Modulating electron flow along a donor-connector-acceptor system by control of the central torsion angle of a bridging group could lead to improved communication systems.

The control of electron movement along carbon-based wires of nanometre length could soon be accomplished. The insertion of a miniaturised regulator into the molecular-scale wire to achieve this has up to now proven to be a significant challenge. Now, for the first time, researchers from the Molecular Photonics Laboratory in Newcastle, UK, have collected data that may prove useful in tackling this problem. 

Anthony Harriman and his colleagues have synthesised a set of ruthenium(II)-osmium(II) bis(2,2’:6’,2"-terpyridine)s in which a link is incorporated in the ethynylene-substituted biphenyl bridge. This joins the terminal metal complexes together and acts as a variable ratchet, controlling the torsion angle between the rings of the central unit. Molecular dynamics simulations showed that the link constrained the geometry around the biphenyl group and imposed a preferred dihedral angle in the lowest-energy conformation. 



Harriman’s group made time-resolved luminescence measurements after laser excitation of the chromophores. They used low temperatures so that the solvent remained frozen in a glassy matrix, preventing undue rotation. Their results showed that the rate of the intramolecular triplet-triplet energy transfer from Ru(II) donor to Os(II) acceptor, when connected through the organic spacer, depended precisely on the torsion angle of the lowest-energy conformation of the bridge. 

This angular dependence has been anticipated for several decades, but this is the first experimental quantitative data showing how torsion angle affects the extent of long-range electronic coupling has been presented. Learning how to control the movement of electrons on the molecular and nanometre scales could help scientists to construct improved miniaturised communication systems.   

Debora Giovanelli