University of Groningen

Controlling molecular chirality and motionvan Delden, Richard Andreas

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143

Chapter 6

Unidirectional Rotation in a Liquid Crystalline Environment

In this chapter the photophysical aspects of the unidirectionally rotary molecular motor,introduced in the previous chapter, in a liquid crystal environment are described. Liquidcrystals could function as hosts for this chiral molecular motor and amplify its chirality. It isdemonstrated that the chiral properties of the four different forms of this molecular motorallow color induction of the cholesteric phase as well as full color tuning by irradiation.*

* Part of this chapter is in press: R.A. van Delden, N. Koumura, N. Harada, B.L. Feringa, Proc. Nat.

Acad. Sci. 2002.

Chapter 6

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6.1 Introduction

In the previous chapter a unidirectionally rotating motor 6.1 was introduced. This compoundon a molecular level functions as a motor since the energy of the irradiation light is used toexert a mechanical effect, i.e. a unidirectional rotation. Photon energy is converted intokinetic energy with a preferential direction. The next step towards any nanotechnologicalapplication of such a motor would be to actually drive some other function or event. AsGeorge M. Whitesides recently put it: "the age of nanofabrication is here, and the age ofnanoscience has dawned, but the age of nanotechnology--finding practical uses fornanostructures--has not really started yet".1 Whereas the synthesis or nanofabrication of thishydrocarbon compound was accomplished and its basic features (the nanoscience) in solutionwere proven to induce unidirectional rotation, any nanotechnological application remains tobe demonstrated. Combining the properties of our molecular motor 6.1 with liquid crystaltechnology might offer a step in the direction of real nanotechnology by this definition.

(P,P)-trans-6.1diaxial methyl groups

> 280 nm

> 380 nm

> 280 nm

> 380 nm

∆ 20oC60oC

(M,M)-cis-6.1diequatorial methyl groups

(P,P)-cis-6.1diaxial methyl groups

(M,M)-trans-6.1diequatorial methyl groups

Scheme 6.1 Unidirectional rotation of the molecular motor 6.1

The processes involved in the unidirectional rotation of compound 6.1, as they werediscussed in detail in the previous chapter, are depicted in Scheme 6.1. Two photo-inducedisomerization steps are combined with two thermal helix inversion steps to form a four-stagerotation. Both isomerizations force the methyl substituents next to the stereogenic centers toadopt an energetically unfavorable equatorial orientation. The thermal helix inversionsrelease the internal energy and the methyl substituents again adopt an axial orientation. It isimportant to note that in all four discrete steps, the photoisomerizations and the thermalisomerization steps, the helix of the molecule is inverted.

Unidirectional Rotation in a Liquid Crystalline Environment

145

As already noted for the chiroptical molecular switches, but also for future application of thisunidirectionally rotating motor 6.1, preservation of the molecular properties, as demonstratedin solution, in an organized medium is essential.2 Comparing the structural features of ourchiroptical molecular switches with motor 6.1, which are both sterically overcrowdedalkenes, shows a lot of resemblance. Both systems function under the influence of light,where a cis-trans isomerization is the key step. The stereochemistry of the systems is similar.Both systems are intrinsically chiral sterically overcrowded alkenes where photoinducedhelix inversion is essential. Since these unique (chiral) properties of the rotating molecularsystem resemble the features of the chiroptical switches, this might allow photoinducedrotation to have an effect on macroscopic properties. Studies on polymer systems employingmolecular motors and on molecular motors assembled on a surface are already underway inour group but the systems of choice for this research were again liquid crystals for severalreasons. Liquid crystalline matrices were shown to be excellent hosts for our chiropticalmolecular switches. In liquid crystalline surrounding the photochemical properties wereretained and the obtained doped LC materials could be aligned and processed. Moreimportant, the liquid crystal host in all cases functioned as an amplifier of molecular chirality.The dopant chirality was reflected in the chiral packing of the liquid crystalline host materialand chiroptical switching of the dopant resulted in switching of the macroscopic chiralproperties of the liquid crystal. The essence of the research described in this chapter is to seewhether the control of molecular rotation that can be exerted by light irradiation andgoverned by the configuration of two stereogenic centers can be amplified to full control ofmacroscopic (chiral) properties in a liquid crystalline phase thereby allowing indirectmacroscopic visualization of rotary motion.

6.2 Comparison of the Molecular Motor and the Molecular Switch

The first requirement of employing a liquid crystal to amplify the chiral clockwise orcounterclockwise unidirectional molecular rotation is that compound 6.1 is capable ofinducing chirality in a liquid crystalline phase. Next to M15 (4-methoxy-4'-biphenylcarbonitrile), again E7 (a mixture of different biphenylcarbonitrile-based mesogens)was chosen as a nematic LC host because of the useful properties of induced cholestericphases and the essential requirement of liquid crystallinity at room temperature and over abroad temperature range. The molecular structure and properties of the LC host materialswere discussed in Chapter 3. Helical twisting powers of the different forms should besufficiently large to allow formation of cholesteric phases at reasonably low dopant loading.3

An additional requirement is that the different cis- and trans-forms of the molecular motorshow different helical twisting power, since only then would a change in the geometry of themolecular system result in a macroscopic change in the cholesteric properties.

Comparing the motor system 6.1 to the donor-acceptor substituted molecular switches, wherehelical twisting powers in the range of 10 µm-1 and opposite in sign for the twopseudoenantiomers were found, the intrinsic helical structure of the molecular motor asevident from CD spectroscopy is far more pronounced. Therefore stronger inductive effectson the liquid crystalline matrix can be anticipated. In comparing the CD spectra of molecular

Chapter 6

146

motor (P,P)-trans-6.1 and molecular switch (P)-trans-6.2, the distinct circular dichroismbands for 6.1 are apparent (Figure 6.1). The larger ∆ε-values for 6.1 compared to 6.2 reflectthe double helix in this molecule. Helical twisting power and circular dichroism effects areboth related to the chirality of a compound but in general no relation is observed betweenthem due to two important reasons. Where CD depends on the properties of the ground andexcited states of the molecule, the helical twisting power is a consequence of the interactionof a chiral dopant with an anisotropic mesophase. A second important difference is that whilethe helical twisting power is intrinsic to a given chiral substance, the sign and magnitude ofan effect in circular dichroism is strongly dependent on the chosen transition. Nevertheless,despite these intrinsic differences Kuball et al. showed, for a series of structurally similarchiral amino-anthraquinones and amines, that there is a correlation between the twoproperties when the absorption band is well chosen.4 Since the two compounds comparedhere are also structurally related the molecular motor is expected to show high helicaltwisting power.

250 300 350 400

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-50

0

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150

∆ε

wavelength (nm)

(P )-trans-6.2

S

S

NO 2N

(P ,P )-tr ans-6.1

Figure 6.1 Comparison of circular dichroism spectra of (P,P)-trans-6.1 (black line) and (P)-trans-6.2 (dotted line).

6.3 Molecular Motor in a Liquid Crystalline Matrix

6.3.1 Stationary Properties of the Molecular Motor in a Liquid Crystalline MatrixInitial doping of nematic M15 with 3.4 weight% of enantiomerically pure (3R,3’R)-(P,P)-trans-6.1 resulted in a stable cholesteric phase. A distinct pitch of 390 nm was determined forthis sample by the Grandjean Cano technique.5 A corresponding helical twisting power (β) of+75 µm-1 was determined for (3R,3’R)-(P,P)-trans-6.1. This value is dramatically higher thanthe values found for chiroptical switch 6.2. For comparison the β-values for the stable cis-isomer (3R,3’R)-(P,P)-cis-6.1 and the unstable trans-isomer (3R,3’R)-(M,M)-trans-6.1 inM15 were measured. For (3R,3’R)-(P,P)-cis-6.1 a positive β-value of 8 µm-1 was foundwhich is in the same order of magnitude as the values for the chiroptical switches. For(3R,3’R)-(M,M)-trans-6.1 a negative β-value of -18 µm-1 was found, reflecting the helical

Unidirectional Rotation in a Liquid Crystalline Environment

147

structure and again higher than the values found for the chiroptical switches but substantiallylower than the value found for (3R,3’R)-(P,P)-trans-6.1. Furthermore, the β-values of the two(P,P)-stereoisomers were determined in the desired LC host E7 whereas the value for (M,M)-trans-6.1 could be calculated, vide infra. From the data given in Table 6.1 it is evident that(P,P)-trans-6.1 shows a high positive β-value and a large decrease in helical twisting poweris found going to (P,P)-cis-6.1 and (M,M)-trans-6.1. It should also be noted that (P,P)-cis-6.1shows positive β-values (right-handed cholesteric) and (M,M)-trans-6.1 shows negative β-values (left-handed cholesteric).

LC temperature (°C) (P,P)-trans-6.1(µm-1)

(P,P)-cis-6.1(µm-1)

(M,M)-trans-6.1(µm-1)

M15 50 + 75 + 8 - 18E7 20 + 69 + 12 - 5*

Table 6.1 Helical twisting powers (β-values) of three forms of the molecular motor 6.1 in twodifferent liquid crystalline hosts (M15 and E7). * = value calculated from HTP for mixtures ofknown composition.

For 6.1 sufficiently large helical twisting powers are found to efficiently induce cholestericphases for both mesogenic host compounds. The helical twisting of +69 and +75 µm-1 foundfor (P,P)-trans-6.1 in E7 and M15, respectively, open the opportunity of color generation ofthe cholesteric phase. The dramatic decrease in the helical twisting power going to the otherisomers is essential for generating light-induced macroscopic changes by unidirectionalrotation. This dramatically lowered helical twisting power found for the (P,P)-cis-isomer canbe explained by the more sphere-like geometry of this structure. Helical twisting powers aregenerally larger for dopants which structurally resemble the liquid crystalline host.6,7 Theopposite values for the helical twisting powers found for both (P,P)-isomers compared to the(M,M)-isomer is as expected and indicates that the helical structure is mainly responsible forthe induced cholesteric phase rather than the configuration at the stereogenic centers, whichare both (R) in all states of 6.1. The low helical twisting power found for the (M,M)-trans-isomer is not readily explained but must be due to a different packing of the mesogenicmolecules around this diastereomeric form compared to the (P,P)-trans-isomer, which is alsoapparent from the values in M15. It should be noted that the β-value of (M,M)-trans-6.1 in E7was obtained from an approximate calculation from the determined β-values for mixtures ofknown composition, therefore no unequivocal conclusions should be drawn from this value.

6.3.2 Unidirectional Rotation in an Liquid Crystalline EnvironmentIn order to test the rotary process in the more fixed liquid crystalline matrix, a drop-castedsample of E7 doped with 2.4 weight% of (P,P)-trans-6.1 was prepared. As in the case of n-hexane solution, when the (P,P)-trans-6.1 doped LC phase is irradiated at room temperaturewith the appropriate wavelength of light (> 280 nm) a trans to cis isomerization is inducedand as expected (M,M)-cis-6.1 is formed. This energetically unfavorable isomer at roomtemperature readily converts to (P,P)-cis-6.1, which was confirmed by HPLC analysis. Upon

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148

continued irradiation the (P,P)-cis-isomer is also photoisomerized to the corresponding(M,M)-trans-isomer which is stable at room temperature and can be detected by HPLC. Thetotal process in time was monitored by HPLC analysis as shown in Figure 6.2.

0 900 1800 2700 3600 4500 5400 6300 72000

10

20

30

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50

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80

90

100

%

time (s)

Figure 6.2 Percentage of the three detectable forms of 6.1 upon irradiation of a doped film of6.1 (2.4 weight%) in E7 at room temperature in time; (P,P)-trans-6.1 (�), (P,P)-cis-6.1 (�)and (M,M)-trans-6.1 (�) (analysis by HPLC).

The whole light-driven process is less efficient in a liquid crystal than in solution for threeimportant reasons; i) the absorption of the LC material reduces the amount of photons thatwill actually reach the photoisomerizable material, which is evident from the strongdependence of isomerization efficiency on substrate thickness. ii) due to the absorption of theLC material, the wavelength range that actually reaches the compound is > 340 nm whichcauses a shift in the two photoequilibria to the stable side ((P,P)-trans and (P,P)-cis). In thecase of the (P,P)-trans-6.1 to (M,M)-cis-6.1 photoisomerization this wavelength shift is notimportant for the entire process of rotation because of the fact that (M,M)-cis-6.1 is convertedfast to (P,P)-cis-6.1 resulting in a complete shift of the first photochemical equilibrium tocompletion. However, in the second photoisomerization step this will lead to an approximate30 : 70 mixture of (P,P)-cis-6.1 : (M,M)-trans-6.1, compared to a ratio of 10 : 90 in UV-transparent n-hexane solution. iii) the stability of energetically unstable (M,M)-cis-6.1 isincreased in a more rigid liquid crystalline matrix (vide infra).

Next, the doped LC phase, now consisting of (P,P)-trans-6.1, (P,P)-cis-6.1 and (M,M)-trans-6.1, was heated to 60°C for 4 h to allow helix inversion of the thermally unstable (M,M)-trans-6.1, finalizing the rotation cycle. Again testing the composition of the dopant by HPLCshowed that all the (M,M)-trans-6.1 had gone and an equal amount of (P,P)-trans-6.1appeared, indicating that indeed the expected energetically down-hill unidirectional helixinversion had taken place. It should be noted, however, that during the heating process the LCmaterial was in an isotropic stage and as a consequence the orientation is temporarily lost.

Unidirectional Rotation in a Liquid Crystalline Environment

149

Nevertheless, this completes the four-step unidirectional rotation showing that such a process,although less efficient than in solution, is indeed possible in a more rigid LC surroundings.

6.3.3. Color Tuning in a Motor Doped Liquid Crystalline PhaseFor any given application of this molecular motor system the findings that the photochemicalproperties are retained and amplification of chirality in a liquid crystalline matrix is possible.These are essential features for further applications of these molecular motors. Thedevelopment of a color tunable liquid crystal was already illustrated in detail for the donor-acceptor substituted molecular switches in Chapters 3 and 4. A consequence of the highhelical twisting power of (P,P)-trans-6.1 is that in order to reach a cholesteric phase with acertain pitch, compared to the molecular switches discussed in the previous chapters, in thepresent cases the concentration of dopant can be dramatically decreased. This opens thepossibility to generate cholesteric phases with pitch lengths in the range of the wavelength ofvisible light. These phases, as discussed in Chapter 4, are known to have useful opticalproperties. Certain wavelengths (colors) of light are selectively reflected, following Equation6.1.8

×

×××=

××= −−

neecn

npn

αβ

ααλ sinsincos

1sinsincos)( 11

(6.1)

For the color tuning, thin LC films comprising E7 doped with 6.16 weight% (3R,3’R)-(P,P)-trans-6.1, and spin-coated on linearly rubbed polyimide-covered glass plates were used. Thisresulted in a pitch of 234 nm. Measurements on the reflection wavelengths were performedby directly monitoring the reflection of the sample under an angle of 45°. The wavelengththat is reflected by this film was 357 nm. The normal reflection that can be calculated viaEquation 6.1 is 393 nm, which is confirmed by the violet color of the film.

350 400 450 500 550 600 6500

20

40

60

80

100

nor

ma

lize

d r

efle

ctio

n in

tens

ity(%

)

wavelength (nm)

Figure 6.3 Wavelength of reflection at different selected times of irradiation ranging from(dark to dashed to dotted) t = 0, 10, 40, 120 and 180 s.

Chapter 6

150

Upon irradiation of this film at λ > 280 nm a fast bathochromic shift of the reflectionwavelength occurred. Figure 6.3 shows selected reflection wavelength curve of this 6.16weight% sample in time upon irradiation at > 280 nm. The time dependent quantitativechange in reflection wavelength measured at a 45° angle is presented in Figure 6.4.

0 60 120 180350

400

450

500

550re

flect

ion

wav

elen

gth

(nm

)

time (s)

Figure 6.4 Wavelength of reflection at a 45° angle of a molecular motor doped LC phase (6.16weight% in E7) as a function of time starting from (P,P)-trans-6.1 upon irradiation with >280nm.

The change in reflection wavelength can be fully accounted for by the increase in the amountof both (P,P)-cis-6.1 and (M,M)-trans-6.1 together with a slight wavelength dependentchange in net refractive index (n) of the film as is also observed for undoped E7. As aconsequence a decrease in the net helical twisting power of the dopant occurs upon light-induced isomerization. The bathochromic shifts reflect the rather low β-values of (P,P)-cis-6.1 and (M,M)-trans-6.1. Accordingly the color of the film gradually and rapidly changesfrom violet to red as can be readily detected by visual inspection (Figure 6.5).

Figure 6.5 Colors of a molecular motor doped LC phase (6.16 weight% in E7) in time startingfrom pure (P,P)-trans-6.1 upon irradiation with > 280 nm light as taken from actualphotographs of the sample.

After heating the sample to 60°C at any time of irradiation the (M,M)-trans-6.1 dopant isconverted to (P,P)-trans-6.1 with a concomitant hypsochromic shift of the reflectionwavelength. Again during the heating process the LC material was in an isotropic stage and

Unidirectional Rotation in a Liquid Crystalline Environment

151

as a consequence the orientation is temporarily lost. Upon cooling the material reverts to thechiral nematic phase but in most cases the orientation, and thus the color of the material, isnot as pronounced as before heating. Considering that pure E7 is liquid crystalline up to 58°Cthis final helix inversion, although slower might also be induced in a cholesteric orientation.

6.4 Liquid Crystals as a Probe for Molecular Chirality

The results presented above indicate the possibility of using a LC material doped with ourmolecular motor 6.1 to induce colors. The color change is dependent on the irradiation time,but also irradiation intensity and irradiation wavelength changes should have a similar effect.The color of the LC phase can be controlled leading to all the wavelengths of the visiblespectrum required for LCD application. In comparison to the molecular switches, whereextensive research had to be performed in order to increase the compatibility of the systemsin a liquid crystalline surrounding, here, due to substantially higher helical twisting powers,the molecular motor 6.1 already at relatively low concentration is capable of inducing coloredcholesteric phases. Due to the low dopant concentration, at least when compared to thedonor-acceptor substituted switches, these colored LC phases are stable over a period ofhours. The color of the LC material at its turn can be used to test the chirality and in this casethe exact (chiral) nature of the dopant material. When HPLC is used to measure the exactratios of (P,P)-trans-6.1, (P,P)-cis-6.1 and (M,M)-trans-6.1 in time and one accounts for thefact that during every HPLC measurement all the (M,M)-cis-6.1 that is built up in thephotochemical process is fully converted to energetically favored (P,P)-cis-6.1, one can comeup with a quantitative kinetic model for all the steps, photochemically as well as thermally, inthe process. In this model, five steps with their respective rate constants have to be taken intoaccount. These include the two reversible photochemical steps and the thermal helixinversion step of (M,M)-cis-6.1 to (P,P)-cis-6.1. As stated before, the rate of the entirerotational process is highly dependent on the thickness of the LC sample. The relative rates ofthe photoisomerization steps are expected to be influenced only to a small extent. For ourcalculation we used the sample for which the ratio was determined by HPLC and the datashown in Figure 6.2 to verify the rate constants. The theoretical scheme used together withfive rate constants is depicted in Scheme 6.2.

(P,P)-trans-6.1 (M,M)-cis-6.1 (P,P)-cis-6.1 (M,M)-trans-6.1

k1

k -1

k2 k3

k -3

Scheme 6.2 Rotation scheme and rate constant used for a kinetic model.

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152

The total rate of the process can be measured from the decrease in the concentration of (P,P)-trans-6.1 as shown in Figure 6.2. In the presented HPLC experiment, however, time wasallowed for the full helix inversion of (M,M)-cis-6.1 to (P,P)-cis-6.1. Taking this intoaccount, in a trial and error fashion adjusting for the relative rate constants, where especiallythe rate of the photochemical steps are sensitive to the exact nature of the sample, gives arough approximation of the measured values for the relative ratios of the four forms.However, more thorough examination of the experimental curves, especially at the initialtime, shows a delay in the entire process. An explanation for the observed delay could be thatthe first couple of excitations of the molecules are needed to generate some space in the fixedLC surroundings and that repeated excitation would then induce the space-demandingisomerization step. Alternatively, one can also think of a temperature effect where the initialirradiation slightly rises the temperature of the material slowly increasing the isomerizationrate. Even without detailed knowledge on the exact reasons, one can account for this effect byintroducing a delay in all four photochemical steps. It can be assumed that the initial rateconstant in the first 90 seconds of this process is different (by a factor of 5 x 105 for the firstisomerization, for example) from the final rate constant. Theory then accounts precisely forthe observed ratios of the three forms of the molecular rotor (Figure 6.6).

0 900 1800 2700 3600 4500 5400 6300 72000

10

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100

0 900 1800 2700 3600 4500 5400 6300 72000

25

50

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%

time (s)

(M,M)-cis-6.2(P,P)-cis-6.2

(M,M)-trans-6.2

(P,P)-trans-6.2%

time (s)

Figure 6.6 Correlation of calculated ratio (gray) with measured ratio (black) of the threedetectable forms of 6.1 upon irradiation of a doped film of 6.1 (2.4 weight%) in E7 in time;(P,P)-trans-6.1 (�), (P,P)-cis-6.1 (�) and (M,M)-trans-6.1 (�). Inset: Ratio of all four formscalculated from this kinetic model in case no analysis was performed during irradiation.

Unidirectional Rotation in a Liquid Crystalline Environment

153

The rate constants (defined according to Scheme 6.2) were fitted as k1 = 5.0 × 10-3 s-1; k-1 =4.3 × 10-2 s-1; k2 = 8.0 × 10-4 s-1; k3 = 8.3 × 10-4 s-1 and k-3 = 1.9 × 10-3 s-1. As an inset the ratioof all four forms of the molecular motor in time, which would be the actual values when notime was given for reflection measurements or HPLC analyses are shown. This indicates anextreme example of a measurement itself interfering with the observation, that is the outcomeof the measurement. Since different aspects of the liquid crystalline matrix play an importantrole for these rate constants, no absolute molecular properties can be deduced. Someinformation on the two photoequilibria however can directly be calculated, since the ratio ofthe rate of the forward and back reaction in any unimolecular equilibrium equals the ratio ofthe concentrations of the initial and the final state. A photostationary state for the (P,P)-trans-6.1 to (M,M)-cis-6.1 isomerization can be calculated to consist of 10% of (P,P)-trans-6.1 and90% of (M,M)-cis-6.1 which is the same ratio as found from solution measurements. Acalculation on the second photoequilibrium gives a ratio of 30% (P,P)-cis-6.1 and 70%(M,M)-trans-6.1 as was also determined independently by HPLC analysis as indicated above.The rate constant for the helix inversion from (M,M)-cis-6.1 to (P,P)-cis-6.1 of 8.0 × 10-4 s-1

corresponds to a value for the Gibbs energy of activation (∆Gk) of 90.6 kJ mol-1 and a half-life of 866 s at room temperature. This shows that the thermally unstable (M,M)-cis-6.1 isdramatically more stable in a rigid liquid crystalline host compared to solution; a similarstabilization effect has also been reported, for example, for thermally unstable cis-azobenzenes where due to the rigidity of the environment thermal isomerization is sloweddown.9

0 600 1200 1800350

400

450

500

550

refle

ctio

n w

avel

engt

h (n

m)

time (s)

Figure 6.7 Calculated wavelength of reflection at a 45° angle of a molecular motor doped LCphase (6.16 weight% in E7) as a function of time starting from pure (P,P)-trans-6.1 uponirradiation with >280 nm light.

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Using the helical twisting powers determined for (P,P)-trans-6.1 and (P,P)-cis-6.1 andestimating the helical twisting power of (M,M)-trans-6.1 in E7 (which is -5 µm-1, the valuealso mentioned in Table 6.1), one can also calculate the change in pitch and correspondinglythe change in the wavelength of reflected light in time (Figure 6.7). The calculated curve for6.16 weight% of dopant nicely correlates in shape with the observed curve for a spin-coatedsample except for the time axis. It should be noted that the rate constants were estimated forthe drop-casted sample used in the initial HPLC experiments and due to the larger layerthickness the entire process but especially the photoisomerization is slowed down. This doesnot affect the equilibria but the lower intensity of light that actually reaches the sample has adramatic influence on the color tuning. This effect can however be accounted for by usingdifferent k-values adjusted to the other circumstances. With data and the correlation shownone can use a graph like the one in Figure 6.7 to deduce the ratio of the three forms of thechiral dopant and thus as a means to non-destructively read-out the states of the molecularmotor.

This type of experiment requires a strictly defined thickness of the liquid crystal sample andalso of the light intensity employed. In the presented experiments these properties were ofminor importance. Light intensity was more or less constant during all the experiments sincethe same irradiation equipment was used under the same experimental conditions. Two typesof liquid crystalline layers were employed, first for the ratio determination by HPLC analysis,samples generated by simple evaporation of solvent were used, and for the color tuning spin-coated samples were used. The thickness of the samples was not determined. In the presentcase therefore these kinetic consideration function as an illustration of a concept rather than aquantitative description. It should, however, be noted that for any LCD application the sameuniformity requirements hold. That is for the current LC doped system to function in an LCDapplication uniformity of LC layer thickness and light intensity is absolutely essential and inthat case a similar kinetic approach can lead to real quantitative data for the processes in theliquid crystalline film at hand. Considering the current state of LCD technology suchuniformity should be readily feasible.

6.5 Conclusion

The results presented in this chapter showed that unidirectional rotary motion could beperformed in a LC matrix, as is schematically illustrated in Figure 6.8. Furthermore, the light-driven motion in the dopant induces the motion of a large ensemble of rod-like moleculesduring the reorganization in the LC film. This indirectly allows visual observation of therotary motion. The high helical twisting power of (P,P)-trans-6.1 in combination with thelarge change in β going to the other stages makes it possible that the reflection wavelengthcan readily be tuned throughout the entire visible spectrum simply by changing the irradiationtime. These findings not only demonstrate that a macroscopic effect, i.e. a change of thephysical properties of a material (in the present case an LC film), can be induced by a rotarymolecular motor but also that color pixels in an LC film can be generated using thissupramolecular approach. The mechanism of the pitch increase in the LC phase and inparticular the intriguing question if the molecular motor indeed drives the unidirectional

Unidirectional Rotation in a Liquid Crystalline Environment

155

unwinding of the helical packing of several molecules in the LC matrix is a subject of furtherstudies.

Figure 6.8 Schematic representation of unidirectional rotation of the guest molecular motor6.1, the induced elongation of the pitch of the LC host matrix and the change in reflectionwavelength of the light.

6.6 Experimental Section

For general remarks, see Section 2.6. For details on the liquid-crystalline materials, the preparation ofaligned cholesteric phases and measurement of helical twisting powers, see Section 3.7. For all detailsconcerning compound 6.1, see Chapter 5.

Reflection WavelengthsReflection measurements were performed on a JASCO J715 Spectrophotometer equipped with afluorescence extension (a photomultiplier perpendicular to the direction of the light). Thisspectrophotometer was adapted to hold liquid crystalline covered glass plates in such a way that boththe incident light beam as well as the photomultiplier tube were at an angle of 45° to the surface.Actual color photographs of the aligned cholesteric structures were taken with a Minolta 404Si singlelens reflex camera perpendicular to the LC-covered surface.

Irradiation and AnalysisIrradiations were performed using the same method as presented in Chapter 2 employing a 180 WOriel Hg-lamp adapted with a Pyrex filter to obtain light with a wavelength longer than 280 nm.Ratios of the different forms of the molecular motor were determined using HPLC on a silica column(Econosphere Silica; 5 µm; 250 � 4.6 mm) and pure n-heptane as eluent. The three different formsthat are observable at room temperature ((P,P)-trans-6.1, (P,P)-cis-6.1 and (M,M)-trans-6.1) are

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readily separated, (tR ((M,M)-trans-6.1) = 11.0 min; tR ((P,P)-trans-6.1) = 11.5 min; tR ((P,P)-cis-6.1)= 12.2 min) from each other and the LC material which is only eluted when n-heptane/EtOH 95/5 isused as the eluent. The ratio of the three forms was checked at their isosbestic point at 305.9 nm (for(P,P)-trans-6.1 and (P,P)-cis-6.1) and 333.2 nm (for (M,M)-trans-6.1 and (P,P)-cis-6.1) by PDAdetection using a Waters 996 Diode Array Detector (DAD).

6.7 References and Notes

1 G.M. Whitesides, J.C. Love, Sci. Amer. sept. 2001, 33.2 See Chapter 3 and references therein.3 The helical twisting power (β) is defined as (p×ee×c)-1, with p = pitch; ee = enantiomeric excess

and c = concentration of the dopant in weight%, as discussed in detail in Chapter 3.4 H.-G. Kuball, H. Brüning, Chirality 1997, 9, 407.5 G. Heppke, F. Oestreicher, Mol. Cryst. Liq. Cryst. 1977, 41, 245.6 G. Solladié, R.G. Zimmermann, Angew. Chem. Int. Ed. Engl. 1984, 23, 348.7 a) 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.

8 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.

9 See for an example: S. Morino, A. Kaiho, K. Ichimura, Appl. Phys. Lett. 1998, 73, 1317.