Controlled Synthesis of a Helical Conjugated Polythiophene

Two new polymer systems, poly­(3-phenylenevinylene)­thiophene (P3PVT) and poly­(3-phenyl)­thiophene (P3PT), were designed with the aim of obtaining a helical conjugated polymer via a living polymerization. The polymerization proceeded without transfer and termination reactions via the Kumada catalyst transfer condensative polymerization (KCTCP) mechanism, confirming the living nature of the polymerization. Solvatochroism and circular dichroism (CD) experiments showed the helical nature of P3PVT and the stacking behavior of P3PT in poor solvent conditions. Block copolymers of 3-alkyl-substituted polythiophenes and helical P3PVT were prepared to determine the aggregation behavior of such systems. Solvatochroism, CD, and AFM measurements showed that the blocks influence each other’s behavior. If the P3AT block stacks before the helical P3PVT block organizes, one-handed helix formation is hindered. If helix formation occurs first, the stacking behavior is not influenced

Nanoscale Control over the Mixing Behavior of Surface-Confined Bicomponent Supramolecular Networks Using an Oriented External Electric Field

Strong electric fields are known to influence the properties of molecules as well as materials. Here we show that by changing the orientation of an externally applied electric field, one can locally control the mixing behavior of two molecules physisorbed on a solid surface. Whether the starting two-component network evolves into an ordered two-dimensional (2D) cocrystal, yields an amorphous network where the two components phase separate, or shows preferential adsorption of only one component depends on the solution stoichiometry. The experiments are carried out by changing the orientation of the strong electric field that exists between the tip of a scanning tunneling microscope and a solid substrate. The structure of the two-component network typically changes from open porous at negative substrate bias to relatively compact when the polarity of the applied bias is reversed. The electric-field-induced mixing behavior is reversible, and the supramolecular system exhibits excellent stability and good response efficiency. When molecular guests are adsorbed in the porous networks, the field-induced switching behavior was found to be completely different. Plausible reasons behind the field-induced mixing behavior are discussed

Periodic Functionalization of Surface-Confined Pores in a Two-Dimensional Porous Network Using a Tailored Molecular Building Block

We present here the periodic functionalization of a two-dimensional (2D) porous molecular network using a tailored molecular building block. For this purpose, a dehydrobenzo[12]­annulene (DBA) derivative, <b>1-isoDBA</b>, having an isophthalic acid unit connected by an azobenzene linker to a C₁₂ alkyl chain and five C₁₄ chains, was designed and synthesized. After the optimization of monolayer preparation conditions at the 1,2,4-trichlorobezene (TCB)/graphite interface, scanning tunneling microscopy (STM) observation of the self-assembled monolayer of <b>1-isoDBA</b> revealed the formation of extended domains of a porous honeycomb-type molecular network, which consists of periodically located nanowells each functionalized by a cyclic hexamer of hydrogen-bonded isophthalic acid units and those without functional groups. This result demonstrates that the present strategy based on precise molecular design is a viable route to site-specific functionalization of surface-confined nanowells. The nanowells of different size can be used for guest coadsorption of different guests, coronene <b>COR</b> and hexakis­[4-(phenylethynyl)­phenylethynyl]­benzene <b>HPEPEB</b>, whose size and shape match the respective nanowells. STM observation of a ternary mixture (<b>1-isoDBA</b>/<b>COR</b>/<b>HPEPEB</b>) at the TCB/graphite interface revealed the site-selective immobilization of the two different guest molecules at the respective nanowells, producing a highly ordered three-component 2D structure

Structural Insights into the Mechanism of Chiral Recognition and Chirality Transfer in Host–Guest Assemblies at the Liquid–Solid Interface

Understanding structure–efficiency relationships in chiral recognition and chirality transfer constitutes an important step toward the rational design of improved chiral probes and chirality auxiliaries or inducers. Recently discovered enantioselective host–guest adsorption opened a new pathway toward the enantioselective reconstruction of on-surface monolayers. In this study, we explored the importance of size matching between host cavity and chiral guest for the efficiency of chiral recognition and subsequent chirality induction in the initially racemic host

Conjugated Covalent Organic Frameworks via Michael Addition–Elimination

Dynamic covalent chemistry enables self-assembly of reactive building blocks into structurally complex yet robust materials, such as covalent organic frameworks (COFs). However, the synthetic toolbox used to prepare such materials, and thus the spectrum of attainable properties, is very limited. For π-conjugated COFs, the Schiff base condensation of aldehydes and amines is the only general dynamic reaction, but the resulting imine-linked COFs display only a moderate electron delocalization and are susceptible to hydrolysis, particularly in acidic conditions. Here we report a new dynamic polymerization based on Michael addition–elimination reaction of structurally diverse β-ketoenols with amines, and use it to prepare novel two-dimensional (2D) π-conjugated COFs, as crystalline powders and exfoliated micron-size sheets. π-Conjugation is manifested in these COFs in significantly reduced band gap (1.8–2.2 eV), solid state luminescence and reversible electrochemical doping creating midgap (NIR absorbing) polaronic states. The β-ketoenamine moiety enables protonation control of electron delocalization through the 2D COF sheets. It also gives rise to direct sensing of triacetone triperoxide (TATP) explosive through fluorescence quenching

Odd–Even Effects in Chiral Phase Transition at the Liquid/Solid Interface

Chiral selection on inorganic crystalline surfaces represents one of the most promising avenues to the separation of enantiomers. However, there are competing influences at play: on the one hand, the confinement to the surface seems to enhance chiral discrimination between enantiomers; on the other hand, racemic patterns tend to possess higher packing density therewith higher stability. A clear picture on the delicate balance between these two opposing factors is missing. We address this issue in monolayers of alkylated dehydrobenzo[12]­annulene (DBA) derivatives at the liquid/solid interface by a detailed investigation of the relationship between packing density and 2D chirality. We report on chiral phase transitions, evolving from homochiral low density porous networks to enantiomeric excess, racemic, or homochiral densely packed structures, by using scanning tunneling microscopy (STM) as a visualization tool. The changes in monolayer chirality in response to increased packing density, however, are strongly correlated with molecular structural features such as the length of the alkyl chains and in particular their parity. While heterochiral lattices are indeed denser than its enantiomorphous counterparts, close packing does not necessarily favor racemic crystallization: the azimuthal orientation of building blocks in a domain may play a decisive role. In light of the popularity of using alkyl chains to adhere molecules onto a surface, we believe that our findings may have implications for predictive chiral recognition and resolution processes

Preprogrammed 2D Folding of Conformationally Flexible Oligoamides: Foldamers with Multiple Turn Elements

Controlling the molecular conformation of oligomers on surfaces through noncovalent interactions symbolizes an important approach in the bottom-up patterning of surfaces with nanoscale precision. Here we report on the design, synthesis, and scanning tunneling microscopy (STM) investigation of linear oligoamides adsorbed at the liquid–solid interface. A new class of extended foldamers comprising as many as four turn elements based on a structural design “rule” adapted from a mimic foldamer was successfully synthesized. The self-assembly of these progressively complex oligomeric structures was scrutinized at the liquid–solid interface by employing STM. Submolecularly resolved STM images of foldamers reveal characteristic in-plane folding and self-assembly behavior of these conformationally flexible molecules. The complexity of the supramolecular architectures increases with increasing number of turn elements. The dissimilarity in the adsorption behavior of different foldamers is discussed qualitatively in light of enthalpic and entropic factors. The modular construction of these oligomeric foldamers with integrated functionalities provides a simple, efficient, and versatile approach to surface patterning with molecular precision

Complex Chiral Induction Processes at the Solution/Solid Interface

Two-dimensional supramolecular chirality is often achieved by confining molecules against a solid surface. The <i>sergeants–soldiers</i> principle is a popular strategy to fabricate chiral surfaces using predominantly achiral molecules. In this method, achiral molecules (the <i>soldiers</i>) are forced to assemble in a chiral fashion by mixing them with a small percentage of structurally similar chiral molecules (the <i>sergeants</i>). The full complexity of the amplification processes in chiral induction studies is rarely revealed due to the specific experimental conditions used. Here we report the evolution of chirality in mixed supramolecular networks of chiral and achiral dehydrobenzo[12]­annulene (DBA) derivatives using scanning tunneling microscopy (STM) at the solution/solid interface. The experiments were carried out in the high <i>sergeants–soldiers</i> mole ratio regime in relatively concentrated solutions. Variation in the sergeants/soldiers composition at a constant solution concentration revealed different mole ratio regimes where either amplification of supramolecular handedness as defined by the <i>sergeant</i> chirality or its reversal was observed. The chiral induction/reversal processes were found to be a convolution of different phenomena occurring at the solution-solid interface namely, structural polymorphism, competitive adsorption and adaptive host–guest recognition. Grasping the full complexity of chiral amplification processes as described here is a stepping-stone toward developing a predictive understanding of chiral amplification processes

Self-Assembly Behavior of Alkylated Isophthalic Acids Revisited: Concentration in Control and Guest-Induced Phase Transformation

The engineering of two-dimensional crystals by physisorption-based molecular self-assembly at the liquid–solid interface is a powerful method to functionalize and nanostructure surfaces. The formation of high-symmetry networks from low-symmetry building blocks is a particularly important target. Alkylated isophthalic acid (ISA) derivatives are early test systems, and it was demonstrated that to produce a so-called porous hexagonal packing of plane group <i>p</i>6, i.e., a regular array of nanowells, either short alkyl chains or the introduction of bulky groups within the chains were mandatory. After all, the van der Waals interactions between adjacent alkyl chains or alkyl chains and the surface would dominate the ideal hydrogen bonding between the carboxyl groups, and therefore, a close-packed lamella structure (plane group <i>p</i>2) was uniquely observed. In this contribution, we show two versatile approaches to circumvent this problem, which are based on well-known principles: the “concentration in control” and the “guest-induced transformation” methods. The successful application of these methods makes ISA suitable building blocks to engineer a porous pattern, in which the distance between the pores can be tuned with nanometer precision

Flow-Assisted 2D Polymorph Selection: Stabilizing Metastable Monolayers at the Liquid–Solid Interface

Controlling crystal polymorphism constitutes a formidable challenge in contemporary chemistry. Two-dimensional (2D) crystals often provide model systems to decipher the complications in 3D crystals. In this contribution, we explore a unique way of governing 2D polymorphism at the organic liquid–solid interface. We demonstrate that a directional solvent flow could be used to stabilize crystalline monolayers of a metastable polymorph. Furthermore, flow fields active within the applied flow generate millimeter-sized domains of either polymorph in a controlled and reproducible fashion