Hongfei Gao

Columnar mesophases of luminescent polycatenar liquid crystals incorporating a 1,3-substituted benzene ring interconnecting two 1,3,4-oxadiazoles

New polycatenar liquid crystals containing two 1,3,4-oxadiazole rings interconnected by a bent 1,3-substituted benzene ring as the central linking unit and three alkoxy chains at each terminal have been synthesized and investigated by polarizing microscopy, DSC, XRD scattering, UV-vis spectroscopy and photoluminescence measurements. All compounds form enantiotropic hexagonal columnar phases and have broader mesophase ranges than the corresponding para substituted analogues and show strong blue fluorescence emission with a large Stokes shift in solution.

Transition between triangular and square tiling patterns in liquid-crystalline honeycombs formed by tetrathiophene-based bolaamphiphiles

A series of 5,5′′′-diphenyl tetrathiophenes with polar glycerol groups at each end and two lateral flexible chains self-assemble into a series of liquid-crystalline honeycombs, formed by the π-conjugated rods which enclose polygonal prismatic cells filled by the lateral chains. With increasing chain length a discontinuous transition from triangular to square honeycombs takes place. At this transition a periodic honeycomb composed of a mixture of square and triangular cells in a ratio 1 : 2 was formed at low temperature, whereas at higher temperature a hexagonal columnar phase composed of triangular and randomly distributed rhombic cells, a new kind of cybotactic nematic phase, and also a cybotactic isotropic phase, both composed of square honeycomb fragments, represent the intermediate states. This provides an example of a dynamic self-assembled system where, depending on the molecular mobility, the transition between two periodic structures with different symmetry either leads to an increase of complexity, or to a chaotic regime with reduced order.

Synthesis and self-assembly of photoresponsive and luminescent polycatenar liquid crystals incorporating an azobenzene unit interconnecting two 1,3,4-thiadiazoles

Novel polycatenar liquid crystals containing two 1,3,4-thiadiazole rings interconnected by an azobenzene central linkage have been synthesized and investigated by polarizing microscopy, DSC, X-ray scattering, SEM, UV-vis spectroscopy and photoluminescence measurements. These compounds can self-assemble into SmC, Colhex/p6mm and CubI/Pmn liquid crystalline phases in the bulk states and form multistimuli responsive organogels in organic solvents. They have reversible photoresponsive properties in solution, liquid crystalline states and gel states. They also show fluorescence emission with large Stokes shift in solution and binding selectivity to Cu2+ among a series of cations in CH3CN–CH2Cl2 solution.

Effect of central linkages on mesophase behavior of imidazolium-based rod-like ionic liquid crystals

Two series of phenylbenzylether and benzanilide based rod-like imidazolium bromides and their nonionic precursors, the 1-phenyl-1H-imidazoles have been synthesized and the influence of the number and length of the alkyl chain(s) and the structure of the linking group in the aromatic core (–CH2O–, –COO–, –CONH–) on their mesophase self-assembly in ionic liquid crystalline phases were studied by POM, DSC and XRD. Upon decreasing the length of the N-terminal chain or by enlarging the number and length of the C-terminal chains, the sequence smectic (SmA)–hexagonal columnar (Colhex)–micellar cubic (CubI/Pm3n) was found for the ether based imidazolium salts; while only SmA and Colhex phases were observed for the related amides. The influence of the polarity of the central linkages, namely –CH2O– and –CONH–, on the mesophase structure and stability is discussed and compared with related –COO– connected ILC.

Synthesis and self-assembly of luminescent hexacatenar molecules incorporating a 4,7-diphenyl-2,1,3-benzothiadiazole core

A new polycatenar liquid crystal containing a 4,7-diphenyl-2,1,3-benzothiadiazole core and three alkoxy chains at each end has been synthesized via a copper-catalyzed azide–alkyne click reaction and investigated by polarizing microscopy, DSC, XRD scattering, SEM, UV-vis spectroscopy and photoluminescence measurements. This compound can self-assemble into an oblique columnar liquid crystalline phase in the bulk state and form organogels in different organic solvents. It shows highly fluorescent emission with large Stokes shift in solution and has binding selectivity to Li+ among a series of cations in DMSO–CH2Cl2 solution.

Dithiophene based X-shaped bolaamphiphiles: liquid crystals with single wall honeycombs and geometric frustration

A series of 5,5′-diphenyl-2,2′-dithiophene based X-shaped polyphiles with two long lateral alkyl chains and terminal glycerol groups was synthesized and the liquid crystalline phases formed by these compounds were investigated by polarizing microscopy, DSC and XRD. These compounds form square (p4mm and p4gm) and hexagonal (p6mm) columnar LC phases. In these mesophases the molecules organize into polygonal honeycombs where the π-conjugated cores form the walls, fused at the edges by the hydrogen bonding networks between the glycerol units and filled by the lateral alkyl chains. By elongation of these chains, a series of polygonal honeycomb phases with a “single wall” structure, ranging from triangular via square and pentagonal to hexagonal was observed. Most triangular honeycombs appear to be defective and can be considered as mixtures of triangular cylinders with orientationally randomized rhombic cylinders. The transition from this improper triangular honeycomb to the square honeycomb takes place via a disordered isotropic phase. Addition of water to this isotropic phase gives rise to a true triangular honeycomb LC phase. Replacing one of the long lateral chains by a small methyl group leads to honeycombs formed by double walls instead of single walls. UV investigations indicate π-stacking of the aromatic cores organized in the honeycomb walls, which is of interest for the potential application of these materials in self assembled arrays of organic electronic material.

Amphotropic azobenzene derivatives with oligooxyethylene and glycerol based polar groups

A series of amphiphilic azobenzenes with one to three lipophilic alkyl chains at one end and polar groups with oligooxyethylene (EO) and racemic 3-glyceryl units at the opposite end was synthesized and their thermotropic and lyotropic liquid crystalline self-assemblies were studied by POM, DSC and XRD. Tilted and non-tilted lamellar phases with interdigitated double layer structures (SmCd and SmAd, respectively) were found for the compounds with a single alkyl chain, whereas hexagonal columnar phases were formed by the compounds with two or three alkyl chains. The effect of protic solvents, like formamide, ethylene glycol and water, was investigated for representative examples. For the compounds with the single chain, induction and stabilization of SmA phases were observed, though broad regions of lyotropic SmC phases were retained in most cases. Depending on the structure of the polar group, the hexagonal columnar phases were either removed or drastically stabilized by the solvents. Photoisomerisation of an azobenzene chromophore was also studied.

Synthesis and self-assembly of 5,5′-bis(phenylethynyl)-2,2′-bithiophene-based bolapolyphiles in triangular and square LC honeycombs

A series of X-shaped 5,5′-bis(phenylethynyl)-2,2′-bithiophene-based bolaamphiphiles bearing two long lateral alkyl chains and two terminal glycerol groups has been synthesized by using Kumada and Sonogashira coupling reactions as key steps. The thermotropic liquid crystalline behavior of these compounds was investigated by POM, DSC and X-ray scattering. With elongation of the lateral alkyl chains two different kinds of liquid crystalline phases with honeycomb structures, ColhexΔ/p6mm, formed by (defective) triangular honeycomb cells, and Colsqu/p4mm with square cells were observed for these compounds. UV and PL measurements indicate fluorescent properties making them potential candidates for application in fluorescence sensor devices.

Tolane-based bent bolaamphiphiles forming liquid crystalline hexagonal honeycombs with trigonal symmetry

Bolaamphiphiles consisting of a bent 1,3-bis(phenylethynyl)benzene core with two terminal glycerol units, a lateral n-alkyl chain in the bay-position and a methyl group at the apex of the central 1,3-substituted benzene ring have been synthesized via Sonogashira coupling reactions as key steps. The thermotropic and solvent-modified or solvent-induced liquid crystalline (LC) phases of these compounds were investigated by POM, DSC and X-ray scattering. Compounds with medium alkyl chain length form a hexagonal honeycomb LC phase with non-centrosymmetric trigonal p3m1 symmetry as a monotropic LC phase. The addition of water increases the LC phase range, leading to enantiotropic p3m1 phases and induces LC phases for the non-mesomorphic compounds with shorter and longer lateral alkyl chains.

Syntheses and Properties of meso-Substituted Porphyrin Mesogens with Triazole Linkages and Peripheral Alkyl Chains

Discotic mesogens P/n-M (n=12, 16, 18, M=2 H, Zn and Cu) bearing a porphyrin core, triazole linkages and peripheral 3,4,5-trialkoxybenzyl units have been synthesized by a click-chemistry approach. The thermal behavior, photophysical properties and morphologies of these compounds were investigated by polarized optical microscopy (POM), differential scanning calorimetry (DSC), XRD, UV and PL, SEM and TEM. These compounds can self-assemble into hexagonal columnar phases in their pure states and form organogels in 1,4-dioxane with unusually flower-like sphere morphology. The supramolecular complexes of P/18-Zn with C70 or 4,7-di-4-pydriyl-2,1,3-benzothadiazole can display hexagonal columnar phases too. Additionally, zinc porphyrin compounds P/n-Zn show binding selectivity to Cu2+ among a series of cations in THF/H2 O.