Chapter 6

Light-Driven Chiral Molecular Switches or Motors in Liquid Crystal Media

Yan Wang and Quan Li

Liquid Crystal Institute, Kent State University, Kent, OH, USA

6.1 Introduction

Thorough understanding and/or mimicking nature's art of expressing and augmenting chirality from microscopic to mesoscopic levels remains elusive. However, the ubiquitous bio-molecular self-assembly into helical structures such as the double helix of DNA, α-helix of peptides, and the elegant colors of butterfly wings, bird feathers, and beetle exoskeletons [1–4] has inspired chemists to develop novel materials not only to reveal the structure–property correlation but also to explore their usage in diverse technological applications. The foremost objective of such studies has been the design and synthesis of chiral molecular systems capable of yielding complex large scale helical structure originating from the manifestation of chirality in the constituent molecules through non-covalent supramolecular interactions. Among the self-organized supramolecular systems, liquid crystals (LCs) represent a promising class of materials which might exhibit stable supramolecular helical organizations if the mesogens are chiral. The fascinating helical superstructure of chiral nematic LCs, i.e., cholesteric LCs (CLCs), undoubtedly is a striking example of such self-organization owing to its unique property of selective reflection of light and its consequent potential applications. However, large scale production of chiral LCs with desired properties is discouraging because of the high cost of chiral starting materials, synthetic difficulties and purification challenges, etc. The search for alternative ways of obtaining chiral nematic phase has led to the observation that when small quantities of chiral materials, i.e., chiral dopants, are dissolved in an achiral nematic LC (NLC), this results in a chiral nematic phase. One of the hallmarks of such systems is the amplification of molecular chirality by the anisotropic medium. To further elaborate its scope and add another dynamic quality to the LC system, the incorporation of switchable chiral dopants capable of shape change under the influence of external stimuli has attracted tremendous attention in the recent years. Such dopants are known as chiral molecular switches or motors [5], where molecules should own bistable structures, normally two isomers, which can be driven easily to convert from one state to another by various external stimuli [6]. Thereby the handedness or/and pitch length of the induced helical organization by chiral molecular switches or motors can be tuned and controlled. Compared with molecular switches or motors driven by electric and magnetic field, heat, chemical or electrochemical reaction, those capable of being driven by light possess advantages of ease addressability, fast response time, and potential for remote control in a wide range of ambient environment. Hence, the subject of this chapter is confined to the use of light as the controlling stimulus to accomplish dynamic reflection wavelength changes including the inversion of helical handedness in induced cholesteric LCs. The LC materials can be applied not only in novel LC photo displays but also in various non-display photonic applications, such as optical switches, optical storage, optical computing, and energy-saving devices. Effective materials for molecular switches or motors with chiral component(s) are being sought comprehensively as viable dopants for LCs in order to achieve complete light-driven systems for the applications above.

In this chapter, we will focus on light-driven chiral molecular switches or motors in LC media for the induction and manipulation of photoresponsive cholesteric LC system and their consequent applications.

6.2 Photoresponsive Cholesteric Liquid Crystals

Historically, chiral nematic LCs were called cholesterics because the first materials observed exhibiting this phase were cholesterol derivatives. Nowadays this is not the case and there exist many different types of chiral materials that exhibit chiral nematic (cholesteric) phase and most of them have no resemblance to cholesterol whatsoever. Cholesteric LCs have similar orientational order as nematics but differ from the fact that the molecules are locally oriented in a plane which rotates around a perpendicular direction (called helical axis) that repeats itself within a length called pitch. The pitch characterizes the distance across the helical axis where the director in each “plane” completes a full 360° rotation. For this reason, cholesterics may be picturized as a layered structure where the layer separation corresponds to half pitch, which is easily observed in the “finger print” texture of cholesterics.

As will be discussed later, the light reflections as well as any other applications are directly related to the pitch. Ever since the first application of cholesterics was discovered, being able to tune the pitch has been a major goal, as it would allow dynamic change in the system, for example, continuously changing the wavelength of reflected light. However, a direct tuning has always been an issue. Perhaps the easiest and most widely used manner is by taking advantage of photoresponsive CLC materials where light-driven molecules suffer structure change under irradiation leading to change in the helical superstructure and therefore a shift in the pitch length. There are three basic methods to obtain photoresponsive cholesteric LCs. The first way, also the most direct way, is to use photoresponsive chiral mesogens which can furnish the chiral nematic LC phase [7]. However, this method has a major problem that the pitch in such single molecular system is usually tuned over a relatively narrow range and cannot match its physical properties required for device applications, so it is considered as the oldest but not a very useful strategy. The second way is to photosensitize either nematic LC host/system or chiral doped LCs, i.e., dope both chiral molecules and achiral photoresponsive molecules in a nematic host, or dope both photoresponsive achiral/chiral molecules and non-photoresponsive chiral molecules in a nematic host. This method uses the photoresponsive cholesteric LC with more than one dopant in the nematic host, which makes the CLC system more complicated and may alter the desired physical properties of LC host. It is worth noting here that commercial LC blends are often composed of many components. The third and most commonly used method is to dope a small amount of photoresponsive chiral trigger molecules (light-driven chiral dopant) into an achiral nematic LC host, which can self-organize into a helical superstructure. The changes of concentration or shape of chiral dopant upon light irradiation can easily induce pitch change (Figure 6.1). When the chiral dopant and the LC host are mixed together, they will self-organize into a helical superstructure, i.e., CLC phase, and most of the LC properties will not change significantly if the amount of the trigger dopant is small. Currently the third strategy is being studied widely, and the most important aspect of this method is that it is the chiral dopant on which the sign and the magnitude of the CLC pitch strongly depend. Regardless of the method of how the cholesteric phase is obtained, when light propagates through the CLC medium, it selectively reflects light of specific wavelength according to Bragg's law. The average wavelength λ of the selective reflection is defined by λ = np, where p is the pitch length of the helical structure and n is the average refraction index of the LC material. Hence by varying the pitch length of the CLCs upon light irradiation, the wavelength of the reflected light can be tuned, providing opportunities as well as challenges in fundamental science that are opening the door for many applications such as tunable color filters, tunable LC lasers, optically addressed displays, and biomedical applications.

6.2.1 Helical Twisting Power of Chiral Dopants

As discussed above, while the cholesteric LC phase can be observed in single component molecular system, these materials are mostly formed by adding a chiral dopant to an achiral nematic LC host/system. When a chiral dopant is dispersed into a nematic LC media, the system self-organizes into a unique helical superstructure. The ability of a chiral dopant to twist an achiral nematic phase is expressed by the equation: β = (pc)⁻¹ where β is helical twisting power (HTP), p is the pitch length of the helical structure, and c is the concentration of the chiral dopant in LC. Different dopant molecules have different capability to twist the NLC. Therefore, HTP is an important parameter for the applications of CLC systems.

So far, many different techniques have been developed to quantitatively measure the HTP of different dopant materials. However, there are two conventional techniques that are widely used nowadays. One is spectroscopic method, and the other is the Grandjean–Cano method [8–11]. The spectroscopic technique is mainly based on the unique reflection wavelength, which is governed by the equation: λ = np. Typical NLC host has an average refractive index that is around 1.6. Thus pitch length can be obtained by measuring the reflection wavelength of CLC. With known concentration c, β can be easily calculated here according to the equation: β = (pc)⁻¹. The non-spectroscopic technique is usually based on a wedge cell, where the alignment is planar and substrates are rubbed parallel. The total twist inside the cell must be a multiple of half pitch in order to follow the boundary conditions. Thus the pitch is discrete and only certain pitch lengths are allowed. As the cell thickness change in the wedge cell, more half pitch turns are formed, but only when the cell gap and the boundary conditions allow it, as shown in Figure 6.2. This arrangement produces disclination lines between areas that contain a different number of layers. The disclination lines of CLCs in the wedge cell can be observed through a polarizing optical microscope. The pitch can be determined according to the equation: p =2R tan θ, where R represents the distance between the Grandjean lines and θ is the wedge angle of wedge cells. The inverse of pitch proportionately increases with increasing the concentration of the chiral dopant and HTP value.

6.3 Light-Driven Molecular Switches or Motors as Dopants

Chiral dopants for LC research have been developed mainly for two different purposes. The first purpose focuses on the development of chiral dopants with persistent shape, and the research mainly aims at achieving high HTP and investigation of the interaction between chiral dopant and LC host molecules [12–14]. Another purpose, currently attracting more attention, is to develop switchable chiral dopants, whose shapes are changed by external stimulus such as light or heat [15–18]. Such molecular switches can act much as an electronic “on and off” switch under light-driven condition. These molecules can exist in at least two stable states and the equilibrium of the transition between these two states can be achieved upon light irradiation as shown in Figure 6.3. Moreover, light-driven switching requires that the photoresponsive molecule employed as chiral dopant either reverses its intrinsic chirality or forms different switching states capable of inducing the helical superstructure including handedness inversion of cholesteric helix upon light irradiation.

Many different molecular switchable systems based on azobenzene, spiropyran, fulgide, diarylethene, etc. have been developed [19, 20]. These chiral molecular switches can be applied as bistable dopants for switching in LC media to create different helicity and pitch length in cholesteric states. As mentioned above, a variety of external stimuli, including pH, pressure, magnetic field, solvent, chemical reactions, electric field, heat, and light can induce the switching process [21]; however, heat and light are most commonly applied for these LC systems due to their non-destructive, reversible nature. Light especially has advantages over other stimuli, and can be used at selected wavelengths, distinct polarizations, and different intensities as well as for remote, spatial, and temporal controls. Moreover, the use of photoresponsive chiral dopants in optically addressed displays would require no drive electronics or control circuitry and can be made flexible. Furthermore, it gives the possibility of laser and mask applications, as the radiation pattern and intensity distribution can be accurately controlled. As a result, most of the molecular switches are designed as light-driven switches, which are doped into an LC media to achieve the change in helical pitch or order upon irradiation with the appropriate wavelength of light. Light-driven chiral switches or motors doped in LC media can be classified and distinguished by the different radiation triggered processes.

The first report on modulation of CLC properties by doping photoresponsive materials was reported by Sackmann in 1971 [22], where azobenzene was chosen as the photo trigger molecule. After that initial study, many molecular switches or motors were applied as light-controlled dopants in LC media. All these switches or motors exist as bistable structures; however, molecules with bistable states cannot necessarily be used as chiral dopants. As a result, the molecules that are regarded as light-driven chiral molecular switches or motors in LCs should possess the following properties. First and foremost, the chiral switch or motor must be soluble in LC host. It must maintain light stability as well as light sensitivity in the host material. The switch or motor must have an adequately high HTP to induce a Bragg reflection since its high concentration can often lead to phase separation, coloration, and alter the desired physical properties of the LC host. The excitation and relaxation in the host material must be tunable with fatigue resistance. Accordingly, many molecular switches or motors have been developed especially over the past decade and are widely used as light-driven chiral dopants in LC media to induce the photoresponsive CLCs, which are illustrated and discussed in the following sections.

6.3.1 Chiral Azobenzenes as Dopants

Azobenzenes have the unique feature of reversible trans–cis isomerization upon light irradiation, which can cause the large conformational and polarization changes intramolecularly. The trans-form of azobenzene has a rod-like structure that can stabilize the LC superstructure, whereas its cis-form is bent-like structure and generally destabilizes the LC superstructure by generating disorder in the aligned systems. This property has been used in photochemical orientation of nematic films [23–25], pitch change in cholesterics [23–29], and phase transitions from nematic to isotropic states [30]. The dopant containing an azobenzene core which effects a change in cholesteric pitch upon irradiation was first reported in 1971 [22]. However, the azobenzene moiety is still the most widely used photoactive bistable group in LC research today because of its easy synthesis and having a good compatibility with LC phase especially in its trans-form (its elongated structure). Besides, due to the dramatic difference of molecular geometry of trans- and cis-forms, the HTPs of these states typically have large difference, which in turn makes a large change of the cholesteric pitch.

Generally in a CLC mixture containing chiral azobenzene, the HTP of chiral azobenzene dopant depends on its molecular structure, the nature of chirality, and the interaction with host molecules [31]. It is interesting that azobenzene with axial chirality usually shows much more efficient ability to induce the cholesteric LC phase than azobenzene with tetrahedral chirality. For example, the highest HTP (β) values reported for azobenzenes with tetrahedral chirality are around 15 μm⁻¹ [31–36], whereas azobenzenes with axial chirality can have β value over 300 μm⁻¹ [36–39] (Figure 6.4).

It is known that the trans-isomer of chiral azobenzene normally shows more efficient cholesteric induction than its cis-form, whereas even small amounts of its cis-forms can destabilize the LC phase into an isotropic phase. For example, Li et al. reported chiral azobenzene 3 with tetrahedral chirality as a mesogenic dopant in nematic LC 5CB (Figure 6.5) [40]. As expected, its HTP is low, which is approximately 13 μm⁻¹. The chiral mesogenic dopant 3 needs to dope 25 wt% into an achiral nematic 5CB (or K15) to induce phase chirality with characteristic fingerprint texture (Figure 6.5A). Within 10 s under UV irradiation, this sample transits to isotropic phase as evidenced by a texture change as shown in Figure 6.5B. This experiment exhibited that the conversion from trans to cis configuration of the chiral dopant resulted in destabilization of the LC phase of the mixture. Removal of UV light immediately led to reverse process of chiral nematic domain formation from isotropic phase appearing as droplet nucleation followed by coalescence (Figure 6.5C). The reversion to the original polygonal fingerprint texture in Figure 6.5A was reached within approximately 2 h at room temperature in the dark.

However, Ichimura et al. reported that the cis-forms of chiral azobenzenes 4–6 exhibited higher “intrinsic” HTPs than their corresponding trans-isomers (Figure 6.6) [41, 42], which might result from the cis-isomers having a more rod-like shape compared with their trans-forms.

As mentioned above, HTP value of trans-chiral azobenzene is usually larger than that of its cis-chiral azobenzene. The combination of chiral azobenzene and non-photoresponsive chiral compound in nematic LC host can provide some interesting mechanisms for photochemical control of the helical structure such as phototuning helical pitch in any direction longer or shorter, phase transition between nematic and cholesteric phase, and handedness change of helical superstructure. Kurihara et al. reported photo-controlled switching of the photoresponsive CLCs consisting of chiral azobenzene (S)-7 and non-photoresponsive chiral dopant (S)-8 or its enantiomer (R)-8 (Figure 6.7) [35]. Chiral azobenzene (S)-7 induced a left-handed helix from an achiral nematic E44 whereas (S)-8 and (R)-8 induce a left- and right-handed helix, respectively. Figure 6.7a shows transmittance spectra of the CLC mixture of 17 wt% (S)-7 and 16 wt% (S)-8 in nematic LC E44 before and after UV irradiation, where the selective reflection wavelength was red-shifted upon UV irradiation. Contrary to the result shown in Figure 6.7a, the selective reflection of the CLC mixture composed of 5 wt% (S)-7 and 28 wt% (S)-8 in nematic LC E44 was blue-shifted upon UV irradiation. The results demonstrated that the helical pitch can be tuned and controlled in both directions to longer and shorter wavelengths by the combination of light-driven chiral switch or motor and non-photoresponsive chiral material as co-chiral dopant.

Kurihara et al. reported a combination of chiral azobenzene 9 and non-photoresponsive chiral compound 10 with LC host E44 to provide an effective photochemical modulation of the helical structure of CLCs (Figure 6.8) [43]. Non-photoresponsive chiral compound 10 was used for adjusting the initial reflection wavelength. Figure 6.9 (top) shows the colors reflected from the resulting CLC with different UV irradiation time. Before UV irradiation, the CLC was purple, and it turned to green, and then gradually to red with increasing irradiation time. The color could also be adjusted by varying the light intensity with a gray mask, as seen in Figure 6.9a and b. The resolution of the color patterning was estimated to be 70–100 μm by patterning experiments with the use of a photomask. The limitation of the resolution may be related to the diffusion of the low-molecular-weight compounds.

As mentioned previously, azobenzene with axial chirality usually induces short pitch cholesteric LCs due to high HTP. Many efforts were made to obtain a photo-controllable visible light reflector by doping axially chiral azobenzenes into a nematic LC media [44, 45]. The reflection wavelengths can be changed reversibly by photoisomerization of these azobenzenes [37, 38], normally red-shift upon UV irradiation and blue-shift upon visible light irradiation. Li et al. reported four reversible photoswitchable axially chiral azo dopants 11–14 with high HTPs as shown in Figure 6.10 [46]. These light-driven chiral switches were found suitable for dopants in nematic host for applications in novel optical addressed displays, i.e., photodisplay. For example, an image was created on the display cell filled with chiral switch 11-based CLC using UV light with a negative photo mask made of 10 mil PET placed on the top of the cell and exposed to UV light (637 μW/cm² at λ_max =365 nm) for 20 min. Depending on the optical density of the mask, certain areas were exposed with different intensities of light, resulting in an image composed of a variety of colors due to the various shifts in pitch length. Figure 6.10 (bottom) shows the photo of an original image (A), the negative mask (B), and the resulting image on the display cell (C). The light-driven switches in LC media were sufficiently responsive to an addressing light source that a high resolution image with gray scale could be imaged in a few seconds of irradiation time. It was further found that an image could be retained on the screen at room temperature for 24 h before being thermally erased. The high solubility of these materials in nematic host is also of commercial interest for stability in display applications.

A flexible optically addressed photochiral display is shown in Figure 6.11A [47, 48]. This photochiral display is also based on reversibly photoswitchable axially chiral azobenzene 11 with high HTP and the ability for molecular conformational changes upon light irradiation [46]. This display is flexed and based on flexible cholesteric LC display technology [49, 50]. As shown in Figure 6.11, two identical displays were driven by different energies. One is electrically addressed with the standard multiplexing electronics, while the other one is optically addressed. Relatively, the overall size of the display module is reduced in case of the light-driven one and the cost can potentially be saved up to six times compared to the cost of the electric-driven one. The simplification of the final product can make markets such as security badges, small point of sale advertisements, and other applications that require a very low cost module that is updated infrequently now possible. It is worth noting here that the photo display device can display a high resolution image without the need of attached drive and control electronics, substantially reducing the cost of the display unit for use in applications where paper is currently used.

Phototuning reflection wavelength over 2000 nm was demonstrated by White et al. in an azobenzene-based CLC consisting of a high HTP axially chiral azobenzene 11 (Figure 6.12) [51]. Phototuning range and rate are compared as a function of chiral dopant concentration, light intensity, and thickness. CLCs composed of 11 maintain the CLC phase regardless of intensity or duration of exposure. The time necessary for the complete restoration of the original spectral properties (position, bandwidth, baseline transmission, and reflectivity) of 11-based CLC is dramatically reduced from days to a few minutes by polymer stabilization of the CLC helix.

Green et al. reported two light-driven chiral molecular switches 15 and 16 with tetrahedral and axial chirality (Figure 6.13) [52]. When chiral switch 15 was doped in nematic LC host E31 at 15 wt% concentration, phototuning the reflection color over the entire visible region was observed. An amazing feature of this photoresponsive CLC system is quick relaxation. After 1 min of exposure to bright white light, it has surprisingly returned to the original ambient color. Unfortunately, the mixture in such high concentration is near saturation level and visible signs of phase separation after several phototuning cycles were observed due to their poor solubility in LC host.

As noted before, light-driven switch 2 with axial chirality exhibited the highest HTP value for any light-driven switch reported so far [39]. The switch was found to be able to impart its chirality to a commercial nematic LC host, at low doping levels, to form a self-organized, optically tunable helical superstructure capable of fast and reversible phototuning of the structural reflection across entire visible region. This was the first report on reversible phototuning reflection color truly across entire visible region by employing light-driven chiral molecular switch or motor as the only chiral dopant in a LC media. For example, a mixture of 6.5 wt% 2 in nematic LC E7 was capillary filled into a 5 μm thick glass cell with a polyimide planar alignment layer and the cell was painted black on one side. The reflection wavelength of the cell was able to be tuned starting from UV region across the entire visible region to near infrared region upon UV irradiation at 365 nm (5.0 mW/cm²) within approximately 50 s, whereas its reversible process starting from near infrared region across the entire visible region to UV region was achieved by visible light at 520 nm (1.5 mW/cm²) or dark thermal relaxation. The reflection colors across the entire visible region were uniform and brilliant as shown in Figure 6.14A and B. Its ability to reversibly phototune the reflection color truly across entire visible region is further evidenced in Figure 6.14C and D. The reversible process with visible light is much faster than dark thermal relaxation. For instance, the phototuning time of 6.5 wt% 2 in E7 with a visible light at 520 nm (1.5 mW/cm²) from near IR region back across entire visible region to UV region is within 20 s whereas its dark thermal relaxation back through the entire visible region took approximately 10 h. Each reflection spectrum in Figure 6.14C and D has no drawback such as the dramatic change of the peak intensity and bandwidth compared with electric field-induced color tuning [53]. The reversible phototuning process was repeated many times without degradation. It is worth noting here that the reversible phototuning process across the entire visible region was achieved in seconds with the increase of light exposure intensity.

Furthermore, this chiral switch 2 was used in a color, photo-addressed liquid crystal display driven by light and hidden as well as fixed by application of an electric field from thermal degradation. Like conventional cholesteric LCs, the chiral switch doped in LC media is able to be electrically switched to bistable display by using polymer stabilized or surface stabilized chiral nematic texture. Even though the optically switched azo compounds are not thermally stable, an image can be made thermally stable and be retained indefinitely by electrically switching either the image or the image background to the focal conic state before it thermally relaxes. The image or its background is electrically selected by shifts in the electro-optic response curve that result from a change in the twisting power of the photosensitive chiral compound. An advantage of this display is that a thermally stable high resolution image can be captured without patterned electrodes or costly electronic drive and control circuitry, and retained indefinitely until electrically erased. Here such a light-driven device was made using the chiral switch 2. The phototunable cholesteric layer sandwiched between two simple unpatterned transparent electrodes is sufficient. For example, an optical writing took place within seconds in a planar state through a photomask by a UV light. The reflective image (Figure 6.15A) can be hidden in focal conic texture by applying a 30 V pulse and revealed by applying a 60 V pulse (Figure 6.15C). Moreover, by applying a 30 V pulse to an optically written image so as to make the UV irradiated region going to the focal conic texture and the UV un-irradiated region going to the planar texture, an optically written image can be stored indefinitely because the planar and focal conic textures are stable even though the light-driven switch relaxes to the un-irradiated state.

Chiral cyclic azobenzene switches have also been used to investigate the light-driven twisting behaviors for CLC system [54–56]. It was reported that some chiral cyclic compounds showed a reversible inversion in the handedness of CLC by means of their photoisomerization upon light. Manoj et al. recently reported a fast photon mode reversible handedness inversion of a self-organized helical superstructure, i.e., cholesteric LC phase, using light-driven chiral cyclic dopants (R)-17 and (R)-18 [56]. The two light-driven cyclic azobenzenophanes with axial chirality show photochemically reversible trans to cis isomerization in solution without undergoing thermal or photoinduced racemization (Figure 6.16A). The switches exhibited good solubility, high HTP, and a large change in HTP due to photoisomerization in three commercially available structurally different achiral LC hosts. Therefore, reversible tuning reflection colors from blue to near IR by light irradiation from the induced CLC was observed. More interestingly, the different switching states of the two chiral cyclic dopants were found to be able to induce a helical superstructure of opposite handedness. For example, a typical CLC texture observed for the N* phase of the CLC mixture containing 10 wt% (R)-17 in nematic LC ZLI-1132 under planar alignment conditions was quickly transformed into a planar N texture upon UV irradiation (Figure 6.16, A and B). As the sample in the N phase was rotated between fixed crossed polarizers, an extinguishing orientation of the cell was found when the orientation of the molecular director was along one of the polarizer directions (Figure 6.16C). This transient N phase was quickly transformed into an N* phase upon continued UV irradiation for a few more seconds (Figure 6.16D). The whole switching process was reversible with 440 nm irradiation. This provides clear evidence on the reversible handedness inversion upon light irradiation.

The induced helical pitch and photo-tunability of chiral cyclic dopants (R)-17 and (R)-18 in nematic LC media were measured using Cano's wedge method and the corresponding change in HTP values was summarized in Table 6.1. (R)-17 in its trans-form shows a high HTP value in E7 and K15 while the corresponding value in ZLI-1132 was found to be low. Its analog (R)-18 exhibits a lower HTP in E7 and K15 LC hosts compared to (R)-17. On the contrary, the HTP value of (R)-18 in ZLI-1132 was found to be higher than what was obtained for its lower homologue compound. Compared with its analog at ortho-substitution, the chiral switch (R)-17 with meta-substitution exhibited a higher HTP and a higher change in HTP, which might result from the intrinsic nature of its molecular structure and having a more dramatic geometrical change upon photoisomerization. Different LC hosts result in the different intermolecular associations between dopants and hosts. These results clearly reveal the subtle dependence of HTP on the molecular structures of both the dopant and the NLC hosts. Interestingly, Kawamoto et al. reported that (R)-17 can behave uniquely for non-destructive erasable chiroptical memory through its photoinduced switching in neat film [57].

Table 6.1 Helical twisting powers (β) of light-driven chiral molecular switches (R)-17 and (R)-18 in different nematic LC hosts as determined by Cano's wedge method and the observed change in values by irradiation. Positive and negative values represent right- and left-handed helical twists respectively.

6.3.2 Chiral Olefins as Dopants

Chiral olefins are the typical compounds with the capability of trans–cis isomerization similar to chiral azobenzenes, which can be used as light-driven chiral switches in LC media. Such compounds with exocyclic double bond should be chemically stable and do not form photo-dimers. However, to date only a few of these molecules have been reported to induce photoresponsive CLC system [58]. Yarmolenko et al. reported menthone-based chiral dopant 19 with high HTP and efficient cholesteric pitch modulation (Figure 6.17) [59]. Its cis-isomer was rather stable, and no thermally excited cis–trans isomerization was observed upon heating to 80°C, in contrast to azobenzene. As seen from Figure 6.17, the HTP value at its trans- and cis-form exhibited a considerable difference, which results from their dramatically different shape, similar to the change observed in azobenzene isomers. Chiral dopant 19 doped in nematic host MBBA exhibited a handedness inversion upon light irradiation, whereas no such handedness inversion of the resulting CLC was observed when using 5CB instead of MBBA as the nematic host. These results clearly reveal the subtle dependence of HTP on the molecular structure of nematic LC host since different LC host results in the different intermolecular association between dopant 19 and its host. The high HTP of 19 is probably due to its better compatibility and interaction in the LC medium owing to its very similar structure to the host LC molecules. Later, Lub et al. synthesized menthone derivatives 20 and 21 and observed moderate HTP in E7 mixture [60]. Moreover, in order to investigate the effect of chemical structure on HTP two new photoisomerizable compounds that are structurally related to menthone derivative 20 were designed and synthesized (Figure 6.17). However, the trans-isomers of nopinone and camphor derivatives 22 and 23 exhibited much lower HTPs than 20. It is possible that the chiral groups of the cage-like structure of 22 and 23 show less interaction with the LC host and hence lower HTP. It is interesting to note that the twist sense of the CLCs induced by 22 and 23 are opposite to the twist sense of 20. Furthermore, the twist sense of trans- and cis-isomers of 22 and 23 are also opposite. Though the HTPs are less for these compounds, their studies led to better understanding of structure–property relationship of chiral photoisomerizable dopants.

Stilbene derivatives are another class of olefins which undergo cis–trans isomerization upon photoirradiation. Therefore by linking chiral moieties, stilbenes can be made photoresponsive chiral dopants to induce chiral nematic phase and the pitch of the resulting phase can be modulated upon photoirradiation owing to their photoisomerization. Lub and co-workers have reported several chiral stilbene derivatives 24, 25 and 26 containing different chiral auxiliaries [61, 62]. Their structures and HTPs in achiral nematic liquid crystal hosts are shown in Figure 6.18.

Similar to menthone and stilbene derivatives, cinnamic esters are also capable of exhibiting photoinduced cis–trans isomerization and hence are potential candidates for photoswitchable dopants. Accordingly several chiral cinnamate esters 27–30 (Figure 6.19) containing isosorbide as the chiral moiety have been synthesized and investigated as efficient chiral dopants in nematic LC media [63, 64].

6.3.3 Chiral Diarylethenes as Dopants

Photochromic diarylethenes undergo a reversible 6-electron cyclization upon irradiation, leading to distinct change in structure and electronic configuration of the molecule [65]. This switching unit has been applied for reversible cholesteric to nematic transition and vice versa as well as photomanipulation of the cholesteric pitch [66–73]. Figure 6.20 shows some structures of these chiral diarylethenes. Feringa et al. reported the reversible cholesteric to nematic transition using open and closed form diarylethene 31 as shown in Figure 6.20 (top) [66]. When 1.4 wt% 31 in LC ZLI-389 was heated up under crossed polarizing microscope, a stable cholesteric phase was observed close to the N–I transition temperature. When the temperature was kept within the range of 51–54°C, the cholesteric phase with identical fingerprint texture was stable (Figure 6.20A). When it was irradiated with UV light at 300 nm for 50 s, the cholesteric phase disappeared and a nematic phase texture was observed (Figure 6.20B). Irradiation of the sample with visible light for 30 s resulted in the reappearance of the cholesteric fingerprint texture. This results from the fact that the open form of chiral diarylethene 31 facilitates the formation of a stable cholesteric phase in ZLI-389, while its HTP in the closed form is too low to effectively stabilize a cholesteric phase. Yamaguchi et al. reported photochromic diarylethene 32 with axial chirality which can induce a stable photoswitching between the nematic and cholesteric phase due to its very weak HTP (β_M ~ 0 μm⁻¹) at open form [68–72]. Cholesteric induction by this type of switch was supposed to be not very efficient because of extremely low HTP [74]. More recently, van Leeuwen et al. reported diarylethene 33 with a high HTP value of 50 μm⁻¹ [73]. In contrast to the other diarylethene dopants reported previously, its ring-closed form 33 can induce CLC phase as well.

Rameshbabu et al. reported three photochromic chiral LC diarylethenes with tetrahedral chirality 34–36 which were found not only to be able to self-organize into a phototunable helical superstructure, but also to be able to induce a photoresponsive helical superstructure in an achiral LC host (Figure 6.21) [75]. For instance, 10 wt% 34 as a mesogenic dopant in a conventional achiral nematic 5CB exhibited a cholesteric polygonal fingerprint texture, as shown in Figure 6.21A. The transition from cholesteric to isotropic phase was observed. With UV irradiation at 310 nm (30 mW/cm²) for 30 s, it transformed into isotropic phase (Figure 6.21B) whereas upon visible irradiation at 670 nm the reverse process was observed, as evidenced by the formation of the chiral nematic domain from isotropic phase appearing as droplet nucleation followed by coalescence (Figure 6.21C). The reverse process upon visible light irradiation was reached within 30 min.

Very recently Li et al. reported three light-driven dithienylcyclopentene switches (S,S)-37, (R,R)-37, and (S,S)-38 (Figure 6.22) [76]. These chiral molecular switches with axial chirality were found not only to be able to act as a chiral dopant and induce a helical superstructure in an achiral nematic LC host, but also to be able to reversibly and dynamically tune the transmittance and reflection of the resulting cholesteric phase upon light irradiation. Light-driven chiral switch 37 exhibited an unusually high HTP which is significantly larger than those of the known chiral diarylethenes reported so far.

6.3.4 Chiral Spirooxazines as Dopants

Spirooxazine has been known as a promising photochromic compound with good photo-fatigue resistance for a long time [77]. Typical examples of photochromic reactions of spirooxazines are the reversible photochemical cleavage of the CߝO bond in the spirooxazine rings. Because the spiro-carbon of a spirooxazine molecule has potential as a chiral center, spirooxazines could be used as chiroptical molecular switches [78]. However, spirooxazines are usually racemic mixtures as shown in Figure 6.23. Therefore, if spirooxazines are to be used as chiroptical molecules in nematic LC system, modification of the spirooxazine with a chiral group is required. There are a few examples of spirooxazines used as the dopants in LC systems [78, 79]. Recently Jin et al. reported some novel thermally reversible photochromic axially chiral spirooxazines 40–43 [79]. These axially chiral spirooxazines showed ability to twist the nematic host LC E7 to form the cholesteric phases and the HTPs were relatively high (Figure 6.24). Additionally, the result illustrated that the chiral spirooxazines containing bridged binaphthyl moiety exhibit higher HTP than the corresponding unbridged ones either for the initial state (ring-closed form) or for the photostationary state (ring-opened form, irradiated with 365 nm UV light). Furthermore, this bifunctional system exhibited excellent thermally reversible photochromic behavior together with the chiral induction capability in LC hosts.

6.3.5 Chiral Fulgides as Dopants

Chiral fulgides are an interesting class of thermally irreversible photochromic materials with 6-electron cyclization upon light irradiation [80], which can be used as a light-driven trigger for LC systems. The photochromism of fulgides occurs between one of the colorless open forms and the photocyclized colored form. Yokoyama et al. reported that fulgides 44 and 45 with axial chirality acted as chiral dopants in nematic LC 5CB to induce cholesteric phase (Figure 6.25) [81–83]. The incorporation of an axially chiral binaphthol moiety into fulgide structure resulted in a bistable system with an enormous difference in HTP between the open and closed forms of the switch [81–83]. For example, chiral fulgide 45 at open form has a β_M of −28.0 μm⁻¹ in 5CB whereas its ring-closed isomer has an impressive β_M of −175 μm⁻¹. This allows photoswitching between cholesteric phases with a long and a short pitch, respectively, using small amounts of light-driven chiral dopant. The resulting CLC did not exhibit a handedness inversion upon light irradiation. However, this was circumvented with addition of non-photoresponsive chiral dopant (S)-dinaphtho [2,1-d:1′,2′-f] [1,3]dioxepin with opposite HTP (β_M = +92 μm⁻¹), resulting in reversible switching between a positive and negative handedness of cholesteric helix [83].

6.3.6 Chiral Overcrowded Alkenes as Dopants

Chiral overcrowded alkenes as dopants are much more likely to show inversion of the cholesteric helix sign upon switching. This kind of compounds is originally pioneered by Feringa and co-workers who continue to champion these materials for applications as molecular switches, molecular motors, and as enablers to photogenerate dynamic optical effects in CLCs. They reported some asymmetric overcrowded alkenes for chiroptical switches or motors [45, 84–87]. Take light-driven chiral motor 46 as example (Figure 6.26, top) [45, 87]. Its initial HTP at (P,P)-trans-form in nematic E7 is +99 μm⁻¹, but generation of a cholesteric helix with an opposite sign of similar pitch is impossible, as the (M,M)-trans-form possesses a minor negative HTP (β_M = −7 μm⁻¹, E7). As a result of the high HTP at (P,P)-trans-form, colored LC films were easily generated using this dopant. Photochemical and thermal isomerization of the motor leads to irreversible color change in the LC film as shown in Figure 6.26 (bottom) [45].

A breakthrough in this area was achieved with the introduction of fluorene-derived molecular motors. Possibly due to the structural compatibility of the fluorene group with the LC host's biphenyl core, motor 47 was found to possess very large helical twisting powers for both stable and unstable forms (Figure 6.27, top). Moreover, these two forms induce cholesteric phases of opposite signs, making it possible to switch efficiently between cholesteric helicities. As the thermal isomerization step (from unstable to stable form) occurs readily at room temperature, these motors were found to be able to induce fully reversible color change of a liquid crystalline film across the entire visible spectrum [88, 89]. Moreover, switching of this molecular motor in a liquid crystalline environment induced an unprecedented rotational reorganization of the LC film, which was applied in the light-driven rotation of microscale glass rods (Figure 6.27, bottom) [90, 91].

Besides, other groups also reported some chiral overcrowded alkenes as the dopants in LC media [92, 93]. Bunning et al. showed the polarized optical microscopy (POM) images of light-driven chiral motor 47 in nematic LC media (Figure 6.28). As shown in Figure 6.28a, the CLC consisted of 4.2 wt% 47 in LC1444 exhibiting a characteristic Grandjean texture expected of a short-pitch CLC. After exposure to 10 μW/cm² UV light, the texture of the CLC remained in this state but undergoes color change, indicating a change in pitch. As the CLC pitch unwinds, a texture shown in Figure 6.28b was observed for the nematic phase. Continued UV exposure generates the fingerprint texture apparented in Figure 6.28c, characteristic of a long-pitch CLC. With continued UV exposure, the CLC again shows the Grandjean texture (Figure 6.28d–g). As evident in these panels, the number of defects in the Grandjean texture was initially low and then became larger. Continued light exposure seemed to annihilate some of these defects, as evident in Figure 6.28f and g. After UV exposure, POM images were also captured in the dark. As expected, the texture of the CLC evolves from Grandjean (Figure 6.28h) to nematic (Figure 6.28i) to fingerprint (Figure 6.28j) as the helix inverts.

Furthermore, overcrowded alkenes have another pathway to show a switchable process in LC media which is caused by the chiral isomerization. This series of bistable switches of the overcrowded alkenes with an enantiomeric relationship between the two switch states can be interconverted by using circularly polarized light (CPL). It can be considered as a new type dopant, which exhibits the partial photoresolution under irradiating with CPL of one handedness. During the CPL process, the two enantiomers have different capability for absorbing the left-handed CPL (l-CPL) or right-handed CPL (r-CPL). As a result, one enantiomer is excited preferentially by either l-CPL or r-CPL within a racemic system, which will convert into the other enantiomer. However, this CPL being used has almost no effect to another enantiomer. On this occasion, the amount of the enantiomer will accumulate until an equilibrium or photostationary state (PSS) is reached. The enantiomeric excess (ee) value of this PSS (ee_PSS) at a certain wavelength of irradiation depends on the Kuhn anisotropy factor g_λ, defined as the ratio of the circular dichroism (Δε) and the extinction coefficient (ε) (Eq. (6.1)) [94].

(6.1)

Normally, as g-value do not exceed 0.01, CPL photoresolution rarely leads to ee values over 0.5%. This ee values cannot be easily determined by the common methods. However, because the conversion from nematic to cholesteric is essentially thresholdless, theoretically these ee values are high enough to induce a nematic to cholesteric transition and can be determined from the cholesteric pitch via Equation (6.1). Similarly, the helicity of a cholesteric phase for this system can be controlled by only using the chiral information in the CPL. At last, the transition from cholesteric to nematic phase can be caused by irradiation with unpolarized light (UPL) or linearly polarized light (LPL), to lead to the racemization of chiral switch or motor [74].

Feringa et al. proved this concept by adopting the inherently dissymmetric overcrowded alkene 48 (Figure 6.29) [95]. They applied l-CPL irradiation at 313 nm to 20 wt% racemic 48 in a nematic LC K15 that can obtain the (M)-48 with 0.07% ee as a cholesteric phase. Then, irradiating the (M)-48 with LPL, the cholesteric LC phase gradually disappeared with the racemization. In the same way, the irradiation with r-CPL resulted in the cholesteric LC phase with opposite handedness, which still can go back to racemic state through LPL or UPL. Though the HTP (β) and the anisotropy factor (g) were both very low in this result, it did show the potential of this system for amplification of chirality via a chiral molecular switch to a macroscopic nematic to cholesteric phase transition by using a handedness CPL. In addition, this 3-stage LC switching system also presented how to control and develop between the positive and negative cholesteric LC phase.

6.3.7 Axially Chiral Bicyclic Ketones as Dopants

Another series of reversible photoswitching of racemic bistable axially chiral bicyclic ketones irradiated by CPL, as mentioned previously in Section 3.6, was investigated by Schuster et al. [96–99]. Racemic axially chiral bicyclic ketone 49 was irradiated with l-CPL leading to the partial photoresolution (Figure 6.30) [96]. After irradiating for 6.7 h, a photostationary state was achieved with 0.4% ee, which is in good agreement with the calculated ee value from the anisotropy factor (g₃₀₅ = 0.0105 at 305 nm). However, the enantiomeric enrichment cannot effectively cause the nematic to cholesteric phase transition, probably due to the low helical twisting power.

Several chiral bicyclic ketones 50–53 were designed as the photochemical molecular switches and applied as the triggers for the control of the LC phases (Figure 6.31) [96–99]. The structures of their rigid bicyclic core and ketone chromophore generally possess large g-values. Unfortunately, both the helical twisting power and solubility in nematic LC media are often low for most of them, which make it difficult to induce the nematic to cholesteric phase transition. Finally, they found the chiral bicyclic ketone 53 with a mesogenic unit, which resulted in a system capable of reversible nematic to cholesteric phase transition using the CPL resource (Figure 6.31) [99]. Ketone 53 contains a mesogenic moiety similar to the LC host ZLI-1167 resulting in a helical twisting power of 15 μm⁻¹, a high g-value (g₃₀₀ =0.016), and the good solubility. CPL irradiation (λ > 295 nm) of a nematic mixture containing 13 mol% racemic 53 resulted in a cholesteric phase with a pitch of 190 μm. This was more than twice the pitch obtained when a photo-resolved sample at the photostationary state was doped in the mesogenic host, probably due to scattering of the CPL by the LC mixture.

6.4 Conclusion

In this chapter, we have presented a brief overview about the dynamic behaviors and the properties of light-driven chiral molecular switches or motors in LC media. This kind of chiral molecular switches or motors doped into the LC media can be used as optical memory, optical display, and optical switching in the field of optical devices. As guest molecules, they can induce helical superstructures in an achiral LC host to obtain cholesteric LC and dynamically phototune the superstructures to achieve reversible reflection color or/and handedness inversion etc. Moreover, the phenomenon of cholesteric induction is a remarkable example of how the chiral information at the molecular level can be transmitted through amplification in self-organized stimuli-responsive soft matter. From the above discussions, it is clear that adding small quantities of chiral dopants to achiral liquid crystals have become the method of choice for helicity induction in liquid crystals. Furthermore, liquid crystals can serve as model systems in the development of supramolecular assemblies with controlled chiral architectures induced by stimuli-responsive chiral triggers.

The continuous efforts on finding new efficient photoswitchable and soluble chiral dopants are expected to provide better understanding of chiral induction in soft matter and could provide future smart materials and devices with improved properties and performance. Although the calamitic nematic phase has been largely exploited in this endeavor, the nematic phases exhibited by discotic and bent-core liquid crystals are still left to be explored. Finally the development of novel switchable chiral dopants with very high HTP in very small quantities as low as parts per million (ppm) and which can aid fast and reversible phototuning of reflection colors over the entire visible spectrum is urgently required to fully explore the potential of these intriguing materials. Open research fields also include other LC phases with induced chirality, like blue phases and smectic C* phases, as well as chiral doped micelles.

Acknowledgments

The preparation of this chapter benefited from the support to Quan Li by the Ohio Board of Regents under its Research Challenge program, the Air Force Office of Scientific Research (AFOSR FA9950-09-1-0193 and FA9950-09-1-0254), the Department of Energy (DOE DE-SC0001412), the Department of Defense Multidisciplinary University Research Initiative (AFOSR MURI FA 9550-06-1-0337 and FA9550-12-1-0037), and the National Science Foundation (NSF IIP 0750379).

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