Design,Synthesis And Application Of Stimuli-responsive Covalent Organic Frameworks

Covalent organic frameworks(COFs),as a new class of crystalline porous polymers,allow crystallographically precise integration of building blocks into periodic structures.Due to their excellent crystallinity,high structural stability,high specific surface areas,diverse pore structures,and easily modified pore systems,COFs have a wide range of applications in gas adsorption and separation,energy storage,photoelectron,catalysis and other fields.Over the past decade,most of the reported COFs are still two-dimensional(2D)frameworks with eclipsed stacking structures because of their simpler synthesis.Three-dimensional(3D)COFs have received increasing attention due to their higher specific surface areas and unique pore structures.Scientists have attempted to develop diverse strategies to achieve the synthesis of COFs.However,as an advanced functional platform,the functionalization of COFs remains largely undeveloped up to now,and especially architectures with stimuli-responsive functions have been barely reported.Stimuli-responsive materials are capable of producing flexible adaptive responses by external stimuli,such as light,temperature,pH,pressure,humidity,solvent,electric field,magnetic field,etc.In recent years,these materials have attracted increasing attention due to their wide applications in oil and water separation,biosensor,drug delivery,intelligent window and various fields.Because COFs have definite structure,orderly channel and can be predesigned and adjustable,which are conducive to the stimuli-responsive material to achieve accurate and reversible control of the structure.Therefore,it is of great significance to prepare stimuli-responsive COFs for the enrichment of their structures and the expansion of their applications.Based on these considerations,we designed and synthesized three novel responsive COFs and studied their possible applications.The research content of this thesis is mainly divided into the following three parts:(1)According to literature review,diverse structural units contribute to enriching the structural types of materials,and the specificity of functional groups endow COFs with new functionality.Therefore,we designed and synthesized a new hydrazone building unit,and then prepared a series of three-dimensional hydrazone-functionalized COFs(JUC-556-[HZ]X(E))with pH response through a multi-component bottom-up method.JUC-556-[HZ]X(E)exhibited high crystallinity,good chemical stability and reversible E/Z isomerization at different pH values.Remarkably,after loading cytarabine(Ara-C)as a model drug molecule,these pH-responsive COFs showed an excellent and intelligent sustained-release effect,without distinct reduction in their drug delivery capacity after five cycles.It is worth noting that the drug release rate of JUC-556-[HZ]X(E)at pH 4.8 was nearly four-fold higher than that at pH 7.4,which can effectively improve drug targeting and reduce drug side effects.Thus,these results open a way toward designing stimuli-responsive functionalized COF materials and promote their potential application as drug carriers in the field of disease treatment.(2)One of the most popular photoresponsive organic moieties is the azobenzene group.When triggered by UV irradiation or heating,the azobenzene can undergo trans-to-cis isomerization reversibly,along with changing of its geometry and related physical and chemical properties.Since it is challenging to simultaneously crystallize,stabilize,and functionalize in the same material,the introduction of azobenzene into the COFs skeleton is not so easy.Therefore,we started from the stable two-dimensional framework and synthesized JUC-501 with light/thermal response by a mild post-modification functionalization method.The specific synthesis strategy is as follows: a COF(JUC-500)based on enol-keto tautomerism was prepared first,and then was modified with the azobenzene functional group,to successfully synthesis a stimuli-responsive COF(JUC-501).The obtained COFs possessed high crystallinity,permanent porosity and ultra high chemical stability.The reversible capture and release of dye pollutant MO was realized by using JUC-501 in the process of alternating UV and heating treatments.In addition,JUC-501 still maintained good structural and chemical stability after repeated use,which paved the way for the practical application of JUC-501 as a new stimuli-responsive material.(3)After exploring the optical response of 2D COF,we study the preparation of azobenzene functional 3D COFs,because 3D materials have unique pore structures,high specific surface areas,and excellent performances.In order to optimize functionalization of the materials,we first designed and synthesized a novel linear ligand containing azobenzene(AZO-CHO),and it can directly condensated with the corresponding tetrahedral organic units to prepare the first type of three-dimensional light-responsive COFs(3D-AZO-COFs).These obtained COFs exhibited high crystallinity,excellent chemical stability,and rapid,efficient as well as reversible photoisomerization.More importantly,the 3D-AZO-COFs displayed fully reversible photo-responsive specific surface areas and pore size distribution under the alternating treatment of different light sources.Furthermore,we also explored the influence of stimuli-responsive 3D-AZO-COFs on the gas adsorption.Under UV irradiation,the gas uptake(CO2 and H2)of fresh samples increased,which demonstrated remarkable stimuli-responsive adsorption effect,and the gas adsorption performance would undergo multiple reversible cycles with the change of external stimuli.This study not only enriches the functional development of 3D COFs,but also promotes their potential applications in intelligent adsorbents and logic gates.