Electrochromism is the property that some chemical species possess to change color reversibly when an electric charge is applied to them.

A good example of an electrochromic material is polyaniline, which can be synthesized by chemical [1] or electrochemical oxidation of aniline. If an electrode is immersed in hydrochloric acid containing a low concentration of aniline, a polyaniline film may grow on the electrode. According to the redox state, polyaniline can be either pale yellow or shaded black-green. Other electrochromic materials for which technological applications have been found are viologens and polyoxotungstates. We can cite tungsten oxide (WO) as another example of an electrochromic material, which is the chemical species used in the manufacture of electrochromic windows (called smart windows).

Since color change is persistent and energy is needed to make a color change, electrochromic materials are used to control the amounts of light and heat that can pass through a smart window, and are also applied in the automotive industry to tint mirrors according to different lighting conditions. Viologens are used in combination with titanium dioxide (TiO) to make smaller digital displays. This combination could replace liquid crystal screens since the viologen (normally a dark blue color) contrasts strongly with the bright white of the titanium, thus giving great readability to the screen.