Positionally isomeric organic gelators: Structure-gelation study, racemic versus enantiomeric gelators, and solvation effects

DOI: 10.1002/chem.200902342

Positionally Isomeric Organic Gelators: Structure–Gelation Study, Racemic

versus Enantiomeric Gelators, and Solvation Effects

[a]

ˇ

Vesna Caplar, Leo Frkanec, Natasˇa Sijakovic´ Vujicˇic´, and Mladen Zinic´*

Abstract: Low molecular weight gela-

tor molecules consisting of aliphatic

acid, amino acid (phenylglycine), and

w-aminoaliphatic acid units have been

designed. By varying the number of

methylene units in the aliphatic and w-

aminoaliphatic acid chains, as defined

by descriptors m and n, respectively, a

series of positionally isomeric gelators

having different positions of the pepti-

dic hydrogen-bonding unit within the

gelator molecule has been obtained.

The gelation properties of the position-

al isomers have been determined in re-

lation to a defined set of twenty sol-

vents of different structure and polarity

and analyzed in terms of gelator versa-

tility (G_ver) and effectiveness (G_eff).

The results of gelation tests have

shown that simple synthetic optimiza-

tions of a “lead gelator molecule” by

variation of m and n, end-group polari-

ty (carboxylic acid versus sodium car-

boxylate), and stereochemistry (race-

mate versus optically pure form) al-

lowed the identification of gelators

with tremendously improved versatility

(G_ver) and effectiveness (G_eff). Dramat-

ic differences in G_effvalues of up to 70

times could be observed between pure

racemate/enantiomer pairs of some ge-

lators, which were manifested even in

the gelation of very similar solvents

such as isomeric xylenes. The combined

results of spectroscopic (¹H NMR,

FTIR), electron microscopy (TEM),

and X-ray diffraction studies suggest

similar organization of the positionally

isomeric gelators at the molecular

level, comprising parallel b-sheet hy-

drogen-bonded primary assemblies that

form inversed bilayers at a higher or-

ganizational level. Differential scan-

ning calorimetry (DSC) studies of se-

lected enantiomer/racemate gelator

pairs and their o- and p-xylene gels re-

vealed the simultaneous presence of

different polymorphs in the racemate

gels. The increased gelation effective-

ness of the racemate compared to that

of the single enantiomer is most likely

a consequence of its spontaneous reso-

lution into enantiomeric bilayers and

their subsequent organization into

polymorphic aggregates of different

energy. The latter determine the gel

fiber thickness and solvent immobiliza-

tion capacity of the formed gel net-

work.

Keywords:

enantiomers

·

fatty acids · gelation · racemates ·

self-assembly

Introduction

neering, food processing, and pharmacology.^[2]Organogela-

tors are molecules capable of self-assembling into long, fi-

brous structures of nanosized diameters through highly spe-

cific noncovalent interactions such as hydrogen bonding,

van der Waals, p-stacking, electrostatic, and charge-transfer

interactions.^[3]Noncovalent cross-links between the nanofib-

ers and/or mechanical entanglements create a three-dimen-

sional network, which entraps the solvent inside the intersti-

ces leading to gelation and loss of fluidity of the system.

Hence, the observation of gelation indicates the occurrence

of the self-assembly of gelator molecules and represents a

macroscopic manifestation thereof. The basic structural re-

quirement for a molecule to exhibit gelation properties is its

self-complementarity and capability of undergoing unidirec-

tional self-assembly into fibrous aggregates instead of disso-

lution or crystallization.^[3]However, despite the considerable

advances in the understanding of gelation as a supramolec-

The gelation of water and various organic fluids by low mo-

lecular weight organic gelators, giving in most cases thermal-

ly reversible gels, has emerged as a fascinating supramolec-

ular phenomenon.^[1]The topic has received much attention

due to potential applications in many areas, such as in the

preparation of viscoelastic materials, the cosmetics industry,

the design of drug delivery systems and sensors, tissue engi-

ˇ

ˇ ´

´

[a] Dr. V. Caplar, Dr. L. Frkanec, N. S. Vujicic, Prof. Dr. M. Zinic

Division of Organic Chemistry and Biochemistry

Laboratory for Supramolecular and Nucleoside Chemistry

Ruder Boskovic Institute, P.O.B. 180, 10002 Zagreb (Croatia)

¯

ˇ

´

Fax : (+385)1-4680-195

E-mail: zinic@irb.hr

Supporting information for this article is available on the WWW

under http://dx.doi.org/10.1002/chem.200902342.

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ular phenomenon, it is still scarcely possible to predict its

appearance on the basis of structural characteristics of a

candidate molecule and it is even more difficult to predict

which solvents might be gelled and with what degree of effi-

ciency.^[4]Hence, structure–gelation relationship studies

based on systematic structural variations of a selected “lead

gelator molecule”, followed by a determination of gelation

properties, the morphological characteristics of the gel

fibers, and the self-assembly motifs of its structural variants,

would seem necessary. Such studies may provide a deeper

understanding of the gelation mechanism and also shed

more light on the role of the solvent as the second supra-

molecular partner of each gelation system.^[3a,5]

group polarity could be altered by transformation of the car-

boxylic acid into sodium carboxylate, and the influence of

stereochemistry could be assessed by comparing the gelation

properties of the racemates and optically pure forms.

In our earlier work on low molecular weight gelators of

water and organic solvents, we

prepared chiral gelators con-

structed from 11-aminoundeca-

noic, lauric, and amino acid

units.^[6]This combination of

structural fragments was as-

sumed to be favorable for gela-

tion.

Indeed, some of the representatives of this series in the

free-acid or sodium carboxylate forms exhibited excellent

gelation properties, being capable of gelling both highly

polar solvents (water, DMSO) as well as highly lipophilic

solvents, including some common hydrocarbon fuels (gaso-

line, diesel). Spectroscopic studies revealed that intermolec-

ular hydrogen-bonding interactions between the peptidic

and carboxylic acid units are involved in the stabilization of

the gel assemblies. For gelators with terminal sodium car-

boxylate groups, an additional strong stabilization of the ag-

gregates was attributed to the electrostatic and ion-dipole

interactions between the carboxylate moieties and the

sodium cations.^[6d]

As a further step in the investigation of this class of versa-

tile gelators, we decided to study their structure–gelation re-

lationships by preparing a set of isomers with variable

lengths of the terminal and carboxylic acid methylene

chains. The chain lengths are defined by descriptors m and

n, which correspond to the number of methylene units in

the terminal and carboxylic acid chains, respectively. Keep-

ing the sum of m + n constant at 20, a set of positionally

isomeric gelators was obtained, having the same hydrophil-

ic/lipophilic balance (ratio of lipophilic and polar groups in

the gelator molecule), but with variable position of the core

Figure 1. Positionally isomeric carboxylic acid gelators with varying num-

bers of methylene units m and n in their terminal and carboxylic acid

alkyl chains, respectively. The isomers are denoted as m-n-H or m-n-Na

using the m and n descriptors.

In this work, we report on the synthesis of six positionally

isomeric free-acid and sodium salt gelators in racemic and

optically pure forms and present the results of a compara-

tive gelation study employing a selected set of twenty sol-

vents of different structures and polarities. The gelation

properties of each positional isomer have been analyzed in

terms of its versatility (G_ver) and effectiveness (G_eff). G_verin-

dicates the number of different solvents from the defined

set that could be gelled by the tested positional isomer,

which in turn describes its capacity to gel solvents of differ-

ent structure and polarity. G_effis expressed in mL of the

tested solvent and shows how great a volume could be im-

mobilized by 10 mg of the tested positional isomer. Hence,

the G_effvalues reveal the gelation capacity of each positional

isomer toward the selected solvent.

À

-CONH CHPhCONH- hydrogen-bonding unit within the

main chain of the gelator molecule (Figure 1). Such posi-

tionally isomeric gelators are expected to self-assemble by

using the same type of intermolecular forces (H-bonding

and lipophilic alkyl chain interactions); however, the varia-

tions of alkyl chain lengths (m and n) may strongly influence

their gelation properties through affecting the strength of in-

termolecular interactions, solubility in various solvents and

their organization at the supramolecular level which could

be reflected in different fiber morphology and thermal prop-

erties of gels. Besides the variation of m and n, the end-

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M. Zinic et al.

Herein, we demonstrate that simple synthetic optimiza-

tions of a “lead gelator molecule” by the variation of alkyl

chain lengths (m and n), end-group polarity (carboxylic acid

versus sodium carboxylate), and stereochemistry (racemate

versus optically pure form) can lead to the identification of

gelators with tremendously improved versatility (G_ver) and

effectiveness (G_eff). The described gelator optimization ap-

proach may be of great benefit in the search for highly effi-

cient gelators of a targeted solvent. We also show that dra-

matic differences in G_effvalues of up to 70 times could be

observed even in the gelation of very similar solvents such

as isomeric xylenes. The combined results of spectroscopic

(¹H NMR, FTIR), electron microscopy (TEM), X-ray dif-

fraction, and differential scanning calorimetry (DSC) studies

suggest that the observed G_effdifferences are not the conse-

quence of different organization of the positional isomers at

the molecular level, which was found to be very similar in

each case, but more likely the consequence of different or-

ganization at the supramolecular level, affected by specific

solvation effects, the latter depending even on the solvent

structure, as found in the gelation of isomeric xylenes.

The methyl ester of 16-aminohexadecanoic acid was pre-

pared by initial esterification of 16-bromohexadecanoic acid.

Subsequent substitution of the bromo substituent by an

azido group and hydrogenolysis of the azido derivative gave

methyl w-aminohexadecanoate, which was used in the prep-

aration of compounds with m=5, n=15. N-Heptanoylphe-

nylglycine was prepared by acylation of phenylglycine with

heptanoyl chloride and then coupled with methyl 16-amino-

hexadecanoate under activation by Ph₃P, CCl₄, and Et₃N

(Scheme 2). Subsequent alkaline hydrolysis of the ester gave

the sodium salt 15–5-Na and after acidification the free acid

15–5-H.

Results and Discussion

Scheme 2. a) CH₃OH,

H₂SO₄;

b) NaN₃/MeCN;

c) H₂-Pd/MeOH;

d) NaOH/H₂O; e) Ph₃P, Et₃N, CCl₄/MeCN; f) NaOH, HCl; g) NaOH.

Synthesis: A set of candidate gelator molecules was pre-

pared, each containing a core racemic or optically active

phenylglycine fragment, by varying the number of methyl-

ene units in the terminal (m) and interposed (n) aliphatic

chains (Figure 1). The general synthetic route involved cou-

pling of an N-protected racemic or (R)-phenylglycine reac-

tive ester with a selected w-aminoalkyl carboxylic acid

methyl ester, followed by deprotection and acylation of the

phenylglycine alkylamide with an appropriate acyl chloride

(Scheme 1). The obtained methyl esters were hydrolyzed to

free acids or sodium salts. Methyl esters of glycine, 4-amino-

butyric acid, and 6-aminohexanoic acid are commercially

available; methyl esters of 8-aminooctanoic acid and 11-ami-

noundecanoic acid were prepared by esterification of the

corresponding acids using MeOH and SOCl₂.

In the case of the long terminal alkyl chain derivative 19–

1-H, the m=19 fragment was prepared starting from 1-bro-

moeicosane, the bromo substituent of which was replaced

by cyanide and then the cyano derivative was hydrolyzed to

heneicosanoic acid (Scheme 3). The synthetic route using

methyl phenylglycylglycinate for the preparation of the phe-

nylglycylglycine derivative (n=1) failed due to formation of

the respective diketopiperazine. Therefore, the glycine frag-

ment had to be introduced in the last step. Heneicosanoic

acid was activated by succinimide ester formation and cou-

pled with phenylglycine methyl ester; this intermediate was

hydrolyzed and finally coupled with glycine methyl ester

under activation with Ph₃P, CC l₄, and Et₃N. The resulting

ester was hydrolyzed to give the corresponding free-acid

and sodium salt derivatives.

Gelation properties of racemic

positionally isomeric carboxylic

acid gelators: Racemic free-

acid derivatives 19–1-H, 17–3-

H, 15–5-H, 13–7-H, 10–10-H,

and 5–15-H were tested with

regard to their gelation proper-

ties toward water and selected

polar and apolar organic sol-

vents (Table 1 and Figure 2).

However, all of the tested free-

acid gelators were insoluble in

water so that no hydrogelation

Scheme 1. a) Et₃N/dioxane; b) TFA/CH₂Cl₂; c) Et₃N/CH₂Cl₂; d) NaOH, H⁺; e) NaOH.

was observed. Only the posi-

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using the public domain software ALOGPS 2.1.^[7]The aver-

age logP values showed all of the positionally isomeric gela-

tors to be highly lipophilic compounds, albeit with slightly

different logP values that decrease with decreasing m and

increasing n descriptors (19–1-H: 8.39, 17–3-H: 8.16, 15–5-

H: 8.05, 13–7-H: 8.04, 10–10-H: 8.03, and 5–15-H: 7.93).

Hence, the more lipophilic positional isomers with m=19,

17, 15, and 13 appear to be insufficiently soluble in highly

polar and hydrogen-bond competitive solvents (water,

DMSO, DMF, EtOH, acetone, EtOAc) or tend to crystallize

from hot solutions, presumably due to inefficient solvation

of their long terminal alkyl chains. In addition, the extensive

gelator–solvent hydrogen bonding competes with the self-as-

sembly process, so that crystallization becomes a favored

Scheme 3. a) KCN/DMF, b) 50% H₂SO₄; c) HOSu, DCC/dioxane;

d) Et₃N/dioxane; e) NaOH/MeOH, H⁺; f) Ph₃P, Et ₃N, CCl₄/MeCN;

g) NaOH, MeOH, H⁺; h) NaOH.

tional isomers 17–3-H and 15– Table 1. Gelation efficiencies (G_eff, mL) and MGC values (in parentheses; 10^À3moldm^À3) of racemic acid gela-

tors 19–1-H, 17–3-H, 15–5-H, 13–7-H, 10–10-H, and 5–15-H.

5-H with longer terminal alkyl

Solvent

19–1-H

17–3-H

15–5-H

13–7-H

10–10-H

5–15-H

chains (m=17, 15) gelled small

volumes of DMSO to give

milky opaque gels, while the

other isomers were soluble.

DMF and EtOH were not

gelled, nor were other solvents

of medium polarity. It was ob-

served that 17–3-H crystallized

from acetone, ethyl acetate,

acetonitrile, and cyclohexane to

produce a network of very long,

microcrystalline fibers resem-

bling cotton-wool, which was

capable of trapping large vol-

umes of solvents. The results of

gelation experiments collected

in Table 1 show that some of

the investigated positional iso-

mers exhibited excellent gela-

tion of aromatic solvents, deca-

lin, and two common hydrocar-

bon fuels, while others were in-

water

DMSO

DMF

ins.

t.v.s.

cr.

ins.

cr.

ins.

fl.

ins.

fl.

cr.

ins.

sol.

cr.

ins.

sol.

sl. sol.

cr.

0.7^[a](27.6)

cr.

0.5^[a](38.7)

fl.

EtOH

cr.

fl.

cr.

sol.

ins.

cr.

acetone

EtOAc

MeCN

THF

CH₂Cl₂

CCl₄

cyclohexane

benzene

toluene

o-xylene

m-xylene

p-xylene

tetralin

decalin

gasoline

diesel

c.w. (6.45)

c.w. (6.91)

cr.

sol.

emuls.

sol.

cr.

sol.

–

sol.

w.

t.v.s.

ins.

–

ins.

sl. sol. fl.

c.w. (9.68)

11.8^[a](1.64)

8.5^[b](2.28)

26.7^[b](0.72)

15.0^[b](1.19)

5.8^[b](3.34)

10.2^[b](1.90)

fl.

4.4^[a](4.40)

3.5^[a](5.53)

5.9^[a](3.28)

6.0^[b](3.22)

14.7^[b](1.32)

4.5^[b](4.30)

2.6^[a](7.44)

11.6^[a](1.67)

2.0 (9.68)^[b]

sol.

cr.

t.v.s.

ins.

31.5^[b](0.61)

9.3^[b](2.08)

1.4^[a](13.8)

1.8^[a](10.7)

8.6^[b](2.25)

fl.

w.

–

ins.

32.2^[b]

w.

fl.

2.0^[a](9.68)

1.0^[a](19.3)

ins.

[a] Colorless opaque gel. [b] Colorless transparent gel. Abbreviations: t.v.s. =turbid viscous solution, fl.=floc-

culent precipitate, cr.=crystals, ins.=insoluble, sol.=soluble, w.=wax-like precipitate, c.w.=cotton wool-like

precipitate.

capable of gelling any of the twenty tested solvents. The po-

sitional isomers 5–15-H, 10–10-H, and 13–7-H with shorter

terminal chains (m=5, 10, and 13) and longer carboxylic

acid chains (n=15, 10, and 7) lacked gelation ability toward

both polar and apolar solvents, although 13–7-H was capa-

ble of gelating a very small volume of tetralin (Table 1, Ex-

perimental Section, and Figure 2).

Racemic 17–3-H and 15–5-H (G_ver=9 and 10) appeared

to be the most versatile gelators among the tested positional

isomers, being capable of gelling nine and ten of the twenty

solvents tested, including two common hydrocarbon fuels

(gasoline, diesel). The isomer 19–1-H, with the longest ter-

minal alkyl chain (m=19) and the shortest carboxylic acid

chain (n=1), proved to be much less versatile, displaying a

G_vervalue of only 5. To account for the observed differences

in G_verbetween the positional isomers, their relative lipophi-

licities were estimated by calculation of average logP values

process. The presence of shorter terminal chains in 10–10-H

and 5–15-H (m=10, 5) enables their dissolution in highly

polar DMSO and DMF or results in crystallization from sol-

utions in less polar and apolar solvents. However, apolar sol-

vents are capable of efficiently competing with the self-as-

sembly of 10–10-H and 5–15-H due to weaker intermolecu-

lar lipophilic interactions between their shorter alkyl chains.

The more lipophilic isomers 19–1-H, 17–3-H, and 15–5-H

are sufficiently soluble in lipophilic aromatic solvents, deca-

lin, and hydrocarbon fuels, which also favor strong intermo-

lecular hydrogen bonding, so that the self-assembly leading

to gelation is strongly favored.

It can be noted, however, that 19–1-H, the most lipophilic

of the positional isomers, is less versatile (G_ver=5) than 17–

3-H and 15–5-H (G_ver=9 and 10), both of which are slightly

less lipophilic than 19–1-H but capable of gelling a wider

range of lipophilic aliphatic and aromatic solvents and even

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M. Zinic et al.

solubilities in toluene, they cannot account for the different

G_effvalues. It is particularly surprising that 19–1-H, 17–3-H,

and 15–5-H also exhibited very different G_effvalues in the

gelation of the isomeric xylenes, with these solvents having

very similar physical properties. For o-xylene, the obtained

G_effvalues were: 19–1-H: 9.3 mL, 17–3-H: 26.7 mL, and 15–

5-H: 5.9 mL, making 17–3-H the most efficient gelator of

this solvent. The same gelator also proved to be the most ef-

ficient in the gelation of m-xylene. However, 15–5-H

showed a much higher G_effvalue (14.7 mL) for p-xylene

compared to those of 19–1-H (G_eff=1.8 mL) and 17–3-H

(G_eff=5.8 mL).

Gelation of isomeric xylenes: Recently, Smith and Mirave-

t^[5a]showed that the efficacies of members of their class of

toluene gelators were dependent on the solubility of gelator

assemblies in this solvent. They presented convincing evi-

dence that the aggregation behavior of these gelators is co-

operative^[9]and that the alternative isodesmic model,^[10]in

which there is a continuous distribution of monomers and

oligomers, is not valid. Therefore, gelator molecules are dis-

tributed between a solid-like gel network (c_crit), in which

Figure 2. Gelation efficiencies, G_eff, of racemic 19–1-H, 17–3-H, 15–5-H,

and 13–7-H for selected solvents.

1

they cannot be observed by H NMR, and dissolved NMR-

a small volume of highly polar DMSO. It should also be

noted that 15–5-H, 13–7-H, 10–10-H, and 5–15-H, despite

having very similar average logP values, displayed very dif-

ferent G_vervalues (15–5-H: G_ver=10, 13–7-H: G_ver=1, 10–

10-H and 5–15-H: no gelation). Hence, there is no simple

relationship between the lipophilicity of a gelator and its

versatility. Spectroscopic and powder XRD investigations

(vide infra) suggest similar organization of the positionally

isomeric gelators at the molecular level, forming assemblies

stabilized by intermolecular hydrogen bonding and lipophil-

ic interactions between aliphatic chains. As recently report-

ed by van Esch et al.,^[8]the contributions of lipophilic and

hydrogen-bonding interactions to the overall stabilization of

gel assemblies are different in solvents of different polari-

ties. In polar and hydrogen-bonding competitive solvents,

lipophilic interactions usually provide the dominant contri-

bution, while in lipophilic solvents hydrogen bonding pro-

vides the major stabilizing contribution. Hence, it can be

concluded that the versatility of a gelator depends in part

on gelator–solvent interactions that determine its solubility

in solvents of different polarity, but also on the interplay of

contributions from lipophilic and hydrogen-bonding interac-

tions, which should provide sufficient stability of the assem-

blies in solvents of different polarities.

Figure 2 shows that the racemic positionally isomeric gela-

tors exhibit large differences in gelled volumes of the same

solvent, defined here as gelator effectiveness (G_eff). Toluene

is by far the most efficiently gelled by the most lipophilic

19–1-H (G_eff=31.5 mL) compared to the less lipophilic 17–

3-H (G_eff=8.5 mL) and 15–5-H (G_eff=3.5 mL); thus, 19–1-H

is a four and nine times more efficient gelator of toluene

than the positionally isomeric 17–3-H and 15–5-H, respec-

tively. Since the similar average logP values calculated for

the positional isomers imply that they should have similar

observable gelator assemblies (c_diss) present in the immobi-

lized solvent. Hence, the experimentally determined mini-

mal gelation concentration (MGC) actually represents the

sum of c_critand c_diss. When c_dissis equal to zero, c_critcorre-

sponds to the minimal gelator concentration sufficient to im-

mobilize the relevant volume of a solvent. It was also shown

that the saturation point in the gel-to-sol transition diagrams

was determined by c_diss, as our group found earlier for other

types of gelators.^[11]Hence, for gels containing a significant

concentration of dissolved gelator assemblies (significant

c_diss), the experimentally determined MGC is higher to com-

pensate for these. According to these results, higher experi-

mentally determined MGC values should correspond to

lower gelator effectiveness G_effdue to higher solubility of

gelator assemblies (c_diss). In an attempt to explain the large

differences in G_efffound for 19–1-H, 17–3-H, and 15–5-H in

the gelation of isomeric xylenes as a consequence of differ-

ent c_diss, we assumed that the solubilities of the gelator as-

semblies should depend on the relative lipophilicities of the

positionally isomeric gelators. Hence, the most lipophilic ge-

lator should also give the most soluble assemblies (the high-

est c_diss) in the specified solvent and consequently should be

the least effective gelator.^[12]The relationship between the

calculated average logP values of the gelators and the ex-

perimentally determined G_effvalues for toluene and isomer-

ic xylene gels is shown in Figure 3. In marked contrast to

the above predictions, 19–1-H, as the most lipophilic gelator

(average logP 8.39), showed the highest G_effof 31.5 mL in

the gelation of toluene compared to the values for the less

lipophilic 17–3-H and 15–5-H. The relationships between

the lipophilicities of 19–1-H, 17–3-H, and 15–5-H and their

G_effvalues determined for o-, m-, and p-xylene gels are also

at variance with the prediction that the highest G_effshould

be observed for the least lipophilic gelator, this only being

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Gelation of Solvents with Isomeric Organic Gelators

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are stronger for smaller network compartments. Conse-

quently, the specific solvation effects exhibited by structural-

ly different isomeric xylenes may stem from the formation

of fibers of different morphology, which in turn give net-

works of variable density having different gelation capaci-

ties. It can be concluded that gelator effectiveness may in

some cases be dominated by the solubility of gelator assem-

blies,^[5a]as well as by specific solvation effects determining

the fiber and network morphology and ultimately the sol-

vent immobilization capacity.

Carboxylic acid versus sodium salt gelators: Table 2 shows

the results of gelation studies with racemic sodium salt gela-

tors 19–1-Na, 17–3-Na, 15–5-Na, 13–7-Na, 10–10-Na, and 5–

15-Na and the same set of twenty different solvents. Gener-

ally, the salts gelled a similar set of solvents as the acid-type

gelators. Ethanol and other solvents of medium polarity

were not gelled by the salts nor by the acids. Considering ge-

lator versatility (G_ver), 19–1-Na appeared to be much less

versatile (G_ver=2) compared to its free-acid counterpart

(19–1-H, G_ver=5), but, in contrast, the salt was capable of

gelling a small volume of DMSO, presumably due to its

somewhat better solubility in this solvent. G_vervalues

Figure 3. Graphical presentation of G_effvalues determined for toluene,

and o-, m-, and p-xylene gels of 19–1-H, 17–3-H, and 15–5-H, with their

average logP values indicated by arrows.

true for p-xylene gels, in which case the most lipophilic 19–

1-H showed the lowest effectiveness.

(Tables 1 and 2) of 17–3-Na (G_ver=11) and 15–5-Na (G_ver

=

Apparently, the observed large G_effdifferences among the

positionally isomeric gelators in the gelation of toluene and

o-, m-, and p-xylenes, solvents of very similar lipophilicities,

cannot be explained by considering only the solubility of the

gelator assemblies. Rather, the observed gelator effective-

ness seems to be dominated by specific solvation effects in-

volved in the self-assembly processes at different levels of

supramolecular organization. Recently, Meijer et al.^[3a]pre-

sented convincing evidence that co-organization of solvent

at the periphery of the gel aggregates plays a direct role in

the assembly process, even

9) are similar to those of the free-acid gelators 17–3-H (9)

and 15–5-H (10) and both types of gelators showed prefer-

ential gelation of lipophilic aromatic solvents. However, in

contrast to 13–7-H and 10–10-H, which showed practically

no gelation, 13–7-Na and 10–10-Na showed increased gela-

tion ability towards aromatic solvents and decalin, giving

G_vervalues of 5 and 3, respectively.

Distinct differences in gelator effectiveness (G_eff) between

the salts and the corresponding acids were observed in the

gelation of selected apolar solvents. Figure 4 enables com-

during the formation of pre-ag-

gregates. The influence of sol-

vent structure on the length of

the aggregates was clearly dem-

onstrated.

Table 2. Gelation efficiencies (G_eff, mL) and MGC values (in parentheses; 10^À3moldm^À3) of sodium salt gela-

tors 19–1-Na, 17–3-Na, 15–5-Na, 13–7-Na, 10–10-Na, and 5–15-Na.

Solvent

19–1-Na

17–3-Na

15–5-Na

13–7-Na

10–10-Na

5–15-Na

water

DMSO

DMF

t.v.s.

2.0^[a](9.28)

fl.

t.v.s.

fl.

cr.

1.0^[a](18.6)

2.0^[a](9.28)

cr.

1.4^[a](13.3)

fl.

cr.

0.8 (23.2)

1.1 (16.9)

0.6 (30.9)

cr.

t.v.s.

fl.

0.8^[a](23.2)

ins.

Hence, in the case of the iso-

meric xylenes, which have very

similar physical properties, the

structure of the solvent is likely

to influence the self-assembly

and ultimately determine the

gel fiber morphology.^[13]Evi-

dence has also been provided

that a gel network consisting of

thinner and flexible fibers has

higher solvent immobilization

capacity than a network com-

posed of thick and rigid

fibers.^[14]This can be explained

by the fact that the immobiliza-

tion of solvent is predominantly

EtOH

cr.

acetone

EtOAc

MeCN

THF

ins.

cr.

ins.

fl.

ins.

CH₂Cl₂

CCl₄

ins.

t.v.s.

2.1^[b](8.84)

sol.

t.v.s.

fl.

ins.

–

ins.

w.

16.1 (1.15)

12.0 (1.55)

18.0 (1.03)

ins.

0.9^[a](20.6)

4.4^[a](42.2)

5.3^[b](35.0)

2.0^[b](9.28)

2.2^[b](8.44)

2.1^[b](8.84)

20.6^[b](0.90)

7.9^[b](2.35)

1.5^[b](12.4)

2.1^[b](8.84)

3.4^[b](5.46)

cyclohexane

benzene

toluene

o-xylene

m-xylene

p-xylene

tetralin

decalin

gasoline

diesel

6.8^[b](2.73)

5.8^[b](3.20)

2.1^[b](8.84)

4.5^[b](4.12)

8.2^[b](2.26)

17.0^[b](1.09)

ins.

0.7^[b](26.5)

1.6^[b](11.6)

4.3^[b](4.32)

7.3^[b](2.54)

0.7^[a](24.7)

ins.

1.2^[b](15.5)

w.

ins.

t.v.s.

w.

2.8^[b](6.63)

ins.

[a] Colorless opaque gel, [b] Colorless transparent gel. Abbreviations: t.v.s. =turbid viscous solution, fl.=floc-

due to capillary forces, which culent precipitate, cr.=crystals, ins.=insoluble, sol.=soluble, w.=wax-like precipitate.

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M. Zinic et al.

parison of the G_effvalues of the free-acid and sodium salt

gelators. The acid gelator 19–1-H (light-blue bar) displayed

a 15 times higher G_eff(31.5 mL) for toluene than the corre-

sponding sodium salt (19–1-Na, red bar, G_eff=2.1 mL).

Optically pure versus racemic gelators in the gelation of iso-

meric xylenes: The influence of stereochemistry on the gela-

tion properties of chiral gelators is very well documented.^[15]

In the majority of the published results, the pure enantiomer

was seen to be a more effective

gelator than the racemate, al-

though several exceptions have

also been reported.^[14]However,

symmetrical meso forms of vari-

ous gelators have as a rule been

found to lack any gelation abili-

ty.^[15d,16]Since racemic 17–3-H,

17–3-Na, 15–5-H, and 10–10-Na

exhibited surprisingly different

G_effvalues in the gelation of

isomeric xylenes, we decided to

investigate whether these prop-

erties were a consequence of

their positional isomerism or a

result of their stereochemical

form. We prepared their opti-

Figure 4. Comparison of gelator effectiveness (G_eff) values of free-acid gelators 19–1-H, 17–3-H, 15–5-H, and

13–7-H (10–10-H and 5–15-H are not shown due to their lack of gelation ability) and the corresponding

sodium salts 19–1-Na, 17–3-Na, 15–5-Na, 13–7-Na, 10–10-Na, and 5–15-Na.

cally active forms and com-

pared the gelling properties of

the enantiomers and racemates

toward isomeric xylenes. The

Thus, transformation of the carboxylic acid end-group of

19–1-H to the more polar sodium carboxylate of 19–1-Na re-

sults in a dramatic decrease in the gelation effectiveness of

the latter toward toluene, presumably due to its lower solu-

bility. Comparison of the G_effvalues for 17–3-H (yellow bar)

and 17–3-Na (light-blue bar) shows that the former is much

more efficient in the gelation of o- and m-xylenes (17–3-H:

optically active free-acid and sodium salt derivatives were

prepared starting from (R)-phenylglycine following the reac-

tion sequences outlined in Scheme 1.

Figures 5a and b compare the G_effvalues of rac- and (R)-

free-acid and sodium salt gelators determined for isomeric

xylenes and reveal dramatic differences in some cases.

While rac-17–3-H showed high effectiveness in the gelations

of o-xylene (G_eff=26.7 mL) and m-xylene (G_eff=15.0 mL),

it was only capable of gelling a much smaller volume of p-

xylene (G_eff=5.8 mL). In contrast, the (R)-enantiomer

showed similarly high degrees of effectiveness toward all

three isomeric xylenes (G_efffor o-, m-, and p-xylenes: 19.3,

22.2, and 19.0 mL, respectively), thus being a better gelator

than the racemate for the latter two solvents (Table 3 and

G

_eff=26.7 and 15.0 mL; 17–3-Na: G_eff=2.2 and 2.1 mL, re-

spectively), while the salt gelled p-xylene much more effi-

ciently (17–3-Na: _eff=20.6 mL; 17–3-H: _eff=5.8 mL).

G

Hence, the simple end-group transformation can in some

cases dramatically change the gelator effectiveness, even

toward very similar solvents such as o- and p-xylenes. Large

differences in G_effwere also observed for 15–5-H/15–5-Na in

the gelation of p-xylene and decalin (Table 2, Figure 4).

While 10–10-H was found to be incapable of gelling p-

xylene, tetralin, and decalin, 10–10-Na exhibited very effi-

cient gelation of these solvents, giving G_effvalues of 16.1,

12.0, and 18.0 mL, respectively.

Table 3. Gelation efficiencies (G_eff, mL) and MGC values (in parenthe-

ses; 10^À3moldm^À3) of racemic and optically active acids and sodium salts

in the gelation of isomeric xylenes.

o-Xylene

m-Xylene

p-Xylene

As in the case of some acid-type gelators, large differen-

ces in gelator effectiveness, particularly of 17–3-Na toward

o-, m-, and p-xylenes (G_eff=2.2, 2.1, and 20.6 mL, respec-

tively), were observed, showing that this gelator is capable

of immobilizing a tenfold larger volume of p- than of o- or

m-xylene. The results of this gelation study show that trans-

formation of the carboxylic acid end-group to the sodium

carboxylate in these positionally isomeric gelators may gen-

erate a much more effective gelator for specific solvents

(compare 17–3-Na/17–3-H for gelling p-xylene; 15–5-Na/15–

5-H for gelling decalin, and 10–10-Na efficiently gelling p-

xylene, tetralin, and decalin in contrast to non-gelling 10–10-

H).

17–3-H

(R)-17–3-H

15–5-H

(R)-15–5-H

10–10-H

(R)-10–10-H

17–3-Na

(R)-17–3-Na

15–5-Na

(R)-15–5-Na

10–10-Na

(R)-10–10^[6c]

26.7 (0.72)^[b]

19.3 (1.00)^[a]

5.9 (3.28)^[a]

0.5 (38.7)^[a]

cr.

15.0 (1.29)^[b]

22.2 (0.87)^[b]

6.0 (3.22)^[b]

0.8 (24.2)^[a]

cr.

5.8 (3.34)^[b]

19.0 (1.02)^[b]

14.7 (1.32)^[b]

0.2 (96.8)^[a]

cr.

0.2 (96.8)^[a]

2.2 (8.44)^[b]

1.3 (14.3)^[b]

5.8 (3.20)^[b]

v.s.

0.2 (96.8)^[a]

2.1 (8.84)^[b]

1.8 (10.3)^[b]

2.1 (8.84)^[b]

17.0 (1.09)^[b]

v.s.

0.6 (32.2)^[a]

20.6 (0.90)^[b]

1.2 (15.5)^[a]

4.5 (4.12)^[b]

16.8 (1.10)^[b]

16.1 (1.15)

1.0 (18.6)

v.s.

[a] Colorless opaque gel. [b] Colorless transparent gel. Abbreviations:

cr.=crystals, w.=wax-like precipitate, v.s.=viscous solution.

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Gelation of Solvents with Isomeric Organic Gelators

FULL PAPER

Figure 5a). Very large differences in G_effwere also found for

racemic and optically active 15–5-H; racemic 15–5-H gelled

moderate volumes of o-xylene (G_eff=5.9 mL) and m-xylene

times more effective gelator of p-xylene than its (R)-enan-

tiomer. In contrast, racemic 15–5-Na gelled only moderate

volumes of isomeric xylenes (G_effvalues for o-, m-, and p-

xylenes: 5.8, 2.1, and 4.5 mL, respectively), while (R)-15–5-

(G_eff=6.0 mL) and a large volume of p-xylene (G_eff

=

14.7 mL), while (R)-15–5-H proved to be a very poor gelator

of all three solvents (G_effvalues for o-, m-, and p-xylenes:

0.5, 0.8, and 0.2 mL, respectively). In the latter case, the

racemate was seen to be a much more effective gelator for

all three isomeric xylenes than the (R)-enantiomer. Indeed,

considering only the gelation of p-xylene, the racemate ex-

hibited a G_effvalue more than 70 times higher than that of

the (R)-enantiomer.

Na exhibited excellent gelation of m- and p-xylenes (G_eff=

17.0 and 16.8 mL, respectively), but only gelled a moderate

volume of o-xylene (G_eff=5.8 mL). Racemic 10–10-Na

showed excellent gelation of p-xylene (G_eff=16.1 mL) but

gave waxy precipitates with the other two xylenes; in con-

trast, its (R)-enantiomer proved to be capable of only weak

gelation of p-xylene (G_eff=1.0 mL). These results clearly

show that the effectiveness of a gelator in the gelation of

even very similar solvents can be highly dependent on its

stereochemical form. Hence, in searching for highly efficient

gelators for a targeted solvent, both the racemic and optical-

ly pure forms of each gelator should be prepared and tested.

As shown in Figure 3, the G_effvalues of the racemic 19–1-

H, 17–3-H, and 15–5-H obtained for the gelation of isomeric

xylenes are not related to their lipophilicities or the solubili-

ties of the gelator assemblies. Hence, the most plausible ex-

planation for the large difference in G_effvalues observed for

the enantiomer and the racemate in the gelation of the

same solvent seemingly originates from different supra-

molecular organization of the two stereochemical forms.

In cases in which the racemate appeared to be more effec-

tive than the enantiomer (rac-15–5-H/(R)-15–5-H and rac-

17–3-Na/(R)-17–3-Na p-xylene gels), it may have undergone

spontaneous resolution into enantiomeric assemblies, which

may then have interacted at a higher level of supramolec-

ular organization giving diastereomeric assemblies.^[14]In sup-

port of this interpretation, polyACHTNUTRGENNUG(d-lysine) and polyACHTUNGTRENN(UGN l-lysine)

were found to show a much higher propensity to form ag-

gregated b-sheet structures in water compared to the pure

enantiomers.^[17]The thermodynamic driving force for such

diastereoisomeric aggregation appears to be the increased

water entropy gained by the diastereoisomeric packing

mode. Hence, similar entropy gain can be assumed for the

gelation of certain organic solvents by racemic gelators as a

result of diastereoisomeric aggregation. The formation of

more densely packed diastereoisomeric assemblies may

result in thinner, more tightly packed gel fibers compared to

those formed in the enantiomer gel. Indeed, as reported for

chiral bis(amino acid)oxalamide gelators, much thinner

fibers were observed by TEM in toluene gels with the race-

mate than with the enantiomer and the increased effective-

ness of the racemate was explained in terms of the forma-

tion of a denser gel network capable of immobilizing a

larger volume of toluene.^[14]However, the opposite was also

observed in another case, in which the racemate organized

in much thicker fibers than the enantiomer.^[18]Apparently,

in the same solvent the racemic and optically active gelators

may give gel fibers of different morphologies. As was also

recently shown, racemic and pure enantiomer bola-amphi-

philic gelators exhibited very similar molecular level organi-

zation in hydrogels; however, these were transformed into

final assemblies of distinctly different morphologies, such as

flat ribbons in the racemate gels and chirally twisted or heli-

Figure 5. Gelator effectiveness G_eff[mL] values obtained in the gelation

of xylene isomers by racemic and optically active acids a) and sodium

salts b).

Comparison of the G_effvalues of the racemic and optically

active sodium salt gelators (Figure 5b) shows that rac-17–3-

Na is capable of gelling an almost tenfold larger volume of

p-xylene (G_eff=20.6 mL) than of o-xylene (G_eff=2.2 mL) or

m-xylene (G_eff=2.1 mL), while the (R)-enantiomer showed

much less efficient gelation of all three isomeric xylenes

(G_effvalues for o-, m-, and p-xylenes: 1.3, 1.8, and 1.2 mL,

respectively). Thus, racemic 17–3-Na was found to be a 17

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M. Zinic et al.

cal ribbons in the enantiomer gels.^[19]The largest differences

in G_effbetween the racemic 15–5-H and 17–3-Na and their

enantiomeric forms (R)-15–5-H and (R)-17–3-Na were

found in the gelation of p-xylene, the differences being less

pronounced (15–5-H) or insignificant (17–3-Na) in the gela-

tion of o- or m-xylenes. This suggests that the processes of

spontaneous resolution into enantiomeric assemblies fol-

lowed by diastereomeric aggregation could also be strongly

influenced by the solvent structure. Recently obtained evi-

dence for the co-organization of solvent molecules at the pe-

riphery of gelator pre-aggregates having a direct influence

on the self-assembly process supports this conclusion.^[3a]

the signals. In this interval, large temperature coefficients

(Dd_NH/DT) of 13ꢁ10^À3and 17.8ꢁ10^À3ppmK^À1were deter-

mined for -(CO)NH-(CH₂) and (C*)-NHACTHNUTRGNEUNG(CO-), respectively,

these being characteristic of amido NH protons engaged in

hydrogen bonding. A significant upfield shift (Dd_C*H/DT=

6ꢁ10^À3ppmK^À1) was also observed for the C*H proton,

which is closest to the strong hydrogen bond formed by the

neighboring amide group. A plausible explanation would

seem to be that the C*H proton is deshielded in the aggre-

gate by the anisotropic effect of the nearby amide carbonyl

oxygen of the second molecule and hence becomes shielded

upon disaggregation.

The signals of some of the methylene protons (-CH₂-CH₂-

CONH- and -CH₂-CH₂-CH₃) were also slightly downfield

shifted on increasing the temperature from 25 to 958C;

¹H NMR and FTIR investigations: organization at the mo-

lecular level: In some cases, heating of gel samples for

¹H NMR analysis resulted in the appearance and increase of

gelator signals due to gel network disassembly into smaller

NMR-observable aggregates.^[11,16,20]On the other hand,

some gels are thermally stable in the accessible temperature

range for common NMR solvents and no increase of the ge-

lator signals could be observed upon heating of the samples.

Analysis of the chemical shift changes that accompany ther-

mally induced disassembly may give valuable information

on the intermolecular interactions stabilizing the gelator ag-

gregates and consequently on the self-assembled motifs in-

volved.^[11]The temperature dependences of the ¹H NMR

spectra of 15–5-H and 15–5-Na in their gels with [D₈]toluene

were investigated. For this purpose, 15–5-H and 15–5-Na

[D₈]toluene gel samples (c_tot=0.0058 and 0.0031 moldm^À3,

respectively) were prepared with 1,1,2,2-tetrachloroethane

(0.5 mol/mol of the gelator) as an internal standard. The

concentrations of the NMR-observable gel aggregates were

calculated by comparing the intensities of selected gelator

signals with that of the internal standard. In the spectrum of

the 15–5-H [D₈]toluene gel acquired at room temperature,

none of the gelator signals could be observed. On increasing

the temperature to 358C, 8% of the total gelator concentra-

tion was detected; at 458C, this value increased to 80%, and

at 558C it increased to 90–100%, showing that at the latter

temperature most of the network had been disassembled

into NMR-observable aggregates. In contrast, heating of the

15–5-Na [D₈]toluene gel sample (c_tot=3.1ꢁ10^À3moldm^À3)

from 25 to 958C failed to produce any significant appear-

ance of the gelator signals. This showed that the gel was

thermally stable in the specified temperature range and that

no significant disaggregation of the network into dissolved

NMR-observable aggregates occurred. The temperature de-

pendence of the chemical shifts of 15–5-H (Figure 6) was

studied at a gelator concentration in the NMR tube slightly

below its experimentally determined MGC for [D₈]toluene.

At c_tot=5.8ꢁ10^À3moldm^À3, the sample was highly viscous,

indicative of the presence of highly aggregated species that

were expected to be similarly organized as in the gel state.

At up to 558C, the signals of both amide NH protons, some

phenyl protons, NH-(CH₂), and the methine proton (C*H)

were shifted downfield (Figure 6a); a further increase of the

temperature from 55 to 958C caused upfield shifts of all of

Figure 6. Temperature-induced variations in the ¹H NMR shifts of select-

ed protons of 15–5-H in [D₈]toluene.

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Gelation of Solvents with Isomeric Organic Gelators

FULL PAPER

these shifts may also be explained in terms of shielding of

the relevant protons in the aggregates and their deshielding

upon disaggregation. These observations indicate that the

intermolecular lipophilic interactions between alkyl chains

also contribute to the stabilization of the aggregates.

In the FTIR spectra of xerogels obtained by solvent re-

moval from 17–3-Na and 15–5-H o- and p-xylenes (Table 4),

the NH, amideI, and amide II bands appear at 3308–3299,

1637–1638, and 1532–1560 cm^À1, respectively, and corre-

spond to hydrogen-bonded peptidic units.

b-sheet dimer.^[24]This result, together with the fact that the

amide I bands in the FTIR spectra of each of the xerogels

appeared at 1637–1638 cm^À1(Table 4) and no band at

1695 cm^À1characteristic of an antiparallel b-sheet^[25]could

be observed, strongly suggests the formation of parallel b-

sheets.

Powder XRD and TEM investigations of 17–3-Na and 15–5-

H o- and p-xylene gels: Powder X-ray diffraction (PXRD)

patterns obtained for xerogels of 17–3-Na or 15–5-H with o-

and p-xylenes were seen to be

very similar (Figure 8a, b). In-

Table 4. Selected bands [cm^À1] in the FTIR spectra of 17–3-Na, 15–5-H, and (R)-15–5-H xerogels prepared

tense peaks at 2qꢀ5.7, 7.6, 9.5,

10.9, 19.5, and 22.78, corre-

sponding to distance periodici-

ties d of 15.6, 11.6, 9.2, 8.1, 4.5,

and 3.9 ꢂ, respectively, were

common to the PXRD spectra

of each of the gelator mole-

cules, implying a similar layered

organization in each case.

from their o- and p-xylene gels.

À

Xerogel/gelled

solvent

n NH

n_as

n_sC H

n C=O;

amide I

d NH

amide II

n C=O CO(O )

or -CO(OH)

À

C H

17–3-Na o-xylene

15–5-H o-xylene

17–3-Na p-xylene

15–5-H p-xylene

(R)-15–5-H p-xylene

3299

3308

3299

3308

3304

2918

2915

2918

2916

2849

2848

2849

1637

1638

1637

1638

1637

1560

1532

1560

1533

1534

1717

1719

1715

1719

1731

À

The asymmetric and symmetric C H stretching vibrations

appear at low wavenumbers, 2915–2918 cm^À1and 2848–

2849 cm^À1, respectively, which are characteristic of con-

strained alkyl chains in the aggregates^[21]and are also in

accord with the intermolecular alkyl chain interactions

postulated on the basis of NMR data. In the case of the car-

boxylic acid gelator 15–5-H, the vibration of the carboxylic

acid group appears at 1719 cm^À1. This indicates a possible

lateral type of hydrogen bonding, as found previously in

some polycarboxylic acid derivatives.^[22]

1

The H NMR and FTIR results show that the gel aggre-

gates of both the acid 15–5-H and the sodium salt 17–3-Na

gelators are stabilized by cooperative hydrogen bonding be-

tween peptidic units and lipophilic interactions between

alkyl chains. It should also be noted that the respective

FTIR vibrations of both gelators in the spectra of their p-

xylene and o-xylene gels are practically identical, despite

the fact that both showed much more efficient gelation of

the former solvent. The same can be concluded from com-

parison of the FTIR spectra of racemic 15–5-H and (R)-15–

5-H p-xylene gels, which differ only in the position of the

carboxylic acid vibration at 1719 and 1731 cm^À1, respectively.

The band at 1731 cm^À1indicates the formation of carboxylic

acid dimers,^[21,23]as opposed to the lateral carboxylic acid hy-

drogen bonding that seems to dominate in the assemblies of

the racemates.

Figure 7 shows possible parallel and antiparallel b-sheet

hydrogen-bonding motifs of 15–5-H. It should be noted that

the antiparallel b-sheet arrangement is less favorable com-

pared to the parallel one due to weaker lipophilic interac-

tions between the terminal alkyl chains. This appears to be

particularly significant for 15–5-H and 17–3-H, which have

peptidic hydrogen-bonding units closer to their carboxylic

acid termini. Molecular modeling showed the 17–3-H anti-

parallel dimer to be 8 kcalmol^À1less stable than the parallel

Figure 7. Intermolecular hydrogen bonding of 15–5-H giving parallel and

antiparallel b-sheet organization. ¹H NMR and FTIR results suggest for-

mation of the parallel b-sheet.

It should be emphasized that despite the structural differ-

ence of 17–3-Na and 15–5-H and the significantly more effi-

cient gelation of p-xylene than of o-xylene, the PXRD pro-

files of all four xerogels appeared very similar. The PXRD

profiles of o- and p-xylene xerogels of 13–7-Na, having a

much shorter terminal chain than its positional isomer 17–3-

Na, were also determined. Again, the PXRD patterns for

13–7-Na o- and p-xylene xerogels were found to be practi-

cally identical, with the most intense peaks corresponding to

d spacings of 16.3, 12.2, 8.06, 4.5, and 4.08 ꢂ. These distance

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M. Zinic et al.

Figure 9. Possible packing of 17–3-H parallel b-sheets proposed on the

basis of ¹H NMR and FTIR investigations, PXRD distance periodicities

in the xerogels, and molecular modeling.

The morphologies and fiber dimensions in o-, m-, and p-

xylene gels were examined by TEM. An image of the 17–3-

H o-xylene gel network (Figure 10a) shows the presence of

fiber bundles with diameters in the range 50–240 nm, which

have a clearly visible inner structure made up of many

aligned thinner fibers with diameters between 10 and 14 nm.

The m-xylene gel (Figure 10b) comprises somewhat shorter

and narrower tape-like bundles with diameters of 45–

130 nm, which also consist of thinner fibers with a similar

range of diameters as in the o-xylene gel. However, a TEM

image of the p-xylene gel (Figure 10c) shows a distinctly dif-

ferent morphology characterized by the presence of relative-

ly short bundles of pronounced tape-like shapes having

widths in the range 60–90 nm. The large tapes are well struc-

tured and consist of smaller stacked tapes or fibers with di-

ameters in the range 9–18 nm.

Figure 8. Powder X-ray diffraction (PXRD) patterns for a) 17–3-Na and

b) 15–5-H xerogels prepared from o-xylene (thinner line) and p-xylene

(thicker line) gels.

periodicities for the most intense peaks are close to those

obtained for the 17–3-Na and 15–5-H xerogels. It was shown

that, in contrast to 17–3-Na and 15–5-H, 15–5-Na gelled sim-

ilar volumes of o- and p-xylenes (Table 2); the periodic dis-

tances d found in the PXRD profiles of 15–5-Na o- and p-

xylene xerogels were again very similar, 12.2, 9.8, 8.1, 4.5,

and 4.06 ꢂ and 11.7, 9.2, 8.1, 4.5, and 3.97 ꢂ, respectively,

these d spacings being similar to those found for the 15–5-H

xerogels.

Taking into account the antiparallel b-sheet-type of mo-

1

lecular organization suggested by the H NMR and FTIR in-

vestigations, a molecular modeling study was undertaken.

The 17–3-H antiparallel hydrogen-bonded dimer was gener-

ated by docking of two molecules of the gelator. A model

containing two parallel stacked dimers was generated, which

gave intermolecular distances of 4.5 and 4.0 ꢂ, in accord

with the periodic distances and their multiplicities found in

the PXRD profiles of each of the examined xerogels

(Figure 9).

The results obtained from PXRD studies of selected xero-

gels strongly suggest that the positionally isomeric sodium

salt gelators 17–3-Na and 15–5-Na, as well as the free-acid

gelator 15–5-H, organize into very similar layered structures

in both their o- and p-xylene gels, despite the fact that large

differences in G_effwere observed in the gelation of these sol-

vents. The results strongly suggest that the differences in

G_effare most likely a consequence of their different assem-

blies at a higher level of supramolecular organization, in-

volving interactions between fibrous assemblies that ulti-

mately give rise to fiber bundles of different dimensions and

morphologies.

Figure 10. TEM images of 17–3-H gels (dipotassium polytungstate

(PWK)-stained; bar=500 nm): a) o-xylene, b) m-xylene, and c) p-xylene.

As described above (Table 1 and Figure 2), 17–3-H is a

much more efficient gelator of o-xylene (G_eff=26.7 mL)

than of m-xylene (G_eff=15.0 mL) or p-xylene (G_eff

=

5.8 mL). TEM analyses of the o-, m-, and p-xylene gel mor-

phologies showed that each gel contained thinner fibers

with similar diameters in the range 10–20 nm, but that these

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Gelation of Solvents with Isomeric Organic Gelators

FULL PAPER

gave rise to different higher assemblies with a preference

for tape-like aggregates in the m- and p-xylene gels.

lar organization consisting of aggregated b-sheet chains

(Figure 9), and considering the length of the gelator in the

fully extended form of around 3 nm, the diameters of the

finest fibers observed by TEM could be rationalized in

terms of the aggregation of two to three reversed bilayers

each of width around 6 nm.

The latter morphology could explain the observed less ef-

ficient gelation of m- and p-xylenes, since a less dense gel

network with lower solvent immobilization capacity can be

expected to be formed by wider and less flexible tape-like

assemblies. In contrast, the TEM image of the 17–3-H o-

xylene gel shows the presence of large fiber bundles consist-

ing of many tiny fibers and an absence of any tape-like as-

semblies. It should be emphasized, however, that the ob-

served large dimensions of the bundles may not reflect the

actual gel state and could be a consequence of the adher-

ence of many tiny fibers during the TEM experiment.

TEM images of the 17–3-Na o- and p-xylene gels (Fig-

ure 11a, b) show different morphologies compared to the

17–3-H gels with the same solvents. The sodium salt gelator

in o-xylene forms tiny flexible fibers with a rather uniform

distribution of diameters in the range 16–20 nm. In the p-

xylene gel, flexible but longer fibers with the same distribu-

tion of diameters could also be observed. Hence, compared

to the 17–3-H gels, the corresponding sodium salt shows a

lower tendency for bundling and aggregation into tape-like

assemblies. The fact that 17–3-Na gelled p-xylene much

more efficiently than o-xylene is again reflected in the fiber

morphology, which shows the formation of longer and flexi-

ble fibers in the former gel that are capable of efficient net-

working. The positionally isomeric 15–5-Na (G_eff=4.5 mL)

is a much less efficient gelator of p-xylene than 17–3-Na

(G_eff=20.6 mL) and a TEM image (Figure 11c) showed the

presence of short and thick fiber bundles (diameters 70–

120 nm) in 15–5-Na p-xylene gel, in sharp contrast to the

presence of long and thin flexible fibers in the 17–3-Na p-

xylene gel.

Gel-to-sol transition diagrams and DSC investigation: The

concentration dependences of the gel-to-sol transition tem-

peratures (T_g) of 17–3-H and 15–5-H o-, m-, and p-xylene

gels are shown in Figures 12a and b, respectively. The T_g

temperatures determined by the common tube inversion

method increased with increasing concentration of 17–3-H

in each of the isomeric xylene gels, reaching the same pla-

teau T_gvalue of about 728C at 12.5 mm. Hence, the thermal

stabilities of the 17–3-H o-, m-, and p-xylene gels are simi-

lar, despite the fact that gelator effectiveness is much higher

for o-xylene (G_eff=26.7 mL) than for m-xylene (G_eff

=

15.0 mL) or p-xylene (G_eff=5.8 mL). In contrast, at a gelator

concentration of 15 mm, the T_gof the 15–5-H p-xylene gel,

with a gelator effectiveness (G_eff) of 14.7 mL, is about 78C

higher than those of the less efficiently gelled m- and o-xy-

lenes (G_effvalues of 6.0 and 5.9 mL, respectively). Clearly,

the positionally isomeric 17–3-H and 15–5-H show different

thermally induced gel-to-sol transition behavior in the sense

that the higher G_effof 15–5-H for p-xylene is also reflected

in higher gel thermal stability.

The o- and p-xylene gels of the racemic 17–3-H, 17–3-Na,

and 15–5-H and optically active (R)-17–3-Na and (R)-15–5-

H were also investigated by DSC (Figure 13a, b, and c, re-

spectively; Table 5). In the DSC traces of the heating and

cooling cycles, distinct differences between the o- and p-

xylene gels could be observed. In most cases, multiple tran-

sitions were observed, indicating the polymorphic character

of the present assemblies. In the heating cycles of the 17–3-

H o- and p-xylene gels, enthalpy changes of 20.93 and

39.94 kJmol^À1, respectively, were measured for the first tran-

sitions, T_m1=78.6 and 77.88C, which correspond to the T_g

temperatures determined independently by tube inversion.

The difference between the first transition DH_mvalues for

the o- and p-xylene gels amounted to 19 kJmol^À1, suggesting

different organization of the assemblies formed in the o-

and p-xylene gels. In contrast, for the 17–3-Na o- and p-

xylene gels, different DSC characteristics were observed.

The measured DH_mdifference of only 3 kJmol^À1between

the first transitions in the 17–3-Na o- and p-xylene gels is

much smaller than that obtained for the 17–3-H gels.

Figure 11. TEM images of 17–3-Na gels with a) o-xylene and b) p-xylene

(PWK-stained; bar=500 nm) and c) 15–5-Na p-xylene gel (Pd shad-

owed).

Here, the predominant formation of assemblies of similar

structure and thermal stability could be postulated in both

gels. In the 17–3-H p-xylene gel, the presence of two addi-

tional polymorphic assemblies with small contributions to

the total DH_mcould be observed (Figure 13a and Table 5).

However, for the 17–3-Na p-xylene gel, a significant DH_m

contribution of 28.33 kJmol^À1from the second transition

and a minor contribution of 2.02 kJmol^À1from the third

transition were observed, which were absent from the DSC

trace of the o-xylene gel. In the latter case, significant for-

Thus, TEM investigations of the gel morphologies re-

vealed distinct differences in the shapes and dimensions of

gel fibers formed in o-, m-, and p-xylenes, which could be

related to gelator effectiveness. By careful analysis of the

TEM images, the diameters of the thinnest fibers involved

in bundling could be ascertained to be in the range 10–

20 nm. Taking into account the proposed model of molecu-

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3077

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M. Zinic et al.

the first transitions in the 15–5-H o- and p-xylene gels an

even larger DH_mdifference of 31 kJmol^À1was measured

(Table 5). While for the 15–5-H p-xylene gel three different

polymorphs could be observed, DSC showed only one tran-

sition for the corresponding o-xylene gel, indicating the

presence of only one type of assembly. It should be noted

that the enthalpy changes for the first transitions measured

for the (R)-15–5-H o- and p-xylene gels (DH_m=41.91 and

52.19 kJmol^À1, respectively) are much closer to that for the

racemic 15–5-H o-xylene gel (DH_m=55.73 kJmol^À1) than to

that for the p-xylene gel (DH_m=87.19 kJmol^À1).

The DSC investigations allowed deeper insight into the

rather complex organization of the racemic gelators in their

o- and p-xylene gels. The results showed that with the race-

mic gelators and very similar solvents, different numbers of

polymorphic assemblies of different energy could be

formed. It should be noted that for all three racemic gela-

tors, 17–3-H, 17–3-Na, and 15–5-H, three different poly-

morphs could be observed in their p-xylene gels (Table 5),

as compared to two or only one in o-xylene. Moreover, in

contrast to the three polymorphs observed in the racemic

15–5-H p-xylene gel, its optically active form gave only a

single polymorph in its p-xylene and o-xylene gels. Hence,

the racemic gelators showed a higher tendency to self-as-

semble into polymorphic assemblies than the optically pure

gelators, and this tendency was clearly stronger for p- than

for o-xylene. The presented results strongly bring into focus

the role of solvent in the gelation process. It can be conclud-

ed that both in the case of spontaneous resolution and in

the formation of different polymorphs, specific solvation ef-

fects have a high impact on the observed self-assembly pro-

cess, even for very similar solvents such as o- and p-xylenes.

Figure 12. Gel-to-sol transition diagrams of 17–3-H a) and 15–5-H, b) o-

~

^

*

(

), m- ( ), and p-( )-xylene gels.

mation of polymorphic assemblies in the p-xylene gel may

contribute to more efficient networking, thereby resulting in

the formation of a denser network with higher solvent im-

mobilization capacity compared to that formed in the o-

xylene gel. This offers a likely explanation for the much

higher G_effof 17–3-Na toward p-xylene compared to that for

o-xylene, with which such polymorphic assemblies are

absent. The DSC results for the 15–5-H o- and p-xylene gels

resemble those obtained for the 17–3-H gels, although for

Enantiomer and racemate polymorphic assemblies: Taking

into account the similar molecular level organization of ge-

lator molecules in o- and p-xylene gels, the formation of

polymorphic assemblies differing in energy could be ration-

alized by considering the chirality and directionality of the

primary bilayer-type assemblies interacting at a higher level

of supramolecular organization.

As shown schematically in Figure 14i), a gelator of (R)-

configuration may organize into head-to-head bilayers with

the same or opposite intermo-

lecular hydrogen-bonding direc-

Table 5. Total enthalpy changes^[a]and transition temperatures in heating (DH_m, kJmol^À1; T_m, 8C) and cooling

(T_c, 8C) cycles obtained by DSC measurements of 17–3-H, 17–3-Na, and (R)-17–3-Na o- and p-xylene gels at a

concentration of each gelator of 20 mm.

tionality; in principle, for the

interacting bilayers four differ-

ent polymorphs differing in hy-

drogen-bonding directionality

could be constructed (Figur-

e 14ii). In the case of a racemic

DH_m(T_m1

)

DH_m(T_m2

)

DH_m(T_m3

)

DH_mtotal

T_c1

T_c2

T_c3

17–3-H p-xylene

17–3-H o-xylene

20.93 (78.6)

39.94 (77.8)

43.60 (75.2)

40.59 (73.3)

28.35 (78.5)

34.06 (74.6)

87.19 (86.6)

55.73 (64.9)

52.19 (59.8)

41.91 (53.1)

7.11 (105.3)

–

28.33 (97.9)

–

5.8 (103.0)

–

1.37 (92.1)

–

2.8 (116.3)

8.38 (114.4)

2.02 (106.2)

–

30.85

48.32

73.96

40.59

34.18

38.47

104.23

55.73

52.19

41.91

37.6

41.6

38.4

36.8

38.6

41.7

27.9

21.7

21.3

7.9

78.7

–

68.4

–

79.8

–

42.9

–

112.2

–

104.5

–

17–3-Na p-xylene

17–3-Na o-xylene

(R)-17–3-Na p-xylene

(R)-17–3-Na/o-xylene

15–5-H p-xylene

15–5-H o-xylene

(R)-15–5-H p-xylene

(R)-15–5-H o-xylene

gelator,

a larger number of

–

polymorphic assemblies could

be constructed considering the

possible formation of meso-bi-

layers built from (R)- and (S)-

gelator molecules (Figure 15a, i

and ii) or considering interac-

4.40 (113.7)

15.67 (111.6)

–

109.6

–

[a] “DH_mtotal” represents the sum of enthalpies for all transitions observed in the heating cycle.

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Gelation of Solvents with Isomeric Organic Gelators

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Fuhrhopꢃs

“chiral

bilayer

effect” formulated for amphi-

philic gelators.^[15i,26]In the case

of interacting enantiomeric bi-

layers, five different poly-

morphs could be constructed,

each lacking intra-bilayer sym-

metry, three of them possessing

inter-bilayer symmetry, and one

lacking both intra- and inter-bi-

layer symmetry (Figure 15b, ii).

One might speculate that the

formation of diastereoisomeric

assemblies from interacting

enantiomeric bilayers in such a

way that they conserve the

inter-bilayer symmetry is favor-

able for packing, and that this

should result in the formation

of thinner fibers that are capa-

ble of forming denser networks

with higher solvent immobiliza-

tion capacities.

Figure 13. DSC diagrams showing heating (upper curves) and cooling (lower curves) cycles of a) 17–3-H,

b) 17–3-Na, and c) (R)-17–3-Na p-xylene (solid line) and o-xylene (dotted line) gels.

Conclusion

A series of chiral, positionally

isomeric gelators has been pre-

pared by variation of the termi-

nal and carboxylic acid alkyl

chain lengths defined by de-

scriptors m and n, respectively

(Figure 1). Their gelation prop-

erties (Tables 1–3 and Fig-

ures 2–5) have been analyzed in

terms of gelator versatility

(G_ver) and gelator effectiveness

(G_eff). The results of extensive

gelation studies have shown

that simple synthetic optimiza-

tion of a “lead gelator mole-

cule” by variation of the lengths

of its alkyl chains, end-group

polarity (carboxylic acid versus

sodium carboxylate), and ste-

reochemical form (racemate

versus optically pure form) can

have a tremendous influence on

the G_verand G_effvalues. The re-

sults have also shown that de-

Figure 14. Schematic representation of single enantiomer interacting bilayers differing in directionality of inter-

molecular hydrogen bonding.

tions between enantiomeric bilayers each consisting of a

single enantiomer (Figure 15b, i and ii). In the first case of

the meso-bilayer, three polymorphs with intra- and inter-bi-

layer symmetry and one lacking inter-bilayer symmetry may

be postulated. The formation of such highly symmetric as-

semblies is favorable for crystallization, as predicted by

spite the identical lipophilic/hydrophilic balance common to

all of the positionally isomeric gelators, a different arrange-

ment of the apolar (alkyl chains) and polar (peptidic and

carboxylic acid) moieties within the gelator molecule is

highly important and has a strong impact on their gelation

properties. The positional isomerism has been found to have

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5–15-H were practically incapable of gelling any of the

tested solvents.

Modification of the gelator end-group polarity by trans-

formation of the carboxylic acid into a sodium carboxylate

group resulted in the gelation of a similar set of lipophilic

aromatic solvents as gelled by the free-acid-type gelators. It

thus appears that this end-group modification has limited

impact on gelator versatility, making the sodium salt gela-

tors with m=17, 15 and n=3, 5 methylene units in their

alkyl chains the most versatile, as in the case of the free-acid

gelators (Table 2 and Figure 3).

Much more pronounced effects of positional isomerism

and end-group modification were observed with respect to

gelator effectiveness (G_eff). For example, 19–1-H was by far

the best gelator of toluene (G_eff=31.5 mL) compared to the

other positional isomers (Table 1); however, 17–3-H proved

to be superior in the gelation of o- and m-xylenes, while 15–

5-H was seen to be the most effective in the gelation of p-

xylene (Table 1, Figures 2 and 4). Some of the free-acid-type

racemic positional isomers showed up to tenfold more effi-

cient gelation of specific solvents than other isomers. Unex-

pectedly, such efficiency differences appeared even in the

gelation of very similar solvents such as isomeric xylenes.

Evidence has been obtained (Figure 3) that, in contrast to

some other types of toluene gelators, the effectiveness of

the tested positional isomers cannot be correlated with the

solubility of their assemblies in the gelled solvents.^[5a]

Large differences in G_effwere also found for the position-

ally isomeric sodium salt gelators (Table 2, Figure 3). While

17–3-H proved to be the most efficient gelator of o-xylene

(G_effvalues for o-, m-, and p-xylenes: 26.7, 15.0, and 5.8 mL,

respectively), its Na salt (17–3-Na) showed the most effi-

cient gelation of p-xylene (G_effvalues for: p-, m-, and o-xy-

lenes: 20.6, 2.1, and 2.2 mL, respectively). These results

show that for some of the positional isomers, a simple trans-

formation of the carboxylic acid group into the correspond-

ing sodium carboxylate may result in a dramatic change in

G_efftoward the same solvent (compare also the G_effvalues

for 19–1-H/19–1-Na toluene gels, 17–3-H/17–3-Na m-xylene

gels, 15–5-H/15–5-Na decalin gels, and 10–10-H/10–10-Na p-

xylene, tetralin, and decalin gels).

Comparison of the efficacies of the racemic and optically

pure gelators in the gelation of isomeric xylenes showed in

some cases large or very large differences in G_effvalues

toward the same solvent (Table 3). For example, (R)-17–3-H

and its racemate displayed G_effvalues of 19.0 and 5.8 mL,

respectively, in the gelation of p-xylene. In contrast, the rac-

emic 15–5-H was found to be about 70 times more efficient

in the gelation of p-xylene than its pure enantiomer (R)-15–

5-H (G_effvalues of 14.7 and 0.2 mL, respectively); this result

represents the largest difference in G_efffound in this study.

Large differences in G_effwere also found for (R)-15–5-Na/

15–5-Na (G_effratios 8.0 and 3.7 for m- and p-xylene gels)

and rac-17–3-Na/(R)-17–3-Na (G_effratio 17.1 for p-xylene

gels). These results clearly demonstrate that the effective-

ness of a gelator in the gelation of certain solvents is highly

dependent on its stereochemical form. In some cases, the

Figure 15. Schematic representation of a) meso-bilayers (i) and possible

polymorphs (ii) and b) enantiomeric bilayers (i) and the polymorphs (ii)

with intra- and inter-bilayer symmetry.

an impact on the relative lipophilicity of the isomers, as re-

flected in different calculated average logP values; they de-

crease with decreasing length (m) of the terminal alkyl

chains and increasing length (n) of the carboxylic acid alkyl

chains, making 19–1-H the most lipophilic and 5–15-H the

least lipophilic in the series. Considering gelator versatility,

of the series of racemic positionally isomeric free-acid gela-

tors, 17–3-H and 15–5-H were found to be the most versa-

tile, being capable of gelling ten of the twenty solvents

tested (G_ver=9 and 10), including two highly lipophilic hy-

drocarbon fuels (gasoline and diesel fuel) (Table 1 and

Figure 2). The most lipophilic isomer, 19–1-H, having the

longest terminal alkyl chain (m=19) and the shortest car-

boxylic acid chain (n=1), proved to be much less versatile

than 17–3-H and 15–5-H, giving a G_vervalue of only 5, while

the isomers with m smaller than 15 and larger n such as 13–

7-H (G_ver=1, only slight gelation of tetralin), 10–10-H, and

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Gelation of Solvents with Isomeric Organic Gelators

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racemic gelator was found to be much more efficient than

the pure enantiomer, which indicates their different organi-

zation in the final gel fiber assemblies.

results show that, even for very similar solvents such as iso-

meric xylenes, specific solvation effects may result in sponta-

neous resolution into enantiomeric assemblies, which is then

followed by different diastereomeric aggregation leading to

the formation of polymorphic assemblies. Hence, these spe-

cific solvation effects have a high impact on the self-assem-

bly process, determining both the morphology and stability

of gel fibers and consequently the solvent immobilization

capacity of the network. This is reflected in sometimes dra-

matically different gelator effectiveness values G_eff, as seen,

for example, for racemic 15–5-H and its (R)-enantiomer, the

former being a 70 times more effective gelator of p-xylene

than the latter.

The results of ¹H NMR, FTIR, and PXRD studies re-

vealed similar organization of the positionally isomeric gela-

tors at the molecular level, characterized by the formation

of stacked parallel b-sheet assemblies (Figures 7 and 9).

TEM investigations revealed the formation of fiber bundles

composed of fine fibers with diameters in the range 10–

20 nm. The observed fiber dimensions suggested possible ag-

gregation of two to three reversed bilayers each containing

two extended gelator molecules of length 3 nm. The TEM

images also showed clear morphology differences between

the 17–3-H and 17–3-Na o- and p-xylene gels, the first sol-

vent being much more efficiently gelled by 17–3-H and the

second by 17–3-Na (Figures 10 and 11). Previous investiga-

tions have shown that thin and flexible fibers are capable of

forming denser gel networks containing smaller compart-

ments, which are capable of immobilizing larger volumes of

solvent than networks built from thicker and more rigid

fibers.^[14,18]

Experimental Section

All synthetic procedures, equipment used, and characterizations of the

prepared compounds are summarized in the Supporting Information.

Determination of gelling properties: A sample (10 mg) of the tested sub-

stance was placed in a test tube and the requisite solvent was added by

means of a microsyringe in 100–500 mL portions. After each addition, the

mixture was gently heated until the substance dissolved, then allowed to

cool naturally to room temperature. The formation of a gel was checked

by inversion of the test tube. The procedure was repeated until sample

fluidity was restored at room temperature.

DSC investigations of 17–3-H, 17–3-Na, 15–5-H, and opti-

cally pure (R)-17–3-Na and (R)-15–5-H gels allowed deeper

insight into the rather complex organizations coexisting in

their o- and p-xylene gels. Distinct differences in the DSC

traces for the o- and p-xylene gels were observed, as mani-

fested in the number of observed transitions in both the

heating and cooling cycles, as well as in the measured en-

thalpy changes (Table 5 and Figure 13). The large difference

between the first transition enthalpy changes (DH_m) mea-

sured for the o-xylene and p-xylene gels of racemic 15–5-H

shows that the polymorphic assemblies formed in two simi-

lar solvents can be significantly different in energy.

Acknowledgements

Financial support from the Croatian Ministry of Science, Education, and

Sport (Project 098-0982904-2912) is gratefully acknowledged. We are

ˇ ´

grateful to Dr. N. Ljubesic and L. Horvat for acquiring the TEM images

´

and to Dr. D. Lyons and Dr. D. Medakovic for performing the PXRD

measurements.

Using hydrogen-bonding directionality and symmetry ar-

guments for the racemate gels, four possible head-to-head

meso-polymorphs could be constructed, three of them

having intra- and inter-bilayer symmetry (Figure 15a). Such

highly symmetrical organizations are favorable for crystalli-

zation, in accord with the chiral bilayer effect.^[15i,26]In the

case of spontaneous resolution followed by interactions be-

tween enantiomeric bilayers, five different polymorphs

could be constructed (Figure 15b), three of them having

inter-bilayer symmetry. It can be assumed that diastereo-

meric interactions with conserved inter-bilayer symmetry

are energetically favored and lead to the formation of thin-

ner fibers capable of efficient networking. Such fibers are

capable of forming denser gel networks with higher solvent

immobilization capacity, which is in turn reflected in high

gelator effectiveness (G_eff) values. It seems that the forma-

tion of polymorphic assemblies is more pronounced in p-

xylene than in o-xylene; the racemic 17–3-H, 17–3-Na, and

15–5-H gave rise to three polymorphs in p-xylene gels, while

in the o-xylene gels two or only one polymorph could be ob-

served (Table 5). The presented results, together with other

most recent work,^[3a,5a]strongly bring into focus the role of

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the least lipophilic 4z had a low MGC and the lowest c_dissand

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Received: August 25, 2009

Published online: January 29, 2010

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Chem. Eur. J. 2010, 16, 3066 – 3082

Article Doi

Positionally isomeric organic gelators: Structure-gelation study, racemic versus enantiomeric gelators, and solvation effects

DOI: 10.1002/chem.200902342

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Full text of DOI:10.1002/chem.200902342

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