Chemically-powered molecular scissors and tweezer-like triangles offer new ways to manipulate structures on the nanoscale, claim Japanese and German researchers.

A Japanese team from the University of Tokyo has made a scissor-like nanomachine. Kenichi Tanaka and Kazushi Kinbara say their machine runs continually in the presence of molecular fuels.¹ Meanwhile, a separate study into self-assembling molecular triangles could be adapted to make adjustable molecular tweezers, say Michael Schmittel and Kingsuk Mahata at Siegen University in Germany.²

Shortening the third side and attaching it part way down the two arms, the structure could form adjustable molecular tweezers

The engine in Kinbara's molecular machine is a rhodium complex, which is attached to two molecular arms by a ferrocene pivot. Two 'fuel' molecules trigger the rhodium to continually switch its geometry, which opens and closes the molecule's arms like the blades of a pair of scissors. 

The first fuel, diphenylphosphoryl azide, plucks a carbonyl ligand from the rhodium complex, converting it from a tetrahedral to a square planar geometry, opening the arms. An aldehyde - the second fuel - replaces the carbonyl on the rhodium and switches the structure back to tetrahedral, which re-closes the arms. As long as both fuels are present in solution, molecular motion continues.

'Possible applications of such molecular machines go beyond switching devices,' says Kinbara, who adds that harnessing unidirectional motion would lead to molecular transportation systems or molecular pumps. 'The next step is to develop a system where the mechanical motion of the ligand can be extracted as mechanical force. We would also like to develop much larger systems including transition metal catalysts as a power-generating unit.'

In another study, Schmittel made self-assembling molecular triangles, overcoming the tendency of such systems to form less-constrained squares. Each side of the structure is made from a rod-like molecule with a metal ion binding site at each end. Schmittel pre-formed two sides of the triangle by attaching two arms to a copper ion 'corner' - and then attached the third side using silver ions to form the remaining two corners. By shortening the third side and attaching it part way down the two arms, Schmittel suggests the structure could form adjustable molecular tweezers.

'The development of nanomechanically operated devices represents a huge challenge,' says Schmittel. 'As chemists we are used to seeing molecules and their reactions being influenced by polar, steric and solvent effects. The question is whether we can equally influence them through nanomechanics.'

James Mitchell Crow