Smart glass can quickly change color when exposed to electricity. A new material developed by chemists at the Ludwig-Maximilians-Universität (LMU) in Munich set a speed record for such a change.

Imagine driving on a highway at night. It is raining, and the bright headlights of the car behind you are dazzling. How convenient it is to have an auto-dimming rearview mirror. Technically, this useful supplement is based on electrochromic materials. When voltage is applied, their light absorption and color change. In this way, the rearview mirror controlled by the ambient light sensor filters out glare.

Recently, experts have discovered that, in addition to the already known inorganic electrochromic materials, a new generation of highly ordered lattice structures – covalent organic frameworks (COFs) – can be equipped with this capability. Such materials are composed of synthetic organic building blocks. In suitable combinations, they form crystalline and nanoporous networks. Here, the color change can be caused by the applied voltage. It causes oxidation or reduction of the material.

A team of scientists from LMU led by Thomas Bain has developed COF structures, the switching speed and staining efficiency of which are many times higher than that of inorganic compounds. COFs are attractive in that their material properties are controlled over a wide range, as soon as their molecular building blocks are changed. Scientists at the LMU in Munich and the University of Cambridge have taken advantage of this to develop ideal COFs.

The scientists used the COF modular design principle and designed the perfect building block for our purposes with a specific thienoisoindigo molecule. Included in COF, the new component demonstrates how much it can improve its properties. For example, the new material not only absorbs shorter wavelengths of ultraviolet light or small portions of the visible spectrum, but also achieves good near-infrared photoactivity.

At the same time, the new COF structures are much more sensitive to electrochemical oxidation. Even a low applied voltage is sufficient to cause a color change, which is also completely reversible. It also happens at a very high speed: the response time for a complete and distinct color change as a result of oxidation is about 0.38 seconds, recovery to original state is 0.2 seconds. This makes the electrochromic organic structures of the e-conversion team one of the fastest and most efficient in the world.

The study is driving the development of a new class of high performance electrochromic coatings. An obvious need for this is evident in the current applications of smart glass such as switchable sunscreens and sun-shaded windows for entire building facades.