New method shows how molecular switches are influenced by their neighbors

Researchers at Friedrich Schiller University Jena and the Helmholtz Centre Berlin (HZB) have, for the first time, been able to directly observe how the environment surrounding a molecular switch influences its electronic structure. Molecular switches are molecules that can be switched between two states by external influences such as changes in temperature—similar to a switch with the positions »On« and »Off«. Such molecules are being investigated as potential building blocks for future data storage devices or sensor materials. Using magnetic-field-dependent terahertz spectroscopy, the researchers have now, for the first time, been able to distinguish whether neighbouring molecules in a material are in the same or different states. The results have been published in the journal »Angewandte Chemie International Edition«.

When molecules »notice« what their neighbours are doing

In the iron compounds studied, the molecules can switch between two electronic states. In doing so, the electrons around the iron ion arrange themselves differently: in the so-called high-spin state, there are unpaired electrons—this makes the molecule magnetic. In the low-spin state, by contrast, the electrons are paired; this state is non-magnetic.

»If we want to make targeted use of molecular switches, we need to know how many molecules have to ›communicate‹ with one another to produce a specific switching behaviour«, says Prof. Dr Birgit Weber from the Institute of Inorganic and Analytical Chemistry at the University of Jena. »With the new method, we can, so to speak, see what the neighbouring molecules are doing.«

Until now, it has been possible to determine what proportion of the molecules are in which state. It was more difficult to find out how these states are distributed within the material: do the molecules switch largely independently of one another, or do larger groups change their state together? It is precisely this information that is important for understanding the properties of the material as a whole.

A fingerprint of the molecular environment

For their study, the researchers investigated two iron(II) compounds whose switching states can be influenced by the rate of cooling. During rapid cooling, a state present at higher temperatures can be »frozen«. Slow cooling, on the other hand, gives rise to different or mixed states. This enabled the researchers to compare how a molecule behaves when its environment changes.

THz-EPR spectroscopy reveals very small changes in the electronic structure. »The electronic structure bears, so to speak, a ›fingerprint‹ of its environment«, says Prof. Dr Winfried Plass from the Institute of Inorganic and Analytical Chemistry at the University of Jena. »When neighbouring molecules change their state, this ›fingerprint‹ also changes —and that is precisely what we can measure.«

In the case of one of the compounds, it was already known from previous studies that magnetic and non-magnetic states alternate regularly at low temperatures. The changes now measured were consistent with this known pattern and were confirmed by quantum chemical calculations.

How many molecules are needed for a stable state?

This method can now be used to investigate whether molecules switch individually or form larger areas, known as domains, in which many molecules change their state together. This is also relevant for potential applications: if two stable states, »0« and »1«, are to be utilised, the smallest possible storage unit depends on how many molecules must interact to achieve this.

»If hundreds or thousands of molecules always have to switch together for this, it sets a limit on how small such systems can be made«, says Weber. »If, on the other hand, even very few or even individual molecules can be switched independently into a stable state, significantly higher storage densities would be conceivable.«

Independent confirmation at BESSY II

According to the researchers, this is the first application of magnetic THz EPR spectroscopy to such switchable iron compounds. The measurement facilities at BESSY II at the Helmholtz Centre Berlin played a central role. There, terahertz measurements can be combined with strong magnetic fields and very low temperatures. The decisive energy shifts involved are exceptionally small. The fact that they actually originate from the material and are not a measurement artefact was proven by measurements using synchrotron radiation, which independently reproduced the observations on separate samples. Their high brilliance also opens up access to systems with weaker signals and to transitions at even lower energies.

[This page has been machine translated.]