Molecules that can reversibly switch between at least two states in response to an external trigger are essential for the development of molecular electronics. Azobenzene — a molecule composed of two phenyl rings linked by a nitrogen–nitrogen double bond — has been of particular interest in this regard because it can reversibly isomerize between trans and cis forms under the influence of light, an electric field or tunnelling electrons. However, future devices will require functional molecules, integrated into larger architectures that can be addressed selectively. Now Stefan Hecht of Humboldt University in Berlin, Leonhard Grill of the Free University of Berlin and colleagues have shown that molecules can be collectively switched with spatial selectivity (page 649).

Credit: C. Dri et al.

The researchers used an asymmetric azobenzene derivative containing a single methoxy group (–OCH3), which was adsorbed on a gold surface and examined with a scanning tunnelling microscope (STM). Voltage pulses from the STM tip were used to transform the molecules from the relatively flat trans configuration to the three-dimensional cis configuration. The team found that the probability of switching was significantly influenced by the surrounding molecules and supporting surface. As a result, they were able to create the same lattices of cis isomers in repeated switching cycles. The image here shows a STM image of such a lattice, with the cis isomers in yellow surrounded by the trans isomers in blue.