Designing Amphotropic Smectic Liquid Crystals Based on

J. Phys. Chem. B 2009, 113, 4209–4217

4209

Designing Amphotropic Smectic Liquid Crystals Based on Phosphonium Salts for Partial Ordering of Solutes as Monitored by NMR Spectroscopy Astghik A. Shahkhatuni,† Kefeng Ma, and Richard G. Weiss* Department of Chemistry, Georgetown UniVersity, Washington, D.C. 20057-1227 ReceiVed: December 8, 2008; ReVised Manuscript ReceiVed: January 17, 2009

The ordering parameters of selected solutes from NMR spectroscopic measurements have been assessed in the thermotropic and amphotropic smectic liquid-crystalline phases of a wide structural range of phosphonium salts with three equivalent long n-alkyl chains, one shorter chain, and various anions. The nature of the added liquids that convert the salts to amphotropic phasessalcohols and other small organic moleculessand their concentrations have been determined. These factors are correlated with the NMR-derived parameters in order to understand how the phases can be optimized to maximize information about the solutes. The various salts cover a range of liquid crystallinity from -40 to 100 °C. The phosphonium salts are easily aligned in the strong magnetic fields of the spectrometers. In several of the systems, a coexistence of isotropic and anisotropic phases is observed over a wide range of temperatures. The order parameters of the amphotropic liquidcrystalline phases vary from high to very low values, and some of the systems provide good spectral resolution for the solute molecules. Also, structural and orientational parameters of a model molecule, 13C-enriched acetonitrile, have been calculated in various systems in order to evaluate more precisely the applicability of the host systems for determining solute structures by NMR. The results, in toto, indicate that several of the phosphonium salts are very promising as hosts to determine solute structures. Introduction Nonaqueous, nonpolymeric amphotropic ionic liquid crystals (ILCs) based on phosphonium salts containing three long, equivalent n-alkyl chains and a fourth, shorter chain,1-3 have been shown to form smectic A2 phases4 that are surprisingly easy to orient in strong magnetic fields by cooling directly from their isotropic phases.5 As such, they are interesting as potential media for studies utilizing NMR spectroscopy.6-12 Among these studies are explorations of how smectic phases form and orient and whether their weak alignment will allow 3D structure determinations of organic solutes. In addition, they may be useful to mediate the stereo- and regiochemistries of reactive solutes.13 We report here the amphotropic14 nature of the phosphonium salts with the general structure 1PnA (Figure 1) induced by alcohols and acetonitrile. Previously, we reported that water can induce liquid-crystallinity in some 1PnA salts that are not mesomorphic when neat.3,11,12 At least one equivalent of hydroxyl groups (or water) of the solutes must be present the solutes to convert fully a 1PnA salt into its amphotropic and/ or lyotropic liquid-crystalline phase.11,12,15 In our studies thus far, n and A have been 10, 14, or 18 and one of a wide range of anions (usually a simple one such as halide, NO3-, C1O4-, etc.), respectively. As mentioned, these phosphonium salts have low order parameters and their amphotropic smectic phases exist at or near room temperature over fairly wide temperature ranges for the most part.8,9,11,12 In addition, the salts are easily synthesized from the corresponding tri-n-alkylphosphines with a variety of n lengths and A groups and are very stable in air to at least 200 °C.2,3,16 * E-mail: [email protected]. † Permanent address: Molecule Structure Research Center, Yerevan, 0014, Armenia.

Figure 1. Molecular structures of 1PnA and 2PnOHA salts.

Salts with one or two long n-alkyl chains have been studied intensely because they are surfactants and the latter are models for phospholipids. They and their combinations can form various lyotropic micellar systems when dissolved in a solvent, especially water. Although phosphonium salts with four equivalent long n-alkyl chains, do not form liquid-crystalline phases,17 many of those with three long chains and one short one do.1-3 The manner in which the packing of the phosphonium salts with one, two, three, and four long chains pack in their solid and liquid-crystalline phases is instructive in learning how to design mesophases with specific bulk properties that might be useful for designated tasks. Recently, the order parameters of mixtures of 1P10Cl or 1P10Br and various concentrations of water or different alcohols were calculated and correlated with the phase structures.11,12 The order parameters were found to be near 10-2, much lower than the values commonly encountered for nonionic, thermotropic liquid-crystalline phases. With the development of weak alignment techniques for three-dimensional structure determination of molecules by NMR spectroscopy,18-21 it has became increasingly important to find weakly aligning media for molecules soluble in organic solvents. Most of the applications to date have concentrated on the structures of biomolecules, and most of the weakly aligning systems currently in use are aqueous. Recently, the successful application of the weak alignment approach was demonstrated for a number of organic molecules.22-24 Although several polymer and gel systems have been developed as media for weak alignment of organic molecules,25-28 the

10.1021/jp810793n CCC: $40.75  2009 American Chemical Society Published on Web 03/03/2009

4210 J. Phys. Chem. B, Vol. 113, No. 13, 2009 search for weakly aligning systems with better properties and wider applicability is ongoing; none of which we are aware employs salts like those investigated here. The principal objectives of this study are to explore the potential of amphotropic 1PnA LCs as aligning media for 3D structure determinations of organic molecules by NMR spectroscopy and to assess the molecular design criteria for maximizing the effectiveness of the 1PnA for this purpose. Thus, the range of various phosphonium salts has been expanded by using anions such as I-, NO3-, BF4-, PF6-, and (CF3SO2)2N- (hereafter Tf2N-) with alkyl chain lengths, n ) 10, 14, and 18 (Figure 1). Previously, we found that the neat 2PnOHBr and 2PnOHCl salts form smectic liquid-crystalline phases and argued that their hydroxymethylene groups serve a similar purpose to that of methanol in converting the nonmesomorphic, neat 1PnBr and 1PnCl salts into amphotropic liquid crystals.11 The phase diagrams, transition temperatures, and order parameters of the thermotropic liquid-crystalline phases of the neat compounds, as well as of the amphotropic phases with added methanol, dimethyl sulfoxide, acetonitrile, chloroform, 1-butanol, 1-hexanol, and 1-decanol have been studied by NMR spectroscopy and optical microscopy. From these experiments, it has been possible to discern some general relationships between the phase behavior of these systems with respect to temperature, their long n-alkyl chain length, anion structure, and solute type and concentration. Systematically, the acquired data have been used to develop systems with lower degrees of orientation, a necessary criterion for weak alignment of solutes.29,30 Experimental Section Phosphonium salts and several deuterated salts, [H(CH2)n]3P+CD3NO3-, where n ) 10, 14, and 18, and [H(CH2)14]3P+CD3I- were synthesized by literature procedures or were available from previous studies.8,9,11,16 Deuterated methanol-d4 (99.8 atom % D), acetonitrile-d3 (99.8 atom % D), chloroform-d (99.8 atom % D), dimethyl sulfoxide-d6 (99.9 atom % D), and 2-13C enriched acetonitrile (99 atom % 13C) were obtained from Cambridge Isotope Laboratories, Inc. N-(4Ethoxybenzylidene)-4′-n-butylaniline (EBBA, 98%) and cetyltrimethylammonium bromide (CTAB, g98%) were purchased from Sigma-Aldrich Chemical Co. and were used as received. Phase transition temperatures were measured using a Leitz 585 SM-LUX-POL optical microscope with crossed polarizers, a Leitz 350 heating stage, and an Omega HH503 microprocessor thermometer connected to a J-K-T thermocouple. 2H NMR spectra were recorded on a Varian Unity Inova 500 spectrometer with a switchable probe. Depending on sample composition and resolution, 64-512 free induction decays (FIDs) were accumulated. Procedures for sample preparation for NMR and optical microscopy, as well as detailed heating and cooling protocols, were reported previously.11 Optical micrographs, mesophase temperature ranges, and mesophase types for various mixtures of 1PnA, and quadrupolar splittings of solutes and order parameters obtained by NMR spectroscopy are collected in Tables S1 and S2 of the Supporting Information. Results and Discussion General Considerations for NMR Spectra. We have examined five variables that affect the phase properties and order parameters of our phosphonium salts: temperature, type of solute, solute concentration, long n-alkyl chain length, and type of anion. Unfortunately, changing one of these (such as solute

Shahkhatuni et al.

Figure 2. Stack plot of 2H NMR spectra with quadrupolar splittings of methyl group deuterons of methanol in 1P14BF4 · CD3OD. The isotropic (I), the coexistence of the isotropic and liquid-crystal phases (I + LC), liquid-crystal phase (LC), and crystal phase (K) of the salt with some phase-separated liquid methanol (L) and remaining LC phase were observed while cooling the sample in 5 °C increments.

type or concentration) can affect others (such as the onset and range of temperatures of the mesophases), making direct comparisons difficult. Despite this, it has been possible to correlate important alterations in the phase properties caused by one of the variables while maintaining the others almost constant. NMR spectroscopy and, in particular, quadrupolar interactions, of oriented molecules are the main tools employed in this research to discern the phase changes. They are supplemented with observations from polarizing optical microscopy that provides more precise data about bulk phase transitions (because observations can be made continuously over a temperature range), but do not help to define the ordering of the phases or solute molecules within them. For example, if deuterated solute molecules in an amphotropic liquid crystal are oriented, additional splitting(s) characterizing dipolar and quadrupolar interactions appear in the 2H NMR spectra of solute molecules, instead of a singlet observed in the corresponding isotropic spectrum. The coexistence of isotropic and anisotropic phases also can be monitored by 2H NMR spectra; commonly, the coexistence of singlet and a quadrupolar doublet in a spectrum indicates that the molecules in which the deuterium nuclei exist are in two separate environments (possibly an isotropic and an anisotropic one) and that they interchange slowly between the two. A typical stack plot of NMR spectra of 1P14BF4 · CD3OD (i.e., methyl-tri-n-tetradecylphosphonium tetrafluoroborate and 1 equiv of perdeuterated methanol) recorded at 5 °C intervals is shown in Figure 2. The spectra span temperatures where the sample is isotropic, liquid crystal, and biphasic (phase-separated solid 1P14BF4 and some liquid CD3OD). When the resolution of the 2H NMR spectra is not very high, doublet(s) characterizing quadrupolar interactions of deuterons in groups in the solute or salt are observed. For example, the 2 H-spectrum of oriented methanol-d4 consists of two sets of doublets from quadrupolar splittings in the OD and CD3 environments. However, each of the signals is a complicated multiplet, with additional splittings, arising from dipolar interactions of deuterons of the solute.31 When resolution is relatively high (as in the LC phase of 1P14BF4 · CD3OD (Figure 2)), additional splittings are observed within each peak of the doublet. The quadrupolar splittings were estimated from the difference between chemical shifts of the centers of multiplets.

Smectic Liquid Crystals Based on Phosphonium Salts

J. Phys. Chem. B, Vol. 113, No. 13, 2009 4211

TABLE 1: Dependence of Quadrupolar Splitting and Order Parameters of Methyl Group Deuterons on Added Perdeuterated Methanol and Acetonitrile for 1P14NO3 and 1P18NO3 (1 equiv of Solute) and 2P14OHBr (0.2 equiv of Solute) methanol

acetonitrile

salt

T (°C)

∆ (KHz)

SCD

∆ (KHz)

SCD

1P14NO3 1P18NO3 2P14OHBr

25 60 50

4.3 3.2 4.8

0.019 0.014 0.021

1.0 1.2 1.5

0.004 0.005 0.007

Usually the OD signals are very broad so that it is not possible to use them in the precise calculation of order parameters. In some samples, when two sets of signals from CD3 exist with different intensities, methanol molecules are probably in two different exchangeable orientations with respect to the phosphonium center and anion.11,12 Quadrupolar splittings (∆) from the 2H NMR spectra of deuterated solute molecules and, in some cases, of selectively deuterated, host phosphonium salts were used with eq 1 to analyze the degree of orientation, SXD, along an X-D bond (where X ) C or O) with respect to the external magnetic field of the spectrometer.

∆)

( )

3 e2qQ SXD 2 h

(1)

In this equation, (e2qQ/h) is the deuteron quadrupolar coupling constant along the X-D bond, eQ is the electric quadrupole moment of a deuteron, and eq is the electric field gradient at the deuteron.32 The quadrupolar coupling constants for methyl groups were taken to be 155 (methanol),33 148.3 (acetonitrile), and 165 (DMSO), and 170 KHz for the R-methylene groups of 1-butanol and longer 1-alkanols.34,35

1 SXD ) (3 cos2 β - 1)SC3 2

(2)

Then, from eq 2, order parameters of a methyl group along its 3-fold symmetry axis, SC3, were calculated from SXD and taking β, the angle between a C-D bond and the C3-axis of methyl, to be 70°.36 For purposes of utilizing these liquid-crystalline phases to determine the conformations of guest molecules, the lowest (nonzero) order parameters, preferably ∼10-3, are sought.22,23 In such media, the residual dipolar couplings from protons or 13 C atoms are smaller than the chemical shift differences and easily interpretable spectra are obtained. Temperature Dependence. Samples were studied over wide temperature ranges in order to determine the lower onset and clearing temperatures of the liquid-crystalline phases. Some samples formed no liquid-crystalline phase and underwent transitions directly between their isotropic and crystalline phases. Phase characterization and transition temperature data from optical microscopy are summarized in Table S1 of the Supporting Information, and the NMR data from liquid-crystalline samples are collected in Table S2 of the Supporting Information. The transition temperatures, as determined by solute orientations from 2H NMR experiments, are normally lower than those measured by optical microscopy (which are based on changes in macroscopic textures resulting from the packing arrangements of the salts). The disparities are especially pronounced for the

transitions to isotropic phases. The reason for the discrepancy is that disturbances of the orientations of the solute molecules can be detected before changes in the lamellar packing of the salts are observed visually. Usually, the degree of orientation (and the order parameter) decreased with increasing temperature. However, in some samples, the opposite and unexpected behavior was found. Examples of the dependencies of quadrupolar splittings on temperature for some solutes and salts are shown in Figure 3. Although the quadrupolar splittings of methanol in 1P14NO3 and the methyl groups of the 1P10A salts decrease with increasing temperature, an “abnormal” dependence was found with acetonitrile. It will be discussed later. Solute Dependence. The structure and concentration of a solute is known to have a dramatic influence on the formation and properties of the amphotropic liquid-crystalline phases of the 1PnA salts.8,9 As mentioned earlier, at least 1 equiv of methanol or other alcohol is needed to convert a nonmesomorphic 1PnA salt completely into a mesophase.11,12 For this reason, all studies here were conducted on salts that contained either no or at least 1 equiv of added solute. Our choice of added solutes was based on several considerations, including the data in hand concerning the loss or creation of liquid-crystalline phases by solutes, the abilities of solutes to act as solvents for a wide range of other organic compounds, and the perceived requisites of possible future applications. Upon the basis of its demonstrated ability to induce mesomorphism in 1PnA salts,11,12 methanol was selected as a principal solute for our studies (Table S2 of the Supporting Information). Note in Figure 3B the large decreases in the quadrupolar splittings of the deuterated methyl group of [H(CH2)14]3P+CD3I- as well as in the onset and clearing temperatures of the liquid-crystalline phase when 1 equiv of methanol is added. In some cases, such as with 1P10NO3 · DMSO and 1P14Tf2N · DMSO, addition of a solute did not induce liquid crystallinity and only solid-isotropic phase transitions were detected by optical microscopy and NMR. However, the coexistence of isotropic and liquid-crystalline phases was detected at some temperatures for several of the 1PnA salts to which a solute had been added. Regardless of the heating, cooling or mixing protocols employed, none of these samples formed a complete liquid-crystalline phase (as indicated by the presence of a central singlet peak between the ordered doublet in each of the 2H NMR spectra) before phase separating to a solid and a liquid upon cooling or before reaching a totally isotropic state on heating (Figure 4). Even when only 1/2 equiv of DMSO was added to 1P10NO3, only biphasic behavior was found. However, 1P14NO3 · CD3CN seems to be a promising medium for structural determinations because it exhibits a liquidcrystalline phase in a convenient temperature range, 25-40 °C, and the 2H NMR spectra have small quadrupolar splittings with narrow lines. The data included in Table 1 provide quantitative comparisons of salts with added acetonitrile and with methanol. The quadrupolar splittings (and, therefore, order parameters) with the former solute are lower. As mentioned above, these comparisons are not as simple as they appear because the magnitudes of the quadrupolar splittings of the two solutes exhibit different dependencies on temperature: the splittings of methanol decrease with increasing temperature and those of acetonitrile increase with increasing temperature (Figure 3A). Because the same, unexpected dependence of the quadrupolar splitting on temperature is observed for 1P14BF4 · CD3CN, 1P14NO3 · CD3CN, 1P18NO3 · CD3CN,

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Figure 3. Temperature dependence of quadrupolar splittings of CD3 deuterons of (A) methanol (b) and acetonitrile (9) in 1P14NO3 · CD3OD and 1P14NO3 · CD3CN, respectively, and of (B) neat salts ([H(CH2)14]3P+CD3I- (b) and [H(CH2)14]3P+CD3NO3- (O)) and solute-salt mixtures ({[H(CH2)14]3P+CD3I-} · CD3OD (9; data for the methyl on phosphorus shown) and {[H(CH2)14]3P+CD3NO3-} · CH3(CH2)3OH (∆)).

Figure 4. Mesophase temperature ranges for 1PnA with various solutes. The gray regions of the bars represent liquid-crystalline temperature ranges and the cross-hatched regions mark the biphasic temperature regimes. Solids (or phase separated solids and isotropic liquids) are below the bars and isotropic regions are above them. The unfilled bar marks the liquid-to-crystal phase transition temperature range.

1P14I · CD3CN, and 2P14OHBr with various concentrations of CD3CN (vide infra), it is clear that the behavior of acetonitrile, itself, is responsible rather than a specific salt. We conjecture that acetonitrile molecules exchange rapidly between two different orientations (with respect to the direction of the external magnetic field). The dependence of the temperature ranges of the induced liquid-crystalline phases of 1P10Cl and 1P10Br (that are not mesomorphic in the absence of a solute) on the alkyl chain lengths of a series has been reported recently.12 In general, the longer the alcohol chain, the narrower the liquid-crystalline range. Here, we study the influence of alkyl chain length of 1-alkanols on 1PnNO3 (n ) 10, 14, and 18) salts which are mesomorphic in the absence of solute (Figure S2 of the Supporting Information). Phase transition temperature data from optical microscopic studies showed no linear correlation between alcohol chain length and changes in onset temperatures or temperature ranges of the liquid-crystalline phase (Table S1 of the Supporting Information). In most of the samples examined, longer-chain alcohols narrowed the liquid-crystal range and decreased the clearing temperatures. However, there are excep-

tions, such as 1P14NO3 in the presence of 1-butanol (Figure 4). The 2H NMR spectra of 70/30 1P14NO3/[H(CH2)14]3P+CD3NO3- demonstrate that addition of 1 equiv (9.52 wt %) of 1-butanol does reduce the orientation of the phosphonium salt itself (Figure 3B). Of the organic solutes examined in this study, methanol seems most able to induce or widen the range of liquid crystallinity in the 1PnA salts. However, the existence of two orientations for its methyl groups within the liquid crystal11,12 and the relatively high degree of ordering of the liquid-crystalline phases compromise some of its potential applications (N.B., molecular structure determinations using residual dipolar couplings). However, polar, nonhydroxylic solutes such as acetonitrile seem better able to perturb the ordering of the salt matrices. We conjecture that they are located, on average, farther from the ionic headgroup regions of the smectic bilayers than hydroxylic solutes such as methanol. Methanol is known to interact strongly with the positively charged phosphorus atom and the anion within the headgroup regions, and it is assumed to be localized there in two specific orientations.8,9 Similar interactions appear to occur between the head groups and higher alkanol homologues, at least up to 1-butanol. The hydrophobic tails of still longer alcohols result in a stabilization of the lipophilic chain ordering and near or complete loss of liquid crystallinity. Alkyl Chain Length Dependence. The phase properties of some amphotropic liquid crystal phases of the 1PnA with the same anion (Cl-, Br-, NO3-, BF4-) were studied as a function of chain length n. Clearing temperatures increased both for the neat compounds and their mixtures with solutes as the alkyl chain length increased (Figure 4 and Figure S3 of the Supporting Information). Systems with n ) 18 usually had the highest onset temperatures and the narrowest ranges for their liquid-crystalline phases; those with n ) 10 usually had the lowest onset temperatures and the broadest ranges for their liquid-crystalline phases. Because liquid-crystalline temperature regimes of systems with different alkyl chain lengths often did not overlap, it is difficult to compare the influence of different n on the degree of ordersordering depends on temperature as well. In an attempt to normalize the data, they have been compared in Figure 5 at their reduced temperatures, Tred (eq 3), where Tc is the clearing temperature of a liquid-crystalline phase. The trend for methanol as solute is clearsincreasing the length of the long chains decreases the quadrupolar splitting (and, therefore, the ordering);

Smectic Liquid Crystals Based on Phosphonium Salts

J. Phys. Chem. B, Vol. 113, No. 13, 2009 4213

Figure 5. Dependence of quadrupolar splittings of methyl group deuterons of solutes in 1PnA at reduced temperatures for CD3OD and A ) NO3- (2, b, 9) or Br- (∆,O) and for CD3CN and A ) NO3- ([, ]) for n ) 10 (2, ∆), 14 (b, O, [), and 18 (9, ]).

the trend for acetonitrile is not, and its anomalous behavior has been mentioned above.

Tred ) T/Tc

(3)

Anion Dependence. The structures and electronic properties of the anions have a large influence on the ability of 1PnA salts to form liquid-crystalline phases. Figure 6A presents the mesophase transtition temperatures for 1P14A · CD3OD with NO3- (1.79 Å), Cl- (1.81 Å), Br- (1.96 Å), I- (2.20 Å), and BF4- (2.2 Å); reported crystal radii of the anions37,38 are in parentheses. Those 1P14A salts whose anions have larger crystal radii, such as PF6- (2.95 Å)39 and Tf2N- (3.46 Å),40 do not form liquid-crystalline phases, even upon addition of a solute. The dependence of quadrupolar splittings of methyl group deuterons on the anion at the one temperature is shown in Figure 6B; their dependence on temperature for the same systems is shown in Figure S4 of the Supporting Information. As can be seen, the anionic crystal radii do not correlate clearly with the liquid-crystalline ranges, onset temperatures, clearing temperatures, or quadrupolar splittings. These data are suggestive of a trend in which there is an optimal anion size and the order parameters may decrease as the halides become larger, but there are too few examples to make definitive statements at this time. However, for the systems with n ) 10, the dependence of temperature range and clearing temperature on the anions is different (Figure S5 of the Supporting Information). Systems with NO3- and BF4- have smaller order parameters compared to those of salts with the other anions investigated. Systems with Tf2N- and PF6- do not form LC phases, probably due to their large crystal radii. Among the three halides, the systems with Cl- exhibit the narrowest LC phase ranges and highest degrees of orientation; they are, therefore, the least interesting as hosts for solute structure determinations by NMR (vide infra). Solute Concentration Dependence. To determine the influence of a solute on salts that are liquid-crystalline when neat, we have added various amounts of methanol to the 1PnNO3 homologues.2 The n ) 10, 14, and 18 homologues of 1PnNO3 are liquid crystalline over wide temperature ranges and orient in a magnetic field (Figure S2 of the Supporting Information). Addition of 0.25 or 0.6 equiv of methanol-d4 to 1P10NO3 resulted in 2H NMR spectra throughout the mesophase range (as determined from optical microscopy) with “powder patterns”, indicating the absence of a homogenously oriented liquid-

crystalline phase. Additional 2H NMR experiments with [H(CH2)10]3P+CD3NO3- and 0.25 equiv of CH3OH confirmed that the 1P10NO3 molecules are randomly oriented as well at