Effects of different thiophene π-bridge units on electrochemical and electrochromic properties of di-nitrated D-π-A-π-D polymers

In this paper, two di-nitrated D-π-A-π-D type monomers with di-nitrated benzothiadiazole as the acceptor unit, EDOT as the donor unit, and EDOT and 3-dodecylthiophene as the π-bridge units, respectively, were successfully synthesized by Stille coupling reactions, and their corresponding di-nitrated polymers (P(2EDOT-2NO₂-BT) and P(EDOT-C₁₂Th-2NO₂-BT)) were obtained by electrochemical polymerization method. Through optical, cyclic voltammetry, spectroelectrochemistry and kinetic studies, the effects of π-bridge units on the optoelectronic properties, electrochemical and electrochromic properties of di-nitrated monomers and their corresponding D-A polymers were systematically discussed, and the structure-property relationship was further revealed. Compared to the monomer 2EDOT-2NO₂-BT that using EDOT as the π-bridge unit, the absorption and fluorescence spectra of EDOT-C₁₂Th-2NO₂-BT that using 3-dodecylthiophene as the π-bridge unit are blue-shifted, with a larger Stokes shift (196 nm) and optical band gap (1.76 eV). Both of these di-nitrated monomers have very low fluorescence quantum yields and onset oxidation potentials (0.74–0.77 V), which make them ideal for electrodepositing high-quality electrochromic polymer films. Due to the loose and porous surface morphology, both these two di-nitrated polymers exhibit favorable redox activity and redox stability (the retained electroactivity of them were both over 80 % after 1000 cycle), as well as favorable electrochromic performance with obvious and reversible multicolor color change, good optical stability, decent optical contrast (33.24 %), short response time (as low as 1.5 s), good coloration efficiency (152.47 C⁻¹ cm²) and memory effect. Overall, both these two di-nitrated polymers have good redox stability and electrochromic properties, opens new possibilities for electrochromic devices based on the di-nitrated polymers.