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Journal ofMaterials Chemistry C
PAPER
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aDepartamento de Quımica Organica, Facul
Materiales de Aragon (ICMA), CSIC - Univ
50009 Zaragoza, Spain. E-mail: rgimenez@bServicio de Difraccion de Rayos X y Analisis
Universidad de Zaragoza, Pedro Cerbuna 12cInstituto de Nanociencia de Aragon (IN
Esquillor s/n, 50018 Zaragoza, SpaindDepartamento de Quımica Inorganica, Fac
Quımica y Catalisis Homogenea (ISQCH),
Cerbuna 12, 50009 Zaragoza, Spain. E-mai
† Dedicated to the memory of Prof. Drmemory of Dr Christian G. Claessens.
‡ Electronic supplementary informationdensity Fourier maps for PzA0$MeOH inHO3 and HO4. CCDC 909498. For ESI anelectronic format see DOI: 10.1039/c3tc30
Cite this: J. Mater. Chem. C, 2013, 1,3119
Received 28th January 2013Accepted 4th March 2013
DOI: 10.1039/c3tc30174e
www.rsc.org/MaterialsC
This journal is ª The Royal Society of
Self-assembly of 4-aryl-1H-pyrazoles as a novel platformfor luminescent supramolecular columnar liquidcrystals†‡
Sandra Moyano,a Joaquın Barbera,a Beatriz E. Diosdado,b Jose Luis Serrano,ac
Anabel Elduque*d and Raquel Gimenez*a
Supramolecular liquid crystals containing the unprecedented 4-aryl-1H-pyrazole unit are reported. This
moiety is able to self-assemble by H-bonding to give columnar mesophases and also display luminescent
properties in the visible region. The molecular structures consist of a polyalkoxybenzamide group
substituted at the nitrogen atom by the 4-(3,5-dimethyl-1H-pyrazol-4-yl)phenyl unit. A family of
compounds was synthesized to evaluate the supramolecular structures and the mesomorphic properties.
The 3,4,5-tri-n-decyloxybenzamide derivative is a non-discoid molecule that displays a hexagonal
columnar mesophase and a rectangular columnar mesophase. Powder X-ray diffraction studies show that
these molecules are able to self-assemble in columns, and that a columnar stratum of the hexagonal
columnar mesophase contains five molecules on average. A model for the columnar organization is
proposed in which a self-assembled aggregate is formed by an antiparallel arrangement of the molecules
partially interdigitated and interacts by hydrogen bonds. The amide group was changed by an ester
group, in order to asses the role of the amide and pyrazole groups in the self-assembly. The ester
compound also exhibited a columnar mesophase with similar cell parameters. This means that, together
with the tapered shape of the molecule, the 1H-pyrazole moiety, and not the amide group, is essential for
the aggregation leading to the mesomorphism. In addition, the single crystal structure of a model
compound N-(4-(3,5-dimethyl-1H-pyrazol-4-yl)phenyl)benzamide was solved. The structure was a
methanol solvate in which each pyrazole is engaged in hydrogen bonding to form a dimer through
methanol bridges. The pyrazole molecules stack along the a axis and interact by hydrogen bonding
through the amide groups to yield parallel infinite chains. Benzamide compounds are luminescent in the
solid state and in the liquid crystalline state, in the blue region of the visible spectrum.
Introduction
Complex supramolecular structures can be built fromsimpler molecular components by self-assembly processesinvolving hydrogen-bonding interactions.1–3 Proteins or DNA
tad de Ciencias - Instituto de Ciencia de
ersidad de Zaragoza, Pedro Cerbuna 12,
unizar.es
por Fluorescencia, Facultad de Ciencias,
, 50009 Zaragoza, Spain
A), Universidad de Zaragoza, Mariano
ultad de Ciencias - Instituto de SıntesisCSIC - Universidad de Zaragoza, Pedro
Enrique Melendez Andreu and to the
(ESI) available: Difference electronorder to locate and rene HN1, HN4,d crystallographic data in CIF or other174e
Chemistry 2013
are outstanding examples in which the non-covalent inter-actions between hydrogen-bond donor and acceptor groupsplay an essential role in forming a fully functionalsuperstructure.
Among the hydrogen-bonding groups, 1H-pyrazole is a ve-membered heterocycle that contains a hydrogen-bond acceptorand a donor in adjacent positions. This characteristic meansthat 1H-pyrazoles are able to self-replicate and form a widevariety of superstructures. In fact, in the crystal structuresdimers, trimers, tetramers or polymeric chains (named cat-emers) are formed as a consequence of the intermolecularNH/N interactions.4–6 Hydrogen bonding has also provedcrucial in the aggregation scheme of 1H-pyrazoles that showcolumnar liquid crystalline phases. In these columnar phases asupramolecular entity formed by hydrogen-bonded dimersgives rise to the columnar aggregation.7,8 Columnar liquidcrystal phases are being intensively investigated as theycombine the ability to behave as one-dimensional charge orenergy transport pathways with the self-healing properties of aso self-assembled system.9,10 Moreover, if the molecules are
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luminescent, their emissive properties can be transferred tothe liquid crystalline phase.7 This combination of self-organi-zation and optical properties is the starting point to developnovel functional materials from supramolecular liquidcrystals.11–13
As part of our ongoing research on functional organicmaterials we are interested in the synthesis of 4-aryl-1H-pyrazoles and the study of their self-organizational ability.In this context, we have recently described a study ofintrinsic luminescent supergelators that can have enhancedemission upon aggregation in organic solvents and showsupramolecular chirality.14 The compounds were trialkoxy-benzamides substituted at the nitrogen atom by the 4-(3,5-dimethyl-1H-pyrazol-4-yl)phenyl group. Their excellentproperties were due to the H-bonding interactions between1H-pyrazole and amide moieties, which were able to act in acooperative way.
In this work we extend this study to the bulk material andreport on the synthesis, the liquid crystalline properties and thesupramolecular structures of these and other related benza-mides with different number and position of alkoxy chains(Scheme 1, PzA compounds). We found columnar meso-morphism for the 3,4,5-trialkoxy substituted compounds, whichwere studied by powder X-ray diffraction. An ester analogue(PzE3) was also studied in order to assess the role of the amidegroup in the columnar self-organization. The single crystalstructure of a model compound, N-(4-(3,5-dimethyl-1H-pyrazol-4-yl)phenyl)benzamide (PzA0), was solved and the hydrogenbonding interactions were identied. In addition, the absorp-tion and uorescent spectra were recorded in solution and inbulk, nding that the benzamide compounds exhibit an emis-sion band in the visible region in the crystal phase and in theliquid crystal phase.
Scheme 1 Synthetic routes and chemical structures of the pyrazoles studied in th
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Results and discussionSynthesis and characterization
The nal compounds were prepared by the routes depicted inScheme 1. The 1,3-diketone intermediates 3-(4-nitrophenyl)-2,4-pentanedione (DiNO2) and 3-(4-hydroxyphenyl)-2,4-pentano-dione (DiOH) were synthesized by optimizing a copper-cata-lyzed arylation of acetylacetone15,16 with p-iodonitrobenzene orp-iodophenol respectively. The precursory pyrazoles 4-amino-3,5-dimethyl-1H-pyrazole (PzNH2) and 4-hydroxyphenyl-3,5-dimethyl-1H-pyrazole (PzOH) were obtained by a condensationreaction of the 1,3-diketones with hydrazine hydrate in quan-titative yields. Hydrazine was used as the condensation andreducing agent simultaneously to obtain PzNH2 from thenitrodiketone DiNO2, with graphite added to the reactionmedium. This aminopyrazole PzNH2 was the common reagentto prepare all the benzamide compounds PzA. Amide formationproceeded selectively at the amino group by reaction with theappropriate benzoyl chlorides. Finally, the 3,4,5-tri-n-decyloxy-benzoic acid was esteried with PzOH selectively at the phenolgroup to obtain PzE3.
Compounds of the PzA family and PzE3 were characterizedby elemental analysis, NMR spectroscopy, IR and mass spec-trometry and the data are consistent with the proposedchemical structures. In each case themass spectra displayed the[M + 1]+ molecular ion peak. In the 1H NMR spectra of allcompounds the proton signal for the NH pyrazole was notobserved and the NH corresponding to the amide groupappeared as a broad singlet at around 7.7–7.8 ppm in CDCl3solution. In the 13C NMR spectra the signals for C3 and C5 ofthe pyrazole ring were broad and these signals were lost in thebaseline. In some cases only a weak and broad signal at140.4 ppm was observed. The appearance of this signal was
is work.
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attributed to a slow prototropic exchange rate.17 The C4 signal ofthe pyrazole appears as a weak signal at 117.9–118.2 ppm.
Liquid crystal properties and self-organization model
The thermal stability and the liquid crystalline behavior of thepyrazole compounds were studied by thermogravimetric anal-ysis (TGA-DTA), polarized optical microscopy (POM) anddifferential scanning calorimetry (DSC). Table 1 collects thephase transitions observed by POM and DSC, the TGA onsettemperatures and the DTA maximum rate of the decompositionprocess.
All compounds were obtained as crystalline solids at roomtemperature. As a general rule, the amide derivatives PzA meltdirectly to the isotropic liquid, with lower melting temperatureson increasing the number of alkoxy chains. PzA1, with one n-decyloxy chain, is obtained as a mixture of two crystallinephases that melt at nearly 220 �C, and on cooling crystallizes asa unique crystal phase with a pseudoisotropic texture.
Compound PzA2, with 3,4-di-n-decyloxy chains, is alsoobtained as a mixture of crystal phases and shows a complexcrystalline polymorphism on cooling. Compound PzA2b, with
Table 1 Thermal and liquid crystal properties
Compound ProcessPhase transitiona
T/�C (DH/kJ mol�1)TGATonset/�C
DTATmax/�C
PzA0 1st heating Cr 250 (39.6) I 310 359PzA1 1st heating Cr + Cr0 214–219
(47.7)b I318 369
1st cooling I 211 (32.4) Cr0
2nd heating Cr0 219 (31.8) IPzA2 1st heating Cr 103 (3.8) Cr0 124
(10.4) Cr0 0 174 (40) I378 412
1st cooling I 156–154 (38.9)b
Cr0 0 113 (10.2) Cr0
PzA2b 1st heating Cr 64 I + Cr0 94(9.6) I
341 388
1st cooling I 29 g2nd heating g 29 I
PzA3 1st heating Cr 104 (28.4) I 353 3841st cooling I 83c Colh 63
(1.9) Colr2nd heating Colr 69 (1.3)
Colh 86 (0.8) IPzA3S 1st heating Cr 64–90 (18.8)b I 369 405
1st cooling I 66c Colr(local order)
2nd heating Colr (local order)72c I
PzA3R 1st heating Cr 61–89 (20.9)b I 377 4081st cooling I 66c Colr
(local order)2nd heating Colr (local order)
71c IPzE3 1st heating Cr 51 (61.5) I
1st cooling I 37 (1.9) Colh2nd heating Colh 38 (1.5) I 244 270
a Cr, Cr0, Cr0 0: crystal phases, I: isotropic liquid, g: glassy state, Colh:hexagonal columnar mesophase, Colr: rectangular columnarmesophase. b Enthalpy sum of overlapped transitions. c Temperatureas observed by POM.
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3,5-di-n-decyloxy substitution, exhibits lower melting point thanthe isomeric PzA2. This compound does not crystallize oncooling, but vitries with a glass transition observed at 29 �C.
Compound PzA3, with three n-decyloxy chains, is obtained asa crystalline solid that becomes isotropic at 104 �C. In thecooling process a liquid crystal phase appears from theisotropic liquid at 83 �C showing at the POM a pseudo-focalconic texture with homeotropic regions (black under crossedpolarizers). These features can be assigned to a columnarmesophase (Fig. 1a). On further cooling, at around 63 �C, aphase change occurs with a subtle blurring in the texture andthe homeotropic areas becoming slightly brighter, suggesting achange to a different mesophase (Fig. 1b). The sample remainsin this phase at room temperature without crystallizing.
To conrm the liquid crystallinity and to assign the type ofmesophase a powder X-ray study of PzA3 was performed at twodifferent temperatures. The diffractogram obtained at 80 �C ischaracteristic of a hexagonal columnar (Colh) mesophase(Fig. 2a). The pattern contains a set of three sharp maxima inthe low-angle region (Table 2) in the reciprocal ratio of 1 : O3 : 2and these can be indexed as the (10), (11) and (20) reections ofa two-dimensional hexagonal lattice with a lattice constant of a¼ 42.2 A. In addition, at wide angles there is a broad diffusehalo at 4.6 A and this is characteristic of the conformationaldisorder of the hydrocarbon chains in the liquid crystal phase.
The X-ray experiment performed aer cooling the sample to20 �C showed a different diffractogram, with the reection at O3absent and only two reections, slightly deviated from thereciprocal ratio 1 : 2, observed in the low angle region (Fig. 2b).The diffuse halo at 4.6 A is maintained, indicating that thephase change corresponds to another liquid crystal phase andnot to a crystalline phase. In the absence of more reections,which would be required for an unambiguous assignment, thetwo observed spacings can be tentatively assigned to the (20)and (02) reections of the 2D-rectangular lattice (Table 2). Thiswould give lattice parameters a ¼ 74.8 A and b ¼ 39.6 A. Theseparameters are in fair agreement with the dimensions of thehexagonal lattice constant found for the high-temperaturemesophase of this compound (a ¼ 42.2 A). Indeed a hexagonallattice with constant a ¼ 42.2 A is equivalent to a C-centredorthorhombic (rectangular) lattice with constants a ¼ 73.0 Aand b ¼ 42.2 A (a/b ratio equal toO3). Under this assumption,the Colh–Colr transition on cooling involves a distortion of thelattice through a slight lengthening along the a axis and a slightshortening along the b axis.
The parameter for the hexagonal network found experi-mentally (42.2 A) is much larger than the calculated length of amolecule in a fully extended conformation (28.2 A). This ndingcan be considered together with the fact that compound PzA3has an unconventional molecular shape for a liquid crystal,being neither rod-like nor disk-like but rather a taper shapedmolecule, and has the ability to interact by hydrogen bonding.On the basis of this information it is evident that the columnarstratum is formed by more than one molecule, as hydrogenbonding is known to induce and stabilise supramolecularaggregates leading to columnar mesophases from non-discoidmolecules.3,7,8,11,12,18–23
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Fig. 1 POM microphotographs of the textures observed for (a) PzA3 at 78 �C, (b) PzA3 at 42 �C, and (c) PzE3 at 33 �C.
Fig. 2 PzA3: (a) X-ray diffractogram for the Colh mesophase at 80 �C. The (20)reflection is very weak but clearly visible in the original pattern. (b) X-ray dif-fractogram for the Colr mesophase at 20 �C.
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To estimate the number of molecules in the repeating unit ofthe hexagonal cell we performed standard density calculations.Columnar packing is characterized by two structural parame-ters, the columnar cross-section Scol (for a hexagonal networkScol ¼ a2O3/2) and the stacking periodicity c along the columnaraxis. Both parameters are analytically linked through the rela-tionship cScol ¼ ZVm, in which Z is the number of moleculeswithin a columnar stratum (disk) and Vm is the molecularvolume.24 The diffractogram of PzA3 does not show any reec-tion corresponding to the stacking periodicity c. In this situa-tion it is difficult to determine Z and only rough estimations canbe made. From the previous equation, the thickness of the disk
Table 2 X-ray diffraction data for the liquid crystalline phasesa
Compd T/�C dobs/A hk dcalc/AMesophase andlattice constants
PzA3 80 36.5 10 36.5 Colh21.5 11 21.1 a ¼ 42.2 A18.0 20 18.34.6 (br) — —
PzA3 20 37.4 20 Colr19.8 02 a ¼ 74.8 A4.6 (br) — b ¼ 39.6
PzA3S 20 36.5 (diff) 20 Colr (local order)18.6 (diff) 02 a ¼ 73.0 A4.6 (br) — b ¼ 37.2 A
PzE3 20 37.1 10 37.1 Colh21.5 11 21.4 a ¼ 42.8 A18.5 20 18.54.6 (br) —
a dobs, dcalc: observed and calculated spacing; hk: Miller indexes; br:broad maximum; diff: diffuse maximum.
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can be estimated by considering the number of molecules perdisk Z as a variable. In this respect, we can consider that thestacking is not periodic and c is the mean distance betweenneighboring aggregates. A reasonable value for this distance isdeduced from the diffuse scattering maximum found around4.6 A. According to these calculations, a density of about 1 gcm�3 is estimated assuming that ve molecules, on average,occupy the columnar cross-section.
Therefore, we can propose a model for the columnar orga-nization in which PzA3 forms aggregates through hydrogenbonding that involve, on average, ve molecules that are able tostack in columns. This column could be formed by taking intoaccount the tapered shape of the molecule and considering anantiparallel side-by-side aggregation of the aromatic cores withhydrogen-bonding interactions between amide and pyrazolegroups, both acting as hydrogen bond donors and/or acceptors.This would lead to an aggregate with an inner region ofaromatic cores that interact by hydrogen bonding and an outerregion surrounded by aliphatic chains (Fig. 3). It must bestressed that this is a simplied model because the moleculesprobably aggregate in such a way that hydrogen bonding formsbetween molecular units that do not have to be in the samedisk. In such a way, the columns are built up from the micro-segregation of the hydrogen-bonded amide-pyrazole innercolumnar region and the outer hydrophobic columnar regionconsisting of the alkoxy chains.
Compounds PzA3R and PzA3S are isomers of PzA3 and bearthree branched 3,7-dimethyloctyloxy chains of opposite
Fig. 3 Proposed model for the columnar aggregate leading to the Colh meso-phase of PzA3.
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Table 3 IR data
Compd PhaseaAmideN–H st/cm�1
pzN–H st/cm�1
AmideI/cm�1
AmideII/cm�1
PzA0 Crb 3275 3173 1647 1529PzA1 Crb 3300 3171 1644 1531
Cr0c 3300 3177 1646 1535PzA2 Crb 3266 3172 1637 1531
Cr0c 3273 3173 1643 1529PzA2b Crb 3285 3179 1650 1532
gc 3273–3233–3198 (br) 1649 1529PzA3 Crb 3296sh, 3256 3179 1645 1526
I (123 �C) 3288 3194 (br) 1650 1526Colh (80 �C) 3278 3192 (br) 1648 1528Colr (50 �C) 3272 3181 1646 1524
PzA3S Crb,d 3298, 3278 3227–3196 1645 1530Colr(local order)c
3271 3192 (br) 1645 1530
PzA3R Crb,d 3296, 3278 3227–3193 1645 1530Colr(local order)c
3273 3190 (br) 1645 1530
PzE3 Crb — 3182 — —I (70 �C) — 3357 (br), 3192 — —Colh
c — 3357 (br), 3191 — —
a Based on DSC and POM data. b As-obtained solid samples at roomtemperature. c Aer a thermal treatment consisting in heating up tothe isotropic liquid and cooling at room temperature. d Partiallycrystalline solid.
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chirality. The study of these compounds is interesting in thatthe introduction of chiral terminal chains could generatesupramolecular structures leading to chiral columnar phasesand helical structures.25 We found that the thermal behavior ofPzA3R and PzA3S was inferior to that of PzA3, as they wereobtained as partially crystallized solids that aer a complexmelting tend to vitrify on cooling with isotropic textures at POM.Although not observable by optical microscopy, slow cooling ofPzA3S yielded surprisingly an X-ray diffractogram that is similarto the low-temperature phase of PzA3 (Table 2) but with verydiffuse and weak reections. This suggests that the materialpossesses a local order with a similar structure to that of PzA3but with only short-range extent of the packing. The twoobserved spacings can be temptatively assigned to the (20) and(02) reections of the 2D-rectangular lattice, slightly smallervalues than for PzA3. No chiral features were observed from thisstudy.
Finally, compound PzE3, analogous to PzA3 but with an estergroup instead of an amide group, was synthesized in order toassess the inuence of the amide group on the mesomorphicproperties. It was found that the compound melts at lowertemperatures than PzA3, due to the weaker intermolecularinteractions expected from the lack of the amide group. Oncooling the isotropic liquid at 37 �C a mesophase was observedwith a texture similar to the hexagonal columnar mesophase ofPzA3 (Fig. 1c). The mesophase was stable on cooling to roomtemperature and in the second DSC heating cycle only thetransition from the mesophase to the isotropic liquid wasobserved.
The X-ray study of the mesophase shows a diffractogramwith a set of reections in the low-angle region that iscompatible with a hexagonal columnar mesophase (Table 2)with a parameter of a ¼ 42.8 A. In this case there is a clearanalogy with the hexagonal columnar mesophase of PzA3. Byperforming similar calculations we can also propose that thecompound self-organizes in columns in which ve moleculeson average are contained in a columnar stratum. A similarmodel to that proposed for PzA3 can be envisaged. In this casethe ester group can only act as a hydrogen-bond acceptor of theNH-pyrazole in the lateral interactions leading to the columnarorganization. This means that the 1H-pyrazole moiety, and notthe amide group, is essential for the aggregation leading to thecolumnar mesomorphism.
IR study
The infrared (IR) spectra of the compounds were recorded inKBr pellets, as this technique is a sensitive tool for studying thepresence of intermolecular hydrogen bonding in the bulkmaterial (Table 3). For the benzamide compounds PzA0, PzA1,PzA2, PzA2b and PzA3 the crystal phases at room temperatureshow two different bands in the N–H stretching region, onemain band corresponding to the N–H stretch of the associatedamide groups at 3300–3266 cm�1,14,26 and another band corre-sponding to the N–H stretch of the pyrazole ring (around 3175cm�1), which corresponds to the associated 1H-pyrazoles.7,27
The presence of these two stretching bands suggests the
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formation of a well-organized hydrogen-bonded network. Inaddition, the C]O stretch (amide I band) was observed at 1637–1650 cm�1 and the N–H bend (amide II band) appeared at 1530cm�1. All of these bands are typically attributed to amidesassociated by intermolecular hydrogen bonding. Therefore, theexistence of pyrazole–pyrazole interactions and amide–amideinteractions is probable. Interestingly, for PzA2b in the glassystate the spectra display a broad band with three maxima inwhich there is a shi of the NH-pyrazole band towards higherwavenumbers, similar to what happens to compound PzA3 asdiscussed below.
With the family of compounds with three alkoxy terminalchains (PzA3, PzA3S, PzA3R) the IR spectra could be alsoanalyzed in the liquid crystal phases. Differences with respect tothe crystal phase were found as the NH stretching region wasbroader and more complex. In particular, a variable tempera-ture IR study was performed with PzA3 by recording the spectrain the crystal phase of the as-obtained product at roomtemperature, and then heating up to the isotropic liquid andrecording the spectra on cooling in the two columnar meso-phases. In all cases the NH stretching for the amide and pyr-azole groups appeared in the associated region (3300–3150cm�1), which supports the model for the self-organization inthe mesophase discussed above. The crystal phase shows twobands, one at 3179 cm�1 related to the pyrazole group and astronger band centered at 3256 cm�1 related to the amidegroup. In the columnar mesophase and in the isotropic liquidthis region broadens gradually and shis toward higher wave-numbers, thus indicating different hydrogen bonding modesbetween pyrazole and amide functions (Fig. 4a). Probably, this
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Fig. 4 VT-IR spectra of (a) PzA3 and (b) PzE3.
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is a consequence of the tapered shape of these molecules withthe three terminal alkoxy chains, which favor antiparallelinterdigitation, and it is in agreement with the model proposedfor the aggregation of PzA3 in the Colh mesophase.
In the case of compound PzE3, a similar study showed thatthe NH stretching for the pyrazole ring appears in the associ-ated region in the crystal phase (3182 cm�1). In the columnarmesophase the band shis towards higher wavenumbers, andat a similar value, than for the columnar mesophase of PzA3(3191 cm�1; Fig. 4b). In addition, a weak and broad signalaround 3357 cm�1 is observed. This signal increases in theisotropic liquid. This is indicative of the weakening of thehydrogen bonds and the appearance of monomeric pyrazoles,which could be in accordance with the fact that the pyrazole–ester (NH/O]C–O) interactions are weaker than the pyrazole–amide (NH/O]C–NH) interactions. These observations are inagreement with the model proposed for the self-organization ofPzE3 in the mesophase, and conrm that together with thethree n-decyloxy chains, the 1H-pyrazole moiety, and not theamide group, is responsible for mesomorphism.
Fig. 5 UV-Vis absorption (dashed line), and emission spectra for PzA3 in THFsolution (solid line) and as a thin film at room temperature in the columnar phase(dotted line).
Optical propertiesUV-Vis absorption and emission spectra were recordedwith THFsolutions and with thin lms at room temperature (Table 4). All
Table 4 UV-Vis absorption and emission data
Compdlabs(THF)/nm 3/L mol�1 cm�1
lem(THF)/nm
labs(lm)/nm
lem(lm)/nm
PzA0 286 1.80 � 104 354 290 442PzA1 286 2.75 � 104 352 299 427PzA2 297 2.58 � 104 354 300 426PzA2b 291 2.21 � 104 350 296 440PzA3 292 2.59 � 104 358 296 437PzA3S 295 2.26 � 104 358 294 433PzA3R 295 2.26 � 104 358 294 433PzE3 274 2.13 � 104 340 278 402
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compounds absorb in the UV region displaying a broad bandwith amaximumbetween 274 and 297 nmdue to the two-phenylring p-system. This is in accordance with other 3,5-dimethyl-pyrazoles substituted at the 4-position with conjugated aromaticsubstituents.28 It is observed that the amide connecting groupexerts a bathochromic shi with respect to the ester group.
The uorescence spectra of the PzA compounds were recor-ded in diluted tetrahydrofuran (THF) solutions (at concentra-tions between 10�5 M and 10�6 M). The spectra show a band inthe near UV region (350–358 nm) when excited at the absorptionmaxima. Under these conditions the emission band is veryweak. An estimation of the quantum yield relative to quininesulphate for PzA3 is 0.0002. The emission of the estercompound PzE3 appeared at shorter wavelengths than theemission of the amide compounds PzA.
The uorescence spectra were also recorded with thin lmsat room temperature. Whereas PzE3 showed a weak emissioncentered in the near UV region, the benzamide compoundsdisplayed a signicant red-shi (30–40 nm) with respect to PzE3and they were luminescent in the visible region. The spectradisplay a broad uorescence band with a maximum at around427–442 nm. A representative example is plotted in Fig. 5, whereit is shown that compound PzA3 is luminescent as a thin lm at
Fig. 6 ORTEP view of the two independent molecules found in the structure ofPzA0$MeOH and plane names indicated in Table 5.
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room temperature in the columnar phase, displaying a broadband centered at 437 nm. The emission band in the lm has notbeen normalized with respect to the solution in Fig. 5, beingsignicantly larger in the lm with respect to the one in THFsolution. A rough estimation of the enhancement in thequantum yield is about three orders of magnitude. The emis-sion appears to the naked eye as a light cyan color (see TOCgure).
Single-crystal structure of PzA$MeOH
Colourless single crystals of PzA0$MeOH were obtained at roomtemperature by slow evaporation of a previously ltered solu-tion of the compound in acetone/methanol. Fragile but well-formed crystals of a methanol solvate were quickly set up in anitrogen ow at 150 K to avoid crystal damage as a consequenceof loss of the solvent. X-ray diffraction data collection wascarried out at 150 K. The unit cell belongs to the triclinic spacegroup P(�1). A molecular drawing of the asymmetric unit is
Table 5 Angles formed by the two mean planes of the structures ofPzA0$MeOH described schematically in Fig. 6
Plane Angle/� Plane Angle/�
A–B 68.2 A0–B0 54.4A–C 45.6 A0–C0 36.0A–D 83.0 A0–D0 2.9B–D 28.8 B0–D0 59.5
Fig. 7 Intermolecular H-bonds found for PzA0$MeOH.
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shown in Fig. 6. The unit cell contains two independent mole-cules of PzA0 and two molecules of methanol. Both moleculesshow different rotation angles between planes. The anglesformed by mean planes of the pyrazolyl rings, the phenyl ringsand the amide group are collected in Table 5. The pyrazole ringis rotated with respect to the phenyl substituent by 45.6� and36�, respectively (planes A–C and A0–C0 respectively). This is inaccordance with the structure reported for 4-(4-hydroxyphenyl)-3,5-dimethyl-1H-pyrazole (PzOH),17 and indicates little conju-gation between the phenyl and the pyrazolyl ring.
Each pyrazole is engaged in hydrogen bonding to form adimer through methanol bridges. The dimers of the indepen-dent molecules are arranged in a parallel manner that allowsthe formation of an additional interaction through hydrogenbonding between the amide groups to form an innite chainalong the a axis (Fig. 7). Hydrogen atoms involved in thehydrogen bonding were conrmed and their positions werelocated from difference electron density maps. Bond distancesare shown in Table 6 and values are in the typical range for thistype of supramolecular interaction. The two independentmolecules, with different rotation angles between the ringplanes, alternate along the chain formed by amide–amideinteractions. The presence of the methanol molecules does notmake possible a comparison between the supramolecularstructures found in this crystal structure and the organizationin the liquid crystal phases described above. However, it showsthe enormous possibilities of the supramolecular design withthe combination of amide and 1H-pyrazole functions.
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Table 6 Intermolecular H-bond data for PzA0$MeOHa
D–H d(D–H)/A d(H/A)/A DHA angle/� d(D/A)/A A Symmetry code
N1–HN1 0.880 1.896 172.65 2.772 O4 [�x + 1, �y, �z + 1]N4–HN4 0.920 1.860 176.12 2.779 O3 [�x + 1, �y, �z + 1]N3–HN3 0.923 1.953 160.63 2.841 O2 [x + 1, y, z]N6–HN6 0.912 2.005 162.21 2.887 O1 —
a D: donor atom, A: acceptor atom, and d: distance.
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Conclusions
The synthesis of supramolecular structures derived from 4-aryl-1H-pyrazole is described. The molecular design has led to mate-rials with columnar mesophases and luminescent properties inthe visible region. Liquid crystalline behavior was observed forthe trialkoxy-substituted derivative PzA3. In this case a pyrazole–amide intermolecular interaction that gives rise to a supramo-lecular aggregate is proposed to account for the self-organizationinto columnar mesophases. An analogous compound to PzA3with an ester group instead of the amide group PzE3 also showsmesomorphism with the same aggregation scheme, meaningthat the 1H-pyrazolemoiety, andnot the amide group, is essentialfor the aggregation leading to mesomorphism.
The introduction of chirality in the terminal chains of thebenzamide (compounds Pz3S and Pz3R) yielded compoundswith no supramolecular chiral features and inferior mesomor-phic properties, as only a columnar rectangular mesophase withshort local order was deduced by a powder X-ray study per-formed on Pz3S.
The benzamide compounds with one (PzA1) and two alkoxychains (PzA2, PzA2b) were not liquid crystalline despite the factthat they contain the same aromatic region and the amide andpyrazole rings were associated by hydrogen bonding in theircrystal phases. The decyloxy chain seems to be too short tostabilize a smectic mesophase in PzA1 or to ll the space for acolumnar phase in PzA2 or PzA2b.29,30 Therefore the three alkoxyterminal chains giving an overall tapered shape to the moleculeare of fundamental importance to induce interdigitation andlateral H-bond interactions between amide/ester and pyrazole.
A model compound (PzA0) was studied by single crystal X-raydiffraction. The structures could not be directly compared withthe long alkoxy chain compounds as it was a methanol solvatein which the methanol molecules participated in the hydrogen-bonded network by acting as a bridge between two pyrazolerings.
In summary, the 4-aryl-1H-pyrazole unit exhibits a richsupramolecular chemistry. It has been demonstrated that it canbe exploited to obtainmaterials with columnar mesophases andluminescence of interest as novel hierarchized luminescentmaterials for functional so materials.31
Experimental sectionGeneral methods
Compounds were characterized by 1H-NMR, 13C-NMR, IRspectroscopy, mass spectrometry and elemental analysis
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techniques. 1H-NMR and 13C-NMR experiments were per-formed on a Bruker ARX 300 or a Bruker AVANCE 400 spec-trometer. Chemical shis are given in ppm relative to TMS,and the solvent residual peak was used as an internal stan-dard (CDCl3 dH ¼ 7.26 ppm, dC ¼ 77.0 ppm). Infrared spectrawere obtained in a Nicolet Avatar 360 FTIR spectrophotometerin the 400–4000 cm�1 range. MS analyses were performedusing a Bruker Microex spectrometer. Elemental analyseswere performed using a Perkin-Elmer 240C microanalyzer. UV-vis and uorescence spectra were obtained using ATI-UnicamUV4-200 and Perkin-Elmer LS50B spectrophotometers respec-tively. The quantum yield was calculated relative to quininesulfate (0.58 at the excitation wavelength of 350 nm in anaqueous solution of 0.1 M H2SO4). The mesogenic behaviorand transition temperatures were determined using anOlympus BH-2 polarizing microscope equipped with a LinkamTMS91 hot-stage and a CS196 hot-stage central processor. DSCTA Instruments Q-20 and Q-2000 systems were used to carryout differential scanning calorimetry experiments. Sampleswere sealed in aluminum pans and a scanning rate of 10 �Cper minute under a nitrogen atmosphere was used. Temper-atures are taken from the onset of the peak unless otherwisenoted. Thermogravimetric analyses (TGA) were performedusing a TA Instrument TGA Q5000 at a rate of 10 �C perminute under a nitrogen atmosphere. X-ray diffraction exper-iments of the mesophases were performed in a pinholecamera (Anton-Paar) operating with a point focused Ni-lteredCu-Ka beam. Lindemann glass capillaries with 0.9 mm innerdiameter were used to contain the sample and heated, whennecessary, with a high-temperature attachment. The capillaryaxis was held perpendicular to the X-ray beam and the patternwas collected on a at photographic lm. Bragg's law was usedto calculate the d spacings.
Synthetic procedures
Synthesis of 3-aryl-2,4-pentanodiones (DiNO2, DiOH). Amixture of 4-nitroiodobenzene or 4-iodophenol (35 mmol),K2CO3 (175 mmol, 24.4 g), CuI (3.5 mmol, 0.7 g), (L)-proline (7mmol, 0.8 g) and dry dimethylsulfoxide (50 mL) was stirred atroom temperature under an argon atmosphere. Finally, acety-lacetone (105 mmol, 11.6 g) was added. The mixture was stirredat 60 �C for 24 hours for DiNO2, or at 70 �C for 14 hours forDiOH. The suspension was poured into aqueous HCl (1 M)(100 mL). The product was extracted with ethyl acetate andwashed with water. The organic layer was dried with MgSO4 andthe solvent was evaporated.
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Table 7 Crystal data resolution and refinement parameters
Compound reference PzA0Chemical formula C19H21N3O2
Formula mass 323.39Crystal system Triclinica/A 9.7487(11)b/A 13.1235(18)c/A 15.369(2)a/� 107.494(13)b/� 98.048(11)g/� 110.541(12)Unit cell volume/A3 1688.5(4)Temperature/K 150(1)Space group P�1No. of formula units per unit cell, Z 4Radiation type MoKaAbsorption coefficient, m/mm�1 0.084No. of reections measured 10 814No. of independent reections 5942Rint 0.0604Final R1 values (I > 2s(I)) 0.0738Final wR(F2) values (I > 2s(I)) 0.1430Final R1 values (all data) 0.1605Final wR(F2) values (all data) 0.1831Goodness of t on F2 0.995
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For 3-(40-nitrophenyl)-2,4-pentanodione (DiNO2) the productwas puried by column chromatography (hexane/ethyl acetate15/1). It was obtained as a yellow solid (74%). 1H NMR (400MHz, CDCl3) d, ppm 16.75 (s, 1H), 8.26–8.24 (m, AA0XX0, 2H),7.40–7.38 (m, AA0XX0, 2H), 1.89 (s, 6H). 13C NMR (100 MHz,CDCl3) d, ppm 190.4, 147.3, 144.0, 132.2, 124.0, 113.6, 24.1. FTIR(KBr) n (cm�1) 1594, 1568, 1512, 1348, 1102. MS (ESI+)m/z 222.2[M + H]+.
For 3-(40-hydroxyphenyl)-2,4-pentanodione (DiOH) theproduct was puried by column chromatography (hexane/ethylacetate 10/1), dissolved in dichloromethane and precipitatedwith hexane. It was obtained as a white solid (44%). 1H NMR(400 MHz, CDCl3) d, ppm 16.60 (s, 1H), 7.04–7.01 (m, AA0XX0,2H), 6.87–6.85 (m, AA0XX0, 2H), 5.54 (s, 1H), 1.90 (s, 6H). 13CNMR (100 MHz, CDCl3): d, ppm 191.4, 155.1, 132.3, 129.0, 115.7,114.6, 24.1 FTIR (KBr) n, cm�1 3286, 1610, 1576, 1520, 1212. MS(ESI+) m/z 193.0 [M + H]+.
Synthesis of 4-(40-hydroxyphenyl)-3,5-dimethyl-1H-pyrazole(PzOH). A solution of DiOH (10mmol, 1.92 g) in ethanol (50 mL)was stirred at room temperature and hydrazine monohydrate(12.5 mmol, 0.6 g) was added dropwise. Once the addition wascomplete the mixture was heated to 90 �C. When the reactionwas nished the mixture was cooled to room temperature, thesolvent was evaporated and the product was puried byrecrystallization in ethanol. It was obtained as a white solid(78%). 1H NMR (400 MHz, Acetone-d6) d, ppm 7.13–7.11 (m,AA0XX0, 2H), 6.90–6.88 (m, AA0XX0, 2H), 2.21 (s, 6H). 13C NMR (75MHz, acetone-d6): d, ppm 156.7, 131.1, 126.3, 118.4, 116.1, 11.5.FTIR (KBr) n, cm�1 3410, 3322, 3211, 1613, 1575, 1532, 1232. MS(MALDI+, dithranol) m/z 189.3 [M + H]+.
Synthesis of 4-(40-aminophenyl)-3,5-dimethyl-1H-pyrazole(PzNH2). A mixture of DiNO2 (4.52 mmol, 1.00 g) and graphite(2.26 g) in ethanol (50 mL) was stirred at room temperatureunder an Ar atmosphere. Hydrazine monohydrate (36.2 mmol,1.85 g) was added dropwise to the solution. Once the additionwas nished the mixture was heated to 90 �C. Aer 12 hours thereaction was complete. The mixture was hot ltered throughCelite�. The solvent was evaporated and a solid was obtained.The product was puried by recrystallization in water to yield awhite solid (72%). 1H NMR (400 MHz, CDCl3) d, ppm 7.07–7.05(m, AA0XX0, 2H), 6.75–6.73 (m, AA0XX0, 2H), 3.78 (s, 2H), 2.27 (s,6H). 13C NMR (100 MHz, CDCl3) d, ppm 144.7, 130.2, 123.7,118.4, 115.2, 11.5. FTIR (KBr) n, cm�1 3401, 3290, 3177 (br) 1618,1585, 1537, 1268. MS (MALDI+, dithranol) m/z 188.2 [M + H]+.Elemental analysis calcd (%) for C11H13N3: C 70.56, H 7.00, N22.44: found: C 70.63, H 7.11, N 22.30%.
Synthesis of benzamides (PzA). Compound PzNH2 (10 mmol,1.87 g) was dissolved in dry THF (100 mL) under an Ar atmo-sphere. The solution was cooled down to 0 �C and triethylamine(NEt3) (20 mmol, 2.02 g) was added. Finally, a THF solution ofthe appropriate benzoyl chloride (10 mmol in 10 mL) was addeddropwise. The polyalkoxylated benzoyl chlorides were preparedby hydrolysis of the benzoates to form the benzoic acids andreaction with oxalyl chloride. The chiral alkoxy chains requiredfor alkylation reactions were prepared by catalytic hydrogena-tion of the commercially available (R)- or (S)-citronellylbromide.32 The reaction was stirred for 12 hours at room
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temperature. The solvent was evaporated and the residue wasdissolved in ethyl acetate and water to extract the product. Theorganic layer was dried with MgSO4 and the solvent wasremoved under vacuum. The product was obtained as a whitesolid, which was puried by recrystallization withmethanol anddried under vacuum.
N-(4-(30,50-Dimethylpyrazol-40-yl)phenyl)benzamide (PzA0)was obtained as a white solid (71%). 1H NMR (400 MHz, CDCl3)d, ppm 7.90–7.88 (m, 2H), 7.80 (s, 1H), 7.70–7.68 (m, AA0XX0,2H), 7.57–7.49 (m, 3H), 7.30–7.28 (m, AA0XX0, 2H), 2.30 (s, 6H).13C NMR (100 MHz, CDCl3): d, ppm 165.7, 136.3, 135.1, 131.9,130.2, 130.0, 128.9, 127.0, 120.4, 117.9, 11.6. FTIR (KBr) n, cm�1
3275, 3173, 3139, 1647, 1596, 1579, 1529–1517 (br). MS(MALDI+, dithranol) m/z 292.2 [M + H]+. Elemental analysiscalcd (%) for C18H17N3O: C 74.20, H 5.88, N 14.42; found: C73.97, H 5.59, N 14.40%.
N-(4-(30,50-Dimethylpyrazol-40-yl)phenyl)-4-decyloxybenzamide(PzA1) was obtained as a white solid (71%). 1H NMR (400 MHz,CDCl3) d, ppm 7.86–7.84 (m, AA0XX0, 2H), 7.77 (s, 1H), 7.68–7.66(m, AA0XX0, 2H), 7.28–7.26 (m, AA0XX0, 2H), 6.99–6.97 (m,AA0XX0, 2H), 4.02 (t, J ¼ 6.6 Hz, 2H), 2.30 (s, 6H), 1.85–1.78 (m,2H), 1.51–1.43 (m, 2H), 1.43–1.26 (m, 12H), 0.89 (t, J ¼ 6.6 Hz,3H). 13C NMR (75 MHz, CDCl3): d, ppm 165.2, 162.2, 141.6,136.4, 129.9, 129.8, 128.8, 126.8, 120.2, 117.5, 114.5, 68.3, 31.9,29.6, 29.5, 29.4, 29.3, 29.1, 26.0, 22.7, 14.1, 11.5. FTIR (KBr) n(cm�1) 3300, 3171, 3134, 1644, 1608, 1593, 1577, 1531, 1504,1253. MS (MALDI+, dithranol) m/z 448.4 [M + H]+. Elementalanalysis calcd (%) for C28H37N3O2: C 75.13, H 8.33, N 9.33;found: C 74.83, H 8.15, N 9.16%.
PzA2, PzA2b, PzA3, PzA3S and PzA3R were prepared fromPzNH2 in a similar way as for PzA1. The procedure and char-acterization data were the same as reported previously.14 PzE3was prepared from PzOH as reported previously.14
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Single crystal characterization
Single crystal X-ray diffraction data for PzA0$MeOH werecollected at 150 K on an Oxford Diffraction Xcalibur diffrac-tometer equipped with a graphite monochromator utilizingMoKa radiation (l ¼ 0.71073 Ǻ). The diffraction frames wereintegrated and corrected for absorption using the CrysAlisPro,Oxford Diffraction Ltd, Version 1.171.35.15 (release 03-08-2011CrysAlis171 .NET, compiled Aug 3 2011, 13:03:54). The structurewas solved by direct methods using SUPERFLIP.33 All rene-ments were carried out using SHELXL-9734 against the F2 datausing full-matrix least squares methods. All non-hydrogenatoms were rened with anisotropic displacement parameters.All hydrogen atoms were placed at idealized positions andassigned isotropic displacement parameters 1.2 times the Uiso
value of the corresponding bonding partner (1.5 times formethyl hydrogen atoms). HN3 and HN6 were located in adifference Fourier map and freely rened; HN1 and HN4 werelocated, their positions were improved from difference electrondensity maps and rened riding on bonded atoms. HO3 andHO4 were located in difference Fourier maps, but the distancesO–H were long and disorder and delocalization were observedin difference electron density maps, so these hydrogen atomswere removed from themodel (Fourier maps are included in theESI‡). A summary of crystal data and renement parameters isgiven in Table 7, and ORTEP plots and density maps areincluded in the ESI.‡ The CCDC 909498 contains the supple-mentary crystallographic data (excluding structure factors).
Acknowledgements
We thank the following institutions for nancial support:Gobierno de Aragon (project PI109/09, research group E04), theEuropean Union (FSE and FEDER funds) and the MINECO(Spain) (projects CTQ2011-22516, CTQ2009-09030 andMAT2009-14636-CO3-01).
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