Molecular weight and linking position (2,7 vs 3,6) effects on electrochromic properties of alternating carbazole-EDOT copolymers obtained by direct (hetero) arylation polymerization

Abstract

Two carbazole-EDOT electrochromic conjugated polymers (ECPs), namely P(2,7-Cbz-E)-HW and P(3,6-Cbz-E) have been developed by direct (hetero)arylation polymerization (DHAP). The effects of molecular weight and linked position on electrochromic properties for the above ECPs were investigated in detail. The 2,7-linked high molecular weight ECPs of P(2,7-Cbz-E)-HW exhibited excellent optical contrast and electrochemical stability. Meanwhile, both P(2,7-Cbz-E)-LW and P(2,7-Cbz-E)-HW performed short response times at 450 nm (<1 s) and P(2,7-Cbz-E)-LW showed outstanding coloration efficiency (up to 370 cm2 C−1 at 450 nm). However, 3,6-linked ECPs of P(3,6-Cbz-E), whose conjugation are sort of interruption, resulted in a low molecular weight and presented a highly blue-shifted absorption edge and unsatisfactory electrochromic performance compared with 2,7-linked ECPs. Further, flexible and patterned electrochromic devices (ECDs) were presented employing 2,7-linked ECPs, which achieved a reversible color change between yellow and dark blue with acceptable optical contrast (26% for P(2,7-Cbz-E)-LW and 32% for P(2,7-Cbz-E)-HW both at 605 nm). The research indicates that the molecular weight and linked position are crucial to achieve high-performance electrochromism, which provides guidance for designing ECPs and flexible ECDs.