Metal–Organic Frameworks

Promising Materials for Use in Optical Devices

  • Fig. 1: View along the a axis in the iron(III)-based MOF showing one type of pores and the connectivity of the linkers. Solvent molecules inside the pores are omitted for clarity.Fig. 1: View along the a axis in the iron(III)-based MOF showing one type of pores and the connectivity of the linkers. Solvent molecules inside the pores are omitted for clarity.
  • Fig. 1: View along the a axis in the iron(III)-based MOF showing one type of pores and the connectivity of the linkers. Solvent molecules inside the pores are omitted for clarity.
  • Fig. 2: Visualization of birefringence.
The last five years have witnessed a huge breakthrough in the creation and study of the properties of a new class of compounds, namely, metamaterials. The next stage of this technological revolution will be the development of active, controllable and non-linear metamaterials, surpassing natural media as platforms for optical data processing and quantum information applications. However, scientists are constantly faced with the need to find new methods that can ensure the formation of quantum and non-linear metamaterials with higher resolution. One such method of producing metamaterials in the future, which will provide scalability and availability, is chemical synthesis. Meanwhile, the chemical synthesis of organized 3D structures with a period of a few nanometres and a size of up to a few millimetres is not an easy task and is yet to be resolved [1,2].
Producing Metamaterials
A most promising avenue seems to be the use of highly porous structures based on metal–organic frameworks (MOFs) [3,4]. Scientists have only recently turned their attention to the study of the optical properties of MOFs. Over the past two decades, MOFs were extensively investigated with a main focus on potential applications in catalysis, separation and storage of gases, chromatography and drug delivery. Recently, however, there is a tendency to employ the unique physical properties of MOF microcrystals for applications as promising non-linear optical and quantum metamaterials. MOFs are porous networks consisting of metal ions or clusters (nodes) which are connected by organic ligands, so-called linkers. Besides the successful combination of synthetic possibilities of organic and inorganic chemistry, which leads to a huge number of potential materials, in some cases post-synthetic processing is also possible, which allows further modification of the resulting material. For example, "soaking" a crystalline MOF in various solvents may eventually lead to a change in the crystal structure, thereby affecting its symmetry. Changing the symmetry of a system can lead to the emergence of certain physical properties, such as piezoelectric, ferroelectric, pyroelectric and even optical [1].
Chemical Synthesis of Metal–Organic Frameworks (MOF)
In a collaboration between ITMO University, St.

Petersburg, Russia, and Leipzig University, Germany, a new crystalline MOF (fig. 1) made of iron(III) ions connected by organic aromatic carboxylato ligands was obtained [5]. Here, a “metallic” part consisting of metal ions with electron conductive properties alternates with dielectric organic bridges resulting in dielectric channels on the nanoscale. This material shows strong nonlinear optical effects potentially common to metamaterials: specific man-made structures that are capable of controlling the propagation of light in extraordinary ways. Specifically, a birefringence of 0.17 < Δn < 0.68 in the optically visible range was observed, which is enormous compared to those of commercially available materials. Whereas metamaterials are normally made by complex manufacturing processes, this new material was the result of chemical synthesis. Furthermore, it can be assumed that the birefringence can be further increased if the crystal axes can be aligned in a strictly perpendicular direction to the light source.

Optical Properties
The optical properties of this new material could aid the development of a new class of polarizers, that are often used for visualization of 3D objects. This results in much more interesting and useful properties than previously thought. One of the unique outcomes of this study, for instance, was the demonstration of a giant birefringence over the entire visible range. This effect is widely used in devices to control the light polarization (in polarizers or polarization splitters), as well as in nonlinear crystals to transform optical frequencies. Thus, crystals can convert a beam of light of undefined or mixed polarization into a beam of well-defined polarization. The resulting effect is that the light passing through the crystal forms two identical images, one of which is shifted in parallel relative to the other (fig. 2). Therefore, a polarization change is observed when the ordinary and extraordinary rays travel along one path, but with different velocities [6]. 
Typically, the effect of birefringence is used in optical communication systems such as fibre optical isolators, circulators, beam displacers, Glam polarizers and other polarizing devices. Furthermore, polarizers have long been firmly established in the field of chemistry for the study of chiral molecules. Polarizers also play an important role in materials science in studies on structural defects.
The optical materials used at the moment, such as calcite, beryl, quartz, rutile, sapphire and others, are naturally occurring crystals; however, industrial production has some problems. For example, the most famous material, calcite, for which the phenomenon of birefringence was discovered, is obtained under hydrothermal conditions at high pressure [7], but this method is not advantageous due to the very low yield. In contrast, the advantage of MOFs is rapid crystal growth, which furthermore does not require constant monitoring. 
Conclusions and Outlook
A transition from physical methods of production to chemical ones would allow simplification of the process for obtaining specific materials, as well as diversification of their final architectures. Attempts to produce metamaterials by chemical synthesis have been made, but it appears that this not at all simple. However, more recently, it was possible to demonstrate that potential metamaterials can be obtained by chemical methods. In this respect, MOFs are very promising for obtaining metamaterials by chemical methods. However, for introducing MOFs as promising materials for use in optical devices, processes for obtaining sufficiently large single crystals that exhibit the physical characteristics required for applications must be developed.
L. R. Mingabudinova1, A. V. Vinogradov1, E. Hey-Hawkins2


1 ITMO University, St. Petersburg, Russian Federation 
2 Leipzig University, Faculty of Chemistry and Mineralogy, Institute of Inorganic Chemistry, Leipzig, Germany 
Leipzig University
Faculty of Chemistry and Mineralogy
Institute of Inorganic Chemistry
Leipzig, Germany


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