"Inverse" thermoresponse: heat-induced double-helix formation of an ethynylhelicene oligomer with tri(ethylene glycol) termini

Anion-coordination-driven single-double helix switching and chiroptical molecular switching based on oligoureas

Synthetic foldamers with helical conformation are widely seen, but controllable interconversion amongst different geometries (helical structure and sense) is challenging. Here, a family of oligourea (tetra-, penta-, and hexa-) ligands bearing stereocenters at both ends are designed and shown to switch between single and double helices with concomitant inversion of helical senses upon anion coordination. The tetraurea ligand forms a right-handed single helix upon chloride anion (Cl-) binding and is converted into a left-handed double helix when phosphate anion (PO4 3-) is coordinated. The helical senses of the single and double helices are opposite, and the conversion is further found to be dependent on the stoichiometry of the ligand and phosphate anion. In contrast, only a single helix is formed for the hexaurea ligand with the phosphate anion. This distinction is attributed to the fact that the characteristic phosphate anion coordination geometry is satisfied by six urea moieties with twelve H-bonds. Our study revealed unusual single-double helix interconversion accompanied by unexpected chiroptical switching of helical senses.

Molecular bases for temperature sensitivity in supramolecular assemblies and their applications as thermoresponsive soft materials

Thermoresponsive supramolecular assemblies have been extensively explored in diverse formats, from injectable hydrogels to nanoscale carriers, for a variety of applications including drug delivery, tissue engineering and thermo-controlled catalysis. Understanding the molecular bases behind thermal sensitivity of materials is fundamentally important for the rational design of assemblies with optimal combination of properties and predictable tunability for specific applications. In this review, we summarize the recent advances in this area with a specific focus on the parameters and factors that influence thermoresponsive properties of soft materials. We summarize and analyze the effects of structures and architectures of molecules, hydrophilic and lipophilic balance, concentration, components and external additives upon the thermoresponsiveness of the corresponding molecular assemblies.

Modulation of the Self-Assembly of π-Amphiphiles in Water from Enthalpy- to Entropy-Driven by Enwrapping Substituents

Depending on the connectivity of solubilizing oligoethylene glycol (OEG) side chains to the π-cores of amphiphilic naphthalene and perylene bisimide dyes, self-assembly in water occurs either upon heating or cooling. Herein, we show that this effect originates from differences in the enwrapping capability of the π-cores by the OEG chains. Rylene bisimides bearing phenyl substituents with three OEG chains attached directly to the hydrophobic π-cores are strongly sequestered by the OEG chains. These molecules self-assemble at elevated temperatures in an entropy-driven process according to temperature- and concentration-dependent UV/Vis spectroscopy and calorimetric dilution studies. In contrast, for rylene bisimides in which phenyl substituents with three OEG chains are attached via a methylene spacer, leading to much weaker sequestration, self-assembly originates upon cooling in an enthalpy-driven process. Our explanation for this controversial behavior is that the aggregation in the latter case is dictated by the release of "high energy water" from the hydrophobic π-surfaces as well as dispersion interactions between the π-scaffolds which drive the self-assembly in an enthalpically driven process. In contrast, for the former case we suggest that in addition to the conventional explanation of a dehydration of hydrogen-bonded water molecules from OEG units it is in particular the increase in conformational entropy of back-folded OEG side chains upon aggregation that provides the pronounced gain in entropy that drives the aggregation process. Thus, our studies revealed that a subtle change in the attachment of solubilizing substituents can switch the thermodynamic signature for the self-assembly of amphiphilic dyes in water from enthalpy- to entropy-driven.

Formation and dissociation of synthetic hetero-double-helix complex in aqueous solutions: significant effect of water content on dynamics of structural change

A 1 : 1 mixture of the ethynylhelicene pseudoenantiomers (M)-tetramer and (P)-pentamer, which possess hydrophilic terminal tri(ethyleneglycol) (TEG) groups, changes their structures in the water-THF (10 μM) solvent system between dissociated random-coils and an associated hetero-double-helix upon heating and cooling. A small change in water content between 30 and 33% significantly affects the dynamics of structural changes. At 30% water content, heating to 60 °C causes rapid formation of random-coil and cooling to 10 °C causes the rapid formation of hetero-double-helix, accompanied by repeated changes in Δε at 369 nm between 0 and -2000 cm-1 M-1. Heating and cooling experiments at constant rates between 60 and 10 °C resulted in sigmoidal curves in Δε/temperature profiles, which indicate rapid structural changes. Different phenomena occurred at 33% water content. Heating to 60 °C and cooling to 0 °C initially induced changes in Δε between 0 and -2000 cm-1 M-1, and repeated cycles gradually reduced the range between 0 and -500 cm-1 M-1. Heating and cooling experiments at constant rates between 60 and 10 °C caused small changes in Δε, and repeated cycles at 10 °C gradually increased Δε to -500 cm-1 M-1. These phenomena involved rapid changes in molecular structure and slow structural changes in the water-THF solvent system. The sharp switching of the dynamics of structural changes at water content between 30 and 33% indicated discontinuous structural changes in the hydration of TEG and/or in water clusters in the vicinity of oligomer molecules.

Hierarchy of Asymmetry in Chiral Supramolecular Polymers: Toward Functional, Helical Supramolecular Structures

The formation of helical structures through the supramolecular polymerization of a variety of self-assembling units is reviewed. These scaffolds are usually obtained by efficient transfer or amplification of chirality phenomena, in which the starting self-assembling molecules possess different elements of asymmetry, such as point or axial chirality. Relevant examples of helical supramolecular structures investigated under thermodynamic control are reviewed, and the helical outcome of remarkable examples of chiral entities obtained through kinetic control are also highlighted. Finally, selected examples of flexible macroscopic chirality and catalysis are described to illustrate the applicability of helical aggregates.

Host–guest interaction enabled chiroptical photo-switching and enhanced circularly polarized luminescence

A supra-gelator from γ-CyD and a cyanostilbene gelator showed enhanced circularly polarized luminescence and enabled a reversible chiroptical as well as morphological photo-switching.

Construction of Various Supramolecular Assemblies from Rod-Coil Molecules Containing Biphenyl and Anthracene Groups Driven by Donor-Acceptor Interactions

Rod-coil amphiphilic functional molecules, comprising a rigid aromatic building block and hydrophilic oligoether dendrons as the coil segments, were synthesized. These compounds exhibit a powerful self-organizing ability to form supramolecular nanoparticles and long nanofibers in tetrahydrofuran/water solution, by controlling the intermolecular interaction of the rigid blocks. These molecules are able to form supramolecular polymers and, subsequently, to form sheetlike nanoaggregates, through charge-transfer interactions by the addition of a guest molecule, tetracyanoquinodimethane. Notably, upon addition of water-soluble 2,4,6-trinitrophenol, the self-assembly of these molecules exhibits the antagonistic effect owing to donor-acceptor and hydrophobic-hydrophilic interactions among the molecules. The experimental results reveal that various morphologies of rod-coil molecular assemblies can be obtained by tuning the molecular interaction and the hydrophilicity of guest electron-acceptor molecules. Interestingly, the cross-coupling reaction between phenylboronic acid and chlorobenzene occurs within the charge complexes of these molecular aggregates. This occurs in the nanoenvironment that affords an extremely concentrated reaction zone and reduces the activation energy barrier required for the cross-coupling reaction.

Enhanced circular dichroism at elevated temperatures through complexation-induced transformation of a three-layer cyclophane with dualistic dynamic helicity

When two planes stacked one above the other are twisted, they provide a dynamic pair of helical conformations with (M)- or (P)-helicity. We designed a three-layer cyclophane that consists of two such dynamic pairs: the top and middle planes, and the middle and bottom planes. Hence, several global conformations could be created for the overall molecule, e.g., double-helical forms with a pair with the same helicity [(M,M) or (P,P)], and a meso-like form with a pair with a different helicity (M,P). These conformations dynamically interconvert to each other in solution. Chiroptical properties were given by the helical-sense preference of the double-helical forms, which was brought about through complexation with a chiral hydrogen-bonding guest. In terms of the conformational energy in a complexed state, when a desirable relationship between double-helical and meso-like forms was attained, complexation-induced circular dichroism was enhanced at elevated temperatures and decreased at lowered temperatures.

Synthesis and Self-Assembly of Chiral Cylindrical Molecular Complexes: Functional Heterogeneous Liquid-Solid Materials Formed by Helicene Oligomers

Chiral cylindrical molecular complexes of homo- and hetero-double-helices derived from helicene oligomers self-assemble in solution, providing functional heterogeneous liquid-solid materials. Gels and liotropic liquid crystals are formed by fibril self-assembly in solution; molecular monolayers and fibril films are formed by self-assembly on solid surfaces; gels containing gold nanoparticles emit light; silica nanoparticles aggregate and adsorb double-helices. Notable dynamics appears during self-assembly, including multistep self-assembly, solid surface catalyzed double-helix formation, sigmoidal and stairwise kinetics, molecular recognition of nanoparticles, discontinuous self-assembly, materials clocking, chiral symmetry breaking and homogeneous-heterogeneous transitions. These phenomena are derived from strong intercomplex interactions of chiral cylindrical molecular complexes.

Proton triggered circularly polarized luminescence in orthogonal- and co-assemblies of chiral gelators with achiral perylene bisimide

Acid–base exposure switched circularly polarized luminescence was achieved in a coassembled gel which is composed of a chiral gelator and achiral perylene bisimide.

[Synthesis, Aggregation, Self-assembly, and Dynamic Properties of Helicene Oligomers]

 Biological systems exhibit dynamic phenomena at the macroscopic level as a result of the hierarchical integration of phenomena at the molecular level. For example, a number of amino acids compose actin proteins, which form three-dimensional structures determined by the sequence of amino acids. They form fibers by self-assembly, which then form ordered structures such as meshes, lyotropic liquid crystals (LCs), and bundles. The dynamic and reversible polymorphism between these nano- to centimeter-sized ordered structures is essential for biological functions such as cell division, contraction, and locomotion. To understand biological systems and create new functional materials, it is essential to develop a methodology to integrate phenomena at the molecular level into those at the macroscopic level using synthetic molecules. In this research, synthetic oligomers containing helicenes, which exhibit reversible structural transitions between cylindrical double helices and random coils in response to thermal stimuli, were employed as building blocks for the development of such a methodology. The properties of homo- and hetero-double helices at the molecular level were first controlled by taking advantage of the diversity of their molecular structures. Then, nano- to micrometer-sized structures were constructed by the self-assembly of hetero-double helices, which include fibers/gels, vesicles, and lyotropic LCs, and their dynamic properties were controlled by molecular design.

Pendant-Type Helicene Oligomers with p-Phenylene Ethynylene Main Chains: Synthesis, Reversible Formation of Ladderlike Bimolecular Aggregates, and Control of Intramolecular and Intermolecular Aggregation

Pendant-type (P)-helicene oligomers with p-phenylene ethynylene main chains up to a tetramer were synthesized by a building block method. The (P)-tetramer reversibly formed a ladderlike bimolecular aggregate upon cooling and disaggregated upon heating in (trifluoromethyl)benzene. Two bis(tetramer)s, in which two (P)-tetramers were connected by hexadecamethylene linkers, were also synthesized. The head-to-tail bis(tetramer) formed an intramolecular aggregate, and the head-to-head bis(tetramer) formed an intermolecular aggregate in toluene. The results suggest the antiparallel aggregation structure of the pendant-type (P)-tetramers. The structure of the linker was proven to be effective in controlling intramolecular and intermolecular aggregations.

Zwitterionic [4]helicene: a water-soluble and reversible pH-triggered ECD/CPL chiroptical switch in the UV and red spectral regions

The chiroptical switching properties of a readily accessible and water-soluble diaza [4]helicene are disclosed.