Covalent organic frameworks (COFs) are a novel class of porous crystalline organic materials with organic small molecule units connected by strong covalent bonds and extending in two- or three-dimension in an ordered mode. The tunability, porosity, and crystallinity have endowed covalent organic frameworks the capability of multi-faceted functionality. Introduction of fluorophores into their backbones or side-chains creates emissive covalent organic frameworks. Compared with common fluorescent organic solid materials, COFs possess several intrinsic advantages being as a type of irreplaceable fluorescence materials mainly because its highly developed pore structures can accommodate various types of guest analytes by specific or non-specific chemical bonding and non-bonding interaction. Developments in fluorescent COFs have provided opportunities to enhance sensing performance. Moreover, due to its inherent rigidified structures and fixed conformations, the intramolecular rotation, vibration, and motion occurred in common organic small molecules, and organic solid systems can be greatly inhibited. This inhibition decreases the decay of excited-state energy as heat and blocks the non-radiative quenching channel. Thus, fluorescent COFs can be designed, synthesized, and precisely tuned to exhibit optimal luminescence properties in comparison with common homogeneous dissolved organic small molecule dyes and can even compete with the currently mainstream organic solid semiconductor-based luminescence materials. This mini-review discusses the major design principle and the state-of-the-art paragon examples of fluorescent COFs and their typical applications in the detection and monitoring of some key explosive chemicals by fluorescence analysis. The challenges and the future direction of fluorescent COFs are also covered in detail in the concluding section.

Covalent organic frameworks (COFs) are highly porous and crystalline polymeric materials, constructed by covalent bonds and extending in two or threedimensions. After the discovery of the first COF materials in 2005 by Yaghi et al., COFs have experienced exciting progress and exhibitedtheirpromising potential applications invarious fields, such as gas adsorption and separation, energy storage, optoelectronics, sensing and catalysis. Because of their tunablestructures, abundant, regular and customizable pores in addition to large specific surface area, COFs can harvest ultraviolet, visible and near-infrared photons, adsorb a large amount of substrates in internal structures and initiate surface redox reactions to act as effective organic photocatalysts for water splitting, CO2 reduction, organic transformations and pollutant degradation. In this review, we will discuss COF photocatalysts for the degradation of aqueous pollutants. The state-of-the-art paragon examples in this research area will be discussed according to the different structural type of COF photocatalysts. The degradation mechanism will be emphasized. Furthermore, the future development direction, challenges required to be overcome and the perspective in this field will be summarized in the conclusion.

With their outstanding diversities in both structures and performances, newly emerging metal-organic frameworks (MOFs) materials are considered to be the most promising artificial catalysts to meet multiple challenges in the fields of energy and environment. Especially in absorption and conversion of solar energy, a variety of MOFs can be readily designed to cover and harvest the sun irradiation of ultraviolet (UV), visible and near-infrared region through tuning both organic linkers and metal nodes to create optimal photocatalytic efficiency. Nowadays, a variety of MOFs were successfully synthesized as powerful photocatalysts for important redox reactions such as water-splitting, CO2 reduction and aqueous environmental pollutants detoxification. MOFs applications in indoor-air VOCs pollutants cleaning, however, are less concerned partially because of limited diffusion of both gaseous pollutant molecules and photo-induced active species in very porous MOFs structures. In this mini-review, we focus on the major breakthroughs of MOFs as photocatalysts for the effective removal of indoor-air VOCs such as aldehydes, aromatics and short-chain alcohols. According to their nature of photoactive centers, herein MOFs photocatalysts are divided into two categories to comment, that is, MOFs with variable valence metal nodes as direct photoactive centers and MOFs with non-variable valence metal nodes but after combining other photoactive variable valence metal centers as excellent concentrated and concerted electron-transfer materials. The mechanisms and current challenges of the photocatalytic degradation of indoor-air VOC pollutants by these MOFs will be discussed as deeply as possible.

Lewis- and Brönsted-acid catalyzed 1,3-dipolar cycloaddition between azomethine ylides and unsaturated compounds is an important strategy to construct five-membered N-heterocycles. However, such catalytic route usually demands substrates with electron-withdrawing-group (EWG)...

Fluorescence sensing has exhibited its power in analytical sciences due to its sensitivity, selectivity and convenience. However, it is still challenging to further improve the sensitivity to picomolar (pM) and sub-pM level as LPRS- and SERS-enhanced electrochemical, ELISA and aptamer-based highly sensitive methods. The main obstacle lies that even if strongly absorbing fluorophores with 100% photoluminescence quantum yield, the signals it generates at pM level still cannot exceed the background noise by solvent and particle Rayleigh and Raman scattering and detector dark current and a satisfactory signal to noise ratio cannot be achieved. To overcome this issue, non-linear amplification strategies were undertaken such as enzyme catalyzed processes such as ELISA and aptamer-based fluorescence sensing. However, due to the rigorous physiological conditions enzyme required, it is highly desired to develop artificial catalytic amplification method to non-linearly enhance the weak fluorescence signals generated by the fluorophores, which responds to pM level analytes. Here, we developed a TAPA-BTD-COF, which complexes with NH3 by its C=N-H bond, and the as-formed strong interaction strengthen the C=N imine bond and fix the trans-configuration. Given the infinite extending two-dimensional network by strong covalent bond, the fixation of a single imine bond to trans-configuration leads to a domino effect rendering the C=N bond in the two-dimensional plane to be locked one by one to trans-configuration. Since trans-configuration bears stronger fluorescence than cis-configuration because of more rigidified and conjugated structures. This domino amplification effect by a single NH3 molecule drives about 10 million TAPA-BTD-COF C=N-H bond be locked due to the whole rigid two-dimensional COF structure not allowing the flexible configuration isomerization and blocked the non-radiative channel realizing unprecedented pM concentration fluorescence sensing by common chemical sensors without pursuing to biologically enzyme catalyzed amplification process such as ELISA or aptamer.