Artur Ciesielski

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.

Dynamers at the Solid–Liquid Interface: Controlling the Reversible Assembly/Reassembly Process between Two Highly Ordered Supramolecular Guanine Motifs

Complex systems and phenomena in nature are dominated by reversible noncovalent interactions. Nucleobases, which are essential components of the genetic material in all living organisms, encode sophisticated programs for self-assembly into highly ordered and complex architectures, such as the fascinating double helix of DNA. Alongside Watson–Crick base pairing, which directs the formation of the helical structure of DNA, nucleobases can interact through other hydrogen-bonded motifs to form various complex supramolecular architectures, such as guanine (G) quadruplexes. The G quartet G4 , an hydrogen-bonded macrocycle typically formed by cation-templated assembly, was first identified in 1962 as the basis for the aggregation of 5’guanosine monophosphate (5’-GMP), and fits particularly well with contemporary studies in molecular self-assembly and noncovalent synthesis. Guanine is an extremely versatile building block: depending on its environment, it is able to self-assemble into various discrete architectures including dimers, ribbons, and macrocycles. 12] In the presence of certain cations, either G4-based octamers or columnar aggregates (supramolecular polymers) can be formed, depending on the concentration of the cation and nucleobase. The guanine-based structures are interesting for applications in organic electronics and synthesis of supramolecular hydrogels, whereas G quartets are known to have potential in several biological processes and in anticancer drug design, as they can act as enzyme telomerase inhibitors, and therefore are of importance for controlling tumor immortalization. 16, 17] While the self-assembly of guanines into G4-based architectures (not templated by a metal center) on solid surfaces has been studied by STM under ultrahigh vacuum (UHV), 19] STM explorations at the solid–liquid interface have been primarily carried out on guanosine derivatives. Although the structure of a guanine quadruplex templated by a metal center was introduced over 40 years ago, its visualization by STM once assembled at the solid–liquid interface has not been reported to date. We have studied the metal-templated reversible assembly/ reassembly process of a N-alkylguanine into highly ordered quartets and ribbons. Herein, we present a submolecularresolution STM visualization of such a process at the solid– liquid interface on highly oriented pyrolitic graphite (HOPG) surfaces. We focused our attention on the octadecyl guanine derivative 1 (see the Supporting Information for preparation). The presence of a long aliphatic side chain and the absence of the sugar with respect to previously studied guanosines were expected to promote the molecular physisorption on HOPG. The self-assembly of 1 alone on HOPG has been studied, and, upon subsequent addition of [2.2.2]cryptand, potassium picrate (K(pic) ), and trifluoromethanesulfonic acid (HTf), the reversible interconversion between two different highly ordered supramolecular motifs is triggered (Scheme 1). This process was previously indirectly shown by H NMR and circular dichroism spectroscopy to occur in solution. In general, the generation of hydrogen-bond-stabilized ordered motifs at the solid–liquid interface requires fine tuning of the interplay between interactions that involve solvent molecules, solute molecules, and the substrate. The STM observation of a conformational or assembly switching process that occurs at the solid–liquid interface cannot be obtained by operating when concentrated solutions are used, as dictated by the thermodynamics of physisorption at the solid–liquid interface. In a solution containing a large number of identical molecules that have two or more conformations, the component with a greater affinity for the substrate, that is, the component that offers a minimization of the free interface energy per unit area, will assemble on the surface, whereas the other components will remain in the supernatant solution. 27] To have full control over the switching process and immobilize all the components on the surface, and thus achieve a complete physisorption of all different components at the solid–liquid interface, it is mandatory to tune the stoichiometry of the molecules absorbed at surface. 29] At the solid–liquid interface, the number of molecules that are in [*] A. Ciesielski, Prof. P. Samor ISIS/UMR CNRS 7006, Universit de Strasbourg 8 all e Gaspard Monge, 67000 Strasbourg (France) E-mail: samori@unistra.fr

Concentration-Dependent Supramolecular Engineering of Hydrogen-Bonded Nanostructures at Surfaces: Predicting Self-Assembly in 2D

We report a joint computational and experimental study on the concentration-dependent self-assembly of a flat C3-symmetric molecule at surfaces. As a model system we have chosen a rigid molecular module, 1,3,5-tris(pyridine-4-ylethynyl)benzene, which can undergo self-association via hydrogen bonding (H-bonding) to form ordered 2D nanostructures. In particular, the lattice Monte Carlo method, combined with density functional calculations, was employed to explore the spontaneous supramolecular organization of this tripod-shaped molecule under surface confinement. We analyzed the stability of different weak H-bonded patterns and the influence of the concentration of the starting molecule on the 2D supramolecular packing. We found that ordered, densely packed monolayers and 2D porous networks are obtained at high and low concentrations, respectively. A concentration-dependent scanning tunneling microscopy investigation of the molecular self-assembly at a graphite-solution interface revealed supramolecular motifs, which are in perfect agreement with those obtained by simulations. Therefore, our computational approach represents a step forward toward the deterministic prediction of molecular self-assembly at surfaces and interfaces.

Dynamic covalent chemistry of bisimines at the solid/liquid interface monitored by scanning tunnelling microscopy

Dynamic covalent chemistry relies on the formation of reversible covalent bonds under thermodynamic control to generate dynamic combinatorial libraries. It provides access to numerous types of complex functional architectures, and thereby targets several technologically relevant applications, such as in drug discovery, (bio)sensing and dynamic materials. In liquid media it was proved that by taking advantage of the reversible nature of the bond formation it is possible to combine the error-correction capacity of supramolecular chemistry with the robustness of covalent bonding to generate adaptive systems. Here we show that double imine formation between 4-(hexadecyloxy)benzaldehyde and different α,ω-diamines as well as reversible bistransimination reactions can be achieved at the solid/liquid interface, as monitored on the submolecular scale by in situ scanning tunnelling microscopy imaging. Our modular approach enables the structurally controlled reversible incorporation of various molecular components to form sophisticated covalent architectures, which opens up perspectives towards responsive multicomponent two-dimensional materials and devices.

Supramolecular Approaches to Graphene: From Self-Assembly to Molecule-Assisted Liquid-Phase Exfoliation.

Graphene, a one-atom thick two-dimensional (2D) material, is at the core of an ever-growing research effort due to its combination of unique mechanical, thermal, optical and electrical properties. Two strategies are being pursued for the graphene production: the bottom-up and the top-down. The former relies on the use of covalent chemistry approaches on properly designed molecular building blocks undergoing chemical reaction to form 2D covalent networks. The latter occurs via exfoliation of bulk graphite into individual graphene sheets. Amongst the various types of exfoliations exploited so far, ultrasound-induced liquid-phase exfoliation (UILPE) is an attractive strategy, being extremely versatile, up-scalable and applicable to a variety of environments. In this review, we highlight the recent developments that have led to successful non-covalent functionalization of graphene and how the latter can be exploited to promote the process of molecule-assisted UILPE of graphite. The functionalization of graphene with non-covalently interacting molecules, both in dispersions as well as in dry films, represents a promising and modular approach to tune various physical and chemical properties of graphene, eventually conferring to such a 2D system a multifunctional nature.

Harnessing the Liquid‐Phase Exfoliation of Graphene Using Aliphatic Compounds: A Supramolecular Approach

The technological exploitation of the extraordinary properties of graphene relies on the ability to achieve full control over the production of a high-quality material and its processing by up-scalable approaches in order to fabricate large-area films with single-layer or a few atomic-layer thickness, which might be integrated in working devices. A simple method is reported for producing homogenous dispersions of unfunctionalized and non-oxidized graphene nanosheets in N-methyl-2-pyrrolidone (NMP) by using simple molecular modules, which act as dispersion-stabilizing compounds during the liquid-phase exfoliation (LPE) process, leading to an increase in the concentration of graphene in dispersions. The LPE-processed graphene dispersion was shown to be a conductive ink. This approach opens up new avenues for the technological applications of this graphene ink as low-cost electrodes and conducting nanocomposite for electronics.

A supramolecular strategy to leverage the liquid-phase exfoliation of graphene in the presence of surfactants: unraveling the role of the length of fatty acids.

Achieving the full control over the production as well as processability of high-quality graphene represents a major challenge with potential interest in the field of fabrication of multifunctional devices. The outstanding effort dedicated to tackle this challenge in the last decade revealed that certain organic molecules are capable of leveraging the exfoliation of graphite with different efficiencies. Here, a fundamental understanding on a straightforward supramolecular approach for producing homogenous dispersions of unfunctionalized and non-oxidized graphene nanosheets in four different solvents is attained, namely N-methyl-2-pyrrolidinone, N,N-dimethylformamide, ortho-dichlorobenzene, and 1,2,4-trichlorobenzene. In particular, a comparative study on the liquid-phase exfoliation of graphene in the presence of linear alkanes of different lengths terminated by a carboxylic-acid head group is performed. These molecules act as graphene dispersion-stabilizing agents during the exfoliation process. The efficiency of the exfoliation in terms of concentration of exfoliated graphene is found to be proportional to the length of the employed fatty acid. Importantly, a high percentage of single-layer graphene flakes is revealed by high-resolution transmission electron microscopy and Raman spectroscopy analyses. A simple yet effective thermodynamic model is developed to interpret the chain-length dependence of the exfoliation yield. This approach relying on the synergistic effect of a ad-hoc solvent and molecules to promote the exfoliation of graphene in liquid media represents a promising and modular strategy towards the rational design of improved dispersion-stabilizing agents.

Photoswitching Vertically Oriented Azobenzene Self-Assembled Monolayers at the Solid–Liquid Interface

Due to the large geometrical difference between thestructures of the trans and cis isomers, a necessary prerequi-site for the occurrence of photoisomerization in azoben-zene-containing monolayers is to provide sufficient freevolume to the system. Decoupling the photochromic unitelectronically from the conducting surface by avoiding adirect contact is also crucial to preserve the photoelectronicactivity.

Unraveling unprecedented charge carrier mobility through structure property relationship of four isomers of didodecyl[1]benzothieno[3,2-b][1]benzothiophene

The structural and electronic properties of four isomers of didodecyl[1]-benzothieno[3,2-b][1]benzothiophene (C12-BTBT) have been investigated. Results show the strong impact of the molecular packing on charge carrier transport and electronic polarization properties. Field-induced time-resolved microwave conductivity measurements unravel an unprecedented high average interfacial mobility of 170 cm(2) V(-1) s(-1) for the 2,7-isomer, holding great promise for the field of organic electronics.

Surface‐Induced Selection During In Situ Photoswitching at the Solid/Liquid Interface

Here we report for the first time a submolecularly resolved scanning tunneling microscopy (STM) study at the solid/liquid interface of the in situ reversible interconversion between two isomers of a diarylethene photoswitch, that is, open and closed form, self-assembled on a graphite surface. Prolonged irradiation with UV light led to the in situ irreversible formation of another isomer as by-product of the reaction, which due to its preferential physisorption accumulates at the surface. By making use of a simple yet powerful thermodynamic model we provide a quantitative description for the observed surface-induced selection of one isomeric form.