Synthesis and characterization of Luminescent Metal Organic Frameworks
Introduction-
Materials that are of nanoporous have been of great interest for both academia and industry due to its various applications such as gas storage, gas separation, shape/size catalysis, drug delivery and storage, imaging and sensing.[1] Metal Organic Frameworks (MOFs) are usually compounds consisting of metal ions or cluster coordinated to organic molecules to form one-, two-, or three-dimensional structures that are porous. In MOFs, metal acts as coordination centres and organic ligand acts as linkers between metal centres. It is also regarded as coordination network with organic ligands containing potential voids.[2]
MOFs have been known since 1965, but this field was able to attract interest only in the late 1999, when MOFs were synthesized based on the concept of reticular design.[3]MOFs areexcellent alternatives to the traditional nanoporous materials in many scientific and industrial fields. Thus, MOFs is one of the most exciting recent developments in the field of nanoporous material science, have been also termed as coordination polymers.4 The two major components of MOFs are metal ions or cluster of metal ions and an organic molecule called a linker. This is the reason why MOFs are often referred to as the hybrid Organic-Inorganic materials.Organic molecules are usually mono-, di-, tri-, or tetravalent ligands. The properties and structures of MOFs depend upon the choice of metal and linker molecule.5
The ability to tune the emission properties of a material is of immense interest for manufacturing photovoltaic, light-emitting diodes, non-linear optical (NLO) materials and bio-imaging agents. Luminescent MOFs are promising multifunctional materials in this regard. Although functional MOFs for gas storage, separation and heterogeneous catalysis has been extensively studied and realized at lab and industrial scale, the research on luminescent MOFs is still at an early stage. The practical application of the luminescent MOFs in our daily life warrants in-depth studies on the mechanism of the origin of the luminescence and their structure-luminescent property relationship. However, the enormous diversity of structural topologies and morphologies of MOFs makes it difficult to rationalize any guiding principle of their assembly and thereby deduce meaningful structure-luminescent property correlation. Thus, in view of the difficulties to elucidate structure-luminescent property relationship, we consider it appropriate to attempt to prepare series of topologically and morphologically similar MOFs having pendent functional groups that are amenable to further post-synthetic modifications. Our intention is to explore whether the pendent groups could be modified to incorporate various luminescent organic ligands leading to isostructural luminescent MOFs.
Review of Literature
Luminescent MOFs are an exciting class of materials that can find application in diverse realms of modern science. This materials hold promise as chemical sensors, light emitting devices (LED) and in biomedicine. Several recent studies have shown that with judicious selection organic linkers, these materials can be made to sense potential explosives such as nitroaromatics, an exciting proposition for defence industry. These materials have also shown promise in the emission in the NIR and a wide spectrum of visible lights.Nanometer scale luminescent MOFs is an interesting choice to design theranosticnanomedical devices. This is because their composition can be systematically tuned by judicious choice of building blocks. Some porous luminescent MOFs can secure biological molecules such as anticancer drugs or biogas into their pores.
MOFs are an excellent source for light emitting devices due to their structural diversity and tunable luminescence. Till date large number of MOFs with tunable emission for light emitting devices have been designed and investigated. Work carried out to date by several research groups – those of Qian and Chen,[4]Hupp,[5] Allendorf,2,[6] Lin[7], Horcajada[8] and others[9] on luminescent MOFs has amply demonstrated that these materials can have potential applications in diverse fields such as lighting, display, sensing, and optical devices, NLO materials and as biomedicine as drug delivery agents.
Recently a direct white light emitting MOF [AgL]n. nH2O (L=4-Cyanobenzoate) with tunable yellow to white luminescence by variation of excitation light was reported by Wang et .al.14 Some nanoporous MOFs can secure biological molecules such as anti cancer drugs or biogas into their pores. Lanthanide MOFs possess paramagnetic properties, which helps to increase the relaxation rate of water protons in the tissues being imaged, making them useful as contrast agent in magnetic resonance imaging (MRI) spectroscopy. MOFs have a significant role in drug delivery due to the possibility of adjusting the framework’s functional groups and tuning of the pore size. MOFs contribute to the efficiency of the treatment and reduce side effects. The first group of MOFs considered as potential drug delivery system is the MIL (Materials of Institute Lavoisier) family, which was developed by Ferey and colleagues.15 However, a small pore size of MOFs in the micro porous limits the uptake and drug loading capacity.
Studies on luminescent MOFs till date primarily focuses on investigating the fundamental syntheses and luminescent properties of MOFs; useof luminescence of MOFs to probe local environment, structure, and guest species; and development of multifunctional MOFs by combining luminescence, magnetism, and biocompatibility for biomedical application. However, creating MOFs with tunable luminescence is rare.
In last few years, post-synthetic modification (PSM) has been increasingly used to functionalize MOFs.[10] Covalent transformation of MOFsusing well-established organic reactions that take place on functionalities built into the organic linkers is an effective approach for modification of MOFs. For example, transformation of MOFs via amide couplings by using amine-functionalized SBUs in the construction of porous MOFs has been achieved in recent years.[11] Another facile method for the post-synthetic modification of MOFs is via azide-alkyne Huisgen cycloaddition reaction commonly known as click reaction.[12]The azide-alkyne based “click” reactions are attractive alternatives in this context since they usually involve weakly polarized reactants, minimizing
Objective of present study-:
The main objective of this project is to design isostrutural MOFs with specific pendant functional groups that are amendable to post synthetic modifications. Subsequently we look to modify the synthesized MOFs via surface functionalization to incorporate various luminescent organic ligands leading to luminescent MOFs. Studies would be done to understand the structure-fluorescent relationship. In order to achieve these objectives, we propose to undertake the research in a phased manner and by identifying suitable targets as outlined below;-
Phase-I: - Design, synthesis and characterization of organic linkers for MOF synthesis.
This is the initial stage of the project and will involve the extensive of organic ligands so as to design ditopic or tritopic ligands having suitable pendant moiety that are amendable to further post synthetic modification. These ligands will be the starting material and key building functional group. An important part of this work is the optimization of the different synthetic strategies that will be utilised for the synthesis. It may be mentioned that apart from synthesis, the compound prepared as part of this project will be analyzed and completely characterized.
Phase- II: - Utilization of the synthesized ligand as the building block for the construction of MOF.
The organic linkers successfully would be studied to discover the possibility of organizing them into MOFs by modulating their covalent and non-covalent interactions. This study will need elaborate studies using IR spectroscopy, fluorescence spectroscopy, NMR spectroscopy and mass spectroscopy (LC-MS) and single crystal X-ray diffraction.
Phase-III: - Post synthetic modification of the as synthesized MOFs and their fluorescence studies.
Post synthetic modification studies on the as synthesized MOFs would be carried out. Primarily, we would use covalent Post Synthetic Method (PSM) using well-established organic reaction such as amide coupling, Diel Alders reaction and copper catalysed reaction for the incorporation of fluorescent MOF would be characterized by spectroscopic, analytical and X- ray crystallographic techniques. Detailed studies would be carried out o the fluorescent MOFs.
References
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