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Maintaining fluorescence as dyes crystalize to form solids is a problem that has existed in materials for over 100 years. Now, a novel approach to tackling the issue and a revolutionary material could unlock a new age of optics.
Florescence is used across a wide range of optical technology, from biomedical scanners to solar collecting technology, and even to the OLED screens in our home televisions.
When transformed into a crystalized, solid form, these dyes lose their glow and usefulness in many cases. This has held fluorescence back in terms of its possible applications. Researchers now report in the journal Chem that they may have solved this problem.
The team, led by Amar Flood, a chemist at Indiana University and Bo Laursen of the University of Copenhagen, has formulated positively charged fluorescent dyes into a new class of materials called small-molecule ionic isolation lattices (SMILES). These materials have the highest known brightness per volume ever observed by researchers.
The team believes that the new material has a wide range of potential applications in any tech that calls for bright fluorescence or requires the development of unique optical properties - this includes solar energy harvesting, bioimaging, and lasers.
The team indicates that SMILES could also be applied to light-switchable materials in information storage and photochromic glass, as well as circularly polarized luminescence in 3D display technology.
Something to SMILE About
Of the staggering 100,000+ fluorescent dyes currently available on the market, none can be combined in a predictable way to create a solid fluorescent material. This is predominantly because they have the same problem.
Although dyes can be readily made with bright fluorescence of any color, those properties cannot be transferred to materials without losing this fluorescence and changing their color. This is because when these dyes are packed together in solids, they become electronically coupled to each other. Therefore, they stop acting as individuals, and this strong excitonic coupling quenches their emissions.
SMILES overcomes this hurdle by transferring the optical properties of dyes to solids. They are simple to make, formed by mixing cationic dyes with an anion-binding macrocycle— a molecule comprising of twelve atoms in a ring-like structure—known as cyanostar.
The cyanostar - a star-shaped colorless macrocycle molecule - form well-defined lattices, and the dyes slot into gaps in the matrix, remaining separated but together. This successfully transfers optical properties from the solutions to the solid, preserving emission. As solidification proceeds, crystals begin to form, developing into a dry powder. This product can be incorporated into a thin-film or a polymer.
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The process works with significant classes of commercial dyes, including xanthenes, oxazines, styryls, cyanines, and trianguleniums, solving concentration quenching to impart fluorescence to commercial polymers.
Cyanostar generates a lattice-like structure. This means that the dyes can be inserted into the lattice and used immediately in the material without any adjustments. The material taking on the color of the dye led the team to label SMILES as ‘plug and play’ fluorescents.
The strategy that the team used to space the dye molecules and preserve the individual properties is similar to a past approach. However, the team discovered the use of colorless macrocycles, with previous attempts being based on colored macrocycles.
Fluorescent Materials and a Brighter Future
Among the many possible applications of SMILEs, the most significant is in the harvesting of solar energy.
While solar cells are one of the most essential and significant forms of green and renewable energy, cells and solar panels that contain them are by no means as efficient as they could be.
This is because many frequencies of light emitted by the Sun cannot be absorbed by solar cells, which convert sunlight into electrical energy. An average of 50% of the solar panel’s light that is absorbed does not go towards the production of electricity and is wasted.
Fluorescent materials that can hold their fluorescence could majorly improve this situation by converting infrared and low-frequency light into frequencies that can be absorbed by solar panels.
SMILEs could have a major impact on the solar panel market, which has risen in value from an estimated $86 billion in 2015 to more than $52 billion in 2018, and is expected to reach $422 billion by 2022. A marked rise in efficiency could see this value soar even more rapidly, and increased efficiency persuades more consumers to invest in such technology.
It is still early days for SMILEs, and the researchers point out that they are not certain what properties of this new material will offer the most efficient functionality. The team is also uncertain about the limits of SMILEs, which is a critical factor in scaling up their production.
This means that the next step in their research is the development of a deeper understanding of how SMILEs work.
While it may be a long road to widespread usage, the future could be bright for optical devices and technology thanks to SMILEs.
References and Further Reading
Benson. C.R., Laursen. B. W., Flood. A. H., et al. (2020) Plug-and-Play Optical Materials from Fluorescent Dyes and Macrocycles. Chem, https://doi.org/10.1016/j.chempr.2020.06.029.