Dr Peter Bamfield has a PhD in Chemistry and over 40 years involvement in colours research. He retired as Research Manager for the Colours Business of ICI/Zeneca in 1993 and since then has acted as a freelance consultant to international companies, publishing reviews and presenting papers at colour conferences, whilst maintaining an up to date knowledge of the subject area. He authored a book on Research & Development Management in the Chemical and Pharmaceutical Industry (Wiley-VCH) which is now in its 3rd edition.

Dr Peter Bamfield has a PhD in Chemistry and over 40 years involvement in colours research. He retired as Research Manager for the Colours Business of ICI/Zeneca in 1993 and since then has acted as a freelance consultant to international companies, publishing reviews and presenting papers at colour conferences, whilst maintaining an up to date knowledge of the subject area. He authored a book on Research & Development Management in the Chemical and Pharmaceutical Industry (Wiley-VCH) which is now in its 3rd edition.

Chromic Phenomena

Technological Applications of Colour Chemistry

By Peter Bamfield, Michael G. Hutchings

The Royal Society of Chemistry

Copyright © 2010 Peter Bamfield and Michael G. Hutchings
All rights reserved.
ISBN: 978-1-84755-868-8

Contents

Introduction, 1,
Chapter 1 Phenomena Involving a Stimulated Colour Change, 9,
Chapter 2 Phenomena Involving the Absorption and Reflectance of Light, 141,
Chapter 3 Phenomena Involving Absorption of Energy Followed by Emission of Light, 234,
Chapter 4 Phenomena Involving Absorption of Light and Energy Transfer, 366,
Chapter 5 Chromic Phenomena Involving the Manipulation of Light, 471,
Subject Index, 548,

CHAPTER 1

Phenomena Involving a Stimulated Colour Change

1.1 Introduction

Colour is perceptually conspicuous, a property we can discern directly using our eyes. Therefore any 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, can be easily detected in a direct way by an observer or in a secondary way by the use of simple spectrophotometric instruments. As such, changes in colour provide very important visual signals that can be used to convey useful information to the observer, the most obvious in everyday experience being traffic control signals. 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. Consequently, 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. 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.

Some clarification is required on the terminology used in this area. Chromic materials is a term widely used to cover products that exhibit chromic phenomena, finding applications in what are known as smart or intelligent materials. Chromogenic phenomena, whilst synonymous with chromic phenomena, has been adopted as the preferred one in the automotive and architectural areas, hence chromogenic materials. 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.

These colour change phenomena, whether known as chromic or chromogenic, are classified and named after the stimulus that causes the change. Accordingly, photochromism 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, many of which are very specific and 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 sections below. Among the miscellaneous chromisms some are growing in commercial importance, namely gasochromism, vapochromism, mechanochromism and those due to aggregation or morphological changes, called aggregachromism, and the more recent one due to plasmonic effects in metal nanoparticles.

1.2 Photochromism

Photochromism is a chemical process in which a compound undergoes a reversible change between two states having separate absorption spectra, i.e. different colours, whether the compound is in a crystalline, amorphous or solution state. The change, as illustrated in Figure 1.1, from a thermodynamically stable form A to B occurs under the influence of electromagnetic radiation, usually UV light, and in the reverse direction, B to A, by altering or removing the light source or alternatively by thermal means. When the back reaction occurs photochemically this is known as type P photochromism and when thermally as type T photochromism. The change in colour in the forward direction is usually to longer wavelength, or bathochromic, as shown in Figure 1.1. The reversibility of this distinct colour change is key to many of the uses of photochromism.

In many systems, including spiropyrans, spirooxazines and chromenes, the back reaction is predominantly thermally driven, but in others the photochemically induced state is thermally stable and the back reaction must be driven photochemically, as in fulgides and diarylethenes. The assistance of heat in the reversion of colour can be regarded as an example of thermochromism, but in this text the term is reserved for those systems where heat is the main cause of the colour change (see 1.3).

Photochromism is a vast field and in this section of the book we will only be able to describe in any detail the main classes of photochromic compounds. However, whilst the main emphasis will be on the current commercial applications for these materials, this is a constantly changing field and so some applications of a more speculative nature, especially those having potential in the near future, will also be described. For more detailed accounts of the classes of photochromic materials the reader should consult the books on photochromics edited by Crano and Guglielmetti or the book and review by Bouas-Laurent and Durr.

1.2.1 Main Chemical Classes

For most commercial applications the minimum properties required for practical use from any class of organic photochromic compounds are:

1. Colour development. The material must develop a strong colour rapidly upon irradiation with UV light.

2. Control of return back to colourless state. The fade rate back to the colourless state must be controllable.

3. Wide colour range. The range of colours must be across the visible spectrum.

4. Long life. The response must be constant through many coloration cycles.

5. Colourless rest state. The rest state must have as little colour as possible, preferably colourless.

There are five main classes of compounds which can approach these ideal requirements: spiropyrans, specifically spiroindolinobenzopyrans, spironaphthoxazines, naphthopyrans, fulgides and diarylethenes.

1.2.2 Spirobenzopyrans

Spirobenzopyrans are a very widely studied chemical class of compounds which exhibit photochromism. They consist structurally of a pyran ring,...