Construction of Pyrene-based Covalent Organic Frameworks with D- π -D- π Molecular Structure and Their Electrochromic Properties

Covalent organic frameworks (COFs) have attracted extensive interest in the field of optoelectronic functional materials due to their high design freedom, flexibility in structural modulation, and ease of introducing functional groups. Pyrene, the smallest densely-packed polycyclic aromatic hydrocarbon, possesses excellent thermal stability and high carrier mobility properties. As a building block of COFs, its integration has attracted wide attention. Here, 1,3,6,8-tetrakis(4-aminophenyl)pyrene (Py(NH 2 ) 4 ) and 1,3,6,8-tetrakis((4-(4′-amino-1-ethynylphenyl)pyrene (TAEPy) were selected as amine construction units. Also, 1,3,6,8-tetrakis(4-formylphenyl)pyrene (Py(CHO) 4 ) and 1,3,6,8-tetrakis((4-(4′-formyl-1-ethynylphenyl)pyrene (TFEPy) were used as aldehyde building units. We synthesized two imine-based COF electrochromic materials (Py-Py COF and TATFEPy COF) via a solvothermal method. The acetylene group, with its high electron mobility and electron density, acts as a π bridge. Together with the pyrene group as the donor (D), it forms a D- π -D- π structure. This structure enhances π – π stacking between COF molecules and boosts material stability. Tests show that TATFEPy COF outperforms Py-Py COF in electrochromism. Their contrasts are 0.45 and 0.30, and the coloring/bleaching times are 8.5/12.0 s and 12.0/20.0 s, respectively. After 2000 s cycling, contrast retention is 73.3% for TATFEPy COF and 54.0% for Py-Py COF. Therefore, constructing D- π -D- π molecular structures is an effective method for designing and synthesizing high-performance electrochromic polymers, enhancing their optoelectronic properties.