University of Groningen Controlling molecular chirality

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. 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).

The publication may also be distributed here under the terms of Article 25fa of the Dutch Copyright Act, indicated by the “Taverne” license.More information can be found on the University of Groningen website: https://www.rug.nl/library/open-access/self-archiving-pure/taverne-amendment.

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: 24-03-2022

https://research.rug.nl/en/publications/c8aa352a-47ec-42fd-aa8c-4ad6ee195e33

81

Chapter 3

Controlling Supramolecular Chirality by Photoswitching of CholestericLiquid Crystals

In this chapter the use of the newly developed donor-acceptor switch, introduced in theprevious chapter, as a trigger for liquid crystalline phase transitions is described. A shortintroduction on liquid crystals is given and the importance of the control of supramolecularchirality is outlined. Donor-acceptor substituted switches are shown to induce a nematic tocholesteric phase transition. In the appropriate liquid crystalline hosts, photoswitching of thedopant is reflected in an inversion of the helical pitch of the cholesteric phase. The liquidcrystal matrix is shown to amplify the molecular chirality of a chiroptical molecular switch toa macroscopic chirality of a liquid crystal film.

Chapter 3

82

3.1 Introduction

The control of molecular chirality, as exerted by light in the chiroptical molecular switchsystems described in the previous chapter, is highly important from a fundamental point ofview. The research on these systems in solution forms the basis for the eventual developmentof applicable dense optical data storage systems and data processing units based on molecularlevel switching elements. As discussed in Chapter 2, one molecule is the equivalent of one bitof information, and accordingly a compact disc with the dimension that are used today fullycovered with these switchable systems, assuming 100% efficiency in switching, would resultin about 240 years of music, for example. Although this rough calculation gives an idea of thepower of nanotechnology in general, there are of course severe problems in the developmentof these systems. Next to the fact that 100% efficiency is not reached in our systems, themajor limitation in miniaturization of optical components is the dimension of light itself.Normally a light beam of minimal dimensions has a diameter equal to the wavelength. Thismeans that employing 380 nm light, which was shown in the previous chapter to switch thesystem to the trans state in case of compound 3.6 (Scheme 3.5; 2.4 in the previous chapter),the minimal area of the irradiation beam would at best be 900000 nm2, while the dimensionof compound 3.6 can be roughly estimated to be 1 nm2. This is one of the reasons thatamplification of this chiral switching event at the molecular level is a main topic of research.When the chiral response of a photochromic unit can be amplified by a response of a supra-or supermolecular systems of which this unit is a part then chiroptical switching would resultin an immediate macroscopic change of the material. Although the limitations in thedimensions of light still hold, this amplification of chirality will result in a more efficientprocess. Two other advantages of the use of chiroptical molecular switches in these systemsimmediately arise. All these systems offer the additional advantage of improvedprocessability compared to previously described applications of switches in solution and,furthermore, a matrix or supramolecular effect due to chiroptical switching would result in adirect application of the systems since now chiral macroscopic properties can be triggeredand controlled by light.

Different types of materials for amplification can be envisioned. Next to photochromic lowmolecular weight liquid crystals (LMW LC), which will be the main subject of this chapteralso photochromic polymers or polymer liquid crystal (PLC) are used for this purposewhereas some photoresponsive gels have also been reported.12 Although in Chapter 1 the useof the different chiral switches in these matrices was briefly mentioned, the subject will bediscussed in more detail in the next paragraphs.

3.2 Photochromic Polymers

Polymer-based photochromic systems have been studied extensively and are attractive inview of practical application because of the advantages of stability and processability. Anumber of reviews and chapters dealing with various aspects of photochromic polymers andphotoactive biomaterials have been published.3 Chiral photochromic peptides4 and polymers

Controlling Supramolecular Chirality by Photoswitching of Cholesteric Liquid Crystals

83

for holographic data storage and non-linear optics5 have been reviewed. Specificstereochemical effects in chiral photoresponsive polymers include chiral matrix effects on anachiral photochromic unit, photo-induced changes in the conformation or organization of achiral macromolecule and modulation of the chirality of the polymer by a chiroptical switch.6

Photochemical control of the chirality and organization of dynamic helical polymers has beenshown for peptides4 and chiral polyisocyanates.7 Achiral polyisocyanates are either racemicmixtures of (P)- and (M)-helices or they are composed of equal amounts of these oppositehelical segments in a long polymer chain. In the presence of chiral side groups, thepolyisocyanate chains become diastereomeric and a strong preference for one helical twistsense can already be observed when a small number of chiral side groups are incorporated.This high cooperativity results in amplification of chirality and is denoted the sergeants andsoldiers effect8. A polyisocyanate with an azobenzene photochromic group containing twostereogenic centers 3.1 has been prepared (Scheme 3.1).9 Irradiation of 3.1 at 365 nm resultsin trans-cis photoisomerization and a change in CD and ORD spectra indicating an inversionof the helical twist sense in the polymer chain, although the stereogenic centers do not changeupon photoisomerization (Scheme 3.1).

N

N

O

Cl

N

N

O

Cl

hν

∆

3.1

Scheme 3.1 Photocontrol of polyisocyanate chirality by achiral photoinduced cis-transisomerization.

Only few polymer systems have been described in which the switching unit itself is chiral.One example of a chiroptical switch is a chiral polyazulene where the bistability is based on aphotochemical 10π-electron cycloreversion of dihydroazulenes or the electron transferproperties of azulenes.10 An example from our group involves a polymer-bound stericallyovercrowded alkene 3.2 (Figure 3.1). Irradiation of thin films of this chiral photochromicpolymer results in distinct changes in the CD spectrum. This covalently bound system 3.2suffers from low diastereoselectivity in the switching process and longer irradiation times arerequired to reach the photostationary states. Photochemical switching of polymers doped withsterically overcrowded alkenes revealed that the thermal and photochemical stability wasretained in the polymer matrix. Kinetic studies, dielectric thermal analysis and dynamic

Chapter 3

84

mechanical analysis showed that the isomerization processes critically depend on themobility in the matrix.11

O

S

O O

CCCC

OO

x y

( )n

3.2

Figure 3.1 Polymer-bound chiroptical molecular switch (n = 1 - 5; x : y � ��

Next to relative stability and easy processability, the occurrence of a glass transitiontemperature (Tg) might offer another important advantage of using polymeric materials.Below Tg the segmental motion of the polymer chain is frozen and this effect might be usedto enhance the lifetime of the stored information in a switch system. For instance, in the caseof the structural variation induced by the trans-cis isomerization of azobenzenes, the cis-formcan be stabilized in the glassy state.12 In case of the sterically overcrowded alkenes the use ofpolymer matrices showed a large influence on the speed of the switching process, due to thehigh volume demand of the isomerization process where the sterically demanding upper halfhas to switch in an almost full semirotation. The restrictive polymer matrix slows down thisprocess to a large extent and for real application of the sterically overcrowded alkenes aschiroptical molecular switches it is better to use a more flexible liquid crystalline host toretain or amplify the molecular properties in a processable medium. Before the research onchiroptical switches doped in liquid crystalline matrices is discussed, liquid crystals need ageneral introduction.

3.3 Liquid Crystals as Amplifiers of Chirality

3.3.1 BackgroundLiquid crystalline (LC) materials form an intermediate state of matter between solid(crystalline) and liquid (isotropic) materials.13 As such they show some properties of both aliquid as well as a solid. The individual liquid crystalline moieties (mesogens) in an LCphase, which can be present as individual molecules or as units in, for example, polymericsystems, are ordered to a certain extent, either in their position and / or in their orientation,but not to such an extent that the material would solidify. The LC phase retains a certainliquid-like disorder and still shows flowing behavior although in most cases due to theordering the LC material is a very viscous and turbid substance. There are a lot of differentliquid crystalline phases known which differ in the degree as well as the kind of orientation.These substances show liquid crystallinity only over a limited temperature range. Below thisrange a liquid crystalline substance will show a crystalline (c) phase and above thistemperature an isotropic (i) liquid phase. For this reason the liquid crystalline phase is alsocalled an intermediate or mesomorphous phase (mesophase) but might also be found referredto as the fourth state of matter.14 Liquid crystalline materials are abundant in biological


85

systems. Lipids forming the cell membranes and myelins, a lipid material surrounding andprotecting the nerves are liquid crystalline. Also in technology liquid crystals are veryabundant nowadays, the simplest watches and calculators make use of liquid crystallinedisplays (LCD's) and high-tech color applications in laptop computers and currently alsodesktop monitors and full-size television screens are readily available on the market. Theseelectronic applications will be discussed in more detail in the next chapter.

Liquid crystallinity was first observed for cholesteryl benzoate in 1888 by Reinitzer,15 anAustrian botanist who was puzzled by the apparent double melting point of this compound.Together with Lehman, a German physicist this mysterious behavior was further unraveled.Fascinated by this new fourth state of matter detailed research on liquid crystalline materialwas performed.16 Although different types of liquid crystalline phases are known thematerials that form liquid crystalline phases are all strongly anisotropic in shape. Liquidcrystalline molecules generally have either an elongated rod-like (calamatic) or disc-like(discotic) structure.17 This strong shape anisotropy is reflected in a strong anisotropy in theorganization of the macroscopic liquid crystalline phase and its physical properties. Severalclassifications of liquid crystalline phases can be made. Apart for the distinction betweencalamitic and discotic materials, classifications can be based on amphiphilic or non-amphiphilic, metal containing or non-metal containing and low molecular weight orpolymeric liquid crystals.18 The most generally used classification is based on the fact thatliquid crystalline phases are known based on pure compounds dependent on the temperatureas discussed above (thermotropic liquid crystals) and based on solvent-solute type systemswhere aggregates of molecules result in liquid crystallinity (lyotropic liquid crystals).19 Thediscussion here will focus on thermotropic liquid crystals that are calamatic, non-metalcontaining and non-amphiphilic. Low molecular weight systems are used in the presentedresearch but also polymeric systems will be discussed briefly.

solid liquidliquid crystal

temperature

smectic C nematic cholesteric

Figure 3.2 Molecular arrangement of liquid crystalline phases as intermediate state betweensolid and liquid.

Chapter 3

86

Calamatic thermotropic liquid crystals can be divided into different type of phases that differin the degree of orientational ordering. Dependent on this ordering three major types can bedistinguished: smectic, nematic and chiral nematic (or cholesteric) (Figure 3.2).

3.3.1.1 Smectic Liquid CrystalsSmectic liquid crystals, named after the Greek word σµεκτος (smectos meaning soap-like),were originally found for amphiphilic molecules. The name is now used for liquid crystallinephases where the individual molecules do not only show an orientational order but also apositional order; the molecules are organized in layers. These layers can slide relative to eachother leading to flow characteristics with high viscosity. A lot of different smectic phases areknown, distinguished by a letter and denoted as SA, SB, SC etc. The smectic C phase, wherethe molecules in the layered structure are tilted, can be chiral (SC

*) when it consists of chiralmesogenic molecules. In this chiral smectic C phase the director of the molecules displays ahelical propagation. This phase can exhibit ferroelectric properties and might be useful infuture applications using this property.

3.3.1.2 Nematic Liquid CrystalsNematic liquid crystals are named after the Greek word νεµατος (nematos meaning thread-like). The nematic phase is the simplest liquid crystalline phase that can be envisioned. Theindividual molecules do not show any positional order and only orientational order of thedirector of the individual mesogens is to some extent observed. The molecules can rotatealong the long molecular axis and can move freely, leading to flow characteristics with lowerviscosities than found for smectic phases.

pitch

Figure 3.3 Aligned chiral nematic or cholesteric liquid crystalline phase.

3.3.1.3 Cholesteric Liquid CrystalsThe nematic phase has a chiral counterpart called the chiral nematic or cholesteric phase,named after cholesterol derivatives for which this phase was first observed. In the cholestericLC phase, the molecules show the same ordering but the net director of the long axes of themesogenic molecules is twisted to a certain extent going through the LC sample, resulting ina helicoidal pattern in the material, as schematically illustrated for an aligned sample in


87

Figure 3.3. This orientation is chiral and the chirality of a cholesteric LC material is indicatedby the sign and magnitude of the cholesteric pitch. This pitch is defined as the distance in thecholesteric matrix that is needed for the director of the molecule to rotate a full 360°. Thismeans that the larger the pitch the smaller the chiral information in the LC matrix. An infinitepitch would imply an achiral nematic LC phase.

Two types of cholesteric liquid crystals can be distinguished. Next to mesogenic moleculeswhich themselves are chiral (e.g. cholesteryl benzoate), doped cholesteric phases are known.Doping an achiral nematic liquid crystalline host with a chiral guest molecule results in theformation of a chiral nematic phase and this phenomenon offers the possibility ofamplification of molecular chirality, as desired for our chiroptical molecular switches.20 Ofcourse, the nature of the induced cholesteric phase is highly dependent on the properties ofthe chiral dopant; this is reflected both in the sign as well as the magnitude of the cholestericpitch. An important property here is the helical twisting power, a property intrinsic to everychiral compound, which indicates the ability of a chiral guest molecule to induce a cholestericphase. It reflects the amount of chiral dopant (in weight%) needed to reach a cholestericphase of a certain pitch.21 This cholesteric pitch (p) then is dependent on the concentration (cin weight%) of the dopant, the helical twisting power (β) of the dopant and the enantiomericexcess (ee) of the dopant (Equation 3.1).

eecp

××=

β1

(3.1)

The pitch is generally determined by the Grandjean-Cano technique,22 a method that requiresan aligned LC sample between a plane-convex lens and a flat surface as will be discussed insection 3.8. As such, for a doped LC matrix with two pseudoenantiomers of a chiropticalmolecular switch differing in their helical twisting powers, this pitch measurement offers anon-destructive - though laborious - read-out procedure. Cholesteric materials showinteresting optical properties when the pitch of the liquid crystal comes in the region of thewavelength of visible light (350 - 700 nm). The corresponding phases show bright colorreflections.

3.4 Photocontrol of Liquid Crystalline Phases

The reversible control of the anisotropic properties of liquid crystalline (LC) materials offersan attractive way to amplify the effects of molecular optical switches with the additionalbenefit of non-destructive read-out. Electronic modulation of the LC phases forms the basisfor current LC display technology.23 Photochemical switching of LC phases24 might providematerials with potential advantages for all-optical devices, enhanced speed of data processingand the possibility to modulate reflection and transmission with light.

Switching of a photoactive dopant in a liquid crystal can induce large changes in the LCphase, for instance in the case of an azobenzene as a result of the large configurational

Chapter 3

88

change upon photoisomerization. The trans-form is rod-like and therefore stabilizes the liquidcrystalline phase. The cis-form is bent and generally destabilizes the LC phase (Scheme3.2).25 This has been used for switching different -mostly achiral- liquid crystalline phases.Electro-optical switching has been reported for different phases where again in an achiralfashion the transmittance of a liquid crystalline layer can be modified reversibly by anapplied electric field. This is the way in which simple black-and-white calculator displayswork. Photomodulation of different LC phases, including smectic phases and ferroelectricliquid crystals,26,27 is known. For amplification of molecular chirality, photochemicallytunable doped cholesteric liquid crystals are the most attractive materials.

N

N

R1

R2

N

N

R2

R1

Scheme 3.2 Disruption of nematic liquid crystalline phase by trans to cis photoisomerization ofan azobenzene dopant.

Photochemically induced changes in the structure or stereochemistry of the chiral dopant canlead to significant changes in the organization of the LC phase. Irreversible light inducedconversion of cholesteric to nematic phases was achieved by photodecomposition of a chiralguest and by photoracemization.17,28 Some prototype systems for reversiblephotomanipulation of (colored) LC phases have been reported which employ cholestericpolymer liquid crystals (PLC) but also considerable effort, including the research reportedhere, is devoted to the use of low molecular weight (LMW) liquid crystals.29

3.4.1 Cholesteric Polymer Liquid CrystalsTazuke12,30 and Wendorff31 demonstrated the use of doped polymer liquid crystals withphotochromic guest molecules and polymers with covalently attached photochromic sidechains in the construction of optical data storage systems. Major improvements wereachieved by Ringsdorf 32 using copolymers of acrylates with LC side groups and thermallyirreversible photochromic fulgide side groups. A variety of new photochromic polymer liquidcrystals have been reported in recent years.33 A reversible change in optical rotation in acholesteric LC polymer by the photochromism of a dopant spiropyran was shown.34 Polymercholesteric liquid crystals have further been extensively studied for (color) LCD application(see Chapter 4).

3.4.2 Cholesteric Low Molecular Weight Liquid CrystalsThe use of the circularly polarized light switches as optical trigger for liquid crystalline phasetransitions was already discussed in Chapter 1. Photochromic compounds as chiral fulgides35


89

or chiral diarylethylenes36 have also been used for the manipulation of a LC phase but here nochange in sign of the liquid crystalline packing upon irradiation was observed due to the factthat the stereocenters or chiral groups of the photoswitchable molecules are distant from theactual switching part. An elegant way to overcome this problem employing a chiralbinaphthalene-substituted fulgide 3.3 was recently reported. Switching from a closed to anopen form resulted in a large change in magnitude of the helical twisting power in liquidcrystalline host K15 (4’-pentyl-4-biphenylcarbonitrile) from -28.0 µm-1 to -175.3 µm-1.37

Although no change in sign was observed, by the combination of this switchable compoundwith a non-switchable chiral binaphthol derivative 3.4 showing opposite helical twistingpower (+ 91.8 µm-1), reversible switching between a positive and negative cholesteric phasewas demonstrated.

O

O

O

O

N

366 nm

>450 nmO

O

O

O

N

trans-3.3α 3.3C

O

O(S)-3.4

Scheme 3.3 Combination of fulgide switch 3.3 with a chiral auxiliary dopant 3.4 to allowreversible switching of the chirality of a liquid crystalline phase K15.

3.5 Donor-Acceptor Switches in LC matrices

3.5.1 Liquid Crystalline HostsThe behavior of donor-acceptor substituted chiroptical molecular switches was studied in anumber of nematic liquid crystalline hosts. These compounds consist of either singlemolecules (M15 and pCH7) or are mixtures of compounds (E7 and ZLI-389) (Table 3.1). Allthe liquid crystals show calamatic (rod-like) structures and all materials show nematic liquidcrystalline behavior over a certain temperature range. The structurally related materials areexpected to show similar behavior in functioning as a host for chiroptical molecular switches.Subtle differences between the LC hosts can be expected. First of all, these are geometricaldifferences that will be reflected in the helical twisting power of the differentpseudoenantiomeric forms. Second, the different dipolar nature and UV-VIS absorptioncharacteristics will have an influence on the actual photoswitching in the particular host.

Chapter 3

90

Additionally, different switching times can be anticipated depending on the viscosity of theLC host. However, this aspect was not investigated in detail.

Structure phasetransition*

Name

M15 H11C5O CN C 48 N 67 I4'-pentyloxy-4-

biphenylcarbonitrile

pCH7 H15C7 CN N 48 I4'-heptylcyclohexyl-4-

phenylcarbonitrile

ZLI-389

a.o.:

RO O

O

C5H11C -65 N 35 I

mixture of alkoxyphenyl pentylbenzoates

E7

a.o.:

H11C5 CN

CNH15C7

N 58 Imixture of 4'-substituted 4-

biphenylcarbonitriles

Table 3.1 Nematic liquid crystalline host materials and their structural features (* C =crystalline, N = nematic, I = isotropic; numbers indicate phase transition temperature in °C).

For any technological application, LC materials should, preferably, show liquid crystallinityat room temperature and over a broad temperature range. Of the liquid crystalline hosts inTable 3.1, only pCH7 and to a lesser extent ZLI-389 fulfil this requirement. M15 is onlyliquid crystalline at elevated temperatures.

3.5.2 Dimethylamino Nitro SwitchThe photochemical modulation of the helical screw sense and the pitch of a cholesteric phasewere realized with the combination of a nematic liquid crystalline host and donor-acceptorswitch 3.5 by N. Huck (Scheme 3.4).38

S

S

NO2N

(M)-cis-3.5

S

S

NO2N

(P)-trans-3.5

365 nm

435 nm

Scheme 3.4 Photochemical interconversion of parent donor-acceptor switch 3.5.


91

For example, doping of liquid crystalline compound M15 (4’-(pentyloxy)-4-biphenylcarbonitrile) with (P)-trans-3.5 (2.4 weight%) converts the nematic phase into acholesteric phase. Irradiation at 365 nm or 435 nm of a thin film of this cholesteric phase ledto photostationary states with an excess of (M)-cis-3.5 or an excess of (P)-trans-3.5,comparable to the system in n-hexane solution. These two photostationary cholesteric phasesshow different pitches and opposite screw sense, as expected from the pseudoenantiomericrelationship of the two forms of the photoswitchable dopant (+8.5 Pm and -12.2 Pm,respectively). Different liquid crystals can be used as a host and the most important resultsare summarized in Table 3.2.39

LC Host β (M)-cis-3.5

(µm-1)β (P)-trans-3.5

(µm-1)cis : transPSS 435 nm

trans : cisPSS 365 nm

pitchPSS 435 nm

(2.4 weight%)

pitchPSS 365 nm

(2.4 weight%)

doping limit(weight%)

solution - - 90 : 10 70 : 30 − − −M15 - 3.4 + 5.6 88 : 12 75 : 25 - 12.2 µm + 8.5 µm 9

pCH7 - 5.5* + 3.7 80 : 20 70 : 30 - 10 µm + 36 µm 6ZLI-389 - 5.7 + 0.8 77 : 23 70 : 30 - 25 µm + 22 µm 8

Table 3.2 Properties of parent donor-acceptor switch in a variety of liquid crystals (* = valuecalculated from photostationary state ratios and pitches).

For 3.5 switching efficiencies in a liquid crystalline environment and in solution are more orless equal. It should be noted that the irradiation times have increased. Doping concentrationsare, however, limited for this particular system. When using a liquid crystal as a means ofread-out or just a host for a bistable switching system, this is of limited importance. However,when extending the concept to LCD applications this is an extremely important property. Inthe previous chapter a novel donor-acceptor switch bearing a hexyl chain (3.6) wasintroduced. Although improved switching behavior was found, compound 3.6 was initiallysynthesized to increase the compatibility of the chiroptical molecular switch in liquidcrystalline matrices. Another aspect is that improved compatibility is expected to increasehelical twisting powers.

3.5.3 Hexylmethylamino Nitro SwitchThe newly synthesized analogue of the donor-acceptor switches, 7-(N-methylhexylamino)-2-nitro-9-(1’,2’,3’,4’-tetrahydrophenanthrene-4’ylide)-9H-thioxanthene 3.6 (Scheme 3.5) wasalready introduced in the previous chapter where the photophysical properties in solutionwere discussed. Compatibility and switching efficiencies of this compound in liquidcrystalline surroundings were tested for a variety of LC materials. A material of choice forfurther improvement of the system is E7, a commercially applied mixture of differentbiphenylcarbonitrile-based mesogens which is liquid crystalline up to 58°C. This temperatureis also more or less the limit of applicability of these sterically overcrowded alkenes becauseof racemization interfering at elevated temperatures.

Chapter 3

92

S

S

NO2N

(M)-cis-3.6

S

S

NO2N

(P)-trans-3.6

380 nm

465 nm

Scheme 3.5 Switching of an n-hexyl functionalized donor-acceptor switch 3.6.

3.5.3.1 CompatibilityThe compatibility of compound 3.6 in various LC materials was tested by simply increasingits amount in the LC host material. For this purpose, a mixture of cis- and trans-3.6 was usedrather than one of the pure isomers because for any switching application the switchcompound will be present as a mixture. The given compatibility values are approximateconcentrations where the LC phase still is uniform, as seen through a polarizationmicroscope. Compatibilities in three different liquid crystalline hosts were tested and in twocases comparison with 3.5 was possible. In both these mesogenic hosts the compatibility ofthe switch system was improved.

LC Host phase transitions compatibilitycompound 3.6 (weight%)

phase transitionsdoped with 2.6 weight% 3.6

M15 C 53 N 68 I 17 C 47 N/N* 61 IpCH7 N 59 I 10 N/N* 52 I

ZLI-389 N 35 I - N/N* 34 IE7 N 58 I 25 N/N* 51 I

Table 3.3 Compatibility and phase transition temperature upon doping of donor-acceptorswitch 3.6 in different liquid crystals.

The observed increase in compatibility can be ascribed to the presence of a solubilizing n-hexyl tail. The phase transition temperatures are only effected to a small extent. In all casesthe transition temperatures are lowered by the addition of dopant. This effect can becompared to the lowering of a melting point of a solid by an impurity. Due to the use ofmixtures of cis- and trans-isomers rather than an enantiomerically pure form of compound3.6, (compensated) nematic textures were observed. Doping with enantiomerically pureswitch resulted in cholesteric textures, which is an essential aspect of this research.

3.5.3.2 Chirality AspectsIncreased compatibility was the initial goal with the development of 3.6. Preservation ofchiral properties and switching efficiency are also essential. These requirements werefulfilled in solution, as was discussed in the previous chapter, but preservation of the


93

properties in a liquid crystal is crucial. In all tested nematic liquid crystalline hosts, acholesteric phase was readily induced by doping with either one of the enantiomerically pureforms of 3.5. In all cases the molecular chirality of the dopant was amplified to provide amacroscopic chirality of the liquid crystalline phase. The determining factor for this chiralinduction is the helical twisting power. The helical twisting powers of the different forms of3.6 were determined by pitch measurements using the Grandjean-Cano technique22 (seeExperimental Section) of known concentrations of two pseudoenantiomers of 3.6 in differentliquid crystalline hosts (Table 3.4).40

LC Host pitch upon dopingwith 2.6 weight%

(M)-cis-3.6

pitch upon dopingwith 2.6 weight%

(P)-trans-3.6

β (M)-cis-3.6 β (P)-trans-3.6

M15 - 4.8 µm + 4.1 µm - 8.1 µm-1 + 9.3 µm-1

pCH7 - 11.0 µm + 7.3 µm - 3.5 µm-1 + 5.3 µm-1

ZLI-389 - 25.6 µm + 9.6 µm - 3.5 µm-1 + 4.0 µm-1

E7 - 3.8 µm + 2.9 µm - 10.1 µm-1 + 13.5 µm-1

Table 3.4 Pitches and helical twisting powers of donor-acceptor switch 3.6 in different liquidcrystals.

Although there are differences found in the helical twisting powers for thepseudoenantiomers (P)-cis-3.6 and (M)-trans-3.6, in all cases the sign of the cholestericphase was opposite. This pseudoenantiomeric influence on the cholesteric packing can beused to generate a cholesteric matrix of which the screw sense can be switched under theinfluence of light. In the biphenyl-based LC hosts (M15 and E7) the highest helical twistingpowers were found and the differences between the two pseudoenantiomers were relativelysmall. It is striking that for all hosts tested the helical twisting power of the trans-isomer, inabsolute sense, is higher than that of the cis-isomer. This is probably due to different packingin the LC matrix, as is also reflected in the different β-values. Nevertheless, particularly inM15 or E7 host an efficient photoswitching of the chirality of the cholesteric phase should bepossible provided that photochemical switching is still possible in a liquid-crystalline matrix.

3.5.3.3 Switching EfficienciesThe switching efficiencies of 3.6 were tested in the different host materials employing 380nm and 435 nm light. The latter wavelength, though not the most efficient in solution, waschosen because in this case a high intensity mercury light source could be used, and becausefurther increasing the wavelength would result in longer switching times. It was alreadyknown that in the LC phase the switching time increases with approximately a factor of 3 incomparison with an isotropic solution.39 This was also observed for the present systems,although when spin-coated samples are used irradiation times dramatically decrease. This isprobably due to extinction of the LC layer being dependent on the layer thickness. The dataare summarized in Table 3.5.

Chapter 3

94

Host (M)-cis-3.6

PSS 435 nm

: (P)-trans-3.6

PSS 380 nm

pitchPSS 435 nm

(2.6 weight%)

pitchPSS 380 nm

(2.6 weight%)

n-hexane 73 : 27 30 : 70 − −M15 72 : 28 31 : 69 -12.0 µm + 10.1 µmpCH7 53 : 47 24 : 76 + 60.5 µm* + 12.1 µm*

ZLI-389 62 : 38 22 : 78 + 65.2 µm* + 13.8 µm*

E7 72 : 28 23 : 77 - 10.9 µm + 5.1 µm

Table 3.5 Switching cholesteric liquid crystalline phases, ratio determined by HPLC (* =calculated values).

Photoswitching of 3.6 is clearly still possible in a liquid crystal. The switching selectivitiesvary somewhat with the liquid crystalline host. In all cases, however, employing 435 and 380nm light, switching is possible between a photostationary state with excess cis-3.6 and aphotostationary state with excess trans-3.6. The change in pitch of the cholesteric phase forM15 and E7 nicely reflects the ratio of (M)-cis-3.6 and (P)-trans-3.6 in the photostationarystate. The (M)-cis-3.6 photostationary state results in a negative cholesteric pitch while the(P)-trans-3.6 photostationary state results in a positive cholesteric pitch. By changing theirradiation time (all samples were irradiated overnight to ensure complete photoequilibrium)or the irradiation wavelength, analogous to the solution experiments cholesteric phases withintermediate pitches, including a compensated nematic phase, are addressable. For the othertwo LC hosts pCH7 and ZLI-389, the pitches of the photostationary samples could not bedetermined. A calculation using the ratio at the photostationary state and the helical twistingpowers of the two pseudoenantiomers of 3.6 showed that pitches longer than 60 µm areexpected. Pitches of these dimensions cannot be measured with our technique. It should benoted that theoretically in both the pCH7 and ZLI-389 host all the pitches that can begenerated using the (M)-cis-3.6 / (P)-trans-3.6 switching system are of positive sign as aresult of a combination of low switching efficiencies and higher helical twisting powersobserved for the (P)-trans-isomer. Based on these observations the biphenyl-based liquidcrystals M15 and E7 are favored over pCH7 and ZLI-389 for any application in combinationwith 3.6.

Although in all cases the same irradiation wavelengths are used, the most efficientwavelengths for switching are strongly dependent on the environment. Further optimizationof the systems can be done in a trial and error fashion by just changing the wavelength andmonitoring the effect on the photostationary state. A more elegant way of optimizing thesystem requires detailed knowledge on the UV-VIS absorption characteristics of the chiraldopant. This knowledge can be obtained from experiments closely resembling the ones insolution discussed in the previous chapter. Due to scattering, however, UV-VIS experimentson liquid crystal samples are problematic. An approximate idea of the UV-VIS characteristicsof compound 3.6 in a liquid crystalline host can be obtained by experiments in the isotropicphase. For this purpose the most promising liquid crystalline host E7 was used. In anexperiment closely resembling the fast screening experiments discussed in the previous


95

chapter, the change in UV-VIS absorption upon irradiation was monitored. For this purpose asample of racemic trans-3.6 in E7 in a 1 mm cuvet was heated at 60°C. At this temperatureE7 is isotropic and UV-VIS measurement were performed (Figure 3.4). For theseexperiments undoped isotropic E7, at the same temperature was used as a baseline. Note herethat the E7 host material shows strong absorption up to about 350 nm. The mixture wasirradiated at this temperature at 435 nm for 90 min in the spectrometer to allow substantialisomerization to take place and again a UV-VIS measurement was performed. Comparison ofthe two UV-VIS spectra and more specifically their ratio gives the most ideal wavelengths forswitching. In this case 492 nm and 378 nm were determined to be the most efficientwavelengths for switching.

Irradiation of the photostationary state mixture (obtained after 380 nm irradiation) with 378nm light did not result in a significant change and a photostationary state of 77 : 23 trans-3.6: cis-3.6 was proven the highest possible selectivity toward the trans-isomers. Irradiation firstat 470 nm resulted in a photostationary state of 87 : 13 cis-3.6 : trans-3.6 and prolongedirradiation at higher wavelengths (475, 480, 485 and 490 nm) did not result in a moreselective switching towards the cis-isomer. This can either be due to the small absorption atthese wavelengths leading to extremely slow formation of the photostationary state or to anunreliable ratio of the two UV-VIS absorption curves at higher wavelength where bothabsorptions approach zero.

380 400 420 440 460 480 5000.00

0.02

0.04

0.06

0.08

0.10

375 400 425 450 475 500

0.9

1.0

1.1

1.2

1.3

abso

rban

ce (

a.u.

)

wavelength (nm)

ratio

wavelength (nm)

Figure 3.4 UV-VIS absorption spectrum of racemic trans-3.6 (solid) in isotropic E7 at 60°Cand of the photostationary mixture obtained after 90 minutes of irradiation at 435 nm (dashed).The inset shows the ratio of the two curves.

Selective switching between cis and trans photostationary states of 3.6 has beendemonstrated in a liquid crystalline environment. This experiment was repeated forenantiomerically pure (P)-trans-3.6 doped at 2.6 weight% in E7. This cholesteric phase witha pitch of + 2.9 µm, was irradiated with 470 nm light overnight. This resulted in a

Chapter 3

96

photostationary cholesteric phase with a pitch of -5.5 µm and a 87 : 13 ratio of (M)-cis-3.6 :(P)-trans-3.6. This pitch is in accordance with the determined helical twisting powers andsubstantially lower than found after 435 nm irradiation (-10.9 µm, Table 3.5). Thephotostationary state with the trans-isomer in excess was shown to have a pitch of + 5.1 µm.In the switching of these doped cholesteric phases of opposite screw sense, first the pitchlength is increased to form a compensated nematic phase. Elongated irradiation result in apitch of opposite screw sense with a gradually decreasing length eventually resulting in thephotostationary state. By varying the irradiation time cholesteric phases with intermediatepitches can be induced. In this particular system of 2.6 weight% of 3.6 doped in E7 host,employing 380 nm light cholesteric phases with positive pitches of 5.1 µm and higher can beinduced. Employing 470 nm light, cholesteric phases with negative pitches of 5.5 µm andhigher can be induced. Depending on the irradiation times a large range of cholesteric pitch isreadily accessible, as schematically depicted in Scheme 3.6.

(+) cholesteric phaseexcess (P)-trans-3.6

pitch = + 5.1 µm

(-) cholesteric phaseexcess (M)-cis-3.6

pitch = - 5.5 µm

Compensated nematic phase

380 nm 380 nm

470 nm 470 nm

Scheme 3.6 Schematic representation of the switching of the chirality of a doped cholestericliquid crystal (E7).

3.6 Discussion and Conclusions

In this chapter it was shown that nematic liquid crystals form excellent hosts for the newlydeveloped chiroptical molecular switch 3.6. This compound shows improved compatibilitycompared to parent compound 3.5 in all investigated mesogenic hosts. Switching propertiesare maintained in liquid crystalline surroundings and switching selectivities comparable toexperiments in isotropic solution were found. The nature of the nematic host only has a minor


97

effect on this selectivity. More dramatic differences between the host materials were found inthe chiral induction by 3.6, where large differences in helical twisting power are observed indifferent LC hosts. In all cases, however, the two pseudoenantiomeric forms of 3.6 ((M)-cis-3.6 and (P)-trans-3.6) induce cholesteric phases with pitches of opposite screw sense.Switching between cholesteric phases with opposite handedness was possible for the twobiphenyl-based LC hosts investigated (M15 and E7). Due to a combination of low switchingselectivity and a relatively large difference in helical twisting power for the twopseudoenantiomers in pCH7 and ZLI-389, in principle only the magnitude of the pitch butnot the sign of the cholesteric phase can be switched. No further research was done on thesetwo LC hosts. Upon 380 and 435 nm irradiation two photostationary states withcorresponding cholesteric phases of opposite screw sense are induced. By changingirradiation time and/or wavelength every intermediate state (including a compensated nematicphase) can be induced.

For E7, UV-VIS spectroscopy and photoisomerization experiments in the isotropic phaseshowed that employing higher wavelengths, irradiation would result in more selectiveswitching towards the cis-photostationary state. For a 2.6 weight% sample of 3.6 in E7,employing 380 and 470 nm pitches between -5.5 µm and + 5.1 µm could readily be induced.Switching selectivities are decreased only to a small extent compared to solution but moreefficient switching to the trans-photostationary state was found. The photostationary (M)-cis-3.6 : (P)-trans-3.6 ratio was determined to be 23 : 77 in E7, compared to 30 : 70 found in n-hexane solution. The switching selectivity towards the cis-photostationary state wasdecreased, a photostationary state ratio of 98 : 2 (M)-cis-3.6 : (P)-trans-3.6 was found in n-hexane. Most selective photoswitching in E7 employing 470 nm light resulted in a ratio of 87: 13. The exact reason for this decreased selectivity is not clear. It should be noted, that inChapter 2, by comparison of the systems 3.5 and 3.6 switching towards the cis-photostationary state at the red-end of the UV-VIS absorption of the switchable compoundswas shown to be highly sensitive to any perturbation. A change from a methyl to an n-hexyl-substituent had a dramatic effect on the switching selectivity. The presented results, however,show that a highly efficient photoswitching in a liquid crystalline host is possible usingbiphenyl-based mesogens. More important, the chiroptical molecular switch induces acholesteric LC phase and the chirality of this phase can be switched by photoirradiation. Thecontrol of molecular chirality of the switchable compound 3.6 is efficiently amplified to acontrol of supramolecular chirality of the screw sense of a macroscopic cholesteric phase.

Direct comparison of the results obtained for 3.6 and the results of photochemical switchingof the parent donor-acceptor switch 3.5 in LC matrices is difficult since for system 3.5 noswitching experiments were performed in E7. This E7 host is the most promising liquidcrystal matrix for future application of compound 3.6 both because of liquid crystallinity atroom temperature and the highly selective switching in this material. The only unequivocalconclusion that can be drawn by comparing results for compounds 3.5 and 3.6 is that thecompatibility has been improved. Improved switching of liquid crystalline phases is alsofound for compound 3.6, due to a combination of two effects: i) employing E7 compared tothe other LC hosts increased the helical twisting power of both (M)-cis-3.6 and (P)-cis-3.6,and ii) the UV-VIS absorption characteristics of the guest chiroptical molecular switch in this

Chapter 3

98

host material could be investigated in the isotropic state, these experiment indicated thatemploying higher wavelengths would result in more selective switching towards the cis-photostationary state as was experimentally shown. Where for compound 3.5 maximalswitching between LC samples of + 12.2 and - 8.5 µm was reported,39 for compound 3.6switching between LC samples of + 5.1 and - 5.5 µm was achieved. Further improvement ofboth doped systems should be possible by increasing the dopant concentration. In this respectcompound 3.6 is a more promising chiroptical molecular switch because of the increasedcompatibility. Preliminary experiments on a 3.6-doped sample of higher concentration willbriefly be discussed in the next chapter.

3.7 Experimental Sections

For general remarks and the synthesis of the hexylmethylamino donor-acceptor switch 3.6, seeSection 2.6.

MaterialsThe liquid crystalline materials M15 and ZLI-389 were purchased from Aldrich and Merck,respectively. E7 was received as a gift from Merck, Darmstadt. Nematic liquid crystal pCH7 wasprovided by Dr. V. Vill, Hamburg.

Preparation and Analysis of Liquid Crystalline SamplesThe liquid crystalline samples were prepared by weighing the appropriate amount of dopant guest andliquid crystalline host material that are fully mixed in solution. Upon slow evaporation of the solvent(both dichloromethane and toluene were used) a homogeneously doped liquid crystalline phase wasobtained at an appropriate temperature. Phase transition temperatures were determined by polarizationmicroscopy using an Olympus BX 60 microscope and a Linkam THMS 600 hotstage. Upon heating at5°C min-1 the clearing point of the liquid crystalline phase was monitored. Typical textures41 obtainedfor aligned (compensated) nematic phases and cholesteric phases are depicted in Figure 3.5. Thepictures are obtained between cross polarizers.

Figure 3.5 Schlieren texture42 of (compensated) nematic phase obtained for racemic cis-transmixture of 3.6 in pCH7 (left) and polygonal texture43 of orientated cholesteric liquid crystalobtained for (P)-trans-3.6 doped pCH7 (right).


99

The pitch of the liquid crystalline phases and thus indirectly the helical twisting power of a chiraldopant were determined by the Grandjean-Cano technique.22 The distance between readily visibleGrandjean-Cano lines44 is a measure for the pitch of the aligned cholesteric material. A Grandjean-Cano texture can be obtained by alignment of a cholesteric liquid crystalline material on a polyimide-covered glass surface. For this purpose a glass surface (typically 6.25 cm2) was carefully cleaned withaqueous detergent and with organic solvent (2-propanol). This clean and dry surface was spin-coated(at approximately 3000 rpm for 1 min) with commercially available polyimide AL1051 (purchasedfrom JSR, Belgium). These coated glass surfaces were allowed to harden at 170°C in vacuum for atleast 3 h, but usually overnight. The surface was then linearly rubbed with a velvet cloth to induce aparallel-aligned pattern that could be detected visually and is necessary to induced plane-parallelalignment of the cholesteric LC phase. The LC material doped with the appropriate amount of dopantdissolved in toluene or dichloromethane (± 5 mg ml-1) was slowly poured onto this alignment layer.After evaporation of the solvent at room temperature (for the host materials that were liquidcrystalline at room temperature) or any other suitable temperature (for host materials liquid crystallineonly at higher temperatures) a suitable aligned LC film was obtained.

Figure 3.6 Grandjean-Cano texture as observed through crossed polarizers obtained for (P)-trans-3.6 in E7 (A) and contact method for sign determination: B) microscope picture obtaineddirectly after contact of ZLI-811 in E7 with known negative pitch with (M)-cis-3.6 in E7 wheremixing indicates joint negative helicity; C) microscope picture obtained directly after contact ofZLI-811 in E7 with known negative pitch with (P)-trans-3.6 in E7, clearly showing separatedLC phases indicate opposite pitch handedness.

The Grandjean-Cano texture was obtained by applying a plan-convex converging lens of knownradius (R = 25.119, 30.287, 40.388 or 50.481 mm; Linos Components; Radiometer) onto this liquidcrystalline covered surface. This confinement of the liquid crystalline material leads to concentricrings, the so-called Grandjean-Cano lines, when observed through a (polarization) microscope(Olympus BX 60 microscope) again equipped with a Linkam THMS 600 hot stage if necessary(Figure 3.6 A). The observed distances between these concentric rings by accounting for the radius ofthe plan-convex lens and the magnification of the microscope result in the pitch of the liquidcrystalline phase. The sign of the cholesteric phases was determined using a contact method wheremixing of the samples with a doped cholesteric liquid crystal of known negative screw sense,

Chapter 3

100

consisting of dopant ZLI-811, obtained from Merck in the appropriate liquid crystalline host, wastested. Two cholesteric phases of the same screw sense are known to show complete mixing uponcontact (Figure 3.6 B) whereas for two cholesteric phases of opposite screw sense a clear border isvisible through a microscope (Figure 3.6 C)

Irradiation ExperimentsIrradiation experiments were performed in a similar fashion as described in Chapter 2 at theappropriate wavelengths as given in the text and tables. The composition of the photostationary stateswas determined by HPLC using the same conditions as described already in Chapter 2 for the samedonor-acceptor substituted compound 3.6. Some overlap of the peaks of the liquid crystal materialswith the first or first and second eluted fractions of 3.6 occurs in most cases. By monitoring at anisosbestic point outside of the absorption range of all the liquid crystalline materials used (412 nm, orin some cases 343 nm) still accurate analysis is possible.

The change in cholesteric pitch upon irradiation was mostly determined by two pitch determinationsbefore and after irradiation. A change in the Grandjean-Cano pattern upon irradiation can also bedirectly observed by irradiation under the polarization microscope but this leads to dramaticallyincreased irradiation times and does not give a lot of additional information apart from the visualobservation that indeed the helical pitch gradually increases upon irradiation.

3.8 References

1 K. Murata, M. Aoki, T. Nishi, A. Ikeda, S. Shinkai, J. Chem. Soc., Chem. Commun. 1991, 1715.2 For an example of non-chiral photoswitchable gel: T.J. Effing, I.J. McLennan, J.C.T. Kwak, J.

Phys. Chem. 1994, 98, 2499.3 a) O. Pieroni, A. Fissi, G. Popova, Prog. Polym. Sci. 1998, 23, 81; b) F. Ciardelli, O. Pieroni, A.

Fissi, C. Carlini, A. Altomare, Br. Polym. J. 1989, 21, 97; c) M. Irie, Adv. Polym. Sci. 1990, 94,27; d) Applied Photochromic Polymer Systems; C.B. McArdle, Ed.; Blackie: Glasgow, UK, 1992;e) F. Ciardelli, O. Pieroni, A. Fissi, J.L. Houben, Biopolymers 1984, 23, 1423; f) O. Pieroni, F.Ciardelli, Trends in Polym. Sci. 1995, 3, 282; g) T. Kinoshita, Prog. Polym. Sci. 1995, 20, 527; h)I. Willner, Acc. Chem. Res. 1997, 30, 347.

4 F. Ciardelli, O. Pieroni in Chiroptical Molecular Switches, B.L. Feringa Ed., Wiley-VCH,Weinheim, 2001, Chapter 13, pp. 399-441.

5 J.A. Delaire, K. Nakatani, Chem. Rev. 2000, 100, 1817.6 For an extensive account of chiral optical switches: a) B.L. Feringa, R.A. van Delden, N.

Koumura, E.M. Geertsema, Chem. Rev. 2000, 100, 1789; b) B.L. Feringa, R.A. van Delden,M.K.J. ter Wiel in Chiroptical Molecular Switches, B.L. Feringa Ed., Wiley-VCH, Weinheim,2001, Chapter 5, pp. 123-163.

7 a) M.M. Green, J.-W. Park, T. Sato, A. Teramoto, S. Lifson, R.L.B. Selinger, J.V. Selinger,Angew. Chem. Int. Ed. 1999, 38, 3139; b) S. Lifson, C. Andreola, N.C. Peterson, M.M. Green, J.Am. Chem. Soc. 1989, 111, 8850.

8 M.M. Green, N.C. Peterson, T. Sato, A. Teramoto, R. Cook, S. Lifson, Science 1995, 268, 1860.9 G. Maxein, R. Zentel, Macromolecules 1995, 28, 8438.10 M. Porsch, G. Sigl-Seifert, J. Daub, Adv. Mater. 1997, 9, 635.11 A.M. Schoevaars, Ph.D. Thesis, University of Groningen, 1998.12 T. Ikeda, S. Horiuchi, D.B. Karanjit, S. Kurihara, S. Tazuke, Macromolecules 1990, 23, 36.13 a) Handbook of Liquid Crystals Vol 1, 2A, 2B and 3, D. Demus, J. Goodby, G.W. Gray, H.-W.

Spiess, V. Vill Ed., Wiley-VCH, Weinheim, 1998. b) D. Demus, Mol. Cryst. Liq. Cryst. 1988, 165,45.


101

14 For the early development of liquid crystals, see: a) H. Kelker, Mol. Cryst. Liq. Cryst. 1973, 21, 1;b) H. Kelker, Mol. Cryst. Liq. Cryst. 1988, 165, 1.

15 F. Reinitzer, Monatsh. Chem. 1988, 9, 421; for an English translation, see: Liq. Cryst. 1989, 5, 17.16 V. Vill, Mol. Cryst. Liq. Cryst. 1992, 213, 67.17 D. Demus, Liq. Cryst. 1989, 5, 75.18 a) D.W. Bruce, J. Chem. Soc., Dalton Trans. 1993, 2983; b) A.M. Giroud-Godquin, P.M. Maitlis,

Angew. Chem. Int. Ed. Engl. 1991, 30, 375.19 C. Tschierske, Prog. Polym. Sci. 1996, 21, 775.20 a) G. Solladié, R.G. Zimmermann, Angew. Chem, Int. Ed. Engl. 1984, 23, 348. b) G. Gottarelli,

G.P. Spada, R. Bartsch, G. Solladié, R.G. Zimmermann, J. Org. Chem. 1986, 51, 589; b) G.Gottarelli, M.A. Osipov, G.P. Spada, J. Phys. Chem. 1991, 95, 3879; c) C. Rosini, G.P. Spada, G.Proni, S. Masiero, S. Scamuzzi, J. Am. Chem. Soc. 1997, 119, 506; d) C. Rosini, S. Scamuzzi, M.Pisani Focati, P. Salvadori, J. Org. Chem. 1995, 60, 8289; e) G. Gottarelli, M. Hibert, B. Samori,G. Solladié, G.P. Spada, R.G. Zimmermann, J. Am. Chem. Soc. 1983, 105, 7318. f) G. Gottarelli,G. Proni, G.P. Spada, D. Fabbri, S. Gladiali, C. Rosini, J. Org. Chem. 1996, 61, 2013. g) I. Rosati,C. Rosini, G.P. Spada, Chirality 1995, 7, 353.

21 a) H. Finkelmann, H. Stegemeyer, Ber. Bunsenges. Phys. Chem. 1978, 82, 1302; b) H.-G. Kuball,H. Bruning, T. Muller, O. Turk, A. Schonhofer, J. Mater. Chem. 1995, 5, 2167.

22 G. Heppke, F. Oestreicher, Mol. Cryst. Liq. Cryst. 1977, 41, 245.23 a) D. Kreysig, J. Stumpe, Selected Topics in Liquid Crystalline Research, H.D. Koswig Ed., VCH,

Weinheim, 1990; b) M. Freemantle, Chem. Eng. News 1996, 74 (50), 33; c) P.G. De Gennes,Angew. Chem, Int. Ed. Engl. 1992, 31, 842; d) Liquid Crystals: Applications and Uses, Vol. I-III,B. Bahadur Ed., World Scientific, Singapore, 1991; e) W.M. Gibbons, P.J. Shannon, S.-T. Sun,B.J. Swetlin, Nature 1991, 351, 49.

24 T. Ikeda, A. Kanazawa in Chiroptical Molecular Switches, B.L. Feringa Ed., Wiley-VCH,Weinheim, 2001, Chapter 12, pp. 363-397.

25 T. Ikeda, T. Sasaki, K. Ichimura, Nature 1993, 361, 428.26 T. Kusumoto, K. Sato, K. Ogino, T. Hiyama, S. Takehara, M. Osawa, K. Nakamura, Liq. Cryst.

1993, 14, 727.27 a) M. Negishi, O. Tsutsumi, T. Ikeda, T. Hiyama, J. Kawamura, M. Aizawa, S. Takehara, Chem.

Lett. 1996, 319; b) M. Negishi, K. Kanie, T. Ikeda, T. Hiyama, Chem. Lett. 1996, 583.28 a) R.P. Lemieux, G.B. Schuster, J. Org. Chem. 1993, 58, 100; b) C. Mioskowski, J. Bourguignon,

S. Candau, G. Solladie, Chem. Phys. Lett. 1976, 38, 456.29 N. Tamaoki, Adv. Mater. 2001, 13, 1135.30 a) T. Ikeda, S. Horiuchi, D.B. Karanjit, S. Kurihara, S. Tazuke, Chem. Lett. 1988, 1679; b) H.

Yamaguchi, T. Ikeda, S. Tazuke, Chem. Lett. 1988, 539; c) S. Tazuke, T. Ikeda, S. Kurihara,Chem. Lett. 1987, 911.

31 a) J.H. Wendorf, M. Eich, B. Reck, H. Ringsdorf, Macromol. Chem., Rapid. Commun. 1987, 8, 59;b) J.H. Wendorff, M. Eich, Macromol. Chem., Rapid Commun. 1987, 8, 467.

32 I. Cabrera, H. Ringsdorf, A. Dittrich, Angew. Chem, Int. Ed. Engl. 1991, 30, 76.33 a) T. Ikeda, O. Tsutsumi, Science 1995, 268, 1873; b) S. Hvilsted, F. Andruzzi, C. Kulinna, H.W.

Siesler, P.S. Ramanujam, Macromolecules 1995, 28, 2172; c) O. Tsutsumi, T. Kitsunai, A.Kanazawa, T. Shiono, T. Ikeda, Macromolecules 1998, 31, 355; d) O. Tsutsumi, Y. Demachi, A.Kanazawa, T. Shiono, T. Ikeda, Y. Nagasa, J. Phys. Chem. B 1998, 102, 2869; e) A. Shishido, O.Tsutsumi, A. Kanazawa, T. Shiono, T. Ikeda, N. Tamai, J. Am. Chem. Soc. 1997, 119, 7791; f) Y.Wu, Y. Demachi, O. Tsutsumi, A. Kanazawa, T. Shiono, T. Ikeda, Macromolecules 1998, 31,1104; g) H. Tokuhisa, M. Yokoyama, K. Kimura, J. Mater. Chem. 1998, 8, 889; h) A.Y.Bobrovsky, N.I. Boiko, V.P. Shibaev, Adv. Mater. 1999, 11, 1025.

34 K. Ichimura, A. Hosoki, K. Ozawa, Y. Suzuki, Polym. Bull. 1987, 17, 285.35 a) S.Z. Janicki, G.B. Schuster, J. Am. Chem. Soc. 1995, 117, 8524; b) Y. Yokoyama, T. Sagisaka,

Chem. Lett. 1997, 687.36 C. Denekamp, B.L. Feringa, Adv. Mater. 1998, 10, 1080.

Chapter 3

102

37 T. Sagisaka, Y. Yokoyama, Bull. Chem. Soc. Jpn. 2000, 73, 191.38 B.L. Feringa, N.P.M. Huck, H.A. van Doren, J. Am. Chem. Soc. 1995, 117, 9929.39 N.P.M. Huck, Ph.D. Thesis, University of Groningen, 1997.40 By definition, the relative enantiomers of (M)-cis-3.6 and (P)-trans-3.6 ((P)-cis-3.6 and (M)-trans-

3.6) have opposite helical twisting powers of the same magnitude.41 D. Demus, L. Richter, Textures of Liquid Crystals, VEB, Leipzig, 1978.42 a) G. Friedel, Ann. Physique 1922, 18, 273; b) J. Nehring, A. Saupe, J. Chem. Soc., Faraday

Trans. II 1972, 68, 1; c) A. Saupe, Mol. Cryst. Liq. Cryst. 1973, 21, 211.43 Y. Bouligand, Ann. Physique 1972, 33, 715.44 a) F. Grandjean, C.R. Hebd. Séan. Acad. Sci. 1921, 172, 71; b) R. Cano, Bull. Soc. Fr. Miner.

1968, 91, 20.

Documents

University of Groningen Controlling molecular chirality