Effect of conjugate length of monomeric thiophene backbone on electrochromic performance of benzo[1,2-c:4,5-c']dithiophene-4,8-dione-based D-A polymers

Abstract

In this work, a series of donor (D)-acceptor (A)-donor (D) type monomers, including TBD, TTBD and TTTBD, were designed and synthesized by modulating the conjugated length of the monomer backbone based on the benzo[1,2-c:4,5-c']dithiophene-4,8-dione acceptor unit. The corresponding polymers PTTBD and PTTTBD were successfully synthesized through electrochemical polymerization, whereas this method was ineffective for the synthesis of PTBD from its monomer TBD. As the conjugated length of the monomeric backbone increased, the absorption spectra of these three monomers TBD, TTBD and TTTBD exhibited a progressive red-shift, and their onset potentials (E_onset) gradually decreased. In addition, PTTBD demonstrated superior electrochemical stability and electrochromic performance, changing color from purple in the neutral state to grey in the oxidized state, with a higher optical contrast of 45.65 % and a coloration efficiency (CE) value of 570.84 cm² C⁻¹ at 1100 nm compared to PTTTBD (27.50 % and 265.68 cm² C⁻¹). These results indicate that the strategy of tuning the conjugated length of the monomer backbone can optimize the electrochromic performance, offering a promising approach to the design of high-performance electrochromic polymers.