The Investigation And Applications Of New Organic Systems In Electrochromism

Electrochromism(EC)refers to the stable and reversible change of the optical properties(such as transmittance,absorption,reflectivity,etc.)of organic or inorganic materials under the stimulus of an external electric field.Accompanied by continuous ion intercalation/extraction within the electrolyte,the redox products of the materials can exhibit reversible changes in color or transparency on a macroscopic scale.Among them,organic electrochromic materials possess high scientific and commercial value due to their rich color states,high coloring efficiency and easy structure control.However,the development of electrochromism in the organic field has mostly been reported regarding the structural or functional modification of traditional materials.Therefore,exploring and expanding the practical system of new electrochromic materials has become a challenging and marketable subject.This paper mainly focuses on a series of explorations on the application of new organic systems in electrochromism.The research content involves the introduction of novel Schiff-base structures and the delicate design of organic non-complementary devices.While characterizing the electrochromic properties of these new materials or devices,we also try to provide some innovative ideas for multifunctional expansion and mechanism elaboration,mainly including the following three parts:In the first part of the work,we synthesized and developed a triphenylamine-based salicylaldehyde Schiff-base molecule,referred to here as TPASB,and investigated its electrochromic,photochromic,aggregation-induced emission and self-assembly properties.Photochromic and aggregation-induced emission behaviors are both induced by the excited-state intramolecular proton hydrogen transfer(ESIPT)effect,one of the characteristic of salicylaldehyde Schiff-bases.The fluorescence properties of TPASB are controlled by dual processes of intramolecular twisted charge transfer and aggregation-induced emission,while exhibiting a consistent color change from yellow to orange-red in response to light and electrical stimulation.Furthermore,the selfassembled morphological transition was observed during the aggregation of multifunctional Schiff-base molecule.Salicylaldehyde Schiff-bases are functionally grafted onto the electrochromic-active triphenylamine host,the maximum transmittance change can reach 64.2%,while exhibiting unique UV-shield property.In the second part of the work,we explored the application of metal-organic frameworks(MOFs)and their derivatives in solid-state electrochromic thin films based on novel Schiff-base system.First,we designed and synthesized a series of novel Schiff-base ligands through nucleophilic addition reactions,while applying the abundant nitrogen source within Schiff-base ligands as a heteroatom introduction strategy to prepare hybrid electrochromic materials.Among them,the MOFs precursors obtained by the coordination of Schiff-base ligands and nickel ions are prepared by a novel reductive electrosynthesis method at room temperature,which is beneficial to realize scalable production of large-area electrochromic thin films.Nitrogen-doped carbon encapsulated nickel oxide(N-C@NiO)films can be obtained from MOFs precursors under different pyrolysis procedures with high specific surface area and hierarchically porous heterojunction structure.The N-C@NiO#1 thin film prepared by one-step pyrolysis in air has an optical contrast as high as 68%with the transmittance in the colored state being only 3.6%,and the coloring efficiency also reaches 80.18 cm2/C.The N-C@NiO#2 thin film prepared by the two-step method shows better cycling stability(only 7%contrast degradation after 500 cycles),and faster response time.In addition,different pyrolysis procedures can lead to different carbon/nitrogen atomic ratios and configurations of carbon-nitrogen bonds in the final hybrids,which directly affect the electrochromic performance.All of the above indicate that the novel Schiff-base system served as MOFs precursor is an effective way to introduce nitrogendoped carbon structure into electrochromic materials,and provides a new approach for the development of non-traditional electrochromic materials.In the third part of the work,we moved from Schiff-base structure design to noncomplementary device optimization.Different from the single liquid-state and solidstate devices in the previous two parts,this part of the work mainly relies on the hybrid devices of new organic non-complementary system,in order to maximize the use of the rich color states of organic electrochromic materials.We demonstrated the strategy of non-complementary pairing of two cathodic electrochromic materials,for the first time,enabling the full-color display of single independent device,covering colorless,red,orange,yellow,green,blue,and violet.The realization of the full-color concept comes from the precise complementation of the absorbance of two cathodic electrochromic materials and the flexible design of non-complementary device assembly.The UVshield trait of extended viologen and the introduction of solar cells that selectively harvest near-infrared light together realize the concept of full spectrum modulation,that is,"absorbing infrared light,modulating visible light,and blocking ultraviolet light".The smart window fabricated by the hybrid device is reversibly switchable between colorless and dark states with an optical contrast of around 86%.The introduction of near-infrared solar cell bring up a conceptually selective display mode,providing new ideas for the cutting-edge applications of next-generation smart displays.Meantime,as a novel and promising alternative to break through the limitation of single traditional electrochromic material,the full-color performance is also conducive for the development of related energy-saving technologies.