STM Investigation Of Self-assembled Nanostructures Controlled By Intermolecular Hydrogen And Halogen Bonds At The Solid-liquid Interface

Hydrogen bond and halogen bond have a wide range of in-depth studies in three-dimensional crystal engineering,organic semiconductor devices and supramolecular self-assembly,due to their similar directionality and intensity.The supramolecular self-assembly is the process of forming the ordered structure through various non-covalent interactions between the functional molecular primitives.Therefore,designing reasonable function groups and exploring tthe microscopic intermolecular interaction between the functional units has important significance.In this paper,the optimized conformation,self-assembled mechanism of intermolecular interaction binding sites,and its effect on the film morphology of the pyrrolopyrrolidone-thiophene derivative(TDPP-C16)with electron donor and acceptor units have been investigated by scanning tunneling microscopy(STM)and atomic force microscopy(AFM).In addition,we synthesized the salicylic acid derivatives(5-BHBA and 5-BHDB)with bifunctional system(halogen bonding,hydrogen bonding)with flexible chain.The construction and regulation of two-dimensional self-assembly of the molecular system at the liquid/solid interface have been studied by using STM.The main research work and innovative results of the dissertation are summarized as follows:(1)We studied the self-assembled structure of TDPP-C16 molecule in different solvents(1-phenyloctane,1-octanoic acid and n-hexadecane)by means of STM and density functional theory(DFT).It was found that the self-assembly of TDPP-C16 in 1-octanoic acid are the same as in the 1-phenyloctane and n-hexadecane solvent.By further consideration,we conclude that the self-assembled architecture of TDPP-C16 with pure chiral(R-type,S-type)is driven by the hydrogen bonding of the H…O=C.At the same time,the carbon chains in different strips are interdigitated and form the zigzag self-assembly in close packing principle.In addition,we also observed the surface morphology of TDPP-C16 molecules by AFM,and it also testified strong intermolecular interaction.Both the experimental results of STM and DFT calculations show that the formation of the hydrogen bonding by the carbonyl(O=C)in the DPP unit and the hydrogen atom on the thiophene ring.This innovative work that ensuring the molecular interaction of TDPP derivatives not only provides a new scientific experimental basis and methods in crystal engineering and in the field of organic semiconductor devices,but also helps us develop potential organic molecular materials with specific function groups.(2)The self-assembly of bifunctional system(5-bromo-2-hexadecyloxy-benzoic methyl acid,5-BHBA)has been investigated by STM in two different solvents(1-octanoic acid/n-hexadecane).We have found that by adjusting the concentration of 1-octanoic acid and n-hexadecane solution,different self-assembled structures can be obtained due to the cooperative and competitive intermolecular halogen and hydrogen bonding,such as Br-Br,triangular C=O…Br…H,–OH…Br,H…C=O,and –COOH…HOOC– interactions.Furthermore,two kinds of self-assembled linear patterns can be observed due to the coadsorption,in which the dimers are dominated by intermolecular –COOH…HOOC– hydrogen bonds as well as molecule–solvent,and solvent–solvent van der Waals force.The result shows provides an innovative insight to induce the 2D self-assembled nanostructures by halogen and hydrogen bonds at the liquid-solid interface.(3)A molecule(5-BHDB)with a halogen(–Br)atom and ester functional group was designed and synthesized.The results by Gaussian View simulation show that the most negative charge region of 5-BHDB locates on the ester group.The self-assemblies of 5-BHDB adlayer in solvent such as 1-phenyloctane,n-pentadecane,n-tetradecane and n-decane have been studied by using STM.Moreover,the surface morphology has been studied by using scanning electron microscopy(SEM).The experimental results show that the self-assembly of 5-BHDB adlayer forms linear two-row structure in the above-mentioned solvents at high concentrations.In addition,three kinds of nanoarchitectures composed of dimers,trimers,tetramers are formed due to the attraction C=O…Br halogen bonds between the positive potential at the top of bromine atom(so called "?-hole")and the negative potential of the carbonyl group.By further analysing of the morphology of 5-BHDB,we believe that the self-assemblied conformational types of 5-BHDB in different solvents may be related to the strong intermolecular halogen bonding.