| Surface-confined dynamic covalent reactions are an effective way to construct covalently linked two-dimensional nanostructures.Fully understanding the controlling factors on the dynamic covalent reactions is very important for the construction of long-range ordered covalent nanostructures.So far,the researches in this area mainly focused on exploiting new surface reactions to synthesize structurally stable surface covalent organic frameworks(COF)in a well-defined arrangement.However,few examples concerning the functionalization of surface COF have been reported so far.In this thesis we mainly investigate the reaction process of surface-confined olefin metathesis,Schiff-base coupling,boronic acid self-condensation and boronate esterification reaction by scanning tunneling microscope(STM)on the highly oriented pyrolytic graphite(HOPG)surface,and further deeply explore the functionalization of surface COF materials based on these reversible reactions.The surface-confined nanostructures,linked by carbon-carbon bonds rather than reversible covalent bonds such as imine bonds and B-O bonds,have higher structural stability.Olefin metathesis forms carbon-carbon double bonds in a reversible way.Submolecularly resolved STM provides detailed insights into olefin metathesis at the interface by using conjugated diene monomers 1,3-divinyl-6-decyl-benzene(M1,R =C10H21)and 1,3-divinyl-6-decyl-benzoate(M2,R = COOC10H21)as precursors.The surface-confined olefin metathesis was performed on the HOPG surface.The effects of reaction temperature,monomer substituents and environmental pressure on the distribution of products of surface-confined olefin metathesis were revealed by STM under atmospheric pressure.The direct visualization of the presence of linear oligomers and macrocycles on the surface,for the first time,provides the unambiguous visual evidence for the previously proposed mechanism,which involves initial formation of oligomers with low molecular weight,followed by chain growth and cyclization at elevated temperature to eventually form thermodynamically stable cyclohexamers when in equilibrium.Surface confinement dramatically decreases the translational and rotational freedom of the adsorbate,thus significantly altering the product distribution of a dynamic system.Besides,due to the selective adsorption,surface can behave as an amplifier which promotes the generation of specific intermediate or product,such as dimers.The results indicate that olefin metathesis can take place under mild conditions in the presence of Grubbs' 2nd generation catalyst,which may provide a new path for the preparation of surface covalent nanostructures by designing the monomers in a reasonable way.Olefin metathesis yields thermodynamically stable macrocycles as the target products in equilibrium,coexisting with various linear oligomeric or polymeric by-products atthe same time.It is challenging to separate macrocycles from these linear by-products through conventional separation approaches due to their high similarity in polarity and structure.Therefore,precise design of the pore diameter allows us to maximize the interaction of macrocycles with the Schiff-base surface COF,through forming host-guest architectures the adsorption preference of macrocycles and linear by-products can be completely reversed.Subsequently,the linear by-products can be readily eliminated by cleaning the surface with an appropriate solvent,hence allowing us to further purify the macrocycles.The Schiff-base surface COF was successfully obtained by co-condensation of linear 4,4'-azodianiline(DA)and triangular benzene-1,3,5-tricarbaldehyde(BTA)at room temperature.On-surface purification results show that two-dimensional Schiff-base COF,as a host template,can effectively immobilize macrocycles,and remove linear polymers simply by washing with solvent.An important advantage of this on-surface purification with host-guest architecture is that the separating experiment can be performed step by step in a controlled way.Moreover,this purification strategy can be scale up,which has been proven by using powder graphite instead of HOPG.Schiff-base surface COF with azobenzene group was obtained at the octanoic acid/HOPG interface.However,STM characterization indicates that surface COF exhibits no isomerization behavior possibly due to the self-healing effect of Schiff-base reaction at the solid/liquid interface,which inhibits the photoisomerization of the surface COF.In order to verify that the self-healing effect at the octanoic acid/HOPG interface inhibits the photoisomerization behavior of surface COF,and further enriches the types of photoresponsive surface COF,a new surface COF functionalized with azobenzene group was explored by surface-confined esterification between 4,4'-phenylazobenzoyl diboronic acid(ABBA)and 2,3,6,7,10,11-hexahydroxytriphenylene(HHTP)at the octanoic acid/HOPG interface at room temperature.An important advantage of this preparation method is that it can completely exclude the homocoupling between ABBAs.STM characterizations reveal that both monomer concentration and preferential adsorption of HHTP on the HOPG surface play important roles in the formation of surface COF.The structural evolution from multi-wall porous networks to single-wall hexagonal surface COF was observed by delicate control of monomer concentration.Finally,a regular surface COF was successfully obtained at low monomer concentration.The dynamic self-healing process was demonstrated by in-situ STM observation and manipulation of voltage pulse at the octanoic acid/HOPG interface.The 1H NMR results further confirm the formation of boronic ester bonds under room temperature,and octanoic acid plays an important role in facilitating the boronic esterification reaction.However,the prepared surface COF is not suitable for further studying its photoisomerization behavior due to the structural defects.In order to prepare photoresponsive surface COF,by delicate control of the monomerconcentration,annealing temperature and time,a regular and extended surface-confined photoresponsive single-layer surface COF was prepared based on self-condensation of4,4'-phenylazobenzoyl diboronic acid(ABBA)with azobenzene group in the backbone as precursor at the solid/gas interface.The results indicate that the isomerization from trans to cis can take place after continuous UV irradiation,which gives rise to the destruction of the surface COF.Furthermore,the broken surface COF can recover again with further annealing treatment.This photoresponsive decomposition provides a feasible way for the controlled capture and release of guest molecules by using these periodic nanoporous surface COF as a host template,which has been proven by using copper phthalocyanine(Cu Pc)as a model guest.Therefore,light-induced reversible isomerization,and host-guest chemistry may lead to the development of novel materials,which is potentially applicable in the field of drug delivery and smart surfaces. |
PDF Full Text Request