Study On Self-assembly Of Chiral Hierarchical Nanostructures

Chiral nanomaterials are of great significance in promoting the development of nanotechnologies such as chiral analysis and detection,chiral separation,biological labeling,polarization-based photonics and optoelectronics,among many others.To date,although a variety of organic and inorganic chiral nanostructures have been fabricated,chiral hierarchical nanostructures are much less reported.The self-assembly mechanism of hierarchical structures and the relationship between hierarchical structures and properties still lack systematic investigation.In this thesis,we prepared diverse chiral hierarchical nanomaterials from building blocks of either gold-thiolate nanoplatelets or organic macrocycles via self-assembly.Subsequently,the mechanism of self-assembly,colloidal,chemical and chiroptical properties of these nanostructures were investigated.Major contents and conclusions of these work are listed as follows.1.Construction of hierarchically organized chiral particles from gold-cysteinate(Au-Cys).The reaction between gold chloride and cysteine produced nanoplatelets with a broad lateral size distribution.Addition of surfactant CTAB into the solution counteracted competing and strong interactions(electrostatic repulsion and close-range attractions)between nanoplatelets.Under the circumstance,weaker and secondary factors,such as particle asymmetry or chirality,took over and determined the assembly pathways of chiral nanoparticles.As a result,coccolith-like particles(CLIPs)with hierarchical nanostructures were obtained.Developing graph theory and establishing algorithm of complexity index(CI),we accessed the complexity of CLIPs quantitatively and found their CI was comparable or even higher than many biotic and abiotic sophisticated nanostructures.A phase diagram of hierarchically organized particles(HOPs)with rich morphologies were obtained by changing the enantiomeric excess?and nucleation temperature t_n.Comparing the CI of different structures in the diagram,it was found that the most complex particles formed when?approached±100%.Replacing chiral ligand Cys with achiral molecule of thioglycolic acid(TGA)resulted in formation of much simpler particles.As a comparison,replacing Cys with another chiral ligand of penicillamine gave rise to similar CLIPs.It revealed the vital role of chirality in the preparation of hierarchical and complex nanostructures.Doping silver or cupper into Au-Cys nanoplatelets enhanced the elastic restrictions,changed the morphology and lowered the complexity of the final assembled particles.2.Colloidal,chemical stability and chiroptical activities of Au-Cys HOPs.Au-Cys CLIPs could be well dispersed in a variety of polar and nonpolar solvents,exhibiting good colloidal stability.They could retain their geometry and luminescence from extreme acidic to highly basic solutions,displaying remarkable chemical stability.Au-Cys CLIPs showed strong chiroptical properties,and the dissymmetry factor g(g-factor)of extinction and emission reached high values of 0.055 and 0.01,respectively.Au-Cys materials are flexible in chemical design.Doping with cupper or silver not only changed the morphology,but also blue shifted the red emission to orange and yellowish-green.Despite changes in color,the emission was still circularly polarized.The handedness of emitted light from Au-Cys CLIPs reversed after a physical treatment of sonication.Here,we proposed a new mechanism for circularly polarized light emission(CPLE).The photons emitted from Au-Cys sheets are circularly polarized because they are emitted by chiral electronic states.Differential scattering of emitted photons on chiral particles took place after the emissive event.These two processes resulted in emission with opposite handedness and thus acted as a pair of competing contributions.In CLIPs,differential scattering was more pronounced and dominated the handedness of emission.While in the dispersion of disassembled nanoribbons,scattering was weak and the primary emission from Au-Cys sheets determined the final handedness of emission.3.Construction of diverse chiral hierarchical nanostructures from organic macrocycles.The reaction between a chiral diamine and a diisocyanate produced a combinational library of macrocyclic homologues.The dimeric macrocycle(MCU₂)self-sorted and self-assembled into irregular nanoflakes,further self-separated from the reaction solution.The self-assembly of chiral MCU₂ molecules at different conditions resulted in a variety of chiral nanostructures,including propeller-blade particles(PBPs),supra PBPs,nanoflowers,twisted nanofibers,helices of nanoparticles and ring-banded spherulite particles composed of helical nanofilaments.Self-assemblies prepared from two enantiomeric MCU₂ molecules showed good structural mirror symmetry.Further structural analysis revealed that in all the nanostructures with chiral geometrical features,MCU₂ molecules were crystallized in a simple monoclinic phase.If a simple rectangular phase instead,no large-scale geometrical chirality was identified.