pubs.acs.org/Langmuir © 2009 American Chemical Society
Organogels Based on 1H-Imidazolecarboxamide Amphiphiles† Sang Hyuk Seo, Jun Ha Park, and Ji Young Chang* Department of Materials Science and Engineering, College of Engineering, Seoul National University, Seoul 151-744, Korea Received December 30, 2008. Revised Manuscript Received March 2, 2009 We prepared 1H-imidazolecarboxamide amphiphiles as potential organogelators. Compounds A1-A3, in which an imidazole head was connected to a hydrophobic trialkyloxyphenyl group, showed an ability to gelate nonpolar solvents, including alkanes. The dry gels obtained from compounds A1-A3 had columnar hexagonal structures. Polycatenar 1H-imidazolecarboxamide amphiphile B2, consisting of a 1H-imidazole head connected through a benzene ring to a tridecyloxyphenyl tail, formed an organogel in DMSO. In a concentrated THF solution (30 wt %), compound B2 exhibited a lyotropic liquid-crystalline phase with a columnar hexagonal structure. X-ray diffraction (XRD) results suggested a molecular arrangement consisting of a disk, via hydrogen bonding between successive imidazole moieties, and an assembly of columnar structures.
Introduction Amphiphilic molecules have a tendency to self-assemble into micelles, cylindrical micellar fibers, and bilayers in aqueous solution.1-4 In nonpolar organic solvents, they form reverse aggregates such as reverse micelles and reverse bilayers. Certain amphiphiles can gelate an organic solvent at low concentrations below 5 wt % to produce an organogel.5-10 Organogels are thermoreversible viscoelastic materials consisting of low-molecular-weight gelators and organic solvents. Gelation is believed to proceed through the self-assembly of gelator molecules into fibers and their entanglement. Self-assembly via hydrogen bonding in combination with π-π interactions and solvophobic effects is the most common process of gelation. Typical examples of hydrogen bond-based gelators are amide-,11-16 urea-,17 amino *Corresponding author. Tel: +82-2-880-7190. Fax: +82-2-885-1748. E-mail:
[email protected]. † Part of the Gels and Fibrillar Networks: Molecular and Polymer Gels and Materials with Self-Assembled Fibrillar Networks special issue.
(1) Dan, N.; Shimoni, K.; Pata, V.; Danino, D. Langmuir 2006, 22, 9860– 9865. (2) Jiang, Y.; Wang, Y.; Ma, N.; Wang, Z.; Smet, M.; Zhang, X. Langmuir 2007, 23, 4029–4034. (3) Ghosh, A.; Dey, J. Langmuir 2008, 24, 6018–6026. (4) Marques, E. F.; Brito, R. O.; Silva, S. G.; Rodrigues-Borges, J. E.; do Vale, M. L.; Gomes, P.; Arauio, M. J.; Soderman, O. Langmuir 2008, 24, 11009–11017. (5) Gao, P.; Zhan, C.; Liu, M. Langmuir 2006, 22, 775–779. (6) Ballabh, A.; Trivedi, D. R.; Dastidar, P. Chem. Mater. 2003, 15, 2136– 2140. (7) Hafkamp, R. J. H.; Feiters, M. C.; Nolte, R. J. M. J. Org. Chem. 1999, 64, 412–426. (8) Simmons, B.; Li, S.; John, V. T.; McPherson, G. L.; Taylor, C.; Schwartz, D. K.; Maskos, K. Nano Lett. 2002, 2, 1037–1042. (9) Jung, J. H.; Shinkai, S.; Shimizu, T. Chem.;Eur. J. 2002, 8, 2684– 2690. (10) Jahnke, E.; Weiss, J.; Neuhaus, S.; Hoheisel, T. N.; Frauenrath, H. Chem.;Eur. J. 2009, 15, 388–404. (11) Escuder, B.; Mart, S.; Miravet, J. F. Langmuir 2005, 21, 6776–6787. (12) Escuder, B.; Miravet, J. F. Langmuir 2006, 22, 7793–7797. (13) Palui, G.; Banerjee, A. J. Phys. Chem. B 2008, 112, 10107–10115. (14) Suzuki, M.; Nakajima; Yumoto, M.; Kimura, M.; Shirai, H.; Hanabusa, K. Langmuir 2003, 19, 8622–8624. (15) Hashimoto, M.; Ujiie, S.; Mori, A. Adv. Mater. 2003, 15, 797–800. (16) Kim, C.; Kim, K. T.; Chang, Y.; Song, H. H.; Cho, T. Y.; Jeon, H. J. J. Am. Chem. Soc. 2001, 123, 5586–5587. (17) Dautel, O. J.; Robitzer, M.; Lre-Porte, J. P.; Serein-Spirau, F.; Moreau, J. J. E. J. Am. Chem. Soc. 2006, 128, 16213–16223.
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acid-,18,19 and carbohydrate-based compounds.20-22 In more concentrated solution, some amphiphilic molecules form lyotropic liquid-crystalline structures. Lyotropic columnar liquid-crystalline phases of amphiphilic molecules, which are highly ordered examples of supramolecular assemblies, may be obtained and manipulated by intra- or intermolecular hydrogen bonding. Di-, tri-, or tetramerization of identical half-discotic or dendritic molecules was reported to form columnar mesophases.23-26 Recently, we reported the self-assembly behaviors of 1H-imidazolecarboxylate amphiphiles in an organic solution.27 The amphiphiles, in which an imidazole head was connected to a hydrophobic alkyloxyphenyl group through an ester linkage, formed organogels. The intermolecular hydrogen bonding between 1H-imidazole heads, which was observed by 1H NMR spectroscopy, could be a driving force for gelation. In the dry gel state, the amphiphiles selfassembled into a fibrous aggregate having a reverse micellar cubic structure. However, their amide analogs, in which an ester linkage was replaced by an amide linkage, did not show any gelation ability, probably because the polar amide groups disturbed the assembly of the imidazole molecules. In this work, we prepared 1H-imidazolecarboxamide amphiphiles and studied their gelation in organic solvents. By introducing a trialkyloxyphenyl group into an imidazolecarboxamide, we expected to find strong π-π interactions, in addition to hydrogen bonding between the gelator molecules (18) Yan, X.; Cui, Y.; He, Q.; Wang, K.; Li, J. Chem. Mater. 2008, 20, 1522–1526. (19) Kuang, G. C.; Ji, Y.; Jia, X. R.; Li, Y.; Chen, E. Q.; Wei, Y. Chem. Mater. 2008, 20, 4173–4175. (20) Zhu, G.; Dordick, J. S. Chem. Mater. 2006, 18, 5988–5995. (21) Ghosh, R.; Chakraborty, A.; Maiti, D. K.; Puranik, V. G. Org. Lett. 2006, 8, 1061–1064. (22) Chen, W.; Yang, Y.; Lee, C. H.; Shen, A. Q. Langmuir 2008, 24, 10432–10436. (23) Lim, G. S.; Jung, B. M.; Lee, S. J.; Song, H. H.; Kim, C.; Chang, J. Y. Chem. Mater. 2007, 19, 460–467. (24) Barber, J.; Puig, L.; Romero, P.; Serrano, J. L.; Sierra, T. J. Am. Chem. Soc. 2005, 127, 456–464. (25) Rzepecki, P.; Hochdrffer, K.; Schaller, T.; Zienau, J.; Harms, K.; Ochsenfeld, C.; Xie, X.; Schrader, T. J. Am. Chem. Soc. 2008, 130, 586–591. (26) Peterca, M.; Percec, V.; Imam, M. R.; Leowanawat, P.; Morimitsu, K.; Heiney, P. A. J. Am. Chem. Soc. 2008, 130, 14840–14852. (27) Seo, S. H.; Chang, J. Y. Chem. Mater. 2005, 17, 3249–3254.
Published on Web 03/13/2009
DOI: 10.1021/la804319e
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Letter Scheme 1. Structures of 1H-Imidazolecarboxamide Amphiphiles
Table 1. Gelation Test Resultsa and Critical Gelation Concentrations (in Parentheses, wt %)b of A1-A3 and B2 (2 wt %) in Organic Solvents solvent
in organic solutions. Imidazole and its derivatives have very interesting functionalities, such as playing important roles in biological systems as the proton donor and/or acceptor, ligands of the coordination system, and the base of the charge-transfer processes. Imidazole-containing polymers have been studied as proton solvents, and they show high proton conductivity owing to the amphoteric property of the 1H-imidazole groups.28 In organogels, the nanosized fibers formed by the self-assembly of the gelator molecules are finely distributed. A controlled network structure of the gelator fibrils could be useful in optimizing the functional properties of 1H-imidazole materials.
A1
A2
A3
B2
n-hexane G (0.4) G (0.5) G (0.5) S n-octane G (0.4) G (0.5) G (0.5) S cyclohexane G (0.5) G (0.5) G (0.5) S toluene S S S S THF S S S S DMF S S S S DMSO S S S G (0.5) ethanol S S S P a G, stable gel at room temperature; S, soluble; P, precipitation. b Determined by a inverse flow method.
Results and Discussion Scheme 1 shows the structures of 1H-imidazolecarboxamide amphiphiles. The detailed synthesis and characterization data of 1H-imidazole amphiphiles are provided in Supporting Information. For the gelation test, a weighed amount of 1Himidazole amphiphiles A1-A3 and B1-B3 (2 wt %) in an organic solvent were heated in a septum-capped test tube (5 cm height 1 cm radius) until the solid dissolved. The solution was then left to air cool to room temperature. The state of the phase was confirmed by visual observations. Gel formation was observed during cooling or immediately after the cooling process. Compounds A1-A3 gelated various nonpolar solvents, including n-alkanes and cyclohexane, but did not form a gel in toluene, THF, DMF, DMSO, or ethanol. In contrast, compound B2 formed a gel only in DMSO. Compound B1, with shorter alkyloxy tails, and B3, with longer alkyloxy tails than compound B2, could not form stable gels. The gelation test results are summarized in Table 1. The SEM micrographs of the dry gels of compounds A1, A2, and A3 obtained from cyclohexane exhibited typical images of gel networks consisting of entangled fibers with diameters of 100-200 nm (Figure 1a). For further analysis of these self-assembled gel structures, tapping mode atomic force microscopy (AFM) was performed. The organogels prepared from 2 wt % cyclohexane solutions were carefully spread on a plasma-cleaned Si wafer. Amphiphiles A1-A3 directly selfassembled into fiber bundles with diameters of ∼100 nm. The sizes and morphologies observed by AFM were similar to those visualized by SEM (Figure 1b). The SEM micrograph of the dry gel of compound B2 obtained from DMSO showed a platelike structure (Figure S1a). The structures of the dry gels were investigated by X-ray diffraction (XRD). In the small-angle X-ray diffractogram of the dry gel of A1 obtained from cyclohexane, four reflections corresponding to d spacings of 31.1, 17.9, 15.5, and 11.8 A˚ were obtained. The√relative positions of these reflections were √ √ √ 1, 3, 4, and 7, which are in good agreement with the (100), (110), (200), and (210) reflections, respectively, of a (28) Schuster, M.; Meyer, W. H.; Wegner, G.; Herz, H. G.; Ise, M.; Schuster, M.; Kreuer, K. D.; Maier, J. Solid State Ionics 2001, 145, 85–92.
8440 DOI: 10.1021/la804319e
Figure 1. (a) SEM and (b) AFM height images of the dry gel of A1 (2 wt % in cyclohexane). Table 2. X-ray Diffraction (XRD) Data for the Dry Gels of Compounds A1, A2, A3, and B2 compound
phase
dobs [A˚]
hkl
lattice constants [A˚]
A1
Colh
Colh
A3
Colh
B2
Colr
100 110 200 210 100 110 200 210 100 110 100 010 110 200 120
a = 35.9
A2
31.1 17.9 15.5 11.8 36.8 21.4 18.2 13.8 40.8 23.8 40.9 32.4 25.8 20.2 14.6
a = 42.5
a = 47.1 a = 40.9, b = 32.4
columnar hexagonal phase with a lattice constant of 35.9 A˚. The dry gels of compounds A2 and A3 also showed several reflections of columnar hexagonal structures with lattice constants of 42.5 and 47.1 A˚, respectively. The dry gel of compound B2 obtained from DMSO formed a columnar rectangular structure. Five reflections corresponding to d spacings of 40.9, 32.4, 25.8, 20.2, and 14.6 A˚ were obtained, which were indexed in sequence as the (100), (010), (110), (200), and (120) reflections, respectively, of a columnar rectangular lattice with lattice parameters of a = 40.9 A˚ and b = 32.4 A˚. The XRD data for the dry gels are summarized in Table 2. In a concentrated THF solution (30 wt %), compound B2 exhibited a birefringent mesophase under a polarizing optical microscope, indicating the formation of a lyotropic liquid crystal. The solution (30 wt % in THF) was concentrated by evaporation under ambient conditions to generate a viscous residue for characterization by X-ray analysis. During this process, the molecules possibly further assembled into a more Langmuir 2009, 25(15), 8439–8441
Letter
Figure 2. Schematic representation of (a) the columnar hexagonal structure and (b) the columnar rectangular structure formed by compound B2. Table 3. Small-Angle X-ray Diffraction (XRD) Data for the Lyotropic Mesophases of Compounds B1-B3 compound
dobs [A˚]
phase
hkl
lattice constants [A˚]
na
37.4 100 a = 43.2 6 21.5 110 18.8 200 40.7 100 a = 47.0 7 B2 Colh 23.6 110 20.5 200 52.0 100 a = 60.0 10 B3 Colh 29.6 110 a Number of molecules arranged side by side in a single column slice. B1
Colh
ordered structure. In the small-angle X-ray diffractogram of the mesophase of compound B2 in THF, three reflection peaks appeared corresponding to d spacings of 40.7, 23.6, and 20.5. These reflections were indexed in sequence as the (100), (110), and (200) reflections, respectively, of a columnar hexagonal lattice with the lattice parameter of a = 47.0 A˚. Compounds B1 and B3 also formed lyotropic phases in THF (30 wt %), showing columnar hexagonal structures. The X-ray analysis results are summarized in Table 3. Compounds B1-B3 formed fibrillar bundles with diameters of about 1 μm when dried slowly from the lyotropic mesophases (Figure S1b). Because the structures of the amphiphiles were not disklike, we assumed that a column slice comprised more than one molecule. The number (n) of molecules constituting a single disk of the column can be estimated according to eq 1.29 pffiffiffi NA 3 h F n ¼ ða Þ 2 M þ mMsolvent 2
ð1Þ
Parameter a is the hexagonal lattice parameter, NA is Avogadro’s number, M is the molecular mass of the amphiphile, and mMsolvent is the mass of the solvent molecules surrounding the amphiphile molecule. The thickness (h) of a slice is about 0.45 nm, calculated on the basis of wide-angle X-ray diffractograms. Assuming a density (F) of 1 g cm-3 and neglecting mMsolvent in the case of the highly concentrated sample used for the X-ray measurement, the estimated values of the number of molecules (n) in a disk in the lyotropic columnar hexagonal state were 6 for B1, 7 for B2, and 10 for B3. :: (29) Borisch, K.; Diele, S.; Goring, P.; Kresse, H.; Tschierske, C. J. Mater. Chem. 1998, 8, 529–543.
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Because the lengths of the molecules were calculated to be 20.1 A˚ for B1, 22.2 A˚ for B2, and 23.6 A˚ for B3, using a simple molecular modeling method such as MM2, it is likely that the molecules were arranged in a disk to form a longer mesogen via hydrogen bonding between successive imidazole moieties and further assembled into a columnar structure, as shown in Figure 2. Chain structures of hydrogen-bonded imidazole molecules were found in both the crystal and solution states.30,31 We presumed that the organogels of the 1H-imidazole amphiphiles were formed by a similar hierarchical self-assembly process, considering the columnar structures of their dry gels.
Conclusions We prepared organogelators with a 1H-imidazole group. The 1H-imidazolecarboxamide amphiphiles (A1-A3) in which an imidazole head was connected to a hydrophobic trialkyloxyphenyl group through an amide linkage formed organogels in alkanes. When dried from the gel state, fibrous aggregates with a columnar hexagonal structure were formed. 1H-Imidazolecarboxamide amphiphile B2, consisting of a 1H-imidazole head connected through a benzene ring to a tridecyloxyphenyl tail, showed the ability to gelate DMSO. When dried from the gel state, platelike morphology with a columnar rectangular structure was obtained. In a concentrated THF solution (30 wt %), the compound formed lyotropic liquid crystals that had a columnar hexagonal structure. On the basis of their amphoteric property, the 1H-imidazole compounds exhibit very interesting functionalities that could be optimized through controlled assembly. Acknowledgment. This work was supported by a grant (R01-2007-000-10324-0) from the Korea Science and Engineering Foundation and a grant from the Global Research Laboratory (GRL) program. Supporting Information Available: Synthesis procedures and characterization data for the compounds. Gelation test results for compounds B1-B3. SEM image of the dry gel of compound B2. Small-angle X-ray diffractograms of the dry gels. This material is available free of charge via the Internet at http://pubs.acs.org. (30) Flakus, H. R.; Bryk, A. J. Mol. Struct. 1995, 372, 215–227. (31) Brzezinski, B.; Zundel, G. J. Mol. Struct. 1998, 446, 199–207.
DOI: 10.1021/la804319e
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