Thereof, and Methods Thereof

[0001] This application claims the priority of U.S. Provisional Application No. 61/404,909, filed October 12, 2010.

BACKGROUND OF THE INVENTION

[0002] The invention is supported by the Air Force Office of Scientific Research (FA9550-09-1-0193 and FA9550-09-1-0254). The U.S. Government has certain rights in the invention.

[0003] The present exemplary embodiment relates to a liquid crystal composition containing a chiral dopant, a device thereof, and methods thereof. It finds particular application in conjunction with electronic papers, dynamic reflectors, tunable filters, color filters, tunable color filters, information storage, photodisplays, and color displays, and will be described with particular reference thereto. However, it is to be appreciated that the present exemplary embodiment is also amenable to other like applications.

[0004] Liquid crystals (LCs) represent a fascinating state of matter which combines order and mobility on a molecular and supermolecular level. The unique combination of order and mobility results in that LCs are typically "soft" and respond easily to external stimuli. The responsive nature and diversity of LCs provide tremendous opportunities as well as challenges for insight in fundamental science, and open the door to various applications. For example, the LC display (LCD) industry is based on the electro-optic response of rod-like LCs.

[0005] Cholesteric liquid crystal (Ch-LC) materials have been extensively used for cholesteric display, optical switch, optical communication, and similar applications. In obtaining Ch-LC, one method is using a liquid crystal (LC) exhibiting cholesteric phase in the bulk, while another method involves doping a chiral dopant into a nematic LC to form a helical surperstructure in mixture systems. The chirality of dopants is transferred into the host materials. The resultant helical superstructure, i.e. cholesteric LC phase, can selectively reflect light according to Bragg's law and has the useful property of being tuned by light. The wavelength A of reflected light is decided by λ=ηρ according to Bragg's law, wherein n is the average index and p is the pitch length of the Ch-LC (the distance of the helical structure undergoing a 360° twist). The ability of a chiral dopant to twist the nematic host is called helical twisting power (HTP), which is defined by HTP= (pc) ^' where c is the concentration of the chiral dopant.

[0006] If the HTP or p is dynamically controlled, the selective reflection property of Ch-LC can be tuned. The challenge and opportunity to change HTP or p can be achieved through applying external forces, such as electric fields, to extend, compress, unwind, or tilt the helical structure. However, the reflection peaks are usually broadened or decreased and sometimes light scattering is induced upon the electric filed.

[0007] An increasingly interesting approach to tuning the reflection is the use of light, which overcomes some of the disadvantages induced by an electric field and exploits more electro-optical applications, as disclosed in T. Yoshioka, T. Ogata, T. Nonaka, M. Moritsugu, S.N. Kim, S. Kurihara, Reversible-photon-mode full-color display by means of photochemical modulation of a helically cholesteric structure, Adv. Mater. 17 (2005) 1226-1229; and T.J. White, R.L Bricker, L.V. Natarajan, N.V. Tabiryan, L. Green, Q. Li, T.J. Bunning, Phototunable azobenzene cholesteric liquid crystals with 2000 nm range, Adv. Funct. Mater. 19 (2009) 1-5.

[0008] Several chiral dopant compounds with the function of phototuning have been reported in Q. Li, L. Green, N. Venkataraman, I. Shiyanovskaya, A. Khan, A. Urbas, J.W. Doane, Reversible photoswitchable axially chiral dopants with high helical twisting power, J. Am. Chem. Soc. 129 (2007) 12908-12909; L. Green, Y. Li, T. White, A. Urbas, T. Bunning, Q. Li, Light-driven molecular switches with tetrahedral and axial chirality, Org. Biomol. Chem. 7 (2009) 3930-3933.

[0009] In these reports, an azobenzene group was introduced into the chiral dopant. The undergoing of trans and cis isomerization of the azo-chiral material by light dynamically changes the HTP values, and the reflected light of the Ch-LC is photo- tuned in a desired range. The dramatic change of the HTP enables a light-driving, photo-addressed color Ch-LC display, which makes non-color filter displays possible.

[0010] It is highly desirable to dynamically phototune the reflected color over the entire visible spectrum, or the primary red, green and blue colors, with only small amounts of light-driven chiral switch since a high concentration of chiral dopant can often lead to phase separation, undesirable coloration, and alteration of the desired physical properties of the LC host. This requires the dopant to have high HTP, as well as a significant difference in HTP among the various states of the switch. To date, reports on full range color control in induced chiral nematic LC, without added non photoresponsive chiral co-dopants, are limited to using helically chiral overcrowded alkenes (R. A. van Delden, N. Koumura, N. Harada, B. L. Feringa, PNAS 2002, 99, 4945; R. Eelkema, B. L Feringa, Chem. Asian J. 2006, 1 , 367), planar chiral azobenzenophanes (M. Mathews, N. Tamaoki, J. Am. Chem. Soc. 2008, 130, 1 1409), axially chiral binapthyl azobenzenes (S. Pieraccini, G. Gottarelli, R. Labruto, S. Masiero, O. S. Pandolini, G. P. Spada, Chem. Eur. J. 2004, 10, 5632; Q. Li, L. Green, N. Venkataraman, I. Shiyanovskaya, A. Khan, A. Urbas, J. W. Doane, J. Am. Chem. Soc. 2007, 122, 12908) and binapthyl azobenzenes with axial and tetrahedral chirality (L. Green, Y. Li, T. White, A. Urbas, T. Bunning, Q. Li, Org. Biomol. Chem. 2009, 7, 3930).

[0011] However, problems identified in most of these reports include that the color change is either irreversible, or requires a relatively long thermal relaxation. Some systems require a high concentration of planar chiral azobenzenophanes (12 wt%) or the binapthyl azobenzenes with axial and tetrahydral chirality (15 wt%), in which case the two systems cannot fully cover the entire visible spectrum upon the irradiation of visible light.

[0012] Advantageously, the present invention provides a LC composition, a LC device, a method of controlling the stationary reflection of a LC composition, and a method of processing information, which overcome the aforementioned problems. The invention exhibits merits such as reversible color change, wavelength selectivity, faster phototuning time, lower concentration of the chiral dopant, and wider color spectrum, among others.

BRIEF DESCRIPTION OF THE INVENTION

[0013] An aspect of the invention provides a liquid crystal (LC) composition comprising at least a liquid crystal host and a chiral dopant, wherein the composition can exhibit a stationary reflection of at least red, green, and blue colors.

[0014] Another aspect of the invention provides a LC device including a first transparent electrically conductive substrate, a second transparent electrically conductive substrate, and the composition as defined above, wherein the composition is sandwiched between the first substrate and the second substrate.

[0015] Still another aspect of the invention provides a method of controlling the stationary reflection of a liquid crystal composition comprising: (i) mixing at least a liquid crystal host and a chiral dopant to prepare the composition, and (ii) employing a photo- stationary reflection and an optional initial state reflection of the composition to reflect at least red, green, and blue colors.

[0016] A further aspect of the invention provides a method of processing information including: (i) providing a liquid crystal composition which exhibits a chiral nematic phase, (ii) optically writing information in the planar state of the chiral nematic liquid crystal, (iii) applying a first voltage pulse to hide the written information in the focal conic state, and (iv) reappearing the hidden information in the planar state mechanically or by applying a second voltage pulse. In a preferred embodiment, the first voltage pulse is lower than the second voltage pulse.

BRIEF DESCRIPTION OF THE DRAWINGS

[0017] The application file contains at least one drawing executed in color. Copies of this patent application publication with color drawings will be provided by the Office upon request and payment of the necessary fee.

[0018] Figure 1 shows a liquid crystal device including two substrates between which is sandwiched with a LC composition;

[0019] Figure 2 shows the reflection color images of a LC composition in a planar cell;

[0020] Figure 3A shows the reflective spectra of a LC composition in a planar cell under UV light with different times;

[0021] Figure 3B shows the reflective spectra of a LC composition in a planar cell under visible light with different times;

[0022] Figure 4 shows the reflective image in a LC composition, which can be hidden and reappeared by voltage pulses;

[0023] Figure 5A shows the reflective spectra of a LC composition in a planar cell under UV light with different times; [0024] Figure 5B shows the reflective spectra of a LC composition in a planar cell under visible light with different times;

[0025] Figures 6A, 6B and 6C show the photostationary states of LC compositions with 3 different concentrations of chiral dopant in a planar cell;

[0026] Figure 7 shows the three primary colors at photostationary states after light irradiation of a LC composition in a planar cell;

[0027] Figure 8 shows the thermal relaxation of a chiral dopant of a LC composition in a planar cell from the c/ ^' s-isomer to its frans-isomer;

[0028] Figures 9A and 9B show the response time of a LC composition in a planar cell upon irradiation of the UV light with different intensity and the visible light with different wavelength respectively; and

[0029] Figure 10 illustrates a photo-addressed and multi-switchable Ch-LC display using a LC composition to process the information in an embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

[0030] Certain terms and phrases used throughout the disclosure are intended to have the meaning provided below. Other terms and phrases may be interpreted as having the meaning thereof known and used by one skilled in the relevant field of technology.

[0031] "Photoresponsive" as used herein is the response of a material to light by altering its property such as structure, optical constant and electric constant. "Stationary reflection" as used herein is the reflection state of a material having an equilibrium chemical composition state under a specific kind of electromagnetic irradiation or a relatively stable environmental state. "Photo-stationary reflection" is a reflection state of the material with an equilibrium chemical composition state under a specific kind of electromagnetic irradiation. "An optional initial state reflection" is a reflection state of the material without any optical stimulus. "Red light" is the electromagnetic radiation having a spectrum dominated by energy with a wavelength of roughly 630-740 nm. "Green light" is the electromagnetic radiation having a spectrum dominated by energy with a wavelength of roughly 520-570 nm. "Blue light" is the electromagnetic radiation having a spectrum dominated by energy with a wavelength of roughly 440-490 nm.

[0032] In various embodiments, the liquid crystal composition can exhibit a chiral nematic liquid crystal phase, i.e. cholesteric phase, and the reflection is Bragg reflection at the planar state. The reflection can be dynamically tuned over a spectrum at least from 380 nm to 780 nm or from 780 nm to 380 nm upon an electromagnetic radiation.

[0033] In an embodiment, the stationary reflection comprises a photo-stationary reflection and an optional initial state reflection. For example, the photo-stationary reflection may be achieved with an electromagnetic radiation having a wavelength of 250 to 1500 nm, such as 365 nm, 440 nm, 450 nm, 480 nm, 520 nm, or 550 nm.

[0034] Although any liquid crystal material or mixture thereof may be used in the liquid crystal host, in preferred embodiments, the liquid crystal host comprises a nematic liquid crystal. For example, the nematic liquid crystal may comprise a cyanobiphenyl eutectic mixture, such as mixtures of aliphatic tail cyanobiphenyls, for example, those commercially known as Liquid Crystal E7, E44, and 5CB. Liquid Crystal E44 is a commercial product that can be obtained from EMD Chemicals. The liquid crystal mixture E7 was obtained from Merck Ltd., GB and consists of 51 wt% of 4-cyano 4'- pentyl biphenyl (formula A), 25 wt% of 4-cyano-4'-heptyl biphenyl (formula B), 16 wt% of 4-cyano-4'-octyloxy biphenyl (formula C), and 8 wt% 4-cyano-4'-pentyl-terphenyl (formula D):

(C)

(D)

[0035] In various embodiments, the cyanobiphenyl eutectic mixture may comprise Liquid Crystal 5CB, as shown below, together with its phase transitions:

5CB

C ►N ► I

24 °C 35 °C

[0036] In the liquid crystal composition according to the present invention, the amount of the chiral dopant may be greater than zero and lower than 11%, preferably lower than 8%, by weight based on the total weight of the liquid crystal composition. In various embodiments, the chiral dopant is photoresponsive upon exposure to electromagnetic radiation. For example, the chiral dopant may comprise a compound of formula (I);

wherein R ₃ is any group.

[0038] In specific embodiments, the chiral dopant may comprise a compound of formula (II), such as a compound of formula (III), wherein R ₂ and R ₃ may be, in

[0039] For example, R ₂ and R ₃ may be, independently of each other, selected from the groups of formulas (IV) and (V):

wherein R ₄ and R ₅ may be, independently of each other, any group, such as alkyl groups containing 1 -20 carbon atoms. In exemplary embodiments, both R ₄ and R ₅ are n-propyl or n-heptyl.

[0040] A specific example of the chiral dopant is represented by formula (VI), which is al

[0041] The invention provides new light-driven chiral molecular switches or motors which exhibit good device performance at low doping concentrations, e.g. fast and reversible phototuning of the structural reflection over the entire visible spectrum or the primary red, green and blue colors.

[0042] In embodiments, the liquid crystal composition may further include a polymer network or a polymer matrix. Polymer networks can be formed by ultraviolet light induced polymerization of monomer(s). Polymer networks are formed from a small quantity of reactive monomer(s) and photoinitiator in the cholesteric liquid crystal. The amount of chiral dopant can be adjusted to produce the desired cholesteric pitch. After the desired texture is established through the combination of surface preparations and/or an applied field, the ultraviolet light is used to photopolymerize the sample. The morphology of the resulting polymer network mimics the textures of initial cholesteric mesophases in the preparation. Factors controlling morphology are related to the LC texture, monomer concentration, photopolymerization temperature, UV intensity, and exposure time, and other similar factors. The presence of a polymer network provides similar advantages in enhancing the stability of the structure, aiding in the return of the LC orientation to the desired stable configuration, reducing the switching time, and helping to determine and maintain the chiral nematic domain size.

[0043] The method of controlling the stationary reflection of a liquid crystal composition comprises:

(i) mixing at least a liquid crystal host and a chiral dopant to prepare the composition, wherein for example , the amount of the chiral dopant is greater than zero and lower than 1 1 % by weight based on the total weight of the liquid crystal composition; and

(ii) employing a photo-stationary reflection and an optional initial state reflection of the composition to reflect at least the wavelengths of red light, green light, and blue light.

[0044] In an embodiment, the method of the invention further includes a step of irradiating the composition with an electromagnetic radiation with a wavelength of 250 nm to 1500 nm, such as 365 nm, 440 nm, 450 nm, 480 nm, 520 nm, or 550 nm to induce the photo-stationary reflection at the wavelength of red light, green light, or blue light. [0045] In an embodiment, the method of the invention further includes the step of irradiating the composition with an electromagnetic radiation with a wavelength of 250 nm to 1500 nm, such as 365 nm, 440 nm, 450 nm, 480 nm, 520 nm, or 550 nm to dynamically tune the reflection across the spectrum, at least from 380 nm to 780 nm.

[0046] As shown in Figure 1 , the present invention provides a liquid crystal device 10 including a first transparent electrically conductive substrate 101 , a second transparent electrically conductive substrate 102, and the LC composition 103. The LC composition 103 is sandwiched between, but not necessarily in contact with, the first substrate 101 and the second substrate 102. Examples of the liquid crystal device include, but are not limited to, electronic papers, dynamic reflectors, tunable filters, color filters, tunable color filters, information storage, photodisplays, and color displays.

[0047] The cholesteric LCs of the invention can also be used as photonic band-gap materials. One application of the photonic band-gap materials is to control the spontaneous emission and photo localizations in the cholesteric LC system. Cholesteric LCs, with their natural periodicity and birefringence, are one-dimensional photonic band-gap materials. Mirrorless lasing may be achieved using a dye-doped cholesteric liquid crystal. In this system, the primary role of the cholesteric liquid crystal is to provide distributed cavities that function as host cites for the active dyes, such as DCM laser dyes. The lasing may occur at the reflection band edge of the dye-doped cholesteric LC. Self-lasing at the edge of the reflection band in a pure cholesteric LC may also be achieved. In these systems, the spectral characteristics, such as the center wavelength and band width of the laser from the cholesteric LC system with a constant pitch, is fixed. The photonic band-gap can be electrically tuned, e.g. for to 33 nm, which shows the opportunity of the tuning of the spectrum characteristic of the laser using a Ch-LC system.

[0048] Various embodiments of the invention utilize an azo-chiral dopant to realize the phototuning of the reflection wavelength of the Ch-LC by photo-isomerization. The Ch-LC compound shows photo-sensitivity when exposed to visible light resulting in the selectivity of the reflection wavelength in the Ch-LC. In an embodiment, at least three primary colors (blue, green and red) can be obtained at certain wavelengths of visible light in the photostationary state (PSS), which can remove the need to employ a color filter in a display including the compound. The display may exhibit a multi-switchable property when the materials are included in a homeotropic cell in a bistable display. The information in the cell can be recorded by light and switched by the application of an electric field and/or a mechanical force.

[0049] An embodiment of the invention provides a light-driven and photo-addressed color Ch-LC display using an azobenzene chiral molecular switch QL-4, this compound being represented by:

[0050] The invention demonstrates the phototuning property of the compound to different wavelength light, thermal relaxation, and the response time of the Ch-LC display. The reflection color of QL-4 in a liquid crystal host shows the wavelength selectivity. Three primary colors (blue, green and red) can be achieved and fixed in the photostationary state at a proper wavelength of light stimuli and using a proper concentration of the chiral dopant, thereby expanding the controllability of the display.

[0051] The LC composition in the planar state can exhibit a stationary reflection of at least the wavelengths of red light, green light, and blue light. Information, such as image and data, is able to be optically written in the planar state, and the written information is hidden in the focal conic state by applying a first voltage pulse, and the hidden information may be caused to reappear using a mechanical method or an electric method. In an embodiment, the image in the Ch-LC displays recorded by light can be hidden in the focal conic texture by an electric field and caused to reappear using a mechanical force, which results in novel multi-switchable driving. Without the intension of being bound by any particular theory, it is believed that a chiral nematic LC has three textures, or states. A planar texture, where the director of the helical axis is perpendicular to the cell surface, can selectively reflect light, i.e. its optical state is reflective. If a low voltage pulse is applied normal to the cell surface, the focal conic texture is formed, wherein the director of the helical axis is more or less parallel to the cell surface. A random distribution of the helical axes is the characteristic of the focal conic texture which scatters the incident light in all directions. If an electric field at a value above a threshold value is applied, the focal conic texture is switched into a homeotropic texture, wherein the helical structure becomes unwound, with the liquid crystal director aligned in the cell normal direction.

[0052] In an embodiment, a perfect planar state can be obtained when the Ch-LC layers are in a uniform, anti-parallel, homogeneous rubbing alignment. The alignment layers are located on the inner surface of the substrates (glass or flexible substrates). In this case, the planar state and the reflective optical state of the Ch-LC are stable at zero electric field. To achieve a stable focal conic state, a small amount of polymer is dispersed in the Ch-LC materials to form an energy barrier to make the LC in the focal conic state, which is called a "polymer stabilized cholesteric display". If there are homeotropic alignment layers or weak homogeneous alignment layers located on the inner surface of the substrates, the surface energies of the two textures are about the same, and the energy between the surface energies and the elastic energies of the defects contributed by the LC director distortions is different in the planar and focal conic state. In this scenario, both the planar and the focal conic states are stable at zero field, which is called a "surface stabilized cholesteric display".

Example 1 : Chiral Dopants

[0053] The chiral molecular switch QL-5 was prepared in a facile synthesis. Its chemical structure was well identified by ¹ H NMR, ¹³ C NMR, high resolution MS and elemental analysis. The material is chemically and thermally stable, and exhibits fast, reversible, optically tunable behavior in both an organic solvent and in LC media. For example, dark incubation of a solution of QL-5 in CH2CI2 served to maximize the absorption at 354 nm corresponding to the (trans, frans)-azobenzene chromophore. Irradiation of this solution with 365 nm light resulted in clean photoisomerization to (c/s, c/s)-5, as evidenced by a decrease in the absorbance at 354 nm and an increase in the absorbance at 458 nm. Due to the molecular switch having two azo linkages, UV irradiation leads to reversible trans-cis isomerization of the azo configurations, producing two other isomers containing one or two c/ ^' s configurations, respectively. The sequence of the photochemical switch of the three isomers is (trans, trans)-5 ==> (trans, c/s)-5 ==> (c/s, c/ ^' s)-5. The reverse process from (c/s, c/s)-5 ==> (trans, c/ ^' s)-5 ==> (fr-ans, trans)-5 can occur thermally or photochemically with visible light irradiation.

[0054] The azobenzene chiral molecular switch QL-4 was also prepared and identified, in a manner consistent with that defined above for the preparation and identification of QL-5.

Example 2: LC Compositions

[0055] Doping of the chiral molecular switch QL-5 in an achiral nematic LC host even at a low concentration can induce an optically tunable helical superstructure, i.e. a cholesteric phase, as evidenced by a characteristic oily streak texture. A mixture of 6.5 wt% QL-5 in LC medium E7 (Merck, Δη=0.2246 and n ₀ =1.5216) was prepared. The helical twisting powers were measured by using the Grandjean-Cano method. The HTP value at its initial state is unusually high, and HTP in the various states of the chiral switch exhibits a considerable difference, as shown in Table 1 , which demonstrates the helical twisting powers (β) of QL-5 in the initial state and in the photostationary state (PSS) upon light irradiation of nematic E7.

Table 1 β (inoiar* ¾) μηι ^~ ' β (wt%) I m '

Initial PSSuv PSSvis initial PSSuv PSSvis

304 198 90 26 58

[0056] QL-4 has very good solubility in the commercial nematic LC hosts, such as E7, because of the introduction of two flexible longer alkyloxy chains in the molecule. The change in HTP is from 80 to 28 m ^"1 when exposed to UV light irradiation at 365 nm, and is reversibly driven back to 51 μιτΓ ¹ when exposed to subsequent visible light irradiation at 520 nm, due to the trans-cis isomerization of the azo configuration. Thermal relaxation also can reversibly shift the HTP back to the initial value.

Example 3: Reflection

[0057] A mixture of 6.5 wt% QL-5 in LC medium E7 was capillary-filled into a 5 μιτι thick glass cell with a polyimide alignment layer and the cell was painted black on one side. The wavelength of reflection light of the cell was able to be tuned starting from the UV region across the entire visible region to the near infrared region when exposed to UV irradiation at 365 nm (5.0 mW cm ^"2 ) within approximately 50 s. Its reverse process, starting from the near infrared region across the entire visible region to the UV region, was achieved by exposure to visible light at 520 nm, or dark thermal relaxation. The reflection colors were uniform and brilliant, as shown in Figure 2. The colors were taken from a polarized reflective mode microscope. With reference to Figure 2, the reflection color images were taken from 6.5 wt% QL-5 in commercially available achiral LC host E7 in the 5 μιη thick planar cell. Panel A in Figure 2 is the reflection color upon UV light at 365 nm (5.0 mW cm ^"2 ) with different times from 7 s to 43 s. Panel B in Figure 2 is the reverse back across the entire visible spectrum when exposed to visible light at 520 nm (1.5 mW/cm ² ) at the times shown.

[0058] Figures 3A and 3B show the reflective spectra of 6.5 wt% QL-5 in LC host E7 in a 5 μιη thick planar cell at room temperature. Figure 3A shows the reflective spectra under UV light at 365 nm wavelength (5.0 mW/cm ² ) with different times: 3 s, 8 s, 16 s, 25 s, 40 s and 47 s (from left to right). Figure 3B shows the reflective spectra under visible light at 520 nm wavelength (1.5 mW/cm ² ) with different times: 2 s, 5 s, 9 s, 12 s and 20 s (from right to left). The reversible process with visible light is much faster than the dark thermal relaxation. For instance, the phototuning time of 6.5 wt% QL-5 in E7 with a visible light irradiation at 520 nm (1.5 mW/cm ² ) from the near IR region back across the entire visible region to the UV region is within 20 s, as shown in Figure 3B, whereas its dark thermal relaxation back through the entire visible region took approximately 10 hours. The reflection spectra in Figures 3A and 3B show no drawback, such as the dramatic change of the peak intensity and bandwidth as compared with the electric field-induced color tuning. The reversible phototuning process was repeated many times without degradation, and was able to be achieved (across the entire visible region) in seconds with an increase in light exposure intensity.

[0059] The ability of chiral molecular switch QL-5 to quickly and reversibly phototune the reflection color over the entire visible region may result from the introduction of the mesogenic low-molecular-weight rod-like cyclohexylphenyl building block. This could induce a more dramatic geometrical change upon photoisomerization and result in the observed higher photoinduced change in the HTP and a better c/ ^' s to trans conversion ratio upon visible light irradiation. The cis-trans dramatic geometrical building block change of QL-5 improves the dose-rate in the Ch-LC host for a given concentration of dopant. This allows for lower dopant concentrations to be used, lower phototuning intensities to maintain photostationary states, and faster overall tuning response from the systems.

Example 4: Bistable Display

[0060] Like conventional cholesteric LCs, the chiral switch doped in LC media is able to be electrically switched to a bistable display by using polymer stabilized or surface stabilized chiral nematic texture. The bistability can be used to create an optically addressed display whereby the image is retained indefinitely and then erased electrically when desired. Even though the optically switched azo compounds are not thermally stable, an image can be made thermally stable and be retained indefinitely by electrically switching either the image or the image background to the focal conic state before it thermally relaxes. The image or its background is electrically selected by shifts in the electro-optic response curve due to a change in the HTP of the photosensitive chiral compound. An advantage of this display is that a thermally stable high resolution image can be captured without patterned electrodes or costly electronic drive and control circuitry, and retained indefinitely until electrically erased. A light-driven device was made using QL-5. The phototunable cholesteric layer was sandwiched between two simple, unpatterned transparent electrodes. For example, an optical writing took place within seconds in a planar state through a photomask by a UV light. The reflective image was hidden in focal conic texture by applying a 30 V pulse and was made to reappear by applying a 60 V pulse, as shown in Figure 4. By applying a 30 V pulse to this image to make the UV irradiated region go to the focal conic texture and the UV unirradiated region go to the planar texture, the optically written image was stored indefinitely since both the planar and focal conic textures are stable. With reference to Figure 4, the images are from a 5 Mm thick homeotropic alignment cell with 4 wt% chiral switch 4 in LC host E7. The image was recorded in a planar state through a photomask by a UV light (left). The image was hidden by a low voltage pulse in a focal conic state (middle), and was made to reappear by a high voltage pulse (right). Of course, this could be accomplished by a mechanical force. The image and background color in the cell were able to be adjusted by light.

[0061] The invention provides a light-driven nanoscale chiral molecular switch with high helical twisting power that can reversibly phototune the reflection color across the full visible spectrum. This chiral molecular switch was found to impart its chirality to a commercial LC host at low doping levels to form a self-organized, optically tunable helical superstructure capable of fast and reversible phototuning of the structural reflection across the entire visible region. Reversible, dynamic, full range color tuning was able to be achieved in seconds just by light. This chiral switch may also be used in a color, photo-addressed liquid crystal display driven by light, and hidden as well as fixed by application of an electric field from thermal degradation.

Example 5: Reflection

[0062] QL-4 (6.0 wt%) was doped into E7 and filled into an empty cell. The sample in a homogenous alignment cell with anti-parallel rubbing direction was prepared. The cell thickness was controlled at 5pm.

[0063] The reflection spectra of the sample in the homogenous cell were measured at normal angle by an Ocean Optics spectrometer with a light source irradiation. Figure 5A shows the reflection spectra upon exposure to UV light irradiation (365 nm, e.OmW/cm ² ) with different exposure times: 0, 6 s, 15 s, 18 s and 25 s (from left to right). Figure 5B shows the reflection spectra upon exposure to visible light irradiation (520 nm, 2.7mW/cm ² ) with different time: 0, 2 s, 10 s and 40 s (from right to left). The reflection color was tuned from blue to red color upon exposure to UV light irradiation at 365 nm, i.e., red-shift occurs. The reversible process from red to blue color, i.e. blue- shift, was achieved upon exposure to visible light irradiation such as at 520 nm. These colors were uniform and brilliant under room light condition. The peak and bandwidth of each reflection light showed good properties without obvious drawbacks, such as broadening or decreasing of the peak intensity and bandwidth, compared with that generated by an electrically-induced color tuning method. This phototuning ability offers the opportunity of photo-addressed color display without the requirement of the color filter.

[0064] The photo-tuning property of QL-4 also exhibits wavelength selectivity, i.e., the photostationary state (PSS) reflection wavelength of the Ch-LC is different when exposed to different wavelength light irradiation. Several wavelengths (440 nm, 450 nm, 480 nm, 520 nm and 550 nm) of visible light were used to reversibly drive the reflection color to check the PSS. Three different concentrations of QL-4 (5.5 wt%, 6.0 wt%, and 6.5 wt%) in E7 were investigated. The reflection spectra of PSS of 5.5 wt%, 6.0 wt%, and 6.5 wt% QL-4 in E7, respectively in a 5 pm thick homogenous cell are shown in Figures 6A, 6B and 6C. The intensity of irradiation at 440 nm, 450 nm, 480 nm, 520 nm, and 550 nm are 2.1 , 2.2, 2.9, 2.7 and 3.3 mW/cm ² , respectively. Figure 7 shows the three primary colors at photostationary states (after 550 nm, 450 nm and 440nm light irradiation) of 6.0 wt% QL-4 in E7 in a 5 pm thick homogenous cell, proving the ability to control the Ch-LC to obtain three primary colors and use the same in a full color display.

[0065] By using the Grandjean-Cano method, the HTP of QL-4 under different wavelength of light can be measured, as shown in Table 2. The HTP is different under light with different wavelength.

Table 2

HTP (wt%) under PSS

Wavelength (nm) Initial 365 440 450 550

HTP (pm ) 79 28 38 47 56

[0066] The thermal relaxation of QL-4 occurs when the c/ ^' s-isomerization is switched to the frans-isomerization. The thermal relaxation of 6.0 wt% QL-4 in E7 in a 5 m planar cell in the dark from 800 nm to 430 nm was measured, and the result is shown in Figure 8. It took 3.5 hours from photostationary red color to green color and 6 hours from photostationary green color to blue color.

[0067] The response time of 6.0 wt% QL-4 in E7 in a 5 pm planar cell upon exposure to UV and visible light are shown in Figures 9A and 9B, respectively. Form 380 nm to 780 nm, the response time is about 10 s, 25 s and 120 s for the UV light with the intensity of 18.2, 1 1.8 and 1.7 mW/cm ² . The higher the intensity of UV light, the faster the response time. For the visible light irradiation, it took 42, 39 and 210 s of reflection center wavelength from 800nm to reach PPS under 440 nm, 450 nm and 550 nm light. Compared to the thermal relaxation, the light driving process is very fast, only in seconds or minutes. When used as light-driving displays, the fast light driving process can compensate for the color degradation due to the thermal relaxation. For instance, the color can be written once again by UV or visible light before the color shift occurs due to the thermal relaxation.

Example 6: Bistable Display

[0068] To demonstrate photo-addressed color displays and switchable image information, a homeotropic cell with 5 μιη thickness was used. Figure 10 illustrates the photo-addressed and multi-switchable Ch-LC display of 6 wt% QL-4 in E7 in the cell. In Figure 10, the upper panel schematically shows the Ch-LC textures, and the bottom panel is the information process in the Ch-LC cell. The image "LCI" was recorded in the cell by a UV light at 365 nm through a photomask. The color-shifting upon exposure to UV light derives from the decrease of the HTP value of QL-4. The image can also be recorded by laser.

[0069] Ch-LC with QL-4 exhibits a multi-switchable ability which can be driven by an electric- and/or mechanical driving method. This method does not need costly circuitry or a complex ITO pattern on the surface of the substrate. Rather, only two simple unpatterned transparent electrodes are sufficient. The image recorded in the cell was hidden in the focal conic state by applying a 32 V electric pulse (100 Hz, 1000 ms). The hidden image was caused to reappear by applying a uniform mechanical force, such as a pressure (the thumb), as shown in Figure 10. This hidden-reappearance process can be repeated many times until thermal-relaxation of the azo-molecules occurs. The background color and favorite images can be changed by a light-driving method.

[0070] With regard to the mechanism of the mechanical switchable method, a first, the Ch-LC texture was an imperfect, planar texture because the homeotropic alignment layer gave a weak anchoring, which was a poly-domain structure as schematically shown in Figure 10(a). The helical axes in the domains were not exactly vertical to the substrate surface, but it still exhibited reflection. When a 30 V pulse (100 Hz, 1000 ms) was applied, the texture exhibited a focal conic texture as shown in Figure 10, (b), which was a multi-domain structure within a random alignment. There was no reflection contributed from the focal conic state, except some weak scatterings. The image was hidden. When a mechanical force such as pressure, or a squeeze was applied into the cell, as shown in Figure 10(b), a planar texture with some defects was achieved, again due to the micro-flow and shear-flow induced alignment effect. The helix orientation of the Ch-LC was aligned by the micro-flow effect to form an imperfect planar state of alignment. Although the texture was slightly different than the initial state, the reflection image reappeared as shown in Figure 10, (c).

[0071] In most situations, the micro-flow alignment effect does not lend itself to a display because it causes unwanted display defects for regular display applications. However, the foregoing example exhibits that the effect can bring out new switchable applications combined with azo-chiral molecules. Therefore, a surface-stabilized liquid crystal texture may be more suitable for the driving method.

[0072] A Ch-LC display with an azo-chiral dopant was thus achieved. The azo-chiral dopant enhances the phototuning ability of the conventional Ch-LC display. The wavelength selectivity of the azo-chiral dopant was achieved through the different wavelength light irradiations, and can be used to obtain three primary colors, thus rendering a full color display. The light-driving and photo-addressed Ch-LC display using the multi-switched method through electric and mechanical force was also achieved. It can be used for more intricate LC displays, security information storage, or LCD writing tablets. The azo-chiral dopant Ch-LC may be used as a desirable candidate for the novel light-driving display technologies due to its unique property.

[0073] The exemplary embodiments have been described with reference to the preferred embodiments. Obviously, modifications and alterations will occur to others upon reading and understanding the preceding detailed description. It is intended that the exemplary embodiment be construed as including all such modifications and alterations insofar as they come within the scope of the appended claims or the equivalents thereof.