Optical Properties And Self-assembled Of Several Amphiphilic AIE Molecules

Since the discovery of the phenomenon of aggregation-induced emission（AIE）,its potential application has attracted great interest from researchers.In this paper,three types of fluorescent molecules are studied,including two AIE molecules（Fluorene Azine Salicylaldehyde,FAS and Diphenylketone Azine Salicylaldehyde,DPAS）that do not contain chiral carbon atoms,two molecules FAS-L-Ala and DPAS-L-Ala（L-Aline was introduced into FAS and DPAS,respectively）with excited-state intermolecular proton transfer（ESIPT）,two enantiomers N₁-PIM-L-Ala and N₁-PIM-D-L-Ala synthesized by the N₁-position modification of alanine.Using UV-vis spectroscopy,fluorescence spectroscopy,circular dichroism（CD）,circularly polarized luminescence（CPL）,we studied the optical properties of these three types of luminescent molecules.And we investigated the assembled morphologies and structures of these molecules by atomic force microscopy（AFM）,fluorescence microscopy and scanning electron microscopy（SEM）.（1）Using fluorescence spectroscopy and ultraviolet absorption spectroscopy,we confirmed that both FAS and DPAS have typical AIE characteristics.We investigated the assembled structures of FAS and DPAS,and the results showed that both two AIE molecules without chiral carbon atoms can be assembled to form chiral helical fibers.At the same time,the acidity and alkalinity of the solution,the polarity of the solvent,different interface and substrate can influence the self-assembly process.FAS and DPAS self-assembled into nano-sized helical fibers in low-concentration solutions,increasing the solution concentration,helical fibers became thicker and increased in size to micron.The helical fibers formed by FAS gradually became thicker as the water content of increases,forming closed loops.The intermolecular hydrogen bonds of FAS were destroyed in the strong alkaline solution,and the helical fibers were no longer twined with each other but densely packed horizontally to form thin films.FAS self-assembled to long helical fibers on the surface of hydrophilic mica and short rods on the surface of hydrophobic graphite.DPAS self-assembled to form helical fibers under the induction of poor solvent water,while the formation of lamellar structures under the induction of n-hexane.（2）We investigated the optical properties and polarized light emission properties of FAS-L-Ala and DPAS-L-Ala by fluorescence spectroscopy,CD and CPL.The results of fluorescence spectroscopy showed that both FAS-L-Ala and DPAS-L-Ala had the typical AIE characteristics.The results of CD and CPL spectroscopy indicated both FAS-L-Ala and DPAS-L-Ala had not CD absorption and CPL signal in solution,but had CD and CPL signals,displaying the characteristics of aggregation-induced chirality.Using AFM,fluorescence microscopy and SEM,we systematically studied the factors affecting self-assembly process,found that the poor solvent,the pH value of solution and the concentration of solution can affect the assembled structures of FAS-L-Ala and DPAS-L-Ala.Self-assembly of FAS-L-Ala and DPAS-L-Ala in pure THF solution formed chiral films.After the addition of poor solvent water,the edge of films curled into nano-helical fibers.Increasing the solution concentration to 10^-3M,molecular assembled into micron-sized super-helical fibers.（3）The results of fluorescence spectroscopy indicated both N₁-PIM-L-Ala and N₁-PIM-D-Ala had the typical ACQ characteristics.And we found that two molecules had not CD absorption and CPL signal in solution,but had CD and CPL signals,illustrating the characteristics of aggregation-induced circular dichroism.We studied the assembled structures of N₁-PIM-L-Ala and N₁-PIM-D-Ala,using AFM and fluorescence microscopy,and the results showed that molecules self-assembled into different morphologies structures in different solutions environment.At the same time,the polarity of the solvent and the concentration of solution can affect the process of self-assembly,and N₁-PIM-L-Ala and N₁-PIM-D-Ala are easier to assemble to form helical fibers in highly concentrated solutions.The length of fibers can reach tens of micrometers or even hundreds of micrometers.N₁-PIM-L-Ala assembled to form fibers under the induction of poor solvent water,but films under the induction of n-hexane.N₁-PIM-D-Ala self-assembled into hollow tubes in the solution with water content of 80%.