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Luminescence Amplification by Metal organic framework-Polyoxometalate composites

Final Report Summary - LAMP (Luminescence Amplification by Metal organic framework-Polyoxometalate composites)

During the last decade, organic-inorganic hybrids or metal organic frameworks (MOFs) have become very popular in the materials chemistry world. These hybrids display mixed organic-inorganic behaviours, resulting from synergy of purely organic or purely inorganic features. The resulting materials have potential in energy-efficient technology; sensors, displays, fuel cells, cosmetics, gas adsorption, catalysis etc, are just a few of the options for application. In order to enhance functionality of MOFs, guest molecules such as polyoxometalates (POMs) could be occluded in the pores. POMs are hydrated transition metal clusters with discrete metal-oxygen bonds around a central ion (HxXM12O40 with X=Si4+ or P5+, M=Mo6+ or W6+). They have well defined topologies with intriguing physical and chemical properties and an astounding range of redox transformations with wide range of application in homogeneous catalysis. Immobilization of POMs in the pores of MOFs leads to new materials which easily can be structured for application in catalysis, luminescence and ionic conductivity etc, but especially solves the problem of leaching of the highly active POM from the material. For instance, Cu3(BTC)2.3H2O-type (BTC=1,3,5-benzenetricarboxylic acid) MOF is a classic example of highly porous organic-inorganic hybrid material with potential application in gas adsorption. This material however lacks solvent stability, stability under steam conditions etc, limiting its application in industry. By incorporating POMs in the pores of Cu3(BTC)2.3H2O MOF, we can obtain functionalized POM@MOF composites with greater stability as well as extended applications in ion-exchange, luminescence and heterogeneous catalysis.

The aim of this project was fundamental study leading to development of robust display materials with tunable properties. One of the striking features of the POM@MOF is possibility for ion exchange with lanthanide elements such as Eu3+ and Yb3+. This is due to the presence of extra-framework Cu2+ cations compensating the negative charge introduced by the POM in the MOF pores. Dr. Bajpe, strategically tuned the Cu2+ content in the framework leading to the availability of these extra-framework Cu2+ cations for ion exchange and hence successfully induce luminescence in the MOF. For the first time, a working proof was provided for luminescent hybrid frameworks (ChemPlusChem., 2013, 78(5), 402). This work then progressed into the investigation of the luminescent properties and resulted in a second publication in Dalton Transactions. The work was also selected for front cover of the journal.

Several thousand organic-inorganic hybrid materials have been synthesised to date, with some of them being successfully applied for different innovative applications in industry. In her quest to find more POM@MOF composites for various industrial applications, Dr. Bajpe started investigating the formation of POM@MOFs using Niobium-based polyoxometalates (polyoxoniobates). Polyoxoniobates or Nb-based polyoxometalate compounds show remarkable catalytic properties either as part of a known structure or as an additive. POMs of Nb & Ta despite having similar structure as that of the W and Mo-based POMs offer extreme challenges to polyoxometalate chemists in templating the formation of MOFs due to their highly basic nature. Considering the high activity of Nb for potential applications in industry, Dr. Bajpe spent significant amount of time investigating the interaction mechanisms of these niobates with several organic linkers. While doing this, she discovered two new polyoxoniobates whose crystal structures were solved and are currently being written into a manuscript.

Continuing her investigation of Cu-based POM@MOFs, Dr. Bajpe discovered that Cu2+ cation alone can stabilize the P and Si based POMs forming new POM structures. Two new crystals Cu1.5PW12O40 and Cu2SiW12O40 were synthesized and their structures were solved using single crystal x-ray diffraction. This work is very much at completion and will soon be submitted to European Journal of Inorganic Chemistry.

During the course of her two year Marie-Curie Fellowship, Dr.Bajpe has developed expertise in single crystal x-ray diffraction, luminescence spectroscopy and nanoindentation for mechanical property measurements. She has also successfully established collaborations with various research groups for investigation of the new materials for application in industry. All these collaborations are still ongoing and Dr. Bajpe would soon be able to publish these results.

Despite the ongoing research work that has been discussed above, Dr. Bajpe places priority for exploration of design and application of various other functionalised materials as a long-term objective with sustainability at the crux of her research theme. This would involve design of devices for luminescent materials, materials exhibiting ionic conduction, materials for green chemistry etc. Although her immediate research focus has been on establishing a concrete approach for the design of innovative materials, her long-term goal is to be able to find new pathways and networks to design materials that work in tandem with global energy and sustainability issues.