Light-Driven Liquid Crystalline Materials: From ... - ACS Publications

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Light-Driven Liquid Crystalline Materials: From Photo-Induced Phase Transitions and Property Modulations to Applications Hari Krishna Bisoyi and Quan Li* Liquid Crystal Institute and Chemical Physics Interdisciplinary Program, Kent State University, Kent, Ohio 44242, United States ABSTRACT: Light-driven phenomena both in living systems and nonliving materials have enabled truly fascinating and incredible dynamic architectures with terrific forms and functions. Recently, liquid crystalline materials endowed with photoresponsive capability have emerged as enticing systems. In this Review, we focus on the developments of lightdriven liquid crystalline materials containing photochromic components over the past decade. Design and synthesis of photochromic liquid crystals (LCs), photoinduced phase transitions in LC, and photoalignment and photoorientation of LCs have been covered. Photomodulation of pitch, polarization, lattice constant and handedness inversion of chiral LCs is discussed. Light-driven phenomena and properties of liquid crystalline polymers, elastomers, and networks have also been analyzed. The applications of photoinduced phase transitions, photoalignment, photomodulation of chiral LCs, and photomobile polymers have been highlighted wherever appropriate. The combination of photochromism, liquid crystallinity, and fabrication techniques has enabled some fascinating functional materials which can be driven by ultraviolet, visible, and infrared light irradiation. Nanoscale particles have been incorporated to widen and diversify the scope of the light-driven liquid crystalline materials. The developed materials possess huge potential for applications in optics, photonics, adaptive materials, nanotechnology, etc. The challenges and opportunities in this area are discussed at the end of the Review.

CONTENTS 1. Introduction 1.1. Liquid Crystals 1.2. Photochromic Materials 2. Photochromic Liquid Crystals 2.1. Photochromic Liquid Crystals Based on Spiropyrans and Spirooxazines 2.2. Photochromic Liquid Crystals Based on Dithienylcyclopentenes 2.3. Other Photochromic Liquid Crystals 3. Photostimulated Phase Transitions 3.1. Photoinduced Phase Transitions in Low Molecular Weight Liquid Crystals 3.2. Photoinduced Phase Transitions in Polymeric Liquid Crystalline Systems 4. Photoalignment and Photoorientation of Liquid Crystals 4.1. Photoalignment of Liquid Crystals 4.2. Photoorientation of Liquid Crystals 5. Photomodulation of Chiral Liquid Crystals 5.1. Photomodulation of Cholesteric Liquid Crystals 5.1.1. Azobenzene-Based Dopants in Cholesteric Liquid Crystals 5.1.2. Overcrowded Alkenes in Cholesteric Liquid Crystals 5.1.3. Dithienylcyclopentene Derivatives in Cholesteric Liquid Crystals

5.1.4. Fulgide-Based Chiral Dopants in Cholesteric Liquid Crystals 5.1.5. Bicyclic and α,β-Unsaturated KetoneBased Chiral Dopants in Cholesteric Liquid Crystals 5.1.6. Cinnamate- and Spirooxazine-Based Chiral Dopants in Cholesteric Liquid Crystals 5.2. Photomodulation of Blue Phases 5.3. Photomodulation of Ferroelectric Liquid Crystals 5.4. Light-Driven Chiral Induction in Liquid Crystals 6. Light-Driven Liquid Crystalline Polymers and Elastomers 7. Light-Driven Liquid Crystalline Gels and Bent-Core Liquid Crystals 8. Summary and Outlook Author Information Corresponding Author Notes Biographies Acknowledgments References

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AD AI AL Received: June 30, 2016

© XXXX American Chemical Society

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DOI: 10.1021/acs.chemrev.6b00415 Chem. Rev. XXXX, XXX, XXX−XXX

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1. INTRODUCTION Seeing is believing. We are able to see and applaud the vivid and sparkling natural world that surrounds us due to the natural process of cis−trans photoisomerization of the retinal.1−9 At the outset, light actuates this natural phenomenon followed by a cascade of photomechanical events, eventually leading to “sight” otherwise referred to as “vision”. Therefore, it is aptly stated that “there is no sight without light”. This single example is sufficient to appreciate the role of light-driven processes in the animal world. As far as the plant kingdom is concerned, there operates another analogous light-driven cis−trans photoisomerization process in the photoreceptor phytochrome, a linear tetrapyrrole. This light-driven photochromic phenomenon governs physiological, growth, and developmental processes in plants culminating in certain heliotropic and phototropic motions in addition to stem movements. The above-mentioned two representative paradigms in the natural world convincingly demonstrate the prevalence and criticality of light-triggered processes and phenomena in nature. The elegance of nature performing such light-driven functions has been a great source of inspiration for scientists in the research and development of light-driven materials and systems not only for fundamental scientific studies but also for device applications. Consequently, they have delved into the realm of design, synthesis, and evaluation of properties of dynamic and reconfigurable lightdriven materials and systems. Moreover, the rising potential of nanotechnology and photopharmacology has provided additional thrust to this endeavor since light triggered molecular events can be cooperatively and effectively translated into controllable macroscopic phenomena.3−9 Toward this end, lightdriven liquid crystalline molecular and macromolecular materials have garnered substantial attention, owing to their distinguished and pragmatic attributes. In this Review, we cover the development and progress in light-driven liquid crystalline materials during the past decade. This Review is an attempt to showcase the diverse activities in the field of light-driven liquid crystalline materials under a single umbrella. We note that there are several accounts out there covering specific aspects of the field from different perspectives. We have intended to present the various topics through a unified and overarching approach and have tried to highlight how the basic concepts are applied across different areas of the field to modulate the properties of lightdriven liquid crystalline materials and enable their applications in various devices. Wherever warranted, sufficient background information has been furnished and succinctly discussed. In the following, we concisely introduce the world of liquid crystals (LCs) and photochromic systems in turn.

leading to their adoption in different applications. The ubiquity of LC as the active switching ingredient in flat information display devices such as television, camera, mobile phone, laptop, and computer screens is a testimony to their fast response and low power consumption. Beyond these display applications, the use of LCs in different areas of materials science, biological and biomedical science, and nanoscience has been demonstrated. LCs can act as functional optical materials, charge, energy, and ion transporting soft materials, and thermally, electrically, photochemically, and mechanically driven multifunctional stimuli-responsive materials. Liquid crystalline materials as useful as they are, the term “Liquid Crystals” being an oxymoron, possess an element of mystery and attraction for both the nonspecialists and the general public. At the fundamental level, LCs serve as model supramolecular systems since they involve almost all noncovalent secondary interactions (dipolar and dispersion interaction, hydrogen and halogen bonding, π−π and charge transfer interaction, and metal coordination) in the formation of well-defined self-assembled hierarchical architectures from simple to complex modes. In doing so, LCs convincingly manifest the basic organization principle of matter by maximizing the intermolecular interaction and minimizing free space (i.e., excluded volume). Depending on how the mesophases can been attained, LCs are broadly classified into two different categories namely thermotropic and lyotropic LCs.10,23 Thermotropic LC phases can be achieved either by heating a crystalline solid or by cooling an isotropic liquid. LC phase formation in organic materials made of anisometric molecules can be briefly outlined as follows. When the crystalline state possessing both positional and orientation order of molecules is heated, at its melting point, the molecules lose their positional ordering; however, orientational ordering of the molecules does not vanish. This results in an orientationally ordered dynamic fluid phase which is known as the LC phase. Upon further heating, the material loses the orientational ordering among its molecules and passes into an isotropic liquid without any orientational and positional ordering (Figure 1).

1.1. Liquid Crystals

LC, which thermodynamically prevails between the crystalline solid state and isotropic liquid state, is a unique state of matter that displays the anisotropic properties of crystals and flow properties of ordinary liquids.10−67 Owing to its appearance between two condensed state of matter, the LC phase is often referred to as the mesophase. The constituent entities of mesophases are known as mesogens. LC, being a bonafide thermodynamic stable state of matter, is also referred to as the fourth state of matter after solid, liquid, and gas. The anisotropic characteristics of LCs arise from the orientational ordering of the constituent molecules, whereas the fluid properties are due to the concomitant presence of mobility of the molecules in these intermediate phases. Concurrent existence of order and mobility has entitled LCs as functional stimuli-responsive soft materials,

Figure 1. Molecular organization in crystal, LC, and liquid states of a material made of elongated anisometric molecules.

The molecules in some LC phases possess short-range positional ordering in addition to orientational ordering. Lyotropic LC phases are usually fabricated by dissolving amphiphilic compounds in suitable solvents.36,37 Here the concentration of the amphiphilic compound in the solutions largely determines the kind of LC phase formed. However, for a given concentration of the amphiphilic compounds, phase transitions between different LC phases can also be effected by varying the B


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Calamitics (i.e., rod-shaped compounds), generally exhibit nematic, cholesteric (chiral nematic), and smectic LC phases depending on the organization of molecules in the mesophases. The molecular organizations in the common phases are shown in Figure 3. In the nematic LC phase, the molecules exhibit only

temperature of the system. Nematic, lamellar, columnar, and cubic phases are the commonly observed lyotropic LC phases. Lyotropic LCs are important in soap, detergent, cosmetic, and food industry as well as in biology and in drug delivery. Many biomacromolecules such as polypeptides, polysaccharides, DNA, and some viruses exhibit different lyotropic LC phases under suitable conditions.32 It should be noted that recently the observation of lyotropic LC phases in the dispersions of anisotropic nanoparticles such as nanorods, nanotubes, and nanodiscs is in limelight since they can be facilely processed from their LC phases for the fabrication of functional materials and devices. Liquids are classified into different categories based on different properties such as polar, nonpolar, and ionic, viscous and mobile, volatile and nonvolatile, colored and colorless, odor and odorless, and flammable and nonflammable. Unlike liquids, crystalline materials are generally classified into different groups based on symmetry. Interestingly, LCs, though exist between amorphous liquid and crystalline solid states, have been popularly classified into different categories based on the geometric shape of the constituents (Figure 2). As follows:

Figure 3. Molecular organization in common LC phases formed by rodshaped molecules.

orientational ordering by staying nearly parallel to their neighbors.53 The direction of preferred orientation is denoted by a vector quantity known as the director and is represented by n. The amount of order in a LC phase can be expressed by a quantity known as order parameter, S. This quantity has been defined such that for a perfectly ordered crystal its value is 1 and for an amorphous liquid its value is 0. It therefore follows that the value of the order parameter in LC phases lies between 1 and 0. In the cholesteric phase otherwise referred to as chiral nematic phase, the molecules are slightly twisted with respect to their neighbors, consequently, such molecular organization leads to a helical superstructure.41 In the smectic phase, the molecules are organized in layers. With dependence on intra- and interlayer correlation among the molecules, different smectic phases have been identified. However, the most common smectic phases are smectic A and smectic C. In the smectic A phase, the molecules are upright in the layers (i.e., the director is parallel to the layer normal). In the smectic C phase, the molecules are tilted (i.e., the director makes a finite angle with the layer normal) and the tilt angle is temperature-dependent. Interesting phenomena happens when the molecules of a smectic C phase are chiral due to reduced phase symmetry. One promising consequence of the reduced phase symmetry is the occurrence of polar order in the smectic C phase. Since LCs are dynamic and stimuliresponsive, the polarity of the phase can be switched by external electric fields, which forms the basis of fast switching microdisplays.42−44 Rod-shaped molecues with X- and T-shaped substitution patterns are known to self-organize into complex mesophase morphologies.47,51 Discotic (formed by disc-shaped molecules) LCs generally exhibit nematic and columnar phases. Due to the strong pi−pi interaction, the discotic compound often show columnar phases and nematic phases are rarely encountered.68−96 The molecular organizations in different phases are depicted in Figure 4. Discotic nematic LCs have been employed as optical compensation films to enhance the contrast and widen the viewing angle of LCDs. The columnar phases of discotic LCs behave as one-dimensional (1D) charge and energy migration pathways and hence have been tested in organic field effect transistors, organic light-emiting diodes, and organic photovoltaic solar cells. Bent-core LCs, formed by banana-shaped compounds, are only two decades old, nevertheless these LC compounds exhibit some unique and unprecedented phenomena and properties.97−118 Bent-core compounds exhibit conventional nematic, smectic, and columnar LC phases as well as nonconventional LC

Figure 2. Geometric shapes of organic compounds forming LC phases.

calamitic (formed by rod shaped molecules) LCs, discotic (formed by disc-shaped molecules) LCs, and bent-core (formed by banana-shaped molecules) LCs. It seems more appropriate to classify LC phases based on symmetry. For example, when a compound exhibits multiple LC phases (polymesomorphism), at the phase transition from one LC phase to another LC phase, the molecular geometry does not change but the phase symmetry changes. Moreover, the phase symmetry which takes into account the molecular organization in the phase governs the physical properties of the LC phase. C


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characterize and understand the detailed molecular organization and dynamics at the nanoscale in relatively complex LC phases. 1.2. Photochromic Materials

By definition, photochromism is the phenomenon of reversible transformation of a chemical species between two isomeric forms by the absorption of electromagnetic radiation, where the two forms display different absorption spectra and some distinct physical properties.119−139 If the photochromic chemical compounds are colored, they would exhibit color change upon exposure to light of appropriate wavelength. The reversible structural changes by absorption of appropriate wavelength of light qualify such materials as light-driven molecular switches or motors. Good photochromic materials satisfy the prerequisites such as distinct absorption profiles for selectivity, stability of both isomers, high sensitivity, fast response, fatigue resistance, high quantum yields, and photostationary states predominantly composed of one isomer. Light-driven structural changes of photochromic compounds often lead to modulation of electronic properties, absorption coefficients, polarity, refractive index, electrochemical redox potentials, conjugation, conductivity, molecular geometry, physical dimensions, chirality, solubility, etc. These molecular level photophysical and photochemical changes of the materials can be harnessed to regulate macroscopic properties and functions. As a consequence, compounds with photochromic fragments have been invaluable building blocks for the fabrication of light-driven advanced materials and devices with tunable properties and performances. Photochromic materials are very promising in several scientific research fields, ranging from chemistry, physics, materials science, optics, photonics, and photopharmacology to nanotechnology. Moreover, the use of light as the driving agent to modulate properties of materials has many advantages. The use of light is convenient, and it has noninvasive character. Light with variable wavelength, polarization, and intensity is readily available. Temporal and spatial resolution of light with autonomous, remote, and digital controllability renders it an ideal stimulus to modulate the applicable properties of photochromic materials. Light delivers energy to materials and systems at the speed of light, and due to the possibility of noncontact delivery of energy, light acts as an outstanding orthogonal stimulus. Scheme 1 presents the commonly used photochromic moieties in the fabrication of light-driven LCs and their reversible light-driven transformations between isomeric forms. This Review has been organized into six distinct sections, each section covering different light-driven phenomenon in photoresponsive liquid crystalline materials. Section 2 deals with photochromic liquid crystalline materials which contain photochromic fragments in the compounds and upon light irradiation either maintain its original phase or transform into another LC phase. Photoinduced phase transitions in liquid crystalline materials have been compiled in section 3.140−143 Both pure molecular and macromolecular systems as well as doped LC mixtures have been covered in this section. Photoalignment and photoorientation of different molecular and macromolecular liquid crystalline materials have been collated in section 4.144−165 Section 5 deals with the photomodulation of chiral LC phases. Pitch, polarization, lattice constants, and handedness alteration of cholesteric, ferroelectric, and blue phases by light irradiation are discussed in this section.166−183 Moreover, photochemical chiral induction has also been covered. Light-driven liquid crystalline polymers, polymer networks, and elastomers have

Figure 4. Molecular organization in commonly observed LC phases formed by disc-shaped molecules.

phases which are not found in rod-shaped compounds. The structures and molecular organizations of some LC phases formed by bent-core compounds are depicted in Figure 5. Bent-

Figure 5. Molecular organization in LC phases formed by bent-core compounds.

core compounds in addition to classical nematic phase, exhibit the biaxial nematic phase (i.e., a nematic phase with two mutually orthogonal directors)103−109 and the twist-bend nematic phase (i.e., a nematic phase with heliconical (oblique helicoidal) organization of molecules).110−117 It should be emphasized that the twist-bend nematic phase with promising and unique properties is the new sensation in LC research. Smectic phases with tilted bent-core molecules display polarity and chirality, even though the bent-shaped molecules themselves are achiral (i.e., they are devoid of stereogenic centers). Bent-core compounds are also able to furnish switchable polar columnar phases. The switchability of the polar phases is very attractive both for fundamental studies and their applications. Liquid crystalline materials are often characterized by a set of complementary techniques to avoid ambiguities in the phase identifications. Polarizing optical microscopy (POM) is the first technique employed to detect the presence of LC phases. In the case of thermotropic LCs, the material is heated on a hot stage from its solid crystalline state to classical liquid state and the existence of LC phases are observed. In the case of lyotropic LCs, the LC phase can be studied at room temperature, but the system can exhibit temperature-dependent phase transitions that can be studied by heating or cooling the sample. LC phases display optical anisotropy under POM and often exhibit characteristic optical textures. Following the detection of the existence of LC phases in a material, the second technique known as differential scanning calorimetry (DSC) is used to accurately determine the phase transition temperatures and the associated enthalpy changes involved in the phase transitions. X-ray diffraction (XRD) is the third technique which is used to determine LC phase structures and hence phase symmetries. In addition to the above three general techniques, recently other methods like electro-optical studies, dielectric investigations, scanning probe and electron microscopy techniques have been used to fully D


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as practical applications. Therefore, photochromic LCs have been designed, synthesized, and studied. The other objective in preparing photochromic LCs is to obtain a material responding reversibly to both an electric field and light. The most straightforward design of such a material would be a hybrid molecule composed of covalently linked mesogenic and photochromic units. Accordingly, photochromic LCs have been realized by chemical linking of photochromic groups (Scheme 2) with mesogenic moieties. Functional materials with

Scheme 1. Molecular Structures of Commonly Encountered Photochromic Compounds and Their Reversible Photoisomerizationa

Scheme 2. Commonly Used Photochromic Molecular Moieties for the Development of Photochromic LCs

promising properties have been obtained in this way. In the following, we present the different classes of photochromic LCs. Here we note that photochromic LCs which upon photoirradiation with suitable light wavelength either maintain its phase structure or transform to another LC phase are discussed. In other words, these photochromic materials are in the LC state both before and after photoirradiation process (i.e., both the initial and the photostationary state of the photochromic moieties support the occurrence of LC state though with different phase characteristics). Therefore, photochromic systems, which exhibit LC to isotropic phase transition before the photostationary state is reached, are covered in a different section. Similarly, the fabrication and studies of photochromic systems by doping small quantities of photochromic compounds into room-temperature liquid crystalline hosts are covered in a separate section. 2.1. Photochromic Liquid Crystals Based on Spiropyrans and Spirooxazines

a

The photochromic groups from top to bottom are azobenzene (A), spiropyran (B), dithienylcyclopentene (C), fulgide (D), overcrowded alkene (E), α,β-unsaturated ketone (F), thioindigo (G), and 1,3butadiene (H). The substituents R, R1, R2, and R3 are generally normal, branched, or chiral alkyl chains with different numbers of methylene groups.

Spiropyrans and spirooxazines are an interesting family of photochromic materials due to their unique properties such as excellent photofatigue resistance, strong photocoloration, and fast thermal relaxation. The colorless ring-closed spiro forms of spiropyran and spirooxazine can be transformed into the colored ring-opened merocyanine form upon irradiation with UV light, whereas its reverse process occurs thermally in the dark or photochemically by irradiation with visible light. A unique feature of spiropyran is its significantly increased dipole moment after photoisomerization from the ring-closed spiropyran form to the ring-open charge-separated zwitterionic merocyanine form. The physical and chemical properties of the two forms of spiropyran and spirooxazine are distinctly different; therefore, the thermally reversible photochromic switching has been the basis for the intelligent materials with applications in smart devices. Since the geometric shape changes upon photoirradiation are not drastic and the initial and final isomeric states are more or less elongated, both spiropyrans and spirooxazines have been used in the development of photochromic liquid crystalline materials. Both

been assembled in section 6,184−214 where photomechanics and photomobile characteristics of liquid crystalline polymers have been stressed. Before culminating the article with summary and outlook, the developments of light-driven liquid crystalline gels and bent-core LCs are discussed. In every section, the applications of the light-driven phenomena in photoresponsive LCs have been highlighted as appropriate.

2. PHOTOCHROMIC LIQUID CRYSTALS Combining the optical properties of photochromic molecules and self-assembling and stimuli responsive characteristics of LCs could potentially yield versatile functional materials with novel and/or enhanced properties for both fundamental studies as well E


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Scheme 3. Spiropyran and Spirooxazine-Based Liquid Crystalline Polymers and Oligomers

new applications in imaging technology. Prior to this, they synthesized hybrid molecules containing spiropyran and mesogenic groups; however, these materials show some peculiar phase behavior which was named as quasi-LC. Subsequently, Yitzchaik et al. reported acrylic copolymers 2 with mesogenic and spiropyran side chains which showed photochromic mesophases.217 Upon irradiation with UV light, these copolymers were found to absorb visible light strongly. The color disappears on irradiation with visible light or thermally. Interestingly, in these copolymers, two distinct sites were observed: mesogenic domains and amorphous sites. Main chains and photochromic side chains are presumably located in the amorphous site which expands and causes the mesophase to disappear as the spiropyran content increases in the polymer. Shragina et al. have reported photochromic LCs 3 and 4 containing spironaphthoxazine substituted with mesogenic group. The compound spiroindoline-naphthoxazine 4 with mesogenic substituent showed LC properties. Natarajan et al. synthesized first example of photochromic cholesteric LC siloxane compound 5 containing the spiropyran group.218 This LC compound was observed to

molecular and polymeric liquid crystalline spiropyran and spirooxazine derivatives have been designed and synthesized. Cabrera et al. synthesized photochromic liquid crystalline polysiloxanes 1 containing spiropyran groups (Scheme 3).215,216 The phenylbenzoate copolymers with the spiropyran side chains give a mesophase whose clearing temperature decreases with increasing spiropyran content in the macromolecules. These copolymers were found to exhibit interesting photochromic behavior. Red-, blue-, and yellow-colored polymer films could be obtained by photoirradiation with suitable wavelength at appropriate temperatures. The thermochromic spiropyran-tomerocyanine dye conversion occurs on heating of the copolymers. The copolymer films cast from solution acquire a pink color at room temperature and exhibit strong birefringence. Irradiation with visible light (>500 nm) brings about a pale yellow color, while irradiation of the yellow film with UV light (365 nm) results in a deep red color. A yellow film irradiated with UV light at −20 °C turns blue and can be converted back to yellow by irradiation with visible light. The possibility of controlling the formation of the primary colors with light and temperature may make it possible to tailor these LC polymers for F


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exhibit the selective reflection property of cholesteric LC as well as photochromism in thin films, in fibers, and in solution. Hattori et al. designed and synthesized polymerizable photochromic liquid crystalline materials 6−10 containing biphenylene as the mesogenic moiety (Scheme 4).219 These

Scheme 5. Photochromic Spiropyran-Based LCs

Scheme 4. Polymerizable Spirooxazine-Based LCs

light, they can transform from the colorless open-ring form to the colored closed-ring form. The reverse process is thermally stable and occurs only by visible light irradiation. Since the physical and chemical properties of the two isomeric forms are different, the optically reversible switching has been the basis for generating new functional materials. Various liquid crystalline materials based on dithienyl cyclopentene with and without mesogenic substitution have been reported. The Mehl group made a modular approach toward photochromic LCs by using the 1,2bis(2-methylbenzo[b]thiophen-3-yl)hexafluorocyclopentene system linked to two cyanobiphenyl mesogen groups via flexible spacers of ten methylene units (Scheme 6).223−228 The system is shown in Scheme 6. This is a very versatile approach which can lead to the systematic investigation of the influence of the photochromic group, the spacer lengths, and the position of mesogens on the properties of such systems. POM revealed Scheme 6. Mesogen-Functionalized Liquid Crystalline Dithienylcyclopentenes

hybrid photochromic compounds displayed metastable mesophases as revealed by POM, DSC, and XRD studies. It has been presumed that the appearance of mesophase on cooling was caused by reorientation of the spirooxazine moieties in the molten state. Photochromic polymerizable acrylates containing spirooxazine moieties with a chiral substituent were also prepared which yielded photochromic chiral liquid crystalline systems.220 The photochromic acrylates containing both an undecamethylene group and a (2S, 3S)-2-chloro-3-methylpentanoyloxy group or a (−)-menthoxyacetoxy group gave a supercooled mesophase, the latter compound was found to reflect right-handed blue light at room temperature. Keum et al. have synthesized and studied the LC phase behavior of various spiropyran-based materials 11−13 (Scheme 5).221,222 These compounds were found to exhibit the metastable monotropic nematic LC phase as studied by DSC and POM. 2.2. Photochromic Liquid Crystals Based on Dithienylcyclopentenes

Dithienylethenes are considered to be the most promising photochromic materials for optical memory and photoswitching applications because of their excellent fatigue resistance and thermal stability of both photoisomers (i.e., thermal bistability, fast photocyclization, and electrical conductivity). They undergo a reversible photocyclization reaction between colorless ringopened and colored ring-closed forms. Upon irradiation with UV G


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broken focal conic defect in conjuction with schlieren textures, indicating the formation of a smectic C phase in compound 14. The presence of nematic phase before the material transforms into the isotropic state was confirmed by the observation of a schlieren texture, with two and four brush defects. The sample in the photostationary state (which contains both open and closed ring isomers) exhibited a different phase behavior. It displays only the nematic phase between the crystalline and isotropic state. Repeated heating and cooling cycles of the samples did not have any influence on the transition temperatures and enthalpies, suggesting thermal stability of the system. Different structural isomers of compound 14 have also been realized by changing the position of the mesogens. The LC phase behavior for all of these systems is broadly similar in terms of their phase structures. However, upon photoirradiation the phase behavior of the compounds in the photostationary states are significantly different. This system shows that combining individual functionalities of a photochromic core and mesogens linked by flexible spacers allows the design of materials where mesomorphic phase structures and the phase stability can be modulated. LC phase behaviors of the open-ring isomers 15 and its photostationary state were investigated using POM and DSC. Compound 15 melts at 140.1 °C and on cooling from the isotropic liquid exhibits a nematic phase at 96.1 °C. Irradiation with UV light alters these properties significantly. In the photostationary state, the reduction in flexibility due to ring closure of the system reduces the stability of the nematic phase while the melting point is less altered. In this compound it was shown that the introduction of cyanobiphenyl groups in 2,2′positions via alkyl spacers in a diarylethene derivative enhances the rate of photoswitching without altering the other photochromic properties. This also affords a new alternative in the design of photochromic LCs based on diarylethene derivatives. Frigoli et al. have designed and synthesized photoswitchable liquids exhibiting room temperature nematic phases as shown in Scheme 6.228 The compound 16 has a nematic range over 50 °C, including room temperature, and undergoes a glass transition at about 3 °C. A new series of photochromic LC has been reported by the Mehl group as shown in Scheme 7. These are the first examples of liquid crystalline dithienylcyclopentenes devoid of mesogenic moieties. The thermotropic phase behaviors of the open ring isomers were investigated by DSC and POM. When cooled from the isotropic state, the compounds 17 and 18 exhibit monotropic nematic phases as characterized by typical schlieren textures. The stability of the nematic phases are drastically reduced in the photostationary states which have been attributed to the reduction of flexibility of the central cores. Surprisingly, compound 19 does not exhibit any mesomorphism at all. Chen et al. reported photochromic glassy LCs as a new class of functional optical materials (Scheme 8).229 These materials were synthesized by functionalizing dithienylcyclopentene core with nematogens (nematic phase forming mesogens). With dependence on the type and number of nematogens, three distinct types of thermotropic phase behavior have been observed. Compound 20 on cooling from the isotropic state exhibited the nematic phase, which transformed into glassy liquid crystalline film with smectic ordering. However, compound 21 containing more number of mesogens resulted in a nematic glass. Therefore, morphologically stable glassy nematic LC comprising a dithienylcyclopentene core has been successfully designed and synthesized.

Scheme 7. Liquid Crystalline Photochromic Dithienylcyclopentene Derivatives

Scheme 8. Photochromic Glassy LCs Based on Dithienylcyclopentenes

Rameshbabu et al. reported mesogenic dithienylcyclopentene compounds 22−24 (Scheme 9) which show chiral nematic phases over a broad temperature range as evidenced by typical oily streak textures (Figure 6).230 These compounds do not crystallize but pass into photochromic glassy state, which is beneficial for the fabrication of solid thin films without grain boundaries. It is worth noting that the fabricated thin films are thermally stable (i.e., they do not relax back due to the thermal Scheme 9. Cholesterol-Substituted Photochromic Dithienylcyclopentene Derivatives 22−24

H


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Scheme 11. Fulgimide-Based Photochromic Liquid Crystalline Polymersa

a

26, R = H, R′ = CN; 27, R = CH3, R′ = OCH3.

fulgimide chromophores. The fulgimide moieties in the polymer undergo reversible photocyclization and cycloreversion upon photoirradiation. The ring closure of the fulgimide groups occur on irradiation with 366 nm wavelength light, and the cycloreversion occurs upon shining with visible light of 475 nm wavelength. It was found that the clearing temperature of the liquid crystalline copolymers decreased with increase in the content of the photochromic fulgimide groups in the side chain. Interestingly, it was observed that irradiation of the polymer with UV light leads to a higher clearing point of the LC phases, which clearly indicates that the ring-closed form of the fulgimide photochromic moiety is better compatible in the LC matrix. With the use of the standard photomask technique, optical information recording with thermal stability was demonstrated.

Figure 6. POM texture of the photochromic dithienylcyclopentene compound 22 in the chiral nematic phase (top row), planar textures of the supercooled cholesteric phase at room temperature before and after UV irradiation (bottom). Reproduced from ref 230. Copyright 2011 American Chemical Society.

stability of the dithienylethene isomers). These compounds have been used as chiral dopants to induce photoresponsive cholesteric LCs by adding them into commercially available nematic LC hosts. The photochemical phase transition behaviors of the cholesteric mixture have been studied by successive irradiation with UV and visible light. The reversible photoinduced phase transition process is shown in Figure 7.230

3. PHOTOSTIMULATED PHASE TRANSITIONS As mentioned above, phase transitions in thermotropic LCs take place due to the effect of temperature. However, it has been observed that LCs containing photoisomerizable molecular switches can undergo phase transition by light irradiation in an isothermal manner. Since the molecular switches used in photoindued phase transitions exhibit reversible photoisomerization, the photoindued phase transition process occurs reversibly. In many cases, the photochromic compounds themselves act as mesogens, though they are often used as dopants in LC hosts to modulate physical properties. Both low molecular weight and polymeric LCs have been fabricated, which undergo photostimulated phase transitions. The photoinduced phase transition occurs due to the different type of intermolecular interactions of the photoisomers of photochromic compounds with the LC host material. While one isomer of the photochromic compound is highly compatible in the LC matrix, its photoisomer may not be compatible with the LC matrix. This incompatibility results in destabilization of the LC phase, consequently leading to a phase transition either to a different LC phase or to the isotropic phase (i.e., complete disruption of the LC ordering). It is interesting to note that if the isomer produced upon light irradiation is incompatible with the LC phase, it would cause photostimulated phase transition to the isotropic phase; however, if the photoisomer is compatible with the LC phase, it can cause photosuppression of the phase transition (i.e., a LC phase would appear from the isotropic phase). Therefore, it follows that photoisomers can induce both order-decreasing phase transitions as well as order-increasing phase transitions. In the following, photoinduced phase transitions in low molecular weight and polymeric LCs are discussed along with their applications. In both the cases, pure as well as doped systems are covered.

Figure 7. Photoinduced reversible phase transition process in the induced cholesteric phase. Reproduced from ref 230. Copyright 2011 American Chemical Society.

2.3. Other Photochromic Liquid Crystals

New biindenyldenedione derivatives 25 containing two biphenyl substituents have been synthesized and characterized (Scheme 10).231 All of them exhibit photochromic properties, and the Scheme 10. Photochromic Liquid Crystalline Materials 25

photochromic state may be returned to the original form thermally. The compounds 25 display monotropic smectic A mesophase, which has been confirmed by DSC and POM investigations. These are first examples of liquid crystalline materials exhibiting light-induced radical behavior. Cabrera et al. have designed, synthesized, and studied the photochromic liquid crystalline properties of fulgimide-based copolymers 26 and 27 (Scheme 11).232 These are the first liquid crystalline polymers with thermally irreversible photochromic properties due to the thermal bistability of two isomeric forms of I


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3.1. Photoinduced Phase Transitions in Low Molecular Weight Liquid Crystals

the concentration and geometric shape of the photochromic compound, temperature, and nature of the LC host material. Moreover, it was apparent that the sensitivity of the nematic to isotropic phase transition could be increased by carrying out the photoirradition at a temperature below but close to the phase transition temperature of the initial system. This amplified image storage process was later extended to polymeric LC doped with the azobenzene derivative. This polymeric system 34 also exhibited the photoinduced nematic to isotropic phase transition upon light irradiation. The group around Ikeda carried out extensive investigations on photoinduced phase transitions following these reports.237−249 Through the use of different analysis techniques, they established the factors that greatly affect such photostimulated phase transitions. It was observed that the phase transition temperature of a LC host containing trans azobenzene derivatives is higher than the corresponding LC containing cis azobenzene. This difference arises due to the large variation in the geometric shape of the azobenzene compounds upon photoisomerization, which affects the compatibility of the isomers and modulates the intermolecular interactions with the host LC molecules. It is known that the trans form of azobenzene is rod-shaped, whereas its cis photoisomer is bent in shape. Therefore, while the trans azobenzene form is compatible in the calamitic nematic LC matrix and hence contributes toward phase stabilization, its cis counterpart which results upon photoirradiation is not compatible in the LC matrix and thus destabilizes the phase which is reflected in the lowering of phase transition temperature. With increase in the cis isomer content in a LC mixture, its phase transition temperature progressively decreases. When a LC mixture containing trans azobenzene compounds is irradiated with light of suitable wavelength, trans−cis photoisomerization occurs, and consequently, the phase transition temperature of the mixture gets lowered. If the temperature of the sample is maintained between the temperatures of nematic−isotropic of trans mixture and cis mixture, the system undergoes an isothermal photochemical phase transition from the nematic to isotropic phase. As trans− cis photoisomerization is reversible, photoinduced reversible phase transitions occur. The cis azobenzene isomers can also relax back to its trans form thermally, and this phenomenon enables isotropic to nematic phase transition in dark. Studies on the effect of different parameters such as sample temperature, structure of guest azocompounds and their concentration, and the structure and nature of liquid crystal host materials on the nematic to isotropic photochemical phase transitions have been extensively undertaken. Kurihara et al. studied photochemical phase transition behavior of LC mixtures containing photochromic spiropyran derivatives, which are either stable in the closed spiropyran form or in the open merocyanine form.243 The unprecedented and unique order-increasing phenomenon of the isotropic to nematic phase transition was observed by photoisomerization behavior of the stable, closed spiropyran form. These observations were expressed in terms of change in the shape anisotropy of the spiropyran guest isomers. Ikeda et al. developed new molecular and polymeric azobenzene-based liquid crystalline polymers (Scheme 14) and utilized these materials for optical switching and image storage techniques.244 In these materials, azobenzene moieties play double role of photochromic fragments and rod-shaped mesogen. Upon trans−cis photoisomerization, the LCs undergo isothermal phase transition due to the fact that azobenzenes in

Photostimulated phase transition phenomenon was demonstrated during the mid 1970’s soon after Sackmann et al. disclosed light-driven reflection color modulation of an azobenzene-doped cholesteric LC in the early 1970’s.233 Haas et al. demonstrated UV imaging with nematic liquid crystalline chlorostilbene derivatives 28 (Scheme 12). The image process Scheme 12. Chemical Structures of Liquid Crystalline Compounds Exhibiting Photochemical Phase Transitions

was based on the optical difference between the light scattering liquid crystalline state and nonscattering isotropic state. By UV irradiation, the isotropic transition temperature of the LC was depressed to the experimental temperature thereby inducing phase transition to the isotropic state. Subsequently, Pelzl et al. reported the photochemical phase transition studies on liquid crystalline derivatives of azo- and azoxy-compounds and stilbene and cinnamic acid derivatives 29−32 upon light irradiation (Scheme 12).234 It was found that azo- and azoxy-derivatives exhibit reversible phase transitions between LC and LC states and LC and isotropic states. Experimental results suggested that these photochemical phase transitions are a result of trans−cis photoisomerization of the mesogens, since the elongated rod-shaped mesogens transform into bent geometries during photoirradiation. Ikeda et al. demonstrated amplified image recording by inducing photochemical phase transitions in a nematic LC doped with an azobenzene derivative 33 (Scheme 13).235,236 Owing to the reversible trans−cis photoisomerization of the azobenzene derivative doped in the nematic LC, the amplification process was found to be reversible. They observed that the critical point at which photoinduced phase transition occurs is determined by Scheme 13. Chemical Structure of a Liquid Crystalline Azobenzene Derivative and a Side-Chain Liquid Crystalline Polymer

J


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deduced that the dynamics of the photochemical phase transition process are affected by the number, size, and growth rate of the isotropic domains.

Scheme 14. Azobenzene-Based Molecular and Polymeric LCs

the cis isomer form fail to retain the LC state of their corresponding trans isomers. Remarkable fast optical response (hundreds of microseconds) was recorded for these photosensitive liquid crystalline materials. An optical image was recorded by using a photomask and UV irradiation to affect photochemical nematic to isotropic phase transition in a thin film of the liquid crystalline polymer (Figure 8) below its glass transition Tg.244 The image was stable over Figure 9. Optical textures observed during photoinduced phase transition of a nematic LC mixture containing an azo mesogen. Adapted from ref 245. Copyright 2002 American Chemical Society.

Tamaoki et al. designed, synthesized, and investigated the light-driven smectic to cholesteric phase transition in a liquid crystalline dimer containing cholesteryl group and azobenzene moiety (Scheme 15).250−252 The dimeric compound 38 exhibits smectic and cholesteric mesophases before transforming into the isotropic state. Upon UV irradiation, the smectic phase gradually yields the cholesteric phase isothermally, and upon further irradiation, the cholesteric phase transforms into the isotropic state. This process is schematically depicted in Figure 11.250 When the isotropic state is kept in the dark, thermal back relaxation of the cis azo fragment takes place and the system returns back to the smectic phase through the intervening cholesteric phase. Compound 39 was found to exhibit temperature-dependent distinct pathways for photostimulated phase transitions. Glass-forming photoresponsive LC dimers have been synthesized (Scheme 16) and their photochemical phase transition behaviors have been studied. The compounds with alkyl chains on the azobenzene group exhibit only smectic phases, whereas the related compounds without the terminal alkyl chain furnish cholesteric phases. The smectic compounds directly undergo photochemical phase transition into isotropic state (Figure 12).251 Interestingly, the cholesteric compounds show increase in chirality upon photoisomerization as noticed from the decrease in pitch in these compounds before they yield the isotropic state. Upon rapid cooling, the isotropic liquids, glassy states result in all these dimesogenic compounds. Rewritable photochemical image recording has been demonstrated by using some of these compounds. Light-driven nanophase segregation of an azobenzene derivative in a smectic host has been observed.253 This segregation happens due to the migration of the azobenzene compounds from within the smectic layers to locations between the layers upon trans−cis photoisomerization. Reversible and interconvertible photochemical phase transition and photochemical phase separation have been observed in doped liquid crystalline systems with a high concentration of azo-dye compounds.254,255 The amount of cis isomer in the photoirradiated state is found to control the process of photochemical

Figure 8. Photomask and the recorded optical image by photochemical phase transition in the azobenzene-based polymer LC. Reproduced with permission from ref 244. Copyright 1995 American Association for the Advancement of Science.

time, and the isotropic regions of the image were maintained even though the cis isomers of the azobenzene mesogens returned to their thermally stable trans form. This stability of the isotropic state has been attributed to the lack of orientational dynamics of the trans isomers below the Tg, even though they have relaxed from the bent cis form thermally. Photoinduced change in refractive index of azobenzene-based LCs was used for fast optical switching, and it was registered by reflection mode analysis in contrast to the conventional transmission-mode analysis. These studies established that the refractive index parallel to long molecular axis of the LCs (ne) which is different from the refractive index perpendicular to short molecular axis (no) follow the relation ne > n > no, where n is the refractive index of the isotropic state. This relation enables periodic modulation of refractive index of liquid crystalline films by patterned irradiation, leading to the photochemical isothermal phase transition. Such periodic refractive modulations have formed the basis of fabrication of diffraction gratings by photochemically induced phase transitions in pure and doped liquid crystalline films. Moreover, reversible photoinduced phase transitions have been utilized in the reversible optical control of transmittance in polymer/LC composites. The dynamics and evolution of photostimulated phase transition process was elaborated through transmittance change in LC mixtures containing azobenzene-based photochromic compounds. The formation of cis isomer leading to local isotropic domains in the irradiated site of the sample followed by growth of the isotropic domains causes the photochemical phase transition of the samples (Figures 9 and 10).245 It has been K


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Figure 10. Schematic depiction of the process of photoinduced nematic to isotropic phase transition. (A) Before irradiation, (B) just after irradiation, note that trans−cis photoisomerization of the azo guest sets in, and local nematic to isotropic phase transition commences. (C and D) The growth of isotropic domain by diffusion, (E) the isotropic phase ultimately propagates throughout the entire area upon prolonged irradiation. Reproduced from ref 245. Copyright 2002 American Chemical Society.

Scheme 15. Chemical Structure of Azobenzene-Based Dimers

Figure 12. Optical textures observed after the UV irradiation through a photomask with transparent lines. Reproduced from ref 251. Copyright 2003 American Chemical Society. Figure 11. Schematic illustration of photochemical switching process between smectic, cholesteric, and isotropic phases. Reproduced from ref 250. Copyright 2003 American Chemical Society.

owing to its high incompatibility in the matrix. Inspired by simulation results, Ichimura et al. designed 3,3-disubstituted azobenzenes (e.g., 43) to modulate the compatibility of their photoisomers in nematic LC matrixes (Scheme 17).256 Since

Scheme 16. Chemical Structures of Dimeric LC Compounds

Scheme 17. Chemical Structures of 3,3′-Disubstituted Azobenzene Compound 43 and Its 4,4′-Disubstituted Counterpart 44

both the trans and cis isomers of the compounds are rod-shaped, it was found that nematic LC mixtures containing these compounds do not undergo photochemical phase transition in sharp contrast to their 4,4′-disubstituted counterparts (e.g., 44). This report clarified the critical role of geometric shape of guest molecules in photoinduced phase transitions.

phase transition. Comprehensive investigation has revealed that when the cis isomer concentration increased above a certain value in the system, the azo-dye phase separates from the LC, L


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phase upon light irradiation, and this process was found to be reversible. Another LC mixture was fabricated by adding large amounts of both azo compound and the chiral dopant so that the smectic phase is suppressed and the mixture exhibits only the cholesteric phase. Upon light irradiation, this cholesteric phase exhibits phototunable reflection color before transition to isotropic phase. Layer alignment control of an azo dye doped ferroelectric LC facilitated by photoinduced phase transition has been demonstrated.261 In this case, UV irradiation led to photoinduced chiral smectic C to cholesteric phase transition; however, the system returns to the chiral smectic C phase upon stopping the light irradiation. By conducting this cycle of chiral smectic C-cholesteric-chiral smectic C photoinduced phase transition under an applied external electric field, the control over smectic layer alignment has been achieved. This photoinduced phase transition enabled the layer switching process in the polar LC to be applied to the fabrication of optical devices with multidomain structure. It has been established that LC materials can be obtained through noncovalent interactions between molecules possessing appropriate complementary structural elements. Thus, hydrogen-bonded, halogen-bonded, and ionic supramolecular LCs have been realized even from nonmesomorphic components. In this context, several photoresponsive supramolecular LCs displaying photoinduced phase transitions have been designed, synthesized, and studied. Supramolecular hydrogen-bonded photoresponsive LCs have been fabricated from nonmesomorphic azabipyridine and alkoxy benzoic acid derivatives.262 Photoinduced phase transition behavior of some of these compounds has been examined in thin films. Similarly, halogen-bonded photoresponsive supramolecular LCs have been realized from azapyridine derivatives and molecular iodine and bromine.263 The iodine complexes furnished smectic A phases in higher homologues while all the bromine complexes yield smectic A phases. Upon UV irradiation, the smectic A phase of iodine complexes transforms to the isotropic phase, and the reverse process is driven by visible light. However, the liquid crystalline bromine complexes do not undergo such phase transition under UV irradiation. Ionic LCs have recently emerged as versatile functional materials since they effectively integrate the useful functional properties of ionic liquids and selfassembling attributes of LCs in a single system. Alongside the development of ionic LCs, photoresponsive capability has been conferred to such materials by incorporating photochromic units, and their isothermal photoinduced phase transition behavior has been investigated. Azobenzene-doped imidazolium-based ionic LCs with smectic phases have been found to exhibit reversible isothermal smectic-isotropic phase transition.264 It was found that just by increasing the power density of the light source, the phase transition process can be remarkably expedited. These stimuli responsive ionic soft materials could be employed as switchable ionic conductors and as ionic media with phototunable order/disorder. From the above discussion, it is clear that photoresponsive supramolecular LCs constructed through noncovalent interactions have a lot of interesting phenomena and properties to offer. Almost all the photoinduced phase transition studies in azobenzene derivative-doped LC systems have been carried out with one-photon excitation. This process undoubtedly involves the use of high energy light radiations to drive the process of phase transition. To circumvent the use of high energy light, recently the possibility of two-photon excitation has been demonstrated to drive the photoinduced phase transition

Low molar mass liquid crystalline compounds based on alkyloalkoxy azobenzene derivatives 45 (Scheme 18) exhibiting Scheme 18. Chemical Structures of Azobenzene Derivatives 45 and 46

nematic and/or smectic phases have been recently synthesized and studied.257 The compounds showing polymesomorphism undergo photoinduced phase transitions between nematic, smectic, and isotropic phases, and this phenomenon can be controlled by light irradiation. The isothermal photoinduced phase transition process in these materials has been investigated by recording holographic gratings through interference pattern to better understand the mechanism involved. Real time observation of the dynamic holographic recording process in these photoresponsive materials under POM has indeed yielded some new insights into the mechanism of the photochemical phase transition in pure single component systems. Chiral azobenzene-based single component LCs possessing the chiral smectic C phase along with other chiral LC phases exhibit photoinduced phase transition behavior driven by both light illumination and temperature adjustments.258 In these polymesomorphic chiral compounds, photoirradiation leads to shifts in the phase transition temperatures. The reversible phase transitions can be driven from the ferroelectric smectic C to the isotropic phase via smectic A, twist grain boundary A, cholesteric and blue phase by combination of suitable temperature variation and light irradiation level. This reversible process in these compounds has been elaborated by a new type of “illuminationtemperature” phase diagram. In azobenzene derivative doped photoresponsive LC systems, noncovalent intermolecular interactions between the host and the photochromic dopant have been observed to influence the isothermal photoinduced phase transition process. It was found that the photoinduced phase transition temperatures could be tuned by appropriate molecular design of the azobenzene derivatives and suitable choice of host material.259 Detailed and systematic investigations carried out on different host−guest combinations have shown that linear azobenzene derivatives 46 (Scheme 18) with extended aromatic cores containing four phenyl groups are particularly efficient in modulating the photoinduced phase transition phenomenon. Similar to intermolecular interaction modulation, distinct photoinduced phase transition phenomena can be endowed to systems by suitable adjustment of the concentrations of different components of LC mixtures. In this context, photoresponsive chiral LC mixtures were fabricated and studied by mixing a LC host exhibiting nematic and smectic phases, an azobenzene derivative, and an axially chiral binaphthol.260 The mixture with low quantity of azo compound exhibited the chiral smectic A and cholesteric phases. When this mixture was irradiated by light, the system showed a chiral smectic A to cholesteric phase transition. However, the system could not be driven to isotropic state by light irradiation due to the presence of insufficient amount of the azo compound. By increasing the amount of azo compound, the mixture could be driven to the isotropic state through cholesteric M


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discotic nematic phase behavior, though discotic compounds are more favorable for columnar phase formation. However, compounds 49 and 50 show smectic A and rectangular columnar phases. Upon light irradiation, this compound was found to exhibit interesting reversible shape alteration between disc and rod in the LC phase and undergoes isothermal phase transition into the isotropic state (Figure 13b). Photoinduced isotropic state of a cholesteric LC fabricated from azo-based nematic LC has been found to possess interesting thermodynamic and optical properties.269 With the help of visible light laser beams, photoinduced phase transition between the isotropic state and reflective cholesteric phase was carried out. By changing the wavelength of photoirradiation, complex two-dimensional (2D) patterns with reflective and isotropic regions can be written and erased in thin films (Figure 14), and

phenomenon in azobenzene derivative containing LC mixtures.265 This process is particularly appealing since it involves the use of red or infrared light, and localized phase transition can be effected in the system with a high degree of spatial resolution through the two-photon absorption by dispersed azobenzene derivatives. Norikane et al. designed and synthesized liquid crystalline macrocyclic compounds 47 and 48 tethered by multiple azobenzenes bearing alkoxy chains (Scheme 19).266 The Scheme 19. Chemical Structures of Azobenzene-Containing Discotic Liquid Crystalline Compounds

Figure 14. Reflection color tunability and image writing, erasing, and rewriting by photochemically transforming between isotropic state and reflective cholesteric state of an azobased cholesteric LC mixture. Reproduced with permission from ref 269. Copyright 2007 Wiley-VCH.

the images are stable with respect to time.269 Reflection wavelength tuning across the visible spectrum has been demonstrated by introducing pitch gradient in thin cholesteric films that are restored from the photoinduced isotropic state. Recently, light-induced liquid crystallinity has been demonstrated in a system doped with a naphthopyran derivative (Scheme 20).270 The naphthopyran-based compound 51 exhibit photoinduced conformational changes in molecular shape (i.e., elongated and planar), which enables order-increasing phase

molecular structures promote the formation of columnar and lamellar phases. These photochromic materials exhibit isothermal phase transition from LC to isotropic state upon light irradiation due to shape change of the molecules (Figure 13a). Triphenylene-based discotic LCs bearing six rodlike azobenzene peripheral moieties linked through alkyl chains have been synthesized.267,268 These disc-rod hybrid compounds exhibit

Scheme 20. Photoisomerization of Naphthopyran Derivative 51

Figure 13. Photoinduced phase transition in discotic LCs, containing azobenzene groups. Reproduced with permission from refs 266 and 268. Copyright 2011 and 2012 The Royal Society of Chemistry. N


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Figure 15. Photoinduced reversible phase transition from isotropic to nematic and from nematic to smectic phases. Reproduced with permission from ref 270. Copyright 2012 Macmillan Publishers Limited.

transitions. Figure 15 shows the photoinduced process of disorder to order as well as order-increasing phase transitions when LC mixtures in their isotropic state were exposed to UV light. Isotropic to nematic and nematic to smectic phase transitions have been demonstrated by UV light irradiation. The photoinduced nematic and smectic phases revert back to the isotropic state upon switching off the light irradiation source. The dramatic increase in order upon UV irradiation of the naphthopyran compound is believed to be responsible for this interesting behavior. Moreover, the unprecedented change in order parameter of the dye enables switching the optical transmission through a thin film from clear to strongly absorbing and dichroic, which has appealing implications for applications. Recently, the photothermal effect of nanoparticles is in the limelight due to the remote and noncontact delivery of energy to systems to perform different functions. In particular, gold nanorods upon exposure to laser light of wavelength, which matches with its surface plasmon resonance, strongly absorbs the light and efficiently converts it into heat through a cascade of processes. In this context, organic corona-functionalized gold nanorods have been employed to achieve near-infrared lightdriven phase transitions in LC nanocomposites (Figure 16).271 Here the photo thermal effect resulting from the longitudinal surface plasmon resonance of the gold nanorods has been exploited. Though this method of photochemical phase transition does not involve any photochromic units, it is gaining popularity due to its potential and convenience. Azobenzene-based liquid crystalline dimers have been observed to exhibit parity and length of spacer dependent photoinduced nematic to isotropic phase transitions.272 It was noticed that the odd−even parity of the spacers of the dimers has a profound effect on the magnitude of the light-driven shift in the

Figure 16. Near-infrared (NIR) light-driven phase transition enabled by photothermal effect of gold nanorods. Reproduced with permission from ref 271. Copyright 2015 The Royal Society of Chemistry.

phase transition temperature. Prasad et al. demonstrated dynamic self-assembly of smectic A LC phase from an azo compound doped nematic phase by means of photochemical phase transition.273−279 This is another example of the photoinduced order increasing phase transition from higher to lower symmetry LC phase. The photoinduced smectic A phase is found to be stable as long as the light exposure is on. It has been reasoned that light-driven nanophase segregations promotes such phenomenon (Figure 17).277 Kinetics of thermal back relaxation time of isothermal photoinduced phase transition has been undertaken by studying the temperature dependence of the response time. It has been deduced that both photochemical process and back relaxation time is a smooth function of the temperature of the sample. Several aspects of isothermal photochemical phase transitions have been looked at rather elaborately by the Prasad group. Moreover, DSC and dielectric studies have been undertaken to understand the effect of aerosol dispersions on photochemical nematic to isotropic phase transition. O


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isotropic phase. However, the nonliquid crystalline photoisomers could be driven back by light irradiation to yield the LC phase. The thermal stability of the photoisomers of donor− acceptor-substituted butadienes is largely dependent on the strength of the donor−acceptor groups. Here the authors have isolated the photoisomers and studied their thermal stability. The EE (all trans) isomers of diphenyl butadiene derivatives possess a strong absorption band centered around 360 nm. Photoirradition with 360 nm light causes the decrease of the intensity of this absorption band and formation of a new band centered on 265 nm. Following photoirradiation, it was found that the photostationary state contains a mixture of EZ and ZE isomers together with the EE isomer. The photoisomerization process of these butadienes can be represented as shown in Scheme 21. The lack of LC phase in the photoisomers has been attributed to the bent shape of the photoisomers in the Z state. The photoinduced isotropic phase was found to be thermally stable. Later LC dimers 54 containing diphenyl butadiene and cholesterol units linked through flexible methyl chains were synthesized and studied (Scheme 22). The photochromic Figure 17. Nematic to smectic A phase transition and the nanophase segregation mechanism during photoirradiation. Reproduced from ref 277. Copyright 2007 American Chemical Society.

Scheme 22. Butadiene-Based Liquid Crystalline Dimers and Trimers Showing Photoinduced Phase Transition

Liquid crystalline diphenyl butadiene derivatives 52 have been synthesized and characterized (Scheme 21).280,281 These liquid Scheme 21. Liquid Crystalline Butadiene Derivatives Exhibiting Photoinduced Phase Transitions

property of the butadiene unit was helpful in bringing about an isothermal phase transition from the smectic to the cholesteric phase. The photochemical trans/cis isomerization was used to control the ratio of the cis/trans isomers. This ratio control enabled continuous tuning of the pitch of the cholesteric phase, and the color of the film could be tuned over the entire visible region (Figure 18).284 Interestingly, the color of the films thus obtained could be stabilized by converting the material into a glassy state. The pitch of the intervening cholesteric phase could be tuned over the entire visible region.

crystalline photochromic materials which possess nematic and smectic phases exhibit large birefringence, low viscosity, and high dielectric anisotropy. Though the photochromic behaviors of these materials were not studied initially, the possibility of cis− trans isomerization was indicated. Later Davis et al. carried out the photochromic studies on such materials.282−285 It was observed that upon photoirradiation the E,E butadiene 53 yields a mixture of EZ and ZE isomers (Scheme 21). The EE isomer undergoes isothermal photoinduced phase transition to the

Figure 18. Schematic representation of photoinduced isothermal phase transition from SmA* to isotropic phase via the N* phase. Reproduced from ref 284. Copyright 2006 American Chemical Society. P


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Photoresponsive trimeric compounds 55 containing butadiene core and two cholesteryl moieties on either side were systhesized and studied (Scheme 22). Photoisomerization of the butadiene unit leads to an isothermal phase transition in these materials. The pitch of the cholesteric phase in these compounds could be tuned both photochemically and thermally. Moreover, these materials were able to display circularly polarized photoluminescence in the glassy state. It is apparent that such butadiene-based multifunctional materials can be used for image recording devices and for polarized luminescent materials.

Scheme 24. Chemical Structures of Side Chain LC Polymers with Laterally Attached Mesogens

3.2. Photoinduced Phase Transitions in Polymeric Liquid Crystalline Systems

Ikeda et al. began the study of photochemical phase transitions in doped liquid crystalline polymeric systems and photoresponsive polymeric liquid crystalline systems by incorporating photochromic units in the polymer structures. Polymeric liquid crystalline systems are promising functional materials since they combine the mechanical properties of polymers and selfassembling and stimuli responsive properties of LC phases. In order to gain a fundamental understanding and to enquire about the applicability of polymer LCs that exhibit photochemical phase transitions, the Ikeda group designed, synthesized, and studied a great variety of photoresponsive liquid crystalline polymers and copolymers.286−291 The chemical structures of some of these polymers are shown in Scheme 23. Keller et al. have designed, synthesized, and studied the photoinduced nematic to isotropic phase transition in a side chain polymer LC 60 with laterally attached azomesogens (Scheme 24).292 Recently, Petr et al. reported the synthesis and characterization of the side chain polysiloxane 61, which exhibits LC phase

behavior at room temperature.293 Upon UV irradiation, the nematic phase of the polymer transforms to the isotropic state through the nucleation of isotropic domains and subsequent coalescence. Photoinduced phase transition behaviors of a ribbon-shaped chiral liquid crystalline dendrimer 62 have been investigated.294 However, interesting results have been obtained by using this material as chiral dopant to induce photoresponsive cholesteric LC phases. This compound exhibits high helical twisting power and a large change in helical twisting power upon photoirradiation. These powerful traits of this compound have enabled reversible selective reflection color tuning over the entire visible spectrum (Figure 19).294

Scheme 23. LC Polymers and Copolymers Exhibiting Photochemical Phase Transitions

Figure 19. Reflection color tuning in a photoresponsive cholesteric LC mixture. Reproduced with permission from ref 294. Copyright 2014 Wiley-VCH.

Okano et al. designed and synthesized azotolane-based side chain liquid crystalline polymers (Scheme 25) and evaluated their photoinduced phase transition behavior along with other properties.295 Owing to the direct linking of azobenene groups with tolane groups, the mesogens possess a large conjugation length, which consequently makes the polymers highly birefringent. Special attention was paid to the effect of the location of the tolane moiety and donor−acceptor groups on Q


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Scheme 25. Azotolane-Based Side Chain Polymer LCs

light-driven phase transition behavior. High values of birefringence have been obtained from homogeneously aligned thin films of these polymers. The application of photochemical phase transitions has been tested in the fabrication and characterization of dynamic gratings. For this purpose, the Ikeda group has synthesized a different variety of liquid crystalline polymers and copolymers with donor and acceptor substituents on azobenzene chromophores.296−303 The chemical structures of the side chain polymers used in the fabrication of holographic gratings are shown in Scheme 26.

Figure 20. AFM three-dimensional views of the gratings recorded in the (A) glassy state and (B) nematic phase. Reproduced from ref 297. Copyright 1999 American Chemical Society.

investigated. Supramolecular liquid crystalline polymers based on azobenzene units have also been used for light-driven formation of surface relief gratings.306 Recently, light-driven mass migration process is reported in liquid crystalline dendrimer thin films.307 It is worth noting that the fabrication and study of holographic gratings both in low molecular weight and polymeric LCs by means of photochemical phase transitions are still continuing.308−311

Scheme 26. LC Polymers Used for the Fabrication of Holographic Grating

4. PHOTOALIGNMENT AND PHOTOORIENTATION OF LIQUID CRYSTALS 4.1. Photoalignment of Liquid Crystals

As discussed above, LCs are anisotropic materials. However, to quantify the magnitude of anisotropy of different physical properties, it is necessary to “align” LCs with suitable uniform molecular (director) orientations. LC phases in general consist of numerous microdomains (Figure 22) which are randomly oriented in all possible directions on substrate surfaces in the absence of any external stimulus and surface treatments. The LC sample in such situations is referred to as the “polydomain sample”. The molecular orientation in small microdomains in a polydomain sample is however uniform along a single direction. Therefore, a polydomain sample does not possess a single director (i.e., the directors of the microdomains are randomly oriented). As a consequence, the anisotropic properties of LCs average out in a macroscopic unaligned sample. For both fundamental scientific studies and technological applications, it is necessary to align the LCs along a desired direction over macroscopic areas. The LC samples with macroscopic alignment along a single direction are referred to as “monodomain samples” or single crystal-like samples. To evaluate bulk properties like order parameter and anisotropic physical properties such as birefringence, dielectric anisotropy, magnetic anisotropy, and conductivity anisotropy aligned monodoamin samples are used. Moreover, to identify LC phases, it is also necessary to use

The surface topography characterization of a grating developed from a LC polymer is shown in Figure 20, and the process of grating formation by utilizing photochemical phase transition in liquid crystalline polymers is schematically depicted in Figure 21.297,298 Photochemically induced phase transition of photosensitive liquid crystalline copolymers have been employed by Seki et al. in the fabrication and characterization of surface relief gratings.304 The chemical structures of such copolymers are shown in Scheme 27. Mass migration has been observed to facilitate such surface relief grating formation in liquid crystalline polymer films. Accordingly, they have systematically explored the mass migration mechanism in photosensitive polymer films.305 Similarly, by systematically varying thermal properties of polymer films, mass migration phenomenon have been R


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Figure 22. Different alignment states of LCs. The ellipses represent rodshaped molecules. The arrows indicate the “director” orientation.

and tilted alignment (director is tilted with respect to the substrate surface normal) as shown in Figure 22. However, other types of alignments such as twisted and hybrid alignment are also used in device applications. Due to the criticality of sample alignment in LC studies and applications, varieties of alignment methods and aligning materials have been developed to obtain different desired sample alignments. The commonly used and popular alignment procedure in academic laboratories as well as in industries is the rubbing method. In this procedure, the substrate is first coated with an alignment material like polyimide followed by unidirectional rubbing (buffing) of the polymer coating by a cloth or paper. Due to its simplicity and convenience, the rubbing method is widely adopted. Nevertheless, some serious drawbacks of this method have been encountered and several limitations of this method have been recognized. Physical rubbing of the alignment layer produces and accumulates static charges and dust particles which are detrimental to the LC device operation and performance. Moreover, rubbing can cause damage to the alignment layer coating. As far as the limitations are concerned, the rubbing method cannot be applied to align LCs in enclosed areas, thin micro- or nano gaps and on curved surfaces. High-resolution patterned multidomian alignment on surfaces is difficult to achieve by the rubbing procedure. In order to address the drawbacks and overcome the limitations of rubbing method, many alternative alignment control methods have been proposed and demonstrated. However, among the different techniques, “photoalignment” technique has been found to be very effective and fascinating. In this technique, LCs can be aligned due to the action of light on photosensitive materials. Both photophysical as well as photochemical processes have been exploited for the alignment control of LCs. High quality uniform LC alignment with controllable anchoring energy, pretilt angle, and acceptable stability has been accomplished. Moreover, the photoalignment technique utilizes the unique advantages of light stimulus (spatial, temporal, and remote control) and has enabled new prospects for LC applications. In addition to energy, light effectively transfers the polarization and phase information to LCs. Recent industrial application of photoalignment technique in the production of commercial LC displays has exerted additional thrust to this enabling endeavor. In the following, we discuss the different approaches to photoalignment of LCs. Ichimura et al. in the late 1980’s reported the photoalignment control of a nematic LC between homeotropic and degenerate planar alignment states. When LC cells, with substrates whose

Figure 21. (a) Scheme of the preirradiation method to eliminate cooperative motions. The inset of (a) is the molecular scheme and properties of the material used in this paper. (b) Possible scheme of AZ mesogens in SWGs recorded in the liquid crystalline polymer films. Reproduced with permission from ref 298. Copyright 2008 American Institute of Physics.

Scheme 27. Liquid Crystalline Copolymers Used in the Fabrication of Surface Relief Gratings

aligned samples during X-ray diffraction investigations. LC alignment is critical in the manufacturing of LC displays and other LC devices. Commonly employed LC alignments are homeotropic alignment (director is perpendicular to the substrate surface), homogeneous alignment (director is parallel to the substrate surface and oriented along a single direction), S


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not absorb polarized light. Therefore, during the photoisomerization process, the azobenze compound reorients while relaxing from the cis state and falls perpendicular to the axis of polarization. In this orientation, the trans azobenzene molecules become inactive toward polarized light exposure. This behavior of trans azobenzene has turned out to be a very enabling phenomena in photoaddressable liquid crystalline systems. Subsequently, the Ichimura group used LPL to study the alignment change of LCs on azobenzene-modified command surfaces. It was observed that the LC alignment can be reversibly changed between homeotropic and homogeneous states as well as between orthogonal homogeneous states by changing the polarization direction of the light in perpendicular directions. Azobenzene derivative 72 was used to prepare command surfaces to study the photoalignment behavior of LCs.324 The monolayer of this compound with side-on attached azobenzene group yields planar alignment of LCs in the beginning; however, upon irradiation with LPL, the LC alignment becomes homogeneous in the direction perpendicular to the polarization direction of the light. The direction of the uniaxial homogeneous alignment can be altered in orthogonal directions by changing the polarization direction of the incident light. Uniform and unidirectional photoalignment of LCs have been demonstrated by employing a variety of photosensitive surface layers and by the use of suitable light stimulus. In addition to the use of azobenzene-based monolayers, azo dye doped polymer layers, azobenzene-based polymers, and Langmuir−Blodgett techniques have been tested with respect to different aspects of photoalignment, including the generation of the pretilt angle. Apart from calamitic LCs, the photoalignment of other types of LCs by using command surfaces and surface layers have been accomplished. Polyimides functionalized with azobenzene derivatives have also been synthesized and used as a photoalignment layer. Photoinduced three-dimensional (3D) orientational order of azobenzene-based side chain liquid crystalline polymers both in bulk and on free surface has been investigated and their effect on LC photoalignment has been clarified.326−330 Photoalignment on command surfaces exposed to interference patterns of laser beams have been reported. High efficiency gratings have been fabricated by using photoalignment layers. Recently aminoazobenzene-based monolayers have been used for photoalignment which exhibit high performance. Alignment switch from uniform planar to twisted state was observed in a cell made from monolayer and polyimide modified substrates upon irradiation with polarized light. Colloidal particles and their self-assembled structures in nematic LCs have been manipulated by using the aminoazobenzene monolayers. Rotation, translation, localization, and assembly of spherical and complex-shaped particles in cells have been demonstrated by using polarized light. Ferroelectric and antiferroelectric LCs have been aligned using the photoalignment technique.331−335 Photoalignment studies using azobenzene-based polymers and azo dye doped polymers as command surfaces are still actively pursued. Dyadyusha et al. and Schadt et al. independently but almost simultaneously introduced photoalignment of LCs on polyvinylcinnamates films irradiated by LPL.336−346 Scheme 29 shows the chemical structure of the polyvinyl cinnamate 74. This polymer upon irradiation with linearly polarized UV light undergoes [2 + 2] cycloaddition with its neighbors to yield crosslinked polymer network, and the film becomes anisotropic with uniaxial orientation of the polymers. The cycloaddition takes place only under the condition that the polarization direction of the light is parallel with the elongated side groups; otherwise no

surfaces were modified by a monolayer of azobenzene derivative 71 (Scheme 28), were irradiated with UV light, the initial Scheme 28. Azobenzene Derivatives Used for Photoalignment of LCs

homeotropic alignment of the LC changed into planar alignment (Figure 23).312−324 However, upon visible light irradiation, the

Figure 23. Alignment change of a LC between homeotropic and random planar states upon irradiation with UV and visible light. Reproduced with permission from ref 160. Copyright 2013 Elsevier Ltd.

homeotropic alignment was restored from the planar alignment. This reversible alignment alteration upon UV and visible light irradiation was attributed to the reversible trans−cis photoisomerization of the azobenzene groups, which transforms the rodlike trans isomer into the bent cis isomer. Since a very small number of azobenzene derivatives were able to command the alignment change of a very large number of molecules of the LC compound, the azobenzene-modified surfaces were referred to as “command surfaces”. Soon after this ground breaking report, Gibbons et al. demonstrated surface-mediated alignment of LCs with polarized laser light.325 They used a polyimide film doped with an azo dye compound 73 as a dichroic material in a LC cell. When this film was irradiated with linearly polarized light (LPL), the LC molecules at the irradiated site became perpendicular to the polarization direction of the laser light. As a consequence, the initial planar cell exhibited a twisted organization of the LC at the irradiated site, and since the other substrate in the cell contained only polyimide, it did not get affected by the light irradiation. This light-driven alignment change of the LC is stable and can be erased or rewritten by altering the polarization direction of the incident laser beam. Here the LC molecules align parallel to the reorinted trans azobenzene compound. This is due to the selective absorption of polarized light by trans azobenzene molecules. Polarized light with its axis of polarization parallel to the transition moment of trans azobenzene gets absorbed and causes trans to cis isomerization. When the polarization axis is orthogonal to the long molecular axis of trans azobenzene, it does T


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Coumarin-based side chain polymers 79 (Scheme 31) are another class of materials which have been used for the

Scheme 29. Cycloaddition and Trans-Cis Isomerization of Cinnamate Groups

Scheme 31. Coumaric-Based Polymers for Photoalignment Control of LCs

photochemical reaction takes place. It should be noted that cinnamate groups can also undergo trans−cis photoisomerization in addition to photocycloaddition. On linearly photopolymerized polyvinylcinnamate films, LCs align in the perpendicular direction to the light polarization. However, polyvinyl cinnamates promoting parallel alignment with respect to the polarization direction are also known. In some cases, both parallel and perpendicular alignment can be achieved by controlling the exposure dose of the LPL. Oblique orientation of LCs has also been demonstrated on polyvinyl cinnamate polymers films. Patterned alignments of LCs have been shown on the photopolymerized films of the polymer 75 (Scheme 30). Optical

photoalignment control of LCs.347−351 In these polymer films, linear photopolymerization occurs through [2 + 2] cycloaddition and results in anisotropic surfaces. In contrast to cinnamate materials, coumarin-based materials generally promote alignment of LCs parallel to the polarization direction of light. Unlike cinnamates, coumarins do not show photoisomerization. The Ichimura group reported polymethacrylates 80 with coumarin side chains.348 The polymethacrylates with coumarins linked through flexible spacers promote LC alignment parallel to the polarization direction of the LPL. However, polymethcrylates without spacers promote reversion of photoalignment. This observation of perpendicular alignment was interpreted in terms of the differential intermolecular interactions of LC molecules with unreacted coumarins and photodimerized products. The Chen group synthesized and studied the photoalignment behavior of LCs on polymethacrylate films containing coumarin pendant groups 81−84 (Scheme 31).349−351 It was found that at a high degree of dimerization, LC alignment switches from parallel to perpendicular with respect to the polarization axis of UV light. Polymer 83 induced photoalignment parallel to the polarization direction of the light; however, polymer 84 induced a switching of alignment from parallel to perpendicular direction with respect to the polarization axis of UV light. Detailed photoalignment studies of LC have been carried out on the photopolymerized films of the polymers 81−84 to gain greater insight into the fundamental processes. Chalcone-based photoalignment materials have been developed and studied.352−356 Chalcone films photopolymerized by linearly polarized UV light provide stable LC alignment. Both parallel and perpendicular alignments of LCs with stable pretilt angles have been demonstrated on chalcone-based films irradiated with LPL. Photo cross-linked films of LC polymers 85 and 86 (Scheme 32) with chalcone pendant groups have been investigated in the context of their LC alignment capability. The LC alignment ability of these polymer films arises from the surface anisotropy created by the [2 + 2] cycloaddition between

Scheme 30. Structures of Cinnamate Compounds

retarders and polarization interference filters have been fabricated by using the photopolymerized films. Amphiphilic cationic cinnamate derivative 76 has been photopolymerized, and LC alignment on these films has been studied.345 Linearly photopolymerized films of polymer 77 have been observed to yield alignment of LCs parallel to the polarization direction of the light. Polymers 78 with regioisomeric cinnamate side chains were studied for photoalignment regulation of LCs. It was revealed that both photoisomerization and photocycloaddition of the cinnamate groups contribute toward alignment control of LCs. U


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occurs upon visible light irradiation. The light-driven process is depicted in Figure 24. An optically switchable device based on this reversible light-driven alignment change was demonstrated.

Scheme 32. Polymers Containing Chalcone Moieties for Photoalignment of LCs

chalcone moieties. Polystyrene and polyimide films carrying chalcone groups have also been employed in the photoalignment of LCs. Polyimide films containing stilbene moieties in their backbone has also been found to induce photoalignment of LCs. In the above-discussed cases, photoalignment results either from photoisomerization or photo-cross-linking, which lends anisotropy to the film surfaces. Anisotropic surfaces have also been obtained by photodegradation of polymer films when irradiated with high energy UV light.357−364 It has been clarified that the surface anisotropy on polyimide films is a consequence of selective depolymerization of polyimide main chains parallel to the electric field of linearly polarized UV light. Polyimides with a variety of main chain structures have been investigated for the photoalignment of LCs. Polystyrene films exposed to linearly polarized UV light were found to align LC in preferred directions. Continuous or abrupt transition from homeotropic to planar alignment on polyimide alignment films irradiated with polarized UV light has been reported. Recently, the photoalignment technique has been used to align LCs in cylindrical capillaries.365 A designed polymer with dibenzobarrelene groups has been found to induce photoalignment of nematic LCs.366 The Reznikov group reported the observation of LC alignment by action of light on azo dye compounds present in the bulk of the LC mixture. Here the adsorption of photoisomerized azo dye molecules from the LC bulk onto the aligning surface was proposed to cause the reorientation of LC molecules. The Reznikov group has extensively studied the “bulk alignment” of LCs by using dye-doped LC mixtures.367−378 Evolution of lightinduced anchoring and surface gliding of the nematic director have been studied and accounted for. Several groups have studied this light-driven phenomenon in dye doped LCs.379−384 Later it was observed that the dye molecules phase separate from the bulk mixture and assemble on the surface of the cell substrates. The dye aggregates on substrate surfaces have been characterized by electron microscopy and scanning probe microscopy. The transition from chiral nematic to nematic phase has been observed by light irradiation to a LC mixture containing chiral azo dopant. The dipole moments of azo dyes are known to increase upon photoisomerization from the trans to cis form. Therefore, an increase in molecular polarity occurs upon photoisomerization. The polar molecules have an obvious tendency to adsorb onto the polar alignment layers in LC cells. This phenomenon has been observed in a nematic LC made from an azobenzene derivative. Recently we reported on the light-driven reversible alignment switching of a nematic LC containing azo thiol functionalized gold nanoparticles.384 Trans−cis photoisomerization of the azo groups on gold nanoparticle surfaces causes planar to vertical alignment transition upon UV light irradiation, while the reverse process

Figure 24. Light-driven alignment change in azo thiol functionalized gold nanoparticle doped nematic LC. Reproduced with permission from ref 384. Copyright 2015 Wiley-VCH.

Photoalignment of the nematic phase of discotic LCs have been demonstrated by the Ichimura group. 385−393 The compound 87 (Scheme 33) has been aligned on a command Scheme 33. Discotic Nematic LCs Exhibiting Photoalignment

surface containing the cyano azobenzene group. Tilted alignment was obtained by oblique irradiation with unpolarized light, whereas normal irradiation with LPL yielded homeotropic alignment of the discotic nematic LC. The photoalignment scheme is shown in Figure 25.385 Tilted alignment of the discotic nematic compound 87 was observed on polyvinylcinnamate films. Subsequently detailed V


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presented in Figure 26.394 In a series of studies, they have clarified the photoalignment properties of the columnar phases.

Figure 26. Alignment control of columnar hexagonal phase by polarized infrared light irradiation. Reproduced with permission from ref 394. Copyright 2006 Wiley-VCH.

Figure 25. Photoalignment of a discotic nematic LC. Reproduced with permission from ref 385. Copyright 2000 Wiley-VCH.

investigations were carried out on the photoalignment of discotic nematic phase on photosensitive polymer films. Similarly, the photoalignment behavior of the compound 88 was studied on photosensitive polymer thin films, and photopatterning of the nematic liquid crystalline film was achieved. Recently, a pillarenebased liquid crystalline compound containing azobenzene groups has been found to impart alignment control to the columnar phase of a triphenylene-based discotic LC 89 (Scheme 34).393 The photoresponsive performance of the thin alignment layer of the pillarene LC on the columnar phase was studied in depth.

Complex molecular order of LC polymer networks has been attained by applying the photoalignment technique. The orientation control over the director of LC by photoalignment technique has enabled macroscopic programmable deformation of LC polymer network films. Recently, LC polymer networks with azimuthal, radial, and circular twisted nematic director profile were fabricated.400−402 Upon infrared light irradiation, the dye-doped flat polymer network films exhibit reversible actuation. The film with azimuthal director profile deforms into a conical shape, whereas the film with radial director profile deforms into a saddle shape. The process of actuation and the corresponding circular director profiles in the flat films are shown in Figure 27.400 Similar LC polymer films with voxelated circular director profile have been fabricated by utilizing the photoalignment technique. These films upon heating give rise to the formation of conical deformations, and on cooling down, the deformation disappears yielding the initial flat film (Figure 28).401 LC polymer network films with patterned 3D director profile have been fabricated by using photoalignment technique. In these films, the director rotates by 90 degrees across the film thickness. Upon heating, the polymer film with striped director profile in the plane diplays accordion-like folds; however, the film with a checkerboard director profile in the plane deforms out of plane. The actuation behavior of a film with checkerboard director configuration is shown in Figure 29.402 These above examples show that the judicious use of the photoalignment technique can lead to materials capable of arbitrary and programmable shape morphing. It should be noted that LC photoalignment has been used for the manufacture of commercial LCD devices.403,404 Applications of photoalignment have been recently used to demonstrated interesting phenomena.405−408 Very recently, the photoalignment technique has been used to control the phase of cholesteric LC helices.405 Through this phase control, the phase of the selectively reflected circularly polarized light has been modulated and the effect of handedness of the cholesteric LCs on this phenomenon has been explored. Planar optical devices with interesting properties have been demonstrated through this new phase control method. Dynamic superstructures with arbitrarily controlled orientation of helices have been achieved by photoaligning cholesteric

Scheme 34. Triphenylene-Based Discotic LCs

Shimizu group has studied the alignment control of triphenylene-based discotic LCs 89−91 exhibiting columnar hexagonal, columnar plastic, and helical phases by infrared light irradiation.394−399 They have found that linearly polarized infrared light switches the homeotropic alignment of columnar hexagonal phase into the planar state. The alignment of the columnar hexagonal phase can be switched into homeotropic state by circularly polarized infrared light. Therefore, sequential irradiation with linearly polarized and circularly polarized infrared light enables reversible alignment switching between homeotropic and planar states. This photoalignment process is W


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Figure 27. Photoactuation of polymer network films with azimuthal and radial alignment. Note that depending on the combination of radial and azimuthal deformation, the film adopts a conical or a saddle shape. The LC director profiles are schematically illustrated below. Reproduced with permission from ref 400. Copyright 2012 Wiley-VCH.

optical vortices have been generated in LC gratings by appropriate photopatterning.407 These optical vortices with high efficiency, wide operating spectrum range, and polarization independency possess great potential for a wide range of applications of fork gratings. Furthermore, orthogonal photoalignment has been employed to generate polarizationindependent LC gratings.408 4.2. Photoorientation of Liquid Crystals

In the case of photoorientation of LCs, the liquid crystalline material itself is photosensitive, which upon photoirradiation exhibits molecular reorientation of the mesogens in response to the light beam. Unlike the case of photoalignment, there is no need of any alignment layers and the cells used just act as mechanical boundaries to contain the liquid crystalline materials which are often liquid crystalline polymers containing photosensitive groups. In the case of copolymers which contain both photosensitive and photoinsensitive mesogens, the later mesogens follow the dynamics of former mesogens upon light irradiation through cooperative molecular motion. The Stumpe group has undertaken extensive studies on the phenomenon of photoorientation leading to photoinduced optical anisotropy in the thin films of azobenzene containing liquid crystalline polymer materials.409−413 They have prepared a variety of liquid crystalline copolymers with different amounts of azobenzene contents. The structure of such a polymer 92 is shown in Scheme 35. The films fabricated from these polymers acquire high birefringence upon irradiation with linearly polarized light due to co-operative molecular reorientation. The kinetics of this reorientation process in different photochromic liquid crystalline polymer materials has been studied. The combined effect of photoorientation and photochemically induced order in liquid crystalline polymer films results in a significant amplification of their optical anisotropy. Multifunctional photoresponsive terpolymers have also been designed and synthesized, and their photoorientation attributes have been investigated. These copoly(methacrylates) with different photochromic groups have been compared with respect to their photoorientation behavior. Recently, photorotor materials containing a photosensitive ethane unit flanked by donor and acceptor substituents have been prepared and studied.414 Interestingly, both forward and backward photoisomerization in these materials proceeds through rotation around the C−C double bond with a single irradiation wavelength, and the isomers do not exhibit thermal relaxation. The films of the liquid crystalline polymer have been found to undergo photoorientation upon irradiation with linearly

Figure 28. Deformation of polymer network films with patterned topological defects. Since the director orientation varies azimuthally around the defects, conical shapes pop up from the flat film on heating. Reproduced with permission from ref 401. Copyright 2015 The American Association for the Advancement of Science.

Figure 29. Deformation of polymer network films with twisted alignment in a checkerboard pattern. Infrared light directed shape morphing and simulation results of the polymer film deformation on heating have been illustrated. Reproduced with permission from ref 402. Copyright 2013 Wiley-VCH.

LCs.406 Different kinds of textures and gratings have been realized through this process which holds great promise for LC helical superstructures in advanced photonic devices and lithographic processes. Similarly, switchable and reconfigurable X


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Scheme 35. Structures of Copolymers Undergoing LightDriven Reorientation of the Mesogens

Scheme 36. Chemical Structures of Block Copolymers

polarized light of suitable wavelength. Recently, supramolecular low molar mass liquid crystalline materials have been fabricated through ionic self-assembly between an azobenzene-based anion and tetraalkylammonium cation.415,416 When linearly polarized light impinges on thin films of these ionic photoresponsive materials, effective induction of optical anisotropy occurs. This property has been exploited to realize optical patterns and phase gratings on the thin films. Moreover, a detailed investigation to gain insight into the photoinduced reorientation phenomenon in the lamellar and columnar phases of the ionic complexes has been undertaken. The Ikeda group reported the alignment of azobenzene-based mesogens by irradiating a polydomain LC copolymers 93 (Scheme 35) film with LPL.417−426 Light-driven orientation of the mesogens of copolymer 93 with a nitroazobenzene moiety enabled the fabrication gratings with high efficiency. The copolymer 94 enabled light-driven formation of Bragg gratings with large angular multiplicity through reorientation of azobenzene mesogens. It was found that the mesogens were oriented perpendicular to the polarization axis of UV light and generated optical anisotropy in the polymer film. A breakthrough in the photoorientation of LCs was reported by Yu et al.419 They observed light-driven alignment of ethylene oxide nanocylinders by supramolecular cooperative motion in the liquid crystalline diblock copolymer 95 (Scheme 36). This polymer on annealing produces nanocylinders of poly(ethylene oxide) (PEO) dispersed in the azobenzene LC matrix. The PEO nanocylinders had a hexagonal registry, and they were perpendicular to the substrates as revealed by atomic force microscopy investigations. Upon irradiation with polarized UV light, the homeotropic alignment of the azobenzene LC polymer changed to a homogeneous state and the LC alignment was perpendicular to the polarization of the light. This LC alignment change was found to have a great influence on the PEO domains.

Upon annealing at the LC temperature, the PEO nanocylinders formed regular array, and their orientation was found to be parallel to the LC alignment. Thus, perpendicular to parallel orientation change of the PEO nanocylinders was achieved through supramolecular cooperative motion in the LC copolymer. Figure 30 shows the orientation of PEO nanocylinders in light irradiated and unirradiated areas of the diblock copolymer film.419 Diblock liquid crystalline copolymers with azobenzene moieties and methyl methacrylate groups furnished wormlike nanostructures in bulk films. This report was the first observation of wormlike nanostructures in block copolymer films, though they were known to occur in block copolymer micelles. In the block copolymer, the azobenze forms the minority phase and self-assemble into wormlike domains in bulk films. The diblock copolymer 96 with azobenzene groups exhibit microphaseseparated nanostructures. In this polymer, the mesogenic blocks can be continuous or separated phases owing to varied contents of mesogens. It was also observed that photoalignment and holographic recordings in the films of this copolymer are dependent on mesogen content. Light-driven cooperative motion was also observed in another well-defined triblock copolymer containing a poly(methyl methacrylate) block in addition to azobenzene and cyanobiphenyl mesogen blocks. Recently, block copolymers 97 and 98 have been synthesized, and their photoresponsive and holographic behaviors have been studied. Photoinduced alignment changes in polymers 99 and 100 (Scheme 37) have been used in the fabrication of high performance holographic gratings. Figure 31 shows the 3D object, and its reconstructed image with high resolution.426 Y


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unraveled the mechanism of light-driven orientation change of the nanocylinders. The effect of confinement on photoalignment of azobenzene groups in microphase-separated domains has been investigated by preparing diblock copolymers.440 Cyclic side chain liquid crystalline polymers containing azobenzene groups have been synthesized, and their photoinduced birefringence properties have been investigated. It was found that photoinduced anisotropy in these cyclic polymers is different from their linear counterparts, indicating that the topological constraint arising from the tortuosity of the ring structures plays a significant role.441 High optical anisotropy has been endowed to the films of side chain liquid crystalline ionic complexes obtained from a cationic polyelectrolyte- and azobenzene-based anions derived from the dye methyl orange.442,443 It should be noted that this supramolecular liquid crystalline complex is devoid of any flexible spacers and tails and exhibits mesomorphism even though its constituents are nonmesomorphic. A series of such liquid crystalline ionic complexes yielding photoinduced birefringence and surface relief grating have been thoroughly investigated. Photoorientation has been observed in the films of an azobenzene functionalized liquid crystalline dendrimer.444 Pulsed laser irradiation creates periodic surface microstructures in these films which have been used for alignment control of LCs in cells. Light responsive liquid crystalline supramolecular complexes have been constructed via halogen bonding in low molar mass compounds which exhibit photoinduced high magnitude optical anisotropy upon irradiation with polarized light.445 Interesting surface relief grating formation takes place in thin films of these materials when exposed to a light interference pattern. This study is a beautiful illustration of halogen bonding as an enabling tool in the fabrication of functional photoresponsive superstructures. Recently, 3D orientation control of nanocylinders of PEO blocks was shown in an amphiphilic diblock copolymer exhibiting the nematic LC phase. Macroscopic perpendicular and parallel patterning of ordered nanocylinders could be fabricated by thermal annealing and light-driven alignment control.446 A very fascinating recent development in the photoalignment of LCs is “free surface commanding” by a skin layer of surfacesegregating photoresponsive polymer 110.435−438 This technique has enabled the alignment control of both photoresponsive as well as photoinsensitive liquid crystalline polymers. Liquid crystalline homopolymers and microphase separating block copolymers have been found to respond well to the free surface commanding by polarized photoirradiation and thermal annealing of their films. Figures 34 and 35 show the process of free surface commanding by surface segregating photoresponsive polymer skin layer.437,438 Detailed and systematic studies on free surface commanding is being under taken by the Seki group. Light-driven alignment control of surface grafted photoresponsive LC polymers 111−113 (Scheme 40) has been studied.447−452 The orientation switching of the mesogens by polarized light irradiation has been evaluated by measuring the order parameter on thin films. The effect of molecular parameters like flexible spacers and flexible blocks on the photoorientation behavior have been examined to understand the factors affecting ordered alignment control of these polymers. Figure 36 shows the process of photoalignment in one of the grafted systems.451 The Kawatsuki group has synthesized a variety of LC polymers containing photo cross-linkable cinnamate side chains.453−461

Figure 30. Microphase-separated scheme of the block copolymer, both azobenzenes and PEO cylinders are perpendicular to the substrate (top). Scheme of LC alignment and microphase-separated structures in the irradiated and unirradiated area of the block copolymer film (bottom). Reproduced from ref 419. Copyright 2006 American Chemical Society.

Light-driven alignment change has been observed in azotolane-based LC polymers 101−103.427−429 Due to the presence of the tolane moiety in these compounds, they exhibit high birefringence. Upon photoinduced alignment change, these compounds show a very large change in birefringence. These compounds could find potential application in high performance photonic devices. Seki group synthesized the triblock copolymer 104 (Scheme 38) containing azobenzene side chains and reported on the twodimensional microphase separation driven by light.430−439 Subsequently, they synthesized the diblock copolymer 105 and studied its photoalignment behavior. The PEO block forms microphase-separated nanocylinders in thin films. Three-dimensional orientation control of these nanocylinders has been demonstrated in this study. Out-of-plane orientation control has been achieved by varying the film thickness whereas the in plane orientation of the nanocylinders has been enabled by linearly polarized light illumination. The light-driven orientation control of the nanocylinders is shown in Figure 32.430 Light-driven threedimensional (3D) on-demand orientation control of microphase-separated nanocylinders of polystyrene blocks was demonstrated using the block copolymer 106. This unprecedented control was achieved by judicious optimization of the block copolymer film preparation conditions. The combination of linear polarized light illumination and thermal annealing steps yields the desired orientations of the nanocylinders (Figure 33).431 The Seki group has synthesized many liquid crystalline block copolymers (Scheme 39) capable of microphase separation and investigated the effects of structural parameters as well as Z


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Scheme 37. Chemical Structures of Photoresponsive Polymers Containing Tolane Groups

Scheme 38. Chemical Structures of Photoresponsive Block Copolymers

to enhance the orientation of the mesogens and hence their optical anisotropy. Initially they studied light-driven orientation of LC polymers 114 and 115 (Scheme 41) containing biphenyl groups. The effects of spacer lengths on the photoorientation behavior were investigated. It has been found that both photoisomerization and photo cross-linking of the mesogenic groups occur during polarized light exposure to the polymer films. They also synthesized and investigated the light-driven orientation of the copolymer 116. Subsequently, they extended

Figure 31. A 3D object and its reconstructed image. Reproduced from ref 426. Copyright 2002 American Chemical Society.

These polymer films upon irradiation with polarized UV light undergo axis-selective cross-linking which produces alignment of the mesogens in the polymer film. This light-driven orientation of the mesogens yields high value of birefringence for the polymer films. Thermal annealing of the films has been observed AA


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Scheme 39. Block Copolymers Exhibiting Microphase Separated Morphology

Figure 34. Illustrations of the orientation and alignment process for a homopolymer (upper) and a block copolymer (lower) by free surface commanding. Reproduced with permission from ref 437. Copyright 2013 Wiley-VCH.

Figure 32. Photoinduced in plane orientation control of PEO nanocylinders. Reproduced with permission from ref 430. Copyright 2006 Wiley-VCH.

5. PHOTOMODULATION OF CHIRAL LIQUID CRYSTALS Chiral LCs are interesting materials for basic studies as well as technological applications. Common chiral LC phases, such as the chiral nematic or cholesteric LC (CLC), the blue phases (BP), and the chiral smectic C phase, display quite fascinating optical and electrooptical phenomena compared to nonchiral LC phases. It should be noted that while both CLC and ferroelectric chiral smectic C phases have been widely used in LC displays (LCDs) and microdisplays, the use of blue phases in LCDs has been recently demonstrated. Moreover, many beyond-display applications for these intriguing LC phases have been

their studies to LC polymers carrying tolane groups since it is known that tolane moieties possess larger inherent optical birefringence than biphenyl groups. Accordingly, the polymers 117 and 118 (Scheme 42) were investigated. Polarization holographic gratings with large birefringence were fabricated using thin films of polymer 118. Light-driven orientations of hydrogen-bonded LC polymers 119 have also been studied. Moreover, alignment of low molar mass LCs on these polymer films has been examined.

Figure 33. Process of 3D orientation control of polystyrene nanocylinders in the block copolymer film. Reproduced from ref 431. Copyright 2007 American Chemical Society. AB


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Figure 35. Homeotropic to planar alignment change by photoresponsive surface polymer layer. Reproduced from ref 438. Copyright 2016 American Chemical Society.

Scheme 40. Surface Grafted LC Polymers

Figure 36. Schematic illustration of the photoinduction of orientations of azo mesogens and smectic layers in a grafted polymer film. Reproduced from ref 451. Copyright 2009 American Chemical Society.

Scheme 41. Liquid Crystalline Cinnamate Polymers

demonstrated and many more potential applications have been envisaged. To broaden their scope and diversify their potential applications, these chiral LC phases have been engendered with photoresponsive capability. In the following, photomodulation of the properties of these chiral LCs are discussed. 5.1. Photomodulation of Cholesteric Liquid Crystals

Scheme 42. Liquid Crystalline Polymers Exhibiting LightDriven Molecular Orientation

Chiral nematic or cholesteric LCs are the chiral versions of nematic LCs. In the nematic LC phase, the molecules maintain more or less parallel orientation with respect to their neighbors. However, in the CLC phase the molecules are slightly twisted with respect to their neighboring molecules. The twisted orientation of molecules in CLCs arises due to the presence of chiral molecules in the system. As a consequence of this twisted disposition of the molecules, the director spirals about an axis orthogonal to it, and the molecules in a CLC phase build up a helical superstructure (Figure 3). This helical superstructure of CLCs is characterized by its “pitch” and “handedness”. The distance over which the LC director completes a 360 °C rotation along the helical axis is termed as the pitch of CLCs. It is denoted by P. The twist sense of the molecules in a CLC phase is expressed in terms of its handedness, which can be either left handed or right handed in nature. One of the striking characteristics of CLCs is the “selective reflection” of circularly

polarized light (CPL) of same handedness as its helix according to Bragg’s law. The wavelength of the selectively reflected light, λ, is directly correlated with its pitch length through the equation λ AC


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= nP, where n is the refractive index of the CLC system. Therefore, when the pitch length of the CLCs lies in the ranges of the order of wavelengths of visible light, they appear vividly colored. Many of the practical applications of CLCs are based on this elegant phenomenon. CLCs can be fabricated either from chiral compounds or by doping nonracemic chiral guest compounds into nematic LC phases. For the reasons of convenience, economic, and flexibility, the later procedure is the method of choice which is almost often adopted. The ability of chiral guest (dopant) compounds to induce a CLC from nematic phases is quantitatively expressed in terms of their helical twisting power (HTP), which is denoted by the symbol “β”. The HTPs of the guest compounds govern the pitch of the induced CLCs according to the relation: P = 1/βC, where C is the molar concentration of enantiopure dopants. The HTP values of chiral dopants are assigned signs, positive or negative, depending on whether they induce a right-handed or left-handed helical superstructure from the host nematic LCs. Photoresponsive CLCs result when at least one of the components of the CLC mixtures is photoresponsive. Recently, chiral photoresponsive dopants have been extensively used to fabricate photoresponsive CLCs. The use of photoresponsive chiral dopants facilitates the effective utilization of the distinct advantages of light stimulus to drive and dynamically tune the properties of induced CLCs. Light-driven pitch modulation and handedness inversion in photoresponsive CLCs have been accomplished by employing photochromic chiral dopants based on azobenzene, overcrowded alkene, dithienylcyclopentene, fulgide, spirooxazine derivatives, etc. The photomodulation of CLCs has enabled many applications of these functional stimuli responsive soft materials. In the following, we discuss light-driven property modulations of induced CLCs developed using different chiral molecular switches and motors. 5.1.1. Azobenzene-Based Dopants in Cholesteric Liquid Crystals. The development of and studies on azobenzene doped CLCs began in the early 1970’s where lightdriven pitch modulation of an induced CLC was demonstrated. However, the past decade has witnessed intense activities in this endeavor; consequently, CLC systems containing azobenzenebased chiral dopants are most widely developed and investigated. The relatively straightforward synthesis of chiral azo dopants and high compatibility of their trans forms in the nematic LC matrix provides the necessary edge to these compounds. Moreover, the drastic change in molecular shape of azobenzene derivatives (trans form is rod shaped where as cis isomer is bent) upon photoirradition allows facile tunability of the pitch of the CLCs, though in some cases photoinduced phase transitions have been registered. Recently, azobenzene derivatives with very high HTPs and capable of endowing handedness inversion capability to CLCs have been realized. In addition to tetrahedral chirality (arising from stereogenic center), axial and planar chirality have also been introduced to enhance the HTP of azodopants. Moreover, azobenzene derivatives exhibiting trans to cis and cis to trans photoisomerization exclusively by visible light have also been made available. During early 2000s, Ichimura et al. synthesized azobenzene derivatives 120 (Scheme 43) by varying the substituent positions on the phenyl rings and studied the light-driven pitch modulation phenomenon by adding these compounds to CLCs.462 One of the remarkable observations from this study is that the azobenzene derivatives substituted in their 3,3′ positions of phenyl groups maintain more or less rodlike shape in their cis isomeric forms upon photoisomerization from their rod-shaped

Scheme 43. Azobenzene Derivatives Used As Chiral Dopants

trans form. This fact is reflected in the moderate or no change in the selective reflection wavelength of the CLCs upon photoirradiation. Kurihara et al. reported light-driven switching between a compensated nematic and a cholesteric phase by using compound 121 and R811 in a nematic host.463 Later they pursued the handedness inversion process in the induced CLC by taking advantage of the opposite signs of the HTPs of the photosensitive chiral azo compound 121 and photoinsensitive chiral dopant R811. Compound 122 has been synthesized and tested as a chiral dopant for the induction of photoresponsive CLCs.464 It has been observed that this compound bestows handedness inversion capability to CLCs in addition to pitch modulation. Interestingly, the handedness inversion happens through an interaction-selective manner with compounds possessing terminal alkenyl groups. The handedness inversion has been confirmed by the observation of the sequence of phase transitions, cholesteric-nematic-cholesteric during photoirradiation. The nematic phase appearing between two cholesteric phases of opposite handedness is referred to as a compensated nematic phase. This achiral nematic phase can be thought of as a racemic mixture of equal amounts of left- and right-handed helices. This can be clarified in terms of the change of the HTP of the chiral dopant as follows. Upon photoirradiation, the initial HTP value of the dopant decreases and reaches to zero when the initial HTP of the dopant is equally compensated by the HTP value of its isomer exhibiting an opposite sign of HTP. At this cross over point in the photochemical process as the net HTP value approaches zero, the pitch approaches to infinity, thus resulting in a nematic phase. After all, a CLC phase with infinite pitch is the nematic phase by definition. Further irradiation of the compensated nematic phase yields a CLC phase with opposite handedness to the initial one. It should be emphasized that the whole process is dynamic and reversible in nature as depicted in Figure 37. Therefore, the system can be made to switch in either direction by selecting the appropriate wavelength of light irradiation. Light-driven pitch modulation has been exploited by Tamaoki et al. for rewritable full-color recording in azobezene-based dopant containing CLC glassy film fabricated from a cholesterolbased LC dimer.465 The Gottarelli group introduced the axially chiral azobenzene derivatives 123 and 124 (Scheme 44) and studied their CLC AD


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Scheme 45. Azobenzene-Based Chiral Dopants Exhibiting High HTPs and Large Change in HTPs

Figure 37. Schematic presentation of the process of light-directed chirality inversion in the induced helical superstructure of a CLC containing a chiral dopant that changes molecular shape upon photoirradiation. Note that the handedness inversion process proceeds through an achiral nematic phase.

Scheme 44. Axially Chiral Azobenzene Derivatives

demonstrated. Later systematic study was carried out with related chiral compounds. Our group synthesized photoresponsive cholesterol derivatives 128 (Scheme 46) by linking Scheme 46. Azobenzene Based Chiral Compounds Developed by Our Groupa

a

induction behaviors by doping into nematic LCs.466,467 It was found that compound 123 possess high helical twisting power. This high HTP enables induction of CLCs with low dopant concentrations which is beneficial to avoid coloration of the sample and phase separation and does not compromise or alter the optimized physical properties of the host material. Compound 124 enables photochemical pitch modulation of induced CLCs so that their selective reflection color can be tuned. However, it brings about light-driven handedness inversion in a specific nematic host. It induced a right-handed CLC, which upon photoirradiation changes to a left-handed CLC. The reverse process can be executed by changing the energy of the light source. Similarly, another compound 125 with axial chirality was synthesized, and it was found to enable handedness inversion in induced photoresponsive CLCs.468 Such chiral dopants which bring about light-driven handedness inversion in CLCs are highly sought after compounds, for handedness inversion is a chance event and rare phenomenon but possess great implications in optics and photonics. Compounds 126 and 127 were designed and synthesized which show not only high HTP values but also a great change in their HTP values upon photoisomerization (Scheme 45).469 These traits are hugely beneficial since a small quantity of material is needed to induce CLCs and wide color tunability can be achieved. Wide wavelength tunability in induced CLCs containing these materials have been used to realize red, green, and blue (RGB), the three primary colors, in thin films. Moreover, by using a photomask technique, RGB colors in a single film with good color domain resolution have been

a, R = C8H17; b, R = C10H21; c, R = C12H25; d, R = C14H29.

an azobenzene moiety through a flexible spacer and found that these compounds have better solubility in nematic host LCs.470 These compounds neither show high HTPs nor large variation in HTP values as chiral dopants. Nevertheless, light-driven reversible cholesteric to isotropic phase transition was demonstrated by using these compounds. Subsequently, our group extensively elaborated on the synthesis of axially chiral azobenzene-based molecular switches and comprehensively evaluated their dopant characteristics by fabricating photoresponsive CLCs.471−486 The chemical structures of the early systems 129 developed are shown in Scheme 46. These compounds possess both high HTP values and exhibit large variation in their HTP values upon photoisomerization. Some of these compounds have been employed in the fabrication of chiral photodisplays, which do not require driving electronics usually used in conventional electrooptic LC displays. Devoid of the driving electronics drastically brings down the cost of such displays and makes them lightweight. Such photodisplay has been depicted in Figure 38.472 Light-driven selective reflection tuning of more than 2000 nm of a CLC in a single cell has been achieved. Lasing and hyper reflectivity have been demonstrated with the help of these chiral compounds. Cholesteryl group functionalized axially chiral azoarenes 130 (Scheme 47) have been designed and synthesized to test the effect of both tetrahedral and axial chirality on HTP. It was indeed found that these compounds exhibited very high helical twisting powers and large variation in their HTP values upon photoisomerization. As a consequence, wide range AE


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compounds displayed greater compatibility with the nematic hosts and exhibited exceptionally high HTP values. Electrically switchable, photoaddressable CLC reflectors have been developed by using these materials. Wide range nonmechanical beam steering has been demonstrated by using a cholesteric liquid crystalline film containing one of these materials (Figure 39).474 Other possibilities for these materials have also been assessed. Lasing has been demonstrated in cholesteric liquid crystalline films by using these compounds as light-driven chiral dopants. By combining upconversion nanoparticles (UCNPs) with a CLC induced by chiral switch 131e, near-infrared lightdriven pitch modulation has been achieved (Figure 40).476 Interestingly, red-shifting and blue-shifting of the reflection wavelength was observed just by changing the power density of the near-infrared light source. Similarly, compounds 132 (Scheme 48) also showed better solubility in nematic hosts and exhibited high HTPs. In addition, the large variation in the HTP values upon photoisomerization of these compounds enabled reversible color tuning across the entire visible spectrum from cholesteric liquid crystalline films (Figure 41).478 These compounds were also used to obtain static and dynamic RGB colors in thin cholesteric liquid crystalline films. The static RGB colors here correspond to the selective reflections of cholesteric liquid crystalline films at their photostationary states. Additionally, multistimuli-switchable and photoaddressable displays were demonstrated using these compounds as chiral dopants in CLCs (Figure 42).480 Recently, cholesteric microshells were fabricated using compound 132 as the dopant and omnidirectional lasing was demonstrated from the microshells (Figure 43).481 Compound 133 deserves special mention, as this compound can be photoisomerized by using visible light in both directions and hence there is no need of UV light.483 Besides, this compound possesses high HTP and large difference in HTP values in different states. As a result, reflection colors spanning across the entire visible region has been demonstrated by visible light irradiation (Figure 44). This observation is profound as visible light can be used to control the wavelength of visible light reflection. Azoarenes 134 (Scheme 49) containing multiple axially chiral binaphthyl moieties with opposite handedness have been synthesized to examine if the intramolecular chiral conflict can render handedness inversion capability to induced CLCs fabricated by using these compounds. Indeed it was found that these compounds cause handedness inversion of the CLCs fabricated from different nematic hosts upon photoirradiation. Schematic illustration of the chiral contributions of opposite handed axially chiral groups during this light-driven handedness inversion process is depicted in Figure 45.485 Following this idea, hydrogen-bonded azoarenes containing opposite chirality have been synthesized and found that these compounds can be used for pitch modulation of CLCs, and compound 135 did enable light-driven handedness inversion of induced CLCs. Cyclic azobenene derivatives (Scheme 50) with axial and planar chirality have been designed, synthesized, and studied as light-driven chiral doapnts in the fabrication of photoresponsive induced CLCs. Compound 136 exhibits high HTP and causes light-driven handedness inversion in CLCs fabricated from three different nematic hosts. Similarly, compound 137 is able to confer handedness inversion capability to induced CLCs.487−490 Compounds 138−141 (Scheme 50) have been evaluated as light-driven chiral dopants in the induction and modulation of CLCs. These compounds were found to possess very high HTPs and exhibit large differences in their HTPs upon photo-

Figure 38. Photodisplays fabricated from photoresponsive CLCs: a flexible photodisplay (top) and a conventional electrically addressed display with attached bulky and costly electronics (bottom, left) compared with a photodisplay showing the same image without the added electronics (bottom, right). Reproduced with permission from ref 472. Copyright 2008 Society for Information Display.

Scheme 47. Axially Chiral Azoarenes Showing High HTP and Large Variation in HTP Values Developed by Our Group

reflection colors have been obtained from CLC thin films containing these chiral dopants. Some breakthroughs came along when compounds 131 functionalized with rodlike moieties were synthesized and investigated as chiral dopants. Since the rodlike units in these chiral dopants resemble the molecular structure of nematic hosts made of rod-shaped molecules, it was found that these AF


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Figure 39. Wide range nonmechanical beam steering by a cholesteric liquid crystalline film driven by light. Reproduced with permission from ref 474. Copyright 2014 Wiley-VCH.

46).489 Compound 144 showing high HTP value has enabled RGB reflection colors from CLC thin films. Recently, compound 143 has been synthesized which possesses an exceptionally high HTP value. Reversible tunning of RGB reflection colors as well as micro glass rod motion on CLC surface has been achieved with the help of this compound at very low doping concentrations. Bunning group demonstrated both red-shifted and blueshifted reflection tuning in different CLCs doped with azobenzene-based photochromic compound.491 The photoinduced states in these materials were found to be stable for a long time following irradiation withdrawal. By using continuous wave and nanosecond laser beams, the ability to write information on such cholesteric liquid crystalline films has been demonstrated. CLCs obtained from azobenzene-based nematic hosts by doping light insensitive chiral dopants with high HTPs has been observed to exhibit a widely tunable reflection notch. Reversible tuning of laser wavelengths has been achieved in dye-doped CLC lasers made of azo compounds 145.492 Lightdriven reflection wavelength tuning and cholesteric to nematic phase transition has been reported for CLCs doped with axially chiral azobenzene-based dopant 146 (Scheme 51).493,494 Both rotational and translational motion of microscale objects have

Figure 40. Upconversion-nanoparticle-doped reflection color tuning of CLCs. Reproduced from ref 476. Copyright 2014 American Chemical Society.

isomerization. As a result, full color tuning has been achieved in different CLCs using these compounds as chiral dopants. Some of these compounds can even be driven only by visible light. Moreover, light-driven microscale object rotation has been demonstrated on cholesteric liquid crystalline films (Figure AG


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Scheme 48. Chemical Structures of Axially Chiral Azoarenes Developed by Our Group

Figure 41. Light-driven reflection color tuning across the entire visible spectrum. Reproduced with permission from ref 478. Copyright 2010 The Royal Society of Chemistry.

Azobenzene-based chiral compounds 148 and 149 (Scheme 52) with thermally stable photoinduced cis isomers have been synthesized and characterized. These compounds have also been used as chiral dopants to produce light-driven CLCs with tunable pitch and stability.495,496 Compound 149 along with a nonphotosensitive codopant has been used to induce a photoresponsive CLC. This CLC exhibits light-driven handedness inversion of its helical superstructure. Light-driven rotatable diffraction gratings have been demonstrated using these CLCs in hybrid LC cells. These gratings display clockwise or counter clockwise rotation depending on the irradiation light wavelength. Light-driven generation of rotatable diffraction gratings from lefthanded and right-handed CLCs is shown in Figure 48.497 A cholesteric liquid crystalline polymer composite was developed by using compound 150 as the chiral dopant and a liquid crystalline polymer. The polymer film exhibited light-driven reflection color tuning across the entire visible region, which has enabled color image recording by using the photomask technique. Surface relief gratings have been developed from the films of compound 151 by the photoirradiation process. Design, synthesis, and studies of azobenzene-based chiral dopants in the development of light-driven CLCs are in continuous development along with their different applications.498−508 Moreover, stimuli-responsive dual frequency CLCs are being developed and investigated.509,510

Figure 42. Light-, electric-field-, and mechanical-pressure-driven CLC color photodisplays: (top) schematic illustrations of cholesteric textures; (middle) demonstration of an image; (bottom) POM textures. Reproduced with permission from ref 480. Copyright 2011 Wiley-VCH.

been demonstrated recently on cholesteric liquid crystalline films by using 147 as the light-driven chiral molecular switch. The translational motion of the micro object could be controlled by the irradiation position (Figure 47). AH


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Figure 43. Schematic illustrations of the structure of the photoresponsive monodisperse CLC microshell as a water−oil−water double emulsion (left). Phototuning of the pitch (middle) and confocal microscope images (right) of the CLC microshells. Reproduced with permission from ref 481. Copyright 2014 Wiley-VCH.

Figure 44. Visible-light-driven reflection color tuning in cholesteric liquid crystalline films. Reproduced from ref 483. Copyright 2012 American Chemical Society.

5.1.2. Overcrowded Alkenes in Cholesteric Liquid Crystals. Overcrowded alkenes with helical molecular confomations have been investigated in the context of light-driven chiral dopants toward the induction of CLCs. Feringa group introduced such interesting materials as light-driven molecular switches or motors depending on the photoisomerization pathways, which is partly determined by the substituents present around the carbon−carbon double bond. For some, the developed compounds have been found to possess very high HTPs and display large changes in their HTP values upon photoisomerization.511−523 This has enabled wide reflection band tuning in induced CLCs with the help of small quantities of dopants. However, the most appealing characteristic of these kinds of materials is that upon photoisomerization, they undergo molecular helicity inversion. That means the photoisomers show the opposite sign for their HTP values, consequently the induced CLCs are able to exhibit handedness inversion upon photoirradiation. The Feringa group has synthesized and studied the

Figure 45. Schematic mechanism of handedness inversion in CLCs containing 134. Reproduced with permission from ref 485. Copyright 2013 Wiley-VCH.

CLC induction properties of compounds 152 and 153, (Scheme 53) and found that their HTPs are not very high.514−516 However, these compounds leveraged light-driven handedness inversion to the induced CLCs so that reversible handedness inversion was achieved by altering the irradiation wavelength of light. In another study, CLC of specific handedness was introduced by irradiating the LC mixture containing racemic 154 with CPL.515 This was the result of photoresolution and enrichment of one isomer of the compound depending on the chirality of the CPL. Therefore, by altering the chirality of the CPL, the handedness of the induced CLC was inverted.

Scheme 49. Azoarenes with Opposite Chirality Enabling Handedness Inversion in CLCs

AI


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Scheme 50. Axial and Planar Chiral Cyclic Azobenzenes

Figure 46. Optical micrographs showing the rotation of a glass rod on the surface of a chiral nematic liquid crystalline film formed by doping 139. Rotation of a glass rod upon irradiation at (a−d) 366 nm and (e−h) 436 nm recorded at intervals of 15 s. Reproduced with permission from ref 489. Copyright 2012 Wiley-VCH.

Scheme 51. Chemical Structures of Light-Driven Chiral Dopants 145−147

Figure 47. Optical micrographs of translational motion of a glass rod on the surface of a liquid crystalline film doped with 147. The film was irradiated with UV/vis light from the right side. (a) Initial position of the rod. (b) Upon irradiation with UV light (365 nm), the glass rod moved to the right side, that is, toward the irradiation position. (c) The glass rod moved to the left side, that is, in the opposite direction, upon visible light (436 nm) irradiation. The length of the rod was 30 mm and diameter of the rod was about 7 mm. Reproduced with permission from ref 493. Copyright 2011 Wiley-VCH.

was found that upon light irradiation this compound exhibits unidirectional rotary motion (i.e., one part of the molecule (rotor) undergoes 360 °C rotation about the C−C double bond with respect to the other part (stator) of the molecule).511 This fascinating compound, regarded as a molecular motor, was used

Compound 155 (Scheme 54) was synthesized, and its photoisomerization property was evaluated. Interestingly, it AJ


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Scheme 52. Chemical Structures of Azobenzene-Based Chiral Compounds with Thermally Stable Cis Isomer and Others

Scheme 53. Overcrowded Alkene-Based Molecular Switches

as a chiral dopant to induce photo responsive CLCs. Upon light irradiation, the cholesteric liquid crystalline films display tunable color over the entire visible spectrum. This color tuning has been enabled by the rotary motion of the compound 155 at the molecular level, which imparts molecular reorganization in the cholesteric liquid crystalline film. Later on compound 156 (Scheme 55) was designed and synthesized.512 This molecular motor as a chiral dopant has some remarkable attributes. It has a fluorine unit which acts as the stator part, and its structure resembles the molecular structures of commonly used nematic hosts for the fabrication of induced CLCs. Therefore, this compound has better compatibility in the LC matrix which contributes toward the high HTP of this

material. Moreover, in addition to molecular helicity, it possess a stereogenic center at the rotor part. This combined effect of chirality confers special characteristics to this compound as as light-driven molecular motor. This compound is known to cause handedness inversion of CLCs upon photoisomerization. However, the photoisomer is observed to be not stable thermally and quickly relaxes back to the initial state. This light-driven handedness inversion capability and thermal relaxation of the compound following light irradiation has enabled rotation of a microscale object on a cholesteric liquid crystalline film in opposite directions (Figure 49).520 This is a classic example of conversion of light energy to mechanical work enabled by a nanoscale molecular motor. Owing to its high HTP and large

Figure 48. POM images of (a) right-handed and (b) left-handed CLC gratings in a hybrid LC cell. The light-driven rotation of a grating versus irradiation time is shown in the bottom panel. Reproduced with permission from ref 497. Copyright 2015 Wiley-VCH. AK


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detailed experimental and theoretical investigation have been undertaken to understand and account for the promising observations made in CLC with this compound as the chiral dopant. The ability of this compound to cause handedness inversion in CLCs fabricated from different nematic hosts has been examined by using different techniques. Light-driven surface relief grating formation on polymer films has been demonstrated using this molecular motor. Recently, studies on the thermal relaxation kinetics of this compound and its derivatives have been elaborated. Polyisocyanate-based helical polymers forming lyotropic LC phases have been functionalized with an overcrowed alkenebased light-driven molecular switch 157.517 This functionalization breaks the symmetry and enables the occurrence of chiral nematic phase in the solutions of polyisocyanates. However, upon light irradiation with suitable wavelength, the chiral nematic phase undergoes handedness inversion. Reversible handedness inversion can be dynamically carried out in the system by changing the light irradiation wavelength. The process of handedness inversion is shown in Figure 50. Chen et al. have synthesized suberene-based compounds 158 and 159 (Scheme 56). Photoisomerization studies have revealed that these compounds upon isomerization change their molecular helicity.524,525 Owing to this helicity inversion, these compounds have been found to endow handedness inversion to induced CLCs in addition to their pitch modulations. Compound 158 can be selectively addressed by choosing the light wavelengths corresponding to the two different photoisomerizable groups present in the molecule (i.e., either the azo or the C−C double bond can be independently addressed by light). 5.1.3. Dithienylcyclopentene Derivatives in Cholesteric Liquid Crystals. Dithienyl cyclopentene derivatives as chiral dopants were introduced by the Feringa group. They studied the induction of photoresponsive CLCs by doping compound 160 (Scheme 57) into a nematic host. Light-driven modulation of these CLCs was carried out.526,527 Upon photocyclization of the dopant, the CLC shows pitch variation and even cholesteric to nematic phase transition has been documented. These events are a result of the different HTPs of the open and closed forms of the chiral dopant. Similarly, compound 161 was used as a chiral light-driven molecular switch in the induction of CLC phase. Reversible photocontrol over the pitch of the induced CLC was demonstrated through irradiation

Scheme 54. Chemical Structure of the Molecular Motor 155 and the Light-Driven Rotation Process

Scheme 55. Structure of Molecular Motor 156 and Molecular Switch 157a

a

a, R = Ph; b, R = Me; c, R = iPr.

change in its HTP value upon photoisomerization, full color tuning across the visible region has been achieved. Subsequently,

Figure 49. Rotational reorganization of a cholesteric liquid crystalline film which causes rotation of a microscale object on its surface. Reproduced with permission from ref 520. Copyright 2006 Nature Publishing Group. AL


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Figure 50. Schematic illustration of chirality inversion in the lyotropic CLC made from polyisocyanate polymers terminally functionalized with a molecular switch. Reproduced from ref 522. Copyright 2008 American Chemical Society.

Scheme 56. Suberene-Based Chiral Dopants

Scheme 57. Chiral Dithienylcyclopentene Dopants Developed by the Feringa Group

into the chiral nematic phase by shining visible light. Due to good fatigue resistance of the dopant, CLC to the nematic reversible transition could be repeated many times. Similarly, the CLC induction capability of the cyclohexane derivative was studied. It was found that the CLC phase can be induced either by using a large quantity of the dopant or by a small quantity of dopant followed by photoirradiation. This observation has been attributed to the increase in HTP of the dopant upon photocyclization. Reversible CLC to nematic transition could be carried out by using UV and visible light irradiation. Later, axially chiral diethienylcyclopentene dopants 164 and 165 were synthesized and studied. These compounds enabled large pitch changes in induced CLCs. Thermally irreversible photochromic property of these dopants was investigated by the circular dichroism technique. The above-discussed chiral dopants though exhibited interesting behavior: their HTPs were not very impressive. In order to overcome this bottleneck, our group has undertaken an extensive exercise in the design, synthesis, and study of the CLC induction behavior of dithienyl cyclopentene dopants.532−540 During this endeavor, chiral dopants with high HTPs showing large difference in HTPs in different forms and capability of

with UV and visible light. Moreover, it was found that the open form of this dopant possesses higher value of HTP than its closed photoisomer; the difference in HTPs has been attributed to the molecular structure flexibility. The Irie group designed, synthesized, and studied the CLC induction properties of chiral dithienylcyclopentene-based compounds 162−165 (Scheme 58).528−531 Compound 162 containing two cholesteryl groups was tested as a chiral dopant to induce the CLC phase. The CLC phase upon UV light irradiation transformed into a nematic phase which was then converted back AM


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Scheme 58. Chiral Dithienylcyclopentene Dopants Developed by the Irie Group

Scheme 60. Axially Chiral Dithienyl Cyclopentene Compounds Developed by Our Group

causing handedness inversion in induced CLCs have been realized. Compounds with a dithienyl cyclopentene moiety flanked by two axially chiral binaphthyl groups were synthesized (Scheme 59) and their properties were evaluated. These Scheme 61. Axially Chiral Dopants with Bridged Binaphthyl Moieties Developed by Our Group

Scheme 59. Chemical Structures of Axially Chiral Dopants Developed by Our Group

compounds were found to exhibit very high HTPs. Light-driven reflection notch tuning was studied in CLC using these compounds as chiral molecular switches. Compounds with axially chiral binaphthyl units directly attached to a central dithienylcyclopentene moiety were designed and synthesized. Compounds with different linking positions have been realized to study their CLC induction behaviors. These compounds depicted in Scheme 60 were found to have high HTPs. Therefore, light-driven pitch modulation of CLCs was investigated by using these thermally stable fatigueresistant dopants. Encouraged results of the above compounds led us to synthesize chiral dopants 171 and 172 (Scheme 61) with bridged binaphthyl groups. Bridged binaphthyl groups were chosen due to their known powerful helicity induction capabilities. As anticipated, these compounds exhibited exceptionally high HTP values in different nematic hosts. In addition, the HTP difference between the open and closed forms of the molecular switches was found to be very large. The large difference in HTPs has enabled reflection color tuning across the entire visible spectrum. Moreover, RGB colors devoid of thermal relaxation were demonstrated in single cholesteric liquid

crystalline films (Figure 51).535 Owing to the thermal irreversibility of the dopants, such light-driven photodisplays are stable with respect to temperature. Recently, we fabricated microdroplets from CLCs induced by the chiral dopant 171a and achieved omnidirectional selective reflection of circularly polarized light from these microdroplets.536 Interesting optical cross communication was observed from arrays of microdroplets. Taking advantage of thermally stable reflection colors of the microdroplets, color patterns were realized by carefully addressing selected areas and even to the level of individual droplets. These spectacular observations are reproduced in Figure 52. Compound 171d with a flexible tetramethylene bridge in the binaphthyl units enables not only pitch modulation but also AN


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any arbitrary light irradiated state on account of the thermal irreversibility of the chiral dopant. Moreover, using this lightdriven CLC, wide range 2D beam steering has been demonstrated (Figure 56). The facile 3D control over the helical axis of the CLC has also been utilized in a specially engineered bilayer LC cell to obtain 2D diffraction gratings. Interestingly, reversible transformation between 2D to 1D and 1D to zero-dimensional (off state) diffraction states have been achieved by using UV and visible light irradiation (Figure 57).540 It is interesting to note that the initial and final 2D gratings are formed by CLCs with opposite handedness. The 3D control of the helical axis could be useful in advanced optical and photonic devices. Compounds 173 (Scheme 62) have been synthesized and its CLC induction properties have been studied.541 It has been observed that different members of the compounds are able to show distinct behavior. Some members enable light-driven reversible handedness inversion in CLCs, while other members either cause pitch modulation or CLC to nematic phase transition as depicted in Figure 58. We have designed and synthesized compound 174 to study its CLC induction capability. These compounds exhibited extremely high values of HTPs. Photocyclization results in large variation in their HTP values. All the three primary colors were obtained by tuning the pitch of the CLCs by light irradiation. A CLC-based photodisplay with image recording has been demonstrated by using one of these compounds as a light-driven chiral molecular switch. 5.1.4. Fulgide-Based Chiral Dopants in Cholesteric Liquid Crystals. CLCs that do not show fatigue during lightdriven reversible pitch modulation was reported by the Schuster group by using chiral fulgide 175 (Scheme 63) as CLC inducers.542,543 Such systems are solely driven by light due to thermal irreversibility of the photochromic dopants. The hexatriene group under UV irradiation undergoes cyclization to yield a cyclohexadiene unit in the compound, whereas the reverse process can be driven back by visible light irradiation. The HTP values are different for the open form and closed form which is the basis of light-driven pitch modulation in CLCs. Yokoyama et al. synthesized fulgide 176 with enhanced HTP by covalently linking the axially chiral binaphthyl moiety.544 The pitch modulation of a CLC was investigated by using this compound as the light-driven chiral dopant. They subsequently demonstrated light-driven handedness inversion in a CLC by

Figure 51. Photodisplays with thermal stability. Thermally stable RGB reflection colors in cholesteric liquid crystalline films containing 171a. Reproduced with permission from ref 535. Copyright 2012 American Chemical Society.

handedness inversion of the helical superstructures of CLCs fabricated from different nematic hosts. The reason for this special characteristic of this dopant has been clarified with help of density functional theory calculations of the open and closed ring isomers. The process of light-driven handedness inversion by UV and visible light irradiation is shown in Figure 53.537 Recently, to avoid the use of high energy UV and visible radiation and take advantage of the beneficial characteristics of near-infrared light, we explored the possibility of using near-infrared light to drive handedness inversion and pitch modulation in such thermally stable CLCs. To accomplish this forbidden task, we used upconversion nanoparticles as nanotransducers which are known to efficiently convert low energy radiation into high energy radiation. Using near-infrared laser, we demonstrated handedness inversion in UCNPs impregnated CLCs as illustrated in Figure 54.538 Very recently, we demonstrated an unprecedented phenomenon of light-driven three-dimensional (3D) control of the helical axis of a CLC using compound 171d as the chiral dopant. UV irradiation of a CLC sample in planar state undergoes handedness inversion and yields the planar state of opposite handedness. Upon continuous irradiation with UV light, the standing helix transforms to a lying state and the lying helix rotates in the plane of the LC cell until the system reaches its photostationary state. The system can be driven back in the opposite direction by visible light irradiation. Thus, 3D control over the helical axis was achieved (Figure 55).540 Interestingly, the system can be reversibly driven forward or backward from

Figure 52. Photonic communication in the monodisperse 3D cholesteric microdroplets. POM images and photonic cross-communications resulting linear-, triangular-, diamond-shaped and “lit firecracker” patterns in the cholesteric microdroplet arrays. “Flower-opening” patterns of microdroplets with light-driven iridescent colors. Reproduced with permission from ref 536. Copyright 2015 Wiley-VCH. AO


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Figure 53. Demonstration of helix inversion in cholesteric liquid crystalline films containing 50d in (a−e) a wedge cell and (f−j) a homeotropic cell upon UV irradiation and (k−o) schematic illustrations of the corresponding LC phases. The inset in (h) shows a conoscopic observation. Reproduced with permission from ref 537. Copyright 2013 Wiley-VCH.

Figure 54. Schematic mechanism of wavelength-selective near-infrared (NIR) light triggered reversible handedness inversion of the selforganized helical superstructure incorporated with chiral dithienylcyclopentene switch 171d and core−multishell nanotransducers. Reproduced with permission from ref 538. Copyright 2015 Wiley-VCH.

using a photoinsensitive codopant with compound 176. Recently, compound 177 was synthesized and its CLC induction properties were studied.545 This compound enables optically reconfigurable color-stable cholesteric liquid crystalline films. Due to thermal irreversibility of the dopant, long-lasting reflection colors at arbitrary spectral positions have been demonstrated. 5.1.5. Bicyclic and α,β-Unsaturated Ketone-Based Chiral Dopants in Cholesteric Liquid Crystals. A series of axially chiral bicyclic ketones with exocyclic doule bond was synthesized by Schuster group and was tested as light-driven chiral dopants.546,547 Upon photoirradiation, these compounds undergo photoracemization; however, by using circularly polarized light partial resolution can be established. Therefore, a CLC phase was induced by using compound 178 (Scheme 64) as the chiral dopant and circularly polarized light as the resolving agent. Later the induction of a CLC was reported by using racemic acrylic ester 179 and irradiation with CPL. α,βUnsaturated ketones 180−185 have been reported to act as light-driven chiral dopants owing to their trans−cis photoisomerization about the C−C double bond. Reversible photomodulation of pitch has been observed by light irradiation to the

Figure 55. Schematic illustration of the light-driven 3D control over the helical axis of a CLC containing 171d. Adapted with permission from ref 540. Copyright 2016 Macmillan Publishers Limited.

CLCs induced by these dopants.548−553 Even light-driven reversible handedness inversion has been achieved by judiciously combining some of these dopants with other codopants. 5.1.6. Cinnamate- and Spirooxazine-Based Chiral Dopants in Cholesteric Liquid Crystals. Various cinnamate-based chiral dopants 186−189 (Scheme 65) have been synthesized, and their CLC induction capabilities have been evaluated.553−555 Light-driven pitch variation has been noticed in CLCs containing these dopants. Cholesteric gratings have been fabricated by using cinnamate based light-driven chiral dopants. Chiral spirooxazines have been designed and synthesized (Scheme 66) to study their CLC induction properties.556 Although their HTP values are moderate, they exhibit interesting light-driven phenomena. AP


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Scheme 62. Chemical Structures of Chiral Compounds 173 and 174

Figure 56. Light-controlled 2D in-plane beam steering for spectrum scanning resulting from simultaneous in-plane rotation and pitch modulation of the CLC in the lying state. Reproduced with permission from ref 540. Copyright 2016 Macmillan Publishers Limited.

5.2. Photomodulation of Blue Phases

The blue phases (BPs) exist between the isotropic liquid phase and cholesteric LC phase of highly chiral materials. There are three distinct blue phases namely BP I, BP II, and BP III. The BP I has a body-centered cubic, whereas BP II exhibits simple cubic symmetry. The lattice periodicity of cubic blue phases is of the order of the wavelength of visible light which results in fascinating optical properties of these fluid lattices. They have been regarded as soft self-assembled three-dimensional (3D) photonic bandgap (PBG) materials.557,558 Recently, blue phases have been induced by doping achiral bent-core molecules into CLCs.559−566 Chanishvili et al. introduced photoresponsive blue phases and studied photomodulation of their properties.564 They used host

nematic materials, which undergo trans−cis photoisomerization. Two different chiral dopants were added to these nematic host materials, and blue phases were fabricated and photomodulation of their properties were investigated. It was observed that the selective reflection of the blue phases can increase or decrease upon exposure to UV light. Blue phase to blue phase transitions were also registered. Interestingly, a blue phase was induced by UV irradiation to a system which in itself did not exhibit a blue phase at all. It should be noted that this was the first observation of blue phase induction by trans−cis photoisomerization. Chiral diphenylbutadiene mesogen 193 (Scheme 67) undergoes an isothermal phase transition from the chiral smectic A

Figure 57. Light-driven transformation between 2D, 1D, and off state of diffraction pattern. Reproduced with permission from ref 540. Copyright 2016 Macmillan Publishers Limited. AQ


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Scheme 64. Bicyclic and Unsaturated Ketones Acting as Chiral Dopants

Figure 58. Polarized optical micrographs (POMs) of photoresponsive N*-LCs between the open form (left) and PSS (right) by light irradiation. Along the corresponding POMs are described schematic illustrations of (a) reversible helical inversion between N*-LCs with opposite screw senses, (b) reversible phase transition between N*-LC and N-LC, and (c) reversible change in helical pitch within N*-LC upon alternating irradiations of UV (left) and visible (right) light. Reproduced from ref 541. Copyright 2012 American Chemical Society.

Scheme 63. Fulgide-Based Chiral Dopants

Scheme 65. Cinnamate-Based Chiral Dopants

phase to a blue phase upon photoisomerization. This is the first observation of photoinduced blue phase in a single component liquid crystalline material.561 The blue phase was stable over a wide temperature range, which facilitates tuning of its Bragg reflection just by varying the photoirradiation time. Blue phases were induced in a three component mixture containing a nematic host, a photoinsensitive chiral dopant, and an azobenzene derivative 194. Here the photonic band gap of the blue phase was tuned by trans−cis photoisomerization of the azobenzene derivative. Upon photoirradiation, the blue phase displays shift in its photonic bandgap. Owing to the reversibility of the trans− cis isomerization, the bandgap of the blue phase could be tuned reversibly. Red green and blue reflection colors were demonstrated in addition to an optically addressable blue phase display. The blue phase display can be reproducibly written, erased, and rewritten. We reported a wide tuning of the photonic bandgap of a blue phase enabled by an axially chiral azobenzene dopant 131d. The photonic bandgap was tuned over red, green, and blue wavelengths, and the system here also exhibited the unique

phenomenon of order increasing phase transition from BP II to BP I upon photoirradiation. By adjusting the composition of the blue phase LC system, it was shown that the photonic bandgap can be reversibly tuned across the entire visible region of the spectrum in a single nanostructure film (Figure 59).559 Photoinduced lattice transformations were observed by investigating the resulting Kossel diagrams of the blue phases. Similarly, a hydrogen-bonded chiral azobenzene dopant 195 was synthesized to induce the blue phase. This compound was found to be capable of reflection wavelength tuning of the induced blue phase over the visible wavelength region in a reversible manner. Interestingly, the temperature range of the blue phase was observed to increase in this study. The effect of photoirradiation on the blue phase is shown in Figure 60.563 AR


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Scheme 66. Spirooxazine-Based Chiral Dopants

irradiation conditions. The plausible reason for this counterintuitive observation has been clarified. Photoinduced blue phase has also been obtained by doping an azobenzene derivative 198 containing two cholesteryl groups as substituents into a CLC phase. Here upon UV light irradiation, the CLC phase transforms into a blue phase. The bent cis isomer of the azobenzene dopant is thought to induce the blue phase in this system. Spectroscopic observations suggest that the photoinduced blue phase exists over certain degree of photoisomerization, and a low concentration of the dopant is sufficient to induce the blue phase. A cholesterol and azobenzene couple 199 has been synthesized which is able to induce blue phase into a CLC upon photoirradiation.566 The effect of change of concentration of this compound on the phase transition temperature of the blue phase has been studied and a simple pattern with the blue phase and isotropic phase arranged in an interval was demonstrated by using a suitable photomask. Recently, we demonstrated near-infrared light-driven blue phase to blue phase and blue phase to isotropic phase transitions with the help of the photothermal effect of gold nanorods.567 Gold nanorods with longitudinal surface resonance in the nearinfrared region was synthesized and employed as nanoheaters to effect phase transitions by efficiently converting near-infrared light into heat. The process of near-infrared light-driven phase transitions is presented in Figure 61.

Scheme 67. Blue Phase Inducing Chiral Dopants

The induction and stabilization of blue phases has also been achieved by light irradiation to a bent-core chiral nematic phase formed by compound 196 containing a photoactive achiral bentcore compound 197 (Scheme 68).565 The achiral compound contains an azo-linkage in one of its wings and adopts a cis form upon photoisomerization. During photoirradiation, the CLC phase transforms into a blue phase. This is a convenient method to induce the blue phase. It has been reasoned that the cis isomers cause disordering of the LC phase and phase separation into the defect lines of the blue phase and reduce elastic deformation energy within the defects and thus stabilizes the induced blue phase over a wide temperature range. It is worth noting that the blue phase was stable upon switching off the photoirradiation and did not go back to the CLC phase under thermal or visible light

5.3. Photomodulation of Ferroelectric Liquid Crystals

Ferroelectric LC phases exhibit spontaneous polarization. Due to this spontaneous polarization, they are able to respond fast to applied electric fields. Because of this property, they have been adopted in fast responsive microdisplays as active switching components. FLC shows bistable switching (i.e., upon reversal of the electric field the direction of spontaneous polarization inverts). Recently, photoresponsive FLCs have been fabricated and studied.568−580 Ikeda et al. have demonstrated photochemical switching of the spontaneous polarization of FLC by doping a FLC with a photochromic compound capable of AS


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Figure 59. Light-driven RGB colors from photoresponsive blue phases. Reproduced with permission from ref 559. Copyright 2013 Wiley-VCH.

Figure 60. Effect of photoirradiation on the lattice constant of a blue phase. Reproduced with permission from ref 563. Copyright 2014 The Royal Society of Chemistry.

undergoing trans−cis photoisomerization.574 It was noticed that isomerization of the photochromic compound changes the switching potential of the FLC host. Therefore, by applying two different electric fields in opposite direction, the polarization can be switched at different field strength. Moreover, the photoswitching process was found to be fast (hundreds of microseconds), reversible, and highly reproducible. Subsequently, they unraveled the effect of change of different experimental parameters on photochemical switching of the spontaneous polarization on the irradiated site of the FLC sample.572,573 Much faster response times were achieved by using a chiral azobenzene dopant. The properties of ferroelectric LCs were controlled by using photochromic azobenzene derivatives. The effect of the structure of photoresponsive guest compounds on polarization switching by photochemical means was investigated specifically. Guest compounds with similar chemical structure to ferroelectric host compounds were found to be very effective in polarization switching. Since the spontaneous polarization of the ferroelectric LC could be switched photochemically in addition to electric fields, this study opened up new possibilities for the applications of ferroelectric LCs.

Komitov et al. demonstrated an optical recording, employing photochromic FLC 200 doped with an azobenzene-based dopant 201 (Scheme 69).580 They were able to record a high contrast image and invert its contrast just by reversing the polarity of the applied field. Interestingly, the optically recorded image can be stored under bistable boundary conditions even after the electric field is withdrawn. Additionally, it was shown that for ferroelectric liquid crystalline materials that do not change sign of spontaneous polarization with temperature can be reversed by light which could lead to temperature-independent FLC devices. A chiral azobenzene compound 202 exhibiting a chiral LC smectic C phase was studied under photoirradiation.571 The ferroelectric properties of the compound showed strong dependence on the light intensity. The increase in the intensity of light which brings about trans−cis photoisomerization of the mesogens causes a decrease in the order parameter, director tilt angle, and the magnitude of spontaneous polarization. A change of the nature of the ferroelectric transition was also noticed. The above observations were interpreted by taking into account the AT


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However, the cis isomer which results upon photoirradiation is polar and possesses a transverse dipole moment. This transverse dipole moment can couple with the spontaneous polarization of polar LC phases such as FLCs. Accordingly, the transverse dipole moment of a thioindigo-based chiral dopant was harnessed for the photomodulation of the spontaneous polarization of an induced FLC (Scheme 70).569,570 The thioindigo derivative

Scheme 68. Chiral Dopants Used for the Fabrication of Photoresponsive Blue Phases

Scheme 70. Thioindigo-Based Chiral Dopants

which undergoes photoinduced trans−cis isomerization contains both polar and chiral substituents, which are expected to couple efficiently with the host molecules of FLC 204. The dopant almost doubled the magnitude of the spontaneous polarization upon visible light exposure without significant destabilization of the FLC phase. Similar phenomenon were also documented for the chiral dopant 203. This compound exhibits improved photomodulation and was found to be better compatible in the LC matrix. An ambidextrous chiral thioindigo dopant 206 was designed, and its photomodulation behavior was investigated.576,578,579 It was found that this compound brings about photoinduced polarization inversion of a FLC due to the presence of competing chiral side chains that promote spontaneous polarization of the opposite sign as shown in Figure 62.579

Figure 61. Near-infrared light-driven phase transitions in blue phases. Reproduced with permission from ref 567. Copyright 2015 The Royal Society of Chemistry.

Scheme 69. Photoresponsive Compounds Exhibiting FLCs

intermolecular coupling that arises due to the photogeneration of bent-shaped cis-isomers. Thioindigo is a photochromic scaffold which can undergo photoisomerization and relax back thermally. The trans isomer of thioindigo-based symmetrical chromophores are nonpolar.

Figure 62. Photoswitching the polarization of FLCs. Reproduced from ref 579. Copyright 2003 American Chemical Society. AU


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Subsequently, the unsymmetrical chiral thioindigo dopant was found to cause photoinduced sign inversion of the spontaneous polarization of FLCs. The sign inversion was ascertained experimentally by photoswitching surface-stabilized FLCs. Moreover photoinduced phenomena in FLC mixtures containing 204, 205, and 206 have been investigated. This approach shows that it is possible to manipulate chiral bulk properties of LCs by controlling them at the molecular level. Photochemically bistable FLC photoswitching has been demonstrated by using a photochromic dithienylcyclopentene dopants 207−209 (Scheme 71).568,575 Reversible photomodu-

Scheme 72. Achiral Compounds Capable of Light-Driven Chirality

Scheme 71. Dithienylcyclopentene-Based Dopants for the Photomodulation of FLCs

CPL irradiation of a thin polymer film doped with a W-shaped photochromic compound 213 containing two azobenzene groups has been found to exhibit circular dichroism. By altering the CPL, it was possible to observe reversible photoinduced circular dichroism. The occurrence of photoinduced chirality has been attributed to the preferential enrichment of twisted conformations of the W-shaped molecule upon CPL irradiation. The polymer 211 has been observed to furnish photoinduced chiral nematic phase.583 The dimeric compound 212 forms the B4 phase with helical nanofilaments.584 Due to presence of photochromic azo groups, this material exhibits photoresponse which has enabled photopatterning of the sample by UV light. The formation of propeller-shaped tetrameric units by hydrogen bonding between melamine derivatives and V-shaped molecules 214 (Scheme 73) with the carboxylic acid group have been observed.585−595 These supramolecular entities are found to exhibit columnar LC phases with intracolumnar helical dispositions. Induction of supramolecular chirality in columnar LC phases of azobenzene-containing compounds have been achieved by irradiation with CPL. CPL as an external stimulus controls the chirality of a helical stack of propeller-like complexes. The inductions of supramolecular chirality into polymeric materials have also been demonstrated. They have elaborated on the application of CPL as a chiral stimulus for the formation, modification, and control of supramolecular chiral organizations based on azo-containing materials.585 The handedness of propeller-like hydrogen-bonded complexes can be controlled with CPL irradiation. Chiral induction can be done to achiral columnar systems by irradiation with CPL.589 The V-shaped acids without chiral side chains form tetrameric complexes with a central melamine moiety, and these complexes stack to furnish a columnar phase. These columnar phases do not exhibit any chirality; however, upon shining with CPL, the columnar phase acquires chirality as evidenced from their CD spectrum. The systems which are already chiral, the CPL irradiation amplifies their chirality. Illumination of the chiral systems with CPL led to either an increased CD signal or the opposite sign depending on the handedness of the CPL used. It was possible to transfer the chirality of CPL to achiral systems. The 1:1 complex between the V-shaped acids and melamine derivative led to the formation of a super column. Infrared and vibrational circular dichroism

lation of the spontaneous polarization of the FLC was achieved. It was found that the value of spontaneous polarization increases with an increase in the dopant concentration, and the process of photoswitching is fatigue resistant. Later another similar dopant 208 was used to study the photomodulation of Ps. Photocyclization of the dopants diminishes the value of Ps which has been attributed to the loss of conformational flexibility upon photocyclizationn of the dopants. To enhance the compatibility and thus the intermolecular interaction, dopants 209 with side chains capable of nanaosegregation were designed and synthesized and their effect on FLCs has been investigated. 5.4. Light-Driven Chiral Induction in Liquid Crystals

Achiral bent-core compounds are able to form chiral domains of opposite handedness in equal amounts, which makes the macroscopic sample achiral. However, there are different methods adopted to break the degeneracy and induce net chirality to the system involving chiral influence. Recently, it has been shown that without the involvement of chiral molecular species, it is possible to induce chirality to achiral systems.581−584 Achiral bent-shaped dimers 210 (Scheme 72) containing photochromic units form large chiral domains enantioselectively when irradiated with circularly polarized light (CPL).582 Interestingly, the enantioselectivity is controllable just by altering the handedness of the CPL. The chiral domain formation was confirmed by measuring the circular dichroism spectra of the sample in its low temperature LC phase. Direct observations of chiral domains were carried out under a polarizing optical microscope. AV


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Scheme 73. Bent-Core Compounds Exhibiting Photoinduced Chirality in Columnar Phase

crystalline polymers are generally categorized as side chain polymers and main chain polymers depending on the position of the mesogens with respect to the polymer backbone. In the side chain polymers, the mesogens are attached to the polymer backbone through flexible spacers, whereas in the main chain polymers, the mesogens form integral part of the polymer backbone. Report on mixed chain polymers where mesogens are present in the main chain as well as in the side chain have been documented. Like low molecular weight LCs, polymer LCs can exhibit, nematic, smectic, columnar, etc. phases. Recently, liquid crystalline elastomers (LCEs) have been paid great attention due to their special characteristics and facile stimuli responsiveness. Since LCEs possess the elasticity of polymer networks and anisotropic properties of LCs, they have been observed to undergo large and reversible structural and morphological deformations in response to external stimuli. LCEs can be fabricated by lightly cross-linking side chain or main chain liquid crystalline polymers. The concept of LCEs was introduced by de Gennes.596,597 LCEs exhibit reversible thermomechanical response (i.e., well aligned LCEs upon transformation from nematic to isotropic state contract along the director and while cooling across the phase transition they expand). LCEs can be

spectroscopic methods were recently employed to clarify the origin of supramolecular chirality in azobenzenecontaining columnar LCs. The studies suggest that the azobenzene groups on the periphery of the columns are involved in the photoinduced chirality which confirms the existence of helical organization of azobenzene groups along the column axes. Vshaped acids containing one azobenzene in their side wings were synthesized, and their complexation with melamine was studied. It was found that these complexes 215 exhibit columnar mesophases, and their chirality could be controlled by CPL irradiation. Similar complexes 216 containing both azobenzene and oxadiazole side wings (Scheme 74) were found to exhibit columnar mesophases, and their chirality could also be controlled by CPL irradiation.588

6. LIGHT-DRIVEN LIQUID CRYSTALLINE POLYMERS AND ELASTOMERS Liquid crystalline polymers combine the characteristics of polymers and liquid crystalline materials in a single system. Specifically, the mechanical properties of polymers and the selfassembling and stimuli responsive properties of LCs render liquid crystalline polymers as functional materials. Liquid AW


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Scheme 74. Bent-Core Acids Exhibiting Light-Driven Chirality in Their Columnar Phase

photoresponsive LCEs and have elaborated on the photomechanical properties. Through these studies, the effects of various material and experimental parameters on LCEs have been unraveled. The mechanical response of a LCE doped with an azo dye 226 (Scheme 76) toward visible light illumination has yielded interesting results. Fast and large light-driven mechanical deformation has been registered in the LCE made from compounds 223−225 (Scheme 76). When a floating sample of the dye-doped LCE on water surface was illuminated from above, it was found that the sample swam away from the light source like a flat fish (Figure 63).601 The propulsion mechanism of the system has been analyzed in terms of momentum transfer. Ikeda group fabricated a LC polymer film by using mono- and diacrylates 227−231 containing azobenzene groups (Scheme 77).602−617 This film was shown to bend precisely along controlled directions by irradiation with linearly polarized light (Figure 64).602 Subsequently, they have undertaken detailed

made photoresponsive either by physical mixing with or chemical linking to photochromic compounds. Light-driven LCEs were synthesized and studied by Finkelmann et al. in early 21st century.598−601 Combining a variety of compounds 217−222 (Scheme 75), they synthesized monodomain photoresponsive LCEs by employing a two-stage cross-linking method. Prior observations of thermomechanical responses in LCEs prompted them to investigate the possibility of photomechanical response in these systems. They reported the observation of about 20% contraction in the azobenzene containing LCE upon UV light irradiation. Such optomechanical effect arises due to light-driven trans−cis isomerization of azobenzene derivatives causing dramatic uniaxial deformation of the LCE along the director field. Thus, light-driven reversible contraction and expansion in the photoresponsive LCE was established, and the potential application of LCEs as photocontrollable soft actuators was conceived. The Finkelmann group has synthesized a variety of AX


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Scheme 75. Components of a LCE Showing Optomechanical Effect

Scheme 77. Mono and Diacrylates Containing Azobenzene Goups

Scheme 76. Components of a LCE and Azo Dye Exhibiting Fast and Large Mechanical Deformation upon Visible Light Illumination

Figure 64. Light-controlled bending of a polymer film containing azobenzene groups. Reproduced with permission from ref 190. Copyright 2011 Elsevier Ltd.

light-driven changes in the molecular size and order of the azobenzene moieties cause volume contraction at the film surface leading to bending phenomenon. This process is shown in Figure 65 with films having homogeneous or homeotropic alignment of the azo mesogens in the films.603 Through systematic studies on light-driven bending phenomenon on LCE films, the Ikeda group established that monodomain and polydomain films show different bending properties. While monodomain samples bend along the alignment direction, the polydomain films can bend in any direction. It was learned that initial alignment of photochromic mesogens in the film plays a critical role in dictating the bending behavior of LCE films. Specifically films with homogeneous alignment bend toward the UV light source, whereas their homeotropic counterparts bend away from the light source as shown in Figure 65. This results from contraction in the surface of films with homogeneous alignment and expansion in the surface of the homeotropic films. Ferroelectric LCE films containing azobenzene moieties 229 and 230 were fabricated by photopolymerization under the influence of an applied electric field. Light-driven bending phenomenon in these films was investigated. Interestingly, it was observed that photogenerated mechanical force in these films is comparable to the contraction force of human muscles. This attribute qualifies these materials for potential application in artificial muscles and light-

Figure 63. (a) Photomechanical response of an LCE sample. (b) The shape deformation of an LCE sample upon exposure to light. (c) Mechanism of the locomotion of the dye-doped LCE sample. Reproduced with permission from ref 601. Copyright 2004 Nature Publishing Group.

light-driven bending studies on such films by varying the crosslinking density. These films whose dichroic ratios and order parameters were determined from polarized UV absorbance exhibit light-driven bending and unbending behavior upon exposure to unpolarized UV and visible light, respectively. Moreover, it was observed that the magnitude and rate of bending is different for films with different cross-linking densities. The light-driven bending phenomenon has been explained by taking into account the large absorption extinction coefficient of the azobenzene chromophores. It is suggested that AY


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actions similar to their covalently linked counterparts. These are the early examples of hydrogen-bonded supramolecular photomechanical systems that transduce light to mechanical work. The interrelationship between photoisomerization and geometry changes of azobenzene groups with photoinduced macroscopic bending and stress have been investigated and analyzed in polymer films containing azobenzene-based cross-linkers. These fundamental studies have yielded useful insights into the complex physical processes regulating the photomechanical response of azobenzene-containing cross-linked polymer systems. As mentioned above, the bending behavior of polymer films are governed by the initial alignment of the mesogens in the crosslinked polymer films; however, it has been recently demonstrated that the bending behavior can be modulated by changing the location of the photochromic azobenzene moieties. Accordingly, the bending direction of a sample has been observed to reverse by just altering the location of the azobenzene moieties from crosslinks to side chains under identical photoirradiation conditions. Light-driven liquid crystalline elastomers have been recently developed which possess dynamic covalent bonds. Interestingly, these materials could be reshaped and the realignment of mesogens occurs in the cross-linked polymer network even after polymerization. The monodomain polymer network films exhibit reversible light-driven bending behavior similar to other photoresponsive liquid crystalline elastomer films.616 In addition to azobenzene-containing cross-linked polymers, the Ikeda group has recently explored photomobile polymers containing mesomorphic diarylethenes 232 (Scheme 78).617 The crossFigure 65. Anisotroipc bending phenomenon in azobenzene-containing polymer films with different alignment. Reproduced from ref 603. Copyright 2004 American Chemical Society.

Scheme 78. Dithienylcyclopentene-Containing Precursors of Cross Linked Polymers

driven mechanical devices. A light-driven plastic motor was demonstrated using a laminated azobenzene-containing LCE film (Figure 66).610 The film was fabricated from the

linked liquid crystalline polymers containing dithienylcyclopentene moieties exhibit light-driven bending behavior similar to their azobenzene-containing counterparts. Here the light-driven phenomenon is caused by the reversible cyclization and cycloreversion of the dithienylethene groups upon irradiation to UV and visble light, respectively. One of the remarkable properties of these polymer films is their thermal stability in the bending state. Since the isomerization of dithienyl cyclopentene is thermally irreversible, polymer films in the bent state remain so until irradiation with light of appropriate wavelength. Such films can be left in any arbitrary bent state by controlling the irradiation time. The themal stability of these films is in sharp contrast with the films containing azobenzene groups since the cis azobenzene form can return back to its trans form thermally even in the dark. As far as photomobility and photomechanics are concerned, the cross-linked liquid crystalline polymer films containing dithienylcyclopentene groups, in some sense, complement the azobenzene-containing polymers.

Figure 66. Light-driven plastic motor. Reproduced with permission from ref 610. Copyright 2008 Wiley-VCH.

polymerizable acrylates 230 and 231. A cyclic belt made from the LCE film was able to drive a pair of pulleys when irradiated simultaneously at different positions with UV and visible light.This is a clear demonstration of the conversion of light energy into mechanical work. Interesting 3D movements have been demonstrated in cross-linked LC polymer films by light illumination. Inchworm walk and flexible robotic arm motions have been achieved in homogeneously aligned laminated polymer films. Cross-linked liquid crystalline polymer films formed by hydrogen-bonding interaction among side chain polymers have been found to exhibit light-driven bending and unbending AZ


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The Keller group has reported the studies on side-on nematic elastomers containing azobenzene-based mesogens 233 and 234 (Scheme 79).618−622 UV irradiation of the nematic elastomer

Scheme 80. Azobenzene-Based Monomers and Cross Linkers

Scheme 79. Chemical Structures of Side-on Attached Mesogenic Monomers

tively understand how the intensity and polarization of light affect their photoactuation characteristics. Broer and co-workers have reported enhanced performance of LC network actuators by varying the orientation of the director along the thickness of their films.628−638 Films with splayed and twisted director profile were fabricated and studied. It was shown that these films exhibit faster bending with greater amplitude than uniaxial planar systems of identical composition. Moreover, the bending direction in such films is found to be governed by the director orientation. Two-way bending was demonstrated in samples with composition gradient of mono- and difunctional reactive mesogens. Light-driven artificial cilia were fabricated by the printing technique from LC networks. The LC property of the actuators enables generation of large strain gradients by remote addressing. Microstructures with different subunits were fabricated from multiple inks. These can be conveniently and selectively addressed with light of different wavelengths. The structures of molecular building blocks used for the preparation of LC polymer network are shown in Scheme 81. Compounds 238 and 239 lend the photoresponsive behavior to the network. Scheme 81. Molecular Building Blocks Used for the Fabrication of Artificial Cilia

Figure 67. UV light induced contraction in a nematic elastomer film. Reproduced with permission from ref 618. Copyright 2003 Wiley-VCH.

film of 233 causes its contaction (Figure 67), and the magnitude of contraction was found to be dependent on the light intensity.618 The film returns to its original size thermally following the light irradiation due the thermal back relaxation of the cis isomers of the azobenzene groups to their trans form. Later they have reported on the light-driven bending phenomenon in LC elastomer fibers obtained from compound 234.623 Both cross-linked and copolymers without cross-links were fabricated and investigated. It was found that wires drawn from both polymers exhibit light-driven bending toward the direction of the UV source. The Terentjev group has designed and synthesized a variety of LCEs from azobenzene-based monomers and cross-linkers 235− 237 (Scheme 80) and studied their light-driven characteristics. 624−627 Monodomain samples containing different amounts of photoisomerizable azobenzene derivatives acting either as mesogens or as cross-links have been studied and large uniaxial contractions have been observed upon UV irradiation. Polysiloxane-based LCEs have been investigated to quantitaBA


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Figure 68. Photoresponse of polymer network films with splay-bend configuration. Reproduced with permission from ref 631. Copyright 2009 Macmillan Publishers Limited.

The director orientation of the mesogens has been maintained in splay bend configuration. Since splay molecular alignment governs bending direction and bending axis of the polymer network films, their direction of response is independent of direction of light irradiation. Figure 68 shows the photoresponse of polymer network films under different incident light.631 Polymer networks with chiral nematic phase were fabricated from mixtures containing compounds 238 and 240−243. Dynamic photomodulation of surface topologies have been demonstrated by taking advantage of the anisotropic geometric changes of the polymer network film. Patterned films with alternating chiral nematic and homeotropic alignment upon light irradiation generate modulated surface topology (Figure 69).633 Subsequently, both dynamic and permanent surface topologies were demonstrated in chiral nematic polymer films. It was found that upon trans to cis photoisomerization of azobenzene moieties in the films leads to reduction of the film density and increase in local volume. Such photoactivated volume generation in a liquid crystalline network has been investigated in detail to gain insights into the different factors that contribute toward it. The new insights provide general guidelines for the controllable generation of free volume in LC surface coatings which could facilitate facile formation of dynamic surface corrugations and large macroscopic deformations.637 A LC polymer coating capable of reversible change in its friction under light irradiation has been realized. The coating reversibly switches between a flat and corrugated surface texture. Controllable friction and adhesion has been demonstrated using 3D fingerprints in LC coatings. Flat LC coatings when actuated by light yield 3D fingerprints on the surface, and the process is reversible (Figure 70).634 Such smart coatings may find many applications in nanotechnology. The Bunning and White group has demonstrated light-driven polymer oscillators with high frequency and large amplitude.639−652 Cantilevers were fabricated from azobenzenecontaining LC polymer networks and were actuated by laser. Aligned monodomain LC elastomers have been used to prepare photosensitive cantilevers. The molecular building blocks 244− 247 of the LC elastomers are shown in Scheme 82. Light-driven angular bending of one such cantilever was found to depend on the polarization angle of the source with respect to the long-axis of the cantilever (Figure 71).641 Moreover, comparative studies

Figure 69. Light-driven change in surface topology of polymer network films with alternating cholesteric and homeotropic alignment. Adapted with permission from ref 633. Copyright 2012 Wiley-VCH.

on the bending performance between monodomain samples and polydomain samples have been undertaken. A detailed study on photomechanical mechanism and structure property relationBB


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Liquid crystalline polymer networks with topological defects have been used in the generation of light-driven features. Depending on the strength of the defect, different surface topographies have been demonstrated.642 Figure 72 shows

Figure 72. Photomechanical response of azo-LCN films subsumed with high-order defects. The director field orientation (a, e), polarized optical microscopy image (1 cm diameter) (b, f), image of the film after photomechanical deflection (1.2 cm diameter) (c, g), and illustration of the photomechanical deflection (d, h) for a + 10 defect and a−10 defect, respectively. Reproduced with permission from ref 642. Copyright 2013 Wiley-VCH.

Figure 70. Confocal microscopic images of fingerprints. (a) 3D Image of the initial flat state and (b) 3D image of surface topographies under UV exposure. Adapted with permission from ref 634. Copyright 2014 WileyVCH.

surface topographies created from polymer films with engineered topological defects. Fabrication of diverse topographical features has been demonstrated in liquid crystalline elastomers by adopting spatially complex director orientation in the films by photoalignment control. It should be noted that similar photoalignment technique have been used for obtaining programmed LC elastomers which exhibit tunable thermomechanical actuation strain.651 Photoirradiation causes reversible shape morphing between 2D and 3D shapes within the elastic sheets as a consequence of light-driven deformation and recovery of complex topographical features.652 Yu et al. synthesized the photoresponsive polymer 248 (Scheme 83) containing azopyridyl group and fabricated microparticles by combining these polymers with different dicarboxylic acids.653−657 The supramolecularly assembled microparticles exhibited photoinduced deformations only when they were liquid crystalline. It was found that light-driven phase transition is responsible for the shape change of the microparticles.653,654 Such microparticles with photoresponsiveness could find application in photonics and actuators. Light-driven phase transition has been used in the fabrication of a thin film

Scheme 82. Chemical Structures of Polymerizable Compounds

ships in glassy azobenzene-containing LC polymer networks prepared from monomers 244−247 has been reported.

Figure 71. In-plane bending and out-of-plane twisting observed in photosensitive polymer cantilevers with the nematic director aligned 0°, 30°, 45°, 65°, and −65° to the cantilever long axis (x) when exposed 442 nm light polarized parallel to x. Adapted with permission from ref 641. Copyright 2011 WileyVCH. BC


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Scheme 83. Structures of Compounds 248−250

optical diode with erasable characteristics.656 The photomechanical property of liquid crystalline actuators prepared from compounds 249 and 250 have been used in the demonstration of an optical pendulum generator which enables conversion of light energy into electricity.657 The Yu group reported visible light-driven bending and unbending of a cross-linked liquid crystalline polymer carrying azotolane moieties 251 and 252 (Scheme 84).658−660 It was

Figure 73. (a) Light driven movement of liquid slogs inside a photodeformable microactuator tube. The bottom panel (b) shows the reversible forward and backward movement of silicone oil slug inside a microtube actuated by attenuated light irradiation. Reproduced with permission from ref 661. Copyright 2016 Macmillan Publishers Limited.

Scheme 84. Chemical Structures of Azobenzene-Based Mesogens and Cross Linkers

to have great impact as far as the application of microoptomechanical systems is concerned. 3D Liquid crystalline elastomer microstructures such as rings and woodpiles have been fabricated by direct laser writing.663−665 Systematic studies have been carried out to produce 3D structures with desired shapes and dimensions. Such light controlled structures have huge potential in photonics. The compound 255 (Scheme 85) was used as the photochromic Scheme 85. Chemical Structures of Compounds 255 and 256

found that order parameter and light intensity play a role in determining the bending speed of the polymer films. Lightgenerated mechanical force of the films was found to be proportional to the cross-linking density. Plastic microrobots which are able to manipulate objects under the influence of visible light have been demonstrated.658 Light-driven bending behavior of azobenzene-containing cross-linked LC polymers with a poly(oxyethylene) backbone has been investigated.660 Compounds 253 and 254 were used as mesogen and crosslinker, respectively. Very recently, Yu et al. demonstrated the fabrication of lightdriven tubular microactuators from photoresponsive liquid crystalline polymers and their applications in the translation of fluid slugs inside the tubular actuators.661,662 This interesting and enabling light-driven modulation of capillary forces phenomemon is illustrated in Figure 73.661 By employment of attenuated light, they could prescribe asymmetric deformation of the microtubes leading to capillary force assisted liquid propulsion inside the photoresponsive tubes. Photocontrolled movement of fluids of diverse characteristics has been demonstrated inside a variety of microactuator tube shapes. These studies are expected

moiety which does not interfere with the laser writing process. Microscopic walker driven by light as the fuel has been recently fabricated from LC elastomers. The type of locomotion of the walkers can be regulated by their design as has been demonstrated in this study. In another report, a 3D actuator has been fabricated from a LC elastomer without cross-linking. The LC elastomer was prepared from a specially designed bifunctional monomer 256. Interestingly, actuators in different morphologies can be processed from either melt or from elastomer solution. The actuator films exhibit 3D controlled bending in various directions by incident polarized UV light. Many groups have designed and synthesized liquid crystalline polymers exhibiting photomechanical behaviors.666−677 In addition to nematic polymers and elastomers, photoswitchable smectic elastomers have also been realized and investigated. Springs fabricated from a LC polymer doped with an azobenzene-based molecular switch has been utilized in the conversion of light energy into mechanical work through BD


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crystalline nanoparticles have already been realized.678,679 Carbon nanotubes, graphene derivatives, gold nanoparticles, and upconversion nanoparticles have been embedded into the liquid crystalline polymer and elastomer matrices in the hope of enhancing their existing properties or enabling new properties and phenomena.680−701 In the following, we present the recent developments in this area of LC nanaoscience. Yang et al. fabricated carbon nanotube (CNT) doped LCE which shows reversible actuation by infrared (IR) light irradiation.680 This actuation phenomenon arises due to the fact that carbon nanotubes can efficiently absorb IR light and convert it into heat (photothermal effect) in the elastomer matrix. Moreover due to their good thermal conductivity, the CNTs distributed in the matrix can heat the sample up to the point of phase transition where the decrease in LC ordering causes shape changes in the nanocomposite film. Alignment of carbon nanotubes is a critical requirement in order to exploit their anisotropic properties. In this context, liquid crystalline polymers and elastomers have been employed for the dispersion and alignment of CNTs.682 In addition to the use of IR light, white light has also been used for the actuation of CNT-LC elastomer nanocomposites.685,690 Photomechanical response has also been acquired by using graphene and graphene oxide (GO) as nanoscale heaters in liquid crystalline nanocomposites.694−696 Similar to CNTs and GO, gold nanoparticles (both rod and spheres) have been used as heat transducers in liquid crystalline polymer composites.681,686 With the help of IR laser, reversible and irreversible shape changes have been demonstrated in these nanocomposites. Figure 75 shows shape morphing in a composite film.681 Upconversion nanoparticles have been used for near-infrared light-driven reversible actuation of LCE films (Figure 76).697

controlled and reversible twisting motions. Twist motions such as winding, unwinding, and helix inversion occurring in the springs (Figure 74) are determined by their initial shape.669

Figure 74. Photoactuation modes of polymer springs. Spiral ribbons irradiated for 2 min with ultraviolet light (365 nm) display isochoric winding, unwinding, and helix inversion as dictated by their initial shape and geometry. Reproduced with permission from ref 669. Copyright 2014 Macmillan Publishers Limited.

Moreover, these photoresponsive springs can generate work by translating a macroscopic object. Shape-persistent LC polymer networks have been recently realized by using fluorinated azobenzenes as the photoresponsive moieties.670 By engineering the cross-link density and thermal stability of the photoswitch with optimized molecular orientation, it was demonstrated that the recovery of the photomechanical deformations in these materials could be substantially delayed so that any photogenerated macroscopic shapes can be retained over time. Organic molecular and macromolecular compounds capable of showing photothermal effect in response to NIR light irradiation have been integrated into liquid crystalline elastomers to broaden their scope and applicability.675,677 While the research and development on light-driven liquid crystalline polymers and elastomers are continuing, there is a parallel development of nanocomposites from these fascinating materials. It is worth noting that photoresponsive liquid

7. LIGHT-DRIVEN LIQUID CRYSTALLINE GELS AND BENT-CORE LIQUID CRYSTALS Gels in general and physical gels in particular are outstanding soft and stimuli responsive materials with applications in different areas. Gels are usually fabricated from isotropic liquids by incorporating suitable gelators in appropriate concentrations. In order to complement the gels formed from isotropic liquids, gels from anisotropic liquids such as LCs have been conceived and

Figure 75. Examples of robust reversible morphing of LCE microparticle shapes by means of unidirectional laser beam scanning along blue arrows shown in the insets of (a−f). Reproduced with permission from ref 681. Copyright 2012 American Institute of Physics. BE


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Figure 76. Photographs of the azotolane CLCP/UCNP composite film bending toward the light source along the alignment direction of the mesogens, remaining bent in response to the CW near-infrared irradiation at 980 nm and becoming flat again after the light source was removed. Reproduced from ref 697. Copyright 2013 American Chemical Society.

realized which exhibit interesting properties and phenomena. Both chemical and physical liquid crystalline gels have been fabricated by using different kinds of gelators, and their properties have been evaluated. Moreover gels from molecular as well as macromolecular LCs have been prepared and studied. Owing to synergetic interactions between LCs and fibrous network of gelators, liquid crystalline gels exhibit unique dynamic properties and functions. Among the liquid crystalline gels, photoresponsive gels are particulary appealing.702−709 Ikeda et al. reported the fabrication and light-driven anisotropic bending and unbending of liquid crystalline gels formed by azobenzene derivatives. Liquid crystalline gels formed by the cross-linked network of azomonomers in good solvents such as toluene show anisotropic swelling behavior. Upon UV light irradiation, they exhibit anisotropic bending toward the light direction, whereas they return back to their original state upon visible light irradiations. The reversible bending and unbending phenomenon in a liquid crystalline gel film is shown in Figure 77.704 The bending behavior of the film has been attributed to the absorption gradient between the surface and bulk of the film. The Kato group reported on the fabrication of photoresponsive LC physical gels by using the photoresponsive gelator 257 (Scheme 86) in a room-temperature nematic LC and demonstrated their use in rewritable information recording. The nematic LC gel upon irradiation with UV light transforms into a cholesteric LC. It has been found that upon UV light irradiation, the intermolecular hydrogen bonding existing in the gelator dissociates due to trans−cis isomerization of the azobenzene groups. Upon visible light irradiation, the cholesteric LC phase turns into cholesteric LC gel (Figure 78).706 The photoinduced transitions between the LC gels have been exploited in rewritable patterning. Subsequently, the Kato group has demonstrated photopatterning of liquid crystalline gels fabricated from the columnar hexagonal phase of a triphenylene-based discotic LC.707 Recently, the photoresponsive behavior of microparticle/LC composites gels has been reported. Interesting optically healable characteristics has been demonstrated with these gels. Liquid crystalline gels containing photochromic bent-shaped gelator 258 has been obtained and studied.709 Light irradiation to these gels causes changes in their phase behavior, fluidity, and optical properties. Light-driven gel to solution transition (Figure 79) in these systems has enabled polarized luminescence from nematic and cholesteric mixtures.

Figure 77. Photoinduced bending and unbending in the liquid crystalline gel film. Adapted with permission from ref 704. Copyright 2003 Wiley-VCH.

Scheme 86. Structure of Photoresponsive Gelators

Figure 78. Photoinduced phase transitions in LC gels. Reproduced with permission from ref 706. Copyright 2003 Wiley-VCH.

BF


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liquid crystalline bent-core compounds carrying photochromic groups have been investigated in isotropic solutions to understand their photoresponsive behaviors upon light irradiation. In the following, we discuss the light-driven behaviors of LC phases of bent-core compounds.712−716 Nair et al. reported a study on the light-driven polarization modulation of the B2 mesophase of the bent-core compound 259 (Scheme 87) by doping it with an azobenzene derivative. It was found that the value of spontaneous polarization and polarization switching time of the B2 phase are greatly influenced by UV light irradiation and the polarization modulation efficiency is dependent on the sample temperature. Light-driven reduction of the Frank elastic constants of the nematic phase of the bent-core LC 260 has been observed upon photoisomerization of a doped azobenzene derivative. The reversible changes observed in the nematic phase have been clarified by taking into account the conformational and molecular packing changes upon light irradiation. Recently a dual-frequency addressable optical device has been demonstrated by using bent-core compound 260 and driven by light exposure. Here the photoresponsive bent-core LC mixture exhibits a switch in the sign of the dielectric anisotropy which enables altering the orientation of the molecules. This results in a huge change in the optical transmission of the mixture. The optical switch has also been observed to function as a conductance switch under light irradiation. We have designed and synthesized bent-core compounds 261, which exhibit the chiral nematic phase.715 Since these compounds contain an

Figure 79. Photoinduced liquid crystalline gel to solution transition. Reproduced with permission from ref 709. Copyright 2015 The Royal Society of Chemistry.

LCs formed by banana-shaped or bent-core compounds are known to exhibit phases, properties, and phenomena that are specific to this class of materials. For example, chiral and polar LC phases are furnished by achiral molecules of bent-core compounds. As far as light-driven LC phases of bent-core compounds are concerned, there are not many reports though a large variety of bent-core compounds containing photochromic moieties have been designed and synthesized.710,711 Moreover, Scheme 87. Structure of Photoresponsive Bent-Core Compounds

BG


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and elastomers are starting to be used in biomimicking colorproducing structures, lenses, muscle-like actuators, and sensors. Photomechanical response of liquid crystalline elastomers has the potential to be used in optical device applications. Lightdriven liquid crystalline polymers and elastomers are emerging as new functional materials which have numerous applications such as actuators, sensors, smart surfaces, and biomimetic structures. Cross-linked liquid crystalline polymer materials undergo macroscopic deformations upon light irradiation; however, the self-healing properties of LCs restore the original configurations. Considerable efforts have been dedicated to enhance the photoresponsive and mechanical attributes of such materials through advanced molecular engineering and development of improved fabrication methods toward applications. 2D and 3D Motions in liquid crystalline polymers have been achieved by judicious engineering of the molecular alignments. However, mechanical strength of the films still remains an issue. Ultraviolet, visible, and infrared radiations have been employed to drive different liquid crystalline materials. It will be interesting to design materials that could be driven by utilizing direct sunlight under ambient environments. Azobenzene derivatives have been extensively employed as the photochromic moiety; however, the exploration of other photochromic groups has not been paid adequate attention. Therefore, there is a large scope to play with other photochromic groups in the development of light driven liquid crystalline materials. Similarly, rod-shaped LCs have been the focus of research, it is high time to extend the studies to discotic and bentcore liquid crystalline materials, especially in the context of polymer networks and elastomers. Like photoresponsive cholesteric LCs, other chiral LC phases such as ferroelectric LCs and blue phases deserve more systematic investigations. Supramolecular photoresponsive liquid crystalline materials have been less enquired of, even though they seem to furnish exciting functional attributes. Nanoparticle-doped light-driven LCs could evolve as an independent enterprise. Moreover, the design, synthesis, and study of photoresponsive liquid crystalline nanoparticles whose properties could be reversibly tuned by the action of light through dynamic assembly and disassembly may open up a new direction in the area of nanomaterials research. The prospects for the utilization of upconversion nanoparticles and anisotropic nanorods seem bright. Toward this end, chemists are bound to play a critical role in the design and synthesis of engineered photoaddressable liquid crystalline materials with a gamut of intriguing phenomena and properties which could attract other stake holders into this interdisciplinary vibrant frontier.

azobenzene group in one of their side arms, they undergo lightdriven isomerization. It was observed that these single component photoresponsive cholesteric LCs exhibit phototunable selective reflection wavelengths. Upon UV light irradiation, their selective reflection shows red shift due to the elongation of the pitch of the helical superstructure. Moreover these compounds have been tested as chiral molecular switches to induced photoresponsive cholesteric LCs in nematic hosts. Banana-calamitic hybrid dimers 262 and trimers 263 containing azobenzene groups have been synthesized and their light-driven behavior has been studied.716 One of the trimer compounds and one of the dimer compounds studied show photoorientation of the molecules along the perpendicular direction with respect to the polarization axis of the polarized light.

8. SUMMARY AND OUTLOOK In this Review, we have outlined the recent developments in the design, fabrication, and investigations of light-driven liquid crystalline materials. It is apparent that photoaddressable LCs are emerging as a new generation of multifunctional supramolecular materials in their own right. In other words, such materials have positioned themselves as enabling smart soft materials. Either by incorporating photochromic molecules in LCs as dopants or introducing photoresponsive building blocks into LC molecular structure and polymers, their self-organized superstructures have been efficiently controlled and modulated by light irradiation. Being an elastic anisotropic fluid, photoresponsive LCs can effectively amplify and transmit light-driven information and properties and delicately self-heal morphological defects. This constitutes the basis for the applications of smart photoresponsive liquid crystalline materials in photonics, color filters, polarizers, all-optical reflection displays, lasing and beam steering, holographic optical data storage, sensors, soft actuators, nanotechnology, etc. Light-driven molecular switches or motors such as azobenzene derivatives, diarylethenes, spiropyrans, spirooxazines, and overcrowed alkene-based molecules have been employed as photoresponsive scaffolds in LCs to study the synergetic effect of the two classes (photochromic compounds and LCs) of functional stimuli responsive materials. Photoinduced phase transitions have enabled highly efficient gratings, whereas photoorientation of molecules have been exploited in rewritable holography. Photoalignment technique of LCs has reached such a level that it has been employed in the industrial production of LC display devices for real life use. This moneymaking development has undoubtedly provided additional thrust for research. On the fundamental side, free surface commanding is emerging as a new area to explore. Photoorientation of liquid crystalline polymers in general and microphase segregating block copolymers in particular turns out to be an elegant phenomenon. Photoresponsive cholesteric LCs have been used for the fabrication of high resolution and lightweight photoptical displays. These displays are devoid of the drive electronics with patterned electrodes or complex addressing schemes used in electroptical displays; hence, they are cost-effective and can be made flexible and foldable. Thus, the cholesteric structures have very far-reaching implications in technology, especially in optics and photonics. Through dynamic handedness inversion, circular polarized light of any chirality with arbitrary wavelengths has been realized from the helical superstructures of cholesteric LCs. Similarly, the photonic band gap of blue phases has been driven by light across the entire visible region. Liquid crystalline polymers can achieve extremely high orientational order, which is useful for certain applications. Thus, liquid crystalline polymers

AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest. Biographies Hari Krishna Bisoyi is a Postdoctoral Research Associate in the group of Prof. Quan Li at the Liquid Crystal Institute of Kent State University. He obtained his B. Sc. (2001) and M. Sc. (2003) Chemistry from Berhampur University, Odisha. Subsequently, he received his Ph.D. (2010) from Jawaharlal Nehru University (JNU), India, working at the Raman Research Institute (RRI), Bangalore, under the guidance of Prof. Sandeep Kumar. He was a Marie Curie Fellow (2010−2011) in the BH


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Macroscopic Deformation. Adv. Mater. 2016, DOI: 10.1002/ adma.201602685. (10) Bisoyi, H. K.; Li, Q. Liquid Crystals. Kirk-Othmer Encyclopedia of Chemical Technology; 2014, pp 1−5210.1002/ 0471238961.1209172103151212.a01.pub3. (11) Liquid Crystals Beyond Displays: Chemistry, Physics, and Applications; Li, Q., Ed.; John Wiley and Sons: Hoboken, NJ, 2012. (12) Self-organized Organic Semiconductors: From Materials to Device Applications; Li, Q., Ed.; John Wiley & Sons: Hoboken, NJ, 2011. (13) Intelligent Stimuli Responsive Materials: From Well-Defined Nanostructures to Applications; Li, Q., Ed.;John Wiley & Sons: NJ, 2013. (14) Nanoscience with Liquid Crystals: From Self-Organized Nanostructures to Applications; Li, Q., Ed.; Springer-Verlag, 2014. (15) Kelker, H.; Hatz, R. Handbook of Liquid Crystals; Verlag Chemie: Weinheim, Germany, 1980. (16) Liquid Crystals: Applications and Uses; Bahadur, B., Ed.; World Scientific, Singapore, 1990; Vol. 1−3. (17) Collings, P. J.; Patel, J. S. Handbook of Liquid Crystals Research; Oxford University Press: Oxford, 1997. (18) de Gennes, P. G.; Prost, J. The Physics of Liquid Crystals; Oxford University Press: Oxford, 1993. (19) Chandrasekhar, S. Liquid Crystals; Cambridge University Press: Cambridge, U.K., 1992. (20) Collings, P. J. Liquid Crystals: Natures Delicate Phase of Matter; Princeton University Press: Princeton, NJ, 2002. (21) Collings, P. J.; Hird, M. Introduction to Liquid Crystals: Chemistry and Physics; Taylor & Francis: London, U. K., 1997. (22) Oswald, P.; Pieranski, P. Smectic and Columnar Liquid Crystals: Concepts and Physical Properties Illustrated by Experiments; Taylor & Francis, CRC Press: Boca Raton, FL, 2005. (23) Handbook of Liquid Crystals, 2nd ed.; Goodby, J. W., Collings, P. J., Kato, T., Tschierske, C., Glesson, H., Raynes, P., Eds.; Wiley-VCH: Weinheim, 2014. (24) Oswald, P.; Pieranski, P. Nematic and Cholesteric Liquid crystals: Concepts and Physical Properties Illustrated by Experiments; Taylor & Francis, CRC Press: Boca Raton, FL, 2005. (25) Gray, G. W.; Luckhurst, G. R. (eds.) The Molecular Physics of Liquid Crystals; Academic Press: London, U.K., 1997. (26) Liquid Crystals: Experimental Study of Physical Properties and Phase Transitions; Kumar, S., Ed.; Cambridge University Press: Cambridge, U.K., 2001. (27) Jakli, A.; Saupe, A. One- and Two-Dimensional Fluids: Properties of Smectic, Lamellar and Columnar Liquid Crystals; CRC Press: Boca Raton, FL, 2006. (28) Gray, G. W.; Windsor, P. A. Liquid Crystals and Plastic Crystals; Ellis Horwood Ltd: Chichester, 1974; Vol. 1−2. (29) Fisch, M. R. Liquid Crystals, Laptops and Life; World Scientific: Singapore, 2004. (30) Castellano, J. A. Liquid Gold- The Story of Liquid Crystal Displays and the Creation of an Industry; World Scientific: Singapore, 2005. (31) Chen, R. H. Liquid Crystal Displays: Fundamental Physics and Technology; John Wiley & Sons: NJ, 2011. (32) Brown, G. H.; Wolken, J. J. Liquid Crystals and Biological Structures; Academic Press: New York, 1979. (33) Liquid Crystals: Frontiers in Biomedical Applications; Woltman, S. J., Jay, G. D., Crawford, G. P., Eds.; World Scientific: NJ, 2007. (34) Crystals That Flow: Classic Papers from the History of Liquid Crystals; Sluckin, T. J., Dunmur, D. A., Stegemeyer, H., Eds.; CRC Press: Boca Raton, FL, 2004. (35) Dunmur, D.; Sluckin, T. Soap, Science and Flat-Screen TVs- A History of Liquid Crystals; Oxford University Press: Oxford, 2011. (36) Petrov, A. G. The Lyotropic State of Matter: Molecular Physics and Living Matter Physics; Gordon & Breach Science Pub.: Amsterdam, The Netherlands, 1999. (37) Self-Assembled Supramolecular Architectures: Lyotropic Liquid Crystals; Garti, N., Somasundaran, P., Mezzenga, R., Eds.; Wiley: NJ, 2012. (38) Liquid Crystals: Materials Design and Self-Assembly; Tschierske, C., Ed.; Springer-Verlag: Berlin Heidelberg, 2012.

group of Prof. Juozas Grazulevicius in EU FP7 DENDREAMERS program at Kaunas University of Technology (KTU), Lithuania. Quan Li is Director of Organic Synthesis and Advanced Materials Laboratory at the Liquid Crystal Institute of Kent State University, where he is also Adjunct Professor in the Chemical Physics Interdisciplinary Program. He, as a Principle Investigator and Project Director, has directed research projects funded by the U.S. Air Force Office of Scientific Research, U.S. Air Force Research Laboratory, U.S. Army Research Office, U.S. Department of Defense Multidisciplinary University Research Initiative, U.S. National Science Foundation, U.S. National Aeronautics and Space Administration, U.S. Department of Energy, Ohio Board of Regents under Its Research Challenge Program, Ohio Third Frontier, Samsung Electronics, etc. He received his Ph.D. in Organic Chemistry from the Chinese Academy of Sciences (CAS) in Shanghai, where he was promoted to the youngest Full Professor of Organic Chemistry and Medicinal Chemistry in February of 1998. He was a recipient of CAS One-Hundred Talents Award (BeiRenJiHua) in 1999. He was an Alexander von Humboldt Fellow in Germany. He has won Kent State University Outstanding Research and Scholarship Award. He has also been honored as Guest Professor and Chair Professor by several Universities. Li has edited four Wiley books and three Springer books in the past five years and is the invited author of the entry entitled Liquid Crystals for Kirk-Othmer Encyclopedia.

ACKNOWLEDGMENTS The preparation of this review benefited from the support to Quan Li by the Air Force Office of Scientific Research (AFOSR FA9950-09-1-0193 and FA9950-09-1-0254), the Air Force Research Laboratory (AFRL), the Department of Defense (DoD) Multidisciplinary University Research Initiative (MURI FA9550-12-1-0037), the National Science Foundation (NSF IIP 0750379), the Department of Energy (DOE DE-SC0001412), DoD-Army, the National Aeronautics and Space Administration (NASA), and the Ohio Third Frontier. We thank all Li’s current and former group members as well as his collaborators, whose names are found in the references, for their significant contributions in this project. REFERENCES (1) Ichimura, K. Photoalignment of Liquid-crystal Systems. Chem. Rev. 2000, 100, 1847−1873. (2) Dugave, C.; Demange, L. Cis-trans Isomerization of Organic Molecules and Biomolecules: Implications and Applications. Chem. Rev. 2003, 103, 2475−2532. (3) Gautier, A.; Gauron, C.; Volovitch, M.; Bensimon, D.; Jullien, L.; Vriz, S. How to Control Proteins with Light in Living Systems. Nat. Chem. Biol. 2014, 10, 533−541. (4) Szymanski, W.; Beierle, J. M.; Kistemaker, A. V.; Velema, W. A.; Feringa, B. L. Reversible Photocontrol of Biological Systems by the Incorporation of Molecular Switches. Chem. Rev. 2013, 113, 6114− 6178. (5) Lerch, M. M.; Hansen, M. J.; van Dam, G. M.; Szymanski, W.; Feringa, B. L. Emerging Targets in Photopharmacology. Angew. Chem., Int. Ed. 2016, 47, 2−24. (6) Fehrentz, T.; Schonberger, M.; Trauner, D. Optochemical Genetics. Angew. Chem., Int. Ed. 2011, 50, 12156−12182. (7) Brieke, C.; Rohrbach, F.; Gottschalk, A.; Mayer, G.; Heckel, A. Light-controlled Tools. Angew. Chem., Int. Ed. 2012, 51, 8446−8476. (8) Han, D. D.; Zhang, Y. − L.; Ma, J. − N.; Liu, Y. − Q.; Han, B.; Sun, H. − B. Light-mediated Manufacture and Manipulation of Actuators. Adv. Mater. 2016, 28, 8328. (9) Hu, Y.; Li, Z.; Lan, T.; Chen, W. Photoactuators for Direct Opticalto-Mechanical Energy Conversion: From Nanocomponent Assembly to BI


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