Reversible Photoswitching and Isomer‐Dependent Diffusion of Single Azobenzene Tetramers on a Metal Surface

Abstract

Azobenzene is a prototypical molecular switch that can be reversibly photoisomerized between the nearly planar and apolar trans form, and the distorted, polar cis form. Most studies related to azobenzene derivatives have focused on planar adsorbed molecules. We present herein the study of a three‐dimensional shape‐persistent molecular architecture consisting of four tetrahedrally arranged azobenzene units that is adsorbed on a Ag(111) surface. While the azobenzenes of the tripod in contact with the surface lost their switching ability, different isomers of the upright standing arm of the tetramer were obtained reversibly and efficiently by illumination at different wavelengths, revealing time constants of only a few minutes. Diffusion on the surface was dependent on the isomeric state—trans or cis—of the upright oriented azobenzene group. Hence, molecular mobility can be modulated by its isomeric state, which suggests that molecular growth processes could be controlled by external stimuli.

Light‐Responsive (Supra)Molecular Architectures: Recent Advances

“…Remarkably, the ability of isolated molecules of 2 to diffuse on the metal surface depends on the isomeric configuration of the upward‐pointing azobenzene unit, which in its turn can be switched by external stimulation. This study is significant because it describes a method to install photoactive molecules on surfaces while retaining their switching properties . More generally, it highlights the importance of molecular design and of the possibilities that can arise from a precise control of the arrangement of functional moieties in multicomponent molecular architectures …”

Light on Molecular Devices

“…In the recent past we have described examples of (supra)molecular switches, logic devices, machines and motors (see for instance the supramolecular pump shown in Figure 1), [1][2][3][4] as well as nanoparticles and porousm aterials endowed with light-activated functions. [5][6][7] In our research activity,w hich is supported in part by the European Research Council (https:// site.unibo.it/leaps), we takea dvantage of the full range of processes offered by molecular and supramolecular photophysics and photochemistry,w ith particularr egardt ol uminescence, photoinduced energy-, electron-and proton-transferp rocesses, and photoisomerization reactions.…”

“…These studies are focusedo nf undamental science, and are motivated by the search for innovative solutionstopractical problemsinc hemosensing, molecular delivery,catalysis, energy conversion and storage, and mechanical actuation atd ifferent length scales.In the recent past we have described examples of (supra)molecular switches, logic devices, machines and motors (see for instance the supramolecular pump shown in Figure 1), [1][2][3][4] as well as nanoparticles and porousm aterials endowed with light-activated functions. In this context, light is am ost attractive stimulus because it can be used both to "write" and "read" matter,i tc an be finely tuned in energy,t ime and space domains,a nd it can cause clean reactions.…”

Photoactive Molecular‐Based Devices, Machines and Materials: Recent Advances

“…These compounds, because of their tetrapod structure which resembles that of a caltrop, are promising for probe microscopy studies upon adsorption on surfaces. Indeed, high‐resolution STM experiments performed on 9 deposited on an Ag(111) surface brought about highly interesting results …”