Chemists create light-switchable magnets that remain active for hours

A research team from the University of Chemistry and Technology, Prague (UCT Prague) and the Institute of Organic Chemistry and Biochemistry of the Czech Academy of Sciences (IOCB Prague) has created and described a new type of photoswitch. The molecule, a thienyl-based acylhydrazone, undergoes an unprecedented “closed-to-open-shell” transformation, where light converts it into a stable diradical.

While previously published lifetimes of such triplet states are a few milliseconds, this new molecule’s switched state has a half-life of over six hours. This revolutionary innovation opens the way for optimizing catalytic processes, developing new data storage and spintronic devices, and targeted elimination of antibiotic-resistant pathogens. The work is published in the Journal of Materials Chemistry C.

Photoswitches are molecules that change between two states under the influence of light. This new switch is unique because it transitions from a stable, non-magnetic (closed-shell) state to an exceptionally long-lived magnetic (open-shell triplet) state. In this triplet state, two electrons have parallel spins, making the molecule paramagnetic and highly reactive. This state is crucial for many photochemical processes, including the generation of reactive oxygen species.

“While it is previously known and described in literature that, in molecules, this state lasts on the order of nano- to milliseconds, in the case of our switch, it persists for tens of hours, which is extraordinary,” explains the head of the research, Petr Kovaříček from UCT Prague, adding that his doctoral student, Martin Šetek, made a fundamental contribution to the result. Additionally, the team could not have succeeded without the expert skills of Dana Nachtigalová and Ján Tarábek from IOCB Prague.

The innovation has several immediate applications. In catalysis, the team demonstrated that the photoswitched form acts as a potent radical initiator, successfully driving the radical bromination of toluene. For data storage and spintronics, the molecule forms a complete write-read-erase system.

“The triplet state is paramagnetic, which allows information to be ‘written’ with light, ‘read’ magnetically,” states Dr. Kovaříček. The scientific paper further reveals a crucial third step: The information can be completely and instantly erased by an electrical impulse.

“Such a long-living triplet state is truly surprising. For a long time I did not even consider it possible and rejected this explanation,” he says, explaining why the article is the result of three years of intensive experimental work. “To convince editors and reviewers of this was also a colossal task.”

The team’s most promising application is the photodynamic inactivation of pathogens. “Our new molecules can, after being irradiated with light, generate reactive oxygen species, so-called ROS, which effectively destroy a range of antibiotic-resistant fungi and bacteria, including golden staph (Staphylococcus aureus) and selected pathogens from the WHO list,” says Dr. Kovaříček.

“ROS are extremely effective—they eliminate more than 99.99% of cells, but only where we shine the light. This is key for the safety of the medical application, which of course must be verified in further studies.”

A major advantage of the newly synthesized molecules is their exceptionally low cost and simple production—on a laboratory scale, one kilogram costs approximately 1,000 Czech crowns (about $43 USD).

The idea for medical use was a serendipitous discovery. A bachelor’s student testing the molecules in a biological setting found that they destroyed the sample’s DNA with astonishing efficiency, revealing a powerful new weapon against microorganisms. In collaboration with Dr. Lencová from the Department of Biochemistry and Microbiology, the teams are already preparing a new project to pursue this goal.