Synthesis and Characterization of Monosaccharide-Derived

pubs.acs.org/Langmuir © 2009 American Chemical Society

Synthesis and Characterization of Monosaccharide-Derived Carbamates as Low-Molecular-Weight Gelators† Guijun Wang,* Sherwin Cheuk, Hao Yang, Navneet Goyal, P. V. Narasimha Reddy, and Branden Hopkinson Department of Chemistry, University of New Orleans, New Orleans, Louisiana 70148 Received December 31, 2008. Revised Manuscript Received March 12, 2009 Sugar-based low-molecular-weight gelators are an interesting new class of compounds that are important in supramolecular chemistry and for the preparation of advanced materials. Previously, we synthesized a series of ester and carbamate derivatives of 4,6-O-benzylidene methyl-R-D-glucopyranoside and found that monosubstituted alkynyl esters with five to seven carbons and monosubstituted carbamates with saturated five- and seven-carbon chains are good gelators. To understand the structural requirement for the gelation of the carbamate derivatives (O-linked carbamates), a diverse series of analogs, including alkynyl, aryl, and alkyl halide derivatives, were prepared and analyzed. We found that for gelation the O-linked carbamate derivatives have different structural preferences than the ester derivatives. To exhibit gellation, the ester analogs favor alkyl-containing terminal acetylene groups and the carbamoyl derivatives prefer saturated hydrocarbons. Both the esters and the carbamates showed good gelation properties when they were functionalized with aryl side chains. We also synthesized and screened a new series of carbamates (N-linked carbamates) in which the nitrogen atom of the carbamate group is directly attached to the sugar ring. The N-linked carbamates are good gelators for aqueous DMSO and ethanol solutions, and two of the compounds are also able to form gels in pure water. Optical microscopy and scanning electron microscopy were used to characterize several representative gels. In general, long, narrow, uniform fibrous networks were observed for effective gelators. The structure-gelation correlation obtained here can be used in the design of new sugar-based low-molecular-weight gelators.

Introduction The preparation of functional supramolecular architectures, via the self-assembly of small molecules, is a useful method for designing novel materials with desirable physical properties. Among the many soft materials that can be formed via self-assembly, the reversible gels formed by low-molecular-weight gelators (LMWGs) have shown increasing promise in many areas, including supramolecular chemistry and material science. LMWGs are an interesting new class of small molecules that have gained great attention over the past few decades.1-4 These compounds form reversible physical gels that depend solely on intermolecular, noncovalent forces such as π-π stacking, hydrogen bonding, hydrophobic interactions, and van der Waals forces. These various interactions allow molecules with certain structural features to self-assemble into continuous 3D networks that are capable of entrapping organic solvents or aqueous solutions. The soft materials thus formed have † Part of the Gels and Fibrillar Networks: Molecular and Polymer Gels and Materials with Self-Assembled Fibrillar Networks special issue. *Corresponding author. Phone: 504 280-1258. Fax: 504 280-6860. E-mail: [email protected].

(1) (a) George, M.; Weiss, R. G. Acc. Chem. Res. 2006, 39, 489. (b) Terech, P.; Weiss, R. G. Chem. Rev. 1997, 97, 3133. (c) Abdallah, D. J.; Weiss, R. G. Adv. Mater. 2000, 12, 1237. (2) (a) Gronwald, O.; Snip, E.; Shinkai, S. Curr. Opin. Colloid Interface Sci. 2002, 7, 148. (b) Maitra, N.; Maitra, U. Org. Biomol. Chem. 2008, 6, 657. (c) van Esch, J. H.; Feringa, B. L. Angew. Chem., Int. Ed. 2000, 39, 2263. (3) Estroff, L. A.; Hamilton, A. D. Chem. Rev. 2004, 104, 1201. (4) (a) Yang, Z.; Liang, G.; Xu, B. Acc. Chem. Res. 2008, 41, 315. (b) Yang, Z.; Liang, G.; Xu, B. Soft Matter 2007, 3, 515. (c) Yang, Z.; Xu, B. Adv. Mater. 2006, 18, 3043. (5) (a) Koshi, Y.; Nakata, E.; Yamane, H.; Hamachi, I. J. Am. Chem. Soc. 2006, 128, 10413. (b) Kiyonaka, S.; Sada, K.; Yoshimura, I.; Shinkai, S.; Kato, N.; Hamachi, I. Nat. Mater. 2004, 3, 58. (6) (a) Vintiloiu, A.; Leroux, J.-C. J. Controlled Release 2008, 125, 179. (b) Shaikh, I. M.; Jadhav, K. R.; Kadam, V. J.; Pisal, S. S. Drug Delivery Technol. 2007, 7, 60. (7) Bhuniya, S.; Seo, Y. J.; Kim, B. H. Tetrahedron Lett. 2006, 47, 7153. (8) Ray, S.; Das, A. K.; Banerjee, A. Chem. Mater. 2007, 19, 1633.

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potential applications in enzyme immobilization,5 drug delivery systems,6-9 and advanced nanomaterials.10-12 Low-molecularweight hydrogelators (LMHGs) are particularly interesting because they can form supramolecular hydrogels that may find a variety of biological and biomedical applications.3,4,13-32 Much (9) Cao, S.; Fu, X.; Wang, N.; Wang, H.; Yang, Y. Int. J. Pharm. 2008, 357, 95. (10) Yang, Z.; Gu, H.; Du, J.; Gao, J.; Zhang, B.; Zhang, X.; Xu, B. Tetrahedron 2007, 63, 7349. (11) Matsumoto, S.; Yamaguchi, S.; Ueno, S.; Komatsu, H.; Ikeda, M.; Ishizuka, K.; Iko, Y.; Tabata, K. V.; Aoki, H.; Ito, S.; Noji, H.; Hamachi I. Chem. Eur. J. 2008, 14, 3977. (12) Del Guerzo, A.; Pozzo, J.-L.; Weiss, R. G.; Terech, P. Mol. Gels 2006, 817. (13) Menger, F. M.; Caran, K. L. J. Am. Chem. Soc. 2000, 122, 11679. (14) Suzuki, M.; Yumoto, M.; Kimura, M.; Shirai, H.; Hanabusa, K. Chem. Commun. 2002, 8, 884. (15) Mitra, R. N.; Das, D.; Roy, S.; Das, P. K. J. Phys. Chem. 2007, 111, 14107. (16) Roy, S.; Dasgupta, A.; Das, P. K. Langmuir 2007, 23, 11769. (17) de Loos, M.; Feringa, B. L.; van Esch, J. H. Eur. J. Org. Chem. 2005, 17, 3615. (18) Yang, Z.; Liang, G.; Ma, M.; Gao, Y.; Xu, B. J. Mater. Chem. 2007, 17, 850. (19) Suzuki, M.; Sato, T.; Shirai, H.; Hanabusa, K. New J. Chem. 2007, 31, 69. (20) Wang, G.; Hamilton, A. D. Chem. Comm. 2003, 3, 310. (21) Das, D.; Dasgupta, A.; Roy, S.; Mitra, R. N.; Debnath, S.; Das, P. K. Chem. Eur. J. 2006, 12, 5068. (22) Suzuki, M.; Owa, S.; Shirai, H.; Hanabusa, K. Tetrahedron 2007, 63, 7302.  c, M. Chem. Eur. J. (23) Jokic, M.; Peric, B.; Tomisic, V.; Kojic-Prodic, B.; Zini 2001, 7, 3328. (24) Bhat, S.; Maitra, U. Tetrahedron 2007, 63, 7309. (25) Wang, G.; Hamilton, A. D. Chem. Eur. J. 2002, 8, 1954. (26) Roy, S.; Das, P. K. Biotechnol. Bioeng. 2008, 100, 756. (27) Xing, B. G.; Yu, C. W.; Chow, K. H.; Ho, P. L.; Fu, D. G.; Xu, B. J. Am. Chem. Soc. 2002, 124, 14846. (28) Adarsh, N. N.; Kumar, D. K.; Dastidar, P. Tetrahedron 2007, 63, 7386. (29) (a) Yang, Z.; Ho, P.-L.; Liang, G.; Chow, K. H.; Wang, Q.; Cao, Y.; Guo, Z.; Xu, B. J. Am. Chem. Soc. 2007, 129, 266. (b) Yang, Z.; Liang, G.; Wang, L.; Xu, B. J. Am. Chem. Soc. 2006, 128, 3038. (30) Hwang, I.; Jeon, W. S.; Kim, H.-J.; Kim, D.; Kim, H.; Selvapalam, N.; Fujita, N.; Shinkai, S.; Kim, K. Angew. Chem., Int. Ed. 2007, 46, 210. (31) Koehler, K.; Meister, A.; Foerster, G.; Dobner, B.; Drescher, S.; Ziethe, F.; Richter, W.; Steiniger, F.; Drechsler, M.; Hause, G.; Blume, A. Soft Matter. 2006, 2, 77. (32) Yang, Z.; Liang, G.; Guo, Z.; Guo, Z.; Xu, B. Angew. Chem., Int. Ed. 2007, 46, 8216.

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effort has been devoted to finding new types of LMWGs, and their structures are quite diverse. Among these, amino acid-based gelators13-27 have been very thoroughly studied because of the availability of chiral amino acids and their ease of synthesis. Carbohydrates are readily available, naturally abundant, renewable resources that are useful as chiral pool materials.33 Because sugars contain multiple chiral centers, they are ideal in synthesizing compounds that are able to self-assemble into gellike structures. Carbohydrate-based low-molecular-weight gelators have been reported by several research groups.34-50 The formation of low-molecular-weight gelators from sugar derivatives can have potential impacts in advanced materials and supramolecular chemistry. The intrinsic chirality and biocompatibility of sugar-derived self-assembling systems can lend themselves to many special applications such as forming liquid crystals, separating biomolecules, and as controlled-release drug delivery systems.46,47 Our group has also been interested in synthesizing simple sugar derivatives and understanding their self-assembling behavior in both organic solvents and aqueous solutions.48-50 Previously, we synthesized a series of short-chain acyl derivatives of 4,6-Obenzylidene-R-methyl-glucopyranoside (1) and found that several compounds are good organo/hydrogelators.48 The structures of these good gelators (2-6) are shown in Figure 1. Compounds 2-4 are short-chain alkynyl ester derivatives that are able to gelate both hexane and water. Besides these short-chain derivatives, we had also prepared a series of long-chain diacetylene-containing esters that can form polymerizable gels in ethanol/water mixtures.49 The structure-gelation relationship of the esters has been further studied, and it was found that the terminal alkynyl group is essential in order for the short-chain monoesters to form gels; also, functionalization with an aryl substituent favors gel formation.50 Compounds 5 and 6 are straight-chain carbamate derivatives containing five or seven carbon alkyl chains and were found to be versatile gelators. In general, the carbamates formed more robust gels than did the esters. However, not enough information on the side-chain structures versus gelation properties for this type of carbamate is yet available. To understand the structural requirements of the side chains on the carbamate derivatives for gelation, a diverse series of analogs were synthesized and analyzed. (33) Hollingsworth, R. I.; Wang, G. Chem. Rev. 2000, 100, 4267. (34) Jung, J. H.; Rim, J. A.; Cho, E. J.; Lee, S. J.; Jeong, I. Y.; Kameda, N.; Masuda, M.; Shimizu, T. Tetrahedron 2007, 63, 7449. (35) Jung, J. H.; John, G.; Masuda, M.; Yoshida, K.; Shinkai, S.; Shimizu, T. Langmuir 2001, 17, 7229. (36) Ghosh, R.; Chakraborty, A.; Maiti, D. K.; Puranik, V. G. Org. Lett. 2006, 8, 1061–1064. (37) Friggeri, A.; Gronwald, O.; van Bommel, K. J. C.; Shinkai, S.; Reinhoudt, D. N. J. Am. Chem. Soc. 2002, 124, 10754. (38) Chakraborty, T. K.; Jayaprakash, S.; Srinivasu, P.; Madhavendra, S. S.; Ravi Sankar, A.; Kunwar, A. C. Tetrahedron 2002, 58, 2853. (39) (a) Luboradzki, R.; Gronwald, O.; Ikeda, A.; Shinkai, S. Chem. Lett. 2000, 10, 1148. (b) Gronwald, O.; Shinkai, S. Chem. Eur. J. 2001, 7, 4328. (40) Jung, J. H.; Amaike, M.; Nakashima, K.; Shinkai, S. J. Chem. Soc. 2001, 10, 1938. (41) Yoza, K.; Amanokura, N.; Ono, Y.; Akao, T.; Shinmori, H.; Takeuchi, M.; Shinkai, S.; Reinhoudt, D. N. Chem. Eur. J. 1999, 5, 2722. (42) Amanokura, N.; Yoza, K.; Shinmori, H.; Shinkai, S.; Reinhoudt, D. N. J. Chem. Soc., Perkins Trans. 1998, 2, 2585. (43) Luboradzki, R.; Gronwald, O.; Ikeda, M.; Shinkai, S.; Reinhoudt, D. N. Tetrahedron 2000, 56, 9595. (44) Kobayashi, H.; Friggeri, A.; Koumoto, K.; Amaike, M.; Shinkai, S.; Reinhoudt, D. N. Org. Lett. 2002, 4, 1423. (45) Kiyonaka, S.; Shinkai, S.; Hamachi, I. Chem. Eur. J. 2003, 9, 976. (46) Vemula, P. K.; Li, J.; John, G. J. Am. Chem. Soc. 2006, 128, 8932. (47) Yang, Z.; Liang, G.; Ma, M.; Abbah, A. S.; Lu, W. W.; Xu, B. D. Chem. Commun. 2007, 843. (48) Wang, G.; Cheuk, S.; Williams, K.; Sharma, V.; Dakessian, L.; Thorton, Z. Carbohydr. Res. 2006, 341, 705. (49) Nie, X.; Wang, G. J. Org. Chem. 2006, 71, 4734. (50) Cheuk, S.; Stevens, E.; Wang, G. Carbohydr. Res. 2009, in press.

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Figure 1. Structures of the headgroup used and the esters and carbamates prepared.

In comparison with the esters, the carbamates have an additional hydrogen bond donor, which may improve their gelation properties.

Results and Discussions In this study, we synthesized a series of compounds with the general structure 7 (O-linked carbamate), where R is various hydrophobic groups, and screened them for their gelation properties. To study the structure influence on gelation properties further, a new series of carbamates with the general structure 9 (N-linked carbamate), in which the nitrogen is attached to the headgroup and the side chains are linked to the oxygen atom, were also prepared using D-glucosamine derivative headgroup 8.

To prepare carbamates 7, the corresponding isocyanates were reacted with sugar headgroup 1 in the presence of a catalytic amount of triethylamine. When using about 1 equiv of isocyanate, 2-carbamate 7 is the major product, together with small amounts of 3-carbamate and dicarbamate, which can be separated by silica gel chromatography. Previously, we found that monocarbamates containing saturated alkyl chains with five or seven carbons were good gelators, whereas the dicarbamates typically are not gelators.48 We are interested in determining the role that the R group plays in the structure-gelation relationship of carbamates with general structure 7. This trend may be useful for predicting the 3-carbamate’s gelation behavior. To prepare carbamate 9, glucosamine-derived headgroup 8 was reacted with alkyl or aryl chloroformates under basic conditions. This reaction is usually selective towards the amino group and gives good yields of the desired carbamates. To understand the effect on gelation when changing the R group in carbamate 7, we synthesized a series of compounds (10-19) with different substituents (Figure 2). These include straight-chain six-carbon derivative 10, alkynyl derivatives 11-13, cyclohexyl derivative 14, methacryloyl derivative 15, and chloroethyl carbamate 16. We also prepared three aryl carbamates: the phenyl (17), p-bromophenyl (18), and 1-naphthyl (19) derivatives. To synthesize compounds 10-13, the corresponding isocyanates were prepared from the carboxylic acids by Curtius rearrangement. For the glucosamine-derived carbamates with the general structure of 9, we prepared a few representative compounds using commercially available chloroformates: ethyl derivative 20, allyl derivative 21, isobutyl derivative 22, n-butyl derivative 23, benzyl derivative 24, and phenyl carbamate 25 (Figure 2). After the pure carbamates were obtained, their gelation properties in several solvents were tested. A series of organic solvents DOI: 10.1021/la804337g

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Figure 2. Structures of synthesized carbamoyl derivatives. Table 1. Gelation Test Results for Carbamate Derivatives with the General Structure of 7

For compounds formed from gels, their minimum gelation concentrations (MGCs) are given in mg of gelator/mL of solvent, indicated by * for unstable gels. The appearances of the gels are labeled with subscripted letters: C for clear or transparent, T for translucent, O for opaque. I indicates insoluble, P indicates precipitate, and S indicates soluble at 20 mg/mL.

were screened at 20 mg/mL concentration, and all carbamates (11-25) were found to be soluble in the following organic solvents: chloroform, dichloromethane, acetone, and THF. The gelation test results in several other solvents are given in Tables 1 and 2. The same solvents used in our previous studies48 were also screened here. We are more interested in finding effective LMWGs for water or aqueous solutions because of their potential applications in drug delivery and biological studies. Because ethanol and dimethyl sulfoxide (DMSO) are polar organic 8698 DOI: 10.1021/la804337g

solvents that are miscible with water, aqueous ethanol and aqueous DMSO were tested. DMSO is a very good solvent for solubilizing many organic molecules. Although no longer used in human medicine, DMSO is still widely used in veterinary medicine and is a common industrial solvent. The gelation of aqueous DMSO mixtures may find applications in drug delivery studies. From Table 1, we can see that the O-linked carbamates with the general structure of 7 are efficient gelators for aqueous ethanol and DMSO mixtures. Presumably, the amide bond portion of the Langmuir 2009, 25(15), 8696–8705

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Article Table 2. Gelation Test Results for Carbamate Derivatives with the General Structure of 9

For compound-formed gels, their MGCs are given in mg/mL of solvent, with * indicating unstable gels: I indicates insoluble, P indicates precipitate, and S indicates soluble at 20 mg/mL. The appearances of the gels are labeled with subscripted letters: C for clear or transparent, T for translucent, and O for opaque.

carbamate acting as an additional hydrogen bond donor helps to form a more stable hydrogen bonding network among the carbamate molecules and solvents. Previously, we found that in order for monosubstituted esters to be good gelators, the terminal alkynyl group is important (saturated hydrocarbon chains are not effective).50 However, in the carbamates, the presence of the terminal alkynyl group is not necessary for gelation. Hexyl carbamate 10 and its 3-isomer can form gels in aqueous solutions much like the five- and seven-carbon analogs reported before.48 Compound 11, with the 4-pentynoyl side chain, formed an opaque gel in aqueous DMSO and an unstable gel in aqueous ethanol. Compound 12, which contains a slightly longer six-carbon alkynyl group, formed a transparent gel in ethanol/water at lower concentration but did not form gels in DMSO/water. Compound 13 with an eight-carbon alkynyl group formed translucent gels in the ethanol/water mixture. The comparable efficiency of 13 with the shorter-chain derivatives is probably due to the fact that the longer alkynyl chain can increase hydrophobic interactions and compensate for the presence of the acetylene group, which may disrupt alkyl chain packing. Cyclohexyl carbamate 14 formed translucent gels in EtOH/water and transparent gels in DMSO/ water. The other alkyl derivatives, methacrylate derivative 15 and chloroethyl carbamate 16, were not able to form gels in any of the solvents tested here. However, phenyl derivative 17 and bromophenyl derivative 18 showed excellent gelation properties in aqueous solutions. Naphthyl derivative 19 was able to form gels only in the DMSO/water mixture. This compound is quite hydrophobic, and it is either insoluble or precipitates/crystallizes out of solution if the solvents cannot disrupt the packing of the compound with itself. For the N-linked carbamates derived from glucosamine headgroup 8, we observed some interesting trends. As shown in Langmuir 2009, 25(15), 8696–8705

Table 3. Gelation Temperature of Several Compounds in EtOH/H2O (1/2)a

Tsol (°C) Tgel (°C)

11

12

13

14

17

18

21

22

23

24

72 42

76 43

66 64

>100 76

>100 77

91 22

85 54

91 68

84 62

>100 89

a Tsol is the temperature when the sample is fully dissolved (melted). Tgel is the temperature when the gel is fully formed. This is estimated on the basis of their maximum turbidity. The concentration is 5 mg/mL for samples 11-14, 17, 18, 21, and 22 and 10 mg/mL for 23 and 24. The error in the reading is (1 °C.

Table 2, compounds 20-25 also showed good gelation properties in aqueous solutions, compared to their isomers 10-19. Among these compounds, the short alkyl derivatives are the most effective gelators. Ethyl derivative 20 and isobutyl derivative 22 were able to form gels in pure water at concentrations lower than 1 wt %. Isobutyl derivative 22 is a very efficient gelator for most of the solvents tested; in contrast, n-butyl derivative 23 does not form gels in water. The isobutyl group seems to play an important role in the molecular assembly by disturbing the ordered packing of the straight propyl chain and increasing the interactions with the solvents. The allyl and isobutyl derivatives (21, 22) were both able to form gels in ethanol/water at about 2 mg/mL. Both compounds also formed gels in aqueous DMSO, though isobutyl derivative 22 is a more efficient gelator than allyl derivative 21. Phenyl derivatives 25 and 17 are close analogs and differ only at the nitrogen and oxygen positions. Whereas compound 25 formed a gel only in aqueous DMSO at higher concentration, compound 17 was able to gelate both aqueous DMSO and aqueous ethanol solutions effectively. This result is interesting in terms of understanding the differences in the gelation trends between the N-linked and O-linked carbamates. We measured the IR absorptions of the NH stretching for the two compounds in both the DOI: 10.1021/la804337g

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Figure 3. Optical micrographs of the wet gels formed by compounds 14, 18, 22, and 24. (A, B) Compound 14 in EtOH/H2O (1:2) at 1.0 mg/mL; (C) compound 14 in DMSO/H2O (1:2) at 1.1 mg/mL; (D) compound 18 in EtOH/H2O (1:2) at 2.0 mg/mL; (E) compound 22 in H2O at 4 mg/mL; (F, G) compound 22 in EtOH/H2O (1:2) at 2.5 mg/mL; and (H, I) compound 24 in EtOH/H2O (1:2) at 10 mg/mL. The magnification in B, G, and I is 500; that in A, C-F, and H is 200.

solid form and gel form (both at 20 mg/mL, DMSO/H2O). The two compounds gave similar patterns in the NH stretching regions. The solid of compound 17 has a sharp NH stretching absorption at 3290 cm-1 and a broad shoulder peak at ∼3380 cm-1 for the OH stretching. Compound 25 has a sharp NH absorption at 3323 cm-1 and a broad OH absorption at ∼3421 cm-1. For the gels of both compounds, their NH stretching frequencies stayed the same as for their solid form, but the OH absorptions disappeared. Further temperature-dependent IR studies on the gels may provide some insights into the two types of carbamates.51 We also measured the gelation temperature for several compounds in EtOH/H2O. These are shown in Table 3. It is noted that the gelation temperatures are generally lower than their melting temperatures. The carbamates derived from compound 8 can be synthesized more easily than their analogs, especially on a larger scale because the 2-amino group selectively reacts with electrophiles to give only the N-substituted product. In the case of the carbamates derived from headgroup 1, two to three products can be formed, and the isolation of pure products requires careful chromatography. Also, several of the N-linked carbamates prepared in this study were able to form gels in a wider range of solvents, including nonpolar organic solvents, water, and aqueous solutions. Therefore, the new type of carbamate derived from the glucosamine can be further explored for applications. One possible obstacle is the (51) Pierce, A. M.; Maslanka, P. J.; Carr, A. J.; McCain, K. S. Appl. Spectrosc. 2007, 61, 379.

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commercial availability of the chloroformates; however, they can be synthesized via short steps. Among all of the compounds synthesized here, cyclohexyl derivative 14 is the most efficient gelator for water/ethanol and water/DMSO mixtures, forming gels at about 0.1 wt %. The optical micrographs (OMs) of several wet gels are shown in Figure 3. The optical microscopy studies revealed that the compounds that are able to form stable gels at low concentrations typically showed long fibrous assemblies. The gel morphology of cyclohexyl derivative 14 in an aqueous ethanol mixture had very long fibrous network structures (Figure 3A,B). The fibers have small diameters and much longer lengths and are flexible and homogeneous in morphology. These fibrous assemblies are also birefringent. The gel of compound 14 in DMSO/H2O showed some smooth morphology along with some embedded fibers (Figure 3C). 4-Bromophenyl carbamate 18 was able to form very sturdy translucent gels in aqueous ethanol efficiently. The morphology of this gel showed bundles of small fibers or rods assembled together (Figure 3D). The gel formed by compound 22 in water showed larger tubular and rod-type aggregates (Figure 3E). Its gel in EtOH/water showed smaller fibrous networks (Figure 3F,G), and benzyl carbamate 24 appeared to have narrow fibrous features (Figure 3H,I). In general, the morphologies of the gels are dependent on their structures and the gelation properties. We observed that if the compound forms stable gels in the solvents efficiently then they tend to form very long, soft fibrous networks. For compounds that formed unstable gels or precipitates, we can see crystalline, sheet-type structures, together with some shorter fibrous aggregates. Langmuir 2009, 25(15), 8696–8705

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Figure 4. Scanning electron micrographs of the gels formed by compounds 11, 14, and 18. (A, B) gels formed by 14 in DMSO/H2O (1:2) at 1.1 mg/mL; (C, D) gels formed by 14 in EtOH/H2O (1:2) at 1.0 mg/mL; (E-G) gels formed by 11 in EtOH/H2O (1:2) at 10 mg/mL; and (H, I) gels formed by 18 in EtOH/H2O (1:2) at 2.0 mg/mL.

Figure 5. Scanning electron micrographs of the gels formed by compounds 22 and 24. (A, B) Gels formed by 22 in H2O at 4.0 mg/mL; (C, D) gels formed by 22 in EtOH/H2O (1:2) at 2.5 mg/mL; and (E, F) gels formed by 24 in EtOH/H2O (1:2) at 10 mg/mL.

To characterize the gels more thoroughly, we also obtained scanning electron micrographs (SEMs) of several compounds. Although OMs can give some information on the morphologies of gels, SEMs can give much better resolution for observing the gels in greater detail. These are shown in Figures 4 and 5. Langmuir 2009, 25(15), 8696–8705

Figure 4 shows the SEMs of O-linked carbamates 11, 14, and 18. The gel of carbamate 14 in DMSO/H2O showed nice uniform fibrous structures (Figure 4A,B). At lower magnification, the gel appeared to be composed of long, thin, uniform fibers with a length of >100 μm; at higher resolution, the fibers appeared to DOI: 10.1021/la804337g

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have diameters of 100 μm and diameters of 100 μm and diameters >1 μm. Benzyl carbamate 24 formed a slightly different morphology, as shown in Figure 5E, F. The gel formed more flat sheets or ribbon-type features; however, all of these are composed of a fibrous network with very narrow diameters (Figure 5E, F). The fibrous networks appeared to be intertwined, and it is hard to estimate their length, though the width is typically