Organogel Formation by Coaggregation of Adaptable

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Organogel Formation by Coaggregation of Adaptable Amidocarbamates and Their Tetraamide Analogues Beatriu Escuder,* Santiago Martı´, and Juan F. Miravet* Departament de Quı´mica Inorga` nica i Orga` nica, Universitat Jaume I, 12071 Castello´ , Spain Received March 10, 2005. In Final Form: May 4, 2005 Mixtures of derivatives of Hanabusa’s bolaamphiphilic amidocarbamates containing two Z-valinyl subunits and aliphatic spacers that range from ethylenic to octamethylenic are able to form organogels. A coassembly of them is observed in their acetonitrile and toluene gels; namely, the concentration of a given compound at which a gel is formed is lowered by the presence of equimolar quantities of any other compound in the series. The aggregates were studied by wide-angle X-ray diffraction (WAXD) and the results can be rationalized if the gel fibers are formed by supramolecular copolymers. NMR studies reveal that in solution these molecules adopt folded conformations containing intramolecular H-bonds, but IR studies indicate that these are not present in their aggregates. Additionally, analogues of the amidocarbamates obtained by replacement of the carbamate functionality by amide have been shown to behave in a similar way. For these molecules it can be shown that the central aliphatic subunit is not completely extended in the conformations present in the aggregates according to IR and WAXD studies. The tetraamide-type compounds described are robust organogelators that form gels in a variety of organic solvents with good thermostability and present improved feasibility for the synthesis of envisaged functional organogelators.

Introduction The preparation and study of low molecular weight gelators has received increased attention in recent years.1 This type of molecule forms elongated supramolecular aggregates that further evolve to microscopic fibers that entrap the solvent to yield a gel. The study of physical gels is directly related to the field of supramolecular materials that constitutes a very active front in the development of modern polymer chemistry.2 Driving forces for the study of supramolecular gels include gaining a better understanding of the process of physical gelation at the molecular and supramolecular level and the many applications of polymer gels,3 whose scope can be broadened, taking advantage of their reversible and ordered nature. The physical and chemical properties of organogels are useful to the development of functional materials such as drug delivery systems, responsive materials, catalysts, photonic materials, and in pharmaceutical diagnosis.4 The creation of nano- and microstructures is of great interest in many fields of chemistry and technology. In this context, organogels have been used, for example, as templates for the preparation of nanotubular materials.5 A wide variety of low molecular weight compounds have been described that form gels in aqueous and/or organic solvents.6 The intermolecular interactions responsible of self-assembly include mainly hydrophobic interactions in * Corresponding authors: e-mail [email protected] (B.E.) or [email protected] (J.F.M.). (1) (a) Terech, P.; Weiss, R. G. Chem. Rev. 1997, 97, 3133-3159. (b) Abdallah, D. J.; Weiss, R. G. Adv. Mater. 2000, 12, 1237-1247. (c) van Esch, J.; Feringa, B. L Angew. Chem., Int. Ed. 2000, 39, 2263-2354. (d) Gronwald, O.; Snip, E.; Shinkai, S. Curr. Opin. Colloid Interface Sci. 2002, 7, 148-156. (e) Estroff, L. A.; Hamilton, A. D. Chem. Rev. 2004, 104, 1201-1207. (2) (a) Sijbesma, R. P.; Beijer, F. H.; Brunsveld, L.; Folmer, B. J. B.; Hirschberg, J. H. K. K.; Lange, R. F. M.; Lowe, J. K. L.; Meijer, E. W. Science 1997, 278, 1601-1604. (b) Berl, V.; Schmutz, M.; Krische, M. J.; Khoury, R. G.; Lehn, J.-M. Chem. Eur. J. 2002, 8, 1227-1244. (c) Supramolecular polymers; Ciferri, A., Ed.; Marcel Dekker: New York, 2000. (d) Lehn, J.-M. Science 2002, 295, 2400-2403. (e) Ikkala, O.; ten Brinkle, G. Science 2002, 295, 2407-2409. (f) Ringsdorf, H. Angew. Chem., Int. Ed. 2004, 43, 1064-1076. (3) Polymer gels (Eds.: Bohidar, H. B., Dubin, P., Osada, Y.), American Chemical Society, Washington D. C., 2003.

aqueous media and hydrogen bonding in organic solvents. In the latter case the hydrogen-bonding subunits present in these molecules include commonly amide, carbamate, or urea groups; especially extensive has been the study of amino acid derivatives.7-9 In this type of gelators the hydrogen-bonding centers are usually oriented in such a way that intermolecular bonds produce the growth of linear elongated aggregates. However, a detailed description of the structure of the supramolecular aggregates responsible for gel formation10 is not possible in most cases due to the intrinsic difficulty of analyzing the molecular structure within a gel and the presence of several centers available for intermolecular interactions. (4) (a) Ajayaghosh, A.; George, S. J.; Praveen, V. K. Angew. Chem., Int. Ed. 2003, 42, 332-335. (b) Urabe, H.; Jung, J. H.; Ono, Y.; Shinkai, S.; Soai, K. Tetrahedron Lett. 2003, 44, 721-724. (c) Tiller, J. C. Angew. Chem., Int. Ed. 2003, 42, 3072-3075. (d) Friggeri, A.; Feringa, B.; van Esch, J. J. Controlled Release 2004, 97, 241-248. (e) Roubeau, O.; Colin, A.; Schmitt, V.; Cle´rac, R. Angew. Chem., Int. Ed. 2004, 43, 3283-3286. (f) Yang, Z.; Gu, H.; Fu, D.; Gao, P.; Lam, J. K.; Xu, B. Adv. Mater. 2004, 16, 1440-1444. (g) Sugiyasu, K.; Fujita, N.; Shinkai, S. Angew. Chem., Int. Ed. 2004, 43, 1229-1233. (h) Yao, S.; Beginn, U.; Gress, T.; Lysetska, M.; Wu¨rthner, F. J. Am. Chem. Soc. 2004, 126, 8336-8348. (i) Kiyonaza, S.; Sada, K.; Yoshimura, I.; Shinkai, S.; Kato, N.; Hamachi, I. Nat. Mater. 2004, 3, 58-64. (5) (a) Jung, J. H.; Ono, Y.; Shinkai, S. J. Chem. Soc., Perkin Trans. 2 1999, 1289-1291. (b) Jung, J. H.; Ono, Y.; Shinkai, S. Chem. Eur. J. 2000, 6, 4552-4557. (c) Jung, J. H.; Amaike, M.; Shinkai, S. Chem. Commun. 2000, 2343-2344. (d) Jung, J. H.; Shinkai, S. J. Chem. Soc., Perkin Trans. 2 2000, 2393-2398. (e) Jung, J. H.; Ono, Y.; Shinkai, S. Langmuir 2000, 16, 1643-1649. (f) Jung, J. H.; Kobayashi, H.; Masuda, M.; Shimizu, T.; Shinkai, S. J. Am. Chem. Soc. 2001, 123, 8785-8789. (g) Ono, Y.; Nakashima, K.; Sano, M.; Hojo, J.; Shinkai, S. J. Mater. Chem. 2001, 11, 2412-2419. (h) Jung, J. H.; Shinkai, S. J. Inclusion Phenom. Macrocycl. Chem. 2001, 41, 53-59. (i) Sugiyasu, K.; Tamaru, S.; Takeuchi, M.; Berthier, D.; Huc, I.; Oda, R.; Shinkai, S. Chem. Commun. 2002, 1212-1213. (j) Jung, J. H.; Shinkai, S.; Shimizu, T. Nanolett. 2002, 2, 17-20. (k) Jung, J. H.; Kobayashi, H.; van Bommel, K. J. C.; Shinkai, S.; Shimizu, T. Chem. Mater. 2002, 14, 1445-1447. (l) Jung, J. H.; Yoshida, K.; Shimizu, T. Langmuir 2002, 18, 87248727. (m) Kobayashi, S.; Hamasaki, N.; Suzuki, M.; Kimura, M.; Shirai, H.; Hanabusa, K. J. Am. Chem. Soc. 2002, 124, 6550-6551. (n) Seddon, A. M.; Patel, H. M.; Burkett, S.; Mann, S. Angew. Chem., Int. Ed. 2002, 41, 2988-2991. (o) van Bommel, K. J. C.; Friggeri, A.; Shinkai, S. Angew. Chem., Int. Ed. 2003, 42, 980-999. (p) Numata, M.; Sugiyasu, K.; Hasegawa, T.; Shinkai, S. Angew. Chem., Int. Ed. 2004, 43, 32793283. (q) Barboiu, M.; Cerenaux, S.; van der Lee, A.; Vaughan, G. J. Am. Chem. Soc. 2004, 126, 3545-3550.

10.1021/la050655j CCC: $30.25 © 2005 American Chemical Society Published on Web 06/14/2005

Organogel Formation from Amidocarbamates

Langmuir, Vol. 21, No. 15, 2005 6777 Chart 1

The formation of supramolecular aggregates formed by two or more different molecules is of interest, for example, in the fields of supramolecular synthesis and crystal engineering.11 Since supramolecular gels can be seen as the result of an incomplete crystallization,10b the formation of such gels from mixtures of molecules is expected to take place only in particular cases. For example, some examples of dual or multicomponent organogels have been reported.4f,6b,12 Such systems can be formed either from dissimilar but complementary molecules that interact though H-bonding or ionic interactions or from similar molecules that differ only in structural motifs, such as, for example, the substitution of an aromatic unit. Here we report on the formation of physical gels by mixtures of a family of N,N′-bis[(N-benzyloxycarbonyl)valinyl]diamines (see structures 1a-1e in Chart 1) and the corresponding analogues obtained by substitution of the carbamate subunits by amides (see structures 2a-2e in Chart 1). These compounds contain multiple hydrogenbonding sites and are, a priori, not complementary to each other when they present aliphatic spacers of different length. (6) See, for example, (a) Lin, Y.; Kachar, B.; Weiss, R. G. J. Am. Chem. Soc. 1989, 111, 5542-5551. (b) Oda, R.; Huc, I.; Candau, S. J. Chem. Int. Ed. 1998, 37, 2689-2691. (c) Clavier, G. M.; Brugger, J. F.; Bouas-Laurent, H.; Pozzo, J.-L. J. Chem. Soc., Perkin Trans. 2 1998, 2527-2534. (d) Yoza, K.; Amanokura, N.; Ono, Y.; Akao, T.; Shinmori, H.; Takeuchi, M.; Shinkai, S.; Reinhoudt, D. N. Chem. Eur. J. 1999, 5, 2722-2729. (e) Amanokura, N.; Kanekiyo, Y.; Shinkai, S.; Reinhoudt, D. N. J. Chem. Soc., Perkin Trans. 2 1999, 1995-2000. (f) Hafkamp, R. J. H.; Feiters, M. C.; Nolte, R. J. M. J. Org. Chem. 1999, 64, 412-426. (g) van Esch, J.; Schoonbeek, F.; de Loos, M.; Kooijman, H.; Spek, A. L.; Kellogg, R. M.; Feringa, B. L. Chem. Eur. J. 1999, 5, 937-949. (h) Schoonbeek, F.; van Esch, J.; Hulst, R.; Kellogg, R. M.; Feringa, B. L. Chem. Eur. J. 2000, 6, 2633-2643. (i) de Loos, M.; Ligtenbarg, A. G. J.; van Esch, J.; Kooijman, H.; Spek, A. L.; Hage, R.; Kellogg, R. M.; Feringa, B. L. Eur. J. Org. Chem. 2000, 3675-3678. (j) Ko¨lbel, M.; Menger, F. M. Langmuir 2001, 17, 4490-4492. (k) Ajayaghosh, A.; George, S. J. J. Am. Chem. Soc. 2001, 123, 5148-5149. (l) Beginn, U.; Tartsch, B. Chem. Commun. 2001, 1924-1925. (m) Gronwald, O.; Shinkai, S. Chem. Eur. J. 2001, 7, 4328-4334. (n) van Gorp, J. J.; Vekemans, J. A. J. M.; Meijer, E. W. J. Am. Chem. Soc. 2002, 124, 14759-14769. (o) George, M.; Weiss, R. G. Langmuir 2002, 18, 71247135. (p) George, M.; Weiss, R. G. Langmuir 2003, 19, 1017-1025. (q) Wang, C.; Robertson, A.; Weiss, R. G. Langmuir 2003, 19, 1036-1046. (r) Bu¨hler, G.; Feiters, M. C.; Nolte, R. J. M. Angew. Chem., Int. Ed. 2003, 42, 2494-2497. (s) de Jong, J. J. D.; Lucas, L. N.; Kellogg, R. M.; van Esch, J. H.; Feringa, B. L. Science 2004, 304, 278-281. (t) George, S. J.; Ajayaghosh, A.; Jonkheijm, P.; Schenning, A. P. H. J.; Meijer, E. W. Angew. Chem., Int. Ed. 2004, 43, 3422-3425. (u) Barboiu, M.; Cerneaux, S.; van der Lee, A.; Vaughan, G. J. Am. Chem. Soc. 2004, 126, 3545-3550. (v) Yao, S.; Beginn, U.; Gress, T.; Lysetska, M.; Wurthner, F. J. Am. Chem. Soc. 2004, 126, 8336-8348. (7) Examples of amino acid urea gelators: (a) Wang, G.; Hamilton, A. D. Chem. Eur J. 2002, 8, 1954-1961. (b) Wang, G.; Hamilton, A. D. Chem. Commun. 2003, 310-311. (c) Estroff, L. A.; Huang, J. S.; Hamilton, A. D. Chem. Commun. 2003, 2958-2959. (d) Suzuki, M.; Nakajima, Y.; Yumoto, M.; Kimura, M.; Shirai, H.; Hanabusa, K. Org. Biomol. Chem. 2004, 2, 1155-1159.

Chart 2

Results and Discussion In the course of the preparation of a family of cyclic peptidomimetic organogelators described previously,13 we used Z-protected amino acid derivatives 1a-1e as intermediates and, serendipitously, we noticed that these molecules had the capability of forming physical gels. As a matter of fact, a search on previous work in the field of molecular organogels revealed that closely related compounds had been previously described as organogelators by Hanabusa et al.,14a and studies with similar molecules were also reported later from another group14b (see for example structures 3 and 4). Hanabusa et al. nicely reported that this type of compounds leads to efficient organogel formation in a variety of organic solvents. A model of aggregation such as that shown in Chart 2 was proposed, where every molecule is involved in eight H-bonds, giving place to sheets. This aggregation mode is related, for instance, to that reported for bolaamphiphiles. These molecules, which contain a hydrophobic skeleton and two polar groups on both ends, form ordered aggregates in water such as micelles, vesicles, and fibers. They have been also described to form gels in organic solvents, and aggregation models based on extended conformations, such as that shown in Chart 2, are (8) Examples of amino acid urethane gelators: (a) Hanabusa, K.; Okui, K.; Karaki, K.; Koyama, T.; Shirai, H. J. Chem. Soc., Chem. Commun. 1992, 1371-1373. (b) de Vries, E. J.; Kellogg, R. M. J. Chem. Soc., Chem. Commun. 1993, 238-240. (c) Hanabusa, K.; Matsumoto, Y.; Miki, T.; Koyama, T.; Shirai, H. J. Chem. Soc., Chem. Commun. 1994, 1401-1402. (d) Stock, H. T.; Turner, N. J.; McCague, R. J. Chem. Soc., Chem. Commun. 1995, 2063-2064. (e) Bhattacharya, S.; Ghanashyam Achayra, S. N.; Raju, A. R. Chem. Commun. 1996, 21012102. (f) Hanabusa, K.; Hiratsuka, K.; Kimura, M.; Shirai, H. Chem. Mater. 1999, 11, 649-655. (g) Hanabusa, K.; Matsumoto, Y.; Kimura, M.; Kakehi, A.; Shirai, H. J. Colloid Interface Sci. 2000, 224, 231-244. (h) Nagasawa, J.; Kudo, M.; Hayashi, S.; Tamaoki, N. Langmuir 2004, 20, 7907-7916. (i) Yang, Z.; Gu, H.; Fu, D.; Gao, P.; Lam, J. K.; Xu, B. Adv. Mater. 2004, 16, 1440-1444. (j) Love, C. S.; Hirst, A. R.; Chechik, V.; Smith, D. K.; Ashworth, I.; Brennan, C. Langmuir 2004, 20, 65806585. (k) Hirst, A. R.; Smith, D. K.; Feiters, M. C.; Geurts, H. P. M. Chem. Eur J. 2004, 10, 5901-5910. (l) Koumura, N.; Kudo, M.; Tamaoki, N. Langmuir 2004, 20, 9897-9900. (m) Hirst, A. R.; Smith, D. K. Org. Biomol. Chem. 2004, 2, 2965-2971.

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described for molecules that typically present C10 or longer aliphatic spacers.15 We were interested in the study of other derivatives of this type of molecules as organogelators, and to better understand their mechanism of aggregation, we decided to prepare and study compounds 1a-1e, which contain aliphatic spacers of different lengths. In a preliminary experiment directed toward a possible length-selective supramolecular aggregation mechanism, a mixture of compounds 1a and 1e, that have respectively ethylenic and octamethylenic aliphatic spacers, was found to form a gel in toluene. Intrigued by this finding, the determination of the critical gel concentration (cgc) values for the equimolar binary mixtures of compounds 1a-1e in an apolar solvent such as toluene and in an aprotic polar solvent such as acetonitrile was carried out. As can be seen in Tables 1 and 2, cogelation was observed in all the cases. It can be noticed that the concentration of a given compound in the mixture at which a gel is formed is lowered by the presence of equimolar quantities of any other compound in the series. For example, in toluene (Table 1) the cgc value for pure 1a is 3.8 mM, while this compound forms gels at lower concentrations in its equimolar mixtures with 1b-1e, the cgc values for 1a in these mixtures being respectively 2.8, 2.2, 2.5, and 2.7 (9) Examples of amino acid amide gelators: (a) Hanabusa, K.; Naka, Y.; Koyama, T.; Shirai, H. J. Chem. Soc., Chem. Commun. 1994, 26832684. (b) Jokic, M.; Makarevic, J.; Zinic, M. J. Chem. Soc., Chem. Commun. 1995, 1723-1724. (c) Jayakumar, R.; Murugesan, M.; Asokan, C.; Scibioh, M. A. Langmuir 2000, 16, 1489-1496. (d) Mieden-Gundert, G.; Klein, L.; Fischer, M.; Vo¨gtle, F.; Heuze´, K.; Pozzo, J.-L.; Vallier, M.; Fages, F. Angew. Chem., Int. Ed. 2001, 40, 3164-3166. (e) Luo, X.; Liu, B.; Liang, Y. Chem. Commun. 2001, 1556-1557. (f) Makarevic, J.; Jokic, M.; Peric, B.; Tomisic, V.; Kojic-Prodic, B.; Zinic, M. Chem. Eur. J. 2001, 7, 3328-3341. (g) Yamada, N.; Imai, T.; Koyama, E. Langmuir 2001, 17, 961-963. (h) Suzuki, M.; Yumoto, M.; Kimura, M.; Shirai, H.; Hanabusa, K. Chem. Commun. 2002, 884-885. (i) Makarevic, J.; Jokic, M.; Frkanec, L.; Katalenic, D.; Zinic, M. Chem. Commun. 2002, 22382239. (j) Ihara, I.; Sakurai, T.; Yamada, T.; Hashimoto, T.; Takafuji, M.; Sagawa, T.; Hachisako, H. Langmuir 2002, 18, 7120-7123. (k) Maji, S. K.; Haldar, D.; Banerjee, A.; Banerjee, A. Tetrahedron 2002, 58, 8695-8702. (l) Malik, S.; Maji, S. K.; Banerjee, A.; Nandi, A. K. J. Chem. Soc., Perkin Trans. 2 2002, 1177-1186. (m) Kiyonaka, S.; Shinkai, S.; Hamachi, I. Chem. Eur J. 2003, 9, 976-983; (n) Suzuki, M.; Yumoto, M.; Kimura, M.; Shirai, H.; Hanabusa, K. Chem. Eur J. 2003, 9, 348354. (o) Suzuki, M.; Yumoto, M.; Kimura, M.; Shirai, H.; Hanabusa, K. Helv. Chim. Acta 2003, 86, 2228-2238. (p) van Bommel, K. J. C.; van der Pol, C.; Muizebelt, I.; Friggeri, A.; Heeres, A.; Meetsma, A.; Feringa, B. L.; van Esch, J. Angew. Chem., Int. Ed. 2004, 43, 1663-1667. (q) Caplar, V.; Zinic, M.; Pozzo, J.-L.; Fages, F.; Mieden-Gundert, G.; Vo¨gtle, F. Eur J. Org. Chem. 2004, 4048-4059. (r) Suzuki, M.; Owa, S.; Yumoto, M.; Kimura, M.; Shirai, H.; Hanabusa, K. Tetrahedron Lett. 2004, 45, 5399-5402. (s) Zhan, C.; Gao, P.; Liu, M. Chem. Commun. 2005, 462464. (10) See, for example, (a) Abdallah, D. J.; Sirchio, S. A.; Weiss, R. G. Langmuir 2000, 16, 7558-7561. (b) Menger, F. M.; Caran, K. L. J. Am. Chem. Soc. 2000, 112, 11679-11691. (11) (a) Moulton, B.; Zaworotko, M. J. Chem Rev. 2001, 101, 16291658. (b) Almarsson, O.; Zaworotko, M. J. Chem. Commun. 2004, 18891896. (12) (a) Hanabusa, K.; Miki, T.; Taguchi, Y.; Koyama, T.; Shirai, H. J. Chem. Soc., Chem. Commun. 1993, 1382-1384. (b) de Loos, M.; van Esch, J.; Kellog, R. M.; Feringa, B. L. Angew. Chem., Int. Ed. Engl. 2001, 40, 613-616. (c) Willemen, H. M.; Vermonden, T.; Marcelis, A. T. M.; Sudho¨lter, E. J. R. Langmuir 2002, 18, 7102-7106. (d) Ayabe, M.; Kishida, T.; Fujita, N.; Sada, K.; Shinkai, S. Org. Biomol. Chem. 2003, 1, 2744-2747. (e) Hirst, A. R.; Smith, D. K.; Feiters, M. C.; Geurts, H. P. M.; Wright, A. C. J. Am. Chem. Soc. 2003, 125, 9010-9011. (13) (a) Becerril, J.; Burguete, M. I.; Escuder, B.; Luis, S. V.; Miravet, J. F.; Querol, M. Chem. Commun. 2002, 738-739. (b) Becerril, J.; Bolte, M.; Burguete, M. I.; Galindo, F.; Garcı´a-Espan˜a, E.; Luis, S. V.; Miravet, J. F. J. Am. Chem. Soc. 2003, 125, 6677-6686. (c) Becerril, J.; Burguete, M. I.; Escuder, B.; Galindo, F.; Gavara, R.; Miravet, J. F.; Luis, S. V.; Peris, G. Chem, Eur. J. 2004, 10, 3879-3890. (d) Becerril, J.; Escuder, B.; Gavara, R.; Luis, S. V.; Miravet, J. F. Eur. J. Org. Chem. 2005, 481-485. (14) (a) Hanabusa, K.; Tanaka, R.; Suzuki, M.; Kimura, M.; Shirai, H. Adv. Mater. 1997, 9, 1095-1097. (b) Bhattacharya, S.; Acharya, S. N. G. Chem. Mater. 1999, 11, 3121-3132. (15) (a) Shimizu, T. Polym. J. 2003, 35, 1-22. (b) Fuhrhop, J.-H.; Wang, T. Chem. Rev. 2004, 104, 2901-2937.

Escuder et al. Table 1. Critical Gel Concentrations of Pure Compounds 1a-1e and of the Individual Components in Their Binary Equimolar Mixtures in Toluenea 1a 1b 1c 1d 1e

1a

1b

1c

1d

1e

3.8 (0.8) 2.8 (0.4) 2.2 (0.4) 2.5 (0.6) 2.7 (0.6)

2.8 (0.4) 6.6 (1.8) 3.3 (0.6) 4.3 (1.0) 3.8 (0.6)

2.2 (0.4) 3.3 (0.6) 5.1 (1.0) 3.1 (0.6) 5.1 (1.0)

2.5 (0.6) 4.3 (1.0) 3.1 (0.6) 6.7 (1.4) 4.9 (1.0)

2.7 (0.6) 3.8 (0.6) 5.1 (1.0) 4.9 (1.0) 6.7 (1.6)

a The values in parentheses indicate the difference between the cgc values (millimolar) and the highest concentration where gel formation was not observed in the performed experiments.

Table 2. Critical Gel Concentration of Pure Compounds 1a-1e and of the Individual Components in Their Binary Equimolar Mixtures in Acetonitrilea 1a 1b 1c 1d 1e

1a

1b

1c

1d

1e

6.5 (1.6) 5.5 (1.2) 5.5 (1.2) 3.4 (1.0) 4.8 (1.4)

5.5 (1.2) 7.2 (1.8) 4.3 (0.8) 4.4 (1.0) 4.3 (1.0)

5.5 (1.2) 4.3 (0.8) 8.1 (2.2) 5.3 (1.0) 4.9 (0.5)

3.4 (1) 4.4 (1.0) 5.3 (1) 7.8 (1.8) 4.4 (0.8)

4.8 (1.4) 4.3 (1.0) 4.9 (1.0) 4.4 (0.8) 13.1 (4.2)


mM (the global cgc for these mixtures including both compounds is twice those values). A similar situation is found for the rest of the studied compounds. Furthermore, when a ternary equimolar mixture of 1a, 1c, and 1e was considered, a gel was formed at a concentration for each component of 1.6 mM. When the experiments were carried out in acetonitrile, a polar solvent that at the same time allows for fairly strong hydrogen bonding, cogelation was also found. In this case, the decrease of the cgc values was also remarkable and, for example, compound 1a presents a cgc value of 6.5 mM whereas its equimolar mixture with 1d forms a gel at a concentration of 1a of 3.4 mM. This suggests that the mechanism of aggregation is similar in both solvents. The observed results can be explained if supramolecular copolymers are formed. Three possible assembly mechanisms could be envisaged: random, quasi-random, and phase-separated assembly (see Scheme 1). The first mechanism requires a random assembly that would be the result of the equivalent efficiency for hetero- and homoassociation processes. On the other hand, a mechanism yielding phase separation would imply a selective homoassociation process, giving place to orthogonal gel formation. Finally, other quasi-random aggregation models could be proposed where the formation of homopolymer domains coexists with the presence of other randomly associated regions. This situation would occur in systems where the homoassociation is to some extent favored over the heteroassociation process. Wide-angle X-ray diffraction (WAXD) experiments were informative in this respect. The dried toluene gels of compounds 1a-1e were studied by this technique, which showed that all the materials were microcrystalline (see Figure 1 and Supporting Information). Interestingly, when xerogels of equimolar binary mixtures were analyzed by WAXD, the diffractogram obtained was identical to that corresponding to one of the components of the mixture, while those peaks corresponding to the other component had disappeared. For example, as shown in Figure 1, when an equimolar mixture of 1a and 1c was analyzed by this means, the diffraction pattern matched that obtained for 1a alone, and when a mixture of 1c and 1d (not shown)


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Figure 1. Comparison of the X-ray diffractograms (WAXD) obtained for the dried toluene organogels formed by compounds 1a, 1c, and their binary mixtures. Scheme 1

was studied, the result was very similar to that recorded for pure 1c. It is noteworthy that in both cases the diffraction peaks observed in the binary mixtures correspond to the compound having the lower critical gel concentration, namely, the compound that self-aggregates more efficiently. These findings discard both phase separation and random copolymerization models and

suggest the presence of a quasi-random supramolecular copolymerization. Thus, the formation of supramolecular copolymers would be biased in such a way that noncrystalline domains, as a result of heteroassociation, coexist with homopolymer crystalline domains of the compound that presents stronger self-assembling interactions. For the mixtures of 1a and 1c, 65% of the latter compound is

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Table 3. 1H NMR Chemical Shift for the Amide and Carbamate NHs of Compounds 1a-1e and 5 in CDCl3 and CD3CNa

compd

δ carbamate NH, CDCl3

δ amide NH, CDCl3

δ carbamate NH, CD3CN

δ amide NH, CD3CN

1a 1b 1c 1d 1e 5

5.34 5.36 5.40 5.66 5.48 5.35

6.57 6.87 6.40 6.29 6.31 5.89

5.83 5.83 5.81 5.90 5.82 5.77

6.84 6.86 6.68 6.69 6.61 6.58

a Chemical shift are given in parts per million (ppm). Concentrations were below 1 mM in all cases.

required to obtain a diffractogram corresponding to an amorphous material that can be ascribed to a random copolymer. For higher percentages of 1c, only the diffraction peaks due to the homoassociation of this compound could be observed. Additionally, when a ternary gel formed from 1a, 1c, and 1d was studied by WAXD, only lowintensity peaks corresponding to compound 1a could be detected, suggesting the microcrystalline character of the material was almost completely lost and that random association was the main aggregation mechanism (see Supporting Information). To better understand the relationship between gel formation and structure in these molecules, compounds 5 and 6 were prepared and studied. Compound 5 can be considered as half of molecules 1a-1e and, on the other hand, compound 6 is an analogue that presents a rigid m-phenylene spacer. It was observed that neither compound 5 nor 6 affected the critical gelation concentrations of compounds 1a-1e when the corresponding equimolar mixtures were studied in toluene. Additionally, it was observed that compounds 5 and 6 formed gels in toluene but at higher concentrations than those of 1a-1e (14 and 34 mM, respectively). For compound 5, the results suggest that the reduction of the number of H-bonding centers when compared to 1a-1e affects significantly its capability of homoassociation and also possible heteroassociation. On the other hand, for compound 6, the reduced conformational flexibility when compared to 1a-1e would preclude an optimum fitting for the intermolecular interaction. However, in this case other factors such as steric or electronic effects, associated with the presence of an aromatic amide, cannot be discarded. NMR studies were carried out to obtain structural information on molecules 1a-1e. 1H NMR spectra were recorded in CDCl3 and CD3CN for diluted solutions with concentrations well below the range of gel formation and compared to the spectra obtained for compound 5. The chemical shift values of the amide and carbamate subunits were considered, and the values reported in Table 3 correspond to nonaggregated species since no chemical shift variation was detected upon dilution to the lowest concentrations detectable with this technique. It was found that the chemical shift of the carbamate NH of 5 is, in general, very similar to that of the analogue protons in 1a-1e but the amide NHs of these molecules are considerably shifted downfield in comparison with the same protons in 5. For example, noteworthy differences are observed in CDCl3 when compound 5 is compared with compounds 1a and 1e (Figure 2). The amide signals of the latter compounds are shifted downfield (0.68 and 0.42 ppm respectively) when compared to the amide of compound 5 but, noticeably, the carbamate shift is similar in the three compounds considered. These data suggest the presence of conformations having intermolecular H-

bonding through the amide NHs.16 Additional information supporting intramolecular H-bonding could be obtained from the comparison of the NH chemical shift in CD3CN and CDCl3 (see Table 3). For compound 5, amide and carbamate NHs are shifted respectively 0.69 and 0.42 ppm downfield on going from CDCl3 to CD3CN, reflecting more intense H-bonding interactions with this solvent as previously reported for related molecules.13b,16 In the case of compounds 1a-1e, remarkably, the amide NH chemical shift seems to be much less sensitive to the change of solvent than that of compound 5 (see Table 3 and Figure 3). For example, for compound 1b almost no shift is observed for the amide signals while a downfield shift of 0.47 ppm is measured for the carbamate NH. The observed insensitivity of chemical shift values to changes in solvent polarity can be related to the presence of intramolecular H-bonding that would hide the NH groups from the solvent molecules. With the aim of obtaining a reasonable structural model for the folded conformers of molecules 1a-1e, a study by means of molecular mechanics calculations was performed by use of the AMBER* force field, which has been shown to reproduce H-bonding in related systems.13b,17 After an exhaustive conformational search for isolated molecules 1a-1e, it was observed that several low-energy conformations were found in an interval of a few kilojoules per mole, presenting different intramolecular H-bonding arrangements. Interestingly, for all the molecules, lowenergy conformations (global minimum or close to it) were found that presented a structural arrangement such as that shown in Figure 4. This particular conformation agrees with the previous considerations about intramolecular H-bonding. As can be seen in Figure 4, the selected structures are very similar for compounds 1a-1e, being folded by means of two intramolecular hydrogen bonds between the carbonyl groups of the carbamate functions and NHs of the amide functions, as suggested by the NMR data. Additional experimental evidence of the presence of folded conformers in solution was obtained from 1H NMR one-dimensional nuclear Overhauser effect spectroscopy (NOESY-1D) (500 MHz) studies in CD3CN (see Chart 3 and Table 4). For example, when compound 1c and 1d were studied, large NOE enhancements of signals corresponding to benzylic protons were observed upon irradiation of the aromatic protons (see Supporting Information). Additionally, medium enhancements could be observed for signals corresponding to the methyl groups, the oligomethylene chain, the proton attached to the chiral center, and the amide and carbamate NH protons. These results agree with the presence of folded conformers such as those shown in Figure 4. To study the aggregation process by 1H NMR, spectra were recorded in CDCl3 and CD3CN for samples of 1a-1e at concentrations that ranged from diluted solutions with no aggregation to samples where a gel was formed (ca. 0.2-10 mM). It is noteworthy that although a broadening of the signals took place upon increasing concentration, no chemical shift variation was observed for any signal of the studied molecules. It is especially noticeable that the amide and carbamate NH signals did not shift although H-bonding is a main driving force for aggregation (see IR study below). These results indicate that upon gel formation the NMR signals detected correspond to free orga(16) (a) Gellman, S. H.; Dado, G. P.; Liang, G.-B.; Adams, B. R. J. Am. Chem. Soc. 1991, 113, 1164-1173. (b) Liang, G.-B.; Desper, J. M.; Gellman, S. H. J. Am. Chem. Soc. 1993, 115, 925-938. (17) McDonald, D. Q.; Still, W. C. J. Am. Chem. Soc. 1994, 116, 1155011553.


Figure 2.

Langmuir, Vol. 21, No. 15, 2005 6781

1

H NMR NH signals of compounds 1a, 1e, and 5 in CDCl3 (concentration was below 1 mM in all the samples).

Figure 3. 1H NMR NH signals of compounds 1b and 5 in CDCl3 and CD3CN (concentration was below 1 mM in all the samples). Chart 3

Figure 4. Low-energy conformers found for compounds 1a1e (MACROMODEL 7.0, AMBER* force field).

nogelator molecules that remain in solution and that the aggregates are not detectable in these experiments. However, it has to be mentioned that a significant difference is observed when NMR NOE experiments for diluted solutions and gels are compared. In the first case, positive enhancements were obtained, while negative ones were measured in the gels. A shift from positive to negative NOE can arise by an increase of molecular weight (and therefore of correlation times), and accordingly, it has been reported previously that these negative values can be ascribed to the fact that aggregates are being observed.6h,13c However, here it is shown clearly that the species yielding negative enhancements are not aggregates (as indicated by the NH chemical shift value) and the negative NOE should be ascribed to increased correlation times as a result

of an exchange of the organogelator molecules between the sol and gel phases.18 IR spectroscopic studies confirmed that H-bonding is the main driving force for gelation. As shown in Figure 5, when the aggregation of 1c in chloroform (6 mM) was studied, it was observed that upon cooling a solution from 60 to 20 °C the changes on the IR signals include a significant decrease of the free NH band (ca. 3430 cm-1) at the same time that the associated NH band (ca. 3290 cm-1) increases its intensity. In the same way, the free CdO stretching bands of both the carbamate and amide subunits are progressively transformed on associated Cd O bands upon aggregation. The fact that both carbamate and amide carbonyls are involved in the intermolecular aggregation that results in gel formation can be ascribed to the formation of aggregates from conformations where intramolecular H-bonding is not present. (18) Duncan, D. C.; Whitten, D. G. Langmuir 2000, 16, 6445-6452.

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Table 4. Distances Calculated from Models of 1c and 1e for the Protons that Showed NOE (CD3CN, Gel) upon Irradiation of Aromatic Protonsa enhanced signals of 1c

distance (Å)

enhanced signals of 1e

distance (Å)

B C D F G H I

2.6 (A1-B) 4.2 (A1-C) 4.2 (A1-D′) 2.6 (A1-F′) 3.6 (A1-G′) 2.9 (A2-H′) 3.3 (A3-I′)

B C D E F G H I J, K

2.5 (A1-B) 3.0 (A1-C) 4.4 (A1-D′) 3.6 (A1-E′) 4.2 (A1-F) 3.2 (A1-G′) 3.2 (A3-H′) 4.1 (A2-I′) 2.8 (A2-J′) 2.9 (A2-K′)

a

See Figure 4 and Chart 3.

Figure 5. Variation of IR spectra of 1c (6 mM) in CHCl3 obtained upon cooling the dissolution from 60 to 20 °C. Scheme 2

If all the results are considered, a model of aggregation such as that shown in Scheme 2 could be proposed. The molecules, as shown by NMR, would adopt intramolecularly H-bonded folded conformations in solution, but as reflected by IR, intermolecular aggregation involves all the H-bonding units available, and therefore, a conformational change would occur upon aggregation. The WAXD data point to the formation of quasi-random

Figure 6. Energy-optimized molecular model for the interaction between 1a and 1d (MACROMODEL 7.0, AMBER* force field).

polymers. If a hierarchical assembly as shown in Scheme 1 is considered, a priori, the deviation from randomness of the assembly process could take place both in the formation of the initial one-dimensional aggregates and in the further assembly of those to yield the fibers. However, the second possibility seems unlikely since the interactions at this second level are expected to be much less specific than H-bonding and produce a randomized assembly. Accordingly, the formation of copolymer domains does not fit with the sheetlike packing model with completely extended conformations as described in Chart 2 because it is expected that the different length of the aliphatic spacers would produce mismatches in the assembly process. However, in this case copolymer formation could be rationalized, considering that the flexibility of the aliphatic spacer could afford folded conformations in the largest counterpart that would adapt to the size of the other component. The fact that homoassociation of the component with the shortest length spacer and heteroassociation coexist in the equimolar mixtures seems to fit with this hypothesis and indicates that the required adaptation of the larger component would be somewhat responsible for the reduced efficiency of the heteroassembly. Furthermore, molecular modeling supports the feasibility of the adaptable coassembly proposed. As shown in Figure 6, heteroaggregates formed by compounds with different spacer length such as 1a and 1d could be built where folding of the oligomethylenic chain is present, allowing four H-bonding intermolecular interactions. To gain further insight into the gelation mechanism of this type of molecules, tetramide analogues 2a-2e were synthesized and studied. In these compounds the replacement of the carbamate group by an amide should not preclude association through hydrogen-bonding patterns as shown in Chart 2. Additionally, with the possibility of introducing functional groups for envisaged applications of this type of organogelators in mind, the amide function is advantageous from a synthetic point of view. Compounds 2a-2e have been prepared in good yields by reacting compounds 7a-7e13b with benzoyl chloride in the presence of Et3N in THF solution (Scheme 3). The bis(acetamide) 8 has also been prepared by refluxing 7b with acetic anhydride in glacial acetic acid solution. As already mentioned, the gelation efficiency of amidocarbamates related to 1a-1e has been reported previously, but since compounds 2a-2e constitute a new family of organogelators, their gelation properties were studied in a variety of organic solvents (Table 5). In general, compounds 2a-2e formed gels of different aspect and strength in most of the solvents studied at concentrations of ca. 10 mM (0.5 wt %) or even lower. Transparent or quasi-transparent gels could be obtained in intermediate polarity solvents such as dichloromethane, acetone, dimethoxyethane, dioxane, tetrahydrofuran, and


Langmuir, Vol. 21, No. 15, 2005 6783 Scheme 3

Table 5. Gelation Properties of Bis(amino acyl) Tetraamidesa solvent

2a

2b

2c

2d

2e

methanol ethanol 2-propanol n-butanol chloroform dichloromethane acetone dimethoxyethane dioxane tetrahydrofuran acetonitrile DMSO DMF ethyl acetate hexane cyclohexane toluene benzene

P G (11) WG (11) G (7) P G (5) G (5) WG (5) G (11) WG (11) WG (11) S S I I I G (4) P

WG (10) G (10) G (10) G (10) S G (4) G + P (7) G + P (4) G (7) P G (7) S S G + P (7) I I G (6) G (7)

G (81) WG (10) WG (10) WG (10) WG (7) WG (5) I WG (5) WG (10) G (7) WG (10) S S I I I P P

WG (77) G (77) G (77) WG (77) P S P P WG (10) G (10) P S S I I I G (5) WG (6)

G (73) G (73) G (73) G (73) P G(6) WG (6) G (5) G (6) G (9) G (6) S S G(6) I I G (7) G (5)

a G, gel; WG, weak gel; P, precipitate; I, insoluble; in parentheses: critical gelation concentration (cgc) in millimoles per liter.

acetonitrile. Transparent gels were also formed in some aromatic solvents, but in general, in such apolar solvents as well as in ethyl acetate, the poor solubility of these compounds was a limiting factor. None of them was soluble in hexane and cyclohexane, and all of them were completely soluble at room temperature in aprotic polar solvents such as DMF and DMSO. Tetraamides 2a-2e were soluble in hot alcohols and after cooling to room temperature formed either fibrous precipitates or opaque gels depending on the concentration. Thus, at a concentration of 10 mM, we could find opaque gels of variable strength for compound 2b in methanol and for compounds 2a-2c in the rest of the alcohols, whereas in the case of compounds 2d and 2e, fibrous precipitates were found in all the alcohols. For the latter compounds, concentrations above 75 mM were required in order to obtain opaque gels. This change in the critical gelation concentration (cgc) in alcohols upon a small change in the structure of the gelator is noteworthy. In addition, it is very remarkable that the low cgc values observed are, especially in the case of 2a and 2b, poorly sensitive to changes in the polarity of the solvent. For example, compound 2b formed strong gels in the same range of concentration in ethanol (cgc ) 10 mM), acetonitrile (cgc ) 7 mM), and benzene (cgc ) 7 mM). On the other hand, the length of the aliphatic spacer seems to have an important effect only in the case of alcohols. In general, solubility is a limiting factor for obtaining organogels, and the balance between hydrophilic and lipophilic groups in the molecule is a major issue for success. In this sense, the presence of the benzoyl moiety is important for their solubility in organic media. Indeed,

the related acetate derivative 8 is highly insoluble in most of the studied solvents, and as a consequence, their organogels cannot be formed. The microscopic features of this new family of organogels were studied by scanning electron microscopy. In general, all the gels showed a typical entanglement of fibrillar aggregates. However, differences in the size and shape of the fibers were observed depending on both solvent and compound. For example, Figure 7 shows the electron micrographs of the xerogels for compounds 2c-2e in alcohols. Compound 2c forms a network of fibers with diameters in the range of 50-100 nm in ethanol and slightly thicker (>100 nm) in methanol. The xerogel of compound 2d in ethanol shows elongated objects of different size, namely, rods several micrometers in length and ca. 2 µm in diameter together with thin fibrils less than 100 nm in width. In the case of 2e, its xerogel in methanol shows larger shaving-like aggregates. These results suggest that larger aggregates are found for systems (solvent/compound) with a concentration of ca. 80 mM (2c and 2e in methanol, 2d in ethanol), whereas thinner fibers are predominant in for the lower concentration gels such as that of 2c in ethanol (10 mM). The xerogels in the other of studied organic solvents showed also a crowded fibrillar network formed, in some cases, by ribbons less than 100 nm in width and some micrometers in length, as can be seen in Figure 8. Once again, it seems that lower concentration and higher degree of transparency are related to the appearance of thinner fibrils. Thus, an adequate control of solvent could allow a fine-tuning of the microstructure, a valuable feature especially for materials applications (i.e., templating synthesis of inorganic materials). Interestingly compounds 2a-2e also showed cogelation when binary mixtures were studied (see Table 6). For example, a equimolar mixture of 2b and 2e formed a gel in toluene at a concentration of 3.5 mM of each component, while the isolated compounds formed gels in this solvent at 5.8 and 7.0 mM, respectively. Cogelation was also found when mixtures of amidocarbamates and tetraamides were studied, indicating a similar aggregation pattern for both compounds. For example, an equimolar mixture of 1e and 2b formed a gel at a concentration of 3.8 mM for each compound, while the corresponding concentrations for isolated compounds are respectively 5.8 and 6.7 mM. The Tgel of the gels formed by 2a-2e was roughly estimated by placing the vial containing the gel, at the minimum gel concentration, immersed upside-down in a thermocontrolled silicone bath and heating it at a slow rate until all the gel became a liquid. In most of the cases, the gels showed considerable thermostability and the gelto-sol transition took place at temperature values close to the boiling point of the solvent. This fact and the low solubility of the solids in most of the solvents probably

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Figure 7. SEM of xerogels: (A) 2c-ethanol, (B) 2c-methanol, (C, D) 2d-ethanol, and (E, F) 2e-methanol.

reflect the strong intermolecular interactions between the molecules in the gels as well as in the bulk solids. 1 H NMR data in CD3CN were recorded for the species that remain in solution after gel formation by 2a-2c and 2e, and precipitation in the case of 2d, from 10 mM and for diluted solutions where aggregation is not taking place (ca 0.2 mM). As in the case of amidocarbamates, no significant differences in chemical shift between gels and diluted samples were found, the only difference being that in the gel samples the signals were broader, suggesting that in these cases the species observed by NMR correspond mainly to free organogelator molecules with reduced tumbling rates as a result of gel formation. As can be seen in Figure 9, the NH chemical shift was compared to that of compound 9 (10 mM), which is soluble in that solvent. It is noticeable that the chemical shift of the alkylamide NH is shifted downfield ca. 0.3 ppm for compounds 2a and 2b and ca. 0.15 ppm for 2c-2e when compared to 9. As in molecules 1a-1e, this effect can be ascribed to intramolecular H-bonding taking place in the free organogelator molecules through the alkylamide NH. It is reasonable that the H-bonded conformations are more important for the molecules with shorter aliphatic spacers, as indicated by the larger shift observed for 2a and 2b. In the case of 2d, the benzoylamide NH is shifted ca. 0.2 ppm downfield when compared to 9, suggesting that in

this case both amide NHs could be involved in intramolecularly H-bonded conformations. The presence of H-bonding as the noncovalent interaction responsible for gelation in aprotic organic solvents was also confirmed by IR spectroscopy. However, in this case detailed information regarding the two carbonyl groups of the terminal and central amide units could not be obtained due to their overlapping signals. For example, the gelation process of compound 2e in dichloromethane was accompanied by the appearance of IR bands corresponding to associated N-H and amide CdO bonds at 3286 and 1630 cm-1 respectively, and a decrease of the intensity of the vibrations corresponding to the free N-H at 3433 and 3330 cm-1 and free CdO at 1650 cm-1 (see Supporting Information). Interestingly, the aliphatic spacer of the tetraamide compounds is amenable to IR studies that could not be performed for the amidocarbamates 1a-1e due to the overlapping signals form the benzylic C-H vibrations. The CH2 antisymmetric νas(CH2) and symmetric νs(CH2) stretching vibration bands have been shown to give information about the conformation adopted by an alkylene chain.19 Thus, when these chains are in an all-trans (19) Masuda, M.; Vill, V.; Shimizu, T. J. Am. Chem. Soc. 2000, 122, 12327-12333 and references therein.


Langmuir, Vol. 21, No. 15, 2005 6785

Figure 8. SEM of xerogels of 2a in dioxane (A), acetonitrile (B), and chloroform (C); of 2b in acetonitrile (D) and toluene (E); and of 2c in acetonitrile (F). Table 6. Critical Gel Concentration of Pure Compounds 1a, 1e, 2b, and 2e and of the Individual Components in Their Binary Equimolar Mixtures in Toluenea 1a 1e 2b 2e

1a

1e

2b

2e

3.8 (0.8) 2.7 (0.6) 4.4 (0.6) 3.2 (1.0)

2.7 (0.6) 6.7 (1.6) 3.8 (0.6) 3.7 (0.4)

4.4 (0.6) 3.8 (0.6) 5.8 (1.0) 3.5 (0.4)

3.2 (0.5) 3.7 (0.4) 3.5 (0.4) 7.0 (1.4)


extended conformation, they absorb at lower frequencies than in the case of conformations that include some ethylenic moieties in a gauche disposition. As can be seen in Figure 10, for compounds with short chain lengths (2a2c), bands at 2954 and 2871 cm-1 are present, while for longer chains (2d, 2e), additional lower frequency bands appear at 2930 and 2854 cm-1. This could be explained if both trans and gauche dispositions are present, excluding the possibility of a completely extended conformation. Furthermore, IR spectra of the xerogels of compounds 2a and 2d from three solvents of different polarity (toluene, acetonitrile, and ethanol) were recorded (see Supporting Information). No significant differences between their νas(CH2) and νs(CH2) bands could be observed. Only a small

variation in the intensity of the νs(CH2) band was noticed for 2d in toluene, suggesting a slightly lower percentage of trans conformations in this gel. Circular dichroism is a useful technique in order to gain information about the relative disposition of chromophores in supramolecular systems.20 In the case of organogelators 1a-1e and 2a-2e, since most of the obtained gels were not completely transparent, it was difficult to study them by CD above the minimum gel concentration. Nevertheless, CD spectra could be recorded for methanol and acetonitrile solutions of 2a-2e below the minimum gel concentration, and consequently, we obtained structural information only for the monomeric species present in solution at this concentration. Figure 11 shows the CD spectra of compounds 2a-2e (1 mM) in methanol at 20 °C. For all the compounds a bisignate positive Cotton band could be observed centered at ca. 225-230 nm. For compound 2a, this band was nicely centered with the absorption maximum, indicating a P-helical relative arrangement of the aromatic systems.21 For the rest of the compounds, the shape of the band was maintained (20) For example: Hirst, A. R.; Smith, D. K.; Feiters, M. C.; Geurts, H. P. M. Langmuir 2004, 20, 7070-7077 and references therein. (21) Berova, N., Nakanishi, K., Woody, R. W., Eds. Circular Dichroism. Principles and Applications; Wiley-VCH: New York, 2000.

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Escuder et al. Table 7. X-ray Diffraction Long Distances and Calculated Fully Extended Spacing

Figure 9. 1H NMR spectra of the amide region for 10 mM solutions of compounds 2a-2e and 9 in CD3CN.

Figure 10. CH2 antisymmetric νas(CH2) and symmetric νs(CH2) IR stretching bands for solid KBr pellets of compounds 2a-2e.

Figure 11. CD spectra of 1 mM solutions of compounds 2a2e in methanol.

but the intensity of the high-wavelength half-band was decreasing on going from 2a to 2e, indicating the decrease in the interaction between the dipoles of the aromatic chromophores as a consequence of the increase in length of the central oligomethylene chain together with some conformational changes. To confirm the intramolecular

compd

dmax XRD (Å)

calcd dmax (Å)

1a 2a 1d 2d

14.57 14.30 17.56 17.55

24 22 28 27

nature of the exciton coupling observed, compound 9 was investigated at similar concentrations in methanol. A negative Cotton band could be observed centered at ca. 220 nm but no bisignate effect was present in this case. When the studies were carried out in acetonitrile, similar results were found (Supporting Information), suggesting that there were not important changes in the molecular conformation between those solvents. Therefore, the presence of bisignate bands can be ascribed to the presence of preferred conformations in solution that present the appropriate arrangement of the aromatic groups for this effect to be observed. Having in mind the previous results and the flexible nature of aliphatic spacers, intramolecular H-bonded folded conformations could explain the results obtained. The conformational mobility is expected to increase for longer aliphatic spacers and would explain the reduced bisignate character of the CD bands for the larger compounds. Xerogels of compounds 2a and 2d in ethanol, toluene, and acetonitrile were studied by X-ray powder diffraction (see Supporting Information). As in the case of compounds 1a-1e, all the solids were found to be polycrystalline, showing more or less sharp peaks depending on the compound and solvent. Thus, sharp reflections could be found for the ethanol xerogel of 2a, whereas toluene and acetonitrile xerogels presented a larger amount of amorphous material as reflected by the presence of a broad background in their diffractograms. Compound 2d, on the contrary, showed intense reflections in both toluene and acetonitrile, whereas its ethanol xerogel was less ordered than that corresponding to 2a in that solvent. This difference in the crystalline character of xerogels in ethanol could be related to the described difference in the cgc of both gels (11 mM for 2a and 77 mM for 2d), being a consequence of the concomitant precipitation of amorphous material together with the crystalline phases at high concentration. Nevertheless, a similar diffraction pattern was found in this solvent for both compounds with a shift of ca. 2 Å suggesting a high similarity in their molecular arrangement. Reflections assignable to a lamellar rectangular packing with periodicity in the ratio 1:1/2:1/3 could be found in the diffractograms reported for compounds 2a-2e, while for the analogue amidocarbamtes the diffraction peaks are more difficult to assign. Table 7 collects the long distances (d) found (the possibility of reflections with 2θ < 3° would not be consistent with the periodicity observed for the rest of the peaks) and the distances calculated from molecular models for a fully extended conformation for 2a and 2d and the amidocarbamates 1a and 1d. As can be seen, low-angle reflections are shorter than what could be expected for layers of molecules with the central oligomethylene chain in an all-trans conformation. Although intercalation or tilting cannot be excluded, these data, together with IR, point toward the absence of completely extended conformations in the gels. Conclusions The fact that mixtures of related compounds can form gels is to some point unexpected if gelation is seen as a kind of crystallization. In the case presented here, not


only does the use of binary or ternary mixtures not prevent gelation but also a collaborative effect is observed. However, these findings are, at first glance, striking due to the significant size difference between some of the molecules involved (for example, 1a and 1e), which would predict a mismatch in the hybrid assemblies. Hybrid assemblies are formed also when tetraamides 2a-2e are studied and from mixtures of tetraamides and amidocarbamates, indicating a similar aggregation mechanism for both types of molecules. WAXD analyses demonstrate that there is neither phase separation nor random coassembly in the hybrid gels. The data obtained can be rationalized by considering that the presence of a flexible aliphatic spacer could afford some length adaptation and, consequently, the formation of copolymers. The results also point to the fact that these molecules adopt different conformations in dilute solutions and in the aggregates. Additionally, IR and WAXD studies suggest that the molecules are not completely extended in the aggregates, with the central aliphatic spacer being folded to some extent. Furthermore, a new series of tetraamide compounds capable of forming robust gels has been prepared. This structural scaffold is suited for synthetically feasible modification by the use of different acylating reagents.

Langmuir, Vol. 21, No. 15, 2005 6787

In general, the described results could be of interest in the tailored design of supramolecular aggregates. The possibility of introducing several monomers should allow a fine-tuning of the properties of the supramolecular materials or even give place to new ones. Additionally, the described model may be of interest from a biomimetic point of view because the behavior is reminiscent of processes of peptide and protein aggregation like amyloid fibril formation.22 Acknowledgment. We thank the SCIC of University Jaume I for technical assistance on microscopy studies and the Generalitat Valenciana (Project CTDIA/2002/123) for financial support. Supporting Information Available: Synthetic procedures, characterization of new compounds, technical details, and additional figures (PDF). This material is available free of charge via the Internet at http://pubs.acs.org. LA050655J (22) See, for example, (a) Matsumura, S.; Uemura, S.; Mihara, H. Chem. Eur. J. 2004, 10, 2789-2794. (b) Banerjee, A.; Maji, S. M.; Drew, M. G. B.; Haldar, D.; Das, A. K.; Banerjee, A. Tetrahedron 2004, 60, 5935-5944.

Organogel Formation by Coaggregation of Adaptable

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