| Controlling and unraveling the structural polymorphism has been received special attention in 2D self-assembled monolayers. Solvent effects have been widely studied in solution based molecular self-assembly, and it is commonly reported that different solvents can induce the formation of different polymorphs. Solubility of the solute in a particular solvent plays an important role, but also other parameters like viscosity influence self-assembly. Moreover, molecules with an innate electric dipole moment have promise to be utilized in the manipulation of energy level alignment of interfaces. Up to now, no conclusive theory exists which explains the solvent’s influence, pressing questions must be answered through basic research. These include: How does the dipole moment change upon adsorption to the surface? Can the dipole moment or polarization be manipulated through bond formation? How large of a role do electric dipole moments play in determining self-assembled structures? On considering the excellent opto-electric properties of fluorenone- based materials, besides, the carbonyl group of conjugated moieties has a strong polarity which can also form hydrogen bonds with other electronic groups, we deeply research the structural formation and transformation during the assembly process and realize the organization in a controlled way. Our results furnish the basis for revealing the synergetic mechanism of the weak interactions. This thesis aims to answer above questions through the study of polarization phenomena in single molecules and assemblies of dipolar molecules. The main research works and innovative results are as follows:(1) The two-dimensional hydrogen-bonded networks formed in 1,3,5-tris(10-carboxydecyloxy) benzene(TCDB) show regular solvent- and guest- induced supramolecular structural properties. TCDB acting as a host template can entrap solvent molecules or π-electron-conjugated guest molecules to fabricate the flexible co-adsorption architectures, which are subject to the balance between the hydrogen bonding of the host lattice and the van der Waals forces between the host and the guest molecules. Strong correlation exists between the structural parameters and the physical properties of the solvent. Statistical analysis shows that the unit cell volume of TCDB solved in nonpolar 1-phenylotane and n-tetradecane shrank significantly compared with that in polar solvents. Kinetic parameters like solvent viscosity comes into play in tuning the assembly structures. Mechanical calculations demonstrate that TCDB incline to adsorb with a larger dipole configuration in nonpolar solvents due to its dissolvability.(2) 2D supramolecular self-assembly of F-OC9 at the liquid/solid(l/s) interface was shown to depend on the polarity of solvents. When the polar acids were employed as solvents, the asymmetry environment induced the monotonous structure. With the decrease of F-OC9 concentration, coadsorption was observed attributed to the hydrogen bonding and steric constraints. On the other hand, the fast nucleation rate led to the coexistence of small domains. No concentration effect was found due to the nonpolar environment. In addition, the difference in coadsorption behavior of homologous acids was understood on the basis of dielectric variance among the alkylated acids. Thermodynamics of fluorenone derivative was rationalized in terms of synergetic dipole-dipole and vd Ws forces. Thermodynamic state at the g/s interface was the same as that at l/s interface, which indicated that the ―S‖ shaped structure was dominated by the intermolecular interaction rather than the solvent. Meanwhile, the absence of characteristic peaks and the shifts of π-π* electron transitions in UV-Vis and fluorescence spectra implied that changes in the dipole moment and nature of the solvent can cause significant changes to the excited state of substituted fluorenones.(3) The assembly of symmetric 2,7-bis(n-alkoxy)-9-fluorenone(n = 8-18) dissolved in homologous acids ranging from heptanoic to nonanoic acids was observed. Solvent-responsive 2D morphology of F–OCn(n=9-13) were obtained resulting from molecule-solvent interaction. The molecular size, shape and structure of molecular substituents were key factors in regulating the balance of the interaction and formation of self-assembled structures. Varying the assembly’s symmetries and chain lengths alters the self-assembled monolayer morphology and crystal unit cell properties in a predictable manner. Also a series of comparison in the solvent effects between the homologous alkyl acids were given. As the long side chains varied from n=14 to 18 yielding the large vd Ws interaction of intermolecular(mainly from alkyl-alkyl) and the alkyl chains-substrate, the chain length and odd-even effect appeared. Microscopic origin of the odd-even effect in monolayer of F–OCn(n=14-18) formed on a graphite surface will be considered. However, when the samples were observed more than 3h, zigzag structures showing the stablest linear lamella with dimers appeared. Systematic dipolar and vd Ws calculation indicate that concerned intermolecular interaction of the alkyl chain unit introduce the odd-even effect.(4) We investigated the steric matching and solution concentration controlled structural variety in self-assembly of 2,7-bis(n-alkoxy)-9-fluorenone(F-OCn) at the n-tetradecane and n-tridecane/graphite interface under different concentrations, respectively. Scanning tunneling microscopy revealed that the coadsorbed adlayers of F-OCn and solvents(n = 8-18) were formed and exhibited concentration dependent 2D phases due to the steric matching. The self-assembled monolayer of F-OCn(n = 8-18) evolved from a low-density coadsorbed linear lamellar packing which was formed at low concentrations to higher-density patterns at relatively high concentrations. F-OC14 exhibited complex structural variety, in which a systematic trend of decrease in the molecular density per unit cell with decreasing concentration was obtained. The zigzag structure showing the linear lamella with dimers was observed. The systematic experiments revealed that the self-assembly of F-OCn had chain-length dependence. The results provide insight into the structural variety exhibited by a series of organic molecules and furnish important guidelines to control the morphology by changing the solution concentration.(5) The global hetero- and homochiral polymorphous assemblies from an achiral fluorenone derivative were constructed with multiple intermolecular hydrogen bonds by varying concentration. Scanning tunneling microscopy investigations revealed that a heterochiral supramolecular double rosette-like structure can be fabricated via hydrogen bonding interactions with 1-octanoic acid at a low concentration. When increasing concentration, the self-assembly structure transformed from heterochiral double rosette-like structure to homochiral windmill-like pattern. Interestingly, these two metastable structures ultimately could transform into a most stable zigzag pattern at a bias voltage prompted by the STM tip. At high concentrations, only achiral octamer arrangement could be obtained owing to the changes of intermolecular hydrogen bonding, vd Ws force and dipole–dipole interactions. Coverage-dependent competition between molecule-molecule and molecule-substrate interactions plays key roles. In the low coverage regime, intermolecular interactions determined the short-range order, leading to the formation of molecular chain structures. Long-range ordering of the chains, on the other hand, is dominated by the molecule-surface interaction and leads to chain orientations commensurate with the substrate. In the high coverage regime, intermolecular interactions determine both the short- and long- range order, leading to the molecular arrangements that incommensurate with the HOPG substrate. |
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