The impact of E-Z photo-isomerization on single molecular conductance.

The single molecule conductance of the E and Z isomers of 4,4'-(ethene-1,2-diyl)dibenzoic acid has been determined using two scanning tunneling microscopy (STM) methods for forming molecular break junctions [the I(s) (I = current and s is distance) method and the in situ break junction technique]. Isomerization leads to significant changes in the electrical conductance of these molecules, with the Z isomer exhibiting a higher conductance than the E isomer. Isomerization is achieved directly on the gold surface through photoirradiation, and the STM is used to determine conductance before and after irradiation; reversible switching between the two isomers could be achieved through irradiation of the surface bound species at different wavelengths. In addition, three groups of molecular conductance values [A ("low"), B ("medium"), and C ("high")] have been measured for these carboxylate-terminated molecules. The origin of these conductance groups as well as the increase of the conductance for the Z isomer have been analyzed by comparing the length of the molecules extended in the gap, derived from molecular modeling, with the experimentally observed break-off distance for both isomers.