Structural modulation of covalent organic frameworks based on redox active center of π-conjugated amine unit for enhanced electrochromic properties

The covalent organic frameworks (COFs) possess high designability, which provides a platform for synthesizing novel optoelectric materials, including electrochromic materials. In this work, N,N,N',N'-tetra-(4-aminophenyl)-1,4-phenylenediamine (TPDA) with π-electron conjugate system and redox active centers was used as the main building block. The 4,4′-biphenyl diformaldehyde (BDA) and the 2,2′-thiophene-5,5′-diformaldehyde (TDA) were used as the aldehyde building units. Two imine-based COF electrochromic active materials (COFTPDA-BDA and COFTPDA-TDA) were prepared by Schiff base reaction. The test results reveal that COFTPDA-TDA has superior electrochromic properties than COFTPDA-BDA. Their contrasts are 0.46 and 0.40, and the coloring/fading times are 3.1/10.6 s and 10.9/14.3 s, respectively. The COFTPDA-TDA material possessing a bifunctional group structure was synthesized by replacing the biphenyl group with the bithiophene group with redox activity, which can enhance the optical properties and inter-layer π-π conjugation. Besides the electrochromic properties, the morphology and crystal lattice parameters are different. Therefore, it is expected to develop COFs-based electrochromic materials with rich colors, excellent performance, and wide application by introducing construction units with different redox active centers.