SMILES Brighten Fluorescent Materials

A joint effort between researchers at Indiana University and the University of Copenhagen has yielded a new class of materials, called small-molecule ionic isolation lattices (SMILES), which the researchers say are the brightest fluorescent materials in existence.

To create this material, the researchers mixed a colored dye with a colorless solution of cyanostar, a star-shaped macrocycle molecule that prevents the fluorescent molecules from interacting as the mixture solidified, keeping their optical properties intact. As the mixture became a solid, SMILES formed, which the researchers then grew into crystals, precipitated into dry powders, and then finally spun into a thin film or incorporated directly into polymers.

Glowing 3D-printed gyroids made with bright SMILES materials. Courtesy of Amar Flood.

The materials overcome an old problem: “quenching.” While there are currently more than 100,000 different fluorescent dyes available, very few can be mixed and matched in predictable ways to create solid optical materials. Dyes tend to undergo quenching when they enter a solid state due to how they behave when packed close together, decreasing the intensity of their fluorescence to produce a more subdued glow.

“The problem of quenching and inter-dye coupling emerges when the dyes stand shoulder-to-shoulder inside solids,” said Amar Flood, a chemist at Indiana University and co-senior author on the study, along with Bo Laursen of the University of Copenhagen. “They cannot help but ‘touch’ each other. Like young children sitting at story time, they interfere with each other and stop behaving as individuals.”

Because the cyanostar macrocyles form building blocks that generate a lattice-like checkerboard, the researchers could simply plug a dye into the lattice and, without further adjustments, the structure would take on its color and appearance.

Previous research had explored spacing dyes through the use of macrocycles; however, it relied on colored macrocycles. Flood and colleagues found that colorless macrocycles held an advantage in producing their desired results.

“Some people think that colorless macrocycles are unattractive, but they allowed the isolation lattice to fully express the bright fluorescence of the dyes unencumbered by the colors of the macrocycles,” Flood said.

Because the materials are so new, Flood said, their innate properties that offer superior functionality are not yet known, nor are the limits of the materials. The materials have potential applications in any technology that needs bright fluorescence or calls for designing optical properties, including solar energy harvesting, bioimaging, and lasers, Flood said.

The next step will be to explore the properties of the fluorescent materials to enable work with dye markers to realize the materials’ full potential in a variety of applications.

The research was published in Chem (www.doi.org/10.1016/j.chempr.2020.06.029).