A 2D Substitutional Solid Solution through Hydrogen Bonding of Molecular Building Blocks

Two-dimensional (2D) molecular self-assembly allows for the formation of well-defined supramolecular layers with tailored geometrical, compositional, and chemical properties. To date, random intermixing and entropic effects in these systems have largely been associated with crystalline disorder and glassy phases. Here we describe a 2D crystalline self-assembled molecular system that exhibits random incorporation of substitutional molecules. The lattice is formed from a mixture of trimesic acid (TMA) and terthienobenzenetricarboxylic acid (TTBTA), C₃-symmetric hydrogen-bonding units of very different sizes (0.79 and 1.16 nm, respectively), at the solution-highly oriented pyrolitic graphite (HOPG) interface. Remarkably, the TTBTA substitutes into the TMA lattice at a fixed stoichiometry near 12%. The resulting lattice constant is consistent with Vegard's law prediction for an alloy with a composition TMA_0.88TTBTA_0.12, and the substrate orientation of the lattice is defined by an epitaxial relation with the HOPG substrate. The Gibbs free energy for the TMA/TTBTA lattice was elucidated by considering the entropy of intermixing, via Monte Carlo simulations of multiplicity of the substitutional lattices, and the enthalpy of intermixing, via density functional theory calculations. The latter show that both the bond enthalpy of the H-bonded lattice and the adsorption enthalpy of the molecule/substrate interactions play important roles. This work provides insight into the manifestation of entropy in a molecular crystal constrained by both epitaxy and intermolecular interactions and demonstrates that a randomly intermixed yet crystalline 2D solid can be formed through hydrogen bonding of molecular building blocks of very different size.

Hierarchical molecular self-assemblies: construction and advantages

Hierarchical molecular self-assembly offers many exotic and complicated nanostructures which are of interest in nanotechnology and material science. In the past decade, various strategies leading to hierarchical molecular self-assemblies have been developed. In this review we summarize the recent advances in the creation and application of solution-based self-assembled nanostructures that involve more than one level of arrangement of building blocks. The strategies for construction hierarchical self-assembled structures and the advantages brought up by these assemblies are focused on. The following contents are included: (1) general approaches to fabricate hierarchical self-assembly, including self-assemblies based on supramolecules and specially designed block copolymers; (2) the advantages brought about by the hierarchical self-assembly, including the fabrication of special self-assembled structures, rich responsiveness to external stimuli, and the materials' performance.

Influence of the solvent on the stability of bis(terpyridine) structures on graphite

The effect of solvation on the adsorption of organic molecules on graphite at room temperature has been addressed with force-field molecular dynamics simulations. As a model system, the solvation of a bis(terpyridine) isomer in water and 1,2,4-trichlorobenzene was studied with an explicit solvation model. The inclusion of solvation has a noticeable effect on adsorption energies. Although the results of the various considered force fields differ quite significantly, they all agree that the adsorption of BTP from the TCB solvent is almost thermoneutral. The substrate simply acts as a template to allow a planar arrangement of the network, which is stabilized by the intermolecular interaction. Using an atomic thermodynamics approach, the order of the stability of various network structures as a function of the chemical potential is derived yielding a sequence in agreement with the experiment.

Hierarchically organized bimolecular ladder network exhibiting guided one-dimensional diffusion

The assembly and dynamics of a hierarchical, bimolecular network of sexiphenyl dicarbonitrile and N,N'-diphenyl oxalic amide molecules on the Ag(111) surface are studied by scanning tunneling microscopy at controlled temperature. The network formation is governed by a two-step protocol involving hierarchic interactions, including a novel carbonitrile-oxalic amide bonding motif. For temperatures exceeding ~70 K, more weakly bound sexiphenyl dicarbonitrile molecules carry out one-dimensional diffusion guided by the more stable substructure of the network held together by the carbonitrile-oxalic amide bonding motif. A theoretical investigation at the ab initio level confirms the different binding energies of the two coupling motifs and rationalizes the network formation and the diffusion pathway.

Incorporation dynamics of molecular guests into two-dimensional supramolecular host networks at the liquid-solid interface

The objective of this work is to study both the dynamics and mechanisms of guest incorporation into the pores of 2D supramolecular host networks at the liquid-solid interface. This was accomplished by adding molecular guests to prefabricated self-assembled porous monolayers and the simultaneous acquisition of scanning tunneling microscopy (STM) topographs. The incorporation of the same guest molecule (coronene) into two different host networks was compared, where the pores of the networks either featured a perfect geometric match with the guest (for trimesic acid host networks) or were substantially larger than the guest species (for benzenetribenzoic acid host networks). Even the moderate temporal resolution of standard STM experiments in combination with a novel injection system was sufficient to reveal clear differences in the incorporation dynamics in the two different host networks. Further experiments were aimed at identifying a possible solvent influence. The interpretation of the results is aided by molecular mechanics (MM) and molecular dynamics (MD) simulations.

Peculiar adsorbed phase behaviour of binary mixtures of oligopyridines and extension to a ternary mixture in a host-guest system

Binary mixtures of bis(terpyridine)s show a U shape behaviour in concentration dependent surface coverage at constant molar ratio. The phase separation can be exploited to create ternary mixtures with exclusive adsorption of the third component in one phase.

Septipyridines as conformationally controlled substitutes for inaccessible bis(terpyridine)-derived oligopyridines in two-dimensional self-assembly

The position of the peripheral nitrogen atoms in bis(terpyridine)-derived oligopyridines (BTPs) has a strong impact on their self-assembly behavior at the liquid/HOPG (highly oriented pyrolytic graphite) interface. The intermolecular hydrogen bonding interactions in these peripheral pyridine units show specific 2D structures for each BTP isomer. From nine possible constitutional isomers only four have been described in the literature. The synthesis and self-assembling behavior of an additional isomer is presented here, but the remaining four members of the series are synthetically inaccessible. The self-assembling properties of three of the missing four BTP isomers can be mimicked by making use of the energetically preferred N-C-C-N transoid conformation between 2,2'-bipyridine subunits in a new class of so-called septipyridines. The structures are investigated by scanning tunneling microscopy (STM) and a combination of force-field and first-principles electronic structure calculations.

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.

Bis(terpyridine)-based surface template structures on graphite: a force field and DFT study

Host-guest networks formed by ordered organic layers are promising candidates for applications in molecular storage and quantum computing. We have studied 2-dimensionally ordered surface template structures of bis(terpyridine)-derived molecules (BTPs) on graphite using force field and DFT methods and compared the results to recent experimental observations. In order to determine the force field best suited for surface calculations, bond lengths and angles, torsional potentials, adsorption and stacking energies of smaller aromatic molecules were calculated with different force fields (Compass, UFF, Dreiding and CVFF). Density functional perturbation theory calculations were used to study the intermolecular interactions between 3,3'-BTP molecules. Structural properties, adsorption energies and rotational barriers of the 3,3'-BTP surface structure and its host-guest systems with phthalocyanine (PcH(2)) or excess 3,3'-BTP as guest molecules have been addressed. In addition, STM images of oligopyridine and phthalocyanine molecules were simulated based on periodic and local density functional theory calculations.

A multivalent hexapod: conformational dynamics of six-legged molecules in self-assembled monolayers at a solid-liquid interface

A molecular hexapod having a benzene core and six oligo(p-phenylene vinylene) (OPV) legs is an ideal system to probe various types of (intramolecular) dynamics of individual molecules in physisorbed self-assembled monolayers at a solid-liquid interface. Scanning tunneling microscopy reveals that molecules adsorb in 2D crystalline as well as disordered domains. Strikingly, not all molecules have the six OPV units in contact with the graphite substrate: 4% of the molecules in the 2D crystalline domains and up to 80% of the molecules in the disordered domains have one or two OPV units desorbed. In addition, the presence of such a defect promotes the coexistence of another defect adjacent to it. Time-dependent STM experiments and molecular dynamics simulations reveal in detail the different dynamics involved and the multivalent nature of the interactions between hexapod and surface.

Adsorption of 4-mercaptopyridine on Au(111: a periodic DFT study

We have studied the adsorption of 4-mercaptopyridine (Mpy) on Au(111) using periodic density functional theory calculations. Isolated Mpy molecules adsorb preferentially at near-bridge sites in a tilted configuration. The interaction with water influences the adsorption of Mpy only weakly whereas the binding of ions to Mpy can lead to substantial structural changes in the Mpy adsorption geometry. At higher coverages, the molecules become more upright in order to allow for a denser packing in the self-assembled monolayers (SAMs). Simulated STM images of the 7 x mean square root of 3 and 5 x mean square root of 3 structures compare favorably with experimental results. Using an ab initio thermodynamics approach, we determined the most stable molecular structure as a function of the chemical potential of mercaptopyridine. The stability of the 7 x mean square root of 3 structure is confirmed, but the experimentally observed 5 x mean square root of 13 structure does not appear to be a thermodynamically stable structure. Several possible reasons for the discrepancy between theory and experiment are discussed.