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, usually a 2H-1-benzopyran, linked via a common spiro group to another heterocyclic ring (1.1). Irradiation of the colourless spirobenzopyran (1.1) with UV light causes heterolytic cleavage of the carbon-oxygen bond forming the ring-opened coloured species, often called the "merocyanine" form, which can be present as either cis (1.2) or trans (1.3) isomers (Figure 1.2). They can also be drawn as for the ortho-quinoidal form, as represented by (1.4) for the trans- isomer (1.3). The structure of the ring-opened form is probably best represented by a delocalised system with partial charges on nitrogen and oxygen atoms. For simplicity's sake we will use the equivalent of the trans merocyanine structure (1.4) in this text.

A very large number of possibilities exists for varying the components of the spiropyran ring. The pyran ring is usually substituted benzoor naphthopyran but the heterocyclic component can be chosen from a long list of ring systems including indole, benzthiazole, benzoxazole, benzselenazole, quinoline, acridine, phenanthridine, benzopyran, naphthopyran, xanthene, pyrrolidine and thiazolidine. The thiopyran analogues have attracted much interest, as on ring opening they absorb at longer wavelengths than the corresponding pyrans. Spiroindolinobenzopyrans are readily synthesised typically by reacting Fischer's base or a derivative with aromatic hydroxyaldehydes (salicylaldehydes).

The open-chain form of the spirobenzopyran shows a strong, intense absorption in the visible region of the spectrum typical of merocyanine dyes (see Chapter 2). Because of the thermal instability of the open-chain form it is necessary to use a rapid scanning spectrophotometer to measure the absorption spectrum. The ring-opened form of spiroindolinobenzopyran (1.4) has absorption at λmax 531 nm in toluene. This class of compounds exhibits a strongly positive solvatochromic effect (see Section 1.7), with the shape of the absorption curve changing and its position moving hypsochromically as the solvent polarity increases. A review of these and other closely related spiroheterocycles provides good insight into the origins of their photochromic, thermochromic and solvatochromic properties. From the data given in Table 1.2, it can be seen that substituents in the 3-, 6- and 8-positions of the original spiropyran ring (see 1.1) have the biggest influence on the spectral properties of the coloured form. The large bathochromic shift in (1.4a) versus (1.4b) is credited to steric hindrance caused by the group in position 3, whilst that from the nitro group at position 8 is considered to be due to interaction of the phenolate anion with the oxygen atom of the nitro group. Replacing the isoindoline group in (1.4a) with a benzoxazole ring causes a hypsochromic shift (600 nm) whilst a benzthiazole ring moves the absorption bathochromically (635 nm).

1.2.3 Spironaphthoxazines

Spirooxazines, the nitrogen-containing analogues of the spiropyrans, are very resistant to photodegradation. Known in this field as fatigue resistance, it is an essential property for those photochromic materials designed for applications in solar protection uses, such as sun spectacles. The photochromic ring opening of the benzannelated spironaphthoxazine analogue to its coloured form is shown in Figure 1.3.

Nitrosonaphthols (1.5), the precursors used in the synthetic route to spirooxazines, are much more stable than the nitrosophenols required for the parent benzo analogue and hence all the commercially available products are based on the spiroindolinonaphthoxazines ring structure (1.6).

The spiroindolinonaphthoxazine derivatives became commercially important compounds once detailed research work led to products which overcame many of their initial weaknesses, such as relatively poor fatigue resistance and a poor colour range (550–620 nm). The important positions for substitution in the ring of (1.6) are the 50-position, which has a large effect on the colour; the 60'-position, which has a major effect on both the colour and properties such as optical density (OD) and extinction coefficient; and the alkyl group on position 1, which has a kinetic effect on the rate of loss of colour back to the colourless state.

Another group of commercially important spirooxazines are those where the naphthalene ring is replaced by quinoline to give the spiroindolinopyridobenzoxazines (1.7).

The spectrum of the ring-opened form of the parent spiroindolinonaphthoxazine (1.6) has λmax at 590 nm (acetone). Spiroindolinonaphthoxazines also show a negative solvatochromic shift, the absorption moving hypsochromically (20–60 nm) in less polar solvents (e.g. toluene versus ethanol), the converse of what happens with spiroindolinobenzopyrans (see Section 1.2.2.1).

Introduction of dialkylamino substituents in the 6'-position of (1.6) causes a hypsochromic shift in the absorption maximum of the coloured state and also an increase in its intensity. This hypsochromic shift can also be increased by introducing electron-withdrawing groups into the 5-position of (1.6), whilst electron-donating groups move the absorption maximum in the opposite direction (Table 1.3).

Changing the alkyl substituent on the 1-position of (1.6) has little or no effect on the absorption maxima and no effect on the fatigue resistance. However, there is a very marked and technically important effect on the loss in the initial optical density of the coloured state after activating with UV light. This is frequently reported as the percentage loss in initial optical density ten seconds after removing the UV source, the IODF10 value. The more highly branched the alkyl group the lower the IODF10; methyl shows an IODF10 of 29% whilst for neopentyl this drops to 9%. Additionally, increasing the branching causes a lowering of the temperature dependence of the thermal conversion back to the colourless state.

1.2.4 Benzoand Naphthopyrans (Chromenes)

The photochromic compounds of potential interest, based on the 2H-chromene-ring system, are the 2H-benzopyrans (1.8) or the three isomeric naphthopyrans (1.9–1.11). However, 2H-naphtho[2,3-b]pyrans (1.11) show little or no useful photochromic behaviour and can be discounted from any further discussion. Although R1 and R2 can be part of a carbocyclic spiro ring, they are more commonly unconnected substituents such as gem dialkyl or aryl groups.

The photochromic mechanism for the chromenes is very similar to that for spiropyrans given in Figure 1.1. Under the influence of UV the C–O bond in the pyran ring is broken as in Figure 1.4, where formation of the cis-quinoidal species occurs in picoseconds, followed by isomerisation to the trans-form in nanoseconds.

The two naphthopyrans of interest (1.9) and (1.10) show quite different photochromic behaviour. Isomer (1.10; R1, R2 = Ph) produces a more bathochromic coloured state than (1.9; R1, R2 = Ph), (λmax = 481 nm versus 432 nm in toluene), a large increase in coloration, but a very slow fade rate back to the colourless state. Because of the slow kinetics, coupled with a greater ease in synthesis, most of the work, until the mid 1990s when these problems were overcome, concentrated on the 3H-naphtho[2,1-b]pyrans (1.9).

Simple 2,2-dialkyl-2H-benzopyrans can be synthesised by several well-established routes. 3H-Naphtho[2,1-b]pyrans (1.9) with an amino or alkoxy residue in the 6-position (1.14), which show particularly high colourability, are synthesised from the corresponding 1-aminoand 1-alkoxy-3-hydroxynaphthalenes (1.12 and 1.13) as illustrated in Figure 1.5.

As mentioned in Sections 1.2.2 and 1.2.3, the photochromic reactions of spirobenzopyran and spironaphthoxazines show a marked solvent dependency and this is also the case with benzoand naphthopyrans. Consequently, spectral data collected from the literature are only comparable within any one study or where the same solvent has been used. This accounts for any discrepancies between one set of results and any other one listed in this and related sections of the chapter. The data normally quoted when discussing the properties of photochromic materials relate to the absorption maximum (λmax) of the coloured state, the change in optical density on exposure to the xenon light source (ΔOD) and the fade rate (T1/2), which is the time in seconds for the ΔOD to return to half of its equilibrium value. Two other measurements, often quoted in the literature, are the IOD (initial optical density) at λmax and the IODF10 value already described in Section 1.2.3.

The influences on the absorption spectra and the other photochromic properties of compounds with substituents in the 3H-naphtho[2,1,b]pyran ring and on the 3,3'-aryl groups have been studied in detail. Electron-donating groups in one or both of the 3-phenyl groups, especially in the para-position, show a marked bathochromic shift in the absorption maxima of the coloured state, whilst electron-withdrawing groups have the opposite effect (Table 1.4). Substitutions in the ortho-position have little effect on the absorption maxima but have a very marked effect on the rate of return back to the colourless state, presumably due to stabilisation of the open-chain form (Table 1.4).