Rigidity and Flexibility in Rotaxanes and Their Relatives; On Being Stubborn and Easy-Going

Abstract

Rotaxanes are an emerging class of molecules composed of two building blocks: macrocycles and threads. Rotaxanes, and their pseudorotaxane and polyrotaxane relatives, serve as prototypes for molecular-level switches and machines and as components in materials like elastic polymers and 3D printing inks. The rigidity and flexibility of these molecules is a characteristic feature of their design. However, the mechanical properties of the assembled rotaxane and its components are rarely examined directly, and the translation of these properties from molecules to bulk materials is understudied. In this Review, we consider the mechanical properties of rotaxanes by making use of concepts borrowed from physical organic chemistry. Rigid molecules have fewer accessible conformations with higher energy barriers while flexible molecules have more accessible conformations and lower energy barriers. The macrocycles and threads become rigidified when threaded together as rotaxanes in which the formation of intermolecular interactions and increased steric contacts collectively reduce the conformational space and raise barriers. Conversely, rotational and translational isomerism in rotaxanes adds novel modes of flexibility. We find that rigidification in rotaxanes is almost universal, but novel degrees of flexibility can be introduced. Both have roles to play in the function of rotaxanes.

The Role of Dethreading Process in Pseudorotaxanes and Rotaxanes towards Advanced Applications: Recent Examples

“…[64] Enzyme-responsive interlocked profragrances, which act as molecular logic gates, turned out to be ideal scaffolds for the controlled release of scents. [65] Cucurbit [6]uril (CB6) rotaxane probes for protease detection were prepared by the research group of Francis, in which an enzyme-triggered dethreading process released CB6 allowing interactions between this macrocycle and 129 Xe, thus leading to a protease-responsive hyperpolarization exchange saturation transfer. [66] The abovementioned systems, which operate through the dethreading induced by enzymes, take inspiration from the biological nanomachines working within living organisms.…”

“…Mechanically interlocked molecules (MIMs) are a type of compounds constituted by components which are bonded together through noncovalent interactions, forming a mechanical bond [1–3] . Within the range of intertwined compounds, rotaxanes are postulated as ideal candidates for the design of molecular machines, developing applications in a wide range of fields, [4–6] such as catalysis, [7–14] materials science [15–24] or medicinal chemistry [25–27] …”

Triplet States of Cyanostar and Its Anion Complexes

“…In this form, the two macrocycles are π-stacked and display a ∼36° rotational offset from each other on account of interlocking steric contacts defined by the five tert -butyl substituents. While the number of conformations is reduced in the complex, it retains thermal access to many local minima with dynamically varying torsion angles indicative of a relatively flexible ground state. , Nevertheless, the stability of the 2:1 anion complex with the PF 6 – anion is remarkably high, recorded at 10 12 M –2 . The stability of this complex, CS 2 ·PF 6 – , is generally accepted to increase in less polar solvents, like dichloromethane (DCM). , When studying anion binding, a spectroscopically and redox-inert cation such as tetrabutylammonium (TBA + ) is typically used.…”

“…While the number of conformations is reduced in the complex, it retains thermal access to many local minima with dynamically varying torsion angles indicative of a relatively flexible ground state. 37,39 Nevertheless, the stability of the 2:1 anion complex with the PF 6…”

[2]Rotaxanes featuring minimal helical axles

“…12,16,17,18 A few reports, consider more structurally-elaborate rigid axles, offering a more well-defined ring trajectory along the axle. 19,20,21 The nature of the rigid linker is typically aromatic 20,22 or acetylenic, 21 which can necessitate multi-step synthesis and/or introduce π-conjugated systems which may be redox active and introduce low-lying excited states which may, for instance, result in unwanted rotaxane luminescence quenching. 23 Despite the continued interest in introducing fluorinated groups and sub-units to change the properties of organic compounds, including solubility and pharmacokinetics, integrating fluorinated segments in rotaxanes has been scarcely considered.…”