Experimental Validation of Quantum Circuit Rules in Molecular Junctions

A series of diarylacetylene (tolane) derivatives functionalised at the 4- and 4'-positions by thiolate, thioether, or amine groups capable of serving as anchor groups to secure the molecules within a molecular junction have been prepared and characterised. The series of compounds have a general form X-B-X, Y-B-Y, and X-B-Y where X and Y represent anchor groups and B the molecular bridge. The single-molecule conductance values determined by the scanning tunnelling microscope break-junction method are found to be in excellent agreement with the predictions made on the basis of a recently proposed 'molecular circuit law', which states `the conductance G(XBY) of an asymmetric molecule X-B-Y is the geometric mean root G(XBX)G(YBY) of the conductance of the two symmetric molecules derived from it, G(XBX) and G(YBY).' The experimental verification of the circuit law, which holds for systems in which the constituent moieties X, B, and Y are weakly coupled and whose conductance takes place via off-resonance tunnelling, gives further confidence in the use of this relationship in the design of future compounds for use in molecular electronics research.