Black Electrochromic Materials Using Conjugated Polymers Consisting of Prodot Donor and Biphenyl Acceptor
Authors: Adachi Rika , Aoki Atsushi 
Published: December 2024

Electrochromic (EC) materials, which can change colors by electrochemical switching, are expected to be applied to reflective displays and light-tinting glass, etc. The EC materials have some advantages that they are easy on the eyes because they use reflected light and that they consume little power because power is required only when switching colors. There are various materials for EC devices such as inorganic and organic materials. In particular, conjugated polymers have attractive features about color turning and conductivity. In the previous study, the conjugated polymer consisting of propylenedioxytihophene with dibutoxymethyl groups (ProDOT(dibutoxy)) as a donor and phenyl as an acceptor has been reported to turn from yellow to translucent at the anodic potential applied. In this study, we changed the acceptor unit of the conjugated polymers from phenyl to biphenyl (Bip) to prepare ProDOT(dibutoxy)-Bip conjugated polymer in order to investigate the effect of acceptor on turning color.

ProDOT(dibutoxy)-Bip copolymers were synthesized according to Reynolds et al[1]. First, ProDOT was synthesized by ring-closing reaction of 3,4-dimethoxythiophene with 2,2-bis(bromomethyl)propane-1,3-diol, and boromomethyl group was etherified by butanol to obtain ProDOT(dibutoxy). Then, the copolymerization of ProDOT(dibutoxy) and biphenyl was carried out alternately by Suzuki Miyaura coupling. The synthesized ProDOT(dibutoxy)-Bip copolymer was then deposited on ITO substrates by spray-coating, and in situ spectroelectrochemical analysis and L*a*b* measurements were performed by cyclic voltammetry (CV) and UV-vis spectroscopy.

The UV-vis spectra show that the ProDOT(dibutoxy)-Bip copolymer in the reduced state has π-π* absorption peak at 413 nm at the potential less than 0.6 V vs. Ag/Ag+, indicating a yellow color. It’s similar to the color of ProDOT(Bibutoxy)-Ph copolymer whereas the absorption peak indicates a little bit of blue shift compared to that of ProDOT(dibutoxy)-Ph copolymer at 440 nm. The blue shift would be caused by shorter effective conjugated length in the polymers by rotation of biphenyls which are bonded to two phenyls by a single bond. From the CV measurements, the oxidation potential of the ProDOT(dibutoxy)-Bip copolymer was found to be 0.8 V vs Ag/Ag+, which is anodic shift compared to that of the ProDOT(dibutoxy)-Ph copolymer, 0.45 V vs Ag/Ag+. The reason is that it is more difficult to draw electrons from the copolymer due to the larger proportion of phenyls in the biphenyl acceptor unit relative to ProDOT(dibutoxy) donor unit. On the other hand, when the potential was increased from 0.6 to 1.3 V vs. Ag/Ag+ to oxidize the ProDOT(dibutoxy)-Bip copolymer, the π-π* absorption decreased and a polaron-derived absorption with a broad peak from 900 nm to 1300 nm appeared, resulting in a black color. The ProDOT(dibutoxy)-Ph copolymer also exhibits polaron-derived peaks, but its absorbance in the visible light region is small, so it changes from yellow to transparent. L*a*b* measurements, which measure chromaticity and lightness, show that both ProDOT(dibutoxy)-Bip copolymer and ProDOT(dibutoxy)-Ph copolymer have L* values around 80 in the reduced state, whereas a large difference is observed in the oxidized state. The L* value of the ProDOT(dibutoxy)-Bip copolymer decreased to 30 in the oxidized state, while that of the ProDOT(dibutoxy)-Ph copolymer had 67 in the oxidized state. Repeating cycle between the reduced state and the oxidized state of ProDOT(dibutoxy)-Bip copolymer applied at the potential of 0.4 and 1.1 V vs. Ag/Ag+ , respectively was performed by in-situ measurements of the change in the absorbance at 413 nm as a function of time. It showed that the number of the repeating cycle was achieved to be 113 cycles but the absorbance at the reduced state of the copolymer gradually decreased with increasing cycles.

In summary, we demonstrated that ProDOT(dibutoxy)-Bip copolymer shows good black EC materials. It is expected to apply for reflective display EC devices.

[1] C.M.Amb, J.A.Kerszulis, E.J.Thompson, A.L.Dyer and J.R.Reynolds, Polym.Chem., 2,812(2011).