Electrochromic display device and compositions useful in making such devices

A display device comprising at least two electrodes, one being anodic and one being cathodic, and positioned between those two electrodes in such a manner to be visible a composition comprising:

• (a) a compound that undergoes a reversible redox reaction to generate a pH gradient between the two electrodes,
• (b) at least one indicator dye, and
• (c) a charge transport material wherein components (a), (b) and (c) are different from one another and the standard reduction potential of component (a) is less than the standard reduction potential for the other components.

A composition comprising:

• (a) a compound that undergoes a electron transfer reaction with subsequent change in its protic state
• (b) at least one indicator dye which changes color when a change in pH occurs, and
• (c) an ionically conductive material and optionally component (d), a matrix material, wherein components (a), (b), (c) and (d) are different from one another and component (a) preferentially undergoes the electron transfer reaction when a charge is applied to the composition and provided that if (c) is a fluid the composition further comprises the matrix material (d).