Preparation Of Direct(Hetero) Arylation Coupling Of Electrochromic Polymers And Flexible Devices

Electrochromism（EC）refers to the reversible and stable changes in color or optical properties（absorption/transmission/reflection,etc.）of materials under the influence of an external voltage.Among electrochromic materials,Electrochromic conjugated polymers（ECPs）have attracted attention due to their advantages such as variable coloration,good processability,low cost,fast response speed,high coloring efficiency,etc.ECPs have broad application prospects in fields such as smart windows,electronic paper,thermal control and military camouflage for spacecraft.Developing high-performance ECPs is of great scientific significance.The Direct（Hetero）arylation Polymerization（DHAP）method for preparing ECPs has attracted wide attention and research due to its advantages of fewer synthetic steps,simple purification,and avoiding the hazards of toxic substances to human health and the environment.This article focuses on the synthesis method of DHAP,the relationship between material structure and performance,and the preparation of high-performance ECPs.DHAP was used to prepare ECPs with different structural characteristics and excellent performance,and the structure-performance relationship of ECPs was systematically studied.The synthesized ECPs were then made into flexible electrochromic devices.The specific research work is as follows:（1）Low molecular weight P（2,7-Cbz-E）-LW,high molecular weight P（2,7-Cbz-E）-HW and P（3,6-Cbz-E）ECPs were designed and synthesized by DHAP.The effects of molecular weight and connection site on polymer properties were studied.The results showed that increasing the molecular weight would increase the effective conjugation length,lower the oxidation potential and bandgap,increase the electrochemical stability,and improve the optical contrast and response time of the polymers.P（2,7-Cbz-E）-LW and P（2,7-Cbz-E）-HW exhibited short response times（<1 s）at 450 nm;P（2,7-Cbz-E）-LW had the highest coloring efficiency,reaching 370 cm²·C^-1 at 450 nm.Conjugation at positions 3 and 6 of carbazole led to conjugation breakage and caused multi-color phenomena,but resulted in poor electrochemical and redox stability.（2）Three ECPs with different numbers of thiophene donor units,polybenzo[1,2,5]thiadiazole-thiophene（PBTz-T）,polybenzo[1,2,5]thiadiazole-bithiophene（PBTz-BT）and polybenzo[1,2,5]thiadiazole-terthiophene（PBTz-TT）,were designed and synthesized by DHAP.The effects of the number of thiophene donor units on the color and properties of the polymer were studied.The results showed that as the number of thiophene donor units increased,the absorption peak and absorption edge of the polymer shifted towards longer wavelengths,causing changes in color.The starting oxidation potential decreased while the electrochemical stability increased accordingly.PBTz-T had the highest coloring efficiency and dynamic stability,reaching 916 cm²·C^-1at 530 nm;PBTz-BT displayed the highest thermal stability and optical contrast,reaching28%at 540 nm;PBTz-TT had the fastest response time,with a switching time of 0.5 s/0.7s for coloring/bleaching at 550 nm.（3）Two different electron-withdrawing units,polybenzo[1,2,5]thiadiazole-3,4-ethylenedioxythiophene（PBTz-E）and poly（2-fluoro）-benzo[1,2,5]thiadiazole-3,4-ethylenedioxythiophene（P2F-BTz-E）,were designed and synthesized by DHAP.The effects of difluorination on the properties of the polymers were studied.The results showed that the introduction of two fluorine atoms conferred conformational locking to the polymer,increasing inter-and intra-chain interactions and improving thermal stability.The presence of the two fluorine atoms caused a slight blue shift in the absorption peak and onset point without changing the overall color of the polymer.P2F-BTz-E exhibited better electrochemical stability,transmittance,and response time,while PBTz-E displayed higher transmittance stability and coloring efficiency.PBTz-E had the highest optical contrast at 31%at 570 nm,while P2F-BTz-E had a maximum optical contrast of42%at 570 nm.Both polymers had a reduction time of less than 1 s,and PBTz-E had a coloring efficiency of 774 cm²·C^-1 at 570 nm,which was leading in the BTz-EDOT-based electrochromic polymers.（4）To achieve the preparation of multicolor flexible electrochromic devices,P（2,7-Cbz-E）-HW,PBTz-T,PBTz-BT,PBTz-TT,PBTz-E and P2F-BTz-E were processed into solutions and assembled with a PMMA gel electrolyte on an ITO/PET flexible transparent conductive substrate.The study showed that the flexible devices assembled from the six ECPs could achieve reversible color changes between-2.0 V to 2.6 V,and had long-term cycling stability,maintaining high reversible electrochromic performance after 100repeated bending cycles.