Mechanically Interlocked [2]Rotaxane Aerogels with Tunable Morphologies and Mechanical Properties

Mechanical bonds have been utilized as promising motifs to construct mechanically interlocked aerogels (MIAs) with mechanical adaptivity and multifunctionality. However, fabricating such aerogels with not only precise chemical structures but also dynamic features remains challenging. Herein, we present MIAs carrying dense [2]rotaxane units, which bestow both the stability and flexibility of the aerogel network. Owing to the stable chemical structure of a [2]rotaxane, MIAs possessing a precise and full-scale mechanically interlocked network could be fabricated with the aid of diverse solvents. In addition, the dynamic nature of the [2]rotaxane resulted in morphologies and mechanical performances of the MIAs that can be dramatically modulated under chemical stimuli. We hope that the structure-property relationship in MIAs will facilitate the development of mechanically interlocked materials and provide novel opportunities toward constructing smart materials with multifunctionalities.

Preprogrammed assembly of supramolecular polymer networks via the controlled disassembly of a metastable rotaxane

In our daily life, some of the most valuable commodities are preprogrammed or preassembled by a manufacturer; the end-user puts together the final product and gathers properties or function as desired. Here, we present a chemical approach to preassembled materials, namely supramolecular polymer networks (SPNs), which wait for an operator's command to organize autonomously. In this prototypical system, the controlled disassembly of a metastable interlocked molecule (rotaxane) liberates an active species to the medium. This species crosslinks a ring-containing polymer and assembles with a reporting macrocycle to produce colorful SPNs. We demonstrate that by using identical preprogrammed systems, one can access multiple supramolecular polymer networks with different degrees of fluidity (μ^* = 2.5 to 624 Pa s^-1) and color, all as desired by the end-user.

Monosulfonated dibenzo-24-crown-8 and its properties

In this paper, we describe a method for preparing a monosulfonated dibenzo-24-crown-8 ether, SDB24C8, by direct sulfonation of the parent crown (DB24C8). Since neutral DB24C8 readily interacts with cationic guests, permanently charged SDB24C8 is an advantageous candidate for future supramolecular applications. SDB24C8 can be isolated as a sulfonic acid to be used as it is or converted to a salt of choice. The crystallographic analysis provides the first known host-guest assembly with a DB24C8-based scaffold complexing hydronium and potassium cations. Supramolecular investigations of the interactions of this anionic macrocycle with alkali cations were also performed. According to the expectations, the introduction of the sulfonic group into the DB24C8 scaffold increases the affinities of the receptor. An unusual selectivity of SDB24C8 towards a sodium cation was also observed and further investigated with DFT calculations.

An Operative Electrostatic Slipping Mechanism along Macrocycle Flexibility Accelerates Guest Sliding during pseudo-Rotaxane Formation

A pseudo-rotaxane is a host-guest complex composed of a linear molecule encircled by a macrocyclic ring. These complexes can be assembled by sliding the host over the guest terminal groups. If there is a close match between the molecular volume of the flanking groups on the guest and the cavity size of the macrocycle, the slipping might occur slowly or even become completely hindered. We have previously shown that it is possible to overcome the restraints imposed by steric effects on the sliding process by integrating electrostatic attractive interactions during the slipping step. In this work, we extend our electrostatically assisted slipping approach (EASA) to a new host-guest system featuring a flexible macrocyclic ring and a series of asymmetric guests containing a cyclic tertiary ammonium group. Compelling evidence for pseudo-rotaxane formation is presented, along with thermodynamic and kinetic data. Experimental results suggests that the higher conformational flexibility of 24-crown-8 significantly increases the sliding rate, compared with the more rigid dibenzo-24-crown-8, without affecting complex stability. Furthermore, by combining the EASA and macrocycle flexibility, we were capable to slip a large eight-membered cyclic group across the 24-crown-8 annulus, setting a new limit on the ring molecular size that can pass through a 24-membered crown ether.

Polymerization of a biscalix[5]arene derivative

Recent decades have seen an increased interest in the preparation of polymers possessing host or guest moieties as the end group, which has enabled new polymeric materials such as self-healable, shape-memory, and stimuli-responsive materials. Such polymers are commonly synthesized by tethering the host or guest moieties to polymers. On the other hand, there are limited reports demonstrating the preparation of host- or guest-appended polymers by directly polymerizing the corresponding host- or guest-appended monomers, which is valuable for easy access to diverse polymers from single molecular species. However, reactive host and/or guest moieties of the monomer interfere with the polymerization reaction. Here, we report that a biscalix[5]arene host-appended molecule can be polymerized with various monomers to form the corresponding host-appended polymers. The host–guest complexation behavior of calix[5]arene-appended polymers with fullerene derivatives was studied by ¹H NMR and UV/Vis spectroscopic techniques, which revealed that the long polymer chains did not prevent host–guest complexation even when the fullerene derivative was equipped with a polymer chain. Thus, the present study shows the potential for developing polymers that have various combinations of polymer chains.

A calix[5]arene appended monomer molecule was subjected to polymerization reaction to yield corresponding methacrylate polymers. The calix[5]arene appended polymers showed excellent encapsulation capability for fullerene molecules.

An Anionic Ring Locked into an Anionic Axle: A Metastable Rotaxane with Chemically Activated Electrostatic Stoppers

The use of the electrostatic stoppers concept in the field of mechanically interlocked molecules is reported; these stoppers are chemically sensitive end groups on a linear guest molecule that allows for the conversion of a pseudo-rotaxane species into a rotaxane complex by a change in the medium acidity. The chemical stimulus causes the appearance of negative charges on both ends of the linear component, passing from cationic to anionic, and causing a significant ring-to-axle electrostatic repulsion. This phenomenon has two different and simultaneous effects: 1) destabilizes the complex as a consequence of confining an anionic ring into an anionic axle, and 2) increases the dissociation energy barrier, thus impeding ring extrusion. This newly formed metastable rotaxane species is resistant to solvent and temperature effects and performs as a two-state degenerated molecular shuttle in solution.

[3]rotaxanes composed of two dibenzo-24-crown-8 ether wheels and an azamacrocyclic complex

The azamacrocyclic complex was used as a platform for the construction of [3]rotaxanes containing two DB24C8 macrocycles per molecule. The complex unit incorporates two electron deficient π-bond systems and two N-H hydrogen bond donating groups which facilitated the formation of a 1 : 2 interlocked structure. Synthesis and properties of such compounds are presented. Structures of the obtained compounds were confirmed by NMR spectroscopy, ESI mass spectrometry, elemental analysis and single crystal X-ray diffraction. Both [3]rotaxanes containing two DB24C8 macrocycles per molecule crystallise in P1[combining macron] and P21/n space groups. They have different counterions (PF6- and Cl- anions, respectively) and mostly disordered solvent molecules such as water, methanol and acetone. Both [3]rotaxanes have a flexible axle in which the Cl- salt takes the shape closer to the "S"-letter, while in the PF6- case the axle is more linear. The shape results from respective packing and intra-, and intermolecular interactions among the moieties in the rotaxane and the crystal lattice.