Surface chemistry: Making the right connections.

The idea that electronic circuit boards can be made by connecting designed molecular components has fired the imagination of chemists for many years. Much research has focused on the construction of molecules that mimic the properties of wires, transistors and other devices that are typically associated with electronic circuitry. One of the questions that has always lurked in the shadows of such studies is how are these components, beautiful in their own right, going to be assembled into something that resembles a circuit board? And if a molecular circuit board could be built, then there would need to be a mechanism by which information, electronic or otherwise, could be introduced or read from the array. These are difficult questions to answer but over the years steps have been made towards this challenging goal. Now, writing in Nature Chemistry, Leonhard Grill, Stefan Hecht and co-workers1 have taken a significant step towards addressing some of the challenges in this field by demonstrating a method to rationally construct covalently bonded nanostructures from more than one component on a gold surface. A common strategy that enables molecular arrays to be interrogated with an external probe is to deposit or grow the target system on a surface, facilitating direct molecular-resolution imaging and manipulation using a scanning tunnelling microscope. Back in 2007, Grill and co-workers described a method for creating covalently linked arrays of porphyrin molecules on a gold surface2. This relatively simple system exploited an aryl–aryl coupling reaction that linked bromo-arylfunctionalized porphyrins into twodimensional arrays. The original strategy has been adapted to other molecules3,4 and other coupling processes5, but until now has been generally limited to the construction of arrays built from a single type of molecule. Grill and co-workers have now developed their methodology to enable arrays to be assembled from two different molecular components by exploiting the different reactivities of bromo-aryl and iodo-aryl groups. Although examples of covalently SURFACE CHEMISTRY