Two-dimensional self-assemblies of azobenzene derivatives: effects of methyl substitution of azobenzene core and alkyl chain length.

Elucidating the correlation between the molecular arrangement and physical properties of organic compounds is critical to facilitating the development of advanced functional materials. X-ray structural analyses are generally performed to clarify this relationship. Several attempts have been made to ascertain the links between three-dimensional (3D) crystals and their two-dimensional (2D) structures, which can be revealed by scanning tunnelling microscopy (STM) at the molecular level. Thus, 2D self-assemblies of a series of azobenzene derivatives were investigated in this study, and the effects of methyl substitution of the azobenzene core and alkyl chain length on the 2D molecular arrangements at the solid/liquid interface were revealed. Three types of azobenzene derivatives were prepared; these contained azobenzene (Az), 3-methyl azobenzene (MAz), or 3,3'-dimethyl azobenzene (DAz) as cores and alkyloxy chains of different lengths (C8-13) at their 4,4' positions. The 2D structures of the Az and DAz compounds were found to be modulated owing to the odd-even effect of the alkyl chains in a specific chain-length range; this effect was only weakly exhibited by the MAz compounds. This result suggests that only the methyl-group substitution of the azobenzene core significantly affected the 2D structures. The 2D structural features have been discussed in terms of molecular conformation, as well as their correlation with the photo-melting behaviour of the azobenzene derivatives, particularly the MAz compounds.

Halogen bond-directed self-assembly in bicomponent blends at the solid/liquid interface: effect of the alkyl chain substitution position.

The fabrication of well-organised molecular assemblies on surfaces is fundamental for the creation of functional molecular systems applicable to nanoelectronic and molecular devices. In this study, we investigated the effect of substitution positions of alkyl chains on the formation of halogen-bonded molecular networks. For this purpose, building blocks with different head groups (i.e., pyridine (Py) or tetrafluoro-iodobenzene (FI)) were substituted with hexadecyloxy chains at either the 3,4- or the 3,5-positions. The two-dimensional assembly of each compound as a single-component system was studied using scanning tunnelling microscopy (STM) at the highly oriented pyrolytic graphite (HOPG)/1-phenyloctane interface. All compounds displayed linear structures in which the alkyl chains were aligned along one of the HOPG axes. In the exceptional case of FI bearing hexadecyloxy chains at the 3,5-positions (denoted as FI-3,5), hexagonal arrays were tentatively formed owing to the triangular molecular arrangement induced by halogen bonding. A bicomponent blend of Py-3,4/FI-3,5 (1 : 1 molar ratio) enabled the formation of a honeycomb structure, whereas that of Py-3,5/FI-3,4 (1 : 1 molar ratio) produced a rectangular assembly that was periodically arranged in a zig-zag fashion. Finally, based on the observed blend ratio dependence, the formation of these different two-dimensional structures by variation in the substitution positions of the alkyl chains was discussed in terms of molecule-molecule and molecule-substrate interactions.

Well-organised two-dimensional self-assembly controlled by in situ formation of a Cu(II)-coordinated rufigallol derivative: a scanning tunnelling microscopy study.

The two-dimensional self-assembly of rufigallol derivatives and their metal coordination were studied by scanning tunnelling microscopy. Ex situ Cu(II)-coordinated rufigallol derivatives exhibited columnar structures with some defects, whereas regular and linear structures were formed upon in situ metal coordination at solid/liquid interfaces.

Dynamic host-guest behavior in halogen-bonded two-dimensional molecular networks investigated by scanning tunneling microscopy at the solid/liquid interface.

The fabrication of supramolecularly engineered two-dimensional (2D) networks using simple molecular building blocks is an effective means for studying host-guest chemistry at surfaces toward the potential application of such systems in nanoelectronics and molecular devices. In this study, halogen-bonded molecular networks were constructed by the combination of linear halogen-bond donor and acceptor ligands, and their 2D structures at the highly oriented pyrolytic graphite/1-phenyloctane interface were studied by scanning tunneling microscopy. The bi-component blend of the molecular building blocks possessing tetradecyloxy chains formed a lozenge structure via halogen bonding. Upon the introduction of an appropriate guest molecule (e.g., coronene) into the system, the 2D structure transformed into a hexagonal array, and the central pore of this array was occupied by the guest molecules. Remarkably, the halogen bonding of the original structure was maintained after the introduction of the guest molecule. Thus, the halogen-bonded molecular networks are applicable for assembling guest species on the substrate without the requirement of the conventional rigid molecular building blocks with C 3 symmetry.

Coordination-Driven Construction of Porphyrin Nanoribbons at a Highly Oriented Pyrolytic Graphite (HOPG)/Liquid Interface.

Nanostructures were built at the solid/liquid interface by self-assembly and/or coordination bonds. Metalloporphyrins bearing two external coordination sites and long alkyl chains allowed the self-assembly of the compounds on highly oriented pyrolitic graphite. After addition of a metal ion, long transition-metal linked porphyrin nanoribbons were obtained and visualized by scanning tunneling microscopy. In these porphyrin ribbons electronic delocalization is possible through the d orbitals of the connecting metal ions.

Hexagonal array formation by intermolecular halogen bonding using a binary blend of linear building blocks: STM study.

Hexagonal arrays were fabricated via intermolecular halogen bonding between two linear molecular building blocks in a bicomponent blend. The substitution position of the pyridine N atom involved in the halogen bond plays an important role in the formation of the hexagonal structures.