Polymorphism And Mechanism Of Two-dimensional Self-assembled Structures Induced By Intermolecular Halogen/Hydrogen Bonding

Self-assembled monolayers(SAMs)are a versatile surface coating for applications in wetting and adhesion control,bio-compatibility,chemical resistance,and molecular recognition for sensors and nanofabrication.Studying the various types of weak interaction and mechanism has been a hot issue for scientists to construct two-dimensional ordered molecular selfassembled structures at the solid/liquid interface,and existing in them.Therefore,it is extremely important in the field of nanoscience and technology to design reasonable and unique building blocks and study the influence of their "unique characteristics" on the intermolecular interactions(mainly halogen bonds)and the final self-assembly structures.In this dissertation,three series of organic small molecules are designed and synthesized to study their molecular self-assembly on highly oriented pyrolytic graphite(HOPG)by scanning tunneling microscopy(STM).The main research work and innovative results of this dissertation are as follows:(1)We investigated the concentration-dependent self-assembly phenomenon of iodosubstituted thienophenanthrene derivatives(5,10-DITD)at the 1-octanoic acid/graphite interface was observed by STM.Chiral arrangement and transition phenomena of three twodimensional molecular assemblies mainly driven by halogen bonds were observed.At high concentration,the molecules self-assemble into a honeycomb-like chiral network with the continuous C=O···I···S halogen bond exists in each zigzag chain.At moderate concentration,the dominating C=O···I···H halogen bond which stem from the interaction of molecular and solvent,supplemented by C=O···I···S and I···O=C halogen bond form the chiral kite-like nanoarchitecture together.At low concentrations,the molecule co-adsorb with solvent molecular and form a cyclic chiral network,which is derived from the intermolecular C=O···I···S halogen bond and molecular-solvent C=O···I···H halogen bond.We can conclude that the intermolecular halogen bond and the number of co-adsorbing 1-octanic acids by molecular-solvent C=O···I···H halogen bond work together to determine the formation of selfassembled chiral structures.This research highlights the role of different types of halogen bonds in controlling supramolecular structures and constructing different chiral structures.(2)Bromine substituted benzene and naphthalene derivatives(Br-BC16 and Br-NC16)are explored about their supramolecular self-assemblies draw support from STM.These two molecules are arranged into different linear patterns stabilized by intermolecular H···Br and Br···Br bonds.Owing to the different geometry symmetry of conjugated cores along the C–Br bond,the side chain and conjugated core of Br-BC16 have the same orientation.However,it is different for Br-NC16.Its' side chains extend in different directions along the graphite lattice and the conjugated cores orientation of each Br-NC16 dimer is convers.With the electrostatic potential(ESP)maps and density functional theory(DFT)calculations,it is turn out to be a pair of intermolecular H???Br bonds between the hydrogen atoms with the positive charge and the bromine atoms with the negative charge in each dimer lead the formation of the structure.Besides,the Br???Br bond in each dimer of Br-BC16 and the Br???Br bond between the neighboring dimers for Br-NC16 stable their nanostructures respectively.The results provide a basis for understanding the self-assembly of brominated molecules via hydrogen bonds and halogen bonds at the liquid-solid interface.(3)Self-assembled monolayer of two structural isomers p-DBPy and d-DBPy are investigated on HOPG by STM.These two molecules own a diketopyrrolopyrrole(DPP)unit which relevant to the field of organic photoelectric,and the only difference of them is the position of pyridyl N-atoms.Concentration and solvent effect are existed in their self-assembly pattern.The two molecules exhibit completely different molecular conformations and packing patterns,which also demonstrates that the positional changes of the N atoms on the pyridyl group lead to different intermolecular interactions.For p-DBPy molecule,it bends change to form a banana shape due to the pyridyl N-atoms proximal to the central conjugated core.However,for d-DBPy molecule,it keeps a linear shape in the monolayers result from the pyridyl N-atoms are distal to the central conjugated core.Subtle differences in molecular chemical structure will affect the final morphology of the film significantly.Systematic research work of self-assembly about semiconductors based on DPP on the surface can provide guidance for the rational design of photovoltaic materials and control of film morphology.