Synthesis And Functions Of Two-dimentional Covalent Organic Frameworks

Covalent organic frameworks (COFs) are a new class of crystalline porous polymersthat allow the atomically precise integration of desirable π units to create predesignedskeletons and nanopores in bulky solids and thin films. The field of covalent organicframeworks has been particularly advanced by Yaghi and co-workers, who coined theacronym “COF” in2005. Since this landmark work, COFs have been attracting increasingattentions during the past few years, because their unique features of well-definedcrystalline porous structure, high stability together with tailored functionalities make thempromising materials for gas storage, catalysis, photoelectronic, and energy storage devices.By virtue of strong covalent bond, COFs exhibit excellent thermal stability and do notundergo thermal phase transitions, which make it quite safe in the thermal or solventannealing procedure for optimizing photovoltaic performance. The eclipsed stackingstructure in two-dimensional (2D) COFs provides a unique means to construct ordered πsystems that are difficult to create via traditional covalent and/or noncovalent methods. Incontrast to twisted discotic liquid crystals, the ordered COF skeletons adopt zero-degreedihedral angles between layers and induce a large electronic coupling between theπ-orbitals of the stacking sheets, which can facilitate charge carrier transport through theinborn π-pathways. Last but not the least, the open nanochannels in COFs allow theintroduction of complementary organic or inorganic counterparts for construction verticallyaligned segregated donor-acceptor heterojunction, an ideal structure for photovoltaicdevices. These unique features imply that2D COFs have potentials for developing newtypes of light-emitting diodes, solar cells, and photodiodes.In this thesis, the author mainly focuses on the design, synthesis, and functions ofπ-electronic2D COFs and donor-acceptor COFs. The cotent of this theis is summarized inthe following chapters:In chapter2, a series of novel two-dimensional triazine-based covalent organicframeworks (COFs) with crystalline skeletons and permanent porosity were designed andsynthesized by solvothermal method. By the rational combination of different buildingblocks, the COF skeletons could be precisely designed and the surface areas, pore size andpore volume are totally tunable. By virtue of the robust feature of the imine bond, theseimine-linked COFs show quite high thermal stability and robustness in verious chemical conditions. The stacked π columns in2D COFs efficiently facilitate the transport of chargecarriers through built-in pathways. The charge-carrier behavior of these triazine-basedCOFs were investigated by the well-known flash-photolysis time-resolved microwaveconductivity (FP-TRMC) method.In chapter3, we report a general synthetic strategy for converting these open latticestructure structures into ordered donor-acceptor heterojunctions. A three-componenttopological design scheme was explored to prepare electron-donating intermediate COFs,which upon click reaction were transformed to photoelectric COFs with segregateddonor-acceptor alignments, whereas electron-accepting buckyballs were spatially confinedwithin the nanochannels via covalent anchoring on the channel walls. The donor-acceptorheterojunctions trigger photoinduced electron transfer and allow charge separation withradical species delocalized in the π-arrays, whereas the charge separation efficiency wasdependent on the buckyball content. This new donor-acceptor strategy explores bothskeletons and pores of COFs for charge separation and photoenergy conversion.In chapter4, we report a strategy based on crystalline, porous covalent organicframework for linking porphyrin and phthalocyanine into periodic, self-sorted andsequenced lattice structure. This synthetic approach allows the design of skeleton and pore,controls the stoichiometry and sequence of dye units and is applicable to various metalspecies, as exemplified by12different frameworks. The porphyrin–phthalocyanineframeworks possess low band gaps and show exceptional light-harvesting capability. Theseframeworks are promising photocatalysts using visible and near-infrared lights forsinglet-oxygen generation with one-order-of-magnitude enhanced activity compared withunstructured porphyrin and phthalocyanine controls.