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Researchers at the Universities of Amsterdam and Zurich have developed a molecular system for the controlled release of iron. They integrated ferrocene, a molecular sandwich that encloses an iron atom, with a carbon "nanohoop."

As a result, the system allows for the release of Fe²⁺ ions upon activation with benign green light. The study is in the Journal of the American Chemical Society (JACS).

The research was carried out by the groups of Dr. Tomáš Šolomek at the University of Amsterdam's Van 't Hoff Institute for Molecular Sciences and Dr. Peter Štacko at the University of Zurich (Department of Chemistry). Their expertise is in photocages, molecular photochemical tools that offer precise control over substrate activity in time and space using light as a bio-orthogonal stimulus.

Photocages permit the activation of biologically significant molecules such as proteins, nucleotides, or drugs. Not only are they a great tool to study mechanisms and dynamics of biochemical processes, they also have potential for therapeutic applications such as photoactivated chemotherapy.

In the research now published in JACS, the researchers shifted their focus from controlling the activity of organic molecules to another crucial component in many biological systems: iron. Renowned for its role in oxygen transport in the human body, it also has a pivotal role in the energy-providing redox processes in mitochondria, in the synthesis of deoxyribonucleotides, or in protecting cells from oxidative stress.

Strain-induced photorelease

Nature has developed a protein-based system to tightly regulate iron's uptake and balance. In their paper, the researchers present a less sophisticated yet fully functional synthetic equivalent that stores iron and releases it "on demand."

The system is based upon the use of ferrocene as the iron carrier, and enables controlling its function by integrating it into a carbon nanohoop. Ferrocene is an organometallic "sandwich complex" that tightly holds an iron atom between two cyclopentadienyl rings. By itself, it is chemically rock stable and resistant to light. Incorporating it into a molecular nanohoop, however, changes this.

When the two cyclopentadienyl rings are connected by means of six coupled benzene rings (a cycloparaphenylene nanohoop), a system emerges that enables control over the iron containment.

Although conformationally stable, the integration twists the entire nanohoop structure and exerts a large mechanical stress on the ferrocene. As a result, the system becomes susceptible to irradiation with green light, which results in the release of iron.

In their paper, the researchers describe how the iron can be released with high efficiency upon irradiation. They expect this strategy of introducing mechanical stress in molecules to offer great promise also beyond the realm of photocages.

For instance, it can potentially enable the development of new responsive materials in supramolecular, organometallic, or polymer chemistry.