Synthesis and characterization of D-A type electrochromic polymers based on planar monomers: Cyclopenta[2,1-b;3,4-b′]dithiophene and tris(thienothiophene) as electron donors, diketopyrrolopyrrole as electron acceptor

Abstract

In this work, starting from cyclopenta[2,1-b;3,4-b′]dithiophene (CPDT) and tris(thienothiophene) (3TT) as donor units, and diketopyrrolopyrrole (DPP) as the acceptor unit, three new solution-processable donor-acceptor conjugated copolymers (P1, P2, P3) were successfully synthesized through the palladium-catalyzed Stille coupling reaction. The monomer molar feeding ratios (CPDT:DPP:3TT) of the three polymers are 3:2:1, 2:1:1, 3:1:2, respectively, which are named as PCPT-1, PCPT-2, and PCPT-3. For comparison purpose, we also synthesized the binary copolymers between CPDT and DPP (PCP), CPDT and 3TT (PCT), and compared their properties with the ternary copolymers. The optical band gaps of the five polymers were all lower than 1.9 eV, and they all showed abundant color changes. During the oxidation process, PCPT-1 changed from purple to cyan-gray and turned to earthy brown finally, PCPT-2 changed from purple in a neutral state to blue-gray in an oxidized state. Similarly, PCPT-3 turned from purple to cement gray and finally oxidized to light blue. PCP polymer changed from dark green in a neutral state to cement gray in an oxidized state, PCT polymer changed from purple in a neutral state to Light gray purple in an oxidized state. The electrochromic kinetic performance study showed that the response time of the five polymers was about 1 s within a 4-second interval. In the dynamic stability test of 750 cycles (6000 s), the five polymers demonstrated good stability. The contrast retention of polymer PCPT-2 at 570 nm reached to as high as 96.17%.