pubs.acs.org/Langmuir © 2009 American Chemical Society
Pyromellitamide Gelators: Exponential Rate of Aggregation, Hierarchical Assembly, and Their Viscoelastic Response to Anions† Katie W. K. Tong,‡,§ Sabrina Dehn,‡ James E. A. Webb,§ Kio Nakamura,§ Filip Braet,^ and Pall Thordarson*,‡ ‡ School of Chemistry, The University of New South Wales, Sydney, NSW 2052, Australia, §School of Chemistry, The University of Sydney, NSW 2006, Australia and ^Australian Key Centre For Microscopy and Microanalysis, The University of Sydney, NSW 2006, Australia
Received December 19, 2008. Revised Manuscript Received January 28, 2009 The gelation and aggregation properties of a newly synthesized structurally simplified tetrahexyl pyromellitamide 2 have been studied and compared to the previously reported tetra(ethylhexanoate) pyromellitide 1, indicating that the ester groups in the latter significantly impede its aggregation. Morphology studies (AFM and TEM) on the aggregates formed by tetrahexyl pyromellitamide 2 in cyclohexane revealed highly uniform aggregates with different dimensions at different starting concentrations, suggesting that this molecule aggregates in a hierarchical fashion from a one-dimensional supramolecular polymer through hollow tubes or compressed helices to a network structure and then to a gel. This hypothesis is further supported by viscosity measurements that indicate a crossover point where individual supramolecular fibers get entangled at concentrations above ca. 3 mM in cyclohexane. Addition of 1 equiv of tetraalkylammonium salts of chloride or bromide, however, caused the viscosities of these pyromellitamide solutions to drop by a factor of 2-3 orders of magnitude, demonstrating the sensitivity of these aggregates to the presence of small anions. The sensitivity to anions does depend on the solubility of the salts used as small anion salts with little solubility in cyclohexane did not show this effect. Time-dependent viscosity studies showed that the aggregation of pyromellitamide 2 follows an exponential rate law, possibly related to the columnar rearrangements that are associated with the observed 6 A˚ contraction in d spacing in the XRD pattern of these gels. These results, particularly on the importance of kinetics of aggregation of self-assembled pyromellitamide gels, will be useful for future development of related materials for a number of applications, including tissue engineering and drug delivery.
Introduction Hierarchical self-assembly of molecular building blocks into ordered nanostructures provides a promising approach toward the creation of functional materials through noncovalent interactions.1 Significant interest has been focused on the development of self-assembled gels, which are fibrous three-dimensional networks that are constructed via the self-assembly of small synthetic molecules in a liquid.2,3 The control of the assembly of these reversible structures is governed by noncovalent bonds. Strong and highly directional hydrogen bonds are often used for self-assembled gels which often provide useful anion-binding properties. Selfassembled gels are showing increasing potential for a range of applications including catalysis,4 drug delivery,5 and in † Part of the Gels and Fibrillar Networks: Molecular and Polymer Gels and Materials with Self-Assembled Fibrillar Networks special issue. *To whom correspondence should be addressed.
(1) (a) Aggeli, A.; Nyrkova, I. A.; Bell, M.; Harding, R.; Carrick, L.; McLeish, T. C. B.; Semenov, A. N.; Boden, N. Proc. Natl. Acad. Sci. U.S.A. 2001, 98, 11857–11862. (b) Elemans, J. A. A. W.; Rowan, A. E.; Nolte, R. J. M. J. Mater. Chem. 2003, 13, 2661–2670. (2) (a) Weiss, R. G.; Terech, P. In Molecular Gels: Materials with SelfAssembled Fibrillar Networks; Springer: Dordrecht, The Netherlands, 2006. (b) Terech, P.; Weiss, R. G. Chem. Rev. 1997, 97, 3133–3159. (c) Abdallah, D. J.; Weiss, R. G. Adv. Mater. 2000, 12, 1237–1247. (d) van Esch, J. H.; Feringa, B. L. Angew. Chem., Int. Ed. 2000, 39, 2263–2266. (e) de Loos, M.; Feringa, B. L.; van Esch, J. H. Eur. J. Org. Chem. 2005, 3615–3631. (f) George, M.; Weiss, R. G. Acc. Chem. Res. 2006, 39, 489–497. (3) Estroff, L. A.; Hamilton, A. D. Chem. Rev. 2004, 104, 1201–1217. (4) Wang, Q.; Yang, Z.; Ma, M.; Change, C. K.; Xu, B. Chem.;Eur. J. 2008, 14, 5073–5078. (5) (a) Xing, G. B.; Yu, C. W.; Chow, K. H.; Ho, P. L.; Fu, D. G.; Xu, B. J. Am. Chem. Soc. 2002, 124, 14846–14847. (b) Cao, S.; Fu, X.; Wang, N.; Wang, H.; Yang, Y. Int. J. Pharm. 2008, 357, 95–99.
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tissue engineering.6 It is clear, however, that for researchers to fully realize the potential that these materials have requires better understanding of the thermodynamics and kinetics of gel formation and their response to external stimuli. We have previously reported that tetra(ethylhexanoate) pyromellitide 1, which consists of a rigid aromatic core with tetraamides linked to ethylhexanoate hydrophobic chains, self-assembles into fibers to form organogels in nonpolar solvents.7 This gel formation is driven by formation of extensive hydrogen-bonded arrays (stacks) through intermolecular hydrogen bonds between the amide groups in 1. A remarkable property of these gels is their ability to revert back into solutions upon the addition of small anions such as bromides and chlorides. The anion-induced gel to solution phase transition is caused by the binding of anions to the two anion recognition sites of 1 between the amide hydrogen bonds, which would in turn dissociate the intermolecular aggregation that allows for gel formation, and thus the gels collapse back into solutions.7 Hence, the anion-responsive organogel of 1 provides potential building blocks for the construction of functional materials for applications such as release systems for biomedical purposes. In our ongoing work to understand better the processes, including the thermodynamics and kinetics, that control the formation of self-assembled gels and their response to their (6) (a) Silva, G. A.; Czeisler, C.; Niece, K. L.; Beniash, E.; Harrington, D. A.; Kessler, J. A.; Stupp, S. I. Science 2004, 303, 1352–1355. (b) Rajangam, K.; Behanna, H. A.; Hui, M. J.; Han, X.; Hulvat, J. F.; Lomasney, J. W.; Stupp, S. I. Nano Lett. 2006, 6, 2086–2090. (7) Webb, J. E. A.; Crossley, M. J.; Turner, P.; Thordarson, P. J. Am. Chem. Soc. 2007, 129, 7155–7162.
Published on Web 03/18/2009
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Article Chart 1. Pyromellitamides 1 and 2
Table 1. Gelation Properties of Pyromellitamides 1 and 2 in Various Solventsa solvent
environment, we found that the synthetic effort required for making the pyromellitamide gelator 1 in large quantities via three steps was a significant challenge. Furthermore, although the ester functionalities in 1 were not expected to play a major role in their aggregation, a simpler pyromellitamide derivative that did not include such ester linkages was desired. To address both of these issues, we synthesized the structurally more simplified pyromellitamide 2 that turned out to have superior aggregation properties in a number of solvents compared to the parent pyromellitamide 1. Microscopy studies revealed the different structures that pyromellitamide 2 formed at different concentrations in cyclohexane, ranging from one-dimensional supramolecular polymers to fibers and network of micrometer dimensions. The superior aggregation properties of pyromellitamide 2 were further confirmed by viscosity measurements by comparison to pyromellitamide 1. These viscosity measurements also confirmed the different effects that small and large anions have on the aggregation of 2 as well as the importance that solubility of the anion salts has on their effect. Finally, time-dependent viscosity and X-ray diffraction (XRD) studies showed that aggregation of 2 in cyclohexane seems to follow an exponential rate law, possibly also associated with rearrangement in the columnar structures formed by these pyromellitamide 2 aggregates.
Results and Discussion Synthesis, Gelation, and Solubility Properties of 2. The tetrahexyl pyromellitamide 2 was prepared by the reaction of hexylamine with the pyromellitoyl chloride 38 in 62% yield. The gelation capabilities of this compound was investigated by dissolving it in various solvents by heating and subsequent cooling to room temperature (Table 1, see Experimental Section). For comparison, the gelation properties of the previously reported 17 are also shown in Table 1. The structurally simplified tetrahexyl pyromellitamide 2 showed similar solubility properties to the previously reported tetra(ethylhexanoate) pyromellitamide 1 as it was insoluble in most of the solvents tested at room temperature. Upon heating in nonpolar solvents, 2 gradually dissolved with heating, and after subsequent slow cooling back to room temperature, thermoreversible transparent gels were formed (Table 1). Gel formation in cyclohexane was particularly effective, even at a relatively low concentration of 12.0 mM (0.9% w/w). In more polar solvents, 2 generally dissolved more readily upon heating than 1 and formed opaque gels upon cooling in solvents such as alcohols and dimethylformamide (which formed colloidal precipitates in the latter). The parent tetra(ethylhexanoate) pyromellitamide 1, however, did not form gel-like structure in any of these more polar solvents. In chloroform, 2 formed a viscous solution indicating that significant aggregation had occurred in this solvent. The aggregation properties of 2 made it (8) Almog, J.; Baldwin, J. E.; Crossley, M. J.; DeBernardis, J. F.; Dyer, R. L.; Huff, J. R.; Peters, M. K. Tetrahedron 1981, 37, 3589–3601.
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1
2
cyclohexane TG(9.5) TG(12) hexane TG(10.5) TG(20) diethyl ether TG(84) TG(93) toluene TG(105) TG(124) chloroform S VS 1-butanol C OG ethanol P OG methanol P OG dimethylformamide S PG dimethyl sulfoxide S PG water I I a S: soluble at room temperature; I: insoluble; P: precipitates; C: crystals; VS: viscous solution; OG: opaque gels with slight amount of excess solvent on top; PG: partial gel (opaque gel-colloids precipitated from solvent); TG: transparent gel (minimum gelation concentration in mM).
difficult to obtain well-resolved NMR spectra of this compound, except in d5-pyridine, where, despite relatively poor solubility, 2 did not show any signs of aggregation. Morphology Studies. The structural morphologies of the supramolecular structures formed from tetrahexyl pyromellitamide 2 over a wide concentration range (from dilute to gel) in cyclohexane were investigated using a combination of transmission electron microscopy (TEM) and atomic force microscopy (AFM). The results obtained are in good agreement with the hierarchical aggregation model for gel formation as outlined by Estroff and Hamilton.3 At dilute concentration of 2 in cyclohexane, about 1/4000 of the minimum gelation concentration (2 10-3 mM), samples that had been spin-coated on to mica showed very long (up to several micrometers) thin fiber structures by AFM (Figure 1a), representing the primary structure in the Estroff-Hamilton model. The average height of these fibers was found to be uniformly in the range of 0.8-1.0 nm (except in some places where they cross each other where the height is double this value; see Figure S1 in Supporting Information), suggesting that they are single molecule thick unidirectional supramolecular polymers of 2. Assuming that the stacks of 2 on a mica surface have a similar shape as those observed in the reported X-ray structure of tetra(ethylhexanoate) pyromellitamide 17 with the tail groups sticking to the side, we estimate that these would have thicknesses similar to the two shorter unit cell axis of 1, that is, between 0.7 and 0.9 nm, which is in excellent agreement with the AFM data. The nature of the substrate did not have any visible effect on the structures obtained as essentially identical results were obtained with a highly ordered pyrolitic graphite (HOPG) substrate (see Figure S2 in Supporting Information) instead of mica. Deposition onto carbon-coated copper grids of slightly more concentrated solutions of 2 in cyclohexane (1.5 10-2 mM), about 1/700 of what is required for gelation, resulted in an entangled three-dimensional network of fibrous aggregates (Figure 2a). Closer view of these structures showed intertwining of thinner fibers and thicker fibers of diameters of approximately 20 and 50 nm, respectively (Figure 2b). Individual junctions between fibers were also observed, and these are probably important in stabilizing the supramolecular gel network. Furthermore, the fibers depicted in Figure 2 clearly showed the presence of hollow tubes. We postulate here that these hollow tubes are compressed helices as very faint horizontal bands could be observed on the TEM DOI: 10.1021/la804180h
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Figure 1. Atomic force microscopy (AFM) topography images obtained in tapping mode of (a) dried diluted tetrahexyl pyromellitamide 2 solution (2 10-3 mM) in cyclohexane spin-coated onto mica and (b) dried tetrahexyl pyromellitamide 2 cyclohexane gel (13 mM) on highly ordered pyrolitic graphite (HOPG). The height profiles below images (a) and (b) are obtained from the positions indicated by white arrows in the images above.
Figure 2. Transmission electron microscopy (TEM) micrographs of a dried tetrahexyl pyromellitamide 2 solution (1.5 10-2 mM) in cyclohexane deposited onto carbon-coated copper grids. The micrographs show two different regions representing (a) a dense fibrous network and (b) a region where individual intertwining fibers are clearly visible, and intermediate electron dense faint horizontal bands (arrows) indicate that these might be hollow tubes. (the contrast of these bands is difficult to capture on the TEM digital camera; see arrows in Figure 2b). These tubes or compressed helices (Figure 2b) and resulting network (Figure 2a) represent the secondary and tertiary structures, respectively, in the Estroff-Hamilton hierarchical assembly model.3 In the above discussion, it is assumed that the morphologies observed by AFM and TEM microscopies represent the aggregate structures in solution. Surface-induced aggregation and/or gelation cannot be fully ruled out here; however, contrary to previous reports where an anionic gelator interacted specifically with a cationic surface,9 there is no reason to expect that there will be any specific interactions between (9) Biesner, A. M.; Tiller, J. C. Chem. Commun. 2005, 3942–3944.
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the neutral pyromellitamide 2 and the various substrates in this work (HOPG, mica, and a carbon TEM film). This fact, and the relatively short drying time required for sample preparation, especially in the dilute regime by spincoating (3 h, before they were allowed to cool down under ambient conditions. The viscosities (η) of these solutions were not determined until after at least 24 h, which in combination with the >3 h pre-equilibrium treatment at 60 C ensures that the self-assembly processes had reached an equilibrium (vide supra). The concentration and temperature dependence for the specific viscosities [ηsp = (η/η0) - 1, with η0 = viscosity of the pure solvent] of the cyclohexane solutions of 1 and 2 are shown in Figure 3. The large increases in specific viscosities with increasing concentrations (Figure 3a) of the pyromellitamides 1 and 2 are consistent with formation of supramolecular polymer aggregates.10 The specific viscosities of 2 were more than an order of magnitude greater than 1 over the range of concentrations and temperatures studied. This shows that the ester groups on the “tail groups” in 1 frustrate the aggregation of the pyromellitamides compared to the simpler hexyl tails in 2. The specific viscosities of 1 and 2 decreased exponentially with increasing temperatures (Figure 3b), which is in line with other reports on the viscoelastic temperature dependence of supramolecular aggregates.11 (10) (a) Sijbesma, R. P.; Beijer, F. H.; Brunsveld, L.; Folmer, B. J. B.; Hirchberg, J. H. K. K.; Lange, R. F. M.; Lowe, J. K. L.; Meijer, E. W. Science 1997, 278, 1601–1604. (b) Hirchberg, J. H. K. K.; Beijer, F. H.; van Aert, H. A.; Magusin, P. C. M. M.; Sijbesma, R. P.; Meijer, E. W. Macromolecules 1999, 32, 2696–2705. (c) Xu, J.; Fogleman, E. A.; Craig, S. L. Macromolecules 2004, 37, 1863–1870. (d) Serpe, M. J.; Craig, S. L. Langmuir 2007, 23, 1626–1634. (11) Park, T.; Zimmerman, S. C. J. Am. Chem. Soc. 2006, 128, 11582–11590.
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Figure 3. Specific viscosities (ηsp) of pyromellitamides 1 and 2 in cyclohexane as a (a) function of concentration at 25 C and (b) temperature.
The data have been fitted to an exponential rate law: ηsp = a exp(kT) with an adjusted R2 = 0.96 for 1 (broken dotted line) and 0.9997 for 2 (solid line) and a k = -0.06 ( 0.04 C-1 (95% confidence interval) for 1 and a k = -0.15 ( 0.01 C-1 (95% confidence interval) for 2.
When the results were plotted on a log-log scale (Figure 4), the data did not show a linear behavior for pure 1 and 2 over the range of concentrations studied as expected from the Mark-Houwink relation ηsp [c]n. At concentrations of up to ca. 3 mM a gradual slope of 2-3.5 was observed, whereas at higher concentrations, steeper slopes up to 6 were observed in the case of 2. The steeper slopes imply thickening of the viscous solutions due to the increasing amounts of long-chained aggregates of 1 and 2 overlapping or getting entangled.10,11 Effects of Anions and the Role of Countercations on Viscoelastic Properties. Leading on from our previous observation that small anions such as bromides inhibit gel formation for pyromellitamide 1,7 a detailed study on the viscoelastic response of pyromellitamide 2 to anions was undertaken. The addition of 1 equiv of the tetrabutylammonium bromide (TBA-Br) was shown to dramatically influence the viscosity properties of the cyclohexane solutions of 1 and 2 (Figure 4). In all cases, the decrease in viscosities was in the range of 2-3 orders of magnitude compared to pure pyromellitamides 1 and 2. The slope of the log-log plots for solutions of 1 and 2 with 1 equiv of TBA-Br were uniformly around 2-2.5, indicating that the remaining aggregates were too small and dilute to start to form a network. It should be noted that the viscosities of the solutions of 2 with 1 equiv of TBA-Br present were about 2 times higher than the corresponding solutions of 1 with TBA-Br. To explore further the effects of anions on the aggregation of pyromellitamide 2, the viscoelastic properties of 3.3 mM (0.25% w/w) cyclohexane solutions of 2, to a range of anions in the form of lipophilic salts, were determined (Table 2). These results showed that for the highly lipophilic tetrahexylammonium (THA) and TBA salts of small anions such as chloride and bromide the viscosities of the cyclohexane solutions of 2 dropped by more than 2 orders of magnitude (Table 2, entries 1-3). For large anions, including perchlorate, hexafluorophosphate, and tetraphenylborate (Table 2, entries 6-8), the TBA salts only reduced the viscosities of 2 by around 35-65% compared to the pure solution of 2 (Table 2, entry 12). On the basis of binding studies on the related pyromellitamide 1,7 both the iodide and nitrate TBA salts (Table 2, entries 4 and 5) would have been expected to have a significant effect on the aggregation of 2; however, these two TBA salts showed quite a remarkable difference with the nitrate reducing the viscosity of 2 by 99% while iodide only reduced it by 50%, which is more in line with the effects of the TBA salts of larger anions. Surprisingly, the tetraphenylphosphonium salts of chloride and bromide (Table 2, entries 10 and 11) have relatively little effect on Langmuir 2009, 25(15), 8586–8592
Figure 4. Effect of temperature and the addition of 1 equiv of TBA-Br (at 25 C) on the viscosities of 1 and 2 in cyclohexane solutions shown on a log-log scale. the viscosities of cyclohexane solutions of 2. This may be readily explained by the poor solubility of these salts in cyclohexane (they were largely undissolved after standing for 3 days). The central role that solubility plays on how cyclohexane solutions of 2 respond to the addition of salts is further highlighted by difference between the more soluble (nominally) noncoordinating TBA tetraphenylborate salt compared to the related less soluble ammonium tetraphenylborate salt (Table 2, entries 8 and 9). Kinetics of Pyromellitamide Aggregation. The above viscosity studies were all conducted with cyclohexane solutions of 2 that had been thermally equilibrated for over 3 h and allowed to stand for at least 24 h prior to measurement. Measurements on the kinetics of aggregation were conducted by allowing hot freshly prepared cyclohexane solutions of 2 to cool down to room temperature. To eliminate DOI: 10.1021/la804180h
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Table 2. Viscoelastic Properties of 1:1 Mixtures of Various Salts with Pyromellitamide 2 in Cyclohexane at 3.3 mM (0.25% w/w) Concentration at 25 Ca entry
salt
ηsp
% ηsp of pure 2
- 0.013 0 THA+Cl0.085 0 TBA+Cl+ 5.8 0.9998), resulting in a rate constant of k = (2.7 ( 0.3) 10-5 s-1 (95% confidence interval) from a triplicate measurement. The time and temperature dependence of the increase in specific viscosity was further illustrated when freshly made (t = 30 min) cyclohexane solution of 2 with ηsp = 1.1 was heated briefly to 40 C and then allowed to cool again and measured again (t = 70 min), resulting in ηsp = 2.8, which corresponds to the viscosity in an untreated control solution at least twice that age (t = 180 min). These results illustrate how temperature can be used to speed up the aggregation of pyromellitamide 2. The exponential increase in viscosity seems to be related to exponential growth in the aggregates that 2 form in cyclohexane. Examples of systems that show similar exponential kinetics include the exponential growth observed in the size of highly ordered nanoparticle monolayer islands on surfaces once the islands become significantly larger than surrounding diffusion layer12 as well as the exponential increase in the reaction rate of autocatalytic self-replicating micelles.13 This may suggest that the underlying mechanism for the exponential growth of pyromellitamide aggregates is autocatalytic and governed by size of the aggregates vs diffusion of unbound pyromellitamides 2. Additionally, autocatalytic cooperative changes in the structures of these aggregates cannot be ruled out at this point, especially when comparing the XRD pattern for freshly prepared samples of gels formed from pyromellitamide 2 in cyclohexane (1% w/w) with aged gels (2-7 days) as a significant shift in the lowest 2θ peak from about 3.6 to 4.8 (Figure 6a,b) was observed. The observed XRD shift corresponded to a change in d spacing from about 24 to 18 A˚. The former distance corresponds reasonably well to the calculated distance between the ends of two diagonally opposed and fully extended hexyl chains in pyromellitamide 2 as determined by simple (12) Bigioni, T. P.; Lin, X.-M.; Nguyen, T. T.; Corwin, E. I.; Witten, T. A.; Jaeger, H. M. Nat. Mater. 2006, 5, 265–270. (13) Bachmann, P. A.; Luisi, P. L.; Lang, J. Nature (London) 1992, 357, 57–59.
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Figure 5. A log plot of the specific viscosities (ηsp) on 1.3 mM (0.1% w/w) pyromellitamide 2 solutions in cyclohexane as a function of time (black squares) at 20 C. The data have been fitted to an exponential rate law (ηsp = a exp(kt), solid line).
molecular modeling studies (Hyperchem:14 MM+ force field). Two possible explanations for the contraction in d spacing are (i) as the aggregates formed from pyromellitamide 2 mature, the alkyl chains adopt a less extended conformer than in the initially formed stacks and/or (ii) the tilt angle R of individual molecules relative to the norm of stacks formed from 2 (Figure 6c) changes quite significantly (ΔR up to 40), but further studies are necessary to determine which mechanism is at work here.
Conclusions Compared to our previously reported tetra(ethylhexanoate) pyromellitamide 1, the newly synthesized structurally simplified tetrahexyl pyromellitamide 2 seemed to have superior aggregation properties as it formed gels in a number of nonpolar solvents and partial gels/colloidal suspensions in some of the more polar solvents. Viscosity measurements under equilibrium conditions also indicated that the aggregates formed by 2 are significantly larger than those formed by the previously reported pyromellitamide gelator 1. These results indicate that the carboxylic acid ester groups in tetra(ethylhexanoate) pyromellitamide 1 do significantly inhibit the aggregation of this compound compared to 2. The viscosity measurements also showed that the addition of as little as 1 equiv of small anions such as bromides and chlorides in the form of lipophilic TBA salts lowered the viscosity of pyromellitamide solutions of 1 and 2 in cyclohexane by orders of magnitude. Other TBA salts of larger anions had only a moderate effect on the aggregation of pyromellitamide 2 in cyclohexane; however, the solubility of these salts is equally important as illustrated by the fact that tetraphenylphosphonium bromide and chloride salts had very little effect on the aggregation of 2 in cyclohexane. The combination of microscopy and viscosity measurements also allowed us to propose a detailed hierarchical model, based on the Estroff-Hamilton model,3 for the aggregation of pyromellitamides 1 and 2, starting from one-dimensional supramolecular polymers at lower concentrations, through hollow tubes and to a supramolecular network, and gel formation at higher concentrations in nonpolar solvents (Figure 7). Finally, we looked at the kinetics of aggregation of pyromellitamide 2 in cyclohexane (as measured by viscosity), and to our surprise we found that these seemed to follow an (14) Hyperchem 8.04: Hypercube, Inc., Gainesville, FL, 2007.
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Figure 6. (a) Representative normalized (relative to largest peak in the 2θ range of 3-5) XRD pattern obtained at 1 h, 48 h (2 days), and 7 days intervals of gels formed from pyromellitamide 2 in cyclohexane. (b) Enlargement of the XRD pattern in the 2θ range of 2-7. (c) Schematic model of one possible mechanism for the apparent contraction in columnar width over time by increasing the tilt angle R between the pyromellitamide molecules and the columnar axis resulting in the observed XRD shift; see text for details.
Experimental Section Synthesis. Benzene-1,2,4,5-tetracarbonyl tetrachloride (pyr-
omellitoyl chloride) 38 was synthesized according to literature from pyromellitic dianhydride (97%) obtained from SigmaAldrich. This compound was characterized by comparing the 1H NMR spectrum and melting point values with those reported. Hexylamine (99%) was obtained from Sigma-Aldrich.
N,N0 ,N00 ,N000 -Tetrahexylbenzene-1,2,4,5-tetracarboxamide (Tetrahexyl Pyromellitamide) (2). A solution of benzene-
Figure 7. Schematic representation of the aggregation of 2 in cyclohexane at various concentrations as indicated, proceeding from one-dimensional supramolecular polymers to hollow tubes, networks, and finally a gel. The approximate dimension of the first two structures is also indicated by a scale bar.
exponential rate law, suggesting possibly an autocatalytic and/or slow diffusion vs large nucleation site mechanism. Structural rearrangement, cooperative or otherwise, cannot also be ruled out as time-dependent XRD measurements suggested a significant change in columnar thickness over time. The kinetic aspects of gel formation have not been studied in detail in the past although very recently the group of Escuder et al.15 reported that kinetically (rapidly cooled) trapped metastable gels slowly convert to the thermostable form, identical to that formed by slow cooling. These results and our report here indicate that the kinetics of gel formation can have a significant influence on their structural properties, including stability. Combined with our insight into the hierarchical processes leading to the formation of pyromellitamide gels, it should be straightforward to fine-tune the selfassembly properties of these materials and their response to anions. Work is currently under way in our group to realize these goals and expanding our understanding of both the thermodynamics and kinetics aspects of aggregation in selfassembled gels. (15) Rodrıguez-Llansola, F.; Miravet, J. F.; Escuder, B. Chem. Commun. 2009, 209–211.
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1,2,4,5-tetracarbonyl tetrachloride 38 (1.19 g, 3.62 mmol) in dry tetrahydrofuran (10 mL) was added dropwise to a stirred solution of hexylamine (1.87 g, 18.5 mmol) in dry tetrahydrofuran (10 mL), dichloromethane (10 mL), and pyridine (1.2 mL) at 0 C. The reaction mixture was stirred for 24 h at room temperature. The solvent was removed in vacuo to give the crude product as a yellow gel-like material. The crude product was then washed with methanol and acetone, and the resultant product was filtered to give tetrahexyl pyromellitamide 2 as a white gel-like paste; yield 62% (1.32 g), mp 261.3-262.7 C. 1H NMR (300 MHz, d5-pyridine): δ 0.83 (t, 12H, J = 6.7 Hz), 1.22-1.24 (m, 16H), 1.34-1.45 (m, 8H), 1.66-1.70 (m, 8H), 3.62 (dt, 8H, J = 7.3 and 5.5 Hz), 8.38 (s, 2H), 9.21 (t, 4H, J = 5.5 Hz, NH) ppm. 13C NMR (100 MHz, d5-pyridine): δ 14.22, 22.87, 27.08, 29.97, 31.80, 40.55, (aromatic peaks obscured by pyridine solvent peaks), 168.43 ppm. IR (KBr): ν = 3240, 3071, 2930, 2860, 1636, 1560 cm-1. MS (ESI): m/z 587.27 ([M + H]+). HR-MS (ESI): m/z 587.4538 ([M + H]+, C34H59N4O4 requires 587.4536). Anal. Calcd for C34H58N4O4 C, 69.59; H, 9.96; N, 9.55. Found: C, 69.53; H, 9.68; N, 9.51. Gel Formation: Inversion Test. In a typical experiment, a weighed amount of the tetrahexyl pyromellitamide 2 and the solvent (1.0 mL) was placed in a sealed vial. The vial was then heated until the sample was completely dissolved, forming a clear solution or the boiling point of the solvent, whichever took place first. The solution was allowed to cool slowly at room temperature, and the vial with a closed lid was inverted to determine whether the solution was completely immobilized to achieve gelation. Atomic Force Microscopy (AFM). AFM measurements were performed at the Australian Key Centre for Microscopy and Microanalysis, The University of Sydney, with a Pico SPM II system (Molecular Imaging, now Agilent Technologies) in acoustic alternating current (ACC) mode. Samples were DOI: 10.1021/la804180h
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Article prepared either by spreading a small drop of the gel made from tetrahexyl pyromellitamide 2 in cyclohexane onto a freshly cleaved highly ordered pyrolytic graphite (HOPG) and allowing to air-dry or by placing a dilute solution of 2 in cyclohexane on to freshly cleaved mica or HOPG and then spread the sample by spin-coating at 5000 rpm for 30 s and then allowing to air-dry. All AFM measurements were performed in air at ambient temperature and pressure. The cantilevers used were silicon cantilevers NSC14/AIBS (Mikro Masch, Narva) with a spring constant of 2-12 N/m and resonance frequency of 110-220 kHz. No postprocessing or filtering of the images recorded was done, except a global-plane correction using the PicoScan 5.4 image processing software supplied by Agilent Technologies. Transmission Electron Microscopy (TEM). Carbon-coated copper grids (400 mesh) TEM were prepared by the following procedure: a thin (≈20-50 nm) film of amorphous carbon was created by sputtering carbon from a graphite source in a highvacuum chamber onto a mica target. After removing the substrate from the vacuum chamber, the carbon film was floated off the mica in a large beaker of water. The carbon films were then transferred to a TEM copper grid by dipping them horizontally under the carbon film and then pulling them out of the water. These carbon-coated copper grids were then dried overnight under ambient conditions before the samples were applied. Samples of the gelator tetrahexyl pyromellitamide 2 in cyclohexane at various concentrations were then prepared, and a drop of these solutions was applied onto the carbon-coated copper TEM grids. After ca. 30 s, the excess solvent was drained off the carbon-coated copper TEM grids by blotting with a filter paper and then allowed to dry under ambient conditions for at least 10 min before TEM analysis. The TEM work was performed at the Australian Key Centre for Microscopy and Microanalysis, The University of Sydney, using a Philips CM12 TEM operated at 120 kV. The TEM micrographs were captured using a Morada 11 MegaPixel CCD camera. XRD. XRD analysis was performed at X-ray diffraction laboratory in the Analytical Centre, The University of New South Wales, on a Panalytical X’pert Multipurpose (MPD) X-ray diffractometer with a fixed stage and rotating source. Cu KR X-rays were generated at 40 mA and 45 kV, and the angular (2θ) scanning rate and step size was 0.02/min. For characterization of gels, fresh samples were loaded on to a stainless steel sample holder and analyzed after partially drying out after about 40-60 min over a 2θ range of 2-30. Viscosity Measurements under Equilibrium Conditions. In a typical experiment, a weighted amount of the gelators, tetra(ethylhexanoate) pyromellitamide 1 or tetrahexyl pyromellitamide 2, was heated in cyclohexane (8.0 mL) until a clear solution was obtained (final volume adjusted back to 8.0 mL to compensate for evaporation during heating and the vial then closed). These solutions were then kept at 60 C for about 3 h and then allowed to cool to room temperature under ambient conditions. For measurements on pure 1 or 2, these solutions were allowed to stand for 24 h at room temperature before viscosities were measured after equilibrating in a thermostated ((0.1 C) water bath at 25-55 C for at least 20 min. In experiments involving
8592 DOI: 10.1021/la804180h
Tong et al. the addition of tetrabutylammonium bromide (TBA-Br) or other lipophilic salts, 1 equiv of the neat salt under study was added to the cyclohexane solutions of 1 and 2 after they had been allowed to stand for 24 h. The resulting mixtures were then kept at room temperature for 3 days before their viscosities were measured in the same manner as for the pure cyclohexane solutions of 1 and 2. The viscosities of these solutions were measured using the GILMONT falling-ball viscometer. The solutions were poured into the viscometer (7.0 mL) and were allowed to settle until no air bubbles were observed. A stainless steel ball was dropped into the viscometer, and time of descent between the two red reference lines was measured with a stopwatch. The time recorded was then multiplied with the calibrated viscometer constant and the difference between the density of the falling ball and solvent to obtain the viscosity (η) of the solution according to instructions from the manufacturer. Viscosity Measurements under Kinetic Conditions. In a typical experiment, a weighted amount of tetrahexyl pyromellitamide 2 was heated in cyclohexane (25.0 mL) until a clear solution was obtained. These solutions (volume adjusted back to 25.0 mL to compensate for evaporation during heating) were then allowed to cool to room temperature under ambient conditions in a closed vessel. Samples (2.0 mL) were then removed at certain time intervals, and their viscosities were measured by the time it takes the solution to fall between two marks of a capillary in a Cannon-Ubbelohde Semi-Micro dilution viscometer no. 150 after equilibrating (20 min) in a thermostated ((0.1 C) water bath at 20 C, and the measured time was then multiplied by the viscometer constant (uncertainty
