Fluorescence is achieved in molecular motors powered by light

Rotary molecular motors were first created in 1999, in the laboratory of Ben Feringa, Professor of Organic Chemistry at the University of Groningen. These engines are powered by light. For many reasons, it would be nice to be able to make these kinetic molecules visible. The best way to do this is to make them fluorescent. However, combining two light-induced functions in a single molecule is quite difficult. The Feringa lab has now managed to do just that, in two different ways. These two types of rotary motors driven by fluorescent light were described in Nature communications (September 30) and Advances in Science (November 4).

“After the successful design of molecular motors in recent decades, an important next goal has been to control various functions and properties using such motors,” explains Feringa, who received the Nobel Prize in Chemistry in 2016. “As these work with light in rotary motors , it is particularly difficult to design a system that would have another function controlled by light energy besides rotational motion.”

Feringa and his team were particularly interested in fluorescence, as this is a major technique widely used for detection, for example in biomedical imaging. Usually, two such photochemical events are incompatible in the same molecule. either the light driven motor is running and there is no fluorescence or there is fluorescence and the motor is not running. Feringa says, “We have now demonstrated that both functions can coexist in the same molecular system, which is rather unique.”

Ryojun Toyoda, a postdoctoral researcher in the Feringa group who now holds a professorship at Tohoku University in Japan, added a fluorescent dye to a classic Feringa rotary engine. “The trick was to prevent these two functions from blocking each other,” says Toyoda. It was able to erase direct interactions between the paint and the engine. This was done by placing the paint vertically on top of the engine it was attached to. “This limits the interaction,” Toyoda explains.

Different colors

In this way, fluorescence and rotary operation of the motor can coexist. In addition, it was shown that changing the solvent allows him to tune the system: “By changing the polarity of the solvent, the balance between the two modes can change.” This means the engine has become sensitive to its environment, which could point the way for future applications.

Co-author Shirin Faraji, Professor of Theoretical Chemistry at the University of Groningen, helped explain how this happens. Kiana Moghaddam, a postdoc in her group, performed extensive quantum mechanical calculations and showed how the ground energy governing the photoexcitation dynamics strongly depends on the polarity of the solvent.

Another useful property of this fluorescent motor molecule is that different dyes could attach to it as long as they have a similar structure. “So it’s relatively easy to make engines that glow in different colors,” says Toyoda.

Antenna

A second fluorescence engine was built by Lukas Pfeifer, also while working as a postdoctoral researcher in the Feringa group. He has since joined the École Polytechnique Fédérale in Lausanne, Switzerland: “My solution was based on a motor molecule I had already made, which is driven by two low-energy near-infrared photons.” Motors powered by near-infrared light are useful in biological systems because this light penetrates deeper into tissue than visible light and is less damaging to tissue than ultraviolet light.

“I attached an antenna to the motor molecule that collects the energy of two infrared photons and transfers it to the motor. While we were working on this, we discovered that with some modifications, the antenna could also cause fluorescence,” says Pfeifer. It turned out that the molecule can have two different excited states: in one state, energy is transferred to the motor part and drives the rotation, while the other state causes the molecule to fluoresce.

Power

“In the case of this second motor, the entire molecule fluoresces,” explains Professor Maxim Pshenichnikov, who performed spectroscopic analysis of both types of fluorescence motor and is a co-author of both papers. “This engine is a chemical entity in which the wave function is not localized and, depending on the energy level, it can have two different results. By changing the wavelength of the light and therefore the energy the molecule receives, you have either spin or fluorescence Faraji adds, “Our collaborative approach to principle and practice highlights the interplay between theoretical and experimental studies and demonstrates the power of such combined efforts.”

Now that the team has combined movement and fluorescence in the same molecule, a next step would be to show movement and detect the position of the molecule simultaneously by detecting fluorescence. Feringa says, “This is very powerful, and we could apply it to show how these motors can cross a cell membrane or move inside a cell, since fluorescence is a widely used technique to show where molecules in cells. We could also use it to detect motion caused by the light-driven motor, for example in a nanoscale orbit, or perhaps detect motor-induced transport at the nanoscale. This is all part of tracking research .”