| Electrochromism refers to the phenomenon of materials exhibiting different optical properties under different applied potentials,manifested as reversible changes in color.Donor-acceptor-donor（D-A-D）conjugated polymers are typical electrochromic materials,and the modulation of molecular structure makes it easy to obtain low-bandgap,high-performance electrochromic materials.Research has shown that introducing acceptor units with stronger electron-withdrawing abilities into polymer materials can lead to higher-performance electrochromic materials.The fluorobenzene structure itself has a strong electron-withdrawing ability:due to the high electronegativity and small atomic size of the F atom,substitution with F does not cause significant steric hindrance in the molecular stacking of the corresponding polymer.In addition,the strong electronegativity of the F atom usually leads to a deeper highest occupied molecular orbital（HOMO）energy level,and the small size allows for stronger coplanarity of the molecular skeleton,thereby promoting charge transfer.The incorporation of F atoms can form F-H,which can improve the crystallinity and planarity of the copolymer.Therefore,it is one of the good acceptor unit structures,and its application in the field of electrochromism is expected to achieve new breakthroughs in electrochromic performance.In this study,six D-A-D type electrochromic precursors were designed and synthesized using fluorobenzene structures,such as 2-fluorobenzene（FB）,2,3-difluorobenzene（ODFB）,and 2,5-difluorobenzene（PDFB）,as the acceptor units,and thiophene structures,such as 3-methylthiophene（3MTh）and 3-hexylthiophene（3HTh）as donor units,and the corresponding copolymers were prepared.The specific research content and conclusions are as follows:1.Three D-A-D type polythiophene-based electrochromic materials were synthesized by using fluorobenzene as the acceptor unit and methylthiophene as the donor unit.The electrochemical polymerization behavior and electrochromic properties of the materials were investigated.Three D-A-D monomers,namely3-methylthiophene-2-fluorobenzene-3-methylthiophene（FB-2MT）,3-methylthiophene-2,3-difluorobenzene-3-methylthiophene（ODFB-2MT）,and3-methylthiophene-2,5-difluorobenzene-3-methylthiophene（PDFB-2MT）,were synthesized and corresponding polymer materials PFB-2MT,PODFB-2MT,and PPDFB-2MT were prepared by electrochemical polymerization.The electrochemical polymerization behavior,electrochemical and electrochromic properties of the monomers and polymers were compared and studied.The results showed that the three polymer monomers had lower onset oxidation potentials,with FB-2MT at 0.52V,ODFB-2MT at 0.65 V,and PDFB-2MT at 0.62 V,which were significantly lower than thiophene.The corresponding polymer materials exhibited good adhesion and stability,with PPDFB-2MT showing better stability（93%electrochemical activity after 500 cycles）than PFB-2MT（89.2%electrochemical activity after 500 cycles）and PODFB-2MT（87.3%electrochemical activity after 500 cycles）.All three polymer materials exhibited reversible color changes from yellow to deep blue.It is noteworthy that these simple conjugated polymers exhibited high optical contrast（83%at 1050 nm）throughout the NIR region,the fast response time（1.4 s/0.8 s）,and long-term stability（less than 2%decrease after 250 cycles）.The research on fluorinated benzene-polythiophene materials has further expanded the selection of receptor units and the application prospects of electrochromism.In other words,the potential applications of this type of material in electrochromic technology have been further extended by introducing more receptor units to optimize its performance.2.3-hexylthiophene as a donor unit effectively reduces structural defects in fluorene-thiophene-based D-A-D type polymers,improving their electrochemical stability and electrochromic properties.The introduction of an alkyl chain at theβposition of the thiophene ring can effectively prevent cross-coupling reactions during electro-polymerization,reducing the film-forming potential of the polymer and obtaining more regular and higher quality polymer films.Based on this,three D-A-D type conjugated monomers were synthesized using 3-hexylthiophene as the donor material and FB,ODFB,and PDFB as the acceptor materials,namely3-hexylthiophene-2-fluorobenzene-3-hexylthiophene（FB-2HT）,3-hexylthiophene-2,3-difluorobenzene-3-hexylthiophene（ODFB-2HT）,and3-hexylthiophene-2,5-difluorobenzene-3-hexylthiophene（PDFB-2HT）.The electrochemical behavior of these monomers was studied systematically,as well as the electrochemical behavior and electrochromic properties of the corresponding polymers.The electrochemical behavior study showed that the introduction of the hexyl chain also had a lower oxidation potential（0.49 V/0.51 V/0.69 V）,effectively avoiding structural defects caused by cross-coupling,and the electrochemical activity of the three electro-polymerized polymer films still remained above 90%after 500cycles of scanning.The three polymer materials exhibited a reversible color change from yellow to blue.These simple synthesized conjugated polymers also exhibited high optical contrast（72%at 1090 nm）and high coloring efficiency(456.44 cm2C-1)over the entire NIR region.However,the introduction of a longer alkyl chain may reduce the degree of stability enhancement,possibly due to the larger steric hindrance effect of the longer alkyl chain.3.Based on PPDFB-2MT,an electrochromic device was developed.The poly（3-methylthiophene-2,5-difluorophenylene-3-methylthiophene）（PPDFB-2MT）was chosen as the electrochromic functional layer,and an electrochromic polymer film was obtained through electrochemical polymerization.The electrochromic device was assembled,and the resulting ECDs could undergo reversible color changes between yellow and deep blue.Additionally,long-term continuous cycling by switching the applied potential has shown that electrochromic devices（ECDs）exhibit good stability in electrochromic coloration after 1000 cycles（with optical contrast decay of less than 3%）.When the switching time interval is reduced from 15s to 5 s,theΔT is less than 6%,indicating that the color switching remains stable at different time intervals.The superior optical stability of ECDs under potential switching demonstrates their suitability for practical applications as electrochromic devices.The results showed that the fluorinated phenyl-thiophene D-A-D polymer synthesized by us has a conjugated structure,and reversible color change can be observed.The electropolymerized film also has good electrochemical stability,and shows strong EC performance in terms of coloring efficiency and cycling stability.In addition,this easily prepared electrochromic device also has good EC performance.Therefore,introducing a receptor unit containing F atoms into polymer materials can obtain performance electrochromic materials. |
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