Reversible photochemistry occurring in a single crystal of a simple cyanide complex

When molecules with certain electronic structures (often involving conjugated bonds or chromophores) absorb visible light, they undergo changes, driving key photo-activated processes like photosynthesis, solar energy conversion, and photocatalysis.

In most cases, the photoexcited state is very short, but some systems can be trapped in a stable state for up to months before eventually relaxing back to their ground state.

These materials are known as photoswitchable, and their potential is huge in fields like solar energy harvesting, information processing and storage or even light-controlled drugs.

Most photoswitchable materials studied today are organic molecules, but these rarely change their magnetic properties when exposed to light. Inorganic systems can do this through a process called spin crossover, yet the effect disappears quickly above very low temperatures, making them impractical. Recent findings suggest that tweaking how metal–cyanide compounds respond to light could extend the effect to higher temperatures, offering a new path toward magnetic switches that work closer to room temperature.

Now scientists led by Uniwersytet Jagiellonski in Poland, and with collaborators from the University of Bordeaux and the ESRF, have found metal cyanide compounds that can be switched on and off with light, using beamline ID12 at the ESRF. “None of us thought it could actually be possible”, explains Dawid Pinkowicz, corresponding author of the publication. “A few years ago I found that there was a possibility that this could happen, by doing a careful analysis of photodissociation reported in the literature and was awarded an ERC Consolidator grant to investigate “achieving magnets through visible light excitation at room temperature” (LUX-INVENTA, no. 101045004).

In the study, the team showed that potassium heptacyanomolybdate crystals can reversibly break and reform a metal–cyanide bond: violet light breaks the bond, and red light restores it, all while keeping the crystal intact.

Andrei Rogalev, scientist in charge of the ID12 beamline explains the complexities of the experiment, which combined selected spectroscopy and X-ray diffraction later: “It was technically very challenging, as we needed to perform experiment at low temperature in a bore of superconducting magnet and to introduce visible light at the exact same spot where the X-rays would go in a really confined space”.

This newly discovered rare, non-destructive process could pave the way for materials where magnetic properties are controlled by light, opening new possibilities for polymers for data storage and smart devices.

Reference:

Magott, M. et al. Nat Commun 16, 8377 (2025). https://doi.org/10.1038/s41467-025-63523-x

Text by Montserrat Capellas Espuny