Competition Of The Intermolecuar Weak Bonding Induced2D Self-assembled Nanostructures Based On Fluorenone Derivatives

Fluorenone derivatives are excellent organic light-emitting materials, which have beenreceived broad focus. The carbonyl group of fluorenone derivatives has a strong polarity, inwhich the dipole-dipole interactions can be produced between the groups and also can formhydrogen bonds with other electronic groups. The self-assembly research about suchmolecules has not yet received much attention. Therefore we designed and synthesized aseries of fluorenone derivatives and deeply observed the self-assembled structures on thegraphite surface. By changing the molecular structure, using different solvents, adjusting thesolution concentration, we investigated the structural formation and transformation of thefluorenone derivatives self-assembly on HPOG surfaces and realized from self-assemblyorganization to controlled array, and then revealed the relationship between the variousintermolecular weak bonds and the self-assembled structures. Meanwhile, the result fullyprovided a more powerful support for the weak bonds on the synergy mechanism. The mainresearch works and innovative results are as follows:（1） The self-assembly of2,7-bis（10-ethoxycarbonyl-decyloxy）-9-fluorenone （BEF） hasbeen investigated by scanning tunneling microscopy at the liquid/solid interface. Thecoexistence and reversible transformation of linear structure and cyclic network wereobserved. The observed structural transformation was found to be driven by voltage pulsesapplied to the STM tips. In addition, the solvent with different polarity had little effect on theself-assembly of BEF on the HOPG surface. On the basis of the comparative experiment andcalculation of the simulation results, we could conclude that for the linear structure, the vdWsforces between the molecules were more important than the dipole-dipole interactions; on thecontrary, the intermolecular dipole-dipole interactions induced the cyclic network.（2） Two-dimensional self-assembly of2,7-ditridecyloxy-9-fluorenone (F-OC₁₃) wasinvestigated by scanning tunneling microscopy （STM） in solvents with different polarities andfunctional groups on a high oriented pyrolytic graphite （HOPG） surface. The STM imagesrevealed that the self-assembly of F-OC₁₃was strongly solvent-dependent. The resultsdisplayed that1-Phenyloctane could coadsorb on the self-assembly of F-OC₁₃, and thestructural transformation of the adlayer from the linear structure to alternate lamella could be observed with the decrease of the concentration. The intermolecular hydrogen bondingbetween the1-octanoic acid and the F-OC₁₃molecule was responsive for the formation ofbutterfly configuration. At the1-octanol/HOPG interface, only a well ordered linear patternwas obtained. When n-tridecane or n-tetradecane was used as solvent, a regular alternatepattern was formed under high concentrations, and a coadsorbed lamellar structure wasobserved under low concentrations. In the volatile solvent, with the increasing time, adenser-packed structure and a regular alternate pattern were formed, respectively. The solventinduced self-assembly polymorphism is discussed in terms of factors of the polarity of theF-OC₁₃molecule and the nature of the solvent. The results provide a new objective tofabricate and control molecular nanopatterns based on the polar group in the molecule.（3） Fluorenone derivatives （F-OC_n） with various lengths of peripheral alkyl chains （withcarbon numbers of n=12-18） was synthesized, and their self-assembled adlayers wereobserved in solvents with different polarities and functionalities by scanning tunnelingmicroscopy （STM） on HOPG surface in1-phenyloctane. The chain-length effect on theself-assembly of F-OCevenwas observed. With the decreasing of the chain-length, theself-assembled pattern changed from a dense-and-loose packed pattern to a clip-like structure.The self-assembly of F-OCoddshowed a uniform lamellar pattern. F-OC₁₄formed differentmorphologies on HOPG surface in different solvents due to the solvophilic effect andcoadsorption. In most cases, a mixed linear structure was observed. The self-assembledmorphologies at the interfaces are determined by various parameters, such as thedipole–dipole interaction between the fluorenone groups, the van der Waals interaction ofinterchains and side chain–graphite lattice, and molecule–solvent interaction.（4） The2D self-assembly of various2-hydroxy-7-alkoxy-9-fluorenone（HAF） moleculeshas been investigated by scanning tunneling microscopy（STM） in1-Phenyloctane. Asystematic study revealed that HAF molecules with different numbers of carbon atoms intheir alkoxy chainscould form two or three different kinds of nanostructures, that is,less-ordered, flower-like, and zig-zag patterns, owing to the formation of different types ofintermolecular hydrogen bonds. The observed structural transition was found to be driven bymolecular thermodynamics, surface diffusion, and the voltage pulse that was applied to theSTM tip. The zig-zag pattern was the most stable of these configurations. An odd–even effect on the flower-like structure, as induced by the odd and even number of carbon atoms in theside chain, was observed by STM. Our results are significant for understanding the influenceof hydrogen bonding interactions on the dominant adsorption behavior on the surface andprovide experimental and theoretical basis for observing the influence of the odd–even effecton the phase transition.（5） We investigated the emergence of chirality in2D supramolecular networks formed byachiral molecule of the2-hydroxy-7-pentadecyloxy-9-fluorenone （OH-F-OC15） in theinterface of an achiral solvent of1-octanoic acid at different concentrations. At lowconcentrations, the double-rosette pattern was observed with the double chirality resultingfrom the hydrogen bonding. At middle concentrations, owing to the increasing solubility ofthe solvent, OH-F-OC15molecules could form a chiral tetramer pattern. In addition, thedouble-rosette pattern and tetramer structures only existed for a period of time, which couldswitch to a zigzag structure finally, due to the directionality and cooperativity of hydrogenbonding. In high concentrations, OH-F-OC15molecules could form an octamer structure andthe structural transition was not obtained. Particularly, no chiral structure was observed. Thesolvent mediated chiral induction provides a simple, efficient, and versatile approach for thefabrication of homochiral surfaces using achiral building blocks.