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With the support from ACS-PRF we have investigated the ability of various ionic liquids to affect the gelation of molecular organic solvents. One graduate student, one undergraduate student and two high school students have been involved in various projects, which have been or in the process of being submitted for publications. Details are outlined below.

1. Extending supramolecular networks in ionic liquids

The notion of using ionic liquids to gelate or assist in gelation of molecular organic solvents has been a largely unexplored area of research. We have discovered that certain BF₄-containing ionic liquids could mediate the gelation of dimethylsulfoxide (DMSO) by an organogelator (Figure 1). A is a known gelator for many molecular solvents. We choose the amount of the organogelator A that is 5-times below the minimum gelation concentration, such that no gelation of DMSO takes place. Upon addition of various amounts of ionic liquids transparent gels were realized. We found that only imidazolium and pyridinium BF₄-containing ionic liquids had the ability to assist in the gelation of DMSO; tetraalkylammonium-based ionic liquids had no affect on the gelation of DMSO. This phenomenon was also specific to DMSO, among various molecular solvents screened, including EtOH, CH₃CN, and DMF.

Importantly, the mobility of ions was not impaired by the gel-formation, as the conductivity of the solution and that of the gel were virtually identical. In addition, immobilization of various small molecules, including porphyrins, BODIPY and Congo red dyes, had no impact on the ability of the ionic liquids to induce the gel formation. This should create ample opportunities for designing soft materials with desired properties.

2. Sol-gel transitions in molecular-ionic biphasic systems

We found that the gelation of certain molecular solvents can take place at room temperature using a two-phase gel system composed of a molecular solvent and an ionic liquid (Figure 2). Gelator A (Figure 1) was dissolved in [C₄-mim]NTf₂, allowed to form a gel and on top of the gel an equal volume of cyclohexane (i.e., an immiscible molecular solvent) was placed, a transfer of the organogelator from ionic liquid phase to cyclohexane was observed within a day, which resulted in the gelation of cyclohexane at room temperature, while returning ionic liquid to its liquid state. It is likely that the network of the gelator's molecules drags the entrapped solvent from one phase into the other. Arguably, the gel could be used as a phase transfer carrier of various molecules. In this light we have examined the potential of this system to transfer dyes between the two phases. BODIPY dyes (Figure 3) were chosen as a model. We observed an efficient transfer of these dyes from ionic liquid into cyclohexane, solvent in which these dyes are otherwise completely insoluble.

We have also determined that a very fine balance between the ionic liquid and the gelator exists. For example, the transfer of the gelator's network takes place efficiently in [C₄-mim]NTf₂, and does not place at all in [C₄-mim]PF₆ and [C₄-mim]BF₄, ionic liquids whose structures and properties resemble those of [C₄-mim]NTf₂. Also, we have screened other known organogelators and found out that their phase transfer depends on identity of the ionic liquid as well.

3. Synthesis of novel gel-specific ionic liquids

We have synthesized new chiral, dicationic urea-containing ionic liquid motifs that should be of interest for modulating various supramolecular assemblies (Scheme 1A). Although, some urea-containing ionic liquids are known, their applications are limited exclusively to catalysis in the synthetic transformations.

In addition, we have explored the synthesis of amino acid-based ionic liquids. The preparations of either amino acid-based cation or anion have been reported.

We have made new ionic liquids that contain both chiral anion and chiral cation (Scheme 1B, a L-Pro/L-Pro ionic liquid is shown as an example) via a facile two step process. For example, using L-proline and D-proline, it is possible to make four different ionic liquids, which significantly expands the scope of chiral ionic liquids. We are currently examining the properties of these doubly-chiral ionic liquids.

4. Studies on quadruple hydrogen bonding in ionic liquids

Quadruple hydrogen bonding motifs are known as viable platforms for directing interactions leading to various supramolecular assemblies. We have prepared a novel chiral ureidopyrimidone 1 (Scheme 2) and examined its self-association in molecular and ionic liquids.

In pure DMSO (a solvent known for disrupting intermolecular hydrogen bonding), H_a, H_b, and H_c signals appeared at 8.05, 7.22 and 6.23 ppm (¹H NMR). In non-solvating CDCl₃, 1 H_a, H_b, and H_c were downfield shifted 12.03 and 10.65ppm, respectively; thus confirming the formation of dimer 2.

We examined the ability of several ionic liquids to affect the dimerization, i.e., self-association of 1: a 24 mM solution of 1 in CD₂Cl₂ exhibited resonances at 12.98, 12.03 and 10.65ppm. Addition of [C₄-mim]BF₄, up to 19% (v/v) had no affect on the 1 – 2 equilibrium, which was significantly favored towards 2. Higher content of the ionic liquid precluded the detection of any resonances of 1. Next, 24 mM solution of 1 in CD₂Cl₂ was titrated with various amounts of [C₄-mim]Br, where Br is a much stronger coordinating anion, and therefore, has a higher chance of disrupting the dimerization of 1. However, similarly to [C₄-mim]BF₄, which has a low coordinating BF₄ anion, ¹H NMR resonances indicating the presence of 2 were still observed in the solution of [C₄-mim]Br.

Dissolution of 1 into several ionic liquids produced viscous oils drastically increased viscosity. This suggested that the formation of a supramolecular polymeric network was realized. We used absorbance and circular dichroism (CD) spectroscopy. CD spectra of 1 in several molecular and ionic liquids (Figure 4) revealed that the CD signal, and therefore, the structure of the self-assembly, strongly depends on the nature of the solvent. Remarkably, in [C₄-mim]BF₄ the CD signal was found to be distinct from CH₂Cl₂ (favors the formation of dimmer 2) and DMSO (the solvent that favors the monomer 1). Although preliminary, these studies demonstrate that the self-assembly process of a supramolecular polymer could be studied in ionic liquids.