Spinning in the light

Chemists have developed a material that uses rotatable molecules to emit light faster than has ever been achieved before, in a breakthrough which could pave the way for a new generation of high-efficiency lighting. Their work has been published in the journal Science.

Molecular materials are the driving force behind modern organic light-emitting diodes (OLEDs), which emit light when electricity is applied to the organic (carbon-based) molecules in them. OLED lighting is now widely used in televisions, computers and mobile phones; however, it has to overcome a fundamental issue which has limited efficiency when it comes to converting electrical energy into light.

Passing an electric current through molecular materials puts them into an excited state, but only 25% of these are ‘bright’ states that can emit light rapidly. The remaining 75% are ‘dark’ states that usually waste their energy as heat, similar to an old-fashioned filament light bulb. The underlying reason for this is that the dark states have the wrong type of a quantum property called ‘spin’.

One approach to tackle this problem is to use rare elements, such as iridium, which help the dark states to emit light by allowing them to change their spin. The problem is that this process takes too long, so the energy tied up in the dark states can build up to damaging levels and make the OLED unstable. This effect is such a problem for blue-emitting materials (blue light has the highest energy of all the colours) that, in practice, the approach can’t be used.

Scientists at the University of East Anglia (UEA), working in collaboration with the University of Cambridge and the University of Eastern Finland, have now developed a material where two different organic molecules are joined together by an atom of copper or gold, with the resulting structure resembling a propeller. The compounds, which can be made by a simple one-pot procedure from readily available materials, were found to be surprisingly luminescent.

Most excitingly, by rotating the ‘propeller’, dark states formed on the materials became twisted, which allowed them to change their spin quickly. The process was found to significantly increase the rate at which electrical energy is converted into light, achieving an efficiency of almost 100% and preventing the damaging build-up of dark states.

“It’s amazing that the very first demonstration of this new kind of material already beats the performance of technologies which have taken decades to develop,” said Dr Dan Credgington, from the University of Cambridge’s Cavendish Laboratory. “If the effect we have discovered can be harnessed across the spectrum, it could change the way we generate light.”

The researchers’ next step is to design new molecules that take full advantage of this mechanism, with the ultimate goal of removing the need for rare elements entirely. This would solve the longstanding problem of how to make OLEDs without having to trade off between efficiency and stability.

“Our work shows that excited-state spin and molecular motion can work together to strongly impact the performance of OLEDs,” said Dr Dawei Di, also from the Cavendish Laboratory. “This is an excellent demonstration of how quantum mechanics, an important branch of fundamental science, can have direct consequences for a commercial application which has a massive global market.”

This is a modified version of a news item published by the University of Cambridge under CC BY 4.0.

Image caption: Dr Le Yang holding one of the efficient OLED devices, developed in Cambridge.