J O U R N A L O F C H E M I S T R Y Materials Ionic liquid crystals: hexafluorophosphate salts Charles M. Gordon,*a John D. Holbrey,b Alan R. Kennedya and Kenneth R. Seddonb aDepartment of Pure and Applied Chemistry, University of Strathclyde, Thomas Graham Building, 295 Cathedral Street, Glasgow, UK G1 1XL. E-mail: c.m.gordon@strath.ac.uk bThe QUESTOR Centre, The Queen’s University of Belfast, Stranmillis Road, Belfast, Northern Ireland, UK BT9 5AG Received 5th August 1998, Accepted 19th October 1998 A series of novel hexafluorophosphate salts, based on N,N¾-dialkylimidazolium and substituted N-alkylpyridinium cations, display liquid crystalline behaviour at temperatures above their melting point.The temperature range over which liquid crystalline behaviour is observed increases markedly with increasing alkyl chain length.Alkyl substitution at the 3- and 4-positions on the pyridinium ring results in a decrease in the melting point compared with the equivalent unsubstituted salt, but also leads to a large decrease in the tendency towards liquid crystalline behaviour (or mesogenicity). The salts prepared are fully characterised using a wide variety of techniques, including NMR and IR spectroscopy, DSC, and single crystal X-ray diVraction in the case of 1-dodecyl-3-methylimidazolium hexafluorophosphate. The eVect of preparing mixtures containing diVerent proportions of two cations is also reported.Introduction A large variety of molecular thermotropic liquid crystalline materials have been prepared. In comparison, only a limited range of ionic liquid crystalline species is known.Alkali metal soaps were the first salts identified as displaying this type of behaviour,1 followed more recently by amphiphilic alkylammonium2 and N-alkylpyridinium salts.3 Amphiphilic 1-methyl- 4-alkylpyridinium iodides have also been shown to display thermotropic liquid crystalline behaviour.4 Liquid crystalline and thermochromic behaviour has been observed in 1-methyl- N R H3C N R H3C N R H3C R R = n-C12H25, n-C14H29, n-C16H33, n-C18H37 + 1 + 2 + + 3 4 N N 4-n-alkoxycarbonylpyridinium4 and 1-hexadecyl-4-cyanopyri- Fig. 1 Structures of the organic cations employed. dinium iodide salts.5 Further developments have involved the replacement of the 1-alkyl group with mesogens,6,7 while recent publications have demonstrated thermotropic liquid crystalline phosphate are neutral and extremely hydrophobic.17 By way behaviour in 1-alkyl-4-cholesterylpyridinium,8 1-ethyl-4-(5- of comparison, the eVect of chemical modification of the alkyl-1,3-dioxan-2-yl )pyridinium,9 and 1-alkylstilbazolium cation has been little studied.An increasing chain length in halide salts.10 All of these studies have concentrated on halide the cation will be expected to result in an alteration in the salts; only a few papers report ionic liquid crystalline materials melting point, and an increase in the viscosity and hydrocontaining other anions, notably metallomesogens,11 [MCl4]2- phobicity of the liquids.A further eVect, once chain lengths (M=Co, Ni),12 [ZnBr4]2-,13 [C12H25OSO3]-,14 and hydro- of a suYcient length are present, will be the formation of gentartrate.15 Practical applications of these materials are liquid crystalline phases on melting, as shown by the examples limited, particularly in the case of the simple alkylammonium described in the previous paragraph.salts, by the high melting points of the systems studied. A One ionic liquid which is finding increasing use is 1-butylrecent investigation of transition metal salts based on long 3-methylimidazolium hexafluorophosphate, [bmim][PF6], an chain 1-alkyl-3-methylimidazolium and N-alkylpyridinium cat- easily prepared, almost completely water insoluble liquid, ions showed that these possess relatively low melting points, which has been used as a solvent for two-phase catalysis combined with large mesophase ranges,12 thus showing greater reactions.18 In this study, we report the liquid crystalline potential for further development.properties of some hexafluorophosphate salts of alkyl-substi- Our interest in ionic liquid crystals arose from the study of tuted imidazolium ([R-mim]+ 1), pyridinium ([R-py]+ 2), 3- low melting ionic liquids. Ionic liquids is now a commonly and 4-methylpyridinium ([R-3-Mepy]+ 3 and [R-4-Mepy]+ 4) accepted term for low melting molten salts, and these materials cations,† as indicated in Fig. 1. are finding increasing application as solvents, particularly in Such salts are potentially of great interest as ordered ionic the area of clean technology as controls on conventional solvents, combining the advantages of ionic liquids with those solvents such as chlorinated hydrocarbons become more strin- of liquid crystal solvents.The aim of this study was to identify gent.16 Ionic liquids are prepared by combining bulky organic systems that combined a relatively low melting point with a cations such as 1-butyl-3-methylimidazolium or 1-butylpyridi- large mesophase range. Once the principles influencing the nium with a wide variety of anions.The properties of ionic thermal behaviour of these salts are understood, the intention liquids can be controlled to a large degree by variation in the is to develop more sophisticated systems that can be employed nature of both the cation and the anion. The eVect of altering as ordered solvents. The application of more ‘traditional’ the anion has been quite widely investigated: for example, salts based on aluminium(III ) chloride may be prepared which †Henceforth all cations will be referred to using the shortened form are Franklin acidic or basic, and extremely water sensitive, of the cation name, and the alkyl chain simply by its number of carbon atoms, i.e.[C12-mim]=1-n-dodecyl-3-methylimidazolium. while those based on anions such as triflate and hexafluoro- J.Mater. Chem., 1998, 8, 2627–2636 2627molecular liquid crystalline materials as ordered solvents for Thermal investigations polymerisation19 and stereochemically controlled organic reac- DSC measurements were carried out using a Perkin Elmer tions20 is an area of increasing interest. Only one other related DSC 2 or Perkin Elmer DSC 7.Heating and cooling rates of liquid crystalline salt containing the hexafluorophosphate 10 °Cmin-1 were typically employed. Each sample was pre- anion has been reported: the symmetrically substituted 1,3- heated to its clearing point, allowed to cool to its crystallisation dihexadecylimidazolium hexafluorophosphate, for which point, and then reheated for data collection. Polarised optical transition temperatures and energies were recently reported.21 microscopy (POM) studies on the salts were carried out using an Olympus Vanox microscope under cross-polarised light at Experimental 100× magnification. The samples were placed between two cover slips and heated using a Linkam PR600 hot stage, which Aqueous hexafluorophosphoric acid (60% w/w, ex-Aldrich), permitted control of the temperature to ±0.1 °C.Heating and pyridine, 3-methylpyridine and 4-methylpyridine (ex-Aldrich) cooling rates of 10 °Cmin-1 were once again generally were used as received. 1-Methylimidazole (ex-Aldrich) was employed, except when a very precise determination of trans- purified by distillation from KOH. 1-Chlorododecane, 1- ition temperatures was required, in which case slower rates chlorotetradecane, 1-chlorohexadecane and 1-chlorooctadewere used.As with the DSC measurements, data were only cane (ex-Lancaster or Aldrich) were used as received. Acetonerecorded after the sample had first been heated to its clearing d6 was purchased from Goss Scientific Instruments Ltd. point and then allowed to cool to its crystallisation point.Acetonitrile was dried by distillation from CaH2. Photographs of the mesophase were taken at the same magnification using a Polaroid Micro SLR camera. Synthesis of salts Chloride salts were prepared by mixing equimolar quantities Experimental crystallography of the appropriate amine and chloroalkane in a Carius tube (i ) Powder X-ray diVraction. Powder X-ray diVraction in a dinitrogen-filled drybox.The tube was removed from the studies were made at room temperature with Cu-Ka X-rays, drybox, degassed at -196 °C, sealed under vacuum, and (l=1.542 A° ) using a Siemens D5000 powder diVractometer. heated at 100 °C for 7 days, or until reaction was observed to Data were recorded in the range 2–20° in steps of 0.05°. be complete (the mixture formed a single viscous phase).The tube was then cooled, and opened in the drybox. The crude (ii ) Single crystal X-ray diVraction. Crystals of [C12- solid product was removed, recrystallised from dry CH3CN, mim][PF6] were grown by slow evaporation of a methanolic and then stored in the drybox until use. In general, the halide solution of the salt. Crystal data, data collection and refinement salts became less hygroscopic as the alkyl chain increased in parameters are given in Table 2.Measurements were made at length. The amines employed were 1-methylimidazole, pyri- 123 K with Mo-Ka X-rays, (l=0.71069 A° ), on a Rigaku dine, and 3- and 4-methylpyridine; salts were prepared with AFC7S diVractometer fitted with a graphite monochromator. alkyl chain lengths C12, C14, C16 and C18.Cell dimensions were based on 25 reflections with The hexafluorophosphate salts were prepared by reaction 18.2<2h<34.8°. Intensities, I, were derived from v–2h scans, of the appropriate chloride salt with HPF6 in aqueous solution. and corrections were applied for Lorentz polarisation and As the method was eVectively identical for all salts, the exact absorption eVects, the latter based on averaging several azi- method employed for [C14-py][PF6] will be discussed as a muthal scans.Equivalent intensities were then averaged and representative example: [C14-py]Cl (1.00 g, 3.20 mmol) was unobserved reflections with I<2s(I ) excluded from further dissolved in water (20 cm3), and then cooled to 0 °C in an ice consideration. bath. The mixture was stirred vigorously and 60% w/w HPF6 The structure was solved by direct methods22 and the solution (1.0 cm3, 6.80 mmol) was added using a glass syringe.examination of subsequent diVerence syntheses. All non-hydro- A rapid exothermic reaction then occurred, with the product gen atoms were refined anisotropically. H atoms were pos- forming as an insoluble white solid.This was collected by itioned as found with Biso=1.2Beq of the parent atom. An filtration, washed with a large excess of water to remove any isotropic extinction parameter [5.4(8)×10-7] was also refined. remaining traces of HPF6, then recrystallised from a minimum Final full-matrix, least-squares refinement was on F with w= quantity of methanol, and dried in vacuo. A final yield of 1/s2(F) and converged to give a maximum shift/esd ratio of 0.73 g (54.0%) was obtained. The solubility in methanol 0.003.All calculations were performed on a Silicon Graphics increased with decreasing chain length, generally resulting in Indy R4600 with the teXsan set of programs.23 somewhat lower yields for the shortest chain length products Full crystallographic details, excluding structure factors, and higher yields for the salts with longer alkyl chains.The have been deposited at the Cambridge Crystallographic Data pure recrystallised product formed as white plates, which were Centre (CCDC). See Information for Authors, J. Mater. dried in a vacuum desiccator for at least 24 h. The purity of Chem., 1998, Issue 1. Any request to the CCDC for this all the salts was checked by 1H NMR spectroscopy (in acetonematerial should quote the full literature citation and the d6), microanalysis (see Table 1) and DSC (see later).reference number 1145/1046. Mixtures of two salts were prepared by carefully weighing See http://www.rsc.org/suppdata/jm/1998/2627/for crystal- out the appropriate molar ratios of [C16-mim][PF6] and [C16- lographic details in cif format.py][PF6]. The mixtures were then ground thoroughly, heated to a temperature well above their clearing points, and then Results allowed to cool. This process was repeated three times for the resulting solids, by which stage it was assessed that broadly As stated above, N-alkyl substituted 1-methylimidazolium, homogeneous mixtures had been prepared. pyridinium, 3- and 4-methylpyridinium hexafluorophosphate salts were prepared with alkyl chain lengths of C12, C14, C16, Analytical methods and C18.Halide and tetrachlorometallate salts of some of these cations have been shown previously to display liquid 1H NMR spectra were recorded in acetone-d6 solution on 400 MHz Bruker AMX400 or 250 MHz Bruker WM250 FT crystalline phases at temperatures above their melting points.3,12 The products were prepared in aqueous solution by NMR spectrometers.IR spectra were run as KBr disks on a Nicolet Impact 400D FTIR spectrometer. FAB mass spec- metathesis of the appropriate organic cation halide with HPF6. The white crystalline solid products were insoluble in water, trometry was performed on a JEOL JMS-AX505HA instrument, using a glycerol matrix.and were thus simply collected by filtration. Purification was 2628 J. Mater. Chem., 1998, 8, 2627–2636Table 1 Microanalytical data for anhydrous [Q][PF6 ] salts Q+ C % found (calc.) H % found (calc.) N % found (calc.) C12-mim 48.6 (48.4) 8.2 (7.9) 7.0 (7.1) C14-mim 51.0 (50.9) 8.6 (8.3) 6.6 (6.6) C16-mim 53.1 (53.1) 8.9 (8.7) 6.1 (6.2) C18-mim 54.9 (55.0) 9.1 (9.0) 5.7 (5.8) C12-py 52.0 (51.9) 7.9 (7.7) 3.5 (3.6) C14-py 54.3 (54.1) 8.5 (8.1) 3.3 (3.3) C16-py 55.7 (56.1) 8.5 (8.5) 3.0 (3.1) C18-py 57.6 (57.8) 9.0 (8.9) 3.0 (2.9) C12-3-Mepy 53.0 (53.1) 8.2 (7.9) 3.4 (3.4) C14-3-Mepy 55.2 (55.2) 8.3 (8.3) 3.1 (3.2) C16-3-Mepy 57.0 (57.0) 8.7 (8.7) 3.0 (3.0) C18-3-Mepy 58.4 (58.6) 9.0 (9.0) 2.9 (2.9) C12-4-Mepy 51.8 (53.1) 7.9 (7.9) 3.5 (3.4) C14-4-Mepy 55.3 (55.2) 8.4 (8.3) 3.1 (3.2) C16-4-Mepy 57.0 (57.0) 8.8 (8.7) 3.0 (3.0) C18-4-Mepy 58.9 (58.6) 9.3 (9.0) 2.7 (2.9) Table 2 Crystal parameters for [C12H25-mim][PF6] Table 3 Summary of DSC data and polarising microscope measurements obtained for [Q][PF6] salts under studya Formula C16H31F6N2P Formula weight 396.40 [Q]+ T /°C Energy/kJ mol-1 Transition Crystal system Monoclinic Space group P21/a C12-mim 60 27.3 C�I C14-mim 74 30.9 C�SA a/A° 9.175(2) b/A° 9.849(3) 77 0.4 SA�I C16-mim 75 37.5 C�SA c/A° 22.197(4) b/° 94.132(18) 125 0.5 SA�I C18-mim 80 43.8 C�SA U/A° 3 2000.7(8) Z 4 165 1.1 SA�I C12-py 49 6.2 C�C1 Dc/g cm-3 1.32 Crystal size/mm 0.70×0.40×0.05 89 3.9 C1�C2 104 22.4b C2�C3 Crystal description Colourless plate m/mm-1 0.192 106 22.4b C3�I C14-py 53 5.8 C�C1 2h range/° 5 to 53 Rmerge 0.037 103 4.0 C1�C2 109 16.5 C2�C3 No.of reflections measured 4676 Unique data 4401 124 9.6 C3�I C16-py (86) (-4.9) Observed data 2497 No. of parameters 227 104 (98) 27.7 (-20.9) C�C1 126 8.4 C1�SA Absorption range 0.94–1.00 R, Rw 0.0394, 0.0436 138 0.8 SA�I C18-py (89) (-3.6) S 1.339 Max. Dr/e A° -3 0.219 107 (102) 32.3 (-22.2) C�C1 126 7.6 C1�SA Min.Dr/e A° -3 -0.235 176 1.2 SA�I C12-3-Mepy 55 28.2 C�I C14-3-Mepy 68 25.8 C�I achieved by recrystallisation from methanol. Using this C16-3-Mepy (58) (-35.9) (SA�C) method, the only by-product is hydrochloric acid, which 74 (61) 36.0 (-0.5) C�I (I�SA) remains in the aqueous solution. These salts are thus guaran- C18-3-Mepy 87 38.0 C�SA teed to be free of any traces of metal ions.Residual halide 94 0.5 SA�I C12-4-Mepy 56 20.3 C�I ions were removed by extended washing with water. C14-4-Mepy 71 28.1 C�I Contamination of low melting salts containing anions such as C16-4-Mepy (55) (-34.8) (SA�C) [BF4]- and [CH3CO2]- has been a problem due to the 75 (60) 36.7 (-1.1) C�I (I�SA) requirement to use metal salts, typically silver and lead respect- C18-4-Mepy (77) (-41.8) (SA�C) ively, in their preparation.24,25 88 (84) 33.6 (-0.4) C�I (I�SA) None of the salts containing alkyl chains shorter than C12 aThe entries in parentheses indicate transitions observed on cooling display liquid crystalline behaviour.26 The presence and tem- where these are significantly diVerent from those observed on heating.perature range of liquid crystallinity proved to vary greatly bPeaks were tions, and so each cation will be discussed would not be recorded.separately. The data referred to in the following sections are collected in Table 3. In general the behaviour on cooling was very similar to that observed on heating, except that solidifi- Mepy][PF6], [C16-4-Mepy][PF6] and [C18-4-Mepy][PF6], where the phase is only observed monotropically.cation points were always lower in temperature than melting points, thus giving larger mesophase ranges for the cooling Characteristic POM textures obtained on cooling of isotropic liquids are shown in Fig. 2 for [C16-mim][PF6] and [C16- cycles. The value of the solidification point was very dependent on the cooling rate employed.Therefore, only data recorded py][PF6]. These textures are typical of those observed, which in all cases showed spontaneous formation of homeotropic on heating cycles are reported except for the few examples where monotropic mesophases were observed, which are domains with the optical axis perpendicular to the slide.27 In the POM of [C16-mim][PF6] [Fig. 2(a)], small but characteristic recorded in parentheses in Table 3. The mesophases observed were identified using POM and DSC. For all of the meso- focal conic textures can also be seen in the dark homeotropic regions. The similarity of the textures observed for [R-mim]+ morphic examples, a single enantiotropic smectic mesophase was observed, except for the methylpyridinium salts [C16-3- and [R-py]+ salts indicates that the same type of mesophase J.Mater. Chem., 1998, 8, 2627–2636 2629Fig. 3 DSC traces observed on heating and cooling of (a) [C18- mim][PF6], and (b) [C18-py][PF6]. The peak labels indicate the type of phase transition occurring: C=solid < solid; S=solid < smectic; I=smectic < isotropic. Heating and cooling rates of 10 °Cmin-1 were employed.Fig. 2 Textures observed using POM (100× magnification) of (a) [C16-mim][PF6] at 98° C, and (b) [C16-py][PF6] at 121 °C. Both photographs were taken after cooling from the isotropic phase. is observed for both materials. The textures shown are typical for all salts that displayed liquid crystalline behaviour. In light of the evidence of the POM observations, the mesophases are assigned as smectic A (SA). Further support for the assignment of a smectic A mesophase comes from the fact that the [PF6]- salts displayed contact miscibility in the liquid crystalline state with the corresponding [CoCl4]2- analogues.For these studies small amounts of the two diVerent salts were placed slightly apart between two cover slips.The samples were heated on the microscope heating stage to above their melting points and then rapidly cooled to avoid excessive mixing. Further heating and cooling resulted in the observation of a uniform mesophase across the meeting point of the two samples. Such behaviour is only observed when the same mesophase structure is present. DSC heating and cooling curves for [C18-mim][PF6] and [C18-py][PF6] are illustrated in Fig. 3, from which it can be seen that the transition from the solid to the liquid crystalline phase was of much higher energy than the transition from the liquid crystal to the isotropic liquid. (i) 1-Alkyl-3-methylimidazolium salts The dodecyl-substituted salt showed no mesomorphic behaviour either on heating or cooling, while all the other salts displayed enantiotropic mesophases.The melting and clearing points are displayed graphically in Fig. 4(a). The close agreement between the clearing point on heating and the point at Fig. 4 Plots showing the melting and clearing temperatures observed which the mesophase reforms on cooling is further evidence on heating of (a) [Cn-mim][PF6], (b) [Cn-py][PF6], (c) [Cn-3- Mepy][PF6], and (d) [Cn-4-Mepy][PF6].for the purity of the salts. The melting points of the salts 2630 J. Mater. Chem., 1998, 8, 2627–2636increased only slightly with increasing alkyl chain length, while melting point recorded (55 °C) was slightly low due to the presence of trace amounts of impurities, although no impurity the clearing point increased markedly, ultimately giving a mesophase temperature range of 84 °C on heating and 93 °C peaks could be seen in the 1H NMR spectrum. 1H NMR spectroscopy provided a convenient method for confirming on cooling for the octadecyl substituted salt. Thus, the liquid crystalline range increased greatly with increasing alkyl chain the purity of the cation, and that no solvent residues remained in the salts. The IR spectra simply allowed confirmation of length.the presence of both the appropriate cation and the [PF6]- (ii) 1-Alkylpyridinium salts anion in the salts prepared. The anion displays a characteristic strong peak at 830 cm-1. No mesomorphic behaviour was observed for either the C12- Positive ion FAB mass spectrometry provided a convenient or [C14-py]+ salts. The melting points of all [R-py]+ salts were method for unequivocal characterisation of the salts.All of higher than those of the equivalent [R-mim]+ salts, concurring the data are collected in Table 4. Although the most intense with observations made previously for ionic liquids based on peak was the isolated cation, the next most intense peak such cations.29 The longer chain salts (C16, C18) did display generally corresponded to the species {[Q]2[PF6]}+ (Q= mesomorphic behaviour, and as was the case with the [Rorganic cation).The intensity of such peaks was typically ca. mim]+ salts the clearing temperature increased greatly on 5% that of the Q+ peak, with the intensity generally greater increasing the alkyl chain length by only two carbon atoms. for the shorter chain salts. This type of clustering phenomenon In contrast, the melting point only increased from 124 to has been observed before in similar organic cation salts.30 The 126 °C on increasing the alkyl chain length from 14 to 18.The data for [C18-mim][PF6] was slightly diVerent, with the second melting and clearing points of these species are displayed most intense mass peak occurring at m/z=574 (m/z for graphically in Fig. 4(b). Unlike the [R-mim]+ salts, however, {[C18-mim]2[PF6]}+ is expected at 816). This may correspond solid phase transitions were observed at temperatures below with the species {[C18-mim]3[PF6]}2+ (expected m/z=575), the melting point in all of the [R-py]+ salts studied. It can be although why such a species is only observed for this salt seen from Table 3 that the solid phase behaviour is quite is unclear. diVerent for the two non-liquid crystalline salts (C12 and C14) compared with those which do display liquid crystalline behav- The crystal structure of [C12-mim][PF6] iour (C16 and C18).In the latter case only one solid–solid phase transition was observed on heating and two on cooling, All of the salts investigated were crystalline, and in the case as shown in Fig. 3(b), while in the former there were three of [C12-mim][PF6] it proved possible to produce crystals of transitions both on heating and cooling. suYcient quality for single crystal X-ray diVraction. This is the first long chain 1-alkyl-3-methylimidazolium salt whose (iii) 1-Alkyl-3-methylpyridinium salts crystal structure has been reported, although the structure of 1,3-didodecylbenzimidazolium chloride was reported 3-Methylpyridinium is a more bulky headgroup than the simple pyridinium ring; this was expected to give the dual recently.21 The only other related salt for which a crystal structure has been determined is 1-ethyl-3-methylimidazolium eVects of lowering the melting points and reducing the tendency towards mesomorphic behaviour.This proved indeed to be hexafluorophosphate.31 The much shorter alkyl chain in this species means that the structure is quite diVerent from that the case, with the C16 salt only displaying a liquid crystalline phase on cooling (monotropic behaviour), and over a tempera- reported here. Furthermore, almost all other amphiphilic salts for which crystal structures have been determined are simple ture range of just 3 °C.The C18 salt, however, shows enantiotropic behaviour over a temperature range of ca. 10 °C. This halide salts, and this is one of the first containing a [PF6]- anion. information, and the behaviour observed for the shorter chain salts, is summarised in Table 3 and Fig. 4(c). The C12 and C14 The crystal structure of [C12H25-mim][PF6] consists of discrete cations (Fig. 5) and anions separated by at least van der salts display no mesomorphism, and simply melt directly to the isotropic liquid. Unlike the pyridinium salts, there are no Waals distances (see Fig. 6). The closest contact is 2.950(3) A° for F(2),C(2)*, (*x-1, y, z). Selected geometric parameters solid–solid phase transitions. are given in Table 5 and all are close to the expected values.(iv) 1-Alkyl-4-methylpyridinium salts The imidazolium ring is completely planar, within experimental error, and the bond lengths are very close to those observed As was the case for the [R-3-Mepy]+ salts discussed above, in other 1-alkyl-3-methylimidazolium salts.32 The straight the presence of a 4-methyl substituent on the pyridinium ring chain nature of the alkyl group is disrupted resulted in a lowering of the melting point and a reduction in close to the ring where it adopts a bent conformation, as the stability of the mesophase relative to the equivalent pyridinshown by the torsion angles N(2)–C(5)–C(6)–C(7), ium salt.In this case, both the C16 and C18-substituted salts C(5)–C(6)–C(7)–C(8) and C(6)–C(7)–C(8)–C(9) of displayed only monotropic smectic phases, while the C12 and -66.7(3), 176.4(2) and 60.2(3)° respectively.All other carbon C14 salts showed no mesophases. The other information is chain torsion angles approach 180°. The chain configuration collected in Table 3 and Fig. 4(d). These data show that the and the lack of any disorder in the structure appear to be a melting points for each alkyl chain length are almost identical consequence of the interdigitated molecular packing. The twist to the analogous [R-3-Mepy]+ salt.A further feature in in the alkyl chain occurs over a larger number of carbon common with the [R-3-Mepy]+ salts was the absence of phase atoms than is observed in the [(C12H25)2-benzimidazolium]Cl transitions in the solid phase. salt. The cation in the latter has a geometry described as being like a ‘two-legged stool’,21 whereas the [C12H25-mim]+ cation Characterisation could be described as having a spoon-shaped structure, as illustrated in Fig. 5. The X-ray powder diVraction pattern for Characterisation of the anhydrous salts was largely straightforward, being carried out initially using CHN analysis, 1H NMR the [C12H25-mim][PF6] showed the same unit cell and confirms that the single crystal is typical of the bulk synthetic sample.and IR spectroscopy. CHN analytical data for all salts prepared are summarised in Table 1, all being satisfactory except The crystal structure of the salt was similar to that of liquid crystalline alkylammonium and alkylpyridinium salts reported for [C12-4-Mepy][PF6], for which the carbon analysis was slightly low.This salt had proved very troublesome to purify previously.33,34 It consists of sheets of imidazolium rings and hexafluorophosphate ions, separated by interdigitated satisfactorily by recrystallisation as an oil formed on dissolving the crude solid in methanol. It is possible, therefore, that the alkyl chains; the unit cell is shown in Fig. 6, while Fig. 7 J. Mater. Chem., 1998, 8, 2627–2636 2631Table 4 Peaks observed in the positive ion mass spectra of [R-mim]+, [R-py]+, [R-3-Mepy]+ and [R-4-Mepy]+ salts of [PF6]- Salt m/z (relative intensity) Assignment [C12H25-mim][PF6] 251 (100) [C12H25-mim]+ 648 (14) {[C12H25-mim]2[PF6]}+ [C14H29-mim][PF6] 279 (100) [C14H29-mim]+ 704 (6) {[C14H29-mim]2[PF6]}+ [C16H33-mim][PF6] 307 (100) [C16H33-mim]+ 760 (3) {[C16H33-mim]2[PF6]}+ [C18H37-mim][PF6] 335 (100) [C18H37-mim]+ 574 (7) {[C18H37-mim]3[PF6]}2+ 816 (1.5) {[C18H37-mim]2[PF6]}+ [C12H25-py][PF6] 248 (100) [C12H25-py]+ 642 (10) {[C12H25-py]2[PF6]}+ [C14H29-py][PF6] 276 (100) [C14H29-py]+ 698 (6) {[C14H29-py]2[PF6]}+ [C16H33-py][PF6] 304 (100) [C16H33-py]+ 754 (4) {[C16H33-py]2[PF6]}+ [C18H37-py][PF6] 332 (100) [C18H37-py]+ 809 (4) {[C18H37-py]2[PF6]}+ [C12H25-3-Mepy][PF6] 262 (100) [C12H25-3-Mepy]+ 670 (7) {[C12H25-3-Mepy]2[PF6]}+ [C14H29-3-Mepy][PF6] 290 (100) [C14H29-3-Mepy]+ 726 (9) {[C14H29-3-Mepy]2[PF6]}+ [C16H33-3-Mepy][PF6] 318 (100) [C16H33-3-Mepy]+ 782 (3.5) {[C16H33-3-Mepy]2[PF6]}+ [C18H37-3-Mepy][PF6] 346 (100) [C18H37-3-Mepy]+ 837 (3.5) {[C18H37-3-Mepy]2[PF6]}+ [C12H25-4-Mepy][PF6] 262 (100) [C12H25-4-Mepy]+ 670 (6) {[C12H25-4-Mepy]2[PF6]}+ [C14H29-4-Mepy][PF6] 290 (100) [C14H29-4-Mepy]+ 726 (8) {[C14H29-4-Mepy]2[PF6]}+ [C16H33-4-Mepy][PF6] 318 (100) [C16H33-4-Mepy]+ 782 (3.5) {[C16H33-4-Mepy]2[PF6]}+ [C18H37-4-Mepy][PF6] 346 (100) [C18H37-4-Mepy]+ 837 (5) {[C18H37-4-Mepy]2[PF6]}+ interactions appear to be largely coulombic. This is consistent with earlier work which has shown that hydrogen-bonding is not observed in these and related systems when the charge density (r) on the halide atoms of the anion is <1, following the relation r=z2/x where z is the overall charge on the ion and x is the number of halide atoms in the complex.35 Powder X-ray diVraction studies Fig. 5 Structure of the unique 1-dodecyl-3-methyl cation in [C12- Powder X-ray diVraction studies were carried out on all of mim][PF6].the salts in order to gain further structural information. In all cases, the peak with the lowest value of 2h displayed the highest intensity. The d-spacing calculated from this peak increased relatively regularly with increasing alkyl chain length, as listed in Table 6. In addition, for each class of cation the patterns obtained were similar for all alkyl chain lengths, although with diVerent values of 2h.Thus all salts for each cation are isostructural. These data also indicate that the structures in all cases involve intercalated layers of cations as found for the structure of [C12H25-mim][PF6] discussed above, with a layer spacing corresponding to the lowest angle peak. The very small values of 2h for these peaks mean that there is some margin for error in the layer separation values obtained, owing to limits in the resolution of the diVractometer employed.Assuming that the values are correct to ±0.025°, the layer separations can be regarded as accurate to ±0.3 A° . Fig. 6 Unit cell of [C12-mim][PF6]. Thus the layer separation calculated from powder X-ray diVraction data for [C12H25-mim][PF6] (22.4 A° ) is equivalent demonstrates the interdigitated pattern.Of note is the stepped to that obtained from the single crystal data [22.197(4) A° ], to structure, with the alkyl chains tilted relative to the layers of within experimental error. cations and anions. The spacing between each layer was 22.197(4) A° . Mixtures of salts—binary systems The closest cation C(2),F–PF5 contact (2.950 A° ) is slightly shorter than the analogous closest contact taken from the In an attempt to reduce the melting point of the salts, preferably without decreasing their liquid crystalline tempera- crystal structure of the short chain 1-ethyl-3-methylimidazolium hexafluorophosphate salt (3.206 A° ).31 However, in both ture range, an investigation of the eVect of mixing the salts [C16-mim][PF6] and [C16-py][PF6] was carried out.Such cases, the contacts are close to the van der Waals distance and 2632 J. Mater. Chem., 1998, 8, 2627–2636Table 5 Selected interatomic distances (A° ) and torsion angles (°) for X N(1)–C(1) 1.468(3) N(1)–C(2) 1.322(3) N(1)–C(3) 1.373(3) N(2)–C(2) 1.326(3) N(2)–C(4) 1.374(3) N(2)–C(5) 1.477(3) C(3)–C(4) 1.334(4) P(1)–F(1) 1.591(2) P(1)–F(2) 1.610(2) P(1)–F(3) 1.593(2) P(1)–F(4) 1.602(2) P(1)–F(5) 1.584(2) P(1)–F(6) 1.599(2) N(2)C(5)C(6)C(7) -66.7(3) C(2)N(2)C(5)C(6) 112.4(2) C(4)N(2)C(5)C(6) -66.5(3) C(5)C(6)C(7)C(8) 176.4(2) C(6)C(7)C(8)C(9) 60.2(3) N(2)C(2)N(1)C(1) -176.8(2) procedure was repeated three times, by which stage it was assumed that a homogeneous mixture had been prepared.The melting and clearing points of the binary systems were then investigated by POM and DSC. In all cases a mesophase was observed which was of the same type as that of the individual components, and in no cases was biphasic behaviour observed. The data obtained are summarised in Fig. 8 and Table 7, where it can be seen that although the clearing point varied smoothly as the composition was changed, the melting point varied little between pure [C16-mim][PF6] and the 75% [C16-mim][PF6]–25% [C16-py][PF6] mixture.Thus, this mixture has a larger mesophase range (55 °C) than that of the pure [C16-mim][PF6] (49° C). It should also be noted that the melting transition for the mixed salts was not a sharp transition as observed for the pure salts, but was spread over a range of several degrees.This was reflected in broad peaks in the DSC traces obtained for the 50%–50% and 25% [C16- mim][PF6]–75% [C16-py][PF6] mixtures. It was disappointing that no depression in the melting point was observed, although this may be a reflection of the similarity in the structure of the cations. One additional benefit of carrying out the studies on the mixed salts, however, is that it allowed confirmation of the fact that the mesophase formed was the same for the [Rmim]+ and the [R-py]+ salts.This arises from the ‘miscibility Fig. 7 Overall structure of [C12-mim][PF6] showing the interdigitation and the tilted alkyl chains. methods are routinely used to lower the melting point of both molten salts and molecular liquid crystalline materials, by the formation of eutectic mixtures.The mixtures were prepared Fig. 8 Plot showing the variation in melting and clearing point on by grinding together various proportions of the two constitu- heating salts prepared from various proportions of [C16-mim][PF6] and [C16-py][PF6]. ents, followed by melting, re-cooling, and grinding again. This Table 6 Layer separations obtained from powder X-ray diVraction measurements Layer separation/A° Chain length (n) [R-mim]+ [R-py]+ [R-3-Mepy]+ [R-4-Mepy]+ 12 22.4 22.2 23.5 23.0 14 24.2 23.9 25.9 24.9 16 26.5 26.2 27.4 27.1a 18 27.7 27.4 29.1 27.4 aPoor quality data.J. Mater. Chem., 1998, 8, 2627–2636 2633Table 7 Transitions observed in on heating mixtures of [C16-mim][PF6] comes from the textures observed using POM, as illustrated and [C16-py][PF6], and transition energies obtained from DSC in Fig. 2. As has been stated in the previous section, these measurements were broadly similar for all salts, and were typical of smectic A phases. Smectic A mesophases have previously been ident- mol% [C16-py][PF6] T/°C DHa /kJ mol-1 Transition ified for other amphiphilic materials, for example [Cn-4- 100 104 27.7 C�C1 Mepy]Br (n=16, 18, 22) and [C22-4-Mepy]I.3,37 126 8.4 C1�S Previous investigations of the thermal behaviour of salts 138 0.8 S�I with interdigitated structures suggest that formation of the 75 72L C�C1 liquid crystalline phases results from melting of the alkyl 98K 32.5b C1�S chains, while the ionic layers between remain ordered until the 138 0.7 S�I clearing point.X-Ray studies have shown that in many cases 50 76L C�C1 89K 31.1b C1�S the layer separation actually decreases on moving from the 135 0.8 S�I crystalline solid to the smectic phase. For example, solid n- 25 77 32.2 C�S decylammonium chloride has a layer spacing of 28.1 A° in its 132 0.7 S�I solid phase, and 24.6 A° in its first smectic phase.2 The Raman 0 75 37.5 C�S spectrum of the same compound shows clear signs of loss of 125 0.5 S�I order in the alkyl chains as the temperature is increased.38 aCalculated using an averaged molecular mass for the relative pro- It is possible that the tilted orientation of the alkyl chains portion of the two constituents.bOverlapping of peaks prevented relative to the unit cell observed in the crystal structure of calculation of individual transition enthalpies. [C12-mim][PF6] gives a clue as to the structure of the mesophase in these systems.This solid state arrangement can be compared with the smectic C liquid crystal phase, as all of the alkyl rule’, which states that ‘all liquid crystalline modifications which exhibit an uninterrupted series of mixed crystals in chains are tilted at an angle of ca. 57° to the ab plane containing the anions and the cationic head groups. The binary systems without contradiction can be marked with the same symbol’.36 thermodynamic ordering of liquid crystalline phases allows only the smectic A and nematic phases at higher temperatures than smectic C phases. The relatively large enthalpy (and thus Discussion entropy) change on melting indicates a considerable change in the structure of the system.Thus, it may be postulated that In general, with increasing alkyl chain length the melting points increased only modestly. In the case of the liquid the mesophase structure involves conformational melting of the alkyl chains, accompanied by loss of the tilted orientation crystalline [R-mim]+ and [R-py]+ salts, however, the clearing temperature increased dramatically with increasing chain as indicated in Fig. 9, giving a smectic A phase. Further evidence can only be gained by use of variable temperature length. The widest mesophase range was observed for the salt [C18H37-mim][PF6], from 80 to 165 °C on heating, with an low angle X-ray studies of the salts, unavailable for this study. The enthalpy of the smectic–isotropic transition in the [R- even larger range on cooling (as was observed in all cases).The temperature at which texture was observed on cooling 3-Mepy]+ salts was much smaller than the value for the equivalent pyridinium salt, suggesting that the methyl group from the isotropic liquid (i.e. formation of a liquid crystalline phase) was almost identical to the clearing point in pure was lowering the stability of the mesophase.It is notable in this series that, unlike the [R-py]+ salts, the melting point samples. The solidification temperature varied according to the rate of cooling employed, owing to the supercooling often increases steadily through the series n=12–18. Similar behaviour was noted for the [R-4-Mepy]+ salts, although the observed for the mesophase–solid transition in liquid crystalline materials. The [R-3-Mepy]+ and [R-4-Mepy]+ salts dis- mesophases observed for these compounds were monotropic in nature.Overall, the widest liquid crystalline ranges were played liquid crystalline behaviour only over very small temperature ranges, and for the C16 and C18 salts only. In the observed for the [R-mim]+ salts, followed by those for the [Rpy]+ salts, while the [R-3-Mepy]+ and [R-4-Mepy]+ salts case of the [R-4-Mepy]+ salts the mesophases were exclusively monotropic, while for the [R-3-Mepy]+ salts the C16 salt showed the least tendency to form mesophases.A similar phenomenon was observed for liquid crystalline salts based displayed monotropic behaviour and the C18 salt enantiotropic behaviour. on pyridinium and ethylpyridinium halide salts containing Nsubstituted mesogenic groups.7 It is clear that the [R-mim]+ The data in Table 3 clearly indicate that the enthalpy of melting (DHfus) was always much larger than the enthalpy of salts, whose liquid crystalline behaviour has been little studied to date, present considerable advantages compared with the the clearing transition (DHclear).This is a common observation in thermotropic liquid crystals. The only other similar liquid pyridinium salts for future development in this area. One intriguing observation, which can be seen from the crystalline [PF6]- salt reported to date is [(C16H33)2bzm][PF6] (bzm=benzimidazolium). This forms a lamellar phase at data in Table 3, was that all of the pyridinium salts displayed solid phase polymorphism at temperatures below the melting 103.3 °C on cooling with an enthalpy of 2.0 kJ mol-1, followed by solidification at 61.3 °C with an enthalpy of 46.1 kJ mol-1.21 point.None of the other salts displayed this phenomenon. There was a noticeable diVerence between the non-liquid The equivalent values for [C16-mim][PF6] are the formation of a smectic phase at 122 °C with an enthalpy of 0.5 kJ mol-1, crystalline C12 and C14 sa transitions below their melting point both on heating and cooling, and and solidification at 62 °C with an enthalpy of 37.2 kJ mol-1.Thus, the two long alkyl chains in the benzimidazolium salt the liquid crystalline C16 and C18 salts which displayed just one transition before melting, and two transitions on cooling.result in a less stable mesophase than the single one in the [C16-mim]+ salt, although in the former case the transition These transitions have been shown to appear consistently on repeated heat–cool cycles, indicating that they are not simply enthalpy is somewhat higher. One noticeable observation from the DSC data was that the enthalpies of the melting (DHfus) artefacts. In the case of the C16 and C18 salts, the lowest temperature solid phase transition was ofmuch higher enthalpy and clearing (DHclear) transitions increased with increasing chain length.This suggests that chain interdigitation is of than the higher temperature one. This suggested that a large degree of disorder was occurring in the first transition, presum- increasing importance in stabilising the mesophase as the chain length increases.ably with the formation of a solid of structure more similar to that of the liquid crystalline phase. Clearly variable tempera- In the absence of variable temperature low-angle X-ray studies, the principle evidence for the mesophase structure ture powder X-ray diVraction or vibrational spectroscopic 2634 J.Mater. Chem., 1998, 8, 2627–2636analogous Cl-, [CoCl4]2- and [NiCl4]2- salts.12 The textures observed using POM were very similar, and were interpreted as indicating formation of a smectic A mesophase. The salts based on the pyridinium cation displayed higher melting points and smaller mesophase ranges than imidazolium salts of equivalent alkyl chain length.The pyridinium salts displayed interesting polymorphism, however, with phase transitions observed at temperatures below the melting points of the salts. Both this phenomenon and the structures of the smectic A mesophases would merit further investigation using variable temperature small angle X-ray techniques unavailable in the present study. Substitution of the pyridinium ring at the 3- and 4-positions with methyl groups reduced the melting points of the salts, but reduced the tendency to form liquid crystalline phases still further.Mixtures formed by combining salts with imidazolium and pyridinium cations did not result in depression of the melting point below that of the pure imidazolium salt. A slightly larger mesophase range was obtained for one of the mixtures, suggesting that further investigations, perhaps with cations of greater structural diVerence, might yield interesting results. The intention in future studies is to investigate how the mesophase range and stability are aVected by use of a mesogenic substituent in place of the simple alkyl chains.It is hoped that such a modification will increase the stability of the mesophase, important in any application, without signifi- cantly increasing the melting point of the salts.We also hope to extend the range of anions employed to investigate the eVect this has on mesophase formation. Acknowledgements We would like to thank Dr John Liggat, Margaret Adams and the Queen’s University of Belfast School of Chemistry analytical services for assistance with DSC measurements and CHN analysis; Dr Cecil Burdett and Mark Nieuwenhuyzen for assistance with powder X-ray diVraction studies; the mass spectrometry services of the University of Strathclyde and the Queen’s University of Belfast. We would also like to thank the University of Strathclyde (C.M.G.and A.R.K.), the ERDF N N N N N N N N N N N N N N N N N N N N N N N N (a) (b) Technology Development Programme and the QUESTOR Fig. 9 Schematic representation of (a) the cation structure in the solid Centre (J.D.H.) for financial support, and the EPSRC and state of the mim salts, and (b) a possible cation structure of the Royal Academy of Engineering for the award of a Clean smectic A phase, showing the eVect of conformational melting of the Technology Fellowship (to K.R.S.). alkyl chains.[PF6]- ions have been omitted for clarity. studies would help elucidate any changes of structure. Such References information might also provide valuable information on the 1 A. Skoulios and V. Luzzati, Acta Crystallogr., 1961, 14, 278. structure of the liquid crystalline phases. Behaviour of this 2 See, e.g.: V. Busico, P.Cernicchiaro, P. Corradini and type has been noted previously for liquid crystalline pyridinium M. Vacatello, J. Phys. Chem., 1983, 87, 1631. salts with dodecyl sulfate anions, although the authors also 3 See, e.g.: C. G. Bazuin, D. Guillon, A. Skoulios and J.-F. Nicoud, Liq. Cryst., 1986, 1, 181. did not attempt to explain the phenomenon.14 4 J. J. H. Nusselder, J. B. F. N. Engberts and H.A. van Doren, Liq. Cryst., 1993, 13, 213. Conclusions 5 Y. Kosaka, T. Kato and T. Uryu, Liq. Cryst., 1995, 18, 693. 6 S. Ujiie and K. Iimura, Chem. Lett., 1990, 995. In this paper we have described the properties of a series of 7 D. Navarro-Rodriguez, Y. Frere, P. Gramain, D. Guillon and novel hexafluorophosphate salts, many of which display liquid A. Skoulios, Liq. Cryst., 1991, 9, 321; E.Bravo-Grimaldo, D. Navarro-Rodriguez, A. Skoulios and D. Guillon, Liq. Cryst., crystalline properties on melting. The salts are closely related 1996, 20, 393. to ionic liquids which are finding increasing use as reaction 8 Y. Z. Yousif, A. A. Othman,W. A. Al-Masoudi and P. R. Alapati, solvents. 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