Mechanically interlocked molecules

Incorporating sensitive chromophores into interlocked systems that are mechanically responsive to external stimuli is attractive for tuning of the dye’s optical properties and stability. Here, the authors report the mechanically controlled protection and deprotection of a squaraine dye within a [2]rotaxane, governed by macrocycle shielding/de-shielding upon chloride addition or oxidation.

Singlet oxygen is a highly useful reagent for organic synthesis, disinfection and photodynamic therapy, but its high reactivity calls for systems where its photochemical generation can be switched on and off on demand. Here, the authors report porphyrin-decorated pH-switchable [2]rotaxanes for singlet oxygen photoproduction, finding that molecular folding of the rotaxanes influences the on/off switching in an unforeseen way.

Many templates for accessing hydrogen-bonded amide-based rotaxanes have resulted in modest yields, but a novel templating method has shown significant promise. Conjugated bis(enaminones) serve as an effective template for the formation of rotaxanes, and a post-synthetic double stopper-exchange method further enables the preparation of versatile and complex rotaxane structures.

Mechanically interlocked rotaxanes are typically prepared using covalent bonds to trap a wheel component onto an axle molecule, and rotaxane-type wheel–axle assembly using only noncovalent interactions has been far less explored. Here, a dinickel(II) metallomacrocycle is found to form two different types of wheel–axle assemblies, with a dibenzylammonium axle molecule forming both non-threaded and rotaxane-type threaded assemblies, based only on noncovalent interactions, with formation of one over the other governed by the assembly pathway.

Brønsted basicity can be greatly enhanced by the mechanical entanglement of two or more interlocked molecular subunits within catenanes and rotaxanes. Here, the authors discuss the development of such mechanically interlocked superbases, and outline challenges and opportunities for future directions of research.

Conformational dynamics are integral to enzyme catalysis, yet they are barely explored when designing synthetic catalysts. Now, a catenane-based organocatalyst, which dynamically switches between two conformations, speeds up the catalysis of carbodiimide hydration through spontaneous conformational adaptation for different reaction steps.

Mechanically interlocked supramolecular molecules show potential as imaging probes for biomedical applications. Here, the authors developed synthetic routes based on multicomponent reactions to access rotaxane-based bimodal imaging agents for nuclear and optical detection using the cucurbit[6]uril CB[6]-mediated azide-alkyne click reaction.

Preassembled materials are ubiquitous in our everyday life due to their readiness and functionality; an end-user simply follows instructions to assemble them and harness function. Here, metastable rotaxanes are utilized to approach preassembled materials: a multicomponent, preprogrammed system can be conveniently (via heating) transformed into colorful polymer networks at the end-user’s will.

Polymer beads are used in the core of magnetic particles, and beads functionalised with paramagnetic molecules are promising as agents for dynamic nuclear polarization. Here, the authors use conventional click chemistry to decorate a polymer bead with 10¹⁴ [2]rotaxanes containing paramagnetic {Cr₇Ni} rings.

DNA templating is a useful strategy to control the positioning and aggregation of molecular dyes on a sub-nanometer scale, but sub-angstrom control is desirable for the precise tailoring of excitonic properties. Here, the authors show that templating squaraine dyes functionalized with rotaxane rings promotes an elusive oblique packing arrangement and extended excited-state lifetimes.

Molecular knots are evolving from academic curiosities to a practically useful class of mechanically interlocked molecules, capable of performing unique tasks at the nanoscale. In this comment, the author discusses the properties of molecular knots, and highlights future challenges for chemical topology.