Kan Zhang,Yuqi Liu,Zhikun Shang,Corey J. Evans,Shengfu Yang
A new type of atropisomerism has recently been discovered in 1,3-benzoxazines, where the intramolecular repulsion between negatively charged oxygen atoms on the imide and the oxazine ring hinders the rotation about the C–N bond. The imide group offers a high degree of flexibility for functionalization, allowing a variety of functional groups to be attached, and producing different types of end-caps. In this work, the effects of end-caps on the atropisomerism, thermally activated polymerization ofortho-imide functional benzoxazines, and the associated properties of polybenzoxazines have been systematically investigated. Several end-caps, with different electronic characteristics and rigidities, were designed.1H and13C nuclear magnetic resonance (NMR) spectroscopy and density functional theory (DFT) calculations were employed to obtain structural information, and differential scanning calorimetry (DSC) and in situ Fourier transform infrared (FT-IR) spectroscopy were also performed to study the thermally activated polymerization process of benzoxazine monomers. We demonstrated that the atropisomerization can be switched on/off by the manipulation of the steric structure of the end-caps, and polymerization behaviors can be well-controlled by the electronic properties of the end-caps. Moreover, a trade-off effect were found between the thermal properties and the rigidity of the end-caps in polybenzoxazines.Keywords:benzoxazine;ortho-imide;atropisomerization;thermal properties
Yunqing Lu,Mengmeng Li,Yu Feng,Yuexin Xu,Yongqiang Zhou,Ji Xu,Jin Wang
Optical properties are the most significant properties of optical thin films, which determine its application in optical and photoelectric devices. Designable optical properties are the most challenging goals in optical material engineering. Recently, Metal-organic framework (MOF)-based optical thin films have attracted increasing attention due to their novel optical properties, especially their modular design, which allows fine-tuning of the relevant properties. In this work, a strategy for tuning the optical properties of MOF-based thin films is developed, which is realized by adjusting MOF crystal morphology. A commonly used MOF, ZIF-8 (zeolitic imidazolate framework-8), is selected as the research model. The morphology of ZIF-8 nanocrystal is adjusted using a surfactant/end-capping agent modulation method. The surfactant/end-capping agents are absorbed onto different crystal planes of ZIF-8, slowing the growth of the crystal planes and causing the formation of ZIF-8 crystals with different morphologies. Six different morphologies of ZIF-8 crystals are successfully synthesized in the presence of different amounts of CTAB and/or TRIS: nano-spherical, rhombohedral dodecahedron, nano-cubic, rough octahedron, flake-shaped and hexapod-like morphology. Then, the corresponding ZIF-8 thin films with different crystal morphologies are prepared via spin-coating. The optical properties of the thin films are investigated using UV–vis spectroscopy and ellipsometry in detail. The results indicate that the film with rough octahedron morphology has the best transmittance, reaching 93.35 % at 581 nm. As a key optical parameter of the film, its refractive index can be adjusted between 1.27 and 1.54. The success of tuning of the optical properties of MOF optical thin films by adjusting the crystal morphology will make it meet the different required optical properties in various optical and photoelectric devices.
Siqi Wu,Jianhua Qiu,Jiakai Wang,Li Wang,Chuyang Y. Tang
Covalent organic frameworks (COFs) with excellent solvent stability, high porosity, and well-designed pore size, are competitive candidates for organic solvent nanofiltration (OSN). To manipulate their structures, pre-synthetic and post-synthetic modifications are commonly used. Nevertheless, pre-synthetic modifications may alter the stacking fashion of COF nanosheets ; post-synthetic modifications are constrained by limited pre-functionalities and can hardly achieve complete conversion. In contrast, defect engineering is a facile method to modify the structure of COFs through defects generated by the breakage of linkages or the usage of end-capping molecules. In this study, we adopted defect engineering to improve the OSN performance of TpPa-COF membranes constructed of 2,4,6-triformylphloroglucinol (Tp) and p -phenylenediamine (Pa) monomers with end-capping molecules—aniline (An). The membrane synthesized with 30 percent of An molecules (TpPaAn-30/HPAN membrane) maintained a continuous COF layer with a thickness of ∼20 nm but lower density or more free channels, compared to the control membrane. The optimized TpPaAn-30/HPAN membrane had improved permeance for various organic solvents (e.g., 31.8 L m −2 h −1 bar −1 for methanol, fourfold of the control membrane) but maintained rejection towards methyl blue (>90 %). It also allowed the passage of rhodamine B but blocked methyl blue when filtrating a mixed-dye methanolic solution. Herein, the usage of end-capping monomers is proven to be an efficient method to introduce transport channels and improve the separation performance of COF membranes.
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