Exploring the electrochromic properties of poly(thieno[3,2-b]thiophene)s decorated with electron-deficient side groups

Two novel thieno[3,2-b]thiophene (TT)/3,4-ethylenedioxythiophene (EDOT)-based compounds of 2,5-(EDOT–TT–EDOT) type bearing electron-withdrawing side groups (4-cyanophenyl or 4-pyridyl) at 3,6-positions of the TT moiety have been synthesized. Their electropolymerization leads to electroactive conjugated polymers, P(CNPh-ETTE) and P(Py-ETTE), which possess electrochromic properties changing the color from purple to pale grey-blue or from sand brown to pale grey-green, respectively. Cyclic voltammetry and spectroelectrochemical experiments reveal that functionalization with electron-withdrawing side groups decreases the HOMO and LUMO energy levels and contracts the band gap of materials. Both new polymers demonstrated extremely short response times of 0.9–1.1 s for bleaching and 0.34–0.35 s for coloring. P(CNPh-ETTE) and P(Py-ETTE) polymers showed reasonably good contrast (16–23%) and coloration efficiency (120–190 cm2 C−1) in the visible region (at the maxima of their π–π* transitions, 540/570 nm), and high contrast and coloration efficiency in the near-infrared region (50–62% and 324–440 cm2 C−1 at 1500 nm, respectively). While the stability of the pyridine-functionalized polymer, P(Py-ETTE), was shown to be low (with unstable charge–discharge characteristics, presumably due to the protonation of the pyridine ring during the redox process), P(CNPh-ETTE) demonstrated superior electrochromic performance retaining 91–96% of its electroactivity after 2000 cycles between −0.5 and +1.0 V. DFT calculations on these and related EDOT–TT–EDOT polymers reported by us early have been presented and analyzed to understand the structure–property relationships in this class of electrochromic polymers.