Thiophene-2,5-diesters as electrochromic materials: The effect of ester groups on the device performance and stability

Abstract

Abstract A series of thiophene-2,5-dicarboxylic acid diesters (R–Th, compounds 3–9) with linear and branched alkyl (3–7) and benzyl (8,9) ester groups have been synthesized and characterized by 1H/13C NMR and MS. Their electrochemical reduction in cyclic voltammetry experiments proceed as two-step single-electron transfer process to form radical anions R–Th•– (at −1.87 … −2.00\u202fV vs Ag/Ag+) and dianions R–Th2– (at −2.48 … −2.68\u202fV vs Ag/Ag+). In spectroelectrochemical experiments in Bu4NClO4/N,N-dimethylformamide electrolyte solution, all compounds exhibit reversible color changes from colorless in their neutral state to blue colors in the reduced state, with two characteristic absorptions of radical anions R–Th•– in the short-wavelength (ca. 406–420\u202fnm) and long-wavelength regions (ca. 610–655\xa0nm). The L*a*b* CIE color coordinates of th e sandwich-type electrochromic devices have been evaluated by colorimetry measurements and the devices demonstrated good coloration efficiency of up to 512\xa0cm2\u202fC−1. The cycling stability of the materials on electrochromic coloring/bleaching substantially depends on the structure of the R ester groups and increased with increasing the steric hindrances in the R. In the series of seven studies compounds, the best cycling stability was observed for R\u202f=\u202f2-ethylhexyl and benzyl. The degradation mechanism during electrochromic cycling in an inert atmosphere (under nitrogen) and in air is discussed.