Conductance channels of a platform molecule on Au(111) probed with shot noise

The shot noise of the current $I$ through junctions to single trioxatriangulenium cations $({\mathrm{TOTA}}⁺)$ on Au(111) is measured with a low-temperature scanning tunneling microscope using Au tips. The noise is significantly reduced compared to the Poisson noise power of $2eI$ and varies linearly with the junction conductance. The data are consistent with electron transmission through a single spin-degenerate transport channel and show that ${\mathrm{TOTA}}⁺$ in a Au contact does not acquire an unpaired electron. Ab initio calculations reproduce the observations and show that the current involves the lowest unoccupied orbital of the molecule and tip states close to the Fermi level.