Organic Electrochromic Material Changes Colors Rapidly, Reversibly

Ludwig-Maximilians-Universität researchers developed an electrochromic thin-film material that, by changing colors rapidly, is poised to feature in the design of smart windows, solar energy production/acquisition, and automotive applications. The material, which relies on electricity to change colors, is part of a generation of highly ordered lattice structures, known as covalent organic frameworks (COFs). These are made up of synthetically conceived organic building blocks that, under the right conditions, form crystalline and nanoporous networks.

An applied electrical voltage that causes an oxidation, or reduction of the material, is sufficient to generate a color change.

The team was able to develop COF structures with switching speeds and coloration many times higher than that of inorganic compounds. The material properties of COFs are able to be adjusted over a broad range by modifying their molecular building blocks.

The researchers were able to take advantage of those properties to design COFs ideal for their purposes.

“We have made use of the modular construction principle of the COFs and designed the ideal building block for our purposes with a specific thienoisoindigo molecule,” said Derya Bessinger, first author and Ph.D. student in professor Thomas Bein’s group. “For example, with the new material, we cannot only absorb the shorter-wavelength UV light or small parts of the visible spectrum, but also achieve photoactivity well into the near-infrared spectral regions.”

Additionally, the new COF structures are far more sensitive to electrochemical oxidation; even at a low applied voltage, the materials are able to change color, and completely reversibly so. This happens at very high speeds. The response time for a complete and distinct color change by oxidation is about 0.38 seconds, while the reduction back to the initial state takes only about 0.2 seconds. The technology outperforms previous COFs by at least one order of magnitude.

The high speed is made possible by two factors. First, the conductive framework structure of the COFs allows fast electron transport in the lattice. Due to an optimized pore size, the surrounding electrolyte solution is able to reach every corner quickly, which is essential as the positive charge generated in the oxidized COF structure has to be charge-compensated by negative electrolyte ions.

The technology also showed a high level of stability. Long-term tests demonstrated that the material was able to maintain performance even after 200 oxidation-reduction cycles.

The material has potential applications including smart windows and energy efficient optical displays, due to its high coloration efficiency and fast switching.

The research was published in the Journal of the American Chemical Society (www.doi.org/10.1021/jacs.0c12392).