Programmable hierarchical three-component 2D assembly at a liquid-solid interface: recognition, selection, and transformation

Regulating Rotational Dynamics of Co-Adsorbed Guest Molecules via Halogen Bonds in Functionalized Pores of Self-Assembled Molecular Networks at the Liquid-Solid Interface

Understanding and controlling molecular rotation on surfaces is crucial for the development of molecular-scale artificial motors that operate at interfaces. Herein, it is reported the successful co-adsorption of guest molecules within the functionalized 2D pores of self-assembled molecular networks (SAMNs) through directional halogen bonding, as confirmed by scanning tunneling microscopy. Specifically, the porous SAMN formed by dehydrobenzo[12]annulene derivative DBA-Py with a pyridyl group at the termini of its three alkoxy chains, hosts an iodinated trigonal guest molecule, tris(4-iodophenyl)benzene (TIB), through a halogen bond between the nitrogen and iodine atoms. Within the pores, the TIB molecule exhibits rotational motion, preferentially residing at two locations. In contrast, within the pores formed by a mixture of DBA-Py and DBA-Ph, where DBA-Ph features three phenyl groups instead of pyridyl groups, the guest molecule preferentially resides in a single location. This behavior is attributed to the reduced number of energy minima within the pores owing to the decreased number of pyridyl units. Statistical analysis of the guest orientation suggests that the on-surface arrangement of DBA-Py and DBA-Ph is influenced by the guest molecule. This modular approach using functionalized pores in SAMNs provides an effective strategy for controlling molecular rotational behavior.

Structural Diversity of 2D Molecular Self-Assemblies Arising from Carboxyl Groups Attached to a Molecule: An STM Study at the Liquid-Solid Interface

Understanding the molecular self-assembly behavior, especially at the microscopic level, sheds light on the rational design of artificial supramolecular systems at surfaces. In this work, scanning tunneling microscopy (STM) and force field simulations were utilized to explore two molecular systems where two and four carboxyl groups are symmetrically modified onto a skeleton. The two target molecules are 4,4'-(ethyne-1,2-diyl) dibenzoic acid (EBA) and 1,1'-ethynebenzene-3,3',5,5,'-tetracarboxylic acid (TCA). The former molecular assembly led to robust close packing, whereas the latter resulted in low-density arrangements that present significant adaption, namely, undergoing phase transformations upon external stimulations, e.g., sensitive to STM-polarity switching and guest molecule incorporations.

Formation of Supramolecular Heterostacks at the Liquid-Solid Interface: Impact of Symmetry Mismatching on Structural Growth

The construction of intricate three-dimensional (3D) nanoarchitectures on surfaces through molecular self-assembly attracts attention not only from a crystal engineering viewpoint but also because of its potential in a range of applications, given the current interest in van der Waals heterostructures. We herein report the formation of porous structures on alkane buffer layers on graphite. A dehydrobenzo[12]annulene (DBA) derivative having six decyloxy chains forms hexagonal structures on n-pentacontane and n-hexacontane buffer layers through van der Waals interactions at the 1-octanoic acid/graphite interface. The structural features are very similar to those on the graphite surface, except for the slight structural distortion, which is attributed to the p2 symmetry of the buffer layer underneath. Moreover, based on the observation of small clusters of the DBA molecules, we discussed the nucleation and structural growth of the DBA network on a buffer layer. Finally, a hierarchical multicomponent structure was formed through the coadsorption of a heteromolecular cluster formed by a hydrogen-bonded isophthalic acid cyclic hexamer hosting a coronene molecule on the buffer layer. This study on supramolecular heterostacks provides insights into the construction of intricate 3D nanoarchitectures using self-assembly at interfaces.

Homochiral hierarchical molecular assemblies through dynamic combination of conformational states of a single chiral building block at the liquid/solid interface

We herein report the construction of homochiral, hierarchical self-assembled molecular networks (SAMNs) at the liquid/graphite interface using a single molecular building block, a chiral dehydrobenzo[12]annulene (cDBA) derivative with three chiral alkoxy and three hydroxy groups positioned in an alternating manner on the DBA core. The cDBA molecules form homochiral hierarchical SAMNs consisting of triangular clusters of several sizes, the size of which can be tuned by solvent polarity and solute concentration, reaching periodicities as large as 9.3 nm. We demonstrate the successful transmission of chirality information from the single molecular level to the hierarchical SAMN level, in a process that is mediated by dynamic self-sorting.

Controlled Construction of an Exquisite Three-Component Co-assembly Supramolecular Structure at the Liquid-Solid Interface

A three-component supramolecular co-assembly structure formed at the liquid-solid interface by employing a shape-persistent π-conjugated macrocycle (1_6mer) and two guest molecules (COR and C₆₀) is demonstrated. Scanning tunneling microscopy (STM) observations revealed that 1_6mer can serve as a versatile host molecule that can co-assemble with both COR and C₆₀ guest molecules to form stable two-component structures, where the COR guest molecule filled in the gap between the side chains of adjacent 1_6mer molecules, and the C₆₀ guest molecule entered the inner cavity of 1_6mer. It was found that the adding sequence of COR and C₆₀ guest molecules is crucial to the resulting co-adsorption structure in the three-component system. To obtain the intriguing 1_6mer-COR-C₆₀ three-component co-assembly structure, the 1_6mer and COR two-component co-assembly structure should first be constructed on a HOPG surface, followed by addition of C₆₀. Based on the analysis of the STM results and the density functional theory (DFT) calculations, the formation mechanism of the assembled structures was revealed.

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.

Self-Assembled Two-Dimensional Nanoporous Crystals as Molecular Sieves: Molecular Dynamics Studies of 1,3,5-Tristyrilbenzene-Cn Superstructures

Due to their unique geometry complex, self-assembled nanoporous 2D molecular crystals offer a broad landscape of potential applications, ranging from adsorption and catalysis to optoelectronics, substrate processes, and future nanomachine applications. Here we report and discuss the results of extensive all-atom Molecular Dynamics (MD) investigations of self-assembled organic monolayers (SAOM) of interdigitated 1,3,5-tristyrilbenzene (TSB) molecules terminated by alkoxy peripheral chains Cn containing n carbon atoms (TSB3,5-Cn) deposited onto highly ordered pyrolytic graphite (HOPG). In vacuo structural and electronic properties of the TSB3,5-Cn molecules were initially determined using ab initio second order Møller-Plesset (MP2) calculations. The MD simulations were then used to analyze the behavior of the self-assembled superlattices, including relaxed lattice geometry (in good agreement with experimental results) and stability at ambient temperatures. We show that the intermolecular disordering of the TSB3,5-Cn monolayers arises from competition between decreased rigidity of the alkoxy chains (loss of intramolecular order) and increased stabilization with increasing chain length (afforded by interdigitation). We show that the inclusion of guest organic molecules (e.g., benzene, pyrene, coronene, hexabenzocoronene) into the nanopores (voids formed by interdigitated alkoxy chains) of the TSB3,5-Cn superlattices stabilizes the superstructure, and we highlight the importance of alkoxy chain mobility and available pore space in the dynamics of the systems and their potential application in selective adsorption.

Influence of Magnetic Micelles on Assembly and Deposition of Porphyrin J-Aggregates

Clusters of superparamagnetic iron oxide nanoparticles (SPIONs) have been incorporated into the hydrophobic core of polyethylene glycol (PEG)-modified phospholipid micelles. Two different PEG-phospholipids have been selected to guarantee water solubility and provide an external corona, bearing neutral (SPIONs@PEG-micelles) or positively charged amino groups (SPIONs@NH2-PEG-micelles). Under acidic conditions and with specific mixing protocols (porphyrin first, PF, or porphyrin last, PL), the water-soluble 5,10,15,20-tetrakis-(4-sulfonatophenyl)-porphyrin (TPPS) forms chiral J-aggregates, and in the presence of the two different types of magnetic micelles, an increase of the aggregation rates has been generally observed. In the case of the neutral SPIONs@PEG-micelles, PL protocol affords a stable nanosystem, whereas PF protocol is effective with the charged SPIONs@NH2-PEG-micelles. In both cases, chiral J-aggregates embedded into the magnetic micelles (TPPS@SPIONs@micelles) have been characterized in solution through UV/vis absorption and circular/linear dichroism. An external magnetic field allows depositing films of the TPPS@SPIONs@micelles that retain their chiroptical properties and exhibit a high degree of alignment, which is also confirmed by atomic force microscopy.

Two-dimensional supramolecular crystal engineering: chirality manipulation

Two dimensional (2D) supramolecular crystal engineering, one of the most important strategies towards nanotechnology, is both a science and an industry. In the present review, the recent advances in 2D supramolecular crystal engineering through chirality manipulation on solid surfaces are summarized, with the aid of the scanning tunneling microscopy technique. On-surface chirality manipulation includes surface confined structural chirality formation, chirality transformation, chirality separation as well as chirality elimination, by using component exchange and different external stimuli. Under this principle, host-guest supramolecular interactions, solvent induction, temperature regulation and STM-tip driven orientation control and reorientation effects under equilibrium or out-of-equilibrium conditions, towards the generation of the best-adapted chiral or achiral 2D nanostructures, are mainly described and highlighted. Future challenges and opportunities in this exciting area are also then discussed.

Temperature-Triggered Self-Assembled Structural Transformation: From Pure Hydrogen-Bonding Quadrilateral Nanonetwork to Trihexagonal Structures

Adequate control over the structures of molecular building blocks plays an important role in the fabrication of desired supramolecular nanostructures at interfaces. In this study, the formation of a pure hydrogen-bonding co-assembly supramolecular nanonetwork on a highly oriented pyrolytic graphite surface was demonstrated by means of a scanning tunneling microscope. The thermal annealing process was conducted to monitor the temperature-triggered structural transformation of the self-assembled nanonetwork. On the basis of the single-molecule-level resolution scanning tunneling microscopy images, together with the density functional theory calculations, the formation mechanisms of the formed nanoarrays were proposed. The results have great significance with regard to controlled construction of complex nanostructures on the surface.

Chlorine-assisted fabrication of hybrid supramolecular structures via electrostatic interactions

Supramolecular self-assembly is a spontaneous process relying on non-covalent intermolecular interactions. Among them, electrostatic interactions generated by organic molecules interacting with alkali metals and/or halogens play an important role in structural formation. Herein, we choose cytosine and NaCl as a model system and, from the interplay of STM imaging and DFT calculations, a hybrid network composed of both metal-organic and pure organic motifs interlinked by Cl ions via electrostatic interactions is observed on the Au(111) surface. Moreover, the occasionally missing Cl ions in connections are accompanied by the absence of adjacent organic motifs resulting in defects of the network. This study successfully demonstrates the generality of salt providing both cations and anions simultaneously in the modulation of the structure and provides fundamental knowledge on the formation of hybrid structures as well as the function of halogens in affecting the self-assembly process.

Formation of multicomponent 2D assemblies of C 2v-symmetric terphenyl tetracarboxylic acid at the solid/liquid interface: recognition, selection, and transformation

We report on the two-dimensional self-assembly of C_2v-symmetric [1,1′:3′,1′′-terphenyl]-3,3′′,5,5′′-tetracarboxylic acid (TPTA) at the solid/liquid interface by using scanning tunneling microscopy (STM). Two kinds of different self-assembly structure, i.e. a close-packed and porous rosette structure, are formed by TPTA molecules through intermolecular hydrogen bonds. When adding coronene (COR) as a guest into the TPTA assembly, structural transformation from a densely packed row structure to a rosette network structure is observed. It was found that two kinds of cavities with different sizes in the rosette network structure can be used to realize the selective co-adsorption of guest molecules with appropriate shape and size. Three-component 2D host–guest structures were successfully constructed by using 1,2,3,4,5,6-hexakis(4-bromophenyl)benzene (HBPBE) and copper phthalocyanine (CuPc) as guest molecules.

The formation process of multicomponent 2D assemblies of C_2v-symmetric terphenyl tetracarboxylic acid on a surface.

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.

Bottom-Up Self-Assembled Supramolecular Structures Built by STM at the Solid/Liquid Interface

One of the lines of research on organic devices is focused on their miniaturization to obtain denser and faster electronic circuits. The challenge is to build devices adding atom by atom or molecule by molecule until the desired structures are achieved. To do this job, techniques able to see and manipulate matter at this scale are needed. Scanning tunneling microscopy (STM) has been the selected technique by scientists to develop smart and functional unimolecular devices. This review article compiles the latest developments in this field giving examples of supramolecular systems monitored and fabricated at the molecular scale by bottom-up approaches using STM at the solid/liquid interface.

Template-Driven Dense Packing of Pentagonal Molecules in Monolayer Films

The integration of molecules with irregular shape into a long-range, dense and periodic lattice represents a unique challenge for the fabrication of engineered molecular scale architectures. The tiling of pentagonal molecules on a two-dimensional (2D) plane can be used as a proof-of-principle investigation to overcome this problem because basic geometry dictates that a 2D surface cannot be filled with a periodic arrangement of pentagons, a fundamental limitation that suggests that pentagonal molecules may not be suitable as building blocks for dense films. However, here we show that the 2D covalent organic framework (COF) known as COF-1 can direct the growth of pentagonal guest molecules as dense crystalline films at the solution/solid interface. We find that the pentagonal molecule corannulene adsorbs at two different sites on the COF-1 lattice, and that multiple molecules can adsorb into well-defined clusters patterned by the COF. Two types of these dense periodic packing motifs lead to a five-fold symmetry reduction compatible with translational symmetry, one of which gives an unprecedented high molecular density of 2.12 molecules/nm².

Programmable Multicomponent Self-Assembly Based on Aromatic Amino Acids

Construction of integrated self-assembly with ordered structures from two or more organic building blocks is currently a challenge, since it suffers from intrinsic systematic complexity and diverse competitive pathways. Here, it is reported that aromatic amino acid building units can be incorporated into two- or three-component coassembly driven primarily by hydrogen bonding interactions without the assistance of metal-ligand and macrocycle-based host-guest interactions. The key strategy is to employ a C₃ -symmetric molecule with alternative hydrogen bonding donor/acceptor sites that are able to bind either carboxylic acid or pyridine appended building units. Aromatic amino acids, C₃ -symmetric compound, and bipyridine unit constitute a unique ternary mutual binding system, where three coassembly pathways including two pairwise formations and one ternary combination are unveiled, giving rise to two- and three-component self-assemblies with ordered structures, respectively. The pathway complexity lies in the structural parameter of aromatic amino acids, which can be programmable by controlling substituents at the α-position of amino acids.

Probing functional self-assembled molecular architectures with solution/solid scanning tunnelling microscopy

Over the past two decades, solution/solid STM has made clear contributions to our fundamental understanding of the thermodynamic and kinetic processes that occur in molecular self-assembly at surfaces. As the field matures, we provide an overview of how solution/solid STM is emerging as a tool to elucidate and guide the use of self-assembled molecular systems in practical applications, focusing on small molecule device engineering, molecular recognition and sensing and electronic modification of 2D materials.

Supramolecular Assemblies on Surfaces: Nanopatterning, Functionality, and Reactivity

Understanding how molecules interact to form large-scale hierarchical structures on surfaces holds promise for building designer nanoscale constructs with defined chemical and physical properties. Here, we describe early advances in this field and highlight upcoming opportunities and challenges. Both direct intermolecular interactions and those that are mediated by coordinated metal centers or substrates are discussed. These interactions can be additive, but they can also interfere with each other, leading to new assemblies in which electrical potentials vary at distances much larger than those of typical chemical interactions. Earlier spectroscopic and surface measurements have provided partial information on such interfacial effects. In the interim, scanning probe microscopies have assumed defining roles in the field of molecular organization on surfaces, delivering deeper understanding of interactions, structures, and local potentials. Self-assembly is a key strategy to form extended structures on surfaces, advancing nanolithography into the chemical dimension and providing simultaneous control at multiple scales. In parallel, the emergence of graphene and the resulting impetus to explore 2D materials have broadened the field, as surface-confined reactions of molecular building blocks provide access to such materials as 2D polymers and graphene nanoribbons. In this Review, we describe recent advances and point out promising directions that will lead to even greater and more robust capabilities to exploit designer surfaces.

Hierarchical formation of Fe-9eG supramolecular networks via flexible coordination bonds

From the interplay between high-resolution scanning tunneling microscopy imaging/manipulations and density functional theory calculations, we display the hierarchical formation of supramolecular networks by codeposition of 9eG molecules and Fe atoms on Au(111) based on the flexible coordination bonds (the adaptability and versatility in the coordination modes). In the first step, homochiral islands composed of homochiral G₄Fe₂ motifs are formed; and then in the second step, thermal treatment results in the transformation into the porous networks composed of heterochiral G₄Fe₂ motifs with the ratio of the components being constant. In situ STM manipulations and the coexistence of some other heterochiral G₄Fe₂ motifs and clusters also show the flexibility of the coordination bonds involved. These studies may provide a fundamental understanding of the regulations of multilevel supramolecular structures and shed light on the formation of designed supramolecular nanostructures.

Host-guest chemistry in two-dimensional supramolecular networks

Nanoporous supramolecular networks physisorbed on solid surfaces have been extensively used to immobilize a variety of guest molecules. Host-guest chemistry in such two-dimensional (2D) porous networks is a rapidly expanding field due to potential applications in separation technology, catalysis and nanoscale patterning. Diverse structural topologies with high crystallinity have been obtained to capture molecular guests of different sizes and shapes. A range of non-covalent forces such as hydrogen bonds, van der Waals interactions, coordinate bonds have been employed to assemble the host networks. Recent years have witnessed a surge in the activity in this field with the implementation of rational design strategies for realizing controlled and selective guest capture. In this feature article, we review the development in the field of surface-supported host-guest chemistry as studied by scanning tunneling microscopy (STM). Typical host-guest architectures studied on solid surfaces, both under ambient conditions at the solution-solid interface as well as those formed at the ultrahigh vacuum (UHV)-solid interface, are described. We focus on isoreticular host networks, hosts functionalized pores and dynamic host-guest systems that respond to external stimuli.

Transfer of chiral information from a chiral solvent to a two-dimensional network

Chiral induction in self-assembled monolayers has garnered considerable attention in the recent past, not only due to its importance in chiral resolution and enantioselective heterogeneous catalysis but also because of its relevance to the origin of homochirality in life. Here, we demonstrate the emergence of homochirality in a supramolecular low-density network formed by achiral molecules at the interface of a chiral solvent and an atomically-flat achiral substrate. We focus on the impact of structure and functionality of the adsorbate and the chiral solvent on the chiral induction efficiency in self-assembled physisorbed monolayers, as revealed by scanning tunneling microscopy. Different induction mechanisms are proposed and evaluated, with the assistance of advanced molecular modeling simulations.

Two side chains, three supramolecules: exploration of fluorenone derivatives towards crystal engineering

Structural diversity obtained through two-dimensional molecular self-assembly induced by the chain length effect has gained immense attention, not only because of its significance in crystal engineering but also for its potential application in nanoscience and nanotechnology. Three kinds of fluorenone derivative, named F-C₇C₇, F-C₁₄C₇, and F-C₁₄C₁₄, were synthesized and used for systematic exploration of their crystalline difference. At first, scanning electron microscopy and X-ray powder diffraction were performed to investigate their differences in morphology and three-dimensional crystal structure. Then scanning tunneling microscopy experiments were conducted to compare the self-assembled monolayers. Moreover, different solvents were used to repeatedly investigate the occurrence of structural diversity. F-C₇C₇ could not self-assemble into a stable monolayer on the graphite surface under ambient conditions due to its weak molecule-substrate interaction. F-C₁₄C₇ was observed to self-assemble into twist, plier-like, octamer-curve, and random structures in 1-octanoic acid, 1-phenyloctane, n-tetradecane, and dichloromethane, respectively. However, when the same solvents were used and at similar concentrations, the F-C₁₄C₁₄ molecules were arranged into interval, mixed, linear, and plier-like configurations. These self-assembled nanopatterns formed under the driving forces of dipole-dipole interactions, hydrogen bonds, and chain-chain, molecule-substrate, and molecule-solvent van der Waals interactions. Furthermore, from the viewpoint of thermal analysis, differential scanning calorimetry, as well as polarized optical microscopy, was performed to further elucidate the difference between these three compounds in the solid and liquid crystal states. The present system is believed to provide understanding of how the chain length effect induces different crystalline properties, and to open up the possibility of fabricating diverse self-assembled networks for crystal engineering.

In Situ Atomic-Scale Observation of Droplet Coalescence Driven Nucleation and Growth at Liquid/Solid Interfaces

Unraveling dynamical processes of liquid droplets at liquid/solid interfaces and the interfacial ordering is critical to understanding solidification, liquid-phase epitaxial growth, wetting, liquid-phase joining, crystal growth, and lubrication processes, all of which are linked to different important applications in material science. In this work, we observe direct in situ atomic-scale behavior of Bi droplets segregated on SrBi₂Ta₂O₉ by using aberration-corrected transmission electron microscopy and demonstrate ordered interface and surface structures for the droplets on the oxide at the atomic scale and unravel a nucleation mechanism involving droplet coalescence at the liquid/solid interface. We identify a critical diameter of the formed nanocrystal in stabilizing the crystalline phase and reveal lattice-induced fast crystallization of the droplet at the initial stage of the coalescence of the nanocrystal with the droplet. Further sequential observations show the stepped coalescence and growth mechanism of the nanocrystals at the atomic scale. These results offer insights into the dynamic process at liquid/solid interfaces, which may have implications for many functionalities of materials and their applications.

Host-Guest Chemistry in Integrated Porous Space Formed by Molecular Self-Assembly at Liquid-Solid Interfaces

Host-guest chemistry in two-dimensional (2D) space, that is, physisorbed monolayers of a single atom or a single molecular thickness on surfaces, has become a subject of intense current interest because of perspectives for various applications in molecular-scale electronics, selective sensors, and tailored catalysis. Scanning tunneling microscopy has been used as a powerful tool for the visualization of molecules in real space on a conducting substrate surface. For more than a decade, we have been investigating the self-assembly of a series of triangle-shaped phenylene-ethynylene macrocycles called dehydrobenzo[12]annulenes (DBAs). These molecules are substituted with six alkyl chains and are capable of forming hexagonal porous 2D molecular networks via van der Waals interactions between interdigitated alkyl chains at the interface of organic solvents and graphite. The dimension of the nanoporous space or nanowell formed by the self-assembly of DBAs can be controlled from 1.6 to 4.7 nm by simply changing the alkyl chain length from C₆ to C₂₀. Single molecules as well as homoclusters and heteroclusters are capable of coadsorbing within the host matrix using shape- and size-complementarity principles. Moreover, on the basis of the versatility of the DBA molecules that allows chemical modification of the alkyl chain terminals, we were able to decorate the interior space of the nanoporous networks with functional groups such as azobenzenedicarboxylic acid for photoresponsive guest adsorption/desorption or fluoroalkanes and tetraethylene glycol groups for selective guest binding by electrostatic interactions and zinc-porphyrin units for complexation with a guest by charge-transfer interactions. In this Feature Article, we describe the general aspects of molecular self-assembly at liquid/solid interfaces, followed by the formation of programmed porous molecular networks using rationally designed molecular building blocks. We focus on our own work involving host-guest chemistry in integrated nanoporous space that is modified for specific purposes.

Templated bilayer self-assembly of fully conjugated π-expanded macrocyclic oligothiophenes complexed with fullerenes

Fully conjugated macrocyclic oligothiophenes exhibit a combination of highly attractive structural, optical and electronic properties, and multifunctional molecular thin film architectures thereof are envisioned. However, control over the self-assembly of such systems becomes increasingly challenging, the more complex the target structures are. Here we show a robust self-assembly based on hierarchical non-covalent interactions. A self-assembled monolayer of hydrogen-bonded trimesic acid at the interface between an organic solution and graphite provides host-sites for the epitaxial ordering of Saturn-like complexes of fullerenes with oligothiophene macrocycles in mono- and bilayers. STM tomography verifies the formation of the templated layers. Molecular dynamics simulations corroborate the conformational stability and assign the adsorption sites of the adlayers. Scanning tunnelling spectroscopy determines their rectification characteristics. Current–voltage characteristics reveal the modification of the rectifying properties of the macrocycles by the formation of donor–acceptor complexes in a densely packed all-self-assembled supramolecular nanostructure.

Controlling the self-assembly of oligothiophene complexes that are used in multi-functional thin films can be challenging. Here the authors show a hierarchy of non-covalent interactions for robust self-assembly that orders Saturn-like complexes of fullerenes with oligothiophene macrocycles.

On the formation of concentric 2D multicomponent assemblies at the solution–solid interface

We report on the design and fabrication of a four-component supramolecular network consisting of three concentric shells around a central guest.

Switching the surface homochiral assembly by surface host–guest chemistry

Surface chirality could be switched by host–guest chemistry.

Periodic Functionalization of Surface-Confined Pores in a Two-Dimensional Porous Network Using a Tailored Molecular Building Block

We present here the periodic functionalization of a two-dimensional (2D) porous molecular network using a tailored molecular building block. For this purpose, a dehydrobenzo[12]annulene (DBA) derivative, 1-isoDBA, having an isophthalic acid unit connected by an azobenzene linker to a C12 alkyl chain and five C14 chains, was designed and synthesized. After the optimization of monolayer preparation conditions at the 1,2,4-trichlorobezene (TCB)/graphite interface, scanning tunneling microscopy (STM) observation of the self-assembled monolayer of 1-isoDBA revealed the formation of extended domains of a porous honeycomb-type molecular network, which consists of periodically located nanowells each functionalized by a cyclic hexamer of hydrogen-bonded isophthalic acid units and those without functional groups. This result demonstrates that the present strategy based on precise molecular design is a viable route to site-specific functionalization of surface-confined nanowells. The nanowells of different size can be used for guest coadsorption of different guests, coronene COR and hexakis[4-(phenylethynyl)phenylethynyl]benzene HPEPEB, whose size and shape match the respective nanowells. STM observation of a ternary mixture (1-isoDBA/COR/HPEPEB) at the TCB/graphite interface revealed the site-selective immobilization of the two different guest molecules at the respective nanowells, producing a highly ordered three-component 2D structure.

2D Cocrystallization from H-Bonded Organic Ferroelectrics

The synthesis of 2D H-bonded cocrystals from the room-temperature ferroelectric organics croconic acid (CA) and 3-hydroxyphenalenone (3-HPLN) is demonstrated through self-assembly on a substrate under ultrahigh vacuum. 2D cocrystal polymorphs of varied stoichiometry were identified with scanning tunneling microscopy, and one of the observed structural building blocks consists of two CA and two 3-HPLN molecules. Computational analysis with density functional theory confirmed that the experimental (CA)2(3-HPLN)2 tetramers are lower in energy than single-component structures due to the ability of the tetramers to pack efficiently in two dimensions, the promotion of favorable electrostatic interactions between tetramers, and the optimal number of intermolecular hydrogen bonds. The structures investigated, especially the experimentally found tetrameric building blocks, are not polar. However, it is demonstrated computationally that cocrystallization can, in principle, result in heterogeneous structures with dipole moments that exceed those of homogeneous structures and that 2D structures with select stoichiometries could favor metastable polar structures.

Hydrogen-Bonded Macrocyclic Supramolecular Systems in Solution and on Surfaces

Cyclization into closed assemblies is the most recurrent approach to realize the noncovalent synthesis of discrete, well-defined nanostructures. This review article particularly focuses on the noncovalent synthesis of monocyclic hydrogen-bonded systems that are self-assembled from a single molecule with two binding-sites. Taking advantage of intramolecular binding events, which are favored with respect to intermolecular binding in solution, can afford quantitative amounts of a given supramolecular species under thermodynamic control. The size of the assembly depends on geometric issues such as the monomer structure and the directionality of the binding interaction, whereas the fidelity achieved relies largely on structural preorganization, low degrees of conformational flexibility, and templating effects. Here, we discuss several examples described in the literature in which cycles of different sizes, from dimers to hexamers, are studied by diverse solution or surface characterization techniques.

Water stability of self-assembled peptide nanostructures for sequential formation of two-dimensional interstitial patterns on layered materials

Sequential-assembly of LEY and GrBP5 peptides on a graphite surface.

Selection of conformational states in surface self-assembly for a molecule with eight possible pairs of surface enantiomers

Chiral adsorption of a complex molecule with as many as eight possible pairs of surface enantiomers is investigated by STM and the selection of enantiomers is understood by statistical analysis and DFT modelling.

Formation of Multicomponent Star Structures at the Liquid/Solid Interface

To demonstrate key roles of multiple interactions between multiple components and multiple phases in the formation of an uncommon self-assembling pattern, we present here the construction of a porous hexagonal star (h-star) structure using a trigonal molecular building block at the liquid/solid interface. For this purpose, self-assembly of hexaalkoxy-substituted dehydrobenzo[12]annulene derivatives DBA-OCns was investigated at the tetradecane/graphite interface by means of scanning tunneling microscopy (STM). Monolayer structures were significantly influenced by coadsorbed tetradecane molecules depending on the alkyl chains length (C13-C16) of DBA-OCn. However, none of DBA-OCn molecules formed the expected trigonal complexes, indicating that an additional driving force is necessary for the formation of the trigonal complex and its assembly into the h-star structure. As a first approach, we employed the "guest induced structural change" for the formation of the h-star structure. In the presence of two guest molecules, nonsubstituted DBA and hexakis(phenylethynyl)benzene which fit the respective pores, an h-star structure was formed by DBA-OC15 at the tetradecane/graphite interface. Moreover, a tetradecane molecule was coadsorbed between a pair of alkyl chains of DBA-OC15, thereby blocking the interdigitation of the alkyl chain pairs. Therefore, the h-star structure results from the self-assembly of the four molecular components including the solvent molecule. The second approach is based on aggregation of perfluoroalkyl chains via fluorophilicity of DBA-F, in which the perfluoroalkyl groups are substituted at the end of three alkyl chains of DBA-OCn via p-phenylene linkers. A trigonal complex consisting of DBA-F and three tetradecane molecules formed an h-star structure, in which the perfluoroalkyl groups that orient into the alkane solution phase aggregated at the hexagonal pore via fluorophilicity. The present result provides useful insight into the design and control of complex molecular self-assembly at the liquid/solid interface.

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.

Self-assembly behavior of alkylated isophthalic acids revisited: concentration in control and guest-induced phase transformation

The engineering of two-dimensional crystals by physisorption-based molecular self-assembly at the liquid-solid interface is a powerful method to functionalize and nanostructure surfaces. The formation of high-symmetry networks from low-symmetry building blocks is a particularly important target. Alkylated isophthalic acid (ISA) derivatives are early test systems, and it was demonstrated that to produce a so-called porous hexagonal packing of plane group p6, i.e., a regular array of nanowells, either short alkyl chains or the introduction of bulky groups within the chains were mandatory. After all, the van der Waals interactions between adjacent alkyl chains or alkyl chains and the surface would dominate the ideal hydrogen bonding between the carboxyl groups, and therefore, a close-packed lamella structure (plane group p2) was uniquely observed. In this contribution, we show two versatile approaches to circumvent this problem, which are based on well-known principles: the "concentration in control" and the "guest-induced transformation" methods. The successful application of these methods makes ISA suitable building blocks to engineer a porous pattern, in which the distance between the pores can be tuned with nanometer precision.

Multicomponent self-assembly with a shape-persistent N-heterotriangulene macrocycle on Au(111)

Multicomponent network formation by using a shape-persistent macrocycle (MC6) at the interface between an organic liquid and Au(111) surface is demonstrated. MC6 serves as a versatile building block that can be coadsorbed with a variety of organic molecules based on different types of noncovalent interactions at the liquid-solid interface. Scanning tunneling microscopy (STM) reveals the formation of crystalline bicomponent networks upon codeposition of MC6 with aromatic molecules, such as fullerene (C60) and coronene. Tetracyanoquinodimethane, on the other hand, was found to induce disorder into the MC6 networks by adsorbing on the rim of the macrocycle. Immobilization of MC6 itself was studied in two different noncovalently assembled host networks. MC6 assumed a rather passive role as a guest and simply occupied the host cavities in one network, whereas it induced a structural transition in the other. Finally, the central cavity of MC6 was used to capture C60 in a complex three-component system. Precise immobilization of organic molecules at discrete locations within multicomponent networks, as demonstrated here, constitutes an important step towards bottom-up fabrication of functional surface-based nanostructures.

Formation of hydroxyl-functionalized stilbenoid molecular sieves at the liquid/solid interface on top of a 1-decanol monolayer

Specific molecular tectons can be designed to form molecular sieves through self-assembly at the solid-liquid interface. After demonstrating a model tecton bearing apolar alkyl chains, we then focus on a modified structure involving asymmetric functionalization of some alkyl chains with polar hydroxyl groups in order to get chemical selectivity in the sieving. As the formation of supramolecular self-assembled networks strongly depends on molecule-molecule, molecule-substrate and molecule-solvent interactions, we compared the tectons' self-assembly on graphite for two types of solvent. We demonstrate the possibility to create hydroxylated stilbenoid molecular sieves by using 1-decanol as a solvent. Interestingly, with this solvent, the porous network is developed on top of a 1-decanol monolayer.

Temperature-induced transitions of self-assembled phthalocyanine molecular nanoarrays at the solid-liquid interface: from randomness to order

A promising approach to create functional nanoarrays is supramolecular self-assembly at liquid-solid interfaces. In the present investigation, we report on the self-assembly of phthalocyanine arrays using triphenylene-2,6,10-tricarboxylic acid (H₃TTCA) as a molecular nanotemplate. Five different metastable arrays are achieved in the study, including a thermodynamically stable configuration. Scanning tunneling microscopy (STM) measurements and density function theory (DFT) calculations are utilized to reveal the formation mechanism of the molecular nanoarrays. In general, the transformation process of nanoarrays is regulated by the synergies of a template effect and thermodynamic balance.

Efficient molecular recognition based on nonspecific van der Waals interaction at the solid/liquid interface

A surprising recognition phenomenon based on van der Waals interactions was observed, which proves that the design of the supramolecular assembly from its building blocks represents a highly promising and general strategy.

Bicomponent blend-directed amplification of the alkyl chain effect on the 2D structures

The 2D structures of bicomponent blends in isobutenyl compounds were observed by using scanning tunneling microscopy at the solid/liquid interface. Amplification of the alkyl chain effect was found on the 2D structures.

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.

Odd or even? Monolayer domain size depends on diyne position in alkadiynylanthracenes

1,5-(Alkadiynyl)anthracenes self-assemble single component and multicomponent monolayers at the solution-HOPG interface. An alkadiynyl chain's kinked shape constrains the molecular structures with which it can close-pack. This affords rudimentary molecular recognition that has been used to direct self-assembly of 1-D patterned, multicomponent monolayers. The unit cell building blocks of single- and multicomponent alkadiynylanthracene monolayers repeat with high fidelity for 100s of nanometers along the side chain direction. Unit cell repeat fidelity along the orthogonal, anthracene column direction of the monolayer depends on diyne location within the side chain; even-position diyne side chains produce high fidelity of unit cell repeats and wider domain widths along the anthracene columns, whereas odd-position diyne side chains produce more frequent domain interfaces that disrupt the anthracene columns. Alkadiynylanthracene monolayers may be viewed as stacks of 1-D molecular tapes. 1-D tape molecular composition, sequence, and intratape side chain alignment are dictated by shape complementarity of the kinked alkadiynyl side chains. Stacking alignments of adjacent 1-D tapes are controlled by shape matching of tape peripheries and determine repeat fidelity along the anthracene columns. Tapes stacked with a constant intertape alignment comprise crystalline domains that repeat along the anthracene columns. The 1-D tapes formed by anthracenes with odd-position diynes have triangle wave peripheries that close-pack in multiple stacking alignments. This reduces unit cell repeat fidelity and decreases the widths of crystalline domains along the anthracene columns. Even-position diyne side chains form 1-D tapes with trapezoid wave peripheries that close-pack in only one stacking alignment. This generates higher stacking fidelity, larger domain widths, and fewer domain interfaces along the anthracene columns of even-position diyne monolayers. Even- and odd-position diyne monolayers exhibit comparable densities of interfaces between enantiotopic domains and between domains aligned along different graphite symmetry axes. These interfaces likely arise through collisions of independently nucleated/growing domains and persist for lack of kinetically competent pathways that interconvert or merge the domains.

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.