Solution-processable blue-to-transmissive electrochromic benzotriazole-containing conjugated polymers

A series of donor–acceptor type electrochromic (EC) conjugated polymers P1–P4, poly(4,7-bis(3′,4′-ethylenedioxythiophen-2′-yl)-2-alkylbenzotriazole-5′,5′-diyl-alt-(3,4-dialkoxy-thiophen-2,5-diyl), was synthesized via a Stille coupling reaction. The polymers were characterized by gel permeation chromatography (GPC), nuclear magnetic resonance spectroscopy (NMR), thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). The electrochemical, spectroelectrochemical and EC properties of the polymers were studied. The polymers displayed improved solubility in common organic solvents. Upon oxidation, the polymers switched from a saturated blue to a transmissive light blue state. In particular, P4 exhibited exceptional optical contrasts of 46% and 72% in the visible and near-infrared regions, respectively. Furthermore, the effects of the side chains on the EC properties were examined and the results revealed that polymers with a long alkyl chain in the benzotriazole moiety had faster switching speed both in the visible and near infrared regions in the polymer film state. The absorption–transmission ECDs were fabricated, revealing that under ambient conditions and without any encapsulation of ECDs, the polymers exhibited good optical contrasts and coloration efficiencies with improved stability.