Organogel Formation by a Cholesterol-Stoppered Bistable [2]Rotaxane and Its Dumbbell Precursor

Improbable Rotaxanes Constructed From Surrogate Malonate Rotaxanes as Encircled Methylene Synthons

We have developed a new approach for the synthesis of "improbable" rotaxanes by using malonate-centered rotaxanes as interlocked surrogate precursors. Here, the desired dumbbell-shaped structure can be assembled from two different, completely separate, portions, with the only residual structure introduced from the malonate surrogate being a methylene group. We have synthesized improbable [2]- and [3]rotaxanes with all-hydrocarbon dumbbell-shaped components to demonstrate the potential structural flexibility and scope of the guest species that can be interlocked when using this approach.

The Story of the Little Blue Box: A Tribute to Siegfried Hünig

The tetracationic cyclophane, cyclobis(paraquat-p-phenylene), also known as the little blue box, constitutes a modular receptor that has facilitated the discovery of many host-guest complexes and mechanically interlocked molecules during the past 35 years. Its versatility in binding small π-donors in its tetracationic state, as well as forming trisradical tricationic complexes with viologen radical cations in its doubly reduced bisradical dicationic state, renders it valuable for the construction of various stimuli-responsive materials. Since the first reports in 1988, the little blue box has been featured in over 500 publications in the literature. All this research activity would not have been possible without the seminal contributions carried out by Siegfried Hünig, who not only pioneered the syntheses of viologen-containing cyclophanes, but also revealed their rich redox chemistry in addition to their ability to undergo intramolecular π-dimerization. This Review describes how his pioneering research led to the design and synthesis of the little blue box, and how this redox-active host evolved into the key component of molecular shuttles, switches, and machines.

Controlling Ultra-Large Optical Asymmetry in Amorphous Molecular Aggregations

Although ultra-large optical asymmetry appears in crystalline materials, distractions from the mesoscopic ordering often causes inauthenticity in chiropticity. In amorphous materials, however, it remains challenging and elusive to achieve large chiropticity. Herein, we report the quantitative control of chiral amplification, on amorphous supramolecular structures of cholesteryl-linked bis(dipyrrinato)zinc(II), to an exceptionally high level. A proper chiral packing of the building block at several molecular scale considerably contributes to the absorptive dissymmetry factor g_abs , although the system is overall disordered. The intense and tunable aggregation strength renders a variable g_abs value up to +0.10 and +0.31 in the solution and in film state. On this basis, a superior ON-OFF switching of chiropticity is realized under external stimuli. This work establishes a general design principle to control over ultra-large optical asymmetry on a wider scope of chiral materials.

Pillar[5]arene-Based [2]Rotaxane: Synthesis, Characterization, and Application in a Coupling Reaction

Mechanically interlocked molecules are a class of smart supramolecular species because of their interesting topological structure and application in various areas, such as biology and nanoscience. In this work, we used "multicomponent reaction" to fabricate a new [2]rotaxane based on pillar[5]arene from different small-sized molecules. The molecular structure of the obtained [2]rotaxane R was confirmed by ¹H and ¹³C NMR, high-resolution electrospray ionization mass spectrometry, two-dimensional nuclear Overhauser effect spectroscopy, and density functional theory studies. Interestingly, the [2]rotaxane-based organometallic cross-linked catalyst (Pd@R) was easily constructed via the coordination between triazole groups and Pd(NO₃)₂. Pd@R proved to be a good catalyst for the Suzuki-Miyaura coupling reaction with excellent stability and repeatability.

Theoretical and experimental studies of an oseltamivir-triazole-based thermoresponsive organogel

Low-molecular weight organic gelators have been of significant interest in recent years because of their interesting properties and potential applications in sensing technology, biomedicine and drug delivery. Herein, the synthesis, characterization and gelation properties of new oseltamivir conjugates are reported. The oseltamivir-triazole conjugate 1 was synthesized via a click-reaction in a 75% yield. The key features of this conjugate include the presence of amide, flexible ester linkages and a triazole scaffold linking a hydrophobic alkyl chain. The conjugate 1, possessing a long alkyl chain, showed gelation properties in various apolar organic solvents. This gelation behavior was not observed in the case of the deesterified conjugate 2; this indicated the necessity of the alkyl chain for gelation. The gelator 1 showed thermoreversible gelation properties in a range of linear alkane solvents (from n-pentane to n-dodecane). A scanning electron microscopic study suggests that the gelator 1 exists as cross-linked structures, which are self-aggregated in the range of submicrometers, as supported by extensive ¹H-NMR studies. The rheological parameters supported the occurrence of a soft gelation process, and the gel formed in n-decane was found to be stiffer than that formed in n-hexane. Computational studies suggested that the gelation behavior was indeed due to micelle formation and dependent on the lipophilicity of solvents.

Photocontrolled morphological conversion and chiral transfer of a snowflake-like supramolecular assembly based on azobenzene-bridged bis(dibenzo-24-crown-8) and a cholesterol derivative

A snowflake-like supramolecular clockwise-helical assembly was fabricated via the host–guest interaction, while a snowflake-like supramolecular non-helical assembly can be obtained upon UV-irradiation.

Vesicular self-assembly of a natural ursane-type dihydroxy-triterpenoid corosolic acid

Corosolic acid, a natural ursane-type 6-6-6-6-6 pentacyclic dihydroxy triterpenic acid, is a well known antidiabetic compound extractable from leaves of Psidium guajava. In this manuscript we have reported the self-assembly properties of corosolic acid in different liquids. The compound undergoes self-assembly to give vesicular morphology in different aqueous organic liquids. Supramolecular gels were also obtained in some aqueous binary liquids such as ethanol–water and dimethyl formamide–water. The morphology of the self-assemblies of corosolic acid were characterized by using different microscopic techniques like optical microscopy, field emission scanning electron microscopy, transmission electron microscopy, atomic force microscopy as well as XRD and FTIR studies. We also demonstrated the application of vesicular self-assemblies for the entrapment and release of fluorophores including an anticancer drug.

Corosolic acid, a natural ursane-type 6-6-6-6-6 pentacyclic dihydroxy triterpenic acid, self-assembled in binary liquid mixtures yielding vesicles.

Ionic Liquid Crystals: Versatile Materials

This Review covers the recent developments (2005-2015) in the design, synthesis, characterization, and application of thermotropic ionic liquid crystals. It was designed to give a comprehensive overview of the "state-of-the-art" in the field. The discussion is focused on low molar mass and dendrimeric thermotropic ionic mesogens, as well as selected metal-containing compounds (metallomesogens), but some references to polymeric and/or lyotropic ionic liquid crystals and particularly to ionic liquids will also be provided. Although zwitterionic and mesoionic mesogens are also treated to some extent, emphasis will be directed toward liquid-crystalline materials consisting of organic cations and organic/inorganic anions that are not covalently bound but interact via electrostatic and other noncovalent interactions.

Carbon-Nanotube-Mediated Electrochemical Transition in a Redox-Active Supramolecular Hydrogel Derived from Viologen and an l-Alanine-Based Amphiphile

A two-component hydrogelator (16-A)2 -V(2+) , comprising an l-alanine-based amphiphile (16-A) and a redox-active viologen based partner (V(2+) ), is reported. The formation the hydrogel depended, not only on the acid-to-amine stoichiometric ratio, but on the choice of the l-amino acid group and also on the hydrocarbon chain length of the amphiphilic component. The redox responsive property and the electrochemical behavior of this two-component system were further examined by step-wise chemical and electrochemical reduction of the viologen nucleus (V(2+) /V(+) and V(+) /V(0) ). The half-wave reduction potentials (E1/2 ) associated with the viologen ring shifted to more negative values with increasing amine component. This indicates that higher extent of salt formation hinders reduction of the viologen moiety. Interestingly, the incorporation of single-walled carbon nanotubes in the electrochemically irreversible hydrogel (16-A)2 -V(2+) transformed it into a quasi-reversible electrochemical system.

Diamide-based fullerosteroidal and disteroidal [2]rotaxanes: solvent-induced macrocycle translocation and/or unthreading

Two hydrogen bonded rotaxanes: a template directed synthesis, detailed characterization and shuttling/unthreading processes.

Two-component supramolecular gels derived from amphiphilic shape-persistent cyclo[6]aramides for specific recognition of native arginine

A unique supramolecular two-component gelation system was constructed from amphiphilic shape-persistent cyclo[6]aramides and diethylammonium chloride (or triethylammonium chloride). This system has the ability to discriminate native arginine from 19 other amino acids in a specific fashion. Cyclo[6]aramides show preferential binding for the guanidinium residue over ammonium groups. This specificity was confirmed by both experimental results and theoretical simulations. These results demonstrated a new modular displacement strategy, exploring the use of species-binding hydrogen-bonded macrocyclic foldamers for the construction of two-component gelation systems for selective recognition of native amino acids by competitive host-guest interactions. This strategy may be amenable to developing a variety of functional two-component gelators for specific recognition of various targeted organic molecular species.

Five additional macrocycles that allow Na+ ion-templated threading of guest units featuring a single urea or amide functionality

Five analogues of the macrocycle BPX26C6 are also capable of recognizing single urea and/or amide functionalities in the presence of templating Na⁺ ions.

Enantiomeric organogelators from d-/l-arabinose for phase selective gelation of crude oil and their gel as a photochemical micro-reactor

Easy access via single steps to each enantiomer of a simple chiral organogelator, their gelation abilities and applications are reported.

Light-driven linear helical supramolecular polymer formed by molecular-recognition-directed self-assembly of bis(p-sulfonatocalix[4]arene) and pseudorotaxane

A light-driven, linear, chiral supramolecular polymer was constructed in water by host-guest molecular recognition between bis(p-sulfonatocalix[4]arene) and the α-cyclodextrin-based pseudo[3]rotaxane containing axially chiral 1,1'-binaphthyl and photoresponsive azobenzene moieties. The successful supramolecular polymerization by non-covalent host-guest molecular recognition was confirmed by (1)H NMR spectroscopy and dynamic light scattering (DLS) measurements, and its photoresponsive behavior was investigated by UV-vis absorption spectroscopy, scanning electron microscopy (SEM), atomic force microscopy (AFM), and transmission electron microscopy (TEM). The chirality of this supramolecular polymer was confirmed by circular dichroism spectroscopy. The dramatic morphology change of this chiral polymer driven by light was observed in SEM, AFM and TEM images. More interestingly, dynamically self-assembled, light-driven, single-helical linear supramolecular polymer molecules with lengths of hundreds of nanometers to micrometers in water were directly observed in their native state using cryo-TEM measurements. The observation of considerably lengthy individual supramolecular polymer molecules indicates that the molecular self-assembly in water by non-covalent host-guest molecular recognition is sufficiently strong to form the supramolecular polymer. Moreover, preliminary molecular modeling was performed to substantiate this interesting photoresponsive supramolecular structure.