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Spring 2015

Hosting Light Could Help Clean Water and Save Energy

Light-controlled metal-organic framework creates highly reactive oxygen and possibly much more

Laurent Karim Beland

A schematic representation of the singlet oxygen-generating porous coordination network framework and how it can controllably produce singlet oxygen, 1O2. If ultraviolet light is shined on the framework, singlet oxygen production is inhibited, while shining visible light will promote production (source: Park et al. 2015).

Singlet oxygen, an excited state of di-oxygen, is a highly reactive chemical that, if controllably produced, could be integrated to reduce energy use and improve many industrial processes. To this day, however, the applicability of singlet oxygen-based technology is limited by the lack of such control. A team at the Center for Gas Separations Relevant to Clean Energy Technologies (CGS) may have found an answer to this problem. Led by Hong-Cai "Joe" Zhou, they discovered a porous, crystalline material that can generate this excited state of oxygen if visible light is shed on it, but stop generating the oxygen if ultraviolet light is used instead. This singlet oxygen can then be used for photocatalysis, where light is used to drive reactions, such as treating industrial wastewater, which is an energy-intensive activity.

Singlet oxygen, which is abbreviated 1O2, differs from the ground state of oxygen in the way its outer electronic shell is occupied. In this special configuration, its valence electrons become unpaired, which leads to high chemical reactivity. As such, it can provoke reactions to treat industrial wastewater, sterilize blood, and eliminate cancerous cells, undesired herbs, or pests. To be effective, the 1O2 should be produced in situ, that is, inside its target. By bringing light-harvesting molecules to the target and then using light as an energy source to excite the di-oxygen in its surrounding, it is thought such applications can be developed.

To that objective, efforts were made to activate its production upon interaction with biomolecules or nanomaterials. Up until now, such methods involved irreversible or passive interactions. In other words, they have no "off" switch. This typically leads to imprecise control over 1O2 generation.

Jihye Park and her co-authors, in an article published in Angewandte Chemie International Edition, explain that by combining a molecule well known to activate singlet oxygen when exposed to light—a photosensitizer—to a molecule that changes shape depending if it is exposed to visible or ultraviolet light—a photochromic molecule—they generated 1O2 in a controlled manner.

To be an effective 1O2 generator, a material must efficiently transfer energy from one molecule to another. Typically, ordered crystalline structures transport electric energy more efficiently than disordered, amorphous structures. By adding zinc to the photosensitizer and the photochromic molecules, such an ordered three-dimensional structure was created. This is an example of a metal-organic framework (MOF). The researchers dubbed the material a singlet oxygen-generating porous coordination network.

In this MOF, the photochromic molecule has an open form and a closed form. By shining ultraviolet light, the closed form can be selected. The open form is favored by shining visible light. The other main organic molecule in the MOF, the light sensitizer, combines with zinc, absorbs light and then transfers this energy to its surroundings. In its open form, energy cannot be transferred to the photochromic molecule, enabling the conversion of oxygen in its ground state to singlet oxygen. On the other hand, if the photochromic molecule is in its closed form, most of the energy will be transferred to it, and singlet oxygen production will be significantly reduced, allowing for controlled generation of singlet oxygen.

In the future, Park plans to explore applications of singlet oxygen-generating porous coordination networks. She has shown that a singlet oxygen-generating porous coordination network could be used to catalyze an oxidation reaction and generate singlet oxygen. Such oxidation reactions are part of typical industrial wastewater treatments. Light-induced catalysis of these reactions could significantly reduce the energy required to treat it and facilitate the use of concentrated light as the main energy source for this application.

Further, the material she and her colleagues developed kept its shape and conserved its key functions when put in solution for long periods. This is of particular interest, as many hybrid MOFs exhibit leaching; that is, separation of the different components of the solid when placed in solution. The fact that the network exhibited little leaching, even after 20 hours in solution, makes it that much more interesting for potential applications.

As Park explains, "If this MOF was used as a catalyst, it can easily be recovered from the process—it can just be filtered off." This is a major advantage of using a solid-state catalyst compared to other types that often cannot be filtered out.

More Information

Park J, D Feng, S Yuan, and CH Zhou. 2015. "Photochromic Metal–Organic Frameworks: Reversible Control of Singlet Oxygen Generation." Angewandte Chemie International Edition 54:430-435. DOI: 10.1002/anie.201408862

Acknowledgments

The Center for Gas Separations Relevant to Clean Energy Technologies (CGS), an Energy Frontier Research Center funded by the U.S. Department of Energy (DOE), Office of Science, Office of Basic Energy Sciences, funded the research done by the corresponding author HC Zhou. The Office of Naval Research funded the work of J Park, D Feng, and S Yuan.

About the author(s):

More Information

Park J, D Feng, S Yuan, and CH Zhou. 2015. "Photochromic Metal–Organic Frameworks: Reversible Control of Singlet Oxygen Generation." Angewandte Chemie International Edition 54:430-435. DOI: 10.1002/anie.201408862

Disclaimer: The opinions in this newsletter are those of the individual authors and do not represent the views or position of the Department of Energy.