DOI: 10.1002/asia.201300805

Primary Ammonium Monocarboxylate Synthon in Designing SupramolecularGels: A New Series of Chiral Low-Molecular-Weight Gelators Derived fromSimple Organic Salts that are Capable of Generating and Stabilizing Gold

Nanoparticles

Uttam Kumar Das, Subhabrata Banerjee, and Parthasarathi Dastidar*[a]

Introduction

Supramolecular gels (SGs) are viscoelastic materials thatare generally formed by cooling hot solutions of small mole-cules, known as low-molecular-weight gelators (LMWGs).[1]

Various supramolecular interactions, such as hydrogen-bonding interactions, p�p stacking, van der Waals interac-tions, and charge-transfer interactions, drive the LMWGs toself-assemble into SGs. Owing to their various applicationsin electro-optics,[2] catalysis,[3] biomedical applications,[4]

nanoparticles,[5] conservation of art,[6] cosmetics,[7] etc., therehas been an upsurge in research activity involving SGs. In-terestingly, most of the reported gelators were discovered byan accident rather than by design. However, there havebeen serious efforts towards the design of gelator mole-cules.[8] Over the last decade, we have been trying to devel-op a working hypothesis for the design of gelators by ex-ploiting the “supramolecular synthon” concept.[9] Variousmicroscopy techniques (optical microscopy, SEM, TEM,atomic force microscopy (AFM), confocal microscopy, etc.)on gel/xerogel (dried gel) samples have shown the existence

of gel networks of highly branched and/or entangled 1Dfibers, which are known as self-assembled fibrillar networks(SAFINs).[10] The formation of 1D fibers, which are the in-gredients of SAFINs, may be formed owing to some aniso-tropic interactions that help the gelator molecules to prefer-entially self-assemble in one dimension; on the other hand,the lack of such interactions in the other two dimensionsprevents lateral growth. Thus, 1D fibers may be grown bychoosing molecules that have self-complementary, reasona-bly strong, and directional hydrogen-bonding site(s), whichmight ultimately lead to SAFINs and gelation under suitableconditions. Shinkai and co-workers were the first to proposethis hypothesis.[11] However, the fact that this hypothesis wasindeed based on a logical foundation was most explicitlydemonstrated by ourselves;[12] we exploited a supramolecularsynthon to select a number of organic salts that had the abil-ity to undergo 1D growth and, indeed, many such salts dis-played gelating ability with a wide range of solvents.[1a,13]

Eventually, we discovered many supramolecular synthons,among which the primary ammonium monocarboxylate(PAM) synthon was quite intriguing. PAM salts can formfour types of supramolecular synthon, of which the most fre-quently observed is the 1D synthon in which two ammoniumand two carboxylate anions form a 10-membered hydrogen-bonded ring that propagates in one dimension (synthon W);the other, less-frequently observed 1D synthon is comprisedof alternating 12- and 8-membered hydrogen-bonded rings(synthon X); the least-observed PAM synthons are 2D (syn-thons Y and Z; Scheme 1). We have shown that PAM salts,such as benzylammonium cinnamates, are excellent gelatorsand that many of them can induce the instant gelation of

[a] U. K. Das, S. Banerjee, Dr. P. DastidarDepartment of Organic ChemsitryIndian Association for the Cultivation of Science (IACS)2A and 2B, Raja S. C. Mullick RoadJadavpur, Kolkata—700032, West Bengal (India)Fax: (+91) 33-2473-2805E-mail : ocpd@iacs.res.in

parthod123@rediffmail.com

Supporting information for this article is available on the WWWunder http://dx.doi.org/10.1002/asia.201300805.

Abstract: The primary ammoniummonocarboxylate (PAM) synthon hasbeen exploited to generate a new seriesof PAM salts from the free amine of l-phenylalanine-3-pyridyl amide, (S)-2-amino-3-phenyl-N-(pyridine-3-yl)pro-panamine (designated as “B”), and var-ious substituted benzoic acids (desig-nated as “A(R)”; R=4-Me, 4-Cl, 4-Br,4-NO2, 3-Me, 3-Cl, 3-Br, 3-NO2, 2-Me,2-Cl, 2-Br, 2-NO2). The 4- and 3-substi-

tuted benzoate salts showed moderate-to-excellent gelation ability witha number of polar and apolar solvents.The gels were characterized by DSC,rheology, SEM and TEM, FTIR spec-troscopy, etc. Structure–property stud-

ies based on single-crystal powder X-ray diffraction (PXRD) and FTIR dataprovided insights into the role of thePAM synthon in the formation of thegel networks. Interestingly, some of thegels were capable of forming and stabi-lizing gold nanoparticles at room tem-perature without the use of any exoge-nous reducing agents.

Keywords: gelators · gold · nano-particles · supramolecular chemis-try · X-ray diffraction

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petrol at room temperature.[14] We have also reporteda series of gelating agents based on primary alkylammoniumcinnamates that were capable of gelating selectively com-mercial fuels, like petrol, diesel, and kerosene, from a com-plex oil/water mixture.[15] Recently, we derived a series ofsupramolecular gelators from benzylammonium benzoatesthat were capable of gelating a wide range of organic sol-vents, including instant gelation at room temperature.[16]

More recently, we reported a series of PAM salts that were

derived from various methylesters of l-amino acids and cin-namic acids that displayed anti-solvent-induced gelation anda few of them displayed theslow release of sex pheromonesfor certain pests.[17] Thus far, wehave reported 90 salts, of which42 (46.6%) showed moderate-to-excellent gelation properties.Out of 37 single-crystal struc-tures of the PAM salts (both ge-lators and non-gelators) thatwere reported in these studies,17 adopted the synthon-Wstructure, whereas 14 saltsadopted the synthon-X struc-ture; only 4 salts formed differ-ent types of 2D networks (notshown in Scheme 1) and onesalt, benzylammonium 2-bro-mocinnamate, turned out to bea hydrate that displayed anoverall 3D hydrogen-bondednetwork.[14]

We wanted to study the ro-bustness of the PAM synthon inthe presence of additional hy-

drogen-bonding sites, such as amide and pyridyl moieties, asin the free amine of l-phenylalanine-3-pyridyl amide, aswell as to create supramolecular chirality in the resultinggels that are derived from the PAM salts of this free chiralamine with various substituted benzoic acids. Thus, herein,we report a new series of PAM salts that were derived fromthis chiral free amine and various aromatic acids(Scheme 2); out of the 12 salts reported herein, eight dis-played moderate-to-good gelation ability with various sol-vents, including pure water (hydrogelation, Table 1).

The synthesis and characterization of the resulting gels, aswell as the single-crystal structures of four gelator salts (BA-ACHTUNGTRENNUNG(4-Cl), BAACHTUNGTRENNUNG(4-Br), BA-4-NO2), and BAACHTUNGTRENNUNG(4-Me)) and the typesof supramolecular PAM synthon that were present in thesegelator salts, are discussed. Interestingly, a hydrogel that wasderived from BAACHTUNGTRENNUNG(4-Me) was able to generate Au nanoparti-cles (Au NPs) at room temperature without the use of anexogenous reducing agent.

Results and Discussion

Synthesis

The dicyclohexyl carbodiimide (DCC) coupling of 3-aminopyridine with N-Boc-l-phenylalanine, followed by deprotec-tion of the Boc group and neutralization with an aqueoussolution of NaHCO3, afforded the free amine of l-phenyla-lanine-3-pyridyl amide (B) in an overall yield of about 50 %(see the Supporting Information, Scheme S1). The reactions

Scheme 2. PAM salts that were studied herein.

Scheme 1. Supramolecular PAM synthons that have been reported in the literature.

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of the free amine with various substituted benzoic acids (1:1molar ratio) in MeOH at room temperature generated12 PAM salts, which were characterized by NMR (1H and13C) spectroscopy, FTIR spectroscopy, and elemental analy-sis. In the FTIR spectra, sharp bands within the range 1695–1699 cm�1 were attributed to amide C=O stretches, whilstthe bands within the range 1589–1634 cm�1 (for the COO�

group) clearly confirmed the formation of PAM salts.

Gelation Studies

All of the salts were subjected to gelation tests by using 15representative solvents, including polar and non-polar sol-vents. In a typical experiment, a pre-weighed sample of thesalt (20 mg) was dissolved in the target solvent (0.5 mL) bysonication in an ultrasound bath, followed by heating ona hot plate. Then, the hot solution was kept at room temper-ature, which resulted in gel formation within a few hours.The “tube-inversion” test was employed to confirm gel for-mation; 2/3 of the PAM salts that were investigated hereinshowed gelling capability. Interestingly, salts that contained

4-substituted anionic moieties were versatile gelators thatwere capable of gelling most of the solvents tested herein,whereas all of the 2-substituted benzoic acids failed to showany gelation ability; the 3-substituted benzoate salts werereasonably good gelators. The BAACHTUNGTRENNUNG(3-Cl), BA ACHTUNGTRENNUNG(3-Br), andBAACHTUNGTRENNUNG(4-Me) salts showed hydrogelation ability. Minimum ge-lator concentrations (MGCs) of 1.54–4 wt. % and gel-disso-ciation temperatures (Tgel) of 68–138 8C clearly establishedthe moderate-to-good efficiencies and thermal stabilities ofthese salts (Table 1). All of the gels were thermo-reversibleover several cycles when tested by using the tube-inversionmethod. DSC traces of selected gels of 4-substituted ben-zoate salts also confirmed their reversible behavior.

The thermal behavior of selected gels was also examinedby using “tabletop rheology”.[18] Thus, a plot of Tgel versusthe gelator concentration, [gelator], of 1,2-dichlorobenzene(DCB) gels of these salts (except for BA ACHTUNGTRENNUNG(3-NO2), which wasprepared in nitrobenzene) showed the normal trend ofa steady increase in Tgel with increasing [gelator], which es-tablished the role of supramolecular interactions in gel for-mation. From these experiments, it was clear that the DCB

Table 1. Gelation data.[a]

Solvent BA ACHTUNGTRENNUNG(4-Cl) BA ACHTUNGTRENNUNG(3-Cl) BA ACHTUNGTRENNUNG(2-Cl) BA ACHTUNGTRENNUNG(4-Br) BA ACHTUNGTRENNUNG(3-Br) BA ACHTUNGTRENNUNG(2-Br)MGC

[wt. %]Tgel

[8C]MGC

[wt. %]Tgel

[8C]MGC

[wt. %]Tgel

[8C]MGC

[wt. %]Tgel

[8C]MGC

[wt. %]Tgel

[8C]MGC

[wt. %]Tgel

[8C]

1 MeCN *4 94 4 68 NC – *WP – 1.54 78 L –

2 o-xylene WP – WP – WP – *2.86 96 WP – FC –

3 m-xylene 2.86 118 WP – WP – *2.86 100 GN – WP –

4 p-xylene 2.86 110 WP – WP – *WP – *FN – WP –

5 chlorobenzene 2.86 104 WP – NC – *2.86 105 WP – WP –

6 DCB 2.86 106 1.82 76 WP – 2.86 125 GN – GN –

7 nitrobenzene 2.86 86 GN – FP – 2.86 120 1.82 70 GN –

8 toluene *WP – WP – WP – *WP – *FN – *NC –

9 mesitylene GN – GN – WP – *4 138 GN – *WP –

10 methylsalicylate GN – GN – WP – WP – 4 98 GN –

11 DMSO L – L – L – L – L – L –

12 DMF L – L – L – L – L – L –

13 EG L – L – L – L – L – L –

14 DEG L – L – L – L – L – L –

15 water *WP – 2.86 96 L – *WP – WP – L –

Solvent BA ACHTUNGTRENNUNG(4-NO2) BA ACHTUNGTRENNUNG(3-NO2) BA ACHTUNGTRENNUNG(2-NO2) BA ACHTUNGTRENNUNG(4-Me) BA ACHTUNGTRENNUNG(3-Me) BA ACHTUNGTRENNUNG(2-Me)MGC

[wt. %]Tgel

[8C]MGC

[wt. %]Tgel

[8C]MGC

[wt. %]Tgel

[8C]MGC

[wt. %]Tgel

[8C]MGC

[wt. %]Tgel

[8C]MGC

[wt. %]Tgel

[8C]

1 MeCN *FP – 1.82 78 L – AP – 4 72 *WP –

2 o-xylene 4 118 WP – L – AP – WP – NC –

3 m-xylene WP – WP – WP – AP – WP – WP –

4 p-xylene 2.86 116 WP – WP – AP – WP – WP –

5 chlorobenzene 2.86 110 WP – L – GN – WP – NC –

6 DCB 2.86 114 4 96 WP – 4 65 4 78 NC –

7 nitrobenzene 4 98 WP – L – 4 60 GN – L –

8 toluene *WP – WP – WP – AP – WP – WP –

9 mesitylene FN – WP – WP – AP – WP – WP –

10 methylsalicylate WP – WP – WP – 4 67 WP – NC –

11 DMSO L – L – L – L – L – L –

12 DMF L – L – L – L – L – L –

13 EG L – L – L – L – L – L –

14 DEG L – L – L – L – L – L –

15 water *WP – 2.22 102 L – 4 115 WP – *L –

[a] *=A minimum amount of DMSO was added as a co-solvent to induce dissolution. L= liquid, FC= fibrous crystal, FN = fibrous network, WP =whiteprecipitate, FP = fibrous precipitate, NC =needle-shaped crystals, GN =gelatinous network, AP=amorphous precipitate. EG =ethyleneglycol, DEG =

ethyleneglycol.

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gel of BAACHTUNGTRENNUNG(4-Br) was the most thermally stable, whereas thecorresponding gel of BA ACHTUNGTRENNUNG(4-Me) was the least stable (Fig-ure 1 a); the same observation was also noted in the DSCtraces (Figure 1 b). In fact, the values of Tgel as obtainedfrom these two experiments (DSC and the dropping-ballmethod) were in good agreement (Tgel =152 ACHTUNGTRENNUNG(155) 8C for BA-ACHTUNGTRENNUNG(4-Br), 128 ACHTUNGTRENNUNG(130) 8C for BA ACHTUNGTRENNUNG(4-NO2), 122 ACHTUNGTRENNUNG(120) 8C for BA ACHTUNGTRENNUNG(4-Cl), and 80(82) 8C for BAACHTUNGTRENNUNG(4-Me) ; the values in the paren-thesis were obtained by DSC).

To show the gel-like response of these gels, we performeddynamic rheology on selected samples (6 wt. % DCB gels ofBAACHTUNGTRENNUNG(4-Cl) and BAACHTUNGTRENNUNG(4-NO2)) by employing frequency-sweepexperiments. The elastic modulus, G’, and loss modulus, G’’,were plotted against the angular velocity, w, at a constantstrain of 0.1 %. The G’ values (8.2 and 58.0 kPa for the DCBgels of BA ACHTUNGTRENNUNG(4-Cl) and BA ACHTUNGTRENNUNG(4-NO2), respectively) werealmost-completely frequency invariant and significantlylarger than the corresponding G’’ values (1.5 and 18.7 kPa,respectively), which confirmed the viscoelastic nature ofthese gels (Figure 2).

Microscopy

To study the morphology of the gel networks, we preparedsamples for SEM analysis. For this purpose, samples wereprepared by drop-casting a dilute solution of the gelator inDCB. The gelators that contained 4-substituted benzoatemoieties, that is, BAACHTUNGTRENNUNG(4-Cl), BAACHTUNGTRENNUNG(4-Br), BAACHTUNGTRENNUNG(4-NO2), and BA-ACHTUNGTRENNUNG(4-Me), were chosen for this study. Figure 3 clearly showsthe existence of bunches of think 1D fibers in all of thesecases. Understandably, the solvent molecules were immobi-lized within the fibrous network in the gels. Notably, noneof these fibers showed any macroscopic chirality, such ashelices and twisted fibers, thus indicating that the molecularchirality of the gelators could not enforce supramolecularchirality.

Figure 1. a) Plot of Tgel versus [gelator]. b) DSC traces of 6 wt. % DCBgels; all of the gels were prepared in DCB except for BA ACHTUNGTRENNUNG(3-Br), whichwas prepared in nitrobenzene.

Figure 2. Typical viscoelastic response of two selected gels at a constantstrain (0.1 %).

Figure 3. SEM images show 1D fibrous growth in the DCB xerogels.

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Supramolecular Synthons

As mentioned above, the main aim of this work was tostudy the robustness of the PAM synthon in these salts andits role in the formation of SAFINs and gelation. Becausemost of the salts displayed good gelation behavior, it wasnecessary to study the supramolecular synthons that werepresent in these salts and their role in gelation. For this pur-pose, we tried our best to crystallize all of the salts; howev-er, we only obtained suitable single crystals of four gelatorsalts, namely BAACHTUNGTRENNUNG(4-Cl), BA ACHTUNGTRENNUNG(4-Br), BAACHTUNGTRENNUNG(4-NO2), and BAACHTUNGTRENNUNG(4-Me). The single crystals were grown by slow evaporationfrom MeOH/water. Single-crystal X-ray diffraction revealedthat all four salts were isomorphous, with identical spacegroups (orthorhombic, P212121) and similar cell dimensions(see the Supporting Information, Table S5).

The asymmetric units of these crystals were comprised ofone ion pair in each case. The ion pairs displayed N�H···Ohydrogen-bonding interactions (N···O 2.714(2)–2.772(2) �;N-H···O 149.1–174.38) that involved the ammonium and car-boxylate moieties, which resulted in the formation of 10-membered hydrogen-bonded rings that propagated in 1D, asin the case of PAM synthon W. Additional hydrogen-bond-ing moieties, namely the amide and pyridyl groups, were in-volved in relatively weak hydrogen-bonding interactionswithin the 1D hydrogen-bonded chain; the amide C=Obond was not involved in any hydrogen-bonding interac-tions; the amide N�H bond was involved in hydrogen bond-ing with the pyridyl-N atom (N···N 3.049(5)–3.095(3) �; N�H···N 167.7–170.18 ; Figure 4). Thus, it is clear that, despite

having additional hydrogen-bonding sites, these salts couldstill display PAM synthon W and lead to the formation of anoverall 1D hydrogen-bonding network (HBN). However, itwas necessary to confirm whether this 1D supramolecularsynthon was present in the gel state or not. Followinga method that was originally proposed by Weiss and co-workers,[19] we performed detailed PXRD experiments. Itmay be mentioned here that a hydrogen-bonding networkthat is present in the gel state may not represent the HBNsin the bulk crystalline solid or the xerogel state because, inboth cases, solvent evaporation (during crystallization and

xerogel formation) might induce new nucleation events thatcould lead to different crystalline phases or mixtures of vari-ous crystalline phases. However, obtaining meaningful datafrom a gel sample is difficult because of the severe contribu-tion of scattering from the solvent. Thus, we comparedPXRD patterns that were obtained from the single-crystaldata (simulated PXRD), the bulk solid, and the xerogel ofall four gelators (Figure 5). In all the cases, the simulatedand bulk PXRD patterns were almost superimposable, thusindicating that the single-crystal structures (and, hence, theobserved PAM synthon) truly represented the structures ofthe bulk solids.

Interestingly, in most cases, the xerogel PXRD patternsalso matched quite well with the corresponding simulatedand bulk PXRDs, thereby confirming the existence of ob-served PAM synthon W in the xerogel state. However, it isnot possible to make a definite comment on the supramolec-ular synthon that is present in the gel state for the reasonsmentioned above. In the case of BAACHTUNGTRENNUNG(4-NO2), the PXRD ofthe xerogel was less crystalline, thereby making it difficultto assess the crystalline phase of the xerogel. Interestingly,the morphologies of the gel networks (Figure 3) were foundto be a 1D fibrous architecture, in accord with the currentunderstanding that the 1D HBN promotes 1D growth.[11]

FTIR Spectroscopy

To study the hydrogen-bonding interactions that were re-sponsible for gel formation, we performed a detailed FTIRstudy on the hydrogel of BA ACHTUNGTRENNUNG(4-Me) (Figure 6). Because it isalmost impossible to gain meaningful FTIR data in water,[20]

we performed FTIR experiments in D2O. The single-crystalstructure of BAACHTUNGTRENNUNG(4-Me) clearly established that the ammoni-um cation was involved in hydrogen-bonding interactionswith three neighboring COO� groups and the amide C=Obond did not form any hydrogen-bonding interactions. Inthe solid state, BAACHTUNGTRENNUNG(4-Me) displayed a sharp band at1697 cm�1, which was attributed to the amide I band for theC=O stretch. An identical value was obtained in the FTIRspectrum of the corresponding water xerogel, thus meaningthat the supramolecular environment of the amide C=Ogroup remained unchanged in the xerogel state. Interesting-ly, in the D2O gel of BA ACHTUNGTRENNUNG(4-Me), the amide I band was red-shifted (1681 cm�1), thus meaning that the amide C=Ogroup participated in non-bonding interactions (C=O···D)with D2O. To support the experimental data, we recordedthe FTIR spectrum of a solution of BAACHTUNGTRENNUNG(4-Me) in[D6]DMSO. Understandably, the solvent was unable to par-ticipate in hydrogen-bonding interactions with the amide C=

O group; as expected, the amide I band appeared at1697 cm�1 in this solution. The band for the COO� groupappeared at 1599 cm�1 for the bulk sample, 1601 cm�1 forthe water xerogel, 1589 cm�1 for the D2O gel, and 1608 cm�1

for the solution of the BA ACHTUNGTRENNUNG(4-Me) salt in [D6]DMSO. Theseresults clearly demonstrate that the hydrogen-bonding envi-ronment of COO� remained almost identical in the bulksolid and the xerogel, whereas, in the gel state, the red-shift

Figure 4. Representation of PAM synthon W, as well as additional hydro-gen-bonding interactions in the crystal structure of BA ACHTUNGTRENNUNG(4-Cl). The otherthree salts, namely BA ACHTUNGTRENNUNG(4-Br), BA ACHTUNGTRENNUNG(4-NO2), and BA ACHTUNGTRENNUNG(4-Me) displayed iden-tical HBNs because all four of these salts were isomorphous. The aminoacid side chain is not shown for clarity.

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of 10–12 cm�1 indicated that the COO� moiety must be par-ticipating in non-bonding O···D�O interactions, along withthe observed hydrogen-bonding interactions with the ammo-nium cation.

However, the COO� stretching band was blue-shifted to1608 cm�1 in a solution of BAACHTUNGTRENNUNG(4-Me) in [D6]DMSO, thusmeaning that the S=O of [D6]DMSO must disrupt the hy-

drogen-bonding interactions that involve the ammoniumcation and the carboxylate anions by participating in hydro-gen-bonding interactions of the type N�H···O with the am-monium cation. Thus, these results clearly demonstrate therole of various hydrogen-bonding interactions that involvethe gelator molecule and the gelating solvent in gel forma-tion.

Nanoparticles

Metal nanoparticles (MNPs) display unique physical andchemical properties[21] compared to the bulk metals. Inrecent years, SGs have been used as a template on whichMNPs are generated.[22] MNPs can be generated on gel net-works in three different ways: 1) Independently generatedMNPs can be absorbed onto a gel network; 2) MNPs maybe generated on SGs with the help of an exogenous reduc-ing agent; and c) MNPs may be generated in situ on SGs, inwhich the gel network acts as a template, reducing agent,and a MNP-stabilizing agent. The in situ synthesis of MNPson SGs without the use of an external reducing agent is notso common.[23] However, there are quite a few examples ofthe first two methods.[24]

To study whether the gels reported herein were able toproduce MNPs, we chose a hydrogel that was derived fromBAACHTUNGTRENNUNG(4-Me). In a typical experiment, we prepared a 4 wt. %hydrogel of BAACHTUNGTRENNUNG(4-Me) in a test tube. Then, an aqueous solu-tion of HAuCl4·3 H2O (50 mL, 24 mg gold salt in 300 mLwater) was layered over the hydrogel and the mixture wasallowed to settle at room temperature. After 8–10 h, thecolor of the gel turned light pink, thus indicating the forma-tion of Au NPs. The pink color gradually became more in-tense over time and remained stable for about a month.Subsequent UV/Vis spectroscopy of the disrupted gel sus-pension (after shaking) showed a typical surface Plasmonpeak at about 577 nm, thus indicating the presence ofAu NPs. TEM analysis of the gel sample that containedAu NPs showed the presence of a rod-like morphology, aswell as a small number of spheres, on which the Au NPs hadadhered. The formation of the spherical morphology mightbe due to the generation of highly hydrophobic 4-methylbenzoic acid as a result of the low pH value that was in-duced by the gold salt on the gel bed (see below). The size

Figure 5. PXRD traces under various conditions. All of the xerogels wereprepared from their corresponding 4 wt. % DCB gel except for BA ACHTUNGTRENNUNG(4-Me), which was prepared from a 4 wt. % hydrogel.

Figure 6. FTIR spectra of BA ACHTUNGTRENNUNG(4-Me) under various conditions.

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distribution of the Au NPs that adhered onto the gel fibersrevealed that the majority of the Au NPs had sizes of 20–30 nm (Figure 7). Frequency-sweep experiments on both thehydrogel (BA ACHTUNGTRENNUNG(4-Me)) and the hydrogel that contained

Au NPs displayed a remarkable difference in their viscousmodulus, G’; the Au NPs seemed to have notably increasedthe G’ value (by about 10-fold) compared to its native hy-drogel (see the Supporting Information, Figure S6). A litera-ture survey indicated that the formation of Au NPs on theSGs without the use of an exogenous reducing agent wasnot as common as mentioned earlier. Most interestingly, tothe best of our knowledge, there are no reports on the for-mation of Au NPs on the SG without an external reducingagent in which the gelator is a simple organic salt. There-fore, it was necessary to identify the reducing agent, as wellas the role of the gel network in the formation of Au NPs.To probe this effect, we performed control experiments, asdescribe below.

Black aggregates of Au were formed when a solution ofHAuCl4·3 H2O (1.0 mg) was mixed with an aqueous solutionof BAACHTUNGTRENNUNG(4-Me) (10 mg in 2.0 mL water). This result indicatedthat AuIII was reduced, but that no Au NPs were formed,presumably because the Au NPs could not be stabilized in

the gelator solution. The fact that the Au NPs were easilyformed and remained stable for months on a hydrogel ofBAACHTUNGTRENNUNG(4-Me), the gel network must stabilize the Au NPs insome way. Similar experiments were also carried out on theindividual components of the gelator, that is, 4-methyl ben-zoic acid and amine B, in a water/MeOH mixture. Theamine solution that contained the gold salt did not show anytypical color of Au NPs. TEM and energy-dispersive X-ray(EDX) analysis also supported these findings (see the Sup-porting Information, Figure S1). On the other hand, theacidic solution that contained the gold salt turned a light-pink color, thus indicating the formation of Au NPs. Assuch, UV/Vis spectroscopy of this solution did not result inany characteristic peak for the surface plasmon of Au NPs.The surface of the test tube remained light pink, ever afterdecanting the solution, which indicated that the Au NPswere stabilized on the silica surface of the test tube. Then,the silica-surface-adhered Au NPs were taken into the solu-tion by sonication and mild heating. UV/Vis analysis of thissolution gave a characteristic surface plasmon peak of theAu NPs at 573 nm. A TEM image taken after drop-castingthis solution onto a carbon-coated copper grid confirmedthe presence of relatively larger Au NPs (100–200 nm) ascompared with those obtained within the gel matrix (seeabove). Both the SAED and EDX data also confirmed thatthese NPs were indeed Au NPs (Figure 8). These resultsclearly indicated that the carboxylic-acid component of thegelator salt was the reducing agent and that the gel networkmust stabilize the Au NPs. The pH value of the gold salt ishighly acidic (pH�2.0) and, therefore, it is not surprisingthat the anionic component of the gelator must be protonat-ed to form the corresponding carboxylic acid, which ulti-mately reduces AuIII into Au0. Because a small amount ofthe gold salt (0.1 mmol) was placed over a gel bed that con-tained 0.1 mmol of the gelator salt, only a small part of thegel network at the interface had degraded to produce the re-quired amount of the free acid for reduction and the rest ofthe gel network stabilized the Au NPs.

Conclusions

A new series of PAM salts was prepared from differentlysubstituted benzoic acids and a chiral amine (B), which con-tained an additional hydrogen-bonding functionality, tostudy the robustness of the PAM synthon and its role in ge-lation; about 66 % of the salts showed moderate-to-good ge-lation ability with a number of solvents, both polar andapolar, including water (hydrogelation). The crystal struc-tures of four gelator salts, BAACHTUNGTRENNUNG(4-Cl), BAACHTUNGTRENNUNG(4-Br), BAACHTUNGTRENNUNG(-NO2)and BAACHTUNGTRENNUNG(4-Me), showed the existence of synthon W, whichdisplaying the robustness of PAM synthon W. Structure–property correlation based on single-crystal and powder X-ray diffraction studies, along with FTIR data, clearly estab-lished the existence of PAM synthon W in the xerogel state.The FTIR data also supported the presence of strong non-bonding interactions that involved the gelator molecule and

Figure 7. a) Au NPs adhered to the gel network, thereby displayinga spherical and rod-like morphology; b, c) magnified view of the Au NPsand d) the corresponding size distribution; e) surface plasmon peak ofAu NPs that were harvested from the gel; f) SAED pattern of theAu NPs; g) photograph of black aggregates of Au NPs in the gelator solu-tion.

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the gelating solvent (D2O) in the gel state. One of the gela-tors, BAACHTUNGTRENNUNG(4-Me), was capable of forming and stabilizingAu NPs within the gel network without the use of any exog-enous reducing agent. Control experiments suggested thatthe anionic moiety was protonated in an acidic solution ofthe gold salt, thereby affording the corresponding carboxylicacid, which acted as the reducing agent. Thus, we have dem-onstrated the merit of supramolecular synthons for thedesign of a new series of LMWGs from simple organic salts.To the best of our knowledge, this is the first example of anorganic-salt-based gelator that is capable of forming Au NPswithout the use of an exogenous reducing agent.

Experimental Section

Materials and Physical Measurements

All of the reagents were obtained from commercial sources (Sigma–Al-drich, Rankem, S. D. Fine, etc.) and used without further purification.Solvents were of laboratory reagent grade and used without further distil-lation. 1H and 13C NMR spectroscopy were performed on 500 MHz(Bruker Ultrasheild Plus-500) and 300 MHz spectrometers (BrukerAvance, DPX-300). Standard 5 mm NMR tubes (Aldrich) were used forthe data collection. Chemical shifts (d) are reported in parts per million(ppm) relative to residual solvent peaks for the [D6]DMSO (Aldrich).All of the IR spectra were recorded on an FTIR instrument (FTIR-8300,Shimadzu) by using KBr pallets; IR spectra of the D2O gel and[D6]DMSO solution of BA ACHTUNGTRENNUNG(4-Me) were recorded on a Nicolet 380 FTIRby using a CaF2 cell (path length: 0.05 mm). The elemental compositionsof the purified compounds were confirmed by elemental analysis(Perkin–Elmer Precisely, Series-II, CHNO/S Analyser-2400). TEM analy-sis was performed on a JEOL instrument with a 300-mesh copper TEMgrid at 200 kV. SEM analysis was performed on JEOL, JMS-6700F, and

JMS-2100F field-emission scanningelectron microscopes. Differentialscanning calorimetry (DSC) was per-formed on a Perkin–Elmer DiamondDSC. Rheology studies were per-formed on an SDT Q series advancedrheometer-AR 2000. PXRD datawere recorded on a Bruker-AXS-D8Advance.

Synthetic Procedure

First, pyridin-3-amine was added intoa 250 mL two-necked round-bottomflask and dissolved in dry THF undera nitrogen atmosphere. Then, DCC(1 equiv) was added into the solution.A solution of N-Boc-l-phenylalanine(1 equiv, Boc= tert-butoxycarbonyl)in dry THF was slowly added to thesolution under ice-cold conditionsand the mixture was stirred over-night. Then, it was filtered and thefiltrate was evaporated. The whiteresidue (1) was dissolved in acidicHCl/MeOH (15 % v/v) solution andthe mixture was stirred overnight.Then, dry Et2O was added into thesolution and a white crystalline com-pound (2) precipitated out. Then,these compounds were dissolved inCH2Cl2 by the addition of a saturatedaqueous solution of sodium bicarbon-

ate. The organic layer was collected and dried with anhydrous sodiumsulfate. The mixture was filtered and the filtrate was evaporated over-night to afford a white solid (B ; see the Supporting Information,Scheme S1). Then, these compounds were dissolved in MeOH and addedto a methanolic solution of the substituted benzoic acid, which gave theproducts (BA(R), Scheme 1). All of the final products were characterizedby IR spectroscopy, NMR spectroscopy, and elemental analysis.

BA ACHTUNGTRENNUNG(4-Cl): M.p. 172–176 8C; 1H NMR (300 MHz, [D6]DMSO): d=8.75(d,J=2.4 Hz, 1 H), 8.26 (d, J =4.68 Hz, 1 H), 8.04 (d, J =9.2 Hz, 1 H), 7.95 (d,J =8.48 Hz, 2H), 7.57 (d, J =8.51 Hz, 2H), 7.21 (m, 6H), 3.74 (t, J=

6.8 Hz, 1 H), 3.05 (dd, J =13.47 Hz, J =5.99 Hz, 1 H), 2.85 ppm (dd, J=

13.4 Hz, J =7.62 Hz, 1 H); 13C NMR (300 MHz, [D6]DMSO): d=173.44,167.57, 144.99, 141.72, 138.37, 137.52, 136.13, 135.94, 132.24, 131.69,129.94, 129.04, 128.82, 126.99, 124.23, 57.14, 40.97 ppm; FTIR (KBrpellet): n =3030, 3009, 2988, 2947, 2868, 2805, 2596, 1695, 1601, 1555,1518, 1478, 1427, 1393, 1327, 1271, 1206, 1165, 1134, 1086, 839, 800, 772,745, 700, 685, 665, 640, 592, 525 cm�1; elemental analysis calcd (%) forC21H20ClN3O3: C 63.40, H 5.07, N 10.56; found: C 63.41, H 5.17, N 10.38.

BA ACHTUNGTRENNUNG(3-Cl): M.p. 148–150 8C; 1H NMR (500 MHz, [D6]DMSO): d=8.72 (d,J =2.5 Hz, 1 H), 8.27 (dd, J =5 Hz, 1 H), 8.01 (m, 1H), 7.88 (s, 1 H), 7.86(s, 1H), 7.61 (m, 1 H), 7.48 (m, 1H), 7.25 (m, 7 H), 3.80 (t, J=6.5, 1H),3.085 (q, 1 H), 2.87 ppm (q, 1H); 13C NMR (500 MHz, [D6]DMSO): d=

172.49, 166.55, 144.45, 141.13, 137.55, 135.28, 133.08, 131.70, 130.31,129.35, 128.25, 127.82, 126.45, 123.64, 111.08, 56.37 ppm; FTIR (KBrpellet): n =3250, 3223, 3181, 3032, 2864, 2828, 2687, 1697, 1632, 1597,1560, 1537, 1491, 1476, 1451, 1427, 1379, 1329, 1292, 1265, 1207, 1188,1070, 868, 800, 764, 733, 702, 679 cm�1; elemental analysis calcd (%) forC21H20ClN3O3: C 63.40, H 5.07, N 10.56; found: C 63.38, H 5.19, N 10.46.

BA ACHTUNGTRENNUNG(2-Cl): M.p. 130–135 8C; 1H NMR (500 MHz, [D6]DMSO): d=8.72 (d,J =2 Hz, 1 H), 8.27 (d, J=5 Hz, 1H), 8.01 (d, J =8 Hz, 1H), 7.64 (d, J=

8 Hz, 1 H), 7.30 (m, 9 H), 3.88 (t, J=7 Hz, 1H), 3.08 (dd, J=13.5 Hz, J =

6.5 Hz, 1 H), 2.92 ppm (dd, J =13.5 Hz, J =6.5 Hz, 1H); 13C NMR(500 MHz, [D6]DMSO): d=171.85, 167.71, 144.47, 141.12, 137.22, 135.21,130.97, 130.79, 130.06, 130.04, 129.34, 128.25, 128.12, 126.88, 126.51,126.44, 123.61, 56.09, 40.12 ppm; FTIR (KBr pellet): n =3223, 3183, 3028,2972, 2928, 1699, 1601, 1549, 1532, 1497, 1431, 1427, 1381, 1331, 1294,

Figure 8. Formation of Au NPs in the presence of p-toluic acid. a) Photograph that shows the light-pink colorof the Au NPs in water/MeOH solution. b) Surface plasmon spectrum, c) TEM image, d) SAED pattern, ande) EDX pattern of the Au NPs.

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1271, 1204, 1148, 1125, 1107, 1072, 1049, 1034, 987, 957, 918, 843, 802,746, 702, 646, 627, 588, 457 cm�1; elemental analysis calcd (%) forC21H20ClN3O3: C 63.40, H 5.07, N 10.56; found: C 62.57, H 5.11, N 10.58.

BA ACHTUNGTRENNUNG(4-NO2): M.p. 180–185 8C; 1H NMR (500 MHz, [D6]DMSO): d =8.72(d, J =3.88 Hz, 1H), 8.28 (m, 3H), 8.20 (dd, J =11.5 Hz, J =3.2 Hz, 2H),8.0 (m, 1 H), 7.30 (m, 6 H), 3.91 (t, J =11.39 Hz, 1H), 3.10 (dd, J=

22.36 Hz, J= 10.38 Hz, 1 H), 2.89 ppm (dd, J=22.36 Hz, J =10.38 Hz,1H); 13C NMR (500 MHz, [D6]DMSO): d=171.58, 167.26, 149.61, 148.31,145.18, 141.73, 141.62, 137.34, 135.71, 130.94, 129.95, 128.90, 127.24,127.10, 124.24, 123.82, 56.32 ppm; FTIR (KBr pellet): n =3217, 3179,3032, 2905, 2897, 2173, 1944, 1695, 1613, 1572, 1553, 1561, 1478, 1452,1427, 1389, 1343, 1317, 271, 1223, 1206, 1146, 1103, 1072, 986, 918, 876,826, 799, 745, 721, 700, 671, 658, 588, 509 cm�1; elemental analysis calcd(%) for C21H20N4O5: C 61.76, H 4.94, N 13.72; found: C 61.63, H 4.35,N 13.85.

BA ACHTUNGTRENNUNG(3-NO2): M.p. 164–168 8C; 1H NMR (500 MHz, [D6]DMSO): d =8.70(d, J=2 Hz, 1 H), 8.62 (s, 1 H), 8.35 (dd, J =8.5 Hz, 1H), 8.32 (d, J=

7.5 Hz, 1H), 8.27 (dd, J=6 Hz, 1H), 8.01 (d, J= 8.5 Hz, 1 H), 7.72 (t, J =

7.5 Hz, 1 H), 7.34 (q, 1H), 7.25 (m, 6 H), 3.91 (t, J=7 Hz, 1H), 3.10 (q,1H). 2.94 ppm (q, 1 H); 13C NMR (500 MHz, [D6]DMSO): d=170.94,166.26, 148.25, 141.71, 144.62, 141.15, 136.90, 136.71, 135.32, 135.11,129.81, 129.38, 129.23, 128.32, 126.67, 126.54, 125.62, 123.67, 123.50,55.72 ppm; FTIR (KBr pellet): n=3248, 3034, 2982, 2864, 2692, 2643,2610, 1697, 1634, 1613, 1599, 1570, 1524, 1493, 1474, 1427, 1381, 1350,1331, 1267, 1204, 1074, 826, 800, 789, 745, 716, 700 cm�1; elemental analy-sis calcd (%) for C21H20N4O5: C 61.76, H 4.94, N 13.72; found: C 61.72,H 4.87, N 13.69.

BA ACHTUNGTRENNUNG(2-NO2): M.p. 142–146 8C; 1H NMR (500 MHz, [D6]DMSO): d =8.68(d, J =2.5 Hz, 1 H), 8.26 (dd, J =4.5 Hz, J =1 Hz, 1 H), 7.96 (m, 1 H), 7.71(m, 2 H), 7.61 (t, J =7.5 Hz, 1 H), 7.52 (m, 1H), 7.33 (dd, J =8.5 Hz, J=

5 Hz, 1H), 7.22 (m, 5 H), 4.12 (t, J =7 Hz, 1H), 3.09 (dd, J =14 Hz, J =

7 Hz, 1H), 3.06 ppm (dd, J =14 Hz, J= 7 Hz, 1H); 13C NMR (500 MHz,[D6]DMSO): d= 170.09, 167.28, 149.38, 145.32, 141.78, 136.55, 135.57,132.61, 130.31, 130.24, 130.03, 128.96, 127.43, 127.19, 124.26, 123.31, 55.67,40.72, 38.70 ppm; FTIR (KBr pellet): n=3250, 3221, 3181, 3026, 2996,2915, 2880, 2598, 2171, 1697, 1626, 1586, 1557, 1528, 1505, 1479, 1429,1402, 1370, 1331, 1316, 1292, 1275, 1240, 1223, 1204, 1072, 802, 777, 764,729, 712, 696, 646, 583, 548, 426 cm�1; elemental analysis calcd (%) forC21H20N4O5: C 61.76, H 4.94, N 13.72; found: C 60.66, H 4.80, N 13.51.

BA ACHTUNGTRENNUNG(4-Br): M.p. 177–184 8C; 1H NMR (500 MHz, [D6]DMSO): d=8.71(d,J=2.5 Hz, 1H), 8.24 (dd, J= 5 Hz, J =1.5 Hz, 1H), 8.01 (m, 1H), 7.84 (d,J =7.5 Hz, 2 H), 7.65 (m, 2H), 7.31 (q, 1 H), 7.21 (m, 6H), 3.68 (t, J=

7 Hz, 2H), 3.02 (q, 1 H). 2.79 ppm (q, 1 H); 13C NMR (500 MHz,[D6]DMSO): d= 173.84, 167.44, 144.89, 141.66, 138.56, 135.91, 132.03,131.83, 129.87, 128.75, 126.91, 124.14, 57.28 ppm; FTIR (KBr pellet): n=

3213, 3179, 3009, 2928, 1923, 1695, 1601, 1586, 1553, 1514, 1476, 1427,1393, 1327, 1271, 1207, 1169, 1102, 1072, 1008, 989, 962, 918, 835, 790,768, 700, 626, 588 cm�1; elemental analysis calcd (%) for C21H20BrN3O3:C 57.02, H 4.56, N 9.50; found: C 57.12, H 4.39, N 9.16.

BA ACHTUNGTRENNUNG(3-Br): M.p. 156–158 8C; 1H NMR (500 MHz, [D6]DMSO): d=8.72 (d,J =2.5 Hz, 1H), 8.26 (dd, J= 5 Hz, J= 1 Hz, 1H), 8.01 (m, 2 H), 7.91 (d,J =7.5 Hz, 1H), 7.75 (d, J=7.5 Hz, 1 H), 7.43 (t, J =7.5 Hz, 1H), 7.34 (q,1H), 7.24 (m, 6 H), 3.76 (q, 2H), 3.04 (q, 1H). 2.85 ppm (q, 1H);13C NMR (500 MHz, [D6]DMSO): d =166.57, 144.45, 141.11, 135.25,134.38, 131.74, 130.48, 129.34, 129.23, 128.25, 128.14, 126.48, 126.44,125.29, 123.72, 123.67, 121.45, 56.25 ppm; FTIR (KBr pellet): n =3032,2866, 2604, 1697, 1632, 1597, 1586, 1557, 1537, 1489, 1476, 1427, 1397,1375, 1327, 1265, 1207, 856, 800, 762, 714, 702 cm�1; elemental analysiscalcd (%) for C21H20BrN3O3: C 57.02, H 4.56, N 9.50; found: C 57.05,H 4.28, N 9.76.

BA ACHTUNGTRENNUNG(2-Br): M.p. 118–120 8C; 1H NMR (500 MHz, [D6]DMSO): d=8.73 (d,J =2.5 Hz, 1 H), 8.27 (dd, J =5 Hz, J =1 Hz, 1H), 8.01 (d, J =8.5 Hz, 1H),7.59 (dd, J=13 Hz, J=7.5 Hz, 1H), 7.32 (m, 9H), 3.91 (t, J =6.5 Hz,2H), 3.31 (q, 1 H), 2.95 ppm (q, 1H); 13C NMR (500 MHz, [D6]DMSO):d=171.93, 168.20, 144.45, 141.11, 137.16, 135.20, 133.13, 130.82, 129.80,129.34, 128.22, 127.34, 126.49, 126.40, 123.57, 119.37, 56.03 ppm; FTIR(KBr pellet): n=3220, 3180, 3028, 2926, 1698, 1589, 1548, 1497, 1483,1426, 1380, 1330, 1270, 1204, 1155, 1146, 1073, 1041, 1025, 957, 918, 837,

802, 744, 701, 642, 626, 608, 588, 409 cm�1; elemental analysis calcd (%)for C21H20BrN3O3: C 57.02, H 4.56, N 9.50; found: C 56.16, H 4.56,N 9.17.

BA ACHTUNGTRENNUNG(4-Me): M.p. 130–135 8C; 1H NMR (500 MHz, [D6]DMSO): d =8.72(d, J =2.5 Hz, 1 H), 8.24 (d, J =5.5 Hz, 1 H), 8.02 (d, J =8 Hz, 1 H), 7.81(d, J =8 Hz, 2 H), 7.25 (m, 9 H), 3.74 (t, J= 5.5 Hz, 1H), 3.03 (dd, J =13.5,J =6, 1H), 2.76 (dd, J =13.5, J =7.5, 1H), 2.48 ppm (s, 3 H); 13C NMR(500 MHz, [D6]DMSO): d=174.35, 167.99, 144.81, 143.26, 141.63, 138.83,135.95, 129.85, 129.58, 128.69, 126.85, 126.78, 124.10, 57.47, 41.28,21.65 ppm; FTIR (KBr pellet): n=3215, 3183, 3028, 2988, 2868, 1697,1599, 1551, 1512, 1478, 1427, 1393, 1269, 1207, 849, 766, 698 cm�1; ele-mental analysis calcd (%) for C22H23N3O3: C 70.01, H 6.14, N 11.13;found: C 69.62, H 5.88, N 10.64.

BA ACHTUNGTRENNUNG(3-Me): M.p. 135–139 8C; 1H NMR (500 MHz, [D6]DMSO): d =8.72(d, J =2.5 Hz, 1H), 8.24 (d, J =5 Hz, J=1.5 Hz, 1H), 8.02 (m, J= 1H),7.74 (s, 1 H), 7.72 (d, J=7.5 Hz, 1 H), 7.26 (m, 9H), 3.64 (dd, J =7.5 Hz,J =6 Hz, 1 H), 3.02 (q, 1 H). 2.77 (q, 1H), 2.34 ppm (s, 3H); 13C NMR(500 MHz, [D6]DMSO): d=174.24, 168.21, 144.88, 141.68, 138.79, 138.34,135.99, 133.74, 130.32, 129.90, 128.94, 128.76, 127.02, 126.92, 126.86,124.17, 57.47, 41.25, 21.43 ppm; FTIR (KBr pellet): n=3225, 3179, 3026,2988, 2926, 2868, 2812, 2602, 1695, 1616, 1584, 1549, 1497, 1454, 1425,1375, 1275, 1220, 1207, 1153, 1067, 980, 959, 929, 916, 895, 760, 700, 671,627, 606, 584 cm�1; elemental analysis calcd (%) for C23H24N2O3: C 70.01,H 6.14, N 11.13; found: C 69.70, H 5.90, N 10.60.

BA ACHTUNGTRENNUNG(2-Me): M.p. 106–108 8C; 1H NMR (500 MHz, [D6]DMSO): d =8.72(d, J= 2.5 Hz, 1H), 8.25 (d, J =5 Hz, 1 H), 8.03 (d, J=8 Hz, 1 H), 7.77 (d,J =7 Hz, 1 H), 7.39 (t, J =7.25 Hz, 1 H), 7.32 (dd, J =8 Hz, J =4.5 Hz,1H), 7.25 (m, 6 H), 7.19 (t, J=6.75 Hz, 1H), 3.65 (t, J =7 Hz, 1 H), 3.02(dd, J =13.5 Hz, J =5.5 Hz, 1H). 2.78 (dd, J =13.5 Hz, J =7.5 Hz, 1H),2.50 ppm (s, 3 H); 13C NMR (500 MHz, [D6]DMSO): d=173.69, 168.96,144.26, 141.06, 138.66, 138.20, 135.38, 131.35, 131.31, 129.99, 129.29,128.14, 126.31, 126.24, 125.72, 123.56, 56.87, 40.66, 21.15 ppm; FTIR (KBrpellet): n =3219, 3179, 3026, 2928, 2362, 1697, 1604, 1584, 1545, 1495,1478, 1456, 1452, 1383, 1329, 1271, 1206, 1150, 1126, 1099, 1072, 1045,1026, 990, 961, 918, 849, 802, 785, 741, 700, 658, 627, 588, 538, 496,475 cm�1; elemental analysis calcd (%) for C23H24N2O3: C 70.01, H 6.14,N 11.13; found: C 69.67, H 6.10, N 10.89.

Single-Crystal X-ray Diffraction

Single crystals suitable for X-ray diffraction of BA ACHTUNGTRENNUNG(4-Cl), BA ACHTUNGTRENNUNG(4-Br), BA-ACHTUNGTRENNUNG(4-NO2), and BA ACHTUNGTRENNUNG(4-Me) were obtained by the slow evaporation fromMeOH/water solution at RT. Single-crystal X-ray data were collected byusing MoKa (l= 0.7107 �) radiation on a SMART APEX-II diffractome-ter that was equipped with CCD area detector. Data collection, data re-duction, structure solution, and refinement were performed by using thesoftware package of SMART APEX-II. All of the structures were solvedby using direct methods and refined in a routine manner. In all cases,non-hydrogen atoms were treated anisotropically. Whenever possible, thehydrogen atoms were located on a difference Fourier map and refined.In other cases, the hydrogen atoms were geometrically fixed at their ide-alized positions.

Acknowledgements

U.K.D. and P.D. thank the CSIR, New Delhi, for a Senior Research Fel-lowship (SRF) and financial support, respectively. S.B. thanks the IndianAssociation for The Cultivation of Science (IACS) for a Senior ResearchFellowship (SRF). All of the single-crystal X-ray diffraction data werecollected at the National Single-Crystal Diffractometer Facility of theDepartment of Inorganic Chemistry, IACS, Kolkata, funded by the DST.

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Received: June 19, 2013Published online: && &&, 0000

Chem. Asian J. 2013, 00, 0 – 0 � 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim10&&

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www.chemasianj.org Parthasarathi Dastidar et al.

FULL PAPER

Gelators

Uttam Kumar Das,Subhabrata Banerjee,Parthasarathi Dastidar* &&&&—&&&&

Primary Ammonium MonocarboxylateSynthon in Designing SupramolecularGels: A New Series of Chiral Low-Molecular-Weight Gelators Derivedfrom Simple Organic Salts that areCapable of Generating and StabilizingGold Nanoparticles

Xero to hero : The primary ammoniummonocarboxylate (PAM) supramolec-ular synthon was exploited to accessa new series of organic-salt-based gela-tors from a chiral amine and substi-tuted benzoic acids. One such gel dis-played the ability to form Au NPswithout the use of exogenous reducingagents. A structure–property correla-tion based on simulated and experi-mental PXRD data established thepresence of the 1D PAM synthon inthe xerogels.

Chem. Asian J. 2013, 00, 0 – 0 � 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim11 &&

These are not the final page numbers! ��