Using a strategy that should be applicable to many fluorescent dyes, chemists have created the brightest-ever fluorescent materials. The approach combines commonly available cationic dyes and a colorless macrocycle that sequesters anions. The resulting fluorescent materials could be used in medical diagnostics, photochemistry, and lasers.
In what dye makers might describe as a cruel quirk, quenching causes most dyes that fluoresce brightly in solution to discolor, fade, or lose their fluorescence altogether when incorporated into solids. In the solid state, the dye molecules pack together and quench one another so that they no longer emit photons.
The solution to this problem is to move the fluorophores further apart, but results vary, says Indiana University chemistry professor Amar H. Flood. To distance the dyes, chemists have taken a bespoke approach, using long synthetic sequences to add bulky groups to them. This process works through trial-and-error, so there’s no guarantee that the bulked-up molecule will fluoresce in the solid state.
Now, a team led by Flood and the University of Copenhagen’s Bo W. Laursen have come up with a way to keep cationic fluorescent dye molecules apart without the extra bulk. The method uses macrocycles known as cyanostars, which are large, neutral molecules. The cyanostars sequester small counter anions that accompany the cationic dye molecules and keep the cationic fluorophores apart (Chem 2020, DOI: 10.1016/j.chempr.2020.06.029).
When the researchers mix the dyes and the cyanostars and then crystallize them, the macrocycles determine the structure, because they are much bigger than the other components, Flood explains. And the method works for virtually all the major classes of cationic dyes. As long as the dye is cationic and its counter anion small, the cyanostar places the dyes at a set distance from one another every time. The chemists incorporate fluorescent dyes into polymers and resins without changing their color or quenching them. Based on volume, the materials they made have the brightest fluorescence reported to date.
The focus on complexing the counterion is “very innovative,” says University of Würzburg’s Frank Würthner, an expert on functional dyes.
“It was not at all obvious that the anion would have such a tremendous impact on the solid-state fluorescence intensity,” says Gerald J. Meyer, who studies light-induced electron transfer reactions at the University of North Carolina at Chapel Hill. The work, he says, “has clear implications for applications that require highly emissive materials, such as OLEDs and fluorescent sensors.”