Name: From Molecules to Materials: Engineering New Ionic Liquid Crystals Through Halogen Bonding (Video) | JoVE Journal
Uploaded: 2018-03-24T13:00:00.000+00:00
Duration: 6 min 44 s
Description: Learn to synthesize supramolecular ionic liquid crystals using halogen bonding from non-mesomorphic building blocks. This method enables creation of antitropic mesogens with unique thermal properties.

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5 minutes

We gratefully acknowledge support from the grant, ERC-2012-StG_20111012 FOLDHALO (Grant Agreement no. 307108).

Note: Reactions were carried out in oven-dried glassware under nitrogen atmosphere. Please follow the safety measures described in the MSDS of each chemical.
1. Synthesis of 1-Alkyl-3-methylimidazolium Iodide Salts (1-m, Figure 3)
<ol>
	<li>In a three-necked, round-bottomed flask equipped with a reflux condenser fitted with a nitrogen inlet, a thermometer, a magnetic stirring bar, and a heating oil bath, dissolve 1.5 g (0.018 mol, 1 equiv) of freshly distilled 1-methyl-imidazole and 1.3 equiv of the 1-iodoalkane (chain length from C2 to C12) in 10 mL of acetonitrile.</li>
	<li>Heat the reaction mixture to reflux (75 - 80 &#176;C internal temperature) overnight under nitrogen atmosphere and then cool the mixture down to room temperature.</li>
	<li>Remove the volatile materials from the resulting mixture under reduced pressure at 50 &#176;C, using a rotatory evaporator. 
	NOTE: Imidazolium salts are highly hygroscopic and they quickly absorb water from the air. Thus, it is recommended to store them in a desiccator until use.</li>
</ol>
2. Synthesis of 1-m &#8226; 2-n XB Complexes
<ol>
	<li>Procedure 1: From solution (only for 1-2 &#8226; 2-8 in Figure 1)
	<ol>
		<li>Prepare a solution of the 1-ethyl-3-methylimidazolium iodide (1-2, MW = 238.07 g/mol) by dissolving 50 mg (0.21 mmol, 1 equiv) of 1-2 in 0.5 mL of acetonitrile.</li>
		<li>Prepare a solution of the perfluorooctyl iodide (2-8, MW = 545.96 g/mol) by dissolving 229 mg (0.42 mmol, 2 equiv) of 2-8 in 0.5 mL of acetonitrile.</li>
		<li>Mix the two solutions in a vial and keep the vial in a jar filled with paraffin oil.</li>
		<li>Let the solvent diffuse slowly in the paraffin oil at room temperature. After a period ranging from 3 to 7 days, good-quality single crystals suitable for X-ray diffraction analysis were obtained.</li>
	</ol>
	</li>
	<li>Procedure 2: Melt methodology 
	NOTE: This methodology requires an air-sealed system to mitigate against the volatility of the iodoperfluoroalkanes.
	<ol>
		<li>Mix each 1-alkyl-3-methylimidazolium iodide (1-m) with the appropriate iodoperfluoroalkanes (2-n) in a 1:2 molar ratio in a clear borosilicate glass vial equipped with a magnetic stirring bar.</li>
		<li>Close the vial and put it into an oil bath under vigorous stirring.</li>
		<li>Heat at 70 &#176;C using a hot plate for 15 min then cool the mixture down to room temperature.</li>
	</ol>
	</li>
</ol>
3. 1H and 19F NMR Experiments
NOTE: 1H NMR and 19F NMR spectra were recorded at 25 &#176;C on a NMR spectrometer operating at 499.78 MHz for 1H and 470.21 MHz for 19F, and using CDCl3 as the solvent. TMS and CFCl3 were used as internal standards for calibrating the chemical shifts in 1H NMR and 19F NMR, respectively. Bis-(2,2,2-trifluoroethyl) ether that contains both hydrogen and fluorine nuclei, was used as internal standard for establishing the ratio between 1-m and 2-n in the cocrystals.
<ol>
	<li>Preparation of the sample for NMR analysis and spectra calibration
	<ol>
		<li>Dissolve about 10 mg of each complex 1-m &#8226; 2-n in 0.5 mL of CDCl3.</li>
		<li>Add 1 &#181;L of 2,2,2-trifluoroethyl ether using a micropipette.</li>
		<li>Lock the sample, set the acquisition parameters as reported in Table 1, and record 1H and 19F NMR consecutive spectra, without removing the NMR tube from the magnet, in order to operate under the same operating conditions.</li>
		<li>Calibrate the integration parameters so that in the 1H NMR spectra the CH2O quartet of bis(2,2,2-trifluoroethyl) ether (&#948; = 4.3 ppm) corresponds to four, and in the 19F NMR spectra the CF3 triplet (&#948; = &#8722;74.5 ppm) of 2,2,2-trifluoroethyl ether corresponds to six.</li>
		<li>Calculate the ratio of the -CF2-I signal area deriving from 2-n (&#948; = &#8722;61.2 ppm in the 19F NMR spectra) and the -CH3 signal area deriving from 1-m (&#948; &#8776; 4.0 ppm in the 1H NMR spectra), in order to evaluate the ratio between 2-n and 1-m in each complex.</li>
	</ol>
	</li>
</ol>
4. Preparation of the Sample for Hot Stage Polarized Optical Microscopy (POM)
<ol>
	<li>Prepare a thin layer of the sample by placing a spatula tip amount of the supramolecular complex 1-m&#8226; 2-n between two micro cover glasses (18 mm x 18 mm).</li>
	<li>Place the sample in the hot stage between two crossed polarizers, and heat the sample to melt.</li>
	<li>Submit the sample to repeated heating/cooling cycles in order to promote phase transitions. Crystalline and liquid crystalline phases will interact with polarized light leading to brightness and color gradient, while the isotropic phase will appear dark.</li>
</ol>

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</ol>

The authors have nothing to disclose.

We have reported an easy and versatile protocol for the synthesis of XB fluorinated ILCs in which mesomorphicity is driven by the supramolecular anions [CnF2n+1-I&#8729;&#8729;&#8729;I&#8729;&#8729;&#8729;I-CnF2n+1]-.
It is well-established that ionic liquids based on 1-alkyl-3-methylimidazolium salts can exhibit liquid crystalline behavior. However, whatever the anion, no mesophases were observed for compounds with alkyl chains shorter than 12 carbon atoms. Therefore, the most striking aspect of the reported materials is that it is the anisotropy of the XB anion that determines mesophase formation. In fact, the strict geometrical constraints of XB impose that the intermolecular interaction occurs along the extension of the C-I bond, at the sigma hole, with angles close to 180&#176;. The iodide anions act as bidentate XB acceptors, binding two perfluorinated modules that show linear arrangement around the central iodide anion. Moreover, the fact that perfluoroalkyl chains are stiffer than equivalent HC chains, and adopt a twisted helical structure due to repulsion between 1,3-disposed CF2 groups9,52, further contributes to the generation of rigid, rod-like superanions.
This perfectly agrees with the observation that complexes containing the iodoperfluorodecane (2-10) show transition temperatures higher than those containing the iodoperfluorooctane (2-8) as the former is more anisotropic. Furthermore, the known tendency45,53 of fluoroalkanes to pack into lamellar phases determines the obtainment of SmB and SmA phases.
The superfluorinated ILCs reported in this paper demonstrate for the first time the application of the XB in the construction of ILCs based on imidazolium salts. Thanks to an accurate supramolecular design based on the high directionality of the XB and the fluorophobic effect, it is possible to obtain enantiotropic liquid crystals based on a rigid, non-aromatic, XB supramolecular synthon as the mesogenic core. The liquid crystalline behavior is independent from the length of the alkyl chains.
The supramolecular approach presented here represents an attractive platform for the design of new liquid crystalline materials and can provide new opportunities for the development of sophisticated functional materials such as room temperature ionic conductors, photoresponsive ILCs, and liquid crystalline electrolytes for application in energy devices.

Intermolecular interactions play a very important role in determining physical and chemical properties of bulk materials. When a new interaction becomes available, new structures, namely, new functions, become available. Therefore, exploring novel, specific non-covalent interactions underlying an intermolecular recognition process may open new perspectives in different fields such as materials sciences, catalysis, drug design, supramolecular chemistry, and crystal engineering. This was the case for the XB that recently became a routine tool for controlling aggregation and self-assembly phenomena1,2,3,4,5.
According to the IUPAC definition6: "A halogen bond occurs when there is evidence of a net attractive interaction between an electrophilic region associated with a halogen atom in a molecular entity and a nucleophilic region in another, or the same, molecular entity." A schematic representation of the XB is given in Figure 1, where X is the electrophilic halogen atom (Lewis acid, XB donor) and Y is a donor of electron density (Lewis base, XB acceptor).
<img alt="Chemical reaction diagram; R-X + Y forms R-X-Y; halogens, nitrogen involved in process." class="xfigimg" src="/files/ftp_upload/55636/55636fig1.jpg" data-altupdatedat="1788250015000" data-alt="Figure 1"> 
Figure 1: Schematic representation of the halogen bond. 
The electrophilic halogen atoms (XB-donors) are covalently linked to a wide variety of molecular scaffolds (R), while the XB-acceptors (Y) can be neutral or anionic species. This figure has been reprinted from reference2. <a href="//cloudflare.jove.com/files/ftp_upload/55636/55636fig1large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
At first sight, it may seem that this definition tackles somewhat puzzling aspects, since halogen atoms, because of their high electronegativity, are typically considered as sites of high electron density. However, the behavior of halogen atoms as electrophiles is quite general and has been elegantly rationalized by Politzer et al. with the introduction of the "σ-hole" concept7.
When a halogen atom is involved in a covalent bond, an electron redistribution occurs and the electronic density becomes anisotropic8,9,10,11. The shape of the atom becomes oblate and a region of positive electrostatic potential (the so-called σ-hole) develops along the extension of the covalent bond, on the outermost surface of the halogen atom (Figure 2A). This positive region is surrounded by a belt of negative electrostatic potential, orthogonal to the covalent bond. This model seems to solve the "enigma" of the XB since it explains the non-covalent interaction pattern of covalently-bonded halogen atoms, and the associated directional preferences, i.e., linear interactions with nucleophiles and lateral interactions with electrophiles.
<img alt="Nucleophilic and electrophilic interactions diagram, chemical bonding and molecular structure analysis." class="xfigimg" src="/files/ftp_upload/55636/55636fig2.jpg" data-altupdatedat="1788250015000" data-alt="Figure 2"> 
Figure 2: Anisotropic distribution of electron density around halogen atoms. (A) Schematic representation of the anisotropic distribution of the electron density around covalently bound halogen atoms and the pattern of the resulting interactions. (B) Molecular electrostatic potential maps at the isodensity surface with 0.001 au for CF4, CF3Cl, CF3Br, and CF3I. Color ranges: red, greater than 27 kcal/mol; yellow, between 20 and 14 kcal/mol; green, between 12 and 6 kcal/mol; blue, negative. This figure has been modified from reference2. <a href="//cloudflare.jove.com/files/ftp_upload/55636/55636fig2large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
XB has rapidly grown from a scientific curiosity to one of the most interesting noncovalent interactions in designing functional supramolecular materials12,13,14,15,16, thanks to its unique features, i.e., high directionality, tunable interaction strength, hydrophobicity, and donor atom dimensions1. The high directionality of XB can be easily understood by considering the focused nature of the σ-hole, while the interaction strength frequently correlates to the magnitude of the σ-hole. Both the size (the spatial range) and the magnitude (the value of the maximum electrostatic potential VS,max) of the σ-hole are affected by the polarizability and electronegativity of the halogen atom and the electron-withdrawing ability of the substituents in the neighborhood of the halogen17 (Figure 2B). As a general trend, as the halogen atom becomes more polarizable and the substituents on the molecule become more electron-withdrawing, the halogen σ-hole becomes more positive. Therefore, for a given R, VS,max increases in the order F &lt; Cl &lt; Br &lt; I, namely it increases with the halogen atom polarizability and decreases with its electronegativity. For a given halogen, VS,max becomes more positive when the electron-withdrawing ability of R is increased. Therefore, haloperfluorocarbons, specifically iodo-perfluoroalkanes and arenes, can act as powerful XB-donors forming particularly strong interactions18,19,20 and particularly hydrophobic adducts.
A case of PFC-HC self-assembly to give cocrystals has been reported in the late 1990s21. Upon mixing equimolar amounts of 1,2-diiodotetrafluoroethane and N,N,N',N'-tetramethyl-ethylenediamine in chloroform, a supramolecular complex was isolated as white crystals stable in air at room temperature. Single crystal X-ray analysis revealed that the two components alternate in a 1-dimentional (1D) infinite chain and the interaction between the nitrogen and iodine atoms is largely responsible for keeping the perfluorinated bis-iodide in place. This proved that the nitrogen-iodine XB interaction is strong enough to overcome the low affinity existing between PFC and HC compounds and suggested that XB can be successfully exploited as an easy route to introduce fluorinated moieties into any new supramolecular materials22,23,24, including liquid crystalline materials.
Fluorinated substituents, in fact, have been successfully incorporated into liquid crystalline molecules because of the excellent stability that is offered by the C-F bond as well as the small size and the low polarizability of the fluorine atom, which gives rise to very low intermolecular dispersion interactions25,26,27. Moreover the segregation occurring between PFC and HC chains allowed control over supramolecular liquid-crystalline phases, enhancing the smectic character of a mesomorphic material28,29,30. It is generally understood that both the physical/chemical properties and the mesomorphism are strongly affected by the molecular architecture29,31. Thus, an appropriate molecular design is essential to generate new supramolecular liquid crystals with tailored properties. The basic motif of these soft materials lies in a fairly rigid, rod-like moiety linked to one or two flexible aliphatic chains32,33,34. The conventional design of mesomorphic compounds was mainly performed with neutral species but studies have shown that also ionic pairs can exhibit a mesomorphic behavior, yielding materials with properties at the interface between ionic liquids and liquid crystals35,36,37.
Taking into account recent results on XB liquid crystals24,38,39, and the specific ability of anions to act as XB-acceptors, it seemed particularly interesting to exploit this non-covalent interaction in designing new types of ILCs.
1-Alkyl-3-methylimidazolium iodides (1-m) of various chain-length, which are known as ionic liquids40, were used for the synthesis of the complexes (Figure 3)41,42. Note that only 1-12, bearing a C12 alkyl chain, exhibits liquid crystallinity with smectic A (Sm A) phase (crystal (Cr) to SmA transition temperature = 27 °C; Sm A to isotropic liquid (ISO) transition temperature = 80 °C; Cr • 27 • SmA • 80 • Iso)43. Imidazolium salts were reacted with iodoperfluorooctane (2-8) and iodoperfluorodecane (2-10), which are well-known XB-donors44,45.
<img alt="Chemical reaction diagram; halogen bonding; formula: 1-m + 2 2-n = 1-m·2-n; m, n variables." class="xfigimg" src="/files/ftp_upload/55636/55636fig3.jpg" data-altupdatedat="1788250015000" data-alt="Figure 3"> 
Figure 3: Synthetic scheme. Chemical formulae of starting ionic liquids (1-m), iodoperfluoroalkanes (2-n), and related XB-complexes 1-m•2-n. <a href="//cloudflare.jove.com/files/ftp_upload/55636/55636fig3large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
The iodide anions are expected to participate readily as XB-acceptor in the formation of supramolecular structures with haloperfluoroalkanes44. However, it remains challenging to anticipate the number and topology of the coordinated XB-donor species. In fact, halide anions typically are involved in two or three XBs46,47,48, but higher coordination numbers were also observed49,50.
Herein, we describe a procedure for obtaining highly fluorinated XB ILCs and a detailed procedure for the identification and characterization of the occurrence of XB. The sequence of analysis reported here and utilized to identify the XB can be considered as a general procedure and can be applied to the characterization of any type of XB system.

<table><tbody><tr><td>1-Methyl-imidazole&amp;nbsp;</td><td>Sigma-Aldrich</td><td>M50834-500G</td><td>liquid reagent</td></tr><tr><td>1-Iodoethane</td><td>Sigma-Aldrich</td><td>I7780-100G</td><td>liquid reagent</td></tr><tr><td>1-Iodobutane</td><td>Sigma-Aldrich</td><td>167304-100G</td><td>liquid reagent</td></tr><tr><td>1-Iodohexane</td><td>Sigma-Aldrich</td><td>238287-100G</td><td>liquid reagent</td></tr><tr><td>1-Iodooctane</td><td>Sigma-Aldrich</td><td>238295-25G</td><td>liquid reagent</td></tr><tr><td>1-Iododecane</td><td>Sigma-Aldrich</td><td>238252-100G</td><td>liquid reagent</td></tr><tr><td>1-Iodododecane</td><td>Sigma-Aldrich</td><td>238260-100G</td><td>liquid reagent</td></tr><tr><td>Acetonitrile</td><td>Sigma-Aldrich</td><td>271004-1L</td><td>organic solvent</td></tr><tr><td>Perfluorooctyl iodide&amp;nbsp;</td><td>Apollo Scientific</td><td>PC6170</td><td>fluorinated reagent/halogen-bond donor&amp;nbsp;</td></tr><tr><td>Perfluorodecyl iodide</td><td>Apollo Scientific</td><td>PC5970</td><td>fluorinated reagent/halogen-bond donor&amp;nbsp;</td></tr><tr><td>Bis(2,2,2-trifluoroethyl) ether&amp;nbsp;</td><td>Sigma-Aldrich</td><td>287571-5G</td><td>Internal standard for NMR</td></tr><tr><td>Chloroform-d</td><td>Sigma-Aldrich</td><td>151823-100G</td><td>Solvent for NMR</td></tr></tbody></table>

from molecules to materials: engineering new ionic liquid crystals through halogen bonding

From Molecules to Materials: Engineering New Ionic Liquid Crystals Through Halogen Bonding

Herein, we demonstrate that a bottom-up approach, based on halogen bonding (XB), can be successfully applied for the design of a new type of ionic liquid crystals (ILCs). Taking advantages of the high specificity of XB for haloperfluorocarbons and the ability of anions to act as XB-acceptors, we obtained supramolecular complexes based on 1-alkyl-3-methylimidazolium iodides and iodoperfluorocarbons, overcoming the well-known immiscibility between hydrocarbons (HCs) and perfluorocarbons (PFCs). The high directionality of the XB combined with the fluorophobic effect, allowed us to obtain enantiotropic liquid crystals where a rigid, non-aromatic, XB supramolecular anion acts as mesogenic core.
X-ray structure analysis of the complex between 1-ethyl-3-methylimidazolium iodide and iodoperfluorooctane showed the presence of a layered structure, which is a manifestation of the well-known tendency to segregation of perfluoroalkyl chains. This is consistent with the observation of smectic mesophases. Moreover, all the reported complexes melt below 100 &#176;C, and most are mesomorphic even at room temperature, despite that the starting materials were non-mesomorphic in nature.
The supramolecular strategy reported here provides new design principles for mesogen design allowing a totally new class of functional materials.

Differential scanning calorimetry (DSC), POM, and thermogravimetric analysis (TGA), as well as 1H and 19F NMR were used to characterize the obtained complexes. Good-quality single crystals of 1-2•2-8 were obtained from a 1:2 solution of the starting components in acetonitrile by slow evaporation of the solvent at room temperature.
DSC analyses on mixtures of 1-2 and 2-8 with different stoichiometric ratios, allowed us to derive that the right coordination stoichiometry between the imidazolium salt and the iodoperfluoroalkane is 1:2. In fact, the DSC thermogram of the 1:1 complex in Figure 4A shows the presence of the uncomplexed imidazolium salt 1-2 (peak at 57 °C), while in the 1:3 complex there is an excess pure 2-8 (melting point 24 °C). On the other hand, the thermogram of the 1:2 complex shows a single peak distinct from those of the starting compounds, demonstrating that a new pure crystalline species was formed.
TGA analyses confirmed this result. In fact, only upon mixing 1-12 and 2-10 in 1:2 ratio, a homogenous sample was obtained, which released quantitatively the fluorinated building block at a temperature higher than the uncomplexed iodoperfluorodecane (Figure 5A and 5B).
19F NMR has been applied as a simple, effective, and sensitive tool to detect the formation of XB adducts. The XB between the electron deficient iodine of the iodoperfluoroalkanes and the I- anion strongly influences the signals of the -CF2-I group (δ = −60,0 ppm) and gives rise to detectable upfield shifts of the 19F NMR resonances. A Δδ value of 1.2 ppm was measured for complex 1-2•2-8, confirming the occurrence of attractive noncovalent interactions involving halogen atoms as electrophilic species (Figure 6). Moreover, by recording consecutive 1H and 19F NMR analyses in the presence of 2,2,2-trifluoroethyl ether as the internal standard for signal integration, the 1:2 stoichiometry between the XB-acceptor and donor modules has been further confirmed.
Single crystal X-ray analysis of 1-2•2-8 confirmed that the XB drives the formation of a trimeric supramolecular complex where the iodide anion acts as a bidentate XB-acceptor, binding two fluorinated chains. The I···I- distances are almost the same [3.4582(9) Å and 3.4625(10) Å] and roughly correspond to 27% shortening with respect to the sum of the vdW and Pauling radii of the iodine atom and iodide anion, respectively51. The high directionality of XB is confirmed by the C-I···I- angles of 174.1(1)° and 175.4(2)°, respectively (Figure 7A). Two equally-populated conformations are observed for the imidazolium cations, which lie approximately on the same plane and are related by a non-crystallographic twofold axis. Figure 7B shows the separation between the ionic and neutral parts, as well as between the HC and PFC chains, within the crystal (disorder removed).
DSC and POM studies revealed that all complexes melted at temperatures lower than 100 °C (Figure 8A), as typical for ionic liquids. Furthermore, they all show an enantiotropic liquid crystalline behavior, some at room temperature, with SmB and SmA phases (Figure 8A). On cooling, the SmA-SmB transition was identified by the characteristic striations across the back of the fans (Figure 8B,left). The striations disappeared on cooling further below the transition.
<img alt="Differential scanning calorimetry graph showing melting and clearing points of compounds 1-2-2-8." class="xfigimg" src="/files/ftp_upload/55636/55636fig4.jpg" data-altupdatedat="1788250015000" data-alt="Figure 4"> 
Figure 4: Differential Scanning Calorimetry. DSC thermograms of samples prepared by mixing 1-2 and 2-8 in different stoichiometric ratios: 1:3 sample (top), 1:2 sample (mid), and 1:1 sample (bottom). <a href="https://cloudflare.jove.com/files/ftp_upload/55636/55636fig4large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Thermogravimetric analysis graphs depicting weight loss vs temperature and exothermic peaks." class="xfigimg" src="/files/ftp_upload/55636/55636fig5.jpg" data-altupdatedat="1788250015000" data-alt="Figure 5"> 
Figure 5:Thermal Analysis. DSC heating/cooling cycles (A) and TGA analysis (B) on a sample prepared by mixing 1-12 and 2-10 in a 1:2 molar ratio. This figure has been modified from reference37. <a href="https://cloudflare.jove.com/files/ftp_upload/55636/55636fig5large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="NMR spectrum chart; chemical shift analysis; fluoropolymer structure annotation; spectroscopic study." class="xfigimg" src="/files/ftp_upload/55636/55636fig6.jpg" data-altupdatedat="1788250015000" data-alt="Figure 6"> 
Figure 6:19F NMR Analysis. 19F NMR spectra in CDCl3 of 2-8 (top) and related halogen-bonded complex 1-2•2-8 (bottom). <a href="https://cloudflare.jove.com/files/ftp_upload/55636/55636fig6large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Polymer structure diagram; molecular arrangement A, B, C showing increasing complexity." class="xfigimg" src="/files/ftp_upload/55636/55636fig7.jpg" data-altupdatedat="1788250015000" data-alt="Figure 7"> 
Figure 7: X-ray single crystal analysis. (A) The asymmetric unit of the complex 1-2•2-8 showing the formation of a trimeric superanion where two fluorinated chains are bonded to the central I- anion (a single conformer of the disordered cation is reported); (B) The imidazolium salt 1-2 and the iodoperfluorooctane 2-8 segregate along the crystallographic b-axis; (C) Side view showing ionic and fluorocarbon areas of the structure. Color code: grey, carbon; blue, nitrogen; magenta, iodine; green, fluorine; white, hydrogen. Halogen bonds depicted as grey lines. <a href="https://cloudflare.jove.com/files/ftp_upload/55636/55636fig7large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Phase transition diagram and microscope images, showing smectic to crystalline phases in compounds." class="xfigimg" src="/files/ftp_upload/55636/55636fig8.jpg" data-altupdatedat="1788250015000" data-alt="Figure 8"> 
Figure 8: Polarized Optical Microscopy. (A) Chart of thethermal transitions of halogen-bonded complexes 1-m•2-n measured by hot-stage POM (blue is solid, red is SmB, and orange is SmA phases). (B) Optical textures of the smectic phases observed for 1-12•2-10 on cooling from the isotropic state. Left: SmB phase at 77 °C; Right: SmA phase at 83 °C.Scale bar: 100 µm. This figure has been modified from reference37. <a href="https://cloudflare.jove.com/files/ftp_upload/55636/55636fig8large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<table border="1" fo:keep-together.within-page="1" fo:keep-with-next.within-page="always">
	<tbody>
		<tr>
			<td></td>
			<td>Spectral width (KHz)</td>
			<td>Scans number</td>
			<td>Pulse delay (s)</td>
		</tr>
		<tr>
			<td>1H</td>
			<td>7</td>
			<td>16</td>
			<td>1</td>
		</tr>
		<tr>
			<td>19F</td>
			<td>95</td>
			<td>16</td>
			<td>1</td>
		</tr>
	</tbody>
</table>
Table 1: Acquisition parameters for 1H and 19F NMR experiments

A protocol for the synthesis of a new type of mesogens, based on the halogen-bonded supramolecular anion [CnF2n+1-I&#183;&#183;&#183;I&#183;&#183;&#183;I-CnF2n+1]&#8722;, is reported.

Herein, we demonstrate that a bottom-up approach, based on halogen bonding (XB), can be successfully applied for the design of a new type of ionic liquid ...

The overall goal of this protocol is to provide a procedure for preparing new supramolecular ionic liquid crystals based on halogen bonding starting from non-mesomorphic building blocks. The halogen bond has become a tool for controlling aggregation self-assembly phenomena, and they can play a very important role in determining physical and chemical properties of biomaterials. The halogen bond is strong enough to overcome the lower field existing between perfluorocarbon and hydrocarbon compounds and can be exploited to introduce any fluorinated modules into a new supramolecular material.Generally, people new to this method are suspicious because halogens are electron rich species. However, in covalently bound halogen atoms a region of positive electrostatic potential appears along the extension of the covalent bond. The ability of an ion to be involving in halogen bonding in the ideal directionality of the interaction gave us this idea of applying this method to anionic liquid crystal design.To prepare the halogen-bonded complexes from solution, first dissolve 50 milligrams of the the imidazolium salt in 0.5 milliliters of acetonitrile. Next, dissolve 229 milligrams of perfluorooctyliodide in 0.5 milliliters of acetonitrile. Mix the two solutions in a vial.Then, place the vial in a jar filled with paraffin oil and let the solvent diffuse slowly in the oil at room temperature. It remains challenging to anticipate the composition and the architecture of the final supramolecular structure, because the iodine anions can act as multidentate halogen bonding acceptor. However, the AC and x-ray diffraction analysis revealed that iodine anion preferentially binds to perfluorooctyl chains.To prepare halogen-bonded complexes using the melt methodology, mix the imidazolium salt with appropriate iodo-perofluoroalkane in a clear borosilicate glass vial equipped with a magnetic stirring bar. Close the vial and place into an oil bath under vigorous stirring. Heat the reaction mixture at 70 degrees Celsius for 15 minutes.Then, cool the mixture to room temperature. Perfluorinated molecules are highly volatile, therefore this methodology requires a sealed system to mitigate against the volatility of iodo-perfluoroalkane. Prepare a thin layer of the sample by placing a spatula tip amount of the supramolecular complex between two micro-cover glasses.Place the sample in the hot stage of a polarized optical microscope between two cross-polarizers and heat the sample to melt. Finally, submit the sample to repeated heating and cooling cycles in order to promote phase transitions. DSC analysis of the complex obtained from the imidazolium salt and iodo-perfluorooctane was consistent with the preferred 1:2 stoichiometry.The thermogram of the 1:1 complex shows the uncomplexed imidazolium salt. While, in the 1:3 complex there is excess pure iodo-perfluorooctane. The thermogram of the 1:2 complex shows a single peak distinct from the starting compounds, demonstrating that a new, pure crystalline species was formed.The fluorine NMR spectrum of the complex between the imidazolium salt and iodo-perfluorooctane shows an up-field shift of 1.2 ppm for the difluoromethylene group adjacent to iodine, confirming non-covalent interactions involving halogen atoms as electrophilic species. Single crystal x-ray analysis of the complex between 1-ethyl-3-methylimidazolium iodide and iodo-perfluorooctane confirm that halogen bonding drives the formation of a trimeric supramolecular complex, where the iodide anion acts as a bidentate halogen bond acceptor binding two fluorinated chains. DSC and polarized optical microscopy revealed that all complexes melted at temperatures lower than 100 degrees Celsius and showed an antitropic liquid crystalline behavior with smectic B and smectic A phases.Upon cooling, the smectic A, smectic B transition was identified by characteristic striations across the back of the thins. The supramolecular system reported in the paper demonstrates for the first time, the application of halogen bonding in the construction of supramolecular ionic liquid crystals. Thanks to an accurate supramolecular design based on highly directionality of the halogen bond and the fluoro-phobic effect, it is possible to obtain an antitropic liquid crystals based on a rigid, non-aromatic supramolecular halogen bonding synthons as a mesogenic core.The supramolecular approach presented here represents an attractive platform for the design of new liquid crystalline materials and can provide the new opportunities for the development of sophisticated functional materials.

from-molecules-to-materials-engineering-new-ionic-liquid-crystals

Research

JoVE Journal

Chemistry

Ionic liquids (ILs) mediate fast, simple and cheap access to 1,6-ketoesters in high diastereoselectivities and good yields. The reaction protocol is robust and the 1,6-ketoesters can be obtained in gram scale after a simple filtration protocol. Moreover, the 1,6-ketoesters are potent gelators in hydrocarbon solvents.

Highly Stereoselective Synthesis of 1,6-Ketoesters Mediated by Ionic Liquids: A Three-component Reaction Enabling Rapid Access to a New Class of Low Molecular Weight Gelators

A novel methodology is presented to synthesize and program main-chain liquid-crystalline elastomers using commercially available starting monomers. A wide range of thermomechanical properties was tailored by adjusting the amount of crosslinker, while the actuation performance was dependent on the amount of applied strain during programming.

Synthesis of Programmable Main-chain Liquid-crystalline Elastomers Using a Two-stage Thiol-acrylate Reaction

We demonstrate the preparation of siloxane-based and epoxy-based liquid crystal elastomers (LCEs) and LCE nanocomposites. The LCEs are characterized with respect to reversible strain, liquid crystal ordering, and stiffness. As a potential application, we demonstrate their use as shape-responsive substrates in a custom device for active cell culture.

Preparation of Monodomain Liquid Crystal Elastomers and Liquid Crystal Elastomer Nanocomposites

Here, we describe protocols to prepare phosphonium-based ionic liquid and lithium bis(trifluoromethane)sulfonimide salt electrolytes, and assemble a non-flammable and high temperature functioning lithium-ion coin cell battery.

Synthesis of Ionic Liquid Based Electrolytes, Assembly of Li-ion Batteries, and Measurements of Performance at High Temperature

Here, we present a protocol to trigger an orientational transition of a liquid crystal in response to temperature. Methodologies are described for preparing a sample in order to observe the transition and the detailed transitional evolution.

Orientational Transition in a Liquid Crystal Triggered by the Thermodynamic Growth of Interfacial Wetting Sheets

The goal of the protocol is to create liquid crystalline polymer films that can mechanically oscillate under continuous light irradiation. We describe in great detail the conception of free-standing films, from the liquid crystal alignment method to the photo-actuation. The experimental protocol applied to prepare this material is broadly applicable.

Preparation of Liquid Crystal Networks for Macroscopic Oscillatory Motion Induced by Light

Here, we present the protocols of differential-detection analyses of time-resolved infrared vibrational spectroscopy and electron diffraction which enable observations of the deformations of local structures around photoexcited molecules in a columnar liquid crystal, giving an atomic perspective on the relationship between the structure and the dynamics of this photoactive material.

Novel Techniques for Observing Structural Dynamics of Photoresponsive Liquid Crystals

This article describes the microfluidic process and parameters to prepare actuating particles from liquid crystalline elastomers. This process allows the preparation of actuating particles and the variation of their size and shape (from oblate to strongly prolate, core-shell, and Janus morphologies) as well as the magnitude of actuation.

Microfluidic Preparation of Liquid Crystalline Elastomer Actuators

A simple, reproducible, and versatile approach for the synthesis of intergrown, polycrystalline metal-organic framework membranes on a wide range of unmodified porous and non-porous supports is presented.

Electrophoretic Crystallization of Ultrathin High-performance Metal-organic Framework Membranes

The present protocol describes the crystallization of microscopic ice crystals and clathrate hydrates in microfluidic devices, enabling liquid exchange around the formed crystals. This provides unparalleled possibilities to examine the crystallization process and binding mechanisms of the inhibitors.

A Microfluidic Approach for the Study of Ice and Clathrate Hydrate Crystallization

This protocol discusses standardization for the preparation of CAGE-based ionic liquid systems to ensure reproducibility and streamline the overall workflow across different scientific labs.

Development, Characterization, and Evaluation of CAGE-based Ionic Liquid Systems for Transdermal Delivery

Learn to synthesize supramolecular ionic liquid crystals using halogen bonding from non-mesomorphic building blocks. This method enables creation of antitropic mesogens with unique thermal properties.

From Molecules to Materials: Engineering New Ionic Liquid Crystals Through Halogen Bonding

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