Controlling competing outcomes of a singlemolecule reaction

The ability to programme matter on the scale of individual molecules is a
long-standing goal in nanoscience. The atomic resolution of scanning tunnelling
microscopy and its ability to excite a molecule locally can give control over the
probability of inducing a single-molecule reaction. Moreover, selectivity in the
reaction outcome can be ensured by exciting different electronic states [1–3],
changing the excitation location [4–9], or exciting different molecular vibrations
[10, 11]. These schemes are used to synthesize normally unstable molecular structures [12, 13]. In these examples, the reaction outcome is controlled by changing
the initial conditions of the excitation: only one reaction outcome is possible, but
that outcome can be chosen by tuning the excitation properties. Here we show it
is possible to control the outcome of a single molecule reaction that has multiple
competing outcomes. By precise injection of electrons from an STM tip, toluene
molecules chemisorbed on the Si(111) − 7 × 7 surface at room temperature are
induced to react one at a time with two outcomes: the molecules either desorb,
or switch to an adjacent surface site. The branching ratio between these two
outcomes is dependent on the excess energy the exciting electron carries. Using
known values and ab initio DFT calculations to support our findings, we conclude
that the excess energy of the exciting electron leads to a heating of the intermediate physisorbed state and hence gives control over the two reaction outcomes
via their energy barriers and pre-factors [14]. This opens the way to control more
complex single-molecule reactions via manipulation of the excited state