One Solvent Induces a Series of Structural Transitions in Monodendron Molecular Self-Assembly from Lamellar to Quadrangular to Hexagonal

Detection and Stabilization of a Previously Unknown Two-Dimensional (Pseudo)polymorph using Lateral Nanoconfinement

We report on the detection and stabilization of a previously unknown two-dimensional (2D) pseudopolymorph of an alkoxy isophthalic acid using lateral nanoconfinement. The self-assembled molecular networks formed by the isophthalic acid derivative were studied at the interface between covalently modified graphite and an organic solvent. When self-assembled on graphite with moderate surface coverage of covalently bound aryl groups, a previously unknown metastable pseudopolymorph was detected. This pseudopolymorph, which was presumably "trapped" in between the surface bound aryl groups, underwent a time-dependent phase transition to the stable polymorph typically observed on pristine graphite. The stabilization of the pseudopolymorph was then achieved by using an alternative nanoconfinement strategy, where the domains of the pseudopolymorph could be formed and stabilized by restricting the self-assembly in nanometer-sized shallow compartments produced by STM-based nanolithography carried out on a graphite surface with a high density of covalently bound aryl groups. These experimental results are supported by molecular mechanics and molecular dynamics simulations, which not only provide important insight into the relative stabilities of the different structures, but also shed light onto the mechanism of the formation and stabilization of the pseudopolymorph under nanoscopic lateral confinement.

Growth of a self-assembled monolayer decoupled from the substrate: nucleation on-command using buffer layers

Structural polymorphism is ubiquitous in physisorbed self-assembled monolayers formed at the solution-solid interface. One of the ways to influence network formation at this interface is to physically decouple the self-assembled monolayer from the underlying substrate thereby removing the influence of the substrate lattice, if any. Here we show a systematic exploration of self-assembly of a typical building block, namely 4-tetradecyloxybenzoic acid at the 1-phenyloctane-graphite interface in the presence and in the absence of a buffer layer formed by a long chain alkane, namely n-pentacontane. Using scanning tunneling microscopy (STM), three different structural polymorphs were identified for 4-tetradecyloxybenzoic acid at the 1-phenyloctane-graphite interface. Surprisingly, the same three structures were formed on top of the buffer layer, albeit at different concentrations. Systematic variation of experimental parameters did not lead to any new network in the presence of the buffer layer. We discovered that the self-assembly on top of the buffer layer allows better control over the nanoscale manipulation of the self-assembled networks. Using the influence of the STM tip, we could initiate the nucleation of small isolated domains of the benzoic acid on-command in a reproducible fashion. Such controlled nucleation experiments hold promise for studying fundamental processes inherent to the assembly process on surfaces.

Crystalline Domain Battery Materials

The development of next-generation energy storage materials for secondary batteries relies more and more on the delicate design and tailoring of their local structures and properties. Crystalline domain battery materials (CDBMs) are defined as a family of materials that are hierarchically engineered primarily by bonding selective atoms in certain space groups with short-range order to form nanoscale crystal domains as fundamental constructive and functional units, secondarily by integrating these interactive crystal domains under certain configurations into grains to implement electrochemical synergy, and finally by optimizing grains through nanoengineering toward advanced electrode materials. In CDBMs, adjacent crystal domains can undergo structural co-transformations with noticeable interrelationships, and the overall electrochemical performance is determined not only by the intrinsic structure of each crystal domain (element, bonding, valence, stacking, orientation, etc.) but also by the configuration of crystal domains (size, ratio, interface, distribution, interaction, etc.). Pioneering studies have shown significant enhancement of electrochemical performance by controlling crystal domains, suggesting the prospect of developing novel electrode materials through crystal-domain engineering. However, fundamental understanding and delicate fabrication of this material family, in terms of structural identification, electrochemical structure evolution, reaction mechanism, design and adjustment, and structure-performance relationship, among others, still face great challenges to meet the compelling requirements of high-performance electrode materials for secondary batteries. This Account systematically introduces the structure and electrochemistry of CDBMs. The efficient structural identification of crystal domains, which is still challenging due to their structural complexity, is demonstrated using prototype materials by advanced characterization techniques such as high-energy X-ray diffraction combined with Rietveld refinement and spherical aberration-corrected transmission electron microscopy. Investigations on the structural evolution of CDBMs in electrochemical reactions by ex-situ and in-situ techniques provide insights into reaction scenarios such as how ions migrate in and across crystal domains and how these crystal domains transform synergistically. A crystal-domain reaction mechanism is thus proposed to explain the electrochemistry of these materials. Design principles and adjustment strategies for designated crystal-domain structures including their components, ratios, distributions, and interfaces are deduced from the structural identification, evolution and reaction mechanism. The relationship between crystal-domain structures and electrochemical performance can further be elucidated, inspiring us to explore efficient strategies for optimizing the electrochemical performance, as validated by examples of high-performance batteries using materials with controlled crystal-domain structures. Based on these systematic studies, the trends in the rapid enrichment, deep investigation, and practical application of CDBMs are envisioned to promote continuous studies on this nascent energy storage material family.

Mesomorphic Behavior in Silver(I) N-(4-Pyridyl) Benzamide with Aromatic π⁻π Stacking Counterions

Organic semiconductor materials composed of π–π stacking aromatic compounds have been under intense investigation for their potential uses in flexible electronics and other advanced technologies. Herein we report a new family of seven π–π stacking compounds of silver(I) bis-N-(4-pyridyl) benzamide with varying counterions, namely [Ag(NPBA)2]X, where NPBA is N-(4-pyridyl) benzamine, X = NO₃⁻ (1), ClO₄⁻ (2), CF₃SO₃⁻ (3), PF₆⁻ (4), BF₄⁻ (5), CH₃PhSO₃⁻ (6), and PhSO₃⁻ (7), which form extended π−π stacking networks in one-dimensional (1D), 2D and 3D directions in the crystalline solid-state via the phenyl moiety, with average inter-ring distances of 3.823 Å. Interestingly, the counterions that contain π–π stacking-capable groups, such as in 6 and 7, can induce the formation of mesomorphic phases at 130 °C in dimethylformamide (DMF), and can generate highly branched networks at the mesoscale. Atomic force microscopy studies showed that 2D interconnected fibers form right after nucleation, and they extend from ~30 nm in diameter grow to reach the micron scale, which suggests that it may be possible to stop the process in order to obtain nanofibers. Differential scanning calorimetry studies showed no remarkable thermal behavior in the complexes in the solid state, which suggests that the mesomorphic phases originate from the mechanisms that occur in the DMF solution at high temperatures. An all-electron level simulation of the band gaps using NRLMOL (Naval Research Laboratory Molecular Research Library) on the crystals gave 3.25 eV for (1), 3.68 eV for (2), 1.48 eV for (3), 5.08 eV for (4), 1.53 eV for (5), and 3.55 eV for (6). Mesomorphic behavior in materials containing π–π stacking aromatic interactions that also exhibit low-band gap properties may pave the way to a new generation of highly branched organic semiconductors.

Competitive chiral induction in a 2D molecular assembly: Intrinsic chirality versus coadsorber-induced chirality

Noncovalently introducing stereogenic information is a promising approach to embed chirality in achiral molecular systems. However, the interplay of the noncovalently introduced chirality with the intrinsic chirality of molecules or molecular aggregations has rarely been addressed. We report a competitive chiral expression of the noncovalent interaction-mediated chirality induction and the intrinsic stereogenic center-controlled chirality induction in a two-dimensional (2D) molecular assembly at the liquid/solid interface. Two enantiomorphous honeycomb networks are formed by the coassembly of an achiral 5-(benzyloxy)isophthalic acid (BIC) derivative and 1-octanol at the liquid/solid interface. The preferential formation of the globally homochiral assembly can be achieved either by using the chiral analog of 1-octanol, (S)-6-methyl-1-octanol, as a chiral coadsorber to induce chirality to the BIC assembly via noncovalent hydrogen bonding or by covalently linking a chiral center in the side chain of BIC. Both the chiral coadsorber and the intrinsically chiral BIC derivative can act as a chiral seeds to induce a preferred handedness in the assembly of the achiral BIC derivatives. Furthermore, the noncovalent interaction-mediated chirality induction can restrain or even overrule the manifestation of the intrinsic chirality of the BIC molecule and dominate the handedness of the 2D molecular coassembly. This study provides insight into the interplay of intrinsically chiral centers and external chiral coadsorbers in the chiral induction, transfer, and amplification processes of 2D molecular assembly.

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

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

Self-Assembly under Confinement: Nanocorrals for Understanding Fundamentals of 2D Crystallization

Nanocorrals with different size, shape, and orientation are created on covalently modified highly oriented pyrolytic graphite surfaces using scanning probe nanolithography, i.e., nanoshaving. Alkylated diacetylene molecules undergo laterally confined supramolecular self-assembly within these corrals. When nanoshaving is performed in situ, at the liquid-solid interface, the orientation of the supramolecular lamellae structure is directionally influenced by the gradual graphite surface exposure. Careful choice of the nanoshaving direction with respect to the substrate symmetry axes promotes alignment of the supramolecular lamellae within the corral. Self-assembly occurring inside corrals of different size and shape reveals the importance of geometric and kinetic constraints controlled by the nanoshaving process. Finally, seed-mediated crystallization studies demonstrate confinement control over nucleation and growth principles.

The impact of grafted surface defects and their controlled removal on supramolecular self-assembly

We demonstrate the use of covalently modified graphite as a convenient and powerful test-bed for the versatile investigation and control of 2-D crystallization at the liquid solid interface. Grafted aryls act as surface defects and create barriers to supramolecular self-assembly. An easily tunable grafting density allows for varying the effect of such defects on supramolecular self-assembly. Finally, the defects can be locally removed, triggering monolayer reconstructions and allowing in situ investigations of thermodynamically unstable or metastable morphologies.

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

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

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.

Efficient screening of 2D molecular polymorphs at the solution-solid interface

Formation of multiple polymorphs during two-dimensional (2D) crystallization of organic molecules is more of a routine occurrence than rarity. Although such diverse crystalline structures provide exciting possibilities for studying crystal engineering in 2D, predicting the occurrence of polymorphs for a given building block is often non-trivial. Moreover, there is scarcity of methods that can experimentally verify the presence of such crystalline polymorphs in a straightforward fashion. Here we demonstrate a relatively simple experimental approach for screening of 2D polymorphs formed at the solution-solid interface. The strategy involves use of solution flow produced by contacting a piece of tissue paper to the sample to generate a lateral density gradient along the substrate surface. In situ generation of such gradient allows rapid discovery and nanoscale separation of multiple 2D polymorphs in a single experiment. The concept is demonstrated using three structurally different building blocks that differ in terms of intermolecular interactions responsible for 2D crystal formation. The method described here represents a powerful tool for efficient screening of 2D polymorphs formed at the solution-solid interface.

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.

Steric matching and the concentration induced self-assembled structural variety of 2,7-bis(n-alkoxy)-9-fluorenone at the aliphatic solvent/graphite interface

Controlling and unraveling structural polymorphism has received special attention in 2D self-assembled monolayers. In this work, we investigated the steric matching and solution concentration controlled structural variety in the self-assembly of 2,7-bis(n-alkoxy)-9-fluorenone (F-OCn) at the n-tetradecane and n-tridecane/graphite interface under different concentrations, respectively. Scanning tunneling microscopy (STM) revealed that the coadsorbed adlayers of F-OCn and solvents (n = 12 to 16) were formed and exhibited concentration dependent 2D phases due to the steric matching. The self-assembled monolayer of F-OCn (n = 12 to 16) evolved from a low-density coadsorbed linear lamellar packing, which was formed at low concentrations, to higher-density patterns at relatively high concentrations. F-OC14 exhibited a complex structural variety, in which a systematic trend of decrease in the molecular density per unit cell with decreasing concentration was obtained. Except for F-OCn (n = 13, 15, 17), the zigzag structure showing the linear lamella with dimers was observed. Systematic experiments revealed that the self-assembly of F-OCn was chain-length dependent. The results provide insight into the structural variety exhibited by a series of organic molecules and furnish important guidelines to control the morphology by changing the solution concentration.

Biaxial aromatics with face-on/edge-on stacking adaptability: an STM/STS study of 1D nanowires assembled via rotatable ethynyls

Biaxial aromatics with face-on/edge-on stacking adaptability.

Two-dimensional self-assemblies of telechelic organic compounds: structure and surface host-guest chemistry

Guiding the self-assembly of different types of functional molecules into well-defined structures on surfaces is beneficial for both fundamental surface and interface study and emerging application fields, especially molecular and organic electronics. This review focuses on understanding the two-dimensional self-assembly process of telechelic organics, which feature alkoxylene chains terminated with carboxyl groups. With the combined flexibility of alkyl chains and directionality of carboxyl groups, telechelic organics show unique assembly behaviour on two-dimensional surfaces. By increasing the length of the alkoxylene chains, the cavities in the nanoporous networks of telechelic trimesic acid (1,3,5-benzene tricarboxylic acid) derivatives change from hexagonal cavities to irregular cavities on a highly oriented pyrolytic graphite surface. The nanoporous networks provide a flexible host template for host-guest supramolecular chemistry because the cavities framed by the flexible alkoxylene chains can be changed in accordance with the sizes/shapes of the guest molecules. Furthermore, the terminal carboxylic group can form a hydrogen bond with another hydrogen bond partner, leading to multi-component structural motifs and hierarchical assemblies. The unique assembly behaviour of telechelic organics makes them promising structures as important building blocks for the design and construction of complex self-assembled nanoarchitectures.

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.

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.

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.

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

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

One building block, two different nanoporous self-assembled monolayers: a combined STM and Monte Carlo study

With the use of a single building block, two nanoporous patterns with nearly equal packing density can be formed upon self-assembly at a liquid-solid interface. Moreover, the formation of both of these porous networks can be selectively and homogenously induced by changing external parameters like solvent, concentration, and temperature. Finally, their porous properties are exploited to host up to three different guest molecules in a spatially resolved way.

Hydrogen bond partner reorganization in the coadsorption of a monodendron and pyridylethynyl derivatives

Hydrogen bonds with high selectivity and directionality are significant in harnessing molecules to form 2D supramolecular nanostructures. The competition and reorganization of hydrogen bond partners determine the ultimate molecular assembly and pattern in a 2D supramolecular system. In this study, multicomponent assemblies of a monodendron (5-benzyloxy-isophthalic acid derivative, BIC) and pyridylethynyl derivatives [1,4-bis(4-pyridylethynyl)-2,3-bis-dodecyloxy-benzene (PBPC12) and 1,4-bis(4-pyridylethynyl)-2,3-bis-octadecyloxy-benzene (PBPC18)] have been studied by scanning tunneling microscopy (STM) on a graphite surface. BIC molecules are able to associate with PBPC12 and PBPC18 molecules to induce the rearrangement of hydrogen bond partners and form coassembly structures. Interestingly, BIC acts as a template molecule in the coassembly process, and these multicomponent structures exhibit similar structural features to the assembly structures of BIC itself. The structural details of the coassembled structures are revealed by high-resolution STM images, and their relationship with the original BIC assemblies is discussed. These results provide important insights into the design and fabrication of hydrogen-bond-directed multicomponent molecular nanostructures on solid surfaces.

Engineering of linear molecular nanostructures by a hydrogen-bond-mediated modular and flexible host-guest assembly

The formation of a desired nanostructure with concomitant patterns and functions is of utmost importance in the field of surface molecular engineering and nanotechnology. We here present a flexible host-guest assembly, which steers the formation of linear molecular nanostructures on surfaces by a hydrogen-bond-mediated assembly process. A linear monodendron molecular template with periodic hydrogen-bond binding sites is shown to accommodate a variety of molecules with pyridylethynyl terminals. The unit cell parameters in the transverse direction of the linear pattern can be tuned from 3.4 to 7.3 nm in response to the packing of the guest molecules with different sizes, shapes, and aggregation number. The introduction of hydrogen-bonding partners into the host template and into guest molecules is responsible for the steering of the linear pattern of guest molecules. The modular approach could greatly facilitate the ordering of guest molecules with desired functional moieties.