Synthesis, Formation Mechanism And Properties Of Chiral Mesoporous Materials

Chirality is high prevalent in nature. Design and synthesis of chiral compoumds, supramolecular assemblies and functional materials has been an important project in scientific research. During the last decade, ordered mesoporous materials have attracted much attention due to their high surface area, large pore volume, controllable pore size and flexible functionalization. Chiral mesoporous material is a kind of porous materials with highly ordered two-dimensional hexagonal chiral mesostructure. Since their first synthesis in 2004, much work has been preceded in our group. The present thesis made a systematic summary of the studies on synthesis, formation mechanism and properties of chiral mesoporous materials that finished by the author in these five years. There are eight chapters, including the literature reviewing in Chapter 1, the research on synthesis and formation mechanism in Chapter 2-4, derived materials in Chapter 5 and 6, and properties in Chapter 7, and the conclusions and research proceedings in Chapter 9.In Chapter 2, a detailed investigation was made on the mechanism that governing the enantiopurity of chiral mesoporous silicas by studing the effects of reaction temperature, bacisity and the substitutes on the chiral center of the N-acyl-amino acids. Chiral mesoporous silicas of over 90% ee were obtained with the use of large group substituted N-acyl-amino acids at lower temperature. The chiral mesostructure was supposed to origin from the helical stacking of surfactants in the rod-like micelles. The enantiopurity would be controlled by temperature and substitutes size through the conformational changes of the N-acyl-amino acids. The dependence of ee on temperature and molecular structure was further discussed by thermal dynamical analysis. Besides, it was found that the ee of the chiral mesoporous silicas formed with C16-L-Phe dramatically increased with decreasing basicity of the reaction solution. Tighter packing of the aromatic group substituted C16-L-Phe molecules in the micelles at lower basicity significantly enhances the intermolecular steric hindrance between the adjacent amphiphiles which helps the formation of the CMSs with higher enantiopurity.In Chapter 3, a serial of racemic chiral mesoporous silicas were synthesized with various achiral cationic and anionic amphiphiles. The formation, enantiopurity and helicity of the samples were well discussed, leading to a comprehensive understanding of the formation mechanism of achiral amphiphile-templated chiral mesoporous silicas. The chiral mesostructue may origin from the asymmetric molecular shapes of the achiral surfactants as indicated by the diversity of the templates and their similarity to chiral templates. It was shown by the mechanical analysis that the pitch length should be in direct proportion to the rod diameter but inversely proportional to the moment of the helical propeller-like micelle, which was essentially monitored by the templating molecules. On the other hand, counter-charged chiral amphiphiles were effectively used to generate chirality in achiral amphiphile-templated chiral mesoporous silicas.In Chapter 4, by using C18MIMBr as template, it is demonstrated that a systematic structural change from 2D-hexagonal chiral to 2D-rectangular p2gg to lamellar mesostructure can be controlled by precisely adjusting the basicity of the reaction mixture. The HRTEM image gave direct evidence for the new particular 2D-rectangular p2gg mesostructural constructions with peculiar packing of elliptical mesopores arranged on 2D-hexagonal lattice.In Chapter 5, chiral periodic mesoporous organosilicas with–CH₂CH₂- and–C₆H₄- bridges were successfully obtained by using chiral anionic surfactant C₁₆-L-Ala and achiral nionic surfactant SDS as templates. The chiral chiral periodic mesoporous organosilicas formed with C₁₆-L-Ala exhibited an ee of about 40% and the BTEB based hybrid showed crystal-like walls. The organic brigding groups inside the framework were supposed to be chiral arranged in the framework as indicated by the diffuse reflectance circular dichroism spectra.In Chapter 6, a simple method was reported to assemble chiral porphyrin assemblies into chiral mesostructured porphyrin-silica hybrids. TMAPS, a cationic organosilane bearing a quaternary ammonium group, was utilized to assemble the anionic water-soluble porphyrins, e.g., meso-tetra (4-sulfonatophenyl) porphyrin (TSPP), into cylindrical helical stackings, which led readily to the formation of chiral mesostructured porphyrin-silica hybrids. Furthermore, the ee of the hybrids were dramatically enhanced by the addition of a small amount of chiral dopants, such as (R)- and (S)-1,1'-bi-2-naphthols, leading to a considerable amplification of chirality.In Chapter 7, as confirmed by the diffuse reflectance circular dichroism spectra, chiral conformations of achiral linear conjugated polymers and chiral stackings of discotic conjugated molecules were effectively induced by the extracted chiral mesoporous silicas, indicating a supramoleular chiral imprinting inside the material. During the formation process of chiral mesoporous silicas, the cationic quaternary ammonium groups of the co-structure directing agent (TMAPS) electrostatically interact with the anionic head groups of the chiral amphiphiles. Due to the pairing effect, these functional groups may be helically aligned on the mesopore surface surrounding the helical propeller-like micelle. Thus, the chirality of the primary supramolucular structure of the helical propeller-like micelles is expected to be memorized and immobilized in the helical arrangement of the functional groups on the surface of each mesopore upon removal of the template by extensive extraction. Moreover, it was found that such supramolecular chiral imprinting would be well recognized by B-DNA.