Novel functionalization methods of 1D and 2D materials and their applications

Resumen

In the last decades, new materials with excellent electronic, mechanical and optical properties have been discovered and studied, such as single-walled carbon nanotubes (SWNTs), formed by carbon atoms arrange in a cylindrical hexagonal network and Transition Metal Dichalcogenides (TMDCs), constituted by layers connected by Van der Waals interactions of a transition metal with two chalcogenide atoms (usually S, Se and Te) covalently bonded. These materials have unique properties: in the case of SWNTs, some of them are semiconductors and used in devices. Likewise, TMDCs, such as WS2 and MoS2, are photoluminescence and used as photodetectors. However, in both cases obtaining these properties and applying them is a problem due to the difficulty to handle and/or obtaining: for instance, SWNTs are insoluble in any solvent and tend to aggregate with each other; and TMDC, it is necessary to exfoliate them because the most important properties are only obtained in the monolayer form. To take advantage of all these incredible properties, it is necessary to modify and improve the methods of handling them. In this point the chemistry field appears. This field is responsible for the chemical functionalization in order to improve the properties and increase the manageability. In general terms, the chemistry has had two methods to functionalize materials: through non-covalent interaction, for instance Van der Waals interaction as π-π interactions, and covalent functionalization with covalent bonds. Chapters 1 and 2 focus on carbon nanotubes and their functionalization through mechanical bond (forming species called MINTs) and their applicability in catalysis, due to their electron-giving character and their long active surface. First of all, Mechanically Interlocked Nanotubes (MINTs) are species created by encapsulating SWNTs with macrocycles by ring closing metathesis (RCM) between two alkenes, thus forming a mechanical bond between them. This procedure takes advantages of the two methods of functionalization, forming stable species without modifying the structure of the nanotube. Specifically in chapter 1, with the synthesis of different MINTs (AQ, exTTF and pyr) and their use as catalysts for the nitroarenes reduction reaction, is possible to appreciate an electronic effect of the macrocycle in the SWNTs and a regulation of the catalytic activity: either enhancing it (n-doping) or slowing it down (p-doping). With these results, it is possible to regulate the catalytic behaviour of SWNTs by encapsulating macrocycles by mechanical bond (MINT) without damaging their structure and in non-allosteric pathway. In the chapter 2 the advantages of joining together two electrochemically active species, such as AQ and SWNT by mechanical bonding (MINT-AQ) are shown. Using MINT-AQ as an electrode reveals great electrochemical stability in both aqueous and organic solvents with respect to the isolated species of AQ. In addition, MINT-AQ offers an improvement in the oxygen reduction reaction (ORR) over SWNT. Chapters 3 and 4 focus on the functionalization of TMDCs (MoS2 and WS2) with maleimides through thiol-ene “click” reaction. The thiol-ene reaction is a “click” reaction between thiol group and olefin yielding S-C bond. This reaction is characterized by being robust, efficient and orthogonal. In addition, they are versatile reactions which the reaction conditions are mild. Bring together all of these characteristics, it is widely used in multiple fields such as organic synthesis, biochemistry, polymers and chemistry of materials. There are several kinds of thiol-ene reaction depending on how the bond S-H from thiol is broken, but we will focus on the nucleophile method where thiolate are generated thanks to a base. Specifically, using α,β- unsaturated carbonyls (electron poor enes) is called thiol-Michael Addition. An example of these molecules is the maleimide group. This molecule is characterized by having a electron poor double bond capable of reacting with soft nucleophiles such as thiols. In fact, these molecules react orthogonally and specifically with these groups, offering a great applicability in biology field with cysteine groups and in polymer chemistry. In the chapter 3 we have developed a simple method of covalent functionalization between TMDCs (MoS2 and WS2) and maleimides via thiol-Michael Addition in mild conditions where the structure of TMDCs is preserved, exploiting the soft nucleophilicity of sulfur atom. Extensive characterization proves that the reaction occurs through Michael addition. In the chapter 4, we have described an exhaustive study of the covalent functionalization of MoS2 with maleimides via thiol-Michael Addition, previously described in the chapter 3. During this study of conditions, we have discovered that, in the presence of base, MoS2 is functionalized with a polymer generated from N-benzylmaleimide. In contrast, in the absence of base, it is functionalized with isolated N-benzylmaleimide molecules. In addition, we have performed a complete study of the reaction changing temperature, time, different kind of solvent and using different maleimides derivatives.