Formation Mechanism And Induced Optical Activity Of Chiral Mesoporous Silica

Chirality is found universally in nature and performs as an inherent feature of themolecular and macromolecular components in biological life-forms. Fabrication of chiralchemicals, chiral materials, and understanding the rules that govern their formation, aremajor topics in scientific research and contribute greatly to the fields of pharmacy,biochemistry, optical devices, etc. Highly ordered chiral mesoporous silica (CMS), whichexhibits a novel helical mesostructure with hierarchical chirality, represents a new fashionfor the design and application of chiral materials. Since the first discovery of CMS in2004, many efforts have been made on the synthesis and formation mechanism of CMSby our group. However, the formation mechanism and properties of CMS are not fullyunderstood. Herein, on the basis of CMS, the formation mechanism of CMS has beeninvestigated in details and induced optical activity of CMS by introducing achiral metalnanoparticles has been exploited for the first time.Chapter2. Formation mechanism of chiral mesoporous silica. It is known that themorphology and chirality of CMS are affected by different reaction conditions. In thischapter, the influence of temperature and solvent on the formation of CMS wereinvestigated. Firstly, in the N-myristoyl-L/D-alanine sodium salt (C14-L/D-Ala) templatingsystem, helical ribbon and CMS with opposite handedness were synthesized at0oC and20oC, respectively. By scanning electron microscopy (SEM) images, X-ray diffraction(XRD) patterns and diffuse-reflectance circular dichroism (DRCD) of the products, it wasfounded that (i) chiral arrangement of surfactants was induced by their hydrogen bondingin ribbon-like CMS at0oC;(ii) the hydrogen bonding was destroyed at20oC, which ledto the formation of CMS with rod-like micelle arrangement, the handedness of which wasdetermined by the chiral steric effect of the substituent methyl group of C14-L/D-Ala.Secondly, in the sodium D-12-hydroxystearate (D-HS-Na) templating system, the left-andright-handed organic helical ribbons were obtained by cooling the pure and alcoholicaqueous solutions, respectively. The rigid corresponding mesoporous silica have beenprepared through co-structure-directing agent method. The left-and right-handed mesoporous silica ribbons replicated the antipodal helical D-HS-Na lipid assemblies,while the initial lamellar bilayers were transformed into rod-like micelles with the helicalprofile and its handedness well preserved.Chapter3. Chirality of metal nanoparticles in chiral mesoporous silica. Isotropicmetal nanoparticles (NPs) arranged in a chiral geometry is capable of inducing chiropticalactivity. Chiroptical properties of pure inorganic material have been achieved bydispersing small amounts of achiral Ag NPs into highly ordered CMS. There are threetypes of chirality in CMS:(i) the helical hexagonal surface,(ii) the helical poreorientation, and (iii) the helical arrangement of aminopropyl groups on the surface of themesopore, all of which impart plasmonic circular dichroism (PCD) and have beeninvestigated by introducing Ag NPs into the as-made, calcined and extracted CMS,respectively. The three types of PCD signals originate from asymmetric plasmon-plasmoninteractions of achiral Ag NPs in three types of chiral environments. Of the three sourcesof chirality, the helical pore orientation was considered to be predominantly responsiblefor the PCD response owing to the high efficiency of nanoscale chirality. Interestingly,large Ag NP aggregation in CMS as a result of calcination still resulted in a strong PCDresponse, even the chiral mesostructure was destroyed and the helically oriented organicmolecules were removed completely, probably because the pore chirality of chiral porousfragments remained intact. Rather than the pitch length, the length of helical channel wasmore effective for increasing the PCD intensity due to longitudinal propagation of NPsalong helical channel.Chapter4. Chirality of metal nanowires with a distinct multi-helix. Assemblinganisotropic nanowires into a distinct multi-helix may induce novel optical activity, whichcould create bio-inspired materials and be applied for chiral sensing and photonicmaterials. Ag nanowires (AgNWs) arrays were fabricated inside of CMS based onnano-cast method, producing a multi-helix with a helical channel orientation and helicalarrays of opposite handedness. The AgNWs@CMS complex exhibited strong and tunablePCD signals in the visible and near-infrared (IR) regions due to collective dipole couplingbetween the anisotropic AgNWs along transverse and longitudinal direction of CMS,simultaneously. This behavior differs from the single helix-induced CD response. Basedon the coupled dipole and exciton coupling theory, the optical activity of theAgNWs@CMS were not only dependent on the helical arrays along the direction perpendicular to the rod axis, but also the inherent winding orientation of the helicalchannels. The DRCD signals of AgNWs@CMS were dramatically changed with theincrease of helical pitch, due to weakening coupling along longitudinal direction.