The Modulation Of Two-Dimensional Supramolecular Assemblies By Substituted Group

Organic molecules physiadsorb at liquid/solid interface, forming different assemblies, which are dependent on molecular structures, molecule-molecule interaction, molecule-substrate interaction. Compared with the studies of organic molecular assemblies at liquid/solid, the investigation of the modulation and controlling of the 2D structure is more important, and bigger challenge。In this dissertation, small substituents and different length of alkyl chains were introduced on molecular backbone, which modulated intermolecule and molecule-substrate interaction and controlled 2D structure. Based on the concept of the molecule engineering, we have designed and synthesized a series of Schiff base derivatives, and studied their structural features in two-dimensional(2D) and three-dimensional (3D) states by combining scanning tunneling microscopy (STM) and X-ray diffraction experiments. The Schiff-base derivatives with short alkyl chains crystallize easily, which allows a detailed structural analysis by X-ray diffraction. Due to the strong adsorbate-substrate interactions, those bases with long alkyl chains easily form 2D assemblies on highly oriented pyrolytic graphite (HOPG). The STM images indicate also that the introduction of two methoxy groups into the molecule can change the structure of these 2D assemblies as a result of the increased steric hindrances, for example: the Schiff-base derivative, bearing both methoxy groups and C16H33 tails, forms 2D Moire patterns, and an alignment of pairing Schiff-base molecules may be easily resolved. Conversely, the Schiff base derivative, bearing solely C16H33 tails, forms 2D non-Moire patterns. It is demonstrated that the 3D structural features result from the compromise of intermolecular interactions of different molecular moieties. However, there is one more factor, which also governs the 2D structure: the adsorbate-substrate interaction. The 3D crystal structure may thus help to understand many factors involved in the formation of 2D structures, and would be helpful for designing new molecular assemblies with tailoring functions. To further investigate the influence of steric hindrances, we studied the assemblies of N,N′-dialkyl-substituted quinacridone derivatives at liquid/solid interface. The experimental results indicate that the 2D structure can be controlled by introducing substituents and different length of alkyl chain. We also studied the 2D assemblies of the coadsorptions of quinacridone derivatives and fatty acid on HOPG observed by STM, and focused on how minor structural variations can influence the supramolecular organization of the compounds. For quinacridone derivatives with or without methyl groups only the same enantiotopic faces of the quinacridone cores are observed with high resolution depending on the length of the substituted alkyl chains. For coadsorptions of quinacridone derivatives and fatty acids at the liquid/graphite interface, we have found interestingly that quinacridone derivatives with methyl groups form chiral racemates, whereas the quinacridone derivative without methyl groups forms chiral domains. The chiral 2D structures formed at the interface are only observed when fatty acids are present as guest molecules and for quinacridone derivatives substituted with long...