Electrochromism

[2] In this way, a smart window made of an electrochromic material can block specific wavelengths of ultraviolet, visible or (near) infrared light.

[11] As organic materials, viologens are seen as promising alternatives for electronic applications, compared to metal-based systems, which tend to be expensive, toxic, and a problem to recycle.

[16] Possible advantages of viologens include their optical contrast, coloration efficiency, redox stability, ease of design, and potential to scale up for large-area preparation.

[18] Viologens have been used with phenylenediamine by Gentex Corporation, which has commercialized auto-dimming rearview mirrors[18] and smart windows in Boeing 787 aircraft.

By applying a lower voltage (2.5 V for example) the concentration of Li-ions in the electrochromic layer decreases, thus activating (N)IR-active tungsten oxide.

[25][3] This activation causes reflection of infrared light, thus lowering the greenhouse effect, which in turn reduces the amount of energy needed for air conditioning.

[26] Depending on the electrochromic material used, different parts of the spectrum can be blocked, this way UV, visible and IR light can be independently reflected at the will of a user.

[27] NiO materials have been widely studied as counter electrodes for complementary electrochromic devices, particularly for smart windows.

[28][29] ICE 3 high speed trains use electrochromic glass panels between the passenger compartment and the driver's cabin.