Induced Amphotropic and Thermotropic Ionic Liquid Crystallinity in

Feb 16, 2008 - Department of Chemistry, Georgetown University, Washington, D.C. 20057-1227, and Center of Biomedical Magnetic Resonance, Sanjay Gandhi...
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Induced Amphotropic and Thermotropic Ionic Liquid Crystallinity in Phosphonium Halides: “Lubrication” by Hydroxyl Groups Kefeng Ma,† B. S. Somashekhar,‡ G. A. Nagana Gowda,‡,§ C. L. Khetrapal,‡ and Richard G. Weiss*,† Department of Chemistry, Georgetown UniVersity, Washington, D.C. 20057-1227, and Center of Biomedical Magnetic Resonance, Sanjay Gandhi Post Graduate Institute of Medical Sciences, Lucknow 226 014, India ReceiVed October 12, 2007. In Final Form: December 11, 2007 The influence of covalently attaching hydroxymethylene to the methyl groups of methyl-tri-n-alkylphosphonium halides (where the alkyl chains are decyl, tetradecyl, or octadecyl and the halide is chloride or bromide) or adding methanol as a solute to the salts on their solid, liquid-crystalline (smectic A2), and isotropic phases has been investigated using a variety of experimental techniques. These structural and compositional changes are found to induce liquid crystallinity in some cases and to enhance the temperature range and lower the onset temperature of the liquidcrystalline phases in some others. The results are interpreted in terms of the lengths of the three n-alkyl chains attached to the phosphorus cation, the nature of the halide anion, the influence of H-bonding interactions at the head group regions of the layered phases, and other solvent-solute interactions. The fact that at least 1 molar equiv of methanol must be added to effect complete (isothermal) conversion of a solid methyl-tri-n-alkylphosphonium salt to a liquid crystal demonstrates a direct and strong association between individual methanol molecules and the phosphonium salts. Possible applications of such systems are suggested.

Introduction The negligible vapor pressure, low flammability, high electrochemical and thermal stabilities, and high ionic and thermal conductivities have made ionic liquids (ILs) a very popular class of solvents for a wide range of catalytic and synthetic reactions,1 biotechnological,2 electrochemical,3 and solar cell applications,4 and many other innovative uses in engineering.5 The importance of “tuning” the structural properties of ionic liquids to optimize their effectiveness for one of these applications has been explored as well.6 Liquid crystals (LCs) are another class of materials that * Corresponding author. E-mail: [email protected]. † Georgetown University. ‡ Sanjay Gandhi Post Graduate Institute of Medical Sciences. § Current address: Department of Chemistry, Purdue University, West Lafayette, IN 47907, U.S.A. (1) (a) Earle, M. J.; Seddon, K. R. Pure Appl. Chem. 2000, 72, 1391-1398. (b) Ionic Liquids: Industrial Applications for Green Chemistry; Rogers, R. D., Seddon, K. R., Eds.; ACS Symposium Series 818; American Chemical Society: Washington, DC, 2002. (c) Rogers, R. D.; Seddon, K. R.; Volkov, S. Green Industrial Applications of Ionic Liquids; Kluwer Academic Publishers: Norwell, MA, 2003. (d) Wasserscheid, P.; Welton, T. Ionic Liquids in Synthesis; WileyVCH: Weinheim, 2003. (e) Ionic Liquids as Green SolVents: Progress and Prospects; Rogers, R. D., Seddon, K. R., Eds.; ACS Symposium Series 856; American Chemical Society: Washington, DC, 2003. (f) Ionic Liquids III: fundamentals, progress, challenges, and opportunities; Rogers, R. D., Seddon, K. R., Eds.; ACS Symposium Series 901-902; American Chemical Society: Washington, DC, 2005. (g) Weinga¨rtner, H. Angew. Chem., Int. Ed. 2007, 46, 2-19. (2) (a) Kragel, U.; Eckstein, M.; Kaftzik, N. Curr. Opin. Biotechnol. 2002, 13, 565-571. (b) Park, S.; Kazlauskas, R. J. Curr. Opin. Biotechnol. 2003, 14, 432437. (c) Hudson, E. P.; Eppler, R. K.; Clark, D. S. Curr. Opin. Biotechnol. 2005, 16, 637-643. (3) (a) Ohno, H. Electrochemical Aspects of Ionic Liquids; Wiley-Interscience: New York, 2005. (b) Katase, T.; Kurosaki, R.; Murase, K.; Hirato, T.; Awakura, Y. Electrochem. Solid-State Lett. 2006, 9, C69-72. (c) Appetecchi, G. B.; Scaccia, S.; Tizzani, C.; Alessandrini, F.; Passerini, S. J. Electrochem. Soc. 2006, 153, A1685-1691. (d) Chen, P.-Y. Huaxue 2006, 64, 235-259. (4) (a)Wang, P.; Zakeeruddin, S. M.; Comte, P.; Exnar, I.; Gratzel, M. J. Am. Chem. Soc. 2003, 125, 1166-1167. (b) Chen, J.; Officer, D. L.; Pringle, J. M.; Too, C. O.; Wallace, G. G. Electrochem. Solid-State Lett. 2005, 8, A528-530. (5) Zhao, H. Chem. Eng. Commun. 2006, 193, 1660-1677. (6) (a) Yoshizawa, M.; Xu, W.; Angell, C. A. J. Am. Chem. Soc. 2003, 125, 15411-15419. (b) Xu, W.; Cooper, E. I.; Angell, C. A. J. Phys. Chem. 2003, 107, 6170-6178. (c) Fraser, K. J.; Izgorodina, E. I.; Forsyth, M.; Scott, J. I.; MacFarlane, D. R. Chem. Commun. 2007, 3817-3819.

has been used for a myriad of applications.7-9 Ionic liquid crystals (ILCs) may be useful where materials that are either ILs or LCs are not effective.10 Previously, we reported the phase characteristics of a large number of phosphonium salts, [H(CH2)n]3P+(CH2)mH X(mPnX), where n ) 10, 14, 18, m ) 0-5, and X- can be any of a variety of anions such as a halide, NO3-, ClO4-, BF4-, PF6-, etc.11 Many of the mPnX exhibit enantiotropic, bilayer smectic A (SmA2) liquid-crystalline phases (Figure 1). The completely saturated nature of the cations and the simplicity of the anions make these salts very attractive ILC candidates for the applications mentioned above. Attempts to find phosphonium salts with very simple anions, such as Cl- and Br-, which have broad and lowtemperature mesophase ranges have not been very successful.11 However, several thermotropic, liquid-crystalline mPnX also show amphotropic behavior12 (i.e., their LC ranges are broadened and onset temperatures are lowered upon the addition of one or more equivalents of a solute such as water or an alcohol13), and others without an LC phase develop one when an appropriate alcohol is added.13 Preliminary observations from infrared absorption spectra of some mPnX with added alcohol led us to hypothesize that noncovalent interactions between the hydroxyl groups and the (7) (a) Weiss, R. G. Tetrahedron 1988, 44, 3413-3475. (b) Kansui, H.; Hiraoka, S.; Kunieda, T. J. Am. Chem. Soc. 1996, 118, 5346-5352. (c) Ichimura, K. Chem. ReV. 2000, 100, 1847-1873. (8) (a) Brennan, T.; Hughes, A. V.; Roser, S. J.; Mann, S.; Edler, K. Langmuir 2002, 18, 9838-9844. (b) Lin, H.-P.; Mou, C.-Y. Acc. Chem. Res. 2002, 35, 927-935. (9) (a) Li, L.-S.; Walda, J.; Manna, L.; Alivisatos, A. P. Nano Lett. 2002, 2, 557-560. (b) Antonietti, M.; Faul, C. F. J. AdV. Mater. 2003, 15, 673-683. (c) Wang, L.; Chen, X.; Zhan, J.; Chai, Y.; Yang, C.; Xu, L.; Zhang, W.; Jing, B. J. Phys. Chem. B 2005, 109, 3189-3194. (10) Binnemans, K. Chem. ReV. 2005, 105, 4148-4204. (11) (a) Abdallah, D. J.; Robertson, A.; Hsu, H.-F.; Weiss, R. G. J. Am. Chem. Soc. 2000, 122, 3053-3062. (b) Chen, H.; Kwait, D. C.; Go¨nen, Z. S.; Weslowski, B. T.; Abdallah, D. J.; Weiss, R. G. Chem. Mater. 2002, 14, 4063-4072. (12) Singh, S. Liquid Crystals: Fundamentals, World Scientific: Singapore, 2002; p 3. (13) Ma, K.; Nagana Gowda, G. A.; Khetrapal, C. L.; Tong, Y. Y.; Weiss, R. G. Unpublished work.

10.1021/la703175x CCC: $40.75 © 2008 American Chemical Society Published on Web 02/16/2008

Ionic Liquid Crystallinity in Phosphonium Halides

Figure 1. Representation of a 2-dimensional slice of the proposed packing arrangement for the mPnX in their layered solid and LC phases (R is an H(CH2)m- group where m , n).

charged centers in the head group regions of the phosphonium salt layers create a “molecular lubrication”, which induces mesomorphism and attenuates ion pairing.6,14 Here, we demonstrate that such interactions can be initiated, as well, by hydroxyl groups that are covalently linked to the m parts of the mPnX salts when X ) Cl- or Br- and we compare the properties of the ILC phases of 1PnX containing 1 molar equiv of methanol (1PnX‚H3OH)15 with those of the neat covalent analogues in which the methyl group has been changed to 2-hydroxyethyl, [H(CH2)n]3P+(CH2)2OH X- (2PnOHX). A comprehensive comparison of the mesomorphic properties of 1PnX, 1PnX‚ CH3OH, and 2PnOHX with X ) Br and Cl is presented. Experimental Section Instrumentation. 1H, 31P, and 13C (proton decoupled) NMR solution spectra in CDCl3 were recorded on a Varian 300 MHz spectrometer interfaced to a Sparc UNIX computer using Mercury software. Chemical shifts were referenced to an internal tetramethylsilane (TMS) (1H) standard, an external 85% H3PO4 standard (31P), or the 77.7 ppm peak of CDCl3 (13C). Samples for the 2H NMR experiments were placed in 5 mm tubes which were then flamesealed under dry conditions. 2H NMR spectra were recorded at various temperatures on a Bruker AVANCE 400 spectrometer operating at 61.4 MHz. A spectral width of 10 kHz, a relaxation delay of 1 s, and 32 K data points were used. Thirty-two free induction decays (FIDs) were accumulated.Elemental analyses were conducted on a Perkin-Elmer 2400 CHN Elemental Analyzer equipped with a PerkinElmer AD-6 autobalance. IR spectra were obtained on a PerkinElmer Spectrum One FTIR spectrometer interfaced to a PC; samples were applied neat to the surface of a Miracle ATR accessory bar. Phase transition temperatures (corrected) were measured and optical micrographs (POMs) were recorded on a Leitz 585 SMLUX-POL microscope with crossed polarizers, a Leitz 350 heating stage, a Photometrics CCD camera interfaced to a computer, and an Omega HH503 microprocessor thermometer connected to a J-K-T thermocouple. In a drybox, a small amount of sample was placed in the center of a cover glass and another was placed gently on top; the edges around the cover slip sandwich were then wrapped with a narrow strip of Teflon film and “super glue” was brushed over the edges of the Teflon film to seal the samples and avoid methanol loss via evaporation or hydration on the heating stage of the microscope. In this way, transition temperatures could be reproduced in at least three consecutive heating and cooling cycles. Selected subambient temperatures were maintained by altering the flow of dry nitrogen gas (which had passed through a 2 m long copper coil that was immersed in liquid nitrogen) through the hollow block of the stage. To avoid moisture condensation on the cooling stage and optics, the (14) Abdallah, D. J.; Bachman, R. E.; Perlstein, J.; Weiss, R. G. J. Phys. Chem. B 1999, 103, 9269-9278. (15) The amounts will be specified along with the acronym in those cases where an amount of methanol not equal to 1 molar equiv has been added.

Langmuir, Vol. 24, No. 6, 2008 2747 microscope was enclosed in a plastic container through which a positive pressure of dry nitrogen gas was slowly passed. X-ray diffractometry (XRD) was conducted on a Rigaku RAPID/ XRD image plate system with KR X-rays of Cu (λ ) 1.54 Å) from a Rigaku generator. Temperature control, based on flowing nitrogen gas which passed over a heated filament and a thermocouple with a feedback loop, was estimated to be (0.5 °C. Samples were loaded into 0.5 mm glass capillaries (W. Mu¨ller, Scho¨nwalde, FRG) in a drybox, and then the capillaries were flame-sealed. Diffractions in degrees (θ) were converted to distances (d) using Bragg’s equation (λ ) 2d sin θ).16 Differential scanning calorimetry (DSC) was performed on a TA 2910 DSC cell base interfaced to a TA Thermal Analyst 3100 controller; samples were placed in open aluminum pans under a nitrogen atmosphere. Thermal gravimetric analyses (TGA) were performed under a nitrogen atmosphere at a 5 °C/min heating rate on a TA 2050 thermogravimetric analyzer interfaced to a computer. Materials. The physical and spectroscopic properties of the materials are reported in the Supporting Information file. Solvents (HPLC grade, Fisher Scientific), bromomethane (2.0 M in anhydrous tert-butyl methyl ether, Aldrich), methanol-d4 (99.8%, Aldrich), hydrochloric acid (ACS reagent, 37%, Aldrich), 2-bromoethanol (96%, Alfa Aesar), and potassium ethyl xanthate (98%, Lancaster) were used as received. Tri-n-octadecylphosphine, tritetradecylphosphine, and tridecylphosphine were gifts from Dr. Allan Robertson, Cytec Industries, Niagara Falls, Ontario, Canada. Over time, they formed some oxides, which could be separated easily from the phosphonium salt products via selective crystallization. Syntheses. 1PnBr and 1PnCl (n ) 10, 14, and 18) were synthesized from the phosphines according to reported procedures.11 The same synthetic procedures for each set of 2PnOHCl and 2PnOHBr salts were employed. Details for one of each are described in the Supporting Information file. Tridecyl-(2-hydroxyethyl)phosphonium Bromide (2P10OHBr) in 70% yield of a deformable, hydroscopic solid: TK-SmA2 ) 46.949.2 °C, TSmA2-I ) 66.5-68.1 °C; K ) crystal, I ) isotropic. 1H NMR (CDCl3, δ): 4.09 (dt, 2H, 2JPCH2CO ) 21.2 Hz; 3JCH2CH2O ) 5.6 Hz), 2.68 (m, 2H), 2.36 (m, 6H), 1.52 (m, 12H), 1.27 (m, 36H), 0.88 (t, 9H) ppm. 31P NMR (CDCl3, δ): 33.55 ppm. 13C NMR (CDCl3, δ): 55.55 (d, 2JPCCO ) 7.0 Hz), 32.48, 31.43 (d, 3JPCCC ) 14.6 Hz), 30.10, 29.96, 29.90, 29.61, 23.71 (d, 1JPCCO ) 48.8 Hz), 23.28, 22.47 (d, 2JPCCC ) 5.0 Hz), 20.60 (d, 1JPCCC ) 46.8 Hz), 14.73 ppm. IR: 3257 cm-1 (broad, O-H). Anal. Calcd for C32H68BrPO: C, 66.29; H, 11.82. Found: C, 66.52; H, 11.75. Tritetradecyl-(2-hydroxyethyl)phosphoniumBromide(2P14OHBr) in 67% yield of a white solid: TK-SmA2 ) 58.4-62.0 °C, TSmA2-I ) 97.6-99.0 °C. 1H NMR (CDCl3,δ ): 4.10 (dt, 2H, 2JPCH2CO ) 21.2 Hz; 3JCH2CH2O ) 5.6 Hz), 2.66 (m, 2H), 2.35 (m, 6H), 1.52 (m, 12H), 1.26 (m, 60H), 0.88 (t, 9H) ppm. 31P NMR (CDCl3, δ): 33.94 ppm. 13C NMR (CDCl3, δ): 55.45 (d, 2JPCCO ) 7.0 Hz), 32.53, 31.42 (d, 3JPCCC ) 14.6 Hz), 30.29 (broad singlet from overlapping peaks), 30.16, 29.98, 29.60, 23.78 (d, 1JPCCO ) 48.8 Hz), 23.30, 22.46 (d, 2JPCCC ) 5.0 Hz), 20.58 (d, 1JPCCC ) 46.8 Hz), 14.73 ppm. IR: 3246 cm-1 (broad, O-H). Anal. Calcd for C44H92BrPO: C, 70.64; H, 12.40. Found: C, 70.95; H, 12.21. Trioctadecyl-(2-hydroxyethyl)phosphoniumBromide(2P18OHBr) in 67% yield of a white solid: TK-I ) 77.1-78.8 °C. 1H NMR (CDCl3, δ): 4.10 (dt, 2H, 2JPCH2CO ) 21.1 Hz, 3JCH2CH2O ) 5.6 Hz), 2.65 (m, 2H), 2.34 (m, 6H), 1.53 (m, 12H), 1.26 (m, 84H), 0.88 (t, 9H) ppm. 31P NMR (CDCl3, δ): 33.98 ppm. 13C NMR (CDCl3, δ): 55.56 (d, 2JPCCO ) 7.0 Hz), 32.57, 31.45 (d, 3JPCCC ) 14.6 Hz), 30.32 (broad singlet from overlapping peaks), 30.20, 30.12, 29.64, 23.80 (d, 1JPCCO ) 48.5 Hz), 23.34, 22.39 (d, 2JPCCC ) 5.0 Hz), 20.61 (d, 1J -1 (broad, O-H). Anal. PCCC ) 46.3 Hz), 14.77 ppm. IR: 3246 cm Calcd for C56H116BrPO: C, 73.40; H, 12.76. Found: C, 73.64; H, 12.86. Tridecyl-(2-hydroxyethyl)phosphonium Chloride (2P10OHCl) in 61% yield of a deformable, hydroscopic solid: TK-SmA2 ) 43.5(16) Giacovazzo, C. Fundamentals of Crystallography, 2nd ed.; Oxford University Press: New York, 2002; Chapter 3.

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Figure 2. Polarized optical micrographs (POMs) of liquid-crystalline samples after applying lateral force on their cover slips: (A) 2P10OHCl at 58.2 °C, streak texture; (B) 1P10Cl‚CH3OH (6.0 wt %, 1.0 equiv, methanol) at -9.5 °C, streak texture. Table 1. Transition Temperature Ranges by Optical Microscopya transition temperatures (°C) 1PnX n

X

10 Cl 14 18 10 14 18

K-SmA2

SmA2-I

2PnOHX K-I

K-SmA2

SmA2-I

1PnX‚H3OH K-I

103.2-105.0 43.5-45.1 73.4-75.0

Cl 106.2-107.1 65.5-66.7 97.8-98.8 Cl 100.8-101.6 83.3-85.0 92.5-93.4 Br 98.8-100.7 46.9-49.2 66.5-68.1 Br 103.6-104.7 113.2-114.0 58.4-62.0 97.6-99.0 Br 97.5-98.9 106.2-107.2 77.1-78.8

SmA2-BiP

SmA2-K

(-16.5)(-26.5) 59.6-64.9

SmA2-I

K-I

27.5-34.2 75.7-81.5 79.3-84.6 (-15.0)-(-22.2) 37.4-41.4 49.7-54.9 76.5-82.5 81.5-85.4

All transitions were observed on heating except the SmA2 f BiP(K + I) and SmA2 f K transitions, which were determined by cooling samples from room temperature. Key: K ) solid; SmA2 ) smectic A2; I ) isotropic; BiP ) biphasic (K + I). a

45.1 °C, TSmA2-I ) 73.4-75.0 °C. 1H NMR (CDCl3, δ): 6.26 (H), 4.10 (dt, 2H, 2JPCH2CO ) 21.0 Hz, 3JCH2CH2O ) 5.6 Hz), 2.60 (m, 2H), 2.35 (m, 6H), 1.52 (m, 12H), 1.26 (m, 36H), 0.88 (t, 9H) ppm. 31P NMR (CDCl3, δ): 34.07 ppm. 13C NMR (CDCl3, δ): δ 55.44 (d, 2J 3 PCCO ) 7.1 Hz), 32.47, 31.43 (d, JPCCC ) 14.6 Hz), 30.10, 29.95, 29.89, 29.60, 23.75 (d, 1JPCCO ) 48.8 Hz), 23.28, 22.45 (d, 2JPCCC ) 4.5 Hz), 20.46 (d, 1JPCCC ) 46.9 Hz), 14.72 ppm. IR: 3151 cm-1 (broad, O-H). Anal. Calcd for C32H68ClPO: C, 71.80; H, 12.80. Found: C, 71.70; H, 12.50. Tritetradecyl-(2-hydroxyethyl)phosphoniumChloride(2P14OHCl) in 60% yield of a white solid: TK-SmA2 ) 65.5-66.7 °C, TSmA2-I ) 97.8-98.8 °C. 1H NMR (CDCl3, δ): 6.31 (H), 4.10 (dt, 2H, 2J 3 PCH2CO ) 21.0 Hz, JCH2CH2O ) 5.6 Hz), 2.56 (m, 2H), 2.34 (m, 6H), 1.50 (m, 12H), 1.26 (m, 60H), 0.88 (t, 9H) ppm. 31P NMR (CDCl3, δ): 34.13 ppm. 13C NMR (CDCl3, δ): 55.49 (d, 2JPCCO ) 7.1 Hz), 32.59, 31.46 (d, 3JPCCC ) 14.6 Hz), 31.33 (broad singlet from overlapping peaks), 30.19, 30.04, 30.00, 29.65, 23.81 (d, 1JPCCO ) 49.4 Hz), 23.36, 22.48 (d, 2JPCCC ) 4.5 Hz), 20.49 (d, 1JPCCC ) 47.3 Hz), 14.79 ppm. IR: 3157 cm-1 (broad, O-H). Anal. Calcd for C44H92ClPO: C, 75.11; H, 13.18. Found: C, 75.05; H, 13.48. Trioctadecyl-(2-hydroxyethyl)phosphoniumChloride(2P18OHCl) in 55% yield of a white solid: TK-SmA2 ) 83.3-85.0 °C, TSmA2-I ) 92.5-93.4 °C. 1H NMR (CDCl3, δ): 6.24 (H), 4.09 (dt, 2H, 2J 3 PCH2CO ) 21.2 Hz; JCH2CH2O ) 5.6 Hz), 2.57 (m, 2H), 2.34 (m, 6H), 1.52 (m, 12H), 1.26 (m, 84H), 0.88 (t, 9H) ppm. 13C NMR (CDCl3, δ): 55.55 (d, 2JPCCO ) 7.1 Hz), 32.60, 31.46 (d, 3JPCCC ) 14.6 Hz), 30.34 (broad singlet from overlapping peaks), 30.21, 30.04, 30.00, 29.66, 23.80 (d, 1JPCCO ) 48.8 Hz), 23.36, 22.48 (d, 2JPCCC ) 4.5 Hz), 20.48 (d, 1JPCCC ) 46.9 Hz), 14.79 ppm. 31P NMR (CDCl3, δ): 34.12 ppm. IR: 3144 cm-1 (broad, O-H). Anal. Calcd for C56H116BrPO: C, 77.14; H, 13.41. Found: C, 77.29; H: 13.33. Sample Preparations. Aliquots of 1PnX with different weight percents of methanol were prepared in a drybox under nitrogen atmosphere (Caution: 1P10Cl is especially hygroscopic) and then flame-sealed in glass tubes. The tubes were immersed in a hot water bath at temperatures corresponding to the isotropic phases for at least 5 h (to ensure homogeneity), allowed to cool slowly to room temperature with the bath water (except as noted), and kept for at least 2 days at room temperature. The tubes were opened in a drybox

and aliquots were removed for different experiments. Samples of the 1PnX and 2PnOHX were also handled in a dry atmosphere.

Results Textures and Phase Transition Temperatures from Polarized Optical Microscopy (POM). When formed initially upon cooling from their isotropic phases, liquid-crystalline samples sandwiched between glass cover slips were homeotropically aligned; no birefringence was observed by polarized optical microscopy. Fan-shaped textures, a common feature of smectic A phases,17 appeared when isotropic samples were sheared and then cooled (Figure 2 and Figure S1 of Supporting Information). The images became oily streak textures when the liquid-crystalline phases with either fan-shaped or homeotropic patterns were laterally sheared. The mesophases of mPnX salts were identified previously as smectic A phases (SmA2).11a Combined evidence from POMs and X-ray diffractograms (vide infra) marks the liquid-crystalline phases of the 1PnX‚CH3OH and 2PnOHX as SmA2 as well. Below its SmA2-biphasic phase transition temperature (TSmA2-BiP, where biphasic (BiP) is liquid and solid), 1PnCl‚ CH3OH separated into a solid and small pools of liquid, presumably methanol. The single phase could be re-established only when the cooled samples were heated to above the melting temperature of the solid 1PnCl component. For that reason, smectic-biphasic transition temperatures for the same samples were measured by optical microscopy only upon cooling from room temperature. Although the LC phases are enantiotropic, the clearing temperatures of the 1PnCl‚CH3OH in Table 1 were obtained upon heating. The same cooling and heating protocols were employed to examine the 1PnBr‚CH3OH although they do not show phase separation. As will be shown later, 2H NMR spectra of 1P10X‚CD3OD are somewhat more sensitive to the (17) Dierking, I. Textures of Liquid Crystals; Wiley-VCH: Weinheim, 2003; pp 167-212.

Ionic Liquid Crystallinity in Phosphonium Halides

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Figure 3. Polarized optical micrographs of samples after being cooled from their isotropic phases: 1P10Cl‚CH3OH (6.0 wt %, 1.0 equiv, CH3OH) (A) at -22.1 °C (LC + K) and (B) at -26.6 °C (K); 1P10Br‚CH3OH (5.5 wt %, 1.0 equiv, CH3OH) (C) at -18.8 °C (LC + K) and (D) at -22.8 °C (K); (E) 1P10Cl at 24 °C (K); (F) 1P10Br at 24 °C (K). The rounded, nonbirefringent objects in (C) and (D) are air bubbles. In (C), the round, birefringent objects are solid and the background is liquid crystal; the micrograph in (C) was recorded with the polarizers parallel to enhance the contrast between the solid and liquid-crystalline components.

presence of coexisting phases. Thus, the ranges of the transition temperatures in Table 1 are lower limits to the actual values, but they are accurate indicators of the processes occurring. The optical micrographs reveal that different processes occur when 1P10Cl‚CH3OH (Figure 3A,B ) and 1P10Br‚CH3OH solidify (Figures 3C and 4D). Panels A and B of Figure 3 show liquid crystal with a small amount of solid and solid, respectively. Although both 1P10Cl and 1P10Br form fan-shaped textures when cooled from their isotropic phases and become solids (K, panels E and F of Figure 3, respectively), solid 1P10Cl‚CH3OH and 1P10Br‚CH3OH show large and small (respectively) fanshaped focal conic textures. The changes from fan-shaped to focal conic textures suggest that the 1P10X and 1P10X‚CH3OH have somewhat different molecular packing arrangements in their solid phases. From the results of NMR and XRD experiments (vide infra), at least some of the methanol has phase-separated from the 1P10Cl at -26.6 °C. The image in Figure 3C is that of a mixture of solid and liquid-crystalline 1P10Br‚CH3OH while the one in Figure 3D is that of its complete solid phase; we find no NMR or XRD evidence for the phase separation of methanol from 1P10Br at these temperatures.

Solid 1P14Cl‚CH3OH and 1P18Cl‚CH3OH exhibit plateletlike textures like those of their corresponding 1PnCl compounds when the samples are cooled to room temperature from their isotropic phases; as mentioned above, the texture of solid 1P10Cl‚ CH3OH is slightly different from the fan-shape texture observed for solid 1P10Cl. However, all of the homologues of 1PnBr‚ CH3OH show focal conic textures as solids that are very different from the fan-shaped textures of solid 1P10Br and the plateletlike textures of solid 1P14Br and 1P18Br (Figure S2 of Supporting Information). These data suggest that methanol is retained within the solid phases of 1P14Br‚CH3OH and 1P18Br‚ CH3OH, as in the case of the solid phase of 1P10Br‚CH3OH, but becomes separated from the 1PnCl as they are cooled from their smectic to solid phases. As expected, on the basis of what is known about the transition temperatures of solid n-alkanes, the data in Table 1 indicate that liquid crystallinity and lower melting temperatures are favored by shorter n chains. Additionally, where comparisons of salts with the same X are possible, the ranges of the smectic phases are larger and their onsets temperatures are lower for the 1P10X‚ CH3OH than for the 2P10OHX. Most notably, the SmA2 onset

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Table 2. Enthalpiesa (∆H) and Entropies (∆S) of K-SmA2 and SmA2-I Phase Transitions for 2PnOHX (n ) 10, 14, 18; X ) Cl, Br) and 1PnBr (n ) 14, 18) from DSC Thermograms phosphonium salts 2P10OHCl 2P14OHCl 2P18OHCl 1P10Cl 1P14Cl 1P18Cl 2P10OHBr 2P14OHBr 2P18OHBr 1P10Br 1P14Br 1P18Br

first heating

first cooling

transitions

∆H (kJ/mol)

∆H (kJ/mol)

∆H (kJ/mol)

∆S (J/mol·K)

K-SmA2 SmA2-I K1-K2 K2-SmA2 SmA2-I K-SmA2 SmA2-I K1-K2 K2-I K1-K2 K2-K3 K3-I K1-K2 K2-K3 K3-I K-SmA2 SmA2-I K1-K2 K2-SmA2 SmA2-I K1-K2 K2-K3 K3-I K1-K2 K2-I K1-K2 K2-SmA2 SmA2-I K1-K2 K2-SmA2 SmA2-I

22.3 7.6 26.3 185.1 7.6 255.1 5.6 15.6 12.5 25.8 89.3 41.0 34.8 14.4 57.5 18.3 7.0 183.4 53.0 6.7

18.7 7.6

21.5 7.6

69.6 21.9

35.9 74.6

115.5 7.7 163.3 6.5

344.7 20.5 463.9 16.1

59.0 98.1 78.4 93.7

12.6 (9.9)11b

114.5 7.6 163.1 5.9 3.0 12.4 (10.5)11b

33.0

102.9

66.0 (39.5) 41.0 (7.7)

63.2 (44.2) 40.9 (9.7)

191.8 (126.7) 107.9 (24.1)

56.4 106.0

113.9 57.5 21.3 6.6

111.8 57.5 21.9 6.6

311.6 153.8 69.1 19.5

85.7 100.6 44.0 65.8

91.7 6.8

85.9 6.8 122.3 25.4 47.3 12.6 9.0 (9.4)11b 78.8 (41.6) 19.5 (9.7) 8.0 (3.9) 131.4 (62.0) 31.6 (12.1) 6.7 (20.2)

266.4 18.4 363.0 74.6 138.2

49.3 96.9 63.8 67.3 69.1

88.9 31.1 12.4 8.9 80.5 19.9 7.4 134.6 32.0 6.9

195.6 8.9 (8.2)11b 79.8 (28.2) 19.7 (9.1) 8.0 (3.7) 134.3 (66.1) 32.1 (12.0) 7.1 (3.3)

second heating

24.3 243.6 51.9 (25.3) 20.8 (10.1) 376.3 (183.5) 85.0 (32.7) 17.7 (8.1)

T (°C)b

97.7 50.3 102.6 119.0 76.0 98.8 105.8

a Enthalpies of observed solid-solid (K1-K2) transitions are included as well. Data in parentheses are from ref 10a. b Average of the onset temperatures for the phase transitions by DSC measurements from the second heating and cooling cycle.

temperatures of the two 1P10X‚CH3OH are significantly below room temperature and their mesophases persist for more than 40 degrees. However, although the onset temperatures of the 1P14X‚ CH3OH and 2P14OHX are approximately the same, the latter salts (with their covalently attached hydroxyl groups) have the broader mesophase ranges. Of the salts with n ) 18, only 2P18OHCl forms a mesophase, albeit over a narrow temperature range. Thus, mesomorphism is enhanced by the covalent attachment of hydroxyl groups near ionic head group regions for the longer n chains. Heats of Transition and Transition Temperatures from Thermogravimetric Analyses (TGA). On the basis of TGA data, neat 1PnX and 2PnOHX are thermally stable up to at least 200 °C. Table S1 in the Supporting Information includes the calculated (i.e., for 1 equiv of methanol) and actual weight losses from the 1PnX‚CH3OH and the temperature ranges where the weight reductions were observed. In general, the weight losses occur at lower temperatures and the discrepancies between the theoretical and observed losses of methanol increase with increasing alkyl chain lengths (n). For 1P10X‚CH3OH, a weight loss corresponding to 0.93 equiv (X ) Cl) and 0.92 equiv (X ) Br) of methanol and occurring at temperatures exceeding the boiling point of the alcohol, 65 °C, was measured. The weight loss has been attributed to strong H-bonding interactions between halide anions and hydroxyl protons of methanol as well as between phosphorus cations and hydroxyl oxygen atoms. The loss drops to 0.55 and 0.33 equiv for 1P14Cl‚CH3OH and 1P18Cl‚CH3OH, respectively, and the same trend is found with the bromides. We conjecture that the loss of methanol at lower temperatures (and in smaller amounts) from the longer-chained 1PnCl‚CH3OH is a consequence of the materials being crystalline at room

temperature and their ionic layers not being able to accommodate well the volume of an added methanol molecule on an equimolar basis. Because the methanol molecules are not held tightly within these solid matrices, a large fraction of them probably escapes prior to the onset of measurement in the open pans of the TGA instrument. Evidence for this type of phase separation of methanol from the phosphonium chloride salts is found in XRD, 2H NMR (vide infra), and POM data obtained from their solid phases. For example, when 1P10Cl‚CH3OH is cooled sufficiently, it separates into micropools of liquid interspersed within a solid (as indicated by NMR and XRD measurements). Also, the much lower SmA2 onset temperatures of the 1P10X‚CH3OH and the much higher clearing temperatures of the 2P14OHX (and 2P18OHCl) indicate the advantages and disadvantages of introducing hydroxyl groups either noncovalently or covalently near the ionic layers of the phosphonium salts. Differential Scanning Calorimetry (DSC). In most cases, the heating thermograms from solvent crystallized and melt solidified 2PnOHX were not the same (Table 2),18 indicating the formation of different solid morphs. However, strong evidence for the thermal stability of these molecules above the clearing temperatures (and confirmation of the conclusions reached from the POM data that the SmA2 phases of the 2PnOHX are enantiotropic) was found from the reproducibility of thermograms upon repeated heating and cooling cycles. As expected, the heats of transition for the K-SmA2 transitions are much larger than the corresponding ones for the SmA2-I transitions: the K-SmA2 transitions involve primarily melting of the long alkyl chains (18) As a result of the evaporation of methanol in our open sample pans, DSC thermograms of 1P10Cl‚CH3OH could not be recorded.

Ionic Liquid Crystallinity in Phosphonium Halides

Langmuir, Vol. 24, No. 6, 2008 2751

Figure 4. Vertically offset powder X-ray diffractograms of (A) 1P10Cl, (B) 1P10Br with different concentrations of methanol at 24 °C, and (C) 1P10Cl‚1/2CH3OH (3.0 wt %, 0.5 equiv, CH3OH) at various temperatures. (D) Calculated lamellar spacings (1P10Cl, 9; 1P10Br, b) and fwhh (1P10Cl, 0; 1P10Br, O) from the lowest-angle peak as a function of the number of equivalents of added methanol. The lines linking points are a visual aid only and have no physical significance.

(and some disruption to the ordering at the ionic interfaces); the conversions to isotropic phases require primarily loss of the remaining ionic interactions which maintain the layers. The entropies of the thermodynamically reversible transitions, ∆S ) ∆H/T, were calculated using the average of the onset temperatures from heating and cooling thermograms. For each series of phosphonium salts, the magnitudes of the ∆S of the K-SmA2 transitions increase and those of the SmA2-I transitions decrease with increasing length of the long alkyl chains (n). These trends are consistent with the large increase in entropy expected for chain melting (i.e., K f SmA2) and the lesser inhibition to dissociation of ionic layers (i.e., SmA2 f I) because the motions of the melted longer alkyl chains provide a greater momentum for disrupting layering. Within a series of phosphonium salts which differ only in the nature of their halide anion, the entropies of transition of the chlorides are larger than those of the bromides. Chloride, being a harder and smaller anion than bromide,19 interacts more strongly with phosphonium head groups. As a result, it is more difficult to disrupt the layering from P+-Cl- electrostatic interactions than from P+-Br- interactions. The covalently attached hydroxyl groups of the 2PnOHBr and the noncovalently attached ones of 1PnBr‚CH3OH behave electronically in a manner similar to their solvation of the ionic head groups. Both facilitate ionic separation, loss of salt crystallinity, and disruption of ordering between cationic and (19) Subramanian, V.; Ducker, W. A. Langmuir 2000, 16, 4447-4454.

anionic centers within ionic layers. The resultant entropy change is smaller for the 2PnOHBr than for the corresponding 1PnBr due to the covalently attached hydroxyl group not being able to separate itself from the proximity of the charged head group; the loss of the layered structures in the 2PnOHX need not be accompanied by a loss of solvation. Since interactions between methanol and the head group of the 1PnBr in 1PnBr‚CH3OH are intermolecular, their loss can also involve physical separation in space and an additional increase in entropy. X-ray Diffraction (XRD). Figure 4 shows the influence of added methanol on the room-temperature XRD patterns of 1P10Cl and 1P10Br. Based on the higher angle regions of the diffractograms (Note the residual sharp diffraction peaks at 2θ > 10° in panels A and B of Figures 4 when less than 1 equiv of methanol is present.), the samples with less than 1 molar equiv of methanol appear to exist as mixtures of 1P10X and 1P10X‚CH3OH and are not fully transformed into liquid crystals until 1 molar equiv of methanol is present. The presence of two low-angle peaks and changes in their relative intensities up to the melting temperature of the salt in diffractograms of 1P10Cl‚ 1/2CH3OH at different temperatures provide additional strong evidence for the coexistence of discrete domains of 1PnX and 1PnX‚CH3OH in samples containing less than 1 molar equiv of methanol (Figure 4C). In addition, the strong, low-angle peaks of the diffractograms in panels A and B of Figure 4 shift to slightly higher angles with increasing concentrations of methanol. These shifts correspond to decreases in the lamellar d spacings16 of 10.7% for 1P10Cl and 7.7% for 1P10Br. The absolute value

2752 Langmuir, Vol. 24, No. 6, 2008

Ma et al.

Table 3. Comparisons of Lamellar Spacings (d, (0.1 Å) from X-ray Diffraction Data for 1PnX, 2PnOHX, and 1PnX‚CH3OH in Their Crystalline (K) Phase at 24 °C and SmA2 Phase at Different Temperaturesa phosphonium salt (n ) 10) 1P10Cl 2P10OHCl

1P10Cl•H2O 1P10Cl•CH3OH

1P10Br 2P10OHBr 1P10Br•H2O 1P10Br•CH3OH

T (°C)

phase

d (Å)

-35.5 24 24 51 59.5 67 24 -29.5 -14 3.5 24 -35.5 24 24 52.5 63.5 24 -24.5 -12.5 0.5 24

K1 K2 K SmA2 SmA2 SmA2 SmA2 BiP SmA2 SmA2 SmA2 K1 K2 K SmA2 SmA2 SmA2 K SmA2 SmA2 SmA2

21.1 23.4 22.7 21.0 20.8 20.7 21.3 27.0/21.0 23.7 22.9 20.8 21.0 22.1 22.0 20.4 20.3 21.1b 20.5 21.8 21.6 20.4

phosphonium salt (n ) 14)

T (°C)

phase

d (Å)

1P14Cl

24 90 24 68 82 93.5 24 65 75.5 24 78.5 109 24 62 79.5 91 24 58 74.5 77

K1 K2 K SmA2 SmA2 SmA2 BiP SmA2 SmA2 K1 K2 SmA2 K SmA2 SmA2 SmA2 K SmA2 SmA2 SmA2

28.3 31.7 26.5 27.8 27.1 26.1 28.3 26.9 25.9 28.1 29.9 26.3 25.6 27.3 26.5 25.9 27.7 27.2 25.8 25.5

2P14OHCl

1P14Cl•CH3OH 1P14Br 2P14OHBr

1P14Br•CH3OH

phosphonium salt (n ) 18)

T (°C)

phase

d (Å)

1P18Cl

24 95 24 90.5 24 24 86 103 24 24

K1 K2 K SmA2 K K1 K2 SmA2 K K

35.5 39.8 32.8 31.0 37.0 35.0 38.3 31.1 34.9 34.5

2P18OHCl 1P18Cl•CH3OH 1P18Br 2P18OHBr 1P18Br•CH3OH

a To increase precision, higher order diffraction peaks (when detectable) were used to calculate the d-spacings. The data above room temperature were recorded on samples which were heated after being cooled to room temperature from their isotropic phases. b 21.4 ( 0.2 Å in ref 23a.

of d for 1P10Cl is larger than that for 1P10Br because the smaller ionic radius of chloride (1.69 Å) than bromide (1.82 Å)20 (and stronger electrostatic attraction) translates to a smaller crosssectional area allocated to each 1P10Cl molecule as projected onto an ionic layer plane. Consequently, each of the three decyl chains is splayed out over more lateral space and the lamellar spacing decreases.11a,21 Similarly, the total head group of a 2PnOHX molecule is larger than that of the corresponding 1PnX molecule and the d spacings for the former are smaller than those of the latter within the same phase type (Table 3). Consistent with this explanation, the reduction in d of a 1PnX phase upon addition of 1 molar equiv of water is less than that observed when methanol, a larger molecule, is added. Small, but clearly discernible, increases of the full-width-athalf-height (fwhh) of the low angle peaks for both 1P10X are also noted as methanol is added incrementally up to a concentration of 1.0 molar equiv. Those increases are consistent with our model in which molecules of methanol reside preferentially in the ionic regions and “loosen” the correlated motions of cations and anions residing therein. The lamellar spacing d reported here is an average layer thickness; the fwhh represents the distribution of that average. The distribution increases with increasing concentrations of methanol (up to a point) because of the coexistence of the domains of 1P10X and 1P10X‚CH3OH with different d spacings. Each molecule of phosphonium salt becomes associated with one molecule of methanol at a 1:1 molar ratio; we consider the species present to be weak complexes. However, even when somewhat more than 1 molar equiv of methanol is present, the 1P10X are able to retain their liquid crystallinity, albeit at transition temperatures22 lower than those corresponding to the 1P10X with 1 molar equiv of methanol. (20) Shannon, R. D. Acta Crystallogr. 1976, A32, 751-767. (21) There is an exception to this trend. The lamellar spacing of 2P18OHBr is larger than that of 2P18OHCl in the solid phase at 24 °C (Table 4). Unfortunately, we have been unable to grow single crystals and determine from their X-ray derived structures the details of the interactions within the ionic layers of these salts. We suspect that the stronger interaction expected between the chloride and hydroxyl proton of the hydroxyethyl group of 2P18OHCl is a major factor for this difference along with the fact that, unlike 2P18OHCl, 2P18OHBr passes directly from a solid to its isotropic phase when heated; it forms no liquidcrystalline phase.

Figure 5. Stack plots of X-ray diffractograms of (A) 1P14Br, (B) 2P14OHBr, and (C) 1P14Br‚CH3OH (4.3 wt %, 1.0 equiv, CH3OH) at various temperatures and in different phases. The spacing ratios (marked by arrows), assuming a lamellar structure, are indicated. LC ) SmA2.

Since these phosphonium salts are lamellar in both their solid11a and SmA2 phases, the changes in the layer thicknesses of most of the 1PnX, 2PnOHX, and 1P10X‚CH3OH were explored as a function of temperature using powder X-ray diffraction (Figure 5 and Figures S3-S11 of Supporting Information). All of the salts reported here have peak progressions in the XRD diffractograms of their solid phases which are consistent with lamellar structures; although some of the higher order reflections are missing in some of diffractograms, spacing ratios corresponding (22) Ma, K.; Weiss, R. G. Unpublished work. For example, the liquid-crystalline phase of 1P10Cl with 10.0 wt % (1.8 equiv) CH3OH becomes biphasic (solid and isotropic liquid) when cooled to -37.7 to -26.9 °C and isotropic when heated to 2.4-7.9 °C; the corresponding transitions for 1P10Br with 10.0 wt % (1.9 equiv) CH3OH are -24.8 to -17.8 °C and 7.8-13.2 °C.

Ionic Liquid Crystallinity in Phosphonium Halides

to 1, 1/2, 1/3, etc.23 can be found. The XRD patterns of the phosphonium salts are virtually independent of temperature within each crystalline phase; several of them exhibit more than one solid phase (e.g., Figure 5A), and significant changes in d can be discerned at the solid-solid phase transitions. The XRD patterns of the liquid-crystalline phases of all of the salts consist of one narrow low-angle peak and one broad highangle peak, as expected for a smectic phase. In the corresponding isotropic phases, the low-angle peak is weak and broadened, but its presence indicates that some residual ordering is retained. These observations and the aforementioned homeotropic patterns noted in the POMs for the 2PnOHX, and 1P10X‚CH3OH are compelling evidence that their molecular directors are orthogonal to the bilayer planes of the smectic phases. Furthermore, the distances between a phosphorus nucleus and the van der Waals edge of the outmost hydrogen atom on the terminal carbon of a fully extended long chain (n) of these salts are calculated11a to be 15.4, 20.5, and 25.6 Å when n ) 10, 14, and 18, respectively. Because the d spacings in the LC phases are between one and two of these molecular lengths, the smectic phases are designated to be smectic A2, as were those of the 1PnX.11a Precipitous changes in the lamellar spacings are noted at the phase transition temperatures. The lamellar thicknesses decrease noticeably with increasing temperature in the SmA2 phases (Figure 6) as a result of increased chain motions (including more gauche bends), which also allow each molecule to occupy a larger projected area on the plane defined by the lamellae. These changes are illustrated by the dependence of the lamellar thickness d on temperature for 2PnOHX, 1PnX‚CH3OH, and 1PnX (Figure 6 and Figure S11 of Supporting Information).24 The rate of decrease within liquidcrystalline phases, ∆d/∆T, ranges from -0.019 to -0.20 Å/°C for the phosphonium salts investigated here, depending on n and X. As n and, thus, the number of degrees of freedom for chain motion increases, so does ∆d/∆T. Furthermore, chloride salts suffer larger ∆d/∆T than the corresponding bromide salts within the liquid-crystalline phases, and the rates for the 1PnX are larger than for the corresponding 2PnOHX. A smaller size of X (or the headgroup) is expected to result in a more tightly packed ionic layer and, thus, a more compact packing structure for the long alkyl chains (i.e., making them more extended with fewer gauche bends). Finally, the larger ∆d/∆T of the 1PnX‚ CH3OH than the corresponding 2P10OHX can be rationalized, again, on the basis of more motional freedom by (noncovalently associated) methanol molecules than the hydroxyl groups that are covalently attached through ethylene links to phosphorus atoms. For the same reasons, the TK1fK2 transitions of the nonmesomorphic 1P10X occur at higher temperatures than the TSmA2fK transitions of the corresponding 1P10X‚CH3OH. The same trends are observed also in 2PnOHX, where the OH groups are covalently attached to the head groups. In Figure 6D, the low-angle peak of 1P14Cl‚CH3OH splits into two peaks as it is cooled and passes through the SmA2 f BiP transition. As indicated by the OM measurements, the two peaks are from a coexisting mixture of solid 1P14Cl (in which the methanol molecules have been expelled) and 1P14Cl‚CH3OH complexes. 1P14Br‚CH3OH, which remains monophasic throughout the same cooling regime, retains a single low-angle peak; this result is consistent with the observations mentioned above (23) Stout, G. H.; Jensen, L. H. X-ray Structure Determination: A Practical Guide, 2nd ed.; Wiley: New York, 1989; pp 24 and 28. (24) The dependence of d on temperature for 2P18OHBr and 1P18X‚CH3OH has not been examined; neither forms a liquid-crystalline phase.

Langmuir, Vol. 24, No. 6, 2008 2753

Figure 6. Heat flow from DSC thermograms (second heating, 5 °C/min) (solid lines) and lamellar spacings (d) by X-ray diffraction (ο) as a function of temperature for the salts which form liquidcrystalline phases: (A,B) 2PnOHX, (C) 1PnBr, (D) 1P14Br‚ CH3OH (4.3 wt %, 1.0 equiv CH3OH) and 1P14Cl‚CH3OH (4.5 wt %, 1.0 equiv CH3OH). The two points at one temperature for 1P14Cl‚CH3OH come from two low-angle peaks during the K f SmA2 transition (see Supporting Information). Lines between points are drawn as a visual aid and are not based on a physical model. LC ) SmA2.

in that 1P14Br and methanol remain associated in its solid phase. The data in Figure 6D are also consistent with our assertion that methanol is retained more effectively at higher temperatures by the 1PnBr‚CH3OH salts than by the corresponding chlorides; most of the methanol appears to be lost from 1P14Cl‚CH3OH below the boiling point of methanol. Neat 1P10Cl remains a single solid phase throughout the temperature range investigated and at the temperature where 1P10Cl‚CH3OH exhibits an SmA2 phase and is biphasic (Figure

2754 Langmuir, Vol. 24, No. 6, 2008

Figure 7. X-ray diffractograms of 1P10X‚CH3OH (upper: 1.0 equiv methanol, 6.0 wt % in 1P10Cl (A) and 5.5 wt % in 1P10Br (B)) and their neat 1P10X analogues (lower) below their solidification temperatures. The spacing progressions, based on a lamellar phase, are marked by arrows.

S9 and S12 of Supporting Information). Two low-angle peaks appear as 1P10Cl‚CH3OH enters its BiP phase: one (d ) 21.0 Å) is obviously from the neat solid 1P10Cl (Figure 7A) and the other (d ) 27.0 Å) is from a less interdigitated phase, perhaps as a result of the loss of specific intermolecular interactions between the head groups of 1P10Cl and methanol molecules (vide infra). In contrast, 1P10Br‚CH3OH remains monophasic throughout when being cooled from its LC phase and has a smaller lamellar spacing than 1P10Br in its K phase because the associated methanol molecules increase the headgroup area projected onto a layer plane and, thus, allow the long alkyl chains to bend more (Figure 7B). Solidification of 1P10Br‚CH3OH involves only “freezing” of the motions of the three decyl chains, since methanol molecules remain situated at the ionic head groups. As indicated by the TGA data noted above, the 1PnCl‚CH3OH salts undergo phase separation (to solid 1PnCl and methanol) while the 1PnBr‚CH3OH do not when they are heated to the boiling point of methanol. Additional evidence for this conclusion is found in the XRD diffractograms of the 1PnX‚CH3OH (n ) 14 or 18) that had been heated previously to their isotropic phases and cooled to room temperature (Figure S13 of Supporting Information). Consistent with the loss of methanol from the 1P14Cl‚CH3OH complex, its diffractograms and those of 1P14Cl are nearly the same. In contrast, the XRD pattern and d spacing of 1P18Cl‚CH3OH (which also suffers a loss of methanol at elevated temperatures) are clearly different from those of 1P18Cl. Although the reason for the difference between the two diffractograms in Figure S13B of Supporting Information is not obvious at this time, we conjecture that the solidifying salt is influenced by the presence of methanol molecules to adopt a different packing arrangement. Apparently, London dispersion interactions among the longer chains of the chloride series are more important energetically than the head group interactions with methanol, and the latter are expelled from the ionic planes of the lamellar phases to maintain a more compact packing structure within each layer. By contrast, 1PnBr‚CH3OH loses methanol above its boiling point and the d spacing of solid 1PnBr‚ CH3OH is smaller than that of the corresponding 1PnBr. Deuterium Magnetic Resonance (2H NMR) of 1P10X‚ CD3OD. From the XRD data presented above, molecules of methanol are associated directly with the ionic parts of the 1PnX salts. In addition, dipolar splittings from 2H NMR experiments conducted on various neat (partially deuterated) 1PnX and in the presence of various deuterated solute molecules demonstrate that their smectic phases and the solute molecules therein are aligned easily in the strong magnetic fields employed (in the present work, ≈9.4 T).25 The magnitudes of the dipolar and the quadruploar splittings from deuterium spectra can provide quantitative information about the molecular geometry and

Ma et al.

orientation of the solute molecules.26 For the most part, prior solute structural studies of this sort have relied upon lyotropic liquid crystals (i.e., phases comprised of mostly an isotropic liquid and a small amount of an ordering species);27 our liquid crystals, with added methanol, are at the opposite end of the compositional phase diagram. We find that samples of 1P10X‚CD3OD are also aligned in the employed magnetic field. The quadrupolar splittings of the -CD3 deuterons are much smaller than those of the -OD deuterons within the smectic phases of 1P10Cl‚CD3OD and 1P10Br‚CD3OD (Figure 8).28 The quadrupolar splittings increase with decreasing temperature as expected, although the signal of the -OD deuteron is weak (broadened) or undetectable at ca. e-5 °C due to exchange rate changes.29 The higher temperature spectra for the -CD3 deuterons in Figure 8, consisting of singlets, indicate rapidly reorienting CD3OD molecules in the isotropic phase of their hosts. With decreasing temperature, a quadrupolar doublet of -CD3 deuterons appears and its intensity grows while the central singlet (from residual disordered methanol molecules) disappears. The appearance of the doublet coincides well with the phase transition temperature from the isotropic to the liquid-crystalline phase as measured by POM and XRD (Figure 9). Spectra were recorded by cooling from the highest to the lowest temperatures and then with heating from 25 °C to the highest temperature. The spectra recorded on heating were the same as those obtained at the same temperatures upon cooling. Although 2H NMR is a more sensitive technique than optical microscopy to locate phase transitions, especially those leading to biphases, the 5 °C intervals between our NMR spectra limit the precision of the transition temperatures to (2.5 °C. However, transition temperatures from the two techniques are consistent insofar as they can be compared. The ordered content represents the percent of the sample which is in a liquid-crystalline or solid phase of 1P10X‚CD3OD based on the relative peak areas of the quadrupolar doublets of -CD3 deuterons and the central singlet. At even lower temperatures, a new central single peak appears as the quadrupolar doublet decreases in intensity, broadens, and increases its splitting. From the POM (e.g., Figure 3A,B), XRD (e.g., Figure 7A), and TGA evidence cited above, the singlet at low temperatures is a result of the separation of 1P10Cl‚CD3OD into a solid (presumably 1P10Cl) and an isotropic liquid phase (presumably methanol). The absence of a singlet component in the spectra for 1P10Br‚ CD3OD below its LC f K transition is consistent with the conclusions derived from the POM (e.g., Figure 3C,D), TGA, and XRD data (e.g., Figure 7B) in that phase separation does not (25) (a) Nagana Gowda, G. A.; Chen, H.; Khetrapal, C. L.; Weiss, R. G. Chem. Mater. 2004, 16, 2101-2106. (b) Nagana Gowda, G. A.; Khetrapal, C. L.; Lu, L.; Wauters, H. C.; Abdallah, D. J.; Weiss, R. G. Proc. Ind. Nat. Sci. Acad. 2004, 70A, 627-634. (26) Jacobsen, J. P.; Schaumburg, K. J. Magn. Reson. 1977, 28, 191-201. (27) (a) Sobajima, S. J. Phys. Soc. Jpn. 1967, 23, 1070-1078. (b) Panar, M.; Phillips, W. D. J. Am. Chem. Soc. 1968, 90, 3880-3882. (c) Czarniecka, K.; Samulski, E. T. Mol. Cryst. Liq. Cryst. 1981, 63, 205-214. (d) Abe, A.; Yamazaki, T. Macromolecules 1989, 22, 2145-2149. (e) Tjandra, N.; Bax, A. Science 1997, 278, 1111-1114. (f) Sarfati, M.; Lesot, P.; Merlet, D.; Courtieu, J. Chem. Commun. 2000, 2069-2081. (g) Merlet, D.; Emsley, J. W.; Jokisaari, J.; Kaski, J. Phys. Chem. Chem. Phys. 2001, 3, 4918-4925. (h) Gronenborn, A. M. C. R. Biol. 2002, 325, 957-966. (i) Bax, A. Protein Sci. 2003, 12, 1-16. (j) Ginzburg, B. M.; Shepelevskii, A. A. J. Macromol. Sci. B: Phys. 2003, B42, 1-56. (k) Thiele, C. M.; Berger, S. Org. Lett. 2003, 5, 705-708. (l) Verdier, L.; Sakhaii, P.; Zweckstetter, M.; Griesinger, C. J. Magn. Reson. 2003, 163, 353-359. (m) Pham, T. N.; Hinchley, S. L.; Rankin, D. W. H.; Liptaj, T.; Uhrinl, D. J. Am. Chem. Soc. 2004, 126, 13100-13110. (n) Yethiraj, A.; Weber, A. C. J.; Dong, R. Y.; Burnell, E. E. J. Phys. Chem. B. 2007, 111, 1632-1639. (28) 2H NMR investigations have been restricted to the 1P10X‚CD3OD because their mesophases exist over much broader temperature ranges and commence at much lower temperatures than those of the longer-chained homologues. (29) Liu, K.; Ryan, D.; Nakanishi, K.; McDermott, A. J. Am. Chem. Soc. 1995, 117, 6897-6906.

Ionic Liquid Crystallinity in Phosphonium Halides

Figure 8. 2H NMR stack plots of spectra showing quadrupolar splittings of -CD3 (left and center) and -OD (right) deuterons of (A) 1P10Cl‚CD3OD (6.7 wt %, 1.0 equiv CD3OD) and (B) 1P10Br‚ CD3OD (6.2 wt %, 1.0 equiv CD3OD) at various temperatures and in different phases. The A(II) plot is an expansion of the lower field portion of the spectra in A(I).

occur in this solid phase. Additionally, the doublets in the solid phase at e-25 °C become much broader and weaker than in the LC phase, as expected if methanol molecules are still associated with 1P10Br and suffer increasingly restricted motion (and very rapid spin-lattice relaxation29). Although only one quadrupolar doublet from the -CD3 deuterons in the smectic phase of 1P10Br‚CD3OD can be detected (Figure 8B), two different quadrupolar doublets for the -CD3 deuterons are evident in the spectra of the LC phase of 1P10Cl‚ CD3OD (Figure 8A(II)). Since all of our evidence from other physical measurements indicates that there is only one (homogeneous) smectic phase, the coexistence of two sets of doublets requires that the -CD3 deuterons be distributed in two distinctly different types of chemical environments and that exchange between them be slow on the NMR time scale. Although the magnitude of the order parameter of the solute depends upon the order parameter of the liquid-crystalline matrix, the number of distinguishable CD3OD species and the rate at which they interconvert do not. One possible explanation for the results with 1P10Cl‚CD3OD is that the molecules of methanol are arranged in different orientations with respect to the phosphonium center and chloride groups in the ionic plane of 1P10Cl as a result of strong headgroup interactions; an example of possible orientations is shown in Figure 10.

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Figure 9. Dependence of 2H NMR quadrupolar splittings of deuterons (-CD3, b and 2; -OD, 9) of (A) 1P10Cl‚CD3OD (6.7 wt %, 1.0 equiv CD3OD) and (C) 1P10Br‚CD3OD (6.2 wt %, 1.0 equiv CD3OD) and lamellar spacings (d, 0) of (B) 1P10Cl‚CH3OH (6.0 wt %,1.0 equiv CH3OH) and (D) 1P10Br‚CH3OH (5.5 wt %, 1.0 equiv CH3OH) on temperature. The transition temperatures along the x axis are from optical microscopy. The ordered content (O) is the peak areas of the quadrupolar doublets of -CD3 deuterons divided by the sum of the areas of the quadrupolar doublets and the central singlet. Lines between points are drawn as a visual aid and are not based on a physical model.

Figure 10. Two possible exchanging orientations for CD3OD molecules at the head group region of 1P10Cl in the SmA2 phase of 1P10Cl‚CD3OD.

The relatively weaker interactions between methanol molecules and bromide anions of 1P10Br allow less specific and more rapid interconverting orientations, which are detected as an average in the spectra of Figure 8B. As the temperature is decreased within the smectic phase of 1P10Cl‚CD3OD, the fraction of the -CD3 groups in the lower energy orientation increases and the dipolar doublet peaks from this orientation become broader (as a consequence of decreased motional averaging30) until they are no longer detectable.

∆)

( )

3 e2qQ SXD 2 h

1 SXD ) (3cos2(β) - 1)SC3 2

(1) (2)

The quadrupolar splitting of a deuteron in Hz (∆) can be related to the degree of orientation of its X-D bond (where X

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Table 4. Molecular Orientation Parameters of Methanol Molecules in the Smectic Phases of 1P10X‚CD3OD group -CD3

-OD

a

parameter T range (°C) D (KHz) SCD SC3 T range (°C) D (KHz) SOD

1P10Cl‚CD3OD 30.0 -(-30.0) 5.3 - 8.0 0.023 - 0.034 (-0.070) -(-0.11) 26.0 -(-15.0) 29.3 - 38.1 0.093 - 0.12 a

1P10Br‚CD3OD

15.0 - (-20.0) 7.0 - 12.0 0.030 - 0.052 (-0.093) -(-0.16) b

45.0 -(-20.0) 4.6 - 6.4 0.020 - 0.028 (-0.061) -(-0.086) 40.0 -(-15.0) 30.0 - 36.1 0.096 - 0.12

Figure 8A(I). b Figure 8A(II).

) C or O) with respect to the applied magnetic field direction, SXD, by eq 1, where (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 experienced by the deuteron.31 The quadrupolar coupling constant has been experimentally determined to be 155 ( 5 KHz at 31 °C for the methyl group26 and 209 kHz at 28 °C for the hydroxyl group32 of methanol; both vary slightly with temperature and are assumed to be constant throughout the temperature ranges explored here. The order parameter of the methyl group along its three-fold symmetry axis, SC3, can be calculated by eq 2, where β, 69.86° for methanol,26,33 is the angle between a C-D bond and the C3 axis. The orientation factors, SCD, SOD, and SC3 for 1P10X‚CD3OD in their smectic phases are summarized in Table 4. The negative sign of SC3 reveals a negative diamagnetic susceptibility anisotropy34 of these liquid crystals. Although the quadrupolar splitting for the methyl deuterons of 1P10Cl‚CD3OD (and, therefore, larger SCD and SC3 values) is larger than that of 1P10Br‚CD3OD, both are very low when compared to the typical magnitudes for other thermotropic liquid-crystalline phases.25,35 We attribute the larger value in the chloride phase to the combination of strengths of interactions by the two salts with methanol and the relative sizes of the anions, as described above.

Discussion Packing Arrangements. Attempts to grow single crystals of the phosphonium salts investigated here that are suitable for X-ray analyses have not been successful thus far, and the peaks from X-ray diffractograms of the solid salts with n ) 10 and 2P14OHX can be indexed to several unit cells, almost all of which are layered monoclinic. Although the other salts cannot be indexed as well, we assume that they are also layered. The lamellar thicknesses of all of the phosphonium salts in their solid phases, assuming layered arrangements, are included in Table 3. To reduce error, the d values were calculated from one of the higher order peaks of the lowest-angle peak. In the single crystal of a related compound, benzyltrioctadecylammonium bromide ([H(CH2)18]3N+CH2(C6H5) Br-), the spacing between neighboring interdigitated ionic planes is 19.8 Å and between alternating non-interdigitated bilayers is 30.4 Å.14 The single crystals of tetra-n-alkylphosphonium halides ([H(CH2)n]4P+X-) are packed in non-interdigitated lamellar arrangements for which the layer thicknesses are 23.3 Å (n ) (30) Olsson, U.; Wong, T. C.; So¨derman, O. J. Phys. Chem. 1990, 94, 53565361. (31) (a) Diehl, P.; Khetrapal, C. L. In NMR Basic Principles and Progress, Vol. 1; Diehl, P., Fluck, E., Kosfeld, R., Eds.; Springer-Verlag: New York, 1969. (b) Meiboom, S.; Snyder, L. C. Acc. Chem. Res. 1971, 4, 81-87. (32) Wendt, M. A.; Zeidler, M. D.; Farrar, T. C. Mol. Phys. 1999, 97, 753756. (33) Chen, D. M.; Reeves, L. W.; Tracey, A. S.; Tracey, M. M. J. Am. Chem. Soc. 1974, 96, 5349-5356. (34) Lu, L.; Sharma, N.; Nagana Gowda, G. A.; Khetrapal, C. L.; Weiss, R. G. Liq. Cryst. 1997, 22, 23-28. (35) (a) Lu, L.; Sharma, N.; Nagana Gowda, G. A.; Suryaprakash, N.; Khetrapal, C. L.; Weiss, R. G. Liq. Cryst. 1998, 25, 295-300.

10, X ) Br) and 40.2 Å (n ) 18, X ) I).14 From these and related data, a 31.4 Å layer thickness for the n ) 14, X ) I salt14 can be interpolated. Considering the sizes of the chloride and bromide anions used here and that of iodide, the lamellar thicknesses of our 1PnX salts (n ) 10, 14, or 18) in their K2 phases are slightly less than the layer thicknesses of the tetra-n-alkylphosphonium halides with the same n. The discrepancy is most easily explained if the 1PnX layers are slightly interdigitated. Also, the smaller d values in the lower temperature K1 phases of the 1PnX imply more interdigitation than in the corresponding higher temperature K2 phases. The difference in d between the K1 and K2 phases of the 1PnX is the largest for n ) 18. Also, the d values in the K phases of the 2PnOHX are closer to those of the K1 phases than to those of K2 phases of the 1PnX. This may be due to a closer relationship between the packing arrangements of crystalline 2PnOHX and K1 phases of the 1PnX. The X-ray diffractograms of the BiP phases of what were initially the other two 1PnCl‚CH3OH (Figure 7 and Figure S13 of Supporting Information) clearly show the presence of solid 1PnCl; contractions of the matrix during the SmA2 f BiP transition must force methanol molecules of 1PnCl‚CH3OH to leave the ionic areas. Although the X-ray diffractogram of 1P10Cl‚CH3OH in its BiP phase is rather complicated, it can be interpreted as the sum of reflections from two lamellar structures. One set of peaks is the same as that found for neat, solid 1P10Cl at the same temperature (Figure 7A). The d value of the second structure is 27.0 Å. On the basis of the small size of the headgroup of 1P10Cl‚CH3OH, its lamellar structure is expected to adopt a packing arrangement like that of benzyltrioctadecylammonium bromide.36 The d value calculated on the basis of this model is 27.9 Å.14 By contrast, the spectral and diffraction data indicate that the 1PnBr‚CH3OH salts do not undergo a similar phase separation as they pass from the SmA2 to their solid phases (vide supra) and their d spacings are only 1-2% smaller than those of the corresponding solid (K1) 1PnBr at the same temperatures. More importantly, the 2H NMR data in Figure 8B give no indication of expulsion of methanol molecules from 1P10Br‚CH3OH as it is cooled to its solid phase. Whether the n ) 14 and 18 homologues are equally robust will be determined in future studies. However, we do note that heating the solid phases of all of the 1PnBr‚CH3OH to their liquid-crystalline phase temperatures results in (what appears to be) some solid which disappears slowly. On the basis of the solid structures and the information provided above, we have assigned the packing within the liquid-crystalline phases of the salts to be smectic A2 phases, as shown in Figure 1.11,25a Two alternative packing arrangements, both with molecules in which the long molecular axes are orthogonal to the layer planes, are shown in Figure 11.11a Although we favor the packing structure in Figure 1 based on our structural analyses of solid (36) Abdallah, D. J.; Lu, L.; Cocker, T. M.; Bachman, R. E.; Weiss, R. G. Liq. Cryst. 2000, 27, 831-837.

Ionic Liquid Crystallinity in Phosphonium Halides

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Figure 11. Cartoon representation of two alternative lamellar packing arrangements for the 1PnX (R ) CH3) and 2PnOHX (R ) CH2CH2OH) salts: non-interdigitated bilayer (left) and interdigitated monolayer (right).

Figure 12. Possible intra- and intermolecular interactions between hydroxyl groups and charged centers of 2PnOHX (R ) -(CH2)nH where n ) 10, 14, or 18). Possible orientations for the interaction of methanol near the head groups of the 1PnX in 1PnX‚CH3OH are shown for a specific salt in Figure 10.

structures of a series of related ammonium and phosphonium salts11,14,36 and considerations related to maximization of electrostatic interactions, alternatives such as those shown in Figure 11 cannot be discounted at this time especially considering the very large heats of the K1 f SmA2 transitions for the n ) 14 and 18 salts (Table 2), which suggest that significant packing changes may attend passage to the smectic phase. Regardless, the smectic phases consist of ionic interfaces separating layers of alkyl chains. The Influence of Inter- and Intramolecular Interactions on 1PnX Phases. Within the temperature range explored, the 1PnX form two solid morphs, each with lamellar organization. The lamellar thicknesses increase between the lower temperature K1 and higher temperature K2 phases (Table 3, Figure 6, and Figure S12 of Supporting Information). The increased lamellar thickness may be a consequence of decreased interdigitation, more compact packing arrangements within a layer, or both. Decreased interdigitation is expected to promote liquid-crystallinity, while denser layering should have the opposite effect. Thus, the less densely packed 1P14Br and 1P18Br are able to become liquid crystals upon melting and the corresponding chlorides are not. The lack of liquid crystallinity in 1P10Br is probably a result of the ionic layering not being retained above the melting point; the London dispersion forces responsible for maintaining the layering of the alkyl chains are weakest for the shortest homologue. Here, two structural modifications, the addition of methanol molecules at the ionic head group regions and the covalent attachment of hydroxyl groups near the ionic centers, loosen the layering and induce liquid crystallinity of the 1PnX. Indirect evidence supports our hypothesis that methanol molecules arrange themselves at the head groups within an ionic layer so that the hydroxyl protons make H bonds with the halide anions and the hydroxyl oxygens interact electrostatically with the positively charged phosphorus atoms (Figure 12):13 most of the methanol loss in 1P10X‚CH3OH occurs at temperatures ca. 30 °C above its boiling point; the favored stoichiometry for the 1PnX and methanol is 1:1; and phase separation is observed at the SmA2 f K transitions of 1PnCl‚CH3OH but not 1PnBr‚ CH3OH. (Note that in the bromides, methanol molecules are retained in the ionic regions of the phases at both high and low temperatures.)

Attempts to gain additional information about the interactions between hydroxyl groups of methanol or covalently attached hydroxyl groups and the charged centers of the salts by infrared spectroscopy were unsuccessful. No bands in Raman and IR absorption spectra could be ascribed definitively to H bonding or torsional motions of the OH groups (Figures S14-S17 of Supporting Information file).37 However, the quadrupolar splittings in the 2H NMR spectra of the 1P10X‚CD3OD demonstrate that methanol molecules remain oriented within the liquidcrystalline phases. The most probable cause of that orientation is the specific interactions with the ionic head groups, because if none existed, it is difficult to understand how 1 molar equiv of methanol is needed to induce complete transformation of the solid salts to their liquid-crystalline phases. Incorporation of a 2-hydroxyethyl group on phosphorus of the 2PnOHX may also increase the average distance between Xand P+ within one molecule and impose conformational constraints that impede effective interactions of the hydroxyl oxygen atom and its positively charged phosphorus atom (Figure 12). In the latter case, intermolecular interactions between neighboring molecules may be favored. Regardless, several pieces of experimental evidence attest to the effectiveness of the covalently attached hydroxyl groups to loosen the ionic layering. For example, the ∆H and ∆S calculated from DSC measurements of the SmA2 f I transitions of the 1P14Br are larger than those of 2P14OHBr (or 2P14OHCl); comparisons among the 1P10X salts are not appropriate because they are not liquid crystalline and K f I thermodynamic parameters are intrinsically larger in magnitude than those of SmA2 f I transitions. Loosening the specificity of ionic layering can also increase the head group area and, consequently, decrease the d spacings of the lamellae (Note the d spacings of 1P10X‚CH3OH and 2PnOHX in their crystalline and liquid-crystalline phases in Table 3.) and lower the clearing temperatures (1P10X‚CH3OH, Table 1). The flexibility of the long alkyl chains within the ordered phases of the phosphonium salts is a very important factor in determining the temperatures at which the isotropic phases appear; greater conformational freedom results in lower clearing temperatures. Thus, 1PnX have the highest clearing temperatures. An additional factor mentioned above is the entropic constraints imposed on the covalently attached hydroxyl groups of the 2-hydroxyethyl moieties of 2PnOHX. Covalent attachment also requires that the hydroxyl groups remain in the proximity of the charged head group atoms even in the isotropic phases. Empirically, the 2PnOHX have higher clearing temperatures than those of the corresponding 1PnX‚CH3OH, as anticipated, but they do not consistently extend the mesophase temperature ranges. (37) (a) Iwaki, L. K.; Dlott, D. D. J. Phys. Chem. A 2000, 104, 9101-9112. (b) Joesten, M. D.; Schaad, L. J. Hydrogen Bonding; Marcel-Dekker: New York, 1974; p 295.

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Table 5. Comparisons of Reductions in the Lamellar Spacings (d) of “Modified” Phosphonium Salts in Their SmA2 Phases and in d values of the Corresponding 1PnX Solid Phases at Same Temperatures

a

sample

d reductiona (%) in SmA2 phase

T range (°C)

2P10OHCl 1P10Cl‚CH3OH 2P14OHCl 1P14Cl‚CH3OH

9.4-11.2 6.0-11.1 12.9-18.1 17.0-18.6

45.5-74 14-24 71-97.5 70.5-75.5

sample

d reductiona (%) in SmA2 phase

T range (°C)

2P10OHBr 1P10Br‚H3OH 2P14OHBr 1P14Br‚CH3OH

7.2-8.1 4.5-10.0 10.4-12.8 12.0-13.7

50.5-66.5 14.5-33.5 71-97 69-77

Comparisons of liquid-crystalline 2PnOHX and 1PnX‚CH3OH are limited to temperatures within the K2 phase of corresponding 1PnX.

Thus, the greater disorder of methanol molecules around the head groups of the 1PnX induces liquid crystallinity at lower temperatures than in the 2PnOHX. In both series, the salts with the larger and softer bromide anions, allowing more flexible packing, have lower clearing temperatures than the salts with the smaller and harder chloride anions. The Influence of n on Liquid-Crystal Phase Properties. Although incorporation of a hydroxymethyl group in the 2PnOHX salts or addition of a small solute such as methanol (or water) to the 1PnX can change the range of or even induce mesomorphism, the size and type of such perturbations on the P+/X- electronic interactions are limited.13 An even more important variable is the long alkyl chain length (n). Longer alkyl chains require more space (i.e., distance between P+ and X-) to be disturbed sufficiently to initiate liquid crystallinity. Consequently, the phosphonium salts with larger n are expected to have less well-defined ionic layering in their liquid-crystalline phases. A manifestation of this factor is the magnitude of the differences in the d values between the crystalline and liquidcrystalline phases (Table 5). Since the maintenance of P+-Xelectronic interactions limits the extent to which head group areas can change, modifications of the head groups have the smallest impact on the differences in the d values between the crystalline and liquid-crystalline phases of the salts with n ) 18 and the largest impact on the salts with n ) 10. Additionally, the ionic (and H bonding) interactions at the head group regions of layers within crystalline 2P18OHCl must be retained beyond the temperature where the octadecyl chains melt because it has a discernible narrow mesophase; neither the 1P18X‚CH3OH nor the 2P18OHBr salt (each with very strong London dispersion forces among neighboring octadecyl chains but somewhat weaker headgroup interactions) does.

Conclusions The presence of a hydroxyl group near the 1PnX head groups influences enormously the phase behavior of the salts and their derivatives. Some salts which are not mesomorphic develop liquid-crystal phases, and the temperature ranges of those which are intrinsically liquid crystalline are altered (i.e., are amphtropic12). The covalently attached hydroxyl groups play a similar role to that of added methanol molecules in that both appear to decrease the specificity (extended ion pairing6,14) and increase the spatial diversity of the electrostatic interactions between the positively charged phosphorus centers and halide anions within the ionic layers of the lamellar structures. This conclusion is supported by the quadrupolar splitting data from 2H NMR spectra and the necessity of at least 1 molar equiv of methanol to effect complete (isothermal) conversion of a solid 1PnX salt to a liquid

crystal. The presence of the hydroxyl groups (and the methyl or ethylene groups to which they are attached at the ionic layers of the liquid-crystal phases) leads to decreased lamellar thicknesses as a result of the expanded head group areas and, thereby, to more conformations to be explored by the alkyl chains. The influence of the larger head group areas induced by the hydroxylcontaining moieties is expressed most acutely in the salts with the shortest chains explored here because of their weaker London dispersion forces between neighboring chains. Consequently, the difference between the mesophase temperature ranges of the thermotropic 2PnOHX and the amphotropic 1PnX‚CH3OH is much larger for n ) 10 than for n ) 14 when X is the same anion. In addition, the lamellar spacings of the phosphonium bromides are smaller than the corresponding chlorides when analogous phases and values of n are compared as a result of the larger size and softer nature of the bromide and the larger head group areas that it induces. These observations constitute the basis for approaches to the design of other ionic liquid crystals, and they demonstrate the importance of secondary interactions in inducing or attenuating liquid crystallinity.6 Finally, we note that the magnitudes of the order parameters for the 1PnX‚CH3OH salts (as deduced from the 2H NMR data with deuterated methanol) are very low compared to values usually encountered for thermotropic smectic phases and that the salts are surprisingly easy to align in magnetic fields. Both of these attributes make our liquid-crystalline phases potentially useful media to determine the molecular geometries and orientations of a wide variety of solute molecules (using their residual dipolar coupling constants25b,27,31a,35) and to effect selective transformations of reactive guest molecules.7,38 Results from experiments which explore the limits of the applications of the 1PnX‚CH3OH and 2PnOHX phases will be reported in the future. Acknowledgment. The Georgetown group is grateful to the U.S. National Science Foundation for financial support. Supporting Information Available: Synthetic procedures for 2P10OHCl and 2P10OHBr, instrumental analyses for 1PnX, thermal gravimeter analyses for 1PnX‚CH3OH, X-ray diffractograms and lamellar spacings at various temperatures and in different phases, the change of lamellar spacings with temperature in each phase, and optical micrographs, X-ray diffraction patterns, and infrared and Raman spectra for compounds investigated. This material is available free of charge via the Internet at http://pubs.acs.org. LA703175X (38) Weiss, R. G. In Photochemistry in Organized and Constrained Media; Ramamurthy, V., Ed.; VCH Publishers: New York, 1991; Chapter 14.