[Show abstract][Hide abstract]ABSTRACT: A 1:1 mixture of pseudoenantiomeric aminomethylenehelicene oligomers, (P)-tetramer and (M)-pentamer, in fluorobenzene show a self-catalytic phenomenon in the formation of hetero-double helices from random coils. This study visualizes the spatially heterogeneous nature of the self-catalytic reaction in dilute solution. UV/Vis imaging analysis of the mixture at 70 °C, containing random coils, exhibits a homogeneous bright area. When the solution is cooled from 70 to 30 °C and held at that temperature, dark domains of approximately 1 mm in size appear, which move approximately at a rate of 1 mm min−1. The dark domains indicate that weaker UV/Vis absorption results from the formation of hetero-double helices, which is supported by circular dichroism (CD) imaging experiments. Then a homogeneous mixture is regenerated upon heating to 55 °C, as shown by CD imaging. Under self-catalytic conditions, a homogeneous solution spontaneously changed to a heterogeneous solution in the process of hetero-double-helix formation.
[Show abstract][Hide abstract]ABSTRACT: Racemic mixtures of aminomethylenehelicene (P)- and (M)-pentamers exhibited deterministic and stochastic chiral symmetry breaking during hetero-double-helix formation and self-assembly in solution. Heating a 50:50 mixture of (P)- and (M)-pentamers at 90 °C, and then cooling the mixture to 70 °C resulted in hetero-double-helix formation; a Cotton effect with negative Δε at λ=315 nm appeared. Chiral self-assembly occurred when the mixture was cooled to 25 °C. A strong tendency of deterministic chiral symmetry breaking appeared at the molecular and self-assembled levels, which was indicated by the negative Δε at λ=315 nm that appeared in most cases in repeated experiments. Mixtures containing 60:40 and 40:60 (P)-/(M)-pentamers also self-assembled with the same chirality. When a homo-double-helix (P)-/(M)-pentamer and a random coil (M)-/(P)-pentamer were mixed, the chiral self-assembly formed stochastically, and heating and cooling resulted in deterministic chiral symmetry breaking.
Article · Jul 2016 · Chemistry - A European Journal
[Show abstract][Hide abstract]ABSTRACT: Molecular switching is a phenomenon in which the molecular structure reversibly changes in response to external stimulation. It is crucial in biology and is used in various biological sensing and responses. In contrast to the well-studied molecular switching involving two or more thermodynamically stable states, switching involving metastable states exhibits notable non-equilibrium thermodynamic properties. Synthetic chiral helicene oligomeric foldamers that exhibit molecular thermal hysteresis in dilute solution are an example. Molecular switching can be used for sensing environmental changes, including temperature thresholds, temperature decreases/increases, rates of temperature decrease, counting the numbers 1 and 2, and concentration increases.
[Show abstract][Hide abstract]ABSTRACT: Chemical reactions involving noncovalent bond formation are discussed with regard to the equilibrium and nonequilibrium states: An equilibrium-to-equilibirum chemical reaction involves change of equilibrium states by changing the environment; a nonequilibrium-to-equilibrium reaction involves change of metastable state to equilibrium. Complex nature of the chemical reactions, especially in the latter, is shown in terms of the multiple-path nature in the microscopic molecular structure changes and macroscopic concentration changes. Irrversible and reversible nonequilibrium-to-equilibrium chemical reactions are also compared in terms of the multiple-path. Helicene oligomers, which reversibly form double-helix and random-coil by temperature changes, are discussed with regard to the reversible nonequilibrium-to-equilibrium chemical reaction with self-catalysis, where notable chemical phenomena appear under nonequilibrium conditions.
Article · Jan 2016 · Bulletin of the Chemical Society of Japan
[Show abstract][Hide abstract]ABSTRACT: Oxymethylenehelicene (P)- and (M)-oligomers up to a nonamer were synthesized by a building block method. The oligomers formed dimeric homoaggregates in trifluoromethylbenzene. A mixture of the pseudoenantiomeric (P)-pentamer and (M)-hexamer formed a heteroaggregate, which self-assembled into one-dimensional fibril films at the liquid-solid interface. Discontinuous heterogeneous nucleation occurred, which involved the formation of particles that were 50 nm in diameter and subsequent fibril growth from these particles. The fibril film was formed on the solid surface and the molecules remained dissociated in solution. The fibril film formation was affected by seeding and the solid surface materials.
Article · Oct 2015 · Chemistry - A European Journal
[Show abstract][Hide abstract]ABSTRACT: The sulfonamidohelicene tetramer changes its structure between a random-coil and a helix-dimer, by which molecular thermal hysteresis appears. Mixing experiments on helix-dimer and random-coil solutions and kinetic analysis show that the transition from the random-coil to the helix-dimer involves self-catalysis by the helix-dimer.
[Show abstract][Hide abstract]ABSTRACT: When a toluene solution of ethynylhelicene (M)-nonamer is cooled and heated between 95 and 5 °C at a constant rate, molecular thermal hysteresis. i.e., different courses of the structure change between a double-helix and a random-coil, occurs. Note that the structure change from the random-coil to the double-helix during heating crosses equilibrium and results in excessive concentrations of the double-helix. This phenomenon is named equilibrium crossing. Our experiment revealed an accelerated rate of formation of the double-helix from the random-coil while moving away from equilibrium. The acceleration of the reaction was also observed in constant-temperature experiments at 20 and 25 °C, where the reactions are faster at the intermediate states than at the initial states. A mechanistic model is provided, which involves self-catalysis and a temperature-dependent shift of equilibrium.
[Show abstract][Hide abstract]ABSTRACT: The sulfoneamidohelicene tetramer in solution exhibits different molecular responses to the same cooling stimulus delivered once and twice under thermal hysteresis conditions. Its random-coil state at a high temperature was cooled and maintained at a given temperature for which its molecules remained in a random coil (first cooling); the resulting solution was heated and cooled, after which a helix dimer formed (second cooling). Such a property can be regarded as a molecular function of counting the numbers 1 and 2.
Article · Sep 2014 · Chemistry - A European Journal
[Show abstract][Hide abstract]ABSTRACT: Aminomethylenehelicene oligomers up to the (M)-heptamer were synthesized by reductive amination from a formylhelicene building block. The oligomers containing more than three helicenes formed a double helix in 1,3-dilfuorobenzene. The (M)-tetramer and (M)-pentamer unfolded into a random coil by heating to 60 °C, whereas the (M)-hexamer only slightly unfolded at the same temperature. A two-sided thermal hysteresis was detected in the structural change of the (M)-tetramer and (M)-pentamer during cooling and heating. The (M)-pentamer and (M)-hexamer unfolded with the addition of trifluoroacetic acid and regenerated a double helix with the addition of triethylamine.
Article · Jul 2014 · Asian Journal of Organic Chemistry
[Show abstract][Hide abstract]ABSTRACT: Polycyclic aromatic compounds are an important group of substances in chemistry, and the study of their properties is a subject of interest in the development of drugs and materials. We have been conducting studies to develop chiral polycyclic aromatic compounds, i.e., helicenes and equatorenes. These helical molecules showed notable aggregate-forming properties and the capability for chiral recognition exerted by noncovalent bond interactions, which were not observed in compounds with central chirality. Homo- and hetero-double-helix-forming helicene oligomers were developed, and the latter self-assembled to form gels and vesicles. In this article, we describe such hierarchical studies of polycyclic aromatic compounds, which were started from polyketide aldol synthesis.
[Show abstract][Hide abstract]ABSTRACT: A 1:1 mixture of pseudoenantiomeric aminomethylenehelicene (P)-tetramer and (M)-pentamer formed three states, namely, the hetero-double helices B and C and the random coil A. At high temperatures, A is the most stable. At low temperatures, C is the most stable, and the structural changes from A to the metastable state B to the product C occur, where B and C have enantiomeric helical structures. Heating then converts C to A. Essentially all the molecules change their structure in a one-direction manner from A to B to C to A. Various nonequilibrium reversible thermodynamic responses appeared depending on thermal conditions: The metastable states A and B can be interconverted with thermal hysteresis without forming C in a far-from-equilibrium manner; they can be frozen at low temperatures and defrosted by warming; three-state hysteresis occurs. An energy and population model for nonequilibrium thermodynamic phenomena is given, involving two metastable states and thermal hysteresis.
Article · May 2014 · Journal of the American Chemical Society
[Show abstract][Hide abstract]ABSTRACT: Metal/semiconductor interfaces govern the operation of semiconductor devices
through the formation of charge injection barriers that can be controlled by
tuning the metal work function. However, the controlling ability is typically
limited to being static. We show that a dynamic nature can be imparted to the
interfaces using electrode surface modification with a structurally disordered
molecular monolayer. The barrier height at the interfaces is altered
significantly in a reversible way by an external electric field. As a result, a
dramatic change in the carrier transport properties through the interfaces is
observed, such as a reversible polarity reversion of
[Show abstract][Hide abstract]ABSTRACT: Sulfonamidohelicene oligomers up to the nonamer level were synthesized by the repeated coupling reactions of a building block. A tetramer formed a helix dimer in 1,3-difluorobenzene, which unfolded to a random coil with heating. This structural change exhibited thermal hysteresis in which different thermal responses were observed in the course of temperature increase and decrease. The feature of the hysteresis was examined under different heating/cooling modes, and the mechanisms are discussed on the basis of the population change and the presence of an induction period. A proposal regarding the use of thermal hysteresis for sensing a temperature increase/decrease is also given.
Article · Jul 2013 · Chemistry - A European Journal
[Show abstract][Hide abstract]ABSTRACT: An (amido-ethynyl)helicene bidomain compound and an (amido-ethynyl-amido)helicene tridomain compound were synthesized. The multidomain compounds were designed on the basis of previous findings that amido and ethynyl oligomers form dimeric aggregates with properties orthogonal to each other. Four aggregate states of multidomain compounds, namely, all-dimer, amido-dimer, ethynyl-dimer, and random-coil states, were obtained in different solvents, which were analyzed by circular dichroism (CD), UV/Vis, (1) H NMR, and IR spectroscopy; vapor pressure osmometry (VPO); dynamic light scattering (DLS); and atomic force microscopy (AFM). The amido and ethynyl domains independently aggregated and disaggregated in a two-state manner. Reversible structural changes occurred for a tridomain compound between the ethynyl-dimer/random-coil state and the all-dimer/amido-dimer state with heating and cooling. Two structural change processes with different properties were obtained using a single compound.
Article · Oct 2012 · Chemistry - A European Journal
[Show abstract][Hide abstract]ABSTRACT: A methodology for the formation of fibers/gels and vesicles by molecular assembly and for controlling their properties is presented. Two-component systems of pentamer (P)-5 and tetramer (M)-4 pseudoenantiomeric ethynylhelicenes with decyloxycarbonyl (D) and 4-methyl-2-(2-methylpropyl)-1-pentyloxycarbonyl (bD) side-chains have been examined. Distinct aggregates were formed by changing the solvent for the three combinations of (P)-bD-5/(M)-bD-4, (P)-D-5/(M)-bD-4, and (P)-D-5/(M)-D-4. In toluene, (P)-bD-5/(M)-bD-4, (P)-D-5/(M)-bD-4, and (P)-D-5/(M)-D-4 all formed gels and fibrous assemblies were observed by AFM. The minimum gel-forming concentration (MGC) decreased in the order (P)-bD-5/(M)-bD-4>(P)-D-5/(M)-bD-4>(P)-D-5/(M)-D-4. In diethyl ether, vesicular formation was observed by dynamic light scattering (DLS), AFM, and TEM, and the size of the vesicles decreased in the order (P)-bD-5/(M)-bD-4>(P)-D-5/(M)-bD-4>(P)-D-5/(M)-D-4. Both fiber/gel and vesicle formation were accompanied by enhanced CDs and redshifted UV/Vis absorption bands with a change in color to deep yellow. These are novel two-component oligomeric systems that form assemblies of fibers/gels or vesicles depending on the solvent, and the structures and properties of the assemblies can be fine-tuned by changing the combination of oligomers. In m-difluorobenzene, a homogeneous solution was obtained with (P)-D-5/(M)-bD-4, which again exhibits enhanced CDs and redshifted UV/Vis absorptions. Vapor pressure osmometry analysis showed the formation of a bimolecular heteroaggregate. The study has indicated that pseudoenantiomeric oligomers form hetero-double-helices that hierarchically assemble to form fibers/gels and vesicles.
Article · Jul 2012 · Chemistry - A European Journal
[Show abstract][Hide abstract]ABSTRACT: Organic gel-liquid two-layer systems were constructed by two-liquid-ultrasonication-diffusion-controlled gelation using the chiral aminohydroxyhelicene derivative (M)-1. Gelation took place by the diffusion of (M)-1 and THF from the bottom layer to the upper hexane layer. This method provided multilayer liquid-gel systems.
[Show abstract][Hide abstract]ABSTRACT: The reverse amidohelicene (P)-tetramer containing (P)-helicenediamine and m-phenylenedicarboxylate was synthesized. The circular dichroism (CD) and vapor pressure osmometry (VPO) analysis revealed the dimeric aggregate formation in THF. It is notable that the reverse amide tetramer formed a duplex, as well as the original amidohelicene oligomers. In various solutions, mixtures of the reverse (P)-tetramer and amidohelicene (P)/(M)-tetramer formed homoaggregates instead of heteroaggregates.