Direct Measurement of Electron Transfer through a Hydrogen Bond
between Single Molecules

Understanding electron transfer (ET) from
a single molecule to another single molecule holds
essential importance to realize bottom-up molecular
devices in which constituent molecules are self-assembled
via noncovalent interactions between each other. However,
rather little is currently known about the ET properties at
the single-molecule interface. Here we employ molecular
tips to quantify the ET through a H-bond between single
molecules. We found that a H-bond conducts electrons
better than a covalent σ bond at short-range. Its
conductance, however, decays steeply as the chain length
of the H-bonded molecules increases. First-principle
calculations were performed to reveal the electronic origin
of the facile ET through the H-bond. Our results
demonstrate that H-bonding in a molecular junction
significantly affects its transport property.