Piperidinium, Piperazinium and Morpholinium Ionic Liquid Crystals

Jun 18, 2009 - Karel Goossens , Kathleen Lava , Christopher W. Bielawski , and Koen ... Thomas Cardinaels , Kathleen Lava , Karel Goossens , Svetlana ...
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9506

J. Phys. Chem. B 2009, 113, 9506–9511

Piperidinium, Piperazinium and Morpholinium Ionic Liquid Crystals Kathleen Lava, Koen Binnemans, and Thomas Cardinaels* Katholieke UniVersiteit LeuVen, Department of Chemistry, Celestijnenlaan 200F - bus 2404, B-3001 LeuVen, Belgium ReceiVed: April 21, 2009; ReVised Manuscript ReceiVed: May 20, 2009

Piperidinium, piperazinium and morpholinium cations have been used for the design of ionic liquid crystals. These cations were combined with several types of anions, namely bromide, tetrafluoroborate, hexafluorophosphate, dodecylsulfate, bis(trifluoromethylsulfonyl)imide, dioctylsulfosuccinate, dicyclohexylsulfosuccinate, and dihexylsulfosuccinate. For the bromide salts of piperidinium containing one alkyl chain, the chain length was varied, ranging from 8 to 18 carbon atoms (n ) 8, 10, 12, 14, 16, 18). The compounds show a rich mesomorphic behavior. High-ordered smectic phases (crystal smectic E and T phases), smectic A phases, and hexagonal columnar phases were observed, depending on the type of cation and anion. The morpholinium compounds with sulfosuccinate anions showed hexagonal columnar phases at room temperature and a structural model for the self-assembly of these morpholinium compounds into hexagonal columnar phases is proposed. Introduction Ionic liquid crystals are a fascinating class of materials that combine the properties of both ionic liquids and liquid crystals.1-3 It has been known for a long time that cationic surfactants with long alkyl chains not only exhibit lyotropic mesophases upon addition of a solvent, but that they can also form thermotropic mesophases upon heating.4-12 These quaternary ammonium or phosphonium compounds form typically smectic phases, reflecting their layerlike structure with microphase segregation between the polar parts of the molecules (with the counteranions in their vicinity) and the nonpolar parts formed by the molten alkyl chains.13 Although most of these smectic phases have been identified as smectic A phases, highly ordered smectic phases and crystal smectic phases have been identified as well. Later on, research on ionic liquids has revealed that mesophases are observed for these organic salts if they bear an alkyl chain of a suitable length, typically a dodecyl chain or a longer chain.14-19 The work on liquid-crystalline ionic liquids has shown that also the choice of the anion has a major influence on the mesophase behavior.1 Indeed, cationic surfactants contain in general chloride or bromide anions to make them water soluble, whereas for ionic liquids a much wider choice of anions is available. Many ionic liquids contain fluorinated anions like tetrafluoroborate, hexafluorophosphate, or bis(trifluoromethylsulfonyl)imide, which make these solvents hydrophobic. There are also many examples of ionic liquids with metal-containing anions. Until recently, most studies on ionic liquid crystals have been restricted to a limited number of cationic cores. Imidazolium and pyridinium salts have been by far the most popular classes of ionic liquid crystals. It has been realized that variation of the type of cationic core is a very valuable approach for tuning the mesophase behavior of these compounds. For instance, whereas most imidazolium and pyridinium salts do exhibit smectic A phases, pyrrolidinium salts show a rich mesomorphism, including highly order smectic phases like the crystal smectic E and T phases.20 A major difference between the imidazolium and pyridinium salts on one hand and the pyrrolidinium salts on the other hand * To whom correspondence should [email protected].

be

addressed.

E-mail:

is that the positive charge is delocalized over the aromatic ring in imidazolium and pyridinium salts, whereas the positive charge is localized on the nitrogen atom of the five-membered heterocyclic pyrrolidinium ring. In order to explore whether also ionic liquids crystals based on other aliphatic heterocycles exhibit unusual mesophases, we investigated piperidinium, piperazinium, and morpholinium salts, which are all composed of a six-membered heterocycle. To the best of our knowledge, only one study has been previously published on liquid-crystalline piperazinium salts,21 whereas no examples of liquid-crystalline piperidinium and morpholinium salts have been described in the literature yet. Ionic liquids based on piperidinium, piperazinium, and morpholinium salts have received only limited attention.22-28 A second objective of this work was to develop strategies to lower the melting points of ionic liquid crystals to such an extent that they exhibit a mesophase at ambient temperatures. Therefore, dodecylsulfate and different dialkylsulfosuccinates like dioctylsulfosuccinate, dicyclohexylsulfosuccinate, and dihexylsulfosuccinate have been combined with the cationic cores, besides the more classical anions bromide, tetrafluoroborate, hexafluorophosphate, and bis(trifluoromethylsulfonyl)imide. It has been shown in the literature that replacement of chloride ions by dodecylsulfate anions can lead to a dramatic reduction in the transition temperatures of ionic metallomesogens.29 Bis(2-ethylhexyl)sulfosuccinate has successfully been used to transform a perylene dye into a liquid-crystalline material with a very wide mesophase range.30 Materials that show mesophase behavior at room temperature have distinct advantages compared to compounds that melt at high temperatures. These materials are accessible for study by different physical methods and there are no problems with thermal decomposition of the compounds at room temperature. Moreover, ionic liquid crystals with low melting points can find application as anisotropic ionic conductors.31 In the case of columnar phases, there will be a higher conductivity along the column axis than perpendicular to it. Room-temperature ionic liquid crystals are also being investigated as electrolytes in dye-sensitized solar cells (Gra¨tzel cells).32

10.1021/jp903667e CCC: $40.75  2009 American Chemical Society Published on Web 06/18/2009

Piperidinium, Piperazinium and Morpholinium

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SCHEME 1: Overview of the Piperidinium, Piperazinium and Morpholinium Compoundsa

a Dodecylsulfate, bis(trifluoromethylsulfonyl)imide (or bistriflimide), dioctylsulfosuccinate, dicyclohexylsulfosuccinate, and dihexylsulfosuccinate are abbreviated as DOS, NTf2, DOSS, DcHSS, and DHSS, respectively.

Experimental Methods General Information. Defect textures of the mesophases were observed with an Olympus BX60 polarizing microscope equipped with a LINKAM THMS600 hot stage and a LINKAM TMS93 programmable temperature controller. DSC traces were recorded with a Mettler-Toledo DSC822e module (heating/ cooling rate of 10 °C min-1; He atmosphere). Powder X-ray diffractograms were recorded on a Bruker AXS D8 Discover diffractometer mounted with a copper X-ray ceramic tube, working at 1.6 kW. The emitted Cu KR radiation (λ ) 1.5418 Å) was focused on the sample by a Go¨bel mirror. All the samples (without a thermal history) were prepared by spreading the powders on a thin cleaned silicon wafer. Diffractograms were collected using the Bragg-Brentano reflection geometry (θ/2θ setup) at an angular resolution (in 2θ) of 0.03° per step. The deviation between the temperature on the surface of the sample holder and the set temperature was about 3%. The scattering signal was recorded with a one-dimensional detector (LynxEye detector). Indexation of the powder X-ray diffractograms was performed with the WinXPOW program package with the Index and Refine program using Werner’s TREOR algorithm program (allowed 2θ error in matching the experimentally observed peaks ) 0.05°).33,34 The synthesis of the ionic liquid crystals and their characterization are described in the Supporting Information. Results and Discussion An overview of all the piperidinium, piperazinium, and morpholinium compounds that were investigated is given in Scheme 1. The thermal properties of all the compounds were examined by polarizing optical microscopy (POM), differential scanning calorimetry (DSC), and X-ray diffraction on powder samples (PXRD). The transition temperatures and thermal data for all these compounds are summarized in Table 1. For the sake of brevity, dodecylsulfate, bis(trifluoromethylsulfonyl)imide (or bistriflimide), dioctylsulfosuccinate, dicyclohexylsulfosuc-

cinate, and dihexylsulfosuccinate will be abbreviated as DOS, NTf2, DOSS, DcHSS, and DHSS, respectively. For some compounds, two or more crystalline phases were observed. This is not unusual as crystal polymorphism and the existence of conductive plastic crystal phases (rotationally disordered phases) of pyrrolidinium-based ionic liquids have been reported by several authors in the past.35-39 For the piperidinium compounds containing one alkyl chain, the bromide salts with short alkyl chains (n ) 8-12; 1a-3a, respectively) were not liquid-crystalline. On heating the compounds, a plastic crystalline phase was observed, which transformed into an isotropic liquid upon further heating. The bromide salts with long alkyl chains (n ) 14-18; 4a-6a, respectively) showed a crystal smectic T phase. The tetrafluoroborate (4b) and DOS (4e) salts showed a crystal smectic T phase. The hexafluorophosphate (4c) and all the sulfosuccinate (4f, 4g, and 4h) salts were not liquid-crystalline. For the piperidinium compounds containing two alkyl chains, the bromide (7a), tetrafluoroborate (7b), and hexafluorophosphate (7c) salts showed a crystal smectic E/T phase; the DOS salt (7e) showed a hexagonal columnar phase, while the NTf2 (7d) and all the sulfosuccinate (7f, 7g, and 7h) salts were not liquid-crystalline. For the morpholinium compounds, the bromide (8a), tetrafluoroborate (8b), and the DOS (8e) salts showed a crystal smectic E/T phase at lower temperatures and a smectic A phase at higher temperatures (the bromide salt 8a also showed a plastic crystalline phase below the crystals smectic E/T phase). The NTf2 salt (8d) was not liquid-crystalline. The hexafluorophosphate salt (8c) showed a crystal smectic E/T phase, while all the sulfosuccinate salts (8f, 8g, and 8h) showed a hexagonal columnar phase at room temperature and melted between 120-147 °C to an isotropic liquid. For the piperazinium compounds, the bromide (9a), tetrafluoroborate (9b), hexafluorophosphate (9c), and NTf2 (9d) salts were not liquid-crystalline. The DOS (9e) and all the sulfos-

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TABLE 1: Transition Temperatures and Thermal Data for the Piperidinium, Piperazinium and Morpholinium Salts compd

cation

1a

piperidinium

2a 3a 4a 5a

6a

4b

4c 4d 4e

4f 4g 4h 7a

7b 7c 7d 7e 7f 7g

na

anionb

8 Br-

transitionc

Cr f X XfI piperidinium 10 BrCr f X XfI piperidinium 12 BrCr f X XfI piperidinium 14 BrCr f T TfI piperidinium 16 BrCr1 f Cr2 Cr2 f T TfI piperidinium 18 Br Cr1 f Cr2 Cr2 f T TfI piperidinium 14 BF4Cr1 f Cr2 Cr2 f Cr3 Cr3 f T TfI piperidinium 14 PF6 Cr1 f Cr2 Cr2 f I piperidinium 14 NTf2- Cr f I piperidinium 14 DOS Cr1 f Cr2 Cr2 f Cr3 Cr3 f T TfI piperidinium 14 DOSS Cr f I piperidinium 14 DcHSS Cr f I piperidinium 14 DHSS Cr1 f I Cr2 f I piperidinium 14 BrCr1 f Cr2 Cr2 f SmX SmX f I piperidinium 14 BF4 Cr f SmX SmX f I piperidinium 14 PF6Cr f SmX SmX f I piperidinium 14 NTf2- I piperidinium 14 DOS Cr f Colh Colh f I piperidinium 14 DOSS Cr1 f I Cr2 f I piperidinium 14 DcHSS Cr f I

T ∆H ∆S (J K-1 (°C)d (kJ mol-1) mol-1) compd 104 198 98 203 82 206 88 213 76 94 220 51 100 225 41 62 84 109 82 86 41 51 63 85e 170 51e 56e 41e 65e 70 89 181 77 127 80 119 rt 114 163e 23e 53e 45e

30.5 38.2 34.5 34.6 56.0 30.4 61.9 28.3 11.9 27.3 12.2 18.7 75.4 25.0 5.4 42.8 35.9 10.2 64.7 3.7 32.3 20.6 19.2 41.7 22.8 7.0 39.1 2.0 0.68 6.9 68.2 16.7 128.0 14.4 36.4 10.2

24 24 30 23 55 22 65 22 14 30 10 25 87 22 7 49 39 10 80 4 59 36 32 66 29 15 78 4 1 11 105 21 207 20 64 16

13.2 16.8 2.6 4.1 5.8

25 29 8 11 15

cation

na

anionb

7h 8a

piperidinium 14 DHSS morpholinium 14 Br-

8b

morpholinium

8c

morpholinium

8d 8e

morpholinium morpholinium

8f 8g 8h 9a

morpholinium morpholinium morpholinium piperazinium

9b

piperazinium

9c

piperazinium

9d

piperazinium

9e

piperazinium

9f

piperazinium

9g

piperazinium

9h

piperazinium

transitionc

Cr f I Cr f X X f SmX SmX f SmA SmA f I Cr f SmX 14 BF4 SmX f SmA SmA f I 14 PF6Cr1 f Cr2 Cr2 f SmX SmX f I 14 NTf2 Cr f I 14 DOS Cr1 f Cr2 Cr2 f Cr3 Cr3 f SmX SmX f SmA SmA f I 14 DOSS Colh f I 14 DcHSS Colh f I 14 DHSS Colh f I 14 BrCr1 f Cr2 Cr2 f Cr3 Dec 14 BF4Cr1 f Cr2 Cr2 f Cr3 Dec 14 PF6 Cr1 f Cr2 Cr2 f Cr3 Dec 14 NTf2- Cr1 f Cr2 Cr2 f I 14 DOS Cr1 f Cr2 Cr2 f Cr3 Cr3 f SmA Dec 14 DOSS Cr f SmA SmA f I 14 DcHSS Cr f SmX SmX f SmA SmA f I 14 DHSS Cr f SmA SmA f I

T ∆H ∆S (J K-1 (°C)d (kJ mol-1) mol-1) 106e 97 138 221 230e 82 120 137 100 119 162 36e 53 80 108 181 202 137 120 147 72e 104e >250 53 92 >250 82e 105 >250 60 197d 33 44 66 >250 32 102e 95e 126e 158e 74 169

16.4 121.2 4.9 18.1 66.9 120.9 3.5 1.6 43.5 48.8 19.9 26.1 10.5 28.3 44.7 4.9 4.6 17.3 13.2 17.2 7.0 62.5

38 124 4 14 50 131 3 1 52 55 20 49 18 45 66 6 5 30 22 27 14 111

15.7 5.1

33 10

10.6 14.5

24 31

26.3 19.6 4.9 8.2 1.8

84 45 17 27 5

0,3 1,3 29.0 1.2 2.1 13.0 2.2

1 5 101 4 6 60 8

a n is the number of carbon atoms in the terminal alkyl chain(s). b DOS, dodecylsulfate; DOSS, dioctylsulfosuccinate; DcHSS, dicyclohexylsulfosuccinate; DHSS, dihexylsulfosuccinate. c Abbreviations: Cr, Cr1, Cr2, Cr3 ) crystalline phase, T ) crystal smectic T phase; Colh ) hexagonal columnar phase; SmX ) crystal smectic E or T phase; SmA ) smectic A phase; I ) isotropic liquid; Dec ) decomposition. d Onset temperatures obtained by DSC at heating/cooling rates of 10 °Cmin-1 (He atmosphere). For 4c, 7c, 7f, 7h, 8d, and 8i, values were taken from the second heating run. For all other compounds, values were taken from the first heating run. During the first cooling run, 4f, 4g, 4h, 7f, 7g, 7h, 9e, 9f, 9g, and 9h were cooled to -20 °C, and all other compounds were cooled to 25 °C. e Peak temperature.

uccinate (8f, 8g, and 8h) salts showed smectic A phases and the DcHSS salt (9g) showed an additional crystal smectic E/T phase at lower temperatures. The smectic A phases and the hexagonal columnar phases were identified on the basis of their optical defect textures, observed by POM. The smectic A phases showed oily streak or focal conic textures (Figure 1); the hexagonal columnar phases showed beautiful pseudo focal conic fan textures with nonmerging extinction crosses (Figure 1). The crystal smectic T phases were identified using POM observations combined with PXRD measurements (see below). The crystal smectic T phases showed lancet textures with large homeotropic domains (Figure 1). The high-ordered mesophases of compounds 7a, 7b, 7c, 8a, 8b, 8c, 8e, and 9g could not be unequivocally identified. The mesophases of these compounds were assigned as being crystal smectic E or T phases (no distinction could be made between the two possibilities). The reason why we assigned the phases as being crystal smectic E/T is as follows. Using POM, lancet, and/or mosaic textures with large (pseudo) homeotropic domains were observed (Figure 1). Together with the fact that these phases were highly viscous, this points at crystal smectic B, E, or T phases. Because of the appearance of large (pseudo)

homeotropic domains, tilted high-ordered smectic phases (SmI, SmF, H, G, J, and K phases) could be excluded. In addition, analogous pyrrolidinium ionic liquid crystals only showed crystal smectic T or E phases.20 A hexagonal two-dimensional ordering of the ions seems to be less favorable for such compounds, while ordering of ions in a square or rectangular two-dimensional network with cations and anions at the corners and centers allows a strict alternation of positive and negative charges. Therefore, the phases are most probably crystal smectic E or T (and not hexatic, such as crystal smectic B). Unfortunately, the powder X-ray diffractograms of these compounds did not show a sufficient number of well-resolved diffraction peaks to make a clear distinction between a crystal smectic E or T phase possible (see below). It should be noted that there is only a small difference between these two high-ordered phases. In both cases, the molecules are arranged into layers. Within the layers, the ions are arranged into a rectangular lattice for the crystal smectic E phase, while for the crystal smectic T phase, the ions are arranged into a square lattice (Figure 2). All the liquid-crystalline piperidinium, morpholinium, and piperazinium compounds were studied by X-ray diffraction on powder samples for further identification of the mesophases and

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Figure 1. Mesophase defect textures observed by POM. Top left: oily streak texture of the SmA phase of 8e at 187 °C (100× magnification). Top right: pseudo focal conic fan texture of the Colh phase of 8h at 112 °C (200× magnification). Bottom left: lancet texture of the T phase of 6a at 157 °C (100× magnification). Bottom right: mosaic texture of the E/T phase of 4e at 159 °C (100× magnification).

Figure 2. Arrangement of the ions within the smectic layers for the crystal smectic E (left) and crystal smectic T (right) phases. The charges and the shape of the ions are arbitrarily chosen.

to obtain more information about the molecular packing in the mesophases. Table 2 gives an overview of the Bragg reflections collected from the X-ray diffractograms of all the mesomorphic compounds. The diffractograms of the smectic mesophases show several sharp and equidistant reflections at small angles in the case of the high-ordered crystal smectic phases or only one or two sharp and equidistant reflections at small angles in the case of the SmA phases. These reflections are related to the consecution of the smectic layers of a specific layer thickness d (Table 2). In the wide-angle region, a diffuse signal, centered at about 4.7 Å, was observed, corresponding to the lateral short-range order of the disordered (molten) aliphatic chains. In addition, several relatively sharp peaks were seen in the wide-angle region for the high-ordered crystal smectic phases. The crystal smectic T phases were identified by the presence of three or four, rather sharp peaks in the wide-angle region, which could be indexed as reflections of a tetragonal lattice (for a tetragonal lattice, 1/dhkl2 ) (h2 + k2)/a2 + l2/c2; see Table 2). For the crystal smectic phases of compounds 7a, 7b, 7c, 8a, 8b, 8c, 8e, and 9g, several peaks were observed in the wide-angle region, but these were not well resolved and/or could not be indexed so that no reasonable cell parameters were obtained.

The diffractograms of the hexagonal columnar mesophases show several sharp reflections at small angles, which could be indexed as reflections of a hexagonal lattice (for a hexagonal lattice: dhk ) a/(4/3(h2 + k2 hk))1/2; see Table 2). In the wideangle region, a diffuse signal, centered at about 4.7 Å, was observed, corresponding to the lateral short-range order of the disordered (molten) aliphatic chains. In addition, a weak signal at about 5.0 Å was observed, which was assigned to the stacking periodicity along the columnar axis, h (see Table 2). The most interesting part of this study was the observation of room temperature hexagonal columnar phases for the morpholinium compounds with sulfosuccinate counterions. Therefore, the hexagonal columnar phases of the morpholinium compounds deserves a further discussion. Upon the basis of the parameters obtained by PXRD, a molecular model is presented in the following section. Providing the correct repeating distance along the column (h) is known, the effective number of molecules (Z) per repeat distance (or slice) can be determined by the relationship between the columnar cross-section area (S) and the molecular volume (VM), according to h · S ) Z · VM. VM can be calculated as VM ) (M/0.6022)f, where M is the molecular mass (g mol-1) and f is a temperature correction factor (f ) 0.9813 + 7.474 × 10-4T; T is in °C).40 S can be deduced from the lattice parameter a via the relation S ) a231/2/2. Thus, for the hexagonal columnar phases of the morpholinium sulfosuccinate salts 8f, 8g, and 8h, the equivalent of about three molecules are found in a slice that is about 5 Å thick (see Table 2). A molecular model for the hexagonal columnar phase of the morpholinium DHSS salt 8h is shown in Figure 3. By assembling three molecules, a disklike shape is obtained. These discs stack into a column, and the columns are arranged into a hexagonal lattice to form a hexagonal columnar mesophase. The dimensions of the slices are in good agreement with the molecular dimensions of morpholinium compound 8h. The diameter of a column 2r ) a is 27.5 Å (see Table 2), and for the molecular model shown in Figure 3, a value of about 28 Å was

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TABLE 2: Bragg Reflections Collected from the X-ray Diffractograms of the Different Enantiotropic Mesophases compd 4a

4b

4e

7a

cation

a

n

anion

piperidinium 14 Br-

piperidinium 14 BF4-

piperidinium 14 DOS

piperidinium 14 Br-

7b

piperidinium 14 BF4-

7c

piperidinium 14 PF6-

7e

b

piperidinium 14 DOS

c

d

dmeas/Å

I

30.64 15.40 10.32 7.75 6.19 5.17 4.65 4.54 3.22 2.86 27.29 13.11 9.16 6.87 5.50 4.91 4.82 4.60 4.32 4.00 23.83 11.92 7.95 4.79 4.60 4.54 4.07 25.02 12.57 8.37 4.79 4.42 22.57 11.29 7.54 5.07 4.67 4.54 20.71 10.40 6.94 5,02 4.86 4.55 26.57 10.08 8.92 7.71

VS W S S S M br M VW VW VS M M M M M M br W W VS M W br W VW VW VS S W br VS S W VW VW br VS S W VW br VW VS W W VW

e

c

hkl dcalc/Å 001 002 003 004 005 006 hCH 110 200 210 001 002 003 004 005 110 111 hCH 113 114 001 002 003 hCH 110 111 114 001 002 003 hCH W 001 002 003

30.89 15.45 10.30 7.72 6.18 5.15 4.54 3.21 2.87 27.20 13.60 9.07 6.80 5.44 4.91 4.84

parameters of the mesophase at Tf,g,h,i,j T: T ) 151 °C d ) 30.89 VM ) 684 Å3 AM ) 22.1 Å2 L ) 22.75 Å a ) 6.42 Å

T: T ) 96 °C d ) 27.20 VM ) 629 Å3 AM ) 23.1 Å2 L ) 22.75 Å a ) 6.95 Å

hCH

4.32 3.98 23.84 T: T ) 121 °C 11.92 d ) 23.84 7.95 VM ) 1000 Å3 AM ) 41.9 Å2 4.62 L ) 22.75 Å 4.53 a ) 6.53 Å 3.65 25.09 SmX: T ) 131 °C 12.54 d ) 25.09 8.36 VM ) 678 Å3 AM ) 27.0 Å2 L ) 22.68 Å 22.59 SmX: T ) 101 °C 11.29 d ) 22.59 7.53 VM ) 993 Å3 AM ) 43.9 Å2 L ) 22.68 Å 4.54 20.78 SmX: T ) 100 °C 10.39 d ) 20.78 6.93 VM ) 1094 Å3 AM ) 52.7 Å2 L ) 22.68 Å

100 210 300 220

26.67 10.08 8.89 7.70

hCH 001 002 003

Colh: T ) 125 °C VM ) 1328 Å3 S ) 821 Å2 a ) 30.8 Å

compd

cation

a

n

anion

8a

morpholinium 14 Br-

8b

morpholinium 14 BF4-

8c

8e

8f

b

morpholinium 14 PF6-

morpholinium 14 DOS

morpholinium 14 DOSS

c

d

e

c

hkl dcalc/Å

parameters of the mesophase at Tf,g,h,i,j

dmeas/Å

I

7.39 5.01 4.76 30.95 15.48 10.32 7.75 6.19 5.17 4.54 4.48 28.06 14.03 9.39 7.04 5.64 5.12 4.76 30.02 15.01 4.69

VW W br VS M M S S W VW br VS M M M M br VW VS W br

310 h hCH 001 002 003 004 005 006

30.97 15.49 10.32 7.74 6.19 5.16

SmX: T ) 181 °C d ) 30.97 VM ) 673 Å3 AM ) 21.7 Å2 L ) 21.67 Å

hCH 001 002 003 004 005 hCH

30.97 15.49 10.32 7.74 6.19 5.16

SmX: T ) 105 °C d ) 30.97 VM ) 678 Å3 AM ) 24.1 Å2 L ) 21.67 Å

26.11 13.11 8.79 6.60 5.28 5.11 4.92 4.86 4.63 3.67 23.27 11.73 7.81 4.73 4.65 30.64 15.48 4.97

VS M S S M W M br M VW VS S M br W VS W br

001 002 003 004 005

24.41 14.17 9.36 8.19 7.11 6.85 4.96

VS M W W W W VW

100 110 210 300 310 220 h

001 002 hCH

7.40 Z ) 3.1 L ) 22.68 Å

30.03 SmA: T ) 220 °C 15.01 d ) 30.03 VM ) 691 Å3 AM ) 23.0 Å2 26.29 SmX: T ) 141 °C 13.15 d ) 26.29 8.76 VM ) 800 Å3 6.57 AM ) 30.4 Å2 5.26 L ) 21.67 Å

hCH 001 002 003 hCH 001 002 hCH

23.39 SmX: T ) 121 °C 11.69 d ) 23.39 7.80 VM ) 1003 Å3 AM )42.9 Å2 L ) 21.67 Å 30.80 SmA: T ) 191 °C 15.40 d ) 30.80 VM ) 1052 Å3 AM )34.17 Å2 L ) 21.67 Å 24.60 Colh: T ) 81 °C 14.20 VM ) 1246 Å3 9.30 S ) 699.0 Å2 8.20 a ) 28.4 Å 7.10 Z ) 2.8 6.82 L ) 21.67 Å

a n is the number of carbon atoms in the terminal alkyl chain(s). b DOS, dodecylsulfate; DOSS, dioctylsulfosuccinate; DcHSS, dicyclohexylsulfosuccinate; DHSS, dihexylsulfosuccinate. c dmeas and dcalc are the measured and calculated diffraction spacings, respectively. d I is the intensity of the reflections: VS, very strong; S, strong; M, medium; W, weak; VW, very weak; br, broad reflection. e hkl are the Miller indices of the reflections, h is the stacking periodicity along the columnar axis and hCH the periodicity corresponding to the liquidlike order of the molten chains. f T is the temperature at which the X-ray diffractogram was recorded. g VM is the molecular volume and AM the molecular area. h a is the lattice parameter of the unit cell of the T phase or Colh phase, S is the columnar cross-section and Z the number of molecules within a volume fraction of a column. i L is the calculated length of the relevant cation in its most extended conformation (estimated with Chem3D; the structure of the cation was energy-minimized by an MM2 calculation within Chem3D). j Abbreviations: T ) crystal smectic T phase; Colh ) hexagonal columnar phase; SmX ) crystal smectic E or T phase; SmA ) smectic A phase. dcalc and the mesophases parameters d, VM, AM, a, S, and N are deduced from the following mathematical expressions. For the smectic phases, d ) ) [Σld00l · l]/N00l and the molecular area AM ) VM/d. For the T phase, the lattice parameter a is related to dhkl as 1/dhkl2 ) (h2 + k2)/a2 + l2/c2 (a ) b, c ) d); for the Colh phase, dhk ) a/(4/3(h2 + k2 hk))1/2, and the lattice area (i.e., columnar cross-section) S ) a231/2/2. Z is the number of molecules and is defined as Z ) h · S/VM. VM is the molecular volume and can be calculated as VM ) (M/0.6022)f, where M is the molecular mass (g mol-1) and f is a temperature correction factor (f ) 0.9813 + 7.474 × 10-4T; T is in °C).40

obtained. The molecular packing of the morpholinium compounds 8f and 8h in the hexagonal columnar phases is analogous. Conclusions We have synthesized the first series of thermotropic ionic liquid crystals based on the piperidinium, morpholinium, and piperazinium cores. These compounds show a rich mesomor-

phism, including high-ordered smectic phases (crystal smectic T and E phases), disordered smectic A phases, and hexagonal columnar phases. It was shown for the piperidinium core that a minimum alkyl chain length of 14 carbon atoms was necessary to induce mesomorphism. In general, both the cationic core and the anion type have a great influence on the mesomorphic behavior. For the piperidinium compounds, only high-ordered crystal smectic phases were observed (except for the dodecyl-

Piperidinium, Piperazinium and Morpholinium

Figure 3. Molecular model for the packing of the morpholinium compounds 8h in the hexagonal columnar phase.

sulfate salt of the piperidinium compound with two tetradecyl chains, which showed a hexagonal columnar phase). For the piperazinium compounds, only disordered smectic A phases were observed (except for the DcHSS salt, which also showed a high-ordered crystal smectic phase below the smectic A phase). The best results were obtained for the morpholinium compounds, which all showed liquid crystal phases (except the bistriflimide salt). For the large sulfosuccinate salts of the morpholinium compounds, hexagonal columnar phases were observed at room temperature, which were stable over a wide temperature range. Via powder X-ray diffraction and consecutive molecular modeling, it was shown that via self-assembly of three molecules, a disklike shape is obtained, which is suitable for the formation of hexagonal columnar phases. Acknowledgment. T.C. is a postdoctoral fellow of the FWOFlanders. Financial support by the K.U.Leuven (projects GOA 08/05 and IDO/05/005) is gratefully acknowledged. CHN elemental analyses were performed by Dirk Henot. Mass spectra were measured by Bert Demarsin. Powder X-ray diffractograms were recorded by Dr. Jan D’Haen and Bart Ruttens (UHasselt, Institute for Materials Research). Supporting Information Available: Synthesis and characterization of the precursors and the piperidinium, piperazinium, and morpholinium salts. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Binnemans, K. Chem. ReV. 2005, 105, 4148–4204. (2) Kato, T.; Mizoshita, N.; Kishimoto, K. Angew. Chem., Int. Ed. 2006, 45, 38–68. (3) Lin, I. J. B.; Vasam, C. S. J. Organomet. Chem. 2005, 690, 3498– 3512. (4) Abdallah, D. J.; Lu, L. D.; Cocker, T. M.; Bachman, R. E.; Weiss, R. G. Liq. Cryst. 2000, 27, 831–837. (5) Abdallah, D. J.; Robertson, A.; Hsu, H. F.; Weiss, R. G. J. Am. Chem. Soc. 2000, 122, 3053–3062.

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