Synthesis, Electrochemistry, and Optoelectronic Properties of Biphenyl-EDOT-Based Electrochromic Polymers

In this study, two novel hybrid monomers (<b>4BD-EDOT</b> and <b>3BD-EDOT</b>) containing a biphenyl group and a 3,4-ethylenedioxythiophene (EDOT) unit were synthesized and polymerized electrochemically in a CH₂Cl₂-Bu₄NPF₆ electrolyte solution. Characterizations of the resulting <b>P4BD-EDOT</b> and <b>P3BD-EDOT</b> were studied by CV, scanning electron microscopy (SEM), and spectroelectrochemistry in order to examine the effect of different substitution positions of biphenyl on the electrochromic performance of the resultant hybrid polymers. Both polymers have favorable redox activity (a distinct redox peak) and good redox stability (55-49% electroactivity was retained after 1000 cycles). The spectro-electrochemistry study found that both show a distinct color change from reddish brown to blue/purple for <b>P4BD-EDOT</b> with a lower band gap (1.54 eV) and from transparent color to light blue for <b>P3BD-EDOT</b> with a larger band gap (1.73 eV). These electrochromic polymer films also have fast switching speed (0.5-0.2 s), with the favorable optical contrast (22.6% at 1100 nm for <b>P4BD-EDOT</b>) and decent coloration efficiency (250.4 cm² C^-1 at 780 nm for <b>P3BD-EDOT</b>). All these results show that both monomers have important values related to the electrochromic field. This work also shows that the different substitution positions of the biphenyl unit affect the spectroelectrochemistry and electrochromic characteristics of the resultant hybrid polymers.