Jinqiao Dong

Chiral NH-controlled supramolecular metallacycles

Chiral NH functionalities-based discrimination is a key feature of Natures chemical armory, yet selective binding of biologically active molecules in synthetic systems with high enantioselectivity poses significant challenges. Here we report the assembly of three chiral fluorescent Zn6L6 metallacycles from pyridyl-functionalized Zn(salalen) or Zn(salen) complexes. Each of these metallacycles has a nanoscale hydrophobic cavity decorated with six, three, or zero chiral NH functionalities and packs into a three-dimensional supramolecular porous framework. The binding affinity and enantioselectivity of the metallacycles toward α-hydroxycarboxylic acids, amino acids, small molecule pharamaceuticals (l-dopa, d-penicillamine), and chiral amines increase with the number of chiral NH moieties in the cyclic structure. From single-crystal X-ray diffraction, molecular simulations, and quantum chemical calculations, the chiral recognition and discrimination are attributed to the specific binding of enantiomers in the chiral pockets of the metallacycles. The parent metallacycles are fluorescent with the intensity of emission being linearly related to the enantiomeric composition of the chiral biorelevant guests, which allow them to be utilized in chiral sensing. The fact that manipulation of chiral NH functionalities in metallacycles can control the enantiorecognition of biomolecular complexes would facilitate the design of more effective supramolecular assemblies for enantioselective processes.

Ultrathin two-dimensional porous organic nanosheets with molecular rotors for chemical sensing

Molecular rotors have played an important role in recent materials chemistry. Although several studies on functional materials containing molecular rotors have been reported for fluorescence sensing, this concept has yet to be realized in two-dimensional (2D) materials. Here we report the preparation of all-carbon, π-conjugated 2D porous organic nanosheets, named NUS-24, which contain flexible tetraphenylethylene (TPE) units as the molecular rotors. NUS-24 nanosheets exhibit high stability, large lateral size, and ultrathin thickness (2–5 nm). The dynamic TPE rotors exposed on the surface of NUS-24 nanosheets can be restricted in the aggregated state with different water fractions, which is reminiscent of the aggregation-induced emission mechanism, thereby leading to the size-selective turn-on fluorescence by volatile organic compounds. Significantly, the ultrathin 2D nanosheets and its composite membranes show much higher sensitivity and selectivity toward Fe3+ ions and nitro-containing compounds sensing, suggesting their potential applications in explosive detection and environmental monitoring.Molecular rotors that fluoresce upon restriction are useful components in functional materials. Here, the authors incorporate molecular rotors into 2D porous organic nanosheets, creating sensitive and selective fluorescent sensors for volatile organic compounds and metal ions.

Isoreticular covalent organic frameworks for hydrocarbon uptake and separation: the important role of monomer planarity

Covalent organic frameworks (COFs) represent a new class of crystalline porous organic materials with huge potential in gas storage and separation. Here we present three isoreticular COFs sharing identical two-dimensional layered structures but with different planarity of the building monomers, leading to various crystallinities, porosities and hydrocarbon uptake and separation performances. This study reveals the importance of monomer design in the long-range stacking of COFs, which can be used to tailor COFs with target functionalities for specific applications.

Confinement of Aggregation-Induced Emission Molecular Rotors in Ultrathin Two-Dimensional Porous Organic Nanosheets for Enhanced Molecular Recognition

Despite the rapid development of molecular rotors over the past decade, it still remains a huge challenge to understand their confined behavior in ultrathin two-dimensional (2D) nanomaterials for molecular recognition. Here, we report an all-carbon, 2D π-conjugated aromatic polymer, named NUS-25, containing flexible tetraphenylethylene (TPE) units as aggregation-induced emission (AIE) molecular rotors. NUS-25 bulk powder can be easily exfoliated into micrometer-sized lamellar freestanding nanosheets with a thickness of 2-5 nm. The dynamic behavior of the TPE rotors is partially restricted through noncovalent interactions in the ultrathin 2D nanosheets, which is proved by comparative experimental studies including AIE characteristics, size-selective molecular recognition, and theoretical calculations of rotary energy barrier. Because of the partially restricted TPE rotors, NUS-25 nanosheets are highly fluorescent. This property allows NUS-25 nanosheets to be used as a chemical sensor for the specific detection of acenaphthylene among a series of polycyclic aromatic hydrocarbons (PAHs) via fluorescent quenching mechanism. Further investigations show that NUS-25 nanosheets have much higher sensitivity and selectivity than their stacked bulk powder and other similar polymers containing dynamic TPE rotors. The highly efficient molecular recognition can be attributed to the photoinduced electron transfer (PET) from NUS-25 nanosheets to acenaphthylene, which is investigated by time-resolved photoluminescence measurements (TRPL), excitation and emission spectra, and density functional theory (DFT) calculations. Our findings demonstrate that confinement of AIE molecular rotors in 2D nanomaterials can enhance the molecular recognition. We anticipate that the material design strategy demonstrated in this study will inspire the development of other ultrathin 2D nanomaterials equipped with smart molecular machines for various applications.

Process-Tracing Study on the Postassembly Modification of Highly Stable Zirconium Metal–Organic Cages

Metal-organic cages (MOCs) are discrete molecular assemblies formed by coordination bonds between metal nodes and organic ligands. The application of MOCs has been greatly limited due to their poor stability, especially in aqueous solutions. In this work, we thoroughly investigate the stability of several Zr-MOCs and reveal their excellent stability in aqueous solutions with acidic, neutral, and weak basic conditions. In addition, we present for the first time a process-tracing study on the postassembly modification of one MOC, ZrT-1-NH2, highlighting the excellent stability and versatility of Zr-MOCs as a new type of molecular platform for various applications.

Design and self-assembly of hexahedral coordination cages for cascade reactions

The search for supramolecular reactors that contain no catalytically active sites but can promote chemical transformations has received significant attention, but it remains a synthetic challenge. Here we demonstrate a strategy of incorporating bulky and electro-rich aromatic linkers into metallocages to induce cascade reactions. Two hexahedral cages with a framework formula [(Zn8L6)(OTf)16] are assembled from six tetrakis-bidentate ligands derived from tetraphenylethylene and eight zinc(II)tris(pyridylimine) centers. The cage cavities can accommodate different molecules such as anthranilamide and aromatic aldehyde through supramolecular interactions, allowing for a cascade condensation and cyclization to produce nonplanar 2,3-dihyroquinazolinones. The reaction is highly efficient with high rate enhancements (up to kcat/kuncat = 38,000) and multiple turnovers compared to the bulk reaction mixture. Control experiments and molecular simulations suggest that the acceleration is attributed to inherent strength of binding affinity for reactants and the release of products to establish catalytic turnover is due to the host−guest geometry discrepancy.Supramolecular containers are promising enzyme mimics, but they currently accommodate only a limited range of chemical transformations. Here, the authors describe coordination cages that catalyze two-component cascade reactions without relying on an external or encapsulated catalytic species.

Luminescent Metal–Organic Frameworks for the Detection and Discrimination of o-Xylene from Xylene Isomers

Differentiation of xylene isomers remains as one of the most important challenges in the chemical industry, mainly due to the similar molecular sizes and boiling points of the three xylene isomers. Fluorescence-based chemical sensors have attracted wide attention due to their high sensitivity and versatile applications. Here, we report a novel fluorescent metal-organic framework named NUS-40, which is able to selectively detect and discriminate o-xylene from other xylene isomers. Suspension of NUS-40 in o-xylene produces a distinct red shift in the fluorescence emission compared to that in either m-xylene or p-xylene. Moreover, the extent of peak shift is dependent on the concentration of o-xylene in xylene isomer mixtures, and the observed linear correlation between fluorescence intensity and o-xylene concentration is beneficial for quantitative detection. The possible mechanism of such responsive fluorescence behavior was investigated by Fourier transform infrared spectroscopy, proton nuclear magnetic resonance, and vapor sorption experiments. In addition, facile metalation of the porphyrin centers with metal ions provides an additional route to fine-tune the sensing properties.

In Situ Formation of Micropore-Rich Titanium Dioxide from Metal–Organic Framework Templates

Phase and porosity control in titanium dioxide (TiO2) is essential for the optimization of its photocatalytic activity. However, concurrent control over these two parameters remains challenging. Here, a novel metal-organic framework templating strategy is demonstrated for the preparation of highly microporous anatase TiO2. In situ encapsulation of Ti precursor in ZIF-8 cavities, followed by hydrolysis and etching, produces anatase TiO2 with a high Brunauer-Emmett-Teller surface area of 335 m2·g-1 and a micropore surface area ratio of 48%. Photocatalytic hydrogen generation catalyzed by the porous TiO2 can reach a rate of 2459 μmol·g-1·h-1. The measured photocatalytic activity is found to be positively correlated to the surface area, highlighting the importance of porosity control in heterogeneous photocatalysts.