjected to cyclic voltammetry,chronoamperometry,and in situ spectroscopy tests.The effects of different molecular weights on the electrochemical and electrochromic properties of metal-supramolecules were investigated.The details are as follows:1.Equal molar ratio of bidentate terpyridyl-based ligand and metal ions(Fe²⁺)are coordinated to form pyridyl-based metal-supramolecules（Fe-L（10））withπ-π,π-d conjugation,and intramolecular pores.In particular,the coloring and bleaching times of Fe-L（10）thin film are less than 0.5 s under the condition of optical modulation of 67%,and the coloration efficiency is as high as 2099.2 cm² C^-1.The optical modulation of Fe-L（10）thin films prepared by electrostatic spraying is only 12%attenuated after 3000 cycles,and the 57%of initial state can still be maintained after 6000 cycles.This result is superior to the cycling stability of thin-film electrodes prepared by the interfacial method with similar materials.At the same time,the Fe-L（10）film can be well charged and discharged at current densities from 1.2 A g^-1 to 6.0 A g^-1 and has a gravimetric capacitance greater than 13 m Ah g^-1.Based on its excellent electrochromic performance,we further assembled a 25 cm² ECD,which has a fast switching speed with a bleaching time of 6.8 s and a coloring time of 1.8 s.2.The tridentate terpyridyl-based ligand and the metal ion(Fe²⁺)are complexed at a molar ratio of 1:1.5 to form a two-dimensional metal-supramolecule（Fe-L（6））.We systematically studied the electrochromic performance of films with different thicknesses.The study showed that the optical modulation of the film with a thickness of 641 nm is 71.7%,the bleaching time is 1.5 s,the coloring time is 0.5 s,and the coloration efficiency is 1103.9 cm² C^-1.In particular,the optical modulation of the film is unchanged after 10000 cycles,which was attributed to the good contact between the film and the substrate and the poor solubility of macromolecular materials in the electrolyte.Based on the excellent electrochromic performance of this material,we fabricated large-area devices（225 cm²）,which exhibited excellent performance due toπ-πandπ-d conjugation and intramolecular porous structure to facilitate ion and electron transport electrochromic properties.