Ordinary transistors act as switches because they are based on materials whose resistances change depending on the voltage applied to them – that is, their electrical behaviour can be modified electrically. In contrast, the electrical properties of optoelectronic devices – such as light-dependent resistors – can be adjusted by light, and vice versa. And in the emerging field of spintronics, the electrical behaviour of a device can be changed by a magnetic field that lines up the spins of the electrons.

But scientists have so far failed to make a material in which optical, electrical and magnetic characteristics are linked. “As far as we know, our material is the first organic compound that combines all three,” says Haddon.

The compound created by Haddon’s team consists of two phenalenyl ring systems that are connected by a boron atom and chemical groups such as hexyl, butyl or ethyl. The electrons in this compound are ‘delocalized’ from their parent atoms and – above a certain temperature – they migrate to the outer regions of the molecule, making the compound paramagnetic. Below this temperature, the electrons drift into the centre of the molecule, and the compound becomes diamagnetic.

The researchers set out to study the electrical properties of the substance, and found that it is an insulator in its paramagnetic state and a conductor in its diamagnetic state. “We were working to develop organic conductors and superconductors,” Haddon told PhysicsWeb. But they were surprised to discover also that it is transparent to infrared light when it is an insulator, and opaque when it is a conductor.

The exact characteristics of the compound depend on the choice of chemical group, and on the direction of temperature change. For a butyl group, the transition temperature is about 50 °C if the temperature is rising and about 74 °C if it is falling. The transition between opaque and transparent states occurs for several wavelength bands.

Haddon and co-workers are optimistic that this flexibility will make their ‘multi-functional’ material suitable for a variety of applications. They are now developing molecules that can be directly switched with light.