Optical Properties And Supramolecular Assemble Of Tetraphenylethylene Derivatives With Cholesterol Groups

Tetraphenylethylene(TPE)has been a star molecule in aggregation-induced emission(AIE)materials due to its simple synthesis and high fluorescence quantum yield.It has been widely used in the preparation of various luminescent materials in recent years.Functionalized TPE derivatives not only have AIE properties,but also are endowed with multi-stimulus responsive properties and chirality.It is of great importance to develop simple,new and multi-functional smart materials.AIE molecules with circularly polarized luminescence properties solve the problem of aggregation-caused quenching of most fluorescent materials,and chirality broaden the applications of multifunctional materials.In this thesis,by choosing TPE as the scaffold and modified it with cholesterol,we have designed and synthesized four cholesterolsubstituted TPE derivatives.The chiral properties and multi-stimulus responsive properties of TPE derivatives were systematically characterized with photoluminescence(PL),differential scanning calorimeter(DSC),circular dichroism(CD),circular polarized luminescence(CPL),polarized optical microscope(POM),atomic force microscope(AFM)and powder X-ray diffraction(PXRD).The main contents are as follows:(1)The cholesterol groups were linked by amide bonds or ester groups at para,or ortho position and four cholesterol-substituted TPE derivatives 4TPE-NC,4TPE-OC,3TPE-NC and 3TPE-OC were obtained,respectively.The results show that the four molecules all have typical AIE characteristics and excellent thermal stability.The CD spectra of the solution indicated that the molecular chirality was not significant.By analyzing the differences in optical properties,thermal stability and chirality caused by different binding sites and linking groups on TPE derivatives,it provides an experimental basis for the construction of supramolecular systems and multi-stimulus response in the next two chapters.(2)TPE derivatives were then blended with liquid crystal medium 5CB to have their chirality to be amplified by 5CB.Four supramolecular liquid crystal systems were formed and all have significantly improved CD and CPL signals.The absolute value of the dissymmetry factor in the system was as high as 0.397,indicating that chirality of the TPE derivatives has been successfully transferred and amplified by 5CB.POM results showed that 4TPE-NC,4TPE-OC,and 3TPE-OC as the chiral dopant supramolecular liquid crystals all form fingerprint textures.The molecules with linear geometry had better matching effect with the liquid crystal medium 5CB.The flexibility was also pivotal,and the greater the flexibility of the dropped molecular chain segment,the greater the helical twisting power.Chirality of flexible molecules can be more effectively transferred to the nematic liquid crystal medium,forming a fingerprint texture with a narrower pitch.By studying the self-assembly structure of supramolecular liquid crystals,it is found that supramolecular liquid crystals followed the "Sergeant-soldier" rule that a small amount of TPE derivatives acted as sergeants to guide a majority of liquid crystal medium 5CB soldiers to arrange in helical order.AFM images showed that beaded chiral helical structures were formed with TPE derivative molecules as the central axis and 5CB liquid crystal coated in the exterior.(3)We investigated the mechanochromism and photochromism of the 4TPE-NC and 4TPE-OC.The results showed that these two molecules had multi-state,reversible,high contrast and fast responsive fluorescence characteristics under external stimuli.This was due to their stable stacking mode and weak intermolecular interaction forces,which had good multi-stimulus response characteristics.Both heat-annealed powder and crystal had characteristics of self-recovering photochromism due to the formation of photocyclization intermediate of the TPE scaffold as verified by single crystal analysis and DFT calculation.Both 4TPE-NC and 4TPE-OC molecules provide good examples of multi-state force discoloration of single small organic molecules.4TPEOC molecule has more functional stimulus response properties than the amide-bonded 4TPE-NC molecule,and exhibited the unique properties of multi-state mechanochromism and photochromism.This reversible multi-state,high-contrasted and rapid responsive mechanochromic and photochromic property cooperatively provided double enhancement of a multimode guarantee in advanced anticounterfeiting.