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Self-assembly and luminescence of pyrazole supergelators†

Sandra Moyano,a Jos�e Luis Serrano,ac Anabel Elduque*b and Raquel Gim�enez*a

Received 28th March 2012, Accepted 16th April 2012

DOI: 10.1039/c2sm25726b

Functional supramolecular organogelators containing a luminescent 4-arylpyrazole unit are described.

Moreover, the compounds show supergelator behaviour. The synthesised molecules combine

a 1H-pyrazole ring, an amide group and a trialkoxyphenyl group. The study of the gelation process and

the structure–property relationship reveals that both the pyrazole and the amide groups are essential

for the H-bonding driven gelation process. Chiral derivatives with a similar chemical structure are able

to amplify molecular chirality through the formation of aggregates and gel fibers with supramolecular

chirality by a cooperative self-assembly mechanism which follows a nucleation–elongation model. The

compounds are luminescent in solution and they are able to enhance this property in the gel, exhibiting

a remarkable aggregation-induced enhancement of emission (AIEE effect). Therefore, these novel

molecules are able to efficiently amplify properties such as chirality and luminescence from the

molecular level to the macroscopic level.

Introduction

Supramolecular organogels are nowadays intensely investigated

for varied applications such as drug delivery, oil waste remedi-

ation or functional materials.1,2 Advantages are the reversible

manipulation of the sol–gel state and their easy preparation, as

they consist of a low-molecular-mass organic gelator (LMOG)

and organic solvents.

Gelation involves the immobilization of the solvent by a low

concentration of gelator molecules (usually 2–5 wt%) due to their

self-assembly primarily into 1D nanoscopic structures such as

fibrils, strands or tapes, which form fibers that become entangled.

If the LMOG is able to gel in concentrations below 1 wt%, then it

is regarded as a supergelator.1

Organogelation is also seen as an efficient method for con-

structing self-assembled fibrillar networks (SAFINs) for diverse

applications such as sensing of surface engineering.3 There is

considerable interest in the development of intrinsic luminescent

organogelators as these systems can transfer their molecular

properties and even amplify them in their gel state yielding novel

luminescent soft materials that can be tailored for a variety of

applications like optical materials or sensors.4–6

aDpto Qu�ımica Org�anica, Instituto de Ciencia de Materiales de Arag�on(ICMA) – Facultad de Ciencias, CSIC – Universidad de Zaragoza,50009 Zaragoza, Spain. E-mail: rgimenez@unizar.esbDpto Qu�ımica Inorg�anica, Instituto de S�ıntesis Qu�ımica y Cat�alisisHomog�enea (ISQCH) – Facultad de Ciencias, CSIC – Universidad deZaragoza, 50009 Zaragoza, Spain. E-mail: anaelduq@unizar.escInstituto de Nanociencia de Arag�on (INA), Universidad de Zaragoza,50018 Zaragoza, Spain

† Electronic supplementary information (ESI) available: Syntheticprocedures and characterisation data, Table S1 and Fig. S1–S7. SeeDOI: 10.1039/c2sm25726b

This journal is ª The Royal Society of Chemistry 2012

Self-assembly by hydrogen bonding in combination with p–p

interactions and dispersive forces or solvophobic effects are the

most common supramolecular interactions that act as the driving

force for gelation. Chirality is another factor often intimately

associated with the stability of fibers and formation of complex

architectures mimicking helical structures, like the ones found in

proteins or DNA. Attempts to design LMOGs in a logical

manner have been carried out, but the difficulty in understanding

and controlling the delicate balance of non-covalent forces has

led to the exploration of molecules with very heterogeneous

structural diversity covering amphiphiles, sugars, polyaromatic

compounds, metal complexes, etc.7–10

We report here on the self-assembly of novel luminescent

supergelators derived from pyrazoles. The study of the gelation

properties of 1H-pyrazole derivatives is unexplored to date,

despite the 1H-pyrazole ring possessing a rich supramolecular

chemistry. It has been shown that 1H-pyrazole is able to form

cycles or catemers through highly directional intermolecular

hydrogen bonding in both the solid state11,12 and the liquid

crystal state.13 This property is favourable to design novel

H-bonded supramolecular gelators.

The structures synthesised combine the luminescent unit

1H-4-phenylpyrazole, an amide group and a trialkoxyphenyl

group (Fig. 1) and yielded compounds that are able to gel

organic solvents with strong gelation power, with supergelator

behaviour identified in dodecane. An achiral gelator with three

n-decyloxyl chains (compound 1) and two chiral gelators

with (S)-3,7-dimethyloctyl terminal chains (compound 1S) or

(R)-3,7-dimethyloctyl terminal chains (compound 1R) were

prepared. This allowed us to study the amplification of chirality

in addition to luminescence from the molecular level to the soft

material.

Soft Matter, 2012, 8, 6799–6806 | 6799

Fig. 1 Structures of the novel supergelators.

Fig. 2 Structures of the non-gelator compounds.

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The presence of both amide and 1H-pyrazole functions has

proved to be essential for the formation of gels. This conclusion

was supported by the preparation of two compounds with only

one H-bonding donor group (Fig. 2): compound 2 in which the

amide function was replaced by an ester functional group and

compound 3 with a methyl substituent at the N1 position.

Compounds 2 and 3 both do not behave as organogelators. The

number of alkoxylic terminal chains is also contributing to the

delicate balance of supramolecular interactions. Therefore,

compounds 4 and 5 were also prepared in order to assess the

influence of the number and position of the terminal chains.

Experimental

Synthesis of the compounds

The synthesis of compounds 1, 1S and 1R, 4 and 5 was achieved

by condensation of 4-(40-aminophenyl)-3,5-dimethyl-1H-pyr-

azole with di- or tri-alkoxybenzoyl chlorides as the amidation

reaction is selective for the amino group. Compound 2 was

prepared by esterification of 3,4,5-tri-n-decyloxybenzoic acid

with 4-(40-hydroxyphenyl)-3,5-dimethyl-1H-pyrazole by using

DCC/DPTS, and compound 3 was synthesised by methylation of

1 with methyl iodide. The compounds were obtained as white

solids at room temperature. General methods and full descrip-

tion of the synthetic procedures and molecular characterisation

data are collected in the ESI†.

Gelation tests

The gelating ability of the final compounds was assessed by

weighing a certain amount of the solid compound and the

organic solvent, heating the suspension to dissolve the solid and

Table 1 Gelation testa results in organic solvents at 5 wt%

Compd CH2Cl2 CHCl3 THF EtOAc

1 S S S G1S S S S G1R S S S G2 S S S S3 S S S S4 S S S S5 S S S S

a S ¼ solution, G ¼ transparent gel, OG ¼ opaque gel, P ¼ precipitate.

6800 | Soft Matter, 2012, 8, 6799–6806

cooling down to room temperature. Complete gel formation was

checked by the tube inversion method.14 Comparative data of the

test results in different organic solvents at 5 wt% are shown in

Table 1. The gel-to-sol transition was estimated by heating the

gel in an oil bath until a complete fluid solution was observed.

Sample preparation for morphology studies

Morphology in the xerogel state was studied by FE-SEM using

a Carl Zeiss MERLIN� microscope, by TEM using a JEM-

2000FXII microscope, and by CryoTEM using a FEI TECNAI

T20 device. Samples for FE-SEM studies were prepared by

placing a drop of the gel onto a quartz plate and after complete

evaporation of the solvent the xerogel was covered with plat-

inum. TEM samples were prepared by placing a drop of the

solution onto carbon-coated copper grids. Then a drop of uranyl

acetate was placed over the grid and after 30 seconds the solution

excess was removed with filter paper. CryoTEM samples were

prepared according to a previously described method for samples

in organic solvents in carbon-coated copper grids.15,16 A drop of

the gel was placed onto the grid and then it was fast frozen in

liquid nitrogen using a guillotine-type quenching device. Solvent

was freeze-dried in the microscope by heating the sample until

0 �C and allowed to cool back to �178 �C.

Results and discussion

Gelation properties and self-assembly behaviour

Compounds 1, 1S and 1R were able to gel a variety of nonpolar

or low polarity organic solvents such as dodecane, hexane,

cyclohexane, diethyl ether and ethyl acetate, and were soluble in

dichloromethane, chloroform and THF (Table 1). In all cases,

gel formation was instantaneous and it remained stable at room

temperature. The lowest critical gelling concentration (CGC) was

observed for dodecane, with values of 0.1 wt% for 1 and 0.5 wt%

for 1S and 1R.

Et2O Cyclohexane Hexane Dodecane

OG G G OGG G G GG G G GS S S SS S S SP P P PS P P P

This journal is ª The Royal Society of Chemistry 2012

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Compound 1 in dodecane formed opaque gels with higher

stability than those of the chiral compounds 1S and 1R, which

yielded more transparent gels with lower gel-to-sol transition

temperatures at the same concentrations (Fig. 3). The gel-to-sol

transition for 0.5 wt% gelator in dodecane was estimated to be

around 86 �C for 1, and 73 �C for 1S and 1R. This effect provides

evidence of the subtle influence that dispersive forces between

molecules have on gel stability as the chiral compounds, with

branched and shorter tails than compound 1 (and therefore

weaker interactions), yielded less favourable packing and less

stable fibrous aggregates. The effect of the terminal chain

structure is crucial in observing such gelling properties, as is the

number of tails, as evidenced by compounds 4 and 5, which have

two n-decyloxyl tails at the 3,4 or 3,5 positions of the aromatic

ring and did not form the gel state. The structures of the gels in

dodecane were investigated by FE-SEM, TEM and CryoTEM

(Fig. 4). Xerogels of compound 1 observed by FE-SEM consisted

of a network of entangled fibers that were several micrometres

long, and formed by bunches of parallel thinner filaments

(approximately 25 nm). TEM observations showed the same

fibrillar network as observed by SEM. Compounds 1S and 1R in

the xerogel state gave rise to a different morphology, in both

cases an interpenetrated network formed by highly twisted fibers

with a cylindrical diameter was obtained. In these preparations

we observed that the xerogel tends easily to collapse as a thin film

as a consequence of solvent removal. To avoid drying effects as

much as possible, samples were also studied by the CryoTEM

technique. In this case the microphotographs showed that the

gels were formed by a network of less homogeneous fibers wider

than for compound 1 but twisted.

To investigate the origin of gel formation FTIR spectra were

recorded in solution and in the gel state (Fig. S1, ESI†). A

molecular solution of compound 1 in chloroform shows a single

N–H stretching band at 3460 cm�1 and the amide I band (C]O

stretching band) at 1672 cm�1 indicating that association by

H-bonding does not occur.17,18 However, the spectra of the

dodecane gel display a broad band at 3280 cm�1 for the N–H

stretching and the amide I band at 1643 cm�1, which is charac-

teristic of H-bonded amides. In addition, a band at 3178 cm�1 is

also observed, and this corresponds to the associated NH pyr-

azole. The amide bands and the pyrazole bands indicate that

strong H-bonding in both groups leads to the formation of the

self-assembled system.

In addition the 1H NMR spectra in cyclohexane-d12 show that

the NH amide peak, which appears at 8.3–8.4 ppm, is concen-

tration dependent (Fig. S2, ESI†). This signal is shifted downfield

Fig. 3 Left: opaque gel of 1 in dodecane. Right: transparent gels

obtained form 1S and 1R in dodecane. All concentrations are 0.5 wt%.

This journal is ª The Royal Society of Chemistry 2012

with increasing concentrations, an observation that indicates the

presence of intermolecular H-bonded aggregates. In contrast, the

chemical shifts of the aromatic signals barely change indicating

that p–p interactions are not as relevant in this gel formation.

Amphiphiles containing the five-membered nitrogen ring

1H-imidazole were recently reported to form organogels through

intermolecular H-bonding between the azole rings.19,20 In our

case, aggregation through cooperative H-bonding between both

the pyrazole and amide groups takes place as removal of one

H-bond donor functionality does not yield gelation (compounds

2 and 3 are not organogelators). Therefore, we propose that the

H-bond between amide and pyrazole functional groups, apart

from amide–amide interactions and pyrazole–pyrazole interac-

tions, should essentially occur to yield the gel.

To shed light on the aggregation process a NOESY experiment

has been performed in an aggregated sample (Fig. 5a) and in

a molecularly dissolved sample (Fig. 5b). By comparing the two

spectra we observe two correlations that only appear in the

aggregated sample. They are correlations between the proton

signal which corresponds to the methyl groups of the pyrazole

ring (2.05 ppm) and the aromatic signals corresponding to the

protons situated at the ortho position of the amide group of both

phenyl rings (7.65 ppm and 7.26 ppm). These spatial couplings

can only be explained by considering interdigitation between

closest neighbours due to the presence of pyrazole–amide inter-

actions. A possible model of self-organisation that would explain

the two correlations includes aggregation by interdigitation of

the taper-shaped molecules interacting by H-bonds between

pyrazole and amide bonds in an antiparallel way (Fig. 5c) and

a parallel way (Fig. 5d). This would lead to aggregates with an

hydrophobic periphery that would stack leading to the fibril

aggregates that eventually form the entangled network able to

immobilize a large amount of low polar solvents.

Chirality amplification

CD measurements provide valuable information on the gel

formation of the chiral molecules 1S and 1R. For this we

compared the UV absorption and CD spectra of the chiral

compounds dissolved in THF with those of the organogels in

dodecane. The absorption spectra of a diluted solution in THF

display a broad band in the UV region at 295 nm corresponding

to ap–p* transition of the molecularly dissolved compound. The

maximum blue-shifts to 290 nm on increasing the concentration

in THF to 0.5 wt%, indicating a partial aggregation (Table S1,

ESI†). Blue-shift on aggregation is also shown more markedly in

the case of sol–gel formation, as in dodecane the UV spectrum at

0.5 wt% shows a broad band at 287 nm in the sol state at 80 �Cthat shifts to 280 nm on cooling at 20 �C in the gel state (Fig. 6).

Based on this we compared the CD spectra of the compounds in

THF with those in organogels in dodecane. Although the

compounds are chiral, they do not give any signal in THF, either

in dilute solutions or at 0.5 wt%, indicating that at the molecular

level the chromophoric aromatic rings are too far from the chiral

centre, and the aggregates that may form in THF on increasing

concentrations are not able to amplify molecular chirality.

However, CD spectra in dodecane do show optical activity. In

this case a Cotton effect at 305 nm and a bisignated signal with

a zero crossing at 244 nm were observed (Fig. 6). We can

Soft Matter, 2012, 8, 6799–6806 | 6801

Fig. 5 NOESY experiment (a) in cyclohexane-d12, (b) in CDCl3, (c) amide–pyrazole antiparallel aggregation model, (d) amide–pyrazole parallel

aggregation model.

Fig. 4 Images of xerogels from dodecane. (a) FE-SEM (5 wt%), compound 1; (b) TEM (0.1 wt%), compound 1; (c) FE-SEM (0.5 wt%), compound 1S;

(d) FE-SEM (0.5 wt%), compound 1R; (e) CryoTEM (0.5 wt%), compound 1S; (f) CryoTEM (0.5 wt%), compound 1R.

6802 | Soft Matter, 2012, 8, 6799–6806 This journal is ª The Royal Society of Chemistry 2012

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Fig. 6 Up: CD spectra of compounds 1R (a) and 1S (b) at 293 K.Middle

and bottom: CD spectra andUV absorption change for 1S on the cooling

process: (a) 353 K, (b) 343 K, (c) 293 K.

Fig. 7 Aggregation models. a ¼ degree of association.

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postulate the formation of helical assemblies. A CD study at

variable temperature at 0.5 wt% shows that the gel state retains

the optical activity and gradually diminishes until it disappears

on surpassing through the gel-to-sol transition. The chiral

aggregation/dissociation behaviour is fully reversible and could

be reproduced over many cycles. The molecular chirality of the

compounds is transferred to the macroscopic level through the

formation of ordered chiral aggregates able to gel the organic

solvent. The transference and amplification of the molecular

chirality were confirmed by comparing the CD spectra of the gels

formed by the two enantiomers 1S and 1R. These spectra are

mirror images that depend solely on the relative configuration of

the chiral centre of the terminal chain (Fig. 6). For the achiral

compound 1 a similar experiment did not give rise to any optical

activity.

Molecular self-assembly processes yielding supramolecular

aggregates are generally explained according to twomainmodels,

the isodesmic model and the cooperative model (Fig. 7). The

isodesmic model (or equal K-model) is a non-cooperative model

ruled by a single aggregation constant, Ke, that shows no depen-

dence on the aggregate size. The second model is a cooperative

one, in which self-assembly begins with a nucleation step, ruled by

an association constant,Ka, and a second step, ruled by a different

constant, Ke, in which elongation takes place.21 Monitoring the

aggregation process at different concentrations or temperatures

gives rise to graphswith different shapes according to the followed

model. In the case of an isodesmicmodel a sigmoidal type graph is

obtained, while in the case of a cooperativemodel this dependence

does not follow a sigmoidal shape (Fig. 7).

This journal is ª The Royal Society of Chemistry 2012

There are several techniques that allow us to study the

aggregation mechanism through concentration or temperature

dependent studies.22 Among these techniques, nuclear magnetic

resonance (NMR), circular dichroism, or UV-vis and fluores-

cence spectroscopy are of special interest. The study of the

aggregation mechanism of compounds 1, 1S and 1R by 1H NMR

was not possible as very low concentrations or too high

temperatures are required to observe a single pattern of signals

in the NMR spectra. As indicated, the compounds are

supergelators, therefore, they are aggregated at very low

concentrations. We did observe that the 1H NMR spectra in

cyclohexane-d12 are both concentration dependent (Fig. S2,

ESI†) and temperature-dependent (Fig. S3, ESI†). An increase of

temperature at a fixed concentration gives rise to a change in the

spectrum which indicates that aggregation reduces when the

temperature increases, but it is not possible to obtain a complete

melt of the aggregates. When the sample is cooled down to room

temperature the initial spectrum is recovered, with a complex

spectral pattern, so we can confirm that aggregation is

a temperature dependent process and is completely reversible.

We then used circular dichroism to study the mechanism by

which the chiral aggregates grow. This technique is very sensitive

and allows the study of diluted samples. Recently it has been

described that temperature-dependent studies are more appro-

priate than concentration-dependent studies to study the aggre-

gation process because they allow fast and efficient data

acquisition to study the sigmoidal or non-sigmoidal dependence

of the degree of association22 finally resulting an almost contin-

uous curve.

As it has been previously shown, the CD signal for 1S and 1R

at 0.5 wt% is temperature-dependent and it gradually increases as

we cool down the sample (Fig. 6). With the purpose of studying

the continuous CD signal variation with temperature we heated

the sample at high temperature to fully depolymerise the sample

and registered the CD signal at 305 nm (maximum of the Cotton

effect) on cooling the sample to track the aggregate formation

with the signal growth (Fig. 8). A slow cooling rate (0.5 K min�1)

was applied to prevent kinetic effects on the aggregation process

and to ensure that it is thermodynamically controlled.23 The

Soft Matter, 2012, 8, 6799–6806 | 6803

Fig. 8 Dependence of the CD signal at 305 nm on temperature for 1R

(up) and 1S (down).

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resolution of the data is one data point per 0.1 K, so that we

obtain an almost continuous curve that enables non-linear data

fitting.

The graphs shown in Fig. 8 are non-sigmoidal, hence the

aggregation is not isodesmic. The curve shows the characteristic

form of a cooperative aggregation process for a supramolecular

polymerisation. This means that, at high temperatures, there is

a first step of activation or nucleation, in which all molecules are

molecularly dissolved (CD silent) and, upon cooling to a certain

temperature Te, a large enough achiral nucleus is formed to allow

the chiral aggregation. Below this temperature, there is a second

step, called the elongation regime, in which the chiral aggregate

rapidly grows (CD signal grows).

The same study was performed with four different concen-

trations, all of them under the critical gelling concentration

(CGC) (see the caption of Fig. 9 for the exact values). In these

cases also a CD effect of the same sign as for the gels is observed,

and the intensity of the CD signal decreases with decreasing

concentration. Non-sigmoidal curves were obtained in all cases

indicating a non-isodesmic but a cooperative aggregation process

involving similar chiral aggregates also takes place at these low

concentrations (Fig. 9). The decrease in concentration causes

a decrease in the temperature at which chiral aggregation begins,

Fig. 9 CD signal variation at 305 nm with temperature in dodecane at

different concentrations. 1R (up): 5.2 � 10�4 M (a), 2.4 � 10�4 M (b), 9.8

� 10�5 M (c), 4.9� 10�5 M (d); 1S (down): 4.9� 10�4 M (a), 2.5� 10�4 M

(b), 9.9 � 10�5 M (c), 5.1 � 10�5 M (d).

6804 | Soft Matter, 2012, 8, 6799–6806

Te, indicative of the later formation of the chiral aggregates when

cooling down the solutions.

Quantitative data on the self-assembly process can be obtained

by fitting the data to a model. The behaviour observed follows

the Oosawa–Kasai theory for the helical assembly of proteins in

solution.24a Particularly, we apply a mathematically more trac-

table version of this model, developed by van der Schoot,23,24b for

nucleation–elongation processes that can be applied to many

chemical self-assembly processes of ordered quasi-one dimen-

sional stacks. According to this, the fraction of molecules in

assemblies (fn) obeys approximately eqn (1) (ref. 23), which

correlates the dependence of aggregation on temperature in the

elongation regime of the cooperative model.

fn ¼ 1 � exp[DHe(T � Te)/RTe2] (1)

T: temperature in K, Te: temperature at which helical aggrega-

tion begins, R: gas constant, DHe: molecular enthalpy of the

elongation process.

Curve fitting to eqn (1) of the cooling curves of normalised CD

data (related to fn) shows good agreement with this aggregation

model for the four different concentrations under the CGC

(Fig. S4, ESI†). This allows Te and DHe to be calculated, yielding

values of Te ¼ 335 K, 324 K, 314 K and 304 K for the concen-

trations used for 1R, and Te¼ 334 K, 327 K, 315 K and 305 K for

the concentrations used for 1S. Mean DHe values of �85 kJ

mol�1 for 1R and �83 kJ mol�1 for 1S are calculated. A dimen-

sionless equilibrium constant Ka can only be estimated from data

due to their low value, being 10�6 to 10�7. This indicates that the

process is highly cooperative.

The temperature at which helical aggregation begins, Te,

decreases upon diluting the sample. The dependence of this

equilibrium process follows the van’t Hoff equation (eqn (2)) in

a similar way to that previously described for other nucleation–

elongation processes.22 A good linear fit is obtained (Fig. 10) that

allows a second method to determine DHe, and, in addition, the

calculation of DSe. Values of DHe ¼ �65 kJ mol�1 and DSe

¼�132 J mol�1 K�1 for 1R and similar data DHe ¼�67 kJ mol�1

and DSe ¼ �136 J mol�1 K�1 for 1S are obtained. Negative

enthalpy and entropy parameters indicate that the process is

enthalpy driven.

ln Ke ¼ �ln(C) ¼ �[(DHe/R)(1/T)] + (DSe/R) (2)

Fig. 10 van’t Hoff plot for 1R (left) and 1S (right). Goodness of fit R ¼0.99.

This journal is ª The Royal Society of Chemistry 2012

Fig. 11 Normalised CD data (Fn) monitored at 305 nm upon cooling 1R

(up) and 1S (down) gels in dodecane (0.5 wt%) and fitting to eqn (1).

Fig. 12 Emission spectra of 1R gel in dodecane (0.5 wt%) on the cooling

process from 353 K to 293 K and photographs of the gel (20 �C) and the

sol (80 �C) under UV light (254 nm) showing the luminescent change.

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For the case of concentrated samples yielding gel formation

(0.5 wt%), the curve fitting to eqn (1) shows a slight difference

between experimental data and theoretical prediction. In this

case Te¼ 344 K is calculated. At temperatures below Te (335–310

K), theory predicts higher normalised CD values than those

Fig. 13 Schematic representation of a plausible mechanism

This journal is ª The Royal Society of Chemistry 2012

experimentally measured (Fig. 11). This difference may be

attributed to a slower rate of chiral elongation in the gel state

versus the solutions of Fig. 9, or to the existence of additional

processes not considered in the mentioned polymerisation

theory.23 The sol-to-gel transition measured on heating by the

inversion method is more or less similar to Te. Therefore, at this

initial concentration the elongation of the chiral aggregate takes

place almost simultaneously to gelation.

Luminescent properties and AIEE effect

The compounds are luminescent in the UV region in diluted

solutions. Molecularly dissolved fluorescent spectra of very

diluted solutions in THF show emission maxima at 358 nm in

THF and 330 nm in dodecane. Dodecane gels at 0.5 wt%, as

a consequence of aggregation, display a broad emission band in

the blue region of the visible spectra, with maxima at 440 nm

(Fig. 12). On heating the gel the emission peak decreases grad-

ually while retaining its shape. On passing through the gel-to-sol

temperature the sample becomes much less luminescent and the

starting emission is recovered when the gel state reforms on

lowering the temperature.

To demonstrate that the enhancement of the emission depends

solely on gel formation and does not depend on the effect of

reducing the non-radiative processes by lowering the tempera-

ture, we have compared the luminescence at room temperature of

two samples with the same concentration (0.5 wt%), a THF

solution and the dodecane gel at room temperature (Fig. S5–S7,

ESI†). It was observed that the emission is multiplied by more

than one hundred times in the gel state. An aggregation-induced

enhanced emission effect (AIEE)25 therefore takes place as

a consequence of the formation of supramolecular fibers.

This property, which is due to aggregation-induced effects, has

been found in a few organogels,20,26–29 and is interesting because

of their potential use as turn-on sensors.

Aggregation structure–luminescence relationship

Combining the results obtained from all spectroscopic studies

about intermolecular interactions, chiral aggregation mechanism

of hierarchical self-assembly of compounds 1R or 1S.

Soft Matter, 2012, 8, 6799–6806 | 6805

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and luminescence spectra, it is possible to postulate a model for

the hierarchical self-assembly of compounds 1R and 1S in

cyclohexane/dodecane (Fig. 13).

From the study of the aggregation mechanism we have

concluded that on cooling a solution of sufficient concentration,

the molecularly dissolved chiral compounds aggregate in helical

quasi-one-dimensional stacks by a highly cooperative mecha-

nism that consists of an achiral nucleation step followed by

a helical elongation step. Given that we have observed by NMR

that pyrazole–amide H-bonding interactions are present in the

aggregate we could infer that in the nucleation step, the isolated

molecules start interacting by H-bonds involving both func-

tional groups to yield a non-chiral aggregate consisting of a few

molecules. When the molecules are molecularly dissolved they

emit at 330 nm, but in the aggregated state they emit at 440 nm.

The non-chiral aggregate formed in the nucleation step is able

to further reorganise in such a way that a chiral nucleus is

formed and starts to grow rapidly by helical stacking. At this

point an enthalpy release takes place and the strong coopera-

tivity is probably due to the formation of additional H-bond

interactions, apart from other weaker non-covalent interactions

and molecular reorganisation. At this stage, the emission

maxima do not change but a gradual increase in the lumines-

cence intensity occurs in the elongation step. In the case of the

sol–gel transition the AIEE effect is clearly shown in the

pictures of Fig. 12.

Conclusions

A novel type of molecules with amide and pyrazole H-bond

functional groups that are able to self-assemble and yield orga-

nogels with luminescence and chiral aggregation has been

described. Molecules in the gel state have proved to be highly

luminescent due to the aggregation-induced enhanced emission

effect and chirality appears in the gel state as a consequence of

the formation of helical aggregates by a cooperative mechanism.

The control and reversibility of the self-assembly are of funda-

mental importance for emerging areas of electronic and optical

nanotechnologies and, as reported here, the compounds

described are able to switch reversibly between a gel state and

a sol state with different properties by using very small amount of

material due to their supergelator power.

Acknowledgements

We thank the following institutions for financial support:

Gobierno de Arag�on (project PI109/09, research group E04),

MICINN-Spain, FEDER and FSE-UE (projects CTQ2011-

22516, CTQ2009-09030 and MAT2009-14636-CO3-01).

6806 | Soft Matter, 2012, 8, 6799–6806

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This journal is ª The Royal Society of Chemistry 2012

Addition and correction Note from RSC Publishing This article was originally published with incorrect page numbers. This is the corrected, final version.

The Royal Society of Chemistry apologises for these errors and any consequent inconvenience to authors and readers.

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