A pair of molecules that combine in an unusual 2:1 ratio to give nano-sized spirals have been brought together by an international research team. 

Recently, much effort has centred on designing pairs of photo-active molecules that assemble spontaneously to give nanostructures held together by interactions such as hydrogen bonds. Structures containing melamine and imide are particularly interesting as their photoresponsive properties could be used in miniaturising optoelectronics. These two components usually form a 1:1 ratio as they have complementary hydrogen-bonding sites. 

 

A melamine and a bisimide combine in an unusual 2:1 ratio to give intriguing spiral structures

 

However, when Shiki Yagai at Chiba University, Japan, and Frank Würthner at Würzburg University, Germany, mixed two of these molecules - an azobenzene-functionalised melamine and a perylene bisimide - in a 1:1 ratio, they didn't get the linear structures they expected. Instead, they got small assemblies with ill-defined morphologies, explains Yagai.

To their surprise, adding more of the azobenzene to this mixture triggered the formation of what Yagai describes as 'tightly-coiled helical nanostructures,' containing the two components in a 2:1 ratio. He suggests that the additional melamine molecules in the chain reduces the number of hydrogen bonds, making it more flexible and able to curve. When this happens, bisimide molecules can sit in adjacent bends of the helix, and are able to pi-stack, making the structure more stable.

Ronald Castellano, an expert in self-assembly at the University of Florida, Gainesville, US, says 'this work reminds us of the beautiful complexity that can be achieved from the assembly of relatively simple synthetic building blocks and that there is still much to learn about predicting supramolecular structure on the nanoscale.' He adds, 'this work shows that to access new functional assemblies we must be willing to resist taking the most comfortable or intuitive approach.'

Yagai comments that these helices are similar to those seen in the light-harvesting systems of photosynthetic organisms, and he expects them to attract significant attention from supramolecular chemists. He says that unconventional systems like these could have exciting prospects, because 'switching such a structure into a new morphology might be accomplished by small changes of solvent, coordination of an analyte, a light pulse or an electrochemical stimulus.' The team are now developing smart nanostructures with morphologies and light-harvesting functions that can be controlled by light pulses.

David Barden

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