Design, Synthesis And Properties Of Biphenyl Derivative Functional PMOs

Periodic mesoporous organosilicas (PMOs), in which the functional organic groups are densely and covalently embedded within the pore walls, meaning that the organic groups are an integral part of the frameworks and without plugging the mesopores. PMOs have remarkable features such as ordered mesoporous structure, tunable pore size, modifiable surface properties, and controllable physicochemical properties of the materials, which ensure their wide applications in adsoprtion, catalysis, sensors, drug delivery and optoelectronic devices. PMOs are ideal optical support materials. On one hand, the optical groups are uniformly and covalently embedded within the silica walls, which could efficiently prevent fluorescence quenching due to aggregation; on the other, silica matrix plays an important role in stability and protection of the fluorophores inside the pore walls. Thus, the introduction of functional fluorophores into the PMO framework has opened a new path for the development of fluorescent sensors, biological imaging and solid-state luminescent materials.This thesis concentrates on the study of the synthesis, properties and applications of fluorescent functional PMOs materials. Three kinds of organosilicas precursors with different structures were synthesized through the further modification of the biphenyl derivatives. By introducing different functional groups, such as urea propyl, macrocycle molecules and π-conjugated system, these organosilicas precursors were endowed with different properties and potential applications. Then, using cationic oligomeric surfactant as the structure-directing agent, these organosilicas precursors were immobilized into the framework of the PMOs by co-condensation method. Moreover, based on the study of the structural and optical properties of these PMOs materials, their potential applications, such as charge transfer, pH sensing, drug release and metal ion detection, were also investigated. The main research contents are as follows:(1) By using bis(3-triethoxysilyl propylureido) biphenyl (BpU) as the organosilane precursors, a series of biphenyl-bridged PMOs (BpUPMOs) were successfully synthesized. The BpUPMOs with different content of biphenyl organosilicas exhibited unique optical properties. The red-shift and enhanced fluorescence emission were exhibited as the contents of BpU increased when the amount of biphenyl organosilicas was less than 30 mol%, which was ascribed to the formation of excimers and the restriction of rotation of the biphenyl groups. Whereas, the fluorescence quenching caused by the interaction of intramolecular hydrogen bond was observed as the amount of biphenyl organosilicas was increased to 40%. The hydrogen bonding interaction between ureido and aromatic π-π interaction between biphenyl groups of organosilicas molecular are crucially important in above change of fluorescence emission. Furthermore, using oligomeric cationic surfactant and decylviologen as structure directors together, a Charge-transfer (CT) donor-acceptor system was built. In this system, the biphenyl groups acted as electron donors in the pore walls and the decylviologen molecules were as electron acceptors in the mesochannels of BpUPMO. In addition, the electron acceptor was placed selectively in the ionic interface by utilizing the surfactant properties of decylviologen. Appropriate distance between donor and acceptor is provided by this configuration, which is highly important for the effective charge transfer. The color of CT complex could be controlled by the doping amount of decylviologen in the charge transfer system. The formation of CT complex could be confirmed by the optical tests and change of external colour.(2) Immobilization of pH-driven molecular shuttles into the frameworks of the PMOs was realized by cocondensation of β-cyclodextrin (β-CD) based rotaxane siloxane and tetraethoxysilane (TEOS). For this P-CD based rotaxane molecular shuttle, β-CDs threaded a symmetrical molecular thread composed of a biphenyl unit, two ureido and propyl groups, and were end-trapped mechanically by two siloxane stoppers. The rigid framework of PMOs creates enough free space that allows the P-CD macrocycles of the rotaxane to shuttle mechanically. Thus, the β-CDs as the shuttles could be reversibly translocated along the molecular thread between biphenyl and propyl groups by the pH stimuli in the solid state system, and the information of the shuttling movement could be exhibited from the fluorescent behaviors of the rotaxane PMOs. Thus, in this molecular shuttle, fluorescent biphenyl groups involved in P-CDs in the rotaxane act as the sensors that provide self-feedback of the position of the β-CDs in the rotaxane through the fluorescent properties of the hybrid materials. Ureido units with biphenyl and propyl groups in the sides are as pH triggers. A mechanism of the molecular shuttles was proposed that the transformation of hydrophilic/hydrophobic properties of the biphenyl groups triggered the movement of the β-CDs when the pH was changed. The fact was proven by the technologies of synchrotron soft X-ray absorption, time-resolved fluorescence and 1H NMR etc. Particularly, the PMOs could be employed as a pH-controllable smart-release platform via the reciprocating movement properties of the molecular shuttle. The accelerated cargo release was achieved after acidification, demonstrating a release process in the "ON" state. If the pH of system was further changed from acidic to neutral, the release of cargo was turned back to the "ON" state. Thus, a repeated response release is realized in the solid state system. Furthermore, the PMOs materials show very low cytotoxicity and fine biocompatibility, which ensure their potential in biomedical applications.(3) In order to further improve the fluorescence emission efficiency, a highly π-conjugated fluorophore (TH) was synthesized by the Schiff-bases reaction of p-hydroxybenzaldehyde. PMOs with aggregation-induced emission enhancement (AIEE) characteristic were synthesized using the TH-derived organosilane precursor (TH-Si4). The TH-Si4 precursor was embedded within framework in monomeric form by four polymerizable silyl groups, and the solid PMO materials displayed enhanced fluorescence emission with the increase of the amount of TH-Si4 precursor. The optical tests indicated that the intramolecular rotation of TH was restricted by the framework of TH-PMOs, resulting in the promoted monomeric fluorescence emission. In addition, such enhanced-fluorescence could be selectively quenched by Cu2+due to the specific binding between TH and Cu2+. The detection limit of TH-PMOs would be reduced by reaping benefits from the enhanced-fluorescence, which gave the detection sensitivity up to 10-8 M level. Furthermore, the X-ray absorption near-edge spectroscopy (XANES) analysis was used to confirm the coordination of TH units with Cu2+. To investigate the metal ions in TH-PMOs material in situ, Soft X-ray Scanning Transmission Microscopy (STXM) was employed in our study to provide direct evidence of the diffusion process of Cu2+ and the competitive effect between Cu2+ and Fe2+. The results demonstrate that the specific binding between TH and Cu2+ plays an important role in the adsorption of metal ions. Such hybrid materials that combine high fluorescence and sensitive detection characteristics are promising candidates for various luminescence applications such as solid-state optical emitters and fluorescence sensing.