Molecular design driving tetraporphyrin self-assembly on graphite: a joint STM, electrochemical and computational study

Self-Assembly of Functionalized Lipophilic Guanosines into Cation-Free Stacked Guanine-Quartets

The hierarchical self-assembly of various lipophilic guanosines exposing either a phenyl or a ferrocenyl group in the C(8) position was investigated. In a solution, all the derivatives were found to self-assemble primarily into isolated guanine (G)-quartets. In spite of the apparent similar bulkiness of the two substituents, most of the derivatives form disordered structures in the solid state, whereas a specific 8-phenyl derivative self-assembles into an unprecedented, cation-free stacked G-quartet architecture.

Solvent-Dependent Core-Modified Rubyrin Self-Assembly at Liquid/Solid Interfaces

Scanning tunneling microscopy (STM) was utilized to disclose four novel core-modified rubyrin self-assembly behaviors on the highly-oriented pyrolytic graphite (HOPG) surface, of which N₂S₄-OR(1)/N₂Se₄-OR(2) had no phenanthrene pyrrole ring and N₂S₄-OR(3)/N₂Se₄-OR(4) had phenanthrene-fused pyrrole rings and meso-aryl substituents. It was discovered that the core-modified rubyrin could self-assemble into either face-on or edge-on monolayer structures selectively at the liquid/HOPG interface in different solvents. There was an obvious solvent-dependent self-assembly for N₂S₄-OR(3)/N₂Se₄-OR(4), which adopted an edge-on and face-on structure in 1-phenyloctane and 1-heptanoic acid solvents, respectively, whereas N₂S₄-OR(1)/N₂Se₄-OR(2) showed no obvious difference in the assembly structure, which both adopted a face-on structure in the two solvents. Density functional theory (DFT) calculations were also utilized to reveal the relevant self-assembly mechanisms. This study shows a typical solvent effect regulating core-modified rubyrin self-assembly, which is essential for porphyrin-based functional devices' design and manufacture.

Ordering self-assembly structures via intermolecular BrS interactions

Recent research studies have shown that the halogenated benzo[1,2-b:4,5-b']dithiophene (DTBDT) unit as a polymer donor exhibits high charge carrier mobility due to the well-ordered molecular packing and high crystallinity, which is meaningful for achieving highly efficient organic solar cells (OSCs). However, it is difficult to acquire the accurate packing information of polymer materials. Herein, we investigated the self-assembled behaviors of two DTBDT derivatives, 4,8-bis(4-octadecylthiophen-2-yl)benzo[1,2-b:4,5-b']dithiophene (H-DTBDT) and 4,8-bis(5-bromo-4-octadecylthiophen-2-yl)benzo[1,2-b:4,5-b']dithiophene (Br-DTBDT), to elucidate the effect of introducing a bromine atom on molecular packing structures by STM at the 1-phenyloctane/HOPG interface. It is observed that the H-DTBDT molecules exhibit a random arrangement along each lamella, while the Br-DTBDT molecules self-assemble into a highly ordered lamellar structure. Density functional theory (DFT) analysis combined with the topological properties of the electron density at the bond critical points revealed that the existence of weak intermolecular interactions of BrS facilitates the regular packing motif of Br-DTBDT molecules. The results helped us to understand that the BrS bond generally acted as the auxiliary force and can play the primary role in the construction of supramolecular nanostructures.

Exploring the write-in process in molecular quantum cellular automata: a combined modelingand first-principle approach

The molecular quantum cellular automata paradigm (m-QCA) offers a promising alternative framework to current CMOS implementations. A crucial aspect for implementing this technology concerns the construction of a device which effectively controls intramolecular charge-transfer processes. Tentative experimental implementations have been developed in which a voltage drop is created generating the forces that drive a molecule into a logic state. However, important factors such as the electric field profile, its possible time-dependency and the influence of temperature in the overall success of charge-transfer are relevant issues to be considered in the design of a reliable device. In this work, we theoretically study the role played by these processes in the overall intramolecular charge-transfer process. We have used a Landau-Zener (LZ) model, where different time-dependent electric field profiles have been simulated. The results have been further corroborated employing density functional tight-binding method. The role played by the nuclear motions in the electron-transfer process has been investigated beyond the Born-Oppenheimer approximation by computing the effect of the external electric field in the behavior of the potential energy surface. Hence, we demonstrate that the intramolecular charge-transfer process is a direct consequence of the coherent LZ nonadiabatic tunneling and the hybridization of the diabatic vibronic states which effectively reduces the trapping of the itinerant electron at the donor group.

Assembly structures and electronic properties of truxene-porphyrin compounds studied by STM/STS

The self-assembly of functional molecules into uniform nanostructures with innovational properties has attracted extensive research interest. In the present work, the assembly structures and electronic properties of a novel type of truxene derivative, e.g. truxene-porphyrin derivatives, were studied, for the first time, on a highly oriented pyrolytic graphite (HOPG) surface. Scanning tunneling microscopy (STM) images revealed that the truxene-porphyrin compounds could be parallelly arranged into long-ranged lamellar patterns. Density functional theory (DFT) calculations helped explain the assembly mechanisms further. Besides, order distribution of the smaller compound 1T1P in the 1,3,5-tris(10-carboxydecyloxy)-benzene (TCDB) host network was achieved, which is a reflection of the dimensional effect in the host-guest assembly. Furthermore, together with theoretical analyses, scanning tunneling spectroscopy (STS) measurements were conducted to investigate the electronic properties of truxene-porphyrin compounds. Results showed that the metalation of the porphyrin units could have a significant effect on the band gap and the position of the gap center. The study enhances our understanding of the assembly mechanism of truxene derivatives at the molecular level and paves the way towards fabricating truxene-based functional nanodevices.

Metal Atom Markers for Imaging Epitaxial Molecular Self-Assembly on Graphene by Scanning Transmission Electron Microscopy

Direct imaging of single molecules has to date been primarily achieved using scanning probe microscopy, with limited success using transmission electron microscopy due to electron beam damage and low contrast from the light elements that make up the majority of molecules. Here, we show single complex molecule interactions can be imaged using annular dark field scanning TEM (ADF-STEM) by inserting heavy metal markers of Pt atoms and detecting their positions. Using the high angle ADF-STEM Z^1.7 contrast, combined with graphene as an electron transparent support, we track the 2D monolayer self-assembly of solution-deposited individual linear porphyrin hexamer (Pt-L6) molecules and reveal preferential alignment along the graphene zigzag direction. The epitaxial interactions between graphene and Pt-L6 drive a reduction in the interporphyrin distance to allow perfect commensuration with the graphene. These results demonstrate how single metal atom markers in complex molecules can be used to study large scale packing and chain bending at the single molecule level.

Diversification of Device Platforms by Molecular Layers: Hybrid Sensing Platforms, Monolayer Doping, and Modeling

Inorganic materials such as semiconductors, oxides, and metals are ubiquitous in a wide range of device technologies owing to the outstanding robustness and mature processing technologies available for such materials. However, while the important contribution of inorganic materials to the advancement of device technologies has been well established for decades, organic-inorganic hybrid device systems, which merge molecular functionalities with inorganic platforms, represent a newer domain that is rapidly evolving at an increasing pace. Such devices benefit from the great versatility and flexibility of the organic building blocks merged with the robustness of the inorganic platforms. Given the overwhelming wealth of literature covering various approaches for modifying and using inorganic devices, this feature article selectively highlights some of the advances made in the context of the diversification of devices by surface chemistry. Particular attention is given to oxide-semiconductor systems and metallic surfaces modified with organic monolayers. The inorganic device components, such as semiconductors, metals, and oxides, are modified by organic monolayers, which may serve as either active, static, or sacrificial components. We portray research directions within the broader field of organic-inorganic hybrid device systems that can be viewed as specific examples of the potential of such hybrid device systems given their comprehensive capabilities of design and diversification. Monolayer doping techniques where sacrificial organic monolayers are introduced into semiconducting elements are reviewed as a specific case, together with associated requirements for nanosystems, devices, and sensors for controlling doping levels and doping profiles on the nanometric scale. Another series of examples of the flexibility provided by the marriage of organic functional monolayers and inorganic device components are represented by a new class of biosensors, where the organic layer functionality is exploited in a functioning device for sensing. Considerations for relying on oxide-terminated semiconductors rather than the pristine semiconductor material as a platform both for processing and sensing are discussed. Finally, we cover aspects related to the use of various theoretical and computational approaches to model organic-inorganic systems. The main objectives of the topics covered here are (i) to present the advances made in each respective domain and (ii) to provide a comprehensive view of the potential uses of organic monolayers and self-assembly processes in the rapidly evolving field of molecular-inorganic hybrid device platforms and processing methodologies. The directions highlighted here provide a perspective on a future, not yet fully realized, integrated approach where organic monolayers are combined with inorganic platforms in order to obtain versatile, robust, and flexible systems with enhanced capabilities.

Assemblies of porphyrin and phthalocyanine derivatives studied by STM

Porphyrins and phthalocyanines are currently a prevalent topic with great potential due to their abundant photonic/electronic properties. The study of porphyrin or phthalocyanine supramolecular architectures on solid surfaces is laying the foundation for the further development of molecular electronics or other structures in applications. Above all, the invention of scanning tunneling microscopy (STM) has opened a new path to explore these concepts on surfaces. Self-assemblies on solid surfaces can be probed with STM at submolecular resolutions to disclose the conformations and arrangements of molecules on an individual molecule basis. In this paper, the progress of STM research on porphyrin and phthalocyanine derivatives over past ten years is reviewed. We introduce the self-assembled structures of different porphyrins or phthalocyanines. Throughout the review, the structures, thermodynamics, and dynamics have been emphasized, which are essential current and future research themes.

Self-Assembled Two-Dimensional Supramolecular Networks Characterized by Scanning Tunneling Microscopy and Spectroscopy in Air and under Vacuum

We combine ambient (air) and ultrahigh vacuum (UHV) scanning tunneling microscopy (STM) and spectroscopy (STS) investigations together with density functional theory (DFT) calculations to gain a subnanometer insight into the structure and dynamic of two-dimensional (2D) surface-supported molecular networks. The planar tetraferrocene-porphyrin molecules employed in this study undergo spontaneous self-assembly via the formation of hydrogen bonded networks at the gold substrate-solution interface. To mimic liquid phase ambient deposition conditions, film formation was accomplished in UHV by electro-spraying a solution of the molecule in chloroform onto an Au(111) substrate, thereby providing access to the full spectroscopic capabilities of STM that can be hardly attained under ambient conditions. We show that molecular assembly on Au (111) is identical in films prepared under the two different conditions, and in good agreement with the theoretical predictions. However, we observe the contrast found for a given STM bias condition to be different in ambient and UHV conditions despite the similarity of the structures, and we propose possible origins of the different imaging contrast. This approach could be valuable for the thorough characterization of surface systems that involve large molecules and are prepared mainly in ambient conditions.

Phenoxyaluminum(salophen) Scaffolds: Synthesis, Electrochemical Properties, and Self-Assembly at Surfaces of Multifunctional Systems

Salophens and Salens are Schiff bases generated through the condensation of two equivalents of salicylaldehyde with either 1,2-phenylenediamines or aliphatic diamines, respectively. Both ligands have been extensively exploited as key building blocks in coordination chemistry and catalysis. In particular, their metal complexes have been widely used for various catalytical transformations with high yield and selectivity. Through the modification of the phenol unit it is possible to tune the steric hindrance and electronic properties of Salophen and Salen. The introduction of long aliphatic chains in salicylaldehydes can be used to promote their self-assembly into ordered supramolecular structures on solid surfaces. Herein, we report a novel method towards the facile synthesis of robust and air-stable [Al(Salophen)] derivatives capable of undergoing spontaneous self-assembly at the graphite/solution interface forming highly-ordered nanopatterns. The new synthetic approach relies on the use of [MeAl^III (Salophen)] as a building unit to introduce, via a simple acid/base reaction with functionalized acidic phenol derivatives, selected frameworks integrating multiple functions for efficient surface decoration. STM imaging at the solid/liquid interface made it possible to monitor the formation of ordered supramolecular structures. In addition, the redox properties of the Salophen derivatives functionalized with ferrocene units in solution and on surface were unraveled by cyclic voltammetry. The use of a five-coordinate aluminum alkyl Salophen precursor enables the tailoring of new Salophen molecules capable of undergoing controlled self-assembly on HOPG, and thereby it can be exploited to introduce multiple functionalities with subnanometer precision at surfaces, ultimately forming ordered functional patterns.

Surface modification and pattern formation by nucleobases and their coordination complexes

This review presents recent progress concerning the organization of nucleobases on highly ordered pyrolytic graphite (HOPG), mica, Cu(110) and Au(111) surfaces, followed by their studies using microscopy methods such as atomic force microscopy (AFM), scanning tunneling microscopy (STM) and transmission electron microscopy (TEM). Interesting research prospects related to surface patterning by nucleobases, nucleobase-functionalized carbon nanotubes (CNTs) and metal–nucleobase coordination polymers are also discussed, which offer a wide array of functional molecules for advanced applications. Nucleobases and their analogs are able to invoke non-covalent interactions such as π–π stacking and hydrogen bonding, and possess the required framework to coordinate metal ions, giving rise to fascinating supramolecular architectures. The latter could be transferred to conductive substrates, such as HOPG and gold, for assessment by high-end tunneling microscopy under various conditions. Clear understanding of the principles governing nucleobase self-assembly and metal ion complexation, and precise control over generation of functional architectures, might lead to custom assemblies for targeted nanotechnological and nanomaterial applications.

This review highlights recent advancements in surface patterning of nucleobases, their analogs including nucleobase-CNT hybrids and metal complexes, using various microscopy techniques for nanotechnological applications.

Surfactant Assemblies on Selected Nanostructured Surfaces: Evidence, Driving Forces, and Applications

Surfactant adsorption at solid-liquid interfaces is critical for a number of applications of vast industrial interest and can also be used to seed surface-modification processes. Many of the surfaces of interest are nanostructured, as they might present surface roughness at the molecular scale, chemical heterogeneity, as well as a combination of both surface roughness and chemical heterogeneity. These effects provide lateral confinement on the surfactant aggregates. It is of interest to quantify how much surfactant adsorbs on such nanostructured surfaces and how the surfactant aggregates vary as the degree of lateral confinement changes. This review focuses on experimental evidence on selected substrates, including gold- and carbon-based substrates, suggesting that lateral confinement can have pronounced effects both on the amount adsorbed and on the morphology of the aggregates as well as on a systematic study, via diverse simulation approaches, on the effect of lateral confinement on the structure of the surfactant aggregates. Atomistic and coarse-grained simulations conducted for surfactants on graphene sheets and carbon nanotubes are reviewed, as well as coarse-grained simulations for surfactant adsorption on nanostructured surfaces. Finally, we suggest a few possible extensions of these studies that could positively impact a few practical applications. In particular, the simultaneous effect of lateral confinement and of the coadsorption of molecular compounds within the surface aggregates is expected to yield interesting fundamental results with long-lasting consequences in applications ranging from drug delivery to the design of advanced materials.