Fabrication And Regulation Of Two-dimensional Molecular Assembly Via Hydrogen Bond And Halogen Bond On Solid Surface:An STM Study

Supramolecular self-assembly is of the utmost importance in bottom-up strategies to fabricate functionalized nanostructure and advanced materials.In this context,the study of molecular self-assembly process reveals the intermolecular interactions between functional groups,which is of great importance to the design and fabrication of supramolecular self-assembly.Hydrogen bonds and halogen bonds have been extensively studied in supramolecular self-assembly and crystal engineering for their relatively high strength and good directionality among non-covalent interactions.As a promising surface imaging tool at the atomic and molecular level,scanning tunneling microscopy(STM)has been widely used as a suitable external stimulant(Electric field,Tip manipulation,etc.)to regulate the interfacial nanostructures and the assembly process.This thesis embarks on the STM investigation of surface 2D molecular assembly based on hydrogen bonds and halogen bonds as well as the effects of external electric fields on these molecular assembly process at the molecular level.The main contents of this thesis are as follows:(1)The formation of 2D supramolecular cocrystals bearing carboxylic acid-pyridine heterosynthon and their response to an oriented external electric field(OEEF)at the liquid/solid interface is investigated by using STM and high-resolution atomic force microscope(AFM).It is demonstrated that 2D cocrystals are obtained on the surface in the absence of an external electric field.These 2D cocrystals are responsive to the OEEF.They are stable at negative bias voltage in STM but collapse at positive bias voltage.The electric filed-responsive phase transition of the 2D cocrystals was revealed by in situ STM.It is found that the response of the 2D cocrystals is very fast and the phase transitionaccomplishes almost simultaneously when switching the polarity of the bias voltage.As for the reason for the response of the 2D cocrystals,it is suggested to be attributed to the effect of the electric field on the robustness of the carboxylic acid-pyridine heterosynthon.This work not only provides another supramolecular system that responds to the electric field but also paves a way to engineer the supramolecular cocrystals through the supramolecular synthon approach.(2)The effect of solvents(Alkyl acids with different chain lengths)on the molecular assembled structures stabilized by carboxylic acid-pyridine hydrogen bonds as well as their response to external electric field is systematically investigated.The solvent molecule act as a dispersant(nonanoic acid)or as a co-adsorbent(hexanoic acid,octanoic acid)in the BIC assembly process depending on the length of the solvent alkyl chain.The molecular assembly structures and the kinetic processes of electric field-induced phase transformation were revealed by in-situ STM.The results demonstrate the assembled structure based on the carboxylic acid-carboxyl hydrogen bond is the kinetically favored structure at positive bias,whereas the 2D cocrystal based on the carboxylic acid-pyridine hydrogen bond is the thermodynamically favored structure.This work provides an in-depth understanding of the role of solvents in the molecular self-assembly and electric-field-induced phase transformation assembly process.(3)Molecular assembly based on C-I···~―O-N~+halogen bonds is studied on 2D solid surface.The tridentate molecule 1,3,5-trifluoro-2,4,6-triiodobenzene and charged molecule4,4’-bipyridine,1,1’-dioxide are employed as the halogen bond donor and acceptor respectively.It is found that C-I···~―O-N~+halogen bonds are formed between the halogen bond donor and acceptor,which gives birth to 2D bicomponent molecular assembly on the surface.The role of halogen bonds and their cooperation and competition with other weak interactions such as hydrogen bonds in the molecular assembly process was revealed at molecular level.On the basis of this result,several porous structures based on C-I···~―O-N~+halogen bond were fabricated on the surface through rational design of the molecular structures.This work firstly identifies the C-I···~―O-N~+halogen bond during the molecular assembly on solid surface,providing a new tool for the fabrication of surface functional molecular nanostructures.