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Effects of Hydrogen Bonding and van der Waals Interactions on Organogelation Using Designed Low-Molecular-Weight Gelators and Gel Formation at Room Temperature Masahiro Suzuki,*,† Yasushi Nakajima,‡ Mariko Yumoto,‡ Mutsumi Kimura,‡ Hirofusa Shirai,‡ and Kenji Hanabusa† Graduate School of Science and Technology and Department of Functional Polymer Science, Shinshu University, Ueda, Nagano 386-8568, Japan Received May 6, 2003. In Final Form: July 14, 2003 We synthesized novel L-lysine-based compounds that were classified into three groups, urea-urea types (1-3), amide-urea types (4-6), and amide-amide type (7), and examined the effects of hydrogen bonding and van der Waals interactions on the organogelation behavior. Among these compounds, gelator 5 has the best organogelation ability. Moreover, the organogelation can be achieved at room temperature through the direct synthesis of gelators in organic solvents.
Organogels, in which organic solvents are gelled by lowmolecular-weight compounds (organogelators), have attracted much interest as a result of their unique features and potential applications for new organic soft materials.1 Many organogelators have been reported in the literature.2 These organogelators create a three-dimensional network by self-organization through noncovalent interactions such as hydrogen bonding, van der Waals interactions, π stacking, and coordination. Furthermore, organogelators and their organogels have been used for the fabrication of templated materials, sensors, and assemblies with molecular recognition and other properties.3,4 For organogelation, both the self-assembling of the gelator molecules into nanofibers via hydrogen bonding and the creation of the three-dimensional network structures via van der Waals interaction are important; therefore, the suitable balance of hydrogen bonding and van der Waals interactions is needed. Here, we describe the effects of hydrogen bonding and van der Waals forces on the organogelation behavior using newly designed L-lysine-type organogelators with different numbers of * To whom correspondence should be addressed. E-mail:
[email protected]. Fax: +81-268-21-5608. † Graduate School of Science and Technology, Shinshu University. ‡ Department of Functional Polymer Science, Shinshu University. (1) Terech, P.; Weiss, R. G. Chem. Rev. 1997, 97, 3133. (b) van Esch, J. H.; Feringa, B. L. Angew. Chem., Int. Ed. 2000, 39, 2263. (2) Abdallah, D. J.; Weiss, R. G. Langmuir 2000, 16, 352-355. (b) de Loos, M.; van Esch, J.; Kellogg, R. M.; Feringa, B. L. Angew. Chem., Int. Ed. 2001, 40, 613. (c) Lescanne, M.; Colin, A.; Mondain-Monval, O.; Heuze´, K.; Fages, F.; Pozzo, J.-L. Langmuir 2002, 18, 7151. (d) George, M.; Weiss, R. G. Langmuir 2002, 18, 7124. (e) van Gorp, J. J.; Vekemans, J. A. J. M.; Meijer, E. W. J. Am. Chem. Soc. 2002, 124, 14759. (f) Friggeri, A.; Gronwald, O.; van Bommel, K. J. C.; Shinkai, S.; Reinhoudt, D. N. J. Am. Chem. Soc. 2002, 124, 10754. (g) Yun, Y. J.; Park, M. P.; Kim, B. H. Chem. Commun. 2003, 254. (3) Li, S.; John, V. T.; Irvin, G. C.; Bachakonda, S. H.; McPherson, G. L.; O’Connor, C. J. J. Appl. Phys. 1999, 85, 5965. (b) Jung, J. H.; Kobayashi, H.; Masuda, M.; Shimizu, T.; Shinkai, S. J. Am. Chem. Soc. 2001, 123, 8785. (c) Kobayashi, S.; Hamasaki, N.; Suzuki, M.; Kimura, M.; Shirai, H.; Hanabusa, K. J. Am. Chem. Soc. 2002, 124, 6550. (d) Llusar, M.; Pidol, L.; Roux, C.; Pozzo, J. L.; Sanchez, C. Chem. Mater. 2002, 14, 5124. (4) Hanabusa, K.; Hiratsuka, K.; Kimura, M.; Shirai, H. Chem. Mater. 1999, 11, 649. (b) Kubo, W.; Murakoshi, K.; Kitamura, T.; Yoshida, S.; Hanabusa, K.; Shirai, H.; Wada, Y.; Yanagida, S. J. Phys. Chem. B 2001, 105, 12809. (c) Sugiyasu, K.; Tamaru, S.; Takeuchi, M.; Berthier, D.; Huc, I.; Oda, R.; Shinkai, S. Chem. Commun. 2002, 1212.
Figure 1. Chemical structures of gelators 1-7.
hydrogen-bonding groups as well as the alkyl chain lengths that can be simply synthesized5 and organogelation at room temperature. These compounds can be classified into three groups (Figure 1): 1-3 are urea-urea-type compounds that have six potential hydrogen-bonding sites (four NsH and two CdO), 4-6 are amide-urea-type compounds that have five potential hydrogen-bonding sites (three NsH and two CdO), and 7 was an amide-amide-type compound that has four potential hydrogen-bonding sites.6 Table 1 lists the organogelation properties of 1-7 in various organic solvents. Among 3, 5, and 7, which have almost the same van der Waals force and hydrogen-bonding force order of 3 > 5 > 7, 5, possessing five potential hydrogen-bonding sites, had the best organogelation ability, and 7 (amideamide type) had no gelation ability. Compared with 5, 3 undergoes the strong hydrogen bonding, and the hydrogenbonding force of 7 is too weak. Under the same hydrogenbonding force condition, the organogelation ability significantly depends on the alkyl chain length (van der Waals force). In the urea-urea types 1-3 (van der Waals force order of 1 < 2 < 3) and the amide-urea types 4-6 (4 < 5 < 6), 2 and 5 have the best organogelation ability. The existence of the optimum number of hydrogen-bonding sites and alkyl chain lengths imply that the balance of the (5) See Supporting Information. (6) In our cases, the ester group is not counted among the hydrogenbonding sites because it does not participate in any intermolecular interactions.
10.1021/la034772v CCC: $25.00 © 2003 American Chemical Society Published on Web 09/19/2003
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Table 1. Gelation Properties of 1-7 in Organic Solvents at 25 °Ca c-C6H12 MeOH EtOH 1-PrOH n-BuOH AcOEt acetone THF dioxane Ph-H Ph-CH3 Ph-Cl Ph-NO2 DMF DMSO CHCl3 CCl4 CH3CN c-hexanone
1
2
3
4
5
6b
7
PG S S S S 30 PG 40 20 20 20 30 8 S S S 20 20 35
PG S S S S 10 35 30 15 12 15 25 4 S S S 30 PG 25
15 PG PG PG PG S 15 PG 15 50 50 50 10 28 27 S 18 P 15
10 S S S S 20 17 30 20 45 25 22 13 25 15 S 30 18 18
12 20 20 PG 40 14 14 16 20 22 14 18 9 28 14 S 20 18 14
30 30 PG PG PG 25 40 PG 23 27 30 30 17 40 25 S PG 23 50
P S S S S P P S P PG VS PG PG P 30 S P P S
a
Values denote the minimum gel concentration (mg/mL) necessary for organogelation. b Reference 4a. S, solution; VS, viscous solution; PG, partial gel; P, precipitation.
Figure 2. FE-SEM image of CCl4 gels formed by 5.
hydrogen-bonding force and van der Waals one is very important for organogelation. Of all the compounds, 5 has the best organogelation ability, indicating that 5 possesses the perfect balance of these forces. The gelation mechanism was elucidated by transmission electron microscopy observation and Fourier transform infrared (FT-IR) and 1H NMR studies. To obtain visual insights into the aggregation mode of these gelators in organogels, we took a field emission scanning electron microscopy (FE-SEM) image of the organogel. Figure 2 shows the FE-SEM image of the CCl4 gel of 5. In the CCl4 gel, 5 creates three-dimensional networks formed by entangling self-assembled nanofibers with fiber diameters of 20-60 nm, which indicates that the organogelation takes place by trapping the solvent molecules into the spaces in the three-dimensional networks. The chemical shifts of the three protons of the NsH groups in CDCl3/CCl4 (3:7) shifted to a lower field than that in CDCl3 (Figure 3, top).7 In the FT-IR spectra, the absorption bands at 3445 and 1658 cm-1 in CHCl3, characteristic of the non-hydrogen-bonded amide A (νNs H) and amide I (νCdO), shifted to a lower wavelength in (7) The 1H NMR spectrum of 5 in CCl4 could not be obtained because 5 formed a gel under the experimental conditions.
Figure 3. 1H NMR spectra of 5 in CDCl3 and CDCl3/CCl4 (3:7 v/v; top). FT-IR spectra of 5 in CHCl3 solution (dashed line) and in CCl4 gel (solid line; bottom). Scheme 1. Organogelation at Room Temperature in Toluene Using Methyl 2,6-Diisocyanatohexanoate and Octylamine
the CCl4 gel (Figure 3, bottom). Such spectral shifts are compatible with the presence of intermolecular hydrogenbonded amide and urea groups. Furthermore, the absorption bands of the antisymmetric (νas) and symmetric (νs) CH2 stretching vibrational modes of 5 are observed at 2929 cm-1 (νas, CsH) and 2856 cm-1 (νs, CsH) in CHCl3, while they shift to 2921 and 2849 cm-1 in the CCl4 gel, respectively. The lower wavenumber shift reveals a decrease in the fluidity of the alkyl chains8 due to the strong organization of the alkyl groups via a van der Waals interaction. Consequently, the driving forces for organogelation followed by entanglement of the self-assembled nanofibers are mainly hydrogen bonding and van der Waals interactions. In many cases, organogelations need a heatingdissolution process. Such a process is a disadvantage for many industrial applications of organogelators. We tried the organogelations at room temperature using methyl 2,6-diisocyanatohexanoate and alkylamines that are precursors of gelators 1-3. The typical experimental procedure is shown in Scheme 1.9 Very interestingly, after the mixing of the amine and diisocyanate, the toluene gel immediately formed. This is the reason gelators 1-3 are directly formed in toluene by the reaction of a highly reactive diisocyanate compound with the alkylamine,10 which leads to organogelation. The organogels show the same structural properties (FT-IR, 1H NMR) as those made by 1-3 and the same gel properties such as the ther(8) Yamada, N.; Imai, T.; Koyama, E. Langmuir 2001, 17, 961. (9) Procedure: To a toluene solution (1 mL) of methyl 2,6-diisocyanatohexanoate (13.6 mg) was added a toluene solution (1 mL) of octylamine (16.6 mg) at room temperature. (10) The products were characterized by 1H NMR, FT-IR, and elemental analysis.
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moreversibilities. Furthermore, this procedure can apply for all solvents that gelators 1-3 can gel, as shown in Table 1. In summary, we revealed the effects of hydrogen bonding and van der Waals forces on the organogelation using new L-lysine-based gelators with various numbers of hydrogen-bonding sites as well as alkyl chain lengths and the organogel formation at room temperature. For the L-lysine-based organogelators, gelator 5 possessing the five potential hydrogen-bonding sites and dodecyl groups at both terminals has the best organogelation ability. Using methyl 2,6-diisocyanatohexanoate and alkylamines that are the precursors of organogelators 1-3, organogelation at room temperature can be achieved.
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Acknowledgment. This study was supported by a Grant-in-Aid for The 21st Century CEO Program, a Grantin-Aid for Exploratory Research (14655358), and a Grantin-Aid for Young Scientists (B; 15750117) from the Ministry of Education, Sports, Culture, Science and Technology of Japan. Supporting Information Available: Experimental details concerning the techniques used (gelation test, 1H NMR, FT-IR, FE-SEM), synthetic procedures, and characterization data of 1-7. This material is available free of charge via the Internet at http://pubs.acs.org. LA034772V
