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Automaticallygeneratedrough PDFby Proof Check from River Valley Technologies Ltd DE GRUYTER Physical Sciences Reviews. 2020;20190146 Andrew T owns1 Diarylethene Dyes 1Technical,Lambson Ltd,Clifford House,York Road,Wetherby,West Yorkshire,LS227NSUnited Kingdomof Great Britainand Northern Ireland,E-mail:andrew. towns@lambson. com. https://orcid. org/0000-0002-6772-6119. Abstract: This article introduces the general characteristics of the diarylethene class of photochromic dye and the struc-tural features that make photochromism possible. It touches on the methodologies employed to synthesize these compounds as well as the influences that typical substitution patterns exert on photocoloration. A demon-stration is then given of the great diversity pertaining to the potential applications in which researchers are seeking to exploit them as functional colorants. Keywords: colorant, dye, functional, photochromic, photochromism DOI: 10. 1515/psr-2019-0146 The diarylethene (D AE) class is one of the most intensely scrutinized types of photochromic dye. The serendip-itous discovery of its relatively unusual photochromism [1] attracted great attention from researchers seeking to exploit light responsive switches in a wide variety of fields. As functional colorants, D AEs continue to be the subject of many avenues of research. Despite the interest shown in them, the use of these dyes does not cur-rently extend beyond low-volume niche applications. This Chapter will describe the characteristics that make them such appealing tools for a diverse range of technologies. As well as taking a brief look at the influences on these properties and the chemistry of D AE dyes, it will also give a flavor of the kinds of potential use to which the photochromism of this class may be put. 1 Diarylethene dye characteristics and molecular structure With the right molecular design, the class offers unusual photochromic properties that are of special interest to researchers:-P-type photochromism, whereby colorants can be switched between states possessing different light absorp-tion properties through irradiation with light of specific wavelength ranges [2]. Each state is thermally stable. Most photochromic classes, for example naphthopyrans [3], exhibit T-type behavior, whereby photoacti-vated forms revert in the absence of light to their original state. Consequently, P-type systems are more attractive as functional dyes because they may be switched back and forth on demand between states that would otherwise persist indefinitely. Many D AE dyes behave in this manner for practical purposes. Re-searchers estimate lifetimes of states of certain D AEs at 30°C in terms of millennia [4]!-exhibit photochromism in solid state form. Members of most photochromic dye classes do not undergo changes that are visible to the naked eye unless they are in solution, either in a polymeric matrix or a solvent. In contrast, many D AEs switch color in crystalline form [5].-fatigue resistance enabling cycling between states thousands of times. D AEs are not as hardy as members of other colorant classes, such as those of the naphthopyran and spirooxazine families. However, with the right design they are sufficiently robust to remain of interest as functional colorant switches. Significantly, diarylethene dyes are in general more resilient than members of the other major P-type class (fulgide) [6]. The discovery of these behaviors in D AE compounds during the mid-1980s [1] ignited great interest from tech-nologists working in fields such as optical data storage, nanotechnology, and photonics. During 2000-2016, researchers authored over two thousand academic publications dedicated to D AEs [7]. This amount is more than the total that appeared during the same period which featured the T-type spiropyran, spirooxazine, and naphthopyran classes. The key characteristics that make D AEs attractive as functional colorants originate from specific molecular motifs. The general structure of members of this class is shown in Figure 1. Andrew T owns isthecorrespondingauthor. ©2020Walterde Gruyter Gmb H,Berlin/Boston. 1 | Towns - 2020 - Diarylethene Dyes.pdf |
Automaticallygeneratedrough PDFby Proof Check from River Valley Technologies Ltd Towns DE GRUYTER Figure 1: Generalized structure and photochromism of D AE dye class. An enormous number of variants have been reported [8, 9], but the most commonly used D AEs may be thought of as comprising two crucial design elements: i. an ethene bridge. It is usually substituted in a way that prevents cis-trans photoisomerization from occurring in order to stop this process competing with photocoloration. By far the most well-used strategy is to employ a cycloalkene motif. However, one and even two aromatic rings have been utilized as the unsaturated linking function to produce “terarylene ”[10] and 'tetraarylene “'[11] P-type photochromic colorants. ii. two hetaryl rings. They are linked by the ethene bridge. Not all heterocycles furnish robust P-type pho-tochromism, so these ring systems and their substituents must be selected with care. Nevertheless, a huge number of permutations have been synthesized, which include the insertion of second heteroatoms into the heterocycles [11] as well as attaching or fusing aromatic rings onto them [8, 9]. Like the most industrially important photochromic colorant classes, light-driven color changes in D AE dyes involve photoisomerization by means of pericyclic rearrangement [2]. Decorating a central double bond with two heterocyclic rings, usually thiophene-based (see Figure 1, X = S), and introducing compact groups to block photochemical side reactions (see Figure 1, R ≠ H), affords molecules capable of absorbing light energy to tran-sition reversibly between thermally stable states. The ring-opened forms 1a of D AEs ( “o-D AE ”) are generally colorless, or only weakly colored, because their non-planarity reduces conjugation between the π-systems of the heterocycles. The parallel conformer 1apis not photoactive owing to its geometry, whereas the anti-parallel conformer 1aais capable of cyclisation upon photoexcitation [12]. The latter absorbs ultraviolet and/or short wavelength visible light (see Figure 1, λ1) with the net result that a thermally irreversible conversion to the ring-closed form 1b (“c-D AE ”) occurs: it is intensely colored owing to the presence of an extended planar conjugated π-system, its longest wavelength absorption maximum typically being 200-300 nm longer than that of the cor-responding o-D AE. (With reference to non-photochromic conventional colorants, the extinction coefficients of c-D AEs are comparable to anthraquinonoid and simple azo dyes. ) Reversion of 1b to 1a only occurs when the chromophore of the former absorbs light energy corresponding to wavelengths further into the visible region (see Figure 1, λ2), which the o-D AE 1a cannot absorb. The key to the thermal stability of c-D AE 1b is the relatively low aromatic stabilization energy of the two heterocycles. It minimizes the tendency of the c-D AE form to ring open thermally to 1a, thereby restoring the aromatic nature of the heterocyclic rings. (Replacement of the heterocycles with phenyl rings destroys P-type character. The high aromatic stabilization energy of the carbocycles widens the gap in energy between ring-opened and ring-closed forms. Consequently, the corresponding c-D AE form readily isomerizes thermally to the much lower energy o-D AE photoisomer, thereby leading to T-type photochromism. ) It is important that substitution patterns on the heterocycles do not weaken the central C-C bond linking them in the ring-closed form to cause thermal instability [13, 14], e. g. through steric hindrance in case of R = isopropyl [15]. An ap-propriately substituted c-D AE can be stable for years at 20-30°C when kept away from visible light that would cause it to cyclorevert to the corresponding ring open form [6]. The thermal irreversibility makes it possible 2 | Towns - 2020 - Diarylethene Dyes.pdf |
Automaticallygeneratedrough PDFby Proof Check from River Valley Technologies Ltd DE GRUYTER Towns to resolve and isolate the photoisomers by high performance liquid chromatography. Much more detail con-cerning design principles and the theory behind the mechanics of photoisomerization can be found in [8] and [9]. The change in molecular geometry that accompanies the photoisomerization of D AEs is modest. For in-stance, the long dimension of the simple o-D AE 2(X = Y = 5-Me) shrinks by just 10% on ring-closure while its short dimension grows only 10% and overall planarity increases [8]. Such small alterations enable many D AEs to exhibit pronounced photochromism in environments that completely suppress the photochromic properties of conventional industrial T-type dyes, which undergo greater changes in geometry upon photoisomerization. Numerous D AE colorants transition in polymer substrates with high Y oung 's moduli and relatively inflexi-ble chains that offer too little free volume for commercial T-type naphthopyrans and spirooxazines to operate. Plenty of examples also photoisomerize in their crystalline solid state-an unusual property, which opens up numerous potential applications that are not possible with conventional industrial photochromic colorants. The rate of change is very rapid for D AEs irrespective of whether they are in the form of solutions or crystals: typi-cally switching takes just a matter of picoseconds (10-12s) in either case [6]. The photochemistry of cyclization, as well as cycloreversion, has been much studied: for a good summary, see [8]. By far the most extensively investigated variety of D AE is the dithienylethene (DTE) subclass, specifically dithienylhexafluorocyclopentene derivatives 2as shown in Figure 2. Both the perfluoro substitution and sizing of a five-membered ring increase fatigue resistance compared to that of analogues whose ethene bridges com-prise non-fluorinated rings and/or cyclic structures of other sizes. The remainder of this Chapter will therefore tend to focus on this kind of colorant. DTEs are the sole type of D AE available off-the-shelf on the open market [16]. Only research quantities are offered which is reflected by pricing that is pitched at the level of hundreds of dollars per gram, an order of magnitude or two greater than that for commercial volumes of industrial T-type colorants. Figure 2: T ypical synthetic route to DTE derivatives 2from perfluorocyclopentene. 2 Synthesis of diarylethenes First reported in the early 1990s [1], DTEs 2are typically prepared by reaction of perfluorocyclopentene, a low boiling (27°C) liquid, with thienyllithium compounds. Most DTE derivatives reported tend to be symmetrical for synthetic expediency; only a single step, involving reaction with two equivalents of a single lithiothiophene (see Figure 2, X = Y), is needed. Many asymmetric diarylethene dyes are accessible by modifying this strategy to include sequential reaction of one equivalent each of two different lithioheterocycles (see Figure 2, X ≠ Y). Other coupling chemistries can be employed, e. g. use of hetarylboronic acids instead of organolithium derivatives. Another means of DTE synthesis is construction of the cyclopentene ring through cyclisation by intramolecular Mc Murry coupling of a 1,5-dithienyl-1,5-diketone [17]. For a comprehensive review of pathways to symmetric and asymmetric DTEs, see [18]. A plethora of unsaturated bridging fragments other than hexafluorocyclopentenyl have been explored [19]. Again, synthetic strategies to introduce the ethene bridge include coupling heterocycles to a ring system, or employing cyclisation reactions, creating many D AE examples that comprise heteroatoms and/or a ring of different size. Early instances are the P-type maleic anhydride derivative 4a and its maleimide analogue 4b (see Figure 3). The former is made through alkaline hydrolysis of the dicyano derivative 3[8, 20], which itself exhibits P-type photochromism. 3 | Towns - 2020 - Diarylethene Dyes.pdf |
Automaticallygeneratedrough PDFby Proof Check from River Valley Technologies Ltd Towns DE GRUYTER Figure 3: Properties of dicyano-and maleic acid-derived DTEs in benzene solution [12]. 3 Color and constitution of diarylethene colorants Despite the restrictions imposed on molecular structure to ensure photochromism is P-type (i. e. that ring-closure is thermally irreversible), considerable scope remains for variation in design. The pronounced influence of heterocycle type and substitution on the absorption properties of c-D AEs, coupled with the ingenuity of chemists, means that examples range the whole way across the visible spectrum and beyond. Within the DTE class alone, manipulation of structure generates P-type photocoloration from yellow through to red through to blue and even green (see T able 1). T able 1: Influence on absorption properties by substitution pattern of ring-closed forms of dihetarylhexafluorocyclopentene-derived dyes 5in hexane. 4 | Towns - 2020 - Diarylethene Dyes.pdf |
Automaticallygeneratedrough PDFby Proof Check from River Valley Technologies Ltd DE GRUYTER Towns Expanding the conjugated system of c-DTEs, e. g. fusing ( 5c) or appending ( 5e) phenyl rings to the parent struc-ture produces bathochromic shifts. Introduction of electron donating functions onto pendant phenyl rings (e. g. methoxy groups of 5f) gives further red shifts in absorption as does increasing their strength (e. g. diethylamino groups of 5 g). These changes also increase intensity of coloration: for instance, the extinction coefficients of c-DTEs 5d,5e,5f and 5 ggrow progressively (see T able 1). Further developing the push-pull nature of the chro-mophore by incorporation of electron-accepting motifs leads to pronounced bathochromism ( 5 h-i ): however, this comes at a price — the central C-C bond of c-D AE 1b in the case of 5i is weakened sufficiently that this iso-mer is no longer thermally stable. While the benzodithiole-based donor system opposite the dicyanoethylene-derived acceptor fragment pushes absorption of 5i into the near infrared, the DTE exhibits T-type rather than P-type photochromism. The half-life of its ring-closed form is just ~ 3 h at 60°C whereas that for 5c is > 12 h at 80°C [8]. (Loading electron acceptor groups directly onto the thienyl rings of DTEs tends to lessen their P-type character. ) The absorption maxima and extinction coefficients of o-D AEs respond in a qualitatively similar manner to the aforementioned structural changes in c-D AEs, shifting toward the visible region and becoming larger, respectively. As shall be discussed later, D AEs that can be ring-closed with visible light (i. e. avoiding use of UV radiation) are of much interest as functional colorants for certain applications, provided that P-type behavior and robustness are not compromised. D AEs of this kind remain a focus of research [25-27]. Attaching both thienyl rings to the bridge by their 2-positions, as in the case of 5a, leads to yellow photo-coloration. In contrast, the isomeric colorant 5d in which both thiophene systems are linked to the bridge by their 3-positions becomes red upon exposure to UV. The hypsochromism of 5a is ascribed to the orientations of the hetaryl systems in its ring-closed form largely restricting π-conjugation to within the cyclohexadiene fragment [21]. Greater delocalization over the thienyl rings relative to 5a occurs in the case of 5d upon ring closure, resulting in red-shifted absorption. The absorption maximum of the ring-closed form of analogue 5b, in which one pendant hetaryl ring is 3-thienyl and the other 2-thienyl, lies between these two extremes. P-type photochromism is not sacrificed: the ring-closed forms of each of these three colorants are thermally stable with absorbance in oxygen-free heptane remaining essentially constant for > 500 h at 80°C [21]. Replacement of the thienyl rings with other heterocycles produces major changes in photocoloration in terms of hue, thermal stability, quantum yield, and other properties. Introduction of a nitrogen heteroatom into DTE 6a to create the dithiazolyl analogue 6b leads to pronounced hypsochromism (see T able 2). It also reinforces the thermal stability of its c-D AE photoisomer given the reduced aromatic stabilization energy of the 4-thiazolyl fragments in the o-D AE form. (Note that figures of 525 nm and 10,000 M-1 cm-1are reported for 6b in [32]. ) Substitution of the sulfur heteroatom of 6a and 6b with oxygen yields difuryl and dioxazolyl colorants 6c and 6d, respectively, with even less stabilization energy and greater hypsochromism. Depending upon the nature of the ethene bridge, insertion of nitrogen heteroatoms into the thiophene rings of DTEs brings about other benefits such as improved quantum yield for photocyclization by increasing the proportion of the photoactive anti-parallel isomer 1aain the o-D AE form [11]. T able 2: Influence on absorption properties of the substitution pattern of ring-closed forms of dihetarylhexafluorocyclopentene-derived dyes 6in hexane solution. 5 | Towns - 2020 - Diarylethene Dyes.pdf |
Automaticallygeneratedrough PDFby Proof Check from River Valley Technologies Ltd Towns DE GRUYTER As well as the nature of the pendant heterocycles, the other substituents on the ethene bridge profoundly influ-ence the color of the ring-closed forms. T able 3 illustrates how modifying the hexafluorocyclopentyl fragment of DTE 5c alters shade: breaking the ring to create 7results in a substantial hypsochromic shift, while adjusting ring size ( 8; n = 0, 2) exerts a more modest effect. Ring size affects planarity of the ring-closed form, and thereby π-conjugation within it, which in turn impacts upon absorption band maximum and shape, and ultimately color [22, 33]. Use of stronger electron acceptors as demonstrated by the dicyano derivative 3and maleic acid deriva-tives 4results in bathochromism (see Figure 3) in both ring-opened and-closed forms: the latter are yellowish rather than colorless and thus short wavelength visible light of wavelength ~400 nm as well as near UV triggers color generation. In contrast, the open form of simple hexafluorocyclopentyl DTE 5e responds only to shorter wavelengths, e. g. mid-UV radiation of ~313 nm. Visible light of >500 nm wavelength drives the return of its blue photoisomer 5e back to the colorless form. T able 3: Influence of cycloaliphatic ring system on absorption maximum in benzene solution of ring-closed forms of DTE dyes [22]. Examples of single-molecule P-type DTEs affording dull tertiary shades are known. Dye 9a is brownish when ring-closed [34] owing to two overlapping absorption bands in the visible region. Remarkably its dithiazolyl analogue 9b transitions from colorless to black when irradiated with UV [35] (see Figure 4). This neutral col-oration of the closed ring form, arising from reasonably uniform absorption across most of the visible spectrum, also means that cycloreversion can be effected by irradiation with light of a wide range of wavelengths in the visible region. Figure 4: T wo examples of neutral-colored D AEs and the absorption maxima (in nm) of their ring-closed photoisomers 9 in acetonitrile solution. Relationships between D AE structure and quantum yields for photoisomerization, thermal stability of pho-toisomers, and fatigue resistance are well understood [8]. Removing vulnerable unsubstituted positions on 6 | Towns - 2020 - Diarylethene Dyes.pdf |
Automaticallygeneratedrough PDFby Proof Check from River Valley Technologies Ltd DE GRUYTER Towns thiophene rings of DTEs markedly improves the robustness of dye solutions towards repeated cycling between ring-opened and-closed forms. For exampling, annellation with phenyl rings in the case of 5c greatly curbs loss of photochromism by suppressing reaction with oxygen to create endoperoxide side photoproducts [22]. Over 90% of photocoloration intensity is retained by this dye even after 14,000 cycles of coloration/decoloration in methylcyclohexane solution. The 4-methyl groups of 5e inhibit photochemical cyclisation to non-photochromic colored side products, so fatigue resistance is greatly enhanced compared to the analogous DTE 10 lacking sub-stitution at these positions [36] (see Figure 5). Hexafluorocyclopentene derivatives are relatively hardy D AEs: for example, 5c is over an order of magnitude more fatigue resistant in solution than its maleic anhydride ana-logue 4a [8]. The state of a D AE often makes a big difference to its resilience: the photochromism of such a dye in its solid state is markedly more robust compared to when it is solution. The crystal lattice sterically hinders side reactions and impedes diffusion of oxygen (for example, see Figure 5). Figure 5: Influence of 4-methyl substitution on number of cycles after which 20% loss of DTE molecules occurs in hexane solution under vacuum or in crystalline state [36, 37]. However, since the draw of D AEs lies in them being functional colorants, P-type color change is not neces-sarily of primary interest: variations in properties unrelated to color that accompany the photoisomerization often more greatly concern researchers. These attributes, and the potential applications which exploit them, are covered in the next section. 4 Applications of diarylethene dyes A report of P-type photochromism in DTEs appeared during 1967 [38]. Over twenty years passed before re-searchers seized upon the D AE unit as a tool in applications which required light-activated bistable switches. This interest was prompted by publication of research which demonstrated the high thermal stability and re-silience of DTEs in the late 1980s and early 1990s [8, 39]. V ery soon afterwards the first fruits of work seeking to exploit D AEs as functional colorants were disseminated in the academic literature [40]. The publication of work relating to D AEs, and especially DTEs, blossomed with the annual number of research papers growing year on year over the next two decades [7, 41]. Numerous review papers and books document these extensive efforts: excellent recent accounts giving general overviews include [9] and [42], as well as individual chapters within [43]. T able 4 gives a flavor of the fields in which the exploitation of D AEs has been attempted. It lists a few example applications as well as the nature of the property changes that accompany photoisomerization upon which they depend. The remainder of this section explains some of these entries in more detail. Those outlets involving the solid-state photochromism exhibited by certain D AEs will only be touched on briefly because in such applications these colorants are being employed as pigments rather than dyes [60]. T able 4: Some examples of switchable properties of D AEs and instances of applications in which their exploitation has been attempted. Photoreversible Property Example application Absorbance in mid-infrared Memory element for data storage [44] Absorption of visible light Memory elements [45]; focal plane masks [46]; optical masks [46] Chirality [47, 48] Liquid crystal displays [49], control of reaction stereospecificity [50] Conductance Molecular electronics [45] Crystal dimensions Nanotechnology [51] Cytotoxicity Photodynamic cancer therapy [52] Electronic structure Reaction control [50]; polymerization catalysis [53] Geometry Switchable guest-host receptors [51]; adhesion promoter [54], reaction control [50] 7 | Towns - 2020 - Diarylethene Dyes.pdf |
Automaticallygeneratedrough PDFby Proof Check from River Valley Technologies Ltd Towns DE GRUYTER Ion affinity Biological probe [55] Luminescence Super-resolution microscopy [56]; memory element [45, 57] p Ka Multi-stimulus logic gate construction [55] Refractive index W aveguide optoelectronic component [46]; holography [46]; memory Wettability/surface tension [58] Microfluidics [59] 4. 1 Information technology and optoelectronics Photoreversibility of various optical properties-not just color-serves as the basis for potentially very high-density storage systems consisting of D AEs as binary memory elements. Irradiation with UV or visible light alters the state of an element, i. e. writes data. Reading of the data non-destructively (i. e. without altering it) requires that the state of each element be determined by means of another property which varies between open ring and closed ring forms. For example, absorption in the mid-infrared [44], refractive index [46] or chirality [47] of D AE-doped materials can be probed using electromagnetic radiation of wavelengths that do not induce photoisomerization. Fluorescence is another means of non-destructive readout. An example of this approach relies on a DTE whose fluorescent open ring form is switched to a non-fluorescent closed ring photoisomer by irradiation with light of <400nm and vice versa with visible radiation of >600nm [57]. Probing of the state of an individual element is performed by irradiation with light of ~460nm. An element in open ring form will fluoresce at ~590nm. Neither of these two wavelengths lead to photoisomerization, thereby preserving data. A variation on this strategy employs metal complexes linked to DTEs, enabling non-destructive readout of phosphorescence [61]. D AE-based switches have been designed that are not just bistable, but which can adopt more than the two binary states of a bit. For example, the panchromatic absorption of 9b enables ternary states to be constructed through irradiation with different wavelengths of visible light [35]. In another elegant proof-of-concept study, three DTE units, each of a different structure, were incorporated into a single photochromic colorant that was capable of being switched between yellow, red, and blue hues by particular combinations of UV and visible light for potential use as a multiple state memory element [62]. Photocontrolled switching of electrical current is also of interest for the construction of memory elements that can be read non-destructively by means of current detection, as well as for the manufacture of light-programmable electronic components [63]. Approaches using D AEs may be at bulk or single-molecule scales. In the former case, polymer films doped with appropriately substituted D AEs are sandwiched between elec-trodes and their conductivity switched by irradiation with light-devices have been constructed that yield higher conductivity when the concentration of colored c-D AE form is raised. Such a device has potential in optical memory and electrical circuitry. Studies of photoelectrochemical switching at the molecular scale have a similar application focus [40]. In conductive organic compounds, electrons flow through their conjugated π-systems. By controlling the breaking and re-making of the chain, conductivity can be switched off and back on. The use of DTEs in this manner was investigated only a year or so after their P-type behavior was first reported in the early 1990s [64]. In o-D AE form 1a,π-electrons are localized in the hetaryl rings so the photoisomer is “non-conductive ”or “insulating ”. In c-D AE form 1b, they are delocalized across the molecule, which is thus “conductive ”. Incorporation of DTE units as monomers into conductive polymer chains was undertaken as long as two decades ago [65]. Despite the ingenuity of the work described above, D AE-based memory devices and optoelectronics have not supplanted non-photochromic technologies, nor are they likely to do so in the near future given the pace of advances in current conventional storage and electronics. D AEs have also been at the heart of attempts to explore photochromic materials as components in neuro-morphically engineered constructs to process information, i. e. computing based on biological nervous systems. The authors of a pioneering study [66] use DTE 5c as a memory element in an artificial neural network, likening it to an optical “memristor ”analogue whose absorbance, rather than resistance, is a function of its history. Such systems are many years away from being a practical proposition. Conventional technologies that do not rely on photochromism are unlikely to be displaced any time soon from their position of dominance. 4. 2 Imaging Super-resolution microscopy (SRM) is a clever optical technique that enables features of biological and mate-rial science-related objects to be imaged despite their size being smaller than the so-called “diffraction barrier ” of ~250 nm [67]. The key figures in the development of this “nanoscopy ”technique earned the Nobel Prize for Chemistry in 2014 [68]. SRM 's use of fluorescent molecules visualizes living systems and enables the highlight-8 | Towns - 2020 - Diarylethene Dyes.pdf |
Automaticallygeneratedrough PDFby Proof Check from River Valley Technologies Ltd DE GRUYTER Towns ing of component parts such as specific proteins [69]-something that is out of reach of electron microscopy. Resolution can be even further refined using photoswitchable fluorophores [70]. D AEs which photoisomerize between non-emissive and luminescent states have proved useful for SRM [56], although improvements in per-formance are needed. Researchers are now designing D AE colorants specifically targeted for use in nanoscopy. A priority is enhancing fatigue resistance within the aqueous environments encountered in biological sub-strates [71, 72]. While D AEs could become an entrenched fixture in SRM, their usage will remain high-value and low-volume. An ingenious, yet non-commercialized, macroscopic image creation system relies on D AEs acting as dopants in cholesteric liquid crystals (CLC) [49]. The chirality change that occurs upon irradiation of a dopant D AE affects CLC structure which in turn determines the hue of light reflected by the crystalline phase. In principle, this enables the construction of thermally stable flat displays whose color is photocontrolled-no auxiliary electronic drive circuitry is required unlike conventional LC displays. 4. 3 Process technology Microfluidic technologies, as used for example in bioassays, depend upon accurate manipulation of liquid flowing in microscopic channels of devices [59]. Localized control of this flow with light is of interest since it enables remote operation and is non-invasive. D AE dyes have been explored as the photoresponsive units in such systems. When incorporated into hydrogels, the hydrophobicity/hydrophilicity of these materials can be manipulated by irradiation with light-the accompanying variation in size acts to channel flow. An alternative is to manipulate droplet movement by locally modifying the wettability of the surface on which they reside. D AEs are also being explored as tools to enhance control of chemical synthesis. Using the change in elec-tronic structure and, in some cases, chirality that occurs upon photoisomerization, forays into controlling chem-ical reactions have been made with DTEs and dithiazolylethenes. They play the role of reagents or catalysts, in conjunction with light [73]. The radiation does not serve as an energy source, as it would in traditional photo-chemistry. Instead it dictates when and where chemical transformations occur. In principle, light allows precise remote control in terms of both space and time by influencing reactivity of starting materials, position of re-action equilibria, catalytic activity and product stereochemistry [50]. Dynamic control of polymerization by means of photoswitched catalysts based on DTEs is being explored [53]. The ability to manipulate the conver-sion of monomers with precision in space and time is attractive, enabling unprecedented control over polymer composition, but it remains far from commercial realization for technical and economic reasons. 4. 4 Biotechnology Just as D AEs have formed part of efforts to harness photochromism in the controlled synthesis of small molecules and polymers, this class often finds itself at the center of research seeking to manipulate the course of biochemistry through light [74-76]. D AEs have been explored in connection with photopharmacology (i. e. changing the activity of biological activity of molecules using light [52, 77]), specifically to modulate the cyto-toxicity of drugs by optical means only where required [78, 79]. For example, a photoswitchable analog of the cancer drug cisplatin, modified to incorporate an open-ring DTE unit, is more toxic to cancer cells when it com-prises the ring-closed photoisomer [80]. Conversion to the latter form increases the likelihood of interference with DNA repair, accelerating cell death. Localized application of UV would accomplish the photoisomeriza-tion only in the region of a tumor, thus minimizing damage to surrounding healthy tissue. However, UV is itself toxic, rendering its use clinically unacceptable [79][81]. Developing therapies that respond to light in the 600-1200 nm window is thus of much interest since such wavelengths penetrate biological tissue more deeply than shorter wavelengths and without causing damage. Compatibility towards aqueous environments in terms of solubility and stability is also important in small molecule-based photoswitches. While these goals present major challenges for D AE dye design, avoiding use of UV does appear feasible. An approach that has the big advantage of utilizing visible light depends upon peptides that resemble the highly cytotoxic antibiotic grami-cidin S: incorporation of a ring-closed DTE unit into their backbone reduces cytotoxicity by around an order of magnitude [82, 83]. The premise is that this form has sufficiently low toxicity to enable its systemic application. Irradiation with visible light only in the vicinity of a tumor leads to ring opening of the DTE unit and a localized increase in membrane disruption. Cell death is thus confined to tumor tissue. As well as switching pharmacodynamic characteristics (i. e. the interaction of a drug with its target), D AE-based compounds have also been investigated in relation to pharmacokinetic properties. The latter term refers to how a drug becomes distributed within living tissue, which has a bearing on its efficacy. D AE photochemistry has been employed to regulate uptake of biomolecules into cells. Peptides containing DTE units embedded into 9 | Towns - 2020 - Diarylethene Dyes.pdf |
Automaticallygeneratedrough PDFby Proof Check from River Valley Technologies Ltd Towns DE GRUYTER their backbones more readily penetrate cell membranes when the photochromic species are ring-closed owing to the less rigid o-DTE form promoting greater overall chain flexibility [84]. Ingress of such peptides can thus be controlled by irradiation with tissue-friendly red light. D AEs are regarded as one the most promising types of functional colorant for modulating biological activity [81][85]. Nonetheless, much remains to be done in order to realize their potential in creating efficient photocontrollable drugs [86]. D AEs are of interest in “nanocarrier ”technology to photocontrol drug delivery [26]. For instance, a DTE bearing tri(ethylene glycol) and nonyloxy side-chains in an aqueous environment spontaneously forms color-less submicron-diameter spheres capable of encapsulating bioactive materials [87]. Upon UV irradiation, the relatively flexible o-DTE photoisomerizes into a more rigid c-DTE, leading to conversion of spheres into blue fiber-like aggregates. However, controlled release from D AE-based nanocarriers by photoirradiation has yet to be realized. 4. 5 Photomechanical technologies One of the most striking features of D AE photochromism is the occurrence not only of photochromism in crys-tals, but also the macroscopic changes in shape and orientation which accompany it. The first reports were of square crystals morphing into rhomboid form when irradiated with UV, as well as crystalline rods bending in response to the stimulus [88]. The latter was used to demonstrate the conversion of light energy into mechanical work: the photoinduced motion led to microscopic beads in proximity being batted away. These deformations occur because of planarization of the D AE structure upon ring-closure, driving contraction along one crystallo-graphic axis and expansion along another. These changes at a molecular level become manifest macroscopically because of the precisely arranged D AE units of a crystal lattice exhibiting a concerted photoresponse. (Certain D AE crystals exhibit the rare phenomenon of photosalience [89]: sudden shattering of a solid upon exposure to intense light. ) Pronounced photomechanical responses are more difficult to achieve with polymeric matrices doped with photochromic colorant owing to a lack of long-range ordering of dye molecules in sufficiently high concentration. One means of tackling this difficulty is to employ D AEs to modulate the properties of liquid crystalline polymers. Photomobile materials can be constructed in this way, for example, films that bend to-ward UV sources and return to planarity when exposed to visible light [90]. By careful arrangement of D AE molecules (and in some cases angle of irradiation), photoinduced twisting is also possible [91]. Bi-directional, thermally-stable light-driven geometry changes typical of D AE units are also of great interest to researchers exploring their potential as components of supramolecular systems [76] and nanomachines [51]. Another potential application reliant upon nanoscale interactions concerns use of D AEs as adhesion pro-moters. DTE 5e was found to improve bonding between glass and the polystyrene film into which it had been doped and then activated with UV radiation [54]. The effect remained largely intact following cycloreversion. It was hypothesized that “photochromic annealing ”of the polymer had occurred as a result of the geometry change accompanying ring-closure, leading to localized polymer motion reducing voids in the interface. The overall more intimate contact between glass and polymer promoted adhesion. 5 Summary Like the discovery of several dye and pigment classes of key industrial importance, serendipity played a role in the genesis of D AE colorants. The news of their P-type photochromism, which operated in the solid state for thousands of cycles, was seized upon by technologists and material scientists. They have synthesized many thousands of examples [55], as well as incorporating them into polymeric and supramolecular systems, in an effort to develop applications based upon light-responsive switches. The thermal irreversibility of the pho-tochromic transitions exhibited by suitably designed D AE-based compounds has led to their investigation in many spheres of scientific endeavor, such as cancer therapy and information technology. Some of these applica-tions could revolutionize their fields, making an impact on daily life and well-being of the general population. Whether they will successfully compete with non-D AE approaches on a technical and economic footing re-mains to be seen. 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