Switching individual molecules by light and electrons: From isomerisation to chirality flip

Molecular electronics offers a promising way for constructing nano-electronic devices in future with faster performance and smaller dimensions. For this aim, electronic switches are essential as basic components for storage and logical operations. The main requirements for a molecular switch are reversibility and bistability. This necessitates the existence of at least two different thermally stable forms of a molecule that may be changed repeatedly from one state to the other one through an external stimulus. The transition should then be connected to a measurable change in molecular properties. The development of such molecular switches on the single molecule level is a major challenge on the path towards incorporating molecules as building units into nano-electronic circuits. Since isomers may differ significantly in physical and chemical properties, isomerisation opens a way for a molecular switch. In this article, an overview is provided over those isomerisation reactions of single molecules adsorbed on surfaces that are investigated with a scanning tunnelling microscope and that have a potential as a molecular switch in future molecular electronics. These are mainly, but not exclusively, constitutional, configurational, and geometric isomerisation reactions. The external stimulus is either light or the possible interaction with the tip of a scanning tunnelling microscope, i.e. electrons, electric field, or mechanical force. Some reactions are similar to those observed for the molecule in the liquid phase, but some are observed or even possible only on a surface. The detailed investigation of the isomerisation yield dependence on several parameters gives insight into the underlying processes of the reaction. (C) 2011 Elsevier Ltd. All rights reserved.