High-performance fluorinated D-A-D type electrochromic polymers with different structural types of thiophene donor units

Abstract

In this work, two fluorinated D-π-A-π-D type electrochromic polymer precursors were synthesized by Stille coupling reactions that using mono-fluorine atom substituted benzothiadiazole (F-BT) as acceptor units (A), 3-dodecylthiophene as the π-bridge, and thiophene (Th) and 3,4-ethoxylenedioxythiophene (EDOT) as donor units (D), respectively, and their optical properties were characterized by various analytical techniques. All of them were electropolymerized into the corresponding fluorinated electrochromic polymer films (P(F-Th) and P(F-EDOT)). F-EDOT has red-shifted absorption spectra and emission spectra but with relatively lower fluorescence quantum yield, which is due to the strong electron donating ability of EDTO unit. The lower oxidation potential of F-EDOT (0.72 V) is very beneficial to improve the optoelectronic properties of its electropolymerized P(F-EDOT) with favorable redox activity and better redox stability (remained 73 % redox activity after 1000 cycles). Due to the enhanced π-π* conjugation effect, P(F-EDOT) has shown a red-shifted absorption spectra compared to P(F-Th) and a decreased optical band gap (1.6 eV for P(F-Th); 1.38 eV for P(F-EDOT)). Although P(F-Th) has a higher coloration efficiency (370.4 cm2C−1 at 894 nm) and faster response time (0.04–0.37 s), the transparent green P(F-EDOT) should have more satisfied electrochromic properties due to its favorable coloration efficiency (192 cm2C−1 at 458 nm), higher optical contrast (20.6 % at 458 nm), and also very fast response time (0.2–0.86 s), especially the enhanced optical stability and better memory effect. Overall, these properties highlight the potential applications of the current fluorinated D-π-A-π-D type polymers in displays and smart windows.