Triphenylamine-based covalent organic framework nanospheres: Solvothermal synthesis and electrochromic properties

Covalent organic frameworks (COFs), as a new type of crystalline porous organic polymer, exhibit designable topological structure, highly ordered pore structure and high stability. These advantages of COFs make them suitable for use in optoelectronic materials filed. In this paper, we used Tris (4-aminophenyl) amine (TAPA) as the main building unit, 1,3,5-Benzenetricarboxaldehyde (BT) and 2,4,6-Trihydroxy-1,3,5-Benzenetricarboxaldehyde (THBT) as the aldehyde building units to prepare two types of electrochromic films, which were named as TABT-COFs and TATH-COFs on ITO glass by a solvothermal method. The reversible reaction of imine bonds during the solvothermal process induces the microscopic morphology changing of TABT-COFs and TATH-COFs. Owing to the ordered pore structure of COFs, more active sites of triphenylamine can be exposed to the electrolyte, which is beneficial for the ion transport during the electrochromic process. The large π-electron conjugated system and the inter-layer π-π stacking function can improve the electron conduct of materials. The electrochromic test results show that the coloring and bleaching time of the TABT-COFs and TATH-COFs are 5.6 s/5.2 s and 3.9 s/3.1 s, respectively. Owing to enol ketone isomerization during the synthesis and electrochemical process of TATH COFs, the optical properties are improved. Therefore, it is expected to develop COFs based electrochromic materials with more colorful and novel properties by introducing different construction units with redox activity.