Preparation And Properties Of Multi-Stimuli-Reponsive Liquid Crystal Physical Gels Based On Phenylalanine Derivatives

Liquid crystal（LC）is a kind of self-aggregation materials with long-range order,which is widely used in display devices and has broad potential applications in biosensing,active diaphragm,ordered template,ion transportation,etc.Fluidity gives LCs the ability to response to the electric,magnetic,light,heat,force and other external stimuli rapidly,but also limits their application since the LC fails to self-support.Liquid crystal physical gels（LCPGs）composed of three-dimensional networks self-assembled by gelators through non-covalent interactions and large numbers of anchored LCs,have both the mechanical properties and stimuli reponses.Functional LCPGs with light response,acid-base response and luminescence properties are expected to be used in information storage,electro-optic display,sensor and other fields.However,it is still a chanllenge to prepare LCPGs with good mechanical properties and multi-stimuli-reponse.In this thesis,1,4-benzenedicarboxamide phenylalanine derivatives containing different side groups were synthesized and utilized to gel LCs.Furthermore,LCPGs with photo-response were prepared by the co-assembly of azobenzene compound and the optimized gelator.Similarly,a kind of luminous LCPGs with multi-stimuli-reponses was prepared by simply mixing a cyanostilbene compound containing pyridine ring and the optimized gelator.Some specific investigations and conclusions are shown as follows:（1）In order to prepare LCPGs with high modulus and low driving voltage,the enantiomers（LPF,DPF）of 1,4-benzenedicarboxamide phenylalanine derivatives were synthesized as gelators.The critical gel concentration（CGC）of the single LPF or DPF was0.1 wt%,while the LPF/DPF racemate with molar ratio of 1:1 failed to form a gel.Moreover,the chiral signals for both gels showed mirror images and the supramolecular chirality derived from the chiral gelators by chirality transfer and chirality amplification.When the gelators concentration was 0.5 wt%,the storage modulus（G’）of the gels reached to 10⁴ Pa,meeting the self-supporting requirement,and the threshold voltage downed to 0.5Vμm^-1,slightly higher than pure 5CB(0.4 Vμm^-1).The supramolecular chirality of LPF/DPF aggregates could induce the nematic 5CB to form the typical finger textures of cholesteric liquid crystals（CLCs）.（2）In order to improve the compatibility of 1,4-benzenedicarboxamide phenylalanine derivatives and LCs,while preserving the gelactors’good gelability,the gelators（LPH,DPH,LP2H,DP2H）with flexible side groups were synthesized through esterification reaction between the side carboxyl group of LPF and DPF with the hydroxy group of ethylene glycol or ethylene glycol.The CGC of LPH in the mixed LCs（P0616A）with a wider LC temperature range than 5CB was 0.2 wt%,slightly higher than that of LPF（0.1 wt%）.Additionally,the saturation concentration of LPH could reach 9.0 wt%,and the corresponding G’of the gel went up to 10⁶ Pa.Moreover,the supramolecular chirality of LPH aggregates could induce the nematic P0616A to form the finger textures of CLCs.（3）A kind of photo-responsive LCPG was prepared based on the co-assembly between an azobenzene compound（DHAB）and the optimized gelator LPH.The CGC of LPH/DHAB in P0616A was 0.42 wt%,where hydrogen bond played an important role in the gel formation.When the concentration of LPH reached 0.8 wt%,the G’of the gel was higher than 10⁴ Pa.Upon the UV irradiation,the azophenyl group of DHAB underwent trans-cis isomerization,which destroyed the hydrogen bond between LPH and DHAB molecules,leading to the gel collapse.After reheating the sol and cooling it down,the gel can form again.（4）A kind of multi-stimuli-reponsive and luminescent LCPG was prepared by simply mixing a cyanostilbene compound（CNPPE）containing pyridine ring and the optimized gelator LPH.The results showed that CNPPE had the aggregation induced emission enhancement effect in the gel.CNPPE in the gel underwent trans-cis isomerization under365 nm UV light.During this process,the sample remained the gel state,but the blue-green fluorescence of the CNPPE was quenched.Based on the fluorescence quenching phenomenon,a fluorescent pattern was prepared within the glass box by a mask.The emission wavelength of the gel red-shifted to the green region with trifluoroacetic acid,and the gel collapsed with triethylamine.The gel maintained good thermal stability and mechanical properties within an appropriate range of CNPPE addition.Moreover,the supramolecular chirality induced the formation of finger textures in nematic P0616A.