Self-assembly Investigation Of Fluorene Derivatives On The Graphite Surface By Scanning Tunneling Microscopy

Fluorene derivatives are promising blue light-emitting materials. By introducing different groups in the 2-, 7-, or 9-position of fluorene or fluorenone can yield a series of derivatives. The structures of molecular self-assembly are related to the size and shape of molecules, the functional groups and intermolecular interactions. In this dissertation, we chose two different fluorene derivatives, investigated the self-assembly behavior of these molecules on the liquid/HOPG interface by scanning tunneling microscopy(STM), and discussed the effects of structural motifs of molecules, concentration of solution and solvent on the self-assembled structures. The main research work and innovative results are as follows:1. Firstly, we designed and synthesized three kinds of fluorene derivatives in which the middle tethered spacers were diverse. These three derivatives are N,N’-bis-fluoren-9- ylidene-hexane-1,6-diamine(DFHD), N,N’-bis-fluoren-9-ylidene-dodecane-1,12-diamine(DFDD) and co-crystal consisting of naphthalene-1,5-diamine and 9-fluorenone(NA-FN), respectively. We made a comparison study with the 3D crystal packing and 2D self-assembly at the saturated 1-phenyloctane/HOPG interface. The experiment results highlighted that the 2D assembly patterns had obvious relationship with their crystal alignments. And it was the competition of various weak non-covalent interactions at the liquid/HOPG interface which led to the difference between 2D self-assembly structures and 3D crystal alignments.2. Secondly, we used n-hexadecane as solvent, studied the self-assembly behavior of 2-hydroxy-7-pentadecyloxy-9-fluorenone(HPF) at the n-hexadecane/HOPG interface under different concentrations, and investigated the effect of concentrations on the chiral self-assembled structures. Then we discussed the co-adsorbed solvent effect on the emergence of chiral, the induced mechanism of racemic pattern and the thermodynamic mechanism of polymorphic structures. The phenomenon of concentration-dependence proved that the solvent molecules could not only act as dispersant, but also adjusted chiral self-assembled monolayers by subtly tunning the solvent–molecule and solvent–substrate interactions.3. Finally, we systematically studied the self-assembly behavior of HPF on the graphite surface by use aliphatic solvents with different chains length. The aliphatic solvents included n-pentadecane, n-tetradecane, n-tridecane and n-dodecane. Combined with the self-assembly results of HPF in the n-hexadecane/graphite interface, we summarized the self-assembled patterns, unit cell parameters and stacking density of HPF molecules at different aliphatic solvents/HOPG interfaces under different concentrations. We found that solvent with different chains length could affect the self-assembly formation. The closer the size of solvents to the alkoxy chain length of HPF, the more types the molecular arrangement will exhibit. In addition, the shorter the chains of solvents, the faster of the structural transformation were, and the denser the alignments would be. Our results provide an alternative method for fabricating and manipulating 2D chiral assembled monolayer by choosing appropriate solvents.