Photoswitching Azobenzene Derivatives in Single Molecule Junctions: A Theoretical Insight into the I/V Characteristics

The I/V characteristics of several photoswitching azobenzene derivatives connected to two gold electrodes to form single-molecule junctions are investigated within the nonequilibrium Green’s function formalism coupled to density functional theory. We focus here on the changes in the I/V characteristics as a function of the length and degree of fluorination of the conjugated backbones as well as different coupling strength at the electrodes (chemisorption versus physisorption) upon trans/cis isomerization. The calculations illustrate that the conductance is larger for the trans isomer when the molecule is chemisorbed at both electrodes. However, a larger conduction for the cis isomer is found in the presence of a physisorbed contact at one electrode for specific geometries of the isomer in the junction, in full consistency with the apparent discrepancies observed among experimental measurements. The I/V curves are fully rationalized by analyzing the evolution under bias of the shape of the transmitting molecular orbitals.