Fluorescent Covalent Organic Frameworks: A Promising Material Platform for Explosive Sensing.

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.

Correction: A powerful azomethine ylide route mediated by TiO2 photocatalysis for the preparation of polysubstituted imidazolidines.

Correction for 'A powerful azomethine ylide route mediated by TiO2 photocatalysis for the preparation of polysubstituted imidazolidines' by Anan Liu et al., Org. Biomol. Chem., 2021, 19, 2192-2197, DOI: .

A powerful azomethine ylide route mediated by TiO2 photocatalysis for the preparation of polysubstituted imidazolidines.

Lewis- and Brønsted-acid catalyzed 1,3-dipolar cycloaddition between azomethine ylides and unsaturated compounds is an important strategy to construct five-membered N-heterocycles. However, such a catalytic route usually demands substrates with an electron-withdrawing group (EWG) to facilitate the reactivity. Herein, we report a TiO2 photocatalysis strategy that can conveniently prepare five-membered N-heterocyclic imidazolidines from a common imine (N-benzylidenebenzylamine) and alcohols along the route of 1,3-dipolaron azomethine ylide but without pre-installed EWG substituents on the substrates. Our EPR results uncovered the previously unknown mutual interdependence between an azomethine ylide and TiO2 photo-induced hvb+/ecb- pair. This transformation exhibited a broad scope with 21 successful examples and could be scaled up to the gram level.

Covalent Organic Frameworks: A Promising Materials Platform for Photocatalytic CO2 Reductions.

Covalent organic frameworks (COFs) are a kind of porous crystalline polymeric material. They are constructed by organic module units connected with strong covalent bonds extending in two or three dimensions. COFs possess the advantages of low-density, large specific surface area, high thermal stability, developed pore-structure, long-range order, good crystallinity, and the excellent tunability of the monomer units and the linking reticular chemistry. These features endowed COFs with the ability to be applied in a plethora of applications, ranging from adsorption and separation, sensing, catalysis, optoelectronics, energy storage, mass transport, etc. In this paper, we will review the recent progress of COFs materials applied in photocatalytic CO2 reduction. The state-of-the-art paragon examples and the current challenges will be discussed in detail. The future direction in this research field will be finally outlooked.

Carbon Gels-Modified TiO2: Promising Materials for Photocatalysis Applications.

Carbon gels are a kind of porous organic polymer, which play pivotal roles in electrode, supercapacitor, hydrogen storage, and catalysis. Carbon gels are commonly prepared by the condensation of resorcinol and formaldehyde. The as-prepared polymers are further aged and sintered at a high temperature in an inert atmosphere to form cross-linked and intertwined porous structures. Owing to its large specific area and narrow pore size distribution, this kind of material is very appropriate for mass transfer, substrate absorption, and product desorption from the pores. In recent years, carbon gels have been discovered to function as effective hybrid materials with TiO2 for photocatalytic applications. They could act as efficient deep-traps for photo-induced holes, which decreases the recombination probability of photo-induced carriers and lengthens their lifetime. In this mini-review, we will discuss the state-of-the-art paragon examples of carbon gels/TiO2 composite materials applied in photo(electro)catalysis. The major challenges and gaps of its application in this field will also be emphasized.