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First observation of controlled electron transfer within a single molecule

19 March 2020
First observation of controlled electron transfer within a single molecule
The electron transfer that takes place between two ferrocene units of an isolated molecule is controlled by the oscillating tip of an atomic force microscope. During this process, a certain amount of energy ΔE is released into the surroundings of the molecule.

A team of Czech scientists has demonstrated for the first time the controlled transfer of an electron within a single molecule. Published in the journal Nature Communications, the work presents important knowledge about one of the key processes in physics, chemistry, and biology and also provides inspiration for the construction of quantum computers based on molecular cellular automata and supercapacitors for storing energy in individual molecules.

“We managed to carry out a controlled electron transfer within one isolated molecule and, at the same time, measure the amount of energy released into the environment during this process. Supported by a theoretical model, these measurements provide a detailed understanding of quantum mechanical processes, such as charge transfer and energy conversion at the atomic level,” explains Pavel Jelínek of the Institute of Physics of the Academy of Sciences of the Czech Republic.

To carry out this study, scientists designed a molecule containing two iron atoms chemically bonded in ferrocene units. These so-called redox centres with defined distances then serve as reservoirs between which the electron transfer (i.e. unit charge) takes place.  

“We’re delighted that the design of the model compound has met expectations, allowing physicists to observe fundamental processes at the level of single molecules endowed with the necessary redox properties. Synthesis of the model compound was straightforward yet difficult to obtain in high purity for subsequent physical measurements,” adds Ivo Starý of the Institute of Organic Chemistry and Biochemistry of the Czech Academy of Sciences. 

The molecules were placed on an insulating surface of salt, and measurements were performed under ultra-high vacuum conditions. For controlled electron transfer between ferrocene units and charge positioning, scientists used an atomic force microscope. At the same time, this made it possible to detect energy that was irreversibly released into the environment during electron transfer. 

Subsequent theoretical analysis has shown that the repeated electron transfer induced by the oscillating probe of an atomic force microscope takes the system out of thermal equilibrium, causing a weak temperature dependence of the electron transfer rate between ferrocene units. 

An important prerequisite for the successful advancement of quantum technologies is a detailed knowledge of the basic processes on which they are based, i.e. charge transfer and the associated energy conversion at the atomic level. Electron transport in molecules also plays an important role in many biological and chemical processes, such as photosynthesis, corrosion, and many enzymatic reactions. Despite the crucial importance of these processes and the efforts made to understand them, our current possibilities for studying and managing charge transfer at the level of individual atoms or molecules are still very limited. 

Undertaken by a multidisciplinary team of Czech scientists from the Institute of Physics of the CAS, the Institute of Organic Chemistry and Biochemistry of the CAS, the Faculty of Mathematics and Physics of Charles University, and the Regional Centre of Advanced Technologies and Materials of Palacký University in Olomouc, the work represents a significant step forward in understanding the quantum processes related to electron transfer in single molecules and the associated energy conversion.

Original paper: Berger, J., Ondráček, M., Stetsovych, O. et al. Quantum dissipation driven by electron transfer within a single molecule investigated with atomic force microscopy. Nat Commun 11, 1337 (2020). doi:10.1038/s41467-020-15054-w

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