The optimization of donor-to-acceptor feed ratios with the aim of obtaining black-to-transmissive switching polymers based on isoindigo as the electron-deficient moiety

Isoindigo (iI)-containing donor (D)–acceptor (A)-type polymers have rarely been used as electrochromic materials although their applications in the field of organic photoelectric devices have achieved significant progress in only a few years. Three conjugated polymers, namely, P1–P3, were synthesized via the random copolymerization of three units, including thiophene and 3,3-bis-decyl-3,4-dihydro-2H-thieno[3,4-b][1,4]dioxepine (ProDOT-decyl2) as the D units, and alkyl-substituted isoindigo as the A unit, and the reactions were conducted by the Stille polymerization method. The results showed that the feed ratio of the D units to the A unit had a substantial effect on the electrochemical properties, optical band gap, color presentation, and electrochromic parameters (changes in transmittance, response time, coloration efficiency and persistence of optical contrasts for a long period) of the conjugated polymers. Polymers were easily obtained with the feed ratios (thiophene:iI:ProDOT) of 2:1:1, 4:1:3, and 5:1:4 for P1, P2 and P3, respectively. P1 exhibits a cyan color in the neutral state and a transmissive grey color in the oxidized state. Both P2 and P3 display a change from black to transmissive on conversion from the neutral to the oxidized state. An increase in the ProDOT content was helpful for enabling the extension of the absorption peak of the polymers across the entire spectrum and also improving the switching property of the polymers. In general, the introduction of iI as the A unit for the construction of D–A-type polymers provides the field of electrochromic devices with verifiable performance and deserves further investigation to obtain more promising electrochromic materials.