Secondary Ammonium Dicarboxylate (SAD)A SupramolecularSynthon in Designing Low Molecular Weight Gelators Derived fromAzo-DicarboxylatesPathik Sahoo and Parthasarathi Dastidar*

Department of Organic Chemistry, Indian Association for the Cultivation of Science (IACS), 2A and 2B Raja S C Mullick Road,Jadavpur Kolkata−700032, West Bengal, India

*S Supporting Information

ABSTRACT: The supramolecular synthon namely secondary ammoniumdicarboxylate (SAD) synthon has been exploited to design a new series of lowmolecular weight gelators (LMWGs) derived from azobenzene-4,4′-dicarboxylicacid and azobenzene-4,4′-diacrylic acid, and various secondary amines. Singlecrystal structures of six such salts exclusively established the presence of SADsynthon. Two such salts namely, dicyclohexylammonium azobenzene-4,4′-diacrylate (2.DCHA) and dihexylammonium azobenzene-4,4′-diacrylate(2.DHA) displayed intriguing gelation properties. Powder X-ray diffraction incombination with single crystal X-ray data established existence of SAD synthonin the structure of the gel network of 2.DCHA. UV-irradiation of the salts as wellas the gel did not show any trans−cis isomerization of the azo-moiety.

■ INTRODUCTION

Small molecules (molecular weight <3000) capable ofimmobilizing large amount of solvents within a gel formingnetwork often termed as self-assembled-f ibriller-networks (SA-FINs)1 formed via supramolecular (noncovalent) self-assemblyare known as Low molecular weight gelators (LMWGs).2 Inrecent years, research on LMWGs gained impetus due to theirvarious potential applications such as in the field of electro-optics/photonics,3 sensors,4 cosmetics,5 structure directingagents,6 conservation of art,7 preparation of sculpture,8 drugdelivery,9 and biomedical applications,10 catalysis,11 and soforth. Most often, LMWGs are serendipitously discovered; agrip over the general design of such fascinating materials hasremained an elusive goal. This is mainly because of thestructural diversity of the gelling agents, lack of molecular levelinformation about the structure of the gel-forming-network andpoor understanding of the interactions of the gel-forming-network and the targeted solvents. Nevertheless, there havebeen attempts to design LMWGs.12 Microscopic and X-raydiffraction studies indicated that gel forming networks are oftencrystalline and therefore, have ordered crystal structures; thus,it was argued if the structure of the gel-forming network couldbe determined, designing LMWGs could be a reality. Weiss etal. elegantly demonstrated how the structure of the gel-forming-network could be determined by using X-ray diffraction(combining both powder- and single crystal X-ray dataPXRD and SXRD, respectively).13 Later on Shinkai et al.proposed a hypothesis based on a structure−propertycorrelation approach14 that stated that 1-D hydrogen bondingnetwork (HBN) were important in gelation whereas 2-D and 3-

D networks were not as important. The fact that Shinkai’shypothesis was indeed based on a logical foundation has beendemonstrated by our group by exploiting supramolecularsynthona concept originally proposed by Desiraju15 in thecontext of crystal engineering.16 Among the various supra-molecular synthons that we have exploited in discovering newLMWGs, secondary ammonium monocarboxylate (SAM)synthon is particularly interesting. It can be seen that when asecondary amine is reacted with a monocarboxylic acid, theresultant salt may display either 1D synthon (synthon A) or 0Dsynthon (synthon B) with equal probability (Scheme 1).We have shown with the help of single crystal structures of

various organic salts that 1D SAM synthon, i.e., synthon Aindeed played a crucial role in gelation process, whereas thesalts that displayed SAM synthon B did not show any gelationproperties.17 Thus, the main disadvantage of SAM synthon is

Received: June 18, 2012Revised: October 13, 2012Published: October 18, 2012

Scheme 1. Schematic Illustration of 1D and OD SAMSynthons

Article

pubs.acs.org/crystal

© 2012 American Chemical Society 5917 dx.doi.org/10.1021/cg301245c | Cryst. Growth Des. 2012, 12, 5917−5924

the equal probability of occurring 0D synthon B along with 1Dsynthon A. We have demonstrated that a transition from 0D to1D can indeed be achieved either by choosing suitable cation18

or by exploiting the hydrophobic interactions of the long alkylchain installed on the anionic moiety.19 However, the best wayto ensure 1D supramolecular synthon is to consider secondaryammonium dicarboxyate (SAD) salt comprising a dicarboxylicacid and a secondary amine reacted in 1:2 molar ratio. In thissalt, the 0D synthon B would be expected to propagate in 1DSAD synthon by virtue of the bifunctionality of the anionic part(Scheme 2).

In fact, we have demonstrated it to be true20 and in one suchinteresting case, we could easily generate the organometallicLMWGs that were otherwise synthetically challenging.21

In the present study, we decided to work on SAD saltsderived from a dicarboxylic acid having an azo moiety; thephotosensitivity of the azo moiety prompted us to select suchdicarboxylic acid in order to generate photoresponsive gels.Thus, we considered reacting azobenzene-4,4′-dicarboxylic acidand azobenzene-4,4′-diacrylic acid with various secondaryamines in 1:2 (acid:amine) molar ratio that generated acombinatorial library of SAD salts (Scheme 3). The resultant

SAD salts were characterized by FT-IR, 1H NMR, andelemental analysis. Single crystal structures of as many as sixSAD salts were determined. Gelation tests were also performedon these salts. The gels were characterized by table-toprheology, scanning electron microscopy (SEM), and powder X-ray diffraction (PXRD). A structure−property (gelation)correlation based on the SXRD and PXRD data was attempted.

■ RESULTS AND DISCUSSIONSSupramolecular Synthons. To investigate the supra-

molecular synthon in these SAD salts, we have tried to

crystallize all the salts studied herein. However, we were able tocrystallize 6 salts out of the 8 salts reported (Table 1).

Crystal Structure of Cyclohexylmethylammonium Azo-benzene-4,4′-dicarboxylate (1.CHMA). The salt 1.CHMAcrystallized in the centrosymmetric triclinic space group P1 ̅.The asymmetric unit is comprised of half of the anionic moietysitting on a center of symmetry and a cyclohexylmethylammo-nium cationic moiety. The C−O bond distances [1.250(5)Å]and the presence of 1631 cm−1 and absence of 1685 cm−1 inthe FT-IR clearly indicate the complete deprotonation (saltformation) of the acid moiety. Each COO− functionality of theanionic moiety is involved in hydrogen bonding interactionswith two neighboring cationic species via N−H...O interactions[N...O = 2.735(5)−2.757(6) Å; ∠N−H...O = 171(4)−173(4)°]displaying 0D SAM synthon. By virtue of the bifunctionality ofthe anionic species, the overall network is best described as 1DSAD synthon as depicted in Scheme 2; the 1D chains arepacked in parallel fashion (Figure 1).

Crystal Structure of Cyclohexylmethylammonium Azo-benzene-4,4′-diacrylate (2.CHMA). The crystal of 2.CHMAbelongs to the centrosymmetric monoclinic space group P21/c.The asymmetric unit contains one dicarboxylate species andtwo cyclohexylmethylammoniumall sitting on generalpositions. One of the azo N atoms was found to be disorderover two positions having site occupancy factors (SOFs) of0.51362 and 0.48638. It is clear from the C−O bond distances[1.259(8)−1.265(8) Å] and FT-IR data (presence of 1639cm−1 and absence of 1689 cm−1) that complete deprotonationof the acid moiety indicating salt formation has taken place. Inthe crystal structure, the cationic and anionic species are heldtogether via N−H...O hydrogen bonding [N...O = 2.702(7)−2.727(7)Å; ∠N−H...O = 162.5−174.4°] displaying 1D SADsynthon (Scheme 2); the 1D chains are packed in parallelfashion (Figure 2).

Crystal Structure of Dicyclohexylammonium Azobenzene-4,4′-dicarboxylate (1.DCHA). The crystal of 1.DCHA belongsto the centrosymmetric monoclinic space group P2/c. Theasymmetric unit contains one carboxylate anion and twoammonium cations. Both C−O bond distances [1.229(3)−1.243(3)Å] and FT-IR band at 1633 cm−1 confirmed the saltformation. In the crystal structure, the ionic species are involvedin N−H...O hydrogen bonding [N−H...O = 2.651(3)−2.747(3)Å; ∠N−H...O = 156.2−169.2°] resulting in a 1DSAD synthon as depicted in Scheme 2; the 1D hydrogenbonded chains are further packed in criss-cross fashion (Figure3).

Crystal Structure of Dicyclohexylammonium Azobenzene-4,4′-diacrylate (2.DCHA). The salt 2.DCHA crystallized in thecentrosymmetric monoclinic space group P21/c. The asym-metric unit contains half of the anionic moiety sitting on aninversion center and a cationic moiety located on a generalposition. The N atoms of the azo moiety were found to bedisordered over two positions with site occupancy factors of0.77632 and 0.22368. The presence of 1641 cm−1 and absenceof 1689 cm−1 in the FT-IR spectra and the C−O bonddistances of 1.226(8)−1.230(8)Å confirmed the completeproton transfer (salt formation). Hydrogen bonding inter-actions [N−H...O = 2.727(7)−2.771(7)Å; ∠N−H...O = 164.8−175.7°] involving the NH of the ammonium and COO− of theanionic moieties lead to the formation of 1D SAD synthon(Scheme 2); the 1D chains are further packed in parallelfashion (Figure 4).

Scheme 2. Schematic Illustration of 1D SAD Synthon

Scheme 3. Combinatorial Library of SAD Salts Reported inThis Study

Crystal Growth & Design Article

dx.doi.org/10.1021/cg301245c | Cryst. Growth Des. 2012, 12, 5917−59245918

Table

1.Crystal

Data

crystalparameters

1.CHMA

2.CHMA

1.DCHA

2.DCHA

1.DBuA

2.DBuA

CCDC

No

819370

819373

819374

819372

819369

819371

empiricalform

ula

C28H

40N

4O

4C32H

44N

4O

4C38H

56N

4O

4C21H

30N

2O

2C30H

48N

4O

4C34H

52N

4O

4

form

ulaweight

496.64

548.71

632.87

342.47

528.72

580.80

crystalsize

(mm)

0.28

×0.18

×0.09

0.51

×0.18

×0.18

0.28

×0.28

×0.27

0.40

×0.18

×0.17

0.24

×0.24

×0.24

0.27

×0.22

×08

crystalsystem

triclinic

monoclinic

monoclinic

monoclinic

triclinic

triclinic

spacegroup

P-1

P21/c

P2/c

P21/c

P-1

P-1

a(Å)

6.4485(17)

19.997(3)

22.153(3)

7.721(3)

8.568(10)

8.1405(13)

b(Å)

8.177(2)

19.530(3)

9.7615(12)

14.566(6)

9.238(10)

11.1094(18)

c(Å)

13.989(4)

7.7853(11)

17.488(2)

17.327(7)

10.972(12)

11.6064(19)

α(deg)

101.840(4)

90.00

90.00

90.00

98.503(14)

108.919(3)

β(deg)

95.145(5)

95.823(4)

98.224(3)

94.155(6)

105.534(14)

109.462(3)

γ(deg)

103.562(4)

90.00

90.00

90.00

103.485(14)

101.428(4)

volume(Å

3 )694.4(3)

3024.9(7)

3742.9(8)

1943.5(15)

792.9(15)

879.3(2)

Z1

44

41

1F(000)

268

1184

1376

744

288

316

μMoK

α(m

m−1 )

0.080

0.080

0.073

0.075

0.074

0.072

T(K

)100(2)

293(2)

100(2)

100(2)

100(2)

293(2)

Rint

0.0147

0.0720

0.0458

0.0494

0.0210

0.0291

rangeof

h,k,l

−3/6,

−8/8,

−14/15

−18/18,

−17/17,−7/7

−21/21,

−9/9,

−17/7

−7/7,−13/13,

−16/13

−8/8,

−9/9,

−11/11

−7/8,

−11/11,

−11/10

θmin/m

ax/°

1.50/22.49

1.02/19.07

0.93/20.49

1.83/19.47

1.98/21.00

2.05/21.00

reflectio

nscollected/unique/observed

[I>

2σ(I)]

2071/1635/1278

15639/2465/1783

22125/3750/2761

5416/1680/1470

3767/1693/1383

2612/1831/1283

data/restraints/parameters

1635/0/233

2465/0/357

3750/0/447

1680/0/225

1693/2/162

1831/3/180

goodness

offiton

F21.097

1.085

1.024

1.151

1.578

1.208

finalRindices[I

>2σ(I)]

R1=0.0766

R1=0.0812

R1=0.0429

R1=0.1136

R1=0.1186

R1=0.1536

wR2=0.2081

wR2=0.2257

wR2=0.1133

wR2=0.2861

wR2=0.3442

wR2=0.3830

Rindices(alldata)

R1=0.0932

R1=0.1058

R1=0.0619

R1=0.1261

R1=0.1303

R1=0.1909

wR2=0.2211

wR2=0.2489

wR2=0.1256

wR2=0.2962

wR2=0.3589

wR2=0

.4161

Crystal Growth & Design Article

dx.doi.org/10.1021/cg301245c | Cryst. Growth Des. 2012, 12, 5917−59245919

Crystal Structure of Dibutylammonium Azobenzene-4,4′-dicarboxylate (1.DBuA). The crystal of 1.DBuA crystallized in

the centrosymmetric triclinic space group P1 ̅. The asymmetricunit is composed of half of the azobenzene-4,4′-dicarboxylate

moiety sitting on a center of symmetry and a dibutylammonium

moiety located on a general position. Both the N atoms of the

azo moiety of the anionic part were found to be disordered over

two positions having site occupancy factors of 0.73171 and

Figure 1. Illustration of the single crystal structure of nongelator salt 1.CHMA; (a) Hydrogen bonding interactions forming 1D SAD synthon; (b)parallel packing of the 1D networks.

Figure 2. Illustration of the single crystal structure of nongelator salt 2.CHMA; (a) Hydrogen bonding interactions forming 1D SAD synthon; (b)Parallel packing of the 1D networks.

Crystal Growth & Design Article

dx.doi.org/10.1021/cg301245c | Cryst. Growth Des. 2012, 12, 5917−59245920

0.26829. Two terminal C atoms of one of the butyl chains werealso found to be disordered and were isotropically refined. TheC−O bond distances of 1.235(7)−1.265(7)Å and the presence

of 1629 cm−1 and absence of 1685 cm−1 band in the FT-IRclearly supported the salt formation. The ionic speciesrecognizes its opposite kind via N−H...O hydrogen bonding[N−H...O = 2.701(6)−2.706(6)Å; ∠N−H...O = 160.0−170.0°]resulting in the formation of 1D SAD synthon (Scheme 2). The1D chains are packed in parallel fashion (Figure 5).

Crystal Structure of Dibutylammonium Azobenzene-4,4′-diacrylate (2.DBuA). The salt 2.DBuA crystallized in thecentrosymmetric triclinic space group P1 ̅. The asymmetric unitcontains half of the anionic moiety sitting on a center of

Figure 3. Illustration of the single crystal structure of nongelator salt1.DCHA. (a) Hydrogen bonding interactions forming 1D SADsynthon; (b) packing of the 1D networks in criss-cross fashion.

Figure 4. Illustration of the single crystal structure of gelator salt 2.DCHA. (a) Hydrogen bonding interactions forming 1D SAD synthon; (b)Parallel packing of the 1D networks.

Figure 5. Illustration of the single crystal structure of nongelator salt1.DDBuA. (a) Hydrogen bonding interactions forming 1D SADsynthon; (b) Parallel packing of the 1D networks.

Crystal Growth & Design Article

dx.doi.org/10.1021/cg301245c | Cryst. Growth Des. 2012, 12, 5917−59245921

inversion and a cationic moiety located on a general position.The N atoms of the azo moiety were found to be disorderedover two positions with site occupancy factors of 0.821940 and0.178060. Two terminal C atoms of one of the butyl chainswere also found to be disordered and were isotropically refined.The presence of 1641 cm−1 and absence of 1689 cm−1 in theFT-IR spectra and the C−O bond distances of 1.228(10)−1.254(11)Å clearly supported the complete proton transfer(salt formation). In the crystal structure, the ionic species areinvolved in N−H...O hydrogen bonding [N−H...O =2.684(10)− 2.743(11)Å; ∠N−H...O = 157.7−175.6°] resultingin the formation of 1D SAD synthon as depicted in Scheme 2;the 1D hydrogen bonded chains are further packed in parallelfashion (Figure 6).Gelation. As revealed from SXRD data that all the salts

displayed typical 1D SAD synthon, it was consideredworthwhile to evaluate their gelation behavior. Fifteen selectedsolvents, (from polar to nonpolar) were employed to testify thegelation ability of these salts. It was evidenced from the gelationTable S7 (Supporting Information) that none of the secondaryammonium azobenzene-4,4′-dicarboxylate salts exhibited thegelation property. When the anion was changed to azobenzene-4,4′-diacrylate, it was quiet intriguing to note that, two salts,2.DCHA and 2.DHA showed gelation behavior. It was foundthat the nonpolar solvents were unable to solubilize the organicsalts, whereas, the aprotic solvents having high polarity (e.g.,DMF and DMSO) were able to show gelation behavior withonly two salts, 2.DCHA and 2.DHA. When the salts wereheated in the polar-protic solvent, it was resulted into eitherclear solution or precipitate. It may be mentioned that the salt2.DCHA exhibited the supergelation phenomena in the polaraprotic solvent like DMSO and DMF with the minimumgelator concentration (mgc) 0.88 and 0.80 wt %, respectively.Despite displaying 1D SAD synthon by all six of these SADsalts, only 2.DCHA and 2.DHA showed gelation behavior.Table top rheology was employed to assess the thermal

stability of the gel derived from 2.DCHA in DMF. The gel−soldissociation temperatures (Tgel) at various concentrations of the

gelator were measured by dropping ball method22 and the Tgelversus [gelator] plot was examined (Figure 7). Analysis of the

plot revealed that Tgel steadily increased with the increase inconcentration of the gelators, indicating that the gel networkswere mainly governed by strong supramolecular interactionssuch as hydrogen bonding.To study the morphological features of the gel fibers of

2.DCHA, several microscopic experiments namely, scanningelectron microscopy (SEM), atomic force microscopy (AFM),and optical microscopy (OM) were carried out. (Figure 8).Highly entangled microthin fiber in OM as well as in thecorresponding SEM and AFM micrographs were observed. Itwas evidenced in SEM that several micrometer long fibbersform the 3D matrix wherein the solvent molecules wereunderstandably immobilized to form gel. The diameter of thesefibers was assigned to be 50 nm in AFM.It was curious to note that none of these gels as well as the

single crystals of the SAD salts showed the photoresponsiveness in the presence of ultraviolet light. The failureto undergo trans−cis isomerization of the azo moiety under UVirradiation led us to believe that there were not enough space inthe solid state that allowed such a huge movement as the 1DSAD chains in the crystals remained closely packed. Similar

Figure 6. Illustration of the single crystal structure of nongelator salt 2.DBuA. (a) Hydrogen bonding interactions forming 1D SAD synthon; (b)Parallel packing of the 1D networks.

Figure 7. Tgel vs [gelator] plots of 2.DCHA in DMF.

Crystal Growth & Design Article

dx.doi.org/10.1021/cg301245c | Cryst. Growth Des. 2012, 12, 5917−59245922

observation has also been reported by Hanabusa and co-workers.23

X-ray Powder Diffraction. To study as to what extent the1D SAD synthon is responsible for gelation, we have made anattempt to carry out structure−property correlation usingSXRD and PXRD data data. Near superimposable PXRDpatterns under various conditions (simulated, bulk and gel) of2.DCHAclearly established that the 1D SAD is not onlypresent in the bulk sample but also is the main supramolecularentity in the gel network as well (Figure 9). The bulk PXRD

patterns of other salts also matched quite well with that of thecorresponding simulated pattern establishing the fact that 1DSAD synthon is indeed present in the bulk sample in each salt.

■ CONCLUSIONSIn summary, we have prepared a series of secondaryammonium dicarboxylate salts derived from azobenzene-4,4′-dicarboxylic acid and azobenzene-4,4′-diacrylic acid withvarious secondary amines in 1:2 (acid:amine) molar ratio.Single crystal structures of six such salts revealed the presenceof the 1D SAD synthon. Two such salts (2.DCHA and2.DHA) displayed intriguing gelation ability. The structure−property correlation established the existence of 1D SADsynthon in the gel network of DMF gel of 2.DCHA. The factthat only two such salts out of eight salts synthesized displayedgelation ability indicates that much more efforts are needed tounderstand the crucial role played by gel-network/solventinteractions in the process of gelation.

■ EXPERIMENTAL SECTIONMethods. All of the chemicals (Aldrich) and solvents were (A.R.

grade, commercially available, India) used without any furtherpurification. Petrol used in the gelation experiments had beenpurchased from the local market. Microanalyses were performed ona Perkin-Elmer elemental analyzer 2400, Series II. FT-IR spectra wererecorded using Perkin-Elmer Spectrum GX. Powder X-ray patternswere recorded on XPERT Philips (CuKα radiation, λ = 1.5418 Å)Diffractometer. Scanning Electron Microscopy (FT-SEM) wasperformed on a JEOL; JSM-6700F. Optical microscopy was done inLeica MZ 16. AFM was performed by diCP-II, model FE-0100. SingleCrystal X-ray was done by BRUKER axs, SMART APEX II.

Preparation of Salts. Salts were prepared by reacting azobenzene-4,4′-dicarboxylic acid24 and azobenzene-4,4′-diacrylic acid with varioussecondary amines, namely dicyclohexylamine, dibenzylamine, cyclo-hexylmethylamine, and dihexylamine in 1:2 (acid:amine) molar ratioin methanolic medium. The resultant mixture was subjected tosonication for a few minutes to ensure the homogeneous mixing of thetwo components. An orange precipitate was obtained after completeremoval of MeOH by rotavapor, which were subjected to variousphysicochemical analyses and gelation test.

Single Crystal Preparation. Single crystals were grown frommethanol/methylsalicylate mixtures (∼25 mg of salt in ∼5 mLsolvents in 10 mL beaker) by slow evaporation at room temperature.Typically X-ray quality crystals were appeared after a few weeks.

Crystal Structures. Data were collected using MoKα (λ = 0.7107Å) radiation on a BRUKER APEX II diffractometer equipped withCCD area detector. Data collection, data reduction, structure solution/refinement were carried out using the software package of SMARTAPEX. All structures were solved by direct method and refined in aroutine manner. Nonhydrogen atoms were treated anisotropically,except the atoms where the disorder was observed. All of the hydrogenatoms were geometrically fixed. CCDC 819369 (1.DBuA), 819370(1.CHMA), 819371 (2.DBuA), 819372 (2.DCHA), 819373(2.CHMA), and 819374 (1.DCHA), contain the supplementarycrystallographic data for this paper. These data can be obtained freeof charge via www.ccdc.cam.ac.uk/conts/retrieving.html (or from theCambridge Crystallographic Data Centre, 12 Union Road, CambridgeCB21EZ, UK; fax: (+44) 1223−336−033; or deposit@ccdc.cam.ac.uk).

Photoirradiation. 1.0 mL gel was made in a pyrex glass vial andsubjected to UV irradiation at 350 nm in a Rayonet photoreactor for 3h.

Figure 8. Morphology of the gel fibers of 2.DCHA as observed in various microscopy; (a) AFM of 0.09 wt % DMSO xerogel gel; (b) height profilein AFM; (c) SEM of 0.88 wt % DMSO xerogel gel; and (d) OM of 0.88 wt % DMSO gel.

Figure 9. PXRD comparison plots of 2.DCHA at various conditions.

Crystal Growth & Design Article

dx.doi.org/10.1021/cg301245c | Cryst. Growth Des. 2012, 12, 5917−59245923

■ ASSOCIATED CONTENT*S Supporting InformationPhysico-chemical data for the salts, single-crystal X-ray data,molecular plots and hydrogen bonding parameters for thecompounds, and gelation data (Table S7). This material isavailable free of charge via the Internet at http://pubs.acs.org.

■ AUTHOR INFORMATIONCorresponding Author*E-mail: ocpd@iacs.res.in.NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSP.S. thanks IACS for a research fellowship and P.D. thanksCSIR, New Delhi for financial support.

■ REFERENCES(1) Molecular Gels. Materials with Self-Assembled Fibrillar Networks;Weiss, R. G., Terech, P., Eds.; Springer: Dordrecht, The Netherlands,2005.(2) (a) Dastidar, P. Chem. Soc. Rev. 2008, 37, 2699. (b) Piepenbrock,M.-O. M.; Lloyd, G. O.; Clarke, N.; Steed, J. W. Chem. Rev. 2010, 110,1960. (c) George, M.; Weiss, R. G. Acc. Chem. Res. 2006, 39, 489.(d) Hirst, A. R.; Miravet, J. E.; Escuder, B.; Noirez, L.; Castelletto, V.;Hamley, I. W.; Smith, D. K. Chem.Eur. J. 2009, 15, 372. (e) Sahoo,P.; Puranik, V. G.; Patra, A. K.; Sastry, P. U.; Dastidar, P. Soft Matter2011, 7, 3634. (f) Sahoo, P.; Krishnakumar, D.; Raghavan, S. R.;Dastidar, P. Chem. Asian J. 2011, 6, 1038. (g) South, A. B.; Lyon, L. A.Angew. Chem., Int. Ed. 2010, 49, 767. (h) Jadhav, S. R.; Vemula, P. K.;Kumar, R.; Raghavan, S. R. Angew. Chem., Int. Ed. 2010, 49, 7695.(3) (a) Kato, T. Science 2002, 295, 2414. (b) Ajayaghosh, A.;Praveen, V. K.; Vijayakumar, C.; George, S. J. Angew. Chem., Int. Ed.2007, 46, 6260. (c) Ajayaghosh, A.; Vijayakumar, C.; Praveen, V. K.;Babu, S. S.; Varghese, R. J. Am. Chem. Soc. 2006, 128, 7174.(4) (a) Murata, K.; Aoki, M.; Nishi, T.; Ikeda, A.; Shinkai, S. J. Chem.Soc. Chem. Commun. 1991, 1715. (b) de Jong, J. J. D.; Lucas, L. N.;Kellogg, R. M.; van. Esch, J. H.; Feringa, B. L. Science 2004, 304, 278.(5) Wynne, A.; Whitefield, M.; Dixon, A. J.; Anderson, S. J. Dermatol.Treat. 2002, 13, 61.(6) (a) van Bommel, K. J. C.; Friggeri, A.; Shinkai, S. Angew. Chem.,Int. Ed. 2003, 42, 980. (b) Basit, H.; Pal, A.; Sen, S.; Bhattacharya, S.Chem.Eur. J. 2008, 14, 6534. (c) Ray, S.; Das, A. K.; Banerjee, A.Chem. Commun. 2006, 2816.(7) Carretti, E.; Fratini, E.; Berti, D.; Dei, L.; Baglioni, P.Angew.Chem., Int. Ed. 2009, 48, 8966.(8) Sahoo, P.; Sankolli, R.; Lee, H.-Y.; Raghavan, S. R.; Dastidar, P.Chem.Eur. J. 2012, 18, 8057.(9) Lee, K. Y.; Mooney, D. J. Chem. Rev. 2001, 101, 1869.(10) (a) Yang, Z.; Liang, G.; Wang, L.; Xu, B. J. Am. Chem. Soc. 2006,128, 3038. (b) Muraoka, T.; Koh, C.-Y.; Cui H. (c) Stupp, S. I. Angew.Chem., Int. Ed. 2009, 48, 5946.(11) Rodriguez-Llansola, F.; Miravet, J. F.; Escuder, B. Chem.Commun. 2009, 7303.(12) van Esch, J. H. Langmuir 2009, 25, 8392.(13) Ostuni, E.; Kamaras, P.; Weiss, R. G. Angew. Chem., Int. Ed.1996, 35, 1324.(14) Luboradzki, R.; Gronwald, O.; Ikeda, M.; Shinkaia, S.;Reinhoudtc, D. N. Tetrahedron 2000, 56, 9595.(15) Desiraju, G. R. Angew. Chem., Int. Ed. Engl. 1995, 34, 2311.(16) (a) Desiraju, G. R. Crystal Engineering: The Design of OrganicSolids; Elsevier: Amsterdam and New York, 1989. (b) Desiraju, G. R.Angew. Chem., Int. Ed. 2007, 46, 8342.(17) (a) Trivedi, D. R.; Ballabh, A.; Dastidar, P.; Ganguly, B.Chem.Eur. J. 2004, 10, 5311. (b) Trivedi, D. R.; Ballabh, A.;Dastidar, P. J. Mater. Chem. 2005, 15, 2606. (c) Dastidar, P.; Okabe, S.;

Nakano, K.; Iida, K.; Miyata, M.; Tohnai, N.; Shibayama., M. Chem.Mater. 2005, 17, 741.(18) Trivedi, D. R.; Dastidar, P. Cryst. Growth Des. 2006, 6, 2114.(19) Trivedi, D. R.; Dastidar, P. Cryst. Growth Des. 2006, 6, 1022.(20) Ballabh, A.; Trivedi, D. R.; Dastidar, P. Crys. Growth Des. 2005,5, 1545.(21) Sahoo, P.; Krishna Kumar, D.; Trivedi, D. R.; Dastidar, P.Tetrahedron Lett. 2008, 49, 3052.(22) Raghavan, S. R.; Cipriano, B. H. InMolecular Gels. Materials withSelf-Assembled Fibrillar Networks; Weiss, G., Terech, P., Eds.; Springer:Dordrecht, The Netherlands, 2005; Chapter 8, p 241.(23) (a) Inoue, D.; Suzuki, M.; Shirai, H.; Hanabusa, K. Bull. Chem.Soc. Jpn. 2005, 78, 721. (b) Sahoo, P.; Chakraborty, I.; Dastidar, P. SoftMatter 2012, 8, 2595.(24) Jilani, J. European Pat. EP 1 688 413 A1Hikama PharmaceuticalsCo. Ltd, August, 09, 2006.

Crystal Growth & Design Article

dx.doi.org/10.1021/cg301245c | Cryst. Growth Des. 2012, 12, 5917−59245924