Tunable optoelectronic performances of fluorinated benzene-EDOT based hybrid electrochromic polymers

Abstract

Due to its high electronegativity and small atomic radius without undesirable steric hindrance, the incorporation of fluorine atoms into the benzene unit which started from the commercially available compounds is a feasible way to tune the energy levels of polymers, and its electrochromic application is expected to achieve a new breakthrough. Herein, in this paper, benzene units with different fluorine atom numbers were selected as the core and 3,4-ethylenedioxythiophene (EDOT) as the terminal unit, three fluorinated electrochromic precursors (FPh-EDOT, 2FPh-EDOT, and 4FPh-EDOT) were designed and synthesized by Stille coupling reaction, and the corresponding fluorinated π-conjugated electrochromic polymers (P(FPh-EDOT), P(2Ph-EDOT), and P(4FPh-EDOT)) were prepared by electrochemical polymerization method. The effects of different fluorinated benzene units on the photophysics, electrochemistry, and electrochromic properties of precursors and polymers were systematically investigated, and the structure-property relationship was further revealed. All of them have low initial oxidation potentials {FPh-EDOT (0.65 V), 2FPh-EDOT (0.72 V), and 4FPh-EDOT (0.70 V)}, which makes it easy to obtain high-quality new fluorinated polymers at low polymerization potential. With the increase of the number of F atoms, the UV–vis absorption spectra and emission spectra of fluorinated monomer firstly red-shifted and then blue-shifted. The corresponding fluorinated polymers have good electrochemical activity and stability (the P(2Ph-EDOT) activity remained 82.7% after 500 cycles), making it possible to realize their electrochromic devices. All these three fluorinated polymers show obvious color change from the neutral state to the fully oxidized state (P(FPh-EDOT) changes from brown to grayish blue, P(2FPh-EDOT) changes from brown to gray, and P(4FPh-EDOT) changes from dark blue to light blue). The kinetic study further shows that all of them have distinguish electrochromic properties with good optical stability, very short response time (0.9 s for P(2FPh-EDOT) at 700 nm), and favorable coloration efficiency (130.7 cm2 C−1 for P(FPh-EDOT) at 503 nm). Overall, all these three fluorinated polymers have low optical band gap, good redox stability and electrochromic properties, are very promising electrochromic materials, which is worthy for the further research and development.