Polychromatic electrochromic films via bipolar covalent organic framework design.

• The bipolar COF-GZU1 structure was successfully designed. • The prepared COF-GZU1 films exhibited high specific surface area, crystallinity and porosity. • The ordered porous structure facilitates electron conduction and ion mass transfer. • Great electrochromic properties and regulation. Because of the ordered and loose structure, covalent organic framework (COF) holds broad promise in the field of electrochromism, but the simple color change limits its application and development. A bipolar and polychromatic COF-GZU1 was successfully designed and prepared in this paper. The structure, electrostatic potential and orbital structure of the COF-GZU1 were predicted and calculated using density functional theory, and the calculation results confirmed the feasibility of bipolar construction strategy. XRD, SEM, BET and GIWAXS further mutually established the successful synthesis of COF-GZU1, which exhibited high specific surface area, crystallinity and porosity. Results from electrochemical impedance spectroscopy, density of states analysis, and ion diffusion coefficient measurements verified that the ordered porous structure facilitates electron conduction and ion mass transfer, leading to a significant improvement in electrochromic performance. Moreover, the COF-GZU1 film displayed excellent electrochromic performance, providing polychromatic switching with a rapid response time (7.8/14.5 s and 1.0/12.6 s for coloring/bleaching process) and high coloring efficiency (112.8 and 127.8 cm2∙C-1 at 799 and 570 nm). These results collectively demonstrate the superiority of the COF-GZU1 film and suggest that the bipolar COF design strategy could pave the way for new and high-performance polychromatic electrochromic materials. [ABSTRACT FROM AUTHOR]