Here, we show that two epoxides can be efficiently linked using potassium thioacetate (AcSK) in water even at their imbalanced stoichiometric ratios. We found that the first reaction between epoxide and AcSK gave rise to an intermediate that underwent the second reaction with another epoxide with a reactivity much higher than that of AcSK. Time-dependent 1H NMR measurements revealed that the rate constant of the second reaction was 31 times larger than that of the first reaction. Using this reaction, we succeeded in nonstoichiometric polymerization of a bifunctional epoxide. Furthermore, in the presence of a multifunctional epoxide, we obtained hydrogels and self-standing films. We expect that this straightforward and efficient reaction of versatile reagents, epoxide and AcSK, in water would lead to new applications of epoxides.
Nucleation-elongation is known to give satisfactory descriptions of many supramolecular polymerization systems in thermal equilibrium. Its key feature is the necessity to form a "nucleus" consisting of a certain number of monomer units before being able to grow into a longer polymer chain. The size of the nucleus has significant implications for the understanding of the supramolecular polymerization mechanism. Here we investigate how experiments can give information on the nucleus size by regression analysis of various types of measurements. The measurements of free monomer concentrations, diffusion coefficients, and calorimetric response as functions of concentration or temperature are considered. The nucleation-elongation model with a general value for the nucleus size is used to provide mathematical expressions for these experimental observables. Numerical experiments are performed where experimental errors are simulated by computer-generated random numbers, and it is investigated whether least-squares fitting analyses can give the correct values of the nucleus size in the presence of experimental errors. It is recommended that the calorimetric measurements such as differential scanning calorimetry (DSC) or isothermal titration calorimetry (ITC) be performed under various conditions to correctly determine the nucleus size experimentally.
A single molecular resistive (conductance) switch via control of anchoring positions was examined by using a molecule consisting of more than two same anchors. For this purpose, we adopted the covered quaterthiophene (QT)-based molecular wire junction. The QT-based wire consisted of two thiophene ring anchors on each side; thus, shift of anchors was potentially possible without a change in the binding modes and distortion of the intramolecular structure. We observed three distinct conductance states by using scanning tunneling microscope-based break junction technique. A detailed analysis of the experimental data and first-principles calculations revealed that the mechanism of the resistive switch could be explained by standard length dependence (exponential decay) of conductance. Here, the length is the distance between the anchoring points, i.e., length of the bridged π-conjugated backbone. Most importantly, this effective tunneling length was variable via only controlling the anchoring positions in the same molecule. Furthermore, we experimentally showed the possibility of a dynamic switch of anchoring positions by mechanical control. The results suggested a distinct strategy to design functional devices via contact engineering.
Silica fibrils with a novel double stranded helical structure are prepared by sol-gel transcription of twisted bilayer ribbons formed by cationic gemini surfactants.
Abstract Recent developments in kinetically controlled supramolecular polymerization permit control of the size (i.e., length and area) of self‐assembled nanostructures. However, control of molecular self‐assembly at a level comparable with organic synthetic chemistry and the achievement of structural complexity at a hierarchy larger than the molecular level remain challenging. This study focuses on controlling the aspect ratio of supramolecular nanosheets. A systematic understanding of the relationship between the monomer structure and the self‐assembly energy landscape has derived a new monomer capable of forming supramolecular nanosheets. With this monomer in hand, the aspect ratio of a supramolecular nanosheet is demonstrated that it can be controlled by modulating intermolecular interactions in two dimensions.
Abstract Despite recent advances in mechanistic understanding and controlled‐synthesis methodologies regarding synthetic supramolecular assemblies, it has remained challenging to capture the molecular‐level phenomena in real time, thus hindering further progress in this research field. In this study, we applied high‐speed atomic‐force microscopy (AFM), which has extraordinary spatiotemporal resolution (1 nm and sub‐100 ms), to capture dynamic events occurring during synthetic molecular self‐assembly. High‐speed AFM permitted the visualization of unique dynamic behavior, such as seeded growth and self‐repair in real time. Furthermore, scanning‐probe AFM permitted the site‐specific manipulation and functionalization of a molecular self‐assembly. This powerful combination of bottom‐up and top‐down approaches at the molecular level should enable targeted syntheses of unprecedented functional nanoarchitectures.
Herein, we report on a self-threading polythiophene whose conjugated molecular wire is sheathed within its own cyclic side chains. The defect-free insulating layer prevents electronic cross-communication between the adjacent polythiophene backbone even in the solid film. Notably, the covalently linked cyclic side chains extend the effective conjugation length of the interior polythiophene backbone, which results in an excellent intrawire hole mobility of 0.9 cm(2) V(-1) s(-1).
The influence of added polynucleotide on the gelation ability of nucleobase-appended organogelators was investigated. Uracil-appended cholesterol gelator formed a stable organogel in polar organic solvents such as n-butanol. It was found that the addition of the complementary polyadenylic acid (poly(A)) not only stabilizes the gel but also creates the helical structure in the original gel phase. Thymidine and thymine-appended gelators can form stable gel in apolar solvents, such as benzene, where poly(A)–lipid complex can act as a complementary template for the gelator molecules to create the fibrous composites. Based on these findings, we can conclude that self-assembling modes and gelation properties of nucleobase-appended organogelators are controllable by the addition of their complementary polynucleotide in organic solvents. We believe, therefore, that the present system can open the new paths to accelerate development of well-controlled one-dimensional molecular assembly systems, which would be indispensable for the creation of novel nanomaterials based on organic compounds.
Insulated molecular wires (IMWs) are π-conjugated polymers that are molecularly sheathed with an insulating layer and are structurally analogous to electric power cords at the nanoscale. Such unique architectures are expected in molecular electronics and organic devices. Herein, we propose a new molecular design concept of IMWs, in which the sheaths can be customized, thereby enabling the modulation of the electronic properties of the interior π-conjugated systems. To this end, we focused our attention on the dielectric constant of the sheaths, as it governs the electrostatic interaction between charges. Upon doping, charge carriers, such as polaron and bipolaron, were generated regardless of the dielectric properties of the sheaths. Flash-photolysis time-resolved microwave conductivity measurements revealed that intrawire charge carrier mobility was independent of the sheaths. However, we found that the charge carriers could be stabilized by the sheaths with a high dielectric constant owing to the charge screening effect. We expect that IMWs designed in this way will be useful in a variety of applications, where the nature of charge carriers plays an important role, and particularly when redox switching is required (e.g., electrochromic, magnetic, and memory applications).
In this short review, we provide an overview of multistep molecular and macromolecular assembly in syntheses of higher-order structures that are unobtainable under thermodynamic control. As in the case of organic/macromolecular chemistry, a synthetic scheme is designed such that a series of assembly processes eventually leads to a complex structure. The recent progress in this research field has been made based on the mechanistic understandings from viewpoints of both thermodynamics and kinetics. We also describe relevant systems which make use of advanced experimental apparatuses such as optical tweezers, high-speed atomic force microscopy, and so on. The unprecedented structures obtainable in this way might play a pivotal role in bridging the hierarchical levels from the molecular scale to the macroscopic world, leading to new functional supramolecular materials.
