The long and short of it

The simple rules describing how electrons move through molecules do not apply to cross-conjugated molecules

Credit: © 2008 ACS

Experimentally determined 'rules of thumb' for molecular electron transfer say that electrons should transfer more readily along conjugated molecules than along saturated ones, and through shorter molecules more easily than longer ones. Although exceptions can be found, they are uncommon, and provide new insights into this important phenomenon. Now Gemma Solomon, David Andrews and colleagues at Northwestern University in the US have found¹ that the rules do not describe current flow through cross-conjugated molecules.

Density functional theory was used to examine the electron transport in a cross-conjugated thiol with a long and a short axis. Although all the carbon atoms are sp² hybridised, the short axis provides a pathway that does not contain alternating σ- and π-bonds, but the long pathway does. Contrary to what might be expected, electron transmission through the long system results in more current than through the short one. As a control, they also studied an unconjugated thiol that was the same length as the shorter axis of the cross-conjugated system, finding that although the latter has the greater transmission, the difference is much less than expected.

Another simple rule suggests that, because the energy gap between the HOMO and LUMO is the same for both long and short pathways (because they are in the same molecule), the transmission should be similar. It is not, however, which further indicates that simple rules do not adequately describe electron transport in cross-conjugated molecules. The quantum interference in these compounds has a large effect, which challenges the conventional understanding of these systems.