Insights into the photocatalytic behavior of extended organic materials

Resumen

Covalent Organic Frameworks (COFs) are porous and crystalline materials formed from the assembly of building blocks through covalent bonds of different nature. The formation of these covalent bonds usually presents a certain degree of reversibility, which gives rise to more crystalline and porous structures due to the dynamic correction of structural errors. Similarly, Covalent Triazine Frameworks (CTFs) are built from the formation of triazine rings, but the low reversibility of this process means that, although they present porosity, they are not usually crystalline. Being composed of organic fragments, the structure of new COFs and CTFs can be predicted and designed, thus adapting them to a specific application. For this reason, although they are still at an early age, their popularity in different fields such as adsorption and gas separation or catalysis is increasing. In this doctoral thesis, we have focused on different design strategies for new porous organic materials such as COFs and CTFs for their application as heterogeneous catalysts in a series of organic transformations. Specifically, with the exception of those exposed in chapter 3, all of them are mediated by visible light, which is why they belong to the field of photocatalysis. Therefore, in the first chapter, the general design strategies of these materials are presented, as well as a selection of examples of COFs and CTFs used as heterogeneous photocatalysts in organic reactions. In chapter 2, the covalent functionalization of a simple COF based on imine bonds with Pt(II)-hydroxyquinoline photocatalytic complexes is proposed. This covalent functionalization is achieved through monomer truncation strategy, which consists of punctually altering the structure of the starting monomers that constitute the COF in order to introduce covalent defects in a random manner. The introduction of these complexes in the COF structure results in a dramatic increase in its photocatalytic activity and stability in selective oxidation of sulfides and reduction of organic bromides. This increase in its activity and stability is due to the covalent immobilization and isolation of Pt(II) centers within a porous structure, which avoids deactivation and aggregation pathways that are commonly observed in homogeneous systems. In chapters 3 and 4, the obtention of a imine-linked COF that contains phenanthroline ligands as building blocks (Phen-COF) in its structure is proposed. This material allows the coordination of a variety of metallic molecular centers, giving rise to highly stable, versatile and active heterogeneous systems for different synthetic processes. Specifically, in the third chapter its functionalization with palladium (II) acetate is proposed. This constitutes a simple model system to study the potential of this material as a heterogeneous ligand in cross-coupling reactions with great historical importance, such as the Suzuki-Miyaura or Mizoroki-Heck reactions. In addition, it serves as a starting point for the study of more complex processes, reflected in chapter 4. In this chapter, the functionalization of the material with Ir(III) and Ni(II) complexes allows carrying out dual C(sp3)-C(sp2) couplings mediated by visible light, using different radical precursors and obtaining high TON values. Inspired by the importance of high nitrogen content in organic materials such as COF and CTF observed in the literature, in chapter 5 the design of a pristine CTF with suitable photophysical properties for reduction reactions is proposed. Specifically, the construction of a phenanthroline-based CTF (Phen-CTF) allowed us to carry out different photoreductions of aryl halides. These processes give rise to the corresponding hydrogenated products and, in the presence of suitable radical traps, also to compounds with new C-C, C-P and C-B bonds by capturing the transient aryl radical. Finally, the excellent photoredox activity of Phen-CTF led us to determine what are the structural factors that govern the photocatalytic behavior of a material. To answer this question, an analogous COF was designed that contains the same triazine and phenanthroline fragments as CTF but connected through an imine bond (Phen-Tz-COF). This small variation in the chemical identity of the material triggers large changes in the predominant mechanism. Thus, while Phen-CTF is capable of carrying out electron transfer processes, Phen-Tz-COF presents better activities in energy transfer processes such as olefin photoisomerizations. The support of these observations through theoretical calculations further substantiates the conclusions reached: the imine bond causes a disconnection in electronic delocalization and increases electron-hole recombination, preventing a photoredox mechanism in favor of energy transfer events. Overall, the realization of this doctoral thesis contributes to the design of new heterogeneous systems based on porous organic materials for advanced catalytic applications in modern organic chemistry, with high levels of selectivity, recyclability and sustainability.