Showing papers by "Wenli Deng published in 2015"

Concentration-dependent structure and structural transition from chirality to nonchirality at the liquid–solid interface by coassembly

Abstract: Understanding the formation and structural transition of the two-dimensional chirality of self-assembly is a subject which still gains significant interest in surface or interface chirality studies. Here, we present the solvent-induced chiral structural transition of a 2-hydroxy-7-pentadecyloxy-9-fluorenone (HPF) molecules’ self-assembled adlayer through coassembly with achiral aliphatic solvents under different concentrations. Polymorphic chiral patterns are obtained at low concentrations of aliphatic solvents with different chain lengths. The HPF molecules form coassembled structures with these solvents through van der Waals interactions. At the same time, at high concentrations, HPF molecules uniformly form a nonchiral multimer structure without coadsorbed aliphatic solvent molecules. What is interesting is that these structures under different concentrations will finally change into a zigzag structure, which is the thermodynamically most stable configuration. Especially when using n-hexadecane as the solvent, the adlayer shows perfect steric matching due to the close chain length of HPF and n-hexadecane, which can maximize the molecule–solvent interactions. Thus, HPF molecules in n-hexadecane exhibit the most diversiform configuration. The distinct concentration-dependence has proven that the solvent molecules can act as a coadsorbed component through van der Waals interactions rather than simply a dispersant and further result in the probability and stability of chiral self-assembled monolayers by subtle tuning of the solvent–molecule and solvent–substrate interactions. This result provides a simple and alternative strategy to construct the 2D chiral assembled monolayer.

Chiral Transition of the Supramolecular Assembly by Concentration Modulation at the Liquid/Solid Interface

Abstract: Global hetero- and homochiral polymorphous assemblies from an achiral fluorenone derivative were successfully constructed with multiple intermolecular hydrogen bonds by concentration modulation. Scanning tunneling microscopy investigations reveal that a heterochiral supramolecular double rosette-like structure was fabricated for the first time via hydrogen bond interactions with achiral 1-octanoic acid under low concentrations. When the solution concentration was increased, the structural transition from a heterochiral double rosette-like structure to a homochiral windmill-like pattern was observed. Interestingly, these two metastable structures ultimately could transform into a stable zigzag pattern at a bias voltage prompted by the STM tip. At high concentrations, only an achiral octamer arrangement could be obtained, owing to the changes of intermolecular hydrogen bonding, van der Waals force, and dipole–dipole interactions. The present results provided an important impetus for the induction and contro...

Self-assembly polymorphism of 2,7-bis-nonyloxy-9-fluorenone: solvent induced the diversity of intermolecular dipole-dipole interactions.

Abstract: In this present work, a scanning tunneling microscope (STM) operated under ambient conditions was utilized to probe the self-assembly behavior of 2,7-bis-nonyloxy-9-fluorenone (F–OC9) at the liquid–solid (l/s) interface. On the highly oriented pyrolytic graphite (HOPG) surface, two-dimensional (2D) polymorphism with diversity of intermolecular dipole interactions induced by solvent was found. Solvents ranged from hydrophilic solvating properties with high polarity, such as viscous alkylated acids, to nonpolar alkylated aromatics and alkanes. 1-Octanol and dichloromethane were used to detect the assembly of F–OC9 at the gas–solid (g/s) interface. The opto-electronic properties of F–OC9 were determined by UV-vis and fluorescence spectroscopy in solution. Our results showed that there were tremendous solvent-dependent self-assemblies in 2D ordering for the surface-confined target molecules. When a homologous series of alkanoic acids ranging from heptanoic to nonanoic acid were employed as solvents, the self-assembled monolayer evolved from low-density coadsorbed linear lamellae to a semi-circle-like pattern at relatively high concentrations, which was proven to be the thermodynamic state as it was the sole phase observed at the g/s interface after the evaporation of solvent. Moreover, by increasing the chain length of the alkylated acids, the weight of the carboxylic group, also being the group responsible for the dielectric properties, diminished from heptanoic to nonanoic acid, which could make the easier/earlier appearance of a linear coadsorption effect. However, this was not the case for nonpolar 1-phenyloctane and n-tetradecane: no concentration effect was detected. It showed a strong tendency to aggregate to generate coexistence of separate domains of different phases due to the fast nucleation sites. Furthermore, thermodynamic calculations indicated that the stable structural coexistence of the fluorenone derivative was attributed to synergistic intermolecular dipole–dipole and van der Waals (vdWs) forces at l/s interface. It is believed that the results are of significance to the fields of solvent induced polymorphism assembly and surface science.

Side chain position, length and odd/even effects on the 2D self-assembly of mono-substituted anthraquinone derivatives at the liquid/solid interface

Abstract: The formation of self-assembled adlayers of 1-hydroxyanthraquinone (1-HA) and 2-hydroxyanthraquinone (2-HA) derivatives with various side chain length were investigated using scanning tunneling microscopy for the purpose of determining the influence of chemical structure on 2D molecular arrangement in a self-assembly process. Different structures labeled as Linear I, Linear II, Linear III, Linear IV and Z-like were presented based on their packing modes. Weak O⋯H–C hydrogen bonds existing between adjacent anthraquinone moieties are the key forces driving the formation of ribbon A, A′, B and C, which are the basic rows of the self-assembled structures. The emergence of odd or even numbers of carbon atoms in the alkyl chain inducing structural diversity is an indication that one of the driving forces for 1-HA-OCn (n = 15, 16) and 2-HA-OCn (n = 12, 14–16) molecules to assemble into ordered 2D nanostructures is the van der Waals interactions between interdigitated alkyl chains. 1-HA-OC16 and 1-HA-OC15 exhibited lamellar structures packed in Linear I and Linear II fashions. 2-HA-OC15 and 2-HA-OC16 adopted Linear III structures and Z-like packing modes. Moreover, when the number of carbon atoms in the side chain of 2-HA-OCn molecules was decreased to 12, the self-assembled pattern could present a Linear IV phase. Notably, 2-HA-OC14 showed the coexistence of Z-like and Linear IV phases. Systematic experiments revealed that a better understanding of the alkyl chain position, length and odd/even effects on 2D self-assembly would shed light on better control of assembly patterns and the design of new molecular materials.