Ferrocene is an essential molecule for creating molecular machines, yet it faces a significant hurdle as it tends to decompose on flat noble metal surfaces. Researchers have now found a way to stabilize ferrocene by combining it with ammonium salts and embedding it within a molecular film made of cyclic crown ether molecules. This ammonium-linked molecule shows reversible lateral movement when exposed to electrical voltage, making it the tiniest molecular machine so far.
Artificial molecular machines are tiny devices made up of just a few molecules. They hold great promise for advancing various fields, including catalysis, molecular electronics, medicine, and quantum materials. These machines work by transforming external stimuli, such as electrical signals, into mechanical movement at the molecular scale. Ferrocene, a distinctive drum-shaped molecule featuring an iron (Fe) atom nestled between two five-membered carbon rings, serves as an excellent building block for these machines. Discovered in 1973, ferrocene’s significance in molecular machinery has earned it a Nobel Prize in Chemistry and established it as a crucial element in molecular studies.
Ferrocene is attractive because of its remarkable ability: when the electronic state of the Fe ion shifts from Fe+2 to Fe+3, the two carbon rings rotate approximately 36° around the molecular center. By manipulating this electronic state using an external electrical signal, precise control over molecular rotation may be possible. However, a significant limitation has been that ferrocene quickly decomposes when attached to substrate surfaces, particularly flat noble metal ones, even at room temperature and under ultra-high vacuum. Until now, a reliable method to securely bond isolated ferrocene molecules to a surface without decomposition was not achieved.
A breakthrough study from a research group led by Associate Professor Toyo Kazu Yamada at Chiba University, Japan, alongside Professor Peter Krüger, Professor Satoshi Kera, and Professor Masaki Horie, has now solved this issue. They successfully established the world’s smallest electrically operated molecular machine. “In this work, we managed to stabilize and attach ferrocene molecules to a noble metal surface by initially applying a two-dimensional crown ether film. This represents the first direct experimental proof of ferrocene-based molecular motion at the atomic level,” states Prof. Yamada. Their research was published in the journal Small on November 30, 2024.
The stabilization of ferrocene was achieved by modifying it with ammonium salts, thereby creating ferrocene ammonium salts (Fc-amm). This alteration improved its stability and facilitated secure attachment to the substrate’s surface. The new compound was anchored onto a monolayer film of crown ether cyclic molecules on a flat copper base. Crown ether cyclic molecules have a unique structure that enables them to trap diverse atoms, molecules, and ions. Prof. Yamada elaborates, “We previously discovered that crown ether cyclic molecules can form a monolayer film on flat metal surfaces. This film effectively captures the ammonium ions of Fc-amm within the crown ether’s central ring, shielding ferrocene from decomposition caused by contact with the metal substrate.”
Subsequently, the team utilized a scanning tunneling microscopy (STM) probe to interact with the Fc-amm molecule and applied an electrical voltage, which induced a lateral sliding movement. Specifically, applying a voltage of −1.3 volts introduced a hole in the electron structure of the Fe ion, changing it from Fe2+ to Fe3+. This led to the rotation of the carbon rings alongside the molecule’s lateral sliding motion. Calculations based on density functional theory indicated that this sliding occurs due to Coulomb repulsion among the positively charged Fc-amm ions. Notably, when the voltage was removed, the molecule returned to its original position, demonstrating that the movement is reversible and can be precisely managed via electrical signals.
“This research paves the way for exciting developments in ferrocene-based molecular machinery. Their capacity to execute specific tasks at the molecular level may usher in revolutionary changes across diverse scientific and industrial sectors, including precision medicine, smart materials, and advanced manufacturing,” states Prof. Yamada, emphasizing the vast potential applications of this technology.
