Design,Synthesis And Properties Of Polyaromatic Discotic Liquid Crystals

Polyaromatic organic material is an important branch of functional material chemistry.It is also the important part of supramolecular chemistry.The emergence of calixarene,rufigallol,tetraphenylethylene and other materials have led to the rapid development of polyaromatic chemistry.In recent years,the studies on liquid crystal materials made much progress due to their broad application prospects.In particular,discotic liquid crystals?DLC?with high thermic stability,wide mesogen temperature range and highly ordered liquid crystal phase is one of the hotspots of current research.However,common DLC materials generally have insufficient luminescence performance or defects of aggregation-caused quenching?ACQ?phenomenon.On the other hand,the discovery of the phenomenon of aggregation-induced emission?AIE?by Tang et al in 2001,has drawn much attention because AIE could overcome the ACQ effect caused by the traditional organic fluorescent molecules in the aggregate state.In this paper,calixarene and rufigallol derivatives were used as building blocks to combine with chiral cholesterol derivatives,cyanodistyrene unit,and tetraphenylethylene unit.A series of polyaromatic-based liquid crystal materials were designed and synthesized.The structure was confirmed by means of nuclear magnetic resonance or mass spectrometry.The mesogenic behavior was studied by optical microscopy?POM?,differential scanning calorimetry?DSC?and X-ray diffraction?XRD?.The photophysical properties were systematically investigated by UV-visible absorption spectroscopy and fluorescence emission spectroscopy.The thesis included five parts as following:PART 1:Two novel calix[4]arene-cholesterol derivatives 7a and 7b with a Schiff base bridge were synthesized.The characterization confirmed that the target molecule formed into the columnar liquid crystal,which was based on the molecular arrangement of the calixarene column as the main column,and the Schiff base cholesterol unit as the auxiliary side column.The complexes of 7a and 7b with Ag Cl O₄ were prepared and their effects on the liquid crystal properties were investigated.The mesophase of the target product was found to be regulated by ionic complexation behavior.PART 2:Three rufigallol-cholesterol derivatives 11a,11b and 11c with two cholesteric groups and four alkyl chains were designed and synthesized.Derivatives 11a,11b and 11c adopted the hexagonal columnar?Col_h?phase packing structure and exhibited good liquid crystal behavior and luminescent properties.It was found that an increase in the alkyl chain resulted in an increase in fluorescence but a decrease in the phase transition temperature.It was confirmed by circular dichroism analysis that the chiral characteristic of the cholesteric moiety was successfully transferred to the rufigallol unit.PART 3:By using anthraquinone as parent compound,the trimer derivatives of cyanostilbene-rufigallol-cyanostilbene 15a,15b and 15c were designed and synthesized.Studies on the mesomorphic properties revealed that the derivatives 15a,15b and 15c exhibited DLC behavior and the molecules were all packed in ordered hexagonal columns.The effects of cyanostilbene units on the fluorescence properties of mesophases and derivatives were investigated by photophysical characterization.Derivatives 15a,15b and15c exhibited good AIE properties in THF/H₂O solution.The longer alkyl chain between cyanostilbene unit and rufigallol unit resulted in better AIE properties.The fluorescence intensities of compounds 15a,15b and 15c increased by 8.6,7.6 and 7.8 times in the aggregated state than that in solution,respectively.PART 4:By introducing tetraphenylethylene groups onto the 1,5-position of the rufigallol unit containing four alkyl chains,two novel rufigallol derivatives 18a and 18b with two tetraphenylethylene unit and four alkyl chains were synthesized.After characterization,it was found that the target compounds exhibited good liquid crystal properties.Both compounds 18a and 18b exhibited the discotic liquid crystalline behavior,and compound18b formed an ordered hexagonal columnar stacking.Investigation on photophysical properties suggested that the length of the alkyl chain and tetraphenylethylene unit were favorable for enhancing the luminescent properties.The longer alkyl chains on the rufigallol core caused the higher luminescent properties.The strong AIE effect was observed for compounds 18a and 18b.The derivatives 18a and 18b had different solid luminescent colors.Compound 18a emitted a pale-yellow light and compound 18b showed a light blue-green light.These experimental results indicated that the fluorescent wavelength of target compound could be regulated by the length of the terminal alkyl chain.PART 5:By introducing tetraphenylethylene units with multiple alkyl chains onto4-pentyl-rufigallol,two new rufigallol derivatives 21a and 21b with multiple alkyl tetraphenylethylene units were synthesized.The derivatives 21a and 21b had good liquid crystal properties and columnar liquid crystals.The investigation on photophysical properties implied that the longer alkyl chain on tetraphenylethylene resulted in weaker luminescent properties.Both 21a and 21b had the strong AIE effect.The derivatives 21a and 21b exhibited different luminescent colors.Compound 21a emitted blue-green light,and compound 21b showed pale yellow light.The experimental results confirmed again that the fluorescence wavelength of target compound could be regulated by the length of the terminal alkyl chain,acting the role like the fluorescent switch.