Investigating The Synergistical Effect Of Alkyl Chains And Functional Groups On The Self-assemblies Of Organic Molecules

In this thesis,scanning tunneling microscopy and density functional theory calculation are used to study the self-assembly of organic functional molecules on HOPG interface.From the perspective of factors for TTF molecular self-assembly behaviors,the introduction of guest molecules on TTF molecules and the light response characteristics on the photosensitive molecules are studied.Then the single atoms substituted on the TTF molecules are also investigated.A series of research results have been obtained on the self-assembly of functional molecules and the mechanisms of molecular interactions.The details are summarized as following:The self-assembly process and interactional mechanisms of TTF molecules with different alkyl chain lengths and substituent groups were studied.It is found that benzoic acid substituted TTF molecules can be assembled into stable nanostructures on the HOPG surface no matter with the long or short alkyl chains,while pyridine substituted TTF molecules can only form stable structures with the long alkyl chains,which even have some defects.The van der Waals force,hydrogen bond and the S…S interaction between the TTF molecules play a key role in stabilizing the molecular assembly structures.In view of regulation the TTF derivatives by introducing solvent molecules and guest molecules,it is found that heptanoic acid solvent molecules do not participate in the assembly structures for benzoic acid substituted TTF molecule with short chains.However,for the assembly structure of pyridine substituted,due to the hydrogen bond between N atom in pyridine and-COOH in heptanoic acid,the interaction force is stronger than the S…S interaction between pyridine molecules,the self-assembly behaviors of both C14TTFN and C16TTFN molecules are completely different.In the regulation of C16TTFN by the introduction of guest molecules,it was found that the formation of double-layer structures between the upper and lower molecules was related to the order of the molecules.The interactions between molecules preferentially on the surface of HOPG is the main factor to construct the double/multi-layer supramolecular structures.For para-TTF,the co-assembly structures are still dependent on the preferential adsorption order of the two molecules.When azo-2 is in the lower and para-TTF is in the upper layer,the double-layer structure is relatively loose.Oppositely,a large area of dense and ordered double-layer nanostructures can be formed.According to the same strategy,we can selectively construct two-dimensional nanostructures by designing different positions of carboxyl groups on the molecule,and then realize the isomerization behaviors.For the azobenzene molecules with mono-alkyl and carboxyl group substituted,when-COOH group is at the end of the alkyl chain and the meta-position of benzene rings,it can have obvious photo-isomerization behavior,but when-COOH is at the ortho-position and para-position,it is difficult to observe the molecular isomerization process.For the azo systems substituted by dialkyl chain,the reversible photo response process is easy to occur.The photo response of azo derivatives strongly depends on the position of carboxylic groups.These studies provide the information for the photo-isomerism and the design of molecular optical switch devices.By adjusting the position and number of substitution of F atom and Br atom,the effect of halogen bond interactions on the assembly structures of TTF molecules was investigated,and the purpose of controlling the self-assembly structures with single atomic level was also realized.This research work provides a new way to explore the interactions between organic molecules,the design of molecular optoelectronic devices,and has the significance for regulating the self-assembly nanostructures and the interactional mechanisms for functional molecules.