Covalent organic frameworks (COFs) with redox-active units are a class of ideal materials for electrochemical-energy-storage devices. A novel two-dimensional (2D) PDC–MA–COF with redox-active triazine units was prepared via aldehyde–amine condensation reaction by using 1,4-piperazinedicarboxaldehyde (PDC) and melamine (MA) as structural units, which possessed high specific surface area (S BET = 748.2 m2 g–1), narrow pore width (1.9 nm), large pore volume (1.21 cm3 g–1), and high nitrogen content (47.87%), for pseudocapacitance application. The interlayer C–H···N hydrogen bonding can “lock” the relative distance between two adjacent layers to avoid an interlayer slip, which is more conducive to maintaining the ordered pore structure of the COF and improving a fast charge transfer between the electrode interface and triazine units. The PDC–MA–COF exhibited an excellent electrochemical performance with the highest specific capacitance of 335 F g–1 along with 19.71% accessibility of the redox-active triazine units in a three-electrode system and 94 F g–1 in a two-electrode system at 1.0 A g–1 current density. Asymmetric supercapacitor of PDC–MA–COF//AC assembled using PDC–MA–COF and activated carbon (AC) as positive and negative electrode materials, respectively, exhibited a high energy density of 29.2 W h kg–1 with a power density of 750 W kg–1. At the same time, it also showed an excellent cyclic stability and could retain 88% of the initial capacitance after 20 000 charge–discharge cycles, which was better than those of the most of the analogous materials reported previously. This study provided a new strategy for designing redox-active COFs for pseudocapacitive storage.

The enantioselective synthesis of spirocycles has long been pursued by organic chemists. Despite their unique 3D properties and presence in several natural products, the difficulty in their enantioselective synthesis makes them underrepresented in pharmaceutical libraries. Since the first pioneering reports of the enantioselective construction of spirosilanes by Tamao et al., significant effort has been devoted towards the development of new promising asymmetric methodologies. Remarkably, with the advent of organocatalysis, particularly over six years, the reported methodologies for the synthesis of spirocycles have increased exponentially. The aim of this review is to summarize the latest trends and developments in the enantioselective synthesis of spirocompounds during these last six years.

Fluorescent COFs with large π-conjugated building units and inherent rigid structure have important potential for chemosensing detection of target molecules or ions based on turn-on and turn-off modes.

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Despite increasing efforts in recent years, new methods for the enantioselective formation of chiral compounds by photochemical methods remain rare. The high energy content of an excited photosubstrate results in low activation barriers for further transformations and in rapid relaxation pathways. It is therefore intrinsically difficult to activate a photoexcited compound by a catalyst. Attempts to attain significant enantioselectivities by the use of circularly polarized light have seen little success.[1] Currently, the most frequently used method to achieve catalytic enantioselective photochemical reactions in solution relies on chiral sensitizers, which work by electron or energy transfer. [2,3] Herein we present an as yet unexplored concept for enantioselective photochemical reactions, which is based on the use of chiral Lewis acids. After some optimization we have now found a chiral cationic oxazaborolidine catalyst, which enables enantioselective intramolecular [2+2] photocycloaddition [4] reactions of 4-alkenyl-substituted coumarins. Our preliminary results are disclosed herein.In 1989, Lewis and Barancyk reported that the [2+2] photocycloaddition of 2,3-dimethyl-2-butene to coumarin (1), [5] which is inefficient in the absence of an additive, was promoted by BF 3 ·OEt 2 .[6] By employing 50 mol % of the Lewis acid, product 2 was obtained in 57 % yield upon irradiation in a Pyrex apparatus with a mercury mediumpressure lamp for five hours. Similar reactions were conducted with other alkenes and-based on photochemical and photophysical data-the enhanced reactivity of complexed over uncomplexed coumarin was attributed to its increased singlet-state lifetime and electrophilicity. The observation of catalytic Lewis acid activity in the [2+2] photocycloaddition stimulated the idea to use chiral Lewis acids to achieve an enantioselective process. In search for an optimum Lewis acid catalyst, [7] the irradiation of coumarin in the presence of 2,3-dimethyl-2-butene was performed in CH 2 Cl 2 with different Lewis acids at l = 366 nm (Scheme 1). Without a Lewis acid no reaction was observed after five hours, while 50 mol % of BF 3 ·OEt 2 gave under these conditions a yield of 33 %. The most active catalyst among the tested Lewis acids was found to be AlBr 3 , which promoted an almost complete conversion and a high yield (97 %) of the isolated product 2.Considering the possibility of an enantioselective Lewis acid promoted photochemical reaction it was assumed that Lewis acid coordination was weak and that the association/ dissociation of the coumarin substrate was relatively fast. An intramolecular [2+2] photocycloaddition consequently seemed best suited to achieve high enantioselectivity because the enantioselectivity-determining bond-formation step is more rapid than in an intermolecular reaction. 4-(Pent-4-enyl)coumarin (5) was chosen as a test substrate and was readily prepared from commercially available 4-hydroxycoumarin (3). Conversion into triflate 4 [8] was followed by a Negishi cross-coupling to give the ...

Sensitive turn-off mode fluorescence sensing for Fe3+based polyimide bond linked COFs.

lactide, tannin, tea polyphenols (TP), and many others. [5a,6] Among them, TP are of particular interest as monomers for broadening the scope of functional nanomaterials with interesting physicochemical properties and biological activities. [7] TP, also referred as catechins, represent a typical class of polyphenol mixtures directly extracted from green tea. The unique molecular structures (i.e., catechol and pyrogallol units) and biofunctions (i.e., antioxidation and metal chelation) of TP offer ideal scenarios in establishing a series of polymeric nanomaterials with virtually new possibilities for structural and functional variation. [8] For example, Li's group used Cu 2+ to mediate oxidative coupling assembly of TP molecules for the development of porous and hollow nanoparticles as controlled delivery platforms; [8a] Fei et al. reported the rapid one-pot preparation of autofluorescent TP-based core-shell nanocomposites via Ag + oxidation-inducing self-assembly process under microwave irradiation; [8b] Markova et al. employed the oxidationreduction reaction between TP and Fe 3+ to synthesize the TP-Fe nano-hybrids with significant ecotoxicological impact. [8c] Generally speaking, current approaches for the fabrication of polyphenol nanomaterials mainly involve the oxidation of TP building blocks by using foreign metal ion additives. Despite the proliferation of work via similar strategy, all those established methods have met with profound challenges yet to be resolved. Issues including additional toxicity introduced by foreign additives, as well as the spontaneous metal-catechol/pyrogallol complexation process, might hinder the formation of pure organic polyphenol nanomaterials (note that only metal-organic hybrid materials can be obtained by conventional methods). [5b] Therefore, an environmentally friendly synthetic strategy via greener processes is still highly desirable toward functional polyphenol nanoparticles (PNs) without using any foreign toxic additives.To address this issue, herein we employ theophylline, [9] another kind of biomass compound also extracted from green tea leaves, to copolymerize with TP monomers for the construction of pure organic materials in the presence of air (Scheme 1). This one-pot green method provides a facile access to functional PNs with controlled sizes and desired biofunctions. Results and Discussion Synthesis of Polyphenol NanoparticlesIt was anticipated that the TP molecules could undergo oxidation-mediated self-polymerization under alkaline conditions AntioxidantsThe constant demand for functional nanomaterials from natural biomass polymers usually requires new "green" synthetic strategies without using any foreign additives. Here, the green fabrication of a series of polyphenol nanoparticles (PNs) only from green tea extraction compounds is reported (i.e., tea polyphenols and theophylline). It is found that the nanoparticle formation process involves covalent copolymerization of monomers, as well as noncovalent self-assembly pathways. Additionally, the re...

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