A molecular daisy chain that threads itself together could have future uses in molecular electronic devices, say scientists in Taiwan. 

Sheng-Hsien Chiu and his colleagues at the National Taiwan University, Taipei, and National Chung Hsing University, Taichung, have found a way to interlock pseudorotaxanes and rotaxanes like a daisy chain.  The team are hoping that interlocked rotaxanes will prove useful as molecular actuators and switches for molecular electronic devices, explained Chiu. 

Interlocked rotaxanes form a daisy chain

Daisy chains are typically made by making a hole in the stalk and passing another daisy stalk through this hole, where the flower head acts as a stopper. Chiu's method is akin to passing the flower head through the hole in the stalk. Here the flower head is a divinyl-cyclopropane unit which swells into a larger size at elevated temperatures and then acts as a stopper. 

The daisy monomer used in this study comprises of a hydrogen-bond-donating thread-like unit (a stalk), a hydrogen-bond-accepting macrocycle (the hole in stalk) and a dialkenyl cyclopropane terminal group (a flower head). These components are allowed to self-assemble in solution into daisy-chain-like complexes, before heating the system to swell the terminal groups and trap the molecular daisy chain. 

Chiu and co-workers isolated the daisy chain by chromatography and confirmed its structure using X-ray crystallography. They found a cyclic dimeric daisy chain was formed.  This two-daisy ring is the entropically favoured product, explained Chiu.

'The challenge remains to overcome entropy and assemble larger cyclic and acyclic molecular daisy chains,' said Chiu.  He explained that 'the solution to this problem is to use a more structurally rigid monomer that would disfavour dimerisation and form longer daisy chains'.

In the future Chiu said 'We hope to use this approach to isolate other types of molecular daisy chains and functional interlocked polymers.'  

Emma Shiells