A Kiel research group headed by the chemist, Professor Rainer Herges, has succeeded for the first time in directly controlling the magnetic state of a single molecule at room temperature. The paper was recently published in the journal Science. The switchable molecule, which is the result of a sub-project of the Collaborative Research Centre 677 "Function by Switching," could be used both in the construction of tiny electromagnetic storage units and in the medical imaging.
The scientists at the Kiel University developed a molecular machine constructed in a similar way to a record player. The molecule consists of a nickel ion surrounded by a pigment ring (porphyrin), and a nitrogen atom which hovers above the ring like the tone arm on a record player. "When we irradiate this molecule with blue-green light, the nitrogen atom is placed exactly vertically to the nickel ion like a needle," Rainer Herges explains. "This causes the nickel ion to become magnetic, because the pairing of two electrons is cancelled out," says the chemistry professor. The counter effect is blue-violet light: The nitrogen atom is raised, the electrons form a pair and the nickel ion is no longer magnetic. "We can repeat this switching of the magnetic state over 10,000 times by varied irradiation with the two different wavelengths of light, without wearing out the molecular machine or encountering side reactions," Herges enthuses.
The switch which has been discovered, with its diameter of only 1.2 nanometres, could be used as a tiny magnetic reservoir in molecular electronics. Most of all, hard disk manufacturers may be interested in this, as a higher storage capacity can be achieved by reducing the size of the magnetic particles on the surface of the disks. Professor Herges also believes the use of the magnetic switch in the medical field is feasible: "The record player molecule can be used intravenously as a contrast agent in MRT (magnetic resonance tomography) in order to search for tumours or constricted blood vessels. Initial tests in the University Medical Center Schleswig-Holstein's neuroradiology department were successful."
As the signal-to-noise ratio is improved by the switching process, a smaller amount of the contrast agent is required than for the magnetic salts currently being used. In addition, according to Herges, the molecular machine could also serve as a basis for developing new contrast agents to depict such features as temperature, pH value or even certain biochemical markers in the body in a three-dimensional form. Rainer Herges lists the possible fields of application: "Using contrast agents such as these, it could be possible to localise centres of inflammation, detect tumours and visualise many metabolic processes."
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