J. Phys. Chem. B 2000, 104, 6529-6535
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ARTICLES Conformational Effect on Macroscopic Chirality Modification of Cholesteric Mesophases by Photochromic Azobenzene Dopants Christian Ruslim and Kunihiro Ichimura* Research Laboratory of Resources Utilization, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226-8503, Japan ReceiVed: January 27, 2000; In Final Form: April 4, 2000
Azobenzenes having different positional substituents were dissolved in cholesteric liquid crystals to make cholesteric pitches tunable by photoisomerization. E-to-Z photoisomerization of 3,3′-disubstituted or 2,2′dimethyl-3,3′-disubstituted azobenzenes resulted in a moderate change or even no change in the maximum wavelengths of reflectivity of the cholesteric phase, whereas azobenzene or 4,4′-disubstituted azobenzenes showed much bigger changes. The difference in the conformations between the E- and Z-isomers of the azobenzenes is discussed to explain the observed characteristics. The photoinduced modification of helical pitches by photoisomerization of achiral azobenzenes was identical between a pair of enantiomeric cholesteric solvents, suggesting no preferred intrinsic handedness of the achiral azobenzenes. Experimental results with the corresponding chiral azobenzene suggested that the contribution of the conformation of the azobenzene to the effective helical twisting power is more pronounced than that of the molecular chirality arising from the asymmetric carbon at the terminal alkyl substituents. The photoinduced shortening or lengthening of the helical pitch seems to be determined crucially by the thermal characteristics of the cholesteric liquid crystals.
Introduction An intriguing subject in a variety of researches on cholesteric liquid crystals (LCs) has been the study of their macroscopic chirality exhibiting the handedness and the pitch of the helical array, which is achievable when the mesophasic compounds are intrinsically chiral (optically active) or when a small amount of a chiral compound is added to a nematic LC. The establishment of the macroscopic chirality in the latter case is a good example of the occurrence of a solute-solvent cooperative effect. In the limit of low concentrations, the ability of a chiral dopant to induce helical pitch is quantitatively defined as helical twisting power, β, according to the following equation1
p-1 ) βC
(1)
Here, p and C are helical pitch length and concentration of the dopant, respectively. The origin of this effect has been elucidated as molecular perturbation2 and chirality transfer of the chiral molecules to the neighboring achiral molecules through conformational interactions.3 When the helical pitch of a planar oriented sample is in the range of visible light, circularly polarized visible light will be reflected selectively. A wavelength of the maximum reflection, λmax, is connected to the pitch and the average refractive index n of the mixture, through
λmax ) pn
(2)
* Corresponding author. Tel: +81-45-924-5266. Fax: +81-45-924-5276. E-mail:
[email protected].
The macroscopic chirality of cholesteric LCs is extremely sensitive to physical stimuli such as temperature, pressure, and electric field4 and to chemical modification of the molecules incorporated in the systems including light-induced decomposition, isomerization, and racemization.5 This makes cholesteric LCs significant not only for practical applications such as cholesteric displays, optical data recording, polarizers, and reflectors6 but also for fundamental researches.7-10 The study of solute-solvent interactions is a main subject in the latter case, since the effect of the molecular interactions will emerge directly in the characteristics of the macroscopic chirality of the cholesteric LC systems. Kozawaguchi and Wada7 dealt with the helical pitch of cholesteric-cholesteric and nematic-cholesteric LC mixtures which consist of cholesterol derivatives and concluded that nematic-cholesteric mixtures with a small amounts of cholesterics always give left-handed cholesteric LCs, being independent of the handedness of the cholesterol derivatives. They suggested that the nematic molecules lie in parallel with the long axis of the steroid ring. This alignment diminished partially the right-handed twist of cholesteric LCs found only when the distance of the 3β carbon bond is relatively short (i.e., when substituted with -OH, -Cl, -Br). The handedness of various steroidal molecules in nematic LCs has also been reported by Solladie et al.8 Another example of molecular interactions in cholesteric LCs was reported by Labes, et al.,9,10 who showed nonsymmetrical effects of enantiomeric pairs on the pitches of some cholesteric LCs. They discussed this behavior in term of diastereomeric interactions between the enantiomeric pairs and the cholesteric mixtures.
10.1021/jp000338f CCC: $19.00 © 2000 American Chemical Society Published on Web 06/27/2000
6530 J. Phys. Chem. B, Vol. 104, No. 28, 2000
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Figure 1. Chemical structures of azobenzene compounds used as dopants.
When photochromic dopants such as azobenzene, stilbene, indigo, and so on are dissolved in a cholesteric phase, the photomodulation of the macroscopic chirality can be induced by suitable light. Sackmann has showed that the photoswitching of selective reflection of light to lead to color changes is sometimes achieved by a cholesteric LC doped with azobenzene,11 but there has been no further explanations of the microscopic phenomenon on the basis of molecular interactions. Azobenzene appears to be very attractive as a phototrigger in the study on structure-property relation in cholesteric media, because of its photofatique-resistance, the simplicity of the molecule, as well as the ease in molecular structure modification. Recently, we have designed several novel azobenzene derivatives which have distinct conformational characteristics upon E/Z photoisomerization.12 Photoisomerization of a rod-like E-isomer of a conventional 4,4′-disubstituted azobenzene with UV light results in the formation of a bent-shaped Z-isomer. However, by incorporating suitable substituents to 3- and 3′positions of an azobenzene or a 2,2′-dimethylazobenzene, we have succeeded in creating novel photoisomerizable azobenzenes showing rod-like conformations in both E- and Z-isomers. Such unique characteristics may serve as a good model in exploring how the molecular conformation as well as configuration of dopants affect the macroscopic chirality of cholesteric solvents. In this paper, 3,3′-disubstituted azobenzene (1); 2,2′-dimethyl3,3′-disubstituted azobenzenes (2, 3); 4,4′-disubstituted azobenzenes (5, 6); and unsubstituted azobenzene (4) (Figure 1) were dissolved in cholesteric LCs to form cholesteric mixtures with light-modifiable helical pitches. The effect of E/Z photoisomerization of the dopants on the helical pitch was compared and evaluated to have an insight into the conformation effect on macroscopic chirality, based on the calculated conformations of the E/Z isomers of azobenzenes and the model of induced cholesteric LCs. Thermophysical properties of the photoresponsive cholesteric LCs were also elucidated to support the observed characteristics. Results and Discussion Enantiomeric chiral agents, R-1011 and S-1011, were chosen in preparing cholesteric solvents because they exhibit a symmetrical effect on the induced cholesteric pitch, with R-1011 having right-handed and S-1011 having left-handed sense. The helical twisting power β in nematic DON-103, a mixture of cyclohexanoic acid phenyl esters, is 30.1 (µm wt %)-1. Selective reflection spectra13 with λmax at 670 and 668 nm were confirmed for 7.3 wt % of both chiral agents in DON-103 (Figure 2). Here, λmax were designed to avoid the overlapping of the spectra with that of azobenzene dopants. These two mixtures were used as cholesteric LC hosts and will be addressed as the right-handed cholesteric host and the left-handed cholesteric host in the following section. Into the cholesteric LC hosts were dissolved approximately 2 wt % of azobenzenes, separately, and these mixtures were
Figure 2. Reflection spectra of a right-handed (dotted line) and a lefthanded (solid line) cholesteric host, prepared by mixing chiral agent R-1011 and S-1011 with DON-103, respectively.
TABLE 1 Y (%) at the photostationary state of dopant
λπ-π* (nm)
Z/E × 102 a
UV light (365 nm)
VIS light (436 nm)
1 2 3 4 5 6
b
332 330 318 362 362
4.9b 8.2 2.9 6.3 1.0 1.5
80 86 93 70 95 95
11 4.9 9.4 13 28 26
a The values were measured at maximum wavelength of π-π* bands (λπ-π*) of azobenzenes by HPLC using a mixture of hexane and ethyl acetate (80:20) as eluent. b The λπ-π* is partly superimposed on the absorption of the LC hosts (in hexane the λπ-π* is 314 nm), so that the Z/E is determined at π-π* bands of 354 nm.
heated at 80 °C to obtain homogeneous LC solutions. All mixtures showed excellent homogeneity at room temperature. The injection of cholesteric mixtures into LC cells was performed at their isotropic phases. The cells were subjected to irradiation with UV and visible light successively to give the photostationary states. Absorption spectra of the azobenzene compounds were also probed at the same time to confirm the level of photoisomerization processes. The level of isomerization is defined as Z-fraction, Y (%), at UV or visible light photostationary state, calculated through the following equation.
Y)
1 - (At/A0) 1 - (Z/E)
× 100%
(3)
A0 and At are π-π* absorbances of azobenzenes at initial and photostationary states, respectively, whereas Z and E are molar absorption coefficients of Z- and E-isomers, respectively. Using HPLC, the ratio of Z to E in solutions can be calculated. Table 1 showed the spectroscopic features of the azobenzene compounds in nematic solvents before and after irradiation with light. It is obvious that UV light irradiation produced 70-95% of Z-isomers, whereas visible light induced only 5-30% of Z-isomers in cholesteric hosts. A tendency that Y values of 4,4′disubstituted azobenzenes (5, 6) are higher than those of the other azobenzenes (1-4) can also be observed. The level of the transformation depends on the nature of the compounds, as represented by their absorption spectra, and on the excitation wavelength. Figure 3 (a) depicts the modulation of the right-
Effect of E/Z Photoisomerization on Azobenzenes
J. Phys. Chem. B, Vol. 104, No. 28, 2000 6531
Figure 3. Reflection spectra of right-handed cholesteric LCs containing 2.3 wt % of achiral compound (a) 5 and (b) 3 at initial (solid lines), UV light photostationary (broken lines), and visible light photostationary (dotted lines) states. Notice that these three spectra overlapped with each other in (b). Inset: UV absorption spectra of the azobenzenes at the corresponding states indicated in the figures.
handed cholesteric LC containing 2.3 wt % of 5 upon photoirradiation with UV and visible light, respectively. The initial λmax (698 nm) shifted to 662 and 688 nm after UV and visible light exposure to give photostationary states, respectively. Considering the level of Z-fraction after UV irradiation (in this case 95% as indicated in the inset), the λmax can be interpreted as the apparent effect of the Z-isomer of 5. Surprisingly, in a keen contrast, the right-handed cholesteric LC containing 2.3 wt % of 3 did not show any changes of λmax at all, despite the Z-fraction as large as 93% (Figure 3b). Other azobenzenes in the same host exhibited the modulation of λmax in which the extent depends on the structure of azobenzenes. Quantitative interpretation of the λmax modulation using the concept of effective twisting power β of the azobenzenes is not very authentic, due to the question of whether these achiral azobenzenes possess β, which we shall see in the next section. Therefore, the following relation is used, taking into account that, in the limit of low concentration of a dopant, the λmax of the reflection spectra shifts linearly with the concentration c (in molality).
(λmax)t ) (λmax)0 + kc ∆λmax ) [(λmax)t - (λmax)0] ) kc
(4)
Here, (λmax)0 and (λmax)t indicate λmax of a cholesteric host and
that of a cholesteric mixture with an azobenzene, respectively. The proportionality constant k can be considered as a physical quantity referring to the degree of helical pitch modulation induced by one mole of the azobenzene dopant. Figure 4 provides comparative views on the value of k for the azobenzenes in both right-handed (a) and left-handed (b) cholesteric hosts upon alternate irradiation with UV and visible light for two cycles. Notice that k > 0 indicates that the λmax is shifted to longer wavelength (lengthening of the pitch) with respect to that of the cholesteric host. It is important to notice that the helical pitch modulation induced by E/Z photoisomerization depends significantly on the molecular conformations of the azobenzene dopants. Essentially, it is found here that photochemical E/Z isomerization does not always influence the macroscopic chirality of cholesteric LCs, as shown by dopant 3. It should be stressed here that this phenomenon is not due to the difference in concentration ratios between E- and Z-isomers, since during the whole photoisomerization process the λmax remained unchanged in both cholesteric hosts. A conventional 4,4′-disubstituted azobenzene, 5, showed a much more significant effect on the pitch modulation than azobenzene (4), 3,3′-disubstituted azobenzenes (1), and 2,2′dimethyl-3,3′-disubstituted azobenzenes (2, 3). As has been known, when achiral solutes are dissolved in cholesteric LCs, they may perturb the helical arrangement.11 The distinct photoisomerization effect of the azobenzenes here is attributed to the E/Z conformational interplay of the dopants with the LC molecules arranged in the helical superstructure. On the basis of computational and experimental studies on molecular conformations of geometrical isomers of azobenzenes with different positional substituents reported before,12 the change from rodlike conformation of E-5 to bent conformation of Z-5 after UV light irradiation gave a remarkable impact to the molecular arrangement in the system, leading to a large change of the observed pitch or ∆λmax. However, in the case of 3, rod-like conformation of its E-isomer is retained even in its Z-isomer (see Figure 5). The elongated rod-like conformation of its Z-isomer is energetically much more stable than other possible conformations. Consequently, the E-to-Z transformation of 3 gave no perturbation to the helical arrangement in the cholesteric LCs. This unique behavior of 3 was also confirmed in another cholesteric host consisting of main components of biphenyl and triphenyl LCs. The results were exactly the same, i.e., no change in λmax modification at all during photoisomerization (Figure 6). 3,3′-Disubstituted azobenzene (1) and 2,2′-dimethyl-3,3′disubstituted azobenzene (2) showed a moderate change of pitch as illustrated in Figure 4. The origin seems to stem from the fact that conformations other than the rod-like one of Z-isomer may coexist as has been discussed in our earlier paper.12 Notice that the presence of methyl group at 2- and 2′-positions generating a steric hindrance and the type of linkage (ester) in 3- and 3′-positions are the key points that stabilize the elongated rod-like conformation of E-3 when compared with those of 1 or 2. Additionally the half width of the spectra in all cases remain unchanged, indicating the invariance of the birefringence upon photoisomerization. If we compare the value of k only for achiral azobenzenes (solid lines in Figure 4a and b), we observe that all azobenzenedoped cholesteric LCs show initially λmax shifted to longer wavelengths (k > 0 or enlargement of the pitch) with respect to those of the cholesteric hosts, independent of the handedness of these hosts. Besides, the k values are approximately the same in both right-handed and left-handed hosts. This suggests that there exists no preferred handedness of these achiral azoben-
6532 J. Phys. Chem. B, Vol. 104, No. 28, 2000
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Figure 4. Photoinduced alteration of wavelength of maximum reflection as defined in eq 4 for azobenzene compounds 1 (diamond), 2 (triangle), 3 (inverted triangle), 4 (circle), 5 (square), and 6 (cross) in the (a) right-handed and (b) left-handed cholesteric LCs. I, UV, and Vis of the horizontal axis indicate initial, UV light photostationary, and visible light photostationary states, respectively.
Figure 5. The most stable conformation of E- and Z-isomers of model compounds of 3 and 5, simulated by molecular mechanics MM2 followed by semiempirical MOPAC calculation.
zenes. Our result here is in contrast to that reported by Labes et. al who observed the shortening of the pitch of cholesteric mixtures doped with rod-like molecules.14 However they stated later that this effect could not be reproduced.10 Compound 5 is a typical rod-like molecule in its E-isomers, showing a nematic nature; yet, no indication of the existence of handedness was detected. The results by Kozawaguchi and Wada7 arguing that nematic LCs behave like left-handed cholesteric in right-handed cholesteric LCs (cholesteryl chloride, cholesteryl mercaptan, and cholestery bromide) was understandable only in a region far from the limitation of dilute solutions where a concentration versus pitch curve shows an inflection point at a high nematic LC concentration and consequently, the concept of helical twisting power, thus guest-host interaction becomes ambiguous. Note that they actually observed the lengthening of the pitch when a nematic LC is doped into left-handed cholesteryl oleate, which then may also lead to the interpretation that the nematic is right-handed. Therefore, we come to the conclusion that achiral molecules seem not to possess preferred intrinsic handedness. This behavior becomes more evident when being compared to that of the chiral azobenzene, 6, and after elucidating the effect of the thermal properties of the cholesteric LCs, which are discussed below. When a chiral azobenzene, 6 (dotted lines in Figure 4), having
Figure 6. Reflection spectra of a cholesteric host S-1011/RO-571 (bold line), the same host mixed with 2.2 wt % of 3 before irradiation (solid line), after irradiation with UV light (broken line), and visible light (dotted line) toward the photostationary states. The latter three spectra overlapped with each other because of the photoinert of the helical pitch.
left-handed sense was dissolved in the right-handed host, the λmax before irradiation shifted to even longer wavelength (k )
Effect of E/Z Photoisomerization on Azobenzenes
J. Phys. Chem. B, Vol. 104, No. 28, 2000 6533
0.91) compared to that induced by the achiral compound 5 (k ) 0.47) (see Figure 4a), indicating the role of the opposite molecular chirality of 6 with respect to that of the host. However, when 6 was doped into the left-handed host (dotted line in Figure 4b), no shift to shorter wavelength (shortening of the pitch) occurred, which is beyond our expectation. A shift also to longer wavelength, as seen in the case of dissolving in the right-handed host, was indeed observed. However, it is important to notice that the shift induced by 6 in the left-handed host (k ) 0.10) is much smaller than that in the right-handed host (k ) 0.91). This means that essentially there exists an intrinsic shift to a shorter wavelength when 6 is introduced to the left-handed host due to the microscopic chirality derived from asymmetric carbon at the alkyl substituents of 6. However, this shift was compensated by the shift to longer wavelength attributed to the effect of the conformation of the azobenzene unit, as confirmed in the case of the corresponding 4,4′-disubstituted achiral azobenzenes 5. Here, the shift induced by the conformation of the azobenzene 6 seems to be larger than that by its molecular chirality, so that totally we observed a shift of λmax to longer wavelength in the left-handed host. These results indicate that the effective β of the chiral azobenzenes 6 is the contribution of at least two components, molecular chirality and molecular conformation. This supports the validity of the results reported by Labes et. al, who dealt with more complicated chiral molecules doped in cholesteric LCs.9 Thus, by comparing the behavior of achiral 5 and chiral 6 in both right- and left-handed hosts (Figure 4a and b), it is also indirectly confirmed that there is no intrinsic handedness in achiral azobenzenes. With the increase of the fraction of Z-isomer upon irradiation with UV light, λmax shifted to shorter wavelengths for all mixtures in the above experiments, as though the β of Z-isomers were stronger than E-isomers. However, this appears to be a deceitful interpretation, since first there is no handedness in these achiral azobenzenes, thus no effective β. And second, Z-isomers of the azobenzenes (except for some 3,3′-disubstituted azobenzenes) generally disorder or even destroy LC phases.12,15,16 In fact, Sackmann observed the opposite behavior, i.e., the shift of λmax to longer wavelength upon E-to-Z photoisomerization, using unsubstituted azobenzene in cholesteric LCs.10 An unsubstituted azobenzene (4) was also used in our systems and the behavior, as seen in Figure 4, was opposite to that of Sackmann. After a careful study, we found that this behavior seems to depend crucially on the signs of thermal characteristics of the helical pitch, definable as thermal coefficient R,
R)
1 dp p dT
( )
(5)
where p is the pitch length and T is the temperature. The sign of R is determined only by the sign of (dp/dT). In the righthanded and left-handed hosts used here (Figure 4), we found (dp/dT) < 0. A mixture of cholesteryl chloride and cholesteryl nonanoate (35:65 by weight), which was used by Sackmann, was also prepared to determine its R. We found that this mixture showed (dp/dT) > 0, contrary to our cholesteric hosts (Figure 7). This means that in a cholesteric mixture with (dp/dT) > 0, E-to-Z photoisomerization will end up with shifting of λmax to longer wavelength (lengthening of the pitch) and via versa. This phenomenon can be rationalized by taking into account that the microscopic molecular disordering induced by bent Z-isomer of an azobenzene should give analogous impact to the system (which appears as pitch shifting in this case) with thermally induced disordering of molecular arrangement. However, in some cholesteric mixtures having pretransition to smectic states,
Figure 7. Thermal behavior of the helical pitch of the right-handed cholesteric host (circle), the cholesteric LC consisting of cholestryl chloride and cholesteryl nonanoate at a ratio of 35:65 by weight (square) and the induced cholesteric LC formed by doping 5.8 wt % of chiral 6 in nematic DON-103 (triangle).
Figure 8. UV light-induced lengthening of the helical pitch of a cholesteric LC consisting of 5.8 wt % of 6 in a nematic DON-103.
an anomalous temperature behavior close to the smecticcholesteric transition temperature was observed, due to the formation of smectic clusters.17 Our cases deal only with normal cholesteric states. When chiral compound 6 is dissolved in a nematic LC (DON-103), a cholesteric mixture with a large helical pitch is induced. In this case compound 6 acts as both a chiral agent and a photoresponsive dopant. This mixture exhibited (dp/dT) > 0 (see also Figure 7) and hence E-to-Z photoisomerization resulted in lengthening of the pitch from 6 to 25 µm for the dopant concentration of 5.8 wt % (Figure 8). Again, this supports satisfactorily the validity of the phenomenon mentioned above. Additionally, it has to be stressed that, if a chiral azobenzene is introduced to cholesteric LCs, whether E-to-Z photoisomerization will lengthen or shorten the helical pitch depends competitively on the sign of the thermal coefficient, the handedness, and the magnitude of the effective β of the chiral dopant. Conclusion Photosensitive cholesteric LCs have been achieved by dissolving 3,3′-, 2,2′-dimethyl-3,3′- and 4,4′-disubstituted azobenzenes in cholesteric LCs. It was found that upon photoirradiation the helical pitch was modified markedly in the case of a 4,4′disubstituted azobenzene because of the drastic molecular conformation change between the E- and the Z-isomers. On the other hand, a 3,3′-disubstituted azobenzene or 2,2′-dimethyl3,3′-disubstituted azobenzene showed moderate modification or no modification of the pitches at all, depending on the substituents. A particular phenomenon was observed with
6534 J. Phys. Chem. B, Vol. 104, No. 28, 2000 compound 3, i.e., photoinert of the helical pitch during E/Z photoisomerization. This compound retained its rod-like shape in both E- and Z-isomers as has been confirmed by means of molecular mechanics and molecular orbital calculation. Therefore, E/Z transformation has almost no perturbation to the adjacent molecular arrangement. The achiral azobenzenes showed no preferred handedness in cholesteric LCs within the limit of low dopant concentration. By comparing the behavior of an achiral azobenzene and an analogous chiral azobenzene, it is reasonable to conclude that the effective β of the chiral azobenzene can be considered as the contribution of at least two components, i.e., the molecular conformation of the isomers and the molecular chirality derived from asymmetric carbons. The shrinking or lengthening of the helical pitch upon E-to-Z photoisomerization of achiral azobenzenes seems to depend on the thermal nature of the cholesteric LCs. LC phase-destructive Z-isomer of a conventional 4,4′-substituted azobenzene will induce a shortening of the helical pitch if (dp/dT) of the cholesteric host has a negative sign and vice versa. However, this is not always the case in 2,2′-dimethyl-3,3′-disubstituted azobenzenes since their geometrical isomers, depending on the substituents, may possess similar conformation. In the case where two types of chiral compounds, one of which is a chiral azobenzene, exist in the cholesteric mixtures, the behavior of the helical pitch photomodulation depends then on the competitive strength of the β and handedness of both chiral compounds. The information obtainable from the behavior of the macroscopic chirality in azobenzene containing cholesteric LCs here may serve as a comprehensive model in the interpretation of conformational molecular interaction in LC systems, which is of great importance in molecular modeling to achieve optimum tailored properties. Experimental Section Materials and Characterization. (S)-2-Methylbutanol was purchased from Kanto Chemical and p-toluenesulfonyl chloride was from Tokyo Chemical Industry. All reagents were used without further purification. The synthesis of 4,4′-dihydroxy azobenzene and compounds 1-5 have been reported previously.12b Chiral agents, R1011 and S1011 (Merck) were used as received. DON-103 (TNI ) 74.2 °C), a nematic LC consisting of mixtures of cyclohexanoic acid phenyl esters, and RO-571 (TNI ) 64.1 °C), a nematic LC consisting of mixtures of cyano biphenyl and cyano triphenyl compounds, were kindly donated from LODIC. The chemical structures of products were characterized by 1H spectra, recorded on a Bruker AC-200 NMR spectrometer with TMS as an internal standard, and elemental analysis. Phase transition temperatures were determined by a DSC 22C (Seiko Densi Kogyo) and a polarized optical microscope Olympus BH-2 equipped with a Mettler FP800 hot stage. (S)-2-Methylbutyl-p-toluenesulfonate. p-Toluenesulfonyl chloride (3.00 g; 34.0 mmol), DMAP (2.90 g; 23.7 mmol), and triethylamine (8.00 g; 79.2 mmol) were dissolved in 20 mL of dichloromethane. The solution was stirred and cooled in an icebath, while (S)-2-methylbutanol (3.00 g; 34.0 mmol) diluted in 4 mL of dichloromethane was added dropwise. After 30 min, the reaction was let warm to room temperature and stirred for another 2 h. The product was purified by column chromatography (silica, ethyl acetate/hexane 1:1) to give a product as a light-yellow liquid in a yield of 95%. 1H NMR (200 MHz, CDCl3) δ 0.83 (t, J ) 7.3 Hz, 3 H), 0.88 (d, J ) 6.6 Hz, 3 H), 1.04-1.26 (m, 1 H), 1.29-1.46 (m, 1 H), 1.63-1.79 (m, 1 H), 2.45 (s, 3 H), 3.74-3.93 (m, 2 H), 7.35 (d, J ) 8.0 Hz, 2 H), 7.79 (d, J ) 8.0 Hz, 2 H).
Ruslim and Ichimura (S,S)-4,4′-bis-(2-methylbutyloxy)azobenzene (6). To a 40 mL dehydrated DMF were introduced 4,4′-dihydroxy azobenzene (0.80 g; 3.73 mmol) and potassium carbonate (1.54 g, 11.2 mmol). The suspension was heated at 80 °C and stirred for 30 min before (S)-2-methylbutyl-p-toluenesulfonate (2.71 g; 11.2 mmol) was added dropwise. Three equimolar amounts of the compound were added here in order to obtain also (S)-4-(2methylbutyloxy)-4′-hydroxy azobenzene as another product which is used in other experiments. The products were separated through column chromatography (silica gel, ethyl acetate/hexane 1:10). The product, 6, was purified by recrystallization from ethyl acetate to give yellow crystals (mp 109-110 °C) in a 26% yield (from 4,4′-dihydroxy azobenzene). 1H NMR (200 MHz, CDCl3) δ 0.97 (t, J ) 7.4 Hz, 6 H), 1.04 (d, J ) 6.9 Hz, 6 H), 1.22-1.36 (m, 2 H), 1.50-1.66 (m, 2 H), 1.82-1.98 (m, 2 H), 3.77-3.94 (m, 4 H), 7.00 (d, J ) 9.2 Hz, 4 H), 7.86 (d, J ) 9.2 Hz, 4 H). Anal. Calcd. for C22H30N2O2 (%): C, 74.52; H, 8.55; N, 7.90. Found: C, 74.21; H, 8.45; N, 7.89. Sample Preparation. In nematic LCs, DON-103 and RO571, were dissolved 7.3 wt % of enantiomeric chiral agents, R-1011 (Merck) and S-1011, separately to form right-handed and left-handed cholesteric solvents, respectively. Approximately 2 wt % of azobenzene or substituted azobenzenes was dissolved in the mixtures. LC cells with Grandjean textures were prepared by sandwiching the LC solutions between plates treated with uniaxially rubbed PVA thin films. The thickness of the cells was adjusted by colloidal silica and was approximately 5 µm. Photoirradiation and Cholesteric Pitch Measurement. An LC cell was irradiated with UV and visible light sorted out from high-pressure Hg lamp (USH-500D) passing through a combination of glass filters, UV35/UVD35 and Y43/V44 (Toshiba), respectively. Irradiation was performed toward the photostationary states at about 23 °C. The reflection spectra of shortpitch cholesteric mixtures were recorded on a diode array spectrometer (HP8452A). In the case of large-pitch cholesteric LCs, the pitch was determined by Grandjean-Cano lines in wedge cells.18 The handedness of the cholesteric mixtures was confirmed by contact method.19 Temperature dependence of the cholesteric pitch was measured using a Mettler FP800 hot stage set under an optical microscope. References and Notes (1) (a) Sackmann, E.; Meiboom, S.; Snyder, L. C.; Meixner, A. E.; Dietz, R. E. J. Am. Chem. Soc. 1968, 90, 3567. (b) Baessler, M.; Laronge, T. M.; Labes, M. M. J. Chem. Phys. 1969, 51, 3213. (2) Gottarelli, G.; Samori, B.; Stremmenos, C. Chem. Phys. Lett. 1976, 40, 308. (3) Gottarelli, G.; Hibert, M.; Samori, B.; Solladie, G.; Spada, G. P.; Zimmermann, R. J. Am. Chem. Soc. 1983, 105, 7318. (4) Coles, H. In Handbook of Liquid Crystals; Demus, D., Goodby, J., Gray, G. W., Spiess, H.-W., Vill, V., Eds.; Wiley-VCH: Weinheim, 1998; Vol. 2A, Chapter 4, p 382. (5) (a) Solladie, G.; Zimmermann, R. Angew. Chem., Int. Ed. Engl. 1984, 23, 348. (b) Lemieux, R. P.; Schuster, G. B. J. Org. Chem. 1993, 58, 100. (c) Heppke, G.; Marschall, H.; Nu¨rnberg, P.; Oestreicher, F.; Scherowsky, G. Chem. Ber. 1981, 114, 2501. (d) Bobrovsky, A. Y.; Boiko, N. I.; Shibaev, V. P. Liq. Cryst. 1998, 25, 679. (e) Huck, N. P. M.; Janger, W. F.; de Lange, B.; Feringa, B. L. Science 1996, 273, 1686. (6) See, for instance: (a) Dyer, D. J.; Scho¨der, U. P.; Chang, K. P.; Twieg, R. J. Chem. Mater. 1997, 9, 1665. (b) Tamaoki, N.; Parfenov, A. V.; Masaki, A.; Matsuda, H. AdV. Mater. 1997, 9, 1102. (c) Tamaoki, N.; Song, S.; Moriyama, M.; Matsuda, H. AdV. Mater. 2000, 12, 94. (d) Hikmet, R. A. M.; Kemperman, H. Nature 1998, 392, 476. (e) Van de Vitte, P.; Neuteboom, E. E.; Brehmer, M.; Lub, J. J. Appl. Phys. 1999, 85, 7517. (7) Kozawaguchi, H.; Wada, M. Jpn. J. Appl. Phys. 1975, 14, 651. (8) Hibert, M.; Solladie, G. Mol. Cryst. Liq. Cryst. 1981, 64, 211. (9) Yarovoy, Y. K.; Labes, M. M. Mol. Cryst. Liq. Cryst. 1995, 270, 101. (10) Yarovoy, Y. K.; Patel, V.; Labes, M. M. Mol. Cryst. Liq. Cryst. 1998, 319, 101.
Effect of E/Z Photoisomerization on Azobenzenes (11) Sackmann, E. J. Am. Chem. Soc. 1971, 93, 7088. (12) (a) Ruslim, C.; Ichimura, K. Chem. Lett. 1998, 789. (b) Ruslim, C.; Ichimura, K. J. Mater. Chem. 1999, 9, 673. (13) Selective reflection spectra indicated here were recorded as transmitted spectra of the cholesteric mixtures arranged in Grandjean textures. (14) Labes, M. M.; Shang, W. J. Am. Chem. Soc. 1991, 113, 2773. (15) Sasaki, T.; Ikeda, T.; Ichimura, K. Macromolecules 1992, 25, 3807.
J. Phys. Chem. B, Vol. 104, No. 28, 2000 6535 (16) Fischer, B.; Thieme, C.; Fischer, T. M.; Kremer, F.; Oge, T.; Zentel, R. Liq. Cryst. 1997, 22, 65. (17) Voss, J.; Sackmann, E. Z. Naturforsch. 1973, 28a, 1496. (18) Cano, R. Bull. Soc. Franc. Mineral. 1968, 91, 20. (19) (a) Isaert, N.; Soulestin, B.; Malthete, J. Mol. Cryst. Liq. Cryst. 1976, 37, 321. (b) Gray, G. W.; McDonald, D. G. Mol. Cryst. Liq. Cryst. Lett. 1977, 34, 211.
