Next Article in Journal
Effects of Lipoic Acid on Immune Function, the Antioxidant Defense System, and Inflammation-Related Genes Expression of Broiler Chickens Fed Aflatoxin Contaminated Diets
Previous Article in Journal
Interleukin-6 Receptor rs7529229 T/C Polymorphism Is Associated with Left Main Coronary Artery Disease Phenotype in a Chinese Population
Previous Article in Special Issue
Dynamics of Order Reconstruction in a Nanoconfined Nematic Liquid Crystal with a Topological Defect

Int. J. Mol. Sci. 2014, 15(4), 5634-5648; doi:10.3390/ijms15045634

Article
Liquid Crystalline Assembly of Coil-Rod-Coil Molecules with Lateral Methyl Groups into 3-D Hexagonal and Tetragonal Assemblies
Zhuoshi Wang 1,, Yu Lan 1,, Keli Zhong 2, Yongri Liang 3, Tie Chen 1,* and Long Yi Jin 1,*
1
Key Laboratory for Organism Resources of the Changbai Mountain and Functional Molecules, Department of Chemistry, College of Science, Yanbian University, No.977 Gongyuan Road, Yanji 133002, China; E-Mails: 2013010407@ybu.edu.cn (Z.W.); 2011010505@ybu.edu.cn (Y.L.)
2
College of Chemistry, Chemical Engineering and Food Safety, Bohai University, Jinzhou 121013, China; E-Mail: zhongkeli2000@bhu.edu.cn
3
Beijing National Laboratory for Molecular Sciences, Joint Laboratory of Polymer Science and Materials, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China; E-Mail: liangyr@iccas.ac.cn
These authors contributed equally to this work.
*
Authors to whom correspondence should be addressed; E-Mails: tchen@ybu.edu.cn (T.C.); lyjin@ybu.edu.cn (L.Y.J.); Tel.: +86-433-2733-405 (T.C.); +86-433-2732-316 (L.Y.J.).
Received: 16 October 2013; in revised form: 3 March 2014 / Accepted: 11 March 2014 /
Published: 2 April 2014

Abstract

: In this paper, we report the synthesis and self-assembly behavior of coil-rod-coil molecules, consisting of three biphenyls linked through a vinylene unit as a conjugated rod segment and poly(ethylene oxide) (PEO) with a degree of polymerization (DP) of 7, 12 and 17, incorporating lateral methyl groups between the rod and coil segments as the coil segment. Self-organized investigation of these molecules by means of differential scanning calorimetry (DSC), thermal polarized optical microscopy (POM) and X-ray diffraction (XRD) reveals that the lateral methyl groups attached to the surface of rod and coil segments, dramatically influence the self-assembling behavior in the liquid-crystalline mesophase. Molecule 1 with a relatively short PEO coil length (DP = 7) self-assembles into rectangular and oblique 2-dimensional columnar assemblies, whereas molecules 2 and 3 with DP of 12 and 17 respectively, spontaneously self-organize into unusual 3-dimensional hexagonal close-packed or body-centered tetragonal assemblies.
Keywords:
coil-rod-coil; self-assembly; columnar; hexagonal; tetragonal; small angle X-ray scattering (SAXS)

1. Introduction

Well-ordered supramolecular materials with nanometer-scale architectures have shown a variety of photo-electro or bioactive functionalities in biochemistry, molecular electronics, and materials science [110]. Supramolecular nanostructures of organic or inorganic-organic hybrid materials can be created by self-assembly of these materials via weak intermolecular forces, such as non-covalent forces including hydrophobic and hydrophilic effects, electrostatic interaction, hydrogen bonding, and microphase segregation [1115].

Self-assembling organic molecules, especially, the construction of supramolecular nanostructures with well defined shape and size from conjugated rod-coil molecular architecture is one of the great interesting issues in organic and bioorganic materials chemistry due to the fact that ordered and conjugated rod building blocks can exhibit unique electronic and photonic properties for application in nanomaterials science [1620]. It has been reported that the supramolecular assemblies of coil-rod-coil molecules can be precisely tuned to form various nanostructure by the cooperative effects of several molecular parameters such as volume fraction of rod-to-coil segment, the cross-sectional area of coil segment, the shape of rigid rod segment and rod anisotropy [2123]. For the past ten years, Lee, Percec and other groups reported the formation of 1-D lamellar, 2-D columnar and 3-D discrete bundles supramolecular nanostructures from the self-assembly of a series of rod-coil molecules consisting of three biphenyls connected through vinylene linkages as a conjugated rod segment, and poly(propylene oxide) (PPO), poly(ethylene oxide) (PEO) or aliphatic polyether dendritic wedges with different cross-sections as the coil segments [2426]. For example, Lee reported a wedge-coil block conjugated rod-coil molecules consisting of a rigid wedge and a flexible poly(ethylene oxide) coils with DP (degree of polymerization) of 12, 21, 35 and 45 [27]. The authors demonstrated that the molecules self-assembled into 3D cubic, 2D hexagonal columnar, 3D perforated lamellar, and smectic-like lamellar structures in the liquid crystalline phase, as the length of the PEO chain increased. They also interpreted that compared to other self-assembling rod-coil architecture, the existence of an unusual phase transformation based on dendritic molecules from spherical cubic and columnar structures, into an unusual bilayer structure, was caused by the combination effect of shape complementarity and microphase separation between the conjugated rod and flexible PEO coil segments.

In addition, in our previous work, we have reported the self-assembly of coil-rod-coil triblock molecules consisting of three biphenyls connected through vinylene linkages as a conjugated rod segment and poly(ethylene oxide) with a degree of polymerization of 12 and 17 as the coil segment (molecules 2b and 2c, see Scheme 1) [28]. The results of structural analysis indicate that these molecules spontaneously self-organize into a 3-D hexagonally perforated layer (HPL) structure and a 2-D rectangular or an oblique columnar structure in the crystalline and liquid crystalline phases, respectively.

To further investigate the influence of methyl groups at the surface of rod and coil segments for construction ordered supramolecular nanostructures [15,28,29], in this study, we designed and synthesized coil-rod-coil molecules consisting of three biphenyls linked through a vinylene unit as a conjugated rod segment and poly(ethylene oxide) with a degree of polymerization of 7, 12 and 17, incorporating lateral methyl groups between the rod and coil segment as the coil segment. The self-assembling behaviors of 1a1c was investigated by means of differential scanning calorimetry (DSC), thermal polarized optical microscopy (POM) and X-ray scattering (XRD).

2. Results and Discussion

2.1. Synthesis of Oligomers 1a1c

The synthetic route of coil-rod-coil molecules 1a1c consisting of three biphenyls connected through vinylene linkages as a conjugated rod segment and poly(ethylene oxide) incorporating lateral methyl groups between the rod and coil segment as the coil segment is described in Scheme 1. Molecules 1a1c successfully synthesized through the Wittg-Horner reaction of 4,4′-biphenylmethylbis( diethylphosphonate) (molecule 7) and molecule 6 in the presence of potassium tertbutoxide. Molecules 6 with an aldehyde functional group were prepared from the oxidation reaction of 5 (see Scheme 1 and Figure S1) using pyridinium chlorochromate (PCC) as the oxidation reagent, in the methylene chloride solution at room temperature.

The structures of these molecules were characterized by 1H-NMR spectroscopy and a representative analysis of 1a is described below. The presence of signals of aromatic protons at 7.64–7.53 ppm (m, 20H, o to CHCHphenyl, m to CHCHphenyl, o to phenylOCH2CH, m to phenylOCH2CH, m to OCH2CH2), 7.19 (s, 4H, CHphenyl), 7.02–7.00 (d, J = 8.3 Hz, 4H, o to phenylOCH2), of methylene protons linkaged with phenyl and ether bonds at 4.67–4.56 (m, 2H, CHCH3), 3.75–3.53 (m, 54H, OCH2CH2O), of metoxy protons at 3.38 (s, 6H, OCH2CH2OCH3), and of lateral methyl protons at 1.36–1.34 (d, J = 6.1 Hz, 6H, CHCH3) respectively, indicated that molecule 1a was successfully synthesized (Figure S2). The MALDI-TOF mass spectrum of the molecule 1a exhibit exact signals that can be assigned as the H+ labeled molecular ions; the experimental mass based on peak positions in the spectrum is well matched with the theoretical molecular weight of molecule 1a (Figure S3). The structures of molecules 1b and 1c were also characterized by 1H-NMR spectroscopy and MALDI-TOF mass spectroscopy and were shown to be in full agreement with the structure presented in Scheme 1 (see Figures S4 and S5).

2.2. Self-Assembly of Molecules 1a1c in the Bulk State

The self-assembling behavior of the coil-rod-coil molecule 1 was investigated by means of differential scanning calorimetry (DSC), polarized optical microscopy (POM) and small-angle X-ray scatterings (SAXS). Figure S6 (see Supporting Information) shows the DSC heating, cooling traces and thermal transitions of the oligomers 1a1c. The transition temperatures together with the corresponding enthalpy changes (in brackets) determined from the DSC scans are summarized in Table 1. As shown in Table 1, the melting transition temperatures of the coil-rod-coil molecules decrease as the PEO coil length increase, especially, phase transition temperatures of molecules 1a1c, incorporating lateral methyl groups between the rod and coil segment, significantly decrease than molecules 2b2c (see Table S1) [28]. The results clearly imply that increasing flexible chain length or incorporating side groups at the surface of rod and coil domains leads to reduced phase transition temperatures [30,31].

On slow cooling from the isotropic state of 1a, a pseudo focal conic with arched striation texture was observed on optical polarized microscope, indicating the presence of hexagonally ordered crystalline phase (Figure 1a). To identify the detailed self-assembled nanostructures, the small-angle X-ray scattering experiments of molecules 1a1c were performed in their solid and melt states. Figure 2 presents SAXS diffraction patterns for molecule 1a measured at different temperatures. In the crystalline phases, the SAXS patterns display two main peaks together with several reflections with low intensity, as shown in Figure 2a. The observed reflections can be indexed as the (100), (002), (2–11) and (201) planes for 3-D hexagonal symmetry (P63/mmc space group symmetry) with lattice constants a = 6.48 nm and c = 10.38 nm (Table 2). An interesting point to be noted here is that the peak intensity associated with the (002) reflection appears to be the most intense, implying that the fundamental structure is lamellar. Hence, the supramolecular structure of 1a in the crystalline phases can be characterized as a honeycomb like crystalline layer of the rod segments with in-plane hexagonal packing of coil perforations. From the lattice parameters determined from the X-ray diffraction patterns and the densities of each segment, the perforation sizes in diameter are calculated to be 7.94 nm. Upon cooling from the isotropic liquid, a leaf-like texture with the characteristic of a columnar mesophase can be observed at 160 °C by using an optical polarized microscope (Figure 1b). The detailed liquid crystalline structure is also confirmed by small-angle X-ray scattering experiments.

The SAXS diffraction pattern shows one strong reflection, together with three reflections with low intensity at higher angles (Figure 2b). These reflections can be assigned as the (100), (110), (210) and (300) planes for 2-D rectangular symmetry with lattice parameters of a = 6.49 nm and b = 3.43 nm (Table 2). From the experimental values of the unit cell parameters (a, b, c) and the density (ρ = 1.02), the average number (n) of molecules per cross-sectional slice of the column is calculated as about four, according to following equation [31], where MW is the molecular weight and NA is Avogadro’s number.

n = ( a b c × sin γ ) × ρ × N A / M W

As shown in Figure 1c, the POM pattern measured at 230 °C shows focal conical spherulitic fan texture, also indicating the presence of columnar ordered liquid crystalline phase (Figure 1c). The SAXS scattering of 1a recorded at 230 °C displays one sharp reflection together with three reflections that can be indexed as the (100), (010), (110) and (300) that correspond to a two-dimensional oblique columnar with a characteristic angle γ = 135 of oblique columnar and lattice parameters a = 8.61 nm and b = 4.66 nm (Figure 2c and Table 2). The average number (n) of molecules per cross-sectional slice of the column is calculated as about six, according to Equation (1).

To investigate the cooperative effect between the methyl groups at the surface of rod and coil segments, and coil length for the creation of supramolecular nanostructures, we synthesized molecule 1b and 1c with longer PEO coil chains than molecule 1a. For molecule 1b, X-ray diffraction patterns shown in Figure 3a can be indexed as the (100), (110), (210) and (310) planes that correspond to a two-dimensional rectangular columnar with lattice constants a = 7.9 nm, b = 5.2 nm (Table 3) [32,33]. From the experimental values of the unit cell parameters (a, b, c) and the density (ρ = 1.02), the average number (n) of molecules per cross-sectional slice of the column is calculated as about 8, according to Equation (1). Upon heating the sample, the SAXS diffraction pattern of molecule 1b in the melt state recorded at 160 °C shows one strong reflection, together with three reflections with low intensity at higher angles (Figure 3b), which can be assigned as the (100), (010), (200), (110) and (300) planes for 2-D oblique columnar with lattice parameters of a = 9.04 nm and b = 4.89 nm, characteristic angle γ = 135 (Table 3).

The average number (n) of molecules per cross-sectional slice of the column is calculated as about seven. In sharp contrast to the lower temperature liquid-crystalline phase, a pseudo focal conic with arched striation texture was observed on optical polarized microscope (Figure 1d), and the small-angle X-ray diffraction pattern of 1b measured at 240 °C shown in Figure 3c display two well-resolved reflections with several low intensity peaks, which can be assigned as the (101), (102), (201), (202) and (301) planes for a 3-D hexagonal order (P63/mmc space group symmetry) with lattice constants a = 7.75 nm and c = 15.2 nm (Table 3). The peak intensities indexed as 101 and 102 reflections appeared to be relatively strong, as opposed to a hexagonally perforated lamellar structure where the peak intensity associated with the (002) reflection appears to be the most intense, implying that the fundamental structure of 1b in the liquid crystalline is not lamellar lattice. The results of small angle X-ray scattering analysis, together with optical microscopic observations, confirm that the fundamental structure of the 3-D hexagonal structure is based on discrete bundles. In this 3-D hexagonal order system, molecule 1b self-assembles into discrete rod-bundles that are encapsulated by PEO coils and subsequently organize into a 3-dimensional hexagonal close-packed structure. To describe the detailed supramolecular nanostructure, it is desirable to calculate the number of molecules per micelle. The number of molecules in a single rod bundle can be calculated as about 145, according to Equation (2), where a, c and ρ are the lattice constants and the density of molecule.

n = a 2 c N A ρ 2 M W

For molecule 1c, we also performed SAXS and POM experiments for evaluating the self-assembling nanostructures in the bulk state. As shown in Figure 4a, in the lower temperature crystalline phase, several diffraction peaks with the ratio of 1:√3:√4:√7:√9 in the small-angle region can be indexed as the (100), (110), (200), (210), (300) reflections of a 2D hexagonal columnar structure (Table 4 and Figure S7). From the observed d spacing of the 100 reflection, the lattice parameter of 2D hexagonal columnar phase of 1c is calculated to be 7.4 nm, and the average number (n) of molecules per cross-sectional slice of the column is calculated as about seven, according to Equation (1). In the liquid crystalline phase of 1c, the POM texture of 1c observed at 160 °C shows fern-like domains with rectangular shape growing in four directions (Figure S7). Meanwhile the small-angle X-ray diffraction pattern shows a sharp, high intensity reflection at a low angle together with a number of sharp reflections of low intensity at higher angles (Figure 4b). These reflections can be indexed as the (110), (002), (211), (220), (230) planes for a three-dimensional body-centered tetragonal structure with lattice constants of a = 9.8 nm and c = 9.02 nm (c/a = 0.92) (Table 4) [29]. The average number (n) of molecules per micelle can be calculated as about 132, according to Equation (2).

On the basis of the above discussion, we draw the interesting conclusion that molecules 1a self-assemble into hexagonal perforated lamellar, rectangular columnar and oblique columnar nanostructures in the solid state and the liquid crystalline phase. However, molecules 1b and 1c with a longer PEO coil chains than 1a, self-organize into columnar and hexagonal or tetragonal 3-D nanostructures in the solid state and the liquid crystalline phase. The variation of supramolecular nanostructures constructed by these molecules can be rationalized by considering the microphase separation between the dissimilar parts of the molecules and the space-filling requirements of the flexible PEO chains. Based upon the data presented so far, a schematic representation of the self-assembled structures of 1a1c is illustrated in Figure 5.

It should be pointed out that as mentioned in the introduction, in contrast with molecules 1a1c, molecules 2b and 2c self-assemble into perforated lamellar structures and columnar assemblies in the crystalline phase and in the liquid crystalline mesophase. Hence, it is of great interest that incorporating methyl groups at the surface of the rod and coil segments can create supramolecular structure: from the 2-dimensional columnar assemblies to a 3-dimensional hexagonal close-packed or body-centered tetragonal assemblies of molecules 1b and 1c in the liquid-crystalline mesophase. Therefore, the lateral methyl group at the surface of rod and coil segments is one of the main parameters for influencing the self-assembling behavior of the coil-rod-coil molecular system. The methyl groups at the surface of rod and coil segments can lead to decreased interaction of π-π stacking, and subsequently loosen the packing of the rod segments to produce more stable 3D bundle nanostructures. Thus, the strategy of incorporating lateral methyl groups into the interface coil-rod-coil molecular architecture can produce three dimensional supramolecular nonostructures, as well as adjusting coil volume fraction or chain length of rod-coil molecular system.

3. Experimental Section

3.1. Materials

4′-Hydroxy-4-biphenyl methylene alcohol (99%), pyridinium chlorochromate (PCC, 98%), potassium tertbutoxide (97%), toluene-p-sulfonyl chloride (99%), (−)-Ethyl l-lactate, 3,4-Dihydro-2H-pyran, 3,4-Dihydro-2H-pyran (99%), poly (ethylene oxide) monomethyl ether of DP 7, 12 and 17 (all from J&K CHEMICAL LTD, Shanghai, China, and Sigma-Aldrich China, Inc., Shanghai, China) and the other conventional reagents were used as received. Compounds 35 and phosphonium salt 7 (see Supplementary Information) was synthesized according to the procedure described elsewhere [31,33,34].

3.2. Techniques

1H-NMR spectra was recorded from CDCl3 solution on a Bruker AM 300 spectrometer. Column chromatography (silica gel 100–200) was used to check the purity of the products. A Perkin Elmer Pyris Diamond differential scanning calorimeter (Perkin Elmer, Waltham, MA, USA) was used to determine the thermal transitions with the maxima and minima of their endothermic or exothermic peaks, controlling the heating and cooling rates to 10 °C/min. X-ray scattering measurements were performed in transmission mode with synchrotron radiation at the 1W2A X-ray beam line at Beijing Accelerator Laboratory (Beijing, China). MALDI-TOF-MS was performed on a Perceptive Biosystems Voyager-DE STR (Applied Biosystems, Foster City, CA, USA) using a 2-cyano-3-(4-hydroxyphenyl) acrylic acid (CHCA) as matrix.

3.3. Synthesis Compounds 6a6c

Compounds 6a6c were all synthesized using the same procedure. A representative example is described for 6a. Compound 5a (1 g, 1.97 mmol) was dissolved in 50 mL of methylene chloride. PCC (1.15 g, 5.36 mmol) was added under nitrogen. The reaction mixture was stirred at room temperature under nitrogen for 6 h. The resulting solution was removed in a rotary evaporator, and the crude product was purified by column chromatography (silica gel, ethyl ether eluent) to yield 0.75 g (75%). 1H-NMR (300 MHz, CDCl3, δ, ppm): 10.04 (s, 1H, phenylCHO), 7.94–7.91 (d, J = 8.0 Hz, 2H, o to phenylCHO), 7.73–7.70 (d, J = 7.9 Hz, 2H, m to phenylCHO), 7.59–7.56 (d, J = 8.2 Hz, 2Ar-H, o to phenylCHO), 7.04–7.02 (d, J = 8.3 Hz, 2H, m to phenylCHO), 4.71–4.56 (m, 1H, CHCH3), 3.80–3.55 (m, 36H, OCH2CH2), 3.38 (s, 3H, OCH2CH2OCH3), 1.36–1.34 (d, J = 5.7 Hz, 6H, CHCH3).

Compound 6b: yield: 77%; 1H-NMR (300 MHz, CDCl3, δ, ppm): 10.04(s, 1H, phenylCHO), 7.94–7.91 (d, J = 7.7 Hz, 2H, o to phenylCHO), 7.73–7.70 (d, J = 7.7 Hz, 2H, m to phenylCHO), 7.59–7.56 (d, J = 7.1 Hz, 2Ar-H, o to phenylCHO), 7.04–7.02 (d, J = 7.4 Hz, 2H, m to phenylCHO), 4.69–4.59 (m, 1H, CHCH3), 3.80–3.55 (m, 55H, OCH2CH2), 3.38 (s, 3H, OCH2CH2OCH3), 1.38–1.33 (d, J = 5.7 Hz, 6H, CHCH3).

Compound 6c: yield: 78%; 1H-NMR (300 MHz, CDCl3, δ, ppm): 10.04(s, 1H, phenylCHO), 7.94–7.91 (d, J = 7.6 Hz, 2H, o to phenylCHO), 7.73–7.70 (d, J = 7.7 Hz, 2H, m to phenylCHO), 7.59–7.56 (d, J = 7.1 Hz, 2Ar-H, o to phenylCHO), 7.04–7.02 (d, J = 7.4 Hz, 2H, m to phenylCHO), 4.70–4.60 (m, 1H, CHCH3), 3.79–3.54 (m, 69H, OCH2CH2), 3.38 (s, 3H, OCH2CH2OCH3), 1.37–1.33 (d, J = 5.6 Hz, 6H, CHCH3).

3.4. Synthesis Oligomers 1a1c

Oligomers 1a1c were all synthesized using the same procedure. A representative example is described for 8a. Compound 6a (0.75 g, 1.34 mmol) was dissolved in 30 mL dry THF. A solution of phosphonium salt (0.12 g, 0.27 mmol) and KOC(CH3)3 (0.226 g, 2 mmol) in 30 mL dry THF was added under nitrogen. The reaction mixture was stirred at room temperature under nitrogen for 24 h. The resulting solution was removed in a rotary evaporator. The crude product was extracted with methylene chloride and water. The methylene chloride solution was washed with water, dried over anhydrous magnesium sulfate, and filtered. The solvent was removed in a rotary evaporator, and the crude product was purified by column chromatography (silica gel, ethyl acetate eluent) to yield 0.15 g (42%) of a yellow green solid.

Oligomer 1a: yield: 42%; 1H-NMR (300 MHz, CDCl3, δ, ppm): 7.64–7.53 (m, 20H, o to CHCHphenyl, m to CHCHphenyl, o to phenylOCH2CH, m to phenylOCH2CH, m to OCH2CH2), 7.19 (s, 4H, CHphenyl), 7.02–7.00 (d, J = 8.3 Hz, 4H, o to phenylOCH2- CH2), 4.67–4.56 (m, 2H, CHCH3), 3.75–3.53 (m, 54H, OCH2CH2O), 3.38 (s, 6H, OCH2CH2OCH3), 1.36–1.34 (d, J = 6.1 Hz, 6H, CHCH3). Anal. Calcd for C76H102O18: C, 70.02; H, 7.89. Found: C, 69.95; H, 7.93. MALDI-TOF-MS m/z (M + H)+ 1324.

Oligomer 1b: yield: 40%; 1H-NMR (300 MHz, CDCl3, δ, ppm): 7.64–7.53 (m, 20H, o to CHCHphenyl, m to CHCHphenyl, o to phenylOCH2CH, m to phenylOCH2CH, m to OCH2CH2), 7.19 (s, 4H, CHphenyl), 7.02–6.99 (d, J = 8.8 Hz, 4H, o to phenylOCH2- CH2), 4.66–4.56 (m, 2H, CHCH3), 3.75–3.54 (m, 107H, OCH2CH2O), 3.38 (s, 6H, OCH2CH2OCH3), 1.36–1.34 (d, J = 6.2 Hz, 6H, CHCH3). Anal. Calcd for C96H142O28: C, 66.11; H, 8.21. Found: C, 66.07; H, 8.25. MALDI-TOF-MS m/z (M)+ 1633.

Oligomer 1c: yield: 40%; 1H-NMR (300 MHz, CDCl3, δ, ppm): 7.67–7.53 (m, 20H, o to CHCHphenyl, m to CHCHphenyl, o to phenylOCH2CH, m to phenylOCH2CH, m to OCH2CH2), 7.19 (s, 4H, CHphenyl), 7.02–6.99 (d, J = 8.8 Hz, 4H, o to phenylOCH2-CH2), 4.65–4.58 (m, 2H, CHCH3), 3.76–3.55 (m, 170H, OCH2CH2O), 3.38 (s, 6H, OCH2CH2OCH3), 1.36–1.34 (d, J = 6.2 Hz, 6H, CHCH3). Anal. Calcd for C116H182O38: C, 63.77; H, 8.40. Found: C, 63.68; H, 8.43. MALDI-TOF-MS m/z (M)+ 1985.

4. Conclusions

Coil-rod-coil oligomers 1a1c consisting of a conjugated rod segment and poly(ethylene oxide) incorporating lateral methyl groups between the rod and coil segment as a coil segment were successfully synthesized. Molecules 1a self-assembles into hexagonal perforated lamellar, rectangular columnar and oblique columnar nanostructures in the bulk state. However, molecules 1b and 1c with longer PEO coil chains than 1a, self-organize into columnar and hexagonal or tetragonal 3-D nanostructures in the solid state and the liquid crystalline phase. The experimental results reveal that the lateral methyl groups incorporating at the surface of rod and coil segments, dramatically influence the self-assembling behavior in the solid and in the liquid-crystalline mesophase. Moreover, the strategy of incorporating lateral methyl or other alkyl groups into the interface of coil-rod-coil molecular architecture induced construction of three-dimensional functional molecular micelles in the liquid crystalline phase, compared with rod-coil molecules which lack methyl groups.

Supplementary Information

ijms-15-05634-s001.pdf

Acknowledgments

This work was supported by the National Natural Science Foundation of China (grant number: 21164013, 21304009) and the Open Project of State Key Laboratory of Supramolecular Structure and Materials (Grant Number sklssm201430). We are grateful to Being Synchrotron Radiation Facility (BSRF), Institute of High Energy Physics, Chinese Academy of Sciences for help with the X-ray scattering measurements of molecular structures.

Conflicts of Interest

The authors declare no conflict of interest.

References and Notes

  1. Rosen, B.M.; Wilson, C.J.; Wilson, D.A.; Peterca, M.; Imam, M.R.; Percec, V. Dendron-mediated self-assembly, disassembly and self-organization of complex systems. Chem. Rev 2009, 109, 6275–6540. [Google Scholar]
  2. Ligthart, G.B.; Sijbesma, R.P.; Meijer, E.W. Complementary quadruple hydrogen bonding in supramolecular copolymers. J. Am. Chem. Soc 2005, 127, 810–811. [Google Scholar]
  3. Hoeben, F.; Jonkheijm, P.; Meijer, E.W.; Schenning, A.P. About supramolecular assemblies of π-conjugated systems. Chem. Rev 2005, 105, 1491–1546. [Google Scholar]
  4. Zhao, H.X.; Guo, D.S.; Wang, L.H.; Qian, H.; Liu, Y. A novel supramolecular ternary polymer with two orthogonal host-guest interactions. Chem. Commun 2012, 48, 11319–11321. [Google Scholar]
  5. Roosma, J.; Mes, T.; Leclere, P.; Palmans, A.; Meijer, E.W. Supramolecular materials from benzene-1,3,5-tricarboxamide-based nanorods. J. Am. Chem. Soc 2008, 130, 1120–1121. [Google Scholar]
  6. Wilson, D.A.; Wilson, C.J.; Moldoveanu, C.; Resmerita, A.; Corcoran, P.; Hoang, L.M.; Rosen, B.M.; Percec, V. Neopentylglycolborylation of aryl mesylates and tosylates catalyzed by Ni-based mixed-ligand systems activated with Zn. J. Am. Chem. Soc 2010, 132, 1800–1801. [Google Scholar]
  7. Yue, L.; Ai, H.; Yang, Y.; Lu, W.; Wu, L. Chiral self-assembly and reversible light modulation of a polyoxometalate complex via host–guest recognition. Chem. Commun 2013, 49, 9770–9772. [Google Scholar]
  8. Zhang, X.; Wang, C. Supramolecular amphiphiles. Chem. Soc. Rev 2011, 40, 94–101. [Google Scholar]
  9. Wang, C.; Guo, Y.; Wang, Z.; Zhang, X. Superamphiphiles based on charge transfer complex: Controllable hierarchical self-assembly of nanoribbons. Langmuir 2010, 26, 14509–14511. [Google Scholar]
  10. Bhosale, S.V.; Kalyankar, M.B.; Nalage, S.V.; Lalander, C.H.; Bhosale, S.V.; Langford, S.J.; Oliver, R.F. pH dependent molecular self-assembly of octaphosphonate porphyrin of nanoscale dimensions: Nanosphere and nanorod aggregates. Int. J. Mol. Sci 2011, 12, 1464–1473. [Google Scholar]
  11. Starnes, S.D.; Arungundram, S.; Saunders, C.H. Anion sensors based on β,β′-disubstituted porphyrin derivatives. Tetrahedron Lett 2002, 43, 7785–7788. [Google Scholar]
  12. Kim, H.-J.; Jeong, Y.-H.; Lee, E.; Lee, M. Channel structures from self-assembled hexameric macrocycles in laterally grafted bent rod molecules. J. Am. Chem. Soc 2009, 131, 17371–17375. [Google Scholar]
  13. Percec, V.; Rudick, J.G.; Peterca, M.; Heiney, P.A. Nanomechanical function from self-organizable dendronized helical polyphenylacetylenes. J. Am. Chem. Soc 2008, 130, 7503–7508. [Google Scholar]
  14. Motoyanagi, J.; Fukushima, T.; Ishii, N.; Aida, T. Photochemical stitching of a tubularly assembled hexabenzocoronene amphiphile by dimerization of coumarin pendants. J. Am. Chem. Soc 2006, 128, 4220–4201. [Google Scholar]
  15. Zhong, K.L.; Wang, Q.; Chen, T.; Jin, L.Y. Self assembly of coil-rod-coil molecules into bicontinuous cubic and oblique columnar assemblies depending on the coil chain length. Eur. Polym. J 2013, 49, 3244–3250. [Google Scholar]
  16. Hamley, I.W.; Castelletto, V.; Lu, Z.B.; Imrie, C.T.; Itoh, T.; Al-Hussein, M. Interplay between smectic ordering and microphase separation in a series of side-group liquid-crystal block copolymers. Macromolecules 2004, 37, 4798–4807. [Google Scholar]
  17. Lee, E.; Kim, J.K.; Lee, M. Reversible scrolling of two-dimensional sheets from the self-assembly of laterally grafted amphiphilic rods. Angew. Chem. Int. Ed 2008, 47, 6375–6378. [Google Scholar]
  18. Wu, J.; Pisula, W.; Müllen, K. Graphenes as potential material for electronics. Chem. Rev 2007, 107, 718–747. [Google Scholar]
  19. Rudick, J.G.; Percec, V. Helical chirality in dendronized polyarylacetylenes. New J. Chem 2007, 31, 1083–1096. [Google Scholar]
  20. Liu, L.B.; Moon, K.S.; Gunawidjaja, R.; Lee, E.; Tsukruk, V.; Lee, M. Molecular reorganization of paired assemblies of T-shaped rod-coil amphiphilic molecule at the air-water interface. Langmuir 2008, 24, 3930–3936. [Google Scholar]
  21. Lavigueur, C.; Foster, E.J.; Williams, V.E. Modular assembly of elliptical mesogens. Liq. Cryst 2007, 34, 833–840. [Google Scholar]
  22. Voisin, E.; Foster, J.E.; Rakotomalala, M.; Williams, V.E. Effects of symmetry on the stability of columnar liquid crystals. Chem. Mater 2009, 21, 3251–3261. [Google Scholar]
  23. Hong, D.-J.; Jeong, H.; Lee, J.-K.; Zin, W.-C.; Nguyen, T.D.; Glotzer, S.C.; Lee, M. Solid-state scrolls from hierarchical self-assembly of T-shaped rod-coil molecules. Angew. Chem. Int. Ed 2009, 48, 1664–1668. [Google Scholar]
  24. Lee, M.; Cho, B.K.; Zin, W.C. Supramolecular structures from rod-coil block copolymers. Chem. Rev 2001, 101, 3869–3892. [Google Scholar]
  25. Zhu, J.; Zhong, K.; Liang, Y.; Wang, Z.; Chen, T.; Jin, L.Y. Synthesis and self-assembly of oligomers containing cruciform 9,10-bis(arylethynyl)anthracene unit: Formation of supramolecular nanostructures based on rod-length-dependent organization. Tetrahedron 2014. [Google Scholar] [CrossRef]
  26. Vriezema, D.M.; Aragones, M.C.; Elemans, J.A.A.W.; Comelissen, J.; Rowan, A.E.; Nolte, R.J.M. Self-assembled nanoreactors. Chem. Rev 2005, 105, 1445–1490. [Google Scholar]
  27. Kim, J.-K.; Hong, M.-K.; Ahn, J.-H.; Lee, M. Liquid crystalline assembly from rigid wedge-flexible coil diblock molecules. Angew. Chem. Int. Ed 2005, 44, 328–332. [Google Scholar]
  28. Chen, L.; Zhong, K.L.; Jin, L.Y. Supramolecular honeycomb and columnar assemblies formed by self-assembly of coil-rod-coil molecules with a conjugated rod segment. Macromol. Res 2010, 18, 800–805. [Google Scholar]
  29. Lee, E.; Huang, Z.; Ryu, J.-H.; Lee, M. Rigid-flexible block molecules based on a laterally extended aromatic segment: Hierarchical assembly into single fibers, flat ribbons, and twisted ribbons. Chem. Eur. J 2008, 14, 6957–6966. [Google Scholar]
  30. Wang, Z.S.; Cui, J.J.; Liang, Y.R.; Chen, T.; Lee, M.; Yin, B.Z.; Jin, L.Y. Supramolecular nanostructures from self-assembly of T-shaped rod building block oligomers. J. Polym. Sci. Part A Polym. Chem 2013, 51, 5021–5028. [Google Scholar]
  31. Tian, L.R.; Zhong, K.-L.; Liu, Y.J.; Huang, Z.G.; Jin, L.Y.; Hirst, L.S. Synthesis and self-assembly of coil-rod-coil molecules with lateral methyland ethyl groups in the center of the rod segment. Soft Matter 2010, 6, 5993–5998. [Google Scholar]
  32. The Avogadro constant (symbols: NA) is defined as the number of constituent particles (usually atoms or molecules) in one mole of a given substance. So, the average number (n) of molecules in a single rod bundle can be calculated according to following equation: n = m × NA/MW = v × ρ × NA/MW = (abc × sinγ) × ρ × NA/MW.
  33. Zhong, K.L.; Huang, Z.; Man, Z.; Jin, L.Y.; Yin, B.; Lee, M. Synthesis and self-assembly of rod-coil molecules with n-shaped rod building block. J. Polym. Sci. Part A Polym. Chem 2010, 48, 1415–1422. [Google Scholar]
  34. Zhong, K.; Man, Z.; Huang, Z.; Chen, T.; Yin, B.; Jin, L.Y. Supramolecular columnar nanostructures from self-organization of coil-rod-coil molecules incorporating an anthracene unit. Polym. Int 2011, 60, 845–850. [Google Scholar]
Ijms 15 05634f1 1024
Figure 1. Representative optical polarized micrograph (×100) of the texture exhibited by (a) hexagonal perforated lamellar structure of 1a in the crystal phase; (b) Rectangular columnar structure of 1a and (c) oblique columnar structure of 1a at the transition from the anisotropic liquid crystal phase; (d) 3-D hexagonal close-packed structure of 1b at the transition from the anisotropic liquid crystalline phase; (e) Body-centered tetragonal micelle of 1c at the transition from the anisotropic liquid crystalline phase.

Click here to enlarge figure

Figure 1. Representative optical polarized micrograph (×100) of the texture exhibited by (a) hexagonal perforated lamellar structure of 1a in the crystal phase; (b) Rectangular columnar structure of 1a and (c) oblique columnar structure of 1a at the transition from the anisotropic liquid crystal phase; (d) 3-D hexagonal close-packed structure of 1b at the transition from the anisotropic liquid crystalline phase; (e) Body-centered tetragonal micelle of 1c at the transition from the anisotropic liquid crystalline phase.
Ijms 15 05634f1 1024
Ijms 15 05634f2 1024
Figure 2. Small-angle and wide-angle X-ray diffraction patterns of 1a plotted against q (=4πsin2θ/λ). (a) Hexagonal perforated layer at 30 °C; (b) Rectangular column at 160 °C; and (c) Oblique column at 230 °C.

Click here to enlarge figure

Figure 2. Small-angle and wide-angle X-ray diffraction patterns of 1a plotted against q (=4πsin2θ/λ). (a) Hexagonal perforated layer at 30 °C; (b) Rectangular column at 160 °C; and (c) Oblique column at 230 °C.
Ijms 15 05634f2 1024
Ijms 15 05634f3 1024
Figure 3. Small-angle and wide-angle X-ray diffraction patterns of 1b plotted against q (=4πsin2θ/λ). (a) Rectangular column at 30 °C; (b) Oblique column at 160 °C; and (c) Hexagonal close-packed structure measured at 240 °C.

Click here to enlarge figure

Figure 3. Small-angle and wide-angle X-ray diffraction patterns of 1b plotted against q (=4πsin2θ/λ). (a) Rectangular column at 30 °C; (b) Oblique column at 160 °C; and (c) Hexagonal close-packed structure measured at 240 °C.
Ijms 15 05634f3 1024
Ijms 15 05634f4 1024
Figure 4. Small-angle and wide-angle X-ray diffraction patterns of 1c plotted against q (=4πsin2θ/λ). (a) Hexagonal column at 30 °C; (b) Body-centered tetragonal structure at 160 °C.

Click here to enlarge figure

Figure 4. Small-angle and wide-angle X-ray diffraction patterns of 1c plotted against q (=4πsin2θ/λ). (a) Hexagonal column at 30 °C; (b) Body-centered tetragonal structure at 160 °C.
Ijms 15 05634f4 1024
Ijms 15 05634f5 1024
Figure 5. Schematic representation of self-assembly of (a) hexagonal perforated layer structure for 1a; (b) Rectangular columnar structure for 1a; (c) Oblique columnar structure for 1a; (d) Hexagonal column for 1c; (e) Hexagonal close-packed structure for 1b; (f) Body-centered tetragonal structure for 1c.

Click here to enlarge figure

Figure 5. Schematic representation of self-assembly of (a) hexagonal perforated layer structure for 1a; (b) Rectangular columnar structure for 1a; (c) Oblique columnar structure for 1a; (d) Hexagonal column for 1c; (e) Hexagonal close-packed structure for 1b; (f) Body-centered tetragonal structure for 1c.
Ijms 15 05634f5 1024
Ijms 15 05634f6 1024
Scheme 1. Synthetic route of coil-rod-coil molecules 1a1c (A) and chemical structures of molecules 2b and 2c (B).

Click here to enlarge figure

Scheme 1. Synthetic route of coil-rod-coil molecules 1a1c (A) and chemical structures of molecules 2b and 2c (B).
Ijms 15 05634f6 1024
Table Table 1. Thermal transitions and corresponding enthalpy changes of molecules 1a1c.

Click here to display table

Table 1. Thermal transitions and corresponding enthalpy changes of molecules 1a1c.
MoleculePhase transitions (°C) and corresponding enthalpy changes (in brackets) (kJ/mol)
HeatingCooling
1aHPL 106.4 (2.0) Colrec 243.6 (0.9)
Colob 292.4 (0.4) i
i 274.5 (0.8) Colob 219.7 (1.0)
Colrec 104.8 (1.8) HPL
1bColrec 86.4 (1.9) Colob 224.6 (0.9)
HCP 274.3 (0.4) i
i 249.6 (0.8) HCP 195.9 (1.1)
Colob 80.7 (1.7) Colrec
1cColhex 72.9(1.6) Mtet 225.9 (1.4) ii 205.3 (1.5) Mtet 36.4 (0.8) Colhex

HPL: hexagonal perforated layer; Colrec: rectangular column; Colob: oblique column; Colhex: hexagonal column; HCP: hexagonal close packed; Mtet: body-centered tetragonal micellar; i: isotropic.

Table Table 2. Small-angle X-ray diffraction data for molecule 1a in the bulk state.

Click here to display table

Table 2. Small-angle X-ray diffraction data for molecule 1a in the bulk state.
Mesophase (lattice constants)Reflections/nmMiller indices (h k l)

qobsdqcalcd
HPL at 30 °C;
a = 6.48 nm;
c = 10.38 nm
1.1251.125100
1.2101.211002
2.0372.0362–11
2.3212.321201

Colrec at 160 °C;
a = 6.49 nm;
b = 3.43 nm
0.9680.968100
2.0642.065110
2.6632.662210
2.9042.905300

Colob at 230 °C;
a = 8.61 nm;
b = 4.66 nm;
γ = 135
1.0251.025100
1.9081.908010
2.7332.734110
3.0893.089300

qobsd and qcalcd are the scattering vectors of the observed and calculated reflections; HPL: hexagonal perforated layer; Colrec: rectangular column; Colob: oblique column.

Table Table 3. Small-angle X-ray diffraction data for molecule 1b in the bulk state.

Click here to display table

Table 3. Small-angle X-ray diffraction data for molecule 1b in the bulk state.
Mesophase (lattice constants)Reflections/nmMiller indices (h k l)

qobsdqcalcd
Colrec at 30 °C
a = 7.9 nm
b = 5.2 nm
0.7970.797100
1.4531.453110
1.9941.994210
2.6772.676310

Colob at 160 °C
a = 9.04 nm
b = 4.89 nm
γ = 135
0.9830.983100
1.8221.823010
1.9631.965200
2.6052.605110
2.9452.947300

HCP at 230 °C
a = 7.75 nm
b = 15.2 nm
1.0251.025101
1.2531.253102
1. 9221.922201
2.0502.050202
2.8332.833301

qobsd and qcalcd are the scattering vectors of the observed and calculated reflections; HCP: hexagonal closed-packed; Colrec: rectangular column; Colob: oblique column.

Table Table 4. Small-angle X-ray diffraction data for molecule 1c in the bulk state.

Click here to display table

Table 4. Small-angle X-ray diffraction data for molecule 1c in the bulk state.
Mesophase (lattice constants)Reflections/nmMiller indices (h k l)

qobsdqcalcd
Colh at 30 °C
a = 7.4 nm
0.9830.983100
1.6941.694110
1.9651.965200
2.5902.591210
2.9462.947300

Mtet at 160 °C
a = 9.8 nm
b = 9.02nm
0.9110.911110
1.3961.396002
1.5941. 594211
1.8091.808220
2.3082.307230

qobsd and qcalcd are the scattering vectors of the observed and calculated reflections; Mtet: body-centered tetragonal micelle; Colh: hexagonal column.

Int. J. Mol. Sci. EISSN 1422-0067 Published by MDPI AG, Basel, Switzerland RSS E-Mail Table of Contents Alert