Langmuir 1997, 13, 3575-3578
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A 2D-NMR Investigation of Deuterated Chiral Dopants in Amphiphilic Cholesteric Liquid Crystals Keith Radley* and Gareth J. Lilly Department of Chemical and Biological Sciences, University of Huddersfield, Queensgate, Huddersfield HD1 3DH, England Received June 20, 1996. In Final Form: April 28, 1997X Several R-dideuterated dodecanoyl amino acids are investigated as chiral dopants with the chiral host potassium tetradecanoyl-L-alaninate during the preparation of amphiphlic cholesteric liquid crystals. Temperature dependent laser diffraction twist and 2D-NMR nuclear quadrupolar splittings measurements are made. The temperature gradient of the twist is thought to be a function of the micelle shape and anisotropy. The spectral multiplicity of the prochiral R-deutrons in the enantiomers is not visualized through NMR and hence no direct connection could be made between the molecular and bulk chirality. In two of the chiral dopants, where the precursor amino acids are L-isoleucine and L- leucine, the smaller 2D-NMR quadrupolar splitting moves through zero. This phenomenon is due to the molecular reorientation, where the angle of the axis projected in the CD2 plane, which orients with respect to magnetic field direction, being near to the magic angle in respect to the CD bond.
Introduction The multiplicity of the nuclear quadrupolar splitting doublet in the 2D-NMR assigned to the prochiral deuterium pairs in deuterated hydrocarbon chains of liquid crystals depends upon the molecular and bulk symmetry in liquid crystals.1 When the 2D-NMR spectrum is one doublet, these isochronous deuteriums are said to be enantiotopic with potential to form enantiomers. If either the molecular or bulk symmetry is chiral, the 2D-NMR could be two doublets. These anisochronous deuteriums are said to be diastereotopic with potential to form diastereoisomers. Only the case of the CD2 fragment in a chiral molecule in an nematic liquid crystal has been observed experimentally. Two 2D-NMR doublets have been reported with R-dideuterated dodecanoyl L-alanine and L-valine dissolved in an amphiphilic nematic liquid crystal.2,3 Similar observations have been made with deuterated aero-sol T in an amphiphilic lamellar liquid crystal.4 If the molecular and bulk symmetry are chiral, i.e., when chiral dopants are introduced into a cholesteric liquid crystal, pseudodiastereoisomers can be formed. If the enantiomers of a chiral molecule are dissolved in a cholesteric liquid crystal, the 2D-NMR of the anisochronous deuteriums could be four doublet, two doublets arising from each enantiomer. 2D-NMR experiments where four doublets are observed have potential as a method of the analysis for enantiomeric excess. Equally important, this type of experiment allows the determination of dissymmetry at the molecular level, which would enable the investigation of connections, if any, between molecule and bulk chirality. This case of four doublets has not been observed for prochiral deuteriums on a hydrocarbon chain in a cholesteric liquid crystal. Enantiomers of small molecules have been successfully visualized by NMR in both amphiphilic and polymeric cholesteric liquid crystals.5-10 * To whom correspondence should be addressed: 12 New St., Skelmanthorpe, Huddersfield, HD8 9BL, England. X Abstract published in Advance ACS Abstracts, June 15, 1997. (1) Parker, D. Chem. Rev. 1991, 91, 1441. (2) Tracey, A. S.; Zhang, X. J. Phys. Chem. 1992, 96, 3889. (3) Forrest, B. J.; Reeves, L. W.and Vist, M. Mol. Cryst. Liq. Cryst. 1984, 113, 37. (4) Olson, O.; Wong, T. C.; Soderman, O. J. Phys. Chem. 1990, 94, 5356. (5) Tracey, A. S.; Diehl, P. FEBS Lett. 1975, 59, 131. (6) Tracey, A. S.; Radley, K. J. Phys. Chem. 1984, 88, 6044. (7) Radley, K.; Cattey, H. Mol. Cryst. Liq. Cryst. 1993, 226, 195.
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Amphiphilic cholesteric liquid crystals have been prepared using both chiral hosts11-13 and chiral dopants.14-16 The chiral center facilitates a spontaneously twisted structure characterized by a repeat distant called the pitch, where the reciprocal is called twist. In the present study the 2D-NMR is investigated, using several R-dideuterated dodecanoyl amino acids as chiral dopants. It is hoped to show the apparent connection between NMR quadrupolar splitting and the bulk chirality twist in an amphiphilic cholesteric liquid crystal (ACLC) is superficial.2 It has previously been shown that the magnitude of the twist is derived from the anisotropy and type of micelle and not cis and trans rotamers.15,19 The variation in NMR quadrupolar splittings will be shown to arise from molecular reorientation, which are not related to the bulk chirality twist. It is suggested that chirality can only be measured using NMR if the enantiomers are visualized. Experimental Section The chiral host detergent and the chiral dopant detergents were synthesized as previously described.6,12,13 For the chiral dopants, i.e., the R-dideuterated dodecanoyl amino acids, dodecanoyl aldehyde was R-dideuterated by repeated (twice) refluxing with alkaline D2O, followed by oxidation with acidified KMnO4 to give the R-dideuterated dodecanoic acid. This deuterated acid was converted to the acid chloride using SOCl2. The acid chloride was then used to acylate the amino acid in the presence of cold aqueous NaOH. The R-dideuterated dodecanoyl amino acid was liberated using H2SO4, extracted using diethyl ether, and dried with anhydrous MgSO4. After rotavaporation of the ether, the product was recrystallized from hexane. The amino acid precursors were all 50% DL enantiomer and 50% L-enantiomer, i.e., excess L. The structures of the various R-dideuterated amino acids are presented in Figure 1. (8) Canet, J. L.; Meddour, A.; Courtieu, J. J. Am. Chem. Soc. 1994, 116, 2155. (9) Meddour, A.; Canet, I.; Lowenstein, A.; Pechine J. M.; Courtieu, J. J. Am. Chem. Soc. 1994, 116, 9652. (10) Lesot, P.; Merlet, D.; Meddour, A.; Courtieu, J.; Lowenstein, A. J. Chem. Soc., Faraday. Trans. 1995, 91, 1371. (11) Covello, P. S.; Forrest, B. J.; Marcondes Helene, M. E.; Reeves, L. W. Vist, M. J. Phys. Chem. 1983, 87, 176. (12) Tracey, A. S.; Radley, K. Langmuir 1990, 6, 1221. (13) Radley, K. Liq. Cryst. 1992, 11, 753. (14) Radley, K.; Saupe, A. Mol. Phys. 1978, 35, 1405. (15) Marcondes Helene; M. E.; Figueiredo Neto, A. M. Mol. Cryst. Liq. Cryst. 1988, 162B, 127. (16) Do Aido, T. M. H.; Alcantara, M. R.; Felippe, O., Jr.; Pereira, A. M. G.; Vanin, J. A. Mol. Cryst. Liq. Cryst. 1990, 185, 61.
© 1997 American Chemical Society
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Figure 2. Magnitude of the helical twist determined by laser diffraction, plotted as a function of temperature. 4, b, and 9 denote data for the chiral dopants R-dideuterated L-KDDA, L-DDV, and L-DDPA, respectively. Figure 1. Molecular structures of R-dideuterated dodecanoyl amino acids in the present study: L-DDS, serine; L-DDV, valine; L-KDDA, alanine (potassium salt); L-DDPA, phenylalanine; L-DDIL, isoleucine; L-DDL, leucine.
The temperature dependence of the twist data is plotted out in Figure 2. The gradient of the regression is negative. This is similar to the findings in previous publications.6,12,13 In Figure 2 only the data for the R-dideuterated potassium dodecanoyl-L-alaninate (L-KDDA), dodecanoyl-L-phenylalaninate (L-DDPA) and dodecanoyl-L-valine (L-DDV) have been included. The general trend in all these latter cases is the same. The twist data in each case converges to small values at higher temperatures. A partial phase diagram of the potassium tetradecanoyl-L-alaninate (LKTDA)/decanol/H2O system has been previously published.19 The parameters in the present samples will involve small pertubations of those in the previous study, where there will be a slight difference in salt composition
and a small amount of dopant will have been added. In the previous study as well as the cholesteric-disk micellar ChD phase, the cholesteric-cylindrical micellar ChC phase was investigated, where the twist-temperature gradient was positive.19 Two distinct mechanisms have been proposed for the changes of twist with temperature in cholesteric liquid crystals.20 The first involves the unwinding of the bulk structure, and this appears to be the situation for the ChC phase. The second mechanism involves changes in the micellar spacing, and this appears to be the dominant effect in the ChD phase. X-ray studies have shown in a ChD phase that the twist was proportional to the microscopic shape anisotropy of the micelle.21 It was also suggested in a recent study involving the detergent concentration dependence of twist that the mechanism for the generation of the helical structure involved cis and trans rotamers associated with the peptide link in the detergent headgroup.2 No data to confirm this assertion such as say 13C-NMR visualization of the rotamers has not been presented in this or any other study. It would seem that the generation of the helical structure in amphiphilic cholesteric liquid crystals has very little to do with rotamers, but there is more than likely a strong connection with micelle shape, where the temperature gradient reverses from cylinders to disks and the magnitude of the twist is in proportion to micellar shape anisotropy. The multiplicity of the 2D-NMR and its relationship to the molecular and bulk symmetry have already been discussed. In all of the cases highlighted in the present study four nuclear quadrupolar splitted doublets in the 2 D-NMR should potentially be observed from the prochiral R-dideuterions in the dodecanoyl chain of the enantiomeric material dissolved in the cholesteric liquid crystal.1,9,10 This statement is central for the conclusions drawn. The visualization of the enantiomeric prochiral diastereoisomers is the main aim of investigators working in the field of NMR in cholesteric liquid crystals. From this work a
(17) Boden, N.; Radley, K.; Holmes, M. C. Mol. Phys. 1981, 42, 493. (18) Holmes, M. C.; Boden, N.; Radley, K. Mol. Cryst. Liq. Cryst. 1983, 100, 93. (19) Radley, K. Liq. Cryst. 1995, 18, 151.
(20) Gibson, H. W. Liquid CrystalssThe Fourth State of Matter; Saeve, F. P., Ed.; Marcel Dekker: New York, 1979; p 113. (21) Valente Lopez, M. C.; Figueiredo Neto, A. M. Phys Rev. 1988, 38, 1101.
The samples were prepared as previously described6,7,12,13,19 and after homogeneous mixing were transferred to 5 mm NMR tubes prior to heat sealing. The composition of the host sample was 28.7% potassium tetradecanoyl-L-alaninate, 6.5% decanol, and 64.7% water (9% CsCl and 1% K2CO3). A Bruker AM 250 MHz spectometer fitted with a deuterium probe and a temperature controller was used to acquire the deuterium NMR spectra. The samples were heated to the maximum temperature (325 K) and allowed to align in the magnetic field for at least 6 h. After the temperature decreased, the sample was allowed to equilibrate for at least 30 min at the new temperature before the spectrum was acquired. Spectra were acquired at 1 deg intervals between 295 and 325 K with 10 000 scans and 40 000 Hz spectral width. Temperature dependent laser diffraction measurements of twist were made as previously described13,19 with laser light of wavelength 6.328 × 10-5 cm-1. The phase type with respect to micelle shape in investigated samples was assigned as cholesteric-disks ChD using polarizing microscopy methods previously described.17,18
Results and Discussion
Deuterated Chiral Dopants in Liquid Crystals
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Figure 3. Magnitude of the quadrupolar splittings determined by 2D-NMR plotted as a function of temperature. O, 0, 4, and × denote data for the chiral dopants R-dideuterated L-DDV, L-DDPA, L-KDDA, and L-DDS, respectively. Open shapes denote larger quadrupolar splittings and closed shapes denote smaller quadrupolar splittings.
Figure 4. Ratios of the quadrupolar splittings determined by 2 D-NMR. plotted as a function of temperature. b, 9, 2, and × denote data for the chiral dopants R-dideuterated L-DDV, L-DDPA, L-KDDA, and L-DDS, respectively.
practical method for enantiomeric analysis may evolve. In no case in the present study has four doublets been observed; in fact in the case of R-dideuterated dodecanoylserinic acid, only one doublet was observed. (Although the chiral dopants are denoted as the L enantiomer, the precursor amino acids were in fact 50% L with 50% DL.) This means that in the present study, the enantiomers in these chiral dopants were not visualized through NMR. The present data do not determine chirality in these cholesteric systems and cannot be in any way connected directly with the bulk chiralty. There are several reasons why the enantiomers are not NMR visualized in the present study. In order for NMR to visualize enantiomers the NMR peak resolution must be less than the enantiomeric peak separation. Narrow line widths can only be achieved in liquid crystal NMR with particular chiral liquid crystals, when the sample is homogeneously aligned in a highly homogeneous magnetic field. Sample alignment inhomogeniety can result from concentration and temperature gradients, which are best eliminated by holding the sample horizontal at the required temperature a long time prior to the NMR experiment. If the motional averaging in the amphiphilic entiety, which is part of the chiral micelle, is on the same time scale as the NMR experiment, line broadening would be observed. Line broadening is produced in NMR by dipole-dipole interactions, which are enhanced in an anisotropic environment. It was attempted unsuccessfully to eliminate some of dipole-dipole interaction using proton decoupling. Some of the dipole-dipole interactions cannot be eliminated between adjacent deuteriums. The 2D-NMR data for the chiral dopants R-dideuterated L-KDDA, L-DDV, L-DDPA , and dodecanoyl-L-serinic acid (L-DDS) are presented in Figure 3. In the case of L-DDS it has already been mentioned that there is only one doublet in the NMR, which agrees with previously published data.2 The doublet splitting temperature gradient is slightly negative. In the other three cases there were two doublets. In the case of L-KDDA the temperature gradient for both quadrupolar splitting was negative. With the other two chiral dopants L-DDV and L-DDPA, the gradient for the largest splitting was negative, while that for the
smallest splitting was positive. The data could then be simplified by considering the ratio of the large and small splitting as illustrated in Figure 4. The ratio for serine was of course unity. The ratios for the other three cases were greater than 1. In each of these three cases the temperature gradient was negative. The meaning of the above data may become obvious when the differing data sets associated with the chiral dopants R-dideuterated dodecanoyl-L-leucinic acid (L-DDL) and dodecanoyl-L-isoleucinic acid (L-DDIL) have been discussed. These two cases also give rise to two 2D-NMR quadrupolar splittings, where the largest splitting has a small negative temperature gradient. The smallest splitting in each case passes through zero. See Figure 5. If the ratio of the large and smallest splittings werecalculated as in Figure 6, with rising temperature the magnitude of the ratio approached negative infinity at given temperature and then fliped to positive infinity, where the magnitude of the ratio fell off with rising temperatures. 2D-NMR spectra are illustrated in Figure 7. The sign of the quadrupolar splitting is not measured by the NMR experiment. The sign of the quadrupolar splitting was assigned on a purely arbitary basic. The signs were assigned other than positive only when the magnitude appears to pass through zero. This will become clearer when molecular reorientation is discussed below. The nuclear quadrupolar splittings, as well as being derived from the nuclear quadrupolar moment and resulting from the distortion of the electrical field gradient along the CD bond, are a function of the molecular orientation and structure. An axis in the CD2 plane precesses about the magnetic field. This axis makes angles R and β with each of the CD bonds. R plus β is constant and is close to the tetrahedral angle. When angle R equals angle β, the NMR quadrupolar splittings will be equal resulting in one doublet, as was observed in the case of L-DDS. In other cases where the observed NMR was two doublets, angle R will not be equal to angle β. There is a special case when angle R is not equal to angle β where one of the angles corresponds to the magic angle. In this case one of the smaller quadrupolar splittings will be zero and the other non zero, as was observed with the chiral dopants
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Figure 5. Magnitude of the quadrupolar splittings determined by 2D-NMR plotted as a function of temperature. Open and closed O denote data for the chiral dopants R-dideuterated L-DDIL and L-DDL, respectively.
Radley and Lilly
Figure 7. 2D-NMR spectra of the R-dideuterated L-DDL(leucine) at three different temperatures, 294, 302, and 313 K. Scale is in hertz.
temperature L-DDIL and L-DDL cases. In would seem that in the other cases the same flip from positive to negative would take place if, during the experiment, a substantially lower temperarature could be achieved. Conclusions
Figure 6. Ratio of the quadrupolar splittings determined by 2D-NMR plotted as a function of temperature. Open and closed O denote data for the chiral dopants R-dideuterated L-DDIL and L-DDL, respectively.
L-DDIL and L-DDL. The assignment of sign to low temperature small quadrupolar splitting and the high temperature small quadrupolar splitting assuming the large quadrupolar splitting is positive was made on the basis of converging values when compared to normal cases at the begining of this study. These other cases do not show this flip, but the temperature gradients of the quadrupolar splittings (except in the case of L-DDS) are negative like the high
In five of the R-dideuterated chiral dopants the 2D-NMR of the prochiral deuteriums was two doublets and in the serine case one doublet. In the temperature dependent spectra for the dideuterated L-DDIL and L-DDL, the smallest doublet passed through zero. This divergence from normal behavior takes place because of molecular reorientation where the CD bond angle with respect to the precessing axis about the magnetic field is close to the magic angle. The temperature gradient of twist was thought to be a function of micelle shape and anisotropy. The temperature gradients of quadrupolar splittings on the otherhand could depend upon molecular reorientation, which might move to infinity, when the sign of the quadrupolar splitting passes through zero near the magic angle. In the present study there would seem to be no direct connection between the twist and the NMR quadrupolar splitting in ACLCs. Acknowledgment. Thanks to Professor M. I. Page in the Department of Chemical and Biological Sciences at the University of Huddersfield for providing research facilities. The authors thank the university for providing a small research grant to buy chemicals and also grateful thanks for a postgraduate studentship to G.J.L. Professor J. Courtieu at the University of Paris (Sud) at Orsay France is thanked for helpful discussion and for the use of his 2D-NMR facilities. LA960607M
