Control and induction of surface-confined homochiral porous molecular networks

Alkoxylated dehydrobenzo[12]annulene on Au(111): from single molecules to quantum dot molecular networks

We demonstrate the effective confinement of surface electrons in the pores of molecular networks formed by dehydrobenzo[12]annulene (DBA) molecules with butoxy groups (DBA-OC₄) on Au(111).

Emergence of chirality in hexagonally packed monolayers of hexapentyloxytriphenylene on Au(111): a joint experimental and theoretical study

We investigate the expression of chirality in a monolayer formed spontaneously by 2,3,6,7,10,11-pentyloxytriphenylene (H5T) on Au(111). We resolve its interface morphology by combining scanning tunneling microscopy (STM) with theoretical calculations of intermolecular and interfacial interaction potentials. We observe two commensurate structures. While both of them belong to a hexagonal space group, analogical to the triangular symmetry of the molecule and the hexagonal symmetry of the substrate surface, they surprisingly reveal a 2D chiral character. The corresponding breaking of symmetry arises for two reasons. First it is due to the establishment of a large molecular density on the substrate, which leads to a rotation of the molecules with respect to the molecular network crystallographic axes to avoid steric repulsion between neighboring alkoxy chains. Second it is due to the molecule-substrate interactions, leading to commensurable large crystallographic cells associated with the large size of the molecule. As a consequence, molecular networks disoriented with respect to the high symmetry directions of the substrate are induced. The high simplicity of the intermolecular and molecule-substrate van der Waals interactions leading to these observations suggests a generic character for this kind of symmetry breaking. We demonstrate that, for similar molecular densities, only two kinds of molecular networks are stabilized by the molecule-substrate interactions. The most stable network favors the interfacial interactions between terminal alkoxy tails and Au(111). The metastable one favors a specific orientation of the triphenylene core with its symmetry axes collinear to the Au⟨110⟩. This specific orientation of the triphenylene cores with respect to Au(111) appears associated with an energy advantage larger by at least 0.26 eV with respect to the disoriented core.

Functionalized surface-confined pores: guest binding directed by lateral noncovalent interactions at the solid-liquid interface

We present here the construction of self-assembled two-dimensional (2D) molecular networks that contain pores equipped with functional groups that promote guest-specific binding at the liquid/solid interface. For this purpose, a dehydrobenzo[12]annulene (DBA) derivative, DBA-F, having perfluoroalkyl groups at the end of the three alternating alkoxy chains connected by para-phenylene linkers was synthesized. For comparison DBA-H, having the same carbon backbone without fluorine substituents, was also prepared. STM observations revealed that these molecules formed porous 2D networks whose pores were decorated with either fluoroalkane or simple alkane perimeters. Hexakis(phenylethynyl)benzene, HPEB, and its octadecafluoro derivative, HPEB-F surrounded by 18 fluorine atoms, were employed as planar guest molecules of suitable size. The fluoroalkane-lined pores present in the network of DBA-F exhibited good binding ability toward both guest molecules via fluorophilicity and electrostatic interaction, respectively. In contrast the binding ability of the alkane-lined pore of the network of DBA-H for the fluorinated guest HPEB-F was poor as a result of weaker electrostatic interaction. Interestingly, with HPEB as a guest, this network underwent a periodical structural deformation through an induced-fit mechanism to form a superlattice structure consisting of free and occupied pores. These observations are discussed based on modeling experiments using molecular mechanics and quantum chemical methods to elucidate the roles of lateral noncovalent interactions and size matching between the pore and the guest molecules used for 2D guest binding.

Chiral induction with chiral conformational switches in the limit of low "sergeants to soldiers" ratio

Molecular-level insights into chiral adsorption phenomena are highly relevant within the fields of asymmetric heterogeneous catalysis or chiral separation and may contribute to understand the origins of homochirality in nature. Here, we investigate chiral induction by the "sergeants and soldiers" mechanism for an oligo(phenylene ethynylene) based chiral conformational switch by coadsorbing it with an intrinsically chiral seed on Au(111). Through statistical analysis of scanning tunneling microscopy (STM) data, we demonstrate successful chiral induction with a very low concentration of seeding molecules down to 3%. The microscopic mechanism for the observed chiral induction is suggested to involve nucleation of the intrinsically chiral seeds, allowing for effective transfer and amplification of chirality to large numbers of soldier target molecules.

Synthesis, π-face-selective aggregation, and π-face chiral recognition of configurationally stable C(3)-symmetric propeller-chiral molecules with a π-core

The C3 -symmetric propeller-chiral compounds (P,P,P)-1 and (M,M,M)-1 with planar π-cores perpendicular to the C3 -axis were synthesized in optically pure states. (P,P,P)-1 possesses two distinguishable propeller-chiral π-faces with rims of different heights named the (P/L)-face and (P/H)-face. Each face is configurationally stable because of the rigid structure of the helicenes contained in the π-core. (P,P,P)-1 formed dimeric aggregates in organic solutions as indicated by the results of (1) H NMR, CD, and UV/Vis spectroscopy and vapor pressure osmometry analyses. The (P/L)/(P/L) interactions were observed in the solid state by single-crystal X-ray analysis, and they were also predominant over the (P/H)/(P/H) and (P/L)/(P/H) interactions in solution, as indicated by the results of (1) H and 2D NMR spectroscopy analyses. The dimerization constant was obtained for a racemic mixture, which showed that the heterochiral (P,P,P)-1/(M,M,M)-1 interactions were much weaker than the homochiral (P,P,P)-1/(P,P,P)-1 interactions. The results indicated that the propeller-chiral (P/L)-face interacts with the (P/L)-face more strongly than with the (P/H)-face, (M/L)-face, and (M/H)-face. The study showed the π-face-selective aggregation and π-face chiral recognition of the configurationally stable propeller-chiral molecules.

Synthesis of a covalent monolayer sheet by photochemical anthracene dimerization at the air/water interface and its mechanical characterization by AFM indentation

Covalent monolayer sheets in 2 hours: spreading of threefold anthracene-equipped shape-persistent and amphiphilic monomers at the air/water interface followed by a short photochemical treatment provides access to infinitely sized, strictly monolayered, covalent sheets with in-plane elastic modulus in the range of 19 N/m.

Adaptive reorganization of 2D molecular nanoporous network induced by coadsorbed guest molecule

The ordered array of nanovoids in nanoporous networks, such as honeycomb, Kagome, and square, provides a molecular template for the accommodation of "guest molecules". Compared with the commonly studied guest molecules featuring high symmetry evenly incorporated into the template, guest molecules featuring lower symmetry are rare to report. Herein, we report the formation of a distinct patterned superlattice of guest molecules by selective trapping of guest molecules into the honeycomb network of trimesic acid (TMA). Two distinct surface patterns have been achieved by the guest inclusion induced adaptive reconstruction of a 2D molecular nanoporous network. The honeycomb networks can synergetically tune the arrangement upon inclusion of the guest molecules with different core size but similar peripherals groups, resulting in a trihexagonal Kagome or triangular patterns.

Self-assembled air-stable supramolecular porous networks on graphene

Functionalization and modification of graphene at the nanometer scale is desirable for many applications. Supramolecular assembly offers an attractive approach in this regard, as many organic molecules form well-defined patterns on surfaces such as graphite via physisorption. Here we show that ordered porous supramolecular networks with different pore sizes can be readily fabricated on different graphene substrates via self-assembly of dehydrobenzo[12]annulene (DBA) derivatives at the interface between graphene and an organic liquid. Molecular resolution scanning tunneling microscopy (STM) and atomic force microscopy (AFM) investigations reveal that the extended honeycomb networks are highly flexible and that they follow the topological features of the graphene surface without any discontinuity, irrespective of the step-edges present in the substrate underneath. We also demonstrate the stability of these networks under liquid as well as ambient air conditions. The robust yet flexible DBA network adsorbed on graphene surface is a unique platform for further functionalization and modification of graphene. Identical network formation irrespective of the substrate supporting the graphene layer and the level of surface roughness illustrates the versatility of these building blocks.

Principles of molecular assemblies leading to molecular nanostructures

Self-assembled physisorbed monolayers consist of regular two-dimensional arrays of molecules. Two-dimensional self-assembly of organic and metal-organic building blocks is a widely used strategy for nanoscale functionalization of surfaces. These supramolecular nanostructures are typically sustained by weak non-covalent forces such as van der Waals, electrostatic, metal-ligand, dipole-dipole and hydrogen bonding interactions. A wide variety of structurally very diverse monolayers have been fabricated under ambient conditions at the liquid-solid and air-solid interface or under ultra-high-vacuum (UHV) conditions at the UHV-solid interface. The outcome of the molecular self-assembly process depends on a variety of factors such as the nature of functional groups present on assembling molecules, the type of solvent, the temperature at which the molecules assemble and the concentration of the building blocks. The objective of this review is to provide a brief account of the progress in understanding various parameters affecting two-dimensional molecular self-assembly through illustration of some key examples from contemporary literature.

A tale of tails: alkyl chain directed formation of 2D porous networks reveals odd-even effects and unexpected bicomponent phase behavior

Supramolecular self-assembly of suitably functionalized building blocks on surfaces can serve as an excellent test-bed to gain understanding and control over multicomponent self-assembly in more complex matter. Here we employ a powerful combination of scanning tunnelling microscopy (STM) and molecular modeling to uncover two-dimensional (2D) crystallization and mixing behavior of a series of alkylated building blocks based on dehydrobenzo[12]annulene, forming arrays of nanowells. Thorough STM investigation employing high-resolution spatial imaging, use of specially designed marker molecules, statistical analysis and thermal stability measurements revealed rich and complex supramolecular chemistry, highlighting the impact of odd-even effects on the phase behavior. The methodology and analysis presented in this work can be easily adapted to the self-assembly of other alkylated building blocks.

Temperature-induced structural phase transitions in a two-dimensional self-assembled network

Two-dimensional (2D) supramolecular self-assembly at liquid-solid interfaces is a thermodynamically complex process producing a variety of structures. The formation of multiple network morphologies from the same molecular building blocks is a common occurrence. We use scanning tunnelling microscopy (STM) to investigate a structural phase transition between a densely packed and a porous phase of an alkylated dehydrobenzo[12]annulene (DBA) derivative physisorbed at a solvent-graphite interface. The influence of temperature and concentration are studied and the results combined using a thermodynamic model to measure enthalpy and entropy changes associated with the transition. These experimental results are compared to corresponding values obtained from simulations and theoretical calculations. This comparison highlights the importance of considering the solvent when modeling porous self-assembled networks. The results also demonstrate the power of using structural phase transitions to study the thermodynamics of these systems and will have implications for the development of predictive models for 2D self-assembly.

Steering supramolecular patterns by nucleobase-terminated molecules

Supramolecular patterns formed by adsorption from a liquid of nucleobase-terminated molecular rods on a graphite surface were investigated by scanning tunneling microscopy.

Optoelectronic functional materials based on alkylated-π molecules: self-assembled architectures and nonassembled liquids

The engineering of single molecules into higher-order hierarchical assemblies is a current research focus in molecular materials chemistry. Molecules containing π-conjugated units are an important class of building blocks because their self-assembly is not only of fundamental interest, but also the key to fabricating functional systems for organic electronic and photovoltaic applications. Functionalizing the π-cores with "alkyl chains" is a common strategy in the molecular design that can give the system desirable properties, such as good solubility in organic solvents for solution processing. Moreover, the alkylated-π system can regulate the self-assembly behavior by fine-tuning the intermolecular forces. The optimally assembled structures can then exhibit advanced functions. However, while some general rules have been revealed, a comprehensive understanding of the function played by the attached alkyl chains is still lacking, and current methodology is system-specific in many cases. Better clarification of this issue requires contributions from carefully designed libraries of alkylated-π molecular systems in both self-assembly and nonassembly materialization strategies. Here, based on recent efforts toward this goal, we show the power of the alkyl chains in controlling the self-assembly of soft molecular materials and their resulting optoelectronic properties. The design of alkylated-C60 is selected from our recent research achievements, as the most attractive example of such alkylated-π systems. Some other closely related systems composed of alkyl chains and π-units are also reviewed to indicate the universality of the methodology. Finally, as a contrast to the self-assembled molecular materials, nonassembled, solvent-free, novel functional liquid materials are discussed. In doing so, a new journey toward the ultimate organic "soft" materials is introduced, based on alkylated-π molecular design.

In situ STM investigation of two-dimensional chiral assemblies through Schiff-base condensation at a liquid/solid interface

Nanoscaled two-dimensional (2D) chiral architectures are increasingly receiving scientific interest, because of their potential applications in many domains. In this paper, we present a new method for constructing 2D chiral architectures on surface. Based on in situ Schiff-base reaction of achiral dialdehyde with two types of achiral amines at the solid/liquid interface, two chiral species have been directly formed and confirmed by means of a scanning tunneling microscopy (STM) technique. This work introduces a novel strategy to construct 2D surface chirality, which might be applied in fabricating functional films and nanoelectronic devices.

Photoswitching of chiral supramolecular environments and photoinduced lower critical solution temperature transitions in aqueous media following a supramolecular approach

Photochromic diarylethene is a promising candidate not only for optical memory and switching units in molecular devices, but also as a photoresponsive building block that can regulate supramolecular architectures because of its characteristic changes in the flexibility of the framework and delocalization of π-electrons, along with its photoisomerization ability. Amphiphilic photochromic compounds with oligo-(ethylene glycol) side chains show good solubility in water, and self-assemble into nanostructures in aqueous media. Asymmetric introduction of a methyl group can induce a helical preference in the supramolecular structure, and the photoswitching behaviour of supramolecular chirality was defined by circular dichroism (CD) spectroscopy and density functional theory (DFT) calculations. The effective position of an asymmetric group to induce supramolecular chirality is also discussed. In addition, the self-assemblies of diarylethene derivatives in water show photoresponsive lowest critical solution temperature (LCST). The open- and closed-ring isomers show LCST transitions at different temperatures. In particular, the difference in LCST becomes prominent when amide groups are introduced at the root of hydrophilic side chains. This is attributed to intermolecular hydrogen bonds formed by the amide groups in the hydrophobic region of the supramolecular structure in water.

Globally homochiral assembly of two-dimensional molecular networks triggered by co-absorbers

Understanding the chirality induction and amplification processes, and the construction of globally homochiral surfaces, represent essential challenges in surface chirality studies. Here we report the induction of global homochirality in two-dimensional enantiomorphous networks of achiral molecules via co-assembly with chiral co-absorbers. The scanning tunnelling microscopy investigations and molecular mechanics simulations demonstrate that the point chirality of the co-absorbers transfers to organizational chirality of the assembly units via enantioselective supramolecular interactions, and is then hierarchically amplified to the global homochirality of two-dimensional networks. The global homochirality of the network assembly shows nonlinear dependence on the enantiomeric excess of chiral co-absorber in the solution phase, demonstrating, for the first time, the validation of the 'majority rules' for the homochirality control of achiral molecules at the liquid/solid interface. Such an induction and nonlinear chirality amplification effect promises a new approach towards two-dimensional homochirality control and may reveal important insights into asymmetric heterogeneous catalysis, chiral separation and chiral crystallization.

Solvent-induced homochirality in surface-confined low-density nanoporous molecular networks

Induction of chirality in achiral monolayers has garnered considerable attention in the recent past not only due to its importance in chiral resolutions and enantioselective heterogeneous catalysis but also because of its relevance to the origin of homochirality in life. In this contribution, we demonstrate the emergence of macroscopic chirality in multicomponent supramolecular networks formed by achiral molecules at the interface of a chiral solvent and an achiral substrate. The solvent-mediated chiral induction provides a simple, efficient, and versatile approach for the fabrication of homochiral surfaces using achiral building blocks.

Role of substrate in directing the self-assembly of multicomponent supramolecular networks at the liquid-solid interface

The self-assembly of multicomponent networks at the liquid-solid interface between Au(111) or highly oriented pyrolytic graphite (HOPG) and organic solvents was investigated using scanning tunneling microscopy. Alkoxylated dehydrobenzo[12]annulene (DBA) derivatives form hexagonal nanoporous networks, which trap either single molecules of coronene (COR) or small clusters of COR and isophthalic acid to form multicomponent networks. The pattern of interdigitation between alkyl chains from DBA molecules produces hexagonal pores that are either chiral or achiral. On Au(111) substrates multicomponent networks display an ordered superlattice arrangement of chiral and achiral pores. In comparison, similar networks on HOPG display only chiral pores. The unique superlattice structure observed on Au(111) is related to a lower energetic preference for chiral pores than on HOPG and increased diffusion barriers for guest molecules. The increased diffusion barriers for guests allow them to act as nucleation sites for the formation of achiral pores. Following the initial nucleation of an achiral pore, restrictions imposed by the accommodation of guests within the porous network mean that subsequent growth naturally leads to the formation of the superlattice structure.

Exploring the complexity of supramolecular interactions for patterning at the liquid-solid interface

The use of self-assembly to fabricate surface-confined adsorbed layers (adlayers) from molecular components provides a simple means of producing complex functional surfaces. The molecular self-assembly process relies on supramolecular interactions sustained by noncovalent forces such as van der Waals, electrostatic, dipole-dipole, and hydrogen bonding interactions. Researchers have exploited these noncovalent bonding motifs to construct well-defined two-dimensional (2D) architectures at the liquid-solid interface. Despite myriad examples of 2D molecular assembly, most of these early findings were serendipitous because the intermolecular interactions involved in the process are often numerous, subtle, cooperative, and multifaceted. As a consequence, the ability to tailor supramolecular patterns has evolved slowly. Insight gained from various studies over the years has contributed significantly to the knowledge of supramolecular interactions, and the stage is now set to systematically engineer the 2D supramolecular networks in a "preprogrammed" fashion. The control over 2D self-assembly of molecules has many important implications. Through appropriate manipulation of supramolecular interactions, one can "encode" the information at the molecular level via structural features such as functional groups, substitution patterns, and chiral centers which could then be retrieved, transferred, or amplified at the supramolecular level through well-defined molecular recognition processes. This ability allows for precise control over the nanoscale structure and function of patterned surfaces. A clearer understanding and effective use of these interactions could lead to the development of functional surfaces with potential applications in molecular electronics, chiral separations, sensors based on host-guest systems, and thin film materials for lubrication. In this Account, we portray our various attempts to achieve rational design of self-assembled adlayers by exploiting the aforementioned complex interactions at the liquid-solid interface. The liquid-solid interface presents a unique medium to construct flawless networks of surface confined molecules. The presence of substrate and solvent provides an additional handle for steering the self-assembly of molecules. Scanning tunneling microscopy (STM) was used for probing these molecular layers, a technique that serves not only as a visualization tool but could also be employed for active manipulation of molecules. The supramolecular systems described here are only weakly adsorbed on a substrate, which is typically highly oriented pyrolytic graphite (HOPG). Starting with fundamental studies of substrate and solvent influence on molecular self-assembly, this Account describes progressively complex aspects such as multicomponent self-assembly via 2D crystal engineering, emergence, and induction of chirality and stimulus responsive supramolecular systems.

Chemistry of anthracene-acetylene oligomers XX: synthesis, structures, and self-association of anthracene-anthraquinone cyclic compounds with ethynylene linkers

We have synthesized anthracene-acetylene oligomers, which contained one 10-substituted anthracene unit and one anthraquinone unit, by cyclization with Sonogashira coupling. X-ray analysis revealed an almost-planar framework and significant out-of-plane deformation around the inner carbonyl moiety because of steric hindrance. These compounds underwent self-association in solution and their association constants for monomer-dimer exchange were determined by variable-concentration (1)H NMR measurements in CDCl(3): 8 mol(-1) L (10-substituent: isopropyl), <5 mol(-1) L (methoxy), and 19 mol(-1) L (octyloxy). These results were discussed on the basis of spectroscopic and molecular-orbital analysis. A linear molecular assembly of the octyloxy compound at a liquid/graphite interface was observed by STM measurements.

Two-dimensional chirality: intelligent design

Achiral molecules have now been assembled into a homochiral porous network at a solid–liquid interface. This has implications for practical processes such as separations, but also for understanding how homochirality — crucial in biological systems — arose from achiral or racemic species.