Binglian Bai

Photo-induced fiber–vesicle morphological change in an organogel based on an azophenyl hydrazide derivative

A new low molecular mass organic gelator (LMOG) bearing hydrazide and azobenzene groups, namely N-(3,4,5-octanoxyphenyl)-N′-4-[(4-hydroxyphenyl)azophenyl] benzohydrazide (BNB-t8), was designed and synthesized. The organogelator has shown great ability to gel a variety of organic solvents to form stable organogels with a critical gelation concentration as low as 0.5 mg mL−1. Xerogels from chloroform exhibited entangled and dense fibrous aggregates with diameters of 50–60 nm. The organogel showed photoinduced gel-to-precipitate transition under the irradiation by 365 nm UV-light, which was attributed to trans-BNB-t8 to cis-BNB-t8 photoisomerization. The trans–cisisomerization in BNB-t8 gels further caused the morphological change from fibers to vesicles at the supramolecular level.

Synthesis, liquid crystalline properties and fluorescence of polycatenar 1,3,4‐oxadiazole derivatives

A series of hexacatenar liquid crystals containing the 1,3,4‐oxadiazole group as rigid core, i.e. 1,4‐bis[(3,4,5‐trialkoxyphenyl)‐1,3,4‐oxadiazolyl]‐ benzene (P‐P‐oxd‐n), were designed and synthesised. Based on a detailed study of their thermotropic phase behaviour and mesophase structures, it was revealed that columnar phases are generated in these materials. Furthermore, combination of experimental and calculated results enabled a proposal for the molecular packing in the mesophase. The photoluminescent properties of these materials were examined using P‐P‐oxd‐8 as an example. A strong blue light emission (λmax = 456 nm) was observed in P‐P‐oxd‐8 and a higher quantum yield was obtained in dilute chloroform solution.

Hydrazide-based organogels and liquid crystals with columnar order

We report on the synthesis and self-assembly of a new series of compounds containing a hydrazide unit in the rigid core and three alkoxy chains with varying lengths. The compounds N-(3,4,5-cetyloxybenzoyl)-N′-(4′-nitrobenzoyl) hydrazine (C16) and N-(3,4,5-dodecyloxybenzoyl)-N′-(4′-nitrobenzoyl) hydrazine (C12) exhibited stable columnar phase and strong gelation ability in several apolar organic solvents. The columnar structure was found both in the liquid crystalline state and in the xerogels by wide-angle X-ray diffraction analysis. SEM and TEM images revealed that the molecules self-assembled into twist fibrous aggregates in the xerogels. FT-IR and 1H NMR studies confirmed that the intermolecular hydrogen bonding and van der Waals interactions were the major driving force for the formation of self-assembling both the liquid crystals and gels processes. Further detailed analysis of their aggregation modes were conducted by FT-IR spectroscopy and X-ray diffraction measurement.

Symmetric liquid crystal dimers containing hydrazide groups: parity‐dependent smectic structure, hydrogen bonding and substitution effect

Dimeric hydrazide derivatives with long terminal alkoxy chains, i.e. α,ω‐bis[N‐(4‐hexadecyloxybenzoyl)‐N′‐(benzoyl‐4′‐oxy)hydrazine]alkanes (C16. n. C16, C16 indicates the terminal hexadecyloxy chain; n = 3, 5, 6, 10, indicates the number of the carbon atoms in the spacer) were synthesised. The liquid crystalline properties were investigated by differential scanning calorimetry, polarising optical microscopy and X‐ray diffraction. The dimers with shorter spacer (C16. n. C16, n = 3, 5, 6) exhibit enantiotropic smectic phases, whereas C16.10.C16 with the longest spacer is non‐mesomorphic. Furthermore, the smectic structure is parity dependent, i.e. C16.3.C16 and C16.5.C16 with odd spacer exhibit a monolayer smectic A phase, whereas C16.6.C16 with even spacer exhibits a monolayer smectic C phase, in which the incompatibility between the terminal chains, mesogenic groups and the spacers is considered to be the driving force. The results are compared with those for the terminally nitro‐substituted series, α,ω‐bis[N‐(4‐nitrobenzoyl)‐N′‐(benzoyl‐4′‐oxy)hydrazine]alkanes (N n ), which exhibit an intercalated smectic A phase. Temperature‐dependent IR spectroscopic analysis on these two kind of dimers suggests that intermolecular hydrogen bonding, as well as the dipole–dipole interaction arising from the strong polar substituents (–NO2) synergistically drives the intercalated structure of N n . Although hydrogen bonding exists in the monolayer smectic phase of C16. n. C16, microphase segregation is in favour of the monolayer smectic structure, preventing the formation of an intercalated structure.

Non‐symmetric liquid crystal dimers containing the 4‐nitrobenzohydrazide group: synthesis and mesomorphic behaviour

The synthesis and mesomorphic behaviour are reported of a new series of dimers containing 4‐nitrobenzohydrazide and azobenzene groups as the mesogenic units. These non‐symmetric liquid crystal dimers are found to exhibit a monolayer smectic A phase (SmA1). Lateral hydrogen bonding and strong dipole–dipole interactions are shown to be the major driving forces for the formation of the SmA1 phase. The present study indicates that the intermolecular interactions and thus the mesophase morphology of the liquid crystal dimers can be controlled by the appropriate selection of the molecular fragments capable of forming H‐bonds.

A new series of liquid‐crystalline bi‐1,3,4‐oxadiazole derivatives: synthesis and mesomorphic behaviour

Symmetrical bi‐1,3,4‐oxadiazole derivatives, namely 5,5′‐bis(phenyl 4‐alkoxybenzoate)‐2,2′‐bi‐1,3,4‐oxadiazole (BBOXD‐n, n = 6, 10, 14, 16), were synthesised. All BBOXD‐n exhibited remarkably stable SmC phases by virtue of the high transition enthalpies of SmC–I. In addition, BBOXD‐6 and BBOXD‐10 showed an enantiotropic nemetic phase with enthalpies of the N–I transition up to 5.16 kJ mol−1. As confirmed by wide‐angle X‐ray diffraction analysis and MM2, molecules of BBOXD‐n showed high‐angle tilting (55–57°) within their smectic C phases.

The synthesis and mesomorphic behaviour of tetracatenar and hexacatenar bi‐1,3,4‐oxadiazole derivatives

Symmetrical four‐chained (tetracatenar) and six‐chained (hexacatenar) bi‐1,3,4‐oxadiazole ester derivatives, namely 5,5′‐bis[phenyl(3,4‐dialkoxybenzoate)]‐2,2′‐bi‐1,3,4‐oxadiazole (Cn, n = 6, 10, 12, 14, 16) and 5,5′‐bis[phenyl(3,4,5‐trialkoxybenzoate)]‐2,2′‐bi‐1,3,4‐oxadiazole (Dn, n = 7, 10, 14, 16) were synthesised. The liquid crystalline properties were investigated by differential scanning calorimetry, polarised optical microscopy and wide angle X‐ray diffraction. It was revealed that the hexacatenar derivatives (Dn) are non‐mesomorphic, while the tetracatenar ones (Cn) showed a classic progression from smectic C phases (for C6 and C10) to a hexagonal columnar (Colh) phase (for C12, C14 and C16) with the increase in length of the terminal chains. Molecules of Cn (n = 6, 10) tilt about 55‐56° from the layer normal within their SmC phase.

Observation of intercalated smectic phases in symmetric liquid crystal dimers containing hydrazide groups

Dimeric hydrazide derivatives with nitro, phenyl, and methyl terminal subsistents were synthesized. The liquid crystalline properties were investigated by differential scanning calorimetry, polarizing optical microscopy and wide angle X‐ray diffraction. Interestingly, intercalated smectic phases were observed in these symmetric liquid crystal dimers. The effect of the substituents and the length of the spacer on the mesophase is discussed, confirming that intermolecular hydrogen bonding between the hydrazide groups was the driving force for the formation of the intercalated structures.

Liquid crystalline behaviour of benzoic acid derivatives containing alkoxyazobenzene

Liquid crystal trimers based on the hydrogen bonding dimerization of 4‐{n‐[4‐(4‐m‐alkoxy‐phenylazo)phenoxy]alkoxy}benzoic acid (BAm‐n) have been synthesized and characterized. Temperature‐dependent FTIR spectroscopic studies showed that the carboxylic acid groups in BAm‐n are associated to form H‐bonded cyclic dimers both in their crystalline and liquid crystalline phases. The trimers exhibited enantiotropic liquid crystalline behaviour except for BA1‐3 which showed monotropic behaviour, and the mesophases changed from nematic to smectic phase, with the increase of length of the spacer and the terminal substituents. Pronounced odd–even effects in the melting temperatures, clearing temperatures and nematic–isotropic enthalpy changes were observed.

Control of self-assembly of twin-tapered dihydrazide derivative: mesophase and fluorescence-enhanced organogels

A twin-tapered (six-chained) low-molecular-mass organic gelator bearing hydrazine linkage and end-capped by phenyl, 1,4-bis[(3,4,5-trihexyloxy phenyl)hydrazide]phenylene (TC6) was designed and synthesized. Quadruple hydrogen bonds between the dihydrazide units are the main driving force in forming supramolecular liquid crystals and organogels. The association constants (K) of the quadruple hydrogen bonds between the hydrazide units in chloroform are 7.13 × 103 M−1 and 1.53 × 103 M−1 on the basis of NH-1 and NH-2, respectively. Depending on the environmental conditions, compound TC6 can self-assemble into a hexagonal columnar mesophase in the condensed state, in which two molecules constitute the slice of the column, or into a smectic mesomorphic-like organization in the presence of apolar solvents such as 1,2-dichloroethane, or into a rectangular columnar mesomorphic-like organization in the presence of polar solvents such as ethanol, in which one molecule constitutes the slice of the column. Aggregation-induced emission enhancement (AIEE) has been observed after gelation though conventional chromophore units are not incorporated in TC6.