In their study of rotary molecular motors, researchers in the laboratory of Ben Feringa at the University of Groningen have combined two light-mediated functions — motion and fluorescence — within a single molecule. The advancement is poised to benefit the construction of advanced molecular machines and, according to the researchers, provide prospects toward photoactive multifunctional systems that perform molecular rotary motion while tracking its location in a complex environment. Rotary molecular motors are molecular machines, which can be light-activated and used to power artificial mechanical molecular systems and enable autonomous motion, particularly on the nanoscale. These machines can function in complex bio-environments such as the interior of vessels and cells. They also offer opportunities for the design of responsive materials. The Feringa-led team developed two methods that combined motion and fluorescence in a single molecule. According to Feringa, who is credited with the development of rotary molecular motors in 1999 and who earned the 2016 Nobel Prize in chemistry for the technology, an important next step in the continued development of molecular motors was to use the motors themselves to control their various functions and properties. “As these are light-powered rotary motors, it is particularly challenging to design a system that would have another function that is controlled by light energy, in addition to the rotary motion,” Feringa said. Feringa and his team were interested in fluorescence, which is widely used for detection, such as in biomedical imaging. The team’s achievement overcomes the issue of incompatible photochemical events — motion and fluorescence — within the same molecule. Usually in this case, either the light-driven motor operates and there is no fluorescence, or there is fluorescence and the motor does not operate. In development of the first system in the work, researcher Ryojun Toyoda added a fluorescent dye to a rotary motor. “The trick was to prevent these two functionalities from blocking each other,” Toyoda said. Toyoda quenched the direct interactions between the dye and the motor by positioning a dye perpendicular to the upper part of the motor to which it was attached. This limited the interaction, and it showed that, in this way, the fluorescence and the rotary function of the motor could coexist. Further, changing the solvent allowed Toyoda to tune the system. “By varying the solvent polarity, the balance between both functions can be changed,” he said. This means that the motor became sensitive to its environment, which could point the way for future applications. Different dyes could also be attached to the motor molecular as long as they have a similar structure. “So, it is relatively easy to create motors that are glowing in different colors,” Toyoda said. The researchers based their second fluorescent motor on an already constructed motor that was driven by two low-energy near-infrared photons, according to researcher Lukas Pfeifer. Motors that are powered by near-infrared light are useful in biological systems, as this light penetrates deeper into tissue than visible light and is less harmful to the tissue than ultraviolet light. Pfeifer added an antenna to the motor molecule that collects the energy of two infrared photons and transfers it to the motor. “While working on this, we discovered that with some modifications, the antenna could also cause fluorescence,” Pfeifer said. The team ultimately determined that the molecule could have two different excited states. In one state, the energy is transferred to the motor part and drives rotation. The other state causes the molecule to fluoresce. Researchers in the group of Ben Feringa achieved dual function, including fluorescence, in light-driven molecular motors using two designs. One of the motors was prepared by chemically attaching an antenna to a molecular motor. Rotation and photoluminescence can be controlled using light of different wavelengths. Courtesy of Lukas Pfeifer. Professor Maxim Pshenichniko performed spectroscopic analysis of both types of fluorescent motors. According to Pshenichniko, the second motor is one chemical entity on which the wave function is not localized. Further, depending on the energy level, this motor can have two different effects. By altering the wavelength of the light, and, as a result, the energy that the molecule receives, the motor delivers either rotation or fluorescence. A next step in the work would be to show motility and detect the molecule’s location simultaneously by tracing the fluorescence. Feringa said, “This is very powerful and we might apply it to show how these motors might traverse a cell membrane or move inside a cell, as fluorescence is a widely used technique to show where molecules are in cells. We could also use it to trace the movement that is induced by the light-powered motor, for instance on a nanoscale trajectory or perhaps trace motor-induced transport on the nanoscale.” The research was published in Nature Communications (www.doi.org/10.1038/s41467-022-33177-0) and in Science Advances (www.doi.org/10.1126/sciadv.add0410).
Photosensitizers