Single molecule junctions are of interest for the insight they provide into fundamental attributes of molecular interfaces, such as the alignment between the molecular frontier orbitals and the energy levels of the contact electrodes. They can also provide information on chemical and electrochemical reactions at the single molecule level. We have used transport measurements to study the protonation of Ni - 4,4’-vinylenedipyridine (44VDP) -Ni single-molecule junctions as a function of the applied potential and pH. We use a scanning tunnelling microscope break junction (STM-BJ) apparatus operating under electrochemical control. All measurements are carried out at potentials at which the Ni STM tip and substrate are oxide-free. The conductance of the Ni – 44VDP – Ni junctions shows a transition from approximately 10-3 G0 to 10-4 G0 (where G0 is the conductance quantum) either as the Ni contact electrodes are made more negative or as the solution pH is reduced. Careful analysis shows that the low conductance peak can take on two values, with the lower value being reached at the most negative potentials and lowest pH values. We attribute the high and two low conductance values to unprototonated, singly-protonated and doubly-protonated VDP respectively. Quantitative analysis of the conductance transition further enables us to extract a plausible value for the charge transferred from the singly-protonated molecule to the Ni electrode (0.21±0.06 e). This is a rare example of a single-molecule junction being formed with a charged contact group. The Ni – 44VDP – Ni system is also remarkable because in some traces it is possible to observe the protonation and de-protonation of a single molecule in real time via the associated conductance changes. Furthermore, the Ni – 44VDP – Ni junction is a true 3-terminal pH sensing device as it switches conductance at a value of the pH which can be tuned electrically. This means that unlike previous single-molecule sensors, it not only responds to an external stimulus, but is capable, in principle, of measuring its level.

Dual control of chemical equilibria through pH and potential in single-molecule devices

255th National Meeting (2018)

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