Surface-Induced Diastereomeric Complex Formation of a Nucleoside at the Liquid/Solid Interface: Stereoselective Recognition and Preferential Adsorption

2D Co-crystallization of molecular homologues promoted by size complementarity of the alkyl chains at the liquid/solid interface

Co-crystallization of organic molecules is an important strategy for the fabrication of molecular materials. In this contribution, we investigated the mixing behavior of 5-(benzyloxy)-isophthalic acid homologues (BIC-Cn, n = 6, 8, 10, 12, and 14) at the liquid/solid interface using a scanning tunneling microscope. Deposition of the single component of BIC-Cn always results in typical honeycomb networks, whereas co-deposition of two BIC-Cn homologues leads to hybrid double-walled honeycomb networks or phase separation depending on the difference in the length of their alkyl chains. 2D co-crystallization can only be realized for BIC-C6/BIC-C10 or BIC-C8/BIC-C12 which have a four-methyl unit difference in their alkyl chains. The size complementarity of the alkyl chains in the two components suggests that it is responsible for the 2D co-crystallization, though hydrogen bonding contributes a lot both to the pristine honeycomb network and to the hybrid co-crystal. This result is of importance for understanding the role of van der Waals interaction and its interplay with hydrogen bonding in 2D co-crystallization.

Fabrication of 2D Hetero-Complexes With Nucleic-Acid-Base Adenine and Fatty-Acid Stearic Acid at Liquid/Solid Interface

Designing and fabricating hetero-complexes composed of organic and biological compounds had become an exciting area referring to biological recognition, molecular devices etc. Here, hydrogen-bonded complex of nucleic-acid-base (adenine, A) and fatty-acid (stearic acid, SA) was designed, fabricated and investigated at liquid/solid interface. The interesting striped-shaped structure composed of SA-A-SA trimers was formed after introducing adenine molecules. Meanwhile, the primary lamella-shape characteristic of the assembly of SA molecules was kept because of the collaboration of non-covalent interactions of molecule-molecule and molecule-substrate. With a series of experimental characterization and theoretical simulation, the origination of the as-prepared 2D hetero-complexes was gradually exhibited from the assembled structures of two building blocks of stearic acid and adenine. Our study provides a blueprint for designing additional multi-component complexes based on the existing molecular assembled architectures.

Supramolecular Self-Assembly of Perylene Bisimide Derivatives Assisted by Various Groups

Anthra[2,1,9-def:6,5,10-d'e'f']diisoquinoline-1,3,8,10(2H,9H)-tetraone, namely, perylene bisimides (PBIs), belong to n-type organic semiconductors and possess potential applications in optoelectronic devices. The properties/performance of fabricated nanostructures/devices could be greatly influenced by both molecular structures of PBI building blocks and corresponding arrangement in assembled nanostructures. Many efforts have been made to modify the PBI core and then investigate the nanostructures and properties. However, it is still a great challenge to comprehensively understand the influence of molecular structures on the intermolecular interactions, the self-assembled structures, and the resulting performance. In the present contribution, we mainly summarize recent research aspects on supramolecular assembly behaviors of PBI derivatives assisted by various functional groups. First, a short introduction is given about basic molecular structure, properties, and self-assembly of PBI derivatives. Then, we mainly discuss the modulation of self-assembly of PBIs via introducing various functional groups (flexible or nonflexible chains, and biomolecules especially amino-acid-based groups). After that, the assembly of PBI derivatives from out-of-equilibrium states is described. Finally, a perspective is provided on the design of novel PBI derivatives and the fabrication of unique nanostructures with superior properties.

Role of halogen⋯halogen interactions in the 2D crystallization of n-semiconductors at the liquid–solid interface

The impact of X⋯X interactions on the 2D crystallization of perylene-based n-semiconductors at the liquid–solid interface was investigated.

Role of intrinsic hydrogen bonds in the assembly of perylene imide derivatives in solution and at the liquid–solid interface

The impact of hydrogen bond formation on the supramolecular assembly of two perylene imide-based derivatives was systematically investigated.

Enantiomeric Excess-Tuned 2D Structural Transition: From Heterochiral to Homochiral Supramolecular Assemblies

Spontaneous resolution of enantiomers is an intriguing and important phenomenon in surface chirality studies. Herein, we report on a two-dimensional (2D) structural transition from the heterochiral to homochiral assembly tuned by changing the enantiomeric excess (ee) of enantiomers in the solution phase. Enantiomers cocrystallize as racemates on the surface when the ee of the R-enantiomer (or S-enantiomer) remains below a critical value, whereas chiral segregation is achieved, and globally homochiral surfaces composed of exclusively one enantiomer are obtained as the critical ee is exceeded. The heterochiral-homochiral transition is ascribed to the formation of energetically unfavored homochiral molecular dimers under the control of the majority-rules principle at high ee values. Such results present an intriguing phenomenon in chiral ordering at surfaces, promising a new enlightenment toward understanding chiral resolution and the evolution of chirality.

The formation of right-handed and left-handed chiral nanopores within a single domain during amino acid self-assembly on Au(111)

We report the formation of both right- and left-handed chiral nanopores within a single domain during the self-assembly of an amino acid derivative on an inert Au(111) surface using STM. DFT calculations employed to rationalize this unusual result identified that intermolecular interactions between chiral, windmill-shaped tetramers are crucial for self-assembly.

Self-assembly of Natural and Unnatural Nucleobases at Surfaces and Interfaces

The self-assembly of small organic molecules interacting via non-covalent forces is a viable approach towards the construction of highly ordered nanostructured materials. Among various molecular components, natural and unnatural nucleobases can undergo non-covalent self-association to form supramolecular architectures with ad hoc structural motifs. Such structures, when decorated with appropriate electrically/optically active units, can be used as scaffolds to locate such units in pre-determined positions in 2D on a surface, thereby paving the way towards a wide range of applications, e.g., in optoelectronics. This review discusses some of the basic concepts of the supramolecular engineering of natural and unnatural nucleobases and derivatives thereof as well as self-assembly processes on conductive solid substrates, as investigated by scanning tunnelling microscopy in ultra-high vacuum and at the solid/liquid interface. By unravelling the structure and dynamics of these self-assembled architectures with a sub-nanometer resolution, a greater control over the formation of increasingly sophisticated functional systems is achieved. The ability to understand and predict how nucleobases interact, both among themselves as well as with other molecules, is extremely important, since it provides access to ever more complex DNA- and RNA-based nanostructures and nanomaterials as key components in nanomechanical devices.

Effects of Alkyl Chain Length and Hydrogen Bonds on the Cooperative Self-Assembly of 2-Thienyl-Type Diarylethenes at a Liquid/Highly Oriented Pyrolytic Graphite (HOPG) Interface

An appropriate understanding of the process of self-assembly is of critical importance to tailor nanostructured order on 2D surfaces with functional molecules. Photochromic compounds are promising candidates for building blocks of advanced photoresponsive surfaces. To investigate the relationship between molecular structure and the mechanism of ordering formation, 2-thienyl-type diarylethenes with various lengths of alkyl side chains linked through an amide or ester group were synthesized. Their self-assemblies at a liquid/solid interface were investigated by scanning tunneling microscopy (STM). The concentration dependence of the surface coverage was analyzed by using a cooperative model for a 2D surface based on two characteristic parameters: the nucleation equilibrium constant (Kn) and the elongation equilibrium constant (Ke). The following conclusions can be drawn. 1) The concentration at which a stable 2D molecular ordering is observed by STM exponentially decreases with increasing length of the alkyl chain. 2) Compounds bearing amide groups have higher degrees of cooperativity in self-assembly on 2D surfaces (i.e., σ, which is defined as Kn/Ke) than compounds with ester groups. 3) The self-assembly process of the open-ring isomer of an ester derivative is close to isodesmic, whereas that of the closed-ring isomer is cooperative because of the difference in equilibrium constants for the nucleation step (i.e., Kn) between the two isomers.

Towards enantioselective adsorption in surface-confined nanoporous systems

The adsorption of chiral molecules in surface-confined chiral porous networks shows pronounced selectivity, as a result of complementary host-guest interactions.

Two-dimensional chiral molecular assembly on solid surfaces: formation and regulation

The expression of chirality in 2D molecular assemblies on solid surfaces has unique features compared to the analogous process in 1D and 3D supramolecular assemblies. Understanding the formation of chiral molecular assemblies on surfaces not only provides insight into the origin and transfer of chirality in many enantioselective processes, but also aids rational design and construction of chiral architectures and materials. This present contribution reviews recent studies on how chirality is induced and expressed on the surface at different levels, both from intrinsically chiral and achiral molecules. Furthermore, we discuss the regulation effect of some pivotal factors, for example, the chemical structure, the chiral auxiliary molecules, and the assembled environments, on the expression of chirality in molecular assembly.

Probing a hidden world of molecular self-assembly: concentration-dependent, three-dimensional supramolecular interconversions

A terpyridine-based, concentration-dependent, facile self-assembly process is reported, resulting in two three-dimensional metallosupramolecular architectures, a bis-rhombus and a tetrahedron, which are formed using a two-dimensional, planar, tris-terpyridine ligand. The interconversion between these two structures is concentration-dependent: at a concentration higher than 12 mg mL(-1), only a bis-rhombus, composed of eight ligands and 12 Cd(2+) ions, is formed; whereas a self-assembled tetrahedron, composed of four ligands and six Cd(2+) ions, appears upon sufficient dilution of the tris-terpyridine-metal solution. At concentrations less than 0.5 mg mL(-1), only the tetrahedron possessing an S4 symmetry axis is detected; upon attempted isolation, it quantitatively reverts to the bis-rhombus. This observation opens an unexpected door to unusual chemical pathways under high dilution conditions.

Controlling the stereochemistry and regularity of butanethiol self-assembled monolayers on au(111)

The rich stereochemistry of the self-assembled monolayers (SAMs) of four butanethiols on Au(111) is described, the SAMs containing up to 12 individual C, S, or Au chiral centers per surface unit cell. This is facilitated by synthesis of enantiomerically pure 2-butanethiol (the smallest unsubstituted chiral alkanethiol), followed by in situ scanning tunneling microscopy (STM) imaging combined with density functional theory molecular dynamics STM image simulations. Even though butanethiol SAMs manifest strong headgroup interactions, steric interactions are shown to dominate SAM structure and chirality. Indeed, steric interactions are shown to dictate the nature of the headgroup itself, whether it takes on the adatom-bound motif RS(•)Au(0)S(•)R or involves direct binding of RS(•) to face-centered-cubic or hexagonal-close-packed sites. Binding as RS(•) produces large, organizationally chiral domains even when R is achiral, while adatom binding leads to rectangular plane groups that suppress long-range expression of chirality. Binding as RS(•) also inhibits the pitting intrinsically associated with adatom binding, desirably producing more regularly structured SAMs.

Affecting surface chirality via multicomponent adsorption of chiral and achiral molecules

Here we report on the apparent reduction in surface chirality upon co-assembling a chiral and achiral molecule into a physisorbed self-assembled monolayer at the liquid/solid interface as revealed by scanning tunneling microscopy (STM).