Light-powered electrical switch based on cargo-lifting azobenzene monolayers.

Inspired by the complex molecular machines found in nature, chemists have developed much simpler molecular motors. Among them, several systems incorporating azobenzene have been proposed, which exploit the reversible trans–cis isomerization triggered by light or an electric field for applications such as optical data-storage devices, switchable supramolecular cavities, and sensors. Recently, it has been demonstrated that the photoisomerization process of individual polymer chains incorporating azobenzenes can express mechanical work. In light of these findings, one can foresee self-assembled monolayers (SAMs) of aromatic azobenzenes as molecular systems able to express forces of unprecedented magnitude by exploiting a collective subnanometer structural change. We recently designed a rigid and fully conjugated azobenzene exposing a thiol anchoring group, which was able to form a tightly packed SAM on Au(111) (SAMAZO). Scanning tunneling microscopy (STM) studies revealed that upon light irradiation of the chemisorbed SAMs, a collective isomerization of entire molecular-crystalline domains occurred with an outstandingly high directionality. Based on these results, a cooperative nature of the isomerization of adjacent AZO molecules has been proposed. Furthermore, the joint action of the molecules in the SAM provides an ideal system as a potential “cargo” lifter. Herein, we show that, upon irradiation, azobenzene SAMs incorporated in a junction between an Au(111) surface and a mercury drop are able to 1) lift the “heavy” Hg drop, and 2) reversibly photoswitch the current flowing through the junction (Figure 1). Current–voltage (I–V) characteristics averaged over more than 30 junctions incorporating AZO SAMs in the trans and the cis conformations are shown in Figure 2a. The SAMAZO in the cis conformation was obtained with extremely high yield (98%) upon irradiation by UV light of the SAMAZO initially formed by the trans conformer. The difference in the measured currents, which amounts to about 1.4 orders of magnitude, is in agreement with a through-bond tunneling mechanism described by Equation (1).