Interference Blockade in the Conductance of Organic Molecules

– The conductance of cis/trans isomers of stilbene molecules connected to armchair single wall carbon nanotubes is studied in the Landauer formalism combined with a densityfunctional based approach. For a given arrangement of the electrodes, dramatic differences in the transmission between both isomers are found. For a given isomer, the conductance can be varied by orders of magnitude by changing the molecule-electrode relative orientation. Both effects can be explained by a simple, physically transparent interference rule, which suggests a straightforward conductance control in such molecular systems by different switching mechanisms. Introduction. In recent years rapid progress in the field of molecular electronics has been made. Novel experimental approaches allow the investigation of the electronic transport properties of small molecular groups or even single molecules connected to macroscopic or mesoscopic electrodes [1]. Effects like negative differential resistance (NDR) [2, 3, 4] and rectification [5, 6, 7] have been demonstrated. Common to this class of systems is the subtle interplay between electronic and structural properties in determining the electronic transport. This makes the search for molecules that exhibit controllable conformational changes highly desirable. Thus, recent experiments on conjugated organic molecules have shown reversible conductance switching which can be related to reorientation of single molecules induced by voltage pulses [8], to internal structural modifications induced by charge transfer within the molecular complex [9, 10] or to conformational changes due to interactions with the local environment [11]. Stilbene ((1,2)-diphenyl-ethylene) is a prototypical example of a system with a controllable transition between two stable states. This molecule undergoes a cis-trans isomerization around the central ethylenic bond under, e.g. the influence of a laser field [12], which suggests a natural way to realize a switching mechanism. Many experimental and theoretical investigations have been carried out in order to understand the electronic and structural properties of the molecule, the mechanism leading to isomerization as well as its optimal control [12,13,14,15]. Less theoretical attention has been paid, however, to the electronic transport properties of