Packing of Br(CH2)10COOH and Br(CH2)11COOH on Graphite: An Odd−Even Length Effect Observed by Scanning Tunneling Microscopy

STM investigation of structural isomers: alkyl chain position induced self-assembly at the liquid/solid interface

Investigating and regulating the self-assembly structure is of great importance in 2D crystal engineering and it is also gaining significant interest in surface studies. In this work, we systematically explored the variation of self-assembled patterns induced by the changeable side chain position. Moreover, molecules with different alkyl chain lengths (n = 15, 16) were also synthesized and probed for the purpose of understanding how an odd/even number of carbon atoms in the peripheral chains can affect the molecular adlayers. Structural isomers of bis-substituted anthraquinone derivatives 1,8-A-2OCn, 2,6-A-2OCn, 1,4-A-2OCn and 1,5-A-2OCn (n = 15, 16) were used and investigated by STM. 1,8-A-2OC16 and 1,8-A-2OC15 molecules adopted Z-like I and Linear I structures, respectively. 2,6-A-2OC16 and 2,6-A-2OC15 molecules were severally arranged in Linear II and Linear III configurations. 1,4-A-2OCn (n = 15, 16) molecules were staggered in a Z-like II fashion and 1,5-A-2OCn (n = 15, 16) molecules displayed a Linear IV nanostructure. Therefore, we arrive at a conclusion that self-assembly structures of anthraquinone isomers are chain-position-dependent, and designing isomeric compounds can be taken into consideration in regulating assembled structures. Besides, 2D nanopatterns of 1,8-A-2OCn and 2,6-A-2OCn can be regulated by the odd/even property of the side chains, but this is not the case for 1,4-A-2OCn and 1,5-A-2OCn, ascribed to the difference in driving forces for them. It is believed that the results are of significance to the alkyl chain position induced assembly configurations and surface research studies of structural isomers.

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

This work provides efficient methods for regulating self-assembled structures by changing the position, length and odd/even properties of the side chains.

In situ STM study of potential-driven transitions in the film of a cationic surfactant adsorbed on a Au(111) electrode surface

Electrochemical scanning tunneling microscopy (EC-STM) has been employed to study the structure of a film formed by cationic surfactant N-decyl-N,N,N-trimethylammonium triflate (DeTATf) adsorbed on the Au(111) electrode surface. The film is disordered at potentials corresponding to either large negative charge densities or to positive charge densities. At small negative charge densities, an ordered adlayer of flat-lying DeTATf molecules is formed. High-resolution images of this adlayer reveal that the triflate anion is coadsorbed with the N-decyl-N,N,N-trimethylammonium cation, effectively forming an ion pair at the electrode surface. This is a significant result because it explains why this surfactant behaves like a zwitterionic surfactant at the metal/solution interface.

Scanning Tunneling Microscopy of Prochiral Anthracene Derivatives on Graphite: Chain Length Effects on Monolayer Morphology

The morphology of monolayers formed upon adsorption of prochiral 1,5-substituted anthracene derivatives on highly oriented pyrolytic graphite is investigated using scanning tunneling microscopy at the liquid-solid interface. The adsorption orientation of these prochiral anthracene derivatives positions one of their enantiotopic faces in contact with the graphite. The molecules adsorb in rows with contact between adjacent anthracenes. The anthracene side chains extend perpendicular to the direction of the row repeat. All molecules within a single row adsorb via the same enantiotopic face. Anthracenes with side chains containing an even number of non-hydrogenic atoms (C, S) form monolayers in which molecules in adjacent rows adsorb via opposite enantiotopic faces. Anthracenes with side chains that contain an odd number of non-hydrogenic atoms form two-dimensional chiral domains in which all rows contain molecules adsorbed via the same enantiotopic face. This chain length effect on monolayer morphology represents a generalized example of structural effects previously observed in alkanoic acid monolayers formed on HOPG. The variation of the STM current with position in the vicinity of the anthracenes indicates that the highest occupied molecular orbital is the predominant mediator of tunneling for the aromatic group.