We have observed on−off switching of scanning tunneling microscope current flow to siliconadatoms of the Si(111)-(7 × 7) surface that are enclosed within a bistable dimeric corral of self-assembledchlorododecane molecules. These thermally activated oscillations amounted to an order of magnitudechange in the current. Theory showed that small changes in molecular configuration could cause alterationsin the corralled adatom's electronic energy by as much as 1 eV due to local field effects, accounting for theobserved current switching.