Organogelation and Hydrogelation of Low-Molecular-Weight

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Organogelation and Hydrogelation of Low-Molecular-Weight Amphiphilic Dipeptides: pH Responsiveness in Phase-Selective Gelation and Dye Removal† Tanmoy Kar, Sisir Debnath, Dibyendu Das, Anshupriya Shome, and Prasanta Kumar Das* Department of Biological Chemistry, Indian Association for the Cultivation of Science Jadavpur, Kolkata - 700 032, India Received December 23, 2008. Revised Manuscript Received February 24, 2009 The search for efficient low-molecular-weight gelators (LMWGs) with possible structure-activity correlation is on the rise. The present work reports a novel set of amphiphilic dipeptide-based carboxylic acids capable of efficiently gelating organic solvents. More interestingly, their sodium salts showed enhanced efficiency in organogelation with the additional ability to gelate water. Electrostatic interactions present in the aggregation of the sodium carboxylates of amphiphilic dipeptides seem to be important because some of the nongelator carboxylic acids turned out to be excellent gelators upon salt formation. The combinations and sequence of the amino acids in the dipeptide moiety were systematically altered to understand the collective importance of the nonpolar aliphatic/aromatic substitution in amino acids in the self-assembling behavior of amphiphiles. Almost a 20-fold enhancement in the gelation ability was observed on reversing the sequence of the amino acid residues, and in some cases, nongelators were transformed to efficient gelators. Spectroscopic and microscopic studies of these thermoreversible organo/hydrogels revealed that balanced participation of the noncovalent interactions including hydrogen bonding and van der Waals interactions are crucial for organo/hydrogelation. These dipeptides selectively gelate organic solvents from their mixtures with water, and the xerogels prepared from these organogels showed time-dependent adsorption of dyes such as crystal violet. The most remarkable feature of these gelators is the pH responsiveness, which was aptly utilized for the pH-dependent phaseselective gelation of either solvent in a biphasic mixture of oil and water. The dissimilar gelation ability of the acid and its salt originating from the pH responsiveness of the amphiphilic dipeptide was employed in the instant removal of large amounts of dyes for wastewater treatment.

Introduction Gels, a class of soft materials, are gaining a huge amount of interest owing to their versatile applications in fields such as drug delivery, tissue engineering, cosmetics, optical sensors, templated materials, enzyme-immobilization matrices, and so on.1-4 This surge has amplified the need for the rational design and synthesis † 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. Fax: +(91)-33-24732805. E-mail: [email protected].

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of molecules having versatile gelation ability. In this regard, lowmolecular-weight gelators (LMWGs) have become very important as a result of their supramolecular 3D networks that immobilize a variety of solvents.5-14 Noncovalent interactions (7) (a) Friggeri, A.; Gronwald, O.; van Bommel, K. J. C.; Shinkai, S.; Reinhoudt, D. N. J. Am. Chem. Soc. 2002, 124, 10754. (b) Kiyonaka, S.; Shinkai, S.; Hamachi, I. Chem. Eur. J. 2003, 9, 976. (8) (a) Lin, Y.-C.; Kachar, B.; Weiss, R. G. J. Am. Chem. Soc. 1989, 111, 5542. (b) Murata, K.; Aoki, M.; Suzuki, T.; Harada, T.; Kawabata, H.; Komori, T.; Ohseto, F.; Ueda, K.; Shinkai, S. J. Am. Chem. Soc. 1994, 116, 6664. (9) (a) de Loos, M.; van Esch, J. H.; Kellogg, R. M.; Feringa, B. L. Angew. Chem., Int. Ed. 2001, 40, 613. (b) George, M.; Weiss, R. G. Langmuir 2002, 18, 7124. (c) van Gorp, J. J.; Vekemans, J. A. J. M.; Meijer, E. W. J. Am. Chem. Soc. 2002, 124, 14759. (d) Ajayaghosh, A.; George, S. J. J. Am. Chem. Soc. 2001, 123, 5148. (e) Wang, G.; Hamilton, A. D. Chem. Eur. J. 2002, 8, 1954. (f) Mohmeyer, N.; Kuang, D.; Wang, P.; Schmidt, H. W.; Zakeeruddin, S. M.; Gratzel, M. J. Mater. Chem. 2006, 16, 2978. (g) Bieser, A. M.; Tiller, J. C. Chem. Commun. 2005, 3942. (10) (a) Estroff, L. A.; Hamilton, A. D. Chem. Rev. 2004, 104, 1201. (b) Lee, K. Y.; Mooney, D. J. Chem. Rev. 2001, 101, 1869. (c) Tiller, J. C. Angew. Chem., Int. Ed. 2003, 42, 3072. (d) van Bommel, K. J. C.; van der Pol, C.; Muizebelt, I.; Friggeri, A.; Meetsma, A.; Feringa, B. L.; van Esch, J. Angew. Chem., Int. Ed. 2004, 43, 1663. (e) Bhattacharya, S.; Srivastava, A.; Pal, A. Angew. Chem., Int. Ed. 2006, 45, 2934. (f) Vemula, P. K.; John, G. Chem. Commun. 2006, 2218. (11) (a) von Lipowitz, A. Ann. Chem. Pharm. 1841, 38, 348. (b) Gortner, R. A.; :: :: Hoffman, W. F. J. Am. Chem. Soc. 1921, 43, 2199. (c) Kohler, K.; Forster, G.; Hauser, A.; Dobner, B.; Heiser, U. F.; Ziethe, F.; Richter, W.; Steiniger, F.; Drechsler, M.; Stettin, H.; Blume, A. J. Am. Chem. Soc. 2004, 126, 16804. (d) Debnath, S.; Shome, A.; Dutta, S.; Das, P. K. Chem. Eur. J. 2008, 14, 6870. (e) Herres, A.; van der Pol, C.; Stuart, M.; Friggeri, A.; Feringa, B. L.; van Esch, J. J. Am. Chem. Soc. 2003, 125, 14252. (f) Estroff, L. A.; Leiserowitz, L.; Addadi, L.; Weiner, S.; Hamilton, A. D. Adv. Mater. 2003, 15, 38. (12) van Esch, J. H.; Feringa, B. L. Angew. Chem., Int. Ed. 2000, 39, 2263, and references therein. (13) (a) Yang, Z.; Xu, B. Chem. Commun. 2004, 2424. (b) Kohler, K.; Forster, G.; Hauser, A.; Dobner, B.; Heiser, U. F.; Ziethe, F.; Richter, W.; Steiniger, F.; Drechsler, M.; Stettin, H.; Blume, A. Angew. Chem., Int. Ed. 2004, 43, 245. (c) Wang, G.; Hamilton, A. D. Chem. Commun. 2003, 310. (d) Friggeri, A.; Feringa, B. L.; van Esch, J. J. Controlled Release 2004, 97, 241. (e) John, G.; Vemula, P. K. Soft Matter 2006, 2, 909. (14) (a) Xing, B.; Yu, C. W.; Chow, K. H.; Ho, P. L.; Fu, D.; Xu, B. J. Am. Chem. Soc. 2002, 124, 14846. (b) Shome, A.; Debnath, S.; Das, P. K. Langmuir 2008, 24, 4280.

Published on Web 04/01/2009

DOI: 10.1021/la804235e

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such as electrostatic, van der Waals, π-π, and hydrogen bonding are the driving forces behind these ordered supramolecular arrangements.15 An optimized balance of hydrophilicity and lipophilicity within the LMWG is the prerequisite for its gelation ability. However, it is still difficult to predict the gelation ability of a small molecule.10,11,15 In our earlier reports, we have attempted to establish a rationale behind the synthesis of gelators through a possible structure-property relationship.5c,6b,11d,14b,15f It was found that a hydrogen-bonding dipeptide moiety at the head of an amphiphilic molecule with a long hydrophobic chain at the C terminus and a free amine group at the N terminus (Scheme 1) is an excellent precursor11d for the preparation of both organo- and hydrogelators. Although this precursor amine itself was a nongelator, the quaternization of this amine with methyl iodide yielded a hydrogelator, and coupling this same amine with a fatty acid resulted in an efficient organogelator.5c,11d At this point, we were curious to investigate whether the same dipeptide moiety would still have the ability to gel if the N terminus was swapped with the C terminus (Scheme 1). Hence, the resulting compound in the present work will have a long alkyl chain at the N terminus and a free -COOH group at the C terminus comprising the same structural design of the hydrogen-bonding dipeptide moiety. In this context, Bhattacharya and co-workers previously reported the organogelation of an L-alanine-based gelator with a free carboxylic end.3e The exclusivity of L-alanine as the amino acid component in gelation was highlighted in subsequent work with varying chain length from C-12 to C-16.3e,5d,16 It seemed that only L-alanine could maintain a suitable hydrophilic-lipophilic balance (HLB) within the molecule to be a gelator, but it is always important to have diversity in the gelator structure with versatile gelation ability for the wider exploitation of gels. In this regard, it would be very interesting to determine whether the incorporation of an additional amino acid with an extra hydrogen-bonding amide bond could modulate the supramolecular ordering preferably leading to gelation. Also, the free carboxylic end could provide the flexibility to tune the supramolecular assembly of the molecule by inducing electrostatic interactions through the formation of the sodium salt of the acid.15b,17 In the present work, we have reported the versatile organo/ hydrogelation ability of an amphiphilic dipeptide (Scheme 1) with a long alkyl chain at the N terminus and a free carboxylic group at the C terminus of the dipeptide moiety (Chart 1). Combinations of different L-amino acids with nonpolar aliphatic/aromatic residues and their sequences were judiciously used to delineate their collective role in the gelation behavior. It was observed that the presence of at least one aromatic amino acid in the dipeptide unit is essential for gelation. An almost 20-fold enhancement in the gelation ability was observed on reversing the sequences of the amino acid residues, and in some cases, even nongelators were transformed to efficient gelators. Compound 10 was found to be the best organogelator among the carboxylic acids where the minimum gelation concentration (MGC) was ∼0.45-0.7% w/v in different organic solvents. Furthermore, the gelation ability (15) (a) Mohmeyer, N.; Schmidt, H. W. Chem. Eur. J. 2005, 11, 863. (b) Suzuki, M.; Nanbu, M.; Yumoto, M.; Shiraib, H.; Hanabusa, K. New. J. Chem. 2005, 29, 1439. (c) Yang, H.; Yi, T.; Zhou, Z.; Zhou, Y.; Wu, J.; Xu, M.; Li, F.; Huang, C. Langmuir 2007, 23, 8224. (d) Suzuki, M.; Sato, T.; Shiraib, H.; Hanabusa, K. New. J. Chem. 2006, 30, 1184. (e) Mohmeyer, N.; Schmidt, H. W. Chem. Eur. J. 2007, 13, 4499. (f) Das, D.; Dasgupta, A.; Roy, S.; Mitra, R. N.; Debnath, S.; Das, P. K. Chem. Eur. J. 2006, 12, 5068. (16) Pal, A.; Ghosh, Y. K.; Bhattacharya, S. Tetrahedron 2007, 63, 7334. (17) (a) Caplar, V.; Zinic, M.; Pozzo, J-L.; Fages, F.; Mieden-Gundert, G.; :: Vogtle, F. Eur. J. Org. Chem. 2004, 4048. (b) Khatua, D.; Dey, J. Langmuir 2007, 21, 109. (c) Suzuki, M.; Sato, T.; Kurose, A.; Shirai, H.; Hanabusa, K. Tetrahedron Lett. 2005, 46, 2741.

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Kar et al. Scheme 1. Schematic Representation of the Rationale Behind the Synthesis of Amphiphilic Dipeptide Organo/Hydrogelators

was significantly improved with the conversion of the carboxylic acids to sodium carboxylates (MGC of 14a is ∼0.3% w/v), some of which were capable of efficiently gelating water. These dipeptides selectively gelate organic solvents in the presence of water. However, the most significant feature of these dipeptide-based gelators is the pH responsiveness of the free carboxylic group that was suitably used in pH-triggered phase-selective gelation from hydrogel to organogel. Remarkably, this pH-dependent phaseselective gelation was utilized to wipe out a large amount of dye instantaneously from water simply by altering the pH of the medium.

Experimental Section Materials. All amino acids, palmitic acid, dicyclohexylcarbodiimide (DCC), 4-N,N-(dimethyl)aminopyridine (DMAP), 1-hydroxybenzotriazole (HOBT), and all solvents were purchased from SRL India. 1,10 -Carbonyl diimidazole (CDI), thionyl chloride, and sodium hydroxide were purchased from Spectrochem, India. All deuterated solvents for NMR and FTIR experiments and 8-anilino-1-naphthalene-sulfonic acid (ANS) were obtained from Aldrich Chemical Co. Thin layer chromatography was performed on Merck precoated silica gel 60-F254 plates. 1H NMR spectra were recorded on an Avance 300 MHz (Bruker) spectrometer. Mass spectrometric data were acquired by an electron spray ionization (ESI) technique on a Q-tof-micro quadruple mass spectrometer (Micromass). Elemental analyses were performed on Perkin-Elmer 2400 CHN analyzer. Synthetic Procedure. All of the dipeptide-based gelators were synthesized following very well studied peptide chemistry (Scheme S1, Supporting Information, SI). Briefly, the methyl ester of an L-amino acid was coupled with a C-16 long-chain acid chloride in dry chloroform and dry pyridine. The ester-protected long-chain amide was then purified through column chromatography using 60-120 mesh silica gel and ethyl acetate/hexane as the eluent. The product was hydrolyzed using 1 N NaOH (1.1 equiv) in MeOH for 6 h with stirring at room temperature. Solvents were removed on a rotary evaporator, and the mixture was diluted with water and then washed with ether, followed by acidification by 1 N HCl to get the corresponding carboxylic acid. This acid was then coupled with another methyl esterprotected L-amino acid by using CDI (1 equiv) in dry dichloromethane (DCM). The purified product was obtained by column chromatography using 60-120 mesh silica gel and ethyl acetate/toluene as the eluent. The product was then subjected to hydrolysis by 1 N NaOH (1.1 equiv) in MeOH for 6 h with Langmuir 2009, 25(15), 8639–8648

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Article Chart 1. Chemical Structures of Amphiphilic Dipeptides

stirring at 45-50 °C. Amphiphilic dipeptides with free carboxylic acid ends were obtained following the same procedure as described above. DCC was used as the coupling agent instead of CDI when one of the amino acids was L-tryptophan. The sodium salt of the corresponding carboxylic acid was prepared by dissolving the acid in MeOH, and to that 1 equiv of 1 N NaOH (standardized) was added. After the solution was stirred briefly, the solvent was evaporated and the sample was dried to get the sodium salt. The formation of sodium salt was confirmed via FTIR spectroscopy by the disappearance of the -CdO stretching peak of carboxylic acid at ∼1720-1728 cm-1 and also by the improved solubility of the resultant compound in water. Characterization data of all of the synthesized amphiphilic dipeptides are given in SI. Preparation of the Gel. The required amounts of amphiphiles were slowly heated to dissolve either in organic solvents or in water and then were allowed to cool slowly (undisturbed) to room temperature in a vial with an internal diameter (i.d.) of 10 mm. After 20-30 min, the solid aggregate mass was stable to inversion of the glass vial, and then the compound was recognized to form a gel. Determination of the Gel-to-Sol Transition Temperature (Tgel). The gel-to-sol transition temperature (Tgel) was determined by placing the gel-containing glass vial (i.d. 10 mm) in a thermostatted oil bath and raising the temperature slowly at a rate of 2 °C/min. Tgel was defined as the temperature ((0.5 °C) at which the gel melted and started to flow. Microscopy Study. Field-emission scanning electron microscopy (FESEM) images were obtained on a JEOL-6700F microscope. A drop of gel (at MGC) was placed on a piece of coverslip and dried for few hours under vacuum before imaging. The morphology of the dried gel was also studied by using atomic force microscopy (AFM) on a (Veeco, model AP0100) microscope in noncontact mode. The sample was mounted on a silicon wafer and dried under vacuum for a few hours before imaging. FTIR Measurements. FTIR measurements on gelators in CHCl3 solution, D2O for hydrogels, and dried gels from toluene were carried out in a Perkin-Elmer Spectrum 100 FTIR spectrometer using a KBr cell, CaF2 cell, and KBr pellets, respectively. Langmuir 2009, 25(15), 8639–8648

Circular Dichroism (CD). CD spectra of the aqueous solution of gelators 13a and 14a with varying concentrations were recorded by using a quartz cuvette of 1 mm path length in a Jasco J-815 spectropolarimeter. X-ray Powder Diffraction (XRD). XRD measurements on all of the xerogels were made with a Seifert XRD3000P diffractometer, and the source was Cu KR radiation (R = 0.15406 nm) with a voltage and current of 40 kV and 30 mA, respectively. All of the xerogels were scanned from 1 to 10°. Fluorescence Spectroscopy. The emission spectra of ANS were recorded on a Varian Cary Eclipse luminescence spectrometer by adding the probe molecules in toluene solutions of gelators at varying concentrations at room temperature. ANS was initially dissolved in MeOH to form a 0.002 M superstock solution. Then this solution was diluted 10-fold with toluene to form a 0.0002 M solution. In every experiment, 20 μL of this solution was added to a 380 μL sample solution of toluene. The final concentration of ANS in the cuvette was 1  10-5 M. The ANS solutions were excited at λex = 360 nm. The excitation and emission slit widths were 2.5 and 5 nm, respectively. Phase-Selective Gelation. In a typical procedure, 1 mL of toluene and 1 mL of water were taken in a sample tube to which the desired amount (required to attain at least MGC) gelator was added. The gelator was then solubilized in this two-phase solution by heating and was also shaken vigorously in the dispersion of oil in water. After the mixture was cooled to room temperature, the toluene layer was gelated, keeping the water layer intact in the liquid state. In case of the pH-triggered phaseselective gelation, the required amount of amphiphilic dipeptide sodium salt (which on conversion to the corresponding acid should be present at a concentration capable of gelating the toluene phase) was added to a biphasic mixture containing 1 mL each of water and toluene. The mixture was then heated, vigorously stirred, and subsequently cooled to room temperature, which resulted in the selective gelation of the water phase. Then 1.1 equiv of HCl (from the 1 N solution) was added to break the hydrogel. The system was heated and cooled to get the gel of the toluene phase. Dye Adsorption. In the case of pH-triggered dye adsorption, the required amount of amphiphilic dipeptide sodium salt (which on conversion to the corresponding acid should be DOI: 10.1021/la804235e

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present at a concentration capable of gelating the toluene phase) was added to 900 μL of water and heated. To this solution, 100 μL of 1 mM crystal violet was added (to attain a concentration of 0.1 mM), and the solution was cooled to room temperature to form the dye-entrapped hydrogel. To this gel, 1 mL of toluene was added, with the subsequent addition of 1.1 equiv of HCl (from the 1 N solution). The amount of dye in the water phase was determined from UV-vis spectroscopy. For the dye adsorption with xerogels of acids, the maximum amount of dye absorbed was followed by taking 1 mL of 0.1 mM dye in a sample tube with 5 mg of xerogel. This solution was left for 24 h at room temperature to adsorb the dye. The final concentration of the dye in the solution was determined by UV-vis spectroscopy. However, a time-dependent study on the adsorption of dyes was carried out with 3 mL of a 0.01 mM dye solution (to avoid the off-scale absorbance intensity) in the presence of 5 mg of xerogel.

Results and Discussion LMWGs with varying structural motifs have attracted much interest in recent years because of their distinctive ability to immobilize various types of solvents and their potential applications in multidisciplinary areas of sciences.1-10 Hence, the design and development of LMWGs that have the ability to gelate both organic solvents and water with a possible structure-property relationship is of the utmost importance. In this context, we have shown in a recent past study that a onestep chemical reaction can lead to the development of both hydrogelators and organogelators from an amphiphilic dipeptide precursor amine that itself was a nongelator.5c,11d However, in the present work we were interested in investigating the importance of the free carboxylic group at the C terminus of the amphiphilic dipeptides in modulating their gelation efficiencies (Scheme 1). Hence, we have prepared the amphiphilic dipeptides using different nonpolar aliphatic/aromatic-substituted amino acids with a long C-16 hydrophobic tail at the N terminus, keeping free -COOH at the C terminus (Chart 1). This would also permit us to modulate the aggregation property of these amphiphiles by simply converting them to their corresponding sodium salts.17 In this regard, we have seen from our previous work that the presence of an aromatic amino acid in the dipeptide unit was necessary for gelation.5c,6b,11d,14b,15f By keeping aromatic amino acid residue L-phenylalanine at the N terminus with a C-16 alkyl tail, a series of amphiphilic dipeptides were synthesized by varying the C-terminal amino acid (Chart 1, 1-6). Compound 1 having L-alanine, the smallest chiral aliphatic amino acid with minimum steric bulk at the C terminus, was found to be soluble in different aromatic/aliphatic solvents and insoluble in water. Interestingly, when L-alanine was replaced by other higher aliphatic amino acids such as L-valine (2), L-isoleucine (3), and L-leucine (4), the amphiphilic compounds formed stable gels in various aromatic solvents and carbon tetrachloride (CCl4, Table 1). Thus, switching the terminals of the dipeptide moiety with alkyl chains resulted in the transformation of nongelator free amine to an organogelator with free carboxylic group at the C terminus (Scheme 1). The MGC of each compound is comparable in different aromatic solvents but a little higher in the case of CCl4. MGC gradually increased with larger aliphatic substitution at the amino acids from 2.5% w/v in toluene for 2 to 6.5 and 8.1% w/v for 3 and 4, respectively. L-Valine seems to be the most fitting amino acid component because any other smaller or larger aliphatic residue leads to either nongelation or reduced gelation ability. This result indicates that the balance between the size and the 8642 DOI: 10.1021/la804235e

Table 1. Minimum Gelation Concentration (MGC, % w/v) of 1-16 and Corresponding Sodium Salts 1a-16a in Different Solventsa compounds toluene tetralin o-xylene m-xylene p-xylene CCl4 H2O S S S S S S 1.5 1.55 1.5 1.45 1.5 2.33 2.5 2.6 2.45 2.5 2.55 5 1.67 1.65 1.7 1.7 1.65 Vl 6.5 6.8 6.3 6.5 6.4 10 5.9 5.8 6.1 6 5.9 7.8 8.1 7.6 8.3 8 7.9 S 7.5 7.4 7.6 7.3 7.5 S S S S S S S 1.3 1.35 1.25 1.35 1.2 2.1 1.75 1.8 1.7 1.8 1.9 3.1 1.4 1.5 1.45 1.5 1.35 2.0 S S S S S S S S S S S S S S S S S S S S S S S S S S S S S S S S S S S S 0.62 0.65 0.7 0.6 0.65 0.45 1.7 1.8 1.65 1.75 1.7 2.5 P P P P P P 1.8 1.7 1.75 1.7 1.8 3.1 5.0 5.3 5.1 5.3 5.2 3.3 0.7 0.75 0.8 0.7 0.8 2.3 1.0 1.2 1.1 0.95 1.0 1.25 0.65 0.75 0.7 0.7 0.75 2.5 0.8 0.9 0.8 0.8 0.9 2.67 0.3 0.4 0.3 0.35 0.3 1.6 S S S S S S 0.95 1.0 1.1 1.0 0.9 1.8 5.0 5.3 5.2 5.1 5.1 5.0 0.5 0.6 0.55 0.45 0.5 0.62 a P, precipitate; S, solution; I, insoluble; Vl, viscous liquid.

1 1a 2 2a 3 3a 4 4a 5 5a 6 6a 7 7a 8 8a 9 9a 10 10a 11 11a 12 12a 13 13a 14 14a 15 15a 16 16a

I S I S I S I Vl I 1.7 I Vl I S I S I S I S I 3.2 I Vl I 0.58 I 0.3 I S I 1.7

hydrophobic/hydrophilic properties of the amino acid residues is crucial for gelation. At this point, we thought that the presence of another aromatic amino acid instead of an aliphatic residue at the C terminus might improve the gelation efficiency through increased π-π interaction. Consequently, we prepared compounds 5 and 6 with L-phenylalanine and L-tryptophan residues, respectively, at the C terminus in the dipeptide moiety. Interestingly, 5 and 6 showed contrasting gelation ability (Table 1). Compound 5, having two L-phenylalanine residues, was soluble in different aromatic/aliphatic solvents, and compound 6, having an L-tryptophan residue at the C terminus, showed the best gelation efficiency among 1-6 in aromatic solvents as well as in CCl4 (MGC = ∼1.8% w/v in aromatic solvents, Table 1). This contrasting behavior could be explained by the difference in the hydropathy index of the two aromatic amino acid residues.18 The hydropathy index is a number that represents the relative hydrophilic and hydrophobic properties of the amino acid’s side-chain residues. Hydrophobic amino acids have larger hydropathy indexes. In this case, although both L-phenylalanine and L-tryptophan have aromatic rings, they have contrasting HLB as indicated by the notable difference in their hydropathy indices (L-phenylalanine has a positive hydropathy number of 2.8 whereas that of L-tryptophan is -0.9).18 An extended indole ring containing a hydrophilic -NH group imparts this additional hydrophilicity to L-tryptophan. Thus, it can be inferred that the presence of an extended aromatic residue of L-tryptophan along with its hydrophilicity suitably maintained the appropriate HLB for 6, leading to efficient organogelation. (18) Kyte, J.; Doolittle, R. F. J. Mol. Biol. 1982, 157, 105.

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Besides the amidic intermolecular hydrogen bonding, electrostatic and ion-dipole interactions present in the carboxylate salts are also known to influence the gelation process in lipophilic solvents.15b,17 To impart these features within 1-6, we simply converted them to the corresponding sodium salts (1a-6a, Chart 1). Additionally, lipophilic solvents have a poor capability to solvate ions, which could help the sodium carboxylates to immobilize the solvents. We observed that formation of sodium salts not only improved the gelation efficiency in all cases but also converted nongelators (1 and 5) to efficient organogelators 1a (MGC 1.5% w/v in toluene) and 5a (MGC 1.3% w/v in toluene). The gelation efficiency was also improved for other sodium salt analogues 2a, 3a, and 4a. Moreover, sodium salt 5a acquired the ability to gelate water (MGC = 1.7% w/v) efficiently in addition to its organogelation ability whereas its corresponding acid (5) was insoluble in water. Interestingly, 5a and 6a showed the opposite trend in hydrogelation with respect to the organogelation behavior of the corresponding carboxylic acids. 5a with two hydrophobic aromatic substitutions (L-phenylalanine) efficiently gelated the hydrophilic solvent, water. However, 6a, with a combination of hydrophobic (L-phenylalanine) and hydrophilic substitution (L-tryptophan), did not exhibit gelation efficiency in water (Table 1). Hence, again it is the HLB that plays the crucial role in the gelation efficiency of amphiphilic dipeptides in solvents of different polarities. Because the combination of L-phenylalanine and L-tryptophan in the dipeptide unit (6) exhibited the most efficient organogelation ability, we were interested to determine whether the presence of L-tryptophan at the N terminus instead of L-phenylalanine could enhance the gelation property. Therefore, we synthesized another series of amphiphilic dipeptides with a C-16 hydrophobic tail and L-tryptophan fixed at the N terminus and varied the amino acids at the C terminus (7-11). Surprisingly, 7-9, having L-alanine, L-valine, and L-isoleucine residues, respectively, at their C termini, and their sodium salts 7a-9a did not form gels in organic solvents and water. This observation is in contrast to the gelation nature of the corresponding L-phenylalanine analogues (1-3). As pointed out earlier, the increased hydrophilicity of L-tryptophan probably disturbed the proper HLB and overwhelmed the other essential noncovalent interactions required for gelation whereas 10 with L-tryptophan and L-phenylalanine in the dipeptide unit had the best organogelation ability among 1-10. So far, from the gelation results of 5, 6, and 10 it seemed that the presence of two aromatic residues favors efficient gelation. However, the presence of two L-tryptophan residues (11) in the dipeptide resulted in a nongelator that precipitated from different solvents. Nevertheless, the subsequent sodium salt, 11a, was capable of forming an organogel as well as a hydrogel (Table 1). On the whole, we can conclude that the gelation efficiencies of the amphiphilic dipeptides were enhanced and also some nongelators were converted to efficient gelators upon salt formation. Another interesting observation was the dissimilar organogelation capabilities of 6 and 10 despite having the same structural components including amino acid residues, except for a simple exchange of amino acid position between the N and C termini. An almost 300% change in gelation efficiency was observed between 6 (MGC 1.75% w/v) and 10 (MGC 0.62% w/v) in toluene because of the seemingly innocuous change in the sequence of the dipeptide unit. This intriguing observation indicates that the sequence of amino acid residues is an important factor in modulating the gelation efficiency. Obviously, we became curious to determine whether altering the amino acid sequence in the other amphiphilic dipeptides would further Langmuir 2009, 25(15), 8639–8648

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improve the gelation efficiency. Hence, we synthesized amphiphilic dipeptides 12, 13, and 14 (Chart 1) where the sequence of amino acids was reversed with respect to that of 1, 2, and 3, respectively. Therefore, 12, 13, and 14 had L-phenylalanine at the C terminus with L-alanine, L-valine, and L-isoleucine at the N terminus, respectively. Interestingly, nongelator 1 turned into an organogelator (12) simply by changing the sequence of amino acids. Also, 13 and 14 displayed a significant improvement (∼2.5-10-fold) in gelation efficiency compared to that of 2 and 3. Corresponding sodium salts 12a, 13a, and 14a also turned out to be excellent gelators with 0.3-0.8% w/v MGCs in various aromatic solvents. The MGC values of 12a-14a are ∼5-20-fold lower compared to those observed for corresponding sodium salts 1a-3a. Importantly, 13a and 14a also showed efficient water gelation ability with very low MGC values of 0.58 and 0.3% w/v, respectively. The above result motivated us to reverse the position of amino acids of 7 and 8 (L-tryptophan at the N terminus) to get 15 and 16 with L-tryptophan at the C terminus and L-alanine and L-valine at the N termini, respectively. Even though acid 15 was soluble in different organic solvents, 16 exhibited moderate gelation efficiency, and their salts 15a/16a were excellent organogelators having MGC 0.5-1.8% w/v. Thus, nongelators 7 and 8 and their sodium salts (7a, 8a) turned into efficient gelators (15a, 16a) by altering the sequence of amino acids. On the whole the presence of L-phenylalanine at the C terminus of the dipeptide moiety yields the best gelator, 14a, with versatile organo- and hydrogelation capabilities (Table 1). Thus, it can be summarized that the presence of an aromatic moiety at the C terminus strongly favors the gelation ability of the molecules. In this context, the combination of only nonpolar aliphatic amino acids (in the absence of any aromatic substitution) did not yield any gelation ability for the amphiphilic dipeptides. Also, none of the amphiphilic dipeptides showed gelation ability in polar organic solvents such as ethanol, methanol, DMSO, and so forth because the compounds are soluble in these solvents. Thermoreversible organogels as well as hydrogels melt upon slow heating and again gel on cooling. The gel-to-sol transition temperatures (Tgel) of representative organogels (1a, 6, 10, 12a, 13, 14a, and 16a) in toluene and hydrogels (13a, 14a) having different amino acid residues were measured (Figure 1). In concurrence with the literature, Tgel was found to increase with gelator concentration (Figure 1A).15a,15e,19 The Tgel values of the gelators were comparable at their MGC (39-43 °C, Table S1, SI) except 14a (46 °C) in toluene, which was a little higher than the others. Between the two hydrogelators, 14a had a higher Tgel at comparable concentrations (Figure 1B) obviously because of its better gelation efficiency (Table 1). The Tgel values of these gelators at MGC are given in the Supporting Information (Table S1). Field-emission scanning electron microscopy (FESEM) was done to gain visual insight into the supramolecular arrangement of organogels and hydrogels where most of the xerogels showed a clear fibrous network at their MGC (Figure 2). Differences between the size and nature of the fibers were found, depending on the constituents and sequences of the amino acids. The xerogel (prepared from toluene) of 1a had an entangled fibril network with a thickness of 60-80 nm (Figure 2A), and the corresponding dipeptide with the reverse sequence, 12a, had an interconnected fibril network where two or more fibers were associated with each other to form thicker fibers of 80-100 nm and also a few fibers of 400-500 nm (Figure 2B). The supramolecular arrangement of 12a showed the formation of a better 3D-entangled network with (19) Moniruzzaman, M.; Sundararajan, P. R. Langmuir 2005, 21, 3802.

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Figure 1. Variation of gel-to-sol transition temperatures (Tgel) with gelator concentration.

thicker fibers compared to that of 1a,which is in agreement with the observed superior gelation ability of 12a, having an aromatic moiety at the C terminus. Two other efficient gelators having L-phenylalanine at the C terminus, 13a and 14a, also had entangled fibril networks with average fiber thicknesses of 150180 and 250-500 nm (Figure 2C,D), respectively. The fiber thickness increases with increasing gelation efficiency from 12a to 14a (0.8-0.3% w/v). The xerogel of 5a with L-phenylalanine at both the C and N termini also had an interconnected fibril network with a fiber thickness of 100-120 nm (Figure 2E) with MGC 1.3 wt % w/v. Similar fibril network were found for hydrogels 13a and 14a with average thicknesses of 80-100 and 120-150 nm (Figure 2F,G) respectively, which also reflects the better hydrogelation efficiency of 14a (Table 1). However, the morphology of xerogels of the free carboxylic acid was found to be quite different from that of the corresponding sodium salt. The xerogel of 13 (taken as a representative example) had a porous 3D fibrous network (Figure 2H) in contrast to the entangled fibrous network for its sodium salt, 13a (Figure 2C,F). Similarly, the xerogel of 14 had a very closely packed porous fibril network having a thickness of 100-120 nm (Figure 2I). A microscopy investigation of the 14a organogel in toluene was also done by atomic force microscopy (AFM). The AFM picture (Figure 3A) and its 3D view (Figure 3B) showed the interconnected fibril structure of the 8644 DOI: 10.1021/la804235e

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xerogel with a diameter of ∼500 nm that was comparable to the observed thickness in the SEM image of 14a (Figure 2D). The expression of supramolecular chirality expected to originate from the self-assembled ordered arrangement of chiral monomers was investigated using circular dichroism (CD) spectroscopy.20 CD spectra were recorded for 13a and 14a in water at various concentrations (Figure 4). An aqueous solution of 13a had a negative peak at 225 nm and a positive peak at 200 nm (Figure 4A). However, compound 14a had a strong positive band at 220-225 nm (Figure 4B). The observed Cotton effect in the amide absorption region (i.e., at 220-225 nm) could be attributed to the π-π* transition of the amide20a bond, which is extremely sensitive to coupling with neighboring amides. Interestingly for 13a, the observed negative peak at 225 nm and a positive peak at ∼200 nm are the characteristic peaks for the type II β-turns of a peptide molecule (Figure 4A).20b All CD peaks of 13a and 14a were found to increase with increase in their concentration, which suggests a highly ordered arrangement of monomer at the supramolecular level that probably induces the superior gelation efficiency of the amphiphilic dipeptides. In the quest to determine the driving forces behind the formation of such highly ordered supramolecular self-assembly, we have examined the role of hydrophobic interactions by recording luminescence spectra of ANS, a hydrophobic fluorescent probe, during the gelation of representative dipeptides 13, 13a, and 14a (Figure 5A-C) and 1a, 10, and 14 (Figure S1, SI) in toluene. In each case, the ANS intensity gradually increased with the concentration of gelators almost at a fixed emission wavelength (λem) of ∼472 nm. The observed steady increase in intensity up to a concentration 6-10 times lower than MGC showed its propensity to aggregate with increasing hydrophobicity, leading to the formation of fibers as found in microscopy studies. With further increase in concentration, a decrease in emission intensity was observed along with a blue shift up to 463 nm. It may be possible that in the gel state the probe molecules are aligned in a wellordered network and experience a less hydrophobic environment compared to that in the intermediate state of gelation. FTIR is an important tool for investigating the different noncovalent interactions involved in gelation.6c,15c,17d,21 FTIR spectra of the gel in D2O, xerogels prepared from toluene, and the non-self-assembled state in chloroform were taken by using a Perkin-Elmer Spectrum 100 FTIR spectrometer. In chloroform solution, most of the sodium salt compounds showed a characteristic non-hydrogen-bonded transmittance band at 34103430, ∼1633, and 1520-1530 cm-1 for amide νN-H (amide A), amide νCdO (amide I), and δN-H (amide II), respectively (representative spectra of 10 and 14a, Figure 6).5d,6c,15d The IR spectra of toluene xerogels showed transmission bands at 32903310, 1621-1631, and ∼1540 cm-1, which are the characteristic bands for hydrogen-bonded amide N-H stretching, CdO stretching (amide I), and N-H bending (amide II), respectively (Figure 6, Figure S2, SI).5d,17c However, for free carboxylic acid gelators, in addition to the above bands shifting, the acid CdO stretching band was also shifted from ∼1728 cm-1 in chloroform to 1708-1713 cm-1 in the gel state. These shifts in IR bands indicate the presence of strong intermolecular hydrogen bonding between the amide and carboxylic acid group in the supramolecular gel network. Also, the shifting of antisymmetric (νas,) and symmetric (νs) CH2 stretching frequency bands were observed (20) (a) Friggeri, A.; Pol, C. V. D.; Bommel, K. J. C. V.; Heeres, A.; Stuart, M. C. A.; Feringa, B. L.; vanEsch, J. Chem. Eur. J. 2005, 11, 5353. (b) Lu, Z-X.; Fok, K-F.; Erickson, B. W.; Hugli, T. E. J. Biol. Chem. 1984, 11, 7368. (21) Su, L.; Bao, C.; Lu, R.; Chen, Y.; Xu, T.; Song, D.; Tan, C.; Shi, T.; Zhao, Y. Org. Biomol. Chem. 2006, 4, 2591.

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Figure 2. (A-E, H, I) FESEM images of dried samples of 1a, 12a, 13a, 14a, 5a, 13, and 14 in toluene. (F, G) FESEM images of dried samples of 13a and 14a in water.

Figure 3. (A) AFM image of 14a xerogel from toluene at MGC 0.3% w/v. (B) Three-dimensional view of image A.

from 2927 and 2856 cm-1 in the solution phase to 2919 and 2850 cm-1 in the gel state, respectively. The decrease in fluidity of the hydrophobic chains due to the formation of strong aggregates via van der Waals interaction is evident from this particular shift in the CH2 stretching frequency. A similar shift in IR bands was also observed in the case of hydrogels of 13a and 14a compared to their corresponding CHCl3 solution (Figure S3, SI). Thus, hydrogen bonding and van der Waals interaction are the major contributing factors to the gelation of the amphiphilic dipeptides. In addition to the preceding spectroscopy and microscopy studies, we have measured the X-ray diffraction (XRD) of all of Langmuir 2009, 25(15), 8639–8648

the cast films of dried organogels and hydrogels to investigate the molecular packing and orientation of the gelator molecules in the supramolecular gel state. Representative XRD spectra of the organogels prepared in toluene using an amphiphilic dipeptide with a free -COOH acid group (6) and its corresponding sodium salt (6a) and hydrogel of 13a are shown in Figure 7A. XRD data of the other gelators are given in Supporting Information (Table S2, Figure S4, SI). The ordered arrangement of the gelator molecules in the gel state was indicated by the sharp diffraction peak for all of the dried organogels and hydrogels. The long spacing (D) values of the aggregates obtained in the small-angle DOI: 10.1021/la804235e

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Figure 4. CD spectra of (A) 13a and (B) 14a with varying concentration in water at room temperature.

Figure 5. Luminescence spectra of ANS (1  10-5 M) with varying concentrations of gelators (A) 13, (B) 13a, and (C) 14a in toluene at room temperature. [Amphiphile] (% w/v): a, 0; b, 0.0025; c, 0.005; d, 0.01; e, 0.025; f, 0.05; g, 0.1; h, 0.25; i, 0.5; j, 1; and k, 2.

Figure 6. FTIR spectra of gelators (A) 10 and (B) 14a in CHCl3 (- - -) and the xerogel (-) obtained from toluene.

region for gelators 6, 6a, and 13a were 3.68, 3.54, and 3.30 nm, respectively. However, these D spacings were smaller than twice the fully extended molecular length of each individual molecule (Table S2, calculated by the MOPAC AM1 method, CS Chem. Office) and again larger than the length of one molecule: 3.07, 3.01, and 2.92 nm for 6, 6a, and 13a, respectively. So from the above spectroscopy and microscopy studies, it can be concluded that the gel aggregates are formed by repeating bilayer units in which the molecules are connected by intra- and interlayer (22) Jung, J. H.; Shinkai, S.; Shimizu, T. Chem. Eur. J. 2002, 8, 2684.

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hydrogen bonding and electrostatic interaction. The probable interdigitated bilayer structure is represented in Figure 7B.22,23 So far we have seen that a simple modification of the carboxylic acid group to the sodium carboxylate group not only improved the organogelation efficiency but also allowed the group to acquire an efficient capability to gelate water. This result would find considerable significance simply for the fact that an organogelator can be instantly converted to a hydrogelator and vice versa (23) (a) Kim, C.; Kim, K. T.; Chang, Y.; Song, H. H.; Cho, T.-Y.; Jeon, H.-J. J. Am. Chem. Soc. 2001, 123, 5586. (b) Jung, J. H.; John, G.; Masuda, M.; Yoshida, K.; Shinkai, S.; Shimizu, T. Langmuir 2001, 17, 7229.

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Figure 7. (A) Powder XRD diagrams of the dried gels of (a, b) 6 and 6a in toluene and (c) 13a in water. (B) Schematic presentation of the possible network formed by the gelators during their aggregation.

Figure 8. (A) Hydrogel of 14a containing 0.1 mM of crystal violet (CV). (B) Toluene added to the hydrogel of 14a containing the dye. (C) Status of hydrogel after the addition of 1.1 equiv of HCl. (D) Subsequent heating results in selective gelation of the toluene phase containing CV, leaving the aqueous layer clear within a few minutes of addition. (E) UV-vis spectrum of an aqueous solution of dye (crystal violet) indicating the pH-dependent rapid (∼10 min) removal of the dye from water. (F) 0.01 mM aqueous solution of CV before the addition of xerogel 14. (G) Clear aqueous layer after the adsorption of dye by the xerogel of 14 after ∼24 h. (H) UV-vis spectrum of an aqueous solution of dye (crystal violet) indicating the time-dependent adsorption of the dye from water by the xerogel of 14.

by simply changing the pH. In this regard, phase-selective gelation from a mixture of solvents is always a challenging task, especially when one of the solvents is water. To this end, the research groups of Bhattacharya and Hanabusa have already reported the phaseselective gelation of oil from oil/water mixtures that have potential implications in the case of an oil spill.3e,15d Interestingly, in our case all of the organogelators with the free carboxylic acid end had solvent-specific gelation ability, selectively gelating the organic Langmuir 2009, 25(15), 8639–8648

phase when confronted with a biphasic system of water and organic solvents (Figure S5, SI). More interestingly, the sodium salts of amphiphilic dipeptides (5a, 11a, 13a, 14a, and 16a) having the ability to gelate both organic solvent and water were found to gelate only the aqueous phase in the biphasic mixtures. At this point, we thought of utilizing this intriguing gelation ability of the gelator in its acid/salt forms to gelate different phases of a biphasic mixture selectively by simply changing the pH of the medium. In a DOI: 10.1021/la804235e

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typical procedure, 1 mL of toluene and 1 mL of water were placed in a sample tube to which 10 mg of gelator 14a (for example) was added (Figure S5A, SI). The aqueous layer showed selective gelation (Figure S5B, SI) in the biphasic mixture following the procedure as described in the Experimental Section. When 1.1 equiv of HCl (20 μL of a 1 N solution) with respect to the gelator was added (required to convert the sodium salt to corresponding acid), in the biphasic system there is almost an instantaneous breakup of the aqueous gel mixture (Figure S5C, SI). This breaking of the gel led to the formation of an insoluble dispersion of dipeptide carboxylic acid (14) in water. The resultant mixture was further slowly heated, shaken, and cooled to room temperature. The upper layer (toluene) of the biphasic mixture was found to form an organogel while water remained in its fluid state (Figure S5D, SI). This phenomenon of the facile transition of the hydrogel to the organogel by a simple pH change of the medium can have remarkable implications. The diversity in the networks of gels in the two solvents could further complement the transfer of desired materials from one phase to another. The contamination of water from toxic dyes is a matter of great concern for their harmful environmental repercussions. In this regard, the self-assembly of a gelator that has both hydrophilic and hydrophobic domains can potentially adsorb dyes in its microdomain.24,25 Herein we report the rapid removal of a large quantity of crystal violet (CV) dye from an aqueous solution. At first, a hydrogel was prepared containing 0.1 mM CV dye in 1 mL of water (see Experimental Section for details) with 10 mg of 14a (MGC 0.3% w/v) (Figure 8A). To this gel, 1 mL of toluene was added (Figure 8B). When this system is treated with 1.1 equiv of HCl (20 μL of a 1 N solution), there is an almost instantaneous transition from the gel to sol phase with the concurrent transfer of the gelator molecules toward the interfacial region (Figure 8C). The most striking fact is that the gelator molecules carry the CV dye, with it leaving the aqueous layer clear (monitored by UV-vis spectroscopy, Figure 8E). When the resulting biphasic mixture was gently heated, the free carboxylic acid (14) gelated the toluene layer with the simultaneous entrapment of the CV dye (Figure 8D). Thus, the dye is almost completely removed from the aqueous medium within a few minutes. This pH-dependent dye removal was observed for sodium salt hydrogelators 13a, 14a, and 16a. Furthermore, the xerogels of the free carboxylic acids were employed for the time-dependent removal of dye from water (Figure 8F,G, Figure S6, SI). The adsorption of dye by the xerogels was monitored by UV-vis spectroscopy (Figure 8H), (24) (a) Polubesova, T.; Nir, S.; Zakada, D.; Rabinovitz, O.; Serban, C.; Groisman, L.; Rubin, B. Environ. Sci. Technol. 2005, 39, 2343. (b) Sayari, A.; Hamoudi, S.; Yang, Y. Chem. Mater. 2005, 17, 212. (25) (a) Ray, S.; Das, A. K.; Banerjee, A. Chem. Mater. 2007, 19, 1633. (b) Cho, E. J.; Jeong, I. Y.; Lee, S. J.; Han, W. S.; Kang, J. K.; Jung, J. H. Tetrahedron Lett. 2008, 49, 1076.

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where ∼97% of the dye (0.01 mM) was removed within 24 h. To determine the maximum amount of dye adsorption ability of these organogelators, 5 mg of xerogel was added to 1 mL of a 0.1 mM dye in a sample tube. The best result was obtained for the tryptophan-based compounds (e.g., 6 and 10) where ∼6.3 mg of dye was adsorbed per gram of gelator in 24 h whereas for the other gelators the result varied from 3 to 5.8 mg/g of gelator (Table S3, SI). However, the efficiency observed in the pH-triggered dye adsorption by sodium salt hydrogelators is almost an order of magnitude higher than that by the xerogel of the carboxylic acid organogelators.

Conclusions In summary, a novel set of dipeptide-based long-chain acids/ salts capable of efficiently gelating organic solvents and water were synthesized. Their headgroup structures were varied judiciously to determine a rationale behind the evolution of efficient gelators. The sequence of amino acids plays a crucial role in the gelation of these amphiphilic dipeptides. Nongelators were transformed to excellent gelators, and an almost 20-fold enhancement in gelation efficiency was observed on reversing the sequence. The presence of an aromatic component was also found to be essential for gelation capability. Another interesting finding of this work is the pH-responsive transition of the hydrogels to organogels, which can have tremendous applications. This pH-sensitivity was applied in the phase-selective gelation of any of the solvents in a biphasic mixture of oil and water. Most importantly, this pH-responsiveness of the amphiphilic dipeptides was utilized to wipe out large quantity of crystal violet dye from water almost instantaneously, which has potential application in wastewater treatment. Such amphiphilic fibers of the gelators can also have the ability to adsorb a variety of materials ranging from nanoparticles to proteins in their network. Further work is being carried out to use such self-aggregates as a vehicle for the transfer of desired materials from one phase to another by simply changing the pH of the medium. Acknowledgment. P.K.D. is thankful to the Department of Science and Technology, India, for financial assistance through a Ramanna Fellowship (no. SR/S1/RFPC-04/2006). T.K., S.D., D.D., and A.S. acknowledge the Council of Scientific and Industrial Research, India, for their research fellowships. Supporting Information Available: Generalized synthetic scheme for all of the gelators, characterization of synthesized compounds, X-ray diffraction data, luminescence spectra, FTIR spectra, and dye adsorption figures. This material is available free of charge via the Internet at http://pubs.acs. org.

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