Showing papers by "Christian V. Stevens published in 2020"

Strongly Reducing (Diarylamino)benzene-Based Covalent Organic Framework for Metal-Free Visible Light Photocatalytic H2O2 Generation.

Abstract: Photocatalytic reduction of molecular oxygen is a promising route toward sustainable production of hydrogen peroxide (H2O2). This challenging process requires photoactive semiconductors enabling so...

Elucidating the promotional effect of a covalent triazine framework in aerobic oxidation

Abstract: Synergistic catalysis holds great promise to enhance the catalytic performance of heterogeneous catalysts suffering from sluggish reaction kinetics. Much effort has been dedicated to the development of bimetallic systems in which the two promoter elements display synergistic benefits compared to monometallic counterparts. However, the use of bimetallic catalysts inescapably raises the cost of preparation and environmental issues. This study discovers a synergistic effect when using a bipyridine covalent triazine framework (bipy-CTF) as support for an IrIII complex in the aerobic oxidation reaction. The detailed mechanistic study provides insights into the function of the bipy-CTF in this synergistic catalysis. The EPR and in-situ XANES analyses confirm the applicability of bipy-CTF to activate oxygen and alcohol, resulting in an enhancement of the performance of the IrIII complex to exceed the activity of the homogeneous counterpart. This is an unprecedented report on promoting the activity of a heterogeneous catalyst through its solid support.

Artificial intelligence for computer-aided synthesis in flow : analysis and selection of reaction components

Abstract: Computer-aided synthesis has received much attention in recent years. It is a challenging topic in itself, due to the high dimensionality of chemical and reaction space. It becomes even more challenging when the aim is to suggest syntheses that can be performed in continuous flow. Though continuous flow offers many potential benefits, not all reactions are suited to be operated continuously. In this work, three machine learning models have been developed to provide an assessment of whether a given reaction may benefit from continuous operation, what the likelihood of success in continuous flow is for a certain set of reaction components (i.e. reactants, reagents, solvents, catalysts, and products) and, if the likelihood of success is low, which alternative reaction components can be considered. The first model uses an abstract version of a reaction template, obtained via gaussian mixture modelling, to quantify its relative increase in publishing frequency in continuous flow, without relying on potentially ambiguously defined reaction templates. The second model is an artificial neural network that categorizes feasible and infeasible reaction components with a 75 % success rate. A set of reaction components is considered to be feasible if there is an explicit reference to it being used in continuous synthesis in the database; all other reaction components are considered infeasible. While several cases that are ‘infeasible’ by this definition, are classified as feasible by the neural network, further analysis shows that for many of these cases, it is at least plausible that they are in fact feasible – they simply have not been tested to (dis)prove this. The final model suggests alternative continuous flow components with a top-1 accuracy of 95%. Combined, they offer a black-box evaluation of whether a reaction and a set of reaction components can be considered promising for continuous syntheses.

Single-molecule lamellar hydrogels from bolaform microbial glucolipids.

Abstract: Lipid lamellar hydrogels are rare soft fluids composed of a phospholipid lamellar phase instead of fibrillar networks. The mechanical properties of these materials are controlled by defects, induced by local accumulation of a polymer or surfactant in a classical lipid bilayer. Herein we report a new class of lipid lamellar hydrogels composed of one single bolaform glycosylated lipid obtained by fermentation. The lipid is self-organized into flat interdigitated membranes, stabilized by electrostatic repulsive forces and stacked in micrometer-sized lamellar domains. The defects in the membranes and the interconnection of the lamellar domains are responsible, from the nano- to the micrometer scales, for the elastic properties of the hydrogels. The lamellar structure is probed by combining small angle X-ray and neutron scattering (SAXS, SANS), the defect-rich lamellar domains are visualized by polarized light microscopy while the elastic properties are studied by oscillatory rheology. The latter show that both storage G' and loss G'' moduli scale as a weak power-law of the frequency, that can be fitted with fractional rheology models. The hydrogels possess rheo-thinning properties with second-scale recovery. We also show that ionic strength is not only necessary, as one could expect, to control the interactions in the lamellar phase but, most importantly, it directly controls the elastic properties of the lamellar gels.

N-Rich Porous Polymer with Isolated Tb3+-Ions Displays Unique Temperature Dependent Behavior through the Absence of Thermal Quenching

Abstract: The challenge of measuring fast moving or small scale samples is based on the absence of contact between sample and sensor. Grafting lanthanides onto hybrid materials arises as one of the most promising accurate techniques to obtain noninvasive thermometers. In this work, a novel bipyridine based porous organic polymer (bpyDAT POP) was investigated as temperature sensor after grafting with Eu(acac)(3)and Tb(acac)(3)complexes. The bpyDAT POP successfully showed temperature-dependent behavior in the 10-310 K range, proving the potential of amorphous, porous organic frameworks. We observed unique temperature dependent behavior. More intriguingly, instead of the standard observed change in emission as a result of a change in temperature for both Eu(3+)and Tb3+, the emission spectrum of Tb(3+)remained constant. This work provides framework- and energy-based explanations for the observed phenomenon. The conjugation in the bpyDAT POP framework is interrupted, creating energetically isolated Tb(3+)environments. Energy transfer from Tb(3+)to Eu(3+)is therefore absent, nor energy back transfer from Tb(3+)to bpyDAT POP ligand (i.e. no thermal quenching) is detected.

Chemical Genetics Approach Identifies Abnormal Inflorescence Meristem 1 as a Putative Target of a Novel Sulfonamide That Protects Catalase2-Deficient Arabidopsis against Photorespiratory Stress.

Abstract: Alterations of hydrogen peroxide (H2O2) levels have a profound impact on numerous signaling cascades orchestrating plant growth, development, and stress signaling, including programmed cell death. To expand the repertoire of known molecular mechanisms implicated in H2O2 signaling, we performed a forward chemical screen to identify small molecules that could alleviate the photorespiratory-induced cell death phenotype of Arabidopsisthaliana mutants lacking H2O2-scavenging capacity by peroxisomal catalase2. Here, we report the characterization of pakerine, an m-sulfamoyl benzamide from the sulfonamide family. Pakerine alleviates the cell death phenotype of cat2 mutants exposed to photorespiration-promoting conditions and delays dark-induced senescence in wild-type Arabidopsis leaves. By using a combination of transcriptomics, metabolomics, and affinity purification, we identified abnormal inflorescence meristem 1 (AIM1) as a putative protein target of pakerine. AIM1 is a 3-hydroxyacyl-CoA dehydrogenase involved in fatty acid β-oxidation that contributes to jasmonic acid (JA) and salicylic acid (SA) biosynthesis. Whereas intact JA biosynthesis was not required for pakerine bioactivity, our results point toward a role for β-oxidation-dependent SA production in the execution of H2O2-mediated cell death.

Structural and Photophysical Properties of Various Polypyridyl Ligands: A Combined Experimental and Computational Study

Abstract: Covalent triazine frameworks (CTFs) with polypyridyl ligands are very promising supports to anchor photocatalytic complexes. Herein, we investigate the photophysical properties of a series of ligands which vary by the extent of the aromatic system, the nitrogen content and their topologies to aid in selecting interesting building blocks for CTFs. Interestingly, some linkers have a rotational degree of freedom, allowing both a trans and cis structure, where only the latter allows anchoring. Therefore, the influence of the dihedral angle on the UV-Vis spectrum is studied. The photophysical properties are investigated by a combined computational and experimental study. Theoretically, both static and molecular dynamics simulations are performed to deduce ground- and excited state properties based on density functional theory (DFT) and time-dependent DFT. The position of the main absorption peak shifts towards higher wavelengths for an increased size of the pi-system and a higher pi-electron deficiency. We found that the position of the main absorption peak among the different ligands studied in this work can amount to 271 nm; which has a significant impact on the photophysical properties of the ligands. This broad range of shifts allows modulation of the electronic structure by varying the ligands and may help in a rational design of efficient photocatalysts.

Synthesis and self-assembly of aminyl and alkynyl substituted sophorolipids.

Abstract: Sophorolipids are one of the most important microbial biosurfactants, because of their large-scale production and applications developed so far in the fields of detergency, microbiology, cosmetics or environmental science. However, the structural variety of native sophorolipids is limited/restricted, a limiting fact for the development of new properties and their potential applications. In their open acidic form, C18:1 sophorolipids (SL) are classically composed of a sophorose headgroup and a carboxylic acid (COOH) end-group. The carboxyl group gives them unique pH-responsive properties, but they are a poorly-reactive group and their charge can only be negative. To develop a new generation of pH-responsive, positively-charged, SL and to improve their reactivity for further functionalization, we develop here SLs with an amine (-NH2) or terminal alkyne (-C$ ot\equiv$CH) end-group analogues. The amine group generates positively-charged SL and is more reactive than carboxylic acids, e.g. towards aldehydes; the alkyne group provides access to copper-based click chemistry. In this work, we synthesize (C18:1) and (C18:0) --NH2 and (C18:1) -C$ ot\equiv$CH sophorolipid derivatives and we study their self-assembly properties in response to pH and/or temperature changes by means of static and dynamic light scattering, small angle (X-ray, neutron) scattering and cryogenic electron microscopy. Monounsaturated aminyl SL-C18:1-NH2 sophorolipids form a micellar phase in their neutral form at high pH and a mixed micellar-bilayer phase in their positively-charged form at low pH. Saturated aminyl SL-C18:0-NH2 sophorolipids form a micellar phase in their charged form at low pH and a twisted ribbon phase in their neutral form at high pH and monounsaturated alkynyl SL-C18:1-C$ ot\equiv$CH sophorolipids form a main micellar phase at T> 51.8{\textdegree}C and a twisted ribbon phase at T< 51.8 {\textdegree}C.

Experimental study on the microreactor-assisted synthesis of phosphinic chlorides with varying steric hindrance

Abstract: An efficient and catalyst-free method for the preparation of optically active and racemic mono-substituted t-butyl phosphinic chlorides tBuRP(O)Cl under flow conditions was reported. A variety of mono t-butyl substituted phosphinic chlorides were obtained using this protocol starting from the corresponding phosphine oxide and one equivalent of carbon tetrachloride (CAUTION: hepatotoxic) with reasonable residence times (25–125 min) and excellent conversions (up to 99%). The asymmetric reaction conducted in a glass microreactor chip with an internal volume of 250 μL leads to the corresponding chloride with 96% enantiomeric excess. It is significant that the protocol works effectively when the phosphine oxide has one bulky group such as t-butyl, which prevent the formation of undesired products. The steric hindrance is proven to be important for the stabilization of the P–Cl products. The key results were compared with the results obtained in batch conditions and it can be concluded that the flow method provides a sustainable, efficient alternative to the existing methods for the preparation of phosphinic chlorides. The isolation of the reaction products is straightforward because of the lack of any additives and the high purity of the obtained products. The results of the asymmetric reaction and the computational studies suggest that the reaction occurs through a mechanism involving a pentacoordinated phosphorus TS, with the apical positions occupied by the incoming CCl3− nucleophile and the t-butyl group.