Thermotropic Organization of Hydrogen-Bond-Bridged Bolaform

Department of Chemistry, West Virginia University, Morgantown, West Virginia 26506, United States. § Key Lab of NRCM & FM, ME, Yanbian University, Ya...
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Thermotropic Organization of Hydrogen-Bond-Bridged Bolaform Amphiphiles Jing Zhang,† Mingjun Zhou,† Shan Wang,† Jessica Carr,‡ Wen Li,*,† and Lixin Wu*,†,§ †

State Key Laboratory of Supramolecular Structure and Materials, College of Chemistry, Jilin University, Changchun 130012, China Department of Chemistry, West Virginia University, Morgantown, West Virginia 26506, United States § Key Lab of NRCM & FM, ME, Yanbian University, Yanji 133002, China ‡

bS Supporting Information ABSTRACT: A series of quaternary ammonium amphiphiles (A-n) bearing carboxylic acid groups were designed and synthesized. The branched bolaform structures can be constructed by dimerizations of carboxylic acid groups through intermolecular hydrogen bonding, as demonstrated by the Fourier transform infrared (FT-IR) spectra and the temperature-dependent FTIR spectra. The thermotropic organizations of branched bolaform ammonium dimer complexes were characterized by differential scanning calorimetry, polarized optical microscopy, and X-ray diffraction. We investigated the influence of the spacer between the cationic group and the benzene ring on the thermotropic organization. A-6 with short lateral alkyl chains formed a simple layered structure at room temperature and exhibited smectic A mesophase above 145 °C, whereas A-8 with intermediate lateral chain length organized into smectic A phase over a wide temperature range. A further increase of the length (n = 10, 12) of the lateral chains resulted in the formation of lamellar structure with in-plane layered periodicity, which is rare in the organization of ionic compounds. A packing model of the quasi-2D lamellar was proposed on the basis of the experimental data of X-ray diffraction results. Notably, the quasi-2D lamellar structure could evolve into a simple layer with the increase of temperature. The present results showed a direct relationship in which the branched architecture can be applied to tune the self-assembly behavior of ionic amphiphiles and is allowed to construct new layered superstructure.

’ INTRODUCTION The construction of novel and/or complicated self-assembled structures built from simple molecular components through accurately selecting building blocks and suitably adjusting the driving forces has attracted much attention over the past decades due to the expectation of applying such molecular self-organization systems in nanotechnology and functional materials.1-4 To realize this target, considerable efforts have been devoted to the design of various molecular units.5-9 In this context, ionic amphiphilic molecules have been proved to be particularly useful candidates in fabricating unique aggregations in solutions,10 thin films,11,12 and surface and solid structures13,14 because of the presence of strong microphase segregation of incompatible units, the aggregation of compatible units, and the minimization of volume. Particularly, thermotropic organizations of the ionic amphiphiles, some of which are directed to ionic liquid crystals (ILCs),15-17 can be controlled judiciously by adjusting the driving forces behind the molecular self-assembly. In contrast to the widespread occurrences in adjusting the type of ionic groups, the length of alkyl chains, and the ratio of hydrophobic to hydrophilic blocks, only a few reports are focused on changing the molecular architecture;18-20 most of the synthesized ionic compounds are r 2011 American Chemical Society

limited to linear structure with simple ionic head and hydrophobic tail,21 bolaform,22,23 or Gemini molecules.24-26 It has been confirmed that some nonionic amphiphiles with the branched structure could self-assemble into polygonal honeycombs and thermotropic soft matter with complex structures.27-30 To fabricate novel thermotropic organizations based on ionic amphiphililes, we designed and synthesized a kind of branched quaternary ammonium in earlier work.31 We have demonstrated that branched ILCs, consisting of a rod-like unit linked to an ammonium group at the lateral position by an alkyl chain, could form the simple nematic mesophase. We also found that the lateral chains were in competition with the parallel organization of the rod-like units. One of our persistent interests is concentrated on constructing complex organizations using branched ionic compounds, because the particular molecular shape may provide an additional tool for modulating the self-assembly behavior. ILCs, which combine inherent properties of ionic liquids and self-organization features of liquid crystals,15 are of great interest because of their exhibition in possibilities such as anisotropic ionic Received: January 6, 2011 Revised: January 30, 2011 Published: March 01, 2011 4134

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(97%) were obtained from Fluka without further purification. Trimethylamine aqueous solution (33%) and other reagents used in the sample preparation were commercial products from Sinopharm Chemical Reagent Co. Ltd. Doubly distilled water was used in the experiments. Silica gel (100-200 mesh) was employed for the purification of column chromatography.

Synthesis of Quaternary Ammoniums Containing Benzoic Acid Groups (A-n). The synthetic strategy for the A-n series was

Figure 1. (a) Schematic structures of synthesized quaternary ammonium amphiphiles, A-n; (b) cyclic carboxylic dimer of complex through hydrogen bonding; (c) schematic representation of smectic A phase for A-6 and A-8; and (d) quasi-2D lamellar structure for A-10 and A-12.

conductors,32,33 dye-sensitized solar cells,34 chemical reactors,35 and templates for the synthesis of nanomaterials.36,37 Quite a number of ILCs based on different cationic cores, such as ammonium, phosphonium, imidazolium, pyridinium, pyrrolidinium, viologen salts, etc., have been designed and synthesized, targetting various phase structures and functional properties.38-42 From the practical application point of view, a lower transition temperature of the LC phase could make ILCs possess many more characteristics similar to ionic liquids while maintaining anisotropic feature. Therefore, it is desirable to modify the molecular structure to obtain ILCs that exhibit a mesophase over a wide temperature range including room temperature or close to it.15 The noncovalent synthesis has proved to be an effective strategy to design new building blocks. As a typical example, Zhang and co-workers have developed a novel route to obtain supramolecular ionic amphiphiles through noncovalent synthesis, which can lead to various organized structures and responsive materials.43-45 Inspired by the supramolecular interaction between molecules to design complex building blocks, we herein designed and synthesized a series of quaternary ammoniums (A-n) with the carboxylic acid groups, as shown in Figure 1a. The influence of the spacer between the cationic group and the benzene ring on the thermotropic organization was investigated. Because of the presence of intermolecular hydrogen bonding, the formed cyclic carboxylic acid dimer gave rise to a new kind of ionic amphiphiles with branched bolaform structure (Figure 1b). Interestingly, with lengthening of the lateral alkyl chains from n = 6, 8 to n = 10, 12, the self-organization of A-n changed from simple layer to quasi-2D lamellar structure (Figure 1d) at room temperature, which is a rare case for ionic amphiphiles. Notably, the organized structure of the branched bolaform ammoniums A-10 and A-12 exhibited a transformation from quasi-2D lamellar structure to a simple layer structure with the temperature increasing. The relationship between the molecular architecture and the self-organization structures was discussed in detail.

’ EXPERIMENTAL SECTION Materials. Ethyl 2,4-dihydroxybenzoate (97%) was purchased from Sigma, and 1,6-dibromohexane (96%), 1,8-dibromooctane (97%), 1,10-dibromodecane (95%), and 1,12-dibromododecane

carried out following our previous method,31 as shown in Scheme S1. Details of the synthesis of all synthesized compounds are described in the Supporting Information. A-6. 1H NMR (500 MHz, CDCl3, δ): 0.88 (t, J = 10 Hz, 3H), 1.241.37 (m, 8H), 1.42-1.47 (m, 2H), 1.50-1.56 (m, 2H), 1.58-1.64 (m, 2H), 1.74-1.94 (m, 6H), 3.45 (s, 9H), 3.70-3.73 (m, 2H), 3.99 (t, J = 5 Hz, 2H), 4.18 (t, J = 5 Hz, 2H), 6.50 (s, 1H), 6.56-6.58 (d, J = 10 Hz, 1H), 8.00-8.01 (d, J = 5 Hz, 1H). MALDI-TOF MS (m/z): 408.3, corresponding to [C24H42NO4]þ ion. Anal. Calcd for A-6 (C24H42NO4Br 3 2H2O): C, 54.96; H, 8.84; N, 2.67. Found: C, 54.75; H, 8.98; N, 2.42. A-8. 1H NMR (500 MHz, CDCl3, δ): 0.88 (t, J = 10 Hz, 3H), 1.281.50 (m, 18H), 1.76-1.82 (m, 4H), 1.87-1.92 (m, 2H), 3.45 (s, 9H), 3.66-3.70 (m, 2H), 4.00 (t, J = 5 Hz, 2H), 4.21 (t, J = 5 Hz, 2H), 6.50 (s, 1H), 6.60-6.62 (d, J = 10 Hz, 1H), 8.05-8.06 (d, J = 5 Hz, 1H), 10.80 (s, 1H). MALDI-TOF MS (m/z): 436.3, corresponding to [C26H46NO4]þ ion. Anal. Calcd for A-6 (C26H46NO4Br 3 2H2O): C, 56.51; H, 9.12; N, 2.53. Found: C, 56.34; H, 9.41; N, 2.37. A-10. 1H NMR (500 MHz, CDCl3, δ): 0.89 (t, J = 10 Hz, 3H), 1.291.52 (m, 22H), 1.74-1.84 (m, 4H), 1.87-1.93 (m, 2H), 3.45 (s, 9H), 3.62-3.66 (m, 2H), 4.01 (t, J = 5 Hz, 2H), 4.23 (t, J = 5 Hz, 2H), 6.51 (s, 1H), 6.61-6.63 (d, J = 10 Hz, 1H), 8.06-8.08 (d, J = 5 Hz, 1H), 10.81 (s, 1H). MALDI-TOF MS (m/z): 464.3, corresponding to [C28H50NO4]þ ion. Anal. Calcd for A-10 (C28H50NO4Br 3 2H2O): C, 59.77; H, 9.32; N, 2.49. Found: C, 59.49; H, 9.66; N, 2.23. A-12. 1H NMR (500 MHz, CDCl3, δ): 0.89 (t., J = 5 Hz, 3H), 1.321.53 (m, 26H), 1.73-1.83 (m, 4H), 1.86-1.93 (m, 2H), 3.45 (s, 9H), 3.58 (t, J = 10 Hz, 2H), 4.02 (t, J = 5 Hz, 2H), 4.22 (t, J = 5 Hz, 2H), 6.51 (s, 1H), 6.62-6.64 (d, J = 10 Hz, 1H), 8.07-8.09 (d, J = 10 Hz, 1H), 10.79 (s, 1H). MALDI-TOF MS (m/z): 492.4, corresponding to [C30H54NO4]þ ion. Anal. Calcd for A-12 (C30H54NO4Br 3 2H2O): C, 59.20; H, 9.60; N, 2.30. Found: C, 58.84; H, 9.83; N, 2.15. Characterization. 1H NMR spectra were recorded on a Bruker Avance 500 instrument using CDCl3 as solvent with tetramethylsilane as internal reference (0.00 ppm). Elemental analysis (C, H, N) was performed on a Flash EA1112 from ThermoQuest Italia SPA. The matrix-assisted laser desorption/ionization time-of-flight mass spectra (MALDI-TOF MS) were obtained from an autoflex TOF/TOF (Bruker, Germany) mass spectrometer, equipped with a nitrogen laser (337 nm, 3 ns pulse). The mass spectrometer was operated in the positive ion reflector mode with a detector potential of -4.75 kV. The chloroform solution of the obtained products was used directly for measurement without the matrix because the sample is an ammonium salt. The mass range for data acquisition is from m/z 0 to 1500 Da. Thermogravimetric analysis (TGA) test was carried out on a TA Instruments Q500 thermogravimetric analyzer (TGA) at a heating rate of 10 °C/min with the temperature range from 25 to 900 °C. Infrared spectroscopy (IR) spectra and temperature-dependent FT-IR spectra were recorded on a Bruker Optics VERTEX 80v Fourier transform infrared spectrometer, equipped with a DTGS detector in pressed KBr pellets. A resolution of 4 cm-1 was chosen, and 32 scans were signal-averaged. Differential scanning calorimetric (DSC) measurements were performed on a Netzsch DSC 204 with a scanning rate of 5 °C/min. All the samples were sealed in aluminum capsules in air, and the atmosphere of holder was sustained under dry nitrogen. The optical textures of the mesophases were studied with a Zeiss Axioskop 40 polarizing microscope 4135

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Figure 2. FT-IR spectrum of A-10 at room temperature. equipped with a Linkam THMSE 600 hot stage, a central processor, and a DF1 cooling system. For variable-temperature X-ray diffraction (XRD) experiments, a Bruker AXS D8 ADVANCE X-ray diffractometer using Cu KR radiation of a wavelength of 1.54 Å with a mri Physikalische Ger€ate GmbH TC-Basic temperature chamber was used. 2D XRD was conducted by empolying a Bruker D8Discover diffractometer with a GADDS detector, which was calibrated with silicon powder. The sample was placed on a light etched silicon substrate (the depth of the groove and the spacer between two grooves are all 25 μm), clapped with another silicon substrate, and cooled slowly from the isotropic state with a cooling rate of 5 °C/min. The 2D X-ray diffraction patterns were recorded in reflection mode and transmission mode.

Figure 3. DSC traces of (a) A-6, (b) A-8, (c) A-10, and (d) A-12 complexes on the first cooling and second heating runs.

Table 1. Phase Transition Temperatures and Corresponding Enthalpies for A-n Series Determined by DSC (5 °C/min) first cooling ΔH

’ RESULTS AND DISCUSSION Structural Characterization. In this study, we designed and synthesized a series of quaternary ammonium amphiphiles (A-n, n = 6, 8, 10, 12) with carboxylic acid groups, as shown in Figure 1a. The detailed synthetic routes were described in the Supporting Information (Scheme S1). The structure and purity of the final products were confirmed by 1H NMR spectra, MALDI-TOF mass spectra, elemental analysis, and Fourier transform infrared (FT-IR) measurements (Figures S1-S3). FT-IR spectra demonstrate that all of the samples of the A-n series exist in the state of cyclic carboxylic acid dimers, as shown in Figure 1b. As a representative example, Figure 2 shows the FT-IR spectrum of A-10 at room temperature. A broad band at 3430 cm-1 can be assigned to the hydrogen-bonded O-H stretching mode, which may source from both adsorbed water and carboxyl group, whereas the former can be removed upon heating. The satellite double absorption bands appear at 2476 and 2589 cm-1, which illustrates the formation of hydrogen bonding derived from carboxylic acid distinctly.46-49 The single band with strong absorption intensity at 1675 cm-1 can be assigned to hydrogen-bonding carbonyl stretching vibration in the closed dimeric form.50 Thus, we can conclude the formation of cyclic hydrogen-bonding dimers in A-10, as shown in Figure 1b. Few monomers should still exist in the bulk complex, because the weak band that is attributed to free carbonyl group at 1728 cm-1 emerges as well. The assignments of the major bands of A-n complexes are summarized in Table S1. Thermal Properties of the Branched Bolaform Amphiphiles. The thermal properties of A-n were investigated by thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), temperature-dependent FT-IR measurements, polarized optical microscopy (POM), and variable-temperature

second heating

transitiona A-6

G-X X-SmA SmA-Iso

A-8 A-10 A-12

T (°C)

transitiona

-3.74 -0.68

X-SmA SmA-Iso

2 145 158

ΔH

(kJ/mol)

G-X

T (°C)

(kJ/mol)

1 153 164

4.24 0.69

G-SmA

24

G-SmA

24

SmA-Iso

83

-0.48

SmA-Iso

86

0.41

Cr-Cr0

0

-2.94

Cr-Cr0

25

5.30

Cr0 -Iso

151

-4.03

Cr0 -Iso

157

3.75

G-Cr

12

G-Cr

12

Cr-Iso

83

0.64

a

Cr = crystal; SmA = smectic A phase; X = unknown crystal smectic phase; G = glass transition; and Iso = isotropic phase.

X-ray diffraction (XRD). The results of TGA (Figure S4) reveal that the decomposition temperatures of the as-prepared ionic compounds, A-n, are mostly higher than 180 °C. Moreover, the number (n) of carbon atoms between ionic head and mesogenic group has little effect on the thermal stability. The thermal behaviors of A-n complexes were initially investigated through DSC measurement. To avoid the effect of thermal history on the phase behaviors, we collected the DSC traces of all the A-n series (Figure 3) in the first cooling and second heating processes. The transition temperatures and associated enthalpies were collected in Table 1. The glass transition temperatures were determined as the midpoint of the step change in the heat capacity, and other transition temperatures were taken at the maximum of the transition peaks. As shown in Figure 3a, when first cooling from the isotropic liquid, A-6 exhibited a clear exothermal peak at 158 °C with an enthalpy of 0.68 kJ/mol. The small enthalpy suggests that the state below 158 °C is a low-ordered structure. On further cooling, another exothermal event occurred at 145 °C with an enthalpy of 4136

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Figure 5. Temperature-dependent FT-IR spectra of A-10 in the region of carbonyl stretching vibration (inset: plots of absorption intensity of the bands at 1677 and 1729 cm-1 versus the temperature).

Figure 4. POM images of (a) oily steak texture of A-6 at 150 °C; (b) mosaic texture of A-6 at 60 °C; (c) homeotropically aligned domains of A-8 at 55 °C; (d) birefringent texture of A-8 after mechanical shearing at 55 °C; (e) focal conic texture with Maltese crosses of A-10 at 68 °C; and (f) focal conic texture with Maltese crosses of A-12 at room temperature.

3.74 kJ/mol. The increased enthalpy (relative to 0.68 kJ/mol) implies the formation of a more ordered structure. The ordered state lasted to the glass transition at ca. 2 °C. All the transitions emerged in the cooling process were observed in the second heating run, except a small broad transition occurred at 59 °C, which may be derived from the possible dynamic relaxation. With increasing length of lateral chains, A-8 showed reversible phase transitions between 24 and 86 °C (Figure 3b); the enthalpy at the clearing point during heating is 0.41 kJ/mol. Clearly, the clearing point of A-8 is obviously lower than that of A-6. When further increasing the length of the lateral chains, two phase transitions were observed in A-10 (Figure 3c). It is worth noting that the enthalpy at the clearing point is 3.75 kJ/mol, which is larger than those of A-6 and A-8. In contrast, A-12 did not show a clear exothermal event on the cooling run except a glass transition at 12 °C (Figure 3d). After the sample of A-12 was aged at -20 °C for 5 h, a glass transition (12 °C) and an endothermal event at 83 °C were observed on the second heating scan. We further studied the phase behaviors of the compounds A-n by POM. All the samples were first heated to isotropic liquid states, and the POM textures were observed upon the cooling run. As shown in Figure 4a, A-6 exhibited fluid oily streak texture with large homeotropic domains51-53 at 150 °C. Upon further cooling, the oily streak texture changed to mosaic texture with high viscosity (Figure 4b). In addition to the birefringent texture, large optically isotropic domains can also be observed, indicating an optically uniaxial phase. The mosaic texture lasted to the temperature even below 10 °C. A-8 formed homeotropically aligned domains (Figure 4c) upon cooling from the isotropic state. After a mechanical shearing was applied, birefringent

texture (Figure 4d) appeared, implying the presence of liquid crystal state, although the type of the mesophase can not be assigned unequivocally based on the POM observation alone. For A-10, large Maltese cross textures with high viscosity (Figure 4e) developed upon cooling run, indicating the existence of crystalline state.54-56 However, no textures or birefringent patterns were observed when cooling A-12 from its isotropic liquids directly. Yet after 2 days of delay, focal conic texture with Maltese crosses similar to that of A-10 emerged slowly at room temperature (Figure 4f). On the second heating run, an obvious change in the birefringence was observed, which is consistent with the result of DSC. The color of the textures changed from violet through red and yellow to pale yellow with temperature increasing, but the appearance of the texture did not change until the temperature was over 80 °C (Figure S5). The decreased color intensity indicates that the orientation order parameter of the carboxylic dimer decreases with increasing temperature. From the results of POM and DSC, we can learn that the thermal behaviors of compounds A-n are unusual in comparing with those common ammoniums without branched structures bearing long alkyl chains. The length of the lateral chains had an irregular effect on the phase transition of the ionic compounds. Considering the presence of benzoic acid dimers at room temperature, we suppose that all A-n compounds form the branched bolaform structure, where the carboxylic dimers (Figure 1b) act as rigid units and promote the oriented arrange of ionic molecules. The ammonium groups linked by alkyl chains at the lateral position of carboxylic dimers could reduce the interactions between neighboring molecules due to the steric hindrance, leading to the lower phase transition temperature. With increasing temperature, the branched molecular structure remained unchanged, and the competitive interactions between the lateral chains and the rigid units played an unusual role in the thermal behaviors of the compounds A-n. To verify the existence of the branched hydrogen-bonding dimer with temperature, temperature-dependent FT-IR was performed. As a representative example, Figure 5 displays the FT-IR spectra of A-10 with respect to carbonyl stretching vibration. With temperature increasing, the stretching absorption band of the hydrogen-bonding carbonyl group at 1675 cm-1 shows a slow decrease in peak height, while the band attributed to 4137

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Figure 6. Variable-temperature X-ray diffraction patterns of (a) A-6 and (b) A-8 (inset: wide-angle region).

the free carbonyl group at ca. 1728 cm-1 shows a slow increase in peak height. The sharp changes of these two bands in peak height were found when the sample was heated to higher temperature at ca. 140 °C. The upshift of the band at 1675 cm-1 implies the weakening of hydrogen-bonding strength. The absorbance changes of the hydrogen-bonding carbonyl group at 1677 cm1 and the free carbonyl group at 1729 cm-1 as a function of temperature are depicted in the inset of Figure 5. Below 140 °C, the absorbance at 1677 cm-1 shows little decrease while the absorbance of the free carbonyl group almost does not increase, suggesting the well maintenance of the hydrogen-bonding dimer. In contrast, a dramatic decrease of the absorbance of the hydrogen-bonding carbonyl vibration band and increase of free carbonyl vibration band are observed at ca. 150 °C, indicative of the collapse of the hydrogen-bonding dimer over this temperature. Other compounds A-6, A-8, and A-12 showed similar tendency, as seen from the band intensity change of carbonyl stretching vibration (Figure S6). These results confirm that hydrogen bonding is dominant, and the branched molecular architecture is maintained below the clearing temperature. Thermotropic Organization Structures of the Branched Bolaform Amphiphiles. To understand the thermal behaviors and obtain the detailed information about the molecular arrangement with temperature, A-n complexes were further studied by variable-temperature X-ray diffraction. Figure 6a displays the X-ray diffraction patterns of A-6 at different temperatures during cooling from isotropic state. Two equidistant diffractions at low-angle region and a broad halo at wide-angle region are observed at 150 °C. The equidistant diffractions correspond to a layered structure with a layer spacing

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of 32.7 Å. The wide-angle diffraction centered at 20° corresponds to a spacing of 4.4 Å, indicative of the disordered packing of alkyl chains. Combining the oily streak texture and small enthalpy, a smectic A phase can be identified for A-6. On further cooling to 110 °C, the XRD patterns show three low-angle equidistant diffractions along with several wide-angle diffractions, and the wide-angle diffractions did not show a sufficient number of wellresolved diffractions to make a clear identification of the organization structure, so we assigned the phase to the ordered crystal smectic phase for the moment. Upon further cooling to 30 °C, the XRD patterns show three low-angle equidistant diffractions along with broad wide-angle halos, suggesting the formation of glassy phase. The glassy transition observed from the XRD data is in agreement with the DSC observation. During cooling from the isotropic state, the X-ray diffraction pattern of A-8 (Figure 6b) shows a sharp and intense diffraction peak with two weak subordinate diffraction peaks in the small-angle region with d-spacings in a ratio of 1:1/2:1/3 at 60 °C, suggesting a less-ordered layered structure with the layer spacing of 33.3 Å. In addition, a diffuse scattering halo at about d = 4.4 Å emerges in the wide-angle region, indicative of the disordered packing of alkyl chains, combining the homeotropically aligned domains from the POM observations, and thus a smectic A phase can be identified for A-8. Both A-6 and A-8 reveal a dependence of the observed layer distance on temperature; that is, decreasing the temperature will result in increasing of layer spacing. The temperature-dependent FT-IR spectra have demonstrated that the branched molecular architecture is maintained below the clearing temperature. According to the proposed branched structure, the microphase segregation of incompatible parts, and the aggregation of compatible parts of ionic compounds, the organized structure of the A-n could be speculated. First, the maximum molecular length (l) along the long axis of the cyclic benzoic acid dimer could be estimated to be 36.4 Å for all A-n dimers. Second, the maximum lateral molecular length (Ln) along the short axis is dependent on the lateral carbon number n (for A-6, A-8, A-10, and A-12 dimer, the Ln is ca. 26.6, 31.6, 36.8, and 41.9 Å, respectively). From the results of XRD, the measured layer distance D (32.7 Å) of A-6 locate at L6 < D ≈ l, close to the length of the A-6 dimer along the long axis. Therefore, we confirm that the observed layer periodicity should be derived from the periodic layer structure along the long axis of the A-6 dimer, as shown in Figure 1c. That means the cyclic benzoic acid dimers arrange in a parallel way to form a layered structure along the long axis, where the ammonium groups are situated at the lateral position of the dimers due to the microphase segregation of the incompatible units. The smaller D spacing, in comparison to the ideal value l, is attributed to disorder or a partial intercalation of octyl groups. The steric hindrance of the lateral chains reduced the interactions of parallel arrangement of the benzoic acid dimers obviously, leading to a low melting temperature. As a fact of this, the clearing point of A-8 (86 °C) is much lower than that of A-6 (157 °C), and, likewise, the enthalpy (0.41 kJ/mol) at the clearing point of A-8 is lower than that of A-6 (0.68 kJ/mol). Apparently, the steric requirement of A-8 with the longer lateral chain makes the packing looser in comparison with A-6, making A-8 form a room-temperature liquid crystal state. Notably, with further increase in the length of the lateral alkyl chains, the diffraction patterns of A-10 and A-12 become different from those of A-6 and A-8. As shown in Figure 7a, 4138

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Figure 7. Variable-temperature X-ray diffraction patterns of (a) A-10 and (b) A-12.

A-10 shows two kinds of layer periodicity. The diffraction peak at 2θ ≈ 2.5° indicates the existence of the periodicity with layer spacing D1 of 35.3 Å, a length similar to that found in the case of A-6 and A-8. Although the one-dimensional XRD pattern displays only one diffraction peak, two-dimensional X-ray diffraction patterns (Figure S7) of unaligned sample of A-10 recorded at room temperature confirm a layer structure. Five equidistant diffraction peaks at 2θ ≈ 4.2°, 8.3°, 11.7°, 16°, and 18°, respectively, correspond to another well-defined layered structure with a shorter layer spacing d1 of 21.0 Å. The appearance of several sharp wide-angle diffractions indicates the crystalline state. Upon further cooling, the layer structure with shorter layer spacing d1 remained unchanged, while the periodicity with larger layer spacing D1 shows temperature dependence. When cooled from the isotropic state, A-12 did not show any diffraction peaks in the low-angle region, even at room temperature, which is in accordance with the POM and DSC observations. However, after 2 days of aging, several diffraction peaks emerged, as found in Figure 7b, which can also be assigned to two different lamellar periodicities with spacings of 33.4 and 22.1 Å, respectively. Similar to the case of A-10, the periodicity with longer layer spacing is sensitive to the temperature and disappears upon heating to 70 °C, implying the less stability of this periodicity. Contrarily, the layer structure d1, which can be attributed to longranged periodicity due to the stronger diffraction intensity and narrow half-peak width, is temperature insensitive and remains until the isotropic temperature. On the basis of these facts, it is certain that two layered periodicities with different layer spacings existing simultaneously are derived from the increase of lateral alkyl chain length.

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Considering the branched structures (Figure 1b) of the employed ammoniums, the nonpolar alkyl chains along the long axis of hydrogen-bonding dimer should be located in a cross direction to the lateral chains (alkyl chains terminated by ionic groups along the short axis of hydrogen-bonding dimer) due to the incompatibility between hydrophilic ionic groups and hydrophobic alkyl chains, and the microphase segregation could also make the ionic groups apart from nonpolar alkyl chains and hydrogen-bonding dimer. Therefore, a cross-stacking orientation could be anticipated for the two periodicities, although we could not obtain direct evidence from two-dimensional XRD patterns to prove their dimensionality due to lack of well-aligned sample. On the basis of the above data and the reported results that the T-shaped block molecules with lateral chains exhibit Lam phases where the mesogen is organized parallel to the layer planes,57-60 the organization structure of A-10 and A-12 should correspond to a quasi-2D layered structure as shown in Figure 1d. The microsegregation of lateral chains terminated by the ionic groups from the hydrogen-bonding dimer units results in the long ranged layer structure d, and the microsegregation of dimer units and the nonpolar alkyl chains leads to another period arrangement D. The packing models of the 2D layer structure for A-10 and A-12 are reasonable for the following interpretations: (1) Normally, the longer lateral chains of A-10 and A-12 need comparatively larger space, the steric requirements of which tend to separate the rigid dimer units from each other; as a result, the SmA phase for A-6 and A-8 becomes unfavorable for A-10 and A-12. Besides, the increased van der Waals interactions between the lateral chains and the aggregation of the ionic groups function as stabilizers of the layer structure d. The 2D layer structure is consistent with the high clearing point and large enthalpy obtained from the DSC data. Notably, the enthalpy of A-12 at the clearing point is lower than that of A-10. From the POM observations, we can see that the glass transition of A-12 is a slow dynamic process (about 2 days). We therefore propose that a 5 h balance time at the low temperature is not enough for the glass transition, which results in the low enthalpy. (2) In these ionic compounds, especially when the lateral alkyl chains are long enough, the periodicity derived from the segregation between hydrophilic ionic groups and dimer units should be more stable than that from the microphase segregation between dimer and nonpolar alkyl chains. The periodicity D1 of A-12 disappeared first when the temperature was heated to 70 °C, meaning that it is less stable than another periodicity with shorter layer spacing d, which did not disappear until the clearing point. So it is reasonable that the more stable periodicity d1 derives from the microphase segregation between hydrophilic ionic groups and the dimer units. (3) Because the observed periodicity D1 in A-10 and A-12 dimers is close to the molecular length along the long axis of the hydrogen-bonding dimers, and the periodicity D1 is also temperature-dependent, we believe that the periodicity D1 represents a packing structure similar to that of A-6 and A-8 dimers along the long-axis direction of the benzoic acid dimers. The lateral alkyl chains can be still assigned to extend along the periodicity with short layer spacing d1, when the flexible packing and interdigitation of lateral alkyl chains are considered.

’ CONCLUSIONS We designed a kind of branched quaternary ammoniums through intermolecular hydrogen-bonding interactions. The 4139

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Langmuir branched bolaform molecular architecture can self-assemble into different organization structures. Ionic compound A-6 formed a conventional layer structure at room temperature and exhibited a smectic A mesophase above 145 °C. A-8 formed a smectic A mesophase at room temperature, and the flexibility of lateral chains leads to rather low melting points and also gives rise to broad liquid crystalline ranges, which makes it more suitable for potential applications. A further increase in the length (n = 10, 12) of the lateral chains induced the formation of quasi-2D lamellar structure, where the microphase segregation of lateral chains terminated by the ionic groups resulted in the long ranged layer structure along the short axis of the benzoic acid dimers, and the aggregation of the nonpolar alkyl chains leads to another periodic arrangement along the long axis of the benzoic acid dimers. This kind of lamellar structure with in-plane layered periodicity is a rare case in ionic compounds. The microphase segregation of incompatible components, the aggregation of compatible units, and the steric hindrance effects become the main driving forces controlling the self-organization of branched ammoniums. More interestingly, the organized structure of the branched bolaform ammoniums (A-10 and A-12) exhibited transformation from quasi-2D lamellar structure to simple layer structure with temperature increasing. The results show that the competitive interactions between the steric hindrance and the van der Waals interactions of the lateral chains play an important role in the self-organization behaviors of A-n.

’ ASSOCIATED CONTENT

bS

Supporting Information. Detailed synthesis and characterization of A-n series, and FT-IR, TGA, XRD, and additional POM images. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected] (L.W.); [email protected] (W.L.).

’ ACKNOWLEDGMENT We acknowledge the financial support from the National Basic Research Program (2007CB808003), the National Natural Science Foundation of China (20973082, 20921003, 50973042, 20703019), and the Open Project of State Key Laboratory of Polymer Physics and Chemistry of CAS. We thank the 111 project (B06009) for the visit and helpful discussions with Professor Jung-Il Jin at Korea University. ’ REFERENCES (1) Kato, T.; Mizoshita, N.; Kishimoto, K. Angew. Chem., Int. Ed. 2006, 45, 38–68. (2) Tschierske, C. Chem. Soc. Rev. 2007, 36, 1930–1970. (3) Wang, C.; Guo, Y. S.; Wang, Y. P.; Xu, H. P.; Wang, R. J.; Zhang, X. Angew. Chem., Int. Ed. 2009, 48, 8962–8965. (4) Santoro, A.; Wegrzyn, M.; Whitwood, A. C.; Donnio, B.; Bruce, D. W. J. Am. Chem. Soc. 2010, 132, 10689–10691. (5) Kimizuka, N.; Kawasaki, T.; Hirata, K.; Kunitake, T. J. Am. Chem. Soc. 1998, 120, 4094–4104. (6) Kabanov, A. V.; Bronich, T. K.; Kabanov, V. A.; Yu, K.; Eisenberg, A. J. Am. Chem. Soc. 1998, 120, 9941–9942. (7) Jeon, Y. J.; Bharadwaj, P. K.; Choi, S. W.; Lee, J. W.; Kim, K. Angew. Chem., Int. Ed. 2002, 41, 4474–4476.

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