Difluorinated benzoselenadiazole: a new promising electron withdrawing acceptor unit for building efficient D-A type electrochromic polymers

The benzoselenadiazole-based D-A-D polymers have demonstrated significant potential in organic solar cells, due to its lower optical band gap, more red-shifted absorption spectra, good processing ability, and higher hole transport. However, its application in the electrochromic field is relatively limited so far. Therefore, this work presents the design and synthesis of four difluorinated D-A-D monomers. These monomers incorporate difluorobenzoselenadiazole as their electron-withdrawing segment, with thiophene and 3,4-ethylenedioxythiophene (EDOT) serving as the electron-donor components. These monomers were polymerized into D-A-D polymers via electrochemical deposition method, and their optoelectronic properties were systematically investigated. The elongation of the π-conjugated systems of monomers, along with the stronger electron donating ability of the EDOT unit, further reduced the oxidation potential of Se-Th-EDOT to 0.65 V. This change was accompanied by a blue shift in the electronic spectra and a red shift in the emission spectra. Among the polymers, those derived from EDOT-containing monomers exhibited superior performance, including enhanced redox stability, and a notable shift in hue, evolving from green to a sky-blue shade when oxidized. Notably, the P(Se-Th-EDOT) exhibited remarkable electrochromic performances, featuring an optical contrast of 33.21%, response times within the range of 0.4 to 0.80 s, and a coloration efficiency reaching to 160.08 cm² C^-1. Devices based on P(Se-Th-EDOT) further shown improved coloration efficiency (228.66 cm²C^-1) and rapid response times (0.05-0.28 s). The research outcomes underscore the suitability of D-A polymers that incorporate difluorinated benzoselenadiazole for the advancement of state-of-the-art electrochromic applications.