Bringing out the color of zinc to expand its potential properties

Zinc is an essential element that is widely found in biological systems, is cheap to produce and has low toxicity compared to other metals. However, unlike other similar metals that exhibit various vibrant colors in metal complexes, it was considered impossible to see different colors for zinc materials.

In a study published in Applied Chemistry International EditionResearchers from the University of Tokyo’s Institute of Industrial Science have synthesized a complex with two zinc ions that exhibit color that greatly expands the potential properties of zinc complexes.

Dramatic color changes are used to demonstrate chemical reactions for fun; However, they may also have important applications in indicators, sensing, and smart materials. For some metal complexes these changes occur because visible light has the right energy to move electrons between orbitals—the regions of the atomic structure that accommodate electrons. However, the energy gap between the orbitals of the highly stable ion of zinc is greater than the energy of visible light, so electrons cannot move between the orbitals – and therefore cannot produce color.

Researchers from the University of Tokyo’s Institute of Industrial Science have shown that functionalizing a second zinc atom results in a yellow color as a solid and when dissolved in solution.

The researchers carefully designed two molecules containing silicon atoms that provided perfect docking sites for the zinc ions to penetrate. Both zinc-iron complexes support two zinc atoms, but at different distances.

„We used two systems to show that zinc atoms work together to form a complex that absorbs light in the visible spectrum,” explains Yoshimasa Wada, lead author of the study. „In the first system the zinc atoms were relatively far apart – 5.71 angstroms – and the material was colorless. In the second system, they were much closer – 2.93 angstroms – and the zinc material was yellow.”

In the system where the zinc atoms were close together, they were able to combine their orbitals so that the energy needed to rearrange their electrons was in the visible region. At large, both solid and solution of the second complex appeared yellow.

„The observed interaction between zinc centers expands the potential properties of zinc complexes,” says senior author Yusuke Tsunata. „We believe our findings will open up a new family of interesting materials.”

Zinc can now add visible light interactions to its list of useful properties. Given the prevalence of zinc in biology and its low toxicity, this could open new applications for zinc in biosensing and biocatalysis.