Donor-acceptor type[4+3]covalent organic frameworks:sub-stoichiometric synthesis and photocatalytic application

Three unprecedented 2 D[4+3]covalent organic frameworks(TTCOF-1,TTCOF-2,and TTCOF-3)have been prepared by substoichiometric condensation of tetratopic and tritopic monomers,overcoming the limitations of the design rules of conventional topologies.By reticulating the tetraphenylethylene(TPE)-based and triazine-based moieties into COF frameworks,novel electron donor-acceptor(D-A)type structures were obtained.These TTCOFs have good photocatalytic activity in aerobic C(sp3)–H bond functionalization and arylboronic acid oxidation driven by visible light,with yields up to 94%.This can expedite possibilities of COFs with new structural and topological complexities and can also expand the application of COF-based photocatalysis in synthetic chemistry.

Direct pore engineering of 2D imine covalent organic frameworks via sub-stoichiometric synthesis

Two-dimensional covalent organic frameworks(2D COFs)bearing various topologies can be precisely designed based on the symmetry of monomers.Their pore environment can be further regulated via either pre-functionalization of the monomers or post-modification of the skeleton.Among them,sub-stoichiometric synthesis is an efficient method which can mediate uncondensed functional moieties periodically arranged in the COF skeletons.Herein,a series of imine-linked 2D COFs with specifically decorated formyl or amino groups were selectively synthesized via sub-stoichiometric formyl transamination.The molar ratios of the monomers directly determine the structures of the resulting COFs with periodic vertex vacancies.The COF with polar amino sites could efficiently capture CO_(2) and H_(2)O.This work demonstrates tuning the stoichiometry of the monomers could facilely modulate the functions of the pore channels.It also provides insights into constructing novel COFs with complex structures for targeted applications.

Improving flexural strength of Ti_(2)C-Ti cermet by hot compressive deformation:Microstructure evolution and fracture behaviors

In order to improve the microstructure and mechanical properties,the hot compressive deformation with 50%height reduction at 1100℃was conducted on a Ti_(2)C-Ti cermet.The results showed that the lamellar Ti precipitates in Ti_(2)C grains were transformed to bimodal size distribution,which was approximately 290 nm and 5.8μm in diameter,respectively,after the hot deformation.The bimodal Ti precipitates suppressed{011}cleavage surfaces of Ti_(2)C during flexural fracture,which resulted in an 18.5%increment of strength.This phenomenon can be attributed to the bimodal Ti precipitates that decreased the average crack driving force due to their gentle variation in elastic modulus compared with the monolithic lamellar Ti precipitates.The present work can guide further deformation and mechanical property improvement of Ti_(2)C cermets.