The development of novel functional materials from selforganizing systems is of current interest [1, 2]. Liquid crystals (LCs), representing soft self-assembly materials,are of significant technological importance [3, 4] and provide new routes to increase complexity in soft self-assembly systems . Bent-core LCs composed of a non-linear rigid core with terminal alkyl chains have also attracted significant interest,owing to their ability to exhibit ferroelectric and antiferroelectric properties .
Bolaamphiphiles consisting of a rod-like rigid core,two terminal glycerol groups,and lateral alkyl chains can self-organize into a wide range of polygonal honeycomb columnar phases with different cylinder shapes,ranging from triangular via square,pentagonal to hexagonal etc. [7, 8, 5]. In order to introduce functionality into such soft nanostructures for their potential applications,oligothiophene segments [9, 10, 11, 12, 13] and ethynlene  units have been employed respectively as rod-like building blocks. Till now honeycomb-type columnar phases with triangular,square,and hexagonal cells have been found for functional bolaamphiphiles based on bithiophene [10b, 11],tetrathiophene [10a, 12] and sexithiophene  cores by varying the length and number of the lateral alkyl chains,and recently even the honeycomb-type columnar phase with noncentrosymmetric trigonal p3m1 symmetry (Colhex-3/p3m1),which is of potential interest for nonlinear optics applications  has been found for bolaamphiphiles with π-conjugated 1,3-bis(ethynyl)benzene bent-core [16, 17].
As part of our research work on functional bolaamphiphiles,herein we have further introduced both thienyl and acetylene units intotherigid coresimultaneouslyand obtaineda newseries of bentshaped 2,5-diphenylethynyl thiophene based bolaamphiphiles 1ET/n and compared them with the related compounds 1T/n  without ethynyl units and compounds 1EP/n  with phenyl instead of thienyl as the central unit in the rigid core (see Scheme 1).
|Scheme 1. Structures of compounds 1ET/n under investigation and the related previously reported bent-shaped bolaamphiphiles 1T/n  and 1EP/n  and their tilling patterns and related 2D lattices of their liquid crystalline phases, the views result from cuts through the self-assembled LC polygonal honeycomb phases perpendicular to the cylinder long axis (blue dots: columns incorporating the H-bonding networks, black bold lines: aromatic cores, gray lines: alkyl/alkoxy chain).|
1H NMR and 13C NMR spectra were recorded in CDCl3 solution on a Bruker-DRX-500 spectrometer with tetramethylsilane (TMS) as the internal standard. Elemental analysis was performed using an Elementar VARIO EL elemental analyzer. A Mettler heating stage (FP 82 HT) was used for polarizing optical microscopy (POM,Optiphot 2,Nikon) and DSC were recorded with a DSC 200 F3 Maia calorimeter (NETZSCH) at 10 K min-1. X-Ray diffraction (XRD) of surface aligned samples was performed using a 2D-detector (HIStar,Siemens). UV-vis absorption spectra were recorded on a UV- 240 UV-visible spectrophotometer (Shimadzu,Japan). Fluorescence spectra were recorded using a Hitachi F-7000 fluorescence spectrometer (Hitachi,Japan). The IR spectra were recorded on an AVATAR 360 FT-IR system. These spectral measurements were carried out at room temperature.
Scheme 2 describes the synthesis of the target bent core bolaamphiphiles 1ET/n using Kumada and Sonogashira coupling reactions as key steps. Kumada coupling reaction between 3-bromothiophene and n-alkyl Grignard reagents yielded the 3-alkylthiophenes (2/n) ,which were diiodinated with N-iodosuccinimide (NIS) . The obtained 2,5-diiodo-3-alkylthiophenes 3/n were then coupled with 1,2-O-isopropylidene glycerol substituted phenylacetylene 4 [17, 20] in a Sonogashira coupling reaction,affording the 1,2-O-isopropylidene glycerol substituted 2,5-bis(phenylethynyl)thiophene 5/n. In the last step the 1,2- O-isopropylidene groups were cleaved with pyridinium p-toluenesulfonate (PPTS) in CH3OH [20, 21]. The final compounds 1ET/n were purified by repeated crystallization from petroleum ether/ethyl acetate mixtures (1:2). The detailed procedures and corresponding analysis data are collected in Supporting Information.
|Scheme 2.Synthesis of compounds 1ET/n; Reagents and conditions: (i) CnH2n+1MgBr, [1,3-bis(diphenylphosphino)propane]nickel(Ⅱ) chloride [Ni(dppp)Cl2], tetrahydrofuran (THF); (ii) NIS, CH3COOH, CH2Cl2, 25 ℃, 2 h; (iii) Tetrakis(triphenylphosphine)palladium(0) [Pd(PPh3)4], CuI, THF, triethylamine (Et3N), 25 ℃, 12 h; (iv) PPTS, CH3OH, reflux, 12 h.|
The phase transition temperatures and associated enthalpy values of the synthesized compounds 1ET/n together with the previously reported compounds 1T/n  and 1EP/n  are summarized in Table 1. Compounds 1ET/n are all crystals without liquid crystalline properties in their pure states. However in the presence of water,all of these compounds exhibit LC phases.
Obviously the higher melting points of 1ET/n suppress the formation of LC phases. A comparison of compounds 1ET/n with previously reported related 2,5-diphenyl thiophene based bolaamphiphiles 1T/n  and 1,3-bis(phenylethynyl)benzene based bolaamphiphiles 1EP/n  provides additional insight. As reported,stable higher symmetry LC phases,including Colsqu/ p4mm,Colsqu/p4gm,and Colhex/p6mm phases,are found in 2,5- diphenyl thiophene based bolaamphiphiles 1T/n. In this case,the bent 148° angle in the rigid core induced by the thiophene unit,though it reduces the mesophase stability,does not remove the LC phases. It contributes to the reduction of melting points and crystallization tendency and therefore the broad region of LC phases . While monotropic non-centrosymmetric trigonal p3m1 LC phase is found in bent 1,3-bis(phenylethynyl)benzene based bolaamphiphiles 1EP/n,the bent angel of 120° is essential for the formation of 3-hexagons (p3m1) columnar mesophase. For compounds 1ET/n under investigation here,the bent rigid core is elongated compared with 2,5-diphenyl thiophene based bolaamphiphiles 1T/n by two additional acetylene units,leading to higher melting points,however their bent angle of 148° inhibits the formation of 3-hexagons (p3m1) columnar mesophase as those 1EP/n did . Therefore 1ET/n with the elongated bent core can neither be compensated by rotational averaging along the molecular long axis nor organize into 3-hexagons (p3m1) columnar mesophase as tolane derivatives 1EP/n did,therefore they exhibit only crystal phases in their pure states.
Though no thermotropic LC phases were detected in pure states,room temperature lyotropic LC phases can be induced for all investigated compounds 1ET/n by addition of water,being incorporated into the hydrogen bonding networks of the glycerol groups,increasing the number of attractive intermolecular hydrogen bonding and thus stabilizing these networks (see Table 1) . Under POM,all induced mesophases have spherulitic textures,indicating columnar phases (Fig. 1). All these columnar phases are uniaxial as indicated by the presence of optically isotropic homeotropic regions,where the direction of the cylinder long axis is perpendicular to the substrate surfaces (dark areas in Fig. 1a-d). This means that the investigated columnar phases could either have a hexagonal or a square 2D lattice. Investigation with an additional λ-retarder plate shows that all columnar phases are optically negative,i.e. they have the high index optical axis,which is known to be parallel to the long axis of the π-conjugated cores,perpendicular to the column long axis (see insets in Fig. 1a-d). This is in line with polygonal honeycomb structures in which the π-conjugated rod-like cores are arranged on average perpendicular to the cylinder long axis . In all cases the highest clearing temperature of the mesophase is observed directly at the 1ET/n-water interface and it decreases with decreasing water concentration (see Fig. 2a),indicating that the highest mesophase stability is obtained for the water saturated samples.
|Fig. 1.Representative textures of columnar phases of compounds 1ET/n in water saturated samples as observed between crossed polarizers: (a) compound 1ET/10 at T = 40 ℃; (b) compound 1ET/14 at T = 45 ℃, (c) compound 1ET/16 at T = 45 ℃ and (d) compound 1ET/18 at T = 65 ℃; the insets at the bottom right show sections of the textures with additional l-retarder plates, indicating negative birefringence in all cases; the indicatrix orientation is shown in the top right inset in (a).|
The lyotropic columnar phase of a water saturated sample of compound 1ET/16 was chosen to be investigated by X-ray diffraction at 25 ℃ (Fig. 2b and Fig. S1 and Table S1 in Supporting information). The diffraction pattern is characterized by a diffuse wide angle scattering at d = 0.45 nm confirming the presence of an LC phase. The ratio of the reciprocal spacing in the small angle region is 1:21/2. Based on the intensity distribution of the reflections a p4gm phase [9, 10b, 11] can be excluded and the sharp reflections can be indexed on the basis of a Colsqu/p4mm lattice as 10 and 11 with the lattice parameter asqu = 3.20 nm. This lattice parameter is slightly larger than the molecular length (L = 2.8-3.1 nm),which can be explained by the swelling of the hydrogen bonding networks at the edges by the added water molecules. Fig. 2c shows the proposed arrangement of compound 1ET/16 with added water in the Colsqu/p4mm phase,where rod-like cores are fused together via enhanced hydrogen bonding located at both ends leading to the formation of honeycombs with a square cross sectional area around the lipophilic columns incorporating the lateral alkyl chains. The average number of molecules in the honeycomb walls is nwall = 2.14 (Table S2 in Supporting information),which agrees well with the values observed previously for the square honeycomb LC phases formed by related compounds 1T/n . In addition,IR investigations of bulk compound 1ET/16 and its water saturated sample were carried out (Fig. 3a),focused on the OH stretching frequency region. The broad and intense absorption in the vO-H stretching region indicates extended intermolecular hydrogen-bonding networks in the water-free as well as in the water saturated samples . The intensity of the vO-H vibration is increased by adding water,which is in line with an increase in size of the hydrogen bonded aggregates .
|Fig. 2. (a) Contact region between water and 1ET/16 as observed by POM during cooling at T = 85 ℃; white dotted line indicate the interface between water and 1ET/16; (b) Xray diffraction pattern of water saturated sample of compound 1ET/16 at T = 25 ℃, inset shows the diffuse wide-angle scattering; (c) model of the Colsqu/p4mm phase of water saturated sample of compound 1ET/16; dark gray stick: rigid aromatic core; gray column: alkyl chain region; blue column: enhanced hydrogen bonding network containing water molecule.|
In order to confirm the lyotropic LC phases of the other homologues 1ET/n,a water saturated sample of 1ET/14 was chosen to take the investigations of binary mixtures with water saturated samples of 1ET/16 and pure sample of 1EP/16 respectively. It provided evidence that the columnar phase of water saturated sample of compound 1ET/14 is also a square columnar phase,just like that of the water saturated sample of compound 1ET/16. This was indicated by the continuous growth of the spherulitic texture of the Colsqu/p4mm phase of water saturated sample of compound 1ET/16 into the region of water saturated sample of 1ET/14 without any intermediate minimum,maximum,or visible miscibility gap (Fig. S2 in Supporting Information). In contrast,in the contact region between the LC phases of the water saturated samples of compound 1ET/14 and 1EP/16,in a certain concentration range,there is an isotropic liquid ribbon separating them (Fig. S3 in Supporting Information),indicating that the two LC phases have a different structure and,thus,water saturated sample of compound 1ET/14 cannot form a Colhex-3/p3m1 LC phase. At the same time,there is continuous development of the LC-Iso transition temperatures in all compounds 1ET/n depending on the chain length and the other homologues’ LC textures are very similar to that of compound 1ET/16 (Fig. 1). Overall,it is most likely that all solvent induced LC phases indeed represent Colsqu/p4mm phases,though this cannot be stated with full certainty at present.3.2. Photophysical properties
The UV-vis absorption and fluorescence spectroscopic data in THF solution (c = 10-6 mol/L) and in condensed states (LC state and solid state) are shown in Fig. 3b for compound 1ET/16 which was chosen for investigation. This compound shows the maximum absorption peaks at 357 nm,343 nm,and 341 nm in solution,LC state,and solid state respectively (Fig. 3b left),which may be attributed to the π-π* transitions. This hypsochromic shift of about 14-16 nm in the condensed states indicates the formation of H-aggregates in which the aromatic rigid cores are packed in a molecular arrangement with the transition moments aligned parallel to each other but perpendicular to the line joining their centers (face-to-face arrangement) . This confirms the stacking of the aromatics along the cylinder walls as proposed for the model of the Colsqu/p4mm phase (Fig. 2c). The photoluminescence (PL) spectra of 1ET/16 in LC state and solid state exhibit a broad and red-shifted emission peak at 449 nm and 451 nm respectively,while the solution displays a structured emission with maxima at 422 nm (Fig. 3b right). This is because of the stronger intermolecular interactions produced both in the LC state and solid state. The blue emission of solution can be also observed by the naked eye under irradiation with 365 nm light (insert in Fig. 3b right). The Stokes shift has a remarkably large value of 106 nm in the LC state and 110 nm in the solid state,compared to 65 nm in THF solution. This feature indicates that these compounds may have potential for application in fluorescence sensors  and photoactive functional assemblies .
|Fig. 3. (a) IR spectra of compound 1ET/16 (solid line) and its water saturated sample (dot line) measured at T = 25 ℃ in bulk; (b) UV–vis absorption (left) and PL spectra (right, excited at 357 nm and 341 nm, respectively) of 1ET/16 in THF solution (10-6 mol/L) (monomer state, solid line), in a thin film (solid state, dot line) and in its water saturated film (LC state, short dot line), the insert shows the blue emission of 1ET/16 in THF solution under UV irradiation at 365 nm.|
We have successfully synthesized a series of novel 2,5- diphenylethynyl-thiophene-based bolaamphiphiles. In comparison with related compounds 1T/n ,it is evident that introducing ethynylene units to the thiophene core leads to higher melting point and loss of mesophobic property. The bent angle of 1488 inhibits their formation into Colhex-3/p3m1 as found in the related tolane-based bolaamphiphiles. Such compounds with the elongated bent core cannot be compensated by rotational averaging along the molecular long axis and cannot organize into columnar mesophase as the related bolaamphiphiles. However,water induces Colsqu/p4mm LC phases for all synthesized bolaamphiphiles as confirmed by POM,XRD,IR,UV spectra and binary contact experiments. The approach presented here provides easy access to such LC phases with broad phase ranges within the ambient temperature range,and this encourages the further exploration of this type of self-assembling material in pure state by increasing the hydrogen bonding of the terminal polar groups. Further,the fluorescence properties of the investigated compounds are also of interest for potentially electrooptical applications.
This work was supported by the National Natural Science Foundation of China (Nos. 21364017 and 21274119),the Yunnan Natural Science Foundation (No. 2013FA007) and Scholarship Award for Excellent Doctoral Student of Yunnan Province and Foundation (No. ynuy201418). We thank Dr. Makro Phrem and Prof. Carsten Tschierske from Martin-Luther University HalleWittenberg for XRD measurements.
Appendix A. Supplementary data
Supplementary data associated with this article can be found,in the online version,at http://dx.doi.org/10.1016/j.cclet.2015.05.031.
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