The arrangement of chemical bonds in some molecules can be altered with an external stimulus — such as heat, electricity or light — and this change in connectivity can affect their electronic properties. In particular, a class of molecules known as diarylethenes can be switched from an 'open' structure to a 'closed' one in a process that causes the carbon–carbon double bonds to reorganize into a so-called conjugated system in which electrons are delocalized across the molecule.

Now, Xuefeng Guo, Colin Nuckolls and colleagues1 from Columbia University, New York in the USA have measured the conductance of diarylethene molecules by connecting them to single-walled carbon nanotube (SWNT) electrodes. Each device was made by cutting a SWNT with an electron beam to produce a small gap into which a diarylethene molecule — in its open state — could be bonded. The conductance of a device was found to increase when it was irradiated with ultraviolet light, suggesting that the molecular bridge had been switched from the open to the closed form. Only one type of diarylethene molecule tested — those containing nitrogen rather than sulphur atoms — could be reversibly switched between the two states.

The precise electrical characteristics of each device depended on whether the electrodes were made from a metallic or semiconducting SWNT. Moreover, it is suggested that the high conductance of some devices could result from multiple diarylethene molecules bridging the gap between the electrodes — a hypothesis supported by pulsed ultraviolet switching that reveals discrete conductance jumps.