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 /(s)(/ = current and s is distance) method and the in situ break junction technique]. lsomerization 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.