Fast-Switching Vis–IR Electrochromic Covalent Organic Frameworks

Abstract

Electrochromic coatings are promising for applications in smart windows or energy-efficient optical displays. However, classical inorganic electrochromic materials such as WO 3 suffer from low coloration efficiency and slow switching speed. We have developed highly efficient and fast-switching electrochromic thin films based on fully organic, porous covalent organic frameworks (COFs). The low band gap COFs have strong vis–NIR absorption bands in the neutral state, which shift significantly upon electrochemical oxidation. Fully reversible absorption changes by close to 3 OD can be triggered at low operating voltages and low charge per unit area. Our champion material reaches an electrochromic coloration efficiency of 858 cm 2 C –1 at 880 nm and retains >95% of its electrochromic response over 100 oxidation/reduction cycles. Furthermore, the electrochromic switching is extremely fast with response times below 0.4 s for the oxidation and around 0.2 s for the reduction, outperforming previous COFs by at least an order of magnitude and rendering these materials some of the fastest-switching frameworks to date. This combination of high coloration efficiency and very fast switching reveals intriguing opportunities for applications of porous organic electrochromic materials.

Donor‐Acceptor Type Covalent Organic Frameworks

“…This might be attributed to several reasons: 1) the nearly fixed design custom, that is, the widely adopted donor-linker-acceptor as the repeating unit in almost all reported DÀ A COFs; 2) the required geometric matching of D and A segments; 3) the synthetic complexity of D and A building units. In this context, referring to the extensively studied of small-molecular and polymeric semiconductors with intriguing architectures (e. g., AÀ DÀ A, DÀ AÀ D) and properties, it might be feasible to enrich the family of DÀ A COFs by diversifying the way of positioning D and A moieties, for example, DÀ AÀ D-linker-DÀ AÀ D, [65] DÀ D-linker-AÀ A.…”

Two‐dimensional Covalent Organic Frameworks for Electrochromic Switching

“…Recently, the importance of the D‐A design strategy in electrochromic performance was further investigated by Bein and co‐workers [29] . The specific advantage of the D‐A architecture over the simple π‐conjugated structure consisting solely of the donor (D) or acceptor (A) bridge was addressed.…”

“…Recently, covalent organic frameworks (COFs), a class of crystalline porous organic polymer, have received distinct attention as alternative electrochromic materials [26–30] . COFs with well‐defined two‐dimensional (2D) or three‐dimensional (3D) framework structures have been widely applied in diverse fields starting from gas storage, separation, water purification, sensing, catalysis to energy storage [17,31–39] .…”

“…Electrochromic 2D COFs are obtained by knitting the redox‐active aromatic building blocks with π‐conjugated linkers via covalent bonds [29] . The extensive π‐electron conjugation within the framework leads to a strong light absorption across the visible to NIR regions [40] .…”

“… [41] The rigid network structure and high crystallinity of 2D COFs result in a uniform pore size distribution and large specific surface area, which are beneficial for uninterrupted ion percolation (Figure 3). [26] The faster movement of counter ions through 1D pore‐channels of 2D COFs maintains the charge neutrality during oxidation/reduction processes leading to rapid and efficient electrochromic switching [29,30] . In addition, the processibility of COFs is highly desirable to fabricate an oriented thin film on the electrode surface for the construction of an electrochromic device (Figure 3).…”