Preparation And Bias Regulation Of 2D Molecular Networks At The Solid-Liquid Interface

With increasing interest in stimulus responsive molecular materials in recent years,the phenomenon that molecules response to external stimuli in a predictable manner on the surface has attracted extensive attention.Switchable surfaces have potential utility in biosensing,intelligent nanoelectronics and optics,nanomedicine,etc.The bias voltage applied between a scanning tunneling microscopy（STM）tip and the sample surface can be employed as external stimuli to steer local phase transition in a molecular assembly on surface.In this thesis,we successfully constructed three stimulus responsive 2D networks based on the co-assembly of carboxylic acids,condensation of boronic acid and boronic acid esterification,respectively,and we studied the stimulus responsive behavior of 2D networks.Further,we combined STM,XPS（X-ray photoelectron spectra）,NMR（nuclear magnetic resonance spectroscopy）and theoretical calculations to characterize the structure of 2D networks and process of phase transition.Due to the high directionality and selectivity,hydrogen bonds is widely explored for the construction of cocrystal nanoarchitectures,and co-crystallization from building blocks with distinct symmetries will enable the construction of more complex assembling structures.Firstly,we studied the co-crystallization and bias inducedmanipulationof4,4′,4′′,4′′′-（1,4-phenylenebis（pyridine-6,2,4-triyl））tetrabenzoic acid（PBPTTBA）with two-fold symmetry and 1,3,5-tris（4-carboxyphenyl）benzene（BTB）with three-fold symmetry.We found that when a solution containing a mixture of the two molecules in an appropriate stoichiometry was deposited onto the HOPG surface,two co-crystal,supramolecular porous networks were obtained:a wide and narrow alternate network structure with two-fold symmetry and a flower-like architecture with three-fold symmetry.The formation of a three-fold symmetric cocrystal is interesting,because normally,the symmetry of the cocrystal is controlled by the building block with lower symmetry.Careful inspection of the high resolution STM image reveals that the three-fold cocrystal is sustained by unoptimized R₂²（8）hydrogen bonds,which highlights the flexibility of the hydrogen bonds.Besides,thanks to the bias sensitive characteristic of BTB,we also revealed an electric field-induced switching behavior between the bicomponent structure and the preferential adsorption of BTB.Next,we continued to explored the co-assembly of three C_2Vmolecules that have only one axis of symmetry with BTB at the octanoic acid/HOPG interface respectively.Herein,the three molecules selected are isophthalic acid（ISA）,[1,1’,3’,1’’-terphenyl]-4,4’’-dicarboxylicacid（TPDA）and biphenyl-3,4′,5-tricarboxylic acid（H₃BHTC）,respectively.The deposition of saturated solutions of ISA and H₃BHTC on HOPG resulting closely-packed structures,and there will be a guest induced structure change after the addition of coronene（COR）molecules for both ISA and H₃BHTC.Due to different structures,three different results were obtained when ISA,TPDA and H₃BHTC were mixed with BTB respectively:two kinds of cocrystals（cocrystal A and cocrystal B）were obtained from co-assembly of ISA and BTB,and two kinds of co-assembly structures were obtained from co-assembly of TPDA and BTB.However,when H₃BHTC and BTB were mixed,only phase separation was observed,independent with the mole ratio.Cocrystal A not only had a good responsiveness to the polarity of bias,but also showed a structural transformation behavior induced by guest molecules.Therefore,we realized the construction of 2D networks in response to multiple external stimuli through co-assembly.The two studies above about electric-field-induced phase transitions are based on noncovalent hydrogen bond stabilized supramolecular networks.Both theory and experiments have proven that an oriented external electric fields can facilitate bond formation and bond cleavage,therefore,the large（ca.10⁹V/m）,highly localized electric field generated between the STM tip and the surface also has the potential to regulate surface-confined dynamic covalent networks.Herein,BPDA（biphenyldiboronic acid）were used as the starting molecule,by reversing the direction of the electric field between the STM tip and HOPG,we can locally control the polymerization/depolymerization of BPDA at a liquid/solid interface.Consequently,the reversible transformation between self-assembled molecular networks（SAMs）and boroxine-linked covalent organic frameworks（COFs）can be monitored at the molecular level.What’s more,different from the previous reports that the polymerization of boronic acid requires either stimuli of thermal treatment or the strong,oriented electric field between the STM tip and the surface,we observed a spontaneous formation of well-ordered COFs even at room temperature.Combinined with the characterization of XPS and the comparison of boronic acid condensation reaction rates under different conditions,we demonstrated that the negative sample bias can accelerate the self-condensation reaction of BPDA,but it is not a prerequisite for the formation of COFs.After confirmed the bias-dependent structure transformation characteristic of BPDA,we further studied the boronic acid esterification reaction between BPDA and twopolyhydroxyphenols:5’’-（3’,4’-dihydroxy-[1,1’-biphenyl]-4-yl）-[1,1’:4’,1’’:3’’,1’’’:4’’’,1’’’’-quinquephenyl]-,3’’’’,4,4’’’’-tetraol（HPQT）and 2,3-dihydroxyterephthalic acid（DHTA）,and investigated the effect of boronic acid esterification products on the response behavior of local electric field.When the solution of BPDA and HPQT was dropped onto the HOPG surface,three kinds of products at room temperature were observed:the first generation covalent STs（ST-1）,some oligomers and trapezoidal polymers.When the bias is changed from negative to positive,the polymer structure is destroyed and the self-assembled structure of BPDA is obtained,and the reverse process can also be realized when we switch back the bias polarity to positive.While the boronic ester trimer that formed from the condensation of BPDA and DHTA assembled into a dense lamellar structure through hydrogen bonding between carboxyl group,and showed no response to the polarity of bias.These results illustrated that the bias response of covalent nanostructures is not only related to the properties of covalent bonds,but also to the stability of the assembled structure.