Entropy- and Hydrolytic-Driven Positional Switching of Macrocycle between Imine- and Hydrogen-Bonding Stations in Rotaxane-Based Molecular Shuttles

Synthesis of 'Impossible' Rotaxanes

'Impossible' rotaxanes, which are constituted by interlocked components without obvious binding motifs, have attracted the interest of the mechanically interlocked molecules (MIMs) community. Within the synthetic efforts reported in the last decades towards the preparation of MIMs, some innovative protocols for accessing 'impossible' rotaxanes have been developed. This short review highlights different selected synthetic examples of 'impossible' rotaxanes, as well as suggests some future directions of this research area.

Ring-Over-Ring Deslipping From Imine-Bridged Heterorotaxanes

Ring-over-ring slippage and ring-through-ring penetration are important processes in the construction of ring-in-ring multiple interlocked architectures. We have successfully observed “ring-over-ring deslipping” on the rotaxane axle by exploiting the dynamic covalent nature of imine bonds in imine-bridged heterorotaxanes R1 and R2 with two macrocycles of different ring sizes on the axle. When the imine bridges of R1 were cleaved, a hydrolyzed hetero[4]rotaxane [4]R1′ was formed as an intermediate under dynamic equilibrium, and the larger 38-membered macrocycle M was deslipped over the 24-membered ring (24C8 or DB24C8) to dissociate into a [3]rotaxane [3]R3 and a macrocycle M. The time dependent NMR measurement and the determined thermodynamic parameters revealed that the rate-limiting step of the deslipping process was attributed to steric hindrance between two rings and reduced mobility of M due to proximity to the crown ether, which was bound to the anilinium on the axle molecule.

Distinctive features and challenges in catenane chemistry

From being an aesthetic molecular object to a building block for the construction of molecular machines, catenanes and related mechanically interlocked molecules (MIMs) continue to attract immense interest in many research areas. Catenane chemistry is closely tied to that of rotaxanes and knots, and involves concepts like mechanical bonds, chemical topology and co-conformation that are unique to these molecules. Yet, because of their different topological structures and mechanical bond properties, there are some fundamental differences between the chemistry of catenanes and that of rotaxanes and knots although the boundary is sometimes blurred. Clearly distinguishing these differences, in aspects of bonding, structure, synthesis and properties, between catenanes and other MIMs is therefore of fundamental importance to understand their chemistry and explore the new opportunities from mechanical bonds.

Covalently Templated Syntheses of Mechanically Interlocked Molecules

Mechanically interlocked molecules (MiMs), such as catenanes and rotaxanes, exhibit unique properties due to the mechanical bond which unites their components. The translational and rotational freedom present in these compounds may be harnessed to create stimuli-responsive MiMs, which find potential application as artificial molecular machines. Mechanically interlocked structures such as lasso peptides have also been found in nature, making MiMs promising albeit elusive targets for drug discovery. Although the first syntheses of MiMs were based on covalent strategies, approaches based on non-covalent interactions rose to prominence thereafter and have remained dominant. Non-covalent strategies are generally short and efficient, but do require particular structural motifs which are difficult to alter. In a covalent approach, MiMs can be more easily modified while the components may have increased rotational and translational freedom. Both approaches have complementary merits and combining the unmatched efficiency of non-covalent approaches with the scope of covalent syntheses may open up vast opportunities. In this review, recent covalently templated syntheses of MiMs are discussed to show their complementarity and anticipate future developments in this field.

1 Introduction

2 Tetrahedral Templates

2.1 A Carbonate Template for Non-Rusty Catenanes

2.2 All-Benzene Catenanes on a Silicon Template

2.3 Backfolding from Quaternary Carbon

3 Planar Templates

3.1 Rotaxanes Constructed in a Ring

3.2 Hydrindacene as a Dynamic Covalent Template

3.3 Templating on Tri- and Tetrasubstituted Benzenes

4 Conclusion

[2]Catenane Synthesis via Covalent Templating

After earlier unsuccessful attempts, this work reports the application of covalent templating for the synthesis of mechanically interlocked molecules (MiMs) bearing no supramolecular recognition sites. Two linear strands were covalently connected in a perpendicular fashion by a central ketal linkage. After subsequent attachment of the first strand to a template via temporary benzylic linkages, the second was linked to the template in a backfolding macrocyclization. The resulting pseudo[1]rotaxane structure was successfully converted to a [2]catenane via a second macrocyclization and cleavage of the ketal and temporary linkages.

Macroscopic Change in Luminescent Color by Thermally Driven Sliding Motion in [3]Rotaxanes

Systematic investigation of rotaxane structures has revealed a rational design for thermally driven switching of their macroscopic properties. At low temperature, the luminophore is insulated by the macrocycles and displays monomer emission, whereas at high temperature, the luminophore is exposed owing to a change in the macrocyclic location distribution and interacts with external molecules, affording a thermally driven luminescent color change with high reversibility and responsiveness. This macroscopic switching through efficient thermal sliding was made possible by appropriate tuning of both the macrocycle-luminophore interactions within the rotaxane and the coupling between the excited luminophore and external molecules in an exciplex. The ability to switch properties by a simple and clean thermal stimuli should expand the utilization of rotaxanes as components of thermally driven molecular systems.

LCST phase behavior of benzo-21-crown-7 with different alkyl chains

The introduction of hydrophobic units into crown ethers can dramatically decrease the critical transition temperature of LCST and realize macroscopic phase separation at low to moderate temperature and concentration. Minor modifications in the chemical structure of crown ethers (benzo-21-crown-7, B21C7s) can effectively control the thermo-responsive properties.

Foldamer-based helicate displaying reversible switching between two distinct conformers

A foldamer-based double-stranded helicate displays reversible switching between two distinct helicate conformers in response to temperature.

Redox-responsive host–guest system using redox-active pillar[5]arene containing one benzoquinone unit

Pillar[5]arene with one benzoquinone formed weak complex with a guest, while pillar[5]arene containing one hydroquinone strongly captured the guest.

A pH-driven molecular shuttle based on rotaxane-bridged periodic mesoporous organosilicas with responsive release of guests

A pH-driven molecular shuttle was immobilized into the framework of the PMOs in which the β-CDs could shuttle mechanically.

Energy Aspects of Thermal Molecular Switching: Molecular Thermal Hysteresis of Helicene Oligomers

Molecular switching is a phenomenon by which a molecule reversibly changes its structure and state in response to external stimuli or energy. Herein, molecular switching is discussed from thermodynamic and kinetic aspects in terms of energy supply with an emphasis on the thermal switching exhibited by helicene oligomers. It includes the inversion of relative thermodynamic stability induced by temperature changes and molecular thermal hysteresis in a closed system. The thermal phenomenon associated with the oligomers involves population/concentration changes between metastable states under nonequilibrium thermodynamic control.

Catenanes: fifty years of molecular links

Half a century after Schill and Lüttringhaus carried out the first directed synthesis of a [2]catenane, a plethora of strategies now exist for the construction of molecular Hopf links (singly interlocked rings), the simplest type of catenane. The precision and effectiveness with which suitable templates and/or noncovalent interactions can arrange building blocks has also enabled the synthesis of intricate and often beautiful higher order interlocked systems, including Solomon links, Borromean rings, and a Star of David catenane. This Review outlines the diverse strategies that exist for synthesizing catenanes in the 21st century and examines their emerging applications and the challenges that still exist for the synthesis of more complex topologies.

Multistimuli Responsive and Electroactive Supramolecular Gels Based on Ionic Liquid Gemini Guest

Electroactive supramolecular gels with multistimuli responsiveness were fabricated through host-guest interactions between a host polymer containing β-cyclodextrin (CD) and an ionic liquid (IL) type asymmetric gemini guest, which contains both ferrocene (Fc) and bis(trifluoromethyl-sulfonyl)imide (TFSI^-) as the respective α- and ω-guest groups. Owing to the multiple stimuli-responsiveness of the host-guest interactions, reversible sol-gel phase transition could be triggered by various stimuli, including temperature, electrochemical/chemical redox and anion-exchange reactions. Due to the intrinsically conductive properties of the IL gemini guest, the sol-gel transition behavior could be electrochemically controlled by the applied voltage, without using additional supporting electrolyte.

Induction of motion in a synthetic molecular machine: effect of tuning the driving force

Rotaxane molecular shuttles were studied in which a tetralactam macrocyclic ring moves between a succinamide station and a second station in which the structure is varied. Station 2 in all cases is an aromatic imide, which is a poor hydrogen-bond acceptor in the neutral form, but a strong one when reduced with one or two electrons. When the charge density on the hydrogen-bond-accepting carbonyl groups in station 2 is reduced by changing a naphthalimide into a naphthalene diimide radical anion, the shuttling rate changes only slightly. When station 2 is a pyromellitimide radical anion, however, the shuttling rate is significantly reduced. This implies that the shuttling rate is not only determined by the initial unbinding of the ring from the first station, as previously supposed. An alternative reaction mechanism is proposed in which the ring binds to both stations in the transition state.

Photophysical properties and conformational effects on the circular dichroism of an azobenzene-cyclodextrin [1]rotaxane and its molecular components

The photophysical properties of a multicomponent [1]rotaxane bearing a β-cyclodextrin ring covalently connected to an axle comprising an azobenzene photoisomerisable moiety and a naphthalimide-type fluorescent stopper are investigated by a combined experimental and computational study. The absorption and fluorescence spectra, and particularly the induced circular dichroism (ICD) signal, are determined. The latter shows a sign relation that cannot be rationalised in terms of the simple general rules commonly employed to analyse the ICD spectra of achiral guests encircled by chiral hosts. To assist the interpretation of experimental results, DFT and time-dependent (TD) DFT calculations are performed to explore the availability of low-energy conformations and to model their spectroscopic response. Molecular dynamics simulations performed in water show the interconversion of a number of conformers, the contribution of which to the ICD signal is in agreement with the observation.

Photoreversible switching of the lower critical solution temperature in a photoresponsive host-guest system of pillar[6]arene with triethylene oxide substituents and an azobenzene derivative

A new water-soluble thermoresponsive pillar[6]arene with triethylene oxide groups was synthesized. The pillar[6]arene showed lower critical solution temperature behavior in aqueous solution. Its clouding point was photoreversibly switched based on a photoresponsive host-guest system. The trans form of an azobenzene guest formed a stable 1:1 complex with the pillar[6]arene. Complexation increased the clouding point. Irradiation with UV light induced a conformation change for the azobenzene guest from the trans to cis form, and dethreading occurred because of a size mismatch between the cis form and the pillar[6]arene cavity. This dethreading decreased the clouding point. The photoresponsive host-guest system was reversible, and the clouding point could be switched by alternating irradiation with UV or visible light. We demonstrated photoresponsive reversible clear-to-turbid and turbid-to-clear transitions for the solution based on the reversible switching of the clouding point using the photosensitive host-guest system.

Synthetic molecular machines and polymer/monomer size switches that operate through dynamic and non-dynamic covalent changes

The present chapter is focused on how synthetic molecular machines (e.g. shuttles, switches and molecular motors) and size switches (conversions between polymers and their units, i.e., conversions between relatively large and small molecules) can function through covalent changes. Amongst the interesting examples of devices herein presented are molecular motors and size switches based on dynamic covalent chemistry which is an area of constitutional dynamic chemistry.

Strategies and tactics for the metal-directed synthesis of rotaxanes, knots, catenanes, and higher order links

More than a quarter of a century after the first metal template synthesis of a [2]catenane in Strasbourg, there now exists a plethora of strategies available for the construction of mechanically bonded and entwined molecular level structures. Catenanes, rotaxanes, knots and Borromean rings have all been successfully accessed by methods in which metal ions play a pivotal role. Originally metal ions were used solely for their coordination chemistry; acting either to gather and position the building blocks such that subsequent reactions generated the interlocked products or by being an integral part of the rings or "stoppers" of the interlocked assembly. Recently the role of the metal has evolved to encompass catalysis: the metal ions not only organize the building blocks in an entwined or threaded arrangement but also actively promote the reaction that covalently captures the interlocked structure. This Review outlines the diverse strategies that currently exist for forming mechanically bonded molecular structures with metal ions and details the tactics that the chemist can utilize for creating cross-over points, maximizing the yield of interlocked over non-interlocked products, and the reactions-of-choice for the covalent capture of threaded and entwined intermediates.

Energy landscape of a hydrogen-bonded non-degenerate molecular shuttle

In rotaxane 1, two co-conformations are populated in CDCl(3) at temperatures between 250 and 330 K. The thermodynamic parameters show strong enthalpy-entropy compensation, and a non-negligible heat capacity difference between the two forms.