Construction Of Two-Dimensional Homochiral Porous Nanosheets For Potential Application

Chiral porous materials have attracted grand attention in many scientific fields including chemistry,biology,material science,and physics in recent decades,which have shown a great significance of rapid development for diverse practical applications including asymmetric synthesis,signal amplifications,and optical devices.Compared to the molecular level of homogeneous catalyst,the supramolecular chiral nanostructure-based heterogeneous catalysts can be continuously recovered with high recyclability and hence attract extensive research interest.Thus,many cases of chiral supramolecular architectures have been created and utilized for enantiomeric separations or chiral catalysts through rational design of diverse building blocks.To this end,much efforts have been devoted to develop the chiral porous materials at supramolecular level with remarkable properties.While the efficiency in practical applications is limited,suffering from the low enantioselectivity or low asymmetric conversion in most cases.Among these,two different typical dimensions of 1D and 3D nanostructures for construction of supramolecular chiral porous materials are widely investigated.In the case of 1D nanostructures such as helical tubules,which are easy deformed of the elongated stacking of internal cavities and hence has great influences on the precise performance for applications.However,the 3D structures in bulk state,such as chiral organic frameworks or chiral zeolites,have been extensive utilized in many areas,most of them are rarely well organized in homochirality.In this respect,the 2D porous nanostructures take an intrinsic advantages to endow homochiral properties,which are considered as an excellent candidate to overcome the weakness in the cases of 1D and 3D materials.In the past few decades,there was a rapid development on the construction of the functional 2D porous materials,which offered a new insight and perspective for further significant discovery in this attractive field.However,to create 2D homochiral materials from molecular-level pores with perfect pore performance is still a big challenge due to pore blocking and deformation.With this idear in mind,this thesis describes successful construction of 2D homochiral nanosheets with uniform molecular-level pores through self-assembly of macrocyclic amphiphiles and their excellent performance for enantioselictive separation.The macrocyclic amphiphiles form the chiral pores through dimeric stacking,which consisits of aromatic macrocycle segments and hydrophilic oligoether dendron located at the center part of macrocycle plane to prevent continous aromatic stacks in one dimension.With the macrocyclic amphiphiles as the basic building block,we have summarized all the discoveries and results as mensioned below:1.Construction of Single-Layered porous nanosheets through self-assembly of macrocyclic amphiphiles.The molecular self-assembly of designed building blocks is considered to be a powerful approach to construct different dimensional nanostructures.The various non-covalent interactions can be regulated by the rational design at molecular level.In this chapter,We have successfully constructed two-dimensional nanosheets with uniform pore size by dimeric stacking of macrocyclic amphiphile.In most cases,the flat aromatic amphiphiles grow in one dimension to form nanofibers through face-to-face stacking.To prevent continuous stacking in one dirrection,we employed the macrocyclic amphiphile consisiting of a rigid aromatic segment and a hydrofilic oligoether dendron grafted at the center part of the macrocyclic plane.The macrocyclic amphiphiles first form dimeric micelles by face-to-face stacking through hydrophobic and ?-? interactions in methanol solution.Subsequently,the micelles self-assemble into two-dimentional nanosheets with uniform pore size through lateral association,which were characterized by TEM and AFM porphology study.The rational molecular design is believed to be an efficient strategy to construct single-layered two-dimentional nanosheets with uniform molecular pore size.2.Homochiral porous nanosheets with dynamic gated motion by external stimulationWe design and synthesis the aromatic macrocycle with a chiral oligoether dendron grafted at the cent part of the macrocycle plane.Similarly,the aphiphile first self-assembles into dimeric micelle,then grows laterally by side-to-side solvophobic interaction to form two-dimentional porous nanosheets in methanol solution.Interestingly,the dimeric macrocycle shows a twist stacking with a preferred rotation between each other.The formation preferred twist stacking is due to collective effects of the chirality from hydrophilic oligoether dendrons transfered to achiral macrocyle and the nonplanar saddle-like conformation of the aromatic macrocycle.Thus the twist stacking generates a superstructure with a chiral cavity.The macrocyclic amphiphiles with different chirality of oligoether dendrons were synthesised,the amphiphiles exhibit opposite CD signals with a perfect mirror image,which indicate that the chirality of hydrophilic dendrons is transferred to the dimeric macrocycles,the dimers further grow into homochiral porous nanosheets.Remarkblely,the homochiral pores can de closed by addition of organic salts like ammonium acetate,because the hydrophilic oligoether chains can be desolvated and collapse into the pores.Significantly,the dynamic motion of the homochiral pores achived without deformation of the whole superstructure.This unique structure with uniform chiral cavities might show excellent performace in diverse potential applications including enantioselective separation,asymmetric synthesis.3.Homochiral porous nanosheets for highly efficient enantiomer sievingIn this chapter,we utilize homochiral porous nanosheets for highly efficient ennatioselective separation.We found that the pores are specific binding one enantiomer of hydrophobically protected tryptophan(G1),in which the caculated guest size is well matched with the pore size.It is noteworth that the pores show 96% uptake of the guest,indicating high efficiency and uptake capacity of each pore.Moreover,the CD signal of the nanosheets maintains even after inclusion of the guest,which indicates that the chiral cavity from dimeric macrocycle retains a twist stacking after inclution of the guest.Remarkblely,with addition of organic salt to the guest-trapped nanosheets solution,the captured guest can be pumbed out due to the collapse of oligoether chains inside the pores.To evaluate the enantioselectivity and uptake capacity of the nanosheets,several amino acid derivatives are selected with different size.Similar to G1,the nanosheets exhibit excellent pore performance of enantioselectivity and uptake for the case of tyrosine derivative(G2).However,the nanosheets do not show any obvious uptake activity for smaller guests.The strict selecticity in chirality,size and perfect uptake capacity are considered to be attribute to efficient translation of precise recognization of individual pores to two-dimensional homochiral nanosheets without sacrificing the pore performance.We believe this results will provide inspirations to construct plasema membranes,nanopump materials and artificial intracellular machines.