Enhanced Separation Concept (ESC): Removing the Functional Subunit from the Electrode by Molecular Design

A new concept to improve the reliability of functional single molecule junctions is presented using the E-field triggered switching of FeIIbis-terpyridine complexes in a mechanically controlled break junction experiment as model system. The complexes comprise a push-pull ligand sensing the applied E-field and the resulting distortion of the FeII ligand field is expected to trigger a spin-crossover event reflected in a sudden jump of the transport current. By molecular engineering, the active centre of the complex is separated from the gold electrodes in order to eliminate undesired side-effects. Two aspects are considered to isolate the central metal ion, namely the spacing by introducing additional alkynes, and the steric shielding achieved by bulky isopropyl groups. With this small series of model complexes, a pronounced correlation is observed between the occurrence of bistable junctions and the extent of separation of the central metal ion, affirming the hypothesized Enhanced Separation Concept (ESC).