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Study of the Chromonic Liquid-Crystalline Phases of Bis-(N,N-diethylaminoethyl)perylene-3,4,9,10-tetracarboxylic Diimide Dihydrochloride by Polarized Optical Microscopy and 2H NMR Spectroscopy Suk-Wah Tam-Chang,* Isaac K. Iverson,† and Jennifer Helbley Department of Chemistry, University of Nevada, Reno, Nevada 89557 Received June 23, 2003. In Final Form: September 26, 2003 The chromonic liquid-crystalline properties of bis-(N,N-diethylaminoethyl)perylene-3,4,9,10-tetracarboxylic diimide dihydrochloride in an aqueous solution were investigated by polarized light microscopy and 2H NMR spectroscopy. Both techniques indicate a narrow I + N biphasic region and a broad N phase region at concentrations ranging from ∼6.9 to ∼30 wt % at room temperature. Optical microscopy indicates that a hexagonal M phase exists at higher concentrations. The variation of the I f N + I and N + I f N transition temperatures with concentration was studied by 2H NMR spectroscopy. Finally, the effects of temperature and concentration on the order parameter of the N phase were investigated by 2H NMR using a tetra-deuterated derivative. A value of 0.97 was obtained for the N phase at its upper concentration limit.
Introduction The significance of noncovalent interactions in biological functions, drug actions, materials properties, and the fabrication of devices is well recognized, and, thus, the study of intermolecular interactions has become an important area of research.1-10 Within this broad area of chemistry, the properties and applications of chromonic liquid crystals have gained increasing attention in the last 2 decades.11,12 Chromonic liquid crystals11,12 belong to a class of lyotropic (solvent-dependent) liquid crystals that is distinct from the conventional amphiphilic, lyotropic liquid crystals13 such as soaps, detergents, and lipids. The chromonic mesogens (liquid-crystalline prone molecules) reported included drugs,14 dyes,15,16 and nucleic acids.17 They generally have disk-shaped or plank-shaped aromatic * Author to whom correspondence should be addressed. † Present address: GE Osmonics, 5951 Clearwater Drive, Minnetonka, MN 55343. (1) Lehn, J.-M. Angew. Chem., Int. Ed. Engl. 1988, 27, 89. (2) Philp, D.; Stoddart, J. F. Angew. Chem., Int. Ed. Engl. 1996, 35, 1154. (3) Peczuh, M. W.; Hamilton, A. D. Chem. Rev. 2000, 100, 2479. (4) Ward, M. D.; Pivovar, A. M. Curr. Opin. Solid State Mater. Sci. 1999, 4, 581. (5) Mallouk, T. E.; Gavin, J. A. Acc. Chem. Res. 1998, 31, 209. (6) Gin, D. L.; Gu, W.; Pindzola, B. A.; Zhou, W.-J. Acc. Chem. Res. 2001, 34, 973. (7) Clark, S. L.; Hammond, P. T. Langmuir 2000, 16, 10206. (8) Wu, A.; Chakraborty, A.; Fettinger, J. C.; Flowers, R. A., II; Isaacs, L. Angew. Chem., Int. Ed. 2002, 41, 4028. (9) Xia, Y.; Rogers, J. A.; Paul, K. E.; Whitesides, G. M. Chem. Rev. 1999, 99, 1823. (10) Hulvat, J. F.; Stupp, S. I. Angew. Chem., Int. Ed. 2003, 42, 778. (11) Lydon, J. In Handbook of Liquid Crystals; Demus, D., Goodby, J., Gray, G. W., Spiess, H.-W., Vill, V., Eds.; Wiley-VCH: Weinheim, 1998; Vol. 2B, p 981. (12) Lydon, J. Curr. Opin. Colloid Interface Sci. 1998, 3, 458. (13) Collings, P. J.; Patel, J. S. Handbook of Liquid Crystal Research; Oxford University Press: New York, 1997. (14) Sandquist, H. Berichte 1915, 48, 2054. (15) Harrison, W. J.; Mateer, D. L.; Tiddy, G. J. T. J. Phys. Chem. 1996, 100, 2310. (16) Ruslim, C.; Matsunaga, D.; Hashimoto, M.; Tamaki, T.; Ichimura, K. Langmuir 2003, 19, 3686. (17) Garbesi, A.; Gottarelli, G.; Mariani, P.; Spada, G. P. Pure Appl. Chem. 1993, 65, 641.
rings that are functionalized at the periphery with ionic or other hydrophilic groups for solubility in water. In aqueous solutions, these mesogens self-organize as a result of the enthalpically favored π-stacking interactions between the aromatic rings in addition to entropically driven hydrophobic interactions. The self-organization of these mesogens generates liquid-crystalline phases in which the molecules possess liquid-like mobility and crystal-like ordersorientationally, positionally, or both. Chromonic liquid-crystalline phases observed11,12 include (1) a lamellar phase in which the molecules stack to form a lamellar brick-wall structure, (2) a nematic (N) phase in which the molecules form columnar aggregates (not necessarily simple one-molecule-wide columns) but there is no positional order among the columns, and (3) a hexagonal (M) phase in which the molecules stack into columns and the columns arrange in a hexagonal lattice. When compared with other classes of lyotropic and thermotropic liquid crystals that have been studied, applications of chromonic liquid crystals are relatively unexplored. Potential applications reported for chromonic liquid crystals included the fabrication of light-harvesting devices18 and polarizers by techniques based on induced orientation of the liquid crystals by a photoaligned polymer substrate19-21 or by a shearing force.22-25 In both techniques for polarizer production, an important criterion for generating high-performance polarizing films is the self-organization of dichroic dyes into a highly ordered (18) Gustand, D.; Moore, T. A. Science (Washington, D.C.) 1989, 244, 35. (19) Ichimura, K.; Fujiwara, T.; Momose, M.; Matsunaga, D. J. Mater. Chem. 2002, 12, 3380. (20) Ichimura, K.; Momose, M.; Kudo, K.; Akiyama, H.; Ishizuki, N. Langmuir 1995, 11, 2341. (21) Matsunaga, D.; Tamaki, T.; Akiyama, H.; Ichimura, K. Adv. Mater. 2002, 14, 1477. (22) Tam-Chang, S.-W.; Seo, W.; Iverson, I. K.; Casey, S. M. Angew. Chem., Int. Ed. 2003, 42, 897. (23) Iverson, I. K.; Tam-Chang, S.-W. J. Am. Chem. Soc. 1999, 121, 5801. (24) Carson, T. D.; Seo, W.; Tam-Chang, S.-W.; Casey, S. M. Chem. Mater. 2003, 15, 2292. (25) Iverson, I. K.; Casey, S. M.; Seo, W.; Tam-Chang, S.-W.; Pindzola, B. A. Langmuir 2002, 18, 3510.
10.1021/la030256t CCC: $27.50 © 2004 American Chemical Society Published on Web 12/12/2003
Chromonic Liquid-Crystalline Phases
chromonic liquid-crystalline phase that is subsequently dried to give an anisotropically oriented solid phase. The structure-property relationships of chromonic liquid crystals are not clearly understood despite the identification of many chromonic mesogens. Important questions still remain unanswered. For example, not all amphiphilic or ionic aromatic compounds display chromonic liquid-crystalline properties in aqueous solution. What are the determining structural factors? Why do some chromonic liquid crystals form lamellar structures and others form columnar aggregrates? A further understanding of the structure-property relationships is important for designing novel mesogens, optimizing properties, and developing new applications. We report detailed studies of the chromonic liquid-crystalline properties of bis-(N,Ndiethylaminoethyl)perylene-3,4,9,10-tetracarboxylic diimide dihydrochloride (1) by polarized optical microscopy and 2H nuclear magnetic resonance (NMR) spectroscopy.
Experimental Section Materials. Deuterium oxide (99.9% D) was purchased from Cambridge Isotope or Aldrich Chemical Co. Deuterated trifluoroacetic acid (99.5% D) and deuterated sulfuric acid (D2SO4, 99% D, 96-98% solution in D2O) were purchased from Cambridge Isotope. Perylene-3,4,9,10-tetracarboxylic dianhydride (98%) and N,N-diethylethylene diamine (g98%) were purchased from Acros Chemical Co. Bis-(N,N-diethylaminoethyl)perylene-3,4,9,10-tetracarboxylic diimide dihydrochloride (1). Compound 1 was prepared according to a previously published procedure.23,25 Bis-(N,N-diethylaminoethyl)perylene-3,4,9,10-tetracarboxylic diimide-1,6,7,12-d4 dihydrochloride (2). In a glass vial, 1.004 g (1.71 mmol) of 1 was dissolved in 9.15 g of concentrated sulfuric acid-d2 (96-98 w/w % in 2H2O). The solution was transferred to a 10-mm NMR tube, and it was put into a sand bath at 105 °C for 136 h. The 1H NMR spectra of the solution were obtained periodically during the time period to monitor the integration of signals for the aromatic protons relative to those for the aliphatic protons. The solution was then poured into 60 mL of double-distilled water, and the tube was rinsed with water. An NH4OH(aq) solution [prepared by adding 45 mL of concentrated NH4OH (assay 28-30%) to 55 mL of water] was added to the sulfuric acid solution carefully to give a resulting solution of pH 10. It was sonicated for 5 min and allowed to sit overnight. The precipitate was filtered, washed with about 200 mL of 3.2 M NH4OH(aq), and allowed to dry in air overnight. The solid was dissolved in 100 mL of 1 M HCl(aq), and 75 mL of water was added. The solvent was evaporated to dryness with a rotary evaporator under reduced pressure from an aspirator. The solid recovered was dried under a vacuum to yield 0.992 g (1.49 mmol, 87%) of 2. The regioselectivity indicated for the deuterium labeling
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Figure 1. Photomicrographs of aqueous solutions of 1 observed between crossed polarizers (200×). (Left) The peripheral evaporation experiment showed the liquid-crystalline phase (red droplets) developing out of the 1.7 mM isotropic solution (black regions). (Right) The schlieren texture of a 20 wt % (∼0.3 M) solution of 1 suggested a chromonic N phase. (300 MHz, CF3COOD) δ: 8.80 (s, 4H, Ar-H), 4.77 (t, 4H, J ) 5 Hz, R-CH2), 3.69 (t, 4H, J ) 5.5 Hz, β-CH2), 3.50 (unresolved q, 8H, -NCH2-CH3), 1.45 (t, 12H, J ) 7 Hz, -CH3). HRMS-FAB (m/z): [M2+ - H+] calcd for C36H33D4N4O4, 593.3066; found, 593.3092. mp 290 °C dec. Preparation of Liquid-Crystalline Samples. Liquidcrystalline solutions were prepared by dissolving the desired amount of 1 or 2 in screw-cap vials, which were then tightly sealed with Teflon tape on the threads and Parafilm M sealing film outside the cap. All liquid-crystalline solutions were prepared with double-distilled water from a Barnstead MegaPure 3 water distillation system or deuterium oxide (99.9% D). Solutions of concentrations below 16 wt % were homogenized with shaking for 1 or 2 days at temperatures between 30 and 40 °C. Solutions of concentrations above 16 wt % were heated to ∼45 °C and simultaneously swirled at ∼150 rpm for several weeks. Samples were then transferred to 5-mm NMR tubes and immediately flame-sealed. After the studies were completed, the NMR tubes were broke open and the weights of the solutions were determined. The samples were then dried thoroughly under a vacuum to constant weight. The actual concentrations used for the studies were, thus, determined. Polarized Optical Microscopy. Polarized optical microscopy studies were performed at room temperature with a Nikon PL600 polarizing microscope equipped with a N70 film camera. 2H NMR Spectroscopy. The NMR studies of the liquidcrystalline samples were performed with a Varian Unity Plus 500-MHz NMR spectrometer. For all the samples, the direction of the magnetic field was parallel to the spinning axis of the sample tube. The 2H NMR spectra for the mesophases of 1 in 2H O were collected using the lock deuterium channel. The sample 2 was shimmed on the lock channel, then the lock cable was removed, and the observed cable was connected to the lock for observation of the 2H signal. As a result of the high signal-tonoise inherent in the experiment, only one transient was necessary per spectrum. The sweep width was 7500 Hz. The quadrupolar splitting was measured using the peak pick capabilities of the software. The 2H NMR spectra of 2 in 1H2O solution was measured by tuning the observe channel (5-mm BB probe) to a frequency of 76.727 MHz while ensuring that the normal lock channel was detuned away from 76 MHz. The acquisition time was set to 0.080 s with a sweep width of 100 kHz (the maximum width allowed by the digitizer). The number of transients collected ranged from 5000 for concentrated samples to 60 000 for dilute samples. The dwell time between temperature increments was 10 min.
Results and Discussion
in 2 was inferred from reactivity patterns reported in the literature26 for the perylenetetracarboxylic diimide dyes. 1H NMR (26) Rogovik, V. I.; Shirokii, E. I.; El’tsov, A. V. Zh. Org. Khim. 1980, 16, 867.
Polarized Light Microscopy Studies at Room Temperature. A 1.7 × 10-3 M aqueous solution of 1 was observed to be isotropic (I phase) when examined between crossed polarizers of an optical microscope. When the solvent of this isotropic solution was slowly evaporated from the edge of the cover glass, birefringent droplets (Figure 1, left) gradually appeared in the isotropic bulk
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Figure 2. Photomicrograph of a 37 wt % solution of 1. Both the grainy textures of the M phase and schlieren lines (top right corner) of the N phase were visible. The boundary between the two phases was not well defined. (200×, crossed polarizers)
indicating the formation of a liquid-crystalline phase. Upon further evaporation of solvent, these droplets coalesced to form a bulk liquid-crystalline phase. Examination of the solutions at higher concentrations showed that the transition from the isotropic phase to a liquid-crystalline phase began to occur when the concentration of 1 was at about 6.5 wt % (∼0.1 M); the solution became completely liquid-crystalline at about 6.9 wt % 1. The photomicrograph (Figure 1, right) of a more concentrated solution (20 wt %, ∼0.3 M) of 1 showed that the sample was completely liquid-crystalline. The schlieren texture observed is characteristic of a chromonic N phase (a nematic phase). As described earlier, the molecules aggregate to form columns, but there is no positional order among the columns in this phase.11 At room temperature, the N phase persisted at concentrations above 33 wt % 1. The results of the small-angle X-ray diffraction analysis of 1 in the N phase are previously reported.25 At a concentration of ∼37 wt % 1, the liquid-crystalline sample showed the schlieren texture of the N phase and grainy optical textures. A similar grainy texture was observed for other chromonic liquid crystals and is characteristic of a chromonic M phase11,27,28 in which the columns of molecules are arranged in a hexagonal lattice.29 The photomicrograph of the M + N biphasic solution (37 wt %) is shown in Figure 2. When a 41 wt % aqueous solution of 1 was examined, mainly grainy textures of the M phase (with a few islands of N phase textures) were observed. No further studies were performed at concentrations higher than 41 wt % because the samples became too viscous to handle and dried up too quickly for an accurate phase analysis. To obtain further confirmation of the existence of the M phase, a 23 wt % sample of 1 (in the N phase) was (27) Attwood, T. K.; Lydon, J.; Jones, F. Liq. Cryst. 1986, 1, 499. (28) Turner, J. E.; Lydon, J. Mol. Cryst. Liq. Cryst. Lett. 1988, 5, 93. (29) We attempted to characterize the M phase in a 41 wt % sample by small-angle X-ray diffraction. A diffraction peak between 3 and 4 Å was observed for the intermolecular distance within a column. In addition, a principle diffraction peak was observed at a d spacing of 25.4 Å, but only a very weak peak was observed at 14.4 Å expected for a d/x3 peak characteristic of a hexagonally ordered phase. The d/x3 peak was weak presumably because the disposition of the electron density gives a low value for the structural factor. Another possible reason is the distortion of the hexagonal phase caused by the shearing of this viscous material when transferred to the NMR tube. Previous studies (see ref 25) showed that the mechanical shearing of liquidcrystalline samples of 1 in an aqueous solution led to thin films in which the molecules of 1 were anisotropically oriented in a distorted hexagonal packing. Because samples in the M phase were very viscous to handle and dried up too quickly for accurate weight and phase analysis, no further attempts in X-ray diffraction study were made.
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Figure 3. Peripheral evaporation of a solution of 1 (23 wt %) in a chromonic N phase (bottom left corner) resulted in the formation of herringbone texture (right) characteristics of a hexagonal M phase (400×, crossed polarizers).
concentrated by the peripheral evaporation of the solvent from under the edge of the cover glass. A striated herringbone texture (instead of a grainy texture) is expected when the M phase is allowed to develop gradually from the N phase across a concentration gradient resultant from the slow evaporation of the solvent. The transition of the schlieren texture of the chromonic N phase of 1 into a herringbone texture of the M phase was observed as shown in Figure 3. 2 H NMR Studies of Liquid-Crystalline Solutions. 2 H NMR spectroscopy is a useful technique for studying lyotropic liquid-crystalline samples15,30-32 when the mesogen is deuterium-labeled (e.g., 2) or when a deuterated solvent (e.g., 2H2O) is used. The 2H nucleus has a spin quantum number I ) 1 and, thus, possesses an electric quadrupole moment. In an isotropic medium, the deuterium resonance appears as a single peak. However, in an anisotropic (direction-dependent) environment such as that in a liquid-crystalline phase, the quadrupole moment interacts with the electric field gradients and the deuterium resonance splits into two peaks (2I peaks). From the absence or presence of splitting and the magnitude of splitting, useful information can be gained about the liquid-crystalline system. The phase transition to an isotropic phase, the alignment of the liquid-crystalline domains in the magnetic field, and the extent of order in the liquid-crystalline phase (order parameters) can be probed using 2H NMR.15,30-32 Concentration- and Temperature-Dependent N/I Phase Transition Determined by 2H NMR Studies. Solutions at various concentrations (ranging from 6.7 to 38 wt %) of 1 in 2H2O were examined with NMR spectroscopy by probing the 2H of the solvent. The first spectrum of each sample was acquired within 15 s after depositing the sample into the bore of the spectrometer. At 20 °C, the 2H NMR spectra of all the samples showed two relatively sharp (line width ∼ 3-4 Hz) peaks indicating that the samples were liquid-crystalline and the mesophase was macroscopically aligned within 15 s of exposure to the magnetic field. Evidence for the existence of these samples in a chromonic N phase was provided by the polarized optical microscopy studies discussed in the previous section. No changes in the 2H NMR spectra were observed over several hours as the samples were allowed to sit in the field without spinning or changing the temperature. (30) Halle, B.; Wennerstro¨m, H. J. Chem. Phys. 1981, 75, 1928. (31) Davis, J. H. Biochim. Biophys. Acta 1983, 737, 117. (32) Harrison, W. J.; Mateer, D. L.; Tiddy, G. J. T. Faraday Discuss. 1996, 104, 139.
Chromonic Liquid-Crystalline Phases
Figure 4. Offset 2H NMR spectra of an 11.2 wt % solution of 1 in 2H2O at 68 (top), 65 (middle), and 62 (bottom) °C.
The temperature of the samples in the spectrometer was then increased until only a single resonance peak was observed, indicating the complete transition into an isotopic (I) phase. When the samples were allowed to cool slowly, a doublet emerged gradually on the sides of the singlet peak and the singlet peak at the center slowly decreased in intensity. These changes suggested the transition from the isotopic (I) phase into a biphasic system in which the isotopic (I) and chromonic N phases coexist. Eventually, only the doublet remained, indicating the complete transition into the liquid-crystalline N phase. Shown in Figure 4 are representative spectra showing the effect of temperature on the 2H resonances of 2H2O in an 11.2 wt % solution of 1. The same transition temperatures were observed regardless of whether the sample was heated or cooled through the transition; no hysteresis was observed. The dependence of the clearing temperature (the temperature at which a liquid-crystalline phase is completely converted into an isotropic phase) on the concentration of 1 is shown in Figure 5. For each concentration, the point shown as a circle was the lowest temperature at which only a single resonance peak was observed (indicating that 2H2O molecules were in an isotropic environment) and the point shown as a cross was the highest temperature at which only the doublet was observed (suggesting that 2H2O molecules were in a liquidcrystalline medium). The region between the two points (circle and cross) represents the N + I biphase region. Solutions with concentrations up to 38 wt % in 2H2O were prepared, but the transition to an isotropic phase for these samples was not observed below 100 °C (the practical temperature limit for these experiments) and were, thus, not included in the plot. Effect of Concentration and Temperature on the Magnitude of the Observed 2H Splitting. The magnitude of the observed 2H splitting (νQD) of 2H2O in the chromonic N phase of 1 changes significantly with the concentration of 1. At 25 °C, the observed splitting increased from about 4.6 Hz for an 8.6 wt % solution to about 94 Hz for a 36.7 wt % solution in 2H2O. At a constant concentration of 1, there was no significant change in the observed splitting with increasing temperature.
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Figure 5. Concentration-dependent transition of 1 in the 2H2O solution from the isotropic to the liquid-crystalline chromonic N phase as determined by 2H NMR. The solutions were heated past the clearing point and then slowly cooled through the desired temperature range. The circles represent the onset of the emergence of the doublet (I to N + I phase) while the crosses represent the temperature at which the singlet in the center disappeared upon further cooling (N + I to N phase).
The dependence of the observed 2H splitting (νQD) on the concentration of the chromonic liquid crystals has been observed for other chromonic mesogens and was explained by a two-site model.15,32 In the chromonic liquid-crystalline phase, the 2H2O molecules were assumed to exist in two categories: (1) 2H2O associated with the mesogen aggregates (bound) or (2) 2H2O in bulk solution (free).15,32 Free 2H2O molecules in the bulk were considered to have zero deuterium quadrupole splitting. The observed splitting (νQD) was dependent on the fraction (Fb) of aggregatebound 2H2O molecules that exhibit a deuterium quadrupole splitting of ∆b:
νQD ) Fb∆b
(1)
Assuming a fixed number of bound 2H2O molecules per mesogen molecule in the liquid-crystalline phase, the observed splitting changes linearly with the mole fraction (thus, the concentration) of the mesogen (eq 2)
νQD ) nb(Xd/Xw)∆b
(2)
where nb is the number of 2H2O molecules “bound” to the aggregates per mesogen molecule and Xd and Xw are the mole fractions of mesogen and 2H2O, respectively.15 Assuming that the deuterium quadrupole splitting of ∆b is constant at a fixed temperature, eq 2 was reduced to
νQD ) k(Xd/Xw)
(3)
where k is a constant.15 According to this model, the small change in the observed splitting (νQD) with temperature observed for the liquid-crystalline samples of 1 in 2H2O may suggest that the value of k, which is the product of nb (the number of 2H2O molecules bound to 1 in the chromonic N phase) and ∆b, does not vary significantly with temperature. It has been observed for other liquid-crystalline compounds that the value of k changes with temperature. In some
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Figure 6. Plot of the deuterium quadrupole splitting of D2O as a function of the mole fraction of 1 to D2O at 25 °C.
Figure 8. Effects of concentration and temperature on the order parameter (S0) of the C-2H bonds of 2. The order parameters are calculated from 2H splitting data in Figure 8 and a value of 183 kHz as the static quadrupole splitting constant: (+) 38; (b) 24; (4) 16; (O) 13; and (×) 9.9 wt %.
Figure 7. Temperature dependence of deuterium quadrupole splitting (νQD) for various solutions of 2 in 1H2O: (+) 38; (b) 24; (4) 16; (O) 13; and (×) 9.9 wt %.
examples, the value of νQD increases with temperature initially and then decreases upon further increase in temperature, resulting in a maximum in the νQDtemperature plot.33,34 The effect of the structure of the mesogen on the temperature dependence of k is not understood. As shown by Tiddy and co-workers,15 at a constant temperature the concentration at which the transition of the N + I biphasic region into the I phase occurs can be determined by studying the observed splitting (νQD) of 2H in the 2H2O solution of the mesogen as a function of the mole ratio of the mesogen molecules to 2H2O (Xd/Xw). The plot of the changes in the observed splitting (νQD) of 2H in the 2H2O solution of 1 as a function of the mole ratio of 1 to 2H2O is shown in Figure 6. The mole ratio of 1 to 2H2O at zero observed splitting was found to be 0.0018 ((0.0001): 1, corresponding to a concentration of 6.3 ( 0.4 wt % 1 in 2 H2O. 2 H NMR Spectroscopy of Compound 2 in 1H2O. To further characterize the chromonic liquid-crystalline properties of 1 in aqueous solution, we performed 2H NMR spectroscopy studies of compound 2 in 1H2O. Figure 7 shows the concentration and temperature dependence of the observed 2H splitting for various solutions of compound 2 in 1H2O. At a constant temperature, the splitting observed for deuterium on the aromatic rings of 2 increased as the concentration of 2 in the liquid-crystalline samples increased. However, unlike the splitting observed for the deuterium of the solvent molecules, the splitting for the (33) Kustanovich, I.; Poupko, R.; Zimmermann, H.; Luz, X.; Labes, M. M. J. Am. Chem. Soc. 1985, 107, 3494. (34) Iverson, I. K. Ph.D. Dissertation, Department of Chemistry, University of Nevada, Reno, NV, 2002; p 60.
deuterium on the aromatic rings of 2 at low concentrations was very sensitive to the temperature change. The observed splitting decreased with increasing temperature, and eventually no splitting was observed when the solutions were heated past their clearing point, converting into the isotropic phase. The values of 2H splitting of 2 in 1H2O in Figure 7 were used to calculate the order parameter of the liquidcrystalline system at different concentrations and temperatures. The order parameter is a normalized parameter that indicates the degree of order of a system. An order parameter of 0 indicates disorder; the absolute value in the ordered state is 1. The order parameter (S0) of the particular C-2H bond vector can be related to the quadrupole splitting (νQD) for that deuteron by the equation35,36
νQD ) (3/8)k′S0
(4)
where k′ is the static quadrupole splitting constant of the C-2H bond. Literature values of splitting constants, k′, for Ar-2H bonds vary from 193 kHz (quoted by Seelig for benzene)37 to 183 kHz (used by Goldfarb et al. for deuterated disodium cromoglycate in a chromonic liquidcrystalline phase in water).35 Assuming a value of 183 kHz for the static quadrupole splitting constant of the C-2H bond, the order parameter (S0) values for 2 at different concentrations and temperatures were calculated (Figure 8). At 25 °C, the values of S0 increased from 0.82 for a 9.9 wt % solution to 0.97 for a 38 wt % solution. At low concentrations (e.g., at 9.9 wt % 2), the values of S0 dropped rapidly with increasing temperature. The order parameter decreased more slowly with temperature at higher concentrations of 2. Because the transition temperature of compound 2 is expected to be similar to that of compound 1, a 38 wt % solution of 2 at room temperature should be in the two-phase N + M state. The high value of the order parameter of the 38 wt % sample is presumably resulted from the ordered arrangement of the columns in the M phase regions in addition to the ordered stacking of the mesogens within the columns in both the N phase and the M phase regions. (35) Goldfarb, D.; Luz, Z.; Spielberg, N.; Zimmermann, H. Mol. Cryst. Liq. Cryst. 1985, 126, 225. (36) Rowell, J. C.; Phillips, W. D.; Melby, L. R.; Panar, M. J. Chem. Phys. 1965, 43, 3442. (37) Seelig, J. Q. Rev. Biophys. 1977, 10, 353.
Chromonic Liquid-Crystalline Phases
Conclusions Studies by polarized optical microscopy and NMR spectroscopy showed that the chromonic N phase of 1 existed over a broad range of concentrations and temperatures, while the biphasic N + I region existed over a very narrow composition range only. At room temperature, the emergence of a more ordered chromonic M phase occurred only at high concentrations (∼37 wt %) of 1. The N + M biphasic region persisted at concentrations as high as 41 wt %. The 2H NMR studies of 2 in 1H2O showed that the mesogens at high concentrations (in the N phase or the N + M two-phase region) were highly ordered and the structural order of the columns is stable to heating. The liquid-crystalline properties of 1 (that include the broad composition range of the N phase, high value of the order
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parameter at high concentrations, and high stability of the structural order) offer important advantages in the technological exploitation of this particular chromonic material. In fact, these properties allowed the shearinduced orientation of 1 in the chromonic N phase followed by evaporation of solvent to yield anisotropically oriented solid films.23,25 The oriented films of 1 served as excellent linear polarizers of light with a dichroic ratio as high as 30. Acknowledgment. S.-W.T.-C. is grateful for the CAREER Award granted by NSF (DMR-9876027). We thank Mr. Lew Cary for his assistance in the NMR studies and the helpful suggestions of the reviewers. LA030256T
