University of Groningen

Controlling molecular chirality and motionvan Delden, Richard Andreas

IMPORTANT NOTE: You are advised to consult the publisher's version (publisher's PDF) if you wish to cite fromit. Please check the document version below.

Document VersionPublisher's PDF, also known as Version of record

Publication date:2002

Link to publication in University of Groningen/UMCG research database

Citation for published version (APA):van Delden, R. A. (2002). Controlling molecular chirality and motion. Groningen: s.n.

CopyrightOther than for strictly personal use, it is not permitted to download or to forward/distribute the text or part of it without the consent of theauthor(s) and/or copyright holder(s), unless the work is under an open content license (like Creative Commons).

Take-down policyIf you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediatelyand investigate your claim.

Downloaded from the University of Groningen/UMCG research database (Pure): http://www.rug.nl/research/portal. For technical reasons thenumber of authors shown on this cover page is limited to 10 maximum.

Download date: 19-02-2020

103

Chapter 4

Controlling the Color of Cholesteric Liquid Crystals

This chapter describes research into the doping of colored cholesteric liquid crystals withchiroptical molecular switches. Colored cholesteric phases can be applied in liquid crystaldisplay technology and the combination with the photoswitchable compounds described inprevious chapters might offer an all-optical liquid crystal display. First, in this chapter thebasic theory concerning colored cholesteric liquid crystals and color LCD's is discussed.Details on the tuning of cholesteric phases by light are presented and a locking mechanismbased on photopolymerization of the matrix is introduced.

Chapter 4

104

4.1 Introduction

In the previous chapter we showed that liquid crystals form excellent host materials for ourmolecular switches. These switch-doped cholesteric liquid crystals offer a processableswitching material, essential for any given application of these systems. In their cholestericpacking, the mesogenic host molecules also amplify the molecular chirality of the guestmaterial, in this case the chiroptical switch, in a macroscopic chiral helical packing. As suchthe liquid crystal chirality reflects the state of the chiral switch and this effect can be used in anon-destructive read-out procedure through cholesteric pitch determination. This is only oneadvantage of employing liquid crystals. Also, in all doped liquid crystals described in Chapter3, a liquid crystalline phase transition is induced by the chiral switchable dopant. Here thechiroptical molecular switch as a LC dopant functions as a trigger of a liquid crystal phasetransition. This holds for the initial doping, where a cholesteric phase is induced from anematic phase, which is a common feature for a large variety of chiral guest compounds.1 Ofmuch greater importance, though, is the change in handedness of the cholesteric phaseinduced by the switching process. A transition from a cholesteric phase with a certain screwsense, via a compensated nematic phase, to a cholesteric phase of opposite screw sense istriggered. 2 Upon irradiation at a suitable wavelength, cholesteric phases with intermediatepitches can be induced by varying irradiation time, wavelength, or intensity of the light. Thistriggering of a liquid crystalline phase opens opportunities to use this type of system for anoptically addressable liquid crystalline display, especially when considering the opticalproperties of cholesteric liquid crystals.

4.2 Optical Properties of Cholesteric Liquid Crystals

The molecular features of cholesteric liquid crystals have been discussed in Chapter 3.Cholesteric phases can be assigned by a helical packing of the mesogens with a certain signand a certain pitch.3 This pitch, the distance in a liquid crystal needed for the director of theindividual mesogens to rotate through a full 360 degrees, is a measure of the chirality of thesystem. It was shown in Chapter 3 that cholesteric phases can amplify the molecular chiralityof a switchable compound resulting in pitches in the micrometer range. When the pitch isfurther decreased to values that resemble the wavelengths of visible light, the correspondingcholesteric phases show unique optical properties as schematically depicted in Figure 4.1.4

These cholesteric phases, when illuminated with white light, reflect light of a certainwavelength dependent on the pitch of the LC phase. The reflected light is circularlypolarized: a right-handed cholesteric phase, as depicted in Figure 4.1, is known to reflectright-handed circularly polarized light (r-CPL) of a certain wavelength while left-handedcircularly polarized light (l-CPL) is transmitted.

Controlling the Color of Cholesteric Liquid Crystals

105

Right-handedcholestericphase

r-CPL

α

pitch

l-CPL

Figure 4.1 Schematic representation of a cholesteric phase and its optical properties.

A cholesteric LC layer of one pitch length thickness is depicted in Figure 4.1. The reflectionobserved is of a Bragg-type caused by the repetition of this helical packing.5 The wavelengthof the most important perpendicular reflection (λO) is given by Equation 4.1.

pn ×=⊥λ (4.1)

In this equation n is the average refractive index of the liquid crystalline sample and p is thecholesteric pitch. This wavelength of reflection is strongly angle dependent. With α being theangle of the incident light relative to the normal, this dependency is given by Equation 4.2,which reduces to equation 4.1 for α = 0°.

( )

××= −

npn

ααλ sinsincos 1 (4.2)

The interest in colored doped cholesteric phases in the current research on chiropticalmolecular switches is twofold. When chiroptical molecular switches can induce pitch lengthsresembling the dimensions of the wavelength of visible light, a direct color read-out ofwritten information is possible. On the other hand, direct color-tuning of the cholesteric phaseopens the opportunity to develop color liquid crystal displays addressable by light i.e. color

Chapter 4

106

pixel formation. In order to put the present research in perspective, a brief treatise on liquidcrystalline displays in general and colored liquid crystalline displays in particular isnecessary.

4.3 Liquid Crystalline Displays

Since the feasibility of liquid crystalline displays was proposed by Heilmeier in 1968,6 manyscientists have been engaged in improving the characteristics of such displays.7 This hasresulted in widely known black-and-white LCD's in for example calculators, wristwatches,and mobile phones and color LCD's in laptop computers, and photo and video cameras.Nowadays, desktop monitors and, at a higher price, even flat-screen televisions have becomereadily available to the general public.

4.3.1 Fundamental Features of Liquid Crystal CellsLiquid crystalline displays can be based on different aspects of liquid crystalline matrices.The first LCD to be used in the late 1960s was based on dynamic scattering of a nematicliquid crystalline material.8 This material is aligned between two electrodes in aperpendicular fashion (cf. the parallel alignment of a LC phase discussed in Chapter 3). Thealignment of the material results in a transparent relaxed state of this liquid crystal cell. Whena voltage is applied to this cell the ordered liquid crystalline phase is severely disturbedresulting in strong light scattering and a frosty appearance of the cell in this energized state.This rather primitive LC cell has several disadvantages, mainly contrast problems, anddifferent improved alternatives have been developed. An illustrative example of such animproved system is the twisted nematic (TN) LC cell that is the first type of LCD used forreal applications.9 The basic principle of a TN-LC cell is depicted below in Figure 4.2. Anematic liquid crystal is sandwiched between two perpendicularly oriented alignment layersto which transparent electrodes are attached. These alignment layers are in essence the sameas the rubbed polyimide-covered glass-surfaces discussed in the previous chapter. Thisperpendicular orientation forces the nematic material to adopt a helical packing resulting in a90° twist of the director of these molecules within the cell. Note that the obtained packingclosely resembles the cholesteric packing; the twist of the mesogens here corresponds to aquarter of a pitch in a cholesteric phase. This LC cell is covered with two polarizers orientedperpendicular to each other. Employing unpolarized backlight, the first polarizer onlytransmits linear polarized light. The polarization direction of this light follows the twistedstructure of the cell when transferring because the twisting pitch of the director of the twistedliquid crystal layer is relatively large compared to the wavelength of visible light. The 90°twist of the director of the LC material results in a 90° rotation of the polarization of the light.After passing the LC layer the light can therefore pass the second perpendicular polarizer andthe cell appears white in this case. When a voltage is applied to this system the LC moleculesare forced to twist their director perpendicular to the electrodes, parallel to the electric fieldapplied, due to the dipolar nature of the individual molecules. The plane of the polarized lightin this state is unaffected by the LC material. The linearly polarized light is thereforecompletely blocked by the second polarizer resulting in a black LC cell. Although also thistype of LC cell has some disadvantages, it is generally used in calculators and watches. Also

Controlling the Color of Cholesteric Liquid Crystals

107

super-twisted nematic (STN) cells are known where, instead of rotating the plane polarizedlight over 90°, twist angles of 180-270° are induced in a similar fashion to that illustratedabove for the TN-LC cell. 10

back light

off on

polarizer

electrodepolarizer

electrode

alignment layer

alignment layer

Figure 4.2 Principle of a twisted nematic liquid crystal cell.

In addition to the transmissive mode LCD's, reflective mode LCD's can be employed. Mostof the basic characteristics and properties of these two types of cells are the same. In principleall the described liquid crystal cells can also be used in a reflective mode where backlightingis not necessary but ambient light is used. This makes the LCD less energy consuming but italso has a dramatic effect on the brightness of the LCD and the requirements of the liquidcrystalline phase employed. The principle of a reflective mode LC cell,11 although there are alarge variety of other examples, can easily be explained for a TN LC cell, where thetransmissive analogue was depicted in Figure 4.2. The only necessary extension is a mirror(Figure 4.3).

back light

off off

incident light

reflected light

transmitted light

mirror

Figure 4.3 Reflective mode (right) versus transmissive mode (left) TN LC cell.

Chapter 4

108

The plane polarization of the incident light beam after passing the front polarizer (bottomFigure 4.3) is rotated through 90° and passes the back polarizer. The light is then reflected bythe back mirror where the plane polarization is unchanged. The reflected light passes the backpolarizer and, as with the inward pathway, the polarization is again rotated over 90° andallowed to pass the front polarizer leading to a white pixel for the viewer. Applying a voltageto this reflective mode TN cell has the same effect as for the transmissive analogue; thepolarization of the light is unaffected by the LC matrix, no light passes the back polarizer andmirror, and no light is reflected. This results in a black pixel for the viewer (analogous toFigure 4.2).

Major disadvantages of twisted nematic cells, whether in transmissive or reflective mode,arise when applying this concept to larger displays, where they suffer from contrastdeterioration and long switching times. Another major problem in early examples of LCD'slies in the addressing of the matrix rather than the material properties. An importantimprovement compared to the first systems is the use of thin-film transistors (TFT) to switchthe LC matrix.12 This so-called active matrix addressing can be applied to all electronicallyaddressable displays and is found in all laptop computers. Of course, for this type ofapplication full color displays are nowadays required.

4.3.2 Colored Liquid Crystal Displays

4.3.2.1 Transmissive Mode Colored Liquid Crystal CellsDifferent types of color liquid crystal displays that function in a transmissive mode have beendeveloped. Three types that will be discussed here are a birefringence mode, a twistednematic mode, and a host-guest liquid crystal cell.

Birefringence Mode Liquid Crystal CellsAlthough all known LCD's make use of the birefringence of the mesogenic molecules,birefringence mode LC cells, although hardly used, are based on the fact that when tiltedaligned liquid crystalline phases are employed, two different modes of light propagation existdue to anisotropy of the refractive index of the individual molecules.13 A perpendicularlyaligned nematic phase between crossed polarizers results in a dark LC cell. When a voltage isapplied the individual mesogenic molecules are tilted with respect to the electrodes resultingin a birefringent state. In this birefringent state the velocity and the pathway of the incidentlight is dependent on its polarization relative to the tilted individual mesogenic molecules.The different components of the light interfere with each other and as a result ellipticallypolarized light emerges from the liquid crystal. When applying a polarizer to this ellipticallypolarized light only some components of the spectrum, that is only some colors of light areallowed to pass. The color of the transmitted light is dependent on the tilt angle of themesogenic molecules, which in turn is dependent on the applied voltage. In this way a fullcolor LC cell can be developed.14 The system has severe restriction, however, when appliedto larger displays.

Controlling the Color of Cholesteric Liquid Crystals

109

Twisted Nematic Mode Liquid Crystal CellsAlthough the birefringence cell offers an elegant voltage dependent color display, the liquidcrystal cells most often used in LCD application use color filters as a means to display colors.The most widely used LCD's employ a TN-LC cell as depicted in Figure 4.2 or variationsbased on this concept in combination with colored subpixels to induce colored pixels (Figure4.4).15,16 In the RGB (red, green, blue) mode one pixel consists of three subpixels with a red,green and blue filter. The subpixels can individually be electronically addressed by an active-matrix TFT by a combination of column and row addressing lines where every subpixel hasits own transistor. In the relaxed state each subpixel rotates the plane-polarized light leadingto full transmission of light. This transmitted light falls on a colored polarizer leading to red,green or blue light. A fully relaxed pixel transmits the three basic colors resulting in anobservable white color. The entire visible spectrum, just like in every ordinary cathode raytube used in traditional televisions, can then be induced by activating combinations ofsubpixels. The fully activated state completely absorbs the incident light leading to a blackpixel. The mode of action can be inverted by using parallel placed polarizers: now activatedsubpixels are transmissive and relaxed subpixels fully absorb the incident light.

fluorescentbacklight

pixel

subpixel

liquid crystal

vertical polarizing filter horizontal polarizing filter

glass plate glass plate

subpixel electrode

column addressing line

row addressing line front plate

viewer

red green

blue

color filters

Figure 4.4 Operating mechanism of a standard color active-matrix TFT TN LCD.17

Host-Guest Liquid Crystal CellsAlthough the twisted nematic cells fulfill all requirements of a liquid crystalline display to anacceptable extent, as can readily be seen from any laptop computer, host-guest type cellsoffer an alternative approach.18 On a molecular basis these systems closely resemble thedoped cholesteric phases described in the previous chapter. Nematic LC hosts alignedbetween two electrodes are again used. As guest molecules (dopants) dichroic or pleochroicdye molecules are used.19 These molecules absorb light of a certain wavelength like any dyebut this absorption is anisotropic. They absorb more of a given color of light when thepolarization of the light is parallel to their optical axis than when it is perpendicular to thisaxis. The alignment of the nematic host molecules can be changed by applying a voltage asdiscussed before and this orientational change is followed by the guest material resulting in acolor change of the liquid crystal cell. A full color display can be obtained by combining

Chapter 4

110

three basic color dyes in a row (subtractive mode) or, as illustrated for the TN LC cell,parallel to each other (additive mode).

4.3.2.2 Reflective Mode Colored Liquid Crystal CellNext to applying the above examples of color LCD's in a reflective mode by employingmirrors, cholesteric liquid crystals can be used to generate colors in a reflective mode. Asdiscussed above cholesteric phases show unique light reflection properties when the pitchesare in the range of the wavelength of visible light. Although this is a different type ofreflection compared to reflective TN LC cells, these systems can also be used for LCDapplications. A pitch in the nanometer range can be obtained both by using chiral mesogensand by using nematic phases doped with chiral dopants. When such a LC phase is alignedbetween two electrodes a selective light reflection (color) is observed. Applying a voltagedisturbs the cholesteric packing and the color disappears. The pixel then becomes black whena black absorbing layer is applied underneath. When electronic switching between a coloredpixel and a dark pixel and electronic tuning of the pitch length is possible, the chiralproperties of the cholesteric material can be tuned to obtain all desired colors.20 Bycombining red, green and blue-colored cholesteric cells, as depicted in Figure 4.5, a full colorreflective display can be obtained.21 In the relaxed state as depicted all three basic colors arereflected resulting in a white display. By applying a voltage to either of the stacked three LCcells the cholesteric packing is disturbed and the whole visible spectrum can be generated.

blue green red

blue

green

red

Figure 4.5 Principle of a reflective full color cholesteric liquid crystalline display.

All the discussed devices, although already widely applied, make use of the so-called electro-optical response of a liquid crystalline material. The addressing of the cells or individual(sub)pixels in a LCD application is performed electronically in all cases by applying avoltage. A liquid crystalline matrix addressable by light could offer a useful alternativeconsidering the resolution, where pixel-sizes for a photo-addressable LCD are limited only bythe dimensions of light, and the speed of addressing, although relaxation of the mesogenichost molecules will be the rate determining factor for any application. Cholesteric liquidcrystals with photocontrollable pitches in the range of visible light also offer the advantage ofdirect color control rather than electronic control of the three basic spectral components.Where the currently used and discussed examples still have a switchable on and off state as a

Controlling the Color of Cholesteric Liquid Crystals

111

basis, full pitch control was already realized for chiroptical molecular switches doped inliquid crystals (as discussed in the previous chapter). The rest of this chapter will focus onphotoswitchable colored LC phases, which could in the future lead to full color LCDapplications.

4.4 Photocontrollable Colored Cholesteric Phases

The first observation of direct photochemical color control of a cholesteric liquid crystallinephase was made with a mixture of cholesteryl nonanoate and cholesteryl iodide, a mixturethat displays a cholesteric phase.22 Upon UV irradiation the cholesteryl iodide is decomposedin an irreversible process resulting in an observed shift of the wavelength of reflection from535 nm to about 630 nm (green to red). This wavelength of reflection was monitored at a 30°angle relative to the normal of the sample with an incident light beam at a 60° angle relativeto the normal. An early example of reversible color changes employs the photoisomerizationof azobenzene in a cholesteric mixture of cholesteryl nonanoate and cholesteryl chloride.Irradiation at 420 nm results in a maximal change in normal reflection from 610 nm in thecis-state to 560 nm in the trans-state.23 Also chiral switchable azobenzenes have been used asdopants to change the reflection wavelength properties of a cholesteric liquid crystal.24 Hereagain not the chiral unit itself is switched but merely the geometry of a chiral molecule ischanged upon switching. Maximum reflection shifts reported for these systems areapproximately 100 nm and are dependent on the exact composition of the cholesteric phase.There are relatively few examples of doped low molecular weight cholesteric liquid crystalsfor which the wavelength of reflection could be controlled by light irradiation. Beyond theazobenzene-doped examples, one example based on our chiroptical molecular switches isdiscussed below. Most of the literature examples employ cholesteric polymers.25,26

Polymer liquid crystals have an important advantage over low-molecular-weight systemsbecause stable storage of information is possible in the glassy state of the polymers where thehelical packing is maintained. This can be achieved either by cooling a polymeric liquidcrystalline system below the glass transition temperature27 or by employing(photo)polymerizable units in the liquid crystalline phase.28 A large variety of copolymerswith liquid crystalline properties and switching units, mainly based on menthyl-basedswitches29 or binaphthyl switches30 are described. An illustrative example for which nearlyevery wavelength within the visible spectral range could be achieved by irradiation, waspublished by van de Witte et al.31 A polyacrylate copolymer consisting of 33% ofphotoisomerizable (-)-2-arylidene-p-menthane-3-one substituted monomers 4.1 (Scheme 4.1)was mixed with E7 at 50 weight% to yield a cholesteric polymer-dispersed liquid crystal(PDLC).32 In the stable trans-state the helical twisting power of the chiral unit was 16.7 µm-1

(6.6. µm-1 for the polymer) while upon irradiation at 365 nm for 1 h this value was reduced to2.8 µm-1. For the PDLC with 50 weight% polymer the initial trans-state shows a reflectionwavelength of about 440 nm (blue-green color). Upon irradiation at 365 nm the reflection isbathochromically shifted to the red-end of the visible spectrum to approximately 720 nm(red) after only 4 min of irradiation. In this way, by varying the irradiation time, every color

Chapter 4

112

can be readily generated. It should be noted that a large quantity (50 weight%) of chiralcompound is used and this is rather a mixed system than a doped one. Nevertheless, at roomtemperature, these phase are stable for several months while heating results in regeneration ofthe initial color by thermal cis to trans back isomerization.

( )6 ( )6

OO

O

OO

CN

OO

O

OO

O

0.67 0.33

( )6 ( )6

OO

O

OO

CN

OO

O

OO

O

0.67 0.33

365 nm

4.1

Scheme 4.1 Cholesteric color control by photoisomerization of arylidene-p-methane-3-one unitin copolymer doped system 4.1.

4.5 Donor-Acceptor Switches in Polymerizable Liquid Crystals

Although the results obtained in E7 doped systems (Chapter 3) are promising improvementscompared to results with previously developed switch systems; the generation of colorsimplies pushing the limits of the low molecular weight system. E7 samples were preparedwith high dopant concentration of the n-hexylmethylamine-substituted donor-acceptor switch4.5 (discussed in the previous chapter and depicted in Scheme 4.2). Samples of 18.7 and 25.6weight% (P)-trans-4.5 proved to be stable enough to obtain Grandjean-Cano textures. Fromthe determined helical twisting power (β = + 13.5 µm-1) and the measured concentration,pitches of approximately 396 and 289 nm, respectively, are expected for these samples. Theseproperties might lead to colored LC phases, depending on the morphological influence ofsuch a high concentration of dopant. Pitch measurements, however, showed that the actualpitches obtained were + 2.8 and + 2.5 µm, respectively, for the 18.7 and 25.6 weight%samples. These results indicate non-linearity of the helical twisting power upon increasingconcentration. This might be caused by some kind of saturation of the cholesteric phasewhere increasing the concentration does not have an additional influence which, although notdirectly observed through a polarization microscope, might even lead to phase separation.These chiroptical molecular switch doped low molecular weight liquid crystals might further

Controlling the Color of Cholesteric Liquid Crystals

113

be improved to eventually form colored phases. For instance, the compatibility of the dopantscan be enhanced by structural modification or fine-tuning of the liquid crystalline host, butstability will certainly continue to be a severe problem.

To obtain LC phases stable enough for any given application, a different system has to beused. For two important reasons, the system of choice was a chiral polymerizable cholestericacrylate mixture (4.2 and 4.3) developed by Philips Research.33 First of all, due to thepresence of an achiral monoacrylate and a chiral diacrylate (Figure 4.6), this system has thepossibility to photopolymerize when a suitable photoinitiator is present. Thisphotopolymerization locks the cholesteric helix to generate a stable polymeric matrix with allthe optical properties of the initial liquid crystal matrix. This allows stable storage ofinformation. A second important property of this system is that due to the presence of a chiraldiacrylate the host liquid crystalline phase is already cholesteric. The exact properties aredependent on the ratio of the two components. For the present research a mixture of 40%chiral (S,S)-diacrylate 4.2 with 60% achiral monoacrylate 4.3 was used. This mixture forms agreen cholesteric phase with a maximum reflection wavelength of about 440 nm. Uponpolymerization the material shrinks slightly, thereby decreasing the reflection wavelength by5-15 nm depending on the amount of polymerization inhibitor used.

OO

O

O

OO

O

OO

OO

O

O

OO

O4.3

4.2

Figure 4.6 Photopolymerizable cholesteric mixture of monomeric acrylates 4.2 and 4.3.

An important result of using a cholesteric host is that upon doping with a chiroptical switchthe color of the LC phase only has to be influenced rather than fully induced. This mightreduce the relative amount of dopant needed. It should be noted that the chiral dopant, in thiscase a switch, should be able to compete with the chiral influence of the diacrylate and assuch should still show sufficiently large helical twisting power and compatibility.

4.5.1 Dimethylamine Nitro SwitchThe applicability of our chiroptical molecular switches in this mixture of monomericacrylates was first tested on the parent donor-acceptor switch 4.4 (Scheme 4.2) with adimethylamine electron donor and a nitro electron acceptor moiety by N Huck.34 Both (M)-

Chapter 4

114

trans-4.4 and (P)-trans-4.4 isomers were used and shown to have a minor effect on thecholesteric phase of the monomeric mixture. The initial undoped mixture showed a reflectionat 458 nm whereas after doping with 4.0, 7.0 or 9.5 weight% of (M)-trans-4.4 the reflectionwavelength was reduced to respectively 455, 456 and 446 nm. After doping with 4.0, 7.0 or9.5 weight% of (P)-trans-4.4 the reflection wavelength was surprisingly also reduced torespectively 435, 425 and 420 nm. In almost all cases the pitch was further decreased onformation of the cis photostationary states by irradiation at 435 nm. The effect was mostdramatic for the (M)-trans-4.4 doped phases where a minimum reflection wavelength of 425nm was found for the 9.5 weight% sample. The cis-isomers of 4.4 were not tested separately.

This 30 nm change in reflection wavelength is of course too small for any given applicationbut remarkable behavior was found when polymerizing these photostationary systems. In allcases photopolymerization at 360 nm for 5 min resulted in a distinct red shift of thewavelength of reflection to a maximum of 500 nm for the 9.5 weight% doped sample. A blueshift due to shrinkage of the material was expected. The exact nature of this effect is notknown, but it is likely that irradiation at 360 nm results in simultaneous polymerization andcis to trans switching in the cholesteric phase. This switching of the dopant could elongatethe cholesteric pitch during polymerization. Similar effects were found for both the (M)-trans-4.4 and (P)-trans-4.4 isomers indicating that these effects are mainly caused by ageneral disturbing influence of the dopant rather than a chiral effect. Unequivocal evidence isnot present however since diastereomeric interactions may play a role. With this system,using 9.5 weight% of (M)-trans-4.4 dopant, color tuning between 425 and 500 nm is possibleand it should theoretically be possible to generate polymerized materials with reflections inbetween these two extremes by changing the initial irradiation time. Increasing the amount ofdopant might offer a means to cover a larger part of the visible spectrum but, due to moderatecompatibility of this donor-acceptor system, 9.5 weight% is the maximum dopant loadingaccessible. Again a more compatible system is required and the n-hexylmethylamino nitroswitch 4.5 discussed in the previous chapters seems to be a very promising candidate a priori.

S

S

NO2NR

(M)-cis

S

S

NO2NR

(P)-trans

λ1

λ2

R = Me R = n-hexyl

4.44.5

Scheme 4.2 Donor-acceptor substituted chiroptical molecular switches 4.4 and 4.5.

Controlling the Color of Cholesteric Liquid Crystals

115

4.6 Hexylmethylamino Nitro Switch

4.6.1 Photochemical Properties of Switchable DopantA sample of 2.6 weight% of a racemic cis-trans mixture of 4.5 was tested in the same way aspresented in the previous chapter for a variety of low molecular weight nematic liquidcrystalline host materials. Here, in contrast to the other hosts, doping of a liquid crystallinemixture of 40% 4.2 and 60% 4.3 (forthwith denoted as 4.2/4.3) with racemic cis-transmixture of 4.5 resulted in a large decrease in the solid (C) to cholesteric (N*) phase transitiontemperature from 57°C to 40°C. This decrease in temperature was even more pronounced forsamples of higher concentration, where doped samples with 10 and 12.5 weight% ofenantiomerically pure (M)-trans-4.5 (vide infra) resulted in liquid crystallinity at roomtemperature. This is a very desirable feature of this system since for any LCD applicationliquid crystallinity at room temperature is an absolute requirement.

The switching selectivity for 4.5 was tested on samples of 4.2/4.3 doped with 2.6 weight% ofa racemic mixture of cis- and trans-isomers of 4.5. In a chiral environment such as thischolesteric liquid crystal matrix, the two photoisomerizations are no longer trulyenantiomeric pathways. The exact absorption characteristics of (M)- and (P)-trans-4.5 andalso of (M)- and (P)-cis-4.5 can be different in a chiral environment because ofdiastereomeric interactions. No attention was paid, however, to any effects concerning thisdiastereomeric relationship, which are expected to be small or negligible. The switchingselectivity in this cholesteric host resembled the selectivities found in other nematic liquidcrystalline hosts and in n-hexane solution (see Chapter 3). Upon 435 nm irradiation aphotostationary state was reached with a ratio cis-4.5 : trans-4.5 of 67 : 33. Irradiation at 380nm resulted in a photostationary state of 31% cis-4.5 and 69% trans-4.5. It should be notedthat also in this case these wavelengths are probably not the most efficient wavelengths forswitching.

4.6.2 Controlling the Color of Cholesteric Liquid CrystalsThe first requirement for donor-acceptor system 4.5 is an improved compatibility, comparedto compound 4.4, with the polymerizable liquid crystalline host 4.2/4.3. As described inChapter 3, a better compatibility was indeed found for low molecular weight liquid crystals.Next, compound 4.5 was tested in the cholesteric liquid crystalline acrylate mixture 4.2/4.3.In view of the similar chiral properties of 4.5 compared to the original donor-acceptor switch,(M)-trans-4.5 was used in all the following experiments. Samples were prepared withincreasing weight% of dopant (5 to 20 weight%). Stable samples were obtained with up to 15weight% dopant. The transmission characteristics of these samples were measured on alignedand spin-coated samples. The samples were aligned on a polyimide covered glass surfacefrom toluene solution and spin-coated. In this way, colored thin films of doped LC materialwere obtained, which were used for transmission measurements. The measured transmissioncurves were recalculated to show reflection characteristics.35 A schematic representation ofthe processes involved is depicted in Scheme 4.3.

Chapter 4

116

doping irradiation polymerization

acrylate 4.2/4.3

monomer

acrylate monomer+

(M)-trans-4.5

acrylate monomer+ PSS

(M)-trans-4.5 / (P)-cis-4.5

acrylate polymer+ PSS

(M)-trans-4.5 / (P)-cis-4.5

Scheme 4.3 Schematic representation of processes involved in cholesteric acrylate mixture4.2/4.3 doped with chiroptical molecular switch 4.5.

In all doped cases, the reflection wavelength was red-shifted compared to the undopedmixture. The red-shift increases with increasing concentration up to 12.5 weight% as depictedin Figure 4.7. For the 15 weight% sample the observed reflection wavelength was found tohave blue shifted by 49 nm relative to the 12.5 weight% sample, indicating some instability atthis higher concentration. This might be caused by phase separation but this was not visuallydetected.

400 500 600 7000

20

40

60

80

100

undoped

15 w%

12.5 w%

10 w%

5 w%

refle

ctio

n (%

)

wavelength (nm)

Figure 4.7 Influence of the concentration of dopant 4.5 in 4.2/4.3 on the reflection, wavelengthand intensity of aligned and spin-coated cholesteric phases.

Controlling the Color of Cholesteric Liquid Crystals

117

Switching experiments were performed on an aligned sample with 10 weight% dopant in thepresence of 1 weight% of photoinitiator (Irgacure 651) and 1 weight% of inhibitor (p-methoxyphenol) at 435 nm irradiation. At this wavelength no polymerization is initiated anda trans to cis isomerization of the chiral dopant is the only process observed. Thephotostationary state at this wavelength, determined at lower dopant concentration (2.6weight%), was found to consist of 67% cis and 33% trans (vide supra). Uponphotoisomerization the reflection band of the LC film was gradually shifted to shorterwavelength (Figure 4.8). Starting at a reflection wavelength of 596 nm, a blue shift of thereflection wavelength to 524 nm was observed at the photostationary state after 150 sec ofirradiation.

450 500 550 600 650 7000

20

40

60

80

100 increasing irradiation time

refle

ctio

n (%

)

wavelength (nm)

Figure 4.8 Color tuning by photoisomerization at 435 nm of 10 weight% of switchable dopant(M)-trans-4.5 in cholesteric host material 4.2/4.3, decreasing line thickness indicates increasingirradiation time (t = 0, 30, 60, 90, 120 and 150 sec).

For the 12.5 weight% sample this wavelength shift was more pronounced. Starting at areflection wavelength of 666 nm, under irradiation at 435 nm, a photostationary state wasreached with a reflection wavelength of 541 nm. This represents a blue shift of 125 nm(Figure 4.9). As already indicated in the previous chapter, however, this wavelength of 435nm is not the most efficient switching wavelength for compound 4.5. Indeed, when increasingthe irradiation wavelength using a 450 nm cut-off filter, the wavelength of reflection couldfurther be decreased to a value of 526 nm. This blue shift of 140 nm can also be induceddirectly by > 450 nm irradiation of the initial pure (M)-trans-4.5 doped film. The differencebetween 4.5 and 4.4 is striking, as for the parent compound 4.4, upon 435 nm irradiation(which for that compound is the most efficient wavelength) a maximum wavelength shift ofonly 24 nm is observed.

Chapter 4

118

500 550 600 650 700 7500

20

40

60

80

100 color control by irradiation

refle

ctio

n (%

)

wavelength (nm)

Figure 4.9 Change in reflection wavelength by photoisomerization at 435 nm (thin solid line)and subsequently 450 nm (dashed line) of 12.5 weight% of a thin LC film of switchable dopant4.5 in cholesteric host material 4.2/4.3 at its photostationary states.

Subsequent photopolymerization of the LC film at the photostationary state was effected by 5min irradiation at 365 nm in vacuo. At this wavelength cis to trans back isomerization isexpected to some extent. After the irradiation the liquid crystalline phase is polymerized anda rigid polymer matrix was obtained. The influence of the photopolymerization on theobserved reflection wavelengths is complicated. At low concentration (5 weight%)photopolymerization resulted in a red shift of the reflection band as also observed for theparent compound. The 10 and 12.5 weight% samples did not show this effect. At 10 weight%the cholesteric packing was apparently unaffected by the irradiation, while at 12.5 weight% aslight blue-shift, which is more or less a broadening effect was observed (Table 4.1).

Weight%(M)-trans-4.5

Reflectionwavelength

ReflectionwavelengthPSS 435 nm

ReflectionwavelengthPSS 450 nm

Reflection wavelengthafter photopolymerization

at 365 nm

5 564 nm 536 nm - 618 nm10 698 nm 536 nm - 538 nm

12.5 666 nm 541 nm 526 nm 518 nm15 617 nm - - -

Table 4.1 Influence of irradiation (photoisomerization and subsequent photopolymerization) on thereflection wavelength of the cholesteric LC phase (4.2/4.3)

For the 12.5 weight% sample, which is most promising for any color LC application, theeffect of the photopolymerization was also tested for the initially pure (M)-trans-4.5 dopedphase showing a 666 nm reflection in the monomeric state. Here, photopolymerization at 365nm changed the reflection wavelength to 632 nm, which can be explained by shrinkage of the

Controlling the Color of Cholesteric Liquid Crystals

119

material. This is also observed for undoped samples and caused by the fact that the polymeris of smaller dimensions than the monomer mixture. Combined with the photostationarymixture, which showed a reflection of 526 nm and gave a polymerized matrix with a 518 nmreflection, this system constitutes a "write and lock" mechanism for color information (Figure4.10). Although there is a small degree of cis to trans isomerization on thephotopolymerization process, the obtained polymeric phases in all cases reflect thephotostationary state of the chiral dopant. The polymerized phases are completely inert toprolonged irradiation and the photochemically written color information is locked. Startingfrom 12.5 weight% (M)-trans-4.5 in acrylate mixture 4.2/4.3, writing is done by irradiationwith 450 nm light and cholesteric phases with pitches between 666 nm and 526 nm can beinduced by varying the irradiation time. Color inspection (red to green) offers an easy read-out procedure. Further irradiation of this LC phase will result in a change in the (M)-trans-4.5to (P)-cis-4.5 ratio and consequent change in the wavelength of reflection (the color) of theLC film, as long as the 450 nm photostationary state is not reached. This monomeric state canbe considered a rewritable state. Upon photopolymerization, the LC matrix will harden andthe written information is locked. The polymerization process is accompanied by a slightchange in the chiral properties of the liquid crystalline phase. Polymerized cholesteric filmwith pitches between 632 nm and 518 nm can be obtained, dependent on the (M)-trans-4.5 :(P)-cis-4.5 ratio before photopolymerization. Again color inspection (orange to green) offersan easy read-out procedure which is now absolutely non-destructive. The information inlocked, there is no change in cholesteric pitch observed upon further irradiation.

500 600 700 8000

20

40

60

80

100locking information

color control

refle

ctio

n (

%)

wavelength (nm)

B

A

C

D

Figure 4.10 Color control to write information and photopolymerization as a lockingmechanism for a 12.5 weight% sample of (M)-trans-4.5 in 4.2/4.3. Wavelength of reflection atdifferent stages of the process: A) initial sample; B) photostationary state after 450 nmirradiation; C) polymerized sample after 365 nm irradiation of the initial sample (A); D)polymerized sample after 365 nm irradiation of the photostationary state (B).

Chapter 4

120

For a real switch system these one-direction experiments only constitute half the process,since controlled cis to trans isomerization should also be feasible. The photostationary stateupon irradiation at 380 nm was determined to be 69% (M)-trans-4.5 : 31% (P)-cis-4.5. Thereversibility of the process was also briefly tested. Irradiation at 365 nm of a 10 weight%sample of 4.5 in 4.2/4.3 leads to a competitive irradiation of the switchable dopant and thephotoinitiator for the polymerization process. In vacuo this resulted in polymerization and theobserved changes in reflection wavelength (vide supra). Atmospheric conditions preventpolymerization and irradiation under these conditions resulted in a red shift of the reflectionwavelength indicative of cis to trans back isomerization. This cis to trans switching wastested for the 10 weight% sample of 4.5 in 4.2/4.3 at the 435 nm photostationary state whichshowed a wavelength of reflection of 536 nm (vide supra). The resulting 365 nmphotostationary sample showed a reflection band at 551 nm, a blue shift of only 15 nm. Thisindicates low selectivity for the cis to trans photoisomerization step under these conditions.First of all it should be noted that the 365 nm used is not the most efficient wavelength forswitching. Furthermore, the observed low selectivity is most probably also caused by thecompeting effect of the photoinitiator since for the 2.6 weight% sample of 4.5 reasonableselectivities were found. Extrapolation of the results obtained for low concentration samplesat 380 nm, however, suggest that it should be possible to improve the reversibility by tuningthe exact composition of the LC matrices, especially by varying the amount and kind ofphotoinitiator, and the wavelengths employed for switching and polymerization.

4.7 Conclusion and Future Prospects

In summary, with an aligned and spin-coated sample of 12.5 weight% (M)-trans-4.5 doped ina cholesteric mixture of acrylates 4.2 and 4.3 (40 : 60), photocontrol of the reflection colorbetween red and green is possible. By varying the irradiation time and wavelength all phasesin between should be accessible. The written color information can be stored by subsequentphotopolymerization (Scheme 4.4). The wavelength range covers about one third of thewhole visible wavelength spectrum and as such a real LCD application is farfetched,especially when compared to the PDLC system of photoresponsive polymer 4.1, where thewhole visible spectrum can be generated upon irradiation.31 It should be noted, however, thatthe dopant loading in this system is 50 weight%, four times the amount used for the discussedchiroptical molecular switch 4.5. Due to compatibility problems, such a high dopant loadingis not possible for this system. PDLC systems are fundamentally different from the presentlow molecular weight systems, which the acrylate mixture 4.2/4.3 in the monomeric stateactually is.

Nevertheless, one can readily conceive some improvements for the present system. First ofall, the shrinkage of the material, which is dependent on the degree of polymerization, shouldbe reducible by varying the amount of initiator and inhibitor. This could lead to a broaderaddressable spectral region in the polymerized state. Another improvement that can beenvisioned is the use of a different photoinitiator that can be excited by wavelengths outsidethe absorption range of the switchable dopant. This might also have a dramatic effect on thereversibility of the photoswitching. Adjusting the relative amount of monoacrylate and chiral

Controlling the Color of Cholesteric Liquid Crystals

121

diacrylate should lead to further optimization. Also response times in the present system arestill far from response times required for any LCD application. As noted, for most LCDsystems, relaxation of the mesogenic host molecules is the rate-determining factor. In thepresented system, there is still considerable room for improvement in both the samplepreparation as well as the irradiation techniques applied. The results show however that pixelcolors can be tuned by photoirradiation of a chiroptical molecular switch doped in acholesteric liquid crystal.

photopolymerization

color control

photopolymerization

(RE)WRITABLE INFORMATION

STORED INFORMATION

Scheme 4.4 Schematic representation of color control and storage of information byphotopolymerization of a cholesteric liquid crystal.

Unfortunately, for donor-acceptor system 4.5, the two pseudoenantiomers have similareffects on the cholesteric phase. Both the film doped with pure (M)-trans-4.5, and the 435 or450 nm photostationary samples with (P)-cis-4.5 in excess, show red-shifted reflection curvescompared to undoped host. In an ideal situation the two pseudoenantiomers would showopposite effects on the chiral nature of the matrix where one pseudoenantiomer would inducea red and the other a blue shift of the reflection. The effects observed could in principle alsobe induced by an achiral photoswitch. Instead of focussing on improving all the aspects ofthis particular system under different conditions an improved photosensitive dopant for thistype of application was found. This new system will be the subject of the next two chapters.

Other applications for which the presented system could be used are reflective polarizers andlasers based on cholesteric liquid crystals. The color reflection of a cholesteric phase iscircularly polarized as discussed in the introduction. The polarization of the reflected light isdependent on the handedness of the cholesteric phase. As such, the presented system canfunction as a tunable reflector of circular polarized light at different wavelengths. This was

Chapter 4

122

already reported for a system based on a very similar acrylate mixture.36 Uponphotopolymerization, cholesteric filters of different wavelengths with an approximatebandwidth of about 50 nm could be obtained. Introduction of a gradient in the pitch of thecholesteric helix resulted in a polarization filter by which one of the two components ofcircularly polarized light was reflected over the entire visible spectrum.28b The othercomponent of the light is selectively transmitted. In principle, the LC system presented in thischapter is also useful in such an approach. Cholesteric materials have also been applied intunable mirrorless laser systems. The repetitive structure of a cholesteric phase allows for thepossibility of lasing without using external mirrors when a fluorescent dye is dissolved in thisphase.37 The wavelength of such a laser system is dependent on the fluorescent dye and onthe cholesteric pitch. An example has been published where the wavelength of a laser basedon a cholesteric liquid crystal could be tuned by mechanical modification of the LC phase.38

A phototunable cholesteric liquid crystal, as the one presented in this chapter might offer analternative approach towards phototunable laser sources.

4.8 Experimental Section

For general remarks, see Section 2.6. For preparation of aligned cholesteric phases, see Section 3.7.Spin-coating of the aligned phases was performed at 5 r.p.m. for 2 min resulting in thin LC films withhomogeneous colors. All experiments were performed at Philips Research, Eindhoven.

MaterialsThe cholesteric mixture of 4.2 and 4.3 as well as the photoinitiator (Irgacure 651) and inhibitor (p-methoxyphenol) were kindly provided by Philips Research and used without prior purification. Thesynthesis of chiroptical molecular switch 4.5 was presented in Chapter 2 and was resolved asdescribed there, using preparative chiral HPLC.

Reflection WavelengthsThe reflection wavelengths presented were measured as transmission curves as indicated in the text. APerkin Elmer Lambda 900 UV/VIS/NIR spectrometer was used. The incident light beam travelsthrough a depolarizer and a polarizer before it reaches the sample. After the sample the transmittedlight travels through a λ/4 plate to isolate the circularly polarized components and a second polarizedbefore being analyzed. From the transmission curves, reflection curves were calculated.35

Irradiation ExperimentsIrradiations were performed with a UV lamp equipped with a suitable interference filter (435 nm) orcut-off filter (>450 nm). For the photopolymerization a 365 nm light source was used (PhilipsPL10W/10).

4.9 References and Notes

1 G. Solladié, R.G. Zimmermann, Angew. Chem. Int. Ed. Engl. 1984, 23, 348.2 B.L. Feringa, R.A. van Delden, N. Koumura, E.M. Geertsema, Chem. Rev. 2000, 100, 1789 and

reference cited in Chapter 3.

Controlling the Color of Cholesteric Liquid Crystals

123

3 See for example: a) D. Dunmar, K. Toniyama in Handbook of Liquid Crystals Vol 1:Fundamentals, D. Demus, J. Goodby, G.W. Gray, H.-W. Spiess, V. Vill Ed., Wiley-VCH,Weinheim, 1998, pp. 215-239; b) G. Meier, in Applications of Liquid Crystals, Springer Verlag,Berlin-Heidelberg-New York, 1975, pp. 1-21; c) S. Chandrasekhar, Liquid Crystals, CambridgeUniversity Press, Cambridge, 1977.

4 Next to the references [3], see for example: a) E.B. Priestley in Introduction to Liquids Crystals,Ed. E.B. Priestley, P.J. Wojtowicz, P. Sheng, Plenum Press, New York, 1975, pp. 203-218; b) W.Elser, R.D. Ennulat in Advances in Liquid Crystals Vol. 2, Ed. G.H. Brown, Academic Press, NewYork, 1976, pp. 73-161 and references therein.

5 W.H. Bragg, W.L. Bragg, X-rays and Crystal Structure, G. Bell and Sons, London, 1915.6 G.H.Heilmeier, L.A. Barton, L.A. Zanoni, Appl. Phys. Lett. 1968, 13, 46.7 a) B. Bahadur, Mol. Cryst. Liq. Cryst. 1984, 109, 3; b) P.G. de Gennes, Angew. Chem. Int. Ed.

Engl. 1992, 31, 842; c) E. Kaneko, Liquid Crystal TV Displays: Principles and Applications ofLiquid Crystal Displays, KTK Scientific Publishers, Tokyo, 1987; d) L.A. Goodman inIntroduction to Liquids Crystals, Ed. E.B. Priestley, P.J. Wojtowicz, P. Sheng, Plenum Press, NewYork, 1975, pp. 241-273.

8 G.H. Heilmeier, L.A. Zanoni, L.A. Barton, Proc. IEEE 1968, 56, 1162.9 a) M. Schadt, W. Helfrich, Appl. Phys. Lett. 1971, 18, 127; b) D. de Rossi, J. Robert, J. Appl. Phys.

1978, 49, 1139; c) G. Bauer, Mol. Cryst. Liq. Cryst. 1981, 63, 45; L. Pohl, G. Weber, R.Eidenschink, Appl. Phys. Lett. 1981, 38, 497.

10 a) T.J. Scheffer, Japan Display '83 1983, 400; b) T.J. Scheffer, J. Nehring, Appl. Phys. Lett. 1984,45, 1021.

11 For an extensive account of reflective cells: S.-T. Wu, D.-K. Yang, Reflective Liquid CrystalDisplays, Wiley, Chichester, 2001.

12 Active vs. passive matrix in PC Computing 1993, 6, 197.13 a) G.H. Heilmeier, W. Helfrich, Appl. Phys. Lett. 1970, 16, 155; b) J.G. Grabmeier, W.F. Greubel,

H.H. Krüger, Mol. Cryst. Liq. Cryst. 1971, 15, 95.14 For early examples, see: a) T.J. Scheffer, J. Appl. Phys. 1973, 44, 4869; b) T.J. Scheffer in Non-

Emmisive Electronic Displays, Plenum Press, New York, 1975, pp 45-78; c) T. Uchida, C.Shishido, M. Wada, Mol. Cryst. Liq. Cryst. 1977, 39, 127.

15 For early examples, see: a) T.J. Scheffer, J. Appl. Phys. 1973, 44, 4799; b) I.A. Shanks, ElectronicLett. 1974, 10, 90.

16 S. Musa, Sci. Amer. 1997, 277, 124.17 Figure adaped from reference 16.18 a) G.H. Heilmeier, L.A. Zanoni, Appl. Phys. Lett. 1968, 13, 91; b) G.H. Heilmeier, J.A. Castellano,

L.A. Zanoni, Mol. Cryst. Liq. Cryst. 1969, 8, 293. For the most employed host-guest cell, see: D.L.White, G.N. Taylor, Appl. Phys. Lett. 1974, 45, 4718.

19 a) B. Bahadur, Liquid Crystals Application and Uses, World Scientific, Singapore, 1992, Vol. 3,Chapter 11; b) H.V. Ivashchenko, V.G. Rumyantsev, Mol. Cryst. Liq. Cryst. 1987, 150, 1.

20 See reference 6c, Chapter 3.21 a) F. Vicentini, L.-C. Chien Liq. Cryst. 1998, 24, 483; b) D. Davis, K. Kahn, X.-Y. Huang, J.W.

Doane, SID Intl. Symp. Digest. Tech. Papers 1998, 29, 901.22 a) W. Haas, J. Adams, J. Wysocki, Mol. Cryst. Liq. Cryst. 1969, 7, 371. b) J. Adams, W. Haas, J.

Electrochem. Soc. 1971, 118, 2026.23 E. Sackmann, J. Am. Chem. Soc. 1971, 93, 7088.24 H.-K. Lee, K. Doi, H. Harada, O. Tsutsumi, A. Kanazawa, T. Shioni, T. Ikeda, J. Phys. Chem. B.

2000, 104, 7023.25 N. Tamaoki, Adv. Mater. 2001, 13, 1135.26 see reference 21 and references therein.27 a) H. Finkelmann, J. Koldehoff, H. Ringsdorf, Angew. Chem. Int. Ed. Engl. 1978, 17, 935; b) S.-L.

Tseng, G.V. Laivins, D.G. Gray, Macromolecules 1982, 15, 1262; c) J. Watanabe, T. Nagasse, H.Itoh, T. Ishi, T. Satoh, Mol. Cryst. Liq. Cryst. 1988, 164, 135.

Chapter 4

124

28 a) M. Muller, R. Zentel, H. Keller, Adv. Mater. 1997, 9, 159; b) D.J. Broer, J. Lub, G.N. Mol,Nature 1995, 378, 467; c) P.J. Shannon, Macromolecules 1984, 17, 1873; d) T. Tsutsui, T. Tanaka,Polymer 1980, 21, 1351.

29 a) A.Y. Bobrovsky, N.I. Boiko, V.P. Shibaev, Liq. Cryst. 1998, 25, 393; b) A.Y. Bobrovsky, N.I.Boiko, V.P. Shibaev, Liq. Cryst. 1998, 25, 679; c) N.I. Boiko, A.Y. Bobrovsky, V.P. Shibaev, Mol.Cryst. Liq. Cryst. 1999, 332, 173; d) A.Y. Bobrovsky, N.I. Boiko, V.P. Shibaev, J. Opt. Technol.1999, 66, 574; e) P. van de Witte, J.C. Galan, J. Lub, Liq. Cryst. 1998, 24, 819.

30 a) A.Y. Bobrovsky, N.I. Boiko, V.P. Shibaev, J. Springer, Adv. Mater. 2000, 12, 16; b) F.Vicentini, J. Cho, L.-C. Chien, Liq. Cryst. 1998, 2, 483; c) S. Campbell, Y. Lin, U. Müller, L.-C.Chien, Chem. Mater. 1998, 10, 1652.

31 M. Brehmer, J. Lub, P. van de Witte, Adv. Mater. 1998, 10, 1438.32 D.A. Higgins, Adv. Mater. 2000, 12, 251.33 a) J. Lub, J.H. van der Veen, E. van Echten, Mol. Cryst. Liq. Cryst. 1996, 287, 205; b) J. Lub, J.H.

van der Veen, W. ten Hoeve, Recl. Trav. Chim. Pays-Bas 1996, 115, 321; c) R.A.M. Hikmet, J.Lub, A.J.W. Tol, Macromolecules 1995, 28, 3313; d) R.A.M. Hikmet, B.H. Zwerver, J. Lub,Macromolecules 1994, 27, 6722.

34 N.P.M. Huck, Ph.D Thesis, University of Groningen, 1997.35 Reflection (%) = 100 -%T; the LC material does not show absorption in the wavelength range

used.36 J. Lub, D.J. Broer, R.A.M. Hikmet, K.G.J. Nierop, Mol. Cryst. Liq. Cryst. 1995, 18, 31937 a) B. Taheri, A.F. Muñez, P. Palffy-Muhoray, R. Twieg, Mol. Cryst. Liq. Cryst. 2001, 358, 73; b)

E. Alvarez, M. He, A.F. Muñoz, P. Palffy-Muhoray, S.V. Serak, B. Taheri, R. Twieg, Mol. Cryst.Liq. Cryst. 2001, 369, 75.

38 H. Finkelmann, S.T. Kim, A. Muñoz, P. Palffy-Muhoray, B. Taheri, Adv. Mater. 2001, 13, 1069.