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 surf...

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Supramolecular Assemblies on Surfaces: Nanopatterning, Functionality, and Reactivity

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

Temperature-Induced Structural Phase Transitions in a Two-Dimensional Self-Assembled Network

Self-assembled monolayers are a unique class of nanostructured materials, with properties determined by their molecular lattice structures, as well as the interfaces with their substrates and environments. As with other nanostructured materials, defects and dimensionality play important roles in the physical, chemical, and biological properties of the monolayers. In this review, we discuss monolayer structures ranging from surfaces (two-dimensional) down to single molecules (zero-dimensional), with a focus on applications of each type of structure, and on techniques that enable characterization of monolayer physical properties down to the single-molecule scale.

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From the bottom up: dimensional control and characterization in molecular monolayers

Two-dimensional molecular self-assembly at the liquid/solid interface is a widely employed approach in surface science to pattern surfaces at the nanometre scale. A multitude of supramolecular structures can be realized depending on parameters such as the functionalization of the molecular building blocks, the temperature at which self-assembly takes place, the type of solvent and solute concentration. How these and other parameters influence the kinetics and thermodynamics of the self-assembly process is the subject of this review.

Kinetics and thermodynamics in surface-confined molecular self-assembly

A liquid azobenzene derivative as a solvent-free solar thermal fuel

The self-assembly of 2,7-bis(10-ethoxycarbonyl-decyloxy)-9-fluorenone (BEF) has been investigated by scanning tunneling microscopy at the liquid/solid interface. The coexistence and reversible tran...

Two-Dimensional Self-Assembled Molecular Structures Formed by the Competition of van der Waals Forces and Dipole–Dipole Interactions

Fluorenone derivatives (F–OCn) with various lengths of peripheral alkyl chains (with carbon numbers of n = 12–18) were synthesized, and their self-assembled adlayers were investigated in solvents w...

Dipole-Controlled Self-Assembly of 2,7-Bis(n-alkoxy)-9-fluorenone: Odd–Even and Chain-Length Effects

The self-assembled behaviors of two fluorenone derivatives, 2,7-bis((11-hydroxyundecyl)oxy)-9-fluorenone (BHUF) and 2,7-bis((10-carboxydecyl)oxy)-9-fluorenone (BCDF), were investigated at the liquid–solid interface by scanning tunneling microscopy. Two solvents, 1-octanoic acid and 1-phenyloctane, were employed in consideration of their distinct polarity and solubility. It is observed that the BHUF molecules self-assemble into seven different polymorphs upon adsorption, while only two different polymorphs are observed in the BCDF monolayer. The theoretical calculation is performed to reveal the underlying mechanism. As compared to that of C═O···HO hydrogen bonds, the enhanced binding energy of intermolecular C═O···HOOC hydrogen bonds in the BCDF monolayer would dominate the intermolecular van der Waals (vdWs) interactions and the molecule–solvent interactions, thereby resulting in a limitation of expression of structural polymorphism. In addition, the concentration-dependent polymorphism as well as the re...

Hydroxyl versus Carboxyl Substituent: Effects of Competitive and Cooperative Multiple Hydrogen Bonds on Concentration-Controlled Self-Assembly

Halogen bonding with high specificity and directionality in the geometry has proven to be an important type of noncovalent interaction to fabricate and control 2D molecular architectures on surfaces. Herein, we first report how the orientation of the ester substituent for thienophenanthrene derivatives (5,10-DBTD and 5,10-DITD) affects positive charge distribution of halogens by density functional theory, thus determining the formation of an intermolecular halogen bond and different self-assembled patterns by scanning tunneling microscopy. The system presented here mainly includes heterohalogen X···O═C and X···S halogen bonds, H···Br and H···O hydrogen bonds, and I···I interaction, where the directionality and strength of such weak bonds determine the molecular arrangement by varying the halogen substituent. This study provides a detailed understanding of the role of ester orientation, concentration, and solvent effects on the formation of halogen bonds and proves relevant for identification of multiple h...

Controllable Orientation of Ester-Group-Induced Intermolecular Halogen Bonding in a 2D Self-Assembly.

Two-dimensional self-assembly of 2,7-ditridecyloxy-9-fluorenone (F-OC13) is investigated by scanning tunneling microscopy (STM) in solvents with different polarities and functional groups on a high oriented pyrolytic graphite surface. The STM images reveal that the self-assembly of F-OC13 is strongly solvent-dependent. 1-Phenyloctane can coadsorb on the self-assembly of F-OC13, and the structural transformation of the adlayer from the linear structure to alternate lamella can be observed with the decrease of the concentration. At the 1-octanol/HOPG interface, only a well-ordered linear pattern is obtained. The intermolecular hydrogen bonding between the 1-octanoic acid and the F-OC13 molecule is responsive for the formation of butterfly configuration. When n-tridecane or n-tetradecane is used as solvent, a regular alternate pattern is formed under high concentrations, and a coadsorbed lamellar structure is observed under low concentrations. Furthermore, when the sample with use of the methanol, dichlorome...

Li Xu

Papers

Two-Dimensional Self-Assembled Molecular Structures Formed by the Competition of van der Waals Forces and Dipole–Dipole Interactions

Dipole-Controlled Self-Assembly of 2,7-Bis(n-alkoxy)-9-fluorenone: Odd–Even and Chain-Length Effects

Hydroxyl versus Carboxyl Substituent: Effects of Competitive and Cooperative Multiple Hydrogen Bonds on Concentration-Controlled Self-Assembly

Controllable Orientation of Ester-Group-Induced Intermolecular Halogen Bonding in a 2D Self-Assembly.

Self-Assembly Polymorphism: Solvent-Responsive Two-Dimensional Morphologies of 2,7-Ditridecyloxy-9-fluorenone by Scanning Tunneling Microscopy