Electrochromic materials, which change color or transparency in response to an applied voltage, hold great promise for applications such as smart windows, anti-glare mirrors, and low-energy displays. However, achieving a truly transparent state, especially in metal-based systems, remains a significant challenge. Many existing materials suffer from limited optical contrast, slow switching speeds, or poor cycling stability, hindering their practical use.
In a study published in Angewandte Chemie International Edition, the research group led by Prof. CHEN Zhongning at the Fujian Institute of Research on the Structure of Matter (FJIRSM), Chinese Academy of Sciences (CAS), has developed a novel manganese(II)-based metallopolymer film that exhibits exceptional electrochromic performance. The film switches reversibly among three distinct states (colorless↔brown-yellow↔blue) with high contrast, fast response, and remarkable cycling stability of up to 10,000 cycles.
The researchers designed a hexadentate nitrogen ligand derived from diethylenetriamine, functionalized with two electropolymerizable triphenylamine groups. This ligand coordinates with manganese(II) to form a colorless Mn(II) chelate. The half-filled d5 configuration of Mn(II) suppresses visible-region d-d transitions, enabling a transparent initial state. Through oxidative electropolymerization on an ITO electrode, the Mn(II) chelates were linked to form a uniform metallopolymer film.
Scanning electron microscopy revealed a nanoporous structure conducive to fast ion transport. Spectroelectrochemistry confirmed that the film remains colorless until a voltage is applied, whereupon stepwise oxidation at Mn(II) and triphenylamine sites induces color changes: first to brown-yellow (0.9 V) and then to blue (1.3 V).
The researchers fabricated a solid-state electrochromic device using the metallopolymer film as the active layer, paired with a TiO2 ion storage layer and a UV-cured electrolyte. The device achieved optical contrasts of 40% (brown-yellow) and 73% (blue), with rapid switching times as fast as 1.4 s for coloring and 0.4 s for bleaching. High coloration efficiencies of 337.5 cm2 C-1 and 397.1 cm2 C-1 were achieved for the two colored states, respectively.
Notably, the device demonstrated outstanding long‑term stability. After 10,000 continuous switching cycles between the colorless and brown‑yellow states, the optical contrast retained 80% of its initial value, surpassing most of WO3-based and organic‑polymer devices.
Moreover, thanks to the simple and scalable electrophysmerization process, large‑area devices up to 20 × 20 cm2 were fabricated, which exhibited the same reversible three‑state color switching (colorless↔brown‑yellow↔blue), highlighting the technology’s potential for real‑world applications such as smart windows and large‑format displays.
This study demonstrates a clever molecular engineering strategy to overcome key limitations in electrochromic materials: achieving a transparent state, enhancing stability, and enabling multicolor switching. The use of a stable Mn(II) chelate combined with electropolymerizable units offers a versatile platform for designing durable, high-performance electrochromic devices. This research paves the way for next-generation smart windows and displays that are efficient, low-cost, and easy to fabricate on a large scale.
The UV-vis-NIR absorption spectra and cycling stability of all-solid-state Mn-based electrochromic device. (Image by Prof. CHEN's group)
Contact:
Prof. CHEN Zhongning
Fujian Institute of Research on the Structure of Matter
Chinese Academy of Sciences
Email: czn@fjirsm.ac.cn
