A colour-changing molecular switch that can be monitored by the naked eye has been developed by chemists in Taiwan.

Sheng-Hsien Chiu and colleagues at the National Taiwan University made a molecular switch that consists of a dumbbell-shaped molecule, a ring-like molecule that can move between two sites on the dumbbell, and a molecular clip. 

The switch is controlled by changing the pH. Adding a base makes the ring move from one site on the dumbbell to the other, and adding an acid makes the ring move back to its original position. The molecular clip can attach to only one of the sites. When this site is free the molecular clip attaches itself causing an easy-to-spot colour change in the solution from yellow to green. 

Chiu was keen to explain the advantages of his switch. 'Most acid/base-controllable molecular switches do not provide significant optical outputs, and their operating states are generally monitored using NMR spectroscopic techniques,' he said. 'Although we can use spectroscopic methods to monitor the operation of these switches, if they are to be applied, for example, in displays, it will be necessary to detect this visually.'

Others researchers in the field welcomed the work. 'This is a very nice example of supramolecular chemistry in action,' said Jan Jeppesen, at the University of Southern Denmark. 'The opportunity to monitor the switching process by the naked eye will open up valuable applications in supramolecular chemistry.'

There is more to Chiu's work than just switching, said Cristoph Schalley from Free University of Berlin, Germany. 'One of the significant points in this work is the fact that two inputs can be used to create an output: the presence or absence of the clip and the addition of acids and bases,' said Schalley. 'Only when both inputs are set appropriately, the visible output signal is changed. Consequently, the switch can be operated as a molecular logic gate.'

Now that Chiu has shown his switch working in solution, he plans to investigate whether it could be used molecular devices such as supramolecular nanovalves, where the switch would be attached to a surface.

Susan Batten