The defining characteristic of photochromic colorants is that they change color reversibly in response to variations in the intensity of particular wavelengths of light to which they are exposed. Light-responsive dyes worth millions of dollars are manufactured each year as a result of their successful exploitation over the last quarter century . The bulk is consumed in the production of ophthalmic lenses that darken reversibly when exposed to strong sunshine. Photochromism continues to attract the interest of both industrial and academic researchers, who are looking to harness photochromic colorants in fields like optoelectronics and nanotechnology.
The widely accepted definition of photochromism is that of a reversible color change induced in a compound driven in one or both directions by the action of electromagnetic radiation [2, 3]. Photochromic systems are classified as either “P-type” or “T-type”. The former kind can be switched in each direction with different wavelengths of light. P-type systems change color when irradiated with a specific wavelength range then remain in this state after removal of the stimulus. It is only when they are subjected to light of a different set of wavelengths that they return to their original color. In contrast, T-type behavior is exhibited if light is able to drive the change in just one direction. T-type systems will fade back to their original state, through a thermal back reaction, when they are no longer exposed to the light source. Reversibility of response is a key aspect in both types of photochromism: light-sensitive materials that undergo changes of an irreversible nature cannot be described as photochromic.
The first major application of photochromism was commercialized in the mid-1960s: glass ophthalmic lenses that relied on inorganic halide crystals. These systems have been superseded during the past two decades by organic materials in the form of plastic lenses incorporating T-type dyes [4, 5]. Such colorants are colorless – so to describe them as dyes might initially seem odd! – but they become colored when irradiated with sunlight and fade back thermally to colorless in low levels of light. Developing an economic technology with commercially acceptable durability and performance did not prove easy because photochromic colorants are less robust than conventional dyes and orders of magnitude more expensive. Light-responsive organic compounds that can be switched from one color to another are well known, but because photochromic lens manufacture remains the dominant application, the most industrially important dyes are those which exhibit T-type behavior as depicted in Fig. 1 involving colorless to colored transitions. This kind of color change, in which light causes a shift in absorption to longer wavelengths, is known as “positive photochromism”. The term “negative photochromism” does not mean non-photochromic, but instead covers the converse phenomenon of a colored dye becoming colorless upon irradiation with light, only to return to its original colored state in the dark .
T-Type Photochromic Colorants
Weak visible absorption when unactivated so residual color is low
Quick response to an increase in illumination
Have a strongly absorbing activated state, because even when irradiated with light of high intensity, only a relatively small proportion of colorant will exist in this form
Show a good compromise between depth of activated color and rate of fade to ensure both are acceptable as the properties of high intensity and short half-life tend to be mutually exclusive
Produce satisfactory performance over 2 years by having reasonably lightfast colored and colorless forms which respond well to photostabilizers
Resist the tendency to “fatigue”, whereby during activation, a proportion of the dye is undesirably and irreversibly converted to non-photochromic molecules, leading to gradual weakening of color upon repeated activation
Color up in a manner that is not greatly affected by the temperature of its environment
Exhibit sufficient solubility in lens media to give solutions rather than dispersions because commercial T-type classes do not exhibit useful photochromism in solid form
The wavelengths of light that effect the forward conversion shown in Fig. 2 are normally within the UVA region (315–400nm), but blue light can also play a role for some commercial spiro derivatives. Absorption causes rearrangement of the bonding between the atoms within a colorless or weakly colored molecule to create structures that confer intense color. The colorless form consists of a ring-closed structure, which is made up of two halves that are perpendicular to each other and joined by a spiro carbon atom. The π-systems of these moieties are small, hence the lack of absorption in the visible region. However, absorption of energy in the form of UV light can lead to ring-opening as a result of the bond between the spiro carbon atom and its adjacent oxygen atom breaking. Molecular twisting and bending via intermediates then ensues giving planar species with extended conjugated π-systems whose absorption moves into the visible, generating color. Low light levels result in ring closure back to the colorless form because thermal fading dominates.
Different forms of the dyes exist in a dynamic equilibrium: at a given moment, molecules are isomerizing between colored and colorless species, the concentrations of which are determined by the intensity of incident light. As the flux of UV that the dye is exposed to increases, the proportion of dye that is in its colored state grows through promotion of ring opening relative to ring closure. Removal of the light source leads to the concentration of the colorless ring-closed form rising which is observed as fading. When the intensity of illumination is held constant, the isomer concentrations will settle down into what is known as a photostationary state, where depth of color does not change. These proportions are dependent on the dye, the nature of the illumination and the medium. Because the photostationary state is a dynamic equilibrium, dye molecules will continue to swap between colorless and colored isomeric forms even after it has been attained.
Since a significant proportion of sunlight is made up of radiation in the UV, it is capable of causing pronounced photochromism. In contrast, most commercial T-type dyes do not respond well to artificial light sources, such as tungsten filament bulbs, because the proportion of their UV output is low.
Several classes of T-type compounds have been extensively investigated (e.g., anils, perimidinespirocyclohexadienones, spirodihydroindolizines, etc.) [6, 7], but few have attained commercial significance. The three families of dye that have had the greatest industrial importance will be discussed next.
This class were intensively studied during the 1950s through to the 1970s because they are readily synthesized and photochrome to deep colors, typically violets and blues, that fade at useful rates . However, because members of the spiropyran family generally have poorer photostability than their spirooxazine and naphthopyran counterparts, they are much less important commercially. Nevertheless, spiropyrans are still exploited in research where light stability is not a prerequisite, for example, in the fields of biochemistry and materials science.
Although spirooxazines [6, 9] are similar in structure to spiropyrans, the former are much less prone to fatigue [3, 8]. As a result, they have become well established since they were employed in the production of the first commercial plastic photochromic lenses in the early 1980s .
Another reason for the significance of naphthopyrans is that they simplify the development of colorant recipes for lenses of the most commercially important shades, which are gray and brown [1, 8]. Incorporation of appropriate structural features into 5 produces relatively dull, neutral dyes, enabling use of fewer colorants in a mixture. Given that all of the components of the recipe must activate, fade, and fatigue at the same rate, this is of great advantage to the formulator.
Applications of T-Type Dyes
While the properties of photochromic colorants have been put to purely aesthetic use in artwork, T-type dyes have also been exploited as functional materials, for example, in security printing where light-responsive marks are used as indicators of authenticity. The main outlet for photochromic dyes, lenses for spectacles, falls between these two extremes: variable transmittance is a key function in regulating light intensity reaching the eye, yet color is also an important consideration both for style and comfort. Although T-type colorants have been investigated for many applications, success in developing marketable products has been limited by the challenges that their usage presents. They are not as robust as conventional dyes and pigments, rendering certain methods of application unsuitable. Also, in order to get strong, durable photocoloration, care must be taken to provide T-type dyes with the right environment in which to operate. It must be conducive to the changes in molecular geometry associated with photochromism and not lead to rapid degradation of dye during and/or following application. These considerations will be illustrated for polymers and surface coatings, which are the two most common media for photochromic dyes.
Arresting photochromic effects can be produced by incorporation of dyes into thermoplastics. However, the chemical and physical nature of the polymer (as well as the dye) has a large influence on the kinetics and resilience of the photochromism. For example, spirooxazines and naphthopyrans work well when used in mass coloration of polymers that have relatively low glass transition temperatures with flexible chains, such as polyethylene and polypropylene, giving striking color changes at inclusion levels of 0.3%w/w and less. However, rigid, crystalline materials restrict the necessary conformational changes for photoisomerization, severely inhibiting photochromism. Dyes can tolerate brief exposure to the high temperatures experienced in injection molding or extrusion, but certain polymers, such as polyamides, require processing at elevated temperatures that strongly degrade colorants, leading to discoloration and a lack of observable photochromism. Even when suitable polymers are employed, additives are often needed to enhance dye photostability in order to achieve an effect with a commercially acceptable lifetime. Loss of photochromism is related to cumulative amount of incident UV radiation rather than the number of times that the material is switched between colorless and colored states. Consequently, UV absorbers can usefully shield dyes from excessive radiation provided that they do not strongly absorb the UV wavelengths which activate the dyes. Other additives include hindered amine light stabilizers that scavenge light-generated free radicals, which would otherwise attack colorants, and triplet state quenchers that inhibit photochemical reactions other than the desired one of photoisomerization.
Application to polymers need not involve monomolecular dissolution in the polymer itself. Photochromic dyes can also be used in microencapsulated solvent droplets of typically 1–10μm in diameter. In this form, the dye solution is encased in polymer shells, producing a photochromic powder that can be dispersed like a pigment. The advantage of this material is that its photochromism is much more independent of the medium, i.e., the properties are that of the dye dissolved in the solvent and do not tend to be influenced by the polymer in which the microcapsules are dispersed. In this way photochromic effects can be produced in substrates using the microencapsulate that would not be possible using dye alone. Disadvantages of this technique are that relatively high loadings of pigment are required and the microcapsules can be physically damaged during application. An alternative approach to obtaining photochromism in inhospitable polymers (as well as potentially improving the robustness and responsiveness of the effect) is to attach oligomeric fragments to dyes. These “tails” are thought to provide a favorable microenvironment for the photoactive part of the colorant. All of these methodologies have been commercialized for coloration of polymers. Photochromic materials are found in products as diverse as toys, fashion accessories, fishing line, and motorcycle helmet visors .
T-type colorants have also been used for applications ranging from security printing to cosmetics in the form of photochromic inks and other surface coatings. Commercial dyes work well in nonpolar solvents such as toluene when dissolved at a suitable concentration: solubility and photochromic behavior tend to be good, enabling the formulation of solvent-based inks and varnishes. Aqueous media require an alternative approach since such dyes are not water soluble but must be in solution to exhibit photochromism. One way is to disperse microencapsulated dyes, which were described earlier, into water-based systems like a pigment using commercially available powders or aqueous slurries. Alternatively, micronized particles of an appropriate polymer into which photochromic dye has been incorporated can be employed. It is possible to formulate inks for a variety of printing techniques provided care is taken to ensure that dye is not damaged during formulation or application. Additives may be needed to improve light fastness. Even following optimization, photostability can problematic. This is also true of photochromic textiles: the most effective method of applying dye to fabric is screen printing microencapsulated colorant because typical polymers, such as polyester, inhibit photochromism, while conventional techniques, e.g. exhaustion dyeing, tend to damage typical commercial dyes. Photochromic detail can be added to garments through the use of polypropylene thread that has been melt spun with dye .
The general lack of robustness of photochromic dyes compared to conventional colorants has prevented their use in particularly demanding applications. One example is light-responsive glass for architectural windows. Lifetimes in excess of 10 years are required but such a demand cannot be met by existing dye technology.
T-type colorants are also not suited to potentially important uses which require controlled switching between one or more states (colored and/or colorless) on demand: for these applications, P-type dyes are needed.
P-Type Photochromic Colorants
A significant amount of effort, both in academia and industry, has been invested in P-type colorants over the past three decades because of their potential as molecular switches . Such compounds are converted from one state to another by irradiation with light, remaining so until switched back by other wavelengths. This behavior is of great interest in high technology sectors, but while much time and money has been spent on developing P-type applications, such work has yet to bear commercial fruit. The classes that have been studied most in this connection are discussed next.
With appropriate structural design, the colored cyclized material is essentially thermally stable (owing to a negligibly small reaction rate for thermal reversion) so that the reverse reaction does not occur in the dark. Instead, ring-opening requires the absorption of longer wavelengths of visible light than those that cause activation. Consequently, unlike pyran- and oxazine-derived colorants, diarylethenes do not fade spontaneously once the illumination is removed. A further contrast is that, whereas commercial T-type compounds must be in solution to produce useful photochromism, diarylethenes give photochromic responses in solid form (follow this http://www.rsc.org/suppdata/cc/b5/b505256d/b505256d.mpg for a video). The derivatives that have attracted the greatest attention are members of the dithienylhexafluorocyclopentene subclass 6. A few of them are available on the open market in small quantities. Activated colors can be varied widely by modification of their structure, covering most of the visible spectrum. By linking together different diarylethene skeletons, photochromic molecules have been created that can even be selectively switched between more than one different colored state.
Applications of P-Type Dyes
Fulgides have been used in conventional coloration areas such as textiles and printing inks . However, the greatest interest in P-type photochromic systems stems from their potential use as functional colorants within the fields of optoelectronics, data storage, and nanotechnology [11, 12]. In these contexts, diarylethenes have been the most intensely studied family as they offer a wide scope for the design of molecules whose optical properties can be switched in a controlled manner between persistent states. Much effort has been expended on trying to exploit this behavior, for example, in developing components for all-optical circuitry, such as switches and logic gates. Optical equivalents to electrical components, perhaps reliant on P-type colorants, are needed if the technology is to replace slower, more power-hungry conventional electronics. Molecules that can be switched optically are also being studied in the field of information technology because in theory they could furnish memory systems with much greater densities than those of current commercial devices.
Another avenue of research for P-type colorants is nanotechnology because of their solid phase photochromism. For example, crystals of dihetarylethenes undergo changes in shape, as well as color, as a consequence of molecular geometry altering during photochromic transitions (follow this http://pubs.acs.org/doi/suppl/10.1021/ja105356w/suppl_file/ja105356w_si_002.avi for a video). Such alteration in particle dimensions could form the basis for light-driven actuators in nanomachinery.
The rate of thermal back reaction can be slowed by modification to structure, creating materials with strong P-type behavior where trans to cis photoisomerization occurs with one range of wavelengths and cis to trans with another set . These transformations have been explored for use in optical switching and data storage. The geometry change associated with photoisomerization has also been put to work in creating polymeric materials that undergo reversible photomechanical deformation. Films that contract upon exposure to UV light and then expand when irradiated with visible light have been used to demonstrate the concept of light-driven motors (follow this http://onlinelibrary.wiley.com/store/10.1002/anie.200800760/asset/supinfo/anie_z800760_ikeda_movie1.mov?v=1 for a video).
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