Isomeric chlorination of conjugated thiophene side chain based on quinoxaline to design high-performance electrochromic polymers

The isomeric chlorination of the conjugated side chain is an effective method to improve the photoelectronic properties of polymers, but has not been well investigated in electrochromic performance. Herein, two new D-A-D type isomeric monomers, EQx-α-Cl and EQx-β-Cl, based on 2,3-bis(5-chloro-4-(2-ethylhexyl)thioph-en-2-yl)-6,7-difluoro-5,8-di(thiophen-2-yl)quinoxaline (Qx-α-Cl) and 2,3-bis(4-chloro-5-(2-ethylhexyl)thiophen-2-yl)-6,7-difluoro-5,8-di(thiophen-2-yl)quinoxaline (Qx-β-Cl) as the acceptor units by the isomeric chlorination of the different position of the conjugated thiophene side chain were designed and synthesized. Then their D-A-D type polymers (PEQx-α-Cl and PEQx-β-Cl) were synthesized by electrochemical polymerization. EQx-α-Cl not only exhibits slight red-shifted absorption onset and much stronger absorption with molar extinction coefficient of 1.16×10⁵ M^-1 cm^-1 but also shows lower onset oxidation potential (E_onset) of 0.65 V compared with those of EQx-β-Cl (7.26×10⁴ M^-1 cm^-1 and 0.73 V). And PEQx-α-Cl displayed yellow green at neutral state and dark green at oxidation state, respectively, and showed much better electrochemical stability and electrochromic performance including the higher optical contrast 37.7% and the CE value of 319 cm² C^-1 at 890 nm in comparison with PEQx-β-Cl, which was beneficial to design the electrochromic materials toward adaptive camouflage application. These results demonstrated that the isomeric chlorination strategy of conjugated side chain is a promising approach which may open up a new horizon for designing and synthesizing the high-performance electrochromic polymers.