Mesomorphic Structures of Protonated Surfactant-Encapsulated PolyoxometalateComplexes

Shengyan Yin, Wen Li, Jinfeng Wang, and Lixin Wu*State Key Laboratory of Supramolecular Structure and Materials, Jilin UniVersity, Changchun 130012,People’s Republic of China

ReceiVed: NoVember 16, 2007; In Final Form: December 28, 2007

Keggin-type heteropolyanions, H3PW12O40 (HPW), Na3PW12O40 (NaPW), H4SiW12O40 (HSiW) and K4SiW12O40

(KSiW), were encapsulated by a cationic surfactant, di[12-(4′-octyloxy-4-azophenyl)dodecyloxy]dimethylammonium bromide (L), through the replacement of counterions. The resulting surfactant-encapsulatedpolyoxometalate complexes were characterized by UV-vis, Raman, and NMR spectra in detail. Themeasurement results indicated that some azobenzene groups of the surfactant were protonated in the complexesHL/HPW (HL is the abbreviation of the protonated surfactant), HL/NaPW, and HL/HSiW during the processof encapsulation, whereas the protonation was not observed in L/KSiW. The thermotropic liquid crystalproperties of these complexes were investigated by differential scanning calorimetry, polarized opticalmicroscopy and variable-temperature X-ray diffraction. Interestingly, different smectic mesophases wereobserved between the protonated HL/HSiW and the non-protonated L/KSiW, which suggests that theprotonation of azobenzene groups in HL/HSiW plays a key role in the liquid crystalline organization. However,protonated HL/HPW and HL/NaPW exhibit a similar smectic B phase to that of the de-protonated one, L/HPW.A competitive balance between the phase separation and the volume minimization of surfactants was proposedto explain the self-organized liquid crystal structures of these protonated and non-protonated complexes. Tothe best of our knowledge, the present investigation provides a specific example for protonated hybrid materialswith stable liquid crystal properties.

Introduction

Design and synthesis of organic/inorganic hybrid liquidcrystals (LCs) have been one of the most active research areasin both chemistry and material science over recent years becausethe rich physical properties of inorganic components could beproperly introduced into LC systems.1 The resulting hybridsopen up a new possibility to exploit promising functional LCswith optical,2 electronic,3 and magnetic4 properties. To date,most studies have focused on metallomesogens, in which metalions are chemically integrated into organic ligands, presentinga tunable fluidity of the whole complex in mesophase.5 Veryrecently, the fabrication of self-assembled organic/inorganicnanohybrids, through combining the unique magnetic, electronic,and optical properties of inorganic nanoparticles or clusters withthe soft nature of organic LCs, has given rise to a newperspective in the area of hybrid LCs.1b,6 The ordered liquidcrystalline assemblies are expected to possess synergisticproperties of the hybrids. For example, a gold nanoparticle6c

and ferric oxide nanorod6d have previously been prepared witha large length to diameter ratio, chemically modified withmesomorphic organic molecules. These hybrids exhibit typicalLC properties. However, the reported instances are rare, and itis desirable to design and prepare novel organic/inorganic hybridLCs using different strategies.

Polyoxometalates (PMs) are a kind of nanoscale polyanioncluster possessing potential applications in electrochemistry,proton conduction, magnetism, and optics.7 The realization ofthese various properties strongly depends on their molecular

properties including size, shape, charge number, chemicalstructure, acidity and solubility, etc.7a,8To utilize these functionsin a processable way, many interesting methods have recentlybeen exploited,9 and among them, the enwrapping PMs withorganic molecules through electrostatic interaction has provento be a highly effective route.9c,10 The surfactant encapsulatedPM complexes (SECs) can be well-organized into variousmatrices such as Langmuir-Blodgett film,10a casting film,10b

multilayer vesicles,10c and polymers.10d However, more stableand applicable matrices that can be further functionalized arealso in demand, especially those with precisely designednanostructures. Therefore, it is expected that the introductionof PMs into organic LC systems could lead to novel hybrid LCmaterials. In previous reports,11a we applied a mesomorphiccationic surfactant to enwrap an elliptical PM and found thatthe resulting SEC exhibits interesting thermotropic LC behavior.The effects of the shape, charge number, and charge density ofthe PM, as well as alkyl chain length and rigid group ofsurfactants on the mesophase transitions have been studied indetail.11b,11c Because PMs are common strong acids,7 it is ofinterest to know whether the intrinsic properties of PMs suchas acidity can also affect the LC behavior of the complexes.On the basis of this motivation, we chose an acid-sensitivesurfactant and studied the thermal properties of the resultingcomplexes in detail. It is expected that the mesophases couldbe adjusted by the acidity of PMs, as in some cases, the tuningof liquid crystalline structures by acid stimulation is importantfor the functionalization of LCs.12

Therefore, herein we introduce a representative investigationconcerning PM-containing hybrid LCs. Strong acidic Keggin-* To whom correspondence should be addressed. E-mail: wulx@jlu.edu.cn.

3983J. Phys. Chem. B2008,112,3983-3988

10.1021/jp710940y CCC: $40.75 © 2008 American Chemical SocietyPublished on Web 03/08/2008

type PMs, H3PW12O40 (HPW) and H4SiW12O40 (HSiW), andtheir corresponding salts, Na3PW12O40 (NaPW) and K4SiW12O40

(KSiW), were encapsulated by surfactant L (Figure 1), givingthe SECs, HL/HPW (HL means the surfactant L is protonated),HL/HSiW, HL/NaPW and L/KSiW, respectively. The structuralcharacterization confirmed the protonation of azobenzene groupsof surfactant L in HL/HPW, HL/HSiW, HL/NaPW, rather thanL/KSiW. From the study of the LC properties of thesecomplexes, we obtained a series of stable protonated LCcomplexes, some of which displayed different LC characteristicswhen compared to the neutral ones. To the best of ourknowledge, these stable and reversible mesomorphic transitionsbased on protonated hybrid LC materials have not yet beenreported.

Experimental Section

Materials. HPW and HSiW were purchased from SinopharmChemical Reagent Co., Ltd and used without further purification.NaPW and KSiW were freshly prepared according to theliterature procedure.13 Surfactant L, di[12-(4′-octyloxy-4-azophenyl)dodecyloxy]dimethylammonium bromide (see Figure1) was synthesized as reported previously.11a

Preparation of Complexes. All SECs were synthesizedaccording to a modified procedure previously reported.9c,10

Typically, a yellow chloroform solution of L was addeddropwise with stirring to the HPW aqueous solution under apH of 1.2. The initial molar ratio of L to HPW was controlledat 2.8:1. Notably, the color of the organic phase turned red inseveral minutes. After stirring continuously for 4 h at 35°C,the organic phase was separated and the red powder wasobtained by the evaporation of chloroform to dryness. Theproduct HL/HPW was further dried under vacuum until itsweight remained constant. According to a similar procedure,other complexes were obtained from a mixture of L with otherPMs, corresponding to HL/NaPW, HL/HSiW, and L/KSiW. Themolar ratio of L to PMs was controlled at 3.6:1 for HSiW andKSiW, and 2.8:1 for NaPW, approaching the charge ratio ineach of them. It is noted that PMs are sensitive to the pH value

of the solution, and the chemical structure of these PMs canonly be maintained under the definite pH region.13b Thepreliminary pH values of the aqueous solutions in our case are1.5, 1.1, and 4.3 for NaPW, HSiW, and KSiW, respectively, inthe definite pH region.13b With the exception of L/KSiW, whosecolor is the same as L (yellow), the colors of the obtainedcomplexes are red for HL/HPW and HL/NaPW, and brown forHL/HSiW. The de-protonated complex (L/HPW) of HL/HPW,used in the control experiment, was obtained with HL/HPW asfollows: aqueous ammonia was added to the hot HL/HPWchloroform solution (pH value of aqueous ammonia wasadjusted to∼8) stepwise with stirring until the Raman spectrumof the residue from the solution was the same as that of L, andyellow solid was obtained by removing the solvent underreduced pressure. All five complexes were characterized by IRspectrum, elemental analysis, and TGA as follows.

HL/HPW: IR (KBr, cm-1) for HL/HPW: ν ) 3415, 2953,2919, 2852, 1602, 1581, 1500, 1469, 1247, 1078, 975, 896, 809.Anal. Calcd for HL/HPW (C198H321N15O55PBr3W12, 6268.50):C, 37.94; H, 5.16; N, 3.35. Found: C, 37.71; H, 5.04; N, 3.25.TGA suggests a mass loss of 0.833% in the range of 30-150°C arising from crystal water in HL/HPW, which matchesthe formula: (L)3(HBr)3(PW12O40)(H2O)3 (6268.50).

HL/NaPW: IR (KBr, cm-1) for HL/NaPW: ν ) 3452, 2952,2921, 2850, 1602, 1581, 1500, 1467, 1247, 1079, 975, 894, 840,809. Anal. Calcd for HL/NaPW (C198H319N15O54W12Br3P,6250.49): C, 38.05; H, 5.14; N, 3.36. Found: C, 38.50; H, 5.19;N, 3.03. TGA suggests a mass loss of 0.749% in the range of30-150 °C arising from crystal water in HL/NaPW, whichmatches the formula: (L)3(HBr)3(PW12O40)(H2O)2 (6250.49).

L/HPW: IR (KBr, cm-1) for L/HPW: ν ) 3352, 2952, 2921,2852, 1602, 1581, 1500, 1465, 1249, 1078, 975, 896, 838, 809.Anal. Calcd for L/HPW (C198H320N15O56W12P, 6043.78): C,39.35; H, 5.34; N, 3.48. Found: C, 39.66; H, 5.09; N, 3.11.TGA suggests a mass loss of 1.13% in the range of 30-150°Carising from crystal water in L/HPW, which matches theformula: (L)3(PW12O40)(H2O)4 (6043.78).

Figure 1. Chemical structure of surfactant, coordination polyhedral representations of PMs and schematic drawings of complexes.

3984 J. Phys. Chem. B, Vol. 112, No. 13, 2008 Yin et al.

HL/HSiW: IR (KBr, cm-1) for HL/HSiW: ν ) 3352, 2952,2921, 2852, 1602, 1581, 1498, 1471, 1317, 1247, 1106, 973,919, 844, 794. Anal. Calcd for HL/HSiW (C264H422N20O58W12-SiBr2, 7198.25): C, 44.05; H, 5.91; N, 3.89. Found: C, 44.25;H, 5.99; N, 3.62. TGA suggests a mass loss of 0.506% in therange of 30-150 °C arising from crystal water in HL/HSiW,which matches the formula: (L)4(HBr)2(SiW12O40)(H2O)2(7198.25).

L/KSiW: IR (KBr, cm-1) for L/KSiW: ν ) 3450, 2952,2921, 2852, 1602, 1581, 1500, 1471, 1247, 974, 920, 842, 792.Anal. Calcd for L/KSiW (C264H420N20O58W12Si, 7036.42): C,45.06; H, 6.02; N, 3.98. Found: C, 44.63; H, 6.44; N, 3.57.TGA suggests a mass loss of 0.493% in the range of 30-150 °C arising from crystal water in L/KSiW, just matchingthe formula: (L)4(SiW12O40)(H2O)2 (7036.42).

Measurements.Elemental analysis (C, H, N) was performedon a Flash EA1112 from ThermoQuest Italia S.P.A. Infraredspectra, from pressed KBr pellets, were carried out on a BrukerIFS-66V Fourier transform infrared spectrometer equipped aDTGS detector with a resolution of 4 cm-1. 1H NMR spectrawere recorded on a Bruker UltraShield 500 MHz spectrometerinstrument using CDCl3 as solvent and TMS as internalreference. UV-visible spectra were recorded on a ShimadzuUV-3100 spectrophotometer. Raman spectra were recorded ona Renishaw Model 1000 or Jobin-Yvon T6400 spectrometer andthe 514.5-nm and 488-nm lines of argon ion laser were used asexcitation source, respectively. TGA was conducted with aPerkin-Elmer TG/DTA-7 instrument and the heating rate was10 K/min. The phase behaviors were measured using a polarizedoptical microscope (POM) of Leica DMLP equipped with aMettler FP82HT hot stage and a Mettler FP90 central processor.Differential scanning calorimetry (DSC) measurements werecompleted on a Netzsch DSC 204 at a scanning rate of 5 K/min.The samples were sealed in aluminum capsules in air, and theholder atmosphere was dry nitrogen. Variable-temperature X-raydiffraction (XRD) experiments were carried out on a PhilipsPW 1700 X-ray diffractometer (using Cu KR1 radiation of awavelength of 1.54 Å) with a TTK-HC temperature controller.

Results and Discussion

Characterization of Complexes.In this study, we employedazobenzene-containing surfactant L to encapsulate Keggin-typestrong acids HPW and HSiW. Their corresponding salts, NaPWand KSiW, were also encapsulated as a comparison. Thesynthetic procedure was described in detail in the ExperimentalSection. All of the as-prepared complexes were no longer solublein water, but were soluble in hot organic media such aschloroform, suggesting that the surface of PMs had beeneffectively modified by surfactant L. However, it should bementioned that the solubility of complexes in chloroformsolution is considerably poor, and their colors are different fromthat of the surfactant L, with the exception of L/KSiW, whichevidently differs from those in our previous reports.11a,11bAllof the complexes are thermally stable in air according to theidentical IR spectra before and after the heating cycles.Considering the color darkening of surfactant L in the com-plexes, we propose some change having taken place during theencapsulation process, as the color is unchangeable when weperform the same procedure under the neutral conditions.Spectroscopic measurements were used to examine the colorchange from L to the complexes, and as a representativeexample, UV-vis absorption spectra of L and HL/HPW areshown in Figure 2. Both pure surfactant L and the de-protonatedcomplex L/HPW in their chloroform solution show a typical

absorption band at 360 nm, indicating that L and L/HPW existin their molecularly dispersed states.14 In contrast, HL/HPW inchloroform solution shows multiple absorption bands at 260,360 nm and a broad absorption band in the range of 470-570nm. On the basis of the pH dependence of UV-vis spectra ofazobenzene derivatives,15 the broad band at 535 nm can beclearly ascribed to the absorption of a protonated azobenzenegroup, which is consistent with the color of the complex. Evenin this case, we still observed the normal band of the azobenzenegroup appearing at 360 nm, suggesting the coexistence of neutraland protonated species in the chloroform solution of HL/HPW.

Protonation of Azobenzene Groups.To further confirm theprotonation of azobenzene groups in the complex, Ramanspectra were employed to investigate the protonation of thecomplexes (see Supporting Information), and the results showthat the protonation of azobenzene is present in HL/HPW, HL/NaPW, and HL/HSiW, but not in L/KSiW and L/HPW.Furthermore, elemental analysis and TGA measurements wereapplied to quantitatively calculate the degree of protonation.TGA curves in the range of 30-150 °C show the presence of2-4 crystal water molecules in all of the complexes. The resultsof elemental analysis and the TGA suggest that half of theazobenzene groups have been protonated in HL/HPW and HL/NaPW, and one-fourth in HL/HSiW, but none in L/KSiW andL/HPW, as indicated in the molecular formula of thesecomplexes in the experimental section. Thus, the azobenzenegroups are not protonated completely in the complexes, whichis consistent with the observation of UV-vis and Ramanspectra.

As the encapsulation process is driven by electrostatic forcebetween L and PMs, there are two possible binding positionsof L: one is the cationic ammonium and the other is theprotonated azobenzene group. To identify the exact bindingposition of protonated L with PMs, the complexes werecharacterized by1H NMR spectra. As shown in Figure 3, theproton chemical shift of surfactant L in the complex HL/HPW,

Figure 2. UV-vis spectra of HL/HPW (red), L/HPW (green) and L(black) in chloroform solution.

Figure 3. 1H NMR spectra of (a) surfactant L and (b) HL/HPW inCDCl3, respectively.

Protonated Polyoxometalate Complexes J. Phys. Chem. B, Vol. 112, No. 13, 20083985

in contrast to that of surfactant L alone, shows the followingcharacteristic: (1) the proton peak ofN-methyl (N+-CH3) hassignificantly broadened and shifted to high field by 0.12 ppm;(2) the proton signal ofN-methylene (N+-CH2), which is atriplet in pure L, becomes a considerably broadened singlet andhas shifted toward high field by 0.24 ppm; and (3) other peaksmaintain the same positions as pure L. The peak broadeningshould result from the strong electrostatic interaction betweenthe L and PW cluster, which restricts the mobility of theammonium head group, as reported in the literature.16 Consider-ing that the chemical shift and peak width of the proton signalare related to the local physical and chemical environment, wesuggest that it is the cationic ammonium headgroup of surfactantL that binds to the negative charged cluster electrostatically,even if some azobenzene groups of surfactant L are protonated.Thus, to balance the charge on the protonated azobenzene group,the corresponding counterion should be Br-, which comes fromL. To verify this assumption, an aqueous solution of AgNO3

was added to the chloroform solution of HL/HPW, HL/NaPW,and HL/HSiW. Upon addition, the color of the complexeschanged to yellow, as pure L, and was immediately accompaniedby a white precipitate, while a similar phenomenon was notobserved for the non-protonated L/KSiW, suggesting the pres-ence of Br- as the counterion in the protonated samples.Furthermore, we tried to remove hydrogen bromide from theprotonated azobenzene groups by carefully treating HL/HPWwith aqueous ammonia. Raman spectra confirmed that thedeprotonation of the azobenzene groups was successful (seeSupporting Information). IR spectra proved that the chemicalstructure of the heteropolyanion cluster PW12O40

3- in theprotonated complex L/HPW was well-maintained and elementalanalysis of L/HPW shows that the molar ratio of surfactant Lto PW still retained 3:1. Those results suggest that thePW12O40

3- cluster was not lost during the deprotonation process,confirming the corresponding counterion around the protonatedazobenzene group is Br-, which comes from the surfactant.Therefore, we propose that the hydrogen bromide, resulting fromcounterions of both L and PMs, protonates the azobenzenegroups during the encapsulation process, as presented schemati-cally in Figure 1.

Mesomorphic Behavior of Complexes.The thermal proper-ties of the obtained complexes are investigated by DSC, POM,and variable-temperature XRD. The phase transition tempera-tures, enthalpies and assignments of the phase transitions forall the complexes are summarized in Table 1.

From the DSC curves shown in Figure 4, protonatedcomplexes HL/HPW and HL/NaPW exhibit two phase transi-tions similar to that of the deprotonated L/HPW, which can beassigned to the changes from solid state to liquid crystal phaseand then to isotropic liquid, respectively. In the cases of HL/HSiW and L/KSiW, both display rich transitions, especially forthe former, presenting more phase changes during the coolingcycle from isotropic liquid.

The POM images of the complexes are shown in Figure 5.A smectic B (SmB) phase is affirmed in the case of HL/HPWand HL/NaPW (see Figure 5, parts a and b) because the observedmosaic texture is a common characteristic of SmB phase,particularly when it is obtained through directly decreasingtemperature from isotropic liquid.17 The texture of L/HPW isalso indicative of SmB phase during the cooling process, whichis consistent with that of HL/HPW and HL/NaPW (see Figure5c). In the case of HL/HSiW, the focal conic fan-shaped textureat 201°C (Figure 5d) and broken fan-shaped texture at 185°C(Figure 5e) are observed and can be attributed to Smectic A(SmA) and Smectic C (SmC) phase, respectively. For L/KSiW,the lancet texture, which appeared at 210°C (Figure 5f),suggests the formation of SmB phase. The different liquidcrystalline structures and phase transitions between HL/HSiWand L/KSiW reveal that the protonation of azobenzene groupsplays a key role in the mesomorphic characterization ofcomplexes.

The LC behaviors of all the complexes were further inves-tigated by a variable-temperature XRD measurement. The XRDdata supports the assignments of mesophases. As shown inFigure 6, when HL/HPW and HL/NaPW slowly cool down fromtheir isotropic state, strong equidistant diffractions in the small-angle region emerge along with a single sharp diffraction at 2θ≈ 20° (Figure 6, parts a and b). The equidistant diffractionscorrespond to a layered structure with thed-spacing of 4.01and 4.06 nm calculated from the Bragg equation, respectively.The appearance of a single sharp diffraction at the wide-angleregion for HL/HPW and HL/NaPW indicates the existence ofSmB phase.18 Meanwhile, L/HPW shows an order smecticlayered structure with a layer spacing of 4.11 nm. On the basisof our present results, we believe that the LC structure of theprotonated HL/HPW and HL/NaPW is the same as the de-protonated L/HPW.

HL/HSiW features a disordered smectic phase correspondingto a periodicity of 4.26 nm during its cooling run from theisotropic state, as demonstrated by the presence of equidistantdiffractions in the small-angle region (Figure 6c) and a broadand unconspicuous halo centered at 2θ ≈ 20° (Figure 6c, inset).Considering the fan-shaped texture (Figure 5d), this phase isidentified as SmA phase. When the temperature decreases to197 °C, another lamellar structure appears (Figure 6c). Com-bining the unchanged diffusion peak in the wide-angle region

TABLE 1: Phase Transition Temperatures (°C), Enthalpies(kJ/mol), and Assignments of the Phase Transitions for Allof the Complexesa

first cooling second heating

sample transition T (°C) ∆H (kJ/mol) T (°C) ∆H (kJ/mol)

HL/HPW S-SmB 110 39.85 123 41.59SmB-Iso 198 9.26 207 8.18

HL/NaPW S-SmB 110 34.94 121 34.50SmB-Iso 196 9.85 206 6.47

L/HPW S-SmB 109 42.75 121 45.23SmB-Iso 188 8.45 191 6.04

HL/HSiW S1-S2 109 29.87 111 41.55S2-SmC 164 38.81 170 43.25SmC-SmA 197 31.96SmA-Iso 205 22.49 218 75.01

L/KSiW S1-S2 109 13.30 113 11.61S2-S3 128 4.90 133 6.37S3-SmB 169 47.12 175 49.46SmB-Iso 210 58.24 220 75.29

a (S, SmC, SmB, SmA, and Iso indicate solid, smectic C, smecticB, smectic A, and isotropic phase, respectively.)

Figure 4. DSC curves of HL/HPW, HL/NaPW, L/HPW, HL/HSiW,and L/KSiW on their (a) second heating and (b) first cooling cycles,respectively.

3986 J. Phys. Chem. B, Vol. 112, No. 13, 2008 Yin et al.

(Figure 6c, inset) and the broken fan-shaped texture (Figure 5e),the mesophase appearing at 185°C is identified as SmC phase.With decreasing temperature, the phase transition from SmCto a solid state occurs at 164°C, and the further cooling leadsto a solid-solid transition. Hence, the sequence of mesophasesfor HL/HSiW in the cooling run can be ascribed to the processof Iso-SmA-SmC-(Solid-2)-(Solid-1). In contrast to HL/HSiW,L/KSiW displays a different mesophase sequence. When cooledfrom isotropic liquid, the first lamellar LC phase with a layerthickness of 5.04 nm forms, as illustrated in Figure 6d.Combining the lancet texture (Figure 5f) at the same temperatureregion and the single sharp diffraction at 2θ ≈ 20° (Figure 6dinset), the mesophase can be attributed to SmB phase, and itsstructure keeps until the temperature goes down to 169°C, thenthe phase changes to solid. The solid state can be definitelyassigned from the feature of multiple diffractions in the small-angle region and a number of sharp peaks in the wide-angleregion in the diffraction pattern. The calculated layer spacingsfor all the complexes are listed in Table 2. It is noted that thelayer spacing of L/KSiW is around 5 nm, whereas those of theother complexes are around 4 nm. Combining the radius (0.52nm) of Keggin-type PMs, and the ideal length of L with alltrans-conformation (3.45 nm, calculated by MM2 force field

method), the total thickness of a single complex should bearound 7.9 nm. The simulated molecular length is much largerthan the measured layer distance of all the complexes in theirmesophases, indicating the presence of a partially interdigitatedlamellar structure in L/KSiW and deeply interdigitated lamellarstructures in other complexes.

It is interesting to discuss the reason that HL/HSiW exhibitstwo mesophases, SmA and SmC, whereas L/KSiW forms onlySmB phase. From a quick comparison of the chemical formulas,we can find that the only difference between L/KSiW and HL/HSiW is the presence of protonated azobenzene groups in HL/HSiW. Therefore, it is reasonable that the protonation ofazobenzene groups leads to the different mesomorphic behaviorsin HL/HSiW when compared to with that of L/KSiW. Figure 1shows that all the complexes possess a hydrophobic organicshell and a hydrophilic inorganic core. These complexes arelikely to form lamellar structures due to the phase separationof incompatible molecular components, the aggregation ofcompatible units and the volume minimization of the surfactants.Combining the present results and the crystal structures of somesimilar complexes reported in the literature,19 we propose thatthe mesomorphic structures of the complexes correlate with thematching extent of the volume between the hydrophilic PMcluster and hydrophobic alkyl chains. When the volume ratioof two components in the complex is well-proportioned, anordered smectic phase would be the predominant structure.Otherwise a disordered smectic phase should be the preferentialstructure. In our case, SmB phase of L/KSiW suggests that thevolume of the SiW cluster is well-proportioned to that of thealkyl chains. And in HL/HSiW, as the presence of protonatedazobenzene group increases the incompatibility between thesurfactants, the volume of alkyl chains increases. The enlargedvolume of hydrophobic part makes the volume of SiW clustermismatch with that of alkyl chains, leading to disordered SmAand SmC phases. The explanation can be supported by theresults of XRD. For L/KSiW with a tight packing of alkylchains,

Figure 5. POM images of (a) HL/HPW at 172°C, (b) HL/NaPW at 164°C, (c) L/HPW at 150°C, (d) HL/HSiW at 201°C, (e) HL/HSiW at 185°C, and (f) L/KSiW at 194°C, respectively, during the cooling process (magnification:×400).

Figure 6. Variable-temperature X-ray diffraction patterns of (a) HL/HPW, (b) HL/NaPW, (c) HL/HSiW, and (d) L/KSiW, respectively.(Inset: diffractions in wide-angle region).

TABLE 2: Summary of Layer Spacings (d) for All of theComplexes Calculated from X-ray Diffractions

layer spacing d (nm)

sample SmA SmB SmC solid 3 solid 2 solid 1

HL/HPW 4.01 3.69HL/NaPW 4.06 3.64L/HPW 4.11 3.72HL/HSiW 4.26 4.27 4.17 4.04L/KSiW 5.04 4.81 4.45 4.29

Protonated Polyoxometalate Complexes J. Phys. Chem. B, Vol. 112, No. 13, 20083987

it shows a bit larger layer thickness due to the partiallyinterdigitated layer structure, whereas for HL/HSiW with relaxedalkyl chains, it exhibits a deeply interdigitated structure. Thisimplies that the volume of alkyl chains in HL/HSiW is largerthan that of L/KSiW. As for HL/HPW, in spite of the presenceof protonated azobenzene groups, the complex still exhibits SmBphase. Comparing HL/HPW with HL/HSiW, the followingcharacteristics are obvious: (1) SiW and PW are both Keggintype clusters and have the same size and shape; (2) both HL/HPW and HL/HSiW show deeply interdigitated layers and thelayer spacings of HL/HPW and HL/HSiW are almost the same;(3) the unique difference is in the amount of surfactant moleculesin HL/HPW and HL/HSiW. According to the results ofelemental analysis and TGA, the surfactants around PW are lessthan those covered on SiW, providing more space for theinterdigitation of alkyl chains. Though the deeply interdigitatedlamellar structures are present in HL/HPW, the volume of thePW cluster is still well-proportioned to that of the alkyl chains,thus the SmB phase is well maintained. The current resultimplies that the protonation of azobenzene groups can be usedto tune the LC mesophase of the complexes by the appropriatechoice of PMs.

Conclusions

In this paper, we report mesomorphic structures of a kind ofnovel hybrid LC material with protonated mesogenic groups.Azobenzene-containing surfactant encapsulated polyoxometalatecomplexes, HL/HPW, HL/HSiW, HL/NaPW, and L/KSiW, havebeen prepared. Among them, HL/HPW, HL/HSiW, and HL/NaPW are protonated while L/KSiW is non-protonated. Theintrinsic acidity of the applied PMs is considered to be the mainreason for the protonation of these complexes. Although theprotonation creates additional positively charged sites in thesurfactant molecule, the bonding position between L and PMsis still at the ammonium head group definitely. The bromic anionderived from ion replacement binds to the azobenzene groupas a counterion in these protonated complexes. The protonatedcomplex HL/HSiW reveals SmA and SmC phases, while thecorresponding non-protonated complex L/KSiW exhibits onlySmB phase. The protonated complexes HL/HPW and HL/NaPWself-organize into SmB phase, similar to that of non-protonatedL/HPW. The competitive balance between the phase separationand the volume minimization of surfactants is supposed to playan important role and could be employed to explain the differentself-organized LC structures of these protonated and non-protonated complexes. Both the number of surfactants on thesurface of PMs and the protonation of the azobenzene groupmake the LC phase of SECs become diversified. This allowspotential applications in developing acidity stimulation-responsehybrid LC materials.

Acknowledgment. The authors acknowledge the financialsupport from National Basic Research Program (2007CB808003),National Natural Science Foundation of China (20473032,20574030), PCSIRT of Ministry of Education of China(IRT0422), and Open Project of State Key Laboratory ofPolymer Physics and Chemistry of CAS.

Supporting Information Available: Raman spectra of thecomplexes, assignments of characteristic Raman shift, andadditional references. This material is available free of chargevia the Internet at http://pubs.acs.org.

References and Notes

(1) (a) Tschierske, C.J. Mater. Chem.2001, 11, 2647. (b) Saez, I. M.;Goodby, J. W.J. Mater. Chem.2005, 15, 26. (c) Gin, D. L.; Lu, X.; Nemade,P. R.; Pecinovsky, C. S.; Xu, Y.; Zhou, M.AdV. Funct. Mater.2006, 16,865.

(2) (a) Guillet, E.; Imbert, D.; Scopelliti, R.; Bu¨nzli, J.-C. G.Chem.Mater. 2004, 16, 4063. (b) Bayon, R.; Coco, S.; Espinet, P.Chem. Eur. J.2005, 11, 1079. (c) Pucci, D.; Barberio, G.; Bellusci, A.; Crispini, A.;Donnio, B.; Giorgini, L.; Ghedini, M.; Deda, M. L.; Szerb, E. I.Chem.Eur. J. 2006, 12, 6738.

(3) Matsuo, Y.; Muramatsu, A.; Kamikawa, Y.; Kato, T.; Nakamura,E. J. Am. Chem. Soc.2006, 128, 9586.

(4) Barbera´, J.; Gimenez, R.; Marcos, M.; Serrano, J. L.; Alonso, P.J.; Martınez, J. I.Chem. Mater.2003, 15, 958.

(5) (a) Hudson, S. A.; Maitilis, P. M.Chem. ReV. 1993, 93, 861. (b)Metallomesogens: Synthesis and Applications; Serrano, J. L., Ed.; VCH:Weinheim 1996. (c) Binnemans, K.; Go¨rller-Walrand, C.Chem. ReV. 2002,102, 2303. (d) Donnio, B.; Guillon, D.; Deschenaux, R.; Bruce, D. W. InComprehensiVe Coordination Chemistry; McCleverty, J. A., Meyer, T. J.,Eds.; Elsevier: Oxford, 2003; Vol. 7, pp 357-627.

(6) (a) Kanayama, N.; Tsutsumi, O.; Kanazawa, A.; Ikeda, T.Chem.Commun.2001, 2640. (b) Kanie, K.; Sugimoto, T.J. Am. Chem. Soc.2003,125, 10518. (c) Cseh, L.; Mehl, G. H.J. Am. Chem. Soc.2006, 128, 13376.(d) Kanie, K.; Muramatsu, A.J. Am. Chem. Soc.2005, 127, 11578. (e)Cseh, L.; Mehl, G. H.;J. Mater. Chem.2007, 311.

(7) (a) Pope, M. T.; Mu¨ller, A. Angew. Chem., Int. Ed. 1991, 30, 34.(b) Hill, C. L. Chem. ReV. 1998, 98, 1. The entire issue is devoted topolyoxometalates. (c) Pope, M. T.; Mu¨ller, A. Polyoxometalate Chemistryfrom TopologyVia Self-Assembly to Application; Kluwer Academic Publish-ers: Dordrecht, The Netherlands, 2001.

(8) (a) Pope, M. T.Heteropoly and Isopoly Oxometalates; Springer-Verlag: Berlin, 1983. (b) Pope, M. T.; Mu¨ller, A. Polyoxometalates: fromPlatonicSolids to Anti-retroViral ActiVity; KluwerAcademic Publishers:Norwell, MA, 1994.

(9) (a) Volkmer, D.; Du Chesne, A.; Kurth, D. G.; Schnablegger, H.;Lehmann, P.; Koop, M. J.; Mu¨ller, A. J. Am. Chem. Soc.2000, 122, 1995.(b) Liu, S.; Kurth, D. G.; Bredenko¨tter, B.; Volkmer, D.J. Am. Chem. Soc.2002, 124, 12279. (c) Errington, R. J.; Petkar, S. S.; Horrocks, B. R.;Houlton, A.; Lie, L. H.; Patole, S. N.Angew. Chem., Int. Ed. 2005, 44,1254.

(10) (a) Kurth, D. G.; Lehmann, P.; Volkmer, D.; Mu¨ller, A.; Schwahn,D. J. Chem. Soc. Dalton Trans.2000, 3989 (b) Bu, W.; Fan, H.; Wu, L.;Hou, X.; Hu, C.; Zhang, G.; Zhang, X.Langmuir2002, 18, 6398. (c) Li,H.; Sun, H.; Qi, W.; Xu, M.; Wu, L.Angew. Chem., Int. Ed. 2007, 46,1300. (d) Li, H.; Qi, W.; Li, W.; Sun, H.; Bu, W.; Wu, L.AdV. Mater.2005, 17, 2688.

(11) (a) Li, W.; Bu, W.; Li, H.; Wu, L.; Li, M. Chem. Comm.2005,3785. (b) Li, W.; Yin, S.; Wu, Y.; Wu, L.J. Phys. Chem. B2006, 110,16961. (c) Li, W.; Yin, S.; Wang, J.; Wu, L.Chem. Mater.2008, 20,514.

(12) (a) Pecinovsky, C. S.; Nicodemus, G. D.; Gin, D. L.Chem. Mater.2005, 17, 4889. (b) Tan, B.; Yoshio, M.; Ichikawa, T.; Mukai, T.; Ohno,H.; Kato, T.Chem. Comm.2006, 4703.

(13) (a) Cao, H.Handbook of Inorganic Chemistry Synthesis: BeijingScience Press: Beijing, 1988; Vol. 3, pp 409, 565. (b) Moffat, J. B. Metal-Oxygen Clusters: The Surface and Catalytic Properties of HeteropolyOxometalates; Kluwer Academic/Plenum Publishers: New York, 2001; pp62-68.

(14) (a) Kuiper, J. M.; Engberts, J. B. F. N.Langmuir2004, 20, 1152.(b) Shimomura, M.; Kunitake, T.J. Am. Chem. Soc.1987, 109,5175.

(15) (a) Tian, Y.; Isono, N.; Kawai, T.; Umemura, J.; Takenaka, T.Langmuir1988, 4, 693. (b) Tian, Y.; Umemura, J.; Takenaka, T.Langmuir1988, 4, 1064. (c) Nihei, M.; Kurihara, M.; Mizutani, J.; Nishihara, H.J.Am. Chem. Soc.2003, 125, 2964.

(16) (a) Kurth, D. G.; Lehmann, P.; Volkmer, D.; Mu¨ller, A.; Schwahn,D. J. Chem. Soc. Dalton Trans.2000, 3989. (b) Polarz, S.; Smarsly, B.;Antonietti, M. Chem. Phys. Chem.2001, 2, 457.

(17) (a) Xu, J.; Toh, C. L.; Liu, X.; Wang, S.; He, C.; Lu, X.Macromolecules2005, 38, 1684. (b) Dierking, I. Textrue of Liquid Crystals;Wiley-VCH: Weinheim, Germany, 2003; p 135, and referencestherein.

(18) (a) de Vries, A.Chem. Phys. Lett.1974, 28, 252. (b) Krigbaum,W. R.; Watanabe, J.; Ishikawa, T.Macromolecules1983, 16, 1271. (c) Hsu,C.; Lin, J.; Chou, L.; Hsiue, G.Macromolecules1992, 25, 7126. (d) Xu,J.; Toh, C. L.; Liu, X.; Wang, S.; He, C.; Lu, X.Macromolecules2005,38, 1684.

(19) Nyman, M.; Ingersoll, D.; Singh, S.; Bonhomme, F.; Alam, T.;Brinker, C.; Rodriguez, M.Chem. Mater. 2005, 17, 2885.

3988 J. Phys. Chem. B, Vol. 112, No. 13, 2008 Yin et al.