If you’re reading this now, chances are you’re using a computer display to see Cool Story Bromine in full multicoloured glory. To achieve this vibrant vision of colour, computer monitors use a blend of three primary colours (RGB – red, green and blue) to make any shade in the visible spectrum.
This is realised using additive colour theory.
The gamut (or range) of colours produced is controlled by altering the intensity ratios between the emission of coloured light from RGB pixels.
Currently, these pixels are micrometer sized. However, the smaller the pixel is, the higher the screen resolution, resulting in a better quality image being produced.
What if it was possible to make a pixel that wasn’t micrometer-sized, but the size of a molecule?
All molecules emit and absorb light. The wavelength (and therefore the colour) of light emitted by a moleule is dependent on its structure. A hydrogen molecule will emit light of a different colour to a carbon molecule, or a benzene molecule, or any other molecule in existence. In fact, it is this unique colour signature that allows scientists to determine the chemical composition of stars billions of light years away.
Knowing this, it should be possible to use a “red” molecule, a “green” molecule and a “blue” molecule to make molecular sized pixels that combine together to make any colour in the same way that millimeter-sized pixels do.
However, the problem with using molecules as pixels is that, unlike their lager counterparts, there is an energy transfer between the molecules. This means that if you had a “blue” molecule and a “green” molecule, the energy of the higher-energy blue light will be absorbed by the “green” molecule. The green molecule will only emit green light, so the light appears green and the blue colour is lost.
Overcoming this energy transfer is the subject of the research being done by Soo-Young Park and colleagues at Seoul National University in South Korea.
In an article published in JACS (Journal of the American Chemical Society) last month, the team revealed that they have created a full colour system using molecular pixels.
They use a method called excited-state intramolecular proton transfer (ESIPT), which relies on a chemical process called keto-enol phototautomerisation. This is when a molecule that can have one of two structures (an enol and a keto form) is exposed to UV light, allowing it to become excited and convert to the second structure (known as a tautomer). However, being in the excited state is energetically unfavorable, so the molecule converts back to its original structure and in doing so emits a photon of light of the desired wavelength (i.e. the correct colour).
The shape shifting aspect of the molecule means that light emitted by one molecule isn’t absorbed by a neighbouring one.
Mixing the molecules allowed the team to make films and dyes that produced a range of colours under UV light, but appeared clear under normal light. It is thought that methods such as these will pave the way to a new generation of display systems and biological imaging.