Synthesis, Gelation And Liquid Crystalline Behaviour Of Dihydrazide Derivatives

Self-assembling processes are common throughout nature and technology. Self-assembled materials, such as liquid crystals and organogels, formed through non-covalent bonding have attracted much attention because they are good candidates for the next generation of materials, for which dynamic function, environmental benignity, and low energy processing are required. Hydrogen bonding was most commonly used to direct the self-assembling process among those non-covalent interactions. Many low molecular weight organogels and liquid crystalline materials containing amide and urea groups have been reported where the intermolecular hydrogen bonding was considered to be the driving force. However, it is relatively rare to find low molecular compounds capable of both gelling solvents and exhibiting thermotropic mesomorphic behaviors. In this thesis, five series of dihydrazide derivatives were designed and sythesized, e.g. N-(4-alkoxybenzoyl)- N'-(4'-biphenylcarbonyl) hydrazine｛Cn-ph(n =6,10,12,16)｝, N,N'-Bis-[4-n-alkyloxybenzoyl]-hydrazines｛Cn-Cn (n=7,10,12,16)｝, N-(4-alkoxybenzoyl)-N'-(4'-nitrobenzoyl)hydrazine｛Cn-NO2( n=6,16 )｝,4-decyloxy-N,N'-dimethyl-N'-(4'- biphenylcarbonyl) benzohydrazide (D10-ph) and N,N-(5-alkoxyl-1,3-dicarboxylic benzene) N',N'-di(4-methylbenzoyl) dihydrazide｛En (n=3,7,10)｝. Their molecular structures were confirmed by 1H NMR, FT-IR and Elemental Analysis. The self-assembly behaviors in solvent and at different temperatures were investigated by polarizing optical microscopy (POM), differential scanning calorimetry (DSC), variable temperature X-ray diffraction, variable temperature FT-IR and scanning electron microscopy (SEM).1. Gelation behaviour of dihydrazide derivateivesCn-ph (n=6,10,12,16),Cn-Cn (n=7,10,12,16),Cn-NO2 (n=6,16) and En (n=3,7,10) showed strong gelation ability in organic solvents such as benzene, 1, 2-dichloroethane, chloroform et al. In gel phase, these molecules self-assembled into layered structure, which is the fundamental of fibrous aggregations as observed by SEM. Intermolecular hydrogen bonding was demonstrated to be the major driving force for gelation of organic solvents.The effect of the length of the terminal chains on the gelatinization behaviour was discussed. For example, C10-ph and C10-C10 showed the lowest minimum concentration in Cn-ph and Cn-Cn compounds, and these gels are more stable than the others. Generally, elongation of the terminal flexible alkyl chain, on the one hand, increases the solubility of the compounds in organic solvents, on the other hand, reduces its gelling ability and thus more amount of gelators might be needed to gel the same amount of solvent. The lowest minimum concentration for gelation was achieved in C10-ph and C10-C10 in the present case due to the two opposite effects of terminal alkyl chains on the balance between dissolution and gelation.The polarity of substituent group of molecules plays an important role in stabilizing the organogel. For example, the minimum concentration for C16-ph,C16-C16 and C16-NO2 to gel benzene is 31.1mM,21.0 mM and 14.8mM, respectively. The most stable organogel was observed for Cn-NO2. The effect of lateral hydrogen bonding on the organogel properties was discussed. No gelation was observed for D10-ph which was obtained through substitution of N-H by N-CH3, indicating that intermolecular hydrogen bonding between the hydrazide groups might be the driving force for their gelling properties.2. Liquid crystalline behaviour of dihydrazide derivateivesC6-ph exhibited enantiotropic smectic A phase and nematic phase, while only smectic A phase was observed for C10-ph,C12-ph and C16-ph.Cn-Cn (n=7,12,16) exhibited enantiotropic cubic phases with primitive cubic lattices (Pn3m), while C10-C10 exhibited cubic phase and smectic C phase.Cn-NO2(n=6,16)exhibited enantiotropic smectic A phase with fan-shaped texture, and was further assigned to be SmA1 based on the WAXD results.No mesomorphic phase was observed in D10-ph, whose Tm is 99.9℃.Hydrogen bonding between O=C-NH-NH-C=O groups was confirmed to exist in the mesophases of the derivatives and played an important role in stabilizing the mesophases by variable temperature FT-IR and dilution 1H NMR spectroscopy.The effect of the length of the terminal chains on the liquid crystalline properties was discussed. For example, in compounds Cn-ph, the melting temperatures decreased while the isotropic temperature increased with the increase of length of terminal alkyl chains.This can be understood that enchanced vander waals interaction by elongating the alkoxy chains can stable the mesophase , so the liquid crystalline range broadened.The polarity of substituent group of rod-shape molecules plays an important role in stabilizing the mesophase. For example, the mesophase range of C16-ph, C16-C16 and C16-NO2 with different group polarity of substituent is 66℃,29.1℃and 114.3℃,respectively.The effect of lateral hydrogen bonding on the liquid crystalline properties was also discussed. The melting point of D10-ph decreased compared to that of C10-ph and no mesomorphic phase was observed in D10-ph. Thus it can be concluded that lateral intermolecular hydrogen bonding played an important role in stabilizing the mesophase.