Chemists develop new materials that help smart glass change color in record time

Smart glass can quickly change color through electricity. A new material developed by chemists from Ludwig-Maximilians-Universität (LMU) in Munich has now set a speed record for such a change.

Imagine being on the highway at night. It’s raining, dazzling the bright headlights of the car behind you. How convenient to have an automatic dimming rearview mirror in such a case. Technically, this useful extra is based on electrochromic materials. When a voltage is turned on, their league absorption and color change. The rearview mirror is controlled by a light sensor, and can therefore filter strong brilliant light.

Recently, experts have discovered that, in addition to established inorganic electrochromic materials, a new generation of highly ordered lattice structures can also be equipped with this capability: so-called Covalent Organic Frameworks, or COFs for short. It consists of synthetically produced organic building blocks that, in suitable combinations, form crystal and nanoporous networks. Here, the color change can be caused by an applied electrical voltage that causes an oxidation or reduction of the material.

The LMU team led by Thomas Bein (Physical Chemistry, LMU Munich) has now developed COF structures whose switching speed and color efficiency are many times higher than those of inorganic compounds. COFs are attractive because their material properties can be adjusted over a wide range by adapting their molecular building blocks. Scientists at the LMU Munich and the University of Cambridge used it to design COFs that were ideal for their purposes.

“We used the modular construction principle of the COFs and designed the ideal building block for our purposes with a specific thienoisoindigo molecule,” says Derya Bessinger, first author and Ph.D. student in the group of Thomas Bein. The new component is incorporated into a COF and shows how strongly it can improve the COF’s properties. “For example, with the new material, we can not only absorb the UV light with shorter wavelength or small portions of the visible spectrum, but also achieve good photo activity in the near-infrared spectral regions,” says Bessinger.

At the same time, the new COF structures are much more sensitive to electrochemical oxidation. This means that even a low applied voltage is sufficient to cause a color change of the COFs, which is also completely reversible. In addition, this happens at a very high speed: the response time for a complete and clear color change by oxidation is about 0.38 seconds, while the reduction back to the initial state takes only about 0.2 seconds. This makes the electrochromic organic frameworks of the e-conversion team one of the fastest and most efficient in the world.

There are two things in particular that are responsible for the high speed: the conductive framework structure of the COFs enables fast electron transport in the grid. And thanks to an optimized pore size, the surrounding electrolyte solution can quickly reach every nook and cranny. This is essential because the positive charge generated in the oxidized COF structure must be quickly compensated by negative electrolyte ions. Last but not least, the product of the Munich scientists has a very high stability. Long-term tests showed that the material could maintain its ability even after 200 oxidation-reduction cycles.

With these fundamental findings, the publication promotes the development of a new class of high-performance electrochromic coatings. The obvious demand is shown by the current applications of a ‘smart glass’ as convertible sun protection windows and private screen windows for entire buildings.