Qiang Weng

Improved mesophase stability of benzoxazole derivatives via dipole moment modification

By modifying the molecular dipole moments with lateral monofluorine substituent, improved mesophase stabilities were obtained for novel benzoxazole derivatives, 2-(4ʹ-alkoxy-3-fluorobiphenyl-4-yl)-benzoxazole liquid crystals (coded as nPPF(3)Bx). The series of nPPF(3)Bx with lateral monofluorine substituent ortho to benzoxazole group have larger calculated dipole moments by about 2 D than the corresponding fluorine-substituted analogs (compounds I) with lateral monofluorine ortho to alkoxy group; it is interesting to note that they show lower melting and clearing points but better mesophase stability with wider mesophase ranges for the molecules with both polar (NO2, Cl) and nonpolar (CH3, H) terminal groups. Meanwhile, compounds nPPF(3)Bx show greater red-shifted photoluminescence emissions than compounds I, which suggest that π–π interaction between molecules is reinforced by the enhanced dipole–dipole interaction caused by increased dipole moments. These results suggest that modification of the molecular dipole moment is an effective method to improve the mesophase stability of the classical mesogenic compounds.

N-doped carbon nanocages: Bifunctional electrocatalysts for the oxygen reduction and evolution reactions

Highly efficient metal-free, carbon-based, bi-functional electrocatalysts for the oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) have attracted increased attention for use in electrochemical energy conversion systems, owing to their low cost and high activity. In this work, N-doped carbon nanocages (N-CCs) with a porous self-supported architecture and high specific surface area are synthesized by a facile interfacial assembly synthetic route. The materials are comprehensively characterized by scanning electron microscopy, transmission electron microscopy, nitrogen adsorption–desorption experiments, X-ray diffraction, and X-ray photoelectron spectroscopy. Cyclic voltammetry, chronoamperometry, and linear sweep voltammetry demonstrate that the as-prepared N-CC could serve as an effective metal-free electrocatalyst with excellent catalytic activity, long-term operation durability, and excellent methanol tolerance for the ORR in alkaline media. In the presence of 3 mM methanol, the half wave potential of the N-CCs for the ORR is 190 mV; this is more positive than that of the commercial Pt/C electrocatalyst. Meanwhile, the N-CCs also show an OER activity comparable to that of the commercial Ru/C electrocatalyst, revealing their bifunctional property.

The effect of intermolecular actions on the nematic phase range of tolane-liquid crystals

ABSTRACT An approach to understand the effect of intermolecular actions on the nematic stability, a series of tolane compounds nH containing intermolecular π–π stacking, dipole–dipole and hydrogen bond interactions, are developed and investigated. Their mesophase behaviour were measured by differential scanning calorimetry (DSC) and polarising optical microscopy (POM), the results show that carboxylic acids nH exhibit high melting points and narrow nematic phase intervals, which are attributed to the too strong intermolecular actions. By disrupting the intermolecular hydrogen bond, their corresponding methyl esters nC have been designed and synthesised. It is interesting to note that these compounds exhibit broad nematic mesophase intervals and low melting points. The above results demonstrate that the interruption of intermolecular actions is an effective way to improve the nematic stability. In addition, the effects of the terminal alkyl chains and the terminal polar groups on the nematic stability were also discussed. Finally, DFT calculations of molecular conformation and dipole moment were conducted to better understanding of the molecular structure–mesomorphic property relationship. GRAPHICAL ABSTRACT

Poly(arylene ether sulfone) bearing multiple benzyl-type quaternary ammonium pendants: preparation, stability and conductivity

A new difluoro aromatic ketone monomer, (2,6-difluorophenyl)[3,5-bis[(3,5-dimethylphenyl)methyl]phenyl]methanone, is developed to prepare a series of poly(arylene ether sulfone)s (PAES) bearing two pendant 3,5-dimethylphenyl groups via polycondensation reaction. The desired side-chain type PAESs containing multiple benzyl quaternary ammonium groups (QPAES(x/y)) are obtained by the corresponding bromomethylation and subsequent quaternization. Their properties, such as hydroxide conductivity, dimensional change, water uptake and alkaline stability, are investigated. Side-chain type membranes QPAES(x/y) with ion exchange capacities (IECs) of 0.96–1.22 mequiv. g−1 display anisotropic membrane swelling and hydroxide conductivity of 39.9–49.8 mS cm−1 at 80 °C. The greater length and dense distribution of the benzyl-type quaternary ammonium pendants are probably responsible for the good hydroxide conductivity and alkaline stability. Meanwhile, the corresponding crosslinked membrane (CQPAES(1/4)) shows higher conductivity, lower membrane swelling, better oxidative stability and higher chemical stability in hot alkali solution (80 °C) than the uncrosslinked one with a comparable IEC value.

High electrocapacitive performance of bowl-like monodispersed porous carbon nanoparticles prepared with an interfacial self-assembly process

Bowl-like monodispersed porous carbon nanoparticles (BMPCNs) have been successfully prepared with a facile and scalable approach, and the as-prepared BMPCNs are examined thoroughly by various physical characterizations. Physical measurements display that BMPCNs have the high surface area (1255m2g-1), hierarchical porosity, a certain amount of oxygen-containing groups and high graphitization degree. These unique structure features endow BMPCNs with a high capacitance (281Fg-1 at 0.5Ag-1), excellent rate capability (209Fg-1 at 50Ag-1), and outstanding cycling stability (94.8% capacity retention after 10,000 cycles at 10Ag-1) in electrochemical double-layer capacitors, suggesting its promising applications in field of energy conversion and storage. Further, the studies on formation mechanism of BMPCNs suggest that, the CTAB wormlike micelles assembling with silica/resorcinol-formaldehyde oligomers at the interface of tetraethyl orthosilicate droplets under Stöber condition determines the morphology of BMPCNs.

Preparation and properties of lateral monofluoro-substituted benzoxazole-based mesogenic compounds

A series of 2-(3ʹ-fluoro-4ʹ-alkoxy-1,1ʹ-biphenyl-4-yl)-benzoxazole liquid crystals (coded as nPF(3)PBx) were prepared, where a lateral fluorine substituent, as well as methyl, chlorine and nitro terminal groups, was introduced into the molecules to investigate the effects of different polar substituents on the liquid crystal properties. The mesomorphic and photophysical properties were investigated. The results show that compounds nPF(3)PBx have enantiotropic mesophases; meanwhile, they exhibit UV–vis absorption bands with maxima at 323–326 nm and photoluminescence emission peaks at 389–395 nm, respectively. It is noted that nPF(3)PBx with terminal polar groups or electron-withdrawing groups (NO2, Cl) display higher clearing temperatures and wider mesophase range than those of the corresponding homologues with terminal non-polar groups or electron-donating groups (CH3, H). Meanwhile, compared with two lateral fluorine-substituted analogues containing 3,5-difluorophenyl unit, lateral monofluoro-substituted nPF(3)PBx display enhanced mesophase range both in heating and cooling except for terminal methyl-substituted compounds, as well as show obvious red-shifted UV–vis absorption bands and photoluminescence emission, which are attributed to the enhanced dipole–dipole interaction caused by increased dipole moment.

Synthesis and mesomorphic properties of benzoxazole derivatives with lateral multifluoro substituents

ABSTRACT Fluorinated aromatics is generally chosen as mesogenic cores to design novel liquid crystal compounds. Here, a series of benzoxazole derivatives with laterally multifluorinated biphenyl units, 2-(3′,3-difluoro −4′-alkoxy-1,1′-biphenyl-4-yl)-benzoxazole derivatives (coded as nPF(3)PF(3)Bx), are synthesized and characterized, where methyl and nitro moieties are selected as terminal groups to investigate the effects of different polar substituents on the liquid crystal properties. The compounds nPF(3)PF(3)Bx show enantiotropic mesophases with mesophase ranges of 0–40°C and 0–63°C on heating and cooling for hydrogen-terminated derivatives (nPF(3)PF(3)BH), 43–93°C and 54–123°C for methyl-terminated ones (nPF(3)PF(3)BM), 60–108°C and 74–152°C for nitro terminated ones (nPF(3)PF(3)BN), respectively. They exhibit photoluminescence emission peaks at 390–392 nm and UV–vis absorption bands with maxima at 327–330 nm, respectively. The results reveal that lateral multifluoro substituents lead to a decrease in melting/clearing points, while electron-withdrawing terminal nitro moiety results in increases in both melting point and mesophase range. GRAPHICAL ABSTRACT

Syntheses of new diluents for medium birefringence liquid crystals materials

ABSTRACT New liquid crystals of 3HEHV ((4-trans-propyl-cyclohexyl-1), (4ʹ-vinyl-cyclohexyl-1ʹ)-ethane-1,2) and 3HB2V ((4-trans-propyl-cyclohexyl-1), (4-butenyl-3,4)-benzene) were synthesised and evaluated as the diluents for medium birefringence liquid crystal mixtures. The procedures transferring cis-isomers into trans-isomers of the cyclohexane were described. The results showed that both 3HEHV and 3HB2V decrease the crystallisation temperature and the viscosities by doping them with the medium birefringence mixtures in a fraction of 15%. The characteristics of the structures are correlated with the properties of the mixtures. Graphical Abstract

Preparation and mesomorphic properties of 1-methyl-1H-benzimidazole-based compounds

ABSTRACT A series of 1-methyl-1H-benzimidazole-based compounds, 2-(4ʹ-alkoxy-1,1ʹ-biphenyl-4-yl)-1-methyl- 1H-1,3-benzimidazole derivatives (nPPMx-M) with terminal hydrogen, methyl and nitro moieties (coded as nPPMH-M, nPPMM-M and nPPMN-M, respectively), were prepared and their structures were characterised. The compounds display enantiotropic smectic mesophases for hydrogen and methyl terminated compounds (nPPMH-M and nPPMM-M), and enantiotropic nematic mesophases for nitro terminated compounds (nPPMN-M) with short alkoxy chain below than 10 carbon atoms, where the mesophase ranges are 24–72°C and 74–104°C on heating and cooling processes for nPPMH-M, 90–119°C and 110–135°C for nPPMM-M, and 102–129°C and 113–207°C for nPPMN-M, respectively. It is noted that the compounds nPPMx-M exhibit much lower melting points and much wider mesophase range both in heating and cooling than non-1-methyl substituted analogs, which are ascribed to the disruption of hydrogen bonding among the molecules caused by methyl substitution at 1-position of benzimidazole. Meanwhile, among the compounds nPPMx-M, much wider mesophase ranges are obtained for nPPMM-M and nPPMN-M, indicating a much high mesophase stability for the compounds bearing terminal moiety (CH3 and NO2). Graphical Abstract