Research News

Designing and Defining Pores

Simplified perspective view of the as-synthesized MOF

A new porous metal organic framework developed by Sujit Ghosh’s group transforms its structure and displays luminescence based on easily exchangeable anion molecules. This work has been published in a recent issue ofAngewandte Chemie.

Metal-organic frameworks (MOFs) are porous materials and are made up of metal ions or clusters of metal ions bound to organic molecules. The structural framework of such compounds consists of pores that can sequester molecules enabling MOFs to behave like molecular sponges. Due to this property, MOFs have wide applications in gas storage, chemical separation, drug delivery etc. Since each of these applications requires the “sponges” to hold and release molecules on demand, a good part of research efforts in this area are targeted to identify selective binding and release of guest molecules.

Introducing us to an important feature of MOFs that governs selectivity, Biplab Manna, graduate student in Ghosh’s laboratory and one of the primary authors of the present publication, says, “MOFs, since their development two decades back, have presented a useful alternative to traditional agents used for chemical separation such as zeolites. The early MOFs were porous, but had a rigid structure. More recently, combining the porous nature offered by zeolites with a guest-responsive changes in structure, flexible MOFs have begun to be synthesized. Researchers had noticed that such flexible molecules are a lot more selective in taking up guest molecules.”

Manna, along with a few other members of the Ghosh laboratory, has designed and synthesized one such flexible, structurally dynamic MOF. The team has been able to achieve this by first designing a new chelating organic compound (also called linker or ligand) and letting it bind metal ions (zinc in this case).

To their excitement, they observed that crystal structure of this new compound shows slow opening of the framework when a guest molecule is included into the structure. Importantly, the inclusion was size selective, giving them the first indication that they might be handling a structurally dynamic MOF—the kind of molecules which in recent years have revolutionized the field of chemical adsorption and separation due to their wide potential applications in daily life and in industry.

 Sujit Ghosh (far left) with his lab members

Sujit Ghosh (far left) with his lab members

The authors also found that anions of the new compound could be exchanged and the compound showed a remarkable degree of flexibility in its structure based on the nature of anions present. Yet another useful characteristic of this MOF seems to be the fact that each of these anion exchanged materials showed specific luminescent properties. This further increases their potential for their use in detection- and sensing-based applications.

Summarizing the results of this publication, principal author Sujit Ghosh says “We have been able to show the dynamic nature of the new MOF using a solid state structural transformation based method, which in itself is technically challenging. Two important and immediate applications of this compound are in chemical separation and anion removal.”

The paper titled “Dynamic structural behavior and anion-responsive tunable luminescence of a flexible cationic metal-organic framework” has appeared in Angew. Chem. Int. Ed. 2013, 52:998–1002 and is authored by Biplab Manna, Abhijeet K. Chaudhari, Biplab Joarder, Avishek Karmakar and Sujit K. Ghosh.

The picture above shows Sujit Ghosh (extreme left) with his lab members. Research in Sujit Ghosh’s laboratory is supported by funds from IISER Pune, Department of Atomic Energy (DAE), Department of Science and Technology (DST), and Council of Scientific and Industrial Research (CSIR), India.

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