Bipyridine derivatives at a solid/liquid interface: effects of the number and length of peripheral alkyl chains

Two-dimensional molecular networks at the solid/liquid interface and the role of alkyl chains in their building blocks

Nanoarchitectonics has attracted increasing attention owing to its potential applications in nanomachines, nanoelectronics, catalysis, and nanopatterning, which can contribute to overcoming global problems related to energy and environment, among others. However, the fabrication of ordered nanoarchitectures remains a challenge, even in two dimensions. Therefore, a deeper understanding of the self-assembly processes and substantial factors for building ordered structures is critical for tailoring flexible and desirable nanoarchitectures. Scanning tunneling microscopy is a powerful tool for revealing the molecular conformations, arrangements, and orientations of two-dimensional (2D) networks on surfaces. The fabrication of 2D assemblies involves non-covalent interactions that play a significant role in the molecular arrangement and orientation. Among the non-covalent interactions, dispersion interactions that derive from alkyl chain units are believed to be weak. However, alkyl chains play an important role in the adsorption onto substrates, as well as in the in-plane intermolecular interactions. In this review, we focus on the role of alkyl chains in the formation of ordered 2D assemblies at the solid/liquid interface. The alkyl chain effects on the 2D assemblies are introduced together with examples documented in the past decades.

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.

Chain-Length- and Concentration-Dependent Isomerization of Bithiophenyl-Based Diaminotriazine Derivatives in Two-Dimensional Polymorphic Self-Assembly§

Bithiophenyl-based diaminotriazine derivatives (2TDT-n, n = 10, 12, 16, and 18) with different chain lengths display colhex/p6mm mesophases. Their supramolecular self-assembled mechanism is investigated using scanning tunneling microscopy (STM) at the 1-octanoic acid/graphite interface at various concentrations. The chain length effect on the two-dimensional adlayers is observed in this system, and 2TDT-n molecules show a structural phase transition from the four-leaf arrangement to the two-row linear nanostructure accompanied by the emergence of molecular isomerization with the increase of the side-chain length. The self-assembled structure of 2TDT-10 is composed of a four-leaf pattern with uniform s-cis conformers. In 2TDT-12, three kinds of nanostructures (bamboo-like, two-row linear pattern-I, and flower-like) are observed. These nanostructures are randomly constituted by cis and trans conformers, and the ratios of the s-cis conformer in three kinds of patterns are 55.7, 42.3, and 62.5%, respectively. Furthermore, when n = 16 and 18, the ratio of the s-cis conformer further decreases to 19.0 and 4.3%, respectively. Those molecules mainly form linear nanostructures consisting of s-trans conformers. Therefore, it is reasonable to conclude that the side-chain length has a great effect on the self-assembled patterns and the molecular conformation of bithiophenyl-based diaminotriazine derivatives. Density functional theory calculations are applied to optimize molecular conformers and assess their single-point energies, showing that the s-cis conformation has higher energy than the s-trans conformer. We speculate that the ratio of two conformers in nanostructures might be similar to that of the liquid crystalline phase.

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.

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 nanoelectronics and molecular devices. In this study, we investigated the effect...

Effects of alkyl chain number and position on 2D self-assemblies

Alkyl chain number and position effects are explored via the fabrication and regulation of 2D self-assemblies at liquid/HOPG interfaces.

Odd–even effect in two dimensions induced by the bicomponent blends of isobutenyl compounds

The bicomponent blends in isobutenyl compounds showed 2D structural modulation due to odd–even effect as well as blend ratio-dependent 2D structural change.

Metal dependent motif transition in a self-assembled monolayer of bipyridine derivatives via coordination: An STM study

Low-dimensional molecular motifs with diversity developed via the on-surface chemistry are attracting growing interest for their potential in advanced nanofabrication. In this work, scanning tunneling microscopy was employed to investigate the in situ and ex situ metal coordinations between 4,4'-ditetradecyl-2,2'-bipyridine (bpy) and Zn(ii) or Cu(ii) ions at a highly oriented pyrolytic graphite (HOPG)/1-phenyloctane interface under ambient conditions. The results demonstrate that the bpy adopts a flat-lying orientation with its substituted alkyl chains in a tail-to-tail arrangement in a bpy monolayer. For the in situ coordination, the bpy/Zn(ii) and bpy/Cu(ii) complexes are aligned in edge-on fashions, wherein the bpy stands vertically on the HOPG surface and interdigitates at the alkyl chains. In the two-dimensional arrays of ex situ coordinated complexes, metal dependent motifs have been observed with Zn(ii) and Cu(ii), wherein the bipyridine moieties are parallel to the graphite surface. These results suggest that the desired on-surface coordination architectures may be achieved by the intentional selection of the metal centers.

Structural transition control between dipole–dipole and hydrogen bonds induced chirality and achirality

This study presents efficient strategies on manipulation of hydrogen bonds and dipole–dipole induced chiral and achiral self-assembly nanostructures.

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.

Entropy-controlled 2D supramolecular structures of N,N'-bis(n-alkyl)naphthalenediimides on a HOPG surface

The two-dimensional supramolecular structures of a series of N,N'-bis(n-alkyl)naphthalenediimides (NDIs), whose chain lengths span from C3 to C18, at a liquid-HOPG surface interface, studied by STM and FM-AFM, are assigned with the help of molecular dynamics/molecular mechanics calculations to demonstrate that the C3- and C4-NDIs show lamellar structures, the C4- to C12-NDIs show honeycomb (KAGOME) structures, and the C14- to C18-NDIs show lamellar structures again. The change in supramolecular structure depending on chain length can be explained semiquantitatively by the balance of entropy and enthalpy terms to show the importance of "self-avoiding walk" of the alkyl chain in entropy terms.