STM Investigation And Computational Simulation Of Two-Dimensional Self-Assembled Architectures Mediated By Halogen Bonds

Halogen bond,like hydrogen bond,is a kind of electrostatic attractive interaction.Halogen bond has been widely explored in the field of three-dimensional crystal engineering,drug design and organic luminescence materials due to its high directionality and specificity.Molecular self-assembly is a process to form ordered structures via noncovalent interactions.It is of great importance in designing complicated structures and functional device to combine halogen bond with molecular self-assembly.Hence,we design and synthesize two kinds of organic molecules to study the nature of halogen bond and its binding mechanism in the aspects of two-dimensional molecular self-assembly by scanning tunneling microscopy(STM)and computational calculation.Rationally designing of molecular structure could fabricate different self-assembly patterns by different types of halogen bonds.In this dissertation,we investigated the host-guest self-assembly behavior of 5-bromosalicylic acid derivatives and 1-bromohaxedecane by STM.We employed DFT calculation and QTAIM analysis to reveal the host-guest self-assembly mechanism.We synthesized a thienophenanthrene derivative with high planarity and rigidity,and investigate its self-assembly structure at the solid/liquid interface.It is the first case to observe C–Br···? halogen bond in the field of two-dimensional molecular self-assembly.The main research work in the dissertation are summarized as follows:(1)We design and synthesize a 5-bromosalicylic acid derivative 5-BHDB.We use 1-bromohexadecane(1-BH)as guest molecule to construct host-guest self-assembly patterns by mixing host molecule(5-BHDB).At high concentration,5-BHDB/1-BH mixture powder selfassemble into double-linear structure.At low concentration,self-assembly structures of 5-BHDB/1-BH mixture powder show solvent effect.When octanoic acid is used as solvent,selfassembly pattern is dominated by double-linear structure;when 1-phenyloctane is used as solvent,self-assembly pattern is dominated by double-linear structure with the co-adsorption of 1-phenyloctane;when pentadecane or tetradecane is used as solvent,self-assembly pattern is dominated by wave-like structure;when decane is used as solvent,self-assembly pattern is dominated by single-linear structure.As the scanning time goes,wave-like and single-linear structures will transform into double-linear structure which indicates double-linear structure is the thermodynamic stable structure.By carefully analysis the STM images and construction of molecular model,we find that C–Br···O=C halogen bond formed between host and guest molecules is crucial in forming self-assembly structure and induce the structure transfer.(2)For further study the importance of halogen bond in inducing host-guest selfassembly nanostructure,we employ Gaussian 09 software package to optimize the structures and calculate binding energy of each building blocks.The results indicate that the double-linear structure has the lowest binding energy which can explain why the structural transfer from wave-like and single-linear structures to double-linear structure.We analyze each building blocks with QTAIM,and find that the charge density of C–Br···O halogen bond is the largest one which means this bond is the strongest bond.This indicates C–Br···O halogen bond is the main driven force to induce the formation of self-assembly patterns and structural transfer.Other topological parameters and two-dimensional charge density map reveal that halogen bond is a kind of closed-shell interaction.(3)We synthesize a thienophenanthrene derivative DBPTD-Nap,and investigate its selfassembly structures at the 1-octaonic acid/HOPG and hexadecane/HOPG interface,respectively.This is the first time to observe C–X···? halogen bond in the field of two-dimensional molecular self-assembly.The self-assembly patterns of DBPTD-Nap in different solvents are the same.By analyzing the linear structures,we find four kinds of dimers(N-shape and L-shape dimer).Domains are dominated by the N-shape structure,and domains are doped by single line L-shape structure.By constructing the molecular model,we found two antiparallel DBPTD-Nap molecules form a dimer due to the cooperation of C–H···Br hydrogen bond and C–Br···? halogen bond.We use DFT calculation and QTAIM analysis verify the existence of C–Br···? halogen bond and study the binding properties.