Effects of alkyl or alkoxy side chains on the electrochromic properties of four ambipolar donor-acceptor type polymers

Four donor-acceptor type pi-conjugated polymers, poly[2,3-di(2-furyl)-5,8-bis(2-(4-butoxythiophene))quinoxaline] (PFBOTQ), poly[2,3-di(5-methylfuran-2-yl)-5,8-bis(2-(4-butoxythiophene))quinoxaline] (PMFBOTQ), poly[2,3-di(2-furyl)-5,8-bis(2-(4-butylthiophene))quinoxaline] (PFBTQ) and poly[2,3-di(5-methylfuran-2-yl)-5,8-bis(2-(4-butylthiophene))quinoxaline] (PMFBTQ) containing 2,3-di(2-furyl)quinoxaline or 2,3-di(5-methylfuran-2-yl)quinoxaline moiety in the backbone as the acceptor unit and different thiophene derivatives as donor units are synthesized electrochemically. The four polymers are characterized by cyclic voltammetry (CV), UV-Vis-NIR spectroscopy, scanning electron microscopy (SEM) and step profiler. Both PFBOTQ and PMFBOTQ with strong electron-donating butoxy groups have lower oxidation potentials than PFBTQ and PMFBTQ, respectively. Electrochemical measurements and spectroelectrochemistry analyses demonstrate that all four polymers exhibit both p-and n-type doping processes. Both PFBOTQ and PMFBOTQ exhibit a green color in the neutral state, whereas PFBTQ with two absorption bands at 349 nm and 541 nm shows a light purplish red color and PMFBTQ with one obvious absorption band at 349 nm, as well as a shoulder peak located at around 500 nm shows a light brown-red color in the neutral state. Furthermore, as typical donor-acceptor polymers, all four polymers present robust stabilities, low optical band gaps, excellent optical contrasts in the NIR region, satisfactory coloration efficiencies and fast switching times, which make these four polymers outstanding candidates for electrochromic applications.