High coloration efficiency and fast switching speed of poly(amic acid-imide)s containing triphenylamine in acidic electrolyte
Two novel poly(amic acid-imide)s (PAA-IMs) were prepared from two electroactive diamines, 4,4′-diamino-4′′-methoxytriphenylamine and 4,4′-diamino-4′′-phenoxytriphenylamine, and pyromellitic dianhydride (PMDA) by conventional polycondensation, followed by partial imidization at 150 °C. The resulting PAA-IMs had significant levels of thermal stability associated with 2% weight-loss temperatures between 195 and 370 °C. Flexible PAA-IM films with light color showed excellent adhesion on the surface of an indium–tin oxide (ITO)-coated glass substrate. The electrochromic properties were examined by electrochemical and spectroelectrochemical methods. Cyclic voltammograms of the PAA-IM films cast onto ITO substrate exhibited a reversible oxidation at 1.26–1.35 V vs. Ag/AgCl in acid electrolyte consisting of acetonitrile and 4-toluene sulfonic acid (MBSA), revealed excellent stability of electrochromic characteristics with a color change from neutral yellowish pale form to the green or blue oxidized form at applied potentials ranging from −0.5 to 1.8 V. The anodically electrochromic PAA-IM films not only showed excellent reversible electrochromic stability with extremely high coloration efficiency (CE = 834 and 523 cm2 C−1) but also exhibited fast switching speed which required 1.7 to 3.8 s for color switching and 0.8 to 1.6 s for bleaching. After over 30 cyclic switches, the PAA-IMs films still exhibited stable electrochemical and electrochromic characteristics.