Mono-chlorinated benzothiadiazole: a new electron-deficient unit for constructing 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.