Self-Assembly Polymorphism: Solvent-Responsive Two-Dimensional Morphologies of 2,7-Ditridecyloxy-9-fluorenone by Scanning Tunneling Microscopy

Abstract

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.