Welding, organizing, and planting organic molecules on substrate surfaces--promising approaches towards nanoarchitectonics from the bottom up.

The rapidly evolving field of nanotechnology[1] has so far primarily benefited from the development of instrumentation by the physical community, which enables visualization and manipulation of matter down to the atomic level.[2] These major advances have propelled us chemists into the unique position of gaining fundamental insights into the dynamics and reactivity of single molecules[3] and furthermore to exploit this knowledge for the bottom-up approach to nanofabrication, which promises to revolutionize future device miniaturization and electronics.[1,4] Our ability to custom-design and synthesize programmed molecular objects will be crucial to construct functional nanoscale architectures. These molecules will serve as the functional building blocks of molecular construction sets[5] and inherit the desired electronic or magnetic properties, adopt a well-defined shape, and incorporate specific handles such as reactive groups and recognition motifs. In order to assemble these individual building blocks into larger integrated systems, several issues will have to be addressed: 1) The surface should be locally activated, either by inducing a chemical process in individual molecules or by changing the substrate directly; 2) the molecules should preferably be organized on the substrate by self-assembly; 3) the generated features should be stabilized and permanently fixed to the substrate. Recent advances in these areas utilizing organic molecules will be highlighted here from a synthetic chemist©s point of view.[6] The concept of inducing a chemical reaction by using tunneling electrons of a scanning tunneling microscope (STM) tip was suggested by Ho in the late 1990s.[7] However, there are still few experiments which demonstrate singlemolecule reactivity in the sense of a classical bond-forming event.[8] In a key experiment, Rieder and co-workers recently carried out an TMUllmann reaction∫, which involved the homocoupling of two iodobenzene molecules to afford biphenyl, step by step at the step edge of a Cu(111) surface using a low-temperature, ultrahigh-vacuum STM (Figure 1).[9] First, selective breaking of the weakest bond (the C-I bond) was induced by applying the appropriate sample bias. Single tunneling electrons presumably cause C I bond dissociation and the resulting phenyl radical intermediate is stabilized by p interaction with the surface as well as s interaction