1.1 Introduction

Any changes in the colour of an object, whether this is from white to black, colourless to coloured or from one colour to another, can be easily detected by the human eye or by using simple spectrophotometric instruments. Such changes in colour therefore provide very important visual signals that can be used to convey useful information to an observer, the most obvious being traffic control signals. In addition, by selective absorption or transmission of light by a material, it is possible to restrict the light energy impinging upon an observer, as experienced with the darkened glass in sun spectacles. When a third parameter is introduced, namely an external stimulus, whether this is chemical or physical, that causes the change of colour or the restriction of light transmission, especially when this change is reversible, the potential applications significantly widen. Consequently, research into chemicals that undergo reversible changes in colour upon the application of an external stimulus has been extensive, and the products of this work have found uses in a wide variety of outlets, in both low and high technology areas.

These colour change phenomena are classified and named after the stimulus that causes the change. Accordingly, photochromism is a change in colour, usually colourless to coloured, brought about normally by UV light, electrochromism is a reversible colour change upon oxidation or reduction brought about by an electrical current or potential, thermochromism is a colour change brought about by heat, solvatochromism by solvents and ionochromism by ions, etc.