Liquid Crystalline Assembly of Coil-Rod-Coil Molecules with Lateral Methyl Groups into 3-D Hexagonal and Tetragonal Assemblies

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.

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 [16][17][18][19][20]. 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 [21][22][23]. 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 [24][25][26]. 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 1a-1c was investigated by means of differential scanning calorimetry (DSC), thermal polarized optical microscopy (POM) and X-ray scattering (XRD).

Synthesis of Oligomers 1a-1c
The synthetic route of coil-rod-coil molecules 1a-1c 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 1a-1c 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 1 H-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 phenylOCH 2 CH, m to phenylOCH 2 CH, m to OCH 2 CH 2 ), 7.19 (s, 4H, CHphenyl), 7.02-7.00 (d, J = 8.3 Hz, 4H, o to phenylOCH 2 ), of methylene protons linkaged with phenyl and ether bonds at 4.67-4.56 (m, 2H, CHCH 3 ), 3.75-3.53 (m, 54H, OCH 2 CH 2 O), of metoxy protons at 3.38 (s, 6H, OCH 2 CH 2 OCH 3 ), and of lateral methyl protons at 1.36-1.34 (d, J = 6.1 Hz, 6H, CHCH 3 ) 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 1 H-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).

Self-Assembly of Molecules 1a-1c 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 1a-1c. 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 1a-1c, incorporating lateral methyl groups between the rod and coil segment, significantly decrease than molecules 2b-2c (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 1a-1c 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)(3)(4)(5)(6)(7)(8)(9)(10)(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 M W is the molecular weight and N A is Avogadro's number.
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  = 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)  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) 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)  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).  Table 4. Small-angle X-ray diffraction data for molecule 1c in the bulk state. 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 1a-1c is illustrated in Figure 5.

Mesophase (lattice constants) Reflections/nm Miller indices (h k l) q obsd q calcd
It should be pointed out that as mentioned in the introduction, in contrast with molecules 1a-1c, 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. 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.

Techniques
1 H-NMR spectra was recorded from CDCl 3 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.

Synthesis Compounds 6a-6c
Compounds 6a-6c 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%). 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.