Photochromism Find C&I online at www.soci.org/chemistryandindustry Follow @ChemandInd Photochromism Olympian colour chemistry

also attracted the attention of the security The performance industry, to combat fraud and counterfeiting of passports, travel tickets, and even currency. of photochromic For industrially important dyes, the change is between a colourless and a coloured dyes has much state. The two dominant commercial types, spirooxazines and naphthopyrans, produce in common with colours that span the visible spectrum. Vivimed Labs Europe in Huddersfield, UK, for example, manufactures a Reversacol range of both kinds that of top-class that run the gamut from yellow and orange, to red and purple, through to blue and even athletes, in terms green. The appearance of colour with these compounds is usually driven by ultraviolet of their endurance, (UV) light. When the stimulus is removed, they fade back, eventually to their original speed, strength he summer of 2012 should be a highest and strongest. This summer will also from performance-enhancing substances! couple of decades. Ophthalmic lenses for colourless states. Unfiltered sunlight produces momentous one for the UK whatever see ‘athletes’ of another kind perform in the Photochromic colourants are organic spectacles that darken reversibly in sunlight pronounced photochromism because it has and teamwork, the weather. Years of planning will sunshine on an altogether smaller scale: compounds that change colour reversibly account for most commercial photochromic a significant UV component. Artificial light see the focus of the sporting world photochromic dyes that must demonstrate on exposure to light. In doing so, they must dyes today. However, photochromism is sources, such as compact fluorescent lamps Tcentre on London and the UK for a few weeks world-class speed, strength, endurance and perform an acrobatic change in molecular employed with plastics and surface coatings and tungsten filament bulbs, are much less writes Andy Towns as sportsmen and sportswomen compete teamwork. Like their human counterparts, they structure. The light-induced breaking, to create products with striking colour change effective. Consequently, the dyes colour up well in the spirit of the Olympian motto Citius, have to be given the right environment to coax bending and twisting has been exploited in properties, for example, from toys to clothing in daylight, but much less so indoors or behind Altius, Fortius to prove they are the fastest, the best out of them – and can even benefit a commercially successful way over the last and cosmetics to safety visors. They have glass, which tends to filter out the wavelengths

& & 32 Chemistry Industry • May 2012 Chemistry Industry • May 2012 33 X Me Me Me Me Photochromism Find C&I online at www.soci.org/chemistryandindustry Visit the C&I Facebook page to join the debate N X PhotochromismUV

N O heat N N Y Y alkyl alkyl molecular twisting and bending via Just as no sportsperson can both coloured and colourless forms for mixtures. However, when photochromism O In brief intermediates gives a planar species with an simultaneously be an Olympian sprinter, at least two years so that the wearer P-type photochromismX is involved, formulating blends does not extended conjugated π-system that leads weightlifter and marathonMe runner, the does not readily notice the steady loss in colourless spirooxazine simply involvec ocombiningloure dcolours photo in them erocyanine dye Photochromic colourants are to absorption moving into the visible light design of photochromic dyes involvesMe performance. In addition, the dyes must Me MGreate strides have been made over the past couple of right proportions to achieve the desired organic compounds that change region and the generation of colour. compromise. Typically, a balance must N X resist theU tendencyV to ‘fatigue’. This relates decadesX, Y with = Hcommercial, halog T-typeen, n photochromicitro, etc. dyes shade.(m Thei photochromicxture of ipropertiessomer ofs - others not shown) colour reversibly on exposure to UV The higher the intensity of UV light, be struck between rapid speed of fade to the occurrence of side-reactions that in terms of their colour palette, kinetic control and the individual components of a mixture the greater will be the proportion of dye and high colour strength, since the two compete with photo-isomerisation during robustness. However, because they fade thermally, they’re must also be closely matched. If rates of light X Y The main commercial use is in in its coloured state. As an example, properties tend to be mutuallyN exclusive.O activation.hea Theyt cause dye to be irreversibly N not suitedN to some keenly researchedX uses. These high- activation and half-lives differ too much lenses that darken in sunlight, irradiating oxazine-based systems with Making structuralY changes in order to converted to non-photochromicY molecules, tech applications – for example, as optical switches or between dyes, then the hue of the mixture but the technology is also used to UV favours ring opening of the colourless increase fade rate, often referredal ktoy asl ‘half- leading to a decrease in colour intensity on alinky datal storage - rely on P-type photochromism instead. will vary as it colours and fades. To be create plastics with striking colour- spiro form, shifting the equilibrium life’, which is the time taken for absorbance repeated activation. To get to the top and This phenomenon involves light-driven switching in each commercially acceptable, the dyes must O change properties towards the coloured photomerocyanine to halve, frequently results in a loss in stay there, sportspeople have to be focused direction so that, once activated by UV, a dye remains UV activate as well as fade in unison so that the A reversible light-driven reaction, isomers. Removal of the light source has strength. The chemist mustco lfindour thele smiddles sp irooxazine on what they do. Photochromic dyes coloured until it is switched back to its colourlessY state mixture stays on tone. involving coloured and colourless the opposite effect: the concentration of ground where sufficient depth of shade is must also be selective in theirc behaviour.oloure d photowithm visibleeroc lightyan ratherine dthanye by thermal fading. X Team members should also have the dye forms, is responsible for these the colourless ring-closed form increases, achieved while ensuringX, Y thermal = H bleaching, halo gen, nitro, etc. BecauseMe of the repeated(m ring-openingixture oandf i somersThe - otherdihetarylethenes notO sho class wis n)one of the most same high stamina levels. If components colour-change effects which is seen as fading. If the illumination is acceptably fast. For lenses, half-lives of closing on prolongedMe exposure, fatigue will extensively explored classes of P-typeM ephotochromicMe heat fatigue at different rates, then activated Achieving the desired level is constant, the system will settle the order of tens of seconds are practical. be pronounced if side-reactionsN make up X compoundsUV and has beenR targeted as a source of light- colour will change over the long term. O photochromic effect depends on into a ‘photostationary’ state in which Before activation, So that dyes can perform to their anything other than a tiny proportion of theX actuated elements in Ynanotechnology. Compounds from Additionally, the responses of the dyes balancing the various properties of the isomer concentrations hold steady. above, and below, potential, they need the right arena. X transformations between the different dye this class switch between coloured and colourless forms to changes in environment, such as R Thermodynamics dictates the intensity of after The gymnastic changes in shape forms. even in solid form. The transformation in structure that temperature, must be similar. N O heat N N needed for activation. colour; however, as the equilibrium reached and structure associated with their Y Rate constants for side-reactions for occurs upon photo-isomerisationY alters the dimensions Assembling a successful team is no easy The reverse reaction – from coloured is a dynamic one, dye molecules continue photochromic behaviour require a suitable spirooxazines and naphthopyrans are of crystals of these derivatives. Although the changes task, especially when up to six dyes may alkyl alkyl to colourless – is thermally driven, the to swap between their colourless and microenvironment. Colourants must also small, but evenUV here the constant photo- are small, they have been used to striking effect – be required. Oneco strategyloure isd to d formulateye (m ixture of isomers - Y colourless naphthopyran colourants being said to exhibit T-type coloured forms, despite their concentrations be dissolved, not dispersed, in the medium isomerisation that occurs on exposure X,dihetarylethene Y, R = H, alcrystalskoxy have, am beenino observed, etc. to bend andO mixtures from a single class ooft dyehe wherers n ot shown) photochromism. A key property of this remaining essentially unchanged. in which they are used because they do not to light means that formation of non- then straighten under the influence of light to kick nearby possible. Another commercially successful O colourless spirooxazine phenomenon is its reversible nature. The The ideal photochromic athlete for exhibit useful photochromism as solids. photochromichea byproductst can soon build particles away like miniatureco lfootballs.oured photomerocyaniapproachne dy ehas been to develop dyes that are different forms of the dyes are in dynamic use in lenses would at first glance be a Dyes tend to perform best in plastics Xup, Yover = time H ,reducing halo theirgen, useful n ilifetime.tro, e tc. O (mixture of isomers - others nointrinsicallyt sho wneutraln) in colour. This has been equilibrium, constantly switching between sprinter. Dyes not only need great activation with relatively flexible chains and low glassR Fortunately, performance-enhancing done by incorporating structural features Figure 2 coloured and colourless isomers. Figure speed, colouring up rapidly on exposure transition temperatures, like polyolefins chemicals can be employed – perfectly into a dye that convert it from a bright A strategyR for creating neutral coloured photochromic dyes Groups used here to adjust colour and kinetics 1 shows the interconversion between to strong light, but also a very quick fade and plasticised polyvinyls. In polymers legally – to improve their stamina. Several X colourantY with just a single peak in its visible colourless ring-closed and coloured that races back to colourless as soon as such as poly(styrene–acrylonitriles), types of additives are routinely exploited, absorption spectrum when activated to one X 1.4 1.4 Unactivated ring-opened isomers that occurs with light levels lower to prevent impairment of where the matrix is too rigid or affords including hindered amine light stabilisers 1.2 1.2 that has a more complex absorption curve. spirooxazines and naphthopyrans. Ring- vision. However, colourants face additional insufficient free volume, changes to that mop up damaging free radicalscolo andu re d dye1.0 (mixture of isoActivatedmers - 1.0 UnactivatedFor example, an electron donor function in closed forms consist of two halves that demands. Their ring-opened isomers must molecular shape areco restrictedlourless so that n aphthopyran triplet state quenchers that help prevent 0.8 0.8 Activatedthe right position will furnish a colourant are perpendicular to each other, joined show great strength in terms of absorption, only weak photochromicX, Y, R =effects H, willal beko xy, amino, etc. side reactions. UV absorbers can also others0.6 not shUVown) 0.6 with more than one broad absorption 0.4 0.4 by a central carbon atom. The relatively because even when activated by brilliant observed at best. One solution is to apply usefully shield dyes from destructive Y maximum of similar intensity, conferringElectron on donor group Absorbance (Au) 0.2 Absorbance (Au) 0.2 O small π-systems of these moieties are kept sunshine, only a small proportion of dye is colourant in microencapsulated form. radiation. it a duller, more neutral appearancehere (Figure gives second peak separate by this atom so that molecules not in the colourless ring-closed form. While Provided precautions are taken not to Unfortunately, justO as substances 380 430 480 530 580 630 680 380 430 480 2).530 580 630 680 Wavelengthhea (nm)t Wavelength (nm) absorb only in the UV region and are they have to reach the heights of intense rupture these tiny pigment particles when used illicitly by athletes may produce O Vivimed researchers have designed and Groups here therefore colourless. On absorption of colouration, photochromic colourants Figure 1 they are incorporated into polymers, dye nasty side effects, additivesR intended to Groups used here to adjust colour and kinetics marketed single-molecule greys and browns manipulate UV light, the bond between the central should also possess the stamina to produce Photochromism ofX performance is unaffected by the choice of stabilise photochromic dyes can also have of this kind for use either as self-shades or kinetics R and colour carbon atom and the adjacentMe oxygenMe the same effect time and again on repeated Meimportant industrial plastic. Striking 1.4photochromic transitions 1.4undesired consequences, such as altering with just one or two shading components. Me Unactivated N X UV 1.2 1.2 atom breaks. Following this ring-opening, exposure to light. dye types can be obtained that would not otherwiseActivated photocolourationUnactivated kinetics or generating The time and effort devoted to X have been possible.1.0 1.0residual colour in the non-activated state. nurturing the technologies based on 0.8 0.8 Activated Me Me Me Me Just as many athletes struggle under Absorbers must be chosen carefully so they T-type photochromic materials has paid N O heat N N 0.6 0.6 coloured dye (mixture of isomers - Y N X UV very hot conditions,0.4 the performance co0.4dolo notur soakless up thena wavelengthsphthop yofran UV off. Millions of people around the globe Y Electron donor group alkyl alkyl Absorbance (Au) 0.2 Absorbance (Au) 0.2 O of photochromic dyes suffers when the X, Y, lightR =needed H, toal activatekoxy the, a dye.mi However,no, et c. here gives second peak others not shown)routinely take advantage of the acrobatic O temperature rises. Because ring-closure is when formulated judiciously, cocktails of talents of these dyes. N O heat N N 380 430 480 530 580 630 680 380 430 480 530 580 630 680 Y driven thermally, aWavelength temperature (nm) increase additivesWavelength have (nm) been reported to extend Come the summer sunshine, while colourless spirooxazine Y Groups here Colourless alkyl coalourlkyled photomerocyanine dyepromotes the formation of the colourless the lifetime of photochromic effects by an Olympians are performing on track and (mixture of isomers - others not shown) manipulate spirooxazine X, X, Y = H, halogen, nitro, etc. O isomer. Dyes that colour up intensely in the order of magnitude – without attracting kinetics field, a glance around the stadium crowd Y = H, halogen, and colour colourless spirooxazine mountain sun of the ski slope therefore the possibility of a ban! Despite this, Groups used herewill to revealadjust othercolour teams and kinetics of athletes at work, nitro, etc. coloured photomXerocyanine dye Y do so less effectively in hot climates. current technology is not robust enough for showing off their speed, strength and X, Y = H, halogen, nitro, etc. X (mixture of isomers - others not shown) Naphthopyrans are less susceptible 1.4 applications that require long-term regular 1.4 stamina, albeit on a much smaller scale! Unactivated Coloured to this temperature dependence than 1.2 and prolonged exposure to strong sunlight, 1.2 Activated photomerocyanine X Y spirooxazines, which is one reason for their 1.0 such as smart windows. 1.0 Unactivated X UV Activated dye (mixture of Y commercial dominance. 0.8 As well as the qualities discussed above, 0.8 isomers – others 0.6 0.6 Andy Towns Another basis for their importance is 0.4 photochromic dyes have to work well as 0.4 Addition O Electron donor group is research manager at Vivimed not shown) heat endurance. When used in ophthalmic lens Absorbance (Au) 0.2 a team. The most commercially important Absorbance (Au) 0.2 O UV of electronhere gives second peak R Y O applications, naphthopyrans are expected lens shades are neutral colours – greys Labs Europe Ltd in Huddersfield, 380 430 480 530 580 630 680 380 430 480 530 580 630 680 donor group UK. R to withstand photochemical attack in Wavelengthand browns (nm) – which call for the use of dye Wavelength (nm) O Groups here heat O manipulate R kinetics

colourless naphthopyran coloured dRye (mixture of isomers - and colour Chemistry&Industry • May 2012 Chemistry&Industry • May 2012 34 X, Y, R = H, alkoxy, amino, etc. others not shown) 35

colourless naphthopyran coloured dye (mixture of isomers - X, Y, R = H, alkoxy, amino, etc. others not shown) Groups used here to adjust colour and kinetics

1.4 1.4 Unactivated 1.2 1.2 Activated Unactivated 1.0 1.0 Groups used here to adjust colour and kinetics 0.8 0.8 Activated 0.6 0.6 1.4 0.4 1.4 0.4 Unactivated Electron donor group 1.2 Absorbance (Au) 0.2 1.2 Absorbance (Au) 0.2 O 1.0 Activated 1.0 Unactivated here gives second peak 0.8 380 430 480 530 580 630 680 0.8 Activated380 430 480 530 580 630 680 Wavelength (nm) Wavelength (nm) 0.6 0.6 Groups here 0.4 0.4 Electron donor group manipulate Absorbance (Au) 0.2 Absorbance (Au) 0.2 O here gives second peak kinetics and colour 380 430 480 530 580 630 680 380 430 480 530 580 630 680 Wavelength (nm) Wavelength (nm) Groups here manipulate kinetics and colour