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Due to their high electronegativity and small atomic radius without undesirable steric hindrance, the incorporation of fluorine atoms onto a conjugated backbone has been proven to be a very effective way to tune the energy levels of organic semiconductors. In this paper, 5-fluorobenzo[c][1,2,5]thiadizole was adopted as the electron-deficient unit, thiophene and alkylthiophene as donor units, and further, thiophene and EDOT were taken as peripheral donor units, and one D–A–D and two D–π–A–π′–D fluorinated electrochromic polymer precursors were successfully synthesized via a Stille coupling reaction, namely F-BT, F-BT-Th, and F-BT-EDOT. Due to the elongation of the π-conjugated systems of precursors with the stronger electron-donating ability of the EDOT unit, the oxidation potential of the F-BT-EDOT precursor was further reduced to 0.56 V vs. Ag/AgCl accompanied by red-shifted electronic spectra. As-electropolymerized P(F-BT-EDOT) also showed favorable redox activity and excellent redox stability (<2.33% electroactivity loss after 1000 cycles). At the same time, the optical band gap of P(F-BT-EDOT) (1.37 eV) is much lower than that of P(F-BT-Th) and P(F-BT) (1.62 eV), and a stable reversible electrochromic change from dark green to dark blue was achieved with a fast response time. In the near-infrared region, P(F-BT-EDOT) shows intriguing overall electrochromic performance with a high optical contrast of up to 33% and a coloration efficiency of 196.3 cm2 C−1 at 1100 nm. It can be seen that the introduction of a strong electron-donor EDOT unit could bring about a new breakthrough in the electrochromic performance of 5-fluorobenzo[c][1,2,5]thiadizole-based fluorinated D–A polymers and enhancement in their stability, and it is expected to be applied in near-infrared electrochromic devices. [ABSTRACT FROM AUTHOR]

Chlorination has proved to be an efficient method in tuning energy levels and optoelectronic properties of organic semiconducting materials with cheaper synthesis and low-cost raw materials when compared with the fluorinated analogues. Herein, in this work, 5-chlorobenzo[c][1,2,5]thiadizole (Cl-BT) unit was chosen as a new acceptor unit to design two new D-A type electrochromic conjugated polymer: P(Cl-T) and P(Cl-EDOT). The electrochemical behavior and electrochromic performances of above-mentioned chlorinated polymers were studied by electrochemical polymerization techniques and spectroelectrochemistry. Thanks to the strong electron-withdrawing ability of Cl-BT unit and the strong electron-donating properties of EDOT unit, the resultant Cl-EDOT precursor could electropolymerize into high quality hybrid polymer films at lower potential (0.71 V vs. Ag/AgCl) with favorable redox activity and better redox stability. Spectroelectrochemistry shown that as-formed lower band gaps P(Cl-EDOT) exhibits reversible color-changing nature from pale blue to gray accompanied with the moderate optical contrast (13.9%), fast response time (0.6 s), and favorable coloration efficiency (126.4 cm2 C−1) in the NIR region that was measured from the kinetic studies. By optimizing the tested electrolyte and solution, its electrochromic performance is further improved (optical contrast: 25.6%, response time: 0.25 s, coloration efficiency: 275.9 C−1 cm2). The favorable redox stability and electrochromic performance of P(Cl-EDOT) make it a good candidate to construct electrochromic device, and Cl-BT unit should be a promising building block in designing high performance near infrared electrochromic materials. [Display omitted] • Two electrochromic precursor based on 5-chlorobenzo[c][1,2,5]thiadizole unit were synthesized by Stille coupling reaction. • Different donor brings different influence on the photophysical properties of the chlorinated precursors and its polymers. • The introduction of EDOT unit is beneficial for the preparation of better-quality chlorinated polymers. • P(Cl-EDOT) exhibit better redox activity and stability, and satisfied electrochromic performances. • Chlorinated benzothiadiazole could be a promising building blocks in constructing high performance D-A conjugated polymers. [ABSTRACT FROM AUTHOR]

• Two new D-π-A-π-D type monomers based on mono-chlorinated benzothiadiazole acceptor unit were synthesized. • Types of donor units caused significant effects on the photophysical properties of the chlorinated monomers and its polymers. • Lower oxidation potential of Cl-EDOT was beneficial for the chlorinated polymer's optoelectronic properties. • Lower band gaps P(Cl-EDOT) revealed favorable redox activity and stability, reversible electrochromic nature, and outstanding electrochromic performances. • Chlorinated benzothiadiazole could be a promising building block in designing high-performance D-A electrochromic polymers. Chlorinated benzothiadiazole (Cl-BT) unit, as a new type of electron-accepting building block with the advantages of easy large-scale synthesis in high yields, has been found to be very promising in designing high performance electroactive polymers with decent optoelectronic properties. Most recently, the Cl-BT unit-based D-A polymers have been applied in many organic electronic devices, but little attention was paid to their electrochromic applications. Herein, in this work, to further evaluate the electrochromic performances of the Cl-BT unit-based chlorinated D-A polymers, two new D-π-A-π-D type chlorinated monomers (Cl-Th and Cl-EDOT) which using mono-chlorinated benzothiadiazole unit as the acceptor unit and thiophene/3,4-ethylenedioxythiophene (EDOT) as the donor unit, was successfully synthesized by Stille coupling reaction and then yield their corresponding chlorinated polymer films (P(Cl-Th) and P(Cl-EDOT)) by electrochemical deposition method. Due to the stronger electron donor ability of EDOT unit, the monomer Cl-EDOT has red-shifted UV–Vis and emission spectra and more significant Stoke's shifts. The lower oxidation potential of Cl-EDOT was very beneficial for improving its resultant D-A chlorinated polymer's optoelectronic properties. As a result, as-formed lower band gap P(Cl-EDOT) (∼1.31 eV) showed reversible color change from the neutral dark green to the oxidized dark blue accompanied with outstanding electrochromic properties, including high optical contrast (24.3%), speedy response time (0.6 s), and high coloration efficiency (170.8 cm2/C), as well as favorable redox stability (70% activity retention after 1000 cycles). The favorable electrochromic performances and good optical stability of P(Cl-EDOT) make it a promising candidate in electrochromic devices. Overall, the chlorinated benzothiadiazole and its derivatives were promising building blocks in constructing high-performance D-A electrochromic polymers. [Display omitted] [ABSTRACT FROM AUTHOR]