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Part 1:
Colour Change Phenomena and their
Applications
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CHAPTER 1
Introduction
Colour is a property we can discern directly using our eyes, and hence is described as being perceptually conspicuous. This means that a change in the colour of an object, whether this is achromatic from white to black, or chromatic from colourless to coloured or one colour to another, is easily detected directly by people with normal vision or, in a secondary way, by the use of simple spectrophotometric instruments. Such changes in colour provide very important visual signals that we can use to convey valuable information to an observer, the most obvious in everyday experience being the ubiquitous signals that are used to control traffic flow. When illuminated, red means stop, green go and amber take care, easily seen and unambiguously understood. 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 on sunny days by users of spectacles with darkened glass lenses. When a third parameter, namely an external stimulus, whether this is chemical or physical, is the cause of the change in colour or the restriction of light transmission, especially when this change is reversible, the potential applications significantly widen. Understandably, research into chemicals that undergo changes in colour upon the application of an external stimulus, especially when this change can be effected in real time, has been extensive and ongoing. Chemical and material products of this work have found uses in a wide variety of outlets, in both low and high technology areas, and the number of applications shows no sign of diminishing, as the contents of this Part will show.
Some clarification is required on the terminology used in this area. Chromic materials is a term widely used to cover products, which exhibit chromic phenomena, finding applications in what are known as smart or intelligent materials.1a,b The term chromatic phenomena has also been suggested, but this has been avoided, since this it is already widely used in music, art and the optical sciences. Chromogenic phenomena, whilst synonymous with chromic phenomena, has been adopted as the preferred one in the automotive and architectural areas, hence chromogenic materials.2a,b There is a further complication in inorganic chemistry, especially where metal complexes are involved, as here the topic is called chromotropism, but definitely not chromotropic which is reserved for the naphthalene sulfonic acid of that name.2c
These colour change phenomena, whether known as chromic or chromogenic, are classified and named after the stimulus that causes changes in the differential absorption, and reflection and/or scattering of white light. For this reason, stimuli-responsive materials is a generic name commonly applied to the products used in the various application areas for colour change.2d There are two basic types of chromisms; direct and indirect. Direct is obviously where the stimulus itself causes an almost instantaneous change in colour, e.g. photochromism by light, whereas with indirect chromisms there is not an immediate change in colour but one that only occurs via an intermediate index, which can be either chemical or physical, e.g. chronochromism which involves a passage of time.
Some of the names of these chromisms are immediately obvious. Photochromism, as stated above, is a change in colour, usually colourless to coloured, brought about by light, and the material or chemicals undergoing this change are photochromic. 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. A long list of names, shown in Table 1.1, has been devised to describe such chromic phenomena, from the very specific to others which seem to have been invented on the whim of a particular researcher, e.g. waterchromism for which alternatives like aquachromism and hydrochromism already existed. We have made an attempt to rationalise the nomenclature applied to these chromic phenomena in Table 1.1. To date the most important commercially of these phenomena are photochromism, thermochromism, electrochromism, ionochromism and solvatochromism, and consequently these will be covered in some detail in the chapters below. Among the miscellaneous chromisms several have grown in commercial importance in recent years, namely gasochromism, vapochromism, mechanochromism, and those due to aggregation or morphological changes, called aggregachromism, and the more recent ones due to plasmonic effects in metal nanoparticles.
Table 1.1 List of chromisms.a–s
| Electrochromism | Ionochromism | Mechanochromism | Photochromism | Solvatochromism | Thermochromism | Hybrids (dual stimulus) | Miscellaneous |
| Gasochromism | Acidochromism | Piezochromism | Diastereo-photochromism | Aquachromism | Diastereo-thermochromism | Photoelectro-chromism | Group A |
| | | | Hydrochromism | | Photovolta-chromism | Affinochromism |
| | | | Hygrochromism | | Bioelectro-chromism | Affinitychromism |
| | | | | | | Biochromism |
| Halochromism | Tribochromism | Heliochromism | Rigidochromism | Cryochromism | Thermosolvato-chromism Halosolvato-chromism | Group B |
| | | | Viscochromism | | | Aggregachromism |
| | | | | | | Amorphochromism |
| | | | | | | Crystallochromism |
| | | | | | | Concentrato-chromism |
| Metallochromism | Barochromism | Chirochromism | Sorptiochromism | | Electromechano-chromism | Group C |
| | | | | | Electropiezo-chromism | Cathodochromism |
| | | | | | | Magnetochromism |
| | | | | | | Radiochromism |
| Alkalinochromism | Rheochromism | | Aromachromism | | Chronochromism | Goniochromism |
| | | | Vapochromism | | | |
