Highly transmissive blue electrochromic polymers based on thieno[3,2-b]thiophene†
A series of three polymers based on a thieno[3,2-b]thiophene core is synthesized and polymerized via electrochemical polymerization. The addition of benzene and thiophene rings as two different types of substituents on the 3,6-position of the thieno[3,2-b]thiophene core brings about a variance in color changing, optical contrast and morphological demeanor. Electrochromical studies demonstrate that P1 and P2 switch between deep blue neutral and colorless transparent oxidized states, while P3 switches between violet and light green transmissive states. Amid the three polymers, P1 shows the highest optical contrast (71%) in the visible region with complete coloring and bleaching in just 1.10 s and 1.80 s, respectively. In addition, all three polymers reveal about 60% of the transmittance change in the near-IR region, which endows them with commendable applications in NIR electrochromic devices. AFM images depict an augmented surface roughness due to the introduction of alkyl chains in the thieno[3,2-b]thiophene core, which gives rise to a better stability of the polymer thin film.