Rotaxanes

Shreya Ray. Rahi Reja. Mandar Kulkarni. Surabhi Jirapuri. Prarabdha Jagdhane

20101021

Definition

Supramolecular assemblies consisting of macrocyclic molecules physically

threaded by, but not chemically bonded to, a linear molecule chain or a

linear subchain of a molecule capped with bulky end-groups (referred to as

“stoppers”) which prevent dethreading of cyclic molecules are known as

"rotaxanes"

Ref - Terminology and Nomenclature for Macromolecular Rotaxanes and Pseudorotaxanes, 2011 . IUPAC

Statistical Synthesis

6.00 % yield

Template-Directed Syntheses

Noncovalent forces used in templated synthesis are

1 . acceptor - donor 2. Metal Ligand Interaction 3. Hydrogen bonding

4. Hydrophobic Interactions / cyclodextrin based template synthesis

Clipping – Linear Guest and partial Cyclic host

complex and then end portion is “clipped” by reacting with another molecule .

Template-Directed Syntheses

Threading – Formation of

pseudorotaxane first, followed by stoppering using end bulky groups . Snapping – linear component with

bulky terminal groups dissociates in 2

fragments and one complexes with

cyclic component , then reconstitution of another part . Slippage – Heating of system causing

slippage of linear group with terminal bulky groups through cyclic species .

∆

Metal-Template Synthesis

Wu, C.; Lecavalier, P.R.; Shen, Y.X.; Gibson, H.W. Chem. Mater. 1991, 3, 569-572

Click Chemistry

Org. Lett., Vol. 12, No. 17, 2010

C- C bond formation using Click Chemistry

Polyrotaxane

Harada, A.; Li, J.; Kamachi, M. Nature (London, U. K.) 1992, 356,325.

“Molecular Neckace”

Switchable Rotaxanes

Binding constants between wheel and each station should be in the order: A>B>A’ The two states must be reversible The process should be controllable by external stimuli Applications:

Logic Gates NanorecordingMemory Dots

The switch may be: Chemically Driven (Acid-Base Reaction) Photochemically Driven (Olefin Isomerization) Electrochemically Driven (Reduction-Oxidation Reaction)

Switchable Rotaxanes

Bright Rotaxanes

Photoisomerisation of thread reversibly changes fluorescent properties of molecule. Fluoroscence enhancement or quenching can be used to indicate shuttling of conformations. High sensitivity and specificity.

Chem. Soc. Rev., 2010, 39, 70–80

Nanovalves

A rotaxane shuttle can serve as a molecular device to switch on a surface. Shuttling of the macrocycle closer to and away from the pore orifices of mesoporous silica could close and open the silica nanopores, respectively. The lengths of the linkers determines efficiency of nanovalves.

Chem. Soc. Rev., 2010, 39, 70–80

Dye Delivery

Bolamphiphilic pseudorotaxane built from hydrophobic stoppers and hydrophilic macrocycle. Slow spontaneous dissociation of reverse vesicle can be used to slowly release dye

Langmuir 2012, 28, 14839−14844

Enhanced Reactivity

Interlocked components are forced into close proximity One component may sterically protect the other from chemical attack Alternatively, steric strain may be introduced, enhancing reactivity Macrocycle contraction is obtained by substitution: amideNH residues form stronger H-bonds Stopper group crowding is achieved by shortening thread length

Org. Lett., Vol. 12, No. 21, 2010

Molecular Muscle

(1) Rotaxane-like Interlocked components

(2) Fluoride-sensitive molecular muscles: fluoride weakens N+H…O H-bonding as compared to CH…O H-bonding of pyridinium ions.

Org. Lett., Vol. 11, No. 2, 2009

Conclusion

Rotaxanes comprise of a Macrocycle and a dumbell-shaped molecule Rotaxanes can be prepared using template-directed synthases like clipping, threading, snapping and slippage Switchable Rotaxanes have many applications like logic gates, memory dots, nanovalves Rotaxanes can be used to reduce or enhance reactivity Rotaxanes can be used to built molecular muscles Rotaxanes can be used to make amphiphiles that can self-assemble and release dye slowly