Lanthanide coordination polymeric compounds hitting new niches

by H.Sara

The development and preparation of coordination polymers have been getting more attention over the past few decades, not only for their highly controlled and intriguing topologies but also their possible applications within the areas of luminescent temperature sensing useful nonlinear optical properties and optoelectronics all over the world. Despite the remarkable progress made in the lanthanide coordination polymers field, it is an enormous challenge to develop more efficient luminescent materials in an orderly and systematic manner.

In recent years an enormous amount of research has been given to the development and synthesis of temperature-sensitive materials in response to various demands for temperature within industrial procedures, as well as environmental monitors, and maintenance of equipment. All of these require accurate temperature control. A number of studies have been carried out in the direction of rational development and development of temperature-sensitive materials made of organic metal oxides as well as inorganic polymers. These sensors were developed to measure temperature in “real-world applications” but they are not without certain limitations. In the instance of inorganic materials, long response times are regarded as an issue. For organic polymers, the major drawbacks are instability and hydrophobicity. Additionally, these materials have always been afflicted by a lack of active sites that are hydrophilic, though the practical temperature sensing technique is dependent on the huge number of active sites that allow the substance to soak up more water species and, in turn, increase the sensitiveness. Thus, the development of new lanthanide coordination polymers that are characterized by a broad working temperature range and stability, is important in gaining high-performance temperature sensors.

As a category of organic-inorganic hybrid materials coordination polymers with the limitless options in metal centers and organic linkers make use of the advantageous characteristics of both conventional building blocks. The inorganic component is usually associated with stability, while organic gives materials the ability to be processed. Recently, more is being done on the design and synthesis of lanthanide coordination polymers to be used as active material due to their advantages that include functional groups that are tunable, various size pores, and even non-active sites that are ideal in temperature sensing. Different temperature sensors made of coordination materials have been developed successfully that are primarily based on the working principles of mechanical and thermal stimuli. However, there are only a few studies regarding the production of lanthanide coordination polymers as temperature sensing materials due because the majority of metal-organic assemblies are quite reactive materials. A sensor that is developed for this purpose would be advantageous, as it could benefit from the advantages in the simple process of circuit design, low-cost devices, continuous monitoring and a high-performance performance.

As one of the possible applications, luminescent-based sensing that is based on coordination polymers is a promising option due to its fast response time high sensitivity, simple and also is cost-effectiveness. This technique has been utilized to identify various molecules as well as heavier metal ions (optical heavy-metal ion sensing), and so on. Some of them have been found to be extremely toxic damaging the environment and public health. A completely different thing is sensing temperature. As of now, there are many reports in this area, but the majority of reported sensors focus on lanthanide coordination polymers but the equally high number of reports touch upon materials that comprise transition-metal centers. Another area that is explored so much is related to the intriguing field of nonlinear optical properties of coordination polymers and their applications.

Initially, the nonlinear optical community reported a lot of reports on the nonlinear absorption of coordination polymer. However, the main pitfall here is that researchers employed the Z-scan technique to examine the third-order nonlinear optical properties of these compounds in solution. Such an approach removed a lot of information on effects that influence properties arising from their microcrystalline form, however demonstrated potential that lies in their third-order nonlinearities. Indeed, some examples showed truly outstanding optical limiting performances. Nonlinear optical properties typically observed for solid-state samples, like third- and second harmonic generation, are used in numerous applications in the optical industry, optoelectronics, as well as high-resolution optical microscopy of biological matter. In general, third-harmonic generation can be found in all materials, regardless of their symmetry i.e. regardless of whether the material is non-centrosymmetric or is centrosymmetric. Different structural requirements exist for second harmonic generation, which can only be observed in non-centrosymmetric substances, which include chiral compounds as well. So far it was the second-harmonic generation that was the most investigated property in non-centrosymmetric coordination polymers, however recently nonlinear absorption is getting more and more attention.

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