Mesomorphic Structures of Protonated Surfactant-Encapsulated

Mar 8, 2008 - Interestingly, different smectic mesophases were observed between the protonated HL/HSiW and the non-protonated L/KSiW, which suggests t...
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J. Phys. Chem. B 2008, 112, 3983-3988

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Mesomorphic Structures of Protonated Surfactant-Encapsulated Polyoxometalate Complexes 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]dimethylam monium bromide (L), through the replacement of counterions. The resulting surfactant-encapsulated polyoxometalate complexes were characterized by UV-vis, Raman, and NMR spectra in detail. The measurement results indicated that some azobenzene groups of the surfactant were protonated in the complexes HL/HPW (HL is the abbreviation of the protonated surfactant), HL/NaPW, and HL/HSiW during the process of encapsulation, whereas the protonation was not observed in L/KSiW. The thermotropic liquid crystal properties of these complexes were investigated by differential scanning calorimetry, polarized optical microscopy and variable-temperature X-ray diffraction. Interestingly, different smectic mesophases were observed between the protonated HL/HSiW and the non-protonated L/KSiW, which suggests that the protonation 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 proposed to explain the self-organized liquid crystal structures of these protonated and non-protonated complexes. To the best of our knowledge, the present investigation provides a specific example for protonated hybrid materials with stable liquid crystal properties.

Introduction Design and synthesis of organic/inorganic hybrid liquid crystals (LCs) have been one of the most active research areas in both chemistry and material science over recent years because the rich physical properties of inorganic components could be properly introduced into LC systems.1 The resulting hybrids open up a new possibility to exploit promising functional LCs with optical,2 electronic,3 and magnetic4 properties. To date, most studies have focused on metallomesogens, in which metal ions are chemically integrated into organic ligands, presenting a tunable fluidity of the whole complex in mesophase.5 Very recently, the fabrication of self-assembled organic/inorganic nanohybrids, through combining the unique magnetic, electronic, and optical properties of inorganic nanoparticles or clusters with the soft nature of organic LCs, has given rise to a new perspective in the area of hybrid LCs.1b,6 The ordered liquid crystalline assemblies are expected to possess synergistic properties of the hybrids. For example, a gold nanoparticle6c and ferric oxide nanorod6d have previously been prepared with a large length to diameter ratio, chemically modified with mesomorphic organic molecules. These hybrids exhibit typical LC properties. However, the reported instances are rare, and it is desirable to design and prepare novel organic/inorganic hybrid LCs using different strategies. Polyoxometalates (PMs) are a kind of nanoscale polyanion cluster possessing potential applications in electrochemistry, proton conduction, magnetism, and optics.7 The realization of these various properties strongly depends on their molecular * To whom correspondence should be addressed. E-mail: [email protected].

properties including size, shape, charge number, chemical structure, acidity and solubility, etc.7a,8 To utilize these functions in a processable way, many interesting methods have recently been exploited,9 and among them, the enwrapping PMs with organic molecules through electrostatic interaction has proven to be a highly effective route.9c,10 The surfactant encapsulated PM complexes (SECs) can be well-organized into various matrices such as Langmuir-Blodgett film,10a casting film,10b multilayer vesicles,10c and polymers.10d However, more stable and applicable matrices that can be further functionalized are also in demand, especially those with precisely designed nanostructures. Therefore, it is expected that the introduction of PMs into organic LC systems could lead to novel hybrid LC materials. In previous reports,11a we applied a mesomorphic cationic surfactant to enwrap an elliptical PM and found that the resulting SEC exhibits interesting thermotropic LC behavior. The effects of the shape, charge number, and charge density of the PM, as well as alkyl chain length and rigid group of surfactants on the mesophase transitions have been studied in detail.11b,11c Because PMs are common strong acids,7 it is of interest to know whether the intrinsic properties of PMs such as acidity can also affect the LC behavior of the complexes. On the basis of this motivation, we chose an acid-sensitive surfactant and studied the thermal properties of the resulting complexes in detail. It is expected that the mesophases could be adjusted by the acidity of PMs, as in some cases, the tuning of liquid crystalline structures by acid stimulation is important for the functionalization of LCs.12 Therefore, herein we introduce a representative investigation concerning PM-containing hybrid LCs. Strong acidic Keggin-

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

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

type PMs, H3PW12O40 (HPW) and H4SiW12O40 (HSiW), and their corresponding salts, Na3PW12O40 (NaPW) and K4SiW12O40 (KSiW), were encapsulated by surfactant L (Figure 1), giving the SECs, HL/HPW (HL means the surfactant L is protonated), HL/HSiW, HL/NaPW and L/KSiW, respectively. The structural characterization confirmed the protonation of azobenzene groups of surfactant L in HL/HPW, HL/HSiW, HL/NaPW, rather than L/KSiW. From the study of the LC properties of these complexes, we obtained a series of stable protonated LC complexes, some of which displayed different LC characteristics when compared to the neutral ones. To the best of our knowledge, these stable and reversible mesomorphic transitions based on protonated hybrid LC materials have not yet been reported. Experimental Section Materials. HPW and HSiW were purchased from Sinopharm Chemical Reagent Co., Ltd and used without further purification. NaPW and KSiW were freshly prepared according to the literature procedure.13 Surfactant L, di[12-(4′-octyloxy-4azophenyl)dodecyloxy]dimethylammonium bromide (see Figure 1) was synthesized as reported previously.11a Preparation of Complexes. All SECs were synthesized according to a modified procedure previously reported.9c,10 Typically, a yellow chloroform solution of L was added dropwise with stirring to the HPW aqueous solution under a pH of 1.2. The initial molar ratio of L to HPW was controlled at 2.8:1. Notably, the color of the organic phase turned red in several minutes. After stirring continuously for 4 h at 35 °C, the organic phase was separated and the red powder was obtained by the evaporation of chloroform to dryness. The product HL/HPW was further dried under vacuum until its weight remained constant. According to a similar procedure, other complexes were obtained from a mixture of L with other PMs, corresponding to HL/NaPW, HL/HSiW, and L/KSiW. The molar ratio of L to PMs was controlled at 3.6:1 for HSiW and KSiW, and 2.8:1 for NaPW, approaching the charge ratio in each of them. It is noted that PMs are sensitive to the pH value

of the solution, and the chemical structure of these PMs can only be maintained under the definite pH region.13b The preliminary pH values of the aqueous solutions in our case are 1.5, 1.1, and 4.3 for NaPW, HSiW, and KSiW, respectively, in the definite pH region.13b With the exception of L/KSiW, whose color is the same as L (yellow), the colors of the obtained complexes are red for HL/HPW and HL/NaPW, and brown for HL/HSiW. The de-protonated complex (L/HPW) of HL/HPW, used in the control experiment, was obtained with HL/HPW as follows: aqueous ammonia was added to the hot HL/HPW chloroform solution (pH value of aqueous ammonia was adjusted to ∼8) stepwise with stirring until the Raman spectrum of the residue from the solution was the same as that of L, and yellow solid was obtained by removing the solvent under reduced pressure. All five complexes were characterized by IR spectrum, 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 30150 °C arising from crystal water in HL/HPW, which matches the 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 of 30-150 °C arising from crystal water in HL/NaPW, which matches 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 °C arising from crystal water in L/HPW, which matches the formula: (L)3(PW12O40)(H2O)4 (6043.78).

Protonated Polyoxometalate Complexes 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 (C264H422N20O58W12SiBr2, 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 the range 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 30150 °C arising from crystal water in L/KSiW, just matching the formula: (L)4(SiW12O40)(H2O)2 (7036.42). Measurements. Elemental analysis (C, H, N) was performed on a Flash EA1112 from ThermoQuest Italia S.P.A. Infrared spectra, from pressed KBr pellets, were carried out on a Bruker IFS-66V Fourier transform infrared spectrometer equipped a DTGS detector with a resolution of 4 cm-1. 1H NMR spectra were recorded on a Bruker UltraShield 500 MHz spectrometer instrument using CDCl3 as solvent and TMS as internal reference. UV-visible spectra were recorded on a Shimadzu UV-3100 spectrophotometer. Raman spectra were recorded on a Renishaw Model 1000 or Jobin-Yvon T6400 spectrometer and the 514.5-nm and 488-nm lines of argon ion laser were used as excitation source, respectively. TGA was conducted with a Perkin-Elmer TG/DTA-7 instrument and the heating rate was 10 K/min. The phase behaviors were measured using a polarized optical microscope (POM) of Leica DMLP equipped with a Mettler FP82HT hot stage and a Mettler FP90 central processor. Differential scanning calorimetry (DSC) measurements were completed on a Netzsch DSC 204 at a scanning rate of 5 K/min. The samples were sealed in aluminum capsules in air, and the holder atmosphere was dry nitrogen. Variable-temperature X-ray diffraction (XRD) experiments were carried out on a Philips PW 1700 X-ray diffractometer (using Cu KR1 radiation of a wavelength of 1.54 Å) with a TTK-HC temperature controller. Results and Discussion Characterization of Complexes. In this study, we employed azobenzene-containing surfactant L to encapsulate Keggin-type strong acids HPW and HSiW. Their corresponding salts, NaPW and KSiW, were also encapsulated as a comparison. The synthetic procedure was described in detail in the Experimental Section. All of the as-prepared complexes were no longer soluble in water, but were soluble in hot organic media such as chloroform, suggesting that the surface of PMs had been effectively modified by surfactant L. However, it should be mentioned that the solubility of complexes in chloroform solution is considerably poor, and their colors are different from that of the surfactant L, with the exception of L/KSiW, which evidently differs from those in our previous reports.11a,11b All of the complexes are thermally stable in air according to the identical IR spectra before and after the heating cycles. Considering the color darkening of surfactant L in the complexes, we propose some change having taken place during the encapsulation process, as the color is unchangeable when we perform the same procedure under the neutral conditions. Spectroscopic measurements were used to examine the color change from L to the complexes, and as a representative example, UV-vis absorption spectra of L and HL/HPW are shown in Figure 2. Both pure surfactant L and the de-protonated complex L/HPW in their chloroform solution show a typical

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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 in CDCl3, respectively.

absorption band at 360 nm, indicating that L and L/HPW exist in their molecularly dispersed states.14 In contrast, HL/HPW in chloroform solution shows multiple absorption bands at 260, 360 nm and a broad absorption band in the range of 470-570 nm. On the basis of the pH dependence of UV-vis spectra of azobenzene derivatives,15 the broad band at 535 nm can be clearly ascribed to the absorption of a protonated azobenzene group, which is consistent with the color of the complex. Even in this case, we still observed the normal band of the azobenzene group appearing at 360 nm, suggesting the coexistence of neutral and protonated species in the chloroform solution of HL/HPW. Protonation of Azobenzene Groups. To further confirm the protonation of azobenzene groups in the complex, Raman spectra were employed to investigate the protonation of the complexes (see Supporting Information), and the results show that 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 were applied to quantitatively calculate the degree of protonation. TGA curves in the range of 30-150 °C show the presence of 2-4 crystal water molecules in all of the complexes. The results of elemental analysis and the TGA suggest that half of the azobenzene groups have been protonated in HL/HPW and HL/ NaPW, and one-fourth in HL/HSiW, but none in L/KSiW and L/HPW, as indicated in the molecular formula of these complexes in the experimental section. Thus, the azobenzene groups are not protonated completely in the complexes, which is consistent with the observation of UV-vis and Raman spectra. As the encapsulation process is driven by electrostatic force between L and PMs, there are two possible binding positions of L: one is the cationic ammonium and the other is the protonated azobenzene group. To identify the exact binding position of protonated L with PMs, the complexes were characterized by 1H NMR spectra. As shown in Figure 3, the proton chemical shift of surfactant L in the complex HL/HPW,

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TABLE 1: Phase Transition Temperatures (°C), Enthalpies (kJ/mol), and Assignments of the Phase Transitions for All of the Complexesa first cooling sample HL/HPW

transition

S-SmB SmB-Iso HL/NaPW S-SmB SmB-Iso L/HPW S-SmB SmB-Iso HL/HSiW S1-S2 S2-SmC SmC-SmA SmA-Iso L/KSiW S1-S2 S2-S3 S3-SmB SmB-Iso

second heating

T (°C) ∆H (kJ/mol) T (°C) ∆H (kJ/mol) 110 198 110 196 109 188 109 164 197 205 109 128 169 210

39.85 9.26 34.94 9.85 42.75 8.45 29.87 38.81 31.96 22.49 13.30 4.90 47.12 58.24

123 207 121 206 121 191 111 170

41.59 8.18 34.50 6.47 45.23 6.04 41.55 43.25

218 113 133 175 220

75.01 11.61 6.37 49.46 75.29

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

in contrast to that of surfactant L alone, shows the following characteristic: (1) the proton peak of N-methyl (N+-CH3) has significantly broadened and shifted to high field by 0.12 ppm; (2) the proton signal of N-methylene (N+-CH2), which is a triplet in pure L, becomes a considerably broadened singlet and has shifted toward high field by 0.24 ppm; and (3) other peaks maintain the same positions as pure L. The peak broadening should result from the strong electrostatic interaction between the L and PW cluster, which restricts the mobility of the ammonium head group, as reported in the literature.16 Considering that the chemical shift and peak width of the proton signal are related to the local physical and chemical environment, we suggest that it is the cationic ammonium headgroup of surfactant L 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 from L. 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 complexes changed to yellow, as pure L, and was immediately accompanied by a white precipitate, while a similar phenomenon was not observed for the non-protonated L/KSiW, suggesting the presence of Br- as the counterion in the protonated samples. Furthermore, we tried to remove hydrogen bromide from the protonated azobenzene groups by carefully treating HL/HPW with aqueous ammonia. Raman spectra confirmed that the deprotonation of the azobenzene groups was successful (see Supporting Information). IR spectra proved that the chemical structure of the heteropolyanion cluster PW12O403- in the protonated complex L/HPW was well-maintained and elemental analysis of L/HPW shows that the molar ratio of surfactant L to PW still retained 3:1. Those results suggest that the PW12O403- cluster was not lost during the deprotonation process, confirming the corresponding counterion around the protonated azobenzene group is Br-, which comes from the surfactant. Therefore, we propose that the hydrogen bromide, resulting from counterions of both L and PMs, protonates the azobenzene groups during the encapsulation process, as presented schematically in Figure 1. Mesomorphic Behavior of Complexes. The thermal properties of the obtained complexes are investigated by DSC, POM, and variable-temperature XRD. The phase transition temperatures, enthalpies and assignments of the phase transitions for all the complexes are summarized in Table 1.

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.

From the DSC curves shown in Figure 4, protonated complexes HL/HPW and HL/NaPW exhibit two phase transitions similar to that of the deprotonated L/HPW, which can be assigned to the changes from solid state to liquid crystal phase and then to isotropic liquid, respectively. In the cases of HL/ HSiW and L/KSiW, both display rich transitions, especially for the former, presenting more phase changes during the cooling cycle 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/HPW and HL/NaPW (see Figure 5, parts a and b) because the observed mosaic texture is a common characteristic of SmB phase, particularly when it is obtained through directly decreasing temperature from isotropic liquid.17 The texture of L/HPW is also indicative of SmB phase during the cooling process, which is consistent with that of HL/HPW and HL/NaPW (see Figure 5c). In the case of HL/HSiW, the focal conic fan-shaped texture at 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 liquid crystalline structures and phase transitions between HL/HSiW and L/KSiW reveal that the protonation of azobenzene groups plays a key role in the mesomorphic characterization of complexes. The LC behaviors of all the complexes were further investigated by a variable-temperature XRD measurement. The XRD data supports the assignments of mesophases. As shown in Figure 6, when HL/HPW and HL/NaPW slowly cool down from their isotropic state, strong equidistant diffractions in the smallangle region emerge along with a single sharp diffraction at 2θ ≈ 20° (Figure 6, parts a and b). The equidistant diffractions correspond to a layered structure with the d-spacing of 4.01 and 4.06 nm calculated from the Bragg equation, respectively. The appearance of a single sharp diffraction at the wide-angle region for HL/HPW and HL/NaPW indicates the existence of SmB phase.18 Meanwhile, L/HPW shows an order smectic layered structure with a layer spacing of 4.11 nm. On the basis of our present results, we believe that the LC structure of the protonated HL/HPW and HL/NaPW is the same as the deprotonated L/HPW. HL/HSiW features a disordered smectic phase corresponding to a periodicity of 4.26 nm during its cooling run from the isotropic state, as demonstrated by the presence of equidistant diffractions in the small-angle region (Figure 6c) and a broad and unconspicuous halo centered at 2θ ≈ 20° (Figure 6c, inset). Considering the fan-shaped texture (Figure 5d), this phase is identified as SmA phase. When the temperature decreases to 197 °C, another lamellar structure appears (Figure 6c). Combining the unchanged diffusion peak in the wide-angle region

Protonated Polyoxometalate Complexes

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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).

TABLE 2: Summary of Layer Spacings (d) for All of the Complexes Calculated from X-ray Diffractions layer spacing d (nm) sample HL/HPW HL/NaPW L/HPW HL/HSiW L/KSiW

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).

(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 SmC to a solid state occurs at 164 °C, and the further cooling leads to a solid-solid transition. Hence, the sequence of mesophases for HL/HSiW in the cooling run can be ascribed to the process of Iso-SmA-SmC-(Solid-2)-(Solid-1). In contrast to HL/HSiW, L/KSiW displays a different mesophase sequence. When cooled from isotropic liquid, the first lamellar LC phase with a layer thickness of 5.04 nm forms, as illustrated in Figure 6d. Combining the lancet texture (Figure 5f) at the same temperature region and the single sharp diffraction at 2θ ≈ 20° (Figure 6d inset), the mesophase can be attributed to SmB phase, and its structure keeps until the temperature goes down to 169 °C, then the phase changes to solid. The solid state can be definitely assigned from the feature of multiple diffractions in the smallangle region and a number of sharp peaks in the wide-angle region in the diffraction pattern. The calculated layer spacings for all the complexes are listed in Table 2. It is noted that the layer spacing of L/KSiW is around 5 nm, whereas those of the other complexes are around 4 nm. Combining the radius (0.52 nm) of Keggin-type PMs, and the ideal length of L with all trans-conformation (3.45 nm, calculated by MM2 force field

SmA

SmB

SmC

solid 3

solid 2

solid 1

4.17 4.45

3.69 3.64 3.72 4.04 4.29

4.01 4.06 4.11 4.26

4.27 5.04

4.81

method), the total thickness of a single complex should be around 7.9 nm. The simulated molecular length is much larger than the measured layer distance of all the complexes in their mesophases, indicating the presence of a partially interdigitated lamellar structure in L/KSiW and deeply interdigitated lamellar structures in other complexes. It is interesting to discuss the reason that HL/HSiW exhibits two mesophases, SmA and SmC, whereas L/KSiW forms only SmB 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 of azobenzene groups leads to the different mesomorphic behaviors in HL/HSiW when compared to with that of L/KSiW. Figure 1 shows that all the complexes possess a hydrophobic organic shell and a hydrophilic inorganic core. These complexes are likely to form lamellar structures due to the phase separation of incompatible molecular components, the aggregation of compatible units and the volume minimization of the surfactants. Combining the present results and the crystal structures of some similar complexes reported in the literature,19 we propose that the mesomorphic structures of the complexes correlate with the matching extent of the volume between the hydrophilic PM cluster and hydrophobic alkyl chains. When the volume ratio of two components in the complex is well-proportioned, an ordered smectic phase would be the predominant structure. Otherwise a disordered smectic phase should be the preferential structure. In our case, SmB phase of L/KSiW suggests that the volume of the SiW cluster is well-proportioned to that of the alkyl chains. And in HL/HSiW, as the presence of protonated azobenzene group increases the incompatibility between the surfactants, the volume of alkyl chains increases. The enlarged volume of hydrophobic part makes the volume of SiW cluster mismatch with that of alkyl chains, leading to disordered SmA and SmC phases. The explanation can be supported by the results of XRD. For L/KSiW with a tight packing of alkylchains,

3988 J. Phys. Chem. B, Vol. 112, No. 13, 2008 it shows a bit larger layer thickness due to the partially interdigitated layer structure, whereas for HL/HSiW with relaxed alkyl chains, it exhibits a deeply interdigitated structure. This implies that the volume of alkyl chains in HL/HSiW is larger than that of L/KSiW. As for HL/HPW, in spite of the presence of protonated azobenzene groups, the complex still exhibits SmB phase. Comparing HL/HPW with HL/HSiW, the following characteristics are obvious: (1) SiW and PW are both Keggin type clusters and have the same size and shape; (2) both HL/ HPW and HL/HSiW show deeply interdigitated layers and the layer spacings of HL/HPW and HL/HSiW are almost the same; (3) the unique difference is in the amount of surfactant molecules in HL/HPW and HL/HSiW. According to the results of elemental analysis and TGA, the surfactants around PW are less than those covered on SiW, providing more space for the interdigitation of alkyl chains. Though the deeply interdigitated lamellar structures are present in HL/HPW, the volume of the PW cluster is still well-proportioned to that of the alkyl chains, thus the SmB phase is well maintained. The current result implies that the protonation of azobenzene groups can be used to tune the LC mesophase of the complexes by the appropriate choice of PMs. Conclusions In this paper, we report mesomorphic structures of a kind of novel hybrid LC material with protonated mesogenic groups. Azobenzene-containing surfactant encapsulated polyoxometalate complexes, HL/HPW, HL/HSiW, HL/NaPW, and L/KSiW, have been prepared. Among them, HL/HPW, HL/HSiW, and HL/ NaPW are protonated while L/KSiW is non-protonated. The intrinsic acidity of the applied PMs is considered to be the main reason for the protonation of these complexes. Although the protonation creates additional positively charged sites in the surfactant molecule, the bonding position between L and PMs is still at the ammonium head group definitely. The bromic anion derived from ion replacement binds to the azobenzene group as a counterion in these protonated complexes. The protonated complex HL/HSiW reveals SmA and SmC phases, while the corresponding non-protonated complex L/KSiW exhibits only SmB phase. The protonated complexes HL/HPW and HL/NaPW self-organize into SmB phase, similar to that of non-protonated L/HPW. The competitive balance between the phase separation and the volume minimization of surfactants is supposed to play an important role and could be employed to explain the different self-organized LC structures of these protonated and nonprotonated complexes. Both the number of surfactants on the surface of PMs and the protonation of the azobenzene group make the LC phase of SECs become diversified. This allows potential applications in developing acidity stimulation-response hybrid LC materials. Acknowledgment. The authors acknowledge the financial support 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 of Polymer Physics and Chemistry of CAS. Supporting Information Available: Raman spectra of the complexes, assignments of characteristic Raman shift, and additional references. This material is available free of charge via the Internet at http://pubs.acs.org.

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