P. Van der Voort

Solid-state nuclear magnetic resonance spectroscopy is routinely used in the field of covalent organic frameworks to elucidate or confirm the structure of the synthesized samples and to understand dynamic phenomena. Typically this involves the interpretation and simulation of the spectra through the assumption of symmetry elements of the building units, hinging on the correct assignment of each line shape. To avoid misinterpretation resulting from library-based assignment without a theoretical basis incorporating the impact of the framework, this work proposes a first-principles computational protocol for the assignment of experimental spectra, which exploits the symmetry of the underlying building blocks for computational feasibility. In this way, this protocol accommodates the validation of previous experimental assignments and can serve to complement new NMR measurements.

Porous organic polymers (POPs), and especially covalent triazine frameworks (CTFs), are being developed as the next generation of metal-free heterogeneous photocatalysts. However, many of the current synthetic routes to obtain these photoactive POPs require expensive monomers and rely on precious metal catalysts, thus hindering their widespread implementation. In this work, a range of POPs was synthesized from simple unfunctionalized aromatic building blocks, through Lewis acidcatalyzed polymerization. The obtained materials were applied, for the first time, as heterogeneous photocatalysts for the aromatization of N-heterocycles. With the use of the most active material, denoted as CTF-Pyr, which consists of photoactive pyrene and triazine moieties, a wide range of pyridines, dihydroquinoline-5-ones, tetrahydroacridine-1,8-diones and pyrazoles were obtained in excellent yields (70-99%). Moreover, these reactions were carried out under very mild conditions using air and at room temperature, highlighting the potential of these materials as catalysts for green transformations.

The production of hydrogen via electrocatalytic reduction of water using metal-free nanomaterials as the catalyst is a promising and ultimate green approach. Graphitic carbon nitride, covalent organic frameworks, and covalent triazine frameworks (CTFs) are some of the nanostructured materials that are investigated for this purpose. Currently, these materials still lack the efficiency to compete with other techniques (electrolysis). This is because the reaction mechanism and active sites are, in many cases, still poorly understood. In this work, we report a set of metal-free nanostructure-based electrocatalysts, phosphorus covalent triazine frameworks (PCTFs), for electrocatalytic hydrogen production. The hydrogen evolution reaction (HER) performance of PCTF-based nanomaterials is ascribed to the synergistic effect of isolated single nitrogen and phosphorus sites on the large surface area. By combining both experimental and theoretical studies, we found that especially the pyridinic-nitrogen species are the most active sites for the HER. The presence of phosphorus next to the pyridinic-N enhances the HERs. The present results provide a better understanding of the importance of different heteroatoms in nanomaterials as active sites in HERs. Theoretical studies confirmed that phosphorus, being electron rich, creates high electron densities on the nearby N atoms of the CTF materials and intensifies the HER process.

Four highly porous covalent organic frameworks (COFs) containing pyrene units were prepared and explored for photocatalytic H2O2 production. The experimental studies are complemented by density functional theory calculations, proving that the pyrene unit is more active for H2O2 production than the bipyridine and (diarylamino)benzene units reported previously. H2O2 decomposition experiments verified that the distribution of pyrene units over a large surface area of COFs plays an important role in catalytic performance. The Py-Py-COF, though contains more pyrene units than other COFs, induces a high H2O2 decomposition due to a dense concentration of pyrene in small proximity over a limited surface area. Therefore, a two-phase reaction system (water-benzyl alcohol) was employed to inhibit H2O2 decomposition. This is the first report on applying pyrene-based COFs in a two-phase system for photocatalytic H2O2 generation.