Schiff base type conjugated organic framework nanofibers: Solvothermal synthesis and electrochromic properties

Covalent organic framework is a designable and versatile platform for preparation of various functional materials including conventional gas adsorption and separation materials, catalysis matrix, and photoelectric materials, etc. The covalent organic framework with redox building block and conjugated molecular structure can be viewed as a conjugated polymer with ordered porous structure. In this paper, we synthesized a Schiff base type conjugated organic framework (COF) by a solvothermal method in a common Teflon-lined stainless steel autoclave. The reversible reaction of dynamic imine bonds in solvothermal process induces the morphology transform of COF from nanoparticle to nanofiber. Besides of its structure, morphology, nanofiber-formation mechanism and electrochemical properties, the electrochromic properties of COF were investigated for the first time. Owing to the redox building block of triphenylamine, the COF possesses reversibly electrochromic behavior caused by reversibly redox reaction and band gap change of triphenylamine unit. Under step potentials between −2.5 V and 2.5 V, the COF can switch between bright yellow color and brown color. The stability test shows that the device can reversibly switch with stable contrast after slight decay at the initiated cycles. In consideration of abundant options of building units with redox feature, the COF based electrochromic materials with rich color and novel properties can be expected.