Chemists are developing new material that allows smart glass to change color in record time

Smart glass can quickly change color by means of 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 you are on the highway at night. It’s raining, blinding the bright headlights of the car behind you. How handy to have an automatically dimming rear view mirror in such a case. Technically, this handy extra is based on electrochromic materials. When a voltage is applied, their light absorption and color change. The rear-view mirror is controlled by a light sensor and can thus filter out strong blinding light.

Experts recently discovered that, in addition to established inorganic electrochromic materials, a new generation of highly ordered grid structures can also be equipped with this capability: so-called Covalent Organic Frameworks, or COFs for short. They consist of synthetically produced organic building blocks that form crystalline and nanoporous networks in suitable combinations. Here the color change can be caused by an applied electrical voltage causing 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 speeds and coloring efficiency are many times higher than those of inorganic compounds. COFs are attractive because their material properties can be modified over a wide range by modifying their molecular building blocks. Scientists from LMU Munich and the University of Cambridge took advantage of this to design COFs that were ideal for their purposes.

“We took advantage 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, lead author and Ph.D. student in Thomas Bein’s group. Incorporated into a COF, the new component shows how much it can improve the properties of the COF. “For example, with the new material we can not 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,” 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 enough to cause a color change of the COFs, which is also completely reversible. Moreover, this is done at a very high speed: the reaction time for a complete and clear color change due to oxidation is about 0.38 seconds, while the return to the original state takes only about 0.2 seconds. This makes the e-conversion team’s electrochromic organic frameworks the fastest and most efficient in the world.

Two things in particular are responsible for the high speed: The conductive framework structure of the COFs enables fast electron transport in the lattice. And thanks to an optimized pore size, the surrounding electrolyte solution can quickly reach any corner. This is essential because the positive charge generated in the oxidized COF structure must be quickly compensated for by negative electrolyte ions. Last but not least, the product of the scientists from Munich has a very high stability. Long-term tests showed that the material could maintain its performance 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 for this is evident from the current applications of such “smart glass”, such as switchable solar shading and privacy-resistant windows for facades of entire buildings.