Nucleoside-Assisted Self-Assembly of Oligo(p-phenylenevinylene)s at Liquid/Solid Interface: Chirality and Nanostructures

Asymmetric Molecular Adsorption and Regioselective Bond Cleavage on Chiral PdGa Crystals

Homogenous enantioselective catalysis is nowadays the cornerstone in the manufacturing of enantiopure substances, but its technological implementation suffers from well-known impediments like the lack of endurable catalysts exhibiting long-term stability. The catalytically active intermetallic compound Palladium-Gallium (PdGa), conserving innate bulk chirality on its surfaces, represent a promising system to study asymmetric chemical reactions by heterogeneous catalysis, with prospective relevance for industrial processes. Here, this work investigates the adsorption of 10,10'-dibromo-9,9'-bianthracene (DBBA) on the PdGa:A(1 ¯ 1 ¯ 1 ¯ $\bar{1}\bar{1}\bar{1}$ ) Pd3-terminated surface by means of scanning tunneling microscopy (STM) and spectroscopy (STS). A highly enantioselective adsorption of the molecule evolving into a near 100% enantiomeric excess below room temperature is observed. This exceptionally high enantiomeric excess is attributed to temperature activated conversion of the S to the R chiral conformer. Tip-induced bond cleavage of the R conformer shows a very high regioselectivity of the DBBA debromination. The experimental results are interpreted by density functional theory atomistic simulations. This work extends the knowledge of chirality transfer onto the enantioselective adsorption of non-planar molecules and manifests the ensemble effect of PdGa surfaces resulting in robust regioselective debromination.

Two-Dimensional Hetero- to Homochiral Phase Transition from Dynamic Adsorption of Barbituric Acid Derivatives

Barbituric acid derivative (TDPT) is an achiral molecule, and its adsorption on a surface results in two opposite enantiomerically oriented motifs, namely TDPT-Sp and Rp. Two types of building blocks can be formed; block I is enantiomer-pure and is built up of the same motifs (format SpSp or RpRp) whereas block II is enantiomer-mixed and composes both motifs (format SpRp), respectively. The organization of the building blocks determines the formation of different nanoarchitectures which are investigated using scanning tunneling microscopy at a liquid/HOPG interface. Sophisticated, highly symmetric "nanowaves" are first formed from both building blocks I and II and are heterochiral. The "nanowaves" are metastable and evolve stepwisely into more close-packed "nanowires" which are formed from enantiomer-pure building block I and are homochiral. A dynamic hetero- to homochiral transformation and simultaneous multi-scale phase transitions are demonstrated at the single-molecule level. Our work provides novel insights into the control and the origin of chiral assemblies and chiral transitions, revealing the various roles of enantiomeric selection and chiral competition, driving forces, stability and molecular coverage.

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.

Packing Biomolecules into Sierpiński Triangles with Global Organizational Chirality

Fractals are found in nature and play important roles in biological functions. However, it is challenging to controllably prepare biomolecule fractals. In this study, a series of Sierpiński triangles with global organizational chirality is successfully constructed by the coassembly of l-tryptophan and 1,3-bi(4-pyridyl)benzene molecules on Ag(111). The chirality is switched when replacing l-tryptophan by d-tryptophan. The fractal structures are characterized by low-temperature scanning tunneling microscopy at the single-molecule level. Density functional theory calculations reveal that intermolecular hydrogen bonds stabilize the Sierpiński triangles.

Kinked row-induced chirality driven by molecule-substrate interactions

Combining STM measurements on three different substrates (HOPG, MoS₂, and Au[111]) together with DFT calculations allow for analysis of the origin of the self-assembly of 4-cyano-4'-n-decylbiphenyl (10CB) molecules into kinked row structures using a previously developed phenomenological model. This molecule has an alkyl chain with 10 carbons and a cyanobiphenyl group with a particularly large dipole moment. 10CB represents a toy model that we use here to unravel the relationship between the induced kinked structure, in particular the corresponding chirality expression, and the balanced intermolecular/molecule-substrate interaction. We show that the local ordered structure is driven by the typical alkyl chain/substrate interaction for HOPG and Au[111] and the cyanobiphenyl group/substrate interaction for MoS₂. The strongest molecule/substrate interactions are observed for MoS₂ and Au[111]. These strong interactions should have led to non-kinked, commensurate adsorbed structures. However, this latter appears impossible due to steric interactions between the neighboring cyanobiphenyl groups that lead to a fan-shape structure of the cyanobiphenyl packing on the three substrates. As a result, the kink-induced chirality is particularly large on MoS₂ and Au[111]. A further breaking of symmetry is observed on Au[111] due to an asymmetry of the facing molecules in the rows induced by similar interactions with the substrate of both the alkyl chain and the cyanobiphenyl group. We calculate that the overall 10CB/Au[111] interaction is of the order of 2 eV per molecule. The close 10CB/MoS₂ interaction, in contrast, is dominated by the cyanobiphenyl group, being particularly large possibly due to dipole-dipole interactions between the cyanobiphenyl groups and the MoS₂ substrate.

Template-assisted 2D self-assembled chiral Kagomé network for selective adsorption of coronene

Coadsorbed solvents can serve as a template to fabricate a Kagomé network, which could be used to select adsorption of coronene.

Self-assembly of an oligo(p-phenylenevinylene)-based molecule on an HOPG surface: insights from multi-scale simulation and STM observation

To gain knowledge of the most important weak interactions for supramolecular self-assembly and observe molecular structure for self-assembled architectures, the two-dimensional self-assembly of an oligo(p-phenylenevinylene)-based molecule (AS-OPV) on highly oriented pyrolytic graphite has been investigated. Accurate atomic configuration for the AS-OPV self-assembled pattern has been identified by means of multi-scale simulation combined with scanning tunneling microscopy (STM) experiments. The weak interactions which contribute to the formation of AS-OPV self-assembly are studied by analysis of non-covalent interactions existing in the system and theoretical calculation of their energy values. Investigation of the molecular structure of self-assembly and STM images at a certain temperature range is performed by molecular dynamics and density functional theory simulations. This work paves the way to explore the contribution of weak interactions for the self-assembly system, as well as providing a reference to observe the possible self-assembled structure at temperatures not convenient for direct experimental observation.

Chiral polymorphism in the self-assemblies of achiral molecules induced by multiple hydrogen bonds

Driven by multiple hydrogen bonds, chiral and achiral polymorphs are successfully fabricated at a liquid–solid interface.

Chirality in adsorption on solid surfaces

In the present review we survey the main advances made in recent years on the understanding of chemical chirality at solid surfaces. Chirality is an important topic, made particularly relevant by the homochiral nature of the biochemistry of life on Earth, and many chiral chemical reactions involve solid surfaces. Here we start our discussion with a description of surface chirality and of the different ways that chirality can be bestowed on solid surfaces. We then expand on the studies carried out to date to understand the adsorption of chiral compounds at a molecular level. We summarize the work published on the adsorption of pure enantiomers, of enantiomeric mixtures, and of prochiral molecules on chiral and achiral model surfaces, especially on well-defined metal single crystals but also on other flat substrates such as highly ordered pyrolytic graphite. Several phenomena are identified, including surface reconstruction and chiral imprinting upon adsorption of chiral agents, and the enhancement or suppression of enantioselectivity seen in some cases upon adsorption of enantiomixtures of chiral compounds. The possibility of enhancing the enantiopurity of adsorbed layers upon the addition of chiral seeds and the so-called "sergeants and soldiers" phenomenon are presented. Examples are provided where the chiral behavior has been associated with either thermodynamic or kinetic driving forces. Two main approaches to the creation of enantioselective surface sites are discussed, namely, via the formation of supramolecular chiral ensembles made out of small chiral adsorbates, and by adsorption of more complex chiral molecules capable of providing suitable chiral environments for reactants by themselves, via the formation of individual adsorbate:modifier adducts on the surface. Finally, a discussion is offered on the additional effects generated by the presence of the liquid phase often required in practical applications such as enantioselective crystallization, chiral chromatography, and enantioselective catalysis.

Chiral Self-Assembly of Nonplanar 10,10'-Dibromo-9,9'-bianthryl Molecules on Ag(111)

We report on the low-temperature scanning tunneling microscopy (STM) measurements of the self-assembly of nonplanar 10,10'-dibromo-9,9'-bianthryl (DBBA) molecules on Ag(111) combined with density functional theory (DFT) calculations. DBBA molecules have two enantiomorphous adsorption configurations, from which more chiral structures can be formed. At a low coverage [0.4 monolayer (ML)], DBBA forms racemic netlike islands consisting of molecular chains along ⟨1 2 3̅⟩_Ag. Moreover, the gliding between the molecular chains gives rise to chiral windmill-like patterns in the islands. At 0.8 ML, DBBA forms a racemic row phase and a homochiral hexamer phase. The molecular appearance difference between the two coexisted phases and the DFT calculated molecular adsorption configuration reveal a decrease in the molecular dihedral angle of DBBA, which implies an enhancement in the intermolecular interactions via CH···π and halogen bonds. The transition from a racemic packing mode to a homochiral one suggests that the suitability of steric configurations is dominant in the close-packing mode under enhanced intermolecular interactions.

Co-assembly of donor and acceptor towards organogels tuned by charge transfer interaction strength

Co-assembly of n-type semiconductors NDI and PDI with p-type pyrene derivatives resulted in the formation of stable organogels, which was induced by the strong charge transfer (CT) interactions between acceptors and donors in chloroform. The dimension size of the aromatic core from the acceptors was found to have a significant impact on the organogels. The width of the fibers from CT gels with NDI is about twice that from gels with PDI. It was found that the acceptor NDI preferred an alternate stacking with donors, intercalated with each other via CT interactions. In contrast, the acceptor PDI preferred to stack among themselves within the assemblies and this arose from the stronger π-π interactions because they had larger aromatic cores than the acceptor NDI. The dimension size of the aromatic core has been proved to have a significant impact on the organogels. The substituent impact of the donors was also studied.

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.

Supramolecular chiral host-guest nanoarchitecture induced by the selective assembly of barbituric acid derivative enantiomers

Barbituric acid derivatives are prochiral molecules, i.e. they are chiral upon adsorption on surfaces. Scanning tunneling microscopy reveals that barbituric acid derivatives self-assemble into a chiral guest-host supramolecular architecture at the solid-liquid interface on graphite. The host nanoarchitecture has a sophisticated wavy shape pattern and paired guest molecules are nested insides the cavities of the host structure. Each unit cell of the host structure is composed of both enantiomers with a ratio of 1:1. Furthermore, the wavy patterns of the nanoarchitecture are formed from alternative appearance of left- and right-handed chiral building blocks, which makes the network heterochiral. The functional guest-host nanoarchitecture is the result of two-dimensional chiral amplification from single enantiomers to organizational heterochiral supramolecular self-assembly.

Turning off the majority-rules effect in two-dimensional hierarchical chiral assembly by introducing a chiral mismatch

Understanding the mechanism in chiral transmission from a single molecule to a supramolecular level is fundamentally important to decipher the nonlinear amplification effect in the two-dimensional (2D) chiral assembly process. In this contribution, we report on the dramatically different nonlinear amplification effect in the chiral co-adsorber induced homochiral assemblies constructed by a series of homologous achiral building blocks on the graphite surface under control of the majority-rules principle. Homologous hexagonal networks are formed for 5-(benzyloxy)-isophthalic acid (BIC) derivatives with different alkyl lengths. While globally homochiral monolayers of BIC-C6 or BIC-C16 can be obtained by using a mixture of chiral co-adsorber 2-octanol with a small enantiomeric excess, such majority-rules principle based nonlinear chiral amplification is inoperative for the assembly of BIC-C10. Molecular mechanistic analysis indicates that BIC-C10 assembly can accommodate a chiral mismatched motif to form long-range ordered but short-range disordered crystalline networks, leading to the co-adsorption of enantiomers without enantioselectivity. The present results shed important insights into the significance of chirality mismatch during chiral transmission and benefit the understanding of chiral communication in a surface monolayer.

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.

From nucleobase to DNA templates for precision supramolecular assemblies and synthetic polymers

In this minireview, we report on the recent advances of utilization of nucleobases and DNA as templates to achieve well-defined supramolecular polymers, synthetic polymers, and sequence-controlled polymers.

Concentration-dependent structure and structural transition from chirality to nonchirality at the liquid-solid interface by coassembly

Understanding the formation and structural transition of the two-dimensional chirality of self-assembly is a subject which still gains significant interest in surface or interface chirality studies. Here, we present the solvent-induced chiral structural transition of a 2-hydroxy-7-pentadecyloxy-9-fluorenone (HPF) molecules' self-assembled adlayer through coassembly with achiral aliphatic solvents under different concentrations. Polymorphic chiral patterns are obtained at low concentrations of aliphatic solvents with different chain lengths. The HPF molecules form coassembled structures with these solvents through van der Waals interactions. At the same time, at high concentrations, HPF molecules uniformly form a nonchiral multimer structure without coadsorbed aliphatic solvent molecules. What is interesting is that these structures under different concentrations will finally change into a zigzag structure, which is the thermodynamically most stable configuration. Especially when using n-hexadecane as the solvent, the adlayer shows perfect steric matching due to the close chain length of HPF and n-hexadecane, which can maximize the molecule-solvent interactions. Thus, HPF molecules in n-hexadecane exhibit the most diversiform configuration. The distinct concentration-dependence has proven that the solvent molecules can act as a coadsorbed component through van der Waals interactions rather than simply a dispersant and further result in the probability and stability of chiral self-assembled monolayers by subtle tuning of the solvent-molecule and solvent-substrate interactions. This result provides a simple and alternative strategy to construct the 2D chiral assembled monolayer.

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.

RNA nucleosides as chiral sensing agents in NMR spectroscopy

The study reports chiral sensing properties of RNA nucleosides. A three component derivitazation protocol has been adopted to differentiate chiral amines. All RNA nucleosides exhibit chiral sensing property.

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.

Homochiral and heterochiral assembly preferences at different length scales--conglomerates and racemates in the same assemblies

Length scale dependent formation of conglomerates and racemic compounds has been observed in self-assembled hierarchical supramolecular architectures based on oligo(p-phenylenevinylene)-phenylglycinamide at the liquid-solid interface and in solution.

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).

Self-assembly of N3-substituted xanthines in the solid state and at the solid-liquid interface

The self-assembly of small molecular modules interacting through noncovalent forces is increasingly being used to generate functional structures and materials for electronic, catalytic, and biomedical applications. The greatest control over the geometry in H-bond supramolecular architectures, especially in H-bonded supramolecular polymers, can be achieved by exploiting the rich programmability of artificial nucleobases undergoing self-assembly through strong H bonds. Here N(3)-functionalized xanthine modules are described, which are capable of self-associating through self-complementary H-bonding patterns to form H-bonded supramolecular ribbons. The self-association of xanthines through directional H bonding between neighboring molecules allows the controlled generation of highly compact 1D supramolecular polymeric ribbons on graphite. These architectures have been characterized by scanning tunneling microscopy at the solid-liquid interface, corroborated by dispersion-corrected density functional theory (DFT) studies and X-ray diffraction.

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.

Self-assembled nanostructures of optically active phthalocyanine derivatives. Effect of central metal ion on the morphology, dimension, and handedness

Four optically active {(2,3,9,10,16,17,23,24-octa[(S)-2-methylbutoxy]} phthalocyanine derivatives with different central metal ions, namely (S)-H2Pc(β-OC5H11)8 (1), (S)-ZnPc(β-OC5H11)8 (2), (S)-CuPc(β-OC5H11)8 (3), and (S)-NiPc(β-OC5H11)8 (4) have been synthesized and their self-assembly behaviors systematically investigated by electronic absorption and circular dichroism (CD) spectroscopy, scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD) techniques, and X-ray photoelectron spectroscopy (XPS). Metal free phthalocyanine together with its zinc and copper congeners self-assembles into right-handed helical arrangements in a stack of phthalocyanine chromophores according to the CD spectroscopic result, while the nickel complex does so into the left-handed helical arrangements. These helical arrangements, acting as elemental primary structures, further pack in a hierarchical fashion into either highly ordered right-handed fibrous nanostructures with average 300 μm length, 4.8 μm width, and 4.4 μm helical pitch for 1 and 300 μm length, 2.4 μm width, and 1.8 μm helical pitch for 2 but left-handed fibrous nanostructures with average 4 μm length, 0.48 μm width, and 0.12 μm helical pitch for 3 and 300 μm length, 2 μm width, and 1.2 μm helical pitch for 4, clearly revealing the effect of central metal ion on the distance and relative orientation of neighboring phthalocyanine chromophores and in turn the supramolecular chirality, morphology, dimension, and handedness of the self-assembled nanostructures. The present result not only represents a unique phenomenon in the self-assembly of phthalocyanine compounds but more importantly denotes the transcription and amplification of molecular chirality to supramolecular helicity with different helical bias during the self-assembly processes without changing the chiral handles attached to the phthalocyanine chromophore.

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.

Self-assembly and molecular recognition of adenine- and thymine-functionalized nucleolipids in the mixed monolayers and thymine-functionalized nucleolipids on aqueous melamine at the air-water interface

Self-assembly and molecular recognition of the monolayers composed of an equimolar mixture of adenine- and thymine-functionalized nucleolipids at the air-water interface have been investigated in detail using surface pressure-molecular area isotherms and in situ infrared reflection absorption spectroscopy (IRRAS). Prior to molecular recognition, the adenine moieties in the monolayer were almost oriented on an end-on mode through π-stacking and hydrogen bonding interactions, and the C-C-C planes of the alkyl chains were preferentially oriented perpendicular to the water surface, while the thymine moieties in the monolayer were involved in hydrogen bonding almost with a flat-on orientation. On aqueous subphases containing complementary bases, no significant molecular recognition was observed for the monolayers of individual nucleolipids. In the monolayer of equimolar mixture, molecular recognition occurred between the adenine and thymine moieties through hydrogen bonding probably with the development of cyclic structures of adenine-thymine-adenine-thymine quartets. Although molecular recognition between the monolayer of thymine-functionalized nucleolipids and aqueous melamine took place through triple hydrogen bonds, no melamine binding to the monolayer of equimolar mixture was observed, which reflects the formation of the quartets in the mixed monolayers at the air-water interface. FTIR and small-angle X-ray diffraction (XRD) results of the corresponding Langmuir-Blodgett films support the hydrogen bonding recognition and molecular orientation.