Odd–even effect and metal induced structural convergence in self-assembled monolayers of bipyridine derivatives

Two-dimensional molecular networks at the solid/liquid interface and the role of alkyl chains in their building blocks

Nanoarchitectonics has attracted increasing attention owing to its potential applications in nanomachines, nanoelectronics, catalysis, and nanopatterning, which can contribute to overcoming global problems related to energy and environment, among others. However, the fabrication of ordered nanoarchitectures remains a challenge, even in two dimensions. Therefore, a deeper understanding of the self-assembly processes and substantial factors for building ordered structures is critical for tailoring flexible and desirable nanoarchitectures. Scanning tunneling microscopy is a powerful tool for revealing the molecular conformations, arrangements, and orientations of two-dimensional (2D) networks on surfaces. The fabrication of 2D assemblies involves non-covalent interactions that play a significant role in the molecular arrangement and orientation. Among the non-covalent interactions, dispersion interactions that derive from alkyl chain units are believed to be weak. However, alkyl chains play an important role in the adsorption onto substrates, as well as in the in-plane intermolecular interactions. In this review, we focus on the role of alkyl chains in the formation of ordered 2D assemblies at the solid/liquid interface. The alkyl chain effects on the 2D assemblies are introduced together with examples documented in the past decades.

Chain-Length- and Concentration-Dependent Isomerization of Bithiophenyl-Based Diaminotriazine Derivatives in Two-Dimensional Polymorphic Self-Assembly§

Bithiophenyl-based diaminotriazine derivatives (2TDT-n, n = 10, 12, 16, and 18) with different chain lengths display colhex/p6mm mesophases. Their supramolecular self-assembled mechanism is investigated using scanning tunneling microscopy (STM) at the 1-octanoic acid/graphite interface at various concentrations. The chain length effect on the two-dimensional adlayers is observed in this system, and 2TDT-n molecules show a structural phase transition from the four-leaf arrangement to the two-row linear nanostructure accompanied by the emergence of molecular isomerization with the increase of the side-chain length. The self-assembled structure of 2TDT-10 is composed of a four-leaf pattern with uniform s-cis conformers. In 2TDT-12, three kinds of nanostructures (bamboo-like, two-row linear pattern-I, and flower-like) are observed. These nanostructures are randomly constituted by cis and trans conformers, and the ratios of the s-cis conformer in three kinds of patterns are 55.7, 42.3, and 62.5%, respectively. Furthermore, when n = 16 and 18, the ratio of the s-cis conformer further decreases to 19.0 and 4.3%, respectively. Those molecules mainly form linear nanostructures consisting of s-trans conformers. Therefore, it is reasonable to conclude that the side-chain length has a great effect on the self-assembled patterns and the molecular conformation of bithiophenyl-based diaminotriazine derivatives. Density functional theory calculations are applied to optimize molecular conformers and assess their single-point energies, showing that the s-cis conformation has higher energy than the s-trans conformer. We speculate that the ratio of two conformers in nanostructures might be similar to that of the liquid crystalline phase.

Rationally Designed Semiconducting 2D Surface-Confined Metal-Organic Network

Two-dimensional (2D) surface-confined metal-organic networks (SMONs) are metal-doped self-assembled monolayers of molecules on solid surfaces. We report the formation of uniform large-area solution-processed semiconducting SMONs of Pd and Zn with mellitic acid (MA) on a highly oriented pyrolytic graphite (HOPG) surface under ambient conditions. The microscopic structure is determined using scanning tunneling microscopy (STM), atomic force microscopy (AFM), and X-ray photoelectron spectroscopy (XPS). Using tunneling spectroscopy, we observed a reduction in the band gap of ≈900 and ≈300 meV for MA-Pd and MA-Zn SMONs, respectively, compared to the pure MA assembly. Concomitant density functional theory (DFT) calculations reveal that the coordination geometry and microscopic arrangement give rise to the observed reduction in the band gap. The dispersion of the frontier bands and their delocalization due to strong electronic coupling (between MA and metal) suggest that the MA-Pd SMON could potentially be a 2D electronic material.

One-Dimensional Double Wires and Two-Dimensional Mobile Grids: Cobalt/Bipyridine Coordination Networks at the Solid/Liquid Interface

Various architectures have been generated and observed by STM at a solid/liquid interface resulting from an in situ chemical reaction between the bipyridine terminal groups of a ditopic ligand and Co(II) ions. Large monodomains of one-dimensional (1D) double wires are formed by Co(II)/ligand coordination, with polymer lengths as long as 150 nm. The polymers are organized as parallel wires 8 nm apart, and the voids between wires are occupied by solvent molecules. Two-dimensional (2D) grids, showing high surface mobility, coexist with the wires. The wires are formed from linear chain motifs where each cobalt center is bonded to two bipyridines. 2D grids are generated from a bifurcation node where one cobalt bonds to three bipyridines. Surface reconstruction of the grids and of the 1D wires was observed under the STM tip. As an exciting result, analysis of these movements strongly indicates surface reactions at the solid/liquid interface.

Lyophobicity may not be the main driving force for long chain surfactants from the bulk phase to the interface

According to the Traube rule, a surfactant with a longer alkane chain is more hydrophobic so its tendency to be driven from a polar solvent to a less polar interface is higher. In this work, we revisited this topic by studying the adsorption of quaternary ammonium salts and carboxylic acids with various alkane chain lengths at the hexadecane-water interface. The adsorption free energies of the surfactants at this oil-water interface from the polar (aqueous solution) or nonpolar phase (hexadecane) were estimated from second harmonic generation measurements. The variation of the free energies per methylene group in the bulk phase, at the oil-water interface and at the air-water interface revealed that there are different interactions between the alkane chains of the surfactants in different environments. The chain-chain interaction at the hexadecane-water interface is lower than that at the air-water interface. The driving force for the alkane chains to adsorb at the oil-water interface from the oil phase is close to that from the aqueous phase. This observation reveals that the chain-chain interaction rather than the lyophobicity of the solute with respect to the solvent is the main contributor to the adsorption free energy. This is the first experimental comparison of the free energies of the alkane chains in oil, in water, at the air-water interface and at the oil-water interface. These results provide information for studying the interactions of hydrophobic species in different environments. This work also provides a method for estimating the solvation energy of some head groups in surfactants.

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.

Metal dependent motif transition in a self-assembled monolayer of bipyridine derivatives via coordination: An STM study

Low-dimensional molecular motifs with diversity developed via the on-surface chemistry are attracting growing interest for their potential in advanced nanofabrication. In this work, scanning tunneling microscopy was employed to investigate the in situ and ex situ metal coordinations between 4,4'-ditetradecyl-2,2'-bipyridine (bpy) and Zn(ii) or Cu(ii) ions at a highly oriented pyrolytic graphite (HOPG)/1-phenyloctane interface under ambient conditions. The results demonstrate that the bpy adopts a flat-lying orientation with its substituted alkyl chains in a tail-to-tail arrangement in a bpy monolayer. For the in situ coordination, the bpy/Zn(ii) and bpy/Cu(ii) complexes are aligned in edge-on fashions, wherein the bpy stands vertically on the HOPG surface and interdigitates at the alkyl chains. In the two-dimensional arrays of ex situ coordinated complexes, metal dependent motifs have been observed with Zn(ii) and Cu(ii), wherein the bipyridine moieties are parallel to the graphite surface. These results suggest that the desired on-surface coordination architectures may be achieved by the intentional selection of the metal centers.

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.

STM investigation of structural isomers: alkyl chain position induced self-assembly at the liquid/solid interface

Investigating and regulating the self-assembly structure is of great importance in 2D crystal engineering and it is also gaining significant interest in surface studies. In this work, we systematically explored the variation of self-assembled patterns induced by the changeable side chain position. Moreover, molecules with different alkyl chain lengths (n = 15, 16) were also synthesized and probed for the purpose of understanding how an odd/even number of carbon atoms in the peripheral chains can affect the molecular adlayers. Structural isomers of bis-substituted anthraquinone derivatives 1,8-A-2OCn, 2,6-A-2OCn, 1,4-A-2OCn and 1,5-A-2OCn (n = 15, 16) were used and investigated by STM. 1,8-A-2OC16 and 1,8-A-2OC15 molecules adopted Z-like I and Linear I structures, respectively. 2,6-A-2OC16 and 2,6-A-2OC15 molecules were severally arranged in Linear II and Linear III configurations. 1,4-A-2OCn (n = 15, 16) molecules were staggered in a Z-like II fashion and 1,5-A-2OCn (n = 15, 16) molecules displayed a Linear IV nanostructure. Therefore, we arrive at a conclusion that self-assembly structures of anthraquinone isomers are chain-position-dependent, and designing isomeric compounds can be taken into consideration in regulating assembled structures. Besides, 2D nanopatterns of 1,8-A-2OCn and 2,6-A-2OCn can be regulated by the odd/even property of the side chains, but this is not the case for 1,4-A-2OCn and 1,5-A-2OCn, ascribed to the difference in driving forces for them. It is believed that the results are of significance to the alkyl chain position induced assembly configurations and surface research studies of structural isomers.

The influence of amino acid sequence on structure and morphology of polydiacetylene containing peptide fibres

A systematic study was performed on the influence of charge and steric hindrance on the assembly into fibres of a series of pentameric peptides based on the well-known β-sheet forming sequence Gly-Ala-Gly-Ala-Gly, which were N-terminally acylated with pentacosadiynoic acid. To investigate the effect of steric hindrance and charge repulsion on the fibre structure, either the N-terminal or the C-terminal amino acid in the sequence was replaced by a glutamic acid or lysine residue. Furthermore, peptide amphiphiles (PAs) with an amide or a free acid group at the C-terminus were compared. Steric hindrance and charge repulsion were addressed individually by varying the pH during and after fibre preparation. The self-assembled structures were examined with circular dichroism (CD) spectroscopy and transmission electron microscopy (TEM). UV spectroscopy was used to probe the diacetylene packing in the hydrophobic tail, both by polymerisation behaviour and chromatic properties of the polymers. In brief, the assembly was hindered more if the modification was close to the alkyl tail, and glutamic acid brought about a larger effect than lysine. PAs with two charges yielded assemblies which after polymerisation were found to be the most susceptible towards changes in pH, behaving as a colour-based pH sensor. Typically, TEM and UV showed the same trends, indicating that a distorted morphology as observed with TEM is indicative of a poorer molecular packing of the peptide amphiphile fibres, probed via the changes in absorption of the polydiacetylene backbone.

Side chain position, length and odd/even effects on the 2D self-assembly of mono-substituted anthraquinone derivatives at the liquid/solid interface

This work provides efficient methods for regulating self-assembled structures by changing the position, length and odd/even properties of the side chains.

One plus two: supramolecular coordination in a nano-reactor on surface

The supramolecular coordination of zinc (II) phthalocyanine (Zn-Pc) with V-shaped bi-pyridine in a nano-reactor is probed by scanning tunneling microscopy (STM) at liquid/solid interface. Combined with density functional theory (DFT) calculations, our STM results show that the V-shaped bi-pyridine and Zn-Pc can generate stable “odd-even” patterned architectures in the TCDB network through a two-step coordination process. Moreover, great changes for the size and the shape of the host cavity have happened during the coordination process. In general, the whole coordination process is regulated by the synergies of ligand and template. To the best of our knowledge, this is the first work on imaging of supramolecular coordination in a nano-reactor. Such a template-regulated supramolecular interconversion opens a new avenue towards the crystal engineering and design as well as the generation of controllable nano-patterns.

Molecular patterning at a liquid/solid interface: the foldamer approach

Molecular patterning has received a lot of attention in the past decade; however, the functionalization of these surface-confined 2D patterns on the nanoscale level remains a challenge. Assembling 2D patterns from oligomeric foldamers turns out to be an interesting approach to accomplishing the controlled positioning of functional elements. We designed a family of peptidomimetic foldamers bearing a 2D turn element folding at the liquid/solid interface. The turning element was developed while studying derivatives with one turning unit. Furthermore, folding was found to be induced by the confinement of the surface. This achievement paves the way for the design of foldamers with multiple turns, providing a higher versatility in the functionalization of nanopatterns.

Supramolecular assembly/reassembly processes: molecular motors and dynamers operating at surfaces

Among the many significant advances within the field of supramolecular chemistry over the past decades, the development of the so-called "dynamers" features a direct relevance to materials science. Defined as "combinatorial dynamic polymers", dynamers are constitutional dynamic systems and materials resulting from the application of the principles of supramolecular chemistry to polymer science. Like supramolecular materials in general, dynamers are reversible dynamic multifunctional architectures, capable of modifying their constitution by exchanging, recombining, incorporating components. They may exhibit a variety of novel properties and behave as adaptive materials. In this review we focus on the design of responsive switchable monolayers, i.e. monolayers capable to undergo significant changes in their physical or chemical properties as a result of external stimuli. Scanning tunneling microscopy studies provide direct evidence with a sub-nanometre resolution, on the formation and dynamic response of these self-assembled systems featuring controlled geometries and properties.

Towards supramolecular engineering of functional nanomaterials: pre-programming multi-component 2D self-assembly at solid-liquid interfaces

Materials with a pre-programmed order at the supramolecular level can be engineered with a sub-nanometer precision making use of reversible non- covalent interactions. The intrinsic ability of supramolecular materials to recognize and exchange their constituents makes them constitutionally dynamic materials. The tailoring of the materials properties relies on the full control over the self-assembly behavior of molecular modules exposing recognition sites and incorporating functional units. In this review we focus on three classes of weak-interactions to form complex 2D architectures starting from properly designed molecular modules: van der Waals, metallo-ligand and hydrogen bonding. Scanning tunneling microscopy studies will provide evidence with a sub-nanometer resolution, on the formation of responsive multicomponent architectures with controlled geometries and properties. Such endeavor enriches the scientist capability of generating more and more complex smart materials featuring controlled functions and unprecedented properties.

Bipyridine derivatives at a solid/liquid interface: effects of the number and length of peripheral alkyl chains

Bipyridine derivatives (bpys) with various number and length of peripheral alkyl chains (with carbon numbers of n = 11-17) were synthesized, and their self-assembled monolayers were observed by scanning tunneling microscopy (STM) at a 1-phenyloctane/highly oriented pyrolytic graphite (HOPG) interface. The effects of the number, the substitution position, and the length of alkyl chains on the two-dimensional structures were systematically studied. Bpys substituted by a single alkyl chain in the p-position on each side adopted an almost linear form with zigzag-type alignment of the pi-conjugated unit, whereas, in the case of m-substitution, the bpys showed Z-shaped morphology with interdigitated alkyl chains. In both cases, no odd-even alkyl chain length effects were observed. The bpys with double alkyl chains at m- and p-positions displayed odd-even alkyl chain effects, suggesting that the formation of two-dimensional structure is dominated by the interactions between alkyl chains. Bpys with triple alkyl chains at o-, m-, and p-positions also showed odd-even alkyl chain effects, but only for the higher number of carbon atoms in the alkyl chain unit (n = 14-17). These results indicate that concerted intermolecular interactions of the alkyl chain unit introduce the odd-even chain length effect on the self-assembled two-dimensional structure. After coordination of PdCl(2), odd-even effects were quenched, and bpys were converged into the same lamellar structure, in which the molecules are almost linear. All the structural differences due to the odd-even alkyl chain length effect were explained in terms of intermolecular and molecule-substrate interactions.

Alkyl chain length dependence of the self-organized structure of alkyl-substituted phthalocyanines

The alkyl chain length on alkyl-substituted phthalocyanines (C(n)OPc) dependence of their self-organized structures was examined in this study. STM results indicated that the symmetry of ordered structures decreased as the alkyl chain became longer, with the exception of C(6)OPc, which preferentially formed a quasi-3-fold symmetrical structure. This could be explained by the fact that the C(n)OPc molecules are most likely to form densely packed structures. With C(n)OPc, when n = 4 to 10, the self-organized structures were dependent on the competition between how densely the molecules were arranged and how loose the intermolecular interaction energy was, caused by the formation of the densely packed structure. However, with C(n)OPc, when n = 10-18, the molecules tended to form densely packed structures by reducing the symmetry, even though the C(n)OPc molecules were distorted. When C(12)OPc and cobalt phthalocyanine were coadsorbed, the mixed system exhibited a four-fold symmetrical structure, which is rarely observed in C(12)OPc.