Dynamic colour of chromogenic materials

Abstract

Chromogenic materials change colour due to some stimulus. They are increasingly applied in the so-called smart applications where the change of colour is used. Two examples of such applications are presented, electrochromic windows and thermochromic printing inks. Active component in many switchable smart windows is a thin layer of electrochromic material. Most frequently WO3 is applied, which changes reversibly from colourless to deep blue state, when a constant current is provided in one or reverse direction. It permits the ability to control the light passing through. The variable transmittance influences colour rendering of the incoming daylight. The appearance of coloured objects in the interior could be strongly distorted at higher coloration states of such windows. Thermochromic printing inks have become increasingly important for various smart applications in graphic art such as smart packaging and security printing. The colour of such inks depends reversibly on chemical reactions between active components in pigment capsules. The colour of a sample does not depend only on temperature, but also on thermal history – such a system has a memory. The phenomenon is colour hysteresis. Advanced application of chromogenic materials needs careful colorimetric characterization.