Azobenzene isomerization on a reactive copper surface by efficient decoupling with bulky side groups

• Published in: Surface science 2024-06, Vol.744, p.122468, Article 122468
• Main Authors: Jaekel, Simon, Stoll, Ragnar, Berger, Fabian, Hecht, Stefan, Grill, Leonhard
• Format: Article
• Language: English
• Subjects: Azobenzene: Isomerization; Cu surface; Molecular assembly; Molecular switches; Scanning tunneling microscopy
• ISSN: 0039-6028; 1879-2758
• DOI: 10.1016/j.susc.2024.122468

•A detailed study of molecular switches on a copper surface, resolved at the single-molecule level by scanning tunnelling microscopy, is presented. Molecular switches are not only of interest as a basic principle of chemistry, but also in view of potential applications. Azobenzene has developed as a prototypical switch in this field, undergoing isomerization between trans and cis. In order to visualize the ongoing processes, but also to arrange them in ordered arrays in space, adsorption on – preferentially metallic – surfaces is advantageous. However, although several metals have been used (in particular gold), switching on the more reactive copper surface could never be achieved.•Here, it is shown that azobenzene derivatives can indeed be successfully switched on copper if bulky dimethylphenyl side groups are employed. This also affects the assembly of molecules, due to the strong decoupling from the surface, where various close-packed geometries and multi-molecule unit cells are found. They rather resemble packing in a three-dimensional molecular crystal than the typical adsorption on a flat surface. Switching is found to occur in a very localized fashion, defined by the position of the STM tip and in contrast to previous work, thus allowing to ‘write’ patterns of individual cis isomers into the homogeneous molecular layer.•The results provide an in-depth understanding of molecular isomerization processes in the vicinity of metal surfaces. The achieved precise control over the location of switching events in molecular nanostructures is potentially relevant for future device miniaturization. Azobenzene derivatives, an important class of molecular switches that undergo isomerization between trans and cis states, have been studied on various coinage metal surfaces in the past. However, on reactive copper surfaces switching turned out to be difficult. Here, we show that the use of bulky 2,6-dimethylphenyl side groups enables switching on a Cu(111) surface as studied by scanning tunneling microscopy. By applying voltage pulses from the STM tip, irreversible trans → cis isomerization of individual molecules can be induced where exactly one molecule within each unit cell is isomerized. The absence of backward cis → trans switching illustrates the importance of the adsorption structure on the potential energy landscape of azobenzene. Isomerization, which is activated by inelastic scattering of single tunneling electrons, happens in a very localized fashion, thu