Fluorenone derivatives (F−OC n ) with various lengths of peripheral alkyl chains (with carbon numbers of n = 12−18) were synthesized, and their self-assembled adlayers were investigated in solvents with different polarities and functionalities by scanning tunneling microscopy (STM) on a highly oriented pyrolytic graphite (HOPG) surface. The chain-length effect on the self-assembly of F− OC even was observed in 1-phenyloctane. With the shortening of the side chain, the self-assembled pattern changed from a dense-and loose-packed structure to a pliers-like structure. Self-assembly of F− OC odd showed a uniform lamellar pattern. An even−odd effect was observed resulting from the direction of the end methyl group in the alkyl chain unit. Furthermore, when the samples using dichloromethane as solvent were observed within 3 h, a less ordered lamellar structure appeared in most cases. The pliers-like pattern was observed for self-assembly of F−OC 16 and F−OC 14 . However, F− OC 17 formed a zigzag structure. Observation of the odd−even and chain-length effects on the self-assembled adlayers might provide an analytical method for examining the structural and chemical homogeneities.

The supramolecular patterns of a thienophenanthrene derivative could be switched among dissimilar polymorphs with different halogen-bond densities by solution concentration, which is demonstrated through a combination of STM and density functional theory (DFT) calculations.

Two-dimensional self-assembly of 2,7-ditridecyloxy-9-fluorenone (F-OC13) is investigated by scanning tunneling microscopy (STM) in solvents with different polarities and functional groups on a high oriented pyrolytic graphite surface. The STM images reveal that the self-assembly of F-OC13 is strongly solvent-dependent. 1-Phenyloctane can coadsorb on the self-assembly of F-OC13, and the structural transformation of the adlayer from the linear structure to alternate lamella can be observed with the decrease of the concentration. At the 1-octanol/HOPG interface, only a well-ordered linear pattern is obtained. The intermolecular hydrogen bonding between the 1-octanoic acid and the F-OC13 molecule is responsive for the formation of butterfly configuration. When n-tridecane or n-tetradecane is used as solvent, a regular alternate pattern is formed under high concentrations, and a coadsorbed lamellar structure is observed under low concentrations. Furthermore, when the sample with use of the methanol, dichloromethane, or toluene as solvent is observed within one hour, a denser-packed structure appears. After the sample is placed more than three hours, in methanol and dichloromethane, a regular alternate pattern is formed corresponding to the result using n-tridecane or n-tetradecane as a solvent under high concentration. In toluene, the alternated pattern is similar with that in 1-phenyloctane at low concentration. The solvent induced self-assembly polymorphism is discussed in terms of factors of the polarity of the F-OC13 molecule and the nature of the solvent. The results provide a new objective to fabricate and control molecular nanopatterns based on the polar group in the molecule.

The 2D self-assembly of various 2-hydroxy-7-alkoxy-9-fluorenone (HAF) molecules has been investigated by scanning tunneling microscopy (STM) at the liquid/solid interface. A systematic study revealed that HAF molecules with different numbers of carbon atoms in their alkoxy chains could 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 of intermolecular hydrogen bonds. The observed structural transition was found to be driven by molecular thermodynamics, surface diffusion, and the voltage pulse that was applied to the STM 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 the side chain, was observed by STM. The influence of the odd-even effect on the melting point has a close relationship with the molecular self-assembled pattern. Our results are significant for understanding the influence of hydrogen-bonding interactions on the dominant adsorption behavior on the surface and provide a new visual approach for observing the influence of the odd-even effect on the phase transition.

The self-assembled behaviors of two fluorenone derivatives, 2,7-bis­((11-hydroxyundecyl)­oxy)-9-fluorenone (BHUF) and 2,7-bis­((10-carboxydecyl)­oxy)-9-fluorenone (BCDF), were investigated at the liquid–solid interface by scanning tunneling microscopy. Two solvents, 1-octanoic acid and 1-phenyloctane, were employed in consideration of their distinct polarity and solubility. It is observed that the BHUF molecules self-assemble into seven different polymorphs upon adsorption, while only two different polymorphs are observed in the BCDF monolayer. The theoretical calculation is performed to reveal the underlying mechanism. As compared to that of CO···HO hydrogen bonds, the enhanced binding energy of intermolecular CO···HOOC hydrogen bonds in the BCDF monolayer would dominate the intermolecular van der Waals (vdWs) interactions and the molecule–solvent interactions, thereby resulting in a limitation of expression of structural polymorphism. In addition, the concentration-dependent polymorphism as well as the relative phase transition is discussed in terms of the stability and packing density of different polymorphs. Furthermore, the different self-assembled behaviors of BHUF molecules in these two solvents at lower concentrations are associated with the different energy gain upon solvent coadsorption. The investigation provides a simple and alternative strategy to construct the structural polymorphs by utilizing multiple hydrogen bonds at the liquid–solid interface.