Shape-shifting molecules that emit primary colours have been mixed to create a rainbow of hues for the first time. This morphing ensures that the colours are crisp and clear, potentially giving the molecules a future in high-resolution displays.
Modern displays use bundles of micrometre-sized pixels of each primary colour – red, green and blue – to create a range of colours. But even such small pixels become noticeable on devices that are held close to the user's face, such as smartphones, computer screens and virtual reality headsets. To get round this, researchers are attempting to make pixels the size of a molecule.
All molecules absorb and emit light at certain wavelengths based on their structure. Single molecules that emit red, green or blue light have been made before, but blending them is a challenge. A mixture of blue and red ends up looking red, for example, because the high-energy blue light gets absorbed by red molecules and re-emitted as lower energy red light.
Elegant solution
To fix this, Soo-Young Park of Seoul National University in South Korea and colleagues have made coloured molecules that can each have two structures. Exposing one of these molecules to ultraviolet light boosts its energy level, but this new excited state is unstable so the molecule morphs into its alternate structure. The still-excited molecule then emits a photon at the desired wavelength and relaxes, shifting back to its original form. This duality means light emitted by one molecule won't be absorbed by its neighbours.
By mixing the molecules, the team was able to produce dyes and plastic films that appear clear under normal light but shine in a range of colours under UV light. The films can also fluoresce when a current flows through them, making them suitable for use as organic LEDs, larger versions of which are found in many modern screens.
Since the dyes don't emit colour unless exposed to UV or electricity, they could also prove useful for drawing security marks or even printing hidden photos on credit cards and other objects, says Park.
"It's quite elegant," says John de Mello at Imperial College London, who was not involved in the study. But he believes there is still room for improvement. Red light is at the furthest end of the spectrum from UV, he says, and the current version of the red molecule must absorb 30 times more UV light than the other colours to create equivalent levels of brightness.
Journal reference: Journal of the American Chemical Society, DOI: 10.1021/ja404256s
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