Enhancement of long-cycle electrochromic stability of covalent organic framework films via linkage conversion

Covalent organic frameworks (COFs) with tunable redox activity have emerged as promising electrochromic (EC) materials owing to their structural diversity, tunable bandgaps, interlayer π-π electron delocalization and ordered porosity. However, most imine-linked COF-based EC materials suffer from weak interlayer interactions and inefficient charge transport, leading to poor long-term cycling stability. Herein, we present an efficient linkage conversion strategy, where an imine-linked COF (Im-COF) is oxidized to an amide-linked COF (Am-COF), resulting in a significant improvement in the long-term EC cycling stability of COF films. The Am-COF shows 94.6 % retention after 550 cycles, compared to 81.4 % retention after 480 cycles for Im-COF. Such performance disparity is attributed to differences in interlayer interactions, charge separation, and charge transfer efficiency, as confirmed by characterizations and theoretical calculations. Additionally, at 1050 nm, the Am-COF film exhibits a high optical contrast of 87 %, a rapid response time of 0.6/1.1 s (coloration/bleaching), and a high coloration efficiency of 275 cm² C^-1. This work highlights linkage engineering as an effective approach to optimize the charge transfer and redox reversibility of COF-based EC materials, providing a viable pathway for their practical application in advanced optoelectronic devices.