Yanan Gao

Sophisticated Design of Covalent Organic Frameworks with Controllable Bimetallic Docking for a Cascade Reaction.

Precise control of the number and position of the catalytic metal ions in heterogeneous catalysts remains a big challenge. Here we synthesized a series of two-dimensional (2D) covalent organic frameworks (COFs) containing two different types of nitrogen ligands, namely imine and bipyridine, with controllable contents. For the first time, the selective coordination of the two nitrogen ligands of the 2D COFs to two different metal complexes, chloro(1,5-cyclooctadiene)rhodium(I) (Rh(COD)Cl) and palladium(II) acetate (Pd(OAc)2 ), has been realized using a programmed synthetic procedure. The bimetallically docked COFs showed excellent catalytic activity in a one-pot addition-oxidation cascade reaction. The high surface area, controllable metal-loading content, and predesigned active sites make them ideal candidates for their use as heterogeneous catalysts in a wide range of chemical reactions.

Channel-wall functionalization in covalent organic frameworks for the enhancement of CO2 uptake and CO2/N2 selectivity

A series of tailored covalent organic frameworks (COFs), i.e. [NN]X%–TAPH-COFs and [CC]X%–TAPH-COFs, were synthesized by post-fabrication of [HO]X%–TAPH-COFs with 4-phenylazobenzoyl chloride (PhAzo) and 4-stilbenecarbonyl chloride (PhSti), respectively. Powder X-ray diffraction (PXRD), FT-IR, and solution-state 1H NMR of the digested COFs were applied to clarify the functional groups integrated in the pore channels. Gas sorption isotherms confirmed that the [NN]X%–TAPH-COFs and [CC]X%–TAPH-COFs had moderate surface areas, narrow pore sizes, and good physicochemical stability. Compared with [CC]X%–TAPH-COFs, the [NN]X%–TAPH-COFs exhibited higher CO2 uptake capacities of up to 207 mg g−1 (273 K and 1 bar), isosteric heats of adsorption for CO2 (30.7–43.4 kJ mol−1), and CO2/N2 selectivities of up to 78 (273 K) because of the dipole interactions between the azo group and CO2 as well as the N2-phobic behavior of the azo group. Furthermore, although the decreased pore size was advantageous for increasing CO2 adsorption, the decreased surface area of the COFs would undoubtedly decrease CO2 adsorption if too many functional groups were introduced.

Rational design of photo-responsive supramolecular nanostructures based on an azobenzene-derived surfactant-encapsulated polyoxometalate complex

Using an ionic self-assembly (ISA) approach, photo-responsive surfactant-encapsulated polyoxometalate complexes (SECs) were fabricated in water from an original Keggin-type polyoxometalate (POM) and a cationic surfactant containing an azobenzene group, viz. phosphotungstic acid (H3[PW12O40]) and 4-ethyl-4′-(trimethylaminohexyloxy) azobenzene bromide (ETAB). The driving forces and self-assembly mechanism of the ETAB–POM supramolecular hybrids were investigated by NMR, Fourier transform infrared (FTIR), UV/vis and small angle X-ray scattering (SAXS) characterization methods. Of particular interest is the complex solution which shows an obvious variation upon UV light irradiation. On a macro-scale, its turbidity increases obviously, from a clear solution before UV irradiation to a turbid state. The microcosmic structures of the complex change from coral-like structures to dispersive nanospheres. These phenomena can be ascribed to the transformation of ETAB from trans- to cis-isomers after exposure to UV light. Beyond that, a cyclic voltammetric (CV) method was employed to observe the electrochemical properties of SECs. The results obtained in this work will shed light of the SECs applications in phase separation, heterogeneous catalysis reactions, the detection of environmental pollutants, etc.

Nitrogen ligands in two-dimensional covalent organic frameworks for metal catalysis

Abstract We introduced bipyridine ligands into a series of two-dimensional (2D) covalent organic frameworks (COFs) using 2,2′-bipyridine-5,5′-dicarbaldehyde (2,2′-BPyDCA) as a component in the mixed building blocks. The framework of the COFs was formed by the linkage of imine groups. The ligand content in the COFs was synthetically tuned by the content of 2,2′-BPyDCA, and thus the amount of metal, palladium(II) acetate, bonded to the nitrogen ligands could be manipulated. Both the bipyridine ligands and imine groups can coordinate with Pd(II) ions, but the loading position can be varied, with one ligand favoring binding in the space between adjacent COFs′ layers and the other ligand favoring binding within the pores of the COFs. The Pd(II)-loaded COFs exhibited good catalytic activity for the Heck reaction.

Multi-stimuli Responsive Supramolecular Structures Based on Azobenzene Surfactant-Encapsulated Polyoxometalate

Multi-stimuli responsive materials have attracted intense attention as extensive application prospect in many fields, yet achievement of multi-stimuli responsiveness remains a challenge. Herein, we report a tri-stimuli responsive supramolecular structure fabricated by a cationic surfactant, 4-ethyl-4-(trimethylaminohexyloxy) azobenzene bromide (ETAB), and anionic Eu-containing polyoxometalates (Eu-POM), based on an ionic self-assembly (ISA) strategy. Following different responsive mechanisms, the resultant ETAB/Eu-POM supramolecular materials are responsive to UV light, pH, and Cu(2+), respectively. The response to UV irradiation is based on the configuration change of azobenzene molecules. The response to H(+) can be attributed to the formation of a hydrogen bond W-O···H···O-H among Eu-POM, H(+), and H2O, which blocks the energy transfer pathway from O → W, while the coordination interaction between Cu(2+) and Oc (bridged oxygen of two octahedra sharing an edge in the Eu-POM molecule) causes the response to Cu(2+). The multi-stimuli responsive characteristics for the ETAB/Eu-POM supramolecular structures maybe provide a potential application in ultraviolet detection, optical storage devices, and chemical substance sensors, etc.

A nonionic surfactant-decorated liquid crystal sensor for sensitive and selective detection of proteins.

Proteins are responsible for most biochemical events in human body. It is essential to develop sensitive and selective methods for the detection of proteins. In this study, liquid crystal (LC)-based sensor for highly selective and sensitive detection of lysozyme, concanavalin A (Con A), and bovine serum albumin (BSA) was constructed by utilizing the LC interface decorated with a nonionic surfactant, dodecyl β-d-glucopyranoside. A change of the LC optical images from bright to dark appearance was observed after transferring dodecyl β-d-glucopyranoside onto the aqueous/LC interface due to the formation of stable self-assembled surfactant monolayer, regardless of pH and ion concentrations studied in a wide range. The optical images turned back from dark to bright appearance after addition of lysozyme, Con A and BSA, respectively. Noteworthy is that these proteins can be further distinguished by adding enzyme inhibitors and controlling incubation temperature of the protein solutions based on three different interaction mechanisms between proteins and dodecyl β-d-glucopyranoside, viz. enzymatic hydrolysis, specific saccharide binding, and physical absorption. The LC-based sensor decorated with dodecyl β-d-glucopyranoside shows high sensitivity for protein detection. The limit of detection (LOD) for lysozyme, Con A and BSA reaches around 0.1xa0μg/mL, 0.01xa0μg/mL and 0.001xa0μg/mL, respectively. These results might provide new insights into increasing selectivity and sensitivity of LC-based sensors for the detection of proteins.

Bimetallic docked covalent organic frameworks with high catalytic performance towards tandem reactions

Mn/Pd bimetallic docked covalent organic frameworks were fabricated via a programmed synthetic procedure. Within the framework, MnCl2 could only coordinate with the bipyridine ligands, while Pd(OAc)2 could occupy the rest of the nitrogen sites. Such bimetallic docked materials showed high catalytic activity in a Heck-epoxidation tandem reaction.

A liquid crystal-based sensor for the simple and sensitive detection of cellulase and cysteine

A liquid crystal (LC)-based sensor, which is capable of monitoring enzymatic activity at the aqueous/LC interface and detecting cellulase and cysteine (Cys), was herein reported. When functionalized with a surfactant, dodecyl β-d-glucopyranoside, the 4-cyano-4-pentylbiphenyl (5CB) displays a dark-to-bright transition in the optical appearance for cellulase. We attribute this change to the orientational transition of LCs, as a result of enzymatic hydrolysis between cellulase and surfactant. Furthermore, by adding cellulase and Cu(2+), our surfactant-LCs system performs an interesting ability to detect Cys, even though Cys could not interact with surfactant or LC directly. Alternatively, through the strong binding between Cys and Cu(2+), cellulase was able to hydrolyze surfactant in the presence of Cu(2+), leading to the transition of LCs from dark to bright. The detection limit of the LC sensor was around 1×10(-5)mg/mL and 82.5μM for cellulase and Cys, respectively. The LC-based sensor may contribute to the development of low-cost, expedient, and label-free detection for cellulase and Cys and the design strategy may also provide a novel way for detecting multiple analytes.

Reversible Photoresponsive Molecular Alignment of Liquid Crystals at Fluid Interfaces with Persistent Stability.

This work demonstrates a noninvasive approach to control alignment of liquid crystals persistently and reversibly at fluid interfaces by using a photoresponsive azobenzene-based surfactant dissolved in an ionic liquid (IL), ethylammonium nitrate (EAN). As the first report on the orientational behavior of LCs at the IL/LC interface, our study also expands current understanding of alignment control of LCs at the aqueous/LC interface by adding electrolytes into aqueous solutions. The threshold concentration for switching the optical responses of LCs can be changed just by simply manipulating the ratio of EAN to H2 O. This work will inspire fundamental studies and novel applications of using the LC-based imaging technique to investigate various chemical and biological events in ILs.